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Interpenetrating Nets Ordered Periodic Entanglement

1.Introduction

1.1.Framework Structures

The synthesis and characterization of infinite two-and three-dimensional networks has been an area of rapid growth in recent years.[1±3]The motivation behind much of this activity has been provided by the prospect of generating,by deliberate design,a wide range of purpose-built materials with prede-termined structures and useful properties,for example electronic,magnetic,optical,and catalytic.Implicit or explicit in much of this work has been the idea of self-assembly of specifically designed building blocks.There is still a long way to go in the development of this field before it will be possible to say that these ambitious objectives have been fully realized. Nevertheless,great opportunities for exciting advances are provided by the enormous sweep of chemistry embracing coordination,organic,and main group chemistry,which is at our disposal for creative use in the planned construction of frameworks.

1.2.The Net-Based Approach to

Framework Construction

One conceptual approach to building frameworks is based on the idea of a net.In essence,nets are abstract mathematical entities consisting of a collection of points or nodes with some clearly defined connectivity or topology.A very useful catalogue of nets of relevance to chemistry was compiled by

Interpenetrating Nets:Ordered,Periodic Entanglement

Stuart R.Batten and Richard

Robson*

[*]Dr.R.Robson,Dr.S.R.Batten[ ]

School of Chemistry,University of Melbourne

Parkville,Victoria3052(Australia)

Fax:( 61)3-9347-5180

E-mail:richard±robson@https://www.wendangku.net/doc/694650026.html,.au

[ ]New address:

Chemistry Department,Monash University

Clayton,Victoria3168(Australia)

Fax:( 61)3-9905-4597

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S.R.Batten and R.Robson Wells over twenty years ago.[4]Examples of simple,infinite 2D nets are those given the symbols (6,3)and (4,4)shown in Figures 1and 2,respectively.One way to represent

the Figure 1.Two geometrically different forms of the (6,3)net.topology or connectivity of a given net is in terms of the general symbol (n ,p ),where p is the number of connections to neighboring nodes that radiate from any center or node,and n is the number of nodes in the smallest closed circuits in the net.Thus,the number 6in the symbol (6,3)indicates that the smallest complete circuits in the net are hexagons,and the number 3indicates that each node is connected to three other nodes.The (n ,p )notation applies strictly only to nets in which all p (p à1)/2shortest circuits originating from any node are n -gons;otherwise the more complete Schl?fli notation n p (p à1)/2should be used.Two of the six shortest circuits around any node in the net shown in Figure 2,whose representation as (4,4)is not strictly correct,in fact involve six nodes,and the complete Schl?fli nota-tion should be 4462;this was simplified by Wells to 44or (4,4)by arbitrarily excluding circuits involving colinear connections.Figure 1shows two versions of the (6,3)net which are geometrically different but topologically identical.This example illus-

trates the general point that nets may be geometrically deformed to any extent and yet,provided no connections are broken,the topology is considered to remain unchanged.Some examples of simple,infinite 3D nets are

*the a -polonium (or NaCl)net with six-connected,octahe-dral nodes,

*the diamond,lonsdalite,quartz,feldspar-related,and zeolite-related nets,all of which have four-connected,essentially tetrahedral nodes,

*the NbO net,which has square-planar nodes with a 908twist along every connection,[5]

*a variety of intriguing and little-known three-connected nets described by Wells,[4]

*a number of nets containing more than one type of node,such as the PtS net which contains equal numbers of tetrahedral and square-planar nodes,the rutile net con-taining octahedral and trigonal nodes,the aPt 3O 4onet with square-planar and trigonal nodes,and the Ge 3N 4net with tetrahedral and trigonal nodes.All of these nets provide realistic targets for crystal engineers.One approach to building frameworks is based on the expectation that if molecular building blocks with a chemical functionality and a stereochemistry appropriate to one of the above target nets can be devised,then simply allowing these specifically organized components to react together in the correct proportions may lead to the spontaneous assembly of the intended network.[1,6]Thus,for the sake of illustration,if we had set our sights on a Ge 3N 4-type net,we would require building blocks with tetrahedral and trigonal dispositions of complementary functional groups.This approach has met with some successDfor example,in generating diamond,[6±8]

PtS,[9,10]and a -polonium [11,12]nets by designDbut outcomes of such attempts to build frameworks are often determined by a very fine balance of complex and subtle effects.Predicting such outcomes simply on the basis of the reactants present in the reaction mixture constitutes a major challenge for the theoretician.In the meantime,the empirical,experimental by X-ray crystallography ligand and metal ion.Particular the University of Bristol and his Ph.D.research.

S.R.Batten R.

Robson Figure 2.The (4,4)net.

REVIEWS Interpenetrating Structures

approach,based largely on sensible design and chemical intuition,has a great deal to offer.It has already revealed much new chemistry,including several unprecedented struc-tural types,and there is every indication that it will continue to do so.The idea of topological equivalence is useful in describing networks.For example the 2D network formed when large numbers of benzene-1,3,5-tricarboxylic acid (trimesic acid)molecules associate through hydrogen bonds of the type seen for acetic acid dimers (Figure 3)is said to be

topologically

Figure 3.A hydrogen-bonded sheet of trimesic acid molecules with the (6,3)topology.

equivalent to or to have the topology of the (6,3)net.[13]Nodes within a net do not necessarily have to coincide with a particular atom,and in the trimesic acid sheets they are located at the centers of the benzene rings.The imaginary process of reducing a real chemical structure to the essence of its connectivity is useful for clarifying this fundamental aspect of its nature:It is particularly useful in understanding and interrelating modes of interpenetration,as we shall see below.

1.3.Voids,Interpenetration,and the Scope of This

Review Examination of models of a range of framework structures constructed from molecular building blocks that might realistically be employed in practice leads to the realization that,for many simple nets,connecting units with rodlike character need not be very long to provide structures with relatively large channels,cavities,and windows.In recent years large numbers of framework solids have been structur-ally characterized;the vast majority make use of either hydrogen bonding [3]or ligand-to-metal coordinative bond-ing [2]to link the individual components.In some cases the

frameworks do generate spacious voids,cavities,and channels,which may account for more than half the volume of the crystal.[6,7,10,14]These large spaces are usually occupied by highly disordered,essentially liquid solvent.In other cases remarkable interpenetrating structures are formed in which the voids associated with one framework are occupied by one or more independent frameworks;an inherent feature of such entangled structures is that they can be disentangled only by breaking internal connections.These interpenetrating framework structures are the sub-ject of this article;until recently examples were rare,[15]but they are now being reported with ever increasing frequency.We have restricted the scope of this review to interpenetrating structures in which the building blocks within the individual infinite networks or chains are linked together through either metal-to-ligand bonds or hydrogen bonds.A further restric-tion is that only ordered,structurally regular,and crystallo-graphically characterized systems will be covered.The inter-penetrating polymer networks (IPNs),[16]which are noncrys-talline materials with rubberlike and plastic properties,are therefore not included.1.4.Molecular Entanglement Infinite interpenetrating structures constitute a subgroup within a broader class of entangled systems,which are the subject of intensive current research.[17]A familiar example is provided by the entangled strands in the DNA double helix.This and other helical biological molecules have provided the inspiration for a rapidly expanding range of double-and triple-helical coordination compounds.[18]Dendrimers are another class of tangled compounds of great current inter-est.[19]Catenanes,rotaxanes,and molecular knots,[20]however,occupy positions of special importance in the area of molecular entanglement,because they can be disentangled only by breaking connections (in contrast,for example,to the DNA double helix,for which it is not necessary to disrupt the individual strands in order to disentangle them).Catenanes consist of independent rings that are locked together (Fig-ure 4a)In rotaxanes a ring component encircles the shank of

an Figure 4.Schematic representation of a)a catenane and b)a rotaxane.independent dumbell-like component (Figure 4b).The two astopsoat the ends of the dumbell-like component prevent the ring from escaping.In pseudorotaxanes one or more rings are threaded onto a molecular astringo,but the stops present in true rotaxanes are lacking.Molecular knots are charac-terized by self-entanglement.

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S.R.Batten and R.Robson 1.5.Some General Points about Interpenetration

Some general points about interpenetrating networks can be illustrated by considering the example of zinc cyanide,whose structure was determined over half a century ago.[6,21,22]It consists of two independent infinite frameworks,each with diamondlike topology.The point symbol for the diamond net is 66-a;the superscript indicates the number of different pairs of connections radiating from one node to its neighbors (i.e.,(4?3)/2).The smallest circuits in which each pair participates are hexagons,which is indicated by the main,nonsuperscripted symbol.The àa indicates that this is the most symmetrical of a number of different possible 66nets.In [Zn(CN)2]the metal centers provide the tetrahedral nodes,and the cyanide groups act as linear bridges between the metal atoms.A characteristic structural motif within the diamond net is the aadamantane unito(Figure 5a).The two independ-ent networks interpenetrate in such a way that

every

Figure 5.a)An adamantane unit of a diamondlike net.b)Interlocked adamantane units of two independent diamondlike nets.c)A cyclohexane-like ring of one net with a rod of the other passing through it;the rod is part of six different hexagons.

tetrahedral node is found at the center of an adamantane unit of the other framework,and the four connections to nearest neighbors project through the four cyclohexane-like windows of the surrounding adamantane unit (Figure 5b).We shall refer to connections between nodes within nets in general as rods.Each rod in [Zn(CN)2]forms part of six distinguishable cyclohexane-like rings and is exclusively associated with one particular cyclohexane-like ring of the other framework through which it passes (Figure 5c).This is catenation on the grand scale.Clearly,the two frameworks can be disen-

tangled only by wholesale breaking of connections.The regions of closest contact between the two independent frameworks are at the midpoint of every cyanide unit,and the closest parts of the other framework are the midpoints of the six equivalent cyanide units that surround it.In this review we shall use the term n -fold interpenetration to describe systems with n independent nets;for example,[Zn(CN)2]displays twofold interpenetration.All interpenetrating network structures can be regarded as infinite,ordered polycatenanes or polyrotaxanes.One of their characteristic features is the repeated,orderly appearance of rings belonging to one framework or chain,through which independent components are inextricably entangled.The very interesting possibility,presently unrealized,in which the individual rings in an infinite polycatenane are themselves finite and molecular would not fall in the category of interpenetrating frameworks as we define it here because the units participating in interpenetration are not infinite.Another example of a structure involving infinite entangle-ment which nevertheless does not fall in the category of an interpenetrating structure is the material of composition [IAuP(C 6H 5)2(CH 2)6P(C 6H 5)2AuI]n .This contains 1D zigzag polymeric chains in which neutral digold monomers are linked together through weak gold ±gold interactions.[23]These chains are then interwoven like warp and weft to generate a clothlike 2D sheet in which every chain passes alternately over and then under an infinite number of other chains (Figure 6).This interesting structure is not interpene-trating in the sense we have reserved for the term because it

is Figure 6.Warp-and-weft structure of [IAuP(C 6H 5)2(CH 2)6P(C 6H 5)2AuI]n .The circles represent in order of decreasing size Au,P ,and C atoms.Phenyl groups and iodine atoms have been omitted for clarity.not necessary,in principle,to break any links within the independent chains in order to disentangle them,nor do the components contain rings through which other components can project and become entangled.For similar reasons we exclude from the general class of interpenetrating frameworks microporous structures with linear polymers trapped in their channels.2.Interpenetrating One-Dimensional Polymers We know of only two examples that can be assigned to this category.Ligand 1affords a coordination polymer of compo-

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Interpenetrating Structures

sition [Cd 2(1)3(NO 3)4],which contains infinite,1D,ladder-like chains of the type shown in Figure 7.[24]From the topo-logical point of view each cadmium atom acts as a T-type three-connected center;its seven-coordinate environ-ment is completed by two bidentate nitrate ligands which are pendant,that is,not an essential part of the connectivity of the network.Each ladderlike chain contains an infinite number of rings,all equivalent,defined by two ladder arungsoand segments of two ladder auprightso.In the interpenetration shown in Figure 7the uprights of four separate ladders pass through each such

ring.Figure 7.Interpenetrating ladderlike chains in [Cd 2(1)3](NO 3)4.The T-type three-connected centers represent Cd atoms.Every ring has the uprights of four separate ladders passing through it.

Ligand 2gives a coordination polymer of composition [Ag 2(2)3](NO 3)2which contains 1D chains of the type

shown in Figure 8.[25]We shall later encounter examples of inter-penetrating structures which can correctly be described either as polyrotaxanes or as polycatenanes;deciding which is the better description is not easy.The mode of interpenetration in [Ag 2(2)3](NO 3)2(shown schematically in Figure 8c),however,is unquestionably of the polyrotaxane type,and catenane associations are simply not present.Each individual chain is interlocked with an infinite number of others by multiple rotaxane associations in which every one of its rodlike segments passes through a ring of another chain and every one of its rings encircles a rod of another chain (Figure 8c).In the first example,[Cd 2(1)3(NO 3)4],the interpenetration produces a 3D interlocked structure,whereas in [Ag 2(2)3]-(NO 3)2a 2D interlocked structure

results.

Figure 8.a)A single chain in [Ag 2(2)3](NO 3)2.The large circles represent Ag atoms,and the smaller circles C and N atoms.b)A polyrotaxane sheet in [Ag 2(2)3](NO 3)2.c)Schematic representation of the multiple rotaxane associations.

3.Interpenetrating Two-Dimensional Networks

Two major categories of interpenetrating 2D networks can be discerned:One we shall call aparallel interpenetrationoand the other ainclined interpenetrationo.For parallel interpenetration to be possible,the individual 2D networks must be corrugated or possess some appropriate element of https://www.wendangku.net/doc/694650026.html,ually two such undulating sheets,but occasionally more,are arranged with their average planes parallelDthis is indeed the case in all presently known examplesDso that each is able to pass through the other an infinite number of times (shown schematically in Figure

9).Figure 9.Parallel interpenetration of undulating sheets.

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S.R.Batten and R.Robson The composite structure resulting from this form of inter-penetration is itself two-dimensional,and these composite sheets are then stacked one on top of another.In inclined interpenetration there are two stacks of sheets,one stack inclined,often perpendicular,to the other (Fig-ure 10).Any particular 2D network layer then has an

infinite

Figure 10.Inclined interpenetration of sheets.

number of inclined layers passing through it.In contrast to parallel interpenetration,each sheet passes through an inclined one just once along a line of intersection of the two planes.Also in contrast to parallel interpenetration,the product of inclined interpenetration is a 3D interlocked structure.With only one exception the individual networks involved in 2D interpenetration,whether of the parallel or inclined types,are based on either the (4,4)or the (6,3)topology.

3.1.Parallel Interpenetration of Two-Dimensional

Frameworks 3.1.1.Two Parallel,Interpenetrating Nets Based on the (6,3)Topology

Four topologically different modes of (6,3)parallel inter-penetration which are crystallographically regular and within which all rings are equivalent are shown in Figure 11.By topologically different we mean that one could be trans-formed into another only by breaking connections and making new ones.If we relax the restriction that all rings be equivalent (e.g.,if we allow some rings to have no part of the other net passing through them)then clearly an infinite number of modes of interpenetration become possible.[Ag(tcm)](tcm à tricyanomethanide,C(CN)à3)contains (6,3)nets whose three-connected nodes are provided by alternating central carbon atoms of the tcm ions and three-coordinate Ag I centers.[26,27]The two independent and iden-tical (6,3)networks are corrugated so that each is able to pass through the other an infinite number of times (Figure 12).This is topologically identical to the interpenetration mode shown in Figure 11a.The corrugations in the sheets which make the interpenetration possible arise from a combination of a distinctly nonplanar,pyramidal geometry at the silver centers and a significant bending at the nitrogen centers.It is possible to make derivatives of [Ag(tcm)]which retain the parallel,interpenetrating,double-sheet structure.[Ag(tcm)(CH 3CN)]has basically the same composite layer structure as [Ag(tcm)],but the acetonitrile molecule is

now

Figure 11.a)±d)Four topologically different modes of parallel interpene-tration of (6,3)

nets.Figure 12.Interpenetrating undulating (6,3)nets in the structure of [Ag(tcm)].The circles represent in order of decreasing size Ag,N,and C atoms.coordinated to the metal,which acquires thereby a tetrahe-dral geometry.[28]The acetonitrile methyl groups are directed generally away from the center of the composite sheet and towards the neighboring sheets,causing an increase in the separation between composite layers.In [Ag(tcm)(L)1/2](L phenazine)almost the same double-interpenetrating [Ag(tcm)]layers are present,but they are now connected together through phenazine molecules that bridge silver atoms in one composite sheet to those in a neighboring composite sheet.[27]Two independent but interpenetrating 3D networks are thus formed.Two examples that involve the interpenetration of 3D nets are described at this point because of the close structural relationship,elaborated below,to the tcm compounds above.[Ag{C(CN)2NO}]contains (6,3)nets in the form of undulating sheets,closely analogous to those in [Ag(tcm)],in which the two nitrile nitrogen atoms and the nitroso oxygen atom of each ligand form dative bonds to the silver atom.[29]These undulating sheets participate in twofold parallel interpene-tration in a fashion almost identical to that seen in [Ag(tcm)].

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Interpenetrating Structures

Dative bonds between the nitroso nitrogen atoms in one composite sheet and silver atoms in an adjacent composite sheet then produce two interpenetrating 3D networks (Fig-ure

13).Figure 13.Structure of [Ag{C(CN)2NO}]in which parallel interpenetrat-ing (6,3)nets are joined together into a 3D structure through bonds between Ag atoms in one sheet and the nitroso N atoms of another.The circles represent in order of decreasing size Ag,O,and C/N atoms.

The series of compounds with variable composition [Cu(NH 3)(py)Ag 3àx Cu x (CN)5]′py (py pyridine)form layers of two (6,3)sheets that interpenetrate in the fashion shown in Figure 11a.[30]The three-connected nodes of the sheets are formed by alternating Cu II centers (which also coordinate to pendant pyridine and ammonia groups to give a five-coordinate center)and Ag I centers,and are connected through CN àand Ag(CN)à2bridges.The three-connected

Ag I atoms and the Ag I atom in one of the two crystallo-graphically independent Ag(CN)à2links are partially substi-tuted by Cu I .Again,there are links between the layers.Each set made up of a trigonally coordinated Ag I /Cu I atom and a carbon atom connected to it has a centrosymmetric dimeric partnership with a corresponding pair of atoms in an adjoining layer.The tetrahedral coordination geometry of the metal atom is completed by a carbon atom in the adjoining layer;short metal ±metal contacts are seen (2.641(1)and 2.791(3) in the two phases studied).These interactions produce two interpenetrating 3D nets with a similar topology to that seen in [Ag(tcm)(L)1/2](L phenazine).[Ag{CH 3COCH 2C(CN)2C(CN)2}],produced by the unex-pected reaction of tetracyanoethylene with [Ag(CF 3SO 3)]in H 2O/Me 2CO,has layers of two interpenetrating (6,3)nets in which each ligand is coordinated through three of the nitrile nitrogen atoms to three-connected silver atoms.[31]The inter-penetration of the sheets is topologically identical to that shown in Figure 11a.The semiconductor q -(BEDT-TTF)2[Cu 2(CN){N(CN)2}2](BEDT-TTF bis(ethylenedithio)tetrathiafulvalene)con-

tains anionic [Cu I 2(CN){N(CN)2}2]àlayers with intercalated [(BEDT-TTF)2] ions.[32]The anionic layers consist of two independent and interpenetrating (6,3)nets in which Cu I

provides the three-connected nodes and CN àand N(CN)à2act as two-connectors.A pronounced bend at the central nitrogen atom in N(CN)à2provides the undulating character to each individual sheet that makes the interpenetration possible (Figure 14).This mode of interpenetration is topo-logically identical to that represented schematically in Fig-ure 11b,and is intrinsically different from that observed in the previous

examples.

Figure 14.The parallel interpenetration of (6,3)nets in the [Cu 2(CN){N(CN)2}2]àpolyanion,which is topologically identical to that shown in Figure 11b.The large circles represent Cu atoms,and small circles C and N atoms.The bis(imidazole)ligand 2combines with zinc nitrate to give a coordination polymer of composition [Zn(2)2](NO 3)2′4.5H 2O,which contains layers comprising two independent 2D nets that interpenetrate in the parallel manner;[33]the individual nets,represented schematically in Figure 15a,

are Figure 15.a)Schematic representation of a single 2D net in [Zn(2)2](NO 3)2′4.5H 2O.Zn atoms provide the four-connected nodes,and molecules of 2the connections between nodes.b)Schematic representation of the two interpenetrating nets in [Zn(2)2](NO 3)2′4.5H 2O.c)Schematic representation of the polyrotaxane columns.based on the (6,3)topology,but a second bridging ligand is present within certain pairs of zinc nodes.The zinc atoms thus become four-coordinate.Two such nets then interpenetrate in the manner represented schematically in Figure 15b to generate polyrotaxane columns (Figure 15c).This structure is closely related to that of [Ag 2(2)3](NO 3)2(Figure 8),which unquestionably is a polyrotaxane.However,[Zn(2)2](NO 3)2′4.5H 2O could equally well be considered as a polycatenane,because the smallest rings of one net are involved in catenane associations with the second-smallest rings (Zn 6)of the other net.There is little to be gained by argument as to which

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S.R.Batten and R.Robson description should take precedence;the structure has both polyrotaxane and polycatenane character.

3.1.2.Two Parallel Interpenetrating Nets Based on the (4,4)Topology

A close analogue of the mode of interpenetration seen in [Ag(tcm)]is provided by [Cd(4-pic)2{Ag(CN)2}2](4-pic)(4-pic 4-methylpyridine),which involves two corrugated (4,4)nets (Figure 16)rather than the two (6,3)nets in

[Ag(tcm)]

Figure 16.a)Parallel interpenetrating (4,4)nets in the structure of [Cd(4-pic)2{Ag(CN)2}2](4-pic)(4-pic 4-methylpyridine).The pendant 4-pic molecule is omitted.Therefore,the six-coordinate Cd atoms appear to be four-connected.Each Cd atom is connected to four others through bridging Ag(CN)à2units.The circles represent in order of decreasing size Cd,Ag,and C/N atoms.b)Schematic representation of the mode of interpenetra-tion.

(Figure 12).[34]The cadmium centers,all of which are equiv-alent,are four-connected nodes;they are coordinated by two mutually trans picoline molecules,which can be regarded as mere appendages to the 2D net,and by a roughly square-planar set of nitrogen atoms from four two-connected Ag(CN)à2units.Each (4,4)net is corrugated with its troughs and crests parallel with one diagonal of the Cd 4rhombuses within the net.Two nets then interpenetrate as shown in Figure 16.A topologically different mode of parallel interpenetration of two (4,4)nets is found in [Cu(tcm)(bipy)](bipy 4,4'-bipyridine),in which the tcm ligands act as bent two-connected units (Figure 17).[28]

Clusters of composition [Re 4(CO)12(OH)4],which have a cubanelike arrangement of four Re(CO)3units and four m 3-OH groups at alternating cube corners,cocrystallize with 4,4'-bipyridine to yield crystals of composition Z ′2bipy ′2MeOH (Z [Re 4(CO)12(OH)4])containing hydrogen-bonded (4,4)nets.[35]The four-connected nodes of the net are provided by Z units,which are connected to one another through their OH groups by the hydrogen-bonded system Z(OH)/CH 3OH/bipy/(HO)Z.Two such (4,4)nets interpenetrate in the manner shown schematically in Figure 18.This mode of

interpenetra-

Figure 17.a)Two parallel interpenetrating (4,4)nets in the structure of [Cu(tcm)(bipy)].The large circles represent Cu atoms,and the small circles C and N atoms.b)Schematic representation of the mode of interpenetra-

tion.Figure 18.Schematic representation of the two parallel interpenetrating hydrogen-bonded (4,4)nets in the structure of [Re 4(CO)12(OH)4]′2bipy ′2MeOH.The four-connected nodes shown represent the centers of the Re 4clusters.tion is topologically different from that in [Cd(4-pic)2-{Ag(CN)2}2](4-pic)and [Cu(tcm)(bipy)],as can be appreci-ated by comparison of Figure 18with Figures 16b and 17b;in all of these Figures the structure has been reduced to its topological essence.Answers to questions concerning the number of different ways two (4,4)nets could theoretically interpenetrate in the parallel fashion must await systematic mathematical treatment of the topology of interpenetration.Such an investigation is highly desirable,not only with regard to (4,4)nets,but in general.

REVIEWS Interpenetrating Structures

4,4'-Sulfonyldiphenol crystallizes to give hydrogen-bonded (4,4)nets that interpenetrate in the parallel manner shown in Figure 19.[36]This mode of interpenetration is topologically identical to that in [Cu(tcm)(bipy)](Figure

17).

Figure 19.Two parallel interpenetrating hydrogen-bonded (4,4)nets in the structure of 4,4'-sulfonyldiphenol.The circles represent in order of decreasing size S,O,and C atoms.The thin lines represent hydrogen bonds.

The coordination polymer formed by the ligand N ,N '-p -phenylenedimethylenebis(pyridin-4-one)(3)of composition [Mn(3)3](ClO 4)2consists of two (4,4)-based nets that inter-penetrate in the parallel fashion.[37]As in the case

[Zn(2)2](NO 3)2′4.5H 2O considered in Section 3.1.1,a second bridging ligand is present between certain Mn nodes,whereby Mn 2(3)2rings are formed and the metal centers become six-connected.A schematic representation of the polyrotaxane/polycatenane fashion in which they interpenetrate is shown in Figure https://www.wendangku.net/doc/694650026.html,pounds [Ag 2(2)3](NO 3)2,[Zn(2)](NO 3)2

′

Figure 20.Schematic representation of the two parallel interpenetrating (4,4)nets in [Mn(3)3](ClO 4)2showing the polyrotaxane/polycatenane associations.Six-connected nodes are provided by six-coordinate Mn,and connections between the nodes by ligand 3.

4.5H 2O,and [Mn(3)3](ClO 4)2can be considered as members of a progression in which the connectivity of the metal center increases from three to four to six;in all cases (metal)2-(ligand)2rings participate in rotaxane interactions.3.1.3.Two Interpenetrating Two-Dimensional 8210Nets:Hittorf s Phosphorus The allotrope of phosphorus known as Hittorf s phosphorus is the only example of 2D parallel interpenetration involving nets other those based on (6,3)and (4,4)topologies.Two 2D 8210nets interpenetrate in the manner represented schemati-cally in Figure 21a.[38]The three-connected nodes of the

net Figure 21.a)Schematic representation of the interpenetration of two 2D 8210nets in Hittorf s phosphorus.b)Representation of the columnar rods which constitute the long connections between three-connected phospho-rus nodes in a).The open bonds in b)represent direct P ±P covalent bonds between the three-connected phosphorus nodes of the rod shown to three adjoining rods running at right angles.are located at particular phosphorus atoms.All such nodes are equivalent,and each is connected to one other node through a direct P ±P bond (the short node-to-node connections in Figure 21a)and to two other nodes through columnar rods with a roughly pentagonal cross-section (Figure 21b).3.1.4.Three Parallel Interpenetrating Two-Dimensional Nets There is of course no reason why parallel interpenetration should be restricted to two independent nets.A number of examples of threefold parallel interpenetration are known at present,all of which involve (6,3)nets.Trimesic acid cocrystallizes with 4,4'-bipyridine to form hydrogen-bonded (6,3)networks with voids so large that three identical but independent networks of this type form a structure with parallel interpenetration (Figure 22).[39]This mode of threefold interpenetration is closely related to the hypothetical mode of twofold interpenetration shown in Figure 11c.This can be imagined by removing any one of the three nets in Figure 22,which leaves a twofold inter-penetrating system topologically identical to that in Fig-ure 11c.

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S.R.Batten and R.

Robson Figure 22.Schematic representation of the threefold parallel interpene-tration of (6,3)nets in the cocrystal of trimesic acid and bipy.Three-connected nodes represent the centers of trimesic acid molecules,while the two-connected nodes represent the midpoints of the bipy ligands.

[Cd 2(4)3(NO 3)4]contains three independent (6,3)nets in which the three-connected nodes are provided by cadmium atoms that are bridged by ligand 4.[24]Every Cd 6(4)6ring has a rod from each of the other two nets pass-ing through it (Figure 23).This interpenetration mode is a very close analogue of the twofold interpenetra-tion shown by [Ag(tcm)],as comparison of Figure 23with Figures 12and 11a will reveal.Indeed,removal of any one of the three nets

Figure 23.Three parallel interpenetrating (6,3)nets in the structure of [Cd 2(4)3(NO 3)4].The three-connected nodes are provided by Cd atoms,and ligand 4forms the connections between the nodes.

Figure 23leaves a twofold interpenetrated system topologi-cally identical to that in Figures 12and 11a.The crystal structure of 5contains three interpenetrating (6,3)nets;the nodes of the nets are the centers of the two

isophthalic acid groups in each molecule.[40]One of the links from any three-connected node represents the

covalent spacer between the two isophthalic acid groups of each molecule,and the other two links represent hydrogen-bond-ing associations with adjacent molecules.The interpenetra-tion is shown schematically in Figure 24;a comparison of Figures 23and 24shows that the mode of interpenetration is identical to that in [Cd 2(4)3(NO 3)4

].Figure 24.Schematic representation of the three parallel interpenetrating (6,3)nets in the crystal structure of 5.The three-connected centers represent the centers of the isophthalic acid groups of 5,while the smaller two-connected centers represent the midpoint of the central C 6ring of 5.Similarly,6′2L (L 1,4-benzoquinone)has three inter-penetrating (6,3)sheets in which the three-connected nodes are the centers of the resorcinol groups of the ligand.[41]One link from each node repre-sents the covalent bridging of the resorcinol groups through the anthracene unit,and the other two links rep-resent hydrogen-bonding in-teractions in which resorci-nol groups of two different molecules are bridged by bifunctional 1,4-benzoqui-none molecules.This inter-penetration is shown sche-matically in Figure 25.Removal of any one net would leave a system of two interpenetrating nets topologically identical to that shown in Figure 11c.

REVIEWS Interpenetrating

Structures

Figure 25.Schematic representation of the three parallel interpenetrating (6,3)nets in the crystal structure of 6′2L (L 1,4-benzoquinone).The larger three-connected circles represent the resorcinol groups on 6,and smaller two-connected circles represent the midpoints of the 1,4-benzo-quinone molecules.

3.1.5.Six Parallel Interpenetrating Two-Dimensional Nets

Six parallel interpenetrating (6,3)sheets are seen in the structure of solvated [Ag(teb)CF 3SO 3](teb 1,3,5-tris(4-ethynylbenzonitrile)benzene,7).[42]The three-connected no-des are defined by alternating ligand and silver centers.

The

silver atoms are also coordinated to pendant triflate ions,and hence have tetrahedral geometry.The nets interpenetrate such that any two nets have the same topological relationship as that shown in Figure 11a.

3.2.Inclined Interpenetration of Two-Dimensional

Frameworks In the inclined mode of interpenetration of 2D frameworks any one sheet has an infinite number of others passing through it (see Figure 10).As mentioned earlier,inclined interpenetration of 2D sheets produces an interlocked 3D structure;this is in contrast to parallel interpenetration,which yields a composite 2D product.Any pair of inclined sheets

pierce each other only along the line of intersection of the two planes.In some cases each ring of a sheet has only one rod of an inclined sheet passing through it,but there are examples in which the rings are large enough and the rods slim enough to allow more than one sheet to pass through each ring.All known cases of inclined interpenetration of 2D sheets involve either (4,4)or (6,3)nets.3.2.1.Inclined Interpenetration of (4,4)Nets Crystalline compounds of composition [M(bipy)2(H 2O)2]-(SiF 6)(M Zn,Cd,Cu)and [Cd(bipy)2(H 2O)(OH)](PF 6)have the interlocked,perpendicular,square-grid structure shown in Figure 26.[43,44]The bridging bipy ligands

are Figure 26.Inclined interpenetration (actually perpendicular)of two (4,4)nets in [M(bipy)2(H 2O)2](SiF 6).Four-connected nodes are provided by M atoms,which have two pendant trans H 2O ligands not shown here.arranged around the metal atom in a square-planar fashion,and two water ligands (or,in the Cd/PF 6case,one water and one hydroxide ligand)on opposite sides of that plane complete an octahedral coordination environment.The Cd(PF 6)2/bipy system is interesting in that the use of solvent mixtures only slightly different from those used to obtain the interpenetrating [Cd(bipy)2(H 2O)(OH)](PF 6)structure yields noninterpenetrating [Cd(bipy)2(H 2O)2](PF 6)2′4H 2O ′2bi-py,[44]which contains very similar square-grid sheets in which each corrallike ring encloses two uncoordinated bipy mole-cules.This example demonstrates how fine a balance may dictate whether an interpenetrating structure is formed or not.[Fe(bpe)2(NCS)2]′CH 3OH (bpe is the aextendedobipy analogue trans -1,2-di-(4-pyridyl)ethene)has a structure that is geometrically very similar and topologically identical to that in Figure 26;bipy has been replaced by bpe,and the two trans water ligands have been replaced by thiocyanate ligands.[45]A bipy analogue that is even more extended than bpe,1,4-bis(4-pyridyl)butadiyne (bpb),gives crystals of the composi-tion [M 2(OH)(bpb)4](BF 4)3′H 2O ′EtOH (M Cd,Cu)con-taining adoubled-uposquare-grid sheets which interpenetrate each other (Figure 27).The manner of interpenetration is closely analogous,and in fact topologically identical,to that represented in Figure 26.[46][Cd(NH 3)2{Ag(CN)2}2]also has an interpenetrating sheet structure which is topologically identical to that in Figure 26;

REVIEWS

S.R.Batten and R.

Robson Figure 27.Inclined interpenetrating doubled-up sheets in the structure of [M 2(OH)(bpb)4](BF 4)3′H 2O ′EtOH (bpb 1,4-bis(4-pyridyl)butadiyne).The circles represent M atoms.aShortoconnections here are provided by m 2-OH àand alongoconnections by m 2-bpb ligands.The overall topology of the doubled-up sheets is (4,4),and the nodes are located at the midpoints of the short connections.

there are Ag(CN)à2bridging ligands in place of bipy and two trans amine ligands in place of the two water ligands.[47]

A second,chemically realistic way in which two inclined (4,4)nets could be imagined to interpenetrate is shown in Figure 28.The only example of this mode of

interpenetration

Figure 28.An alternative mode of inclined interpenetration of two (4,4)nets.

exists in the structure of [Cd(4-ampy)2{Ag(CN)2}2]′[Cd(mea)(4-ampy){Ag(CN)2}2]2(4-ampy 4-aminopyridine,mea 2-aminoethanol).[48]This structure contains (4,4)2D sheets that are composed of octahedrally coordinated cadmi-um atoms bound to terminal 4-ampy ligands and bridged by linear Ag(CN)à2moieties.A second type of sheet exists in the structure which contains linear chains of cadmium atoms bridged by Ag(CN)à2groups.These chains are linked to one another through hydrogen bonds between the uncoordinated nitrogen atoms of monodentate Ag(CN)à2ligands,which are also bound to the cadmium atoms,and the oxygen atoms of the mea ligands,which are coordinated to cadmium atoms of adjacent chains (Figure 29).A particularly interesting feature of this structure is that the windows of the two types of sheets have different numbers

Figure 29.Schematic representation of the 2D inclined interpenetration of (4,4)sheets in the structure of [Cd(4-ampy)2{Ag(CN)2}2]′[{Cd(mea)(4-ampy){Ag(CN)2}2](4-ampy 4-aminopyridine,mea 2-aminoethanol).The open bonds represent the coordination polymer nets of composition [Cd(4-ampy)2{Ag(CN)2}2],while the black bonds represent the sheets formed by cross-linking of coordination polymer chains of composition [Cd(mea)(4-ampy){Ag(CN)2}2]through hydrogen bonds.The hydrogen bonds are represented by the almost vertical black bonds.The circles represent Cd https://www.wendangku.net/doc/694650026.html,s passing through them.In the first type of net two sheets of the second type pass through any given window,while in the second type only one net of the first type passes through any given window.The structure of solvated [Cd(py)2{Ag(CN)2}2](py pyri-dine)contains (4,4)sheets composed of octahedrally coordi-nated cadmiums which are bound to pendant pyridine ligands and bridged by linear Ag(CN)à2units.[49]Every ring of any sheet contains parts of two others passing through it (Figure 30).The mode of interpenetration is unrelated to the two previous modes discussed for (4,4)nets (Figures 26and

28).Figure 30.Schematic representation of the 2D inclined interpenetration of (4,4)sheets in the structure of [Cd(py)2{Ag(CN)2}2](py pyridine).Each window of each sheet,all of which are equivalent,has parts of two other sheets passing through it.The circles represent Cd atoms.In principle,inclined interpenetration of (4,4)nets in three mutually perpendicular planes is possible (Figure 31),but no examples of this are known at present.3.2.2.Inclined Interpenetration of (6,3)Nets Two topologically different ways in which two inclined (6,3)nets can interpenetrate are shown in Figure 32.[Cu 2(pz)3]-

REVIEWS Interpenetrating

Structures

Figure 31.Hypothetical mode of interpenetration of (4,4)nets in three mutually perpendicular

planes.

Figure 32.Two topologically different ways inclined (6,3)nets may interpenetrate.

(SiF 6)(pz pyrazine)contains (6,3)nets in which the three-connected nodes are provided by Cu I centers,which are each connected to three neighboring Cu I centers through pyrazine bridges.The mode of interpenetration,shown in Figure 33,is topologically identical to that represented schematically in Figure 32a.

[50]

Figure 33.Inclined interpenetration of (6,3)nets in the structure of [Cu 2(pz)3](SiF 6)(pz pyrazine).The circles represent Cu atoms.The anionic complex [Cu II (opba)]2à(8,opba o -phenyl-enebis(oxamate))can itself act as a bridging unit.With Mn II ions this yields a (6,3)net of composition [Mn II 2(8)3]2àin which the triply chelated Mn II center acts as the three-connect-ed node.The negative charge on the 2D network is counterbal-anced by the organic radical cation 2-(4-N -methylpyridini-um)-4,4,5,5-tetramethylimidazo-lin-1-oxyl-3-oxide.Each (6,3)net has an infinite number of identical sheets passing through it,inclined at 72.78,with one rod from an inclined sheet passing through every hexagon:The essential nature of the inter-penetration is that represented schematically in Figure 32a.The radical cations play an important role in the overall structure,since they provide weak aaxialodative bonds through their oxygen atoms to copper centers in independent inclined sheets and thus act as weak bridging agents between sheets.There are three different paramagnetic centers pres-ent,namely,the radical cation,the copper ions,and the manganese ions;the couplings are such that the material acts as a magnet below 22.5K.[51]One example was reported recently in which every ring has components of two independent inclined (6,3)nets passing through it;[52]in this case the three-connected nodes are provided by Ag I ions which are linked by units of 9.This beautiful structure is an example of a 2D polyrotaxane

REVIEWS

S.R.Batten and R.Robson which the beadlike component encircling the diprotonated dia-mine in 9in rotaxane fashion is furnished by cucurbituril (Fig-ure 34).All rings are equivalent,and every ring has a rod from each of two inclined sheets passing through it.Various forms and derivatives of ben-zene-1,3,5-tricarbox-ylic acid (trimesic

acid)contain hydro-

gen-bonded (6,3)

nets of the type shown in Figure 3,

which interpenetrate so that every hexago-nal ring has compo-nents of three in-clined sheets passing through it.[13]The essential character of the interpenetration,which is related to that in Figure 32a,is represented in Figure

35.

Figure 35.Schematic representation of the mode of inclined (6,3)inter-penetration seen in various derivatives of trimesic acid.Every hexagon has components of three inclined independent nets passing through it in the manner shown in Figure 32a.The circles represent the midpoints of the central C 6rings of the trimesic acid molecules.

Another example of inclined (6,3)interpenetration in which every ring has components of three independent inclined sheets passing through it is [Cu(bipy)X](X Cl,Br,I).[53,54]Binuclear halide-bridged (CuX)2units play the role of the three-connected nodes;each is connected to a neighboring (CuX)2node by two side-by-side bipy ligands and to two other neighbors through single bipy units (Figure 36,the connections consisting of side-by-side bipy units are represented by two parallel lines).This example is of particular interest from the topological point of view because one of the three independent sheets,the central one,passing through any one ring adopts the interpenetration mode represented in Figure 32b,whilst the other two,which are equivalent,adopt a different

mode.

Figure 36.Structure of [Cu(bipy)X](X Cl,Br,I).The circles represent Cu atoms.Each ashortoconnection shown here is provided by two m 2-X àbridges.aLongoconnections are provided by bipy ligands,some of which are present in close side-by-side pairs.The three-connected nodes of the (6,3)net are located at the midpoints of the Cu(X)2Cu rectangles (i.e.,at the midpoints of the ashortoconnections shown here).4.Interpenetrating Three-Dimensional Nets An infinite number of different 3D nets are theoretically possible.Nevertheless,the individual 3D nets involved in interpenetration,as far as is presently known,belong to a restricted number of topological classes with only a few exceptions.This situation may of course change radically in the near future.The diamond net is by far the most common type currently known to participate in interpenetration.We classify the modes of 3D interpenetration below in terms of the topologies of the individual nets.4.1.Interpenetrating Three-Connected Three-Dimensional Nets All known examples of interpenetrating three-connected 3D nets are of either the (10,3)-a type or the (10,3)-b type;there is one exception of the (8,3)-c type.In view of the large number of three-connected 3D nets that are possibleDno fewer than thirty topologically uniform examples were listed by Wells [4]Dit is surprising that so few types are represented in real cases.However these are early days in the generation of framework structures,and perhaps this situation will change as a result of further work in the area.4.1.1.Interpenetrating (10,3)-a Nets Figure 37shows a representation of the (10,3)-a net (some-times referred to as the SrSi 2-related net)in its most regular form possible.It should be recalled that any net,however much it may be geometrically distorted,is considered to retain its topological identity provided connections are not broken.The net is shown in Figure 37in its geometrically most symmetrical form (cubic),and every node shows a trigonal-planar environment with 1208angles.A characteristic feature of the (10,3)-a net is the presence of fourfold helices,all of the same handedness,running parallel with the three cubic axes (Figure 37).The net as a whole is therefore chiral.In the discussion that follows we shall refer to (10,3)-a nets contain-

Figure 34.Representation of the in-terpenetration in the network of composition [Ag I 2(9)3].Every hexa-gon has components of two inclined (6,3)nets passing through it.The circles represent Ag atoms.

REVIEWS Interpenetrating

Structures

Figure 37.The (10,3)-a net in its most symmetrical form,cubic,displayed on a cubic grid.

ing right-handed fourfold helices simply as aright-handed netso.Showing remarkable insight,Wells suggested,[4]well before real examples were discovered,that the (10,3)-a net is in principle capable of interpenetrating not only an identical net of the same handedness,but also a net of the opposite handedness.In the latter case aa three-dimensional race-mateo,as he referred to it,is generated that at the time was purely hypothetical.The crystal structure of cyanamide NH 2CN (Figure 38)can be regarded,because of the

simplicity

Figure 38.Two enantiomorphic interpenetrating (10,3)-a nets in the crystal structure of cyanamide NH 2CN.The circles represent in order of decreas-ing size N,C,and H atoms.

of its components,as the prototype for this sort of racemic interpenetration.[55]The two types of nitrogen centers provide the three-connected nodes for the hydrogen-bonded (10,3)-a net.Two such nets of opposite handedness then interpenetrate in the manner shown in Figure 38.[Ag 2(2,3-Me 2pz)3](SbF 6)2,in which the silver atoms provide the three-connected nodes,is topologically identical.[56]

Both modes of interpenetration proposed by Wells,inter-penetration of a given (10,3)-a net by nets of the same handedness and by nets of the opposite handedness,are simultaneously displayed in cubic crystals of composition [Zn(tpt)2/3(SiF 6)(H 2O)2(CH 3OH)](tpt 2,4,6-tris(4-pyridyl)-

1,3,5-triazine,10).[57]The three-connected nodes are provided by triazine molecules.The zinc cen-tersDwith a coordination environ-ment consisting of two trans tpt-pyridine donors,one SiF 2à6ion bound through one F atom,two water ligands,and one methanol ligandDeffectively act as linear two-connected units.Figure 39a shows one of the eight (10,3)-a nets that are present.Inspection will reveal that this is left-handed;also present are three other left-handed and four right-handed nets.Any net is related to the three others of the same handedness by half-cell translations parallel to the three cubic axes.Helices consequently appear in pairs sharing the same helical axis,an example of which is shown in Figure 39b.Each double helix makes multiple close triazine ±triazine p contacts with four double helices of the opposite handedness,all with their helical axes parallel;a pair of double helices connected by such p interactions is shown in Figure 39c.The tpt units appear in close,centrosymmetrically related pairs (indicated in Figure 39c by arrows).The mineral eglestonite of composition [(Hg 2)3O 2H]Cl 3has been described as consisting of four [(Hg 2)3O 2 2]n nets with the (10,3)-a topology.[58,59]The oxide centers act as the three-connected nodes,and the mercurous units act as two-connected nodes that link one oxide to another.The O ′′′O ′′′O angles of 102.18are close to the tetrahedral angle.This observation led us to the realization that it is possible to construct strain-free (10,3)-a nets using pyramidal three-connected nodes,provided these retain a threefold axis of symmetry,which,like the aparento(10,3)-a net with trigonal nodes,also have cubic symmetry (indeed,the structure of H 2O 2can be described in terms of a (10,3)-a topology with pseudo-tetrahedral centers,and SrSi 2itself displays slight pyramidal distortion at each three-connected node).[4]Such a net consisting of pyramidal nodes with tetrahedral angles of 109.458is shown in Figure 40.For ease of description,(10,3)-a nets comprising trigonal or pyramidal nodes will be referred below to as atrigonal oand apyramidal netso,respectively.As can be seen by comparing Figures 37and 40,the trigonal and pyramidal (10,3)-a nets differ considerably in appearance and geometry.One consequence of substituting the planar-trigo-nal centers by pyramidal ones is that the fourfold helices become markedly flattened.Some of these flattened helices can be seen in Figure 40;their axes are roughly perpendicular to the plane of the page,and,since the structure has cubic symmetry,equivalent flattened helices also run from left to right and from top to bottom.The (10,3)-a nets present in eglestonite,two right-handed and two left-handed (Fig-ure 41),are very similar to the idealized net shown in Figure 40.The flattened helices of two nets of the same handedness share a common axis,and they can be related by a translation along this shared axis of half the helical period combined with a 908rotation (Figure 41).Two pyramidal nets of the same handedness can be derived from a single trigonal net of the same handedness by distorting each individual node into a pyramidal geometry in the appropriate direction along

REVIEWS S.R.Batten and R.

Robson

Figure39.a)One of the eight(10,3)-a nets present in[Zn(tpt)2/3(SiF6)-(H2O)2(CH3OH)](tpt 10).The larger circles represent Zn atoms,and the smaller circles C and N atoms.b)A double helix.c)A p-contacting pair of double helices of opposite handedness.The arrows indicate centrosym-metrically related p-contacting pairs.

the normal to the trigonal plane(Figure42);two directions of distortion are possible,hence the existence of two derived pyramidal nets.Conversely,the trigonal net can be derived from the two pyramidal nets by placing a trigonal node at

the Figure40.A(10,3)-a net constructed from pyramidal nodes with tetrahe-dral

angles.

Figure41.Fourapyramidalo(10,3)-a nets,two right-handed and two left-handed,in eglestonite.The circles represent O

atoms.

Figure42.Relationship between aatrigonalo(10,3)-a net and the two possible derivedapyramidalo(10,3)-a nets.

REVIEWS Interpenetrating Structures

midpoint between two directly neighboring pyramidal nodes of independent nets that share the same threefold axis,as can also be seen in Figure 42.In eglestonite the helices of the second pair of nets with handedness opposite to that of the first pair are then located in the channels generated by the first pair (Figure 41).In many of the structures discussed here it is debatable how independent the various nets are.It is convenient to describe the structure of eglestonite in terms of four aindependento(10,3)-a nets,as we have done above (and as others have done previously,although not in the detail presented here).However,the fact is that the pyramidal oxygen centers from adjacent nets form short hydrogen bonds (O ′′′O %2.5 ;[58]

note the presence of one proton per two oxides in the composition).In cubic RhBi 4all Rh centers are equivalent and provide the nodes for a four-connected 3D net.[60]Bi 2units connect Rh atoms,and every Rh is immediately surrounded by four Bi 2pairs.The four-connected net thus formed has previously and correctly been described as possessing the 32104topology,which in fact is chiral.In RhBi 4two such nets of opposite handedness interpenetrate,as shown in Figure 43.The

units

Figure 43.Two interpenetrating 32′104nets of opposite handedness in the structure of RhBi 4.The circles represent Rh atoms.The midpoints of the Rh 3triangles within a particular 32′104net form a (10,3)-a net.The (10,3)-a net derived from the 32′104net represented here by aopenoconnections is indicated by fine lines and smaller circles.For clarity the (10,3)-a net derived from the other net is omitted.

linked together are triangular Rh 3groups;each triangle is linked to three others through shared Rh vertices.It is helpful to conceptually simplify these nets to the (10,3)-a form in which the midpoints of the Rh 3triangles now provide the three-connected nodes (Figure 43).RhBi 4,which has the same space group as eglestonite (Ia 3d ),can thus be seen as yet another example of the type of 3D racemate predicted by Wells.

4.1.2.Interpenetrating (10,3)-b Nets

The (10,3)-b net (sometimes referred to as the ThSi 2-related net)is tetragonal in its most symmetrical form (Figure 44).As in (10,3)-a nets,the nodes have 1208angles.Each node is connected to two neighbors in a 1D planar zigzag strip.The third connection at every node is to a node belonging to

Figure 44.The (10,3)-b net in its most symmetrical form (tetragonal).zigzag strip running perpendicular to the first.Alternate nodes within a strip are connected to a strip above it,then a strip below it,and so on.An interesting feature of this net is that it can be acollapsedoby concerted rotation around the strip ±strip connections of all the strips in one direction relative to all those in the other direction.This collapse does not interfere with the trigonal geometry of the individual nodes;it merely reduces the angle between connected zigzag strips.This collapse is seen in real cases to extents that vary so as to achieve optimum packing.A characteristic structural motif in the (10,3)-b net that parallels the adamantane motif in the diamond net is the cagelike unit shown in Figure 45a.Indeed,the (10,3)-b

net Figure 45.The imaginary contraction and fusion process relating a a(10,3)-b cageo(a)of the (10,3)-b net to an adamantane cage (b)of the diamondlike net.and the diamond net are very closely related,in that the former can be converted into the latter by contraction of the links between the zigzag strips (i.e.,the links aligned with the tetragonal axis in the most symmetrical form of the (10,3)-b net)to the point where the two three-connected nodes fuse together to become a single four-connected node.Figure 45illustrates this imaginary conversion of a (10,3)-b cage into an adamantane cage.Just as the nature of the interpenetration of

REVIEWS

S.R.Batten and R.Robson diamond nets can most simply be understood by focussing on the way one adamantane unit is interpenetrated by others (as we saw in the example of [Zn(CN)2]in Section 1.5and as we shall see in more complicated examples in Section 4.2.1),the nature of the interpenetration of (10,3)-b nets can be best appreciated by looking at the way one (10,3)-b cage is interpenetrated by others.Examples are known in which two,three,and six inde-pendent (10,3)-b nets interpenetrate.The mineral neptunite,

M I 4M II 2(TiO)2Si 8O 22(M II Fe II ,Mg II ,Mn II ;M I alkali metal),contains two infinite polysilicate (10,3)-b nets,one of which is shown in Figure 46a,that interpenetrate in the

manner Figure 46.a)A single (10,3)-b net in neptunite.The circles represent Si atoms.b)Two interpenetrating nets in neptunite.depicted in Figure 46b.[61]The three-connected nodes are provided by SiO 4tetrahedra that each carry one terminal,nonbridging SiO àunit and are represented as Si 3below.Present in equal numbers are two-connected SiO 4tetrahedra,each carrying two terminal SiO àgroups (represented as Si 2below).The zigzag planar strips that are characteristic of the (10,3)-b net consist in neptunite of alternating long and short bonds between three-connected nodes.The the short bonds consist of direct connections between Si 3centers,and the long bonds have two Si 2centers interposed between Si 3nodes to produce the sequence ′′′Si 3Si 3Si 2Si 2Si 3Si 3Si 2Si 2′′′within a strip (Figure 46a).Zigzag strips of this type run from top right to bottom left and also from top left to bottom right.Also apparent in Figure 46a are channels running perpendicular to the plane of the page,that is,parallel to what would have been the tetragonal axis if the net had not been deformed.The interpenetration is such that the connections between zigzag strips within one net are located along the centers of these channels in the other net (Figure 46b).Two interpenetrating (10,3)-b nets are present in crystals of solvated [(ZnCl 2)3(tpt)2](tpt 10).[28]The three-connected nodes are provided by the triazine molecules.The zinc centers have an approximately tetrahedral coordination environment consisting of two tpt-pyridine and two chloro ligands,and therefore act as angular two-connectors.As a consequence,the resulting (10,3)-b net (Figure 47a)is considerably dis-torted.Figure 47b shows the two interpenetrating (10,3)-b

nets.

Figure 47.a)One of the two (10,3)-b nets in the structure of [(ZnCl 2)3(tpt)2](tpt 10).The circles represent the centers of triazine rings.b)Two interpenetrating (10,3)-b nets.

Three independent,identical,and interpenetrating (10,3)-b nets are seen in [Ag 2(pz)3](BF 4)2.[62]Silver atoms provide the three-connected nodes,which are linked together through bridging pyrazine units.The nets are collapsed,so that the angle between zigzag strips is 55.48.Figure 48a shows three unit cells stacked along the a direction.One net can be generated from another by a (1,0,0)translation;but a shorter translation (1/2,1/2,0)also relates one net to another.Every decagon has one rod from each of the other two nets passing

REVIEWS Interpenetrating

Structures

Figure 48.a)Three unit cells in the structure of [Ag 2(pz)3](BF 4)2.The circles represent Ag atoms.b)Three interpenetrating a(10,3)-b cagesofrom separate nets.

through it.Three interpenetrating (10,3)-b cages from sepa-rate nets are shown in Figure 48b.With regard to the general question concerning the circum-stances under which an interpenetrating structure or a noninterpenetrating alternative form,it is interesting that out of the same reaction mixture a second type of crystal can be obtained with the same [Ag 2(pz)3]2 framework composi-tion but with a different and noninterpenetrating structure:the 2D (6,3)net.This can be viewed as consisting of the same zigzag strips joined together in a coplanar fashion to give the 2D structure,rather than joined together at an angle to give the 3D (10,3)-b net.A second example of threefold interpenetration of (10,3)-b nets is provided by a material of composition [CrP 3S 9 x ](x %0.25),[63]which consists of Cr III centers interconnected through bridging P 2S 2à

6ligands (11),a small fraction of which (approximately 1in 12)have been replaced by P 2S 2à8(12).Both ligands act as bidentate chelating agents at both ends.The Cr III centers are therefore in a triply chelated pseudo-octahedral coor-dination environment of six sulfur atoms and provide the three-connected,essentially trigonal nodes for the (10,3)-b nets.The three independent nets interpenetrate in a manner topologically identical to that described above for [Ag 2(pz)3](BF 4)2.Another system that has been described in terms of three interpenetrating three-connected nets,which we have noticed have the (10,3)-b topology,is [Ag(bipy)]NO 3.[64]Linear polymeric chains consisting of alternating bridging bipy units and linearly coordinated silver centers are cross-linked through Ag ±Ag interactions at a distance of 2.977(1) to give the 3D (10,3)-b net in which the three-connected nodes are the T-shaped metal centers.Three such nets then inter-penetrate (Figure 49a).Three (10,3)-b nets that interpene-trate in a manner topologically identical to that observed for [Ag(bipy)]NO 3are shown in their idealized,most

symmet-

Figure 49.a)Three interpenetrating (10,3)-b nets with T-shaped three-connected nodes in the structure of [Ag(bipy)]NO 3.The circles represent Ag atoms.aLongoconnections represent bridging bipy units,and ashortoconnections are direct Ag ′′′Ag interactions.b)An idealized version of the mode of interpenetration of three (10,3)-b nets,of the type seen in [Ag(bipy)]NO 3.rical form in Figure 49b.This figure was constructed to facilitate comparison with Figure 48b;the two modes of threefold interpenetration are clearly topologically different.Two examples are known of six-fold (10,3)-b interpenetration.One of these is solvated [Ag(teb)]CF 3-SO 3(teb 7),[65]which contains two types of three-connected nodes pro-vided by alternating 7and silver centers.Figure 50shows a represen-tation of the way in which (10,3)-b cages from five other independent nets interpenetrate a given (10,3)-b cage.The central C 6rings of units of 7from independent nets are stacked (with minor deviations)along the direction AB shown in Figure 50(which corresponds to the crystallo-graphic a axis).It is worth noting that this structure is a polymorph of an earlier structure discussed which has six parallel interpenetrating (6,3)2D nets.The second example of sixfold interpenetration of (10,3)-b nets,[Cu 2(bipy)3](NO 3)2′2.5H 2O,[66]dif-fers from the one just described in that all the three-connected nodes are of the same type:Cu I ions.The topology of interpenetration is dis-tinctly different,as can be seen in

TG-NETS3500-52G-4F全千兆管理型交换机-产品资料

S3500-52G-4F全千兆管理型交换机产品概述 S3500-52G-4F交换机是一款超多端口，二层以太网交换机。该机型提供48个10/100/1000Mbps自适应RJ45端口+4个100M/1000Mbps SFP端口，支持所有端口线速转发。基于千兆网络技术，最大效率的避免网络传输颈瓶，并采用业界少有的独立光口设计，突破传统的复用口局限；S3500-52G-4F采用存储转发技术，结合动态内存分配，确保有效的分配到每一个端口，同时具备流量控制，保证节点在传送和接收时，尽可能的避免数据包丢失。该系列产品在安全可靠、多业务融合、易管理和维护等方面为用户提供全新的技术特性和解决方案，是理想的安防网络、酒店网、办公网、业务网和驻地网的汇聚、接入交换机以及中小企业、分支机构的核心交换机。产品外观

产品特点 ?多端口无阻塞高速转发 S3500-52G-4F交换机提供48个10/100/1000Mbps自适应RJ45端口+4个100M/1000Mbps SFP端口，端口利用率极高。 S3500-52G-4F交换机所有端口提供二层线速交换的能力，保证所有端口无阻塞地进行报文转发。 ?完善的安全控制策略支持端口汇聚功能，提升网络带宽的同时，通过链路备份保障网络安全；支持STP、RSTP等多种生成树协议，快速收敛，提高容错能力，保证网络的稳定运行和链路的负载均衡，合理使用网络通道，提高冗余链路利用率；支持用户端口+IP地址+MAC地址绑定，可防御内网ARP攻击和DDoS攻击；支持IP ACL、MAC ACL、Vlan ACL、支持基于三、四层的ACL功能，有效防御ARP攻击和病毒；支持半双工模式下的背压（Back-pressure）流量控制和全双工模式下的IEEE 802.3x流量控制功能，可保障在大流量数据峰值稳定交换，而不会因过度负担而导致网络瘫痪。 ?高效的多业务融合技术提供各种类型网络接入提供完善的端到端的QoS服务质量；提供DHCP监测服务，解决了 DHCP用户的IP和端口跟踪定位问题。提供IGMP组播服务，在安防监控领域大显身手； ?便捷的管理维护通过简单的可视化WEB界面，可对交换机的各种功能进行简单方便的操作；支持SNMP V1/V2C网管对设备进行配置管理，为中小企业客户集中设备管理提供便利；

5脓毒症患者中性粒细胞胞外诱捕网(NETs)的水平及价值

脓毒症患者中性粒细胞胞外诱捕网（NETs）的水平及价值张芳晓章志丹马晓春目的中性粒细胞胞外诱捕网（neutrophil extracellular traps，NETs）是由中性粒细胞核内组分释放到细胞外形成的一种网状纤维结构，可由多种病原体或药物刺激中性粒细胞产生，具有限制病原体扩散，分泌杀菌蛋白等多种作用。因此，中性粒细胞胞外诱捕网可能成为评价脓毒症严重程度的生物学标志物，以及减轻脓毒症的潜在治疗靶点。本研究主要通过检测脓毒症患者血清中性粒细胞胞外诱捕网水平判断其对早期诊断脓毒症的价值。方法采用前瞻性观察性研究2014年11月至2015年3月中国医科大学附属第一医院重症医学可收治的术后患者43例及健康受试者17例，根据1991年美国胸科医师学会（ACCP）和重症医学会（SCCM）制定的脓毒症诊断标准，将患者分为术后非脓毒症组20例及脓毒症组23例。入ICU1h经外周静脉采血3ml，37℃条件下，3000转/分速率离心15分钟。离心后取上清100ul，加入picogreen荧光染料100ul，避光孵育5分钟后应用荧光酶标仪检测样品荧光值，再根据标准曲线计算样品cf-DNA/NETs的浓度。分析血清中NETs的浓度与患者是否诊断脓毒症，白细胞数量，中性粒细胞数量，APACHEII及SOFA评分的关系。结果脓毒症组血清NETs水平（6.15±2.638）较非脓毒症组（3.68±0.726）明显升高（P=0.004），非脓毒症组较健康受试者组无明显差异（P>0.05）。脓毒症组外周血白细胞及中性粒细胞数量较非脓毒症组无明显差异。脓毒症患者NETs水平与外周血白细胞及中性粒细胞数量无相关性，与APACHEII及SOFA评分无相关性。结论脓毒症患者血清NETs水平升高，与白细胞及中性粒细胞数量无明显关系，手术等因素不影响血清NETs水平。而NETs单纯作为早期脓毒症生物学指标，不能评估患者整体器官受损严重程度。血清NETs水平的测定操作简便，与白细胞或中性粒细胞数量相比可以更好的诊断脓毒症的发生。

基于 Deep Belief+Nets+的中文名实体关系抽取

软件学报ISSN 1000-9825, CODEN RUXUEW E-mail: jos@https://www.wendangku.net/doc/694650026.html, Journal of Software,2012,23(10):2572?2585 [doi: 10.3724/SP.J.1001.2012.04181] https://www.wendangku.net/doc/694650026.html, +86-10-62562563 ?中国科学院软件研究所版权所有. Tel/Fax: ? 基于Deep Belief Nets的中文名实体关系抽取陈宇, 郑德权+, 赵铁军 (哈尔滨工业大学计算机科学与技术学院,黑龙江哈尔滨 150001) Chinese Relation Extraction Based on Deep Belief Nets CHEN Yu, ZHENG De-Quan+, ZHAO Tie-Jun (School of Computer Science and Technology, Harbin Institute of Technology, Harbin 150001, China) + Corresponding author: E-mail: dqzheng@https://www.wendangku.net/doc/694650026.html,, https://www.wendangku.net/doc/694650026.html, Chen Y, Zheng DQ, Zhao TJ. Chinese relation extraction based on Deep Belief Nets. Journal of Software, 2012,23(10):2572?2585 (in Chinese). https://www.wendangku.net/doc/694650026.html,/1000-9825/4181.htm Abstract: Relation extraction is a fundamental task in information extraction, which is to identify the semantic relationships between two entities in the text. In this paper, deep belief nets (DBN), which is a classifier of a combination of several unsupervised learning networks, named RBM (restricted Boltzmann machine) and a supervised learning network named BP (back-propagation), is presented to detect and classify the relationships among Chinese name entities. The RBM layers maintain as much information as possible when feature vectors are transferred to next layer. The BP layer is trained to classify the features generated by the last RBM layer. The experiments are conducted on the Automatic Content Extraction 2004 dataset. This paper proves that a character-based feature is more suitable for Chinese relation extraction than a word-based feature. In addition, the paper also performs a set of experiments to assess the Chinese relation extraction on different assumptions of an entity categorization feature. These experiments showed the comparison among models with correct entity types and imperfect entity type classified by DBN and without entity type. The results show that DBN is a successful approach in the high-dimensional-feature-space information extraction task. It outperforms state-of-the-art learning models such as SVM and back-propagation networks. Key words: DBN (deep belief nets); neural network; relation extraction; deep architecture network; character-based feature 摘要: 关系抽取是信息抽取的一项子任务,用以识别文本中实体之间的语义关系.提出一种利用DBN(deep belief nets)模型进行基于特征的实体关系抽取方法,该模型是由多层无监督的RBM(restricted Boltzmann machine)网络和一层有监督的BP(back-propagation)网络组成的神经网络分类器. RBM网络以确保特征向量映射达到最优,最后一层BP网络分类RBM网络的输出特征向量,从而训练实体关系分类器.在ACE04语料上进行的相关测试,一方面证明了字特征比词特征更适用于中文关系抽取任务;另一方面设计了3组不同的实验,分别使用正确的实体类别信息、通过实体类型分类器得到实体类型信息和不使用实体类型信息,用以比较实体类型信息对关系抽取效果的影响.实验结果表明,DBN非常适用于基于高维空间特征的信息抽取任务,获得的效果比SVM和反向传播网络更好. 关键词: DBN(deep belief nets);神经网络;关系抽取;深层网络;字特征 ?基金项目: 国家自然科学基金(61073130); 国家高技术研究发展计划(863)(2011AA01A207) 收稿时间:2011-06-16; 修改时间: 2011-08-09; 定稿时间: 2012-01-16

The Master of the Nets Garden(网师园)

The Master of the Nets Garden The Master of the Nets Garden in Suzhou is among the finest gardens in China. It is located at Gusu District (formerly Canglang District), Dai Cheng Qiao Road, No. 11 Kuo Jia Tou Xiang. It is recognized with other classical Suzhou gardens as a UNESCO World Heritage Site. The garden demonstrates Chinese garden designers' adept skills for synthesizing art, nature, and architecture to create unique metaphysical masterpieces. The Master of the Nets is particularly regarded among garden connoisseurs for its mastering the techniques of relative dimension, contrast, foil, sequence and depth, and borrowed scenery. The Master of the Nets garden was first constructed in 1140 AD during the Southern Song Dynasty (1127 - 1279). Then named the Fisherman's Retreat (Yuyin), it was inspired by the simple and solitary life of a Chinese fisherman. The garden subsequently fell into disarray until six centuries later it was restored by a retired government official of the Qing Dynasty, Qianlong Period (1735 - 1796). He drastically redesigned the garden and added multiple buildings, but retained the humble spirit of the site when renaming it the Master of the Nets. The 5,400 m2 garden is divided into east and west sections. The eastern

NETs在狼疮性肾炎的分布及其相关性研究

NETs在狼疮性肾炎的分布及其相关性研究自从2004年，Brinkmann报道NETs即中性粒细胞胞外诱捕网（Neutrophil Extracellular Traps）现象[1]以来，大量有关NETs的研究犹如雨后春笋般涌现。大量的研究报道提示NETs很有可能是造成狼疮性肾炎的一个重要原因，很可能是一个潜在有效的治疗靶标，因此可见NETs与狼疮性肾炎的关系非常密切。 NETs是新发现的中性粒细胞在固有免疫的一道重要防线。中性粒细胞通过向胞外抛出含有DNA，颗粒蛋白的网状结构粘附并杀死病菌并阻止病菌在体内的扩散[2, 3]。NETs中最主要的物质是DNA，含量最多的蛋白是组蛋白，其他的蛋白还有抗菌肽（LL37）、防御素、转铁蛋白、髓过氧化物酶（MPO）、蛋白酶（PR3）、弹性蛋白酶（NE）、组织蛋白酶G[4]早之前，人们就发现很多狼疮性肾炎的病人血清中含有抗核抗体，甚至87%的病人在发病前就已经产生了抗核抗体[5]。此外还在狼疮性肾炎中检测到了其他ANCA，并且部分ANCA的含量在狼疮性肾炎的病人含量升高并且与疾病的活动性相关[6]。最近，Hakkim研究报道，部分狼疮性肾炎的病人降解NETs的能力下降，因为这部分病人体内含有抑制NETs降解的DNaseI 的抑制剂或是自身抗体和包裹NETs阻止DNaseI对NETs的降解。另有研究表明狼疮肾炎的病人存在DNaseI基因变异[7]。但是，迄今为止，NETs在狼疮性肾炎中的作用在国内外还未见相关研究报道。有报道提示，狼疮性肾炎的种族不同预后结果不同[8-10]，所以很可能出现发病机制和分子水平上的倾向性上存在差异性，因此大力开展针对本国人民狼疮性肾炎的研究显得格外重要。因此，我们课题组假设：NETs与狼疮性肾炎有高度的相关性。本研究拟应用免疫组化、免疫荧光、ELASIA等技术检测NETs在狼疮性肾炎中的表达，再用相关分析软件，分析NETs 在组织中的分布，并对其进行相关分析，如果有证据表明狼疮性肾炎中NETs表达增高，对狼疮性肾炎的早期临床治疗有着重大的实用意义，可产生较大的社会效益和经济效益。国内外尚无相关报道，具有国内领先地位。（二）国内外研究概况 2004年，Brinkmann首先报道NETs现象[1]，大量的研究报道提示NETs很有可能是造成狼疮性肾炎的一个重要原因，随后大量有关NETs和狼疮肾炎的研究展开，最新的研究结果归纳如下：

DriveNets-全新的云端构建网络

DriveNets-全新的云端构建网络关键词：云技术应、运营商、硬件、软件技术产品现如今网络的快速发展，带来了网络云的快速发展，DriveNets通过分离硬件和软件、使用网络白盒和提供一个扁平的软件定价模型，真正地打破了服务提供商的空间。这种强大的组合使运营商能够打破供应商锁定，以成本效益扩大产能，并提高利润率。公司的的旗舰解决方案被称为网络云——一个以软件为中心的路由基础设施，它可以线性增长到前所未有的规模，并且可以在任何规模的中心云中运行任何服务。它将超大规模的云技术应用于SP网络，从根本上简化了SP网络的运营模式，实现了极速增长、快速的服务创新和加速的经济盈利能力。公司的主要产品包括 1. 网络协调器(DNOR)

网络协调器(DNOR)是专门为解决部署、集成和管理分散网络的独特挑战而设计的。DNOR 通过端到端自动化和数据驱动决策，集中协调每个网络云元素的平稳运行，提供了电信级可伸缩的可管理性。DNOR部署了一个完全自动化的过程，将网络云的离散和分离的软件和硬件元素转换为完整的路由实体。从零接触安装到hitless退役，DNOR管理每个网络云元素的完整生命周期，包括单独的和整体的。分散的网络带来了集成和编排方面的挑战。来自多个供应商和位置的硬件和软件需要在正确的时间组合在一起并按照正确的顺序来创造一个可管理的整体。这一挑战对于网络云来说更为重要，因为它具有前所未有的可伸缩性和集群驱动的极端端口密度。DNOR是为这项任务而构建的——从组装一个独立的分体路由器到构建巨大的路由集群。优势： ?DNOR使整个网络生命周期自动化，并提供直观的用户界面来控制和跟踪每个流程。 ?DNOR提供了向北的api，便于与第三方协调器、OSS/BSS和库存管理系统集成。 ?凭借完整的网络可视性，DNOR收集了大量关于网络云的数据——包括性能指标、故障统计和流量数据。 ?为网络及其行为生成独特的端到端可见性，可以通过直观的用户界面访问这些数据。 ?应用先进的人工智能分析工具，并将启用基于机器学习的网络操作。 ?预测故障、拥塞和潜在的优化，并配置网络云以避免或利用这些发展。 ?DNOR利用这种能力来简单而自动地提供网络范围的服务。 ?网络云的整合和统一网络模型支持简化和高度自动化的服务创建链。 2. 网络操作系统(DNOS) DriveNets网络操作系统(DNOS)是一个功能齐全的网络堆栈，可以在任何网络云认证的硬件平台上运行。DNOS创建了一个统一的共享网络基础设施，该基础设施跨越多个服务器和白盒，并作为单个网络实体进行管理。结合云技术和虚拟化技术，DNOS支持在统一的共享基础设施上运行任何服务，并动态地将任何服务附加到任何端口。 DNOS是一个运行在Docker容器上的分布式操作系统，它提供了灵活性、多平台支持、本地虚拟化和快速部署过程。它是一个本地云软件，具有用于自动化、配置和遥测的开放api。优势： ?分类架构DNOS控制平面和数据平面是自然分解的，其中数据平面运行在一组白盒子上，而控制平面可以运行在裸金属上、VM内部或任何其他支持容器的环境中。 ?处理任何规模:DNOS控制和数据平面可以独立地伸缩以处理任何规模要求。 ?没有性能下降:使用虚拟化层，数据平面通过向相同的逻辑网络元素添加更多的白盒子来扩展，没有性能下降或停机时间。控制平面通过向服务器添加更多的资源来扩展。。 ?DNOS构建于集装箱微服务之上，支持在任何端口上运行任何服务。核心路由、聚合和提供者边缘(PE)等关键网络服务是在运行于共享基础设施上的微服务中实现的。然后，DNOS可以动态地将任何服务附加到任何端口。 ?基于DNOS专利的技术，如分布式逻辑、两阶段配置同步和高速消息总线，使网络服务能够在分布式体系结构上以电信级的时间和规模实现。

(精品)NETS是什么考试,和四六级英语有什么关系(四六级英语)

NETS是什么考试，和四六级英语有什么关系？ (本文档下载后可自行修改)

NETS是什么考试，和四六级英语有什么关系？大学英语四六级考试作为学生以后步入社会，投简历时需要具备的面试资格证书之一，其考试内容死板和应试教育的性质，一直被考生所诟病，这个连外国人都拿不到及格的英语等级考试，真的对学生未来的发展有所帮助吗？因此，改革大学英语四六级是迟早的事儿。为此，教育部提出的 “nets测试”正是将影响大学英语四六级的重要改革！你还不知道NETS是什么考试，不知道NETS考试可能会替代大学英语四六级考试，那你一定要收下这碗小编整理的nets考试常识问答集锦！因为这些新政策都将对考研和你的大学生活产生重大的影响！问：NETS是什么考试？答：NETS是国家英语能力等级考试（NETS-5、6级）的考试。问：NETS考试和大学英语四六级有什么关系？答：NETS考试，是教育部考试中心正在进行与全国大学英语四、六级考试对接的国家英语能力等级考试（NETS-5、6级）的测试。NETS-5级测试相当于CET-4水平，NETS-6级测试相当于CET-6级水平。试题由国家教育部考试中心提供。问：NETS考试的考试内容是什么？

答：NETS-5级：听力（35分钟）阅读（60分钟）写作A（30分钟）写作B（30分钟）；NETS-6级：听力（40分钟）阅读（60分钟）翻译（30分钟）写作（35分钟）。问：大学英语四六级考试会被NETS考试替代吗？答：暂时没有官方通知，只是在试点，不排除这个可能性。问：NETS考试比大学四六级英语更难吗？答：是的。新的NETS考试可能会比四六级更难，所以大家一定要在改革前争取高分通过大学英语四六级。问：相对于四六级，NETS考试难在哪里？以下是NETS考试与四六级考察题型及考试时长对比。答：可以看出： 1、考试时长增加了。比如四级听力25分钟，新考试35分钟。再结合之前的四六级听力改革，可以预测，NETS的各题型难度会更大，出题花样更多。 2、写作的比例加大了。新版五级把翻译改成了写作，因此如果写作不好，以后可能更难过。问：NETS考试怎么报名？有什么报名要求吗？答：目前只有个别高校在做试点测试，如果你们学校没有接到邀请你就不能参加这个新的考试。报考要求是已经成功报考2018年6月CET-4、

BENETS试题

选择题（针对以下题目，请选择最符合题目要求的答案。针对每一道题目，所有答案都选对，则该题得分，所选答案错误或不能选出所有答案，则该题不得分。其中1—40题每题分，第41—60题每题2分，共100分） 1.在一台Cisco路由器R1上配置RIPv1协议时，需要用network命令宣告其直连的网络/24，以下命令正确的是（c）。（选择一项） a)R1（config）#network b)R1（config）#network c)R1（config-router）#network 2.R1（config-router）#network 在一台Cisco2811路由器上配置了命令ipnat pool benet prefix-length 27，以下说法错误的是（d）。（选择一项） a)地址池的名称是benet b)地址池的方位是- c)Prefix-length 27表示子网掩码是27位 d)Cisco2811不支持prefix-lengeh 27的配置方法，应改为netmask 3.以下关于Exchange 2007安装部署的说法错误的是（b）。（选择一项） a)在Exchange 2007中，边缘服务器角色可以部署在工作组中 b)可以将中心传输服务器和边缘服务器安装在同一台计算机上 c)建议将村出租的日志文件和数据库分别存放在不同的卷中 d)Exchange 2007根据环境，即可选择升级安装，也可选择全新安装 4.下列协商模式中（c）进行协商后能成功建立以太网通道。 a)PAgP auto 与PAgP auto

b)LACP passive 与LACP on c)PAgP on 与PAgP desirable d)LACP active 与LACP active 5.在RHEL5系统中，以下（d）可正确表示文件在系统中的绝对路径。（选择一项） a)./ b)~/ c)..grub/ d)/boot/grub/ 6.在RHEL5系统中，httpd服务器不支持基于（d）的虚拟WEB主机。（选择一项） a)域名 b)IP地址 c)TCP端口 d)目录 7.在RHEL5系统中，默认使用（c）作为缺省的文件系统类型。（选择一项） a)FAT32 b)NTFS c)EXT3 d)Reiserfs 8.在Exchange 2007 中，可以使用（a）用户在Outlook中创建和管理公