CO2 and SO2 activation by a Cr–Cr quintuple

Cite this:https://www.wendangku.net/doc/7d7805467.html,mun.,2014,50,13127

CO 2and SO 2activation by a Cr–Cr quintuple bond ?

Awal Noor,Sadaf Qayyum,Tobias Bauer,Stefan Schwarz,Birgit Weber and Rhett Kempe*

A quintuply bonded dichromium complex stabilized by amino-pyridinato ligands activates CO 2and SO 2by reducing the (formal)bond order of the metal–metal bonds.Oxygen abstraction is observed during CO 2activation.SO 2activation proceeds via formation of a unique dithionite complex/coordination.Furthermore,N 2O activation was investigated and the formation of a tetrameric Cr–oxo complex was observed.

CO 2and SO 2are important molecules with,for instance,respect to global warming and acid rain formation,respectively.Thus,their emission has to be reduced with the search for utilization protocols of such molecules.Whereas SO 2is diversely used in the food industry and for the production of sulfuric acid,the utiliza-tion of CO 2as a C 1-feedstock is still a highly attractive goal.CO 2is thermodynamically stable and chemically inert and needs to be activated prior to chemical conversions.Stoichiometric CO 2acti-vation has been achieved (for instance)using complexes based on f -block metals,1main group metal compounds,2frustrated Lewis pairs,3and most intensively using transition metal complexes.For the latter,mononuclear 4and bimetallic complexes 5have been investigated.In comparison,SO 2activation has been mainly achieved by mononuclear transition metal complexes.6

Stable molecules containing a quintuple bond have been reported for chromium and molybdenum.7,8The bimetallic platform in such quintuply bonded complexes stores 10electrons and is well suited for small molecule activation.9In addition,the Cr complexes show unusual short metal–metal bonds with the shortest bonds [1.7056(12)?]matching by length with long C–C bonds.10

We have a rather long ongoing interest in CO 2activation 11and quintuple bond reactivity 9and report here on the activation of CO 2and SO 2by a chromium–chromium quintuple bond.

CO 2activation proceeds with C–O-bond cleavage resulting in the formation of a carbonyl complex and a chromium oxo species that differs structurally from the O 2activation product.9d SO 2activation leads to dithionite formation.Attempts to activate CO 2using quintuple bonds have failed so far.9g

The reaction of CO 2with quintuply bonded chromium dimer 1(Scheme 1)leads to C Q O bond cleavage to give a CO bridged complex (2)(Scheme 1).Complex 2shows evident reduction of CO 2to https://www.wendangku.net/doc/7d7805467.html,plex 2can also be prepared by reaction of 1with an excess of carbon monoxide in toluene.Following this reaction by NMR,it was found that the formation of 2is nearly https://www.wendangku.net/doc/7d7805467.html,plex 2is diamagnetic and the 1H NMR spectrum in benzene-d 6shows the resonance of the methyl protons of the isopropyl group as two doublets at 1.06and 1.36ppm with a coupling constant 3

J HH =6.8Hz.The sharp singlet belonging to the methyl group of the 2,6-dimethylphenyl substituents appears at 2.11ppm along with methyl protons of the co-crystallized toluene molecule.The resonance of the isopropyl CH -protons appears as a septet at 3.13ppm.In 13C NMR,in addition to other signals,the peak for the carbon of the CO ligand is observed at 292ppm.The infrared spectrum of complex 2in the range of 4000–400cm à1shows no absorption bands in the region 2300–2100cm à1.The strong absorption bands observed at 1924and 1806cm à1can be attri-buted to the carbonyl ligands.In solution (toluene)signals at 1942and 1857cm à1were recorded.The signals at higher wave numbers can be attributed to semi-bridged CO due to steric crowding,which is in accordance with the electronic structure of 2(vide infra

).

Scheme 1Synthesis of complex 2.In the course of the reaction with CO 2,an oxo–chromium byproduct is formed (vide infra ).

Lehrstuhl Anorganische Chemie II,Universita

¨t Bayreuth,95440Bayreuth,Germany.E-mail:Kempe@uni-bayreuth.de

?Electronic supplementary information (ESI)available:General and synthetic methods,instrumentation,and full crystallographic https://www.wendangku.net/doc/7d7805467.html,DC 1009949(2),1009951(3)and 1009950(4).For ESI and crystallographic data in CIF or other electronic format see DOI:10.1039/c4cc05071a

Received 2nd July 2014,Accepted 31st August 2014DOI:10.1039/c4cc05071a

https://www.wendangku.net/doc/7d7805467.html,/chemcomm

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The molecular structure of 2is shown in Fig.1.In 2,the two Cr atoms are in the plane of the pyridine rings of the aminopyridinato ligands.The two CO ligands are coordinated perpendicular to this plane with a C–O bond length of 1.154(8)?and Cr–C–O angles of 152.9(6)and 153.2(6)1(Fig.1).The Cr–C bond distances of the bridging carbonyl ligand are 2.092(8)and 2.066(8)?.These values are comparable to Cr–Cr bonded complexes with bridging carbonyls.12The Cr–Cr distance of 1.886(19)?is in the range known for Cr–Cr quadruple bonds but a quadruple bond is not in accordance with the electronic structure of 2(vide infra ).

To investigate the potential byproduct resulting from the oxygen released,NMR experiments were carried out.They showed 2to be the major product along with some broad peaks attributable to a paramagnetic compound.This compound di?ers from the oxo complex isolated if 1is reacted with O 2.9d To get more insight we reacted N 2O with 1(Scheme 2).The purple solution of 1in toluene turned green upon exposure to N 2O for half an hour at room temperature.Cooling of the solution under a N 2O atmosphere leads to a cluster compound 3.The X-ray structural analysis of 3shows a chair like conformation for the central [CrO]4fragment,in which two of the oxygens show (m 2-O)and the other two show (m 3-O)bridging coordination modes (Fig.2).The Cr–O bond distances are as expected,longer for the (m 2-O)[1.839–2.050?]coordinated oxo–ligand than the (m 3-O)[1.706–1.965?]oxo-ligated moiety.Similarly,the Cr1–Cr1A bond length [2.7904(10)?]for the Cr–Cr fragment joined by two (m 3-O)is shorter than the two terminal Cr–Cr fragments [2.8375(8)?each]joined by one (m 2-O)and one (m 3-O)ligand.The aminopyridinato ligands are no more coordinated in a m 2-fashion but are coordinated in an Z 2-mode.The N amido –Cr bond lengths for the aminopyridinato ligands coordinated to the two central Cr atoms are comprehensively shorter [1.898(3)?]than N amido –Cr bond lengths [2.081(1)?]for the aminopyridinato ligands coordinated to the two terminal Cr atoms.On the other hand,N pyridine –Cr bond lengths for the two central Cr atoms are longer [2.179(3)?]than N pyridine –Cr bond lengths [2.113(3)?]for the two terminal Cr atoms,highlighting the flexibility of the aminopyridinato ligands.13

The room temperature magnetic moment of 3is 6.16,too low for a Cr III compound with four S =3/2centres (theoretical value:7.74,assuming g =2).The measured values are closer to the theoretically expected values for four S =1centres (m S =5.65m B [g =2])assuming some orbital momentum contribution.This is in agreement with a Cr–Cr-bond with a low bond order.Upon cooling,a slow decrease of the magnetic moment down to 3.84m B at 25K is observed.This behaviour is best explained with weak anti-ferromagnetic interactions between the remaining unpaired electrons of the four chromium centres.Indeed,the magnetic moment at 25K is in the region expected for one unpaired electron per chromium (m S = 3.46m B assuming g =

2).

Fig.1Molecular structure of 2with hydrogen atoms and one toluene molecule omitted for clarity.Select bond lengths [?]and angles [1].Cr1–Cr1A 1.886(19),Cr1–N12.011(4),Cr1–N22.016(4),Cr1–C12.066(8),Cr1–C12.092(8),C1–O1 1.154(8),C1–Cr1 2.066(8);Cr1–Cr1A–N197.56(13),Cr1–Cr1A–N294.04(13),N1–Cr1–N2A 168.39(17),Cr1–Cr1A–C163.7(2),N1–Cr1–C192.0(2),N2–Cr1–C193.5(2),Cr1–Cr1A–C162.3(2),Cr1–C1–O1152.9(6),Cr1A–C1–O1153.2(6),C1–Cr1–C1126.1(2),O1–C1–Cr1152.9(6),Cr1–C1–Cr1A

53.9(2).

Scheme 2Synthesis of complex 3

.

Fig.2Molecular structure of 3with hydrogen atoms omitted for clarity.Select bond lengths [?]and angles [1].N1–Cr11.898(3),N2–Cr12.179(3),N3–Cr22.081(3),N4–Cr22.113(3),O3–Cr11.702(2),O3–Cr21.950(2),O4–Cr11.840(2),O4–Cr1A 1.949(2),O4–Cr22.043(2),Cr1–Cr1A 2.7904(10),Cr1–Cr2 2.8375(8);Cr1–O3–Cr2101.74(10),Cr1–O4–Cr1A 94.80(10),Cr1–O4A–Cr2A 131.82(12),Cr1–O4–Cr290.55(9),O3–Cr1–O4A 122.43(10),O3–Cr1–N1119.07(11),O4–Cr1–N1118.37(11),O3–Cr1–O487.55(10),O4–Cr1–O4A 85.20(10),N1–Cr1–O4A 101.29(10),O3–Cr1–N296.07(10),O4A–Cr1–N2103.41(10),N1–Cr1–N265.75(10),O4–Cr1–N2166.68(10),O3–Cr1–Cr1A 108.90(8),O4–Cr1–Cr1A 44.11(7),N1–Cr1–Cr1A 116.75(8),N2–Cr1–Cr1A 146.37(8),O3–Cr1–Cr242.29(7),O4–Cr1A–Cr2A 115.01(7),N1–Cr1–Cr2112.13(8),N2–Cr1–Cr2134.05(7),Cr1–Cr1A–Cr2A 78.12(2),O3–Cr2–O478.68(9),O3–Cr2–N3162.07(10),O4–Cr2–N3114.10(10),O3–Cr2–N4102.60(10),O4–Cr2–N4178.45(11),O3–Cr2–Cr135.97(6),N3–Cr2–Cr1151.68(8),N4–Cr2–Cr1137.74(8).

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The pronounced drop in the magnetic moment below 25K can be explained using zero field splitting.

The introduction of SO 2into a toluene solution of 1led to an immediate colour change from purple to orange (Scheme 3).Orange crystals of 4were grown at low temperature.X-ray analysis revealed the formation of a dithionite (S 2O 42à)ligand.In compound 4the dithionite ligand lies on top of the Cr 2moiety with a bridging O 2S–SO 22àgroup where the four oxygen atoms coordinate to the two Cr atoms with Cr–O bond distances between 2.0360(14)and 2.0698(14)?(Fig.3).This coordination mode of dithionite to transition metals has not been described yet (to the best of our knowledge).The long S–S bond distance of 2.3786(7)?could be due to the p antibonding interactions and is comparable to that found in Na 2S 2O 4[2.389?].14A Cr–Cr bond distance of 2.3419(5)?was observed.The S–O bond distances are in the range of 1.5045(15)–1.5189(15)?.The O–S–O angles of 107.46(8)and 107.85(8)1are close to tetrahedral but the S–S–O angles (92.21(6)–93.90(6)1)deviate https://www.wendangku.net/doc/7d7805467.html,pound 4is a thiolate analogue of an oxalate,which has been previously observed due to the reductive coupling of CO 2for Fe I and Ni I compounds.15The 1H NMR of 4shows very broad peaks in the diamagnetic range of 0to 8ppm and could

be attributed to the steric hindrance of the molecule.Low tem-perature experiments could not be carried out since the compound precipitates quickly in non-polar solvents like toluene and decom-poses in more polar solvents like https://www.wendangku.net/doc/7d7805467.html,pound 4is stable like 2without an atmosphere of SO 2or CO 2,respectively,and does not convert back to 1as evidenced by NMR studies.

Density functional theory (DFT)calculations were performed using the TURBOMOLE 16programme package.The geometry of 1,2and 4was optimized starting from structural data obtained by X-ray single crystal structure https://www.wendangku.net/doc/7d7805467.html,putational details are given in the ESI.?Calculations indicate the reduction of the formal bond order from five to four for the SO 2activation product (4).Fig.S2(ESI ?)shows the metal centered HOMOs of 1and 4.The formal bond order of the carbonyl complex 2is lower than four.In addition to the consumption of the two d -bonds,one of the p bonds is also significantly involved in CO bonding.The relevant frontier orbitals of 2are shown in Fig.S3(ESI ?).The geometry optimization of 2shows that the CO ligands form stronger bonds with one single Cr center (Table S4,ESI ?).The energy of this non-centrosymmetric struc-ture is 5kcal mol à1lower than the energy of the centrosymmetric structure.Due to the symmetry of the molecule,the crystal structure of 2shows two indistinguishable Cr centers.

In summary,we could show that quintuply bonded dichromium complexes can activate CO 2and SO 2in a controlled fashion.Oxygen removal is observed during CO 2activation leading to a doubly CO bridged chromium complex.As an oxygen scavenger,an oxo-bridging tetrametallic Cr-complex could be identified.SO 2activation proceeds via S–S-bond formation leading to a unique dithionite complex.

Financial support from the Deutsche Forschungsgemeinschaft (DFG KE 756/20-2)is gratefully acknowledged.We thank

Prof.S.Ku

¨mmel for helping with the calculations.Notes and references

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Select bond lengths [?]and angles [1].N1–Cr12.0551(16),N2–Cr22.0594(16),O1–S11.5045(15),O1–Cr12.0360(14),O2–S11.5189(15),O2–Cr22.0698(14),S1–S22.3786(7),Cr1–Cr22.3419(5);S1–O1–Cr1112.80(8),S1–O2–Cr2114.73(8),S2–O3–Cr2112.26(8),S2–O4–Cr1115.36(8),O1–S1–O2107.85(8),O1–S1–S292.21(6),O1–Cr1–N4172.07(6),O1–Cr1–N189.23(6),N4–Cr1–N198.19(6),O1–Cr1–O476.65(6),N4–Cr1–O496.11(6),N1–Cr1–O4165.40(6),O1–Cr1–Cr295.14(4),N1–Cr1–Cr290.47(5),O4–Cr1–Cr287.18(4),O3–Cr2–N2172.16(6),O3–Cr2–N387.64(6),N2–Cr2–N399.76(6),N2–Cr2–O296.29(6),N3–Cr2–O2163.55(6),O3–Cr2–Cr195.27(4),N2–Cr2–Cr187.39(5),O2–Cr2–Cr187.38(4).

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