ISSN:0021-2148    

DOI:10.1002/ijch.199100001

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe activation of CO2by chemical, electrochemical, and photochemical means is discussed. Binuclear transition metal complexes mediate oxygen atom transfers from CO2by three distinct chemical pathways: (i) deoxygenation of CO2, (ii) multiple bond metathesis, and (iii) disproportionation. The complex Ir2(μ‐CNR)2(CNR)2(dmpm),(dmpm = bis(dimethylphosphino)methane) undergoes double cycloaddition of CO2to its μ‐CNR ligands. A subsequent reaction produces the bis(carbamoyl) complex [Ir2(μ‐CO)(μ‐H)(CONHR)2(CNR)2(dmpm)2]Cl. Isotope labelling studies show that the μ‐CO ligand results from net deoxygenation of CO2. In contrast, the binuclear nickel complex Ni2(μ‐CNMe)(CNMe)2(dppm)2(dppm = bis‐(diphenylphosphino)methane) reacts with liquid CO2to give the tricarbonyl complex Ni2(μ‐CO)(CO)2(dppm)2. Isotope labelling indicates that the carbonyl ligands are not derived from CO2deoxygenation, but from C/CO triple bond metathesis. The reaction of CO2with the Ir(0) complex Ir2(CO)3(dmpm)2leads to CO2disproportionation by formation of the carbonate, Ir2(CO3)(CO)2(dmpm)2, and tetracarbonyl, Ir2(CO)4(dmpm)2, complexes. The complex Ir2(CO3)(CO)2(dmpm)2undergoes reversible O‐atom transfers from its carbonate ligand. The electrochemical activation of CO2by the binuclear Ni2(μ‐CNMe)(CNMe)2(dppm)2and trinuclear [Ni3(μ‐CNMe)(μ‐I)(dppm)3][PF6] species is described. The triangular nickel complex [Ni3(μ3‐CNMe)(μ3‐I)(dppm)3][PF6] is an electrocatalyst for the reduction of CO2. The cluster exhibits a reversible single electron reduction at E0(+/0) = −1.09 V vs. Ag/AgCl. In the presence of CO2, the cluster reduces CO2by an EC' electrochemical mechanism. The reduction products correspond to the disproportionation and H‐atom abstraction products of CO2*−, with a partitioning ratio of 10:1. Isotope labelling studies with13CO2indicate that13CO2*−disproportionation produces13CO and13CO32−.Studies of the photochemical activation of CO2by Ni2(μ‐CNMe)(CNMe)2(dppm)2are described. The bimolecular photochemical addition of CO2to the complex was examined by laser transient absorbance spectroscopy. Photolysis at 355nm in the presence of CO2(1 atm) leads to cycloaddition of CO2to the μ‐CNMe ligand and the complex Ni2(μ‐CN(Me)C(O)O)(CNMe)2(dppm)2with Φ355=0.05. The triplet excited state of Ni,(μ‐CNMe)(CNMe)2(dppm)2was determined to react with CO2with the bimolecular reaction rate constantk= 1 × 104M−1s−1. Bridging ligand substituent effects and solvent dependence of the lowest energy electronic absorption spectral bands of the series of complexes, Ni2(μ‐L)(CNMe)2(dppm)2, L = CNMe, CNC6H5, CN‐p‐C6H4Cl. and CN‐p‐C6H4Me, confirm the assignment of di‐metal to bridging ligand charge transfer (M2→μ‐LCT). This assignment is supported by results of extended Hückel calculations which indicate a LUMO of predominantly μ‐isocyanide π* character. A systematic study of the nature of the lowest excited states of related d10–d10binuclear complexes of the type Ni2(μ‐L)(CNMe)2(dppm)2, where L = CNMe(Ph)+, CNMe2+, CNMe(C5H11)+, CNMe(H)+, CNMe(CH2C6H5)+, and NO+reveals dramatic differences in the lowest excited states of the three classes of complexes: μ‐isocyanide, μ‐aminocarbyne, and μ‐nitrosyl. Spectroscopic and extended Hückel MO studies confirm that the μ‐isocyanide complexes are characterized by di‐metal to bridging ligand charge transfer (M2→ μ‐LCT) excited states. However, the μ‐aminocarbyne and μ‐nitrosyl

ISSN:0021-2148    

DOI:10.1002/ijch.199100002

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe mechanism of the loss of stereospecificity in palladium‐catalyzed nucleophilic substitution of allylic substrates has been investigated. Eight substrates (cis and trans isomers of1a‐d)and two nucleophiles (Et2NH and NaCH(SO2Ph)2) were studied. In the animation reactions two pathways are responsible for the formation of anomalous inversion product, viz., isomerization of the starting material (path B, Scheme 2) and isomerization of the π‐allyl intermediate via displacement of palladium by Pd(0) (path C, Scheme 2), the latter of which predominates. In the alkylation the results indicate that loss of stereospecificity is caused only by path C. The use of a more reactive substrate increased the stereospecificity of the reaction and suppressed the isomerization pathway. An analysis of the kinetics is consistent with the hypothesis that path C is the major pathway for the stereochemica

ISSN:0021-2148    

DOI:10.1002/ijch.199100003

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe preparation, X‐ray crystal structure, and conformational analysis of the acetyl complex [(η5‐ C9H7)Fe(CO)(PPh3)COCH3] (5) are described. Deprotonation and alkylation of complex5generates the corresponding propanoyl (6) and butanoyl (7) complexes. Deprotonation and alkylation of complexes6and7to generate complementary diastereoisomers of the corresponding 2‐methylbutanoyl complex occur with high stereoselectivities. The aldol reaction of the diethylaluminium enolate derived from5with acetaldehyde also shows a high stereoselectivity (95:5). The reactions of the indenyl complex5clearly parallel those for the cyclopentadienyl analogue and this is rationalised in terms of the conformational an

ISSN:0021-2148    

DOI:10.1002/ijch.199100004

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractHydrogenation (1 atm) of (PCy2)2Re(μ‐PCy2)2M(1,5‐COD) (M = Rh, Ir; Cy = cyclohexyl; COD = cyclooctadiene), (PCy2)2Re(μ‐PCy2)2Rh(DMPE) (DMPE = [Me2PCH2]2), [(PCy2)2ReH(μ‐PCy2)2Rh(DMPE)]BF4and (PCy2)2ReH(μ‐PCy2)2Pd‐(PPh3) proceeds stepwise with initial additions of H2across the Re = P multiple bonds to give ReH(PCy2H) moieties. Further addition of H2occurs only for M = Rh. The DMPE complex gives the tetrahydride (PCy2H)2ReH3(μ‐PCy2)2RhH(DMPE), which loses PCy2H at 80 °C to give (PCy2H)ReH4(μ‐PCy2)2Rh(DMPE). Hydrogenation of the COD complex affords the stable hexahydride (PCy2H)ReH5(μ‐PCy2)2RhH(PCy2H) via the cyclooctenyl complex. (PCy2)2ReH(μ‐PCy2)2Rh(η3‐C8H13). While the hexahydride is an active homogeneous catalyst precursor for the hydrogenation of alkenes, dienes, and alkynes, attempts to activate the Re center for C‐H bond activation have not yet been successful.31P and1H DNMR spectroscopy have been used to investigate the solution dynamics of the heterobimetallic polyhydrides, and several X‐ray diffraction studies serve t

ISSN:0021-2148    

DOI:10.1002/ijch.199100005

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractMany reactions of transition‐metal hydrides involve H* transfer. With olefins such transfer gives a radical cage, from which escape and collapse lead to product formation. Inverse isotope effects, second‐order kinetics independent of ligand concentration, and CIDNP are diagnostic for this mechanism. Many other reactions of transition‐metal hydrides occur by radical chain mechanisms, in which H* is abstracted by carbon‐centered radicals or by metal radicals.Reasonably accurate values are now available for the M‐H bond strengths of most of the common hydrides, and these values help rationalize the known H* transfer reactions of these hydrides. While the rates of certain H* transfer reactions have been measured by radical clock methods, the measurement of H* transfer rates to a substituted trityl radical has provided the first general comparison of the H* donor abilities of the various hydrides. These relative H* transfer rates are significantly affected by steri

ISSN:0021-2148    

DOI:10.1002/ijch.199100006

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY