Matrix Photochemistry of (η5-Cyclopentadienyl)bis(ethene)rhodium and (η5-Cyclopentadienyl)(ethene)carbonylrhodium: A Test-Bed for Intermediates in C-H Activation

Article abstract of DOI:10.1021/ja00214a025

The photochemical reactivities of CpRh(C2H4)2 (1) and CpRh(C2H4)CO (2) (Cp = η⁵-C5H5) have been examined in low-temperature matrices with IR and UV/vis detection.Photodissociation of ethene from 1 in inert matrices (Ar or CH4) yields the coordinatively unsaturated CpRh(C2H4); the reaction is reversed by long-wavelenght irradiation.The identity of CpRh(C2H4) is demonstrated with (2)H labeling of ethene and cyclopentadienyl groups.The IR results support a bent , ground-state geometry for CpRh(C2H4).UV photolysis of 1 in CO matrices releases ethene and generates 2 and CpRh(CO)2 sequentally.The corresponding reaction in nitrogen matrices leads to CpRh(C2H4)(N2).On photolysis of 1 in CO-doped methane matrices, CpRh(CO)(CH3)H is formed in addition to substitution products.Complex 2 undergoes loss of either CO, yielding CpRh(C2H4), or of C2H4, yielding CpRhCO, on UV photolysis in argon matrices.These reactions are reversed by long-wavelength photolysis, but in more concentrated matrices Cp2Rh2(CO)2 is formed as an additional product.In nitrogen matrices both CpRh(C2H4)(N2) and CpRh(CO)(N2) are generated.In methane matrices CpRh(C2H4) is still detected on UV photolysis, but CpRhCO is replaced by the product of oxidative addition of methane, CpRh(CO)(CH3)H.Long-wavelength photolysis removes CpRh(C2H4) but increases conversion to the (methyl)hydride complex.These experiments demonstrate that oxidative addition of methane must proceed via CpRhCO.The role of CpRhL (L = C2H4, CO) intermediates in solution reactions is discussed.