Structure and reactivity of dimeric rhodium(i) formate complexes: X-ray crystal structure analysis of [{(cod)rh(μ-κ2o,o′-hco2)}2] and phosphane-induced hydride transfer to give an η3-cyclooctenyl complex

Article abstract of DOI:10.1016/0022-328X(95)05969-V

The X-ray crystal structure analysis of [(L₂)Rh(μ-κ²O,O′-HCO₂)₂Rh(L′₂)] 1a (L₂ = L′₂ = cod) is reported. Complex 1a reacts with CO to form 1c (L = L′ = CO) via the intermediate 1b (L₂ = cod, L′ = CO). 1a and 1c do not incorporate ¹³CO₂ or D₂ in the formate moiety and are poor catalysts for CO² hydrogenation to formic acid. Reaction of la with the chelating phosphanes R₂P(CH₂)₂PR₂ (R = Ph, ⁱPr, Cy) 3a-c leads to replacement of the diolefin ligand and cleavage of the dimeric structure under formation of monomeric complexes [(3a-c)₂Rh][HCO₂] 4a-c. 4a was isolated in up to 78% yield and complexes [(3a)₂RhH] 5a and [(3a)Rh(η³-C₈H₁₃)] 6a were detected as side products. Complexes of type 6 are formed exclusively under identical reaction conditions using the bidentate ligand Ph₂P(CH₂)₃PPh₂ 3d or monodentate ligands PAr₃ 3e-f. A possible mechanism for the formation of complexes 4, 5 and 6 is discussed involving hydride transfer from the HCO^-₂ ligand to Rh and subsequently to coordinated cod.