Structure and reactivity of dimeric rhodium(I) formate complexes: X-ray crystal structure analysis of [{(cod)Rh(mu-kappa O-2,O'-HCO2)}(2)] and phosphane-induced hydride transfer to give an eta(3)-cyclooctenyl complex

The X-ray crystal structure analysis of [(L(2))Rh(mu-kappa(2)O,O'-HCO2)(2)Rh(L'2)] 1a (L(2)=L'(2)=cod) is reported. Complex 1a reacts with CO to form 1c (L=L'=CO) via the intermediate 1b (L(2)=cod, L'=CO). 1a and 1c do not incorporate (CO2)-C-13 or D-2 in the formate moiety and are poor catalysts for CO2 hydrogenation to formic acid. Reaction of la with the chelating phosphanes R(2)P(CH2)(2)PR(2) (R=Ph, Pr-i, Cy) 3a-c leads to replacement of the diolefin ligand and cleavage of the dimeric structure under formation of monomeric complexes [(3a-c)(2)Rh][HCO2] 4a-c. 4a was isolated in up to 78% yield and complexes [(3a)(2)RhH] 5a and [(3a)Rh(eta(3)-C8H13)] 6a were detected as side products. Complexes of type 6 are formed exclusively under identical reaction conditions using the bidentate ligand Ph(2)P(CH2)(3)PPh(2) 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 HCO2- ligand to Ph and subsequently to coordinated cod.