Metallacarboranes in Catalysis. 3. Synthesis and Reactivity of exo-nido-Phosphinerhodacarboranes

Article abstract of DOI:10.1021/ja00322a039

The carbon-substituted closo-bis(triphenylphosphine)hydridorhodacarborane , the carbon-substituted exo-nido-bis(triphenylphosphine)rhodacarborane complexes <(PPh3)2Rh(7,8-C2B9H10RR')> , and the salt <(PPh3)3Rh>(1+)(1-) were prepared by the reaction of the carborane anions (1-) (Ia-e) with <(PPh3)3RhCl> in benzene.Complexes IIa,c exhibited a closo-exo-nido equilibrium in solution.The exo-nido complexes can be regarded as being composed of an (1+) cation bound to a (1-) anion cage via two exo polyhedral three-center, two-electron interactions (Rh-H-B bridges) with terminal hydrogen atoms.The (1+) moiety can apparently rotate with respect to and, in some cases, migrate about the polyhedral surface of the cage.Complex IIA reacted with 2 equiv of PCy3 ( Cy = cyclohexyl) to generate the mixed phosphine exo-nido complex <(PPh3)(PCy3)Rh-7,8-μ-(1',2'-CH2C6H4CH2-)-7,8-C2B9H10> (IIIa).Reaction of the exo-nido-bis(triphenylphosphine)rhodacarboranes with good ? donors or CO displaced the rhodium from the carborane cage to give cationic species of the form (1+), (1+), (1+) (L = PPh3, S = solvent); (1+) (L = PEt3); <(L-L)2Rh>(1+) (L-L = dppe); and <(L-L-L)(L')Rh>(1+) (L-L-L = Ph2PCH2CH2P(Ph)CH2CH2PPh2 = triphos, L' = PPh3).The <(PEt3)4Rh>(1+) complexes (Va-e) reacted further to generate closo species of the general formula .The 3,1,2-isomer was obtained when R = R' = Me (VIIc), R, R' = μ-(CH2)3- (VIId), and R, R' = μ-(1',2'-CH2C6H4CH2-) (VIIa); but in the cases of R = Ph, R' = Me and R = 1'-(closo-1',2'-C2B10H11), R' = H, a polytopal rearrangement occurred, resulting in the formation of , R = Me, R' = Ph (VIb) and R = H, R' = 1'-(closo-1',2'-C2B10H11) (VIe).The complexes IIa-d and IIIa underwent oxidative addition of H2 to give dihydrido Rh(III) products in which the <(PPh3)2Rh(H)2>(1+) or <(PPh3)(PCy3)Rh(H)2>(1+) fragment remains bonded to the carborane cage through two three-center, two-electron Rh-H-B interactions.Molecular structures of two representative exo-nido-rhodacarboranes (IIb and IIIa) along with that of closo-rhodacarborane (IIa) have been determined and are formally presented in the following paper of this series.