Synthesis and structure of dimeric Rh-bis(tertiary phosphine) complexes, exceptionally useful synthetic precursors

Removal of the MeOH and hydrogen from the known cis,trans,cis -Rh III -dihydrido complexes 〚Rh(H) 2 (PR 3 ) 2 (MeOH) 2 〛PF 6 (R = Ph, p -tolyl) results in formation of the dimeric species 〚Rh 2 (PR 3 ) 4 〛〚PF 6 〛 2 ; X-ray analysis shows the complexes to be 〚(Ph 3 P)Rh(μ-PhPPh 2 )〛 2 〚PF 6 〛 2 (and the p -tolyl analogue) containing bridged η 6 -arene moieties, while 1 H and 13 C NMR data in CD 2 Cl 2 provide evidence for η 4 -coordination of the arene within the dimer. In more strongly coordinating solvents, formation of cis -〚Rh(PR 3 ) 2 (solvent) 2 〛PF 6 is observed, while formation of 〚(PR 3 ) 2 Rh(η 6 -toluene)〛PF 6 is evident in toluene solution, and this exists in equilibrium with the bis(solvent) species in the presence, for example, of acetone or MeOH. At ambient conditions, none of the arene-containing complexes effected catalytic H 2 -hydrogenation of toluene.