UNUSUAL LOW-VALENT DIRHODIUM COMPLEXES BRIDGED BY BIS(DIMETHYLPHOSPHINO)METHANE LIGANDS

The low-valent complex [Rh2(CO)3(dmpm)2] (1) (dmpm = Me2PCH2PMe2) is prepared by the reaction of trans-[RhCl(CO)(dmpm)]2 with aqueous NaOH and CO. Under a CO atmosphere 1 is in equilibrium with the labile tetracarbonyl species [Rh(CO)(mu-CO)(dmpm)]2 (2). The structure of 1 is believed to be unsymmetrical having a trans arrangement of diphosphine ligands at one metal and a cis arrangement at the other, and a mixed-valence Rh(1+)/Rh(1-) formulation, while 2 is symmetric having the bridging diphosphines cis at both metals, and a corresponding Rh(0)/Rh(0) formulation. Reduction of [Rh2Cl2-(CO)2(mu-RC=CR)(dmpm)2] (R = CO2Me, CF3) yields [Rh2(CO))2(mu-RC=CR)(dmpm)2] (3, 4), which can also be obtained by alkyne addition to 1. The alkyne ligands are apparently bound perpendicular to the Rh-Rh axes in both species. Protonation of 1 with HBF4.OEt2 yields [Rh2(CO)3(mu-H)(dmpm)2][BF4], and under carbon monoxide this yields [Rh2(CO)4(mu-H)(dmpm)2][BF4]. Protonation of 3 yields [Rh2(CO)2-(mu-DMAD)(mu-H)(dmpm)2][BF4], whereas protonation of 4 yields [Rh2(CO)2(CF3C=C(H)CF3)(dmpm)2] [BF4], which apparently has a sigma, pi-vinylic moiety. Reaction of this vinyl complex with CO gives [Rh2(CO)3-(CF3C=C(H)CF3)(mu-CO)(dmpm)2][BF4] in which the vinyl group is sigma-bound to only one metal.