ETA-3-MEC(CH2PPH2)3/RHODIUM COMPLEXES UTILIZE PHOSPHINE ARM DISSOCIATION MECHANISMS AT 25-DEGREES-C

Reaction of RhMe3(triphos) (triphos = MeC(CH2PPh2)3) with CO generates acetone and RhMe(CO)(triphos), which reacts with further CO to give Rh[C(O)Me](CO)(triphos). The structure of RhMe(CO)(triphos) shows one strained P-Rh-P bond angle between equatorial ligands (90.80 (5)°) in a trigonal bipyramid, together with intramolecular steric effects that cause a small equatorial CO-Rh-axial(CH3) C/C angle of 79.09 (25)°. The acetyl and methyl complexes react with H2 at 25 °C to produce acetaldehyde and methane, respectively, together with RhH(CO)(triphos). Reaction of CO with RhH3(triphos) is even faster than with RhMe3(triphos) to give H2 and RhH(CO)(triphos), together with a CO hydrogenation product. These results show that these clean stoichiometric conversions, as well as a variety of isotopic exchange reactions of the Rh(I) and Rh(III) compounds with D2 and 13CO, occur by preequilibrium dissociation of one arm of the triphos ligand at 25 °C. One such species, Rh[C(O)Me](CO)2(η2-triphos), is directly detectable and reveals the mechanism of exchange of Rh[C(O)Me](13CO)(triphos) with 12CO. The coordination of CO to Rh(H)3(η2-triphos) is proposed to generate a dihydrogen complex, thus accounting for the CO-induced elimination of H2. As suggested by these individual reactions, RhH(CO)(triphos) is a catalyst for olefin hydroformylation. The high n:iso selectivity mimics that of RhH(CO)(PPh3)3 in the presence of a large amount of added PPh3, a beneficial consequence of the chelate effect. © 1990, American Chemical Society. All rights reserved.