Homogeneous CO Hydrogenation: Ligand Effects on the Lewis Acid-Assisted Reductive Coupling of Carbon Monoxide

Structure−function studies on the role of pendent Lewis acids in the reductive coupling of CO are reported. Cationic rhenium carbonyl complexes containing zero, one, or two phosphinoborane ligands (Ph 2 P(CH 2 ) n B(C 8 H 14 ), n = 1−3) react with the nucleophilic hydride [HPt(dmpe) 2 ] + to reduce [M−CO] + to M−CHO; this step is relatively insensitive to the Lewis acid, as both pendent (internal) and external boranes of appropriate acid strength can be used. In contrast, whether a second hydride transfer and C−C bond forming steps occur depends strongly on the number of carbon atoms between P and B in the phosphinoborane ligands, as well as the number of pendent acids in the complex: shorter linker chain lengths favor such reductive coupling, whereas longer chains and external boranes are ineffective. A number of different species containing partially reduced CO groups, whose exact structures vary considerably with the nature and number of phosphinoborane ligands, have been crystallographically characterized. The reaction of [(Ph 2 P(CH 2 ) 2 B(C 8 H 14 )) 2 Re(CO) 4 ] + with [HPt(dmpe) 2 ] + takes place via a “hydride shuttle” mechanism, in which hydride is transferred from Pt to a pendent borane and thence to CO, rather than by direct hydride attack at CO. Addition of a second hydride in C 6 D 5 Cl at −40 °C affords an unusual anionic bis(carbene) complex, which converts to a C−C bonded product on warming. These results support a working model for Lewis acid-assisted reductive coupling of CO, in which B (pendent or external) shuttles hydride from Pt to coordinated CO, followed by formation of an intramolecular B−O bond, which facilitates reductive coupling.