118228-22-1Relevant articles and documents
Homogeneous CO hydrogenation: Ligand effects on the lewis acid-assisted reductive coupling of carbon monoxide
Miller, Alexander J. M.,Labinger, Jay A.,Bercaw, John E.
, p. 4499 - 4516 (2011/01/09)
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 (Ph2P(CH 2)nB(C8H14), 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 [(Ph2P(CH 2)2B(C8H14))2Re(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 C6D5Cl 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.
