- Homogeneous CO hydrogenation: Ligand effects on the lewis acid-assisted reductive coupling of carbon monoxide
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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.
- Miller, Alexander J. M.,Labinger, Jay A.,Bercaw, John E.
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p. 4499 - 4516
(2011/01/09)
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- Synthesis and thermolysis of neutral metal formyl complexes of molybdenum, tungsten, manganese, and rhenium
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The possible intermediacy of catalyst-bound formyls in syngas transformations has prompted efforts to prepare and study the chemistry of transition-metal formyl complexes over more than a decade. We have used a mild borohydride in our reactions with metal carbonyl cations and have introduced some variations into the syntheses which allow, in almost all cases, for the pure formyl complex to be precipitated from solution as it is formed. The formyl complexes and their cationic precursors are shown. Seven of the formyls are new; improved procedures have been established for the other four. All but one of the compounds have been isolated.
- Gibson,Owens,Mandal,Sattich,Franco
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p. 498 - 505
(2008/10/08)
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