PHASE-TRANSFER-CATALYZED NUCLEOPHILIC REACTIONS OF HYDROXIDE IONS AT METAL-BOUND CARBON-MONOXIDE CENTERS
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Phase-transfer-catalyzed nucleophilic reactions of hydroxide ions with group 6B metal carbonyls (M(CO)6-nLn, where = 0 or 1 and L = phosphine or phosphite ligands) are discussed. In the presence of H218O, oxygen-18 enriched M(CO)6-nLn species were afforded. These oxygen-exchange processes were monitored by both u(CO) infrared spectroscopy and the small upfield shifts in the 13C NMR resonances caused by oxygen-18. During the (PTC) oxygen-exchange reaction of OH- with the M(CO)6 species, slow formation of highly oxygen-18 labeled m-H[M(CO)5]2- derivatives was observed. Furthermore, when this process was carried out under a carbon monoxide atmosphere, hydrogen gas was produced catalytically; i.e., catalysis of the water-gas shift reaction occurred. The metal dependence of the oxygen-exchange process was found to be W > Mo > Cr, with phosphite-substituted derivatives being more reactive than their phosphine analogues. In substituted metal-carbonyl derivatives, where electronically different carbonyl ligands were present, the oxygen-exchange reaction was shown to occur preferentially at the more electrophilic carbon site, i.e., at the cis carbon monoxide ligands in M(CO)5L derivatives. Several of these thus formed stereoselectively oxygen-18 labeled species were demonstrated to simultaneously undergo intramolecular ligand rearrangement by a non-bond-breaking mechanism. The use of hydroxide ions under (PTC) conditions in the facile CO ligand substitution process in group 6B metal carbonyl complexes was investigated in a 13CO atmosphere as well as in oxygen-18 enriched water. Both processes, CO ligand substitution and oxygen exchange, were greatly enhanced in the presence of phosphine ligands. This observation, coupled with the lack of metal hydride production from the reaction of hydroxide ions with phosphine-substituted metal-carbonyl derivatives, accounts for the quenching of catalytic activity by phosphine ligands during homogeneous catalyzed water-gas shift reactions involving mononuclear and cluster metal-carbonyl compounds. 1980, American Chemical Society. All rights reserved.
