Isolation of the First Metalloxy Ketene Complexes via 'Double Insertion' of Carbon Monoxide into Thorium-Silicon Bonds

摘要

Migratory insertion reactions of carbon monoxide in early-transition-metal compounds have played an important role in development of molecular CO-activation chemistry. In particular, d~0 metal complexes of the transition, lanthanide, and actinide series have often been observed to participate in COCO coupling reactions. The mechanisms of these "double insertion" reactions have been the subject of much speculation, but little mechanistic information has been forthcoming. In some cases, reactivity studies have suggested the possibility that an initial η~2-acyl product of insertion couples with carbon monoxide to form a highly reactive metalloxy ketene intermediate, (L_nMO)(R)CCO; however, despite much effort, such ketene derivatives have never been isolated or even observed. Because of their enhanced reactivity, it has been suggested that these species are activated by intramolecular coordination of the terminal ketene oxygen to the metal center (and bending of the C=C=O linkage) in monometallic or bimetallic structures. In our investigations of the chemistry of d~0 metal—silicon bonds, we have encountered carbonylation reactions which appear to involve (tantaloxy)(silyl)- and (scandoxy)(silyl)-ketene intermediates. We now report the first observation and isolation of metalloxy ketene complexes, which have resulted from attempts to study the chemistry of actinide-silicon bonds.