Exploration of ruthenium -carbon multiple bond complexes: Carbenes, carbynes, and carbides.

Abstract

Complexes that contain a metal-carbon multiple bond have become ubiquitous as catalysts for numerous organic transformations. Arguably the most significant of these transformations include olefin and alkyne metatheses. Ruthenium is an increasingly popular option in the area of olefin metathesis, yet alkyne metathesis is restricted to Mo, W, and Re. Exploration of the interconversion of complexes that contain a metal-carbon multiple bond has led to interesting and unusual transformations. A dehydrochlorination of the first-generation Grubbs catalyst mediated by a subvalent group 14 complex, an alkyl germylene, afforded access to four-coordinate ruthenium carbyne complexes. Treatment of these complexes with acid regenerates carbene complexes. Oxidation of these four-coordinate ruthenium carbynes provided a dichotomy between the small halide complexes, which form six-coordinate ruthenium carbyne complexes, and the large halide complexes, which form five-coordinate ruthenium carbyne complexes. Halide size, phosphine cone angle, and solubility of the resultant products produced a fine balance between five- and six-coordinate, neutral and cationic high oxidation state ruthenium carbyne complexes. Four of these five- and six-coordinate ruthenium benzylidyne complexes were structurally characterized including a rare late metal organometallic trifluoride complex, which showed activity as a terminal alkyne dimerization pre-catalyst to form conjugated enynes. An unprecedented example of spontaneous expulsion of carboxylic acids from Fischer carbene complexes was elucidated. Besides acid expulsion, mechanistic studies of this reaction provided insight into several simultaneous processes, including phosphine exchange and acetate/carbene substituent exchange. The formation of some new and <super>13</super>C-labeled neutral terminal carbides was achieved via acyloxycarbene complexes. The reactivity of the first- and second-generation ruthenium carbides was investigated extensively with the discovery of C-C, C-O, and C-S bond forming reactions. The resulting complexes included an unusual cyclopropenylidene complex and the first NHC/carbyne metal complex.