Alkene Metalates as Hydrogenation Catalysts

Abstract

First-row transition-metal complexes hold great potential as catalysts for hydrogenations and related reductive reactions. Homo-and heteroleptic arene/alkene metalates(1-) (M=Co, Fe) are a structurally distinct catalyst class with good activities in hydrogenations of alkenes and alkynes. The first syntheses of the heteroleptic cobaltates [K([18]crown-6)][Co(eta(4)-cod)(eta(2)-styrene)(2)] (5) and [K([18]crown-6)][Co(eta(4)-dct)(eta(4)-cod)] (6), and the homoleptic complex [K(thf)(2)][Co(eta(4)-dct)(2)] (7; dct=dibenzo[a,e]cyclooctatetraene, cod=1,5-cyclooctadiene), are reported. For comparison, two cyclopentadienylferrates(1-) were synthesized according to literature procedures. The isolated and fully characterized monoanionic complexes were competent precatalysts in alkene hydrogenations under mild conditions (2 bar H-2, r. t., THF). Mechanistic studies by NMR spectroscopy, ESI mass spectrometry, and poisoning experiments documented the operation of a homogeneous mechanism, which was initiated by facile redox-neutral pi-ligand exchange with the substrates followed by H-2 activation. The substrate scope of the investigated precatalysts was also extended to polar substrates (ketones and imines).