A Diiron(I/II) μ-1,2-Carbonyl Complex Relevant to CO Binding on Fe(111)

Abstract

Fischer-Tropsch conversion of syngas to hydrocarbons is proposed to begin with CO binding to the iron surface of the catalyst. CO adsorption on various iron facets of relevance to the Fischer-Tropsch process suggest that the Fe(111) surface is the most active for catalysis, and that CO bound to the penultimate layer of Fe atoms or the b-state is the resting state during catalysis. Notably, a μ-1,2 mode was discarded for the b-state due to a lack of exemplar molecular species and expectation that such a mode would have a higher energy infrared (IR) absorption than observed experimentally (viz. 1735-1860 cm^-1). Here, we report the synthesis of a diiron(I/II) complex in which CO binds μ-1,2: (Fe(OTf))(Fe(THF)(μ-1,2-CO))L where L^2- is a bis(β-diketiminate) cyclophane (1). Surprisingly, the observed ν_CO at 1763 cm^-1 for 1 compares well with that reported for b-state. Electron paramagnetic resonance (EPR), Mössbauer, and density functional theory (DFT) results support a weakly coupled s = 3/2 iron(I) and s = 2 iron(II) pair. Reduction of 1 results in C-O cleavage and C-C bond formation to yield a ketenylidene (CCO) complex as a major product observed spectroscopically.