Carbidocarbonyl clusters of iron

Abstract

Under reducing conditions, at elevated temperatures, coordinated carbon monoxide in transition metal carbonyls may disproportionate to CO 2 and a carbon atom. The carbon atom is trapped in a cage of metal atoms, shielded from further reaction in the core of the resulting carbidocarbonyl clusters. This class of compounds, which has been known for some time, bears some structural resemblance to the binary carbides and may therefore be relevant to Fischer-Tropsch catalysis, in which carbide phases and surface carbon atoms are implicated. The chemical, structural and physical properties of the iron carbidocarbonyIs have been investigated. Reactions are described that lead to C-H and C-C bond formation at the carbide carbon, and these are discussed with regard to the nature of this carbon atom. 13 C n.m.r. spectroscopy reveals large downfield chemical shifts for the carbide carbon, which may be interpreted as either a reflexion of low electron density at the carbon or the influence of paramagnetic contributions to the shift. Reactions of [Fe 4 (CO) 12 C CO 2 CH 3 ]-, [Fe 4 (CO) 12 C C(O)CH 3 ]- and [Fe 4 (CO) 12 C CHO]- with trimethyloxonium fluoborate yield the corresponding vinylidene clusters Fe 4 (CO) 12 - C = C (OCH 3 )R (R = OCH 3 , CH 3 , H).