Bridging hydrogen atoms versus iron–iron multiple bonding in binuclear borole iron carbonyls

Abstract

The geometries and energetics of the binuclear unsubstituted borole iron carbonyls (C4H4BH)2Fe2(CO)n (n=5, 4, 3, 2, 1) have been studied by density functional theory for comparison with the previously studied related substituted borole iron carbonyls (C4H4BR)2Fe2(CO)n [R=CH3, (CH3)2N] having different substituents on the boron atoms. The lowest energy (C4H4BH)2Fe2(CO)n (n=5, 4, 3) structures have terminal borole ligands related to those in the (C4H4BR)2Fe2(CO)n [R=CH3, (CH3)2N] systems as well the isoelectronic (η5-C5H5)2Mn2(CO)n systems. However, the lowest energy structure of the dicarbonyl (C4H4BH)2Fe2(CO)2 is an unusual quintet spin state structure with one of the borole ligands bridging the central Fe2 unit by forming a three-center two-electron BHFe bond to one iron atom as well as functioning as a pentahapto ligand to the other iron atom. For the (C4H4BR)2Fe2(CO)n [R=CH3, (CH3)2N] systems the tricarbonyls (n=3) having formal FeFe triple bonds of lengths ∼2.2Å analogous to the experimentally known (η5-Me5C5)2Mn2(μ-CO)3 structure appear to be favorable structures. An analogous (C4H4BH)2Fe2(μ-CO)3 structure is found for the unsubstituted borole ligand. However, this structure appears to be disfavored relative to disproportionation into (C4H4BH)2Fe2(CO)n+1+(C4H4BH)2Fe2(CO)n−1.