Unsaturation and Variable Hapticity in Binuclear Azulene Iron Carbonyl Complexes

Abstract

The bicyclic hydrocarbon azulene has long been known to form a variety of metal carbonyl derivatives with diverse metals. In this connection the binuclear azulene iron carbonyls C10H8Fe2(CO)n (n = 6, 5, 4, 3, 2, 1) have been investigated herein. For C10H8Fe2(CO)6 the singlet cis and trans structures with the two iron atoms on the same side or opposite sites of the azulene ring system were found to be of comparable energies. The lowest energy structure for C10H8Fe2(CO)5 is the experimentally known singlet cis-η5,η3-C10H8Fe2(CO)5 structure with the C5 ring bonded to the Fe(CO)2 group as a pentahapto ligand and the C7 ring bonded to the Fe(CO)3 group as a trihapto ligand and with a formal Fe−Fe single bond length of 2.83 ± 0.05 Å. This structure is thermodynamically stable with respect to disproportionation into C10H8Fe2(CO)6 + C10H8Fe2(CO)4 by 31 ± 3 kcal/mol and resistant toward CO dissociation by 45 ± 3 kcal/mol, consistent with the fact that it is readily obtained in the laboratory by simple thermal reactions of azulene with iron carbonyls. The lowest energy C10H8Fe2(CO)4 structure is also a singlet cis-η5,η3-C10H8Fe2(CO)4 structure. This compound is predicted to have either a four-electron donor bridging η2-μ-CO group and a formal Fe−Fe single bond of length 2.73 Å by B3LYP or, more likely, all two-electron donor carbonyl groups and a formal Fe═Fe double bond of length 2.48 Å by BP86. The lowest energy structures of the more highly unsaturated C10H8Fe2(CO)n (n = 3, 2, 1) are triplet (n = 3) or quintet (n = 2, 1) spin states with both iron atoms on the same side of the azulene system. In these systems all 10 carbon atoms of the azulene system are within bonding distance of an iron atom with a short enough iron−iron distance to suggest multiple bonding. The lowest energy C10H8Fe2(CO)n (n = 4, 3) structures are predicted to be either thermodynamically unstable or marginally stable with respect to disproportionation into C10H8Fe2(CO)n+1 + C10H8Fe2(CO)n−1. This again is consistent with the experimental observation of cis-C10H8Fe2(CO)5 as the major product from the thermal reaction of azulene with iron carbonyls.