Iron carbonyl complexes of a rigid chelating dicarbene: A density functional theory study

Abstract

The series of vegiR dicarbenes with a phenanthroline-like backbone synthesized by Kunz and co-workers provide rigid chelating bidentate ligands with carbon donor atoms. Such ligands can, in principle, replace two carbonyl groups in a metal carbonyl derivative leading to analogues of metal carbonyls but with weaker-field carbon-based ligands. Density functional theory has been used to investigate the structures and energetics of the iron carbonyl complexes (vegiMe)Fe(CO)n (n = 3, 2) and (vegiMe)Fe2(CO)n (n = 7, 6, 5) of the simplest such ligands with methyl substituents. Replacement of two carbonyl groups in Fe(CO)5 with one vegiMe ligand is predicted to give trigonal bipyramidal and tetragonal pyramidal isomers of the tricarbonyl (vegiMe)Fe(CO)3 having similar energies within ~2 kcal/mol suggesting a fluxional system. Removal of a carbonyl group from (vegiMe)Fe(CO)3 is predicted to give singlet and triplet (vegiMe)Fe(CO)2 dicarbonyl structures with similar energies having a hole in the coordination sphere for the “missing” carbonyl group. A quintet (vegiMe)Fe(CO)2 structure with tetrahedral FeC4 coordination is a higher energy isomer by ~10 kcal/mol. The three lowest energy (vegiMe)Fe2(CO)7 structures, obtained by replacing two carbonyl groups in Fe2(CO)9 with one vegiMe ligand, have the vegiMe ligand bridging a Fe–Fe single bond to form a six-membered Fe2CN2C ring. Isomeric (vegiMe)Fe2(CO)7 structures with the vegiMe ligand chelated to a single iron atom forming a five-membered FeC2N2 ring lie at least 10 kcal/mol above the lowest energy isomer. The lowest energy isomers of the unsaturated (vegiMe)Fe2(CO)n (n = 6, 5) are triplet and quintet spin state structures reflecting the lower field strength of the vegiR ligands relative to carbonyl groups.