Binuclear dimethylaminoborole iron carbonyls: iron–iron multiple bonding versus nitrogen → iron dative bonding

Theoretical studies show that pendant dimethylamino groups can play a significant role in the chemistry of unsaturated binuclear dimethylaminoborole iron carbonyls. For [C4H4BN(CH3)2]2Fe2(CO)5, the lowest energy structures have single CO bridges and Fe–Fe single bonds of lengths ~2.8 A. The lowest energy [C4H4BN(CH3)2]2Fe2(CO) n (n = 4, 3) structures have two bridging CO groups with Fe=Fe double bonds of lengths ~2.5 A for n = 4 and three bridging CO groups with Fe≡Fe triple bonds of lengths ~2.2 A for n = 3. These structures are similar to structures previously found for the corresponding methylborole derivatives (C4H4BCH3)Fe2(CO) n . However, slightly higher energy [C4H4BN(CH3)2]2Fe2(CO) n (n = 4, 3) structures are found in which dimethylaminoborole is a six-electron donor bridging ligand using electron pairs from the nitrogen atom as well as from the two C=C double bonds. For the more highly unsaturated [C4H4BN(CH3)2]2Fe2(CO) n (n = 2, 1), low energy singlet (n = 2) and triplet (n = 1) perpendicular structures are also found with similar bridging six-electron donor dimethylaminoborole ligands. In addition, highly unsaturated [C4H4BN(CH3)2]2Fe2(CO) n (n = 3, 2, 1) structures are found with agostic hydrogen atoms bridging an iron–carbon bond.