Unsaturation and variable hapticity in binuclear azulene manganese carbonyl complexes

Abstract

Azulene is reported to react with Mn₂(CO)₁₀ to give trans-C₁₀H₈Mn₂(CO)₆, which has been shown by X-ray crystallography to have a bis(pentahapto) structure with no metal-metal bond. This structure is found by density functional theory to be the lowest energy C₁₀H₈Mn₂(CO)₆ structure. However, a corresponding bis(pentahapto) cis-C₁₀H₈Mn₂(CO)₆ structure, also without an Mn⋯Mn bond, lies within ∼1 kcal mol⁻¹ of this global minimum. The lowest energy C₁₀H₈Mn₂(CO)₅ structure is singlet cis-η⁵,η⁵-C₁₀H₈Mn₂(CO)₅ with an Mn→Mn dative bond from the Mn(CO)₃ group to the Mn(CO)₂ group. However, a singlet cis-η⁶,η⁴-C₁₀H₈Mn₂(CO)₅ structure with a normal Mn–Mn single bond lies within ∼6 kcal mol⁻¹ of this global minimum. The lowest energy structures of the more highly unsaturated C₁₀H₈Mn₂(CO)_n (n = 4, 3, 2) systems all have cis geometries and manganese-manganese bonds of various orders. The corresponding global minima are triplet cis-η⁵,η³-C₁₀H₈Mn₂(CO)₃(η²-μ-CO) for the tetracarbonyl with a four-electron donor bridging carbonyl group, singlet cis-η⁵,η⁵-C₁₀H₈Mn₂(CO)₃ for the tricarbonyl, and triplet cis-η⁶,η⁴-C₁₀H₈Mn₂(CO)(η²-μ-CO) for the dicarbonyl.