Chemical and Electrochemical Reduction of Polyarene Manganese Tricarbonyl Cations:  Hapticity Changes and Generation of Syn- and Anti-Facial Bimetallic η⁴,η⁶-Naphthalene Complexes

(η⁶-Naphthalene)Mn(CO)₃⁺ is reduced reversibly by two electrons in CH₂Cl₂ to afford (η⁴-naphthalene)Mn(CO)₃^-. The chemical and electrochemical reductions of this and analogous complexes containing polycyclic aromatic hydrocarbons (PAH) coordinated to Mn(CO)₃⁺ indicate that the second electron addition is thermodynamically easier but kinetically slower than the first addition. Density functional theory calculations suggest that most of the bending or folding of the naphthalene ring that accompanies the η⁶ → η⁴ hapticity change occurs when the second electron is added. As an alternative to further reduction, the 19-electron radicals (η⁶-PAH)Mn(CO)₃ can undergo catalytic CO substitution when phosphite nucleophiles are present. Chemical reduction of (η⁶-naphthalene)Mn(CO)₃⁺ and analogues with one equivalent of cobaltocene affords a syn-facial bimetallic complex (η⁴,η⁶-naphthalene)Mn₂(CO)₅, which contains a Mn−Mn bond. Catalytic oxidative activation under CO reversibly converts this complex to the zwitterionic syn-facial bimetallic (η⁴,η⁶-naphthalene)Mn₂(CO)₆, in which the Mn−Mn bond is cleaved and the naphthalene ring is bent by 45°. Controlled reduction experiments at variable temperatures indicate that the bimetallic (η⁴,η⁶-naphthalene)Mn₂(CO)₅ originates from the reaction of (η⁴-naphthalene)Mn(CO)₃^- acting as a nucleophile to displace the arene from (η⁶-naphthalene)Mn(CO)₃⁺. Heteronuclear syn-facial and anti-facial bimetallics are formed by the reduction of mixtures of (η⁶-naphthalene)Mn(CO)₃⁺ and other complexes containing a fused polycyclic ring, e.g., (η⁵-indenyl)Fe(CO)₃⁺ and (η⁶-naphthalene)FeCp⁺. The great ease with which naphthalene-type manganese tricarbonyl complexes undergo an η⁶ → η⁴ hapticity change is the basis for the formation of both the homo- and heteronuclear bimetallics, for the observed two-electron reduction, and for the far greater reactivity of (η⁶-PAH)Mn(CO)₃⁺ complexes in comparison to monocyclic arene analogues.