Unveiling the Unusual Mn(CO)₃ Migration in a Manganese Cyclohexenyl Complex by DFT Computations

Homogeneous catalysis involving a transition metal agostic interaction (TM^…H^…C) is an attractive strategy for C-H bond activation, in which the transition metal agostic intermediates serve as the critical component. To investigate the roles of manganese agostic intermediates in the unusual migration of the Mn(CO)₃ fragment in the (exo-phenyl)(η³-cyclohexenyl)manganese tricarbonyl [(Ph)(η³-C₆H₈)Mn(CO)₃] (complex 1) under the protonation of tetrafluoroboric acid-diethyl ether (HBF₄.Et₂O), a comprehensive density functional theory (DFT) theoretical study was performed. The computational results showed that formation of the [(cyclohex-3-enyl)-η⁶-benzene]manganese tricarbonyl complex [(C₆H₉)(η⁶-Ph)Mn(CO)₃⁺][BF₄] (complex 2) was achieved via a series of mono-agostic and di-agostic intermediates. The overall rate-limiting step for this unusual migration of the Mn(CO)₃ fragment is the formation of the di-agostic (η²-phenyl)manganese complex 8 (4 → 5 → 8) with a Gibbs barrier of 15.4 kcal mol^-1. The agostic intermediates with TM^…H^…C agostic interactions were well-characterized by geometry parameters, Atoms-In-Molecules (AIM) analyses, and the Natural Adaptive Orbitals (NAdOs). The located pathways in the current study successfully explained the experimental observations, and the findings on the TM^…H^…C agostic interaction provided a new aspect of the catalytic reaction with the manganese complex.