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)(η 3 -cyclohexenyl)manganese tricarbonyl [(Ph)(η 3 -C 6 H 8 )Mn(CO) 3 ] (complex 1 ) under the protonation of tetrafluoroboric acid-diethyl ether (HBF 4 .Et 2 O), a comprehensive density functional theory (DFT) theoretical study was performed. The computational results showed that formation of the [(cyclohex-3-enyl)-η 6 -benzene]manganese tricarbonyl complex [(C 6 H 9 )(η 6 -Ph)Mn(CO) 3 + ][BF 4 ] (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 (η 2 -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.