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

## TL;DR

This study uses DFT computations to explain how a manganese complex undergoes unusual migration of its Mn(CO)3 fragment through agostic intermediates.

## Contribution

The study reveals a new catalytic pathway involving manganese agostic intermediates in the migration of Mn(CO)3 fragments.

## Key findings

- The Mn(CO)3 migration occurs via mono-agostic and di-agostic intermediates.
- The rate-limiting step involves a di-agostic complex with a Gibbs barrier of 15.4 kcal mol−1.
- Agostic interactions were confirmed using geometry parameters, AIM, and NAdOs analyses.

## Abstract

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.