The missing agostomer in the fluxionality of cyclohexenylmanganese tricarbonyl

Abstract

Fluxionality of the cyclohexenylmanganese tricarbonyl complex [(C6H9)Mn(CO)3], which has an Mn-H-C 3-center 2-electron agostic interaction, have been analyzed with density functional theory (DFT) computations. The fluxionality of this complex is a classic in organometallic chemistry (as highlighted by its inclusion as a study problem in the textbook by Kegley and Pinhas, Problems and Solutions in Organometallic Chemistry), and we provide new insights into its chemistry. Besides the three previously reported fluxional processes [1,3-endo fast exchange (TS-1), CO ligand equivalence (TS-2), and 1,2-metal migration via Cs η4 diene hydride species (TS-3)], two additional processes were identified: 1,2-agostic isomerization (TS-1-2), and a low-energy hydride transfer process (TS-1-9). This investigation uncovered a previously unidentified agostic interaction in the fluxionality of (C6H9)Mn(CO)3: the 1,2-agostic isomerization leads to a previously uncharacterized “closed” Cs agostomer (structure 2) with a Mn-H distance of 2.04 Å, a Mn-H-C bond angle of 104.3°, and the 1H NMR chemical shift of −4.7 ppm. Energetically-similar agostic isomers have previously been defined as “agostomers”. The variable-temperature 1H NMR spectra were also simulated. Excellent agreement is observed (R2 = 0.9949) when comparing the computed and experimental 1H NMR chemical shifts. Reasonable agreements between the computed activation energies (6.1, 12.9, and 13.8 kcal mol−1) and experimental activation energies (8.3 ± 0.3, 13.1 ± 0.1, and 15.4 ± 0.3 kcal mol−1) were found.