Mechanistic Investigation of the Oxygen-Atom-Transfer Reactivity of Dioxo-molybdenum(VI) Complexes

The oxygen‐atom‐transfer (OAT) reactivity of [LiPrMoO2(OPh)] (1, LiPr=hydrotris(3‐isopropylpyrazol‐1‐yl)borate) with the tertiary phosphines PEt3 and PPh2Me in acetonitrile was investigated. The first step, [LiPrMoO2(OPh)]+PR3→[LiPrMoO(OPh)(OPR3)], follows a second‐order rate law with an associative transition state (PEt3, ΔH ≠=48.4 (±1.9) kJ mol−1, ΔS ≠=−149.2 (±6.4) J mol−1 K−1, ΔG ≠=92.9 kJ mol−1; PPh2Me, ΔH ≠=73.4 (±3.7) kJ mol−1, ΔS ≠=−71.9 (±2.3) J mol−1 K−1, ΔG ≠=94.8 kJ mol−1). With PMe3 as a model substrate, the geometry and the free energy of the transition state (TS) for the formation of the phosphine oxide‐coordinated intermediate were calculated. The latter, 95 kJ mol−1, is in good agreement with the experimental values. An unexpectedly large O‐P‐C angle calculated for the TS suggests that there is significant O‐nucleophilic attack on the PC σ* in addition to the expected nucleophilic attack of the P on the MoO π*. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide with acetonitrile, [LiPrMoO(OPh)(OPR3)] + MeCN → [LiPrMoO(OPh)(MeCN)] + OPR3, follows a first‐order rate law in MeCN. A dissociative interchange (Id) mechanism, with activation parameters of ΔH ≠=93.5 (±0.9) kJ mol−1, ΔS ≠=18.2 (±3.3) J mol−1 K−1, ΔG ≠=88.1 kJ mol−1 and ΔH ≠=97.9 (±3.4) kJ mol−1, ΔS ≠=47.3 (±11.8) J mol−1 K−1, ΔG ≠=83.8 kJ mol−1, for [LiPrMoO(OPh)(OPEt3)] (2 a) and [LiPrMoO(OPh)(OPPh2Me)] (2 b), respectively, is consistent with the experimental data. Although gas‐phase calculations indicate that the MoOPMe3 bond is stronger than the MoNCMe bond, solvation provides the driving force for the release of the phosphine oxide and formation of [LiPrMoO(OPh)(MeCN)] (3). Multiple transition states: The oxygen‐atom‐transfer reactions from a dioxo‐MoVI complex involve more than one transition state, as shown here. The reaction mechanism was probed both experimentally and computationally.