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

The oxygen‐atom‐transfer (OAT) reactivity of [L i Pr MoO 2 (OPh)] ( 1 , L i Pr =hydrotris(3‐isopropylpyrazol‐1‐yl)borate) with the tertiary phosphines PEt 3 and PPh 2 Me in acetonitrile was investigated. The first step, [L i Pr MoO 2 (OPh)]+PR 3 →[L i Pr MoO(OPh)(OPR 3 )], follows a second‐order rate law with an associative transition state (PEt 3 , Δ H   ≠ =48.4 (±1.9) kJ mol −1 , Δ S   ≠ =−149.2 (±6.4) J mol −1  K −1 , Δ G   ≠ =92.9 kJ mol −1 ; PPh 2 Me, Δ H   ≠ =73.4 (±3.7) kJ mol −1 , Δ S   ≠ =−71.9 (±2.3) J mol −1  K −1 , Δ G   ≠ =94.8 kJ mol −1 ). With PMe 3 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 PC σ* in addition to the expected nucleophilic attack of the P on the MoO π*. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide with acetonitrile, [L i Pr MoO(OPh)(OPR 3 )] + MeCN → [L i Pr MoO(OPh)(MeCN)] + OPR 3 , follows a first‐order rate law in MeCN. A dissociative interchange (I d ) mechanism, with activation parameters of Δ H   ≠ =93.5 (±0.9) kJ mol −1 , Δ S   ≠ =18.2 (±3.3) J mol −1  K −1 , Δ G   ≠ =88.1 kJ mol −1 and Δ H   ≠ =97.9 (±3.4) kJ mol −1 , Δ S   ≠ =47.3 (±11.8) J mol −1  K −1 , Δ G   ≠ =83.8 kJ mol −1 , for [L i Pr MoO(OPh)(OPEt 3 )] ( 2 a ) and [L i Pr MoO(OPh)(OPPh 2 Me)] ( 2 b ), respectively, is consistent with the experimental data. Although gas‐phase calculations indicate that the MoOPMe 3 bond is stronger than the MoNCMe bond, solvation provides the driving force for the release of the phosphine oxide and formation of [L i Pr MoO(OPh)(MeCN)] ( 3 ).