Substituent Effect on Oxygen Atom Transfer Reactivity from Oxomolybdenum Centers: Synthesis, Structure, Electrochemistry, and Mechanism

Dioxo molybdenum complexes of general formula Tp*MoO2(S-p-RC6H4) (1), where Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and R = OMe, Me, SMe, NHCOMe, H, Cl, CF3, NO2, were reacted with trimethyl phosphine (PMe3) to convert into complexes of general formula Tp*MoO(S- p-RC6H4)(OPMe3) (2) (where R = OMe, Me, SMe, H, Cl, and CF3). These complexes were isolated and characterized by NMR, IR, UV/vis, and single crystal X-ray crystallography. Electronic and NMR spectra, as well as redox potentials vary as a function of substituent on the thiophenolato ligand. When viewed entirety of the oxygen atom transfer (OAT) reactivity, the reaction of Tp*MoO2(S-p-RC6H4) with PMe3 shows a biphasic behavior, indicating the formation of at least one intermediate. The kinetics of the both steps, that is, the formation of the phosphoryl intermediate and the formation of the solvent coordinated species have been investigated by UV−vis spectroscopy. The first step follows a second order process, first order with respect to both the complex and PMe3, and the overall second order rate constants at 25 °C range from 98.2 (±0.01) × 10−2 M−1 s−1 (for R = OMe) to 223.0 (±0.20) × 10−2 M−1 s−1 (for R = CF3); activation parameters were in the ranges ΔH ‡ = 49.3(±4.1) kJ·mol−1 (for R = OMe) to 34.0 (±7.5) kJ·mol−1 (for R = CF3), ΔS ‡ = −154.0 (±14.2) J·mol−1·K−1 (for R = OMe) to −184.3 (±26.1) J·mol−1 K−1 (for R = CF3), and ΔG ‡ = 95.0 kJ·mol−1 (for R = OMe) to 88.7 kJ·mol−1 (for R = CF3). Formation of the acetonitrile complex from the phosphoryl complex follows a first order process with respect to the complex. The first order rate constants at 25 °C range from 3.60 (±0.01) × 10−4 sec−1 (for R = OMe) to 6.32 (±0.11) × 10−4 sec−1 (for R = CF3), and the enthalpy of activation and entropy of activation show variation; ΔH ‡ = 62.5 (±2.2) to 67.8 (±1.0) kJ·mol−1, ΔS ‡ = −82.5 (±3.3) to −101.3 (±7.5) J·mol−1·K−1, but the free energy of activation remains constant ΔG ‡ ∼ 92 (±1) kJ·mol−1. Large entropies of activation associated with both steps are consistent with associative transition states. The comparable magnitude of the activation energy of the two steps underscores the difficulty in identifying the rate-limiting step in the overall OAT reaction. The first step, however, is more sensitive toward the substituent effects than the second step. Therefore, a change in the substituent can play an important role in deciding the rate-limiting step involved in a two-step OAT reaction.