Remote Charge Effects on the Oxygen-Atom-Transfer Reactivity and Their Relationship to Molybdenum Enzymes

We report the syntheses, crystal structures, and characterization of the novel cis-dioxomolybdenum(VI) complexes [Tpm*MoVIO2Cl](MoO2Cl3) (1) and [Tpm*MoVIO2Cl](ClO4) (2), which are supported by the charge-neutral tris(3,5-dimethyl-1-pyrazolyl)methane (Tpm*) ligand. A comparison between isostructural [Tpm*MoVIO2Cl]+and Tp*MoVIO2Cl [Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate] reveals the effects of one unit of overall charge difference on their spectroscopic and electrochemical properties, geometric and electronic structures, and O-atom-transfer (OAT) reactivities, providing new insight into pyranopterin molybdoenzyme OAT reactivity. Computational studies of these molecules indicate that the delocalized positive charge lowers the lowest unoccupied molecular orbital (LUMO) energy of cationic [Tpm*MoO2Cl]+relative to Tp*MoO2Cl. Despite their virtually identical geometric structures revealed by crystal structures, the MoVI/MoVredox potential of 2is increased by 350 mV relative to that of Tp*MoVIO2Cl. This LUMO stabilization also contributes to an increased effective electrophilicity of [Tpm*MoO2Cl]+relative to that of Tp*MoO2Cl, resulting in a more favorable resonant interaction between the molydenum complex LUMO and the highest occupied molecular orbital (HOMO) of the PPh3substrate. This leads to a greater thermodynamic driving force, an earlier transition state, and a lowered activation barrier for the orbitally controlled first step of the OAT reaction in the Tpm* system relative to the Tp* system. An Eyring plot analysis shows that this initial step yields an O≡MoIV—OPPh3intermediate via an associative transition state, and the reaction is ∼500-fold faster for 2than for Tp*MoO2Cl. The second step of the OAT reaction entails solvolysis of the O≡MoIV—OPPh3intermediate to afford the solvent-substituted MoIVproduct and is 750-fold faster for the Tpm* system at −15 °C compared to the Tp* system. The observed rate enhancement for the second step is ascribed to a switch of the reaction mechanism from a dissociativepathway for the Tp* system to an alternative associativepathway for the Tpm* system. This is due to a more Lewis acidic MoIVcenter in the Tpm* system.