Acid-facilitated product release from a Mo(IV) center: relevance to oxygen atom transfer reactivity of molybdenum oxotransferases.

We report that pyridinium ions (HPyr) accelerate the conversion of [TpMoOCl(OPMe)] (1) to [TpMoOCl(NCCH)] (2) by 10-fold, affording 2 in near-quantitative yield; Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate. This novel reactivity and the mechanism of this reaction were investigated in detail. The formation of 2 followed pseudo-first-order kinetics, with the observed pseudo-first-order rate constant (k ) linearly correlated with [HPyr]. An Eyring plot revealed that this HPyr-facilitated reaction has a small positive value of ∆S indicative of a dissociative interchange (I) mechanism, different from the slower associative interchange (I) mechanism in the absence of HPyr marked with a negative ∆S . Interestingly, log(k ) was found to be linearly correlated to the acidity of substituted pyridinium ions. This novel reactivity is further investigated using combined DFT and ab initio coupled cluster methods. Different reaction pathways, including I, I, and possible alternative routes in the absence or presence of HPyr, were considered, and enthalpy and free energies were calculated for each pathway. Our computational results further underscored that the I route is energetically favored in the presence of HPyr, in contrast with the preferred I-NNO pathway in the absence of HPyr. Our computational results also revealed molecular-level details for the HPyr-facilitated I route. Specifically, HPyr initially becomes hydrogen-bonded to the oxygen atom of the Mo(IV)-OPMe moiety, which lowers the activation barrier for the Mo-OPMe bond cleavage in a rate-limiting step to dissociate the OPMe product. The implications of our results were discussed in the context of molybdoenzymes, particularly the reductive half-reaction of sulfite oxidase.