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

  • Feifei Li
  • Marat R. Talipov
  • Chao Dong
  • Sofia Bali
  • Keying Ding
Original Paper
  • 188 Downloads

Abstract

We report that pyridinium ions (HPyr+) accelerate the conversion of [Tp*MoIVOCl(OPMe3)] (1) to [Tp*MoIVOCl(NCCH3)] (2) by 103-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 obs) 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 (Id) mechanism, different from the slower associative interchange (Ia) mechanism in the absence of HPyr+ marked with a negative ∆S . Interestingly, log(k obs) 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 Id, Ia, 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 Id route is energetically favored in the presence of HPyr+, in contrast with the preferred Ia–NNO pathway in the absence of HPyr+. Our computational results also revealed molecular-level details for the HPyr+-facilitated Id route. Specifically, HPyr+ initially becomes hydrogen-bonded to the oxygen atom of the Mo(IV)–OPMe3 moiety, which lowers the activation barrier for the Mo–OPMe3 bond cleavage in a rate-limiting step to dissociate the OPMe3 product. The implications of our results were discussed in the context of molybdoenzymes, particularly the reductive half-reaction of sulfite oxidase.

Graphical abstract

Keywords

Molybdenum Molybdoenzymes Coordination complexes Oxygen atom transfer reactions Reaction mechanism 

Notes

Acknowledgements

This work was supported by the U.S. National Institute of Health (Project Number: 1SC2GM121183-01), the Extreme Science and Engineering Discovery Environment (XSEDE; Project Number TG-CHE170004), and New Mexico State University.

Supplementary material

775_2017_1518_MOESM1_ESM.pdf (1.6 mb)
Supplementary material 1 (PDF 1636 kb)

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Authors and Affiliations

  1. 1.Department of Chemistry and BiochemistryNew Mexico State UniversityLas CrucesUSA
  2. 2.Department of ChemistryMiddle Tennessee State UniversityMurfreesboroUSA

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