Singh Priya, Lomax Markell J A, Opalade Adedamola A, Nguyen Brandon B, Srnec Martin, Jackson Timothy A
Department of Chemistry and Center for Environmentally Beneficial Catalysis, The University of Kansas, 1567 Irving Hill Road, Lawrence, Kansas 66045, United States.
J. Heyrovský Institute of Physical Chemistry, The Czech Academy of Sciences, Dolejškova 3, Prague 8 18223, Czech Republic.
Inorg Chem. 2024 Nov 18;63(46):21941-21953. doi: 10.1021/acs.inorgchem.4c03254. Epub 2024 Nov 5.
Several manganese-dependent enzymes utilize Mn-hydroxo units in concerted proton-electron transfer (CPET) reactions. We recently demonstrated that hydrogen bonding to the hydroxo ligand in the synthetic [Mn(OH)(PaPyN)] complex increased rates of CPET reactions compared to the [Mn(OH)(PaPyQ)] complex that lacks a hydrogen bond. In this work, we determine the effect of hydrogen bonding on the basicity of the hydroxo ligand and evaluate the corresponding effect on CPET reactions. Both [Mn(OH)(PaPyQ)] and [Mn(OH)(PaPyN)] react with strong acids to yield Mn-aqua complexes [Mn(OH)(PaPyQ)] and [Mn(OH)(PaPyN)], for which we determined p values of 7.6 and 13.1, respectively. Reactions of the Mn-aqua complexes with one-electron reductants yielded estimates of reduction potentials, which were combined with p values to give O-H bond dissociation free energies (BDFEs) of 77 and 85 kcal mol for the Mn-aqua complexes [Mn(OH)(PaPyQ)] and [Mn(OH)(PaPyN)]. Using these BDFEs, we performed an analysis of the thermodynamic driving force for phenol oxidation by these complexes and observed the unexpected result that slower rates are associated with more asynchronous CPET. In addition, reactions of acidic phenols with the Mn-hydroxo complexes show rates that deviate from the thermodynamic trends, consistent with a change in mechanism from CPET to proton transfer.
几种依赖锰的酶在协同质子-电子转移（CPET）反应中利用锰-羟基单元。我们最近证明，与缺乏氢键的[Mn(OH)(PaPyQ)]配合物相比，合成的[Mn(OH)(PaPyN)]配合物中与羟基配体的氢键增加了CPET反应的速率。在这项工作中，我们确定了氢键对羟基配体碱性的影响，并评估了对CPET反应的相应影响。[Mn(OH)(PaPyQ)]和[Mn(OH)(PaPyN)]都与强酸反应生成锰-水配合物[Mn(OH)(PaPyQ)]和[Mn(OH)(PaPyN)]，我们分别测定其p值为7.6和13.1。锰-水配合物与单电子还原剂的反应产生了还原电位的估计值，这些值与p值相结合，得出锰-水配合物[Mn(OH)(PaPyQ)]和[Mn(OH)(PaPyN)]的O-H键解离自由能（BDFE）分别为77和85 kcal/mol。利用这些BDFE，我们对这些配合物氧化苯酚的热力学驱动力进行了分析，观察到了意想不到的结果，即较慢的反应速率与更不同步的CPET相关。此外，酸性苯酚与锰-羟基配合物的反应显示出偏离热力学趋势的速率，这与反应机理从CPET变为质子转移一致。
