Cooney Shannon E, Duggan S Genevieve, Walls M Rebecca A, Gibson Noah J, Mayer James M, Miro Pere, Matson Ellen M
Department of Chemistry, University of Rochester Rochester NY 14627 USA
Department of Chemistry, University of Iowa Iowa City IA 52240 USA.
Chem Sci. 2025 Jan 7;16(6):2886-2897. doi: 10.1039/d4sc06468b. eCollection 2025 Feb 5.
Metal oxides are promising catalysts for small molecule hydrogen chemistries, mediated by interfacial proton-coupled electron transfer (PCET) processes. Engineering the mechanism of PCET has been shown to control the selectivity of reduced products, providing an additional route for improving reductive catalysis with metal oxides. In this work, we present kinetic resolution of the rate determining proton-transfer step of PCET to a titanium-doped POV, TiVO(OCH) with 9,10-dihydrophenazine by monitoring the loss of the cationic radical intermediate using stopped-flow analysis. For this reductant, a 5-fold enhanced rate ( = 1.2 × 10 M s) is accredited to a halved activation barrier in comparison to the homometallic analogue, [VO(OCH)]. By switching to hydrazobenzene as a reductant, a substrate where the electron transfer component of the PCET is thermodynamically unfavorable (Δ = +11 kcal mol), the mechanism is found to be altered to a concerted PCET mechanism. Despite the similar mechanisms and driving forces for TiVO(OCH) and [VO(OCH)], the rate of PCET is accellerated by 3-orders of magnitude ( = 0.3 M s) by the presence of the Ti(iv) ion. Possible origins of the accelleration are considered, including the possibility of strong electronic coupling interactions between TiVO(OCH) with hydrazobenzene. Overall, these results offer insight into the governing factors that control the mechanism of PCET in metal oxide systems.
金属氧化物是用于小分子氢化学的有前景的催化剂，由界面质子耦合电子转移（PCET）过程介导。已表明设计PCET机制可控制还原产物的选择性，为改进金属氧化物的还原催化提供了一条额外途径。在这项工作中，我们通过使用停流分析监测阳离子自由基中间体的损失，对PCET的速率决定质子转移步骤与9,10 - 二氢吩嗪到钛掺杂的POV（TiVO(OCH)）进行动力学拆分。对于这种还原剂，与同金属类似物[VO(OCH)]相比，速率提高了5倍（k = 1.2×10 M⁻¹ s⁻¹）归因于活化能垒减半。通过改用氢化偶氮苯作为还原剂，在这种底物中PCET的电子转移部分在热力学上是不利的（ΔG = +11 kcal mol⁻¹），发现该机制转变为协同PCET机制。尽管TiVO(OCH)和[VO(OCH)]的机制和驱动力相似，但由于Ti(IV)离子的存在，PCET速率加快了3个数量级（k = 0.3 M⁻¹ s⁻¹）。考虑了加速的可能来源，包括TiVO(OCH)与氢化偶氮苯之间强电子耦合相互作用的可能性。总体而言，这些结果深入了解了控制金属氧化物系统中PCET机制的主导因素。
