Department of Chemistry, University of Massachusetts Boston , 100 Morrissey Boulevard, Boston, Massachusetts 02125, United States.
Chemistry Division, Energy & Photon Sciences Directorate, Brookhaven National Laboratory , Upton, New York 11973-5000, United States.
J Am Chem Soc. 2017 Feb 22;139(7):2604-2618. doi: 10.1021/jacs.6b08776. Epub 2017 Feb 10.
Electrocatalytic reduction of CO to CO is reported for the complex, {fac-Mn([(MeO)Ph]bpy)(CO)(CHCN)}(OTf), containing four pendant methoxy groups, where [(MeO)Ph]bpy = 6,6'-bis(2,6-dimethoxyphenyl)-2,2'-bipyridine. In addition to a steric influence similar to that previously established [Sampson, M. D. et al. J. Am. Chem. Soc. 2014, 136, 5460-5471] for the 6,6'-dimesityl-2,2'-bipyridine ligand in fac-Mn(mesbpy)(CO)(CHCN), which prevents Mn-Mn dimerization, the [(MeO)Ph]bpy ligand introduces an additional electronic influence combined with a weak allosteric hydrogen-bonding interaction that significantly lowers the activation barrier for C-OH bond cleavage from the metallocarboxylic acid intermediate. This provides access to the thus far elusive protonation-first pathway, minimizing the required overpotential for electrocatalytic CO to CO conversion by Mn(I) polypyridyl catalysts, while concurrently maintaining a respectable turnover frequency. Comprehensive electrochemical and computational studies here confirm the positive influence of the [(MeO)Ph]bpy ligand framework on electrocatalytic CO reduction and its dependence upon the concentration and pK of the external Brønsted acid proton source (water, methanol, trifluoroethanol, and phenol) that is required for this class of manganese catalyst. Linear sweep voltammetry studies show that both phenol and trifluoroethanol as proton sources exhibit the largest protonation-first catalytic currents in combination with {fac-Mn([(MeO)Ph]bpy)(CO)(CHCN)}(OTf), saving up to 0.55 V in overpotential with respect to the thermodynamically demanding reduction-first pathway, while bulk electrolysis studies confirm a high product selectivity for CO formation. To gain further insight into catalyst activation, time-resolved infrared (TRIR) spectroscopy combined with pulse-radiolysis (PR-TRIR), infrared spectroelectrochemistry, and density functional theory calculations were used to establish the v(CO) stretching frequencies and energetics of key redox intermediates relevant to catalyst activation.
报道了含有四个苯甲氧基取代基的配合物{fac-Mn([(MeO)Ph]bpy)(CO)(CHCN)}(OTf)对 CO 的电化学还原为 CO。其中，[(MeO)Ph]bpy = 6,6'-双(2,6-二甲氧基苯基)-2,2'-联吡啶。除了类似于先前在fac-Mn(mesbpy)(CO)(CHCN)中 6,6'-二取代的 2,2'-联吡啶配体所建立的空间位阻效应[M. D. Sampson 等人，J. Am. Chem. Soc.，2014，136，5460-5471]，阻止了 Mn-Mn 二聚化之外，[(MeO)Ph]bpy 配体还引入了额外的电子影响，同时存在弱的变构氢键相互作用，显著降低了金属羧酸配合物中间体中 C-OH 键断裂的活化能垒。这为迄今为止难以捉摸的质子化优先途径提供了途径，从而最大限度地降低了 Mn(I)多吡啶基催化剂电催化 CO 转化为 CO 所需的过电位，同时保持了可观的周转频率。这里的综合电化学和计算研究证实了[(MeO)Ph]bpy 配体框架对电催化 CO 还原的积极影响，以及它对质子源（水、甲醇、三氟乙醇和苯酚）的浓度和 pK 的依赖性，这对于此类锰催化剂是必需的。线性扫描伏安法研究表明，作为质子源的苯酚和三氟乙醇与{fac-Mn([(MeO)Ph]bpy)(CO)(CHCN)}(OTf)结合时，表现出最大的质子化优先催化电流，与热力学要求的还原优先途径相比，过电位降低了 0.55 V，而批量电解研究证实了 CO 形成的高产物选择性。为了更深入地了解催化剂的活化，使用时间分辨红外（TRIR）光谱结合脉冲辐射（PR-TRIR）、红外光谱电化学和密度泛函理论计算来确定与催化剂活化相关的关键氧化还原中间体的 v(CO)伸缩频率和能学。
