Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States.
J Am Chem Soc. 2022 Nov 16;144(45):20661-20671. doi: 10.1021/jacs.2c07800. Epub 2022 Nov 3.
The synthesis of a piano-stool ruthenium hydride, [(η-CMe)Ru(PmIm)H] (PmIm = (-(1,3,5-trimethylphenyl)-1-(pyrimidin-2-yl)ethan-1-imine), for the dual purpose of catalytic dihydrogen activation and subsequent hydrogen atom transfer for the formation of weak chemical bonds is described. The introduction of a neutral, potentially redox-active PmIm supporting ligand was designed to eliminate the possibility of deleterious C(sp)-H reductive coupling and elimination that has been identified as a deactivation pathway with related rhodium and iridium catalysts. Treatment of [(η-CMe)RuCl] with one equivalent PmIm ligand in the presence of zinc and sodium methoxide resulted in the isolation of the diruthenium complex, [(η-CMe)Ru(PmIm)], arising from the C-C bond formation between two PmIm chelates. Addition of H to the ruthenium dimer under both thermal and blue light irradiation conditions furnished the targeted hydride, [(η-CMe)Ru(PmIm)H], which has a relatively weak DFT-calculated Ru-H bond dissociation free energy (BDFE) of 47.9 kcal/mol. Addition of TEMPO to [(η-CMe)Ru(PmIm)H] generated the 17-electron metalloradical, [(η-CMe)Ru(PmIm)], which was characterized by EPR spectroscopy. The C-C bond forming process was reversible as the irradiation of [(η-CMe)Ru(PmIm)] generated [(η-CMe)Ru(PmIm)H] and a piano-stool ruthenium complex containing an enamide ligand derived from H-atom abstraction from the PmIm chelate. Equilibration studies were used to establish an experimental estimate of the effective Ru-H BDFE, and a value of 50.8 kcal/mol was obtained, in agreement with the observed loss of H and the DFT-computed value. The ruthenium hydride was an effective catalyst for the thermal catalytic hydrogenation of TEMPO, acridine, and a cobalt-imido complex and for the selective reduction of azobenzene to diphenylhydrazine, highlighting the role of this complex in catalytic weak bond formation using H as the stoichiometric reductant.
[(η-CMe)Ru(PmIm)H]（PmIm=（1-(2-嘧啶基）-1-(1,3,5-三甲基苯基)乙-1-亚氨基）的合成是为了双重目的，即催化二氢活化和随后的氢原子转移，以形成弱化学键。引入中性、潜在氧化还原活性的 PmIm 支撑配体旨在消除与相关铑和铱催化剂相关的有害 C(sp)-H 还原偶联和消除的可能性，这已被确定为失活途径。在锌和甲醇钠存在下，用一当量的 PmIm 配体处理[(η-CMe)RuCl]，得到了两个 PmIm 螯合物之间 C-C 键形成的双核钌配合物[(η-CMe)Ru(PmIm)]。在热和蓝光照射条件下，向钌二聚体中添加 H，得到了目标氢化物[(η-CMe)Ru(PmIm)H]，其相对较弱的 DFT 计算 Ru-H 键离解自由能（BDFE）为 47.9 kcal/mol。向[(η-CMe)Ru(PmIm)H]中添加 TEMPO 生成了 17 电子金属自由基[(η-CMe)Ru(PmIm)]，其通过 EPR 光谱进行了表征。C-C 键形成过程是可逆的，因为[(η-CMe)Ru(PmIm)]的辐照生成[(η-CMe)Ru(PmIm)H]和一个钢琴凳式钌配合物，该配合物含有一个酰胺配体，来自 PmIm 螯合物中 H 原子的提取。平衡研究用于确定实验估计的有效 Ru-H BDFE，得到 50.8 kcal/mol，与观察到的 H 损失和 DFT 计算值一致。该钌氢化物是热催化 TEMPO、吖啶和钴亚胺配合物氢化以及选择性还原偶氮苯为二苯肼的有效催化剂，突出了该配合物在使用 H 作为化学计量还原剂形成催化弱键方面的作用。
