Synthesis and Reactivity of Metal–Ligand Cooperative Bifunctional Ruthenium Hydride Complexes: Active Catalysts for β‑Alkylation of Secondary Alcohols with Primary Alcohols

Three unsymmetrical NNN ligands with a 2-hydroxypyridyl fragment were used to react with RuHCl­(PPh3)3(CO), affording the three bidentate ruthenium hydride complexes [(R1-C5H3N-CH2-C5H3N-C5H3N-R2)­RuH­(PPh3)2(CO)]­[Cl] (R1 = R2 = OH, 2a; R1 = OH, R2 = H, 2b; R1 = H, R2 = OH, 2c), respectively. When 2a,b were treated with t-BuOK, the two tridentate products [(O-C5H3N-CH2-C5H3N-C5H3N-R2)­RuH­(PPh3)­(CO)] (R2 = OH, 3a; R2 = H, 3b) were obtained, via selective deprotonation of the −OH group of PyCH2PyOH moiety, indicating that this −OH group is more acidic than that of the PyPyOH moiety. The reaction of 2c with t-BuOK generated the bidentate product [(C5H4N-CH2-C5H3N-C5H3N-O)­RuH­(PPh3)2(CO)] (3c) and the tridentate product [(C5H4N-CH2-C5H3N-C5H3N-O)­RuH­(PPh3)­(CO)] (3d). 3d could be further transformed to the diruthenium complex [(C5H4N-CH-C5H3N-C5H3N-O)­Ru­(PPh3)­(CO)]2 (3e) via C–H activation of the −CH2– group in boiling toluene. The catalytic activity for β-alkylation of secondary alcohols with primary alcohols of these eight ruthenium complexes was tested, and the bidentate complexes 2c and 3c exhibit the highest activity. Complex 3c can be regarded as the intermediate of 2c. These results are important for developing more efficient bifunctional catalysts for such reactions.

以三个带有2-羟基吡啶（2-hydroxypyridyl）片段的不对称NNN配体（NNN ligands），与三(三苯基膦)羰基氯化钌(RuHCl(PPh₃)₃(CO))进行反应，分别得到三种双齿钌氢配合物[(R¹-C₅H₃N-CH₂-C₅H₃N-C₅H₃N-R²)RuH(PPh₃)₂(CO)][Cl]（其中R¹=R²=OH时为2a；R¹=OH、R²=H时为2b；R¹=H、R²=OH时为2c）。当用叔丁醇钾(t-BuOK)处理2a和2b时，通过对PyCH₂PyOH（吡啶基甲基吡啶醇）片段中的-OH基团进行选择性去质子化，得到两种三齿产物[(O-C₅H₃N-CH₂-C₅H₃N-C₅H₃N-R²)RuH(PPh₃)(CO)]（R²=OH时为3a；R²=H时为3b），这表明该-OH基团的酸性强于PyPyOH（联吡啶醇）片段中的-OH基团。2c与叔丁醇钾的反应则生成双齿产物[(C₅H₄N-CH₂-C₅H₃N-C₅H₃N-O)RuH(PPh₃)₂(CO)]（3c）和三齿产物[(C₅H₄N-CH₂-C₅H₃N-C₅H₃N-O)RuH(PPh₃)(CO)]（3d）。3d可在沸腾甲苯中通过对-CH₂-基团的碳氢键（C-H）活化反应，进一步转化为双核钌配合物[(C₅H₄N-CH-C₅H₃N-C₅H₃N-O)Ru(PPh₃)(CO)]₂（3e）。对这八种钌配合物用于伯醇与仲醇的β-烷基化反应的催化活性进行了测试，结果显示双齿配合物2c与3c展现出最优的催化活性，其中配合物3c可被视为2c转化过程中的反应中间体。上述研究结果对于开发用于该类反应的更高效双功能催化剂具有重要意义。