We report here the synthesis and characterization of a series of NNN pincer-nickel complexes of the type (^R2NNN)NiCl₂(CH₃CN) (R = ⁱPr, ^tBu, Cy, Ph, and p-F-C₆H₄) based on bis(imino)pyridine ligands. In solution, these complexes are found to be equilibrium mixtures containing one and two pincer ligands, respectively. While the crystal structure of the former was reported by us recently for R = ⁱPr, we report the crystal structure of the latter in this study for R = p-F-C₆H₄. The considered NNN pincer-Ni complexes have been successfully employed to accomplish the catalytic β-alkylation of several secondary alcohols with a variety of benzyl alcohols at 140 °C with high yields and unprecedented turnovers. A maximum of 92% yield of the β-alkylated product at 18 400 TON was obtained in the reaction of benzyl alcohol with 1-(4-(trifluoromethyl)phenyl)ethane-1-ol in the presence of 0.005 mol % of (^Ph2NNN)NiCl₂(CH₃CN) and 5 mol % of NaO^tBu at 140 °C after 24 h. The reaction exhibits zero-order dependence of rate on catalyst concentration and first-order dependence on the concentration of base, benzyl alcohol, and 1-phenyl ethanol which points to the base-mediated aldol condensation as the rate-determining step. Most of the intermediates involved in catalysis have been identified by HRMS. To the best of our knowledge, this is the first report on a pincer-Ni catalyzed β-alkylation of alcohols and, hitherto, such unprecedented turnovers have not been reported with a homogeneous molecular nickel-based catalyst.

中文翻译：

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钳-镍催化的选择性格尔伯特型反应

我们在此报告了一系列类型为 ( ^R2 NNN)NiCl ₂ (CH ₃ CN) (R = ⁱ Pr、^t Bu、Cy、Ph 和p -FC ₆ H ₄ )的 NNN 钳形镍配合物的合成和表征) 基于双（亚氨基）吡啶配体。在溶液中，这些配合物被发现是分别含有一个和两个钳状配体的平衡混合物。虽然我们最近报道了前者的晶体结构 R = ⁱ Pr，但我们在本研究中报告了后者的晶体结构 R = p -FC ₆ H ₄. 所考虑的 NNN 钳形镍配合物已成功用于在 140°C 下以高产率和前所未有的周转率完成几种仲醇与各种苄醇的催化 β-烷基化。在苯甲醇与 1-(4-(三氟甲基)苯基)乙烷-1-醇在 0.005 mol% ( ^Ph2 NNN)NiCl ₂ (CH ₃ CN) 和 5 mol % NaO ^tBu 在 140 °C 后 24 小时。该反应表现出速率对催化剂浓度的零级依赖性和对碱、苯甲醇和 1-苯基乙醇浓度的一级依赖性，这表明碱介导的羟醛缩合是速率决定步骤。大多数参与催化的中间体已被 HRMS 鉴定。据我们所知，这是关于钳形镍催化醇的 β-烷基化的第一份报告，迄今为止，使用均质分子镍基催化剂尚未报道过这种前所未有的转变。