Catalytic C–S Cross-Coupling Reactions Employing Ni Complexes of Pyrrole-Based Pincer Ligands

A series of catalytic C–S coupling reactions utilizing well-defined Ni­(II) PNP pincer complexes as precatalysts are reported (PNP = anion of 2,5-bis[(dialkyl/aryl-phosphino)­methyl]­pyrrole, abbreviated as P2RPyr). Coupling reactions employing a variety of aryl iodides and thiols in the presence of base and DMF proceed in good to excellent yield at 80 °C with low catalyst loadings. Aryl bromides were found to result in substantially lower yields, and aryl chlorides were found to be unreactive under the catalytic conditions. In an effort to further understand the reactivity of the nickel PNP precatalysts, complexes of Ni­(II) containing amide, alkoxide, hydroxide, thiolate, and hydrosulfide ligands have been prepared and examined in stoichiometric reactions relevant to carbon-heteroatom coupling. Full characterization of each nickel complex is provided, including solid-state structures. The results of stoichiometric reactions implicate a reduced Ni­(I) species as the active catalyst, which forms by reduction of the Ni­(II) precatalyst in the presence of excess thiolate. The facility in forming Ni­(I) species is invoked to rationalize the observed activity among different Ni PNP precatalysts.本工作报道了一系列以结构明确的二价镍PNP钳形配合物（Ni(II) PNP pincer complex）作为预催化剂的催化C–S偶联反应（C–S coupling reaction）体系，其中PNP为2,5-双[(二烷基/芳基膦基)甲基]吡咯的阴离子，缩写为P2RPyr。该偶联反应以多种芳基碘化物与硫醇为底物，在碱与N,N-二甲基甲酰胺（DMF）存在下，于80℃、低催化剂负载量条件下进行，可获得良好至优异的反应产率。研究发现，芳基溴化物参与的反应产率显著降低，而芳基氯化物在该催化条件下无反应活性。为进一步探究该镍PNP预催化剂的反应活性，我们合成了含酰胺基、烷氧基、氢氧根、硫醇根及氢硫化物配体的二价镍配合物，并开展了与碳-杂原子偶联反应（carbon-heteroatom coupling reaction）相关的化学计量反应研究。所有镍配合物均得到完整表征，包括其固态晶体结构。化学计量反应结果表明，还原态一价镍（Ni(I)）物种为活性催化物种，该物种可通过过量硫醇盐存在下还原二价镍预催化剂生成。不同镍PNP预催化剂的活性差异可归因于其生成一价镍物种的难易程度。