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

Abstract

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.