Quadruple cyclometalation of a diphosphine-dicarboxamide ligand at platinum(II)

This work has established the double and quadruple cyclometalation of the ligands 1,3- or 1,4-C6H4(CONHCH2CH2PPh2)2 (dpipa or dppeta respectively) by either reagent [Pt2Me4(μ-SMe2)2], 1, or [Pt(O2CCF3)2(SMe2)2], 2b, to give complexes containing new PNC-pincer ligands. The major product formed depends primarily on the stoichiometry of the reaction, but also on the reaction conditions. Reaction of a 1:1 mixture of dppeta and 1 followed by crystallization from a solvent mixture containing dmso, gave remarkable quadruple metalation of dppeta in the product [Pt2(dmso)2(μ-κ6-C,C′,N,N′,P,P′-C6H2{C(O)N(CH2)2PPh2}2)], which contains two PNC-pincer groups. Reaction of a 1:1 mixture of dpipa and 2b gave double metalation of dpipa in the dimeric product [Pt2{μ-κ4-C,N,P,P′-C6H3(CONH(CH2)2PPh2)(CON(CH2)2PPh2)}2], which contains one PNC-pincer group for each dpipa ligand used. Related reactions were monitored by NMR spectroscopy and gave insight into the reaction sequences involved in the cyclometalation steps. It is argued that phosphine coordination directs a first N–H bond activation and then, in a faster step, that the amido group formed in this step directs an aryl C–H bond activation step, to give double cyclometalation of one arm of the dicarboxamide-diphosphine ligand and that, if stoichiometry allows, this can be followed by similar double cyclometalation of the second arm to give the quadruply cyclometalated ligand.