Coordination of N-heterocyclic phosphine- and phosphenium-containing pincer ligands to copper(I): Evidence for reactive electrophilic metal–phosphenium intermediates

Abstract

The coordination chemistry of a diphosphine pincer ligand incorporating an N-heterocyclic phosphenium cation (PPP+) and its halophosphine precursors [PP(X)P] has been explored using copper(I) reagents. In contrast to the P–X bond cleavage observed when other low valent transition metals are used, simple coordination compounds [PP(X)P]CuY (X=Y=Cl (1); X=Cl, Y=I (2); X=Y=I (3)) are formed when halophosphine precursors are treated with CuY starting materials. Exposure of the N-heterocyclic iodophosphine complex 3 to THF results in ring-opening insertion to form the alkoxyphosphine complex [PP(O-CH2CH2CH2CH2-I)P]CuI (4). Since this reaction does not occur with the chlorophosphine compounds 1 and 2, it is proposed that iodide dissociation exposes the electrophilic metal-bound phosphenium cation to allow THF coordination and subsequent ring-opening. The electrophilicity of the N-heterocyclic phosphenium is also apparent when the halide-free ligand precursor [PPP][BPh4] is employed: CuCl reacts with [PPP][BPh4] to form [PP(Ph)P]CuCl (5), the product of phenyl group abstraction from [BPh4]−. A similar arylphosphine product [PP(Mes)P]CuCl (6, Mes=2,4,6-trimethylphenyl) is obtained via transmetallation between [PP(Cl)P] and mesitylcopper(I). The isolation of complexes 4–6 provide indirect evidence for the formation of an electrophilic copper–phosphenium intermediate {[PPP]CuCl}+.