Ruthenium, copper and ruthenium–copper complexes of an unsymmetrical phosphino pyridyl 1,8-naphthyridine PNNN ligand†

A new unsymmetrical dinucleating phosphino pyridyl 1,8-naphthyridine ligand PNNN is reported. Reaction with CuCl gave the dicopper complex [Cu₂(μ-Cl)₂(PNNN)] (1). In contrast, complexation of [RuCl₂(cymene)]₂ yielded a monometallic species [RuCl(cymene)(PNNN)]Cl ([2]Cl) in which the Ru is bound to the κ²-N,N, rather than κ²-P,N, binding pocket. The selective formation of the monoruthenium complex [2]Cl enabled synthesis of heterobimetallic complexes [RuCuCl₃(cymene)(PNNN)] (3) and [RuCuCl₂(cymene)(PNNN)]₂[PF₆]₂ ([4]₂[PF₆]₂), which both exhibit κ¹-P coordination of Cu. Complexes 1 and [4]₂[PF₆]₂ exhibit reversible dearomatisation–aromatisation behaviour at the metal–ligand cooperative methylene site upon sequential treatment with base (KO^tBu) and acid (HCl). Notably, deprotonation of [4]₂[PF₆]₂ induces a shift in the coordination mode of Cu to κ²-P,N.