Ferromagnetic interactions and polymorphism in radical-substituted gold phosphine complexes

Abstract

A series of gold(I) complexes containing aminoxyl radical-substituted phosphine ligands has been prepared. Complexes chloro{2-[(p-diphenylphosphino)phenyl]-4,4,5,5-tetramethylimidazoline 1-oxyl 3-oxide}gold and [tert-butyl(p-diphenylphosphinophenyl)aminoxyl]chlorogold were prepared by reaction of the free radical phosphine with AuCl·THT (THT = tetrahydrothiophene) or by complexation of the radical precursor of the ligands to AuCl·THT followed by oxidation of the resulting gold complex. The first complex forms two distinct polymorphic crystal phases: in CH₂Cl₂–Et₂O 2 crystallizes as green plates (α) in the triclinic space group P and in CH₂Cl₂–hexanes as blue prisms (β) in the monoclinic space group P2₁/a. The crystal structures of both polymorphs and that of the complex 4 have been solved. The different solid-state arrangement of the molecules in the two polymorphic forms yields different magnetic behaviour; both exhibit intermolecular antiferromagnetic interactions but α exhibits a maximum in χ_m at 3.5 K while β shows no maximum. The favourable overlap of aminoxyl N-oxide groups in α causes this stronger interaction, which was modelled as a 1-D antiferromagnetic Heisenberg chain to give J = –3.2 cm^–1. The magnetic behaviour of the complex 4 was investigated down to 50 mK, showing dominant intermolecular ferromagnetic interactions but long range antiferromagnetic ordering at very low temperatures.