Crystal structures and magnetic properties of mercury(II) bromide complexes with pyridyl-substituted N-oxyl N′-oxides (nitronyl nitroxides)

Abstract

The crystal structures have been determined and magnetic properties investigated for four novel HgBr₂ complexes with pyridyl-substituted ‘nitronyl nitroxides’, 4,4,5,5-tetramethyl-2-(4-pyridyl)-(L¹), -2-(2-pyridyl)-(L²), -2-(3-pyridyl)-(L³) and -2-(6-methyl-2-pyridyl)-2-imidazoline N ¹-oxyl N ³-oxide (L⁴). Complex 1, [HgBr₂L¹], is mononuclear, in which mercury(II) has planar trigonal co-ordination core, from two bromide atoms and one nitrogen atom of the pyridyl group. In 2, [HgBr₂L²] the Hg^II atom has distorted-tetrahedral four co-ordination involving two bromide atoms and chelating by oxygen and nitrogen atoms of the L² ligand. Complex 3, [(HgBr)₃L³₂], is a zigzag polymeric chain with a distorted T-shaped HgBr₂L³ unit and self-assembly involving co-ordination by an oxygen atom of the nitroxide groups. Complex 4, [(HgBr₂)₃L⁴₂], is a quasi-linear chain with HgBr₂L⁴ moieties and HgBr₂ cores. Cryomagnetic susceptibility measurements (4–300 K) showed that 1 and 3 exhibit a weak intermolecular alternating one-dimensional antiferromagnetic exchange interaction, while 3 and 4 possess weak one-dimensional antiferromagnetic and ferromagnetic exchange interactions respectively. A simple spin-polarization model has been used to justify the observed ferromagnetic exchange interaction between the spins of the radical NO group in complex 4.