Syntheses, structures and magnetism of homoleptic complexes of 4-{pyrid-4-yloxy}-2,2,6,6-tetramethyl-1-piperidinoxyl, a new spin-labelled pyridine

Reaction of 4-chloropyridine hydrochloride with 4-hydroxy-2,2,6,6-tetrameth yl-1-piperidinoxyl in the presence of five equivalents of powdered KOH in D MSO at 30 degrees C affords L-1 in 10-50% yield after an aqueous quench. Th e complexes [M(L-1)(4)](BF4)(2) (M=Ni, I; M=Cu, 2; M=Pd, 3) have been prepa red by complexation of hydrated M(BF,), (M=Ni, Cu), or [PdCl2(NCPh)(2)] + 2 AgBF(4), by four equivalents of L-1 in MeCN. Diffusion of Et2O into MeCN so lutions of 2 affords two crystalline polymorphs of this complex, both conta ining square planar [Cu(L-1)(4)](2+) dications [Cu-N = 2.005(7)-2.042(6) An gstrom]. The major, monoclinic form contains axial Cu...NCMe contacts of 2. 394(7) and 2.859(5) A, while the minor, triclinic form exhibits long axial Cu...FBF3 bonds (Cu...F = 2.478(5), 2.551(5) Angstrom). In both structures, the Cu...N(nitroxyl) distances are 9.5-9.7 Angstrom. The e-band EPR spectr um of 2 in 10:1 MeCN:toluene solution at 293 K is a broad featureless line, suggesting \J(Cu-L)1\ greater than or equal to 0.07 cm(-1) Variable temper ature susceptibility measurements on powdered 2 and 3 show weakly antiferro magnetic behaviour. For 2 these data are well reproduced both by the Curie- Weiss law and by a model describing intramolecular superexchange; for 3, th e data could not be fit satisfactorily, suggesting the presence of a signif icant intermolecular superexchange pathway. (C) 1999 Elsevier Science S.A. All rights reserved.