Spin delocalization in the copper(I) complexes of bis(verdazyl) diradicals

1,2',5,5'-tetramethyl-6,6'-dioxobis(verdazyl) (BVD) reacts with copper(I) halides in acetonitrile and copper(II) halides in methanol to give copper(I) coordination polymers of composition [Cu2X2(BVD)]. When X = Cl and X = Br, these polymers crystallize in orthorhombic unit cells with dimensions a = 6.684(1) Angstrom, b = 12.524(3) Angstrom, and c = 8.717(2) a (X = Cl) and a 12.680(2) Angstrom, b = 6.744(1) Angstrom, and c = 8.822(2) Angstrom (X = Br). With X = I, powder diffraction indicates a monoclinic unit cell with dimensions a = 12.669 Angstrom, b = 8.461 Angstrom, c = 7.679 Angstrom, and beta = 91.88 degrees, although poor crystal quality prevented a full structure determination. Magnetic susceptibility measurements taken on the three polymers indicate that the spins couple in one-dimensional chains with alternating exchange parameters: J(1) = -190 cm(-1), J(2) = -116 cm(-1) (X = Cl); J(1) = -200 cm(-1), J(2) = -110 cm(-1) (X = Br); and J(1) = -271 cm(-1), J(2) = -200 cm(-1) (X = I). Variable temperature ESR measurements on the unstable monomeric complex resulting from reaction of BVD with (CH3O)(3)PCuI indicate an interradical exchange parameter J of -230 cm(-1). ESR spectra of all three complexes show extremely broad, featureless lines as a result of very rapid spin-cm(-1). lattice relaxation. The reduction in exchange between the two halves of the bis(verdazyl)ligand upon coordination, and the unusual ESR properties, require a delocalized structure, with significant spin density on the copper atom. Variation of the auxiliary ligands on the copper atom allows tuning of the intramolecular exchange.