SPIN-DENSITY MAPS IN NITROXIDE COPPER(II) COMPLEXES - A POLARIZED NEUTRON-DIFFRACTION DETERMINATION

Low-temperature crystal structures and spin density distributions in t wo copper(II)-nitronyl nitroxide complexes have been determined by neu tron diffraction experiments. The first complex, catena(mu-1,3-(2,4,4, 5,5-pentamethyl-4, 5-dihydro-1H-imidazolyl-1-oxyl 3-oxide)bis(hexafluo roacetylacetonato)copper(II)) ((Cu(hfac)2NITMe)n, 1), is a linear chai n species of alternating radicals and metal ions where the metal-radic al interaction is ferromagnetic. In contrast, 4,5,5-tetramethyl-4,5-di hydro-1H-imidazolyl-1-oxyl 3-oxide)copper(II) chloride (CuCl2-(NITPh)2 , 2) is a discrete centrosymmetric bis-nitroxide adduct exhibiting a s trong metal-nitroxide antiferromagnetic coupling. For 2, the low- and high-temperature molecular structures are identical while in 1, at low temperature, a shortening of the interchain distance and a folding of the metal-acetylacetonato rings are observed. Spin density distributi on maps were determined at 2.5 K for 1 and 13 K for 2; they were found to be in qualitative and quantitative agreement with the ground state of both compounds. In 1, the spin density is positive throughout the molecular backbone in agreement with the observed metal-radical ferrom agnetic interaction. In 2, the antiferromagnetic behavior is reflected in spin densities of.opposite signs on the organic radicals and on th e metal ion; the ratio of these densities (-1/2) allows an unambigous description of the ground doublet state of this three-spin system. Exa mination of the unpaired spin distribution on the various fragments of both compounds clearly indicates that binding induces a shift of the spin density from the oxygen to the nitrogen in the nitroxide group. T his effect is more pronounced in 2 where the radical is stronger bound than in 1 where the metal-oxygen bond is weak. The vanishingly small density on the bonded nitroxyl oxygen in 2 is probably a consequence o f such a redistribution, although compensation between opposite contri butions cannot be excluded. In 1, the copper centers have predominant d(x2-y2) character while in the nitroxide a strong departure from the ideal pi description is observed. The magnetic orbital is tilted from the 2p(z) direction resulting in a favorable arrangement for weak ove rlap and ferromagnetic coupling.