A NEW-TYPE OF THERMALLY-INDUCED SPIN TRANSITION ASSOCIATED WITH AN EQUATORIAL[--]AXIAL CONVERSION IN A COPPER(II)-NITROXIDE CLUSTER

The nitroxide free radical etramethyl-4,5-dihydro-1H-imidazolyl-1-oxy- 3-oxide (L) reacts with Cu(hfac)(2), leading to a complex of formula [ Cu(hfac)(2)](4)L(2). This complex has a cyclic molecular structure whe re two copper ions are bridged by the free radicals. The two remaining metal centers are extracyclic. Both oxygen atoms of the nitronyl nitr oxide ligand are axially bound to both types of copper(II) ions at roo m temperature while, at 50 K, coordination of the NO group to the intr acyclic metal ion is equatorial. The magnetic behavior is strongly cor related to the change of coordination. While at room temperature one o bserves a Curie law corresponding to six independent S = 1/2 spins, at low temperature only two S = 1/2 spins are observed. This behavior is rationalized in the frame of the dependence of the metal-ligand excha nge interaction upon the coordination geometry in nitroxide-copper der ivatives. This spin transition is further characterized by EPR spectro scopy and heat capacity measurements. Relevant structural parameters a t room temperature are as follows: space group <P(1)over bar>, a = 11. 509(2) Angstrom, b = 12.673(2) Angstrom, c = 16.077(2) Angstrom, alpha = 70.29(1)degrees, beta = 81.29(1)degrees, gamma = 84.68(1)degrees, Z = 2.