Copper complexes with mono- and bidentate-bridging nitronyl nitroxide-substituted benzoate ligands

Carboxylate-bridged copper complexes [Cu2(μ-L)4(S)2] 1 and 2 (S=solvent) with pendant nitronyl nitroxide (NIT) radical substituents have been prepared by ligand exchange reaction from copper(II) acetate with 4- and 3-carboxyphenyl-NIT (HL1 and HL2), respectively. Further reaction of 1 with pyridine leads to a monomeric copper complex [Cu(L1)2(py)2(H2O)] (3). Crystal structure determinations of 1 and 2 show a dimeric structure with Cu⋯Cu distances of 2.629(1) and 2.645(3) Å. Four nitronyl nitroxide ligands L1 or L2 are coordinated to the dicopper core. Magnetic susceptibility measurements on powdered samples reveal dominant exchange interactions between the two copper centers with JCu–Cu=−150 cm−1 (1, S=H2O, dmso), and JCu–Cu=–140 cm−1 (2b, S=CH3CN). Weaker intermolecular interactions were accounted for with Weiss constants. The benzoic acid group in L1 and L2 does not mediate effectively an exchange interaction between the pendant nitronyl nitroxide and the copper ions coordinated to the carboxylate group. These experimental findings can be rationalized by DFT calculation of the spin density in L1 and L2, which shows the unpaired electron to be localized on the NO-groups with little contribution from the phenyl ring and the spin density on the carboxylate group to be negligible.