Copper(II) Benzoate Nitroxide Dimers and Chains: Structure and Magnetic Studies^†

Copper(II) benzoate dimers and linear chains have been synthesized and exhibit very different magnetic behaviors. The benzoate dimers, 1a, show typical dimeric singlet−triplet transitions and strong antiferromagnetic coupling (JST = −206 K (−143 cm-1); H = −2JSTSa·Sb). The bromobenzoate dimer can be converted into linear chains of hydrogen-bonded monomers, showing 1-D ferromagnetic coupling (6a, θ = +9 K). Copper(II) sites can also be bridged by nitroxide-substituted benzoates, 1b and 1c, that is, 2-(4‘-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (NNBA, 3a), with JST = −216 K (−150 cm-1), with comparable interactions between the nitroxide and triplet Cu(II) spins, θT = −157 K. A 1-D chain similar to the bromobenzoate monomers can also be produced with NNBA, also exhibiting ferromagnetic coupling (6b, θ = +0.67 K), albeit much weaker. Other nitroxides have been introduced into the Cu(II) dimer system by capping copper(II) acetate with the polydentate 2-(4‘-pyridyl)-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PYNN, 3b), which exhibits almost no coupling to the copper centers when both ends of the dimer are capped (1d, θ = −5.8 K). In contrast, strong coupling is observed when only one PYNN is used (2, θ = −300 K), which is the result of direct coordination of the nitroxide to the copper centers, producing a chain of the dimer units.