Copper(III) 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, la, show typical dimeric singlet-triplet transitions and strong antiferromagnetic coupling (J(ST) = -206 K (-143 cm(-1)); H = -2J(ST)S(a).S-b) The bromobenzoate dimer can be converted into linear chains of hydrogen-bonded monomers, showing 1-D ferromagnetic coupling (6a, theta = +9 K). Copper(II) sites can also be bridged by nitroxide-substituted benzoates, Ib and Ic, that is, 2(4'-carboxyphenyl)-4A,5,5-tetramethylimidazoline-3-oxide-1-oxyl (NNBA, 3a), with JST =;-216 K (-150 cm(-1)), With comparable interactions between the nitroxide and tripler Cu(II) spins, theta (T) = -157 K. A 1-D chain similar to the bromobenzoate monomers can also be produced with NNBA, also exhibiting ferromagnetic coupling (6b, theta = +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 (Id, theta = -5.8 K). In contrast, strong coupling is observed when only one PYNN is used (2, theta = -300 K), which is the result of direct coordination of the nitroxide to the copper centers, producing a chain of the dimer units.