Long-distance magnetic interaction of exchange-coupled copper dimers with nitronyl nitroxide and tert-butyl nitroxide radicals.

To study long-distance magnetic interactions between exchange-coupled metal centers and coordinated radical moieties, two new homodimetallic Cu(II) complexes held together by the chelating ligand L(nPr) = N,N,N',N'-tetrakis(N-propyl-2-benzimidazolyl)-2-hydroxy-1,3-diaminopropane and additionally bridged by either a nitronyl nitroxide (NIT) or a tert-butyl nitroxide (NOA) radical-substituted benzoate have been prepared. The complexes have been investigated by X-ray crystallography, magnetic susceptibility measurements, electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT) calculations. For comparison additionally the related copper dimer with the bridging diamagnetic nitrobenzoate ligand was investigated. Magnetic susceptibility measurements on powdered samples reveal a ferromagnetic interaction between the copper ions of J(Cu-Cu) = +28 cm(-1) for all three compounds. The NIT-bridged dimer additionally shows a metal-radical interaction of J'(Cu-NIT) = -2.3 cm(-1) while in the case of the NOA radical, the coupling becomes weakly ferromagnetic with J'(Cu-NOA) = +1.1 cm(-1). This contrasting behavior is explained by the different effective spin-polarization mechanism present. The magnetic data that show for the first time a significant long distance exchange interaction between a peripheral radical unit and a metal dimer were confirmed by EPR spectroscopy and DFT calculations.