Nitronyl nitroxide radicals linked To exchange-coupled metal dimers - Studies using X-ray crystallography, magnetic susceptibility measurements, EPR spectroscopy, and DFT calculations

To study long-range magnetic interactions between exchange-coupled metal centers and a radical moiety coordinated through a peripheral group, three new homodimetallic complexes with Mn^II, Co^II, and Zn ^II bridged by a nitronyl nitroxide (NIT) substituted benzoate ligand with the structure [(NIT-C₆H₄-COO)M₂(L ^R)](ClO₄)₂ {M = Mn^II, Co ^II, and Zn^II; NIT = nitronyl nitroxide and L^R = N,N,N',N'-tetrakis(2-benzimidazolylalkyl)-2-hydroxy-1,3-diaminopropane} have been prepared and studied by X-ray crystallography, magnetic susceptibility measurements, EPR spectroscopy, and density functional theory calculations. For comparison, related complexes with Mn^II and Co^II bridged by a diamagnetic nitrobenzoate ligand were investigated by using the same methods. In all complexes, the metal centers have a trigonal-bipyramidal N ₃O₂ coordination sphere, with the exception of the Mn ^II-nitrobenzoate, where one of the two manganese centers is in a distorted octahedral environment due to the presence of an additional coordinated acetonitrile molecule. Magnetic susceptibility measurements on powdered samples revealed a dominant antiferromagnetic interaction between the metal ions. For the nitrobenzoate compounds, values of J_Mn-Mn = -5.8 cm^-1 and J_Co-Co = -12.0 cm^-1 are found. Replacement of the bridging ligand by a NIT-substituted benzoate group leads to very similar antiferromagnetic metal-metal interactions with J_Mn-Mn = -7.2 cm^-1 and J_Co-Co = -12.0 cm^-1. A metal-radical interaction could not be observed. CW-X-band EPR measurements at low temperature show the anisotropic NIT signal. This confirms that the radical behaves like an isolated spin center and has no influence on the metal-metal interaction. These experimental data could be confirmed by DFT calculations at the UB3LYP level. (