Nitronyl Nitroxide Radicals Linked to Exchange‐Coupled Metal Dimers – Studies Using X‐ray Crystallography, Magnetic Susceptibility Measurements, EPR Spectroscopy, and DFT Calculations

Abstract

Abstract 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‐C 6 H 4 ‐COO)M 2 (L R )](ClO 4 ) 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 3 O 2 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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)