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Molecular and Electronic Structure of Five-Coordinate Complexes of Iron(II/III) Containing o-Diiminobenzosemiquinonate(1−) π Radical Ligands

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journal contribution
posted on 03.10.2005, 00:00 by Krzysztof Chłopek, Eckhard Bill, Thomas Weyhermüller, Karl Wieghardt
The reaction of the ligand N-phenyl-1,2-benzenediamine (N-phenyl-o-phenylenediamine), H2[LPDI], in dry acetonitrile with [FeIII(dmf)6](ClO4)3 (dmf = N,N-dimethylformamide) affords the dimer (μ-NH,NH)[FeIII(LISQ)(LPDI)]2 (1), where (LISQ)•- represents the π radical monoanion N-phenyl-o-diiminobenzosemiquinonate and (LPDI)2- is its one-electron-reduced, closed-shell form. Complex 1 possesses a diamagnetic ground-state St = 0. Addition reactions of tri-n-butylphosphane, tert-butyl isocyanide, cyclohexyl isocyanide, 4,5-diphenylimidazole, and 4-(1-phenylpentyl)pyridine with 1 in acetonitrile or toluene yields [FeII(LISQ)2(PBu3)] (2), [FeII(LISQ)2(CN-tBu)] (4), [FeII(LISQ)2(CNCy)] (5), [FeIII(LISQ)2(Ph2Im)] (6), and [FeIII(LISQ)(LPDI)(BuPhCH-py)]·BuPhCH-py (7). Oxidation of 1 with iodine affords [FeIII(LISQ)2I] (3), and oxidation of 2 with ferrocenium hexafluorophosphate yields [FeIII(LISQ)2(PBu3)](PF6) (2ox). The structures of complexes 2, 2ox, 3, 5, 6, and 7 have been determined by X-ray crystallography at 100(2) K. Magnetic susceptibility measurements and EPR, UV−vis, and Mössbauer spectroscopy have established that mononuclear complexes containing the [FeII(LISQ)2X] chromophore (2, 4, 5) are diamagnetic (St = 0) whereas those with an [FeIII(LISQ)2X]n chromophore (3, 2ox, 6) are paramagnetic (St = 1/2) and those with an [FeIII(LISQ)(LPDI)X] chromophore (7) possess an St = 1 ground state. It is established that all ferric species have an intrinsic intermediate spin (SFe = 3/2) which is intramolecularly antiferromagnetically coupled to one or two (LISQ)•- ligand radicals yielding an St = 1 (7) or St = 1/2 (2ox, 3, 6) ground state, respectively. In the ferrous complexes 2, 4, and 5 the intrinsic spin at the iron ion is either low spin (SFe = 0) or intermediate spin (SFe = 1). Antiferromagnetic coupling between two radicals (LISQ)•- or, alternatively, between the intermediate spin ferrous ion and two radicals yields then the observed diamagnetic ground state. In 1 two [FeIII(LISQ)(LPDI)] halves with S* = 1 couple antiferromagnetically affording an St = 0 ground state.

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