New route to the mixed valence semiquinone-catecholate based mononuclear FeIII and catecholate based dinuclear MnIII complexes:: First experimental evidence of valence tautomerism in an iron complex

The semiquinone-catecholate based mixed valence complex, [Fe-III(bispicen) (Cl(4)Cat) (Cl(4)SQ)].DMF (1), and catecholate based (H(2)bispictn)[Mn-2(III)(Cl(4)Cat)(4)(DMF)(2)] (2) (bispicen N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine, bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, Cl(4)Cat = tetrachlorocatecholate dianion, and Cl(4)SQ = tetrachloroserniquinone radical anion) were synthesized directly utilizing a facile route. Both the complexes have been characterized by single crystal X-ray diffraction study. The electronic structures have been elucidated by UV-vis-NIR absorption spectroscopy, cyclic voltammetry, EPR, and magnetic properties. The structural as well as spectroscopic features support the mixed valence tetrachlorosemiquinone-tetrachlorocatecholate charge distribution in 1. The ligand based mixed valence state was further confirmed by the presence of an intervalence charge transfer (IVCT) band in the 1900 nm region both in solution and in the solid. The intramolecular electron transfer, a phenomenon known as valence tautomerism (VT), has been followed by electronic absorption spectroscopy. For 1, the isomeric form [Fe-III(bispicen)(Cl(4)Cat) (Cl(4)SQ) is favored at low temperature, while at an elevated temperature, the [Fe-III(bispicen)(Cl(4)SQ)(2)] redox isomer dominates. Infrared as well as UV-vis-NIR spectral characterization for 2 suggest that the Mn-III(Cat)(2)(-) moiety is admixed with its mixed valence serniquinone-catecholate isomer Mn-III(SQ)(Cat)(-), and the electronic absorption spectrum is dominated by the mixed charged species. The origin of the intervalence charge transfer band in the 1900 nm range is associated with the mixed valence form, Mn-III(Cl(4)Cat)(Cl(4)SQ)(-). The observation of VT in complex 1 is the first example where a mixed valence semiquinonecatecholate iron(III) complex undergoes intramolecular electron transfer similar to manganese and cobalt complexes.