New Route to the Mixed Valence Semiquinone-Catecholate Based Mononuclear FeIII and Catecholate Based Dinuclear MnIII Complexes: First Experimental Evidence of Valence Tautomerism

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• 作者：Nizamuddin Shaikh ; Sanchita Goswami ; Anangamohan Panja ; Xin-Yi Wang ; Song Gao ; Ray J. Butcher ; and Pradyot Banerjee
• 刊名：Inorganic Chemistry
• 出版年：2004
• 出版时间：September 20, 2004
• 年：2004
• 卷：43
• 期：19
• 页码：5908 - 5918
• 全文大小：222K
• 年卷期：v.43,no.19(September 20, 2004)
• ISSN：1520-510X

文摘

The semiquinone-catecholate based mixed valence complex, [Fe^III(bispicen)(Cl₄Cat)(Cl₄SQ)]·DMF (1), and catecholatebased (H₂bispictn)[Mn₂^III(Cl₄Cat)₄(DMF)₂] (2) (bispicen = N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine, bispictn =N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, Cl₄Cat = tetrachlorocatecholate dianion, and Cl₄SQ = tetrachlorosemiquinone radical anion) were synthesized directly utilizing a facile route. Both the complexes have beencharacterized 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 asspectroscopic features support the mixed valence tetrachlorosemiquinone-tetrachlorocatecholate charge distributionin 1. The ligand based mixed valence state was further confirmed by the presence of an intervalence chargetransfer (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. For1, the isomeric form [Fe^III(bispicen)(Cl₄Cat)(Cl₄SQ)] is favored at low temperature, while at an elevated temperature,the [Fe^II(bispicen)(Cl₄SQ)₂] redox isomer dominates. Infrared as well as UV-vis-NIR spectral characterization for2 suggest that the Mn^III(Cat)₂^- moiety is admixed with its mixed valence semiquinone-catecholate isomerMn^II(SQ)(Cat)^-, and the electronic absorption spectrum is dominated by the mixed charged species. The origin ofthe intervalence charge transfer band in the 1900 nm range is associated with the mixed valence form,Mn^II(Cl₄Cat)(Cl₄SQ)^-. The observation of VT in complex 1 is the first example where a mixed valence semiquinone-catecholate iron(III) complex undergoes intramolecular electron transfer similar to manganese and cobalt complexes.