New Route to the Mixed Valence Semiquinone-Catecholate Based Mononuclear Fe^III and Catecholate Based Dinuclear Mn^III Complexes:  First Experimental Evidence of Valence Tautomerism in an Iron Complex

Abstract

The semiquinone-catecholate based mixed valence complex, [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] x DMF (1), and catecholate based (H2bispictn)[Mn2III(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 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 [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] is favored at low temperature, while at an elevated temperature, the [FeII(bispicen)(Cl4SQ)2] redox isomer dominates. Infrared as well as UV-vis-NIR spectral characterization for 2 suggest that the MnIII(Cat)2- moiety is admixed with its mixed valence semiquinone-catecholate isomer MnII(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, MnII(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.

Recent Advances on Anilato-Based Molecular Materials with Magnetic and/or Conducting Properties

“…Similar cation- The tetrabutylammonium countercations are crucial for templating the 3D structure, compared with other 1D or 2D hydranilate-based materials [7,108,125,[186][187][188] since were located inside the pores and appear to fill the pores almost completely with no large voids present. Similar cation-dependent morphology changes have been observed for transition metal-oxalate coordination compounds with analogous chemical formula [A + ] 2 M II 2 (ox) 3 [72,81,[189][190][191]. The electronic absorption spectrum broad absorbance extending across the range 4500-14,000 cm −1 , with ν max = 7000 cm −1 .…”

“…44 shows a lower conductivity of 0.0062(1) S/cm at 298 K and a considerably larger activation energy of 180 meV which is consistent with a further divergence of the H 2 An 3− /H 2 An 2− ligand ratio from the optimal mixed-valence ratio of 1:1 ( Figure 31). [72,81,[189][190][191]. The electronic absorption spectrum broad absorbance extending across the range 4500-14,000 cm −1 , with νmax = 7000 cm −1 .…”

Iron oxide and pyrocatechol: a spectroscopy study of the reaction products

“…A more recent paper, however, reports that unlike the previously characterized mixed valence semiquinonecatecholato-iron(III) complexes, the complex [Fe(III) (bispicen) (Cl 4 Cat) (Cl 4 SQ)].DMF (bispicen = N,N-bis (2-pyridylmethyl) -1,2-ethanediamine) is EPR active, presenting at room temperature two broad transitions centered at g = 1.92 and 2.53, and a strong signal appears at 77 K of g = 4.3 typical for the high spin Fe(III) complexes 20 . We believed that the mixed nature of the complex, with SQ -and Cat 2-, resulted in weaker spin-spin coupling at room temperature and iron EPR signal appearing.…”

Mixed‐Ligand Copper Complexes with 8‐Methylthioquinoline and Triphenylphosphane or theo‐Semiquinone/Catecholate Redox System

“…[4] (1) Valence-tautomer (or redox isomer) equilibria are not only of interest for the enzymatic mechanism of copperdependent amine oxidases; [3] (1) has also been implicated in catechol-enhanced Fenton processes for wood decay. [5] In general, valence-tautomer equilibria [6] involving cobalt, [7] manganese, copper, [4,8] nickel, [9] and iron [10] have been discussed with respect to potential applications in molecular electronics ("switching"). [6][7][8][9][10][11] While thioether (methionine) coordination to biological copper(i/ii) is observed for electron transfer proteins [1,2a] and enzymes, [12] there has also been a recent report describing N-S five-membered ring chelate binding of copper in methanobactin.…”