Date of Award

Summer 7-11-2014

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

Cortlandt G. Pierpont

Second Advisor

Wei Zhang

Third Advisor

George Steven

Abstract

Homoleptic first-row transition metal complexes of tetrachloroquinones have been synthesized and characterized. These complexes show unique electrochemical properties, as they have the ability to undergo ligand-based redox chemistry. Mononuclear, tris(tetrachlorocatecholate) complexes of chromium and vanadium have previously been prepared and are well-characterized. Here, the first-row transition metal series is completed by synthesizing and characterizing the manganese, iron, and cobalt analogs.

Synthesis of the tris(tetrachlorocatecholato)manganese(IV) dianion, [Mn(Cl4Cat)3]2-, has previously been described. A synthesis for the mixed-valence bis(tetrachlorocatecholato)tetrachlorosemiquinonatocobalt(III) dianion, [Co(Cl4Cat)2(Cl4SQ)] 2-, is presented, as well as syntheses for tris(tetrachlorosemiquinonato)iron(III), Fe(Cl4SQ)3, and tris(tetrachlorocatecholato)iron(III) trianion, [Fe(Cl4Cat)3]3-. These complexes have been characterized with spectroscopy, X-ray crystallography, and magnetic susceptibility measurements. These confirm mononuclear, homoleptic complexes of MnIV, hs-FeIII, and ls-CoIII.

These complexes undergo ligand-based redox chemistry, which has been studied using cyclic voltammetry and differential pulse voltammetry. The separation in potential between the ligand-based redox couples has been used to gauge the degree of interligand electron coupling though the metal center, which acts as a bridge for electron transfer. The dianion manganese complex, [Mn(Cl4Cat)3]2-, has complicated electrochemical behavior, with strong solvent dependence. The neutral iron complex Fe(Cl4SQ)3 and the trianion complex [Fe(Cl4Cat)3]3- have the same electrochemistry, which occurs over a narrow potential range. The iron complexes have unique electrochemical properties, as three distinct oxidation peaks are observed, but only one broad reduction peak at scan rates greater than 10 mV/s, which resolves into three peaks at slower scan rates. This is observed in multiple solvents. The iron complex redox series, FeIII (Cl4SQ) 3 ↔ [FeIII (Cl4Cat)(Cl4SQ) 2]- ↔ [FeIII (Cl4Cat)2 (Cl4SQ)] 2- ↔ [FeIII (Cl4Cat)3]3-, has been characterized using spectroelectrochemistry. The dianion cobalt complex, [Co(Cl4Cat)2(Cl4SQ)] 2-, is unusual in that it is synthesized as a mixed-valence compound, with ligands in different oxidation states. Cyclic voltammetry of this complex shows two reversible one-electron couples at nearly the same potential, which have been resolved using differential pulse voltammetry.

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