Phenylthiyl radical complexes of gallium(III), iron(III), and cobalt(III) and comparison with their phenoxyl analogues

Three hexadentate, asymmetric pendent arm macrocycles containing a 1,4,7-triazacyclononane-1,4-diacetate backbone and a third, N-bound phenolate or thiophenolate arm have been synthesized. In [L-1](3-) the third arm is 3,5-di-tert-butyl-2-hydroxybenzyl, in [L-2](3-) it is 2-mercaptobenzyl, and in [L-3](3-) it is 3,5-di-tertbutyl-2-mercaptobenzyl. With trivalent metal ions these ligands form very stable neutral mononuclear complexes [(ML1)-L-III] (M = Ga, Fe, Co), [(ML2)-L-III] (M = Ga, Fe, Co), and [(ML3)-L-III] (M = Ga, Co) where the gallium and cobalt complexes possess an S = 0 and the iron complexes an S = 5/2 ground state. Complexes [CoL1]. CH3OH .1.5H(2)O, [CoL3].1.17H(2)O, [FeL1]. H2O, and [FeL2] have been characterized by X-ray crystallography. Cyclic voltammetry shows that all three [(ML1)-L-III] complexes undergo a reversible, ligand-based, one-electron oxidation generating the monocations [(ML1 .)-L-III](+) which contain a coordinated phenoxyl radical as was unambiguously established by their electronic absorption, EPR, and Mossbauer spectra. In contrast, [(ML2)-L-III] complexes in CH3CN solution undergo an irreversible one-electron oxidation where the putative thiyl radical monocationic intermediates dimerize with S-S bond formation yielding dinuclear disulfide species [(ML2)-L-III-(LMIII)-M-2](2+). [GaL3] behaves similarly despite the steric bulk of two tertiary butyl groups at the 3,5-positions of the thiophenolate, but [(CoL3)-L-III] in CH2Cl2 at -20 to -61 degreesC displays a reversible one-electron oxidation yielding a relatively stable monocation [(CoL3 .)-L-III](+). Its electronic spectrum displays intense transitions in the visible at 509 nm (epsilon = 2.6 x 10(3) M-1 cm(-1)) and 670sh, 784 (1.03 x 10(3)) typical of a phenylthiyl radical. The EPR spectrum of this species at 90 K proves the thiyl radical to be coordinated to a diamagnetic cobalt(III) ion (g(iso) = 2.0226; A(iso)(Co-59)= 10.7 G).