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 [L1]3- the
third arm is 3,5-di-tert-butyl-2-hydroxybenzyl, in [L2]3- it is 2-mercaptobenzyl, and in [L3]3- it is 3,5-di-tert-butyl-2-mercaptobenzyl. With trivalent metal ions these ligands form very stable neutral mononuclear complexes
[MIIIL1] (M = Ga, Fe, Co), [MIIIL2] (M = Ga, Fe, Co), and [MIIIL3] (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·1.5H2O, [CoL3]·1.17H2O, [FeL1]·H2O, and [FeL2] have been characterized by X-ray crystallography. Cyclic
voltammetry shows that all three [MIIIL1] complexes undergo a reversible, ligand-based, one-electron oxidation
generating the monocations [MIIIL1•]+ which contain a coordinated phenoxyl radical as was unambiguously
established by their electronic absorption, EPR, and Mössbauer spectra. In contrast, [MIIIL2] 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 [MIIIL2−L2MIII]2+. [GaL3]
behaves similarly despite the steric bulk of two tertiary butyl groups at the 3,5-positions of the thiophenolate,
but [CoIIIL3] in CH2Cl2 at −20 to −61 °C displays a reversible one-electron oxidation yielding a relatively
stable monocation [CoIIIL3•]+. Its electronic spectrum displays intense transitions in the visible at 509 nm (ε
= 2.6 × 103 M-1 cm-1) and 670sh, 784 (1.03 × 103) 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 (giso = 2.0226;
Aiso(59Co) = 10.7 G).