Chelate-ring-dependent shifts in redox isomerism for the Co(Me2N(CH2)(n)NMe2)(3,6-DBQ)(2) (n=1-3) series, where 3,6-DBQ is the semiquinonate or catecholate ligand derived from 3,6-di-tert-butyl-1,2-benzoquinone

Abstract

Intramolecular electron transfer between Co-II(SQ) and Co-III(Cat) redox isomers has been investigated for the series of complexes Co(N-N)(3,6-DBQ)(2), where 3,6-DBQ is the semiquinonate or catecholate form of 3,6-di-tert-butyl-1,2-benzoquinone and N-N is tetramethylmethylenediamine (tmmda), tetramethylethylenediamine (tmeda), or tetramethylpropylenediamine (tmpda). Measurements of magnetic properties and changes in electronic spectra indicate that the transition temperature (T-1/2) for CO(III)/CO(II) redox isomerism in the solid state and in toluene solution is the lowest for Co(tmpda)(3,6-DBQ)(2). This is attributed to the flexibility of the six-membered chelate ring of Co(tmpda) and a positive contribution to Delta S for the equilibrium. Transition temperatures for the tmmda and tmeda analogues are more than 150 deg higher than that for the tmpda species. Structural characterization of Co-III(tmmda)(3,6-DBSQ)(3,6-DBCat), Co-III(tmeda)(3,6-DBSQ)(3,6-D-BCat), and Co-II(tmpda)(3,6-DBSQ)(2) has shown that the Co(tmmda) and Co(tmeda) chelate rings are conformationally ordered, while there is considerable disorder in the Co(tmpda) chelate ring. In toluene solution, T1/2 is approximately 15 deg higher for Co(tmeda)(3,6-DBSQ)(2) than for the tmmda analogue due to the more contracted chelate ring of Co-III(tmmda)(3,6-DBSQ)(3,6-DBCat). For the Co(N-N)(3,6-DBQ)(2) series, T-1/2 varies as tmpda much less than tmmda < tmeda.