Chelate-Ring-Dependent Shifts in Redox Isomerism for the Co(Me2N(CH2)nNMe2)(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
journal contributionposted on 13.10.1998, 00:00 by Ok-Sang Jung, Du Hwan Jo, Young-A Lee, Youn Soo Sohn, Cortlandt G. Pierpont
Intramolecular electron transfer between CoII(SQ) and CoIII(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 (T1/2) 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 ΔS 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 CoIII(tmmda)(3,6-DBSQ)(3,6-DBCat), CoIII(tmeda)(3,6-DBSQ)(3,6-DBCat), and CoII(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 CoIII(tmmda)(3,6-DBSQ)(3,6-DBCat). For the Co(N-N)(3,6-DBQ)2 series, T1/2 varies as tmpda ≪ tmmda < tmeda.