Further Exploration of Aza-Cobalt-Cyclobutenes on CoIII(TIM) Complexes: Reactivity and Spectroelectrochemistry

Abstract

The reaction between [CoIII(TIM)Cl2]PF6 (TIM is 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) and electron-rich terminal alkynes (HC2Y, Y = 4-N,N-bis(4-methoxyphenyl)aniline (TPA) or Fc) yielded a pair of constitutional isomers: 1-aza-2-cobalt-cyclobutene-containing trans-[Co(TIM′)((HC═C)Y)Cl]+ (1a (TPA) and 2a (Fc); TIM′ is the resultant derivative of TIM) and σ-coordinated trans-[Co(TIM)(C2Y)Cl]+ (1b (TPA) and 2b (Fc)). Additionally, the reactivity of the 1-aza-2-cobalt-cyclobutene species was explored based on trans-[Co(TIM′)((HC═C)Fc)Cl]PF6 (2a) because of its good synthetic yield. The reaction of 2a with AgOTf in MeCN resulted in trans-[Co(TIM′)((HC═C)Fc)(NCMe)](PF6)(OTf) (3), and the reaction with KCN in MeOH yielded trans-[Co(TIM′)((HC═C)Fc)CN]PF6 (4). Similarly, trans-[Co(TIM)(C2Ph)(CN)]PF6 (5) was prepared from the reaction between KCN and trans-[Co(TIM)(C2Ph)Cl]PF6. Molecular structures of 1b and 3–5 were established through single crystal X-ray diffraction. Electrochemical studies of 2–4 revealed that these complexes display an irreversible CoIII reduction as well as a reversible one electron ferrocenyl oxidation between 0.11 and 0.18 V (vs Fc+1/0). UV–vis spectroelectrochemical studies revealed a CoIII to Fc+ charge-transfer upon the one electron oxidation of 2a and 2b.