Redox reactions of the boron subhalide clusters BnCln0/˙–/2– (n = 8 or 9) investigated by electrochemical and spectroscopic methods†

Abstract

The redox properties of the electron hyperdeficient boron subhalide clusters octachlorooctaborane(8), B₈Cl₈, and nonachlorononaborane(9), B₉Cl₉, were investigated in solution by cyclic voltammetry at platinum or glassy carbon electrodes, and by ¹¹B NMR as well as ESR spectroscopy. The neutral compounds undergo a spontaneous reduction by traces of moisture usually present even in dried solvents, and the voltammetric experiment starts from B₈Cl₈^˙– or B₉Cl₉^˙–. The radical anions were identified by ESR spectroscopy. Their formation leads to line broadening in NMR spectra of B_nCl_n. Electrochemically, they are quasireversibly reduced to the dianions, but oxidized in an EC_cat (electrochemical step, catalytic chemical step) reaction with an essentially reversible electron transfer step to the neutral compounds. The potential ordering for the two redox processes is “normal” in both clusters, being in accordance with the fact that structural changes accompanying the electron transfer are minor. The radical anion B₈Cl₈^˙– is even more stable against disproportionation than B₉Cl₉^˙–.