Synthesis of zwitterionic cobaltocenium borate and borata-alkene derivatives from a borole-radical anion

Chemical single-electron reduction of 1-mesityl-2,3,4,5-tetraphenylborole (3) gave a stable radical anion [CoCp*2 ][3] as shown in earlier investigations. Herein, we present the reaction of [CoCp*2 ][3] with the 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO), a common radical trap. Instead of radical recombination, the reaction proceeds through a redox pathway involving oxidation of the borole radical anion combined with reduction of TEMPO. This electron-transfer process is accompanied by a deprotonation reaction of the cobaltocenium counterion by the base TEMPO(-) to give TEMPO-H and a neutral cobalt(I) fulvene complex (7). The latter was not observed directly during the reaction, because it instantaneously reacts as a nucleophile attacking at the boron center of the in situ generated borole 3 to give the borate 6. However, 7 was synthesized independently by deprotonation of [CoCp*2 ][PF6 ]. In addition, the obtained zwitterionic cobaltocenium borate 6 undergoes a photolytic rearrangement to form the borata-alkene derivative 9 that thermally transforms to the chiral cobaltocenium borate 12. Our investigations are based on spectroscopic evidence, X-ray crystallography, elemental analysis, as well as DFT calculations.