A Cobalt Adduct of an N-hydroxy-piperidinium Cation

Cooperativity between organic ligands and transition metals in H-atom (proton/electron) transfer catalysis has been an important recent area of investigation. Tetramethylpiperidine-N-oxyl (TEMPO) radicals feature prominently in this area, prompting us to examine cooperativity between its hydrogenated congener, TEMPOH, and Co centers ligated by dihydrazonopyrrole ligands which have previously been shown to also store H-atom equivalents. Addition of TEMPOH to (tBu,TolDHP)CoOTf results in the formation of an unusual Co-adduct of 1-hydroxy-2,2,6,6-tetramethylpiperidin-1-ium (TEMPOH2+) which has been characterized with IR spectroscopy and single crystal X-ray diffraction. This adduct is thermally unstable, and decomposes, putatively via N–O homolysis, to generate 2,2,6,6-tetramethylpiperidine and the Co-hydroxide complex [(tBu,TolDHP)CoOH][OTf]. Computational investigations suggest a proton-coupled electron transfer step to generate the TEMPOH2+ adduct where the Co center serves as an electron acceptor. Despite the prevalence of aminoxyl reagents in catalysis, particularly in aerobic transformations, metal complexes of differently hydrogenated congeners of TEMPO are rare. The isolation of a TEMPOH2+ adduct and investigations into its formation shed light on related transformations that may occur during metal-aminoxyl cooperative catalysis.