A cobalt adduct of an N-hydroxy-piperidinium cation

Abstract

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 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.

Graphical Abstract

Keywords:

• Cobalt
• oxidation
• reaction mechanisms
• redox-active

Author contributions

All authors have given approval to the final version of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the National Institutes of Health (R35 GM133470). We thank the University of Chicago for funding, the 3M Corporation for a NTFA to J.S.A., and the Sloan Foundation for a Research Fellowship to J.S.A. (FG-2019-11497). This work was supported as part of the Inorganometallic Catalyst Design Center, an EFRC funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (DE-SC0012702) We also thank the Research Computing Center at the University of Chicago for providing computing resources. We would like to thank S.J. and M.W. for helpful discussions.