Thiol-Assisted Stepwise Reduction of Nitrite to Nitric Oxide to Nitrous Oxide by a Cobalt(II)-ONO Compound Featuring the Intermediary of Highly Reducing Cobalt(II)-Thiolate Compounds

A detailed study of the stepwise reduction of nitrite (NO₂^-) to nitric oxide (NO) to nitrous oxide (N₂O) by a Co(II)-ONO compound is presented. Reactions of [Co^II(6-COO^--tpa)]¹⁺ (<b>1</b>) and [Co^III(6-COO^--tpa)(DMF)]²⁺ (<b>2</b>) with NO₂^- produced [Co^II(6-COO^--tpa)(ONO)] (<b>3</b>) and [Co^III(6-COO^--tpa)(NO₂)]¹⁺ (<b>4</b>), respectively. Complex <b>4</b> reacts with thiols at -35 °C to produce [Co^III(6-COO^--tpa)(SR)]¹⁺ (R = Ph, <b>5a</b>; <i>p</i>-F-C₆H₄, <b>5b</b>), while the same reaction at RT generated NO (48-58%) along with <b>1</b>. On the other hand, compound <b>3</b> reacts with thiols to generate NO (20-24%) along with [Co^II(6-COO^--tpa)(SR)] (R = Ph, <b>6a</b>; <i>p</i>-F-C₆H₄, <b>6b</b>) and <b>1</b>. Interestingly, mechanistic investigation revealed that the in situ generated, highly reducing (<i>E</i>_1/2 = -0.73 V to -0.75 V vs Cp₂Fe⁺/Cp₂Fe), Co(II)-thiolate compounds, <b>6a</b> and <b>6b</b>, reduced the in situ generated NO further to N₂O. Analytically pure samples of <b>6a</b> and <b>6b</b> demonstrated the direct reduction of NO to N₂O in excellent yields (82-89%), which were improved further to 91-97% upon use of MeCOOH and (TMS)(OTf). Finally, based on IR spectroscopic analysis of the NO reduction reactions, a probable mechanism for the reduction of NO to N₂O involving the generation of a {Co(NO)₂}¹⁰ species has been proposed.