Base-Free and Acceptorless Dehydrogenation of Alcohols Catalyzed by an Iridium Complex Stabilized by a <i>N</i>,<i>N</i>,<i>N</i>‑Osmaligand

The preparation of a <i>N</i>,<i>N</i>,<i>N</i>-osmaligand, its coordination to iridium to afford an efficient catalyst precursor, and the catalytic activity of the latter in dehydrogenation reactions of hydrogen carriers based on alcohols are reported. Complex OsH₂Cl₂(PⁱPr₃)₂ (<b>1</b>) reacts with 3-(2-pyridyl)­pyrazol to give the osmium­(II) complex <b>2H</b>, which contains an acidic hydrogen atom. Deprotonation of the latter by the bridging methoxy groups of the dimer [Ir­(μ-OMe)­(η⁴-COD)]₂ (COD = 1,5-cyclooctadiene) leads to Ir­(<b>2</b>)­(η⁴-COD) (<b>3</b>), where osmaligand <b>2</b> has a free-nitrogen atom. Iridium complex <b>3</b> catalyzes the dehydrogenation of secondary and primary alcohols to ketones and aldehydes or esters, respectively, and the dehydrogenation of diols to lactones. Cyclooctatriene is detected during the catalysis by GC-MS, suggesting that the true catalyst of the reactions is a dihydride IrH₂(<b>2</b>)-species with osmaligand <b>2</b> acting as <i>N</i>,<i>N</i>,<i>N</i>-pincer. The presence of a phenyl group in the substrates favors the catalytic processes. The dehydrogenative homocoupling of primary alcohols to esters appears to take place via the transitory formation of hemiacetals