Base-free and acceptorless dehydrogenation of alcohols catalyzed by an iridium complex stabilized by a N,N,N-osmaligand

The preparation of a N,N,N-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 (1) reacts with 3-(2-pyridyl)pyrazol to give the osmium(II) complex 2H, 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(2)(η4-COD) (3), where osmaligand 2 has a free-nitrogen atom. Iridium complex 3 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(2)-species with osmaligand 2 acting as N,N,N-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.
Financial support from the MINECO of Spain (Projects CTQ2017-82935-P and Red de Excelencia Consolider CTQ2016-81797-REDC), the Diputación General de Aragón (E06_17R), FEDER, and the European Social Fund is acknowledged.