Iridium-(κ-NSi) catalyzed dehydrogenation of formic acid: effect of auxiliary ligands on the catalytic performance.

The iridium(III) complexes [Ir(H)(Cl)(κ-NSi)(κ-bipy)] (2) and [Ir(H)(OTf)(κ-NSi)(κ-bipy)] (3) (NSi = {4-methylpyridine-2-yloxy}ditertbutylsilyl) have been synthesized and characterized including X-ray studies of 3. A comparative study of the catalytic activity of complexes 2, 3, [Ir(H)(OTf)(κ-NSi)(coe)] (4), and [Ir(H)(OTf)(κ-NSi)(PCy)] (5) (0.1 mol%) as catalysts precursors for the solventless formic acid dehydrogenation (FADH) in the presence of EtN (40 mol%) at 353 K has been performed. The highest activity (TOF ≈ 3260 h) has been obtained with 3 at 373 K. However, at that temperature the FTIR spectra show traces of CO together with the desired products (H and CO). Thus, the best performance was achieved at 353 K (TOF ≈ 1210 h and no observable CO). Kinetic studies at variable temperature show that the activation energy of the 3-catalyzed FADH process is 16.76 kcal mol. Kinetic isotopic effect (5 min) values of 1.6, 4.5, and 4.2 were obtained for the 3-catalyzed dehydrogenation of HCOOD, DCOOH, and DCOOD, respectively, at 353 K. The strong KIE found for DCOOH and DCOOD evidenced that the hydride transfer from the C-H bond of formic acid to the metal is the rate-determining step of the process.