An amide-derived N,N,N-Ru(ii) complex catalyzes the conversion of EtOH to 1-BuOH with high activity. Conversion to alcohol upgraded products exceeds 250 turnovers per hour (>50% conversion) with 0.1 mol% catalyst loading. In addition to high activity for ethanol upgrading, catalytic reactions can be set up under ambient conditions with no loss in activity.

An amide-derived NNN-Ru(II) hydride complex catalyzes oxidant-free, acceptorless, and chemoselective dehydrogenation of primary and secondary amines to the corresponding nitriles and imines with liberation of dihydrogen. The catalyst system tolerates oxidizable functionality and is selective for the dehydrogenation of primary amines (-CH2NH2) in the presence of amines without α-CH hydrogens.

The bmpi (1,3-bis(6′-methyl-2′-pyridylimino)isoindoline) pincer Ru(II) hydride complex catalyzes base-free, acceptorless, and chemoselective dehydrogenation of alcohols with liberation of dihydrogen under moderate (<120 °C) conditions. Primary alcohols and diols are converted to ester and lactone products with high conversion efficiencies. The catalyst system is remarkably selective for the oxidation of secondary alcohols in the presence of primary alcohols.

The reversible transformations between ketones and alcohols via sequential hydrogenation−dehydrogenation reactions are efficiently achieved using a single precatalyst HRu(bMepi)(PPh 3 ) 2 (bMepi = 1,3-bis(6′-methyl-2′-pyridylimino)isoindolate). The catalytic mechanism of HRu(bMepi)(PPh 3 ) 2 mediated acceptorless alcohol dehydrogenation (AAD) has been investigated by a series of kinetic and isotopic labeling studies, isolation of intermediates, and evaluation of Ru(b4Rpi)(PPh 3 ) 2 Cl (R = H, Me, Cl, OMe, OH) complexes. Two limiting dehydrogenation scenarios are interrogated: inner-sphere β-H elimination and outer-sphere bifunctional double hydrogen transfer. Isotopic labeling experiments demonstrated that the proton and hydride transfer in a stepwise manner. Catalyst modifications suggest that the imine group on the bMepi pincer scaffold is not necessary for catalytic alcohol dehydrogenation. Evaluation of the kinetic experiments and catalyst modifications suggests a pathway whereby HRu(bMepi)(PPh 3 ) 2 operates via the inner-sphere β-H elimination mechanism. Following a single PPh 3 dissociation, an alcohol substrate can bind and undergo proton transfer followed by a turnover-limiting β-H elimination step. Analysis of the Eyring plot established activation parameters for the β-H elimination reaction as ΔH ⧧ = 15(1) kcal/mol and ΔS ⧧ = −41(3) eu. AAD reactions using a series of Ru(b4Rpi)(PPh 3 ) 2 Cl complexes indicated that the ortho-substituted methyl groups of bMepi slightly impede catalytic activity, and electronic modifications of the pincer scaffold have a minimal effect on the reaction rate. KEYWORDS: acceptorless alcohol dehydrogenation, ruthenium, ligand effects, inner-sphere mechanism, outer-sphere mechanism, metal−ligand cooperativity ■ INTRODUCTIONTransition-metal-catalyzed acceptorless alcohol dehydrogenation (AAD) with the liberation of H 2 is an atom-economical and selective route to generate a variety of organic carbonyl synthons. 1 In the context of the "hydrogen energy economy", AAD also provides a highly desirable strategy for promoting H 2 release from suitable biomass feedstocks for chemical energy storage applications. 2 To achieve high atom economy (no exogenous additives), promoterless AAD reactions are most often mediated by bifunctional catalysts that operate via a metal−ligand cooperative mechanism. This ligand-assisted, transition-metalcatalyzed process differs from the classical inner-sphere mechanism by not requiring coordination of the substrate, thus enabling outer-sphere proton transfer to a ligand-based basic site with concurrent hydride transfer to the metal center (Scheme 1). 3 For example, Milstein's group developed a series of pyridyl PNE (E = PR 2 or NR 2 ) Ru pincer complexes (1, HRu(PNE)(CO)) that employ cooperation of the metal center with the ligand via aromatization−dearomatization of the central pyridinyl group concomitant with protonation− deprotonation of the methylene arm (Scheme 2, left panel). 1a,4 Computational studies revealed that 1 favors an outer-sphere ...

A new series of bifunctional Ru complexes with pendent Lewis acidic boranes were prepared by late-stage modification of an active hydrogen-transfer catalyst. The appended boranes modulate the reactivity of a metal hydride as well as catalytic hydrogenations. After installing acidic auxiliary groups, the complexes become multifunctional and catalyze the cis-selective hydrogenation of alkynes with higher rates, conversions, and selectivities compared with the unmodified catalyst.