Chelating NHC Ruthenium(II) Complexes as Robust Homogeneous Hydrogenation Catalysts

Abstract

A series of ruthenium(II) complexes have been prepared by using bidentate chelating N-heterocyclic carbene (NHC) ligands that feature different donor groups E (E=olefin, thioether, carboxylate, and NHC). Rigid coordination of all donor sites was concluded from NMR spectroscopy, and the electronic impact of the donor group was evaluated by electrochemical analyses. The chelating donor group had a strong influence on the activity of the metal center in catalyzing direct hydrogenation of styrene. A thioether group or a second NHC donor site essentially deactivates the metal center. Complexes comprising a NHC tethered with an olefin or a carboxylate group showed appreciable activity, though only the carboxylate-functionalized system proved to be a precursor for homogeneous hydrogenation. According to in situ high-pressure NMR analyses, complexes featuring a carboxylate chelating group are remarkably resistant toward reductive elimination even under strongly reducing conditions and may, therefore, be used repeatedly.

Transfer Hydrogenation of Ketones and Activated Olefins Using Chelating NHC Ruthenium Complexes

“…The hydrogen transfer rate increased substantially upon degassing the solvent and upon performing the reaction in a gas-tight tube under an inert atmosphere and at 90°C. Under these conditions, complete conversion was reached after less than one hour (Table 1 entry 5). No induction time was noted.…”

“…[15] We therefore extended our studies to the hydrogenation of other functional groups and activated C=C bonds, which were successfully converted by direct hydrogenation using complexes 1-4. [5] …”

“…[5] The chelating group in these complexes has a pronounced effect on the catalytic activity and the stability of the complex. While the olefin donor group in 1 was rapidly hydrogenated, thus inducing complex decomposition and predominantly heterogeneous hydrogenation, the carboxylato group in 2 markedly increased the stability of the complex.…”

Synthesis of palladium(0) and -(ii) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation

“…Bis-NHCs have been coordinated to many transition metals: ruthenium, 4 rhodium, 3,5 iridium, 6 palladium, 7 platinum, 8,9 and more recently also the first row metals such as nickel [10][11][12] and iron. 13,14 We are interested in the properties and reactivity of low-valent late transition metal complexes, notably electron rich zero-valent palladium NHC complexes, as these often play a vital role in the catalytic cycles of many reactions.…”

Transfer hydrogenation of unfunctionalised alkenes using N-heterocyclic carbene ruthenium catalyst precursors

“…We have recently observed that ruthenium complexes comprising a chelating N-heterocyclic carbene (NHC) ligand are catalyst precursors both for the direct hydrogenation of olefins 8 heating gave only a slight increase of hydrogenated product (32% dodecane after 24 h, Table 1). The activity of complex 4 was similar to that of 3 after 5 h (37% hydrogenated product, 63% dodecenes), yet hydrogenation continued and reached 79% after 24 h. Significant higher transfer activity was 55 observed for the olefin-functionalised NHC complex 1, which induced consumption of all starting material within 30 min and complete hydrogenation within 24 h.…”