Synthesis and catalytic alcohol oxidation and ketone transfer hydrogenation activity of donor-functionalized mesoionic triazolylidene ruthenium(ii) complexes

Abstract

We report on the synthesis of a variety of C,E-bidentate triazolylidene ruthenium complexes that comprise different donor substituents E (E = C: phenyl anion; E = O: carboxylate, alkoxide; E = N: pyridine at heterocyclic carbon or nitrogen). Introduction of these donor functionalities is greatly facilitated by the synthetic versatility of triazoles, and their facile preparation routes. Five different complexes featuring a C,E-coordinated ruthenium center with chloride/cymene spectator ligands and three analogous solvento complexes with MeCN spectator ligands were prepared and evaluated as catalyst precursors for direct base- and oxidant-free alcohol dehydrogenation, and for transfer hydrogenation using basic iPrOH as a source of dihydrogen. In both catalytic reactions, the neutral/mono-cationic complexes with chloride/cymene spectator ligands performed better than the solvento ruthenium complexes. The donor functionality had a further profound impact on catalytic activity. For alcohol dehydrogenation, the C,C-bidentate phenyl-triazolylidene ligand induced highest conversions, while carboxylate or pyridine donor sites gave only moderate activity or none at all. In contrast, transfer hydrogenation is most efficient when a pyridyl donor group is linked to the triazolylidene via the heterocyclic carbon atom, providing turnover frequencies as high as 1400 h(-1) for cyclohexanone transfer hydrogenation. The role of the donor group is discussed in mechanistic terms.

RuII, OsII, and IrIII Complexes with Chelating Pyridyl–Mesoionic Carbene Ligands: Structural Characterization and Applications in Transfer Hydrogenation Catalysis

“…Those signals clearly show the formation of metal–hydride species. The most active Ru‐catalysts reported here deliver a better product yield compared to related ruthenium complexes reported in the literature under comparable reaction reactions 7b…”

“…Reports of triazolylidene ligands where the additional donor atoms have been introduced by the azide are rather limited to date 6a. 10, 7b…”

Transalkylation and Migration of N‐Substituent upon Alkylation of 1,2,3‐Triazoles Containing Good Leaving N‐Substituents

“…These compounds are widely used in metal complexes and for organocatalysis, [1][2][3][4][5][6][7] as well as found in biologically active compounds, [1,[8][9][10] chemosensors, [11] and ionic liquids. These compounds are widely used in metal complexes and for organocatalysis, [1][2][3][4][5][6][7] as well as found in biologically active compounds, [1,[8][9][10] chemosensors, [11] and ionic liquids.…”

Transfer hydrogenation of ketones catalysed by half‐sandwich (η6‐p‐cymene) ruthenium(II) complexes incorporating benzoylhydrazone ligands

“…At this point, a comparison in terms of catalytic efficiency and scope of application can be established between our complex 2 and other structurally related arene ruthenium catalysts. Albrecht and co‐workers have reported triazolylidene arene ruthenium(II) complexes for the transfer hydrogenation of acetophenone of high conversions with 0.5 mol% catalyst loading . Ruthenium(II) (arene) complexes of bispyrazolylazines act as catalysts in hydrogen transfer of ketones with low turnover numbers .…”