Synthesis, Stability, and (De)hydrogenation Catalysis by Normal and Abnormal Alkene- and Picolyl-Tethered NHC Ruthenium Complexes

Abstract

A series of p-cymene and cyclopentadienyl Ru(II)-aNHC complexes have been synthesized from 2methylimidazolium salts with either an N-bound alkenyl (1, 3) or picolyl tether (6, 7). The C(5)-Me substituted alkenyl-tethered analogues (2, 4) have also been synthesized. Ag-mediated C(2)dealkylation was a prominent side reaction that led to the formation of normally bound NHC Ru(II) complexes, which in selected cases have been isolated (5, 8). A C(4)-over C(2)-selectivity for ruthenium binding has been established by protecting the C(2)-position with an iPr group on the imidazolium precursor, for which unique p-cymene (9) and cyclopentadienyl (10) Ru(II)-aNHC derivatives have been synthesized. All complexes were applied in the transfer hydrogenation of ketones and in secondary alcohol oxidation, with higher catalytic activity for the p-cymene over the cyclopentadienyl systems, as well as the alkenyl-over the picolyl-containing aNHC complexes.

(para‐Cymene)Ru(II) Complexes with Chelating Benzotriazole Ligands: Application in Oxidation and Reduction Catalysis

“…Interestingly, the other functional groups such as Cl, Br, I, OMe and SMe on the aromatic backbone were found to be tolerated in this catalysis. The efficiencies of these catalysts were found to be comparable to the other reported ( para ‐cymene)Ru II ‐catalysts supported with chelating bipyridine‐based, NHC‐alkene and NHC‐pyridine, and chelating diamine ligands, but relatively lower than the similar Ru catalysts with NHC ligands (see Table S4, Supporting Information) …”

“…Half‐sandwich ( para ‐cymene)Ru(II) complexes having a variety of bidentate robust chelating ligands are well‐known precatalysts explored in several transformations such as olefin oxidation, alcohol oxidation, transfer hydrogenation,– dehydrocoupling reactions, aldehyde‐water shift reaction etc. With the above background in mind, we intended to explore the mono‐chelate coordination of the A ‐type ligands BTZ‐2P and BTZ‐2Q at the ( para ‐cymene)Ru(II) platform, and catalytic application of the resulting half‐sandwich complexes (Figure ).…”

Recent Advances in the Syntheses and Catalytic Applications of Homonuclear Ru‐, Rh‐, and Ir‐Complexes of CNHC^C Cyclometalated Ligands

“…The cyclometalated complexes were synthesized from the corresponding C2-substituted imidazolium salts via transmetalation pathway. [31] Along with the expected aNHC-Ru II -complexes 35 a-d (due to the protection at the C2-position), normal NHC complexes 36 a-b were also formed. Further, the robustness of the C4-bound Ru complexes was tested in acidic medium by adding DCl in different proportions.…”

Bidentate Donor-Functionalized N-Heterocyclic Carbenes: Valuable Ligands for Ruthenium-Catalyzed Transfer Hydrogenation

“…Albrecht and Landman recently reported the synthesis and catalytic activity in ketone TH of rare examples of picolyl-functionalized imidazol-4-ylidene ruthenium(II) complexes [ 45 ]. C(4)/C(5)-bound imidazolylidenes are known to be significantly stronger donors compared to their C(2)-bound counterparts [ 46 ].…”

“…Interestingly, complexes 30 and 31 / 32 proved to be also moderately active in the anaerobic oxidation of 1-phenylethanol to acetophenone in the presence of KO t Bu (5 mol%) in o -dichlorobenzene at 150 °C, with conversions of 73 and 56% after 24 h reaction, respectively [ 45 ]. As no hydride signals of either the p -cymene or Cp catalysts could be observed by 1 H NMR spectroscopy in both the TH and alcohol oxidation reactions, the authors proposed a monohydride mechanism where the monohydride intermediate catalytic species would not be the resting-state intermediate species.…”