Ruthenium complexes with an N-heterocyclic carbene NNC-pincer ligand: preparation and catalytic properties

Abstract

1-Methyl-3-(2-((pyridin-2-ylmethylene)amino)ethyl)-1H-imidazol-3-ium bromide was prepared and used as a Nheterocyclic carbene NNC-pincer ligand precursor. Depending on the coordination strategy, a monometallic [Ru(NNC)(CO3)(PPh3)] complex, or the [Ru(-Cl)(NNC)]2(2Cl -) dimer were obtained. A di-silver complex in which two ligands are monocoordinated to the metal center through the NHC groups was also obtained and characterised. The dimetalic ruthenium complex reacts with alcohols yielding a monohydride species. The preliminary studies on the catalytic activity of the ruthenium dimer indicate that the complex is active in the reduction of ketones and aldehydes under transfer hydrogenation conditions.

Selective Hydrogenation of Amides to Amines and Alcohols Catalyzed by Improved Iron Pincer Complexes

“…On the basis of these findings, we assumed the formation of [(R-PN H P)FeX 2 ]. 43 Especially, for the less bulky ligand Et-PN H P it is important to note that the ligand still exhibits enough steric demand to avoid 2-fold coordination and formation of [(Et-PN H P) 2 Fe](FeX 4 ), as observed for bipyridine-based PNN-type ligands. 44,45 Apart from t Bu-PN H P, all dihalide complexes react with carbon monoxide to give the diamagnetic complexes 1a−d.…”

Probing the effect of heterocycle-bonding in PNX-type ruthenium pre-catalysts for reactions involving H2

“…Furthermore, ruthenium complexes with analogous NNC-type ligands, where the Ph 2 P groups are replaced by carbene donor-groups were shown to be active pre-catalysts for the hydrogenation of ketones. 23 In preliminary experiments, we applied 0.05 mol% of complexes 1-4 as catalysts in the presence of 0.5 mol% KO t Bu in the acceptorless dehydrogenation of benzyl alcohol (reaction (1)). With complex 1, which is known as a highly active catalyst for the dehydrogenation of ethanol, benzyl benzoate was obtained in 60.5% yield after 8 h. The utilization of complexes 2-4 as catalysts under otherwise identical conditions results in very poor conversions, with yields of 2.6% in the case of 2, 7.0% in the case of 3 and less than 0.1% for 4.…”

Synthesis and Reactivity of Remarkably Stable and Nucleophilic Hydroxide-Bridged Dimetallic Nickel NHC Complexes

“…The instability of the resulting complexes and difficulties in characterization often complicated the study of such systems. 1,2 In order to accomplish this goal, various ligands and synthetic approaches have been explored, resulting in complexes with great activity in diverse catalytic applications, such as carbonyl reduction, 3 oxygen activation 4 and C-C and C-X coupling reactions. [5][6][7][8][9] N-heterocyclic carbene (NHC) ligands have become particularly interesting as scaffold for such bimetallic systems due to their electronic flexibility, strong donor ability, and their compatibility with a wide range of transition metals.…”

“…The synthesis of well-defined dimetallic species based on Earth-abundant metals has long been a challenging task. The instability of the resulting complexes and difficulties in characterization have often complicated the study of such systems. , In order to accomplish this goal, various ligands and synthetic approaches have been explored, resulting in complexes with great activity in diverse catalytic applications, such as carbonyl reduction, oxygen activation, and C–C and C–X coupling reactions. − N-heterocyclic carbene (NHC) ligands have become particularly interesting as scaffolds for such bimetallic systems due to their electronic flexibility, strong donor ability, and compatibility with a wide range of transition metals. − Exploration of the coordination of imidazolylidene (imi) ligands to nickel afforded several well-defined homodimetallic NHC-supported complexes with the nickel centers bridged by N, P, or O donors, among others. − Oxo-bridged nickel species containing the Ni–O–Ni motif have some peculiar features, such as a relatively high stability of the resulting complexes imparted by strong Ni–O bonds and a pronounced nucleophilicity of the bridging oxygen. , These two aspects are considered key for their high activity as catalysts: for example, for mimicking enzymatic urease reactivity and for the fixation of atmospheric CO 2 . , Specifically, the enzymatic pocket of urease from Bacillus pasteurii features a Ni–O­(H)–Ni motif as an active site that enables the bridging hydroxide ion to act as a nucleophile for attacking the metal-bound urea …”