Heteroditopic Ru(II)– and Ir(III)–NHC Complexes with Pendant 1,2,3-Triazole/Triazolylidene Groups: Stereoelectronic Impact on Transfer Hydrogenation of Unsaturated Compounds

Abstract

Imidazol-2-ylidene (ImNHC) and 1,2,3-traizol-5-ylidene (tzNHC) have been established as important classes of carbene ligands in homogeneous catalysis. To develop Ru(II)/Ir(III) complexes based on these ligand systems considering their electronic as well as steric profiles for hydride transfer reactions, we employed chelating ligands featuring combinations of ImNHC and triazole-N or mesoionic tzNHC donors bridged by a CH 2 spacer with possible modifications at triazole backbone. In general, synthesized Ru(II) complexes were found to perform significantly better than analogous Ir(III) complexes in ketone and aldimine reduction. Among the Ru(II) complexes, electron-rich complexes 8/9 of the general formula [(p-cymene)(ImNHC−CH 2 −tzNHC)-Ru II (Cl)]BF 4 with two different carbene donors (ImNHC and tzNHC) were found to perform appreciably better in ketone reduction than analogous complexes with a combination of ImNHC and triazole-N-donor ([(p-cymene)(ImNHC−CH 2 −tz−N)Ru II (Cl)]BF 4 ; 4) explaining the electronic fine-tuning of the catalytic systems. No appreciable variation in activity was observed between complexes 8 and 9 having almost similar electronic profiles. However, less bulky Ru(II) complex 9 with a triazole N-phenyl substituent is more suitable for aldimine reduction than is complex 8, having a triazole N-3,5-dimethylphenyl substituent that explains the steric influence in addition to electronic effect on the reduction process.

One‐Pot Transfer Hydrogenation Reductive Amination of Aldehydes and Ketones by Iridium Complexes “on Water”

“…There are many means to achieve this transformation. For example, the reduction of imines, [ 2 ] the reductive amination of aldehydes, [ 3 ] the borrowing hydrogenation of alcohols and amines, [ 4 ] the reduction of amides, [ 5 ] the coupling of halogenated hydrocarbons [ 6 ] and boric acids [ 7 ] to amines, and the addition of amines and alkynes, [ 8 ] etc.…”

[(PPh3)2NiCl2]-Catalyzed C–N Bond Formation Reaction via Borrowing Hydrogen Strategy: Access to Diverse Secondary Amines and Quinolines

“…Recently, we reported Ru-NHC complexes as effective homogeneous catalysts for transfer hydrogenation and C–N bond-forming reactions, which revealed that the ancillary N -heterocyclic carbene (NHC) ligands enhance the activity of the Ru center . Inspired by these results, our interest in an atom-economic and inexpensive catalytic system prompted us to utilize the NHC ligand in a nickel-catalyzed N -alkylation reaction.…”

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

“…Rare examples of pyrazole- and triazole-functionalized imidazole-2-ylidene-Ru(II) complexes were reported by the groups of Messerle [ 33 ], Kühn [ 34 ], and Rit [ 35 ]. Similarly to the pyridine-functionalized NHC complexes 17 – 20 , relatively modest activities were observed for the TH of acetophenone with the pyrazole derivatives 22a , 23 , and 24 that bear both a coordinated and an uncoordinated pyrazole moiety ( Figure 4 ).…”

“…The very modest activity observed with the hydride complex 24 , in particular, was surprising, as the pre-existing hydride ligand was expected to enable a more efficient TH reaction. The triazole derivatives 25 , 26 , and 27 proved slightly more active with maximum TON of 170, 168, and 500, respectively, under milder conditions, i.e., 0.1 ( 27 ) to 0.5 ( 25 , 26 ) mol% catalyst and only 5 ( 25 , 26 ) to 10 ( 27 ) mol% NaO i Pr or KO t Bu as base in isopropanol at reflux for 2–3 h [ 34 , 35 ]. It is noteworthy, however, that full conversion could not be attained in the cases of 25 and 26 under these conditions.…”