N‑Benzyl Substituted N‑Heterocyclic Carbene Complexes of Iridium(I): Assessment in Transfer Hydrogenation Catalyst Sü leyman Gü lcemal,* ,† Aytac ̧ Gü rhan Gö kc ̧ e, ‡ and Bekir C ̧ etinkaya † † Department of Chemistry, Ege University, TR-35100, Bornova, Izmir, Turkey ‡ Department of Physics, Dokuz Eylul University, TR-35160, Buca, Izmir, Turkey * S Supporting Information ABSTRACT: Iridium(I) complexes of N-heterocyclic carbenes (NHCs) (1a-3a) were obtained by transmetalation reactions from the corresponding Ag(I)-NHC complexes. These complexes have been fully characterized by 1 H, 13 C, heteronuclear multiple-bond correlation NMR spectroscopies, and elemental analysis. X-ray diffraction studies on single crystals of 1a and 2a confirm the square planar geometry at the metal center. [IrCl(CO) 2 (NHC)] complexes 1b-3b were also synthesized to compare σ-donor/π-acceptor strength of NHC ligands. Transfer hydrogenation (TH) reactions of various ketones and imines have been studied using complexes 1a-3a as precatalysts. N-Benzyl substituted NHC complexes of Ir(I) proved to be highly efficient precatalysts in the reduction of aromatic and aliphatic ketones to afford the corresponding alcohol products with turnover frequencies values up to 24 000 h -1 . ■ INTRODUCTION During the past decade, N-heterocyclic carbenes (NHCs), often compared to phosphine ligands, have emerged as a versalite class of ligands. For the purpose of catalytic applications, they often give more stable complexes due to the strength of the metal-NHC bond and thus avoid the use of an excess of phosphine ligands. 1 Examples of iridium, rhodium, and ruthenium complexes bearing NHC ligands have been shown to be very good precatalysts for numerous reduction reactions, among those hydrogenation, transfer hydrogenation (TH), and hydrosilylation reactions. 2 TH is a metal-catalyzed process that requires a hydrogen donor atom, typically 2-propanol, in combination with a strong base. 2b This process is preferred for large-scale industrial use in the hope of developing a greener process by reducing waste production, energy use, and lowering toxicity. 3 TH is a safer and more valuable atom-efficient method when compared with the conventional hydrogenation reaction using the highly flammable dihydrogen molecule. In a number of recent examples, NHC complexes of iridium, 4 rhodium, 4c,d,i,m,n,s,5 and ruthenium 6 have been successfully used as precatalysts for TH reactions. The nature of the transition metal is very important in TH; for example, it was found that Ir(I)-NHC complexes show superior activities when compared with their Rh(I) analogue- s. 4e,i,m,n This is preferred for industrial use because of relatively low cost of iridium when compared to rhodium. The steric and electronic nature of the NHC ligand also plays an important role for the catalytic activity. 4i,k In a very recent study, the efficiencies of Rh(I) complexes with symmetrical and unsym- metrical imidazol(in)-2-ylidenes bearing 2,4,6-trimethylphenyl (Mes) or 2,4,6-trimethylbenzyl (CH 2 Mes) substituents on nitrogen atoms have been comparatively investigated in the TH reaction of acetophenone. It has been found that the introduction of flexible benzyl substituent (CH 2 Mes) to the nitrogen atoms enhanced the TH performance. 5a We also noticed that attachment of methyl groups on the 5,6-position of the benzene ring of benzimidazole also played an important role in Rh(I) 5c,7 and Ir(I) 4a catalyzed reduction reactions. In view of these facts, Ir(I) complexes derived from 1,3-bis(2,4,6- triisopropylbenzyl)imidazol-2-ylidene (1a), 1,3-bis(2,4,6- triisopropylbenzyl)-5,6-dimethylbenzimidazol-2-ylidene (2a), and 1,3-dibenzyl-5,6-dimethylbenzimidazol-2-ylidene (3a) li- gands were synthesized and applied as precatalysts for TH reactions. All complexes showed superior activities for the Ir(I)- catalyzed TH reaction of ketones and imines with a variety of substrate scales. ■ RESULTS AND DISCUSSION Synthesis and Characterization of the Compounds. The ligand precursors 1,3-bis(2,4,6-triisopropylbenzyl)- imidazolium bromide 1, 1,3-bis(2,4,6-triisopropylbenzyl)-5,6- dimethylbenzimidazolium bromide 2, and 1,3-dibenzyl-5,6- dimethylbenzimidazolium bromide 3 were synthesized from imidazole or 5,6-dimethylbenzimidazole (Scheme 1). The salts are colorless powders and were obtained in 72-93% yields. The spectral properties are similar to those of other reported imidazolium or benzimidazolium salts. In the 1 H NMR spectrum of 1-3, the NCHN + protons appear at 10.27, 7.74, and 11.72 ppm, respectively, and these downfield signals indicate the formation of azolium salts. Received: June 26, 2013 Published: September 4, 2013 Article pubs.acs.org/IC © 2013 American Chemical Society 10601 dx.doi.org/10.1021/ic401626e | Inorg. Chem. 2013, 52, 10601-10609