Iridium(I) Complexes with Anionic N‐Heterocyclic Carbene Ligands as Catalysts for H/D Exchange in Nonpolar Media

Abstract

A series of neutral iridium(I) complexes of the general type [(WCA−NHC)]IrL(COD)] (COD=1,5‐cyclooctadiene; L=phosphine, pyridine), bearing anionic N‐heterocyclic carbenes (WCA−NHC) with a weakly coordinating anionic (WCA) borate moiety, were prepared by addition of phosphines and pyridine to [(WCA−NHC)]Ir(COD)], in which the available coordination site is stabilized by intramolecular metal‐arene interaction (π‐face donation). The solvent and substrate scope of the neutral complexes as catalysts for H/D exchange was investigated, revealing their suitability for promoting efficient deuteration in nonpolar solvents such as cyclohexane.

Rhodium and Iridium Complexes of Anionic Thione and Selone Ligands Derived from Anionic N‐Heterocyclic Carbenes

“…Surprisingly, the Rh−C ipso distances of 2.579(2) Å ( 9 ) and 2.587(2) Å ( 11 ) are significant longer than the Ir−C ipso distances of 2.381(3) Å ( 10 ) and 2.3990(19) Å ( 12 ); in view of similar Rh and Ir atomic radii, [28] this finding indicates a significantly stronger arene‐iridium interaction. It is also surprising that π‐face donation in complexes 9 – 12 does not involve the borate‐flanking Dipp substituent, which was consistently found in WCA‐NHC complexes, including related rhodium(I) and iridium(I) complexes of the type [(WCA‐NHC)M(COD)] (M=Rh, Ir) [15a,b,g,h] . Theoretical calculations performed for the iridium complexes 10 and 12 (see details below, Table 3) indicate similar energies of the isomeric complexes in which the metal atom is bound to either one of the Dipp substituents, with a slight preference for the experimentally observed anti ‐isomer versus the syn ‐isomer ( 10 : Δ H 298K =−1.5 kcal mol −1 ; 12 : Δ H 298K =−2.5 kcal mol −1 ).…”

Heterobimetallic Coinage Metal‐Ruthenium Complexes Supported by Anionic N‐Heterocyclic Carbenes

“…Synthesis of silver(I) and copper(I) WCA-NHC complexes in THF solution. 175.59 (9)°and C58À Cu-Cl2 = 179.18 (10)°, and the coppercarbon bond lengths of 1.877(3) Å (Cu1À C1) and 1.879(3) Å (Cu2À C58) are in the same but slightly shorter range compared to the neutral analogues [(IDipp)CuCl] [23] and [(IMes)CuCl]. [24] Related copper(I) chloride complexes bearing anionic malonate or enolate functionalized anionic N-heterocyclic carbenes have also been reported.…”

“…With the synthesis and characterization of the complexes [(WCA‐NHC)M( η 2 ‐toluene)] ( 5 , M=Ag, Cu), we have again successfully exploited this class of anionic N‐heterocyclic carbenes with a weakly coordinating borate moiety (WCA‐NHC) for the generation of neutral analogues of otherwise cationic transition metal complexes for applications in nonpolar solvents. [ 7 , 8 , 9 ] Accordingly, the high solubility of the complexes 5 in toluene and other aromatic hydrocarbons provides easy access to these and potentially numerous other silver(I) and copper(I) π‐arene complexes. Attempts to use the complexes 5 as WCA‐NHC transfer reagents were unsuccessful in the case of ruthenium(II); however, the observed transfer of the intact (WCA‐NHC)M units enabled the isolation of the chlorido‐ and iodido‐bridged heterobimetallic Ag/Ru and Cu/Ru complexes 6 and 7 .…”

“…Thus, addition of fluoroboranes afforded so‐called WCA‐NHC systems with a weakly coordinating anionic (WCA) borate moiety such as X=B(C 6 F 5 ) 3 , and the resulting solvated lithium salts [(WCA‐NHC)Li(solv.) n ] ( 1 ) were used as transmetalation reagents for the preparation of late transition metal complexes, for example, Au(I), [7] Rh(I), [8] Ir(I),[ 8 , 9 ] Ni(II), [10] Pd(II), [11] which were preferably used as homogeneous catalysts for applications in nonpolar solvents. Early transition metal cyclopentadienyl‐titanium(IV) and imido‐vanadium(V) complexes were also prepared and used as pre‐catalysts for ethylene copolymerization.…”

Are rate and selectivity correlated in iridium-catalysed hydrogen isotope exchange reactions?

“…Besides the commercial Crabtree's catalyst [Ir(COD)(PCy 3 )(py)][PF 6 ] and Kerr's catalysts, such as [Ir(COD)(IMes)(PPh 3 )][X] (X = PF 6 or BAr F 24 ), which remain the most widely used for isotope labelling in industry (COD = 1,5-cyclooctadiene; IMes = 2,6-bis(2,4,6-trimethylphenyl) imidazol-2-ylidene; Cy = cyclohexyl; py = pyridine; BAr F 24 = tetrakis(3,5-bis(trifluoromethyl)phenyl) borate), numerous complexes of iridium have been developed to overcome the limitations and improve the scope of the ortho-directed H/D exchange; these include neutral [IrCl(COD)(NHC)] catalysts, 13 new generations of iridium catalysts bearing bidentate P,Nligands 14,15 or NHC,N ligand (Burgess's catalyst) 16 and [Ir(COD)(L)(NHC)] complexes with anionic N-heterocyclic carbenes that contain weakly coordinating anionic borate moieties to promote H/D exchange in nonpolar media. 17 Homogeneous iridium-catalysed HIE reactions using D 2 (or T 2 ) gas have proved to be highly efficient for ortho-selective labelling of compounds next to directing groups such as ketones, amides, esters, nitroarenes, and sulfonamides, as well as various heterocycles such as pyridines, pyrimidines, pyrazoles, imidazole(in)es, thiazole(in)es, oxazole(in)es and their benzo-fused analogues. 13,[18][19][20][21][22][23] We have recently constructed an empirical directing group scale that can be used to predict the selectivity of iridium catalysed HIE reactions in substrates with multiple directing groups.…”