Remarkable Base Effect in the Synthesis of Mono- and Dinuclear Iridium(I) NHC Complexes

Abstract

Several [(2-R1-1,3-phenylene)bis(methylene)]bis(1-R2-imidazolium) salts, of general formula R2(CHimid∧CR1∧CHimid)X2, have been synthesized by classical imidazole quaternization and halide metathesis reactions. The reactions of Et(CHimid∧CH∧CHimid)(PF6)2 (1), Et(CHimid∧CH∧CHimid)I2 (2), Mes(CHimid∧CBr∧CHimid)(PF6)2 (3), and Me(CHimid∧CBr∧CHimid)(PF6)2 (4) with [Ir(μ-Cl)(cod)]2 (0.5 equiv) in the presence of Cs2CO3 or NEt3 as a weak base (1.2 equiv) yielded the mono-NHC complexes [IrCl(cod)Et(CHimid∧CH∧CNHC)](PF6) (5), [IrI(cod)Et(CHimid∧CH∧CNHC)]I (6), [IrCl(cod)Mes(CHimid∧CBr∧CNHC)](PF6) (7), and [IrCl(cod)Me(CHimid∧CBr∧CNHC)](PF6) (8), respectively. Doubling the number of equivalents of Ir and base afforded the dinuclear complexes [{IrCl(cod)}2Et(μ-CNHC∧CH∧CNHC)] (11), [{IrI(cod)}2Et(μ-CNHC∧CH∧CNHC)] (12), [{IrCl(cod)}2Mes(μ-CNHC∧CBr∧CNHC)] (13), and [{IrCl(cod)}2Me(μ-CNHC∧CBr∧CNHC)] (14), respectively. A remarkable effect of the nature of the weak base used was observed, since Cs2CO3 allowed the reaction to be carried out at room temperature and with a higher yield than with NEt3. The base effect was even more dramatic when 1 or 2, [Ir(μ-Cl)(cod)]2, and the weak base were reacted in a 1:0.5:2 ratio, since the dinuclear complexes [Ir(cod)Et(μ-CNHC∧CH∧CNHC)]2X2 (9, X = PF6; 10, X = I) were formed only when Cs2CO3 was used. The dinuclear complexes 9−14 could also be synthesized in an elegant stepwise manner from 1−4 via the mono-NHC complexes intermediates 5−8, and Cs2CO3 proved again to be more efficient than NEt3. On the basis of our experimental results, we propose possible reaction pathways leading to the formation of the Ir−NHC bond. In addition to the nature of the base, the influence of the imidazolium counteranions and of the imidazolium ring N substituents on these reactions is also discussed.