Remarkable base effect in the synthesis of mono- and dinuclear iridium(I) NHC complexes

Several [(2-R-1-1,3-phenylene)bis(methylene)]bis(1-R-2-imidazolium) salts, of general formula (R2)(CHimidCR1CHimid)X-2, have been synthesized by classical imidazole quatemization and halide metathesis reactions. The reactions of (Et)(CHimidCHCHimid)(PF6)(2) (1), (Et)(CHimidCHCHimid)I-2 (2), (Mes)(CHimidCBrCHimid)-(PF6)(2) (3), and (Me)(CHimidCBrCHimid)(PF6)(2) (4) with [Ir(mu-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 [IrCI(cod)(Et)(CHimidCHC-NHC)]-(PF6) (5), [IrI(cod)(Et)(CHimidCHC-NHC)]I (6), [IrCl(cod)(Mes)(CHimidCBrC-NHC)]-(PF6) (7), and [IrCl(cod)(Me)(CHimidCBrC-NHC)]-(PF6) (8), respectively. Doubling the number of equivalents of Ir and base afforded the dinuclear complexes [{IrCl(cod)}(2)(Et)(mu-C-NHCCHC-NHC)] (11), [[IrI(cod)}(2)(Et)(mu- C-NHCCHC-NHC)] (12), [{IrCl(cod)}(2)(Mes)(mu-C-NHCCBrC-NHC)] (13), and [{IrCl(cod)}(2)(Me)(mu-C-NHCCBrC-NHC)] (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(mu-Cl)(cod)](2), and the weak base were reacted in a 1:0.5:2 ratio, since the dinuclear complexes [Ir(cod)(Et)(mu-C-NHCCHC-NHC)](2)X-2 (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