Reaction Intermediates in the Synthesis of New Hydrido, N-Heterocyclic Dicarbene Iridium(III) Pincer Complexes

The reactivity of the potential pincer ligands (1,3-phenylene)bis(1-butylimidazolium) diiodide ((n-Bu)(CHimdCHCHimid)I-2, 1) and (4,6-dimethyl-1,3-phenylene)bis(1-butylimidazolium) diiodide ((n-Bu)((CHimidCHCHimid)-C-Me)I-2, 3) toward [Ir(mu-Cl)(cod)](2), in the presence of a weak base, was compared. Performing the reaction with a stoichiometric amount of NEt3 led to three complexes from 1, [IrI2n-Bu(CNHCCCNHC)(NCMe)] (4, 50%), [Ir(H)In-Bu(CNHCCCNHC)(NCMe)] (6, 10%), and {[Ir(mu-I)(n-Bu)((CNHCCCHimid)-C-4)(2)]I-2}(2) (7,40%) and to five complexes from 3, [IrI2n-Bu((CNHCCCNHC)-C-Me)(NCMe)] (8, 30%), [Ir(H)In-Bu((CNHCCCNHC)-C-Me)(NCMe)] (9, 30%), IrI(cod)(n-Bu)((CNHCCHCHimid)-C-Me)I (10, 34%), [{IrI(cod)}(2)(n-Bu)(mu-(CNHCCHCNHC)-C-Me)] (11, < 5%), and [Irn-Bu((CNHCCCNHC)-C-Me)(2)]I (12, < 5%). In both cases, the amount of CNHCCCNHC and C-NHC (CCNHC)-C-Me pincer complexes formed (4 and 6 from 1; 8 and 9 from 3) was almost the same. Replacing NEt3 by Cs2CO3 or using an excess of NEt3 allowed the formation of the pincer iridium(III) hydride complexes 6 and 9 as the main products (isolated yields of ca. 70%). In contrast, base-free oxidative addition of the bis(imidazolium) salt 1 toward [Ir(mu-Cl(cod)](2) unexpectedly yielded the complex [IrI2n-Bu((CNHCCCHimid)-C-4)(NCMe)(2)]I (17), in which aromatic metalation has occurred in the C-4 or C-6 position. Cationic complexes [Ir(H)(n-Bu)(CNHCCCNHC)(NCMe)(2)](PF6) (13) and [Ir(H)(n-Bu)((CNHCCCNHC)-C-Me)(NCMe)(2)](PF6) (14) were prepared by iodide ligand abstraction from 6 and 9, respectively, with TIPF6. The cyclometalated complex [Ir(H)In-Bu((CNHCCCHimid)-C-4)(cod)]I (15) and 10, obtained from 1 and 3, respectively, were shown to be effective intermediates in the synthesis of 6 and 9. On the basis of these experimental results, we suggest possible mechanisms for the formation of 6 and 9 and a plausible explanation of the equilibrium observed between the iridium diiodo pincer complexes (4 and 8) and the iridium hydride complexes (6 and 9). A new dinuclear iridium(II) complex, [Ir-2(mu-I)(mu(2)-eta(1)-CHCH=NEt2)I-2(cod)(2)] (18), containing a ligand derived from activation of NEt3 was obtained, albeit in low yields, which could be involved in the transformation of 4 and 8 into 6 and 9, respectively. Finally, three polyhydride NHC ifidium complexes were generated from [IrI2n-Bu-(CNHCCCNHC)(DMSO)] (21) and 6. All these compounds were characterized by spectroscopic and spectrometric analysis, and the structures of 6, 8, 11, 12, 17, and 18 were determined by X-ray diffraction.