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

Abstract

The reactivity of the potential pincer ligands (1,3-phenylene)bis(1-butylimidazolium) diiodide (n-Bu(CHimidCHCHimid)I2, 1) and (4,6-dimethyl-1,3-phenylene)bis(1-butylimidazolium) diiodide (n-Bu(CHimidMeCHCHimid)I2, 3) toward [Ir(μ-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(μ-I)n-Bu(CNHCC4CHimid)2]I2}2 (7, 40%) and to five complexes from 3, [IrI2n-Bu(CNHCMeCCNHC)(NCMe)] (8, 30%), [Ir(H)In-Bu(CNHCMeCCNHC)(NCMe)] (9, 30%), [IrI(cod)n-Bu(CNHCMeCHCHimid)]I (10, 34%), [{IrI(cod)}2n-Bu(μ-CNHCMeCHCNHC)] (11, <5%), and [Irn-Bu(CNHCMeCCNHC)2]I (12, <5%). In both cases, the amount of CNHCCCNHC and CNHCMeCCNHC 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(μ-Cl)(cod)]2 unexpectedly yielded the complex [IrI2n-Bu(CNHCC4CHimid)(NCMe)2]I (17), in which aromatic metalation has occurred in the C4 or C6 position. Cationic complexes [Ir(H)n-Bu(CNHCCCNHC)(NCMe)2](PF6) (13) and [Ir(H)n-Bu(CNHCMeCCNHC)(NCMe)2](PF6) (14) were prepared by iodide ligand abstraction from 6 and 9, respectively, with TlPF6. The cyclometalated complex [Ir(H)In-Bu(CNHCC4CHimid)(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, [Ir2(μ2-I)(μ2-η1-CHCH═NEt2)I2(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 iridium 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.