title: Iridium complexes containing mesoionic C donors: selective C(sp3)-H versus C(sp2)-H bond activation, reactivity towards acids and bases, and catalytic oxidation of silanes and water.
abstract: Metalation of a C2-methylated pyridylimidazolium salt with [IrCp*Cl2]2 affords either an ylidic complex, resulting from C(sp(3))-H bond activation of the C2-bound CH3 group if the metalation is performed in the presence of a base, such as AgO2 or Na2CO3, or a mesoionic complex via cyclometalation and thermally induced heterocyclic C(sp(2))-H bond activation, if the reaction is performed in the absence of a base. Similar cyclometalation and complex formation via C(sp(2))-H bond activation is observed when the heterocyclic ligand precursor consists of the analogous pyridyltriazolium salt, that is, when the metal bonding at the C2 position is blocked by a nitrogen rather than a methyl substituent. Despite the strongly mesoionic character of both the imidazolylidene and the triazolylidene, the former reacts rapidly with D(+) and undergoes isotope exchange at the heterocyclic C5 position, whereas the triazolylidene ligand is stable and only undergoes H/D exchange under basic conditions, where the imidazolylidene is essentially unreactive. The high stability of the Ir-C bond in aqueous solution over a broad pH range was exploited in catalytic water oxidation and silane oxidation. The catalytic hydrosilylation of ketones proceeds with turnover frequencies as high as 6,000 h(-1) with both the imidazolylidene and the triazolylidene system, whereas water oxidation is enhanced by the stronger donor properties of the imidazol-4-ylidene ligands and is more than three times faster than with the triazolylidene analogue.
authors.affiliation: School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4 (Ireland).
authors.forename: Ana
authors.initials: A
authors.lastname: Petronilho
authors.forename: James A
authors.initials: JA
authors.lastname: Woods
authors.forename: Helge
authors.initials: H
authors.lastname: Mueller-Bunz
authors.forename: Stefan
authors.initials: S
authors.lastname: Bernhard
authors.forename: Martin
authors.initials: M
authors.lastname: Albrecht
mesh_terms: D002384:Catalysis; D056831:Coordination Complexes / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D048289:Imidazolidines / Q000737:chemistry*; D007495:Iridium / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry; D010084:Oxidation-Reduction; D011725:Pyridines / Q000737:chemistry*; D013237:Stereoisomerism; D014230:Triazoles / Q000737:chemistry*
chemical_list: D056831:Coordination Complexes; D007093:Imidazoles; D048289:Imidazolidines; D008024:Ligands; D009942:Organometallic Compounds; D011725:Pyridines; D014230:Triazoles; C000599919:mesoionic C; D007495:Iridium
keywords: CH activation; carbene; iridium; mesoionic ligand; oxidation; stability