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(sp3)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(sp2)H bond activation, if the reaction is performed in the absence of a base. Similar cyclometalation and complex formation via C(sp2)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.

Relevance of Chemical vs. Electrochemical Oxidation of Tunable Carbene Iridium Complexes for Catalytic Water Oxidation

“…The solid‐state structures of complexes 8 – 12 were determined by single‐crystal X‐ray diffraction analyses and confirmed the connectivity pattern deduced from NMR spectroscopy. All complexes show the typical three‐legged piano‐stool geometry around the iridium center as expected for this type of half‐sandwich iridium complexes . Representative molecular structures of complexes 8b , 9a – b and 10 are shown in Figure (see Supporting Information for the structures of complexes 8d , 8e , 9c , 11 , 12a , and 12b ).…”

“…Selected bond lengths and angles are listed in Table . Bond lengths and angles around the iridium center are unsurprising for complexes 8b , 9a – b (Table ) and also for all structures reported in the supporting information, revealing no significant differences to reported triazolylidene iridium(III) complexes . In contrast, the bonding in complex 10 deviates considerably.…”

“…Even though the bonding parameters of complex 10 deviate considerably from average, we note that complex 11 with an –OH substituent does not show similar behavior and the bond lengths and angles are commensurate to the metrics typically observed in such iridium complexes. Therefore, the deviations cannot be attributed to the presence of the oxo substituent but may be a curiosity, possibly induced by packing effects …”

“…General: The metalation reactions were carried out under nitrogen atmosphere using standard Schlenk techniques, and all the reagents and solvents were used as obtained from commercial sources. The precursor compounds 2‐azidopyridine ( 1a ), 1‐(2‐pyridyl)‐1,2,3‐triazole ( 2a ), 5‐methoxy‐2‐(1 H ‐1,2,3‐triazol‐1‐yl)‐pyridine ( 2b ), 4‐methoxy‐2‐(1 H ‐1,2,3‐triazol‐1‐yl)‐pyridine ( 2c ), 4‐ N,N ‐dimethyl‐2‐(1 H ‐1,2,3‐triazol‐1‐yl)pyridinamine ( 2e ), ethyl 1‐(2‐pyridyl)‐1 H ‐1,2,3‐triazole‐4‐carboxylate ( 3a ), triazolium salts 5a , and 6a , as well as iridium complex 8a , and [IrCp*Cl2]2 were prepared according to literature procedures. All other ligand precursors are described in the supporting information.…”

Normal and Abnormal N-Heterocyclic Carbene Ligands

“…Likewise, most X-ray structure analyses of C4-metalated imidazolylidenes reveal a longer C]C bond (1.36-1.38 Å ), 46 which suggests a more delocalized double bond than in imidazolylidenes that bind the metal via C2. 50 This reactivity pattern contrasts the reactivity of complex 13 with a triazolylidene ligand, which comprises a nitrogen (N2) instead of a C2dR group in the heterocycle and which thus belongs to the same abnormal carbene ligand family as imidazol-4-ylidenes. 47 Selective isotope exchange at C5 has also been observed when C4-bound NHC rhodium complexes were used as catalysts for silane activation.…”