Synthesis, stability, and reactivity of mesoionic carbene iridium dihydride complexes

Abstract

Pyridyl-triazolylidene ligands with variable donor properties were used as tunable ligands at a dihydride iridium (III) center. The straightforward synthesis of this type of ligand allows for an easy incorporation of electron-donating substituents in different positions of the pyridine ring or different functional groups such as esters, alkoxy, or aliphatic chains on the C4 position of the triazole heterocycle. The stability of these hydride metal systems allowed these complexes to be used as models for studying the influence of the ligand modifications on hydride reactivity. Spectroscopic analysis provided unambiguous structural assignment of the dihydride system. Modulation of the electronic properties of the wingtip substituents did not appreciably alter the reactivity of the hydrides. Reactivity studies using acids with a wide range of pK a values indicated a correlation between hydride reactivity and acidity and showed exclusive reactivity towards the less shielded hydride trans to the carbene carbon rather than the more shielded hydride trans to the pyridine ring, suggesting that the trans effect is more relevant in these reactions than the NMR spectroscopically deduced hydridic character.