Experimental and theoretical studies of pH-responsive iridium(III) complexes of azole and N-heterocyclic carbene ligands.

A series of nine luminescent iridium(III) complexes with pH-responsive imidazole and benzimidazole ligands have been prepared and characterized. The first series of complexes were of the form [Ir(ppy)2(N^N)]+ or [Ir(ppy)2(C^N)]+ (where ppy is 2-phenylpyridine and N^N is 2-(2-pyridyl)imidazole or 2-(2-pyridyl)benzimidazole and C^N represents a pyridyl-triazolylidene-based N-heterocyclic carbene ligand). For these complexes, the benzimidazole group was either unsubstituted or substituted with electron-withdrawing (Cl) or electron-donating (Me) groups. The second series of complexes were of the form [Ir(phbim)2(N^N)]+ or [Ir(phbim)2(C^N)]+ (where phbim is 2-phenylbenzimidazole and N^N is either 2,2'-bipyridine or 1,10-phenanthroline and C^N is either a pyridyl-imidazolylidene or pyridyl-triazolylidene N-heterocyclic carbene ligand). UV-visible and photoluminescence pH titration studies showed that changing the protonation state of these complexes results in significant changes in the photoluminescence emission properties. The pKa values of prepared complexes were estimated from the spectroscopic pH titration data and these values show that the nature of the pH-sensitive ligands (either main or ancillary ligands) resulted in a significant capacity to modulate the pKa values for these compounds with values ranging from 5.19-11.22. Theoretical investigations into the nature of the electronic transitions for the different protonation states of compounds were performed and the results were consistent with the experimental results.

Luminescent iridium(iii)-boronic acid complexes for carbohydrate sensing.

A family of four Ir(iii) complexes of the form [Ir(ppy)2(L)]Cl (where ppy = 2-phenyl-pyridine and L = a pyridyl-1,2,4-triazole or pyridyl-1,3,4-oxadiazole ligand bearing a boronic acid group) have been prepared as potential luminescent sensors for carbohydrates. A modular eight step procedure was developed to synthesise the complexes, and this was initiated with the preparation of two benzhydrazide and three S-ethylated pyridine-2-thiocarboxamides precursors. Reaction of these precursors produced three new 1,2,4-triazole- and one 1,3,4-oxadiazole-based ligands substituted with boronic acid pinacol ester groups. The boronic acid pinacol esters were then converted to boronic acids in two steps via potassium trifluoroborate intermediates. The boronic acid substituted ligands and their Ir(iii) complexes were fully characterised using a range of techniques including X-ray crystallography in the case of the pyridyl-1,3,4-oxadiazole ligand and two of the Ir(iii) complexes. The capacity of the synthesised Ir(iii) complexes to form boronic acid cyclic esters with the simple sugars glucose and fructose was evaluated using high-resolution mass spectrometry (HRMS) and photoluminescence titration studies. These studies confirm that the Ir(iii) complexes form adducts with both glucose and fructose, with increased levels of boronic acid cyclic esters being formed with fructose at higher pHs. Theoretical calculations were used to gain insight into the nature of the electronic transitions involved in the electronic absorption and emission spectra.