Luminescent iridium(III) complexes bearing boronic acid groups: potential sensors for carbohydrates

A family of eight Ir(III) complexes of the form [Ir(X)₂(L)]⁺ (where X = 2-phenylpyridine, 1-phenylpyrazole, 2-phenylbenzothiazole or 1-phenylindazole and L = 2-(pyrazole-1-yl)pyridine substituted with a boronic acid group at position four of either the pyridyl or the pyrazole ring) have been prepared as potential luminescent sensors for carbohydrate detection. These complexes have been fully characterised via¹H and ¹³C NMR, HRMS and for complexes 11, 14, and 18 by X-ray crystallography. Photoluminescent pH titration studies showed conversion of the boronic acid group to the boronate anionic form occurred within the pK_a range of 4.9-5.3 for complexes 11-14 and 6.4-7.8 for complexes 15-18, with formation of the boronate anion being accompanied by significant changes in the photoluminescence emission properties of these compounds. Similar pH titration studies were undertaken for the complexes in the presence of glucose and fructose sugars, with the binding constants between these sugars and the complexes via cyclic boronate ester formation estimated from the photoluminescent titration data; significantly stronger binding was observed for fructose compared to glucose. Interestingly, the formation of the cyclic boronate ester results in a consistent decrease in the emission intensity for complexes 12, 13, 16, and 17 while complexes 11, 14, 15, and 18 showed enhanced luminescence. Theoretical calculations support the experimental observations.