Interactions of anthracene appended ortho-aminomethylphenylboronic acid 1 with 20 organic and inorganic anions have been studied by fluorescence, 1H and 11B NMR titrations in DMSO. Carboxylate, phosphate and sulphate anions...

The N-quaternized derivative 5 of the James− Shinkai anthracene−boronic acid fluorescence sugar sensor 1 was prepared to probe the role of the bridging nitrogen in the signaling mechanism of 1. Both 5 and 1 contain positively charged bridging groups NMe + or NH + , respectively, but 5 lacks the ability to form the intramolecular ammonium−boronate doubly ionic hydrogen bond present in 1. Receptors 1 and 5 display opposite fluorescence vs pH profiles with a small turn-on effect of the sugar binding to the zwitterion of 5 in contrast to a large effect observed with 1. It is concluded that the ammonium−boronate hydrogen bond is essential for the signaling mechanism of 1. Its possible function is enabling the PET quenching effect by shifting the NH + proton toward boronate anion inside the hydrogen bond, the degree of which is modulated by the ester formation with diols affecting the basicity of boronate anion. This mechanism agrees with observed signaling selectivity of 1 toward a series of di-and polyols of variable structures as well as with the behavior of 1 in buffered D 2 O and methanol solvents at controlled pH and provides an addition to the established "loose bolt" mechanism signaling mode essential for receptors with nonpolar fluorophores.

The binding types (H-bonding or coordinate) and stability constants for complexes of 11 mono- and di-anions with benzoxaborole (1) were determined by 1H and 11B NMR titrations in DMSO or MeCN. Compared to phenylboronic acid (PBA), 1 is a stronger Lewis acid and a poorer H-bond donor with only one B-OH group, which is expected therefore to recognize anions mostly through the coordinate bonding. This is the case however only with F–, HPO4 2–, and PhPO3 2– anions, which are coordinately bonded to 1, and partially with SO4 2–, which forms only the H-bonded complex with PBA, but both H-bonded and coordinate complexes with 1. The majority of tested anions (AcO–, PhPO3H–, (PhO)2PO2 –, Cl–, and Br–) form H-bonded complexes with both 1 and PBA, whereas H2PO4 – changes the binding mode from coordinate for PBA to H-bonded for 1. The preferable binding type for each anion is confirmed by calculations of DFT-optimized structures of the anion complexes of 1. The preferable binding type can be rationalized considering the effects of the steric hindrance, more significant for the coordinate bonding, and of increased anion basicity, which is favorable for both binding types, but enhances the strength of coordinate bonding more significantly than the strength of H-bonding.