pH-Independent Recognition of Polyhydroxy Compounds by Niobium(V) Porphyrin Complex with Unique Sugar Selectivity

The niobium(V) complex with tetraphenylporphin having OH^- as an auxilliay ligand exists as a dimeric complex, [Nb₂(tpp)₂O₃] at a total concentration >10^-4.5 mol dm^-3, and reacts with an aliphatic or aromatic polyhydroxy compound to form a monomeric complex containing chelate rings by coordination of the deprotonated species, and to cause an appreciable UV-Vis spectral change. In contrast to phenylboronic acid (PBA), the reactivity of [Nb₂(tpp)₂O₃] is independent of pH at least between 4 and 8. Aliphatic comounds are more reactive than aromatic compounds in dioxane-water, while the reactivity order is reversed in the two-phase reaction. The sugar selectivity order of [Nb₂(tpp)₂O₃] in dioxane-water (10:1) (sorbose > fructose >> mannose > arabinose, galactose > glucose) is appreciably different from that of PBA (fructose > sorbose >> arabinose > galactose > mannose > glucose). This may be related to the difference in size of the Lewis acidic center.