Re-investigation of optical sensing properties of boronic-acid-appended Re complexes for saccharides

A number of unanswered questions occurred to us upon reading a communication by Yam and Kai (ref. 16) which had reported optical sensing properties of a boronic-acid-appended Re¹ complex for saccharides. Careful re-examination has disclosed that the pK_a-value proposed by them (5.9) is wrong and that the saccharide-binding mode at pH above the pK_a is totally different from that at pH below the pK_a. The absorption spectral change, which reflects an sp²-to-sp³ boron hybridisation change induced by the saccharide complexation, was observed only at pH below the pK_a, and the CD band, which reflects the formation of 1:1 cyclic complexes, appeared only at pH above the pK_a. The results imply that the optimum pH should be carefully selected for the precise optical sensing of saccharides.