No abstract

To establish a detailed reaction mechanism for the condensation between a boronic acid, RB(OH)2, and a diol, H2L, in aqueous solution, the acid dissociation constants (Ka(BL)) of boronic acid diol esters (HBLs) were determined based on the well-established concept of conditional formation constants of metal complexes. The pKa values of HBLs were 2.30, 2.77, and 2.00 for the reaction systems, 2,4-difluorophenylboronic acid and chromotropic acid, 3-nitrophenylboronic acid and alizarin red S, and phenylboronic acid and alizarin red S, respectively. A general and precise reaction mechanism of RB(OH)2 with H2L in aqueous solution, which can serve as a universal reaction mechanism for RB(OH)2 and H2L, was proposed on the basis of (a) the relative kinetic reactivities of the RB(OH)2 and its conjugate base, that is, the boronate ion, toward H2L, and (b) the determined pKa values of HBLs. The use of the conditional formation constant, K', based on the main reaction: RB(OH)2 + H2L (K1)⇌ RB(L)(OH)(-) + H3O(+) instead of the binding constant has been proposed for the general reaction of uncomplexed boronic acid species (B') with uncomplexed diol species (L') to form boronic acid diol complex species (esters, BL') in aqueous solution at pH 5-11: B' + L' (K')⇌ BL'. The proposed reaction mechanism explains perfectly the formation of boronic acid diol ester in aqueous solution.

Structures and stabilization due to stacking of ternary copper(II) complexes containing an aromatic amino acid (AA) and an aromatic diamine (DA), Cu(AA)(DA), have been investigated by potentiometric, spectroscopic, and X-ray diffraction methods. For the systems with AA ) para-X-substituted L-phenylalanine (L-XPhe; X ) H, NO 2 , OH, NH 2 ) and DA ) 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen), the difference absorption spectra in the region 320-400 nm exhibited a peak assignable to the charge transfer interaction between the aromatic rings of DA and L-XPhe, the intensity being in the order NH 2 > OH > H . NO 2 with respect to X. The stability constants of Cu(AA)(DA) determined for AA ) DL-XPhe (X ) F, Cl, Br) and L-XPhe (X ) NH 2 , NO 2 , I) at 25 °C and I ) 0.1 M (KNO 3 ) indicated that stabilization of Cu(L-XPhe)(DA) relative to Cu(L-Ala)(en) (Ala ) alanine; en 3), and [Cu(L-Phe)-(bpy)]ClO 4 ‚H 2 O (4), isolated as crystals, were determined by the X-ray diffraction method: 1, orthorhombic,All of the complexes have a similar distorted square-pyramidal structure around the central Cu(II) ion. While 1 and 2 have a structure involving aromatic ring stacking in the solid state, 4 has a structure without it and 3 has both types of structures in the unit cell.