Influence of the boron atom on the solvatochromic properties of 4-nitroaniline-functionalized boronate esters

Para-nitroaniline derivatives with peripheral 1,2- and 1,3-diol functionalities O(2)N-C(6)H(4)-NR(1)-CH(2)CH(OH)CH(2)OH; O(2)N-C(6)H(4)-NR(1)-CH(CH(2)OH)(2); R(1) = -H, -CH(3)] covalently bonded to the amino group are esterified with various para-substituted phenylboronic acids R(2)-C(6)H(4)-B(OH)(2); R(2) = -OCH(3), -CH(3), -H, -Br, -CHO, -NO(2), -B(OH)(2)], and the solvatochromic properties of these esters are investigated in 33 solvents of different polarity. To interpret the solvent effects, the established linear solvation energy (LSE) multiparameter equations of Kamlet-Taft and the improved Catalán scales are used. Although the boron atom is separated by two or three sp(3)-hybridized carbon atoms from the actual chromophore, solvation effects have a significant positive solvatochromic effect on the nitroaniline unit (R(1) = -CH(3)) as result of the solvent acting as a donor at the boron atom. The influence of the substituent R(2) on the coefficient b of the LSE relationship according to Kamlet-Taft and Catalán, which reflects the quantitative influence of the hydrogen-bonding acceptor or the electron-pair donor capacity of the solvent on the position of the UV-vis absorption maximum, can be determined via a linear Hammett relationship b = f(σ(p))]. The interpretation of the effects is based on the electronic influence of the solvated boronic acid ester unit on the 4-nitroaniline group, predominantly through inductive interactions.