Thermodynamics of several boron acid complexation reactions studied by variable-temperature 1H and 11B NMR spectroscopy

Abstract

Borate ions, RB(OH) − 3, react with polyols to form anionic complexes via the reaction RB(OH) − 3+H 2L⌉har2;Cx −+2H 2O. In the present study thermodynamic parameters of several such reactions were determined by variable-temperature 1H and 11B NMR spectroscopy. The systems studied were: B(OH) − 4/1,2-ethanediol; B(OH) − 4/1,2-propanediol; C 6H 5B(OH) − 3/1,2-ethanediol; CH 3B(OH) − 3/1,2-propanediol; and CH 3B(OH) − 3/1,2-dihydroxybenzene. The first four systems have very similar stability constants and thermodynamic parameters. The reactions are all exothermic (Δ H° ∼ −20 kJ mol −1) and values of Δ S° are quite negative (Δ S° ∼ −60 J mol −1 K −1). The negative entropy is attributed primarily to a loss of configurational entropy in the ligand on complexation. This assertion was further investigated by studying the complexation of CH 3B(OH) − 3 with the rigid ligand 1,2-dihydroxybenzene. The CH 3B(OH) − 3/1,2-dihydroxybenzene reaction is characterized by a stability constant which is greater by four orders of magnitude than those of the other systems and this increase is shown to be entirely due to much more positive values of Δ S°.