THERMODYNAMICS OF SEVERAL BORON ACID COMPLEXATION REACTIONS STUDIED BY VARIABLE-TEMPERATURE H-1 AND B-11 NMR-SPECTROSCOPY

Borate ions, RB(OH)3-, react with polyols to form anionic complexes vi a the reaction RB(OH)3- + H-2L half arrow right over half arrow left C x- +2H2O. In the present study thermodynamic parameters of several suc h reactions were determined by variable-temperature H-1 and B-11 NMR s pectroscopy. The systems studied were: B(OH)4-/1,2-ethanediol; B(OH)4- /1,2-propanediol; C6H5B(OH)3-/1,2-ethanediol; CH3B(OH)3-/1,2-propanedi ol; and CH3B(OH)3-/1,2-dihydroxybenzene. The first four systems have v ery similar stability constants and thermodynamic parameters. The reac tions are all exothermic (DELTAH-degrees is similar to -20 kJ mol-1) a nd values of DELTAS-degrees are quite negative (DELTAS-degrees is simi lar to -60 J mol-1 K-1). The negative entropy is attributed primarily to a loss of configurational entropy in the ligand on complexation. Th is assertion was further investigated by studying the complexation of CH3B(OH)3- with the rigid ligand 1,2-dihydroxybenzene. The CH3B(OH)-/1 ,2-dihydroxybenzene reaction is characterized by a stability constant which is greater by four orders of magnitude than those of the other s ystems and this increase is shown to be entirely due to a much more po sitive value of DELTAS-degrees.