MECHANISTIC STUDY OF THE COMPLEX-FORMATION OF BORIC-ACID

Trigonal boric acid, B(OH)3, reacts with bidentate ligands like 4-isopropyltropolone (Hipt) and chromotropic acid (1,8-dihydroxynaphthalene-3,6-disulfonate, H2cht2-) and a tridentate H-resorcinol (1-((2,4-dihydroxy-1-phenyl)azo)-8-hydroxynaphthalene-3,6-disulfonate, H2res2-) to form the 1:1 complex. The reactions are represented as follows: [GRAPHICS] Kinetic measurements were carried out at various temperatures and pressures by using a high-pressure stopped-flow apparatus with spectrophotometric detection. Rate constants (25-degrees-C) and activation parameters obtained at I = 0.10 M (NaClO4) are as follows: k(f) = 144 M-1 s-1, k(d) = 0.561 s-1, DELTA-H(f)double-ended-dagger = 26.2 +/- 0.7 kJ mol-1, DELTA-H(d)double-ended-dagger = 68.1 +/- 1.2 kJ mol-1, DELTA-S(f)double-ended-dagger = -116 +/- 4 J mol-1 K-1, DELTA-S(d)double-ended-dagger = -21 +/- 4 J mol-1 K-1, DELTA-V(f)double-ended-dagger = -9.9 +/- 0.3 cm3 mol-1 (25-degrees-C), and DELTA-V(d)double-ended-dagger = -7.1 +/- 0.4 cm3 mol-1 (25-degrees-C) for the Hipt reaction; k(f) = 1.38 x 10(3) M-1 s-1, k(d) = 1.61 x 10(4) M-1 s-1, DELTA-H(f)double-ended-dagger = 12.9 +/- 1.5 kJ mol-1, DELTA-H(d)double-ended-dagger = 45.5 +/- 0.6 kJ mol-1, DELTA-S(f)double-ended-dagger = -141 +/- 5 J mol-1 K-1, DELTA-S(d)double-ended-dagger = -12 +/- 4 J mol-1 K-1, DELTA-V(f)double-ended-dagger = -15.3 +/- 1.9 cm3 mol-1 (20-degrees-C), and DELTA-V(d)double-ended-dagger = -6.3 +/- 0.7 cm3 mol-1 (20-degrees-C) for the H2cht2- reaction; k(f) = 1.01 M-1 s-1, DELTA-H(f)double-ended-dagger = 45.2 +/- 0.8 kJ mol-1, DELTA-S(f)double-ended-dagger = -93 +/- 4 J mol-1 K-1, and DELTA-V(f)double-ended-dagger = 3.9 +/- 0.4 cm3 mol-1 (25-degrees-C) for the H2res2- reaction. For the bidentate Hipt and H2cht2-, the rate-determining step is the change in the coordination environment of boron from trigonal to tetrahedral, while for the tridentate, H2res2-, the subsequent chelate-ring closure is rate limiting.