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, H₂cht^2-) and a tridentate H-resorcinol (1-((2,4-dihydroxy-1-phenyl)azo)-8-hydroxynaphthalene-3,6-disulfonate, H₂res^2-) to form the 1:1 complex. The reactions are represented as follows: [formula omited] Kinetic measurements were carried out at various temperatures and pressures by using a high-pressure stopped-flow apparatus with spectrophotometric detection. Rate constants (25 °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, ΔH_f^‡ = 26.2 ± 0.7 kJ mol^-1, ΔH_d^‡ = 68.1 ± 1.2 kJ mol^-1, ΔS_f^‡ = -116 ± 4 J mol^-1 K^-1, ΔS_d^‡ = -21 ± 4 J mol^-1 K^-1, ΔV_f^‡ = -9.9 ± 0.3 cm³ mol^-1 (25 °C), and ΔV_d^‡ = -7.1 ± 0.4 cm³ mol^-1 (25 °C) for the Hipt reaction; k_f = 1.38 × 10³ M^-1 s^-1, = 1.61 × 10⁴ M^-1 s^-1, ΔH_f^‡ = 12.9 ± 1.5 kJ mol^-1, ΔH_d^‡ = 45.5 ± 0.6 kJ mol^-1, ΔS_f^‡ = -141 ± 5 J mol^-1 K^-1, ΔS_d^‡ = -12 ± 4 J mol^-1 K^-1, ΔV_f^‡ = -15.3 ± 1.9 cm³ mol^-1 (20 °C), and ΔV_d^‡ = -6.3 ± 0.7 cm³ mol^-1 (20 °C) for the H₂cht^2- reaction; k_f = 1.01 M^-1 s^-1, ΔH_f^‡ = 45.2 ± 0.8 kJ mol^-1, ΔS_f^‡ = -93 ± 4 J mol^-1 K^-1, and ΔV_f^‡ = 3.9 ± 0.4 cm³ mol^-1 (25 °C) for the H₂res^2- reaction. For the bidentate Hipt and H₂cht^2-, the rate-determining step is the change in the coordination environment of boron from trigonal to tetrahedral, while for the tridentate, H₂res^2-, the subsequent chelate-ring closure is rate limiting.