Mechanistic Study of the Complex Formation of Boric Acid

Koji Ishihara, Yasuko Mouri, Shigenobu Funahashi, Motoharu Tanaka

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Abstract

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: [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: kf = 144 M-1 s-1, kd = 0.561 s-1, ΔHf = 26.2 ± 0.7 kJ mol-1, ΔHd = 68.1 ± 1.2 kJ mol-1, ΔSf = -116 ± 4 J mol-1 K-1, ΔSd = -21 ± 4 J mol-1 K-1, ΔVf = -9.9 ± 0.3 cm3 mol-1 (25 °C), and ΔVd = -7.1 ± 0.4 cm3 mol-1 (25 °C) for the Hipt reaction; kf = 1.38 × 103 M-1 s-1, = 1.61 × 104 M-1 s-1, ΔHf = 12.9 ± 1.5 kJ mol-1, ΔHd = 45.5 ± 0.6 kJ mol-1, ΔSf = -141 ± 5 J mol-1 K-1, ΔSd = -12 ± 4 J mol-1 K-1, ΔVf = -15.3 ± 1.9 cm3 mol-1 (20 °C), and ΔVd = -6.3 ± 0.7 cm3 mol-1 (20 °C) for the H2cht2- reaction; kf = 1.01 M-1 s-1, ΔHf = 45.2 ± 0.8 kJ mol-1, ΔSf = -93 ± 4 J mol-1 K-1, and ΔVf = 3.9 ± 0.4 cm3 mol-1 (25 °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.

Original languageEnglish
Pages (from-to)2356-2360
Number of pages5
JournalInorganic Chemistry
Volume30
Issue number10
DOIs
Publication statusPublished - 1991 May 1

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ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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