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DOI: 10.1039/A906670E (Paper) Reference Section for: J. Chem. Soc., Dalton Trans., 1999, 3809-3816

Reactivity of free and CoIII-co-ordinated phosphite; mechanisms of bromine oxidation and H/D exchange[hair space]†

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Sonya J. Carrington, David A. Buckingham, Jim Simpson, Allan G. Blackman and Charles R. Clark

Abstract

The complexes t- and p-[Co(tren)(NH3){OP(H)(O)2}]ClO4 (1·ClO4 and 2·ClO4, respectively) and syn(OP(H)(O)2), anti(OH2)-[Co(cyclen)(OH2){OP(H)(O)2}]ClO4 (3·ClO4, cyclen = 1,4,7,10-tetraazacyclododecane) have been prepared. Single crystal structures have been determined for 1·ClO4·2H2O and 3·ClO4·3H2O. In weakly acidic aqueous solution 3 equilibrates with its syn(OH2),anti(OP(H)(O)2) isomer and the chelate [Co(cyclen){O2P(H)O}]+. Kinetic studies of acid catalysed H/D exchange in H2DPO3 (4-D) and [Co(NH3)5{OP(D)(OH)O}]2+ (5-D) in water (55.0 °C, I = 1.0 mol dm–3, NaClO4) showed that 4-D (kex = (1.6 ± 0.2) × 10–4 dm3 mol–1 s–1) is four-fold more reactive than 5-D (kex = (4.0 ± 0.5) × 10–5 dm3 mol–1 s–1) and that exchange likely involves P(OH)3 and [Co(NH3)5{OP(OH)2}]2+ intermediates, respectively. Bromine oxidation of [Co(NH3)5{OP(H)(O)2}]+ (5-H, 25 °C, pH 1.22–4.34, I = 1.0 mol dm–3, NaClO4) follows the rate equation kobs = kHKaH [5-H]T/(KaH + [H+])(1 + KBr3[Br–]), where KaH ((4.2 ± 0.5) × 10–4 mol dm–3) corresponds to the independently measured acid dissociation constant of the 5-H conjugate acid, KBr3 (17.6 dm3 mol–1) = [Br3–]/[Br2][Br–], and where kH (1316 ± 20 dm3 mol–1 s–1) represents the specific rate constant for reaction of Br2 with 5-H. For 5-D as substrate kD = 560 dm3 mol–1 s–1 (kH/kD = 2.4). Similar studies with H3PO3 (4-H, 25 °C, pH 1.27–2.55, I = 1.0 mol dm–3, NaClO4) showed that bromine oxidation follows the rate equation: kobs = kHKa1HKa2H[4-H]T/([H+]2 + Ka1H[H+] + Ka1HKa2H )(1 + KBr3[Br–]), with Ka1H (0.108 mol dm–3) and Ka2H (1.67 × 10–6 mol dm–3) corresponding to the independently measured first and second acid dissociation constants of 4-H, and with kH (2.1 × 105 dm3 mol–1 s–1) representing the specific rate constant for reaction of Br2 with HPO32–. For 4-D as substrate kD = 1.22 × 105 dm3 mol–1 s–1 (kH/kD = 1.7). NMR studies showed that there is no loss of 17O label to solvent from H3P17O3, either during H/D exchange or on bromine oxidation. Mechanisms for the reactions are discussed.

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