Reactivity of free and Co-III-co-ordinated phosphite; mechanisms of bromine oxidation and H/D exchange

The complexes t- and p-[Co(tren)(NH3){OP(H)(O)(2)}]ClO4 (1 . ClO4 and 2 . C lO4, 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 prep ared. Single crystal structures have been determined for 1 . ClO4. 2H(2)O a nd 3 . ClO4. 3H(2)O. 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 degrees C, I=1.0 mol dm(-3), NaClO4) showed that 4-D (k(ex)=(1.6 +/- 0.2)x10(-4) dm(3) mol(-1) s(-1)) i s four-fold more reactive than 5-D (k(ex)=(4.0 +/- 0.5)x10(-5) dm(3) 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 degrees C, pH 1.22-4.34, I=1.0 mol dm(-3), NaClO4) fo llows the rate equation k(obs)=k(H)K(a)(H) [5-H](T)/(K-a(H)+[H+])(1+K-Br3[B r-]), where K-a(H) ((4.2 +/- 0.5)x10(-4) mol dm(-3)) corresponds to the ind ependently measured acid dissociation constant of the 5-H conjugate acid, K -Br3 (17.6 dm(3) mol(-1))=[Br-3(-)]/[Br-2][Br-], and where k(H) (1316 +/- 2 0 dm(3) mol(-1) s(-1)) represents the specific rate constant for reaction o f Br-2 with 5-H. For 5-D as substrate k(D)=560 dm(3) mol(-1) s(-1) (k(H)/k( D)=2.4). Similar studies with H3PO3 (4-H, 25 degrees C, pH 1.27-2.55, I=1.0 mol dm(-3), NaClO4) showed that bromine oxidation follows the rate equatio n: k(obs)=k(H)K(a1)(H)K(a2)(H)[4-H](T)/([H+](2)+K-a1(H)[H+]+(Ka1Ka2H)-K-H ) (1+K-Br3[Br-]), with K-a1(H) (0.108 mol dm(-3)) and K-a2(H) (1.67x10(-6) mo l dm(-3)) corresponding to the independently measured first and second acid dissociation constants of 4-H, and with k(H) (2.1x10(5) dm(3) mol(-1) s(-1 )) representing the specific rate constant for reaction of Br-2 with HPO32- . For 4-D as substrate k(D)=1.22x10(5) dm(3) mol(-1) s(-1) (k(H)/k(D)=1.7). NMR studies showed that there is no loss of O-17 label to solvent from (H3 PO3)-O-17, either during H/D exchange or on bromine oxidation. Mechanisms f or the reactions are discussed.