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

摘要

The complexes t- and p-[Co(tren)(NH_3){OP(H)(O)_2}]ClO_4(1.ClO_4 and 2.ClO_4, respectively) and syn(OP(H)(O)_2), anti(OH_2)-[Co(cyclen)(OH_2){OP(H)(O)_2}]ClO_4(3.ClO_4, cyclen = 1,4,7,10-tetraazacyclododecane) have been prepared. Single crystal structures have been determined for 1.ClO_4.2H_2O and 3.ClO_4.3H_2O. In weakly acidic aqueous solution 3 equilibrates with its syn(OH_2), anti(OP(H)(O)_2) isomer and the chelate [Co(cyclen){O_2P(H)O}]~+. Kinetic studies of acid catalysed H/D exchange in H_2DPO_3 (4-D) and [Co(NH_3)_5{OP(D)(OH)O}]~(2+) (5-D) in water (55.0degC, I = 1.0 mol dm~(-3), NaClO_4) showed that 4-D (K_(ex)=(1.6±0.2)×10~(-4) dm~3 mol~(-1) s~(-1)) is four-fold more reactive than 5-D (K_(ex)=(4.0±0.5)×10~(-5) dm~3 mol~(-1) s~(-1)) and that exchange likely involves P(OH)_3 and [Co(NH_3)_(5~-){OP(OH)_2}]~(2+) intermediates, respectively. Bromine oxidation of [Co(NH_3)_5{OP(H)(O)_2}]~+ (5-H, 25degC, pH 1.22-4.34, I=1.0 mod dm~(-3), NaClO_4) follows the rate equation k_(obs)=K~HK_a~H[5-H]_T/(K_a~H+[H~+])(1+K_(Br3)[Br~-]), where K_a~H ((4.2±0.5)×10~(-4) mol dm~(-3)) corresponds to the independently measured acid dissociation constant of the 5-H conjugae acid, K_(Br3)(17.6 dm~3 mol~(-1))=[Br_3~-]/[Br_2][Br~-], and where K~H (1316±20 dm~3 mol~(-1) s~(-1)) represents the specific rate constant for reaction of 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 H_3PO_3 (4-H, 25degC, pH 1.27-2.55, I=1.0 mol dm~(-3), NaClO_4) showed that bromine oxidation follows the rate equation; K_(obs)=K~HK_(al)~HK_(a2)~H[4-H]_T/([H~+]~2+K_(al)~H[H~+]+K+(al)~HK_(a2)~H) (1+K_(Br_3)[Br~-], with K_(al)~H (0.108 mol dm~(-3) and K_(al)~H (1.67×10~(-6) mol dm~(-3)) correspondin to the independently measured first and second acid dissociation constants of 4-H, and with K~H(2.1×10~5 dm~3 mol~(-1) s~(-1)) representing the specific rate constant for reaction of Br_2 with HPO_3~(2-). For 4-D as substrate k~D=1.22×10~5 dm~3 mol~(-1) s~(-1)(K_H/K_D=1.7). NMR studies showed that there is no loss of ~(17)O label to solvent from H_3P~(17)O_3, either during H/D exchange or on bromine oxidation. Mechanisms for the reactions are discussed.