KINETICS AND MECHANISM OF RING-OPENING IN THE HYDROLYSIS OF COBALT(III) CARBONATE CHELATES

By appropriate choice of wavelength (lambda = 470-580 nm) it is shown that decarboxylation of bidentate carbonate in a number of [Co(L)(4)(O 2CO)](n+) chelates follows consecutive first-order reactions in aqueou s acidic solution ([H+] = 0.10-1.0 M). Sometimes the first observed pr ocess follows the rate law k(fast) = k(1)K[H+]/(1 + K[H+]) with the su bsequent slower process k(slow) = k(2) being pH independent, and somet imes k(fast) = k(2) with k(slow) = k(1)K[H+]/(1 + K[H+]). The limiting rate law k(obs) = k(1)K[H+]/(1 + K[H+]) is interpreted mechanisticall y as equilibrium protonation of the exo-O atom of the chelate (K) foll owed by unimolecular cleavage (k(1)) of the Co-O bond of the bicarbona to intermediate [Co(L)(4)(O2COH)]((n+1)+). For (L)(4) = nta(3-), (gly) (2)(2-), and cyclen, direct spectrophotometric measurement of the acid ity of the bicarbonato chelate was possible (K-a/M = 1.1, 0.65, 0.42, respectively). The rate law k(obs) = k(2) is interpreted as unimolecul ar decarboxylation of cis-[Co(L)(4)fOH(2))-(OCO2H)]((n+1)+) to give ci s-[Co(L)(4)(OH2)(2)]((n+2)+) + CO2. For different (L)(4), the followin g data were obtained at 25.0 degrees C and I = 1.0 (NaClO4) [(L)(4) (K /M(-1), k(1)/s(-1), k(2)/s(-1))]: nta(3-) (1.19 +/- 0.08, 85.0 +/- 4.0 , 2.0 +/- 0.2); alpha-trien (0.71 +/- 0.08, 9.7 +/- 0.8, 0.62 +/- 0.01 ); tren (0.61 +/- 0.10, 4.25 +/- 0.48, 0.31 +/- 0.01); (gly)(2)(2-) (1 .37 +/- 0.38, 4.1 +/- 0.5; 4.1 +/- 0.2); (NH3)(4) (1.0 +/- 0.2 +/- 1.6 8 +/- 0.22, 1.50 +/- 0.05); beta-trien (0.80 +/- 0.10, 0.285 +/- 0.02, 0.45 +/- 0.05); cyclen (1.42 +/- 0.28, (1.02 +/- 0.10) x 10(-2), no v alue); N-mecylen (0.43 +/- 0.04, (7.61 +/- 0.50) x 10(-3)), no value). These complexes span the range where ring opening is much faster than (nta(3-), alpha-trien, tren), is similar to [(gly)(2)(2-), (NH3)(4), beta-trien], or is much slower than (cyclen, N-mecyclen) subsequent de carboxylation. For alpha-[Co(trien)(O2CO)](2+), neither k(1) nor k(2) show a significant D2O solvent isotope effect (k1(D2O) = 8.67 +/- 0.38 s(-1), k(2)(D2O) = 0.64 s(-1)), but K does (K-D2O = 1.82 +/- 0.18 M(- 1), K-D2O/K-H2O = 2.6). The large variation in rateconstant k(1) with changing (L)(4) (similar to 10(4)) is interpreted as differing abiliti es to transfer the eno-O proton to the endo-O ring oxygen in the biden tate bicarbonato intermediate [Co(L)(4)(O2COH)]((n+1)+). It is this de chelation of HCO3-, rather than subsequent decarboxylation, that is ve ry sensitive to the ''nonparticipating'' ligand. There is no evidence to suggest a spontaneouspathway for hydrolysis the above carbonate che fates.