Preparation, structures and reactions of isomeric [Co(cyclen)(O2C2O2)]+ and [Co(cyclen)(O2CCH2CO2)]+ complexes (cyclen=1,4,7,10-tetraazacyclododecane)

The syn,anti and syn,syn forms of the complexes [Co(cyclen)(O2C2O2)]ClO4 1 and 2 respectively and [Co(cyclen)(O2CCH2CO2)][ClO4].H2O 3 and 4 respectively (cyclen=1,4,7,10-tetraazacyclododecane) have been prepared and their single crystal structures determined. Protonated forms of 3 and 4 are obtained as their ClO4- salts on crystallisation of the parent complexes from aqueous HClO4, and a crystal structure of the former shows protonation occurs on a carbonyl O atom in the malonato bidentate ligand. Proton exchange (D2O, 25.0 degreesC, I=1.0 M) of the equatorial NH protons (syn and anti) in 1, 2, 3 and 4 is first order in [OD-], with k(H) values in the range (1-8)x10(7) M-1 s(-1). Equilibration between the syn,anti and syn,syn forms of the complexes follows the rate equation k(obs)=k(isom)[OH-] with k(isom)=(1.3 +/-0.1)x10(5) M-1 s(-1) (25.0 degreesC, I=1.0 M NaCl) for both systems, and with final equilibrium distributions of [2]/[1]=0.100 +/-0.005 and [4]/[3]=0.057 +/-0.002. The various equatorial NH sites are very susceptible to inversion, with rate constants for lone pair inversion of the deprotonated centres being in the range 2x10(6) to 1x10(8) s(-1) at 25 degreesC. Alkaline hydrolysis (25.0 degreesC, 1.0 M, NaClO4) of [Co(cyclen)(O2CCH2CO2)](+) gives [Co(cyclen)(OH)(2)](+) in a biphasic reaction, with both paths first order in [OH-], k(OH(1))=26.9 +/-0.6 M-1 s(-1), k(OH(2))=4.9 +/-0.6 M-1 s(-1), whereas alkaline hydrolysis of [Co(cyclen)(O2C2O2)](+) gives [Co(cyclen)(OH)(2)](+) in one step, k(OH)=0.114 +/-0.009 M-1 s(-1). Mechanisms for the reactions are discussed.