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 respective ly (cyclen=1,4,7,10-tetraazacyclododecane) have been prepared and their sin gle crystal structures determined. Protonated forms of 3 and 4 are obtained as their ClO4- salts on crystallisation of the parent complexes from aqueo us 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 co mplexes 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 wi th 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 inver sion, with rate constants for lone pair inversion of the deprotonated centr es being in the range 2x10(6) to 1x10(8) s(-1) at 25 degreesC. Alkaline hyd rolysis (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), wh ereas 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 reac tions are discussed.