Preparation, structures and reactions of isomeric [Co(cyclen)(O2C2O2)](+) and [Co(cyclen)(O2CCH2CO2)](+) complexes (cyclen=1,4,7,10-tetraazacyclododecane)
Aj. Clarkson et al., Preparation, structures and reactions of isomeric [Co(cyclen)(O2C2O2)](+) and [Co(cyclen)(O2CCH2CO2)](+) complexes (cyclen=1,4,7,10-tetraazacyclododecane), J CHEM S DA, (5), 2001, pp. 758-765
Citations number
45
Categorie Soggetti
Inorganic & Nuclear Chemistry
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS
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.