1 Department of Chemistry, Faculty of Science, Københavns Universitet2 Danish Cardiovascular Reseach Academy, Department of Biomedical Sciences, Faculty of Health and Medical Sciences, Københavns Universitet3 unknown4 Department of Chemistry, University of Copenhagen5 Danish Cardiovascular Reseach Academy, Department of Biomedical Sciences, Faculty of Health and Medical Sciences, Københavns Universitet6 Department of Chemistry, Faculty of Science, Københavns Universitet
Amido-bridged dinuclear cobalt(III) complexes with 1,4,7-triazacyclononane (tacn) were synthesized from [Co(tacn)(O3SCF3)3] by treatment with potassium amide in liquid ammonia at 100 degrees C. Two isomeric triply bridged complexes, [(tacn)Co(mu-NH2)3Co(tacn)]3+ and [(tacn)Co(mu-NH2)2[mu-tacn(-H)]Co(NH3)]3+, were isolated as perchlorates, and the crystal structure of the perrhenate of the latter complex was determined by X-ray diffraction. In this compound a nitrogen atom (deprotonated) from one of the tacn ligands forms a third bridge together with two amido bridges. In 1.0 M (Na,H)ClO4 ([H+] 0.1-1.0 M) the two isomers undergo acid-accelerated amido bridge cleavage, as earlier found for chromium(III) analogues, in spite of the fact that such bridges are co-ordinatively saturated. The triamido-bridged isomer is in this acid medium in equilibrium with [(H2O)(tacn)Co(mu-NH2)2Co(tacn)(NH3)]4+. An isolated perchlorate of this complex appeared to be the salt of the trans-ammineaqua isomer as determined by X-ray diffraction. Equilibration from both sides fits the first-order rate constant dependence k(obs)=6.2(3) x 10(-5)[H+] + 2.1(2) x 10(-5)(s(-1)) at 40 degrees C. Prolonged treatment of the two triply bridged isomers in 1.0 M HClO4 at elevated temperature produces primarily triply bridged dinuclear species where one or two amido bridges have been replaced by hydroxo bridges.