Dinuclear 1,4,7-triazacyclononane (tacn) complexes of cobalt(iii) with amido and tacn bridges. Synthesis, characterization and reversible acid-accelerated bridge cleavage

Abstract

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 °C. Two isomeric triply bridged complexes, [(tacn)Co(μ-NH₂)₃Co(tacn)]³⁺ and [(tacn)Co(μ-NH₂)₂{μ-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(μ-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) × 10⁻⁵[H⁺]  + 2.1(2) × 10⁻⁵ (s⁻¹) at 40 °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M HClO₄ at elevated temperature produces primarily triply bridged dinuclear species where one or two amido bridges have been replaced by hydroxo bridges.