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

Abstract

The syn,anti and syn,syn forms of the complexes [Co(cyclen)(O₂C₂O₂)]ClO₄1 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 °C, 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) × 10⁷ 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) × 10⁵ M⁻¹ s⁻¹ (25.0 °C, 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 2 × 10⁶ to 1 × 10⁸ s⁻¹ at 25 °C. Alkaline hydrolysis (25.0 °C, 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.