The coordination chemistry of the pentadentate 2,2,6,6-tetrakis(aminomethyl)-4-azaheptane (ditame)

The protonation and metal complex formation of the branched pentaamine ligand 2,2,6,6-tetrakis(aminomethyl)-4-azaheptane (ditame, L) with Co^II, Co^III, N^II, Cu^II, Zn^II, and CD^II have been studied. The crystal structures of [Ni(ditame)(EtOH)] Br₂, [CO₂(ditame)₂(O₂)Cl₄ ·4H₂O, and [Co₂-(ditame)₂(O₂)]Cl₂ [ZnCl₄][ZnCl₃(H₂O)]·H₂O were determined by X-ray diffraction analyses. The metal complexes showed octahedral coordination geometries with exclusive formation of six-membered chelate rings, the four primary amino groups being in equatorial positions and the secondary amino group (N^sec) apical. This geometry enforces a relatively short M-N^sec bond. Complex formation in aqueous solution was evaluated by potentiometric titration. The ditame ligand forms a surprisingly stable Ni^II complex with logK_NiL = 17.8 (25 °C, 0.1 M KNO₃), and formation of protonated species [Ni(H_nditame)]⁽ⁿ⁺²⁾⁺ (n = 1, 2) is not significant. Different behavior is evident for Zn^II and Cd^II, with which protonated species are extensively formed, while the stability of [ML]²⁺ is relatively low. Complex formation between ditame and Cu^II in the range 3 < pH < 7 resulted in the formation of [Cu(H₂ditame)]⁴⁺. In slightly alkaline solution, the dinuclear [Cu₂(ditame)₂]⁴⁺ appears to be formed. The structural assignment of this dimer is based on its UV/Vis and EPR characteristics, with the EPR data indicating a Cu^II···Cu^II separation of about 5.9 Å. The redox behavior of the Ni and Co complexes was investigated by cyclic voltammetry. The mononuclear species showed quasi-reversible electron transfer for the Ni^II/III and Col^II/III couples. For the dinuclear peroxo/superoxo-bridged Co^II/III complexes, quasi-reversible electron transfer corresponding to oxidation/reduction of the O₂^2-/O₂^- bridge was observed. (