trans-Dioxocyclam (1,4,8,11-tetraazacyclotetradecane-2,9-dione) dihydrate and its Ni^II complex

The crystal structures are reported of trans-dioxocyclam dihydrate, C₁₀H₂₀N₄O₂·2H₂O, a structural isomer of the well known cis-dioxocyclam, and of its novel Ni complex, (1,4,8,11-tetra­aza­cyclo­tetra­decane-2,9-dionato-κ⁴N)­nickel(II) dihydrate, [Ni(C₁₀H₁₈N₄O₂)]·2H₂O, the first example of a trans­ition metal complex of this ligand. Both mol­ecules lie on crystallographic centres of inversion. The free ligand has two of its N atoms turned outwards from the ring and hydrogen bonded to water mol­ecules. A major conformational change takes place in the complex in which the ligand binds in a trans tetradentate fashion, as suggested by the electronic spectrum. The nickel(II) ion is low spin, although the electronic spectrum of the complex in water indicates an equilibrium mixture of low-spin and high-spin species. The irreversible electrochemical oxidation of [NiL¹] (L¹ is deprotonated trans-dioxocyclam, C₁₀H₁₈N₄O₂) in water occurs at a potential of 0.964 V [versus SHE (standard hydrogen electrode)], which is very similar to that for the Ni–cis-dioxocyclam complex.