trans‐Dioxocyclam (1,4,8,11‐tetraazacyclotetradecane‐2,9‐dione) dihydrate and its NiIIcomplex

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 transtetradentate 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] (L1is deprotonated trans‐dioxocyclam, C10H18N4O2) in water occurs at a potential of 0.964 V [versusSHE (standard hydrogen electrode)], which is very similar to that for the Ni–cis‐dioxocyclam complex.