Trans-dioxocyclam (1,4,8,11-tetraazacyclotetradecane-2,9-dione) dihydrate and its NiII complex.

The crystal structures are reported of trans-dioxocyclam dihydrate, C(10)H(20)N(4)O(2).2H(2)O, a structural isomer of the well known cis-dioxocyclam, and of its novel Ni complex, (1,4,8,11-tetraazacyclotetradecane-2,9-dionato-kappa(4)N)nickel(II) dihydrate, [Ni(C(10)H(18)N(4)O(2))].2H(2)O, 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 [NiL(1)] (L(1) is deprotonated trans-dioxocyclam, C(10)H(18)N(4)O(2)) 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.