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, C10H20N 4O2. 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 N-4)nickel(II) dihydrate, [Ni(C10H18N4O2)]. 2H(2) O, the first example of a transition metal complex of this ligand. Both molecules lie on crystallog raphic 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 conf ormational change takes place in the complex in which the ligand binds in a trans tetradentate fashion, as suggested by the electronic spectrum. The n ickel(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] (L-1 is deprotonated transdioxocyclam, C10H18N4O2) in water occurs at a potential of 0.964 V [v ersus SHE (standard hydrogen electrode)], which is very similar to that for the Ni-cis-dioxocyclam complex.