{N,N′-Bis[1-(2-pyridyl)ethylidene]ethane-1,2-diamine-κ4 N,N′,N′′,N′′′}bis(trifluoromethanesulfanato-κO)copper(II)

A discrete neutral CuII complex, [Cu(CF3SO3)2(C16H18N4)], has been derived from the symmetrical tetradentate Schiff base, N,N′-bis[1-(pyridin-2-yl)ethylidene]ethane-1,2-diamine. The copper centre assumes a tetragonally distorted pseudo-octahedral geometry with the O atoms of two trifluoromethanesulfonate anions coordinated weakly in the axial positions. The Cu—N distances lie in the range 1.941 (3)–2.011 (3) Å and the Cu—O distances are 2.474 (3) and 2.564 (3) Å.


Comment
Recently, the coordination chemistry of di-Schiff bases derived from 2-pyridyl ketones or aldehydes has generated a great deal of interest (Hamblin et al., 2002;Gourbatsis et al., 1998;Szklarzewicz & Samotus, 2002;Mentes et al., 2007). These studies have been mostly stimulated by an interest in modelling the enzyme, copper-zinc superoxide dismutase (SOD) (Luo et al., 1993) and also for the synthesis of metal containing polymers with interesting optical, magnetic and electrical properties (Hanack et al., 1988;Marks, 1990). It has also been found that such tetradentate Schiff base ligands may form complexes with different nuclearity according to the coordination preferences of the metal centre (Fielden et al., 2006).
Our interest in the ligand, L, (Scheme 2) was stimulated by the analogy between its donor set and that of the pyridylmethylketazine (L 1 ) and 2-pyridinealdazine (L 2 ) system which form triple-stranded helical complexes with the formula [M 2 (L) 3 ] 4+ (M = Co, Fe and Ni). The helical complexes were shown to undergo exchange reactions on standing to form mono-nuclear complexes [M(L) 2 ] 2+ in which the ligand twists to coordinate as tridentate with non-coordinated imine residue (Hamblin et al., 2002). Mononuclear species are favoured by coordination to octahedral metal centres whose equatorial sites are occupied by N4 donor set of the bis(axial) ligand, and their axial sites being occuppied by solvent molecules or counterions. In addition, dinuclear metal complexes are favoured by the four-coordinate tetrahedral metal centres whose ca 90° twist angle provides good geometric match for the bis(equatorial) ligand (Fielden et al., 2006).
In the free ligand the pyridylimine units adopt a transoid configuration to minimize unfavourable electronic interactions between the lone pairs of pyridine nitrogen and imine nitrogen atoms. However, in the presence of a metal ion, the pyridine rings rotate by 180° with respect to the Aryl-C bond, positioning the two nitrogen atoms of each pyridylimine moiety on the same side of the ligand. Otherwise the geometric parameters in the free ligand are very similar to those of the coordinated moiety.
The pyridylimine units are not ideally planar due to a combined effect of the ring to the metal centre and a twist induced by the ethylene bridge [Cu1, N1, N2, C1>C6 and Cu1, N3, N4, C10, C12>C16 have devaitions from the mean plane of 0.088 (6) Å and 0.106 (6) Å for N2 and N3 respectively]. From puckering analysis (Cremer & Pople, 1975) the ring formed by the metal centre, the imine N atoms and the ethylene bridge has a Q value of 0.283 (3) Å and forms a twisted envelope conformation about the C8-C9 bond. This effect has the result of pushing the methyl groups out of the ring unit plane, with C7 deviating by 0.149 (5) Å and C11 by 0.167 (6) Å and accordingly the pyridylimine units are not coplanar, with the angle formed between these planes being 12.98 (9)°.
The crystal structure does not exhibit any classical hydrogen bonds and is primarily comprised of stacked undulating sheets formed by close packing and C-H···O interactions between the SO 3 group and pyridylimine ring H atoms. and refined using the riding model with U iso (H) set to 1.2 or 1.5U eq (carrier) for CH or CH 2 and CH 3 respectively. When including H atoms, methyl groups were allowed to rotate to enable matching with electron density maxima. Fig. 1. Molecular structure of the title compound (50% probability displacement ellipsoids).