The structural pathways of (dipicolylamine)dinitratocopper(II): an exampleof the uncommon see-saw stereochemistry

The dark blue complex [Cu(dipica)(NO3)(2)] (dipica = dipicolylamine, bis(2- pyridylmethyl)amine) has been isolated and characterized by single crystal X-ray crystallography. The five-co-ordinate CuN3O2 chromophore is located i n a general position and involves a planar tridentate co-ordination of the dipica nitrogen atoms with short Cu-N-py [1.965(4), 1.970(5) Angstrom] and Cu-N-amine [1.973(6) Angstrom] distances. Both the nitrate ions are co-ordi nated in a plane almost perpendicular (90.15 degrees) to the CuN3 plane wit h slightly different Cu-O distances [Cu-O(1), 2.153(4); Cu-O(4), 2.148(4) A ngstrom] and a O(1)CuO(4) angle of 78.4 degrees. The value of the trigonal index tau of 0.33 [(alpha(8) - alpha(1))/60, where alpha(8) = N2-Cu-N1 and alpha(1) = O1-Cu-N3] suggests that the structure is best described as trigo nal bipyramidal distorted square based pyramidal (TBDSBP). Two further remo te oxygen atoms of the nitrato ligands are semi-co-ordinated in the CuO(1)O (4) plane with much longer Cu-O' distances [Cu-O2, 2.698(4); Cu-O6, 2.870(4 ) Angstrom] so that the complex may be considered alternatively to possess a near seven-co-ordinate CuN3O2O2' chromophore. According to the Structural Pathway of the vibronic coupling model the five-co-ordinate structure is t hen best described as an extreme see-saw structure which is best understood in terms of a distortion of the regular five-co-ordinate trigonal bipyrami dal stereochemistry involving a -A + B route distortion. With an O(1)CuO(4) angle of 78.4 degrees this structure is the most extreme example known of the uncommon see-saw stereochemistry of the copper(II) ion.