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

Abstract

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 in 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) Å] and Cu–N_amine [1.973(6) Å] distances. Both the nitrate ions are co-ordinated in a plane almost perpendicular (90.15°) to the CuN₃ plane with slightly different Cu–O distances [Cu–O(1), 2.153(4); Cu–O(4), 2.148(4) Å] and a O(1)CuO(4) angle of 78.4°. The value of the trigonal index τ of 0.33 [(α₈ − α₁)/60, where α₈ = N2–Cu–N1 and α₁ = O1–Cu–N3] suggests that the structure is best described as trigonal bipyramidal distorted square based pyramidal (TBDSBP). Two further remote 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) Å] so that the complex may be considered alternatively to possess a near seven-co-ordinate CuN₃O₂O′₂ chromophore. According to the Structural Pathway of the vibronic coupling model the five-co-ordinate structure is then best described as an extreme see-saw structure which is best understood in terms of a distortion of the regular five-co-ordinate trigonal bipyramidal stereochemistry involving a −A + B route distortion. With an O(1)CuO(4) angle of 78.4° this structure is the most extreme example known of the uncommon see-saw stereochemistry of the copper(II) ion.