Structure of the copper(II) complex of a highly preorganised tetradentate ligand based on bispidine (3,7-diazabicyclo[3.3.1]nonane)

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Abstract
The structure of the complex of the reinforced ligand dpb {3,7-bis(2-pyridylmethyl)-3,7-diazabicyclo[3.3.1]nonane} with copper(II) perchlorate has been determined: space group Pbcn(no. 60), a= 8.904(2), b= 14.366(1), c= 17.402(2)Å, Z= 4, R= 0.0538. The Cu–N bond lengths to the saturated nitrogens are 2.002(4)Å, and those to the aromatic nitrogens of dpb are 1.976(4)Å. The co-ordination geometry around Cu is considerably distorted away from the usual square-planar arrangement found with four nitrogen donor atoms, towards tetrahedral geometry, with the trans N–Cu–N angles being 161.1(2)° instead of the expected 180°. Molecular mechanics calculations showed that the distortion arises because of van der Waals repulsion between that two co-ordinated pyridyl groups of the ligand, particularly the hydrogen atoms ortho to the nitrogen donors. These calculations are also used to support the idea that the much greater thermodynamic stability of the complexes of dpb with smaller metal ions than is found for complexes of analogous unreinforced ligands is due to the rigid bispidine-type bridge of dpb.
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