STRUCTURE OF THE COPPER(II) COMPLEX OF A HIGHLY PREORGANISED TETRADENTATE LIGAND BASED ON BISPIDINE (3,7-DIAZABICYCLO-[3.3.1]NONANE)

The structure of the complex of the reinforced ligand dpb is(2-pyridyl methyl)-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) Angstrom, Z = 4, R = 0.0538. The Cu-N bond lengths t o the saturated nitrogens are 2.002(4) Angstrom. and those to the arom atic nitrogens of dpb are 1.976(4) Angstrom. The co-ordination geometr y around Cu is considerably distorted away from the usual square-plana r arrangement found with four nitrogen donor atoms, towards tetrahedra l geometry, with the trans N-Cu-N angles being 161.1(2)degrees instead of the expected 180 degrees. Molecular mechanics calculations showed that the distortion arises because of van der Waals repulsion between the two co-ordinated pyridyl groups of the ligand, particularly the hy drogen atoms ortho to the nitrogen donors. These calculations are also used to support the idea that the much greater thermodynamic stabilit y of the complexes of dpb with smaller metal ions than is found for co mplexes of analogous unreinforced ligands is due to the rigid bispidin e-type bridge of dpb.