Bispidine copper(II) compounds: Effects of the rigid ligand backbone

Approximative density-functional theory calculations indicate that the tetr adentate ligand L (L = 2,4-bis-(2-pyridyl)-3,7-diaza-[3.3.1]-bicyclononane) enforces an unusual and strong binding of a co-ligand (substrate) to a cop per(II) center. The co-ligand in [Cu(L)(CI)II completes a square-pyramidal coordination around copper(II) and binds in the equatorial plane rather tha n on the apical position. This configuration is a stable geometric isomer f or the model complex [Cu(NH3)(2)(imine)(2)(Cl)](+), but it is disfavored by approximately 10 kJ mol(-1) and not commonly observed for CuN4 chromophore s with a monodentate co-ligand. The equatorial coordination increases the b ond energy of the copper(II) -chloride bond by approximately 80 kJ mol-1, a nd similar results are expected for other copper(II)-L-substrate complexes, some of which show strong catalytic activity or unusual stability. Despite the enforced configuration, L does not impose significant steric strain on the copper(II) center but is well preorganized for the Jahn-Teller labile ion in this unusual geometry. The preorganization extends to the orientatio n of the pyridine donors (torsion angle around the copper-pyridine bond), a nd this seems to be of importance in the reactivity of the copper-L complex es and their derivatives.