Partially encapsulated copper(I) complexes of mono(o-aryl)-substituted derivatives of 2,2′-bipyridine and 1,10-phenanthroline

Abstract

Copper(I) complexes of the potentially terdentate ligands 6-(2-dimethylaminophenyl)-2,2′-bipyridine (L¹) and 2-(2-dimethylaminophenyl)-1,10-phenanthroline (L²), [CuL₂][PF₆](L = L¹1 or L²2) have been prepared. Both complexes have highly distorted tetrahedral structures in which only the α,α′-diimine fragments are co-ordinated; the C₆H₄NMe₂ fragments are pendant, and are twisted with respect to the co-ordinated bipyridine and phenanthroline fragments such that the pendant aromatic ring of one ligand lies stacked with the co-ordinated fragment of the other ligand. This results in ‘partial encapsulation’ of the copper(I) centres which confers chemical stability (no autoxidation in the presence of O₂) and electrochemical stability (wide ‘redox-existence ranges’ for the Cu^I oxidation states) on the complexes. The inter-ligand stacking also results in distortion away from pseudotetrahedral geometry at the metal centres towards square planar (θ, the angle of intersection of the two CuN₂ planes, is 75° for 1 and 76° for 2); ¹H NMR and electronic spectra show that this is maintained in solution. Neither complex is luminescent at room temperature, possibly due to intramolecular quenching via transient co-ordination of the pendant amine groups. Oxidation of 1 and 2 to the copper(II) complexes 1⁺ and 2⁺ with ferrocenium affords stable solutions whose EPR spectra indicate that the metals now have geometries that are much more flattened towards square-planar geometry; this would not be possible with more highly encapsulated complexes of bipyridines or phenanthrolines with two pendant aryl substituents in the ortho positions.