Structures of the Copper(I) and Copper(II) Complexes of 2,9-Diphenyl-1,10-phenanthroline:  Implications for Excited-State Structural Distortion

The syntheses, crystal structures, and electronic absorption spectra of the copper(I) and copper(II) complexes of 2,9-diphenyl-1,10-phenanthroline (dpp) are reported. The complex [Cu(dpp)2](PF6) (1) crystallizes in space group P21/c with a = 11.081(4) Å, b = 25.491(8) Å, c = 14.263(5) Å, β = 92.84(3)°, Z = 4, and V = 4024(2) Å3. For 4813 unique data with F > 4.0σ(F), R = 5.41% and Rw = 6.43%. The coordination geometry about the copper(I) center in [Cu(dpp)2]+ is best described as distorted tetrahedral with approximate C2 symmetry. The structure of [Cu(dpp)2]+ is largely determined by interligand π-stacking interactions that occur between the phenyl groups of one ligand and the phenanthroline moiety of the other ligand. Solution-state absorption and 1H NMR spectra indicate that the [Cu(dpp)2]+ complex is fluxional in solution, rocking between two enantiomeric structures of C2 molecular symmetry through an intermediate of Cs symmetry. The complex [Cu(dpp)2](ClO4)2 (2) crystallizes in space group P1̄ with a = 7.809(3) Å, b = 13.027(6) Å, c = 20.344(10) Å, α = 87.68(4)°, β = 89.16(4)°, γ = 79.26(4)°, Z = 2, and V = 2032(1) Å3. For 4943 unique data with F > 4.0σ(F), R = 5.22% and Rw = 5.37%. The coordination geometry about the copper(II) center in [Cu(dpp)2]2+ is best described as flattened tetrahedral with approximate D2 symmetry. There are no interligand π-stacking interactions in the structure of [Cu(dpp)2]2+. The four-coordinate geometry in [Cu(dpp)2]2+ persists in solution on the basis of solution-state and solid-state absorption spectroscopy. Structural distortion in the metal-to-ligand charge-transfer excited state of [Cu(dpp)2]+ is discussed on the basis of the structures of 1 and 2.