A Twist on the Copper Center in the Crystal Structure of [Cu(dnpp)₂]PF₆ and the Charge-Transfer Excited State? (dnpp = 2,9-Dineopentyl-1,10-phenanthroline)

The X-ray structure of [Cu(dnpp)2]PF6, where dnpp denotes 2,9-dineopentyl-1,10-phenanthroline, reveals a flattened tetrahedral copper complex with a dihedral angle between the least-squares planes of the ligands of only 63.4(1)°. Steric interactions involving γ methyl groups of the substituents have an important role in shaping the complex, but lattice forces are ultimately responsible for the flattened geometry. Crystal data: [Cu(C22H28N2)2]PF6, triclinic, P1̄, a = 10.2755(10) Å, b = 13.9750(12) Å, c = 16.4354(12) Å, α = 79.376(7)°, β = 86.989(7)°, γ = 69.981(7)°, Z = 2. Spectral measurements involving four other Cu(NN)2+ systems, where NN denotes a 2,9-dialkyl-1,10-phenanthroline ligand, reveal that the room-temperature emission maxima fall at shorter wavelengths (20−50 nm) in the solid state as compared with fluid solution. The emission from Cu(dnpp)2+ is unique in that it maximizes at a slightly longer wavelength in the rigid solid (670 nm vs 665 nm in CH2Cl2). The spectral data support the following conclusions regarding structures in fluid solution: (1) The vibrationally relaxed excited state of Cu(dnpp)2+ adopts a structure similar to that observed in the solid. (2) However, the ground state assumes a less flattened, more tetrahedral geometry.