Evaluation of diimine ligand exchange on Cu(I)

Four ligands have been prepared, 8,8-dimethyl-6,7,9-trihydropyrido [1,2-b]acridine and three 4,4',6,6'-tetrasubstituted derivatives of 2,2'-bipyrimidine where the substituents are methyl, phenyl, and p-tolyl. The corresponding [CuL2](+) salts of these ligands evidence nonequivalent NMR signals that allow an estimation of the ligand exchange barrier in both acetonitrile and chloroform solution. Lower barriers are found in the former solvent and attributed to solvent participation in the exchange process. Corresponding differences in the oxidation potentials of the complexes are explained in a similar manner. The electronic absorption properties of the complexes are also consistent with the steric and electronic properties of the ligands. [Cu(2c)(2)](PF6), where 2c = 4,4',6,6'-tetraphenyl-2, 2'-bipyrimidine, was analyzed by X-ray diffraction and found to crystallize in the space group Pccn with a = 14.761(2) Angstrom, b = 15.007(2) Angstrom, c = 24.407(4) Angstrom, and Z = 4. The internal and external phenyl rings are disposed quite differently, with the internal rings interacting strongly with the orthogonal ligand.