Exploring the Optical and Electrochemical Properties of Homoleptic versus Heteroleptic Diimine Copper(I) Complexes

Due to ligand scrambling, the synthesis and investigation of the properties of heteroleptic Cu(I) complexes can be a challenging task. In this work, we have studied the optical and electrochemical properties of a series of homoleptic complexes, such as [Cu(dbda)₂]⁺, [Cu(dmp)₂]⁺, [Cu(Br-dmp)₂]⁺, [Cu(bcp)₂]⁺, [Cu(dsbtmp)₂]⁺, [Cu(biq)₂]⁺, and [Cu(dap)₂]⁺ in solution, and those of their heteroleptics [Cu(dbda)(dmp)]⁺, [Cu(dbda)(Br-dmp)]⁺, [Cu(dbda)(bcp)]⁺, [Cu(dbda)(dsbtmp)]⁺, [Cu(dbda)(biq)]⁺, [Cu(dbda)(dap)]⁺ adsorbed on the surface of anatase TiO₂ (dbda = 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid; dmp = 2,9-dimethyl-1,10-phenanthroline; Br-dmp = 5-bromo-2,9-dimethyl-1,10-phenanthroline; bcp = bathocuproine or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline; dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline; biq = 2,2'-biquinoline; dap = 2,9-dianisyl-1,10-phenanthroline). We show that the maximum absorption wavelengths of the heteroleptic complexes on TiO₂ can be reasonably predicted from those of the homoleptic complexes in solution through a simple linear relation, whereas the prediction of their redox properties is less trivial. In the latter case, two different linear patterns emerge: one including the ligands bcp, biq, and dap and another one including the ligands dmp, Br-dmp, and dsbtmp. We offer an interpretation of the data based on the chemical structure of the ligands. On one hand, ligands bcp, biq, and dap possess a more extended π-conjugated system, which gives a more prominent contribution to the overall redox properties of the ligand dbda. On the other hand, the ligands dmp, Br-dmp, and dsbtmp are all phenanthroline-based containing alkyl substituents and contribute less than dbda to the overall redox properties.