Electronic and Steric Factors Affecting the Ligands Redistribution Reaction of [Cu(2,2′-biquinoline)(PR3)2]+ Systems in Fluid Solutions

Complexes with the formula [Cu(bq)(PR3)2]+ {where bq = 2,2′-biquinoline and R = Ph (Cu-Ph-2), C6H4–4-OMe (Cu-OMe-2), C6H4–4-F (Cu-F-2), cyclohexyl (Cu-Cy-2)} were synthesized and characterized. The absorption and emission spectra of the complexes in dichloromethane indicated the presence of [Cu(bq)2]+, resulting from the ligands redistribution reactions. The solvent-dependence of [Cu(bq)2]+ formation from [Cu(bq)(PPh3)2]+ was investigated, highlighting a decrease in the formation constant in the order: DCM > ACE > DMSO > EtOH > MeCN. The presence of coordinating solvents lowers the formation constant; possibly through the formation of stable adducts with the formula [Cu(bq)2(PPh3)(solvent)]+. Moreover, the electronic and steric aspects of the phosphines were explored for the redistribution reaction of [Cu(bq)(PR3)2]NO3 in DCM. The steric environments of the phosphines in Cu-Ph-2, Cu-F-2 and Cu-OMe-2 are identical; the variable electronic properties caused increases in the homoleptic formation constant in the order: Cu-OMe-2 < Cu-Ph-2 < Cu-F-2. Moreover, comparison between the formation constants of [Cu(bq)2]+ from Cu-Ph-2 and Cu-Cy-2 indicated higher values for Cu-Cy-2. Despite the strong electron-donating nature of PCy3, the high steric bulkiness induces the high formation constant. Calculations suggest that Cu-Cy-2 possesses longer Cu-P bonds, rationalizing the fast dissociation rate of PCy3. Overall, the study highlights that the ligand redistribution equilibrium of [Cu(bq)(PR3)2]+ is dependant on the electronic and steric characteristics of the phosphine ligands.