Copper(II) complexes of symmetric and asymmetric bis(imine) ligands: Tuning the Cu(I)/Cu(II) redox couple

Two new pyridylbis(imine) ligands 2-((2-methyl-2-(pyridin-2-yl)-3-(pyridin-2-ylmethyleneamino)propylimino)methyl)phenol, HL2, and 2-methyl-2-(pyridin-2-yl)-N1,N3-bis(pyridin-2-ylmethylene)propane-1,3-diamine, L3, were synthesized via the Schiff base condensation of 2-methyl-2-pyridin-2-yl-propane-1,3-diamine and either 2-pyridinecarboxaldehyde or 2-hydroxybenzaldehyde. Complexation of Cu(II) ions by HL2 and L3 yielded [Cu(HL2)]2(ClO4)4 (2), and [Cu(L3)](ClO4)2 (3), respectively. The structures and electrochemistry of 2 and 3 were compared to our previously synthesized Cu(II) complex of the ligand 2,2’-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)diphenol (H2L1), which coordinates to Cu(II) as a dianion to form [Cu(L1)(CH3OH)] (1). Whereas (L1)2− and L3 form mononuclear complexes with Cu(II) ions, the asymmetric ligand HL2 produces copper dimers in the solid state with the phenolate O atoms bridging between copper ions. Solution magnetic moment measurements and ESI–MS suggest that all three species exist as monomers in solution, although small amounts of dimeric 2 were detected in solution by ESI–MS and EPR spectroscopy. Complexes of all three ligands show similar EPR properties with typical axial spectra. Cyclic voltammetry reveals that the Cu(I)/Cu(II) redox couples of 2 and 3 are shifted to more positive potentials than that of 1, with 3 having the most positive one-electron reduction potential: E1/2(1)=−1.489V; E1/2(2)=−1.099V; E1/2(3)=−0.438V, all versus Fc/Fc+.