Synthesis, characterization, redox properties, and representative X-ray structures of four- and five-coordinate copper(II) complexes with polydentate aminopyridine ligands

Four copper(II) complexes with the ligands bearing two or three alkylpyridine pendant arms attached to an ethylene diamine framework were isolated in pure form (four-coordinate species as perchlorates, and five-coordinate species as hexafluorophosphates). Three complexes and one tosylated ligand were characterized by X-ray diffraction. In the absence of additional mono- or bidentate ligands, linear tetradentate aminopyridines form distorted square-planar complexes with copper(II). This coordination mode is different from cis-configurations adopted by aminopyridine ligands in octahedral complexes. The degree of the tetrahedral distortion, caused by steric repulsion between pyridine rings, increases with an increase in the chelate ring sizes (555 vs. 656 sequence). Nearly planar arrangement of the two amine nitrogens and two pyridine nitrogens is retained in the five-coordinate copper(II) complex with a pentadentate ligand, in which the fifth pyridine donor occupies an axial position. EPR parameters of the four-coordinated aminopyridine complexes are very similar to those of the tetraamine species, and do not depend significantly on the degree of the tetrahedral distortion. Introducing of a fifth nitrogen donor in the long ethylpyridine pendant arm causes some weakening of the equatorial ligand field, as reflected in EPR parameters (an increase in g∥ and a decrease in A∥). The Cu(II)/Cu(I) redox potentials of the four-coordinate complexes increase with an increase in the chelate ring size, and with the alkylation of the amine nitrogen donors. A relatively weak coordination of the fifth pyridine nitrogen increases the redox potential of the Cu(II)/Cu(I) couple by 85-156 mV.