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

Four copper(II) complexes with the ligands bearing two or three alkylpyridi ne pendant arms attached to an ethylene diamine framework were isolated in pure form (four-co ordinate species as perchlorates, and five-coordinate sp ecies as hexafluorophosphates). Three complexes and one tosylated ligand we re characterized by X-ray diffraction. In the absence of additional mono- o r bidentate ligands, linear tetradentate aminopyridines form distorted squa re-planar complexes with copper(II). This coordination mode is different fr om cis-configurations adopted by aminopyridine ligands in octahedral comple xes. The degree of the tetrahedral distortion, caused by steric repulsion b etween pyridine rings, increases with an increase in the chelate ring sizes (555 vs. 656 sequence). Nearly planar arrangement of the two amine nitroge ns and two pyridine nitrogens is retained in the five-coordinate copper(II) complex with a pentadentate ligand, in which the fifth pyridine donor occu pies an axial position. EPR parameters of the four-coordinated aminopyridin e complexes are very similar to those of the tetraamine species, and do not depend significantly on the degree of the tetrahedral distortion. Introduc ing of a fifth nitrogen donor in the long ethylpyridine pendant arm causes, some weakening of the equatorial ligand field, as reflected in EPR paramet ers (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, t he chelate ring size, and with the alkylation of the amine nitrogen donors. A relatively weak coordination of the fifth pyridine nitrogen increases th e redox potential of the Cu(II)/Cu(I) couple by 85-156 mV. (C) 2001 Elsevie r Science B.V. All rights reserved.