Structure-dependent spectral behavior of five-coordinate ternary copper(II) complexes containing 2,2′-dipicolylamine. X-ray crystal structures of [Cu(dpa)(pic)]ClO4, and [Cu(dpa)(pic)]PF6·H2O (dpa=2,2′dipicolylamine, pic=picolinate)

The X-ray crystal structures and the electronic absorption spectra of the two complexes in the title, [Cu(dpa)(pic)]ClO4 (1) and [Cu(dpa)(pic)]PF6·H2O (2) are described. Complex 1 crystallizes in the triclinic space group P, with a=8.592(2), b=17.18(3), c=8.028(2) Å, α=102.48(5), β=110.24(2), γ=89.43(5)°, and Z=2. Refinement gave final R and Rw values of 0.072 and 0.108, respectively, for 3147 observed reflections. Complex 2 crystallizes in the orthorhombic space group Pbca, with a=21.949(3), b=14.97(2), c=12.964(6) Å, and z=8. Refinement gave final R and Rw values of 0.050 and 0.070, respectively, for 3220 observed reflections. In both complexes, the central copper atom is essentially five-coordinated with a weakly coordinated perchlorate anion or water molecule at the sixth axial position. The terdentate ligand dpa (2,2′-dipicolylamine) coordinates meridionally in the equatorial plane and the bidentate pic (picolinate anion) forms an axial chelate with an apical coordination through its carboxylate group. Intermolecular stackings are observed between the pyridine rings of adjacent complex molecules. The electronic absorption spectra of complexes 1 and 2 varied to a large extent in organic solvents with different coordinating abilities, that is, a solvatochromism was observed for these complexes. Whereas, for a similar five-coordinate complex [Cu(dpa)(l-phe)]ClO4, which has a facially coordinated dpa with an apical coordination of its pyridyl group, such a chromism was not observed. This spectral behavior will be discussed in relation to their complex geometries.