Vibronic Behavior and Single-Crystal EPR Spectra of Cu(II) in Copper-Doped Diaqua(<scp>l</scp>-aspartato)zinc(II) Hydrate

Abstract

We investigate the electronic properties of copper ions substituting zinc ions in diaqua(l-aspartato)Zn(II) hydrate (ZnC4H5O4N·3H2O), to be called Zn(l-asp). In this compound, the Zn ions are in an octahedral coordination with a small distortion in the equatorial plane and the aspartate ion acts as a tridentate ligand. EPR spectra of single crystals of Zn(l-asp) doped with 63Cu(II) were obtained at 9.8 GHz between 77 and 300 K, with the magnetic field applied in the crystallographic planes ab, bc, and ca. The g-factor and 63Cu hyperfine coupling along the crystal axes display large temperature variations. From the single-crystal data we obtained the principal values of these tensors: g1 = 2.0377(5) [2.028(1)], g2 = 2.1701(5) [2.144(2)], g3 = 2.3127(4) [2.351(2)], ACu1 = 61.5(4) × 10-4 cm-1 [58.0(3) × 10-4 cm-1], ACu2 = 20.5(1) × 10-4 cm-1 [3.6(3) × 10-4 cm-1], ACu3 = 87.8(2) × 10-4 cm-1 [114.5(4) × 10-4 cm-1], at 300 K [77 K]. At 77 K, the hyperfine coupling with the 14N ligand is resolved and the principal values of the AN tensor are AN1 = 11(2) × 10-4 cm-1, AN2 = 11(2) × 10-4 cm-1, AN3 = 8(2) × 10-4 cm-1. EPR data in powder samples at X-band and Q-band were compatible with those values. From the Q-band powder spectrum at 5 K we get g1 = 2.03(1), g2 = 2.12(3), g3 = 2.36(3), ACu1 = 55(1) × 10-4 cm-1, |ACu2| = 4(2) × 10-4 cm-1, ACu3 = 117(3) × 10-4 cm-1, which show the low symmetry of the copper site and are assumed to be the “molecular” parameters. The EPR results are discussed in terms of the structure of Zn(l-asp) and the electronic properties of the copper ions. We analyze the temperature dependence of the g and copper hyperfine values considering a dynamic Jahn-Teller effect in terms of a potential surface warped as a consequence of tetragonal and orthorhombic lattice strains. Above 100 K, the behavior is explained in terms of the Boltzmann equilibrium between two isomer configurations distorted along the Cu−water oxygen bond (Silver−Getz model). The energy splitting between the wells are δ23 = 235(5) cm-1 and δ13 ≅ 440(50) cm-1. Below 100 K, the data indicate the restricted validity of the SG model and suggest that the complex is localized in the deepest well.