Synthesis, Structure, Magnetism, and Spectroscopic Properties of Heterobinuclear Copper(II)−Zinc(II) Complexes and Their Copper(II)−Copper(II) Analogues in Asymmetric Ligand Environments

Heterobinuclear copper(II)−zinc(II) complexes and their homobinuclear dicopper(II) counterparts (1−4) of two
asymmetric ligands (H2L1 and H2L2), based on 2-aminocyclopent-1-ene-1-dithiocarboxylate, are reported. The ligands
are capable of providing both donor set and coordination number asymmetry in tandem. Metal centers in these
complexes are connected by a μ-alkoxo and a bridging pyrazolate moiety, as confirmed by X-ray structure analyses
of 1, 3, and 4. The Cu(1) site in the dicopper complex (1) is square planar and so are the copper sites in the
Cu−Zn complexes 3 and 4. The pentacoordinated Zn sites in the latter complexes have distorted TBP geometry
(τ = 0.74), while the corresponding Cu site in 1 has a highly distorted square pyramidal structure (τ = 0.54). The
Cu···Zn separations in 3 and 4 are 3.3782 and 3.3403 Å, respectively, while the Cu···Cu distance in 1 is 3.3687
Å. The dicopper complexes are EPR silent at 77 K, in which the copper(II) centers are coupled by strong
antiferromagnetic coupling (J = ca. −290 cm-1) as confirmed by variable-temperature (4−300 K) magnetic
measurements. These compounds (1 and 2) undergo two one-electron reductions and a single step two-electron
oxidation at ca. −0.26, −1.40, and 1.0 V vs Ag/AgCl reference, respectively, as indicated by cyclic and differential
pulse voltammetry done at subambient temperatures. EPR spectra of 3 and 4 display axial anisotropy at 77 K with
the g⊥ region being split into multiple lines due to N-superhyperfine coupling (AN = 15.3 × 10-4 cm-1). The
observed trend in the spin-Hamiltonian parameters, g∥ > g⊥ > 2.04 and |A⊥| ≪ |A∥| ≈ (120−150) × 10-4 cm-1,
indicates a dx2-y2-based ground state with tetragonal site symmetry for the Cu(II) center in these molecules.