ESR, Zero-Field Splitting, and Magnetic Exchange of Exchange-Coupled Copper(II)−Copper(II) Pairs in Copper(II) Tetraphenylporphyrin <i>N</i>-Oxide^#

The crystal structures of the dimer form of copper(II) tetraphenylporphyrin N-oxide, [Cu(tpp-N-O)]2 (3-dimer), and zinc(II) tetraphenylporphyrin N-oxide, [Zn(tpp-N-O)]2 (4-dimer), were established. The geometry at the copper ion in 3-dimer is essentially square-pyramidal with one oxygen bridge [O(1A)] occupying the apical site, giving a much larger Cu−O bond distance compared to those at the basal plane. The respective Cu···Cu distance and Cu−O−Cu angle in the core of 3-dimer are 3.987(4) Å and 148.1(3)°. The Zn(1) atom in 4-dimer has a distorted square-pyramidal [4 + 1] coordination geometry that gives a τ-value of 0.19. The respective Zn···Zn distance and Zn−O−Zn angle in the dimeric unit of 4-dimer are 4.025(3) Å and 148.1(2)°. The 3-dimer displays axial X-band electron paramagnetic resonance spectral features (Es = 0) in the powder state at 4 K, giving g∥ = 2.51 (A∥,s = (9.6 ± 0.2) × 10-3 cm-1) and g⊥ = 2.11 and in the same powder state at 293 K giving Ds = 0.0731 cm-1 (as derived from ΔMs = 1 lines) or 0.0743 cm-1 (as derived from the ΔMs = 2 lines). In addition, 3-dimer displays a ΔMs = 2 transition at g = 4.17 indicating the presence of spin-exchange coupling. The anisotropic exchange interaction ( = 0.132 cm-1) gives the main contribution to Ds in 3-dimer. The theoretical fit of the susceptibility and effective magnetic moment data of 3-dimer in the temperature range of 5−300 K gives 2J = 68 cm-1, g = 2.01, p = 0.06, and a temperature-independent paramagnetism of 10-6 cm3 mol-1. This magnetic susceptibility data indicates that the copper(II) ions in 3-dimer are coupled in a ferromagnetic manner with the ground-spin triplet stabilized by 68 cm-1 with regard to the singlet.