Electron spin resonance study of dimers of copper(II) octaethylporphyrin, meso-nitro-octaethylporphyrin, and meso-dinitro-octaethylporphyrin in solution

Abstract

The dimer e.s.r. spectra of copper(II)meso-nitro-octaethylporphyrin (1), copper(II)α,β-meso-dinitro-octaethylporphyrin (2), copper(II)α,γ-meso-dinitro-octaethylporphyrin (3), and copper(II) octaethylporphyrin (4) observed in a frozen toluene solution at 77 K are described. The equilibrium constants (K) for dimer formation have been obtained by fitting the observed monomer intensity as a function of the total concentration to a simple monomer–dimer scheme. For (1) and (2) the K values are (in 10⁴ dm³ mol^–1) 27 and 18, respectively, and there is an indication of a higher-order association, while the data for (3) and (4) can be explained only by dimer formation with smaller K values (0.15 × 10⁴ and 0.68 × 10⁴ dm³ mol^–1, respectively). A structural analysis based on the point-dipole approximation shows that the Cu–Cu distance (4.16–4.19 Å) and the lateral shift angle (13–19°) are roughly the same in (1), (3), and (4). Complex (2) is unique in having a large angle (40°) and shorter interplane distance (3.2 Å). The effect of the electron-withdrawing nitro-group on dimerization is clearly demonstrated.