Synthesis, properties and Mössbauer spectra of bisaxially co-ordinated iron(n) phthalocyanine low-spin complexes: The first semi-quantitative explanation of the influence of the character of axial ligands on the spectral parameters

A series of bisaxially co-ordinated low-spin iron(n) phthalocyanine complexes (pc)FeL₂ with different types of nitrogen bases as axial ligands has been prepared and characterised by electronic, ¹H NMR and Mössbauer spectroscopies. The influence of electronic and steric effects of the axial ligands on Mössbauer quadrupole splitting (ΔE_Q) and isomer shift (δ), NMR parameters, and metal-to-ligand charge transfer (MLCT) band position are discussed. Mössbauer partial quadrupole splitting (p.q.s.) and partial isomer shift (p.i.s.) parameters for different ligands have been estimated and rationalised with the overall data for (pc)FeL₂ and (nx)₂FeL₂ (nx = nioxime) complexes. A semi-empirical AM 1 method and the cone angle concept were used to factorise δ- and π-electronic and steric effects of the ligands. A good correlation between the predicted nd experimental MLCT band position of (pc)FeL¹L² complexes, as well as predicted and experimental p.i.s. and p.q.s. values for different ligands, has been observed.