Mössbauer, crystallographic, and density functional theoretical investigation of the electronic structure of bis-ligated low-spin iron(II) phthalocyanines

We present the crystal structure of pcFe(4-methylpiperidine)₂ as a first example of a low-spin iron(II) phthalocyanine (pc) complex bis-axially coordinated by aliphatic amines. It is shown that electronic rather than steric effects are responsible for the elongation of the Fe-N(axial) bond in pcFeL₂ complexes. Using density functional theory, the electronic structures as well as Mössbauer isomer shifts and quadrupole splittings have been investigated for a large number of pcFeL₂ and pcFeL¹L² complexes, in which the axial ligands have varying electronic and steric properties. The electron charge densities and electric field gradients at the iron ion were evaluated using a locally dense basis approach with Wachters' all-electron basis set for the iron ion, the 6-311++G(2d) basis set for atoms directly bonded to the iron ion, the 6-31G(d) basis set for atoms two bonds away from the iron ion, and the 3-21G* basis set for all other atoms. A good correlation between the theoretical and experimental isomer shifts and quadrupole splittings has been observed for all the complexes tested. It has also been shown that the proposed model for the calculation of Mössbauer spectral parameters is adequate for the evaluation of the axial ligand conformation in cases of conformational flexibility in pcFeL₂ complexes.