Mossbauer, 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 coor dinated by aliphatic amines. It is shown that electronic rather than steric effects are responsible for the elongation of the Fe-N(axial) bond in pcFe L(2) complexes. Using density functional theory, the electronic structures as well as Mossbauer isomer shifts and quadrupole splittings have been inve stigated for a large number of pcFeL(2) and pcFeL(1)L(2) complexes, in whic h the axial ligands have varying electronic and steric properties. The elec tron charge densities and electric field gradients at the iron ion were eva luated using a locally dense basis approach with Wachters' all-electron bas is set for the iron ion, the 6-311++G(2d) basis set for atoms directly bond ed to the iron ion, the 6-31G(d) basis set for atoms two bonds away from th e iron ion, and the 3-21G* basis set for all other atoms. A good correlatio n between the theoretical and experimental isomer shifts and quadrupole spl ittings has been observed for all the complexes tested. It has also been sh own that the proposed model for Me calculation of Mossbauer spectral parame ters is adequate for the evaluation of the axial Ligand conformation in cas es of conformational flexibility in pcFeLa(2) complexes.