Mössbauer Spectra of Iron Complexes with Macrocyclic Ligands. Partial Center Shifts and Partial Quadrupole Splittings

Mössbauer spectra have been measured and interpreted in detail for 37 iron complexes with synthetic macrocyclic tetradentate ligands. Most of the complexes are six-coordinate derivatives of the composition Fe^II-III-(MAC)X_2ⁿ⁺. The macrocyclic ligands (MAC) vary in ring size and in the extent and arrangement of unsaturated linkages; the axial ligands X may be CH₃CN, imidazole, NCS^-, CN^-, NO₂ ^-, Cl^-, Br^-, I^-, or CH₃CO₂^-. Two oxo bridged dimers (MAC)FeOFe(MAC)ⁿ⁺ were also studied. The center shift, δ, and quadrupole splitting, ΔE_q, for the ligating groups involved in the six-coordinate low-spin iron(II) complexes have been separated into additive quantities, the partial center shift (pcs) and the partial quadrupole splitting (pqs). These quantities correlate with spectrochemical parameters determined for other ions (Co^III and Ni^II) and with the limited data available for iron(II) itself; from these correlations, spectrochemical parameters or ligand field strengths that are not directly measurable can be estimated for a number of ligands that form important complexes with iron. The pcs and pqs values indicate the importance of π bonding in Fe^II complexes with the α-diimine linkage and support the view that ΔE_q is largely a function of Dq_xy – Dq_z. The relationship of Mössbauer data on iron hemes to that of the relatively simple model system of this report is discussed.