DIMERIC LOW-SPIN IRON(III) PHTHALOCYANINES - SYNTHESIS AND PROPERTIES OF FERROMAGNETICALLY COUPLED MU-OXODI(ACIDOPHTHALOCYANINATOFERRATES(III))

Mu-oxodi(phthalocyaninatoiron(III)) ([FePc2-)2O]) dissolved in pyridine reacts with different Tetra(n-butyl)ammonium salts yielding partly solvated Di(tetra(n-butyl)ammonium)-mu-oxodi(acidophthalocyaninatoferrates(III)) ((nBu4N)2[(Fe(X)Pc2-)2O]; X- = CN-, Im-, NCO-, NCS-, NO2-). The uv-vis. spectra show the typical B, Q, N and L regions of the Pc2- ligand scarcely influenced by the axial ligands X. In comparison with [(FePc2-)2O] mainly the B region is hypsochromically shifted due to strong excitonic coupling (> 3 kK). Two regions of weak absorbance at ca. 7.6-8.7 and 11.4-13.0 kK are assigned to trip-doublet transitions. The m.i.r. and resonance Raman spectra are dominated by the fundamental vibrations of the Pc2- ligand being characteristic for hexa-coordinated low-spin Fe(III) phthalocyanines. Internal vibrations of the ambident axial ligands X are in accordance with the proposed Fe-X bond. The i.r. active asym. (Fe-O-Fe) stretching vibration is observed in the region 631-690 cm-1. Fe-X stretching vibrations are only present in the f.i.r. spectra. The magnetic properties and Mossbauer spectra are interpreted in terms of an electronic model which assumes that a S' = 1 ground state arises from strong ferromagnetic coupling of the low-spin Fe(III) centres. Both spin-Hamiltonian and ligand-field models have been employed to fit the variable temperature susceptibility data. These low-spin mu-oxo Fe(III) dimers are rare compared to the many known examples of coupled high-spin species including the parent, [(FePc2-)2O].