Crystalline isomerism in (?-Oxo)bis[(phthalocyaninato)iron(III)]: Further characterization of the isomer having a linear or quasi-linear Fe-O-Fe bond system (?-Oxo(2))

The crystalline isomer of (?-oxo)bis[(phthalocyaninato)iron(III)], identified previously as ?-oxo(2), has been further investigated by means of magnetic susceptibility measurements and ESR and Mössbauer spectra. Susceptibility data taken on different samples of ?-oxo(2) in the temperature range 300-4 K are indicative of a high-spin (S = 5/2, S = 5/2) strongly antiferromagnetically coupled Fe(III) dimeric species. The observed coupling constant J (-195 cm-1), significantly higher than that found for the alternative isomer, i.e., ?-oxo(1) (characteristic IR absorptions at 852 and 824 cm-1, assigned as ?as(Fe-O-Fe)), is in keeping with the presence of a linear or quasi-linear Fe-O-Fe moiety, as indicated by structural information. The Mössbauer spectra at 4.2 K of samples obtained by different methods of preparation give fairly clean single doublets with ? values (isomer shift) of 0.26 (±0.01) mm s-1 and ?EQ. values (quadrupole splitting) of 1.27 (±0.01) mm s-1. These values are intermediate between those found for the five-coordinate high-spin ?-oxo(1) complex and the related six-coordinate low-spin adduct of formula [(py)PcFe]2O and support the high-spin electronic structure suggested by the magnetic susceptibility data. Discussion and comments are extended to pertinent, partly incorrect, literature reports.