Formation and characterization of five- and six-coordinate iron(III) corrolazine complexes

Electronic structures of five- and six-coordinate iron(III) corrolazine complexes are determined by means of ¹H NMR, ¹³C NMR, EPR, and Mössbauer spectroscopy as well as SQUID magnetometry. A series of five-coordinate complexes, [Fe^III(TBP₈Cz)(L)]* where the axial ligands(L) are cyanide(CN^-), imidazole(HIm), 1-methylimidazole(1-MeIm), 4-(N,N-dimethylamino)pyridine(DMAP), pyridine(Py), 4-cyanopyridine(4-CNPy), and tert-butylisocyanide(^tBuNC), are obtained by the addition of 1 to 2 equiv. of the ligands to the dichloromethane solutions of Fe^III(TBP₈Cz) at 298 K: TBP₈Cz is a trianion of 2,3,7,8,12,13,17,18-octakis(4-tert-butylphenyl)corrolazine. These complexes commonly show the S = 3/2 at 298 K. By contrast, formation of the six-coordinate complexes depends on the nature of the axial ligands. While the addition of 3 equiv. of CN^- has completely converted Fe ^III(TBP₈Cz) to (Bu₄N)₂[Fe ^III(TBP₈Cz)(CN)₂] at 298 K, the conversion to the bis-adduct is only attained below ca. 200 K in the case of HIm, 1-MeIm, and DMAP even in the presence of 50 equiv. of the ligands. If the axial ligand is Py, 4-CNPy, or ^tBuNC, the formation of [Fe^III(TBP ₈Cz)(L)₂] is confirmed only at an extremely low temperature (15 K). Close inspection of the ¹H NMR and EPR spectra has revealed that all the bis-adducts adopt the (d_xy) ²(d_xz, d_yz)³ ground state. While Fe^III(TBP₈Cz) forms paramagnetic bis- and mono-adduct in toluene solution at 298 K in the presence of excess amount of CN^- and ^tBuNC, respectively, the corresponding porphyrazine complex, [Fe^III(TBP₈Pz)]Cl, forms diamagnetic bis-CN and bis- ^tBuNC under the same conditions: TBP₈Pz is a dianion of 2,3,7,8,12,13,17,18-octakis(4-tert-butylphenyl)-porphyrazine. Thus, the iron(III) ion of porphyrazine complex is more easily reduced than that of the corresponding corrolazine complex.