Stabilization of the Less Common (dxzdyz)4(dxy)1Iron(III) Porphyrin Ground Electronic State: 1H NMR Investigations of Iron(III) 5,10,15,20-Tetracyclohexylporphyrin

Abstract

High-spin and low-spin complexes of the iron(III) tetrakis(meso-cyclohexyl)porphyrin ((TCHP)FeIII) have been studied by means of 1-D and 2-D 1H NMR. The complete assignment of porphyrin and R-imidazole 1H resonances has been done on the basis of 2-D COSY and NOESY techniques as well as by selective deuteration of imidazole. The chemical shifts of pyrrole β-Hs have been used as the probe of the electronic state of an iron(III) metal ion. It has been found that cyanide coordinates to the high-spin (TCHP)FeIIICl complex, leading to the formation of the low-spin [(TCHP)FeIII(CN)2]-, with the rare (dxzdyz)4(dxy)1 ground electronic state (the pyrrole β-H resonance at 12.01 ppm at 293 K in CD3OD). A contribution of two electronic configurations, (dxy)2(dxzdyz)3 and (dxzdyz)4(dxy)1, to the ground state of the metal ion has been invoked for the low-spin [(TCHP)FeIII(R-Im)2]+ complexes. Characteristic 1H NMR shifts for these complexes include the pyrrole resonance at 2.81 ppm accompanied by the markedly upfield shi...