ESEEM and mossbauer studies of the ferriheme model compound bis(3- aminopyrazole)tetraphenylporphyrinatoiron(III) chloride, [TPPFe(NH₂PzH)₂]Cl

A model heme complex, bis(3-aminopyrazole)tetraphenylporphinatoiron(III) chloride, [TPPFe (NH₂PzH)₂]Cl, for which the EPR g-values lead to a rhombicity V/Δ = 1.2 if g(zz) is the largest g-value, have been investigated by electron spin echo envelope modulation (ESEEM) and Mossbauer spectroscopies. The ESEEM studies focus on the proton sum frequency peaks at near twice the proton Larmor frequency. Analysis of the distant proton peak (mainly due to the pyrrole-H) at exactly twice the proton Larmor frequency shows conclusively that g(zz) is aligned along the normal to the porphyrin plane, and thus the electron configuration is (d(xy))²(d(xz),d(yz))³, with g(zz)>g(yy)>g(xx). This system is thus another violation to Taylor's 'proper axis system' rule. The near proton (the α-H and N-H of the axial ligands) peaks provide distance information for those protons from the metal. Magnetic Mossbauer studies of the same complex confirm the (d(xy))²(d(xz),d(yz))³ ground state and indicate that, as is the case for cytochrome P450(cam), A(xx) is the largest magnitude A-value, and is negative in sign. Other low- spin iron(III) porphyrinates also have A(xx) of negative sign, but usually the magnitude is only about half that of A(zz), which is always positive in sign.