Methodology to determine the NMR chemical shifts of carbon atoms with radical character: A case of low-spin bis(tert-butylisocyanide) complex of (meso-tetrapropylporphyrinato)iron(III)

Paramagnetic metal complexes sometimes exhibit no appreciable ¹³C NMR signals due to the short relaxation time. By making use of the fast self-exchange electron transfer between paramagnetic [Fe ^III(TPrP)(^tBuNC)₂]ClO₄ and the corresponding diamagnetic [Fe^II(TPrP)(^tBuNC)₂], we have determined the chemical shifts of the carbon atoms that exhibit no signals due to the considerable amount of spin density. We have also determined the spin density of the meso carbon atom in [Fe^III(TPrP)( ^tBuNC)₂]ClO₄ to be 0.076, which is to our knowledge the largest among all the low-spin iron(III) porphyrin complexes reported previously. This method is applicable to all the low-spin iron(III) porphyrinoids if the corresponding diamagnetic complexes are available.