Structural, NMR, and EPR Studies of <i>S</i> = ¹/₂ and <i>S</i> = ³/₂ Fe(III) Bis(4-Cyanopyridine) Complexes of Dodecasubstituted Porphyrins

The NMR and EPR spectra for three complexes, iron(III) octamethyltetraphenylporphyrin bis(4-cyanopyridine) perchlorate, [FeOMTPP(4-CNPy)2]ClO4, and its octaethyl- and tetra-β,β‘-tetramethylenetetraphenylporphyrin analogues, [FeOETPP(4-CNPy)2]ClO4 and [FeTC6TPP(4-CNPy)2]ClO4, are presented. The crystal structures of two different forms of [FeOETPP(4-CNPy)2]ClO4 and one form of [FeOMTPP(4-CNPy)2]ClO4 are also reported. Attempts to crystallize [FeTC6TPP(4-CNPy)2]ClO4 were not successful. The crystal structure of [FeOMTPP(4-CNPy)2]ClO4 reveals a saddled porphyrin core, a small dihedral angle between the axial ligand planes, 64.3°, and an unusually large tilt angle (24.4°) of one of the axial 4-cyanopyridine ligands with respect to the normal to the porphyrin mean plane. There are 4 and 2 independent molecules in the asymmetric units of [FeOETPP(4-CNPy)2]ClO4 crystallized from CD2Cl2/dodecane (1−4) and CDCl3/cyclohexane (5−6), respectively. The geometries of the porphyrin cores in 1−6 vary from purely saddled to saddled with 15% ruffling admixture. In all structures, the Fe−Np distances (1.958−1.976 Å) are very short due to strong nonplanar distortion of the porphyrin cores, while the Fe−Nax distances are relatively long (∼2.2 Å) compared to the same distances in S = 1/2 bis(pyridine)iron(III) porphyrin complexes. An axial EPR signal is observed (g⊥ = 2.49, g∥ = 1.6) in frozen solutions of both [FeOMTPP(4-CNPy)2]ClO4 and [FeTC6TPP(4-CNPy)2]ClO4 at 4.2 K, indicative of the low spin (LS, S = 1/2), (dyzdxz)4(dxy)1 electronic ground state for these two complexes. In agreement with a recent publication (Ikeue, T.; Ohgo, Y.; Ongayi, O.; Vicente, M. G. H.; Nakamura, M. Inorg. Chem. 2003, 42, 5560−5571), the EPR spectra of [FeOETPP(4-CNPy)2]ClO4 are typical of the S = 3/2 state, with g values of 5.21, 4.25, and 2.07. A small amount of LS species with g = 3.03 is also present. However, distinct from previous conclusions, large negative phenyl-H shift differences δm − δo and δm − δp in the 1H NMR spectra indicate significant negative spin density at the meso-carbons, and the larger than expected positive average CH2 shifts are also consistent with a significant population of the S = 2 Fe(II), S = 1/2 porphyrin π-cation radical state, with antiferromagnetic coupling between the metal and porphyrin unpaired electrons. This is the first example of this type of porphyrin-to-metal electron transfer to produce a partial or complete porphyrinate radical state, with antiferromagnetic coupling between metal and macrocycle unpaired electrons in an iron porphyrinate. The kinetics of ring inversion were studied for the [FeOETPP(4-CNPy)2]ClO4 complex using NOESY/EXSY techniques and for the [FeTC6TPP(4-CNPy)2]ClO4 complex using DNMR techniques. For the former, the free energy of activation, ΔG⧧, and rate of ring inversion in CD2Cl2 extrapolated to 298 K are 63(2) kJ mol-1 and 59 s-1, respectively, while for the latter the rate of ring inversion at 298 K is at least 4.4 × 107 s-1, which attests to the much greater flexibility of the TC6TPP ring. The NMR and EPR data are consistent with solution magnetic susceptibility measurements that show S = 3/2 in the temperature range from 320 to 180 K for [FeOETPP(4-CNPy)2]+, while both [FeOMTPP(4-CNPy)2]+ and [FeTC6TPP(4-CNPy)2]+ change their spin state from S = 3/2 at room temperature to mainly LS (S = 1/2) upon cooling to 180 K.