Structural, NMR, and EPR studies of S = (1)/(2) and S = (3)/(2) Fe(III) bis(4-cyanopyridine) complexes of dodecasubstituted porphyrins.

Abstract

The NMR and EPR spectra for three complexes, iron(III) octamethyltetraphenylporphyrin bis(4-cyanopyridine) perchlorate, [FeOMTPP(4-CNPy)(2)]ClO(4), and its octaethyl- and tetra-beta,beta'-tetramethylenetetraphenylporphyrin analogues, [FeOETPP(4-CNPy)(2)]ClO(4) and [FeTC(6)TPP(4-CNPy)(2)]ClO(4), are presented. The crystal structures of two different forms of [FeOETPP(4-CNPy)(2)]ClO(4) and one form of [FeOMTPP(4-CNPy)(2)]ClO(4) are also reported. Attempts to crystallize [FeTC(6)TPP(4-CNPy)(2)]ClO(4) were not successful. The crystal structure of [FeOMTPP(4-CNPy)(2)]ClO(4) reveals a saddled porphyrin core, a small dihedral angle between the axial ligand planes, 64.3 degrees, and an unusually large tilt angle (24.4 degrees ) 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)]ClO(4) crystallized from CD(2)Cl(2)/dodecane (1-4) and CDCl(3)/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-N(p) distances (1.958-1.976 A) are very short due to strong nonplanar distortion of the porphyrin cores, while the Fe-N(ax) distances are relatively long ( approximately 2.2 A) compared to the same distances in S = (1)/(2) bis(pyridine)iron(III) porphyrin complexes. An axial EPR signal is observed (g( perpendicular ) = 2.49, g( parallel ) = 1.6) in frozen solutions of both [FeOMTPP(4-CNPy)(2)]ClO(4) and [FeTC(6)TPP(4-CNPy)(2)]ClO(4) at 4.2 K, indicative of the low spin (LS, S = (1)/(2)), (d(yz)d(xz))(4)(d(xy))(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)]ClO(4) 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 delta(m) - delta(o) and delta(m) - delta(p) in the (1)H NMR spectra indicate significant negative spin density at the meso-carbons, and the larger than expected positive average CH(2) shifts are also consistent with a significant population of the S = 2 Fe(II), S = (1)/(2) porphyrin pi-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)]ClO(4) complex using NOESY/EXSY techniques and for the [FeTC(6)TPP(4-CNPy)(2)]ClO(4) complex using DNMR techniques. For the former, the free energy of activation, deltaG, and rate of ring inversion in CD(2)Cl(2) 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 x 10(7) s(-)(1), which attests to the much greater flexibility of the TC(6)TPP 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 [FeTC(6)TPP(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.