NMR Study of the Interaction of Trinitrobenzene with High-Spin Tetraarylporphinato-Iron(II1) Halides

Abstract

The interaction of several high-spin p-substituted tetraphenylporphinato-iron(III) halide complexes (p-R-TPPFeX) with the aromatic π acceptor, 1,3,5-trinitrobenzene (TNB) has been studied by proton and 13C NMR. The averaged TNB shifts and line-widths are indicative of 1:1 complexes whose stability is decreased by more electron-withdrawing axial halides or phenyl substituents, consistent with the porphyrin acting as a π donor in the adduct. Quantitative evaluation of both proton and 13C relaxation rate ratios in the 1:1 adduct show that a unique structure can be determined solely from relaxation data. The resulting structure is physically reasonable in that it places the TNB ∼ 3 Å from the porphyrin plane on the opposite side of the chloride ligand and approximately over the pyrrole nitrogen, with the NO2 group pointing away from the iron. The absence of any axial interaction in the adduct indicates its structure represents a π complex. The structure, however, does not allow one to differentiate between electronic and steric effects in decreasing the affinity of the iron for a sixth axial ligand.