NMR study of the interaction of trinitrobenzene with high-spin tetraarylporphinato-iron(III) halides: Structure determination of the 1:1 adduct based on dipolar relaxation

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 NO 2 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.