19F Isotropic shifts in paramagnetic iron(III) octaethyltetraphenylporphyrinate and tetraphenylporphyrinate complexes of a variety of electronic ground states: implications for the electron configuration of chloroiron tri-(pentafluorophenyl)corrolate

Abstract

The 19F NMR spectra of three complexes each of two different pentafluorophenyl-substituted iron(III) porphyrinates have been investigated and compared with the respective protonated phenyl-substituted iron porphyrinates. The two series of iron porphyrinates were those of tetraphenylporphyrin (TPP) and octaethyltetraphenylporphyrin (OETPP), and the complexes investigated were the chloroiron (S=5/2 for TPP, S=spin admixed 3/2, 5/2 for OETPP), the bis-4-dimethylaminopyridine (DMAP) complex (S=1/2, dπ unpaired electron) and the bis-t-butylisocyanide (TBIC) complex (S=1/2, dxy unpaired electron). The protonated phenyl compounds have all been well characterized previously. The fluorinated phenyl compounds all show large o-F isotropic shifts (positive for chloroiron and bis-TBIC complexes, negative for bis-DMAP complexes), while the m-F and p-F isotropic shifts are much smaller in magnitude, but the same sign as the o-F isotropic shifts of each complex, even when there is significant spin density at the meso-carbons, as in the cases of the bis-TBIC complexes. Thus, unlike 1H isotropic shifts, which are readily separable into dipolar and contact contributions, the 19F isotropic shifts of the ortho-fluorines have an overriding, large contribution from either a ligand-centered dipolar shift or a through-space contact shift that results from direct electron cloud overlap of the ortho-fluorines with the π-system of the porphyrin ring, but in either case, the large isotropic shift of the ortho-fluorines appears to be related to the spin density at the meso-carbon to which the fluorinated phenyl group is attached. Based upon these findings, and the fact that the 19F isotropic shifts of the tri(pentafluorophenyl)corrolatoiron chloride are negative and only somewhat larger than those of the S=5/2 F20TPPFeCl complex, it is not possible to determine the electron configuration of the corrolate complex from the 19F NMR data, since DFT calculations have shown that there is negative spin density at the meso-carbons of iron corrolates in either the S1=3/2 Fe(III)(corrolate2−), S2=1/2 antiferromagnetically coupled case or the S=1 Fe(IV)(corrolate3−) case. [J. Am. Chem. Soc. 124 (2002) 6636].