NMR Investigation of β-Substituted High-Spin and Low-Spin Iron(III) Tetraphenylporphyrins

Abstract

The NMR spectra of a series of beta-substituted iron(III) tetraphenylporphyrin (2-X-TPP) complexes have been studied to elucidate the relationship between the electron donating/withdrawing properties of the 2-substituent and the (1)H NMR spectral pattern. The electronic nature of the substituent has been significantly varied and covered the -0.6 to 0.8 Hammett constant range. Both high-spin and low-spin complexes of the general formula (2-X-TPP)Fe(III)Cl and [(2-X-TPP)Fe(III)(CN)(2)](-) have been investigated. The (1)H NMR data for the following substituents (X) have been reported: py(+), NO(2), CN, CH(3), BzO (C(6)H(5)COO), H, D, Br, Cl, CH(3), NH(2), NH(3)(+), NHCH(3), OH, and O(-). The (1)H NMR resonances for low-spin dicyano complexes have been completely assigned by a combination of two-dimensional COSY and NOESY experiments. In the case of selected high-spin complexes, the 3-H resonance has been identified by the selective deuteration of all but the 3-H position. The pattern of unambiguously assigned seven pyrrole resonances reflects the asymmetry imposed by 2-substitution and has been used as an unique (1)H NMR spectroscopic probe to map the spin density distribution. The pyrrole isotropic shifts of [(2-X-TPP)Fe(III)(CN)(2)](-) are dominated by the contact term. In order to quantify the substituent effect, the dependence of isotropic shift of all low-spin pyrrole resonances and 3-H high-spin pyrrole resonance versus Hammett constants has been studied. The electronic effect is strongly localized at the beta-substituted pyrrole. The major change of the isotropic shift has also been noted for only one of two adjacent pyrrole rings, i.e., at 7-H and 8-H positions. These neighboring protons, located on a single pyrrole ring, experienced opposite shift changes when electron withdrawing/donating properties were modified. Two other pyrrole rings for all investigated derivatives revealed considerably smaller, substituent related, isotropic shift changes. A long-range secondary isotopic shift has been observed for [(2-D-TPP)Fe(III)(CN)(2)](-). The effect is consistent with a general spin density distribution mechanism due to beta-substitution. A fairly good correlation between the 3-H isotropic shift of (2-X-TPP)Fe(III)Cl and the Hammett constant has been found as well. The observed contact shift pattern of [(2-X-TPP)Fe(III)(CN)(2)](-) reflects spin pi delocalization into the highest filled MO equivalent to the unsubstituted porphyrin 3e(pi) orbital. To account for the substituent contribution, the semiquantitative Fenske-Hall LCAO method has been used to determine the molecular orbitals involved in the spin density delocalization. For low-spin complexes, (13)C pyrrole resonances of carbons bearing a proton have been identified by means of a (1)H-(13)C HMQC experiment. The reversed order of (13)C resonance patterns as compared to their (1)H NMR counterparts has been determined, e.g., the largest isotropic shift of 3-H has been accompanied by the smallest measured (13)C isotropic shift. Analysis of the isotropic shif...

NMR Studies of Paramagnetic Metallocarbaporphyrinoids

“…[14] Actually, the variation of the β-H positions in the 1 H NMR spectrum may be accounted for by specific π-delocalization mechanisms that have been discussed in detail for iron(III) porphyrins. [96,97] One pyrrole 3-H and the 2-NH resonance are evidently different from the other six and show upfield shifts. Thus, the side-on locawww.eurjic.orgtion of the iron(II) with respect to the pyrrole ring primarily affects spin transfer to the modified pyrrole ring.…”

“…meric form, [111] and in the oxidation of [(HCPH)Ni II ] with OsO 4 . [112] Selected spectra that emphasize the utility of 1 [7] iron(III) β-substituted tetraarylporphyrin, [96] and iron(III) N-methyltetraarylporphyrin (δ av for regular pyrrole rings), [13] and the average chemical shift of [(HCPH)Fe…”

Proton NMR Study of Low‐Spin meso‐Unsubstituted β‐Substituted Alkyl Iron Porphyrins: Remarkable Influence of Peripheral Substitution on Spin Density

“…The relative intensities of the resonances and the COSY spectrum allowed the assignment of all the protons except for the two meso protons, α and γ, and the β-pyrrole protons. It was previously reported that HMQC spectroscopy of paramagnetic iron porphyrins provides the scalar heterocorrelation under natural abundance conditions of 13 C. [11] In the present study, this correlation technique offers a route to identify the β-pyrrole resonances by taking advantage of the fact that the full assignment of the proton signals of the β-13 C-1 H pairs has been established. Thus, the signal assignment of the carbons with directly bonded protons such as β-pyrrole protons and meso protons ( 13 C-1 H) was done on the basis of HMQC experiments.…”

Monomeric and Dimeric Iron(III) Complexes of 5‐Hydroxy‐10,15,20‐triphenylporphyrin: Formation of Cyano and Pyridine Complexes of (5‐Oxo‐10,15,20‐triphenylphlorin)iron

“…The 1 H NMR spectra reflect features typical for low-spin (porphyrin)iron(III) complexes. [31,56] ester, and protoporphyrin-IX dimethyl ester, respectively). [22,26,46] Based on the chemical shifts measured for the four (deuterooxophlorin)iron isomers [(DeuteroPO)Fe(py) 2 ], the following values are anticipated for (5-oxophlorin)iron or its β-methylated derivative: pyrrole resonances: δ ϭ Ϫ33 (2-H), 27 (3-H), 13 (12-and 13-H) ppm; methyl resonances: δ ϭ 65 (2-CH 3 ), Ϫ14 (3-CH 3 ) ppm.…”

“…Additionally, the π delocalization locates a considerable amount of the spin density at the meso positions, [16,31,53Ϫ55] and some at the pyrrole positions. [56] The π-spin density at the meso-carbon atoms produces sign alternation of the meso-phenyl resonances. The dipolar contribution, caused only by the ZFS effect in high-spin (porphyrin)iron complexes, is usually notably smaller than the contact contribution, and this observation has been applied in the further analysis of the hyperfine shifts.…”