β-Nitro-substituted free-base, iron(III) and manganese(III) tetraarylporphyrins: synthesis, electrochemistry and effect of theNO₂substituent on spectra and redox potentials in non-aqueous media

Abstract

Two free-base and four metal derivatives of substituted tetraarylporphyrins containing a nitro-substituent on the β-pyrrole position of the macrocycle were synthesized and characterized by UV-vis, FTIR, 1 H NMR and mass spectrometry as well as electrochemistry and spectroelectrochemistry in non-aqueous media. The porphyrins are represented as ( NO 2 TmPP ) M and ( NO 2 TdmPP ) M , where M = 2 H , Fe III Cl or Mn III Cl , m is a CH 3 group on the para-position of the four meso-phenyl rings of the tetraphenylporphyrin (TPP) and dm represents two OCH 3 substituents on the meta-positions of each phenyl ring of the TPP macrocycle. UV-visible spectra of the nitro-substituted porphyrins exhibit absorption bands which are red-shifted by 4–11 nm as compared to bands of the same substituted tetraarylporphyrins lacking a nitro substituent. Three or four reductions are observed for each iron and manganese nitroporphyrin, the first of which is metal-centered, leading to formation of an Fe ( II ) or Mn ( II ) complex. Further reduction at the metal center occurs for the iron porphyrins but this reaction proceeds via an Fe ( II ) π anion radical in the case of the two nitro-substituented derivatives. The β-nitro-substituted porphyrins are easier to reduce and harder to oxidize than the corresponding compounds lacking a nitro group. The effect of NO 2 substituent on reduction/oxidation potentials and the site of electron transfer was also discussed.

Electrochemical and spectroelectrochemical properties of thiadiazole substituted metallo-phthalocyanines

“…5 shows CV and SWV responses of FePc in DMF/TBAP electrolyte. Although there are a few papers on the electrochemistry of Pc bearing Fe 3+ metal centers [22][23][24], Pc bearing Fe 2+ metal centers are rare in the literature due to the difficulty of the syntheses and instability of these complexes [25][26][27]. As shown in Fig.…”

Electrochemistry of Nitrated N‐Confused Free‐Base Tetraaryl‐Porphyrins in Nonaqueous Media

“…For example, the Soret band is red-shifted by 7-12 nm upon going from (Ar) 4 PH 2 to NO 2 (Ar) 4 PH 2 (Ar = p-CH 3 Ph or (m,m'-CH 3 O) 2 Ph) in CH 2 Cl 2 ( Table 1). [22] This resulti ndicates that the nitro substituent in the core of the N-confused porphyrin has al arger effect on the Chem.E ur.J.2015, 21,14579 -14588 www.chemeurj.org UV/Vis spectra than it does in a" regular" porphyrin under the same solution conditions when substitution is at the b-pyrrole positionoft he macrocycle.…”

“…It should be pointedo ut that the E 1/2 for first reduction of the b-nitro substituted porphyrinsi ss hifted more than E 1/2 for the first oxidations as compared to the same redox reactions of the non-nitro "regular" free-base porphyrins in CH 2 Cl 2 . [21,22] An example of this is shown in Figure 13 afor four sets of related compounds,w here the oxidationi sp ositively shifted by approximately 0.12 Va nd the reduction by 0.29-0.38 Va sc ompared to the non-nitro porphyrins in the same solvent. Thus, the potentialdifference betweenthe first oxidation and first reduction( the HOMO-LUMO gap) for the b-pyrrole substituted nitroporphyrinsi ss maller than that of non-nitro porphyrins (Figure 13 a).…”

“…We wished to find conditionsw hich might stabilizet he singly oxidizeda nd singly reduced forms of the N-confused porphyrins and we also wished to examine the electrochemistry of these compounds where an equilibrium was not observedb etween two tautomeric forms of the porphyrin.O ne such group of compounds are the N-confused porphyrins havinganitro group bound to the inner carbon atom of the central cavity. [18][19][20] It is known that additiono fo ne or more highly electron-withdrawing NO 2 groups to the meso-o rb-pyrrole position of a" regular" porphyrin will significantly modify its physichemical and electrochemical properties, [21][22][23][24] with the type and magnitude of the effect being dependent in large part on the positiono ft he added nitro substituent. However, to the besto fo ur knowledge, nothing was known about how the inner carbon bound nitro group would affect the reduction and oxidation properties or the electron-transfer mechanism for these types of porphyrins.…”

A Structural Model of Nitro‐Porphyrin Dyes Based on Spectroscopy and Density Functional Theory

“…Nitro groups drastically affect the electronic properties of the Por when attached to the β‐pyrrole position or even to the meso‐phenyl rings. The strong electron withdrawing nature of the NO 2 moiety polarizes the Por macrocycle, changing its physical and chemical properties . Conversely, nitro groups can also mediate the assembly of supramolecular systems by directly participating in hydrogen bonding with groups such as imidazole or modulating the complexation of the Por with other species in solution .…”