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

Abstract

Four nitratedN -confused free-base tetraarylporphyrinsw ere synthesized and characterized by electrochemistry and spectroelectrochemistry in nonaqueous media. The examined compoundsa re represented as NO 2 (Ar) 4 NcpH 2 , where NO 2 (Ar) 4 Ncp is the dianion of at etraarylN -confused porphyrin with an inner carbonb ound NO 2 group and Ar is a p-CH 3 OPh, p-CH 3 Ph, Ph or p-ClPh substituent on each meso-position of the macrocycle. UV/Vis spectra and NMR spectroscopy data indicate that the same form of the porphyrin exists in CH 2 Cl 2 and DMF whichi su nlike the case of non-NO 2 N-confused porphyrins. The Soret band of NO 2 (Ar) 4 NcpH 2 exhibits a3 0-36 nm red-shift in CH 2 Cl 2 and DMF as compared to the spectrum of the non-NO 2 N-confused porphyrins. The first two reductions and first oxidation of NO 2 (Ar) 4 NcpH 2 are reversible in CH 2 Cl 2 containing 0.1 m TBAP.T he measured HOMO-LUMO gap averages 1.65 Vi n CH 2 Cl 2 and 1.53 Vi nD MF,w ith both values being similart o those of the non-NO 2 substituted compounds. The nitro group on the inverted pyrrole is itself not reducedw ithin the negative potential limit of CH 2 Cl 2 or DMF,b ut its presence significantly affects both the UV/Vis spectra and redox potentials.

Electrochemistry and spectroelectrochemistry of metallohexaphyrins containing bis-copper or bis-zinc central metal ions

“…In contrast to these values, smaller HOMO-LUMO gaps have been reported for the non-planar copper porphyrins, (OETPP)Cu and (DPP)Cu [50], the copper tetrabenzotetraarylporphyrin [51], the N-confused porphyrins [52][53][54] and the non-planar free-base porphyrins [55], where the separation ranges from 1.5-1.8 V under similar solution conditions.…”

Electrochemistry of Methylated N‐Confused Free‐Base Tetraarylporphyrins in Nonaqueous Media

“…N‐confused porphyrins are structurally similar to “normal” porphyrins, but have an inverted pyrrole with a carbon inside the cavity and a nitrogen on the periphery of the π‐system (see structures in Figure ). These compounds have attracted much attention because of their unique spectral and electrochemical properties as compared to “normal” porphyrins …”

“…It is well known that the addition of one or more electron‐withdrawing NO 2 groups to the meso‐ and/or β‐pyrrole positions of a porphyrin macrocycle will have a significant effect on the spectral and redox properties of the compounds . The addition of a single nitro group to the inner carbon of the inverted pyrrole in an N‐confused free‐base porphyrin will also have a large effect on the measured spectral, electrochemical and acid/base properties as compared to what is observed for either the ‘regular’ free‐base porphyrin with the same meso‐ and β‐pyrrole substituents or the related parent N‐confused porphyrin …”

“…We recently described the electrochemistry and protonation reactions of N‐confused porphyrins with and without an inner NO 2 group, (Ar) 4 NcpH 2 and NO 2 (Ar) 4 NcpH 2 , and have now turned our attention to the same type of N‐confused free‐base porphyrins with an exterior N‐methylated pyrrole. The structures of these newly investigated compounds are shown in Figure a and represented as N‐CH 3 (Ar) 4 NcpH 2 , where Ncp is the dianion of the N‐confused porphyrin and Ar is a CH 3 OPh, CH 3 Ph, Ph or ClPh substituent on the four meso‐positions of the macrocycle.…”

“…The structures of these newly investigated compounds are shown in Figure a and represented as N‐CH 3 (Ar) 4 NcpH 2 , where Ncp is the dianion of the N‐confused porphyrin and Ar is a CH 3 OPh, CH 3 Ph, Ph or ClPh substituent on the four meso‐positions of the macrocycle. The previously examined compounds are shown in Figure b and represented as (Ar) 4 NcpH 2 and NO 2 (Ar) 4 NcpH 2 . It has long been known that the outer nitrogen atom of the inverted pyrrole of an N‐confused porphyrin can be easily methylated to form stable N‐methylated compounds, but only two of these compounds have been characterized as to their redox potentials and no mechanistic information about the electrode reactions or spectroscopic characterization of the oxidation and reduction products were given.…”