5,20-Di(pyridin-2-yl)-[28]hexaphyrin(1.1.1.1.1.1): A Stable Hückel Antiaromatic Hexaphyrin Stabilized by Intramolecular Hydrogen Bonding and Protonation-Induced Conformational Twist To Gain Möbius Aromaticity

Abstract

5,20-Di(pyridin-2-yl)-[28]hexaphyrin(1.1.1.1.1.1) 7 was prepared and characterized as a stable Hückel antiaromatic molecule with a dumbbell-like structure stabilized by effective intramolecular hydrogen bonding interactions involving the 2-pyridyl nitrogen atoms. Pd(II) metalation of 7 afforded two bis-Pd(II) complexes, 9-syn and 9-anti, whose structures are rigidly held by Pd(II) coordination, rendering 9-syn to be nonaromatic because of its highly distorted structure and 9-anti to be Hückel antiaromatic because of its enforced planar dumbbell structure. In contrast, protonation of 7 with methanesulfonic acid (MSA) led to the formation of its triprotonated species 7H(3), which has been shown to take on twisted conformations with Möbius aromaticity in CH(2)Cl(2), while the structure was held to be a planar rectangular conformation in the crystal. Excited-state dynamics were measured for 7, 7H(3), 9-syn, and 9-anti, which indicated their electronic nature to be antiaromatic, aromatic, nonaromatic, and antiaromatic, respectively.

Unraveling Excited-Singlet-State Aromaticity via Vibrational Analysis

“…Along with the different geometries between 1 and 2, they showed contrasting magnetic shielding effect and harmonic oscillator model of aromaticity (HOMA) values (Figures S4-S6 and Table S1), [45][46][47] which were well matched with the aromatic and anti-aromatic character of 1 and 2, respectively. 30 Similar geometrical differences was observed for other aromatic and anti-aromatic porphyrinoid congeners, [48][49][50][51][52][53] which illustrates the relationship between the aromaticity and molecular structure in that the inherent instability of the anti-aromatic nature generates distorted and asymmetric structures for energetic stabilization, in contrast with the stable planar conformations of aromatic molecules for effective p conjugation. [2][3][4][5][6]45 The opposite geometries were observed in the S Q state; the planar symmetric structure of 1 in the S 0 state became distorted and asymmetric in the S Q state, whereas the distorted structure of 2 became more planar and symmetric ( Figure 3).…”

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

“…It should be pointed out that up to four reductions have also been reported for other metallohexaphyrins containing different substituents and different central metal ions [11,21,23,25,47]. The potentials of these processes are summarized in Table S1, while structures of the previously examined metallohexaphyrins are shown in Fig.…”

“…Also shown in Chart 1 are structures of a recently investigated series of mono-copper tripyrrinones, represented as (Ar) 3 TPO 2 Cu, where TPO 2 is a dianion of the tripyrrinone and Ar is a 4-CH 3 OPh, 4-CH 3 Ph, Ph or 4-ClPh substituent [46] and the related mono-metallic porphyrins with the same central metal ions and meso-substituents as in the case of HexaPyM 2 . A number of metallohexaphyrins have been characterized in the solid state and in solution [10-14, 16, 20, 21, 25, 47], but not a great deal is known about their electrochemical and spectroelectrochemical properties [11,21,25,47]. This is addressed in the current manuscript where redox properties of a bis-copper and bis-zinc hexaphyrin are examined and the data for the bis-Cu(II) compound compared not only with that for a related mononuclear copper(II) porphyrin having the same mesosubstituents but also with a recently described monocopper(II) tripyrrinone [46] which has three pyrrole units and a structure that corresponds to approximately one half the structure of HexaPyCu 2 as shown in Chart 1.…”

5,20‐Bis(ethoxycarbonyl)‐Substituted Antiaromatic [28]Hexaphyrin and Its Bis‐NiII and Bis‐CuII Complexes

“…As the representative examples, free base [28]hexaphyrins can adopt various conformation including dumbbell, rectangular, Figure‐of‐eight, Möbius strip‐like, and triangular shapes . In recent years, we have reported that free base [28]hexaphyrins become stable antiaromatic molecules with dumbbell conformations when they carry meso ‐substituents such as 1‐alkyl‐2‐imidazolyl, pyridin‐2‐yl, and benzoyl, which act as a hydrogen bonding acceptor toward the inner‐directing pyrrolic protons. As an extension of these works, we report here the synthesis of 5,20‐bis(ethoxycarbonyl)‐10,15,25,30‐tetrakis(pentafluorophenyl) [28]hexaphyrin(1.1.1.1.1.1) 3 and its bis‐Ni II and bis‐Cu II complexes.…”

“…[1] Mosto fe xpanded porphyrins are conformationally flexible, whicha re different from planar porphyrins. As the representative examples, free base [28]hexaphyrins can adopt various conformation including dumbbell, [2][3][4] rectangular, [5]…”