Synthesis, Structural, Spectral, and Electrochemical Studies of Selenabenziporphyrin and Its Pd(II) Complex

Abstract

A new nonaromatic selenabenziporphyrin was synthesized by (3 + 1) condensation of m-benzitripyrrane and 2,5-bis[( p-tolyl)hydroxymethyl] selenophene under mild trifluoroacetic acid-catalyzed reaction conditions. The selenabenziporphyrin was characterized by high-resolution mass spectrometry, one- and two-dimensional NMR spectroscopy, and X-ray crystallography. The crystal structure revealed that the macrocycle was planar with moderately tilted m-phenylene ring and that the phenylene ring completely blocks the macrocyclic π-delocalization. The selenabenziporphyrin exhibits one broad absorption band at 645 nm along with one sharp band at 415 nm, and electrochemical studies revealed that the macrocycle was electron-deficient. The selenabenziporphyrin readily forms organometallic Pd(II) complex when treated with PdCl in CHCN/CHCl at reflux followed by recrystallization. The X-ray structure revealed that the Pd(II) ion was coordinated with two pyrrole "N"s, selenophene "Se", and m-phenylene ring "C" in square-planar fashion, and the complex retained its nonaromatic nature. The Pd(II) complex exhibits ill-defined absorption bands, and it was more electron-deficient than free-base selenabenziporphyrin macrocycle. Time-dependent density functional theory studies supported the experimental observations.

Antiaromatic Carbaporphyrinoids: Fluorene as a Fused Motif toward the Synthesis of meso-Fused Heterobenziporphyrins

“…Comparison of 1 H NMR spectra of fused selena benziporphyrin 1b with that of our recently reported nonaromatic selena m -benziporphyrin I·Se (Figure S20 and Table S1) clearly supports the antiaromatic nature of macrocycle 1b . For example, the inner CH proton (type k) in selena m -benziporphyrin I·Se appeared at 7.20 ppm, whereas the same proton in macrocycle 1b was downfield shifted and appeared at 11.30 ppm.…”

“…Monobenziporphyrins result from replacement of one of the pyrrole rings of tetrapyrrolic porphyrin with a benzene ring. , There are two possible monobenziporphyrins depending on the linking of pyrrole rings with benzene rings via meso -carbon bridges in benziporphyrinoid systems: meta -benziporphyrins , I , in which the pyrrole rings are connected at 1,3-positions of the benzene ring, and para -benziporphyrins II , in which the pyrrole rings are connected at 1,4-positions of the benzene ring (Figure ). The extensive studies indicated that m -benziporphyrins I are nonaromatic, whereas p -benziporphyrins are aromatic. − The complete nonaromatic nature of m -benziporphyrins was attributed to retaining the strong 6π arene aromaticity of the benzene ring relative to the porphyrinoid aromaticity. − Over the years, extensive research has been carried out by Lash et al ,, and other research groups − and showed that by introducing suitable substituent(s) on the benzene ring of m -benziporphyrins, these macrocycles can be transformed into aromatic compounds . For example, the suitably placed hydroxyl group on the benzene ring of m -benziporphyrins such as compound III led to the aromaticity of m -benziporphyrins due to keto–enol tautomerism. ,,, Alternately, the nonaromaticity of m -benziporphyrins can be altered by replacing the m -phenylene ring with a fused aromatic groups such as naphthalene, − anthracene, pyrene, etc., (premodification approach) to result in meso -fused carbaporphyrins such as IV , found to be aromatic.…”

28-Hetero-2,7-Naphthiporphyrins: Horizontal Expansion of the m-Benziporphyrin Macrocycle

“…In this respect, carbaporphyrinoids incorporating phenylene rings, , namely, m -benziporphyrins 1-X , − oxy- m -benziporphyrins, , and p -benziporphyrins 2-X , , were found particularly alluring, giving an access to complexes with metal···arene π, and σ bonds (Figure ). Benziporphyrins demonstrated formation of coordination compounds with variety of metals, metalloids, and nonmetals. − Moreover, in cases of palladium­(II), rhodium­(III), and gold­(III) , complexes of 1-NH and 2-NH the metal···π interactions were envisaged to be stimuli of unusual reactivity resulting in the contraction of benzene unit to cyclopentadiene, leading eventually to complexes of 21-carbaporphyrins.…”

“…Replacement of the m -phenylene subunit of 1-X with the 2,7-naphthalenyl moiety in 13-X clearly influenced the potential coordination properties of the macrocycle. First, the modification distinctively increased dimensions of the coordination pocket, as illustrated by the growing distance between nitrogen atoms of opposite pyrroles from 4.72 Å ( 1-Se ) to 5.62 Å ( 13-Se ) and 5.74 Å ( 13-Te ). This clearly affected possible conformational dynamics, e.g., allowing for subunit rotations.…”

Effects of Core Modification on Electronic Properties of para-Benziporphyrins

“…The electronic properties of m - and p -benziporphyrins can be altered further by replacing one of the pyrrole rings that is across the phenylene subunit of benziporphyrins by other five-membered heterocyclic such as thiophene, furan, selenophene, and tellurophene, and the resulting core-modified benziporphyrins or hetero benziporphyrins with N 2 C 2 X (X = S, O, Se, and Te) cores are expected to exhibit different physicochemical and coordination properties compared to benziporphyrins with a N 3 C 2 core. ,,, A perusal of literature revealed that the all core-modified derivatives of aromatic p -benziporphyrins have not been synthesized, and no systematic investigation of the effect of core modification on electronic properties of p -benziporphyrins has been investigated. Herein, we report the synthesis of the missing hetero p -benziporphyrins such as Selena p -benziporphyrin 5 and tellura p -benziporphyrin 6 by adopting a 3 + 1 synthetic strategy (Chart ).…”