Synthesis and Properties of Acetylene-bridged N-Confused Porphyrin Dimers

Abstract

Acetylene-bridged N-confused porphyrin (NCP) dimers were synthesized by Stille coupling reactions. Moderate electronic interaction between the NCP chromophores was observed in the inner-3H tautomers while strong interaction was observed in the inner-2H tautomers.Cross-coupling reactions are one of the most powerful methods to construct large ³-conjugated molecules.1 Among such molecules, porphyrin dimers as well as oligomers are a particularly important class of compounds, which often show fascinating electronic properties and functions, such as twophoton absorption, light-harvesting, and electron transport. One intrinsic characteristic of NCP is NH-tautomerization, which allows change of the electronic state significantly (Figure 1).5 Thus, the inner-3H tautomer (NCP3H) has an [18]annulenic substructure and is strongly aromatic, while the inner-2H tautomer (NCP2H) loses the [18]annulenic substructure and is moderately aromatic.6 Accordingly, NCP3H is thermodynamically more stable than NCP2H and much difference is found in their photophysical properties.7 Previously, we have demonstrated that intramolecular energy transfer could be controlled by NH-tautomerization in the porphyrinNCP dyad system. 8 Additionally, substitution effect at the 3-position would be much different between NCP3H and NCP2H.9 Along the lines of these findings, we can expect interesting electronic structures in 3,3¤-acetylene-bridged NCP dimers. In this paper, the synthesis and properties of 3,3¤-acetylene-bridged NCP dimers by the Stille coupling are reported.First, the reactions of 3-bromo-NCPs with tributyl(phenylethynyl)tin were studied to prepare 3-alkynyl-NCPs (Scheme 1). The alkynylated NCPs were previously prepared by the Sonogashira coupling 9 or inversion of alkynylated N-fused porphyrins. 10 The reaction of N-confused 3-bromotetraphenylporphyrinatosilver(III) complex 1 proceeded smoothly with Pd(OAc) 2 /AsPh 3 catalyst to give Ag III (NCP3H)CCPh 2 in 63% yield. Transmetalated product, Pd II (NCP2H)CCPh 3, was also obtained in 19% yield. This unique reductive transmetalation from Ag(III) to Pd(II) inside the NCP core was also observed in the Sonogashira coupling reaction. 9 The reaction of N-confused 3-bromotetrakis[4-(trifluoromethyl)phenyl]porphyrinatocopper(II) complex 4 proceeded in a similar manner to give Cu II (NCP2H)CCPh 5 in 48% yield. Transmetalated product 6 was also isolated in 10% yield. The AsPh 3 ligand so far afforded the better yields than the phosphine ligands such as PPh 3 and diphenylphosphinoethane.Then, syntheses of acetylene-bridged NCP dimers were examined by the reactions with bis(tributylstannyl)acetylene (Scheme 2). The Pd-catalyzed reaction of 1 successfully gave Ag III (NCP3H)CCAg III (NCP3H) 7 in 20% yield together with Ag III (NCP3H)CCPd II (NCP2H) 8 in 7% yield. In the 1 H NMR spectrum of 7 in CDCl 3 , the signals due to ¢-pyrrolic protons (12H) were observed in the region of ¤ 8.58.9, suggesting its strong aromatic character. Additionally, the number of proton signals indicated the symmetric stru...

Oligoporphyrins with One-and Two-Atom Covalent Bridges: Synthesis and Reactivity, Solid-State, Solution and Electronic Structures, Spectroscopy and Applications

“…The authors labeled 82 as the "OFF" dimer, having minimal inter-macrocycle conjugation, whereas the 83-type was classified as "ON." 103 Similar trends are shown in the spectra of other classes of conjugated oligoporphyrinoids, and these features will be discussed in detail in Section IV.A.1. Imine-linked diporphyrins are represented by just one report, by Anderson and coworkers in 2002, who described the preparation of 84 in 32% yield by condensation of the aldehyde 85 with primary amine 9e.…”

“…102 The chemistry of C 2 porphyrinoid dimers was extended in 2011 by Furuta and coworkers to core-modified porphyrins of the N-confused type (NCP). 103 These systems can exist as two tautomers, one with strong retention of porphyrinoid aromaticity (NCP3H) and the other with the pyrrolic NH retained on the outside of the molecule, which is cross-conjugated (NCP2H). Precursors of both types were brominated, then subjected to the double Stille coupling with bis(tributylstannyl)ethyne, to yield the C 2 dimers [Ag(III)] 2 82 and [Cu(II)] 2 83a, the former of the 3H class and the latter, the 2H, which were isolated in 20% and 35% yields, respectively.…”

N‐Confused Phlorin‐Prodigiosin Chimera: meso‐Aryl Oxidation and π‐Extension Triggered by Peripheral Coordination

“…The trivalent 3H form has an [18]annulenic substructure with stronger aromaticity, while the divalent 2H form loses full macrocyclic conjugation with reduced aromaticity (Figure c). In fact, the distinct π‐conjugated structure of the later 2H‐form can be advantageous for linear π‐extension through the peripheral α‐pyrrole position of the cross‐conjugated pyrrole moiety, leading to the NIR optical properties (Figure c) . In this regard, the elongation of π‐conjugated oligopyrroles based on the N‐confused phlorin skeleton will be promising because of the interrupted inherent conjugation of the macrocycle.…”

“…In fact, the distinct pconjugated structure of the later 2H-form can be advantageous for linear p-extension through the peripheral a-pyrrole position of the cross-conjugated pyrrole moiety,leading to the NIR optical properties (Figure 1c). [8] In this regard, the elongation of p-conjugated oligopyrroles based on the Nconfused phlorin [9] skeleton will be promising because of the interrupted inherent conjugation of the macrocycle.U pon hybridization with at ypical p-conjugated dye scaffold, dipyrromethene (dipyrrin), effective p-extension could be achieved along the periphery of the a-modified phlorin (Figure 1c). Theresulting N-confused phlorin-dipyrrin hybrid (1;f or structure see Scheme 1) may provide two metal coordination sites at the inner NNNC core and the peripheral prodigiosin-like N3 donor unit, respectively.T he metal coordination could rigidify the structure for p-extension, thereby showing unique photophysical properties and chemical reactivity as exemplified by (sub)porphyrinoid-pincer- type metal complexes (Figure 1b).…”

Regioselective phosphorylation and thiophosphorylation of N-confused porphyrin: a route to hybrid carbaporphyrinoids

“…[16][17][18]22,23 Peripheral C(3) substitutions have also been reported for X-confused porphyrinoids and their silver(III) complexes. 5,7,[24][25][26][27][28][29][30] Recently we have shown that a palladium(II) p-benziporphyrin provides a unique environment that alters the fundamental reactivity of the benzene unit. A remarkable, palladium(II) induced contraction of p-phenylene to cyclopentadiene has been detailed that affords the first complex of a 21-carbaporphyrin.…”