Hydrogen Bonding 1-D Chain Network ofcis-Doubly N-Confused Porphyrins

Creation from Confusion and Fusion in the Porphyrin World─The Last Three Decades of N-Confused Porphyrinoid Chemistry

Confusion is a novel concept of isomerism in porphyrin chemistry, delivering a steady stream of new chemistry since the discovery of N-confused porphyrin, a porphyrin mutant, in 1994. These days, the number of confused porphyrinoids is increasing, and confusion and associated fusion are found in various fields such as supramolecular chemistry, materials chemistry, biological chemistry, and catalysts. In this review, the birth and growth of confused porphyrinoids in the last three decades are described.

Organometallic Group 11 (CuIII , AgIII , AuIII ) Complexes of a trans-Doubly N-Confused Porphyrin: An "Expanded Imidazole" Structural Motif

Complexation of group 11 metal cations with an α-bis(phenylthio)-substituted trans-doubly N-confused porphyrin (trans-N₂ CP_SPh : 4) afforded a series of square-planar trivalent organometallic complexes (i.e., Cu-H4, Ag-H4, and Au-H4). The X-ray crystal structures of the complexes revealed highly planar core geometries along with the presence of peripheral amine and imine nitrogen sites of the pyrrolic moieties. NMR, UV/Vis absorption, and magnetic circular dichroism (MCD) spectroscopies suggested the 18 π-electron aromaticity of the complexes. The aromaticity was also fully analyzed by various theoretical methodologies such as nucleus-independent chemical shift (NICS) and anisotropic induced current density (ACID) calculations. The central metal affects the amphiprotic character of the complexes possessing both pyrrolic amino nitrogen and imino nitrogen atoms at the periphery, which was examined by the photometric titration with trifluoroacetic acid (TFA) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), respectively. The inherent acidity of the complexes was followed in the order; Cu-H4>Au-H4>Ag-H4 and that of basicity was Au-H4>Ag-H4>Cu-H4. The complexes could be considered as an "expanded imidazole" structural motif.

Spectroelectrochemical behavior of N-confused dioxohexaphyrins

Doubly N -confused dioxohexaphyrins were characterized by cyclic voltammetry and spectroelectrochemistry in the -2.5 to +2.0 V vs Ag | Ag + (0.10 M, in CH3CN) range. Six reversible monoelectronic processes were found suggesting that the rich electrochemistry of porphyrin systems was preserved in that expanded macrocycle. However, the electronic and electrochemical properties of the free-base, Cu(II) and Ni(II) species were shown to be sensitive to the coordinated transition metal ions and solvent revealing a quite complex behavior, probably reflecting the higher flexibility and strong conformational effects on the energy levels and stability of those complexes.

N-confused porphyrins as new scaffolds for supramolecular architecture

The peripheral nitrogen of N -confused porphyrins ( NCP ) can serve as a hydrogen bonding donor and acceptor, and a metal coordination site as well. NCP forms versatile dimers with the assistance of metal ions such as Pd II, Pt II, Zn II, Cd II, Hg II, Mn II and Fe II. The square-planar divalent metal complexes of C 6 F 5-substituted NCP with Cu II, Ni II and Pd II act as efficient anion binding receptors. The doubly N -confused porphyrins ( N 2 CP ), possessing both N and NH at the periphery, form self-assembled 1-D hydrogen bonding networks, whose orientations differ in cis (zigzag) and trans (straight) isomers.

Halide-Anion Binding by Singly and Doubly N-Confused Porphyrins

The halide-binding properties of N-confused porphyrin (NCP, 1) and doubly N-confused porphyrins (trans-N2CP (2), cis-N2CP (3)) were examined in CH2Cl2. In the free-base forms, cis-N2CP (3) showed the highest affinity to each anion (Cl-, Br-, I-) with association constants Ka = 7.8x10(3), 1.9x10(3), and 5.8x10(2) M(-1), respectively. As metal complexes, on the other hand, trans-N2CP 2-Cu exhibited the highest affinity to Cl-, Br-, and I- with Ka = 9.0x10(4), 2.7x10(4), and 1.9x10(3) M(-1), respectively. The corresponding Ka values for cis-N2CP 3-Cu and NCP 1-Cu were about 1/10 and 1/2, respectively, of those of 2-Cu. With the help of density functional theory (DFT) calculations and complementary affinity measurements of a series of trisubstituted N-confused porphyrins, the efficient anion binding of NCPs was attributed to strong hydrogen bonding at the highly polarized NH moieties owing to the electron-deficient C6F5 groups at meso positions as well as the ideally oriented dipole moments and large molecular polarizability. The orientation and magnitude of the dipole moments in NCPs were suggested to be important factors in the differentiation of the affinity for anions.

Confusion approach to porphyrinoid chemistry

N-confused porphyrin (NCP) is a porphyrin isomer that is different largely from the parent porphyrin, particularly in the physical, chemical, structural, and coordination properties. Introduction of the confused pyrrole into the normal and expanded porphyrins leads to generation of the confused porphyrinoids having rich structural diversity. In this Account, we introduce a series of N-confused porphyrinoids recently synthesized and highlight their properties such as fusion, peripheral N coordination, supramolecular assemblies, anion binding, and singlet-oxygen sensitization.

Trans Doubly N-Confused Porphyrins: Cu(III) Complexation and Formation of Rodlike Hydrogen-Bonding Networks

A trans type of doubly N-confused isomer of NCP (trans-N2CP) was synthesized via N-confused fused porphyrin (NcFP). The aromatic feature of trans-N2CP due to 18pi electronic system is contrasted to the weak aromaticity of cis-derivative. The solid-state structure of trans-N2CP exhibits pi-stacking column, while the Cu(III) complex shows 1-D rodlike hydrogen bonding chain comparable with the zigzag hydrogen-bonding chain of cis-derivatives.