Regioselective Stepwise Bromination of [14]Triphyrins(2.1.1) and Their Effects on Structural, Spectral, and Redox Properties

Synthesis of triphyrin(2.1.1)-triphyrin(2.1.1) homo dimers and bis-Re(I) triphyrin dimer complex

First examples of covalently linked triphyrin(2.1.1)-triphyrin(2.1.1) homo dimers connected via flexible alkoxy linkers at β-pyrrole positions were synthesized in 28-30% yields by reacting 2-bromo triphyrin(2.1.1) with three different alkane diols such as 1,2-ethanediol, 1,4-butanediol and 1,6-hexanediol in toluene under Pd2(dba)3/DPEphos catalyzed conditions by refluxing for 12-20 h. One of the free base triphyrin-triphyrin homo dimers was metallated by treating the homo dimer with Re(CO)5Cl in toluene under reflux and afforded bis-Re(I) homo dimer complex in 53% yield. Three free base homo dimers and one bis-Re(I) homo dimer complex were thoroughly characterized and studied using HR-MS, 1D and 2D NMR, absorption spectroscopy, cyclic voltammetry and DFT calculations. Our attempts to obtain the X-ray structure of homo dimers were not successful. However, DFT studies revealed that homo dimers prefer to exist in anti-conformation rather than syn-conformation and both triphyrin macrocycles in homo dimers were similar in terms of the deviation of pyrrole rings from the mean plane defined by the four meso carbons of the respective triphyrin unit. Absorption spectra of homo dimers showed similar absorption features such as in triphyrin(2.1.1) with slight changes in their absorption peak maxima. Electrochemical studies revealed that the homo dimers were electron rich, whereas the bis-Re(I) complex was electron deficient in nature.

Synthesis of unsymmetrical 2,3,7,8-tetrabromo meso-5,10,11,16-tetraaryl- triphyrin(2.1.1) and its use in the synthesis of sterically crowded 2,3,5,7,8,10,11,16-octaarylated triphyrin(2.1.1)s

Triphyrin(2.1.1) is a 14π aromatic contracted congener of an 18π aromatic porphyrin(1.1.1.1). An unsymmetrical 2,3,7,8-tetrabromo meso-tetraaryl triphyrin(2.1.1) containing four bromides at the β-pyrrole carbons of two out of three pyrrole rings of the triphyrin core was synthesized for the first time in 90% yield by treating meso-tetraaryl triphyrin(2.1.1) with five equivalents of N-bromosuccinimide in 1,2-dichloroethane (DCE) under reflux for 8 h. The X-ray structure revealed that the triphyrin(2.1.1) macrocycle was significantly distorted in 2,3,7,8-tetrabromo meso-tetraaryl triphyrin compared to planar meso-tetraaryl triphyrin. A series of novel sterically crowded 2,3,5,7,8,10,11,16-octaaryl triphyrin(2.1.1)s were synthesized by coupling 2,3,7,8-tetrabromo meso-tetraaryl triphyrin with six different aryl boronic acids under Suzuki-Miyaura coupling conditions. NMR, absorption, electrochemical and theoretical studies revealed that the structure and electronic properties were drastically altered in the 2,3,5,7,8,10,11,16-octaaryl triphyrin(2.1.1) series due to the presence of four additional aryl groups at the β-pyrrole carbons which caused steric crowding at the periphery of the triphyrin core resulting in a decrease in effective π-conjugation in the triphyrin(2.1.1)s.

Pd(II), Ni(II), and Cu(II) complexes of α,α'-ditolylmethanone dipyrroethene

Dipyrromethenes containing two pyrrole rings connected by one meso-carbon are versatile monoanionic bidentate ligands and form coordination complexes with many metals/nonmetals/metalloids. Dipyrroethenes containing one additional meso-carbon compared to dipyrromethenes have more space between coordinating pyrrole nitrogens and provide a good coordination environment but have not been explored as ligands in coordination chemistry. Dipyrroethenes are dianionic bidentate ligands and by suitable modifications, the coordination environment of dipyrroethenes can be changed further. Herein, we successfully synthesized α,α'-ditolylmethanone dipyrroethene which is a bipyrrolic tetradentate ligand with an ONNO ligand core and used it for the synthesis of novel Pd(II), Ni(II), and Cu(II) metal complexes by treating it with respective metal salts in CH₂Cl₂/CH₃OH at room temperature. The X-ray crystallographic structure of the metal complexes showed that the M(II) ion was coordinated to the ONNO atoms of the ligand in a perfect square planar geometry. The NMR studies of Pd(II) and Ni(II) complexes also supported the highly symmetric nature of the metal complexes. The absorption spectra of the metal complexes showed strong bands in the region of 300-550 nm. The electrochemical studies of metal complexes revealed that only ligand-based oxidation and reduction were observed. The DFT and TD-DFT studies were in agreement with the experimental observations. Our preliminary studies indicated that the Pd(II) complex can be used as a catalyst for the Fujiwara-Moritani olefination reaction.

Effect of Meso-Thienyl and Meso-Furyl Groups on The Electronic Properties of Aromatic 14π Triphyrin(2.1.1)s

A series of triphyrin(2.1.1)s containing five membered heterocycles such as thienyl and furyl groups in place of six membered aryl groups at two or four meso -positions were synthesized by [2+1] condensation of appropriate dipyrroethene diols with pyrrole under acid catalyzed conditions. The meso -thienyl and meso -furyl triphyrin(2.1.1)s were characterized and studied by HR-MS, 1D & 2D NMR spectroscopy, X-ray crystallography, absorption, cyclic voltammetry and DFT/TD-DFT studies. The X-ray structure obtained for one of the meso -thienyl trirphyrins indicated that the macrocycle was nearly planar and showed strong intramolecular hydrogen bonding. The spectral and electrochemical studies revealed that in meso -thienyl and meso -furyl triphyrin(2.1.1)s, the resonances of triphyrin core protons downfield shifted in 1 H NMR, the absorption bands bathochromically shifted and redox potentials shifted cathodically and anodically compared to meso -tetra( p -tolyl) triphyrin(2.1.1). These observations indicated that the thienyl/furyl groups at meso -positions alter the electronic properties of triphyrin(2.1.1)s significantly and the maximum effects were noted in meso -tetra(2-thienyl) triphyrin(2.1.1) and meso -tetra(2-furyl) triphyrin(2.1.1)s. The DFT and TD-DFT studies corroborated experimental observations.