Synthesis of Mono β ‐Pyrrole Substituted Triphyrin(2.1.1)s

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.

Synthesis of the β-dipyrrinyl triphyrin(2.1.1) ligand and its coordination complexes

Functionalized β-formylphenyl triphyrin(2.1.1) was prepared by coupling β-bromo triphyrin(2.1.1) with 4-formylphenyl boronic acid under Pd(0) coupling conditions. β-Formylphenyl triphyrin(2.1.1) was treated with excess pyrrole under acid catalyzed conditions in CH₂Cl₂ to obtain dipyrramethanyl triphyrin(2.1.1), which was subjected to oxidation by treating it with DDQ to obtain the desired ligand, β-dipyrrinyl triphyrin(2.1.1). The dipyrrinyl unit of triphyrin(2.1.1) can act as a bidentate ligand to form interesting coordination complexes. Thus, the dipyrrinyl triphyrin(2.1.1) ligand was treated with BF₃·(OEt)₂ as well as metal salts such as [Ru(p-cymene)Cl₂]₂, Pd(acac)₂ and Zn(CH₃COO)₂ to obtain BODIPY-triphyrin(2.1.1), Pd(II)dipyrrin-triphyrin(2.1.1), Ru(II)-dipyrrin-triphyrin(2.1.1) and bis(Zn dipyrrin)-triphyrin(2.1.1) conjugates in good yields. The ligand and all four conjugates were freely soluble in common organic solvents and thoroughly characterized and studied by HR-MS, 1D & 2D NMR spectroscopy, absorption, cyclic voltammetry and DFT/TD-DFT studies. The optimized structures indicated that the triphyrin(2.1.1) and BODIPY/metal dipyrrin units in the conjugates were oriented w.r.t each other with an angle in the range of 25.18°-77.55°. The spectral studies indicated that the two moieties in the conjugates interact weakly and retain their individual characteristics, whereas electrochemical studies revealed their electron deficient nature. The TD-DFT studies were in agreement with the experimental observations.