Synthesis, properties and coordination chemistry of [14]triphyrins(2.1.1)

Highlights

• The formation of [14]triphyrin(2.1.1) as relatively new congeners among the porphyrinoids seems quite astonishing because porphyrin macrocyclic core itself is perturbed to afford a contracted structure retaining aromaticity, however, exhibiting fundamentally different properties compared to other members in the porphyrin family.

• Although [14]triphyrin(2.1.1) chemistry has been mentioned in the literature, an effort which exclusively focussed on the development of [14] triphyrin(2.1.1) was not made.

• In the current review, we focused on the recent advancement in the chemistry of [14]triphyrin(2.1.1) including its synthesis, structure, coordination chemistry, photophysical and electrochemical properties.

• A fundamental insight in the triphyrin chemistry has immense scope since the reactivity of the molecule is less explored other than using as a ligand to prepare a few metal complexes and coupling reactions.

• The present review will definitely add to the furtherance of the chemistry of [14]triphyrin(2.1.1).

Abstract

Recently, synthetic efforts to prepare novel modified porphyrinoid skeletons have been an important area of research due to its promising applications in various fields ranging from material chemistry to life science. The formation of [14]triphyrin(2.1.1) as relatively new congeners among the porphyrinoids seems quite astonishing because porphyrin macrocyclic core itself is perturbed to afford a contracted structure retaining aromaticity, however, exhibiting fundamentally different properties compared to other members in the porphyrin family. Although [14]triphyrin(2.1.1) chemistry has been mentioned in the literature, an effort which exclusively focussed on the development of [14]triphyrin(2.1.1) was not made. In the current review, we focused on the recent advancement in the chemistry of [14]triphyrin(2.1.1) including its synthesis, structure, coordination chemistry, photophysical and electrochemical properties. A fundamental insight in the triphyrin chemistry has immense scope since the reactivity of the molecule is less explored other than using as a ligand to prepare a few metal complexes and coupling reactions. The present review will definitely add to the furtherance of the chemistry of [14]triphyrin(2.1.1).

Introduction

The family of tetrapyrrolic porphyrins with an aromatic core of 18π electrons and related macrocycles have intrigued chemists as they are a part of many vital biological processes such as photosynthesis (chlorophylls), oxygen transport (hemoglobin), redox processes (cytochromes), enzymatic reactions (vitamin B12), anaerobic oxidation (coenzyme F430) etc., to name a few important processes in biological systems (Chart 1) [1]. Inspired by the importance of tetrapyrrolic macrocycles in several important biological processes, the syntheses of porphyrin model compounds to mimic these biological processes have continuously remained as a challenging area of research [2], [3].

The synthetic explorations on porphyrins and their analogues are thus decades old. Still, porphyrinoid chemistry continues to present fascinating variants namely, expanded porphyrins that contain more than four pyrroles in the π-conjugation or by having additional meso carbons connecting four pyrroles in π-conjugation; contracted porphyrins such as corroles that contain four pyrroles connected by three meso-carbons and thus having one direct pyrrole-pyrrole link; subporphyrins that contain three pyrroles connected via three meso carbons and thus having one pyrrole and one meso-carbon less than porphyrins, and isomeric porphyrins such as N-confused porphyrins and porphycene systems [4]. Thus, porphyrin systems offer synthetic versatility and hence tuning the properties by redesigning the molecular skeleton offers a broad range of applications for porphyrinoids. The use of porphyrinoid macrocycles has grown exponentially over the past few years particularly, in energy transfer/storage materials [5], [6], nonlinear optics [7], photodynamic therapy, bioimaging [8], photonic devices [9], [10], catalysis [11] and so on. The porphyrinoid macrocycles form coordination complexes with various metal ions to exhibit valuable catalytic properties [2], [12]. Hence these macrocycles are quite worthwhile to investigate with tremendous implications in chemistry, material science and life science.

Contracted porphyrinoids came to limelight with the advent of corroles, subporphyrins, and triphyrins which are formed by the removal of a meso carbon or pyrrole ring from porphyrin skeleton, as presented in Chart 2. Corroles are tetrapyrrolic 18π aromatic macrocycles that have a direct pyrrole-pyrrole linkage similar to corrin ring of Vitamin B₁₂ and preserve the aromaticity of porphyrin despite having one less meso-carbon atom. Corroles have three ionizable protons and can stabilize metals in higher oxidation states than porphyrins. The coordination chemistry of corroles [13], [14], [15] has also been extensively investigated. The removal of a pyrrole ring from the 18π-electron corrole gives a more contracted 14π-electron system called subporphyrins which can also be represented as [14]triphyrin(1.1.1). Subporphyrins are 14π aromatic conjugated triphyrins(1.1.1) and can only be synthesized as boron complexes. Osuka and co-workers contributed extensively to the development of subporphyrin chemistry [16], [17], [18], [19], [20], [21]. However, the major limitation of subporphyrins is that it can exist as B(III) complex and attempts to isolate free base by removing B(III) ion led to decomposition. The presence of an additional meso carbon in the structure of [14]triphyrin(1.1.1) gives [14]triphyrin(2.1.1) macrocycle. Thus, triphyrin(2.1.1) is a contracted 14π aromatic macrocycle which contains three pyrrole rings connected via four meso-carbons. Unlike triphyins(1.1.1), [14]triphyrins(2.1.1) can be obtained in free base form. The triphyrins(2.1.1) are monoanionic tridentate ligands with a cavity size larger than triphyrins(1.1.1) and have an ability to form coordination complexes like corroles and porphyrins. However, triphyrins(2.1.1) exhibit different coordination behaviour compared to tetrapyrrolic systems.

The synthetic history of [14]triphyrin(2.1.1) related macrocycle can be traced back to 1972 when the presence of subphthalocyanine 1 was observed during the preparation of boron complex of phthalocyanine (Chart 3) [22]. The next congener namely meso-free tribenzosubporphyrin 2 emerged in 2006 using boric acid as a templating agent [16], [23]. Subsequently, Osuka and coworkers reported meso-substituted tribenzotriphyrin 3 in 2007 [24]. The groups led by Osuka and Kobayashi have independently reported meso-triarylsubporphyrins 4 by adopting different synthetic routes [24], [25]. These subporphyrins 2–4 were synthesized with boron templates [24], [25], [26]. Latos-Grazynski et al. have reported subpyriporphyrin 5, without any boron template and appears as a homologue of [14]triphyrin(1.1.1) [27]. Kobayashi and co-workers prepared the planar, all-pyrrole free-base tribenzotriphyrin 6 with 2.1.1 bridge pattern. Unlike previous reports, the macrocycle 6 was devoid of boron template. The extra meso carbon atom in [14]triphyrin(2.1.1) 6 allows it to act as 14π electron aromatic free-base ligand with a single NH unit.

In recent times, attention is focussed on the synthesis and coordination chemistry of [14]triphyrins(2.1.1) because of their attractive properties and exceptional thermal and photochemical stability. A perusal of literature reveals that there is no thorough review on triphyrins(2.1.1) although Shimizu covered a few aspects of triphyrin(2.1.1) in his recent review on subporphyrins(1.1.1) [28]. In this review, we thoroughly discuss the synthesis, structure, coordination chemistry, spectroscopy, electrochemistry and applications of the recently emerging contracted porphyrinoid, [14]triphyrins(2.1.1).