Abstract
Metalloporphyrins and related macrocycles have been of great interest due to their role in biology and their numerous technological applications. Engineering of the porphyrins by replacing pyrrole nitrogens with other elements is a highly promising approach for tuning properties of porphyrins. To date, numerous efforts have been made to the modification of the porphyrin core with main‐group elements, such as chalcogens (O, S, Se) and phosphorus. Thus, the modification of the porphyrin core by incorporation of heteroatoms instead of nitrogens is a very promising strategy for obtaining novel compounds with unusual optical, electrochemical and coordinating properties as well as reactivity. These novel compounds can be used as building blocks in various nanotechnological applications. Within the framework of this research, the following questions can be formulated: (i) what structures will core‐modified porphyrins adopt? (ii) How will electronic properties of core‐modified porphyrins differ from those of common tetrapyrroles? (iii) Will the core‐modified porphyrins be able to form stacks and other arrays like regular porphyrins? (iv) Can core‐modified porphyrins form complexes with fullerenes? (v) Can core‐modified porphyrins activate small molecules, e.g. O2 or N2? (vi) Will the core‐modified porphyrins be able to form complexes with nanoparticles?
Keywords
- metalloporphyrins
- core modification
- chalcogens
- phosphorus
- structural changes
1. Introduction
Various (metallo)tetrapyrrole compounds, for example, porphyrins (P), porphyrazines (Pz) and phthalocyanines (Pc) (Figure 1), are representatives of the huge class of π‐conjugated (aromatic) organic heterocycles [1−5]. They can be found as cofactors in numerous enzymes: as hemes in various cytochromes, catalases, peroxidases, etc.; as chlorophyll and pheophytin in photosynthetic proteins and as corrin and corphin in other proteins [1−3, 5]. The metalloporphyrins have numerous biological functions such as: (i) O2 transport and storage, (ii) oxidative metabolism, (iii) gas sensing, (iv) antibactericides/microbicides, (v) collection and transport of light energy, (vi) conversion of solar energy to chemical energy, (vii) electron transfer and (viii) NO scavenging and a significant number of other functions [1–3, 5–10]. Numerous technological applications of porphyrins include: catalysis [1, 2, 4, 11, 12], molecular photonic devices [4, 13, 14], medicine [1, 2, 4, 15], artificial photosynthesis [16, 17], sensitizers for dye‐sensitized solar cells [18] and sensor devices [19].
The size, shape, electronic properties and binding ability of porphyrins can be broadly tuned by replacing one or more pyrrole nitrogens with other elements [21–24]. This type of the porphyrin core modification is a highly promising approach for tuning the various properties of porphyrin species. It brings to life the following questions:
(i) What structures will core‐modified porphyrins adopt? (ii) How will atomic charges and other electronic properties (frontier orbital energies, HOMO/LUMO and optical gaps, ionization potentials, electron affinities, etc.) in core‐modified porphyrins differ from regular tetrapyrroles? How can we tune these properties? (iii) What novel properties will core‐modified porphyrins possess?
In recent years, there has been increasing interest in porphyrin core modification with the chalcogens (O, S, Se), which resulted in numerous experimental and computational works in this extremely promising area. Core modification of tetrapyrroles by P has been of long‐lasting interest as well. Of course, it would not be possible to cover all the studies on core modification of porphyrins in this review. Thus, this chapter will cover the most significant and interesting works devoted to the core modification of porphyrins and derivatives with the principal focus on
2. Core modification with different main‐group elements
2.1. Core modification with chalcogens
The first porphyrins
Also, research interest was focused on the core modification of the 20 π‐electron
In 2012, Kon‐no et al. reported the synthesis, structures, optical properties, and electronic structures of the
The results of time‐dependent DFT and ZINDO/s calculations were compared to the observed magnetic circular dichroism (MCD) spectra and the electronic absorption spectra to study the effects of core modification on the electronic structures of S4TPP2+ and S4F20TPP. For [S4TPP2+][ClO4−]2, the MCD spectrum showed correspondence with the weaker bands at 948 and 733 nm in the near‐IR region of the electronic absorption spectrum and a more intense band in the visible region at 491 nm. These bands were assigned as Q00, Q01 and B00 bands, respectively. In contrast to 21‐ and 21,23‐core‐modified porphyrinoids [35], a marked red‐shift of the Q bands into the near‐IR region was observed owing to a narrowing of the HOMO–LUMO gap.
Also in 2012, Rurack and coworkers reported the synthesis of novel
To compare the relative effects of the heavy atoms and core modification on the emission properties of the core‐modified porphyrins, Cl, Br and I were used to generate the compounds denoted as
In 2016, Goto, Shinmyozu and coworkers reported the synthesis, optical and redox properties, and electronic structure of the completely core‐modified tetrakis(pentafluorophenyl)tetrathiaisophlorin dioxide (12) [25]. After the synthesis of the fully core‐modified 5,10,15,20‐tetrakis(pentafluorophenyl)‐21,22,23,24‐tetrathiaisophlorin (11) (Figure 2) [25], the authors aimed to oxidize the S‐atoms of the thiophene moieties of
Within this chapter, it is also of interest to mention the work of Sukumaran, Detty, and coworkers who in 2002 reported their studies of Te‐containing 21‐ and 21,23‐core‐modified porphyrins [39]. Ono and coworkers who also studied the partial core modification of tetrabenzoporphyrins and tetraphenyltetrabenzoporphyrins with O and S observed only minor changes in the optical spectra of 21‐ and 21,23‐core‐modified tetrabenzoporphyrins [35].
Very recently, Anand and coworkers reported extremely interesting synthesis and characterization of the meso‐meso linked antiaromatic tetraoxaisophlorin dimer [40]. It should be noted that antiaromatic units are seldom used as components of functional π‐materials [41], although they can be employed in organic electronics due to their noticeable paramagnetic properties [42]. The chemistry of antiaromatic systems is severely hindered by the very small number of stable antiaromatic compounds. The 4nπ isophlorins offer a rare opportunity to explore novel antiaromatic organic materials for potential applications in optoelectronics. The 20 π‐electrons isophlorin derivatives of thiophene and furan represent the simplest of the stable and planar antiaromatic compounds. Isophlorin can non‐covalently bind to C60 through conventional π‐π interactions, as was shown by the same research group in 2015, thus highlighting the utility of isophlorin as a synthon for supramolecular chemistry [43]. It was found that the compound
Also, it is worthwhile to mention the following several works on
2.2. Core modification with phosphorus
In this section, we will first address the studies on
Now a few words should be said about the phosphole, C5H5P, as the phosphorus isologue of pyrrole, C5H5N. C5H5P has much lower aromaticity than pyrrole due to insufficient π‐conjugation between the cis‐dienic π‐system and the lone electron pair of the P‐atom [49, 50]. The phosphole species possesses the following prominent features affecting its structure, electronic properties and reactivities [50]: (1) the P‐centre adopts a trigonal pyramidal geometry due to insufficient n‐π orbital interaction; (2) the LUMO is located at a lower energy compared to the pyrrole LUMO due to the effective σ*(P–R) – π*(1,3‐diene) hyperconjugation; (3) orbital energies of the C5H5P π‐system are easily tunable by chemical modification at the P‐centre and (4) the P‐bridged 1,3‐diene unit is rigid, electron rich and polarizable. These features of phospholes originate from the intrinsic nature of the P 3s and 3p orbitals. Consequently, phospholes behave both as potential building blocks for the π‐conjugated materials and as ordinary phosphine ligands [51].
In 2003, Delaere and Nguyen [52] reported the DFT study of the structural and optical properties of the core‐modified porphyrins with one or two pyrrole nitrogens replaced by P‐atoms. The geometries of the ground states were optimized using the B3LYP/6‐31G* approach and energies of the lower‐lying excited singlet states of P‐modified porphyrins were computed using the TD‐B3LYP/SV(P) method and compared with those of N‐porphyrins. The substitution of a NH‐ by a PH‐unit did not distort the carbon skeleton which remains essentially planar, whereas replacement of a N‐ by a P‐atom was found to weakly distort (by 15.3o) the P‐containing ring from the porphyrin mean plane. A nearly equal red‐shift of both Q‐ and B‐bands was predicted upon substituting NH‐ by PH‐units, whereas the red shift of Q‐bands was calculated to be much larger than the red shift of B‐bands upon substitution of an N‐atom by a P‐atom.
Later, Matano et al. [53–62] reported syntheses and characterization of various phosphaporphyrins and their derivatives with only one pyrrole nitrogen replaced by a P‐atom. Thus, in their 2010 review [53], the researchers summarized their previous studies on the phosphole‐containing porphyrins and their metal complexes. One of the compounds studied, the porphyrin containing trigonal pyramidal P‐centre was found to possess a slightly distorted 18π‐electron plane, wherein the phosphole and three pyrrole rings were found to be somewhat tilted from the 24‐atom mean plane. It was suggested that the porphyrin 18π‐electron circuit does not involve the lone electron pair of the trigonal pyramidal P‐atom. On the contrary, the 22π‐electron porphyrin containing tetrahedral P‐centre was shown to have a highly‐ruffled structure, with the P‐atom deviated significantly from the porphyrin π‐plane (1.20 Å) to avoid the steric congestion at the core. The Rh(III) and Pd(II) derivatives of these compounds were also shown to possess significant structural distortions. The metal complexes of these P‐modified porphyrins exhibited only a weak antiaromaticity in terms of the magnetic criterion. In the UV/vis absorption spectra of the P‐modified porphyrins, the characteristic two transitions of the porphyrin core, B and Q bands, were clearly observed, with significant red shifts. These results showed that the incorporation of a P‐atom in the porphyrin core considerably reduced both the S0–S2 and S0–S1 excitation energies. The 18π‐electron Rh‐complex also showed characteristic Soret and Q bands, whereas the 20π‐electron Pd‐complex displayed broad and blue‐shifted Soret‐like bands and no detectable Q bands, which is typical of highly ruffled, nonaromatic 4nπ porphyrinoids. It was stated that the observed structures, reactivities, and coordinating properties of the studied P‐core‐modified porphyrins were undoubtedly produced by the P‐atom at the core. In this context, the phosphole‐containing porphyrins were regarded as metal–affinitive macrocyclic π‐systems and could be developed as new classes of metal sensors, sensitizers and catalysts.
Earlier, in the 2009 review [54], Matano and Imahori described the exploration of the utility of phosphole‐containing porphyrins and porphyrinogens as macrocyclic, mixed‐donor ligands. The convenient methods for the synthesis of calixpyrroles, calixphyrins and porphyrins with P and either O or S substitutions (P,X,N2‐hybrids) were described. Also, the effects of varying the combination of core heteroatoms (P, N, S and O) on the coordination properties of the hybrid macrocycles were investigated. The results were summarized to show that: (i) the P,S,N2‐calixpyrroles behave as monophosphine ligands, (ii) the P,X,N2‐calixphyrins behave as neutral, monoanionic or dianionic tetradentate ligands and (iii) the P,S,N2‐porphyrins behave as a redox‐active π‐ligand for group 10 metals (Ni, Pd, Pt), affording a novel class of core‐modified isophlorin complexes. The incorporation of the phosphole subunit into the macrocyclic framework was proved to provide unprecedented coordinating properties for the porphyrin family.
In 2008, the syntheses, structures and coordination chemistry of phosphole‐containing hybrid calixphyrins (P,N2,X‐hybrid calixphyrins) and the catalytic activities of their transition metal complexes were reported [60]. The 5,10‐porphodimethene type 14π‐P,(NH)2,X‐ and 16π‐P,N2,X‐hybrid calixphyrins (where X = O, S, NH) were prepared. The σ3‐P,(NH)2,S‐ and σ3‐P,N2,S‐compounds were shown to produce the same Pd(II)‐P,N2,S‐hybrid complex. In this complex, the calixphyrin ligand was regarded as a dianion. In the complexation with [RhCl(CO)2]2 in CH2Cl2, the σ3‐P,N2,S‐compound was shown to behave as a neutral ligand producing an ionic Rh(I)‐P,N2,S‐hybrid complex. The σ3‐P,N2,NH‐compound was found to behave as an anionic ligand to produce Rh(III)‐P,N3‐hybrid complexes. The complexation of AuCl(SMe2) with the σ3‐P,N2,X‐compounds (X = S, NH) was shown to lead to the formation of the corresponding Au(I)‐monophosphine complexes. The calixphyrin‐Pd and ‐Rh complexes were shown to catalyse the Heck reaction and hydrosilylation reaction, respectively, implying that the metal centre in the core was capable of activating the substrates under appropriate reaction conditions. The study results demonstrated the potential utility of the phosphole‐containing hybrid calixphyrins as a new class of macrocyclic P,N2,X‐mixed donor ligands for designing highly reactive transition metal complexes.
It is also worthwhile to mention the 2009 theoretical investigation of electronic structure and reactivity for oxidative addition for the Pd‐complex of P,S‐containing hybrid calixphyrin [62]. Two kinds of valence tautomers were shown for the Pd‐complex
So far, as can be seen, no computational studies (metallo)porphyrins completely core‐modified with P‐atoms (P(P)4) have been reported, except the 2012 report by Barbee and Kuznetsov on the NiP(P)4 compound [63]. Motivated by the above‐listed works on mono‐P‐core‐modified porphyrins and derivatives, Kuznetsov reported the computational studies of the structures and electronic properties of the fully P‐core‐modified metalloporphyrins, MP(P)4, M = Sc‐Zn [64, 65], along with the computational design of the stacks formed by the ZnP(P)4 species [66]. The prominent structural feature of all the MP(P)4 compounds studied was found to be their significant distortion from planarity (Figure 3) [63–66].
In the 2015 work, the first
In the follow‐up 2016 work [65], the comparative DFT study, including Natural Bond Orbitals analysis, of the binding energies between all the first‐row transition metals Mn+ (M = Sc–Zn) and two ligands of the similar type, porphine, P2−, and its completely P‐modified counterpart, P(P)42−, was reported. The main findings were as follows: (i) generally, for the MP(P)4 compounds the calculated HOMO‐LUMO gaps and optical gaps were shown to be smaller than for their MP counterparts; (ii) the trends in the change of the binding energies between Mn+and P(P)42−/P2− were shown to be very similar for both ligands. The full P‐modification of the porphyrin core was found to decrease the Mn+‐ligand binding energies; however, the MP(P)4 compounds studied were shown to be stable according to the Ebind values and therefore can be potentially synthesized.
Also in 2016, due to motivation by the phenomenon of formation of stacks by regular metalloporphyrins, the computational check of the stack formation between the MP(P)4 species without any linkers or substituents was performed [66]. Three modes of binding or coordination were found to be possible between the monomeric ZnP(P)4 units (Figure 4).
The ‘convexity‐to‐convexity’ dimer I was found to be the most stable compound with the highest binding energy. In the dimer I, the strongly convex shape of both monomer units was demonstrated. The Zn–Zn distances in the dimer I, ca. 3.5 Å, were computed to be significantly shorter than in two other dimers. In the dimer I, significant decrease of the charge was found on the Zn‐centres.
3. Conclusions and perspectives
Thus, as can be seen from this micro‐review, core modification of the porphyrins and their derivatives with other elements than N is a very promising and productive approach to modify and fine‐tune their structures, electronic and coordination properties and reactivities. The research in this area has already been quite productive and brought for our attention numerous compounds with unusual novel structures and properties, which makes these species great candidates for different fields in chemistry and nanotechnology. Without any doubts, studies in this area will be continued and broadened. Based on the considered studies of porphyrin derivatives core‐modified by other elements, we can summarize subareas (or research directions), which would be necessary to focus on to employ the core‐modified porphyrins for the design of building blocks for nanotechnology:
Ability of core‐modified porphyrins to form stacks and other arrays like regular porphyrins. Would be core‐modified porphyrins and their derivatives form stacks/arrays without any linkers or substituents?
Ability of core‐modified porphyrins to form complexes with fullerenes, similar to regular porphyrins. How stable will be such complexes and what properties and potential applications they will have?
Catalytic properties of core‐modified porphyrins. Can they activate, in particular, small molecules, like H2, O2, N2, hydrocarbons?
Ability of core‐modified porphyrins to form complexes with various nanoparticles, including semiconductor NPs. What properties and potential applications with these complexes have? How can we tune structures and properties of these complexes?
Possibility to develop general synthetic strategies for obtaining the core‐modified porphyrins with desired structures and properties. Development of approaches for synthesis of building nanoblocks from these compounds.
Broad and profound application of computational approaches in this area, both to assist the synthesis of novel core‐modified porphyrins and to provide insight in their properties. Also, the computational approaches could be broadly used to assist in obtaining various building nanoblocks from these compounds.
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