1. Introduction
The recently emerged porous materials, metal–organic frameworks (MOFs)—typically formed from metal ions/clusters bridged by multidentate ligands in an extended framework—have provided solutions to tackle challenges in areas such as catalysis
[1][2][3], gas storge/separation
[4][5], biomimetic applications
[6][7][8], drug delivery
[9][10][11], electrochemical applications
[12][13] and biomedical chemistry
[14][15]. Moreover, the structures can be tuned by replacing or incorporating specific linkers or suitable unsaturated metal ions, in addition to adjusting the pore size and/or geometry which substantially influence their catalytic behaviors toward many substances participating in the reaction
[16][17]. Amongst the main categories of MOFs, porphyrin-based MOFs have demonstrated themselves as a tangible material able to provide the properties of both MOFs and metalloporphyrin complexes in one scaffold
[18]. Despite being synthesized less frequently than other types of MOFs and less often explored in research, they have had considerable impact on multiple fields, particularly biomimetic and biomedical ones as catalysts, owing to their resemblance to some molecules discovered in nature
[19].
Tetrapyrrole ligands such as porphyrin (or porphine in the unsubstituted form) and related macrocycles chlorin and corrin are naturally occurring in several bioinorganic metal complexes. The ability of the planar or nearly planar tetradentate ring system to stabilize kinetically labile metal centers (i.e., MgII, NiII, FeII/III, CoII) results in the selective formation of stable metal complexes containing an extensively conjugated π system (Figure 1).
Figure 1. Representative structure of a metalloporphyrin complex with a porphine ligand core.
Thus, metalloporphyrins, due to their abundance in nature, have been explored during the last decades. The synthetic bioinspired complexes resemble in structure, central atoms, and properties the most common naturally occurring biomolecules such as hemoglobin which transports oxygen in animal bodies, chlorophyll which acts as a light-scavenging antenna in photosynthesis inside plants, or vitamin B
12 which is important for metabolism in the cells (
Figure 2,
Table 1)
[20]. The combination of such favorable properties makes artificial metalloporphyrins highly suited for applications in photosynthesis
[21], electrochemical
[22], biosensing
[23], biomedical
[24] applications for tumor therapy
[25] and bioimaging
[26].
Figure 2. Naturally occurred MPs (metalloporphyrins) (
A) iron(II)-porphyrin “Heme B in RBCs” to convey oxygen; (
B) magnesium(II)-porphyrin “chlorophyll a” needed for plant photosynthesis; (
C) cobalt(II)-porphyrin “methylcobalamin (as vitamin B12)” assisted to facilitate nerve system performances; (
D) nickel(II)-porphyrin “Cofactor F430” accelerates methanogenesis in methanogenic archaea). Reprinted with permission from
[20].
Table 1. Naturally occurring metalloporphyrin complexes
[27].
The crystal engineering of MOFs based on metalloporphyrins, or tetrapyrrole ligands in general, began in the early nineties
[28]. The intercalation of tetrapyrrole ligands was recently recognized to be driven mostly by weak dispersive forces and either an offset or a proper π−π stacking with other components of the MOF
[29][30][31][32][33]. Interestingly, there is usually a lack of a stronger specific interaction between the porphyrin sheets themselves. Therefore, the construction of a crystalline MOF depends very much on the additives and their ability to provide suitable interactions and binding with metalloporphyrin and/or other components
[34][35][36][37][38].
On the other hand, further to metallization in their center
[39], metalloporphyrin complexes can also be additionally peripherally functionalized at meso- or β-positions
[40] (in Porphyrin, there are typically 12 positions that can be exchanged in the environs (Pyrrolic rings containing the eight β positions and the other four meso ones which are attributed to the methine substituents)) or even complexed by various (non-) transition metals providing versatile moieties which had led to their use as spacers to construct several porphyrin-based MOFs where they could serve either inside the pores (porphyrin@MOFs)
[41] or as linker merged in the architecture throughout the framework (porphyrinic MOFs)
[42]. The representative structures of synthetized derivatives of porphine are depicted in
Figure 3. They benefit from the extended π conjugated system throughout the planar molecule, which moderates substitution of the metal ions and the functionalities of porphyrin itself
[43]. Therefore, it illustrates exceptional electrochemical and photophysical exploitations while possessing an extraordinary chemical and physical durability. Additionally, porphyrins and their accessories normally have the strongest Soret band (400–450 nm) and a pack of steadily reduced Q-bands located somewhere in the range of 500 to 700 nm in the absorption spectrum
[44]. These features have led them to be considered as one of the most significant organic chromophores with remarkable adsorption bands in the visible region.
Figure 3. Examples of some of the previously fabricated porphyrin linkers.
2. Synthesis Procedures of Porphyrin-Based MOFs
By either integrating porphyrins/metalloporphyrins inside pores freely in situ or by grafting on the surface using post-synthetic methods and/or as part of the network component, porphyrin-based MOFs could be easily constructed. However, downsizing MOFs to the nanoscale will even more profoundly develop their size-dependent properties when encapsulating or accompanying such proactive molecules for any related applications
[14].
2.1. In Situ Method of Porph@MOFs Synthesis
Inspired by these facts, some promising routes to combine porphyrin derivatives into MOF frameworks emerged such as porphyrin@MOFs (porph@MOFs). These include the entrapment of (metallo-) porphyrins into the cavities or the decoration of the surface of MOFs where the former method can be performed using one-pot fabrication in situ, and the latter can be executed post-synthesis. In contrast to in situ assembly in which free porphyrin basis/metalloporphyrin are entrapped by MOF precursors (metal ion and ligand) by self-assembly simultaneously (ship-in-a-bottle)
[45], the post-synthetic method which occurs by anchoring to the exterior or the inclusion of them inside the MOF pores is mainly based on weak chemical interactions such as hydrogen bonding, electrostatics, van der Waals forces and others, between the pre-obtained MOF and porphyrin base/metalloporphyrin
[46]. The simplicity and straightforwardness of porphyrin entrapping by in situ formation led to this path being applied extensively by many researchers working in this field even though the post-functionalization requires the acknowledgement of some issues such as the creation of a suitable interaction between these two materials
[14][20]. Parameters that should be considered before postsynthetic fabrication include the activation of MOF pores and channels by guest solvent removal inside the structure during synthesis to allow for the incorporation of porphyrin instead, the dimensions in terms of shape and size of the porphyrin encapsulated into the cavities should be appropriate, and the stimulus required to initiate bond construction between the porphyrin and MOF structure.
As described above, embedded porph@MOF can be prepared by the in situ mixing of pre-synthesized porphyrins and MOF reactants (metal ion salt and linkers). A series of metalloporphyrin-decorated Cu-based MOFs with a coral-like shape (named as M-TCPP@Cu) were obtained using a one-pot reaction strategy
[47]. Accordingly, the resulting MOFs were developed through intermediate enrichment-enhanced conversion to assist the electrochemical reduction of CO
2 to C
2H
4. The respective porph@MOFs were obtained by dissolving H
3BTC and TCPP/M-TCPP in a mixture of ethanol and DMF followed by the addition of Cu(NO
3)
2·3H
2O in aqueous solution in situ to produce M-TCPP@Cu-MOF (M = Fe, Co, Ni). An ionic Mn-metalloporphyrin was reported which is presented in
Figure 4 that was encapsulated into the interior pores of ZIF-8 by a simple method through which all the precursors in DMF were sealed in a Teflon-lined autoclave and heated at 140 °C for 2 days
[48]. The crystals were then used as heterogeneous catalysts for a cycloaddition reaction of CO
2 with epoxides.
Figure 4. Illustrative demonstration of in situ enveloping of metalloporphyrin into ZIF-8 to conjoin CO
2 to epoxide. Reprinted with permission from
[48].
2.2. Postsynthetic Procedure of Porph@MOFs Fabrication
For the postsynthetic fabrication of functionalized porph@MOFs, they can be either physically absorbed on the exterior or captured into the cavities, and a specific MOF must be prepared in advance; subsequently, the previously formed porphyrins are incorporated to or grafted on the MOF structure. Next, the metal can be exchanged in porphyrin via controlled-immersion of the final material in a solution of metallic salts. A post-synthetic modification (PSM) of a porphyrin-engulfed MOF to enhance the selective adsorption of CO
2 over CH
4 was reported
[49]. The trapped porphyrin used as a structure-directing agent to provide a “ship-in-a-bottle” mode led to template-based Cd-porph@MOM-11 (MOM; metal–organic materials). In
Figure 5a, the effect of the exchange of some cationic organic guests such as H
2ppz
2+ with Li
+ on the selectivity of the adsorption of H
2 over N
2 was assessed. Remarkably, the results presented that ppz (1,1′,4′,1″,4″,1″′-quaterphenyl-3,5,3″′,5″′-tetracarboxylate) demonstrated significant kinetic trap for both the N
2 and H
2 ads-des process whilst Li displayed an increment in the pore volume size and more importantly a relatively higher H
2 isosteric adsorption heat
[50]. In another case, noncatenated hydroxyl-substituted MOF were introduced and replaced by Li
+ and Mg
2+ ions to convert pendant alcohol to metal alkoxides in order to upgrade the H
2 uptake reversibly (
Figure 5b). Exchanging was performed via the immersion of as-fabricated MOF in THF solvent (Tetrahydrofuran) to replace the primarily occupied solvent DMF (N-N-Dimethylformamide). Afterwards, the stirring of the respective MOF in an excess of Li
+[O(CH3)
3−] in CH
3 CN/THF solvents was performed to exchange Li
+ ions which boosted the hydrogen adsorption ability of the MOF significantly
[51]. The illustration in
Figure 5c
[52] indicates that the MOF of the formula Zn
2(NDC)
2(diPyNI) (NDC = 2,6-dicarboxylate, diPyNI =
N,
N′-di-(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide) was reduced by Li
0. The interaction imposed by H
2 − Li
+ inside MOF pores improved its competence to adsorb H
2 which was most likely increased by the augmented ligand polarizability and framework displacement. Furthermore, the experimental work in
[49] with some modifications of a combination of the first two methods mentioned in
Figure 5d submerged single crystals of the prefabricated Cd-porph@MOM-11 into metal chloride salt solutions, with meso-tetra(
N-methyl-4-pyridyl) porphine tetratosylate (TMPyP) in methanol serving as a template for PSM, and formed a basis for MOF formation via single-crystal-to-single-crystal ion exchange processes.
Figure 5. Three basic ways of introduction of open metal sites by PSM synthetic routes to MOFs: (
a) cationic guests or organic cations exchange (blue balls) with metal cations (red balls); (
b) replacement of a hydroxy protons with Li
+ and Mg
2+ ions (red balls); (
c) chemical reduction of MOM with Li (red balls) and (
d) fourth method is a combination of the first two—a collaborative attachment of metal (red balls) chloride (blue ones) salts to anion and cation binding sites. Besides, the sticks and the crescent-shaped bowls attached to sticks are porphyrin-encapsulated inside MOM-11 and cation/anion binding sites. Reprinted with permission from
[49].
2.3. Porphyrinic-Oriented MOFs
With regard to porphyrinic MOFs, porphyrin or metalloporphyrin functioning as an organic linker is one of the main components in the framework, which coordinates with secondary building units (SBUs). Accordingly, the choice of suitable shape, size and geometry that meet the desired pore structure to load substrate molecules and to catalyze several reactions efficiently on the surface depends entirely on the rational selection of porphyrins and SBUs. While the insertion of (metallo-) porphyrins not only equips MOFs with new functionality, they can also maintain or bring about far better stability and diversity across the building blocks. As a consequence, the catalytic performances of these types of porous coordination networks could be simply upgraded by regulating Lewis-acid metal active sites and designing well-qualified circumferential functionalities on metalloporphyrins
[45]. In line with the previous statements, various parameters such as temperature, solvent, reaction time and the method as well as proper metal nodes and porphyrins chosen, could additionally determine the final product
[14].
By applying peripherally functionalized porphyrin/metalloporphyrin as spacer or multidentate ligands directly with metal ions/clusters could lead to the formation of porphyrinic MOFs. Displayed clearly in
Figure 6, the porphyrinic MOF PCN-222/MOF-545 (free base-H
4TCPP and [Zr
6(μ
3-O)
8(O)
8]
8− node) was used to selectively oxidize 2-chloroethyl ethyl sulfide (CEES) to a less toxic 2-chloroethyl ethyl sulfoxide (CEESO) at room temperature and neutral pH. The photooxidation of this mustard-gas simulant under mild conditions by exploiting these porous materials as photosensitizer within a half-life of up to 13 min was found to be one of the most convenient methods for the detoxification of such a poisonous compound
[53].
Figure 6. Comparison of free-base PCN-222/MOF-545 (fb-1). (
a) Tetrakis(4-carboxyphenyl)porphyrin linker, H
4TCPP. (
b) [Zr6(m3-O)8(O)8]8¢ node. (
c) MOF fb-1, shown across the axis a (
d) 3D structure of fb-1, depicted along the c axis. For more clarity hydrogen atoms has been omitted. Reprinted with permission from
[53].
Facile insertion of H
3PW
12O
40 inside the solvothermally prefabricated free-base PCN-222 MOF was investigated for the photocatalytic synthesis of some bioactive N-heterocycles such as Nifedipine, Nicardipine, Nicotinic acid (Vitamin B
3), and Pyridoxine (Vitamin B
6) under visible-LED light irradiation
[54]. The porphyrinic Zr-MOF scaffold was constructed by employing TFA and BA as modifiers followed by post-modification with POM to construct the POM@PCN-222 composite.
This entry is adapted from the peer-reviewed paper 10.3390/molecules27154917