Porphyrin-based Supramolecular Architectures

From Hierarchy to Functions

Shengqian Ma, Gaurav Verma, Shengqian Ma, Gaurav Verma

• Éditeur: Royal Society of Chemistry
• Année: 2021
• ISBN:
  9781839164941
• Édition: 1
• Sujet: Physical Sciences
• Sous-sujet: Organic Chemistry

Porphyrin-based Supramolecular Architectures focuses on the most recent developments in the field, emphasizing the cutting-edge research in a diverse range of applications. Designed for readers considering the unprecedented prosperity of porous materials research, chapters will cover both strategies for structure design (such as MOFs and COFs) as well as emerging applications including CO2 fixation, catalysis and photodynamic therapy. With contributions from global experts, this title will be of interest to graduate students and researchers in supramolecular chemistry, organic chemistry, inorganic chemistry, physical chemistry, organometallic chemistry, solid-state chemistry, catalysis and (porous) materials science.

Chapter 1

Structural Design of Porphyrin-based MOFs

Peiyu Cai^a, Yutao Huang^a, Mallory Smith^a and Hong-Cai Zhou^a

^a Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, USA Email: [email protected]

1.1 Introduction

Metal–organic frameworks (MOFs) are crystalline, porous materials assembled from inorganic nodes linked by multitopic organic ligands. The organic ligands, also known as linkers, typically have a rigid skeleton to prevent the collapse of the framework upon desolvation of the internal pore structure. The inorganic nodes can be monoatomic metal ions or clusters, the latter of which is the basis of the secondary building unit (SBU). SBUs are inorganic polynuclear clusters that have advantages over the use of monoatomic metal nodes, including enhanced stability, modularity, and structural predictability.^1,2 Many reviews have been written to cover the topic of SBUs and their use in constructing polymeric organic/inorganic materials.^3–6

MOFs are attractive materials due to their modularity, tunable porosity, structural diversity, and high crystallinity and have been targeted for various applications including gas storage/separations, catalysis, sensing, biomedicine, and water purification. Furthermore, the incorporation of metalloporphyrins into solid-state materials has long been sought after due to their interesting photophysical properties, catalytic abilities, and biological relevance. Additionally, a variety of metals can be incorporated into the porphyrin core, making metalloporphyrins functionally diverse. Robson et al. reported the earliest example of a porphyrinic coordination network constructed from Cd ions and tetrapyridylporphyrin linkers.⁷ However, as discussed, the use of monoatomic nodes tends to lead to framework collapse, and pyridyl-based linkers are somewhat limited in coordination mode. Encapsulation of porphyrins is another approach, but structural order is difficult to achieve and leeching is inevitable.^8–10 Incorporation of multitopic porphyrin linkers, usually carboxylates, has led to a plethora of stable porphyrinic MOFs with permanent porosity and high crystallinity using a variety of metals.

The following chapter discusses primarily MOFs that have their single-crystal structure reported over the last two decades, most of which use the SBU approach in the construction of permanently porous frameworks. Some porphyrinic MOFs were not included in this chapter as their single crystal structures were not reported. The chapter is divided first into linker types (carboxylate, pyridinyl, etc.) and topicity followed by the type of metal used in the SBU. The chapter begins with tetratopic carboxylate linkers which is the most frequently encountered linker and thus occupies a large portion of the discussion here. The porphyrin linkers depicted in this chapter are also tabulated at the end of the chapter.

1.2 Carboxylate Linkers

1.2.1 Tetratopic Carboxylate Linkers

1.2.1.1 Zr and Hf Clusters

Construction of MOFs based on high oxidation state metals such as Zr⁴⁺ is attractive due to their high stability and rich structural diversity. An interesting feature of Zr₆ clusters is their ability to vary in the number of linker connections while maintaining the robust [Zr₆(μ₃-O)₄(μ₃-OH)₄] core. Tetratopic porphyrin linkers such as tetrakis(4-carboxyphenyl)porphyrin (TCPP, Porphyrin 1), which is the most commonly encountered porphyrinic linker, can also adopt different coordination geometries due to the free rotation of the benzene rings. As a result, TCPP can form various MOFs with different topologies with Zr clusters that can be synthetically controlled. One series of MOFs that demonstrates the structural variation is Zr–TCPP MOFs: PCN-221(MOF-525),^11,12 PCN-222(MOF-545),^12,13 PCN-223,¹⁴ PCN-224,¹⁵ PCN-225,¹⁶ and NU-902.¹⁷ It should be noted that PCN-221 has unique Zr₈ clusters, while the remaining five MOFs contain Zr₆ clusters (Table 1.1).

Table 1.1 Series of MOFs based on TCPP linkers and Zr clusters.

	PCN-221	PCN-222	PCN-223	PCN-224	PCN-225	NU-902
Cluster connectivity	12	8	12	6	8	8
Topology	ftw	csq	shp-a	she	sqc	scu
Porosity	Microporous	Mesoporous	Microporous	Microporous	Microporous	Microporous

PCN-221^11,12 is a microporous MOF with a Brunauer–Emmett–Teller (BET) surface area of 1936 m² g⁻¹ when the free-base (non-metalated) TCPP linker is used to construct the MOF. PCN-222(Fe) exhibits a steep increase in its nitrogen adoption isotherm at P/P₀ = 0.3, suggesting mesoporosity.^12,13 The BET surface area was observed to be 2200 m² g⁻¹ for PCN-222(Fe), while the structurally related PCN-223 is a microporous MOF of surface area ∼1600 m² g⁻¹ and porosity of ∼0.6 cm³ g⁻¹.¹⁴ PCN-224 is a microporous MOF and appears to be the most porous of the six-MOF series and has a surface area of 2600 m² g⁻¹, which is in agreement with reduced connectivity.¹⁵ The calculated total pore volume of PCN-224 is as high as 1.59 cm³ g⁻¹.

In addition to PCN-222, two other 8-connected microporous MOFs have been reported. PCN-225 exhibits an sqc topology and BET surface areas of 1902 and 2080 m² g⁻¹, respectively, for PCN-225 and PCN-225(Zn).¹⁶ NU-902, has a scu topology and a BET surface area of 1580 m² g⁻¹. When HCl was used for sample activation, the BET surface area increased to 2150 m² g⁻¹ likely due to the removal of a residual benzoate modulator. The pore size distribution of NU-902 features a pore diameter of 12 Å. Three distinct distances were found between porphyrins within the framework of 10.5 Å, 18.2 Å, and 20.9 Å.¹⁷

Due to the high charge density and bond polarization of the Zr⁴⁺ cations, the interaction with the carboxylate oxygen atoms is very strong, leading to the high aqueous stability observed in the MOFs listed in Table 1.1. The acid/base stability varies depending on the structure. PCN-222 is highly acid-stable, with no change observed in the PXRD pattern or adsorption isotherm after prolonged treatment with concentrated HCl, but is less stable in basic solutions.^12,13 At the same time, PCN-223, PCN-224, and PCN-225 have demonstrated aqueous stability in the pH range of 1 to 10, 1 to 11, and 1 to 12, respectively.^13–15

The coordinatively unsaturated PCN-224 has the ability to accommodate additional linkers including ...