1. Introduction and Scope
In recent years, various crystalline metal-organic architectures (MOAs) including coordination polymers (CPs) or metal-organic frameworks (MOFs) have been an object of very intense research that spans from the fields of crystal design and engineering to chemistry of functional materials [1
]. In particular, a very interesting research direction concerns the search for new and versatile organic building blocks that can be applied for the design of unusual metal-organic architectures with desirable structural features and notable functional properties [16
]. Despite considerable progress achieved in this field, the assembly of coordination polymers or metal-organic frameworks in a predictable way is often a difficult task. This is mainly because the assembly of such compounds can depend on various factors, such as the nature and coordination properties of metal nodes [20
], connectivity and type of organic building blocks [22
], reaction conditions and stoichiometry [25
], and effects of templates [27
] or supporting ligands [30
A high diversity of aromatic polycarboxylic acids has been extensively applied as multifunctional building blocks in designing novel metal-organic networks [32
]. Among such building blocks, flexible ligands containing biphenyl and phenyl-pyridine cores with a varying number and position of carboxylic groups as well as distinct locations of N
-pyridyl functionality have attracted a special interest [34
]. It can be justified by a possibility of two adjacent phenyl and/or pyridine rings to rotate around the C–C single bond and thus conform to a coordination environment of metal nodes. Besides, the presence of several carboxylic groups with a varying degree of deprotonation in addition to an optional N
-pyridyl functionality can provide multiple and distinct coordination sites, thus leading to different coordination fashions and resulting in the assembly of structurally distinct coordination polymers [36
]. Furthermore, depending on a deprotonation degree and crystal packing arrangement, these aromatic polycarboxylate ligands can behave as good H-bond acceptors and donors, thus furnishing an extra stabilization of metal-organic structures and facilitating their crystallization.
Hence, the main objective of the present work consists in highlighting selected recent examples of coordination polymers that were hydrothermally assembled from a series of multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores (Scheme 1
). These carboxylic acids are still very poorly explored toward the design of CPs or MOFs, but can constitute an interesting type of semi-rigid, thermally stable, multifunctional, and versatile building blocks in crystal engineering research. Thus, the present study briefly discusses the general aspects of hydrothermal synthesis of selected coordination polymers derived from the aromatic carboxylic acids shown in Scheme 1
. Some of them represent isomeric biphenyl tricarboxylate blocks (H3
bptc and H3
btc), while other are isomeric phenyl-pyridine tricarboxylate blocks (H3
dcppa, and H3
cpta). The study also highlights the influence of various parameters (main ligand type, metal node, molar ratio and stoichiometry, temperature, presence of auxiliary ligand or template) on structural diversity of the obtained products. For selected examples of CPs, functional properties and applications are also highlighted.
4. Conclusions and Outlook
In this mini-review, we featured selected recent examples of coordination polymers (CPs) or metal-organic frameworks (MOFs) that were constructed from various multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores (Scheme 1
). Despite being still little explored, these types of semi-rigid, thermally stable, and versatile building blocks appear to be very promising for the hydrothermal synthesis of metal-organic networks with different structural characteristics, topologies, and functional properties. The present work also highlighted an importance of different reaction parameters and conditions on the assembly and structural diversity of coordination polymers. The effects of the type of main carboxylate ligand, kind of metal node, stoichiometry and molar ratio of reagents, temperature, presence or absence of auxiliary ligands or templates were showcased. In addition, some examples of highly porous MOFs, notable luminescent materials, compounds with unusual magnetic properties, and frameworks for selective sensing applications were described.
We believe the application of multifunctional carboxylic acids containing phenyl-pyridine or biphenyl cores toward the design of coordination polymers will be continued, leading to new series of coordination compounds and derived materials with fascinating structural features and notable functional properties. Future research might focus on: (A) widening the family of multicarboxylate building blocks to new members with additional functional groups; (B) diversifying the types of metal nodes; (C) assembling heterometallic metal-organic architectures; (D) optimizing the conditions of the hydrothermal synthesis and crystallization; (E) predicting the structural and topological characteristics; and (F) broadening the types of possible applications of the obtained coordination polymers.