Construction of coordination polymers based on mixed-ligand synthetic strategy

At this stage, the rational construction of structurally-defined CPs is quite successful and a variety of typical examples for such crystalline materials with attractive network architectures and potential applications have been reported. In this regard, their possible applications arising from the remarkable physico-chemical properties of CPs are widely recognized in gas adsorption, molecular/ionic separation, optics, electricity, magnetism, chirality, catalysis, and drug delivery, etc.  As a rule, the optimal implementation for such applications requires good phase purity, framework stability, and in some cases the available porosity of the targeted materials. While their absolute structural control still remains a long-term goal and a worthwhile endeavor.
 
Much work has been devoted to the synthesis, structural characterization, and properties of CPs. In this process, the accumulation of sufficient experimental data allows chemists to proceed beyond the random studies and to derive some useful laws of assembly. Towards this end, several effective synthetic strategies such as ‘node-and-spacer’ and ‘secondary building units (SBUs)’ have been successfully established and developed, based on the notion that molecular precursors (metal ions and organic ligands) can be conveniently conceptualized as objects such as points, lines, polygons, and polyhedra, with CPs as periodic and complementary assemblies of these geometric motifs. Nevertheless, there are still many challenges in practice to perfectly project and regulate the specific crystal packing of such materials, because structural control will be readily thwarted by the intricate and noncovalent nature of the secondary interactions such as H-bonding, aromatic stacking, and van der Waals force, as well as some external physical or chemical factors including counterion, template, temperature, pressure, solvent, and pH value, etc. .
 
To date, it is well known that multi-topic bridging ligands with two or more nitrogen, sulfur, phosphorus, and/or oxygen-involving functional groups can show different binding abilities to metal ions and thus can be applied as effective tectons for constructing diverse coordination networks . However, in the scope of mixed-ligand CPs, ligands such as disulfoxide , dithioether ,diphosphine , and hybrid P,N-type compounds  have seldom been used in practice, probably owing to the lack of competent counterparts as the co-ligands. In contrast, intense research activity towards the rational design and construction of mixed-ligand CPs has shown that the bipyridine, polycarboxyl , and amino compounds represent the most reliable and typical building blocks which can be jointly applied to synthesize a wide range of desired coordination networks. A choice of such connectors in coordination assembly can be rationalized based on the following considerations. On the one hand, the neutral bipyridine ligands normally bind to the metal ions as the rod-like bidentate tectons. On the other hand, polycarboxyl compounds may take the anionic or protonated form to  provide various coordination modes upon binding to metals and  anionic form can obviate small counteranion incorporation, resulting in neutral frameworks with enhanced porosity. While amino acids are well known as excellent building blocks for hydrogen bonded network. Thus, the underlying structures of CPs obtained from a given bipyridine spacer can usually be ascribed to known geometrical networks, at least with hindsight, while assembly of a polycarboxyl bridging ligand with the metal ion under different conditions will lead to considerable structural complexity and diversity of CPs. As a result, by combining the advantages of two such types of ligands, the so-called mixed-ligand synthetic strategy can be rationally proposed. In the broad domain of CP study, coordination frameworks based on mixed ligands have been heavily developed. However, no specific structural prediction and design scheme is available at this stage.
 
SO from the mixed-ligand synthetic point of view, I have proposed to synthesis Monometallic and Heterometallic coordination polymers .
 
-According to the different compositions of mixed-ligand CP systems, they can be roughly classified into three categories:
 
1-Acid–acid mixed-ligand CPs.
2- Base–base mixed-ligand CPs.
3- Acid–base mixed-ligand CPs
 (the concept of acid or base in this review only indicates the Lewis acid/base ligands).
 
 
            * Acid–base mixed-ligand CPs
 
The acid–base system is the most important and flourishing branch of mixed-ligand CPs. Naturally, acid and base ligands are perfect partners that can compensate charge balance, coordination deficiency, repulsive vacuum, and weakly interaction all at once. In this context, it is valuable to propose the rational synthetic strategy to regulate the network structures of mixed-ligand CPs by ligand design or selection, considering the aspects of spacer effect, positional isomeric effect, and substituent effect of the organic building blocks.