Porous materials coordination polymers

Figure 2.4.4 View of a porous cadmium coordination polymer, Cd(N03)2[2,4 -(l,4-phenylene)bispyridine] °° [50], Such materials are a particular target for coordination polymer chemists and require rational, designed, synthetic approaches.

This review deals with the chemistry and coordination complexes of isoelectronic analogues of common oxo-anions of phosphorus such as PO3, POl", RPOl" and R2POy. The article begins with a discussion of homoleptic systems in which all of the 0x0 ligands are replaced by imido (NR) groups. This is followed by an account of heteroleptic phosphorus-centered anions, including [RN(E)P(/<-NR )2P(E)NR]2-, [EP(NR)3]3-, [RP(E)(NR)2] and [R2P(E)(NR )] (E=0,S, Se, Te). The emphasis is on the wide variety of coordination modes exhibited by these poly-dentate ligands, which have both hard (NR) and soft (S, Se or Te) centers. Possible applications of their metal complexes include new catalytic systems, coordination polymers with unique properties, and novel porous materials. 

Approaches to the formation of three new types of micro-porous materials that complement zeolites will be discussed. In each case, whether metal coordination polymers, metal-linked ceramic oxide clusters, or new hybrids containing both coordination and ceramic components, engineering of the Secondary Building Unit (SBU) is of critical importance. Successful examples of these approaches include the first thermally stable 3-D micro-porous coordination polymer with chemical functionalizability [Cu3(TMA)2(H20)3] , as well as a 3-D micro-porous cluster based material [V,2B18O60H8(Cd(en)(H2O) 3]". ... 

The high thermal stability of zeolites and related micro-porous solids is one of their most attractive features. Whilst it Is clear that materials with organic components cannot withstand ultra-high temperatures, quite respectable compositional stability can be achieved. Thus the [Er(TMA)] polymer mentioned above shows no weight loss in its TGA curve before 550°C. However for porous solids another key issue is that of structural stability. Many open framework coordination polymers lose their crystalline structure upon mild heating, or even evacuation, through loss of guest molecules. 

Zeolite networks are the most significant topologies for porous materials, and are extremely scarce in porous coordination polymers. Three new zeolite-type porous coordination polymers, [Zn(mim)2] 2H20 , [Zn(eim)2] 2H20 , and... 

In recent years, great attention has been given to the application of coordination polymers in the development of new porous materials to be applied in gas adsorption and other fields [226-236], Some authors consider that in comparison with usual porous materials, for example, zeolites and activated carbons, these materials are more promissory, as a consequence of the fact that their framework is designable and more flexible because of a diversity of coordination... 

Bilayer architectures formed in M2(2)3(N03)4 n (where M = Co, Ni and Zn) were one of the first systems of coordination polymers to be shown as porous materials [43]. The bilayer architectures interdigitate with each other leaving small channels in the crystal lattice which were occupied by solvated water molecules. Powder X-ray studies indicate that the water molecules can be removed from the network without causing any distortion or decomposition of the network. The adsorption studies of water removed and dried sample indicated that the material is capable of adsorbing CH4, N2 and 02. About 2.3 mmol of CH4 and 0.80 mmol of N2 or 02 are adsorbed per gram of anhydrous material. The adsorption-readsorption followed the same isotherm, indicating the stability of the network throughout the process. Further, the isotherms for the adsorption-readsorption can be classified as type I in the IUPAC classification [48]. 

Fig. 6 Porous coordination polymer (PCP) developed by Kitagawa and coworkers. The pores, which extend throughout the array, can be filled by C02 molecules (grey and red), allowing these materials to employ their high internal surface area as gas adsorbents [13]. Reprinted with permission...

The current focus on highly porous materials has led to a great deal of activity in this field. Kitagawa and coworkers developed what they called porous coordination polymers that were rigid enough to survive loss of the encapsulated solvent from... 

TMA can coordinate to not only Cu2+ to form the typical 3-D framework polymer but also to Co2+ to form a porous host-guest compound Co(TMA) (py) with a different structure. The guest pyridine molecules in this compound may be removed from the channels without the collapse of the framework, and the pyridine-removed porous material adsorbs other guest molecules.[187] Polymerization of TMA with a Ni2+ macro-cyclic complex leads to a porous coordination polymer with a unique structure. In the... 

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