Coordination polymer systems

The requirement for single crystals is perhaps more of a challenge considering the highly insoluble nature of coordination polymers. It is also notable that recrystallisation, often used for molecular systems, is not as relevant for coordination polymer systems, as not only are coordination polymers typically highly insoluble but even if they are dissolved in aggressive solvents, such as DM SO or DMF, the polymer structure will break up and any coordination polymer that reforms may bear no resemblance to the product from the original reaction. 

Approaches to growing crystals of coordination polymers must be found and these may take two major directions. The first involves the judicious choice of building-blocks. Coordination polymer systems that involve the formation of kinetically inert coordinate bonds rarely, if ever, form highly crystalline material. In simplistic terms the reason behind this is that once a bond has been formed in building a coordination polymer, if that bond is inert, it is difficult for that bond to be broken under conventional crystallisation conditions. Thus, mistakes in the coordination polymer formation cannot be readily corrected, typically leading to small crystallites or microcrystalline material. 

The second consideration when attempting to prepare crystals of coordination polymers is the crystallisation condition used. As stated above, recrystallisation approaches are not usually appropriate for the growth of coordination polymer systems, due to solubility reasons. Another method of crystallisation often used for molecular systems, that of vapor diffusion of an anti-solvent into solutions of the product, is also rarely possible for coordination polymer systems. As with traditional recrystallisation this is due to the insolubility of the product material and consequently to the difficulty of obtaining solutions of either the coordination polymer or its constituent parts. 

Materials based on 4,4 -bipyridine are one of most extensively explored coordination polymer systems. Reaction of Ni(N03)2 and 4,4 -bipyridine affords two products [Ni2(N03)4(4,4 -bipyridine)3]oo depending upon whether the reaction is conducted in methanol or ethanol solution[187]. In both materials the charge on Ni(II) is neutralised by coordinated nitrate anions, and the resultant Ni(N03)2 centre is connected by three ditopic 4,4-bipy linkers to form a T-shaped node (Fig. 19a). In the product obtained from methanol solution, T-shaped units form a 1-D ladder structure, and these ladders run parallel (material M). In the case of the product isolated from ethanol, T-shaped units give an interlocked 2-D bilayer product (material E). The common characteristics for these two materials are that the interactions between the ladders or the layers are relatively weak and this results in dynamic stmctures in which the interactions and distances between ladders or layers are modulated as adsorbate molecules enter the framework [187]. Although... 

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. 

Dobry and Boyer-Kawenoki have investigated a number of solvent-polymer-polymer systems, with results which confirm all of the qualitative predictions of the theory. Fig. 125 shows their experimental results for the benzene-rubber-polystyrene system with coordinates expressed in weight percent. The symmetry resulting from the stipulations X2 = Xs and xi2 = Xi3 in the case treated theoretically... 

A polymeric structure can be generated by intermolecular coordination of a metalloporphyrin equipped with a suitable ligand. Fleischer (18,90) solved the crystal structure of a zinc porphyrin with one 4-pyridyl group attached at the meso position. In the solid state, a coordination polymer is formed (75, Fig. 30). The authors reported that the open polymer persists in solution, but the association constant of 3 x 104 M 1 is rather high, and it seems more likely, in the light of later work on closed macrocycles (see above), that this system forms a cyclic tetramer at 10-3 M concentrations in solution (71,73). 

This hypothesis is further supported by the effect on the absorption behavior of molecular oxygen of additives that destroy the a-helix of PLL. When we added to the PLL system non-coordinative polymers, which do not act as ligands but are able to interact with PLL to form polymer aggregates, the cooperative parameter decreased to unity, as shown in Table 11. In these cases, it was found that the a-heli-cal conformation was made unstable by the formation of polymer aggregates111 ... 

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