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Coordination polymers

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Coordination polymer is the term given in inorganic chemistry to a metal coordination compound where a ligand bridges between metal centres, where each metal centre binds to more than one ligand to create a infinite array of metal centres. The majority of common halides and oxides are coordination polymers. More conventionally, the term coordination polymer is reserved for compounds where the metals are bridged by polyatomic ligands, such as cyanide, carboxylates. One of the first coordination compounds ever studied systematically, Prussian Blue, is a coordination polymer based on Fe-CN-Fe linkages.

Contents

Carboxylates

Examples of such materials include many of the metal salts of benzene-1,3,5-tricarboxylic acid (trimesic acid, BTC). This tricarboxylic acid has been one of the most popular bridging ligands used in the synthesis of these polymers. The first major paper on the topic of coordination polymers using BTC was by O. M. Yaghi, Hailian Li, and T. L. Groy in J. Am. Chem. Soc., 1996, 118(38), 9096 - 9101. The senior author on this paper was O. M. Yaghi who has since published a large amount on coordination polymers.

A series of solid polymers have been made via the reaction of this tri-acid with salts of cobalt, manganese, zinc, cadmium, copper, lead, uranium and other metals. Other di- and tricarboxylic acids have been used to form polymers. An example of a research group active in this field can be found at [link]

One rare but important pair of acids are 4,6-dinitro-5-hydroxyisophthalic acid and 2,4-dinitro-3-hydroxybenzoic acid which are formed by the reaction of a mixture of lead nitrate, nickel nitrate, pyridine and 5-hydroxyisophthalic acid inside an autoclave. The resulting acids then form mixed lead/nickel polymers. See Xiaoju Li, Rong Cao, Zhengang Guo and Jian Lü, Chemical Communications, 2006, 1938 - 1940 (DOI: 10.1039/b600684a).

It is the case that by using a non bridging ligand such as 2,2'-bipyridine on a first row transition metal (such as zinc, manganese or cobalt) a less cross linked polymer can be obtained than that which is formed by the reaction of BTC and the simple metal salt in the absence of the non bridging ligand. For example compare the solids in

M.J.Plater, M.R.St.J. Foreman, E. Coronado, C.J. Gomez-Garcia, A.M.Z. Slawin J.Chem.Soc.,Dalton Trans., 1999, 4209 (where 2,2'-bipy is used as an additive for forming solids from BTC and zinc, manganese and cobalt acetates) with the solids formed without bipy for examples see D. Cheng, M.A. Khan and R.P. Houser, Cryst. Growth Des., 2004, 4, 599-604.

A common type of solid formed using BTC is a layered solid where the benzene rings are coplannar with the planes. This is a layered solid which is similar to graphite in the fact that it is colvently bonded layers which are arranged on top of each other.

L.A. Borkowski and C.L. Cahill, Acta Crystallogr.,Sect.E:Struct.Rep.Online, 2004, 60, m198. This is a uranium containing polymer which has uranyl units linked by the BTC units.

Polypyridines

Another common synthetic method is to react a polypyridine whose geometry renders it unable to chelate a metal atoms with all its nitrogen atoms. For instance 4,4'-bipyridine can be used to form polymers with many metals. One of the most developed metals is cadmium, it is the case that cadmium nitrate reacts with a wide array of polypyridines to form different solids. One of the key developments is the use of exotic polypyridines where the pyridine rings are separated by covalent spacers such as prop-1,3-diyl groups.

F.A.A. Paz and J. Klinowski, Inorganic Chemistry, 2004, 43, 3948 described in the literature a coordination polymer which contains a polypyridine and trimesate groups.

Bridging at a single atom

Yet another method of forming coordination polymers is to use a donor atom which bears more than one lone-pair for instance this type of bridging has been seen in uranium fluorides and the lead carboxylates.

It is clear that the UF5 solid is a 1D coordination polymer

References

Illustrative discoveries

Jensen, K. A., "Nickel mercaptides", Z. anorg. Chem., 1944, volume 252, pages 227-33 (pioneering report on thiolato-bridged coordination polymer) Schmitz-Dumont, O.; Pilzecker, J. and Piepenbrink, H. F., "The amides of trivalent Cr and Co", Z. anorg. allgem. Chem., 1941, volume 248, pages 175-207 (discovery of NH2-bridged polymers). Ludi, A., "Prussian blue, an Inorganic Evergreen", Journal of Chemical Education 1981, volume 58, 1013.

Reviews of coordination polymers

 

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