Hofmann clathrates

The history of iaclusion compounds (1,2) dates back to 1823 when Michael Faraday reported the preparation of the clathrate hydrate of chlorine. Other early observations iaclude the preparation of graphite iatercalates ia 1841, the P-hydroquiaone H2S clathrate ia 1849, the choleic acids ia 1885, the cyclodexthn iaclusion compounds ia 1891, and the Hofmann s clathrate ia 1897. Later milestones of the development of iaclusion compounds refer to the tri-(9-thymotide benzene iaclusion compound ia 1914, pheaol clathrates ia 1935, and urea adducts ia 1940. 

Hofmann- and Werner-Type Inclusion Compounds. There is a wide range of clathrates having as the host component inorganic coordination compounds represented by the general formulae M(NH2)2 (CN) and X2Y4. The first formula is typical of Hofmaim-type clathrates... 

Keywords Spin transition Coordination polymers Bistability Hofmann-like clathrates Supramolecular networks... 

Fig. 13 (Left) Perspective view of the 2D Hofmann clathrate Ni(NH3)2[Ni(CN)4] -2G. (Right) Perspective view of the 3D Hofmann clathrate Cd(l,4-diaminobutane) [Ni(CN)4] J-benzene...

Classifying particles, in filtration, 11 326 Class I hybrids, 13 536, 543, 544 Class II hybrids, 13 536, 543 Clastogenesis, 25 206 Clathrate hydrates, 14 170—171 Clathrate receptor chemistry, 16 797 Clathrates, 12 374 14 159, 170-182 formation of, 10 633-635 26 869 Hofmann- and Werner-type, 14 171-172 phenol-type, 14 180 tri-o-thymotide, 14 179 Claus catalysts... 

While this general topic has an ancient lineage, it is an area of intense current research. What is now described as the Hofmann clathrate was first reported in 1897 [8]. However, the structure was not known until 50 years later when reported by Powell and coworkers [9]. Single crystal X-ray diffraction showed the structure in Fig. 5, in which a two-dimensional array of nickel cyanide encapsulates trapped benzene molecules. 

The Hofmann clathrate compounds of general formula Cu(NH3)2 M(CN)4 2G where M = Ni +, Pd " ", or Pt and G is an organic molecule have been studied (523). In the case of the magnetically dilute Cu/Cd complex, h.f.s. are detected and the g values indicate a tetragonal compression with the unpaired electron in the orbital. 

The chemistry of inclusion compounds also looks back on a lively history There are many events of significance in the area of inclusion chemistry till the middle of the twentieth century including the discovery of new inclusion compounds and hosts (Fig. 1), among them the graphite intercalates, the P-quinol and cyclodextrin inclusion compounds, the Hofmann-type clathrates as well as the inclusion compounds of tri-o-thymotide, Dianin s compound, the choleic acids, of phenols, of urea and others specified in comprehensive monographs... 

Fig. 1 A view of Hofmann-type benzene clathrate [M(NH3)2-MXCN)4]-2C6H6 diamminemetalM(II) (right upper) and tetracyanometallateM (n) (right lower) as building blocks of the host.

So-called Werner clathrates are another prominent series of clathrate compounds formed between a metal complex host and an aromatic guest molecule. The name Werner originates from A. Werner, who established the coordination theory of metal complex compounds. The host of the series has a general formula [M(NCS)2(4-Mepy)4], and in fact, the series is obtained conceptually by replacement of C N and NH3 in the Hofmann-type host by SCN and 4-Mepy (4-methylpyridine), respectively. This was done in order to increase the inclusion capability for bulkier substituted aromatic molecules, in particular, those related to the petroleum industry. 

A number of compounds with chemical compositions that are similar to that of the original Hofmann-type clathrates were reported. However, as usual for other inclusion compounds, similarity in composition does not necessarily co relate with similarity of structure. For example, in the [Cd(pn)Ni(CN)4] G series with aliphatic guests, the host structure is entirely different from the Hofmann-pn-type pyrrole clathrate [Cd(pn)Ni(CN)4]. (3/ 2)C4H5N. Two subseries of structures were observed for the channel cavities, one being a snake-like extension similar to chat observed for the urea-host inclusion... 

Numerous instances of isostructurality among Hofmann clathrates. M(NH3)2M (CN)4 2G, are known, the high frequency of occurrence probably owing to the fact that this family of inclusion compounds has been studied extensively over a long period. In addition, isostructurality can manifest itself in different ways, e.g., with either M. M fixed and guest G variable or with M, M variable and a common guest. An example in the former category... 

Kitazawa, T. Sato. Y. Gomi. Y. Takahashi. M. Takeda, M. Crystal structure and Mdssbauer spectra of Hofmann-type pyrrole clathrate, Fe(NH3)2Ni(CN)4 2C4H5N. Mol. Cryst. Liq. Cryst. Sci. Technol. Sect A 1996,286.431-436. 

Concepts in Crystal Engineering, p. 319 Crystal Engineering with Hydrogen Bonds, p. 357 Crystal Structure Prediction, p. 371 Cfystalline Microporous Silicas, p. 380 Dye Inclusion Crystals, p. 497 Hofmann-Type Clathrates, p. 645 Mineralomimetic Structures, p. 868 Molecular Squares, Boxes, and Cubes, p. 909 Organic Zeolites, p. 996 Soft/Smart Materials, p. 1302... 

Chemical Topology, p. 229 Concepts in Crystal Engineering, p. 2 9 Crystalline Microporous Silicas, p. 380 Hofmann-Type Clathrates, p. 645 Interpenetration, p. 135... 

Hofmann-type clathrates Werner clathrates clathrate hydrates inclusion compounds of urea, thiourea, and selenourea cyclodextrins calixarenes gossypol hexa-hosts hydroquinone phenol and Dianin s compounds graphite intercalates natural and pillared clays and others. Such studies contributed to the birth of supramolecular chemistry, with relevance to a new understanding of the world of materials that was emphasized with a Nobel Prize. 

Nishikiori. S. Ratcliffe, C.I. Ripmeester, J.A. " Cd NMR studies of Hofmann type clathrates and related compounds ... 

---