Hofmann inclusion compounds

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... 

Investigations of cyanides and cyano complexes of Cd and Hg have augmented tremendously since about 1990, after detection of inclusion compounds of Cd(CN)2. A thorough review on transition-metal cyanides especially emphasizes the chemistry of inclusion compounds of both the Hofmann type (frameworks dominated by planar Ni(CN)4 building blocks) and the cyanocad-mate type (frameworks with tetrahedral Cd(CN)4 units).87 The structures of these inclusion compounds, but also of cyanides themselves, often topologically resemble the structures of minerals this aspect ( mineralomimetic chemistry ) is dealt with in a simultaneous survey.88 A more generic review of framework structures, with a section on cyanide inclusion compounds, is also to be mentioned.90... 

Hofmann reaction, 76 320. See also Hoffman reaction Hofmann rearrangement, 77 741 Hofmann-type inclusion compounds, 74 171-172... 

The history of inclusion compounds (1,2) dates back to 1823 when Michael Faraday reported the preparation of the clathrate hydrate of chlorine. Other early observations include the preparation of graphite intercalates in 1841, the 3-hydroquinone H2S clathrate in 1849, the choleic acids in 1885, the cyclodextrin inclusion compounds in 1891, and the Hofmann s clathrate in 1897. Later milestones of the development of inclusion compounds refer to the tri-o-thymotide benzene inclusion compound in 1914, phenol clathrates in 1935, and urea adducts in 1940. 

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... 

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... 

Almost all important contributions to the chemistry of Hofmann-type and related inclusion compounds and host cyanometal complexes were reviewed in the lit-erature." - ... 

Iw amoto, T. The Hofmann-Type and Related Inclusion Compounds. In Inclusion Compounds, I Atwood. J.L., Davies. J.E.D., MacNicol. D.D., Eds. Academic Press London, 1984 29-57. 

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... 

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. 

Channel Inclusion Compounds, p. 223 Chiral Guest Recognition, p. 236 Clathrate Hydrates, p. 274 Crown Ethers, p. 326 Crystalline Microporous Silicas, p. 380 Cyclodextrins, p. 398 Hofmann-Type Clathrates, p. 645 Hydrogen Bonding, p. 658 Hydroquinone, p. 679... 

The Hofmann-dabn-type inclusion compounds were prepared by the method described in the previous communication [6] in Table I the results of elemental analyses and the basal spacings estimated from the powder X-ray diffraction patterns are listed. They are not stable under ambient condition but the rates of decomposition are slow enough to be subjected to the elemental analyses if the specimens are kept in tightly capped vials some of them, marked with an asterisk in the Table I, are rather unstabe, however. 

Table I. Hofmann-dabn-type inclusion compounds Cd(CifHi2N2)Ni(CN) i. nG. Elemental analyses are given as Found/Calcd in % d refers to the basal...

T.Iwamoto The Hofmann-type and Related Inclusion Compounds (Inclusion Compounds v. 1, Ed. J.L.Atwood, J.E.D.Davies, and D.D.MacNicol) pp. 29-57. Academic Press (1984). 

One of the problems to be solved for the formation trend of Hofmann-(long-carbon-chain a,oo-diaminoalkane)-type series of inclusion compounds is to interpret the reason why Hofmann-dahxn-type host cannot accommodate xylidines as guest, which Hofmann-dabn-type host can do even with the shorter carbon chain in the full length than dahxn. In order to solve the problem we need more information about the crystal structures of the series. The structure analyses of Hofmann-dahxn-type m-and p-toluidine inclusion compounds are in progress. 

Many further cases of inclusion materials were discovered by happy accident during the next two centuries, for example, additional clathrate hydrates, the Hofmann clathrates, phenol inclusion compounds, Dianin s compound, urea tubulates, choleic acids, cyclodextrins, aud interpenetrated hydroquinone inclusion compounds. These substances remained problematic despite being the object of much painstaking scientific study. Mauy were unstable under ambient conditions and therefore it proved difficult to determine accurate ratios of their two components A and B. Furthermore, the substances did not follow the usual rules of covalent bonding, leading to their representation in the fonn (A)x (B)y. It was surmised that one component somehow trapped the other, but no experimental methods were available to analyze this phenomenon. 

While Wemer complexes are based on discrete coordination complexes, another type of coordination compound with a distinctive extension into 2D layers through utilization of ditopic bidentate ligands was developed and first reported by K. A. Hofmann in 1897, Ni(NH3)2Ni(CN)4 2(C6H6). Hofmann-type compounds also exhibit inclusion properties of suitably sized guest molecules. Iwamoto and coworkers prepared a number of derivatives of Hofmann clathrates (Figure 2). 

The original Hofmann clathrate may be modified in several ways to produce new inclusion compounds. First, the ammonia molecule may be changed for another amine, even a bi- or tridentate one. Second, the square planar... 

T. Iwamoto, The Hofmann-type and Related Inclusion Compounds, ... 

Over the past four years there has been considerable interest in the NMR of Hofmann-type and related metal-cyanide inclusion compounds, with regard to local order or disorder and the dynamics of both guest and host molecules. 

The structure of a Hofmann type inclusion compounds is shown in scheme (C). The reaction of (HI) and (IV) with oxidants such as Fe + in water results in a color change from pale violet to black. When single crystals are used as starting materials, the resulting products, retain the original crystal shape (pseudomorphs), but they lose their single crystal nature and under SEM microscopic examination show small cracks and other defects. The proper formulation of the oxidation products would be [Ni(CN)2NH3] (polymer)(A)x y where polymer=polypyrrole, polyaniline ... 

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. 

The enhanced degradation of polymer in the presence of OMMT is frequently explained in terms of the catalytic activity of acidic sites formed because of the Hofmann degradation of onium compounds [77, 78]. However, it should be emphasized that not only organomod-ifiers and products of their decomposition appear to have a catalytic effect on polymer degradation, but also acidic sites inherently present on the mineral surface and inclusions of transition metals should be considered as active catalytic sites - the latter could significantly enhance the degradation of the polymer matrix and all organic compounds in the nanocomposite material. 

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