Hofmann clathrate

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...

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. 

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... 

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). 

Figure 8.13 Schematic view of the structure of the Ni-Ni Hofmann clathrate with benzene as guest molecule. The planes of the benzene guest molecules are perpendicular to the plane of the metal ions cross-linked by C=N entities. The disordered NH3 groups are represented as a superposition of four crystallographically equivalent configurations. Reproduced with permission of the International Union of Crystallography.

Table 8.1 Free rotor 0> 1> rotational transitions of NH3 in various M-Ni-CgDe Hofmann clathrates at 2 K, after Ref 68, and M N(Hs) stretehing frequencies, after Ref 69.

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... 

PCPs have infinite networks with backbones constructed by metal ions as connectors and ligands as linkers, and form a family of inorganic and organic hybrid polymers. The structural integrity of the building units, which can be maintained throughout the reactions, allows for their use as modules in the assembly of extended structures. Werner complexes, /3-M(4-melhylpyridyl)4(NCS)2 (M = Ni(II) or Co(II)) [48], Prussian blue compounds [49-51], and Hofmann clathrates and their... 

---