Tris-bipy cryptates

Numerous macrocyclic and macropolycyclic ligands featuring subheterocyclic rings such as pyridine, furan or thiophene have been investigated [2.70] among which one may, for instance, cite the cyclic hexapyridine torands (see 19) [2.39] and the cryptands containing pyridine, 2,2 -bipyridine (bipy), 9,10-phenanthroline (phen) etc. units [2.56,2.57,2.71-2.73]. The [Na+ c tris-bipy] cryptate 20 [2.71] and especially lanthanide complexes of the same class have been extensively studied [2.74, 2.75] (see also Sect. 8.2). 

The species that results from the reductive electrocrystallization of the sodium complex of tris-bipy cryptate, the schematic structure of which is shown in figure 2, has been called a "cryptatium." The name was chosen to express the dual nature of its procedence, since it is part cryptate and part sodium metal. It must be stressed that this cryptatium material is electroneutral, thus forming an expanded atom structure. Figure 2 also shows the schematic structure of an electride and that of a simple sodium atom, to illustrate two additional and extreme situations. In one, the electride, the complexation of the metal ion by the cryptand is so strong that the electron is essentially expelled from its interaction with the cationic center. In the other, the simple sodium atom, the outermost electron resides in an s orbital of the metallic center. Cryptatium thus represents an in-between situation, where the electron is not totally expelled from its interaction with the cation but it does not reside on the cation either, but rather on the ligand. The result is an expanded-metal type structure, where the electron is localized in the ligand structure. 

That such cryptate electroreductions are capable of yielding novel materials was shown to be the case with the sodium complex of tris-bipy. The properties of this material will now be briefly summarized. 

The cyclic voltammetric behavior of a series of bipy and related cryptates was recently reported. The structures of the specific complexes that were studied are presented in figure 4. The Na", Ca ", and La " complexes of 1, the tris-bipy cryptand. 

So far five new electrocrystalline materials have been discussed. These are the sodium tris-bipy ciyptate, [1-Na]°, the three [M(bipy)3] materials, with M = Fe, Ru, and Os, and [Ru(binap-2)3](PF5). Although several others have been attempted, only one more has been successfully electrocrystallized. This system is [Ru(terpy)2]°, where terpy stands for the terpyridyl ligand. However, no measurements have yet been performed with this material. No other cryptate has been successfully electrocrystallized, although most of the ones listed in figure 4 have been attempted. 

Since the concept of endohedral fullerenes is important in the context of the current presentation, the CPK structure of C50 is presented in figure 1 next to the structure of a cryptate, in this case the lanthanum complex of tris-bipyridyl cryptand, [La(bipy)3]. For the sake of the argument being presented here, assume that the 60 is really an endohedral complex of some metallic ion or ions. 

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