Maria Neumeier

Stabilization of Tetrahedral P4 and As4 Molecules as Guests in Polymeric and Spherical Environments

Chemistry as a science has originated from the exploration and handling of native elements such as sulfur or noble metals that allowed the formulation of the crucial concept of a chemical element. Despite great achievements of inorganic chemistry in the last century, the structural features of some simple substances are still not explicitly clear. It is remarkable that modern X-ray crystallography succeeded in crystal structure determination of proteins containing several thousands of atoms, yet it still faces obstacles in the characterization of some elementary compounds. For example, the extremely high aggressiveness of fluorine gas that was discovered in 1886 impeded its structural characterization for 78 years. Another barrier is the instability and chemical reactivity of allotropic modifications such as O3, whose crystal structure was not revealed until 2001, or the molecular allotropes of phosphorus and arsenic. The high dynamic motion of tetrahedral P4 molecules in white phosphorus led to a complete disorder of the cubic a-P4 phase under ambient conditions. To overcome this problem and obtain a convincing X-ray structural determination, single crystals of the ordered b-P4 phase have to be grown at temperatures below 77 8C. Arsenic exists in three allotropic modifications of which yellow arsenic, consisting of As4 tetrahedral molecules, is the most toxic and the least stable one. It can be obtained in a time-consuming synthesis by heating gray arsenic to 750 8C. The emerging As4 is taken away by a constant flow of a carrier gas and can be discharged into a hot solvent. In contrast to white phosphorus, yellow arsenic cannot be stored as a solid. It is surprisingly poorly soluble in common organic solvents, and it readily polymerizes under ambient conditions to gray arsenic, especially when exposed to light or X-rays. Hence, until now no solid-state structure of yellow arsenic is known. Moreover, traces of gray arsenic accelerate the polymerization of As4 even in solution. Yet, only scarce facts regarding its reactivity or coordination behavior are known. One of the ways to stabilize such unstable molecules is to include them as a guest in a molecular container or polymeric matrix. Oxidation of P4 in air was shown to be prevented by inclusion into the cavity of a supramolecular arrangement of a tetranuclear iron complex. Moreover, the co-crystallization of P4 in the lattice of solid C60 was reported. [8] Recently, Fujita et al. presented an elegant method for the X-ray structural characterization of organic compounds, only available in nanogram scale, based on their inclusion into singlecrystalline, porous 3D coordination polymers. Furthermore, our group succeeded in the stabilization of the unstable paramagnetic 16-electron complex, [Cp*Cr(h-As5)], embedded as a guest in the giant [Cu20Cl20{Cp*Fe(h -P5)}12] molecule (Cp* = h-C5Me5). [10] We reasoned that the use of host molecules could not only enhance the stability of the E4 (E = P, As) molecules, especially of As4, but would also decrease their molecular motion in the solid state. We have reported that the system [Cp*Fe(h-P5)] and Cu -halides forms either polymeric structures or large fullerene-like spherical molecules capable of encapsulating guest molecules and, thus perhaps, the E4 tetrahedra themselves. Herein we present the synthesis and X-ray molecular and crystal structure of polymeric host compounds that contain intact E4 tetrahedra as guests. Furthermore we show that [Ag(h-As4)2] [pftb] (pftb = {Al(OC(CF3)3)4}) [5e] can be utilized for the release of As4 as remarkably light-stable and highly concentrated solutions, making it an ideal storage medium for yellow arsenic. Finally these As4 solutions, as well as solutions of P4, were used to build up spherical macromolecules containing intact E4 tetrahedra as guest molecules. In the presence of P4 or As4, the reaction of CuCl with [Cp*Fe(h-P5)] leads to the formation of the isostructural compounds [Cu2Cl2{Cp*Fe(h -P5)}2]1·(P4)n (1) and [Cu2Cl2{Cp*Fe(h-P5)}2]1·(0.75As4)n (2), in which the tetrahedral voids are filled by perfectly adjusted E4 molecules. Surprisingly, the crystals of 1 and 2 are lightand air-stable for days and are insoluble in common solvents. Crystal-structure analysis reveals that in 1 all the voids are totally occupied by P4, while in 2 the As4 molecules statistically occupy 75 % of the available sites (Figure 1) probably a result of the low and rapidly decreasing concentration of As4 in the reaction medium. The E4 tetrahedra are fixed between the polymeric chains by four pairs of E···P(P5) intermolecular contacts of 3.98 and 4.00 in 1 and 3.98 and 4.04 in 2 (Figure 1), together with [*] Dr. C. Schwarzmaier, Dr. A. Schindler, C. Heindl, S. Scheuermayer, Dr. M. Neumeier, Prof. Dr. R. Gschwind, Prof. Dr. M. Scheer Universit t Regensburg 93040 Regensburg (Germany) E-mail: Manfred.Scheer@chemie.uni-regensburg.de

Detection of the Elusive Highly Charged Zintl Ions Si44− and Sn44− in Liquid Ammonia by NMR Spectroscopy

Its detected! The existence of the prototypical Zintl ions Tt4 (4-) (Tt=Group 14 element), which are isoelectronic to the P4 molecule, used to be confined to the solid state. Recently, circumstantial evidence for a solution chemistry in liquid ammonia emerged. Direct spectroscopic observation of the dissolved anions has now been achieved. In the case of Si4 (4-) , this is the first solution detection of any homoatomic silicide.

Elongated Gilman cuprates: the key to different reactivities of cyano- and iodocuprates.

In the past the long-standing and very controversial discussion about a special reactivity of cyano- versus iodocuprates concentrated on the existence of higher-order cuprate structures. Later on numerous structural investigations proved the structural equivalence of iodo and cyano Gilman cuprates and their subsequential intermediates. For dimethylcuprates similar reactivities were also shown. However, the reports about higher reactivities of cyanocuprates survived obstinately in many synthetic working groups. In this study we present an alternative structural difference between cyano- and iodocuprates, which is in agreement with the results of both sides. The key is the potential incorporation of alkyl copper in iodo but not in cyano Gilman cuprates during the reaction. In the example of cuprates with a highly soluble substituent (R = Me3SiCH2) we show that in the case of iodocuprates during the reaction several copper-rich complexes are formed, which consume additional iodocuprate and provide lower reactivities. To confirm this, a variety of highly soluble copper-rich complexes were synthesized, and their molecular formulas, the position of the equilibriums, their monomers and their aggregation trends were investigated by NMR spectroscopic methods revealing extended iodo Gilman cuprates. In addition, the effect of these copper-rich complexes on the yields of cross-coupling reactions with an alkyl halide was tested, resulting in reduced yields for iodocuprates. Thus, this study gives an explanation for the thus far confusing results of both similar and different reactivities of cyano- and iodocuprates. In the case of small substituents the produced alkyl copper precipitates and similar reactivities are observed. However, iodocuprates with large substituents are able to incorporate alkyl copper units. The resulting copper-rich species have less polarized alkyl groups, i.e. gradually reduced reactivities.

Stability and Conversion of Tin Zintl Anions in Liquid Ammonia Investigated by NMR Spectroscopy

Homoatomic polyanions of post-transition main-group metals, namely, Zintl anions, are precast in analogous Zintl phases and can react in solution to form new materials. Despite comprehensible reaction approaches, the formed products cannot be planned in advance, as hitherto undetected and therefore disregarded side reactions take place. The outcomes and interpretations of the reactions of Zintl anions are so far based mainly on crystal structures, which only allow characterization of the product that has the lowest solubility. Here we present the results of our investigation of the stability of highly charged tin Zintl anions in liquid ammonia, which is not exclusively based on solution effects but also on the oxidative influence of the solvent. This allows for a deeper understanding of the ongoing processes in solution and opens doors to the directed synthesis of transition metal complexes of Sn4 (4-) , here shown by its reactivity towards MesCu.