Claudia Heindl

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

Size-Determining Dependencies in Supramolecular Organometallic Host–Guest Chemistry

Treatment of the pentaphosphaferrocene [Cp*Fe(η(5)-P(5))] with Cu(I) halides in the presence of different templates leads to novel fullerene-like spherical molecules that serve as hosts for the templates. If ferrocene is used as the template the 80-vertex ball [Cp(2)Fe]@[{Cp*Fe(η(5)-P(5))}(12){CuCl}(20)] (4), with an overall icosahedral C(80) topological symmetry, is obtained. This result shows the ability of ferrocene to compete successfully with the internal template of the reaction system [Cp*Fe(η(5)-P(5))], although the 90-vertex ball [{Cp*Fe(η(5):η(1):η(1):η(1):η(1):η(1)-P(5))}(12)(CuCl)(10)(Cu(2)Cl(3))(5){Cu(CH(3)CN)(2)}(5)] (2 a) containing pentaphosphaferrocene as a guest is also formed as a byproduct. With use of the triple-decker sandwich complex [(CpCr)(2)(μ,η(5)-As(5))] as a template the reaction between [Cp*Fe(η(5)-P(5))] and CuBr leads to the 90-vertex ball [(CpCr)(2)(μ,η(5)-As(5))]@[{Cp*Fe(η(5)-P(5))}(12){CuBr}(10){Cu(2)Br(3)}(5){Cu(CH(3) CN)(2)}(5)] (6), in which the complete molecule acts as a template. However, if the corresponding reaction is instead carried out with CuCl, cleavage of the triple-decker complex is found and the 80-vertex ball [CpCr(η(5)-As(5))]@[{Cp*Fe(η(5)-P(5))}(12){CuCl}(20)] (5) is obtained. This accommodates as its guest [CpCr(η(5)-As(5))], which has only 16 valence electrons in a triplet ground state and is not known as a free molecule. The triple-decker sandwich complex [(CpCr)(2)(μ,η(5)-As(5))] requires 53.1 kcal mol(-1) to undergo cleavage (as calculated by DFT methods) and therefore this reaction is clearly endothermic. All new products have been characterized by single-crystal X-ray crystallography. A favoured orientation of the guest molecules inside the host cages has been identified, which shows π⋅⋅⋅π stacking of the five-membered rings (Cp and cyclo-As(5)) of the guests and the cyclo-P(5) rings of the nanoballs of the hosts.

Giant Rugby Ball [{CpBnFe(η5-P5)}24Cu96Br96] Derived from Pentaphosphaferrocene and CuBr2

The self-assembly of [CpBnFe(η5-P5)] (CpBn = η5-C5(CH2Ph)5) with CuBr2 leads to the formation of an unprecedented rugby ball-shaped supramolecule consisting of 24 units of the pentaphosphaferrocene and an extended CuBr framework, which does not follow the fullerene topology. The resulting scaffold of 312 noncarbon atoms reveals three different coordination modes of the cyclo-P5 ligand including a novel π-coordination. The outer dimensions of 3.7 × 4.6 nm of the sphere approach the range of the size of proteins. With a value of 32.1 nm3, it is 62 times larger in volume than a C60 molecule. Surprisingly, this giant rugby ball is also slightly soluble in CH2Cl2.

A Nano‐sized Supramolecule Beyond the Fullerene Topology

The reaction of [CpBnFe(η5-P5)] (1) (CpBn=η5-C5(CH2Ph)5) with CuI selectively yields a novel spherical supramolecule (CH2Cl2)3.4@[(CpBnFeP5)12{CuI}54(MeCN)1.46] (2) showing a linkage of the scaffold atoms which is beyond the Fullerene topology. Its extended CuI framework reveals an outer diameter of 3.7 nm—a size that has not been reached before using five-fold symmetric building blocks. Furthermore, 2 shows a remarkable solubility in CH2Cl2, and NMR spectroscopy reveals that the scaffold of the supramolecule remains intact in solution. In addition, a novel 2D polymer [{CpBnFe(η5-P5)}2{Cu6(μ-I)2(μ3-I)4}]n (3) with an uncommon structural motif was isolated. Its formation can be avoided by using a large excess of CuI in the reaction with 1.

Tunable Porosities and Shapes of Fullerene‐Like Spheres

The formation of reversible switchable nanostructures monitored by solution and solid-state methods is still a challenge in supramolecular chemistry. By a comprehensive solid state and solution study we demonstrate the potential of the fivefold symmetrical building block of pentaphosphaferrocene in combination with CuI halides to switch between spheres of different porosity and shape. With increasing amount of CuX, the structures of the formed supramolecules change from incomplete to complete spherically shaped fullerene-like assemblies possessing an Ih-C80 topology at one side and to a tetrahedral-structured aggregate at the other. In the solid state, the formed nano-sized aggregates reach an outer diameter of 3.14 and 3.56 nm, respectively. This feature is used to reversibly encapsulate and release guest molecules in solution.

Discrete and extended supersandwich structures based on weak interactions between phosphorus and mercury.

Supersized mercury: Adducts with polymeric (left) or discrete supersandwich structures (right) form from mixtures of the trinuclear mercury complex [(o-C(6)F(4)Hg)(3)] (A) with the triple-decker complex [(CpMo)(2)(μ-η(6):η(6)-P(6))] (B) in the solid state. This arrangement arises from P···Hg interactions between opposing atoms of the P(6) units and the Hg(3) units (see picture; P-purple, Hg-orange, F-green, Mo-red, C-gray).

A cyclo-P6 Ligand Complex for the Formation of Planar 2D Layers.

Abstract The all‐phosphorus analogue of benzene, stabilized as middle deck in triple‐decker complexes, is a promising building block for the formation of graphene‐like sheet structures. The reaction of [(CpMo)2(μ,η6:η6‐P6)] (1) with CuX (X=Br, I) leads to self‐assembly into unprecedented 2D networks of [{(CpMo)2P6}(CuBr)4]n (2) and [{(CpMo)2P6}(CuI)2]n (3). X‐ray structural analyses show a unique deformation of the previously planar cyclo‐P6 ligand. This includes bending of one P atom in an envelope conformation as well as a bisallylic distortion. Despite this, 2 and 3 form planar layers. Both polymers were furthermore analyzed by 31P{1H} magic angle spinning (MAS) NMR spectroscopy, revealing signals corresponding to six non‐equivalent phosphorus sites. A peak assignment is achieved by 2D correlation spectra as well as by DFT chemical shift computations.

Crystal structure of a giant supramolecule encapsulating o-carborane

Abstract A giant quasi-spherical supramolecule [{Cp*Fe(η5-P5)}12(CuBr)18.8] encapsulating o-carborane was prepared by the reaction between CuBr and [Cp*Fe(η5-P5)] (Cp* = pentamethylcyclopentadienyl) and structurally characterized. The crystal structure can be treated as solid solution of various [(Cp*Fe(η5-P5))12(CuBr)n] supramolecules with n = 18–20. Two crystals obtained from different syntheses differ in the location of vacant CuBr positions and solvent molecules. Surprisingly, the supramolecules are packed following the low-density reo (ReO3) packing.

cyclo-P4 Building Blocks: Achieving Non-Classical Fullerene Topology and Beyond

Abstract The cyclo‐P4 complexes [CpRTa(CO)2(η4‐P4)] (CpR: Cp′′=1,3‐C5H3tBu2, Cp′′′=1,2,4‐C5H2tBu3) turned out to be predestined for the formation of hollow spherical supramolecules with non‐classical fullerene‐like topology. The resulting assemblies constructed with CuX (X=Cl, Br) showed a highly symmetric 32‐vertex core of solely four‐ and six‐membered rings. In some supramolecules, the inner cavity was occupied by an additional CuX unit. On the other hand, using CuI, two different supramolecules with either peanut‐ or pear‐like shapes and outer diameters in the range of 2–2.5 nm were isolated. Furthermore, the spherical supramolecules containing Cp′′′ ligands at tantalum are soluble in CH2Cl2. NMR spectroscopic investigations in solution revealed the formation of isomeric supramolecules owing to the steric hindrance caused by the third tBu group on the Cp′′′ ligand. In addition, a 2D coordination polymer was obtained and structurally characterized.

Progress in Polyarsolyl Chemistry

Abstract The synthesis of heteroatom analogues of the cyclopentadienyl anion Cp− is a fascinating and challenging field of research. The replacement of methine moieties by phosphorus is well investigated for the synthesis of mono‐, tri‐ and pentaphospholyl ligands. On the other hand, arsenic derivatives are rare and 1,2,4‐triarsolyl and tetraarsolyl salts are unknown. Herein, we report on the synthesis of Cs[E3C2(trip)2] (1 a: E=P; 1 b: E=As; trip=2,4,6‐triisopropylphenyl) and Cs[E4C(trip)] (2 a: E=P; 2 b: E=As). Compound 1 b represents the first 1,2,4‐triarsolyl and 2 b the first tetraarsolyl anion. All salts are obtained in one‐pot syntheses using E(SiMe3)3, 2,4,6‐triisopropylbenzoyl chloride and CsF. The products 1 a⋅2 C4H8O2, 2 a⋅Et2O and 2 b⋅3 C4H8O2 were characterized by X‐ray structural analysis, which revealed planar heterocycles. Nucleus‐independent chemical shifts (NICS) confirmed the aromaticity of these anions. Notably, compound 2 a⋅Et2O is only the second tetraphospholyl ligand which is structurally characterized.