Cluster Supramolecules

Cluster Supramolecular Chemistry

One of the fundamental underpinnings of the diverse interests of the Zheng Research Group (ZRG) is the chemistry of transition metal and lanthanide cluster species. Specifically, we are interested in preparing and utilizing polynuclear moieties as discrete molecular building blocks in supramolecular assemblies. In the case of the lanthanide cluster species, the stumbling blocks of lability and facile hydrolysis have hindered the development of reliable syntheses, resulting in a large disparity in the understanding of the chemistry of lanthanide clusters relative to their transition metal cousins. The principal challenge in this field remains to determine the "rules" of solution phase cluster synthesis and development of a systematic and reliable means of controlling dimension and nuclearity. The ZRG is at the forefront of this exciting area; we have begun to understand and control lanthanide hydrolysis and have created a number of clusters of predictable nuclearity via hydrolytic reactions. In addition to developing the fundamental chemistry of lanthanide clusters, the ZRG has concurrently pursued the application of a well-known transition metal cluster core, [Re₆(μ₃-Se)₈]²⁺ , as a robust and versatile supramolecular synthon.

Originally prepared as a structural analog to the superconducting Chevrel phases, the [Re₆(μ₃-Se)₈]²⁺ cluster core has recently received considerable attention from both the synthetic and physical chemistry communities. To date, the primary focus has been on the cluster core’s unique electro- and photochemical activity, as well as the preparation of novel solid state materials, including the "expanded" Prussian Blue analogs. Much of this work has used the O_h symmetric cyano-, halido-, and phosphine cluster derivatives, but our own work has been focused the various "site-differentiated" cluster derivatives of the general form [Re₆Se₈(PR₃)_nL_(6-n)]²⁺. These cluster derivatives have proven to be very chemically versatile (L may be anything from a halide ion to complex organic ligands) and particularly useful for generating supramolecular species with predictable dimension and architecture. By isolating a given stereoisomer of the phosphine derived cluster, one has in hand a very large, stereochemically well defined building block with a specific shape (e.g. A right angle for cis-[Re₆Se₈(PR₃)₄L₂]²⁺). We have developed two distinct synthetic methods that take advantage of the shapes of our molecular "bricks"; each is uses a distinct "mortar" and each brick/mortar combination is best suited to specific types of assembly.

The first method is referred to loosely as a direct cluster condensation reaction. This reaction relies on condensing cluster solvates with precise stoichiometric amounts of a pyridyl based ligand, L. The reaction takes the form:

(1) [Re₆(μ₃-Se)₈(PR₃)_n(MeCN)_6-n]²⁺+ L ÷ (6-n)[Re₆(μ₃-Se)₈(PR₃)_n (μ_(6-n)L)]^(6-n)2+ + (6-n) MeCN

The cluster condensation reactions have afforded several novel structural motifs, including discrete molecular squares (for host-guest applications), and star-shaped molecules which are linked by organic chromophores (figure 1). This reaction is quite reliable, and is best suited to the synthesis of molecular entities.

The second synthetic method for creating cluster-supported supramolecular motifs is a true self-assembly approach. Instead of directly binding the cluster derivatives, this method uses a given isomer’s geometry to direct the formation of secondary non-covalent interactions. The two non-covalent forces we have explored are the ubiquitous H-bond and secondary (with respect to primary Re^III coordination) metal ion coordination. These two syntheses are shown in scheme 1. This method allows us to formally synthesize monocluster species in solution, realizing the supramolecular species only upon self-assembly in the solid state. This approach offers the advantage of working with readily purified and soluble monocluster species while still allowing us to realize extended solids of variable dimension and geometry. This "non-covalent" approach appears to be best suited to extended arrays of clusters. An example of a chain of "fused squares" using Cd²⁺ and cis-[Re₆(μ₃-Se)₈(PPh₃)₄(4,4'-dipyridyl)₂]²⁺ is shown in figure 3.

All of these areas are being actively pursued in the ZRG, and several new and promising methods which combine the covalent and non-covalent syntheses are being developed.

Figures

Figure 1

Scheme 1

Figure 2

References

1. Wang, R.; Selby, H. D.; Liu, H.; Carducci, M. D.; Jin, T.; Anthis, J.; Zheng, Z.; Staples, R. J. "Halide-Templated Assembly of Polynuclear Lanthanide- HydroxoComplexes" Inorg. Chem. 2002, 41, 278-286 (featured on cover).

2. Zheng, Z.; Long, J. R.; Holm, R. H. "A Basis Set of Re₆Se₈ Cluster Building Blocks and Demonstration of Their Linking Capability: Directed Synthesis of an Re₁₂Se₁₆ Dicluster" J. Am. Chem. Soc. 1997, 119, 2163-2171.

3. Selby, H. D.; Zheng, Z.; Gray, T. G.; Holm, R. H. "Bridged Multiclusters Derived from the Face-Capped Octahedral [Re₆Se₈]²⁺ Cluster Core" Inorg. Chim. Acta 2001, 312, 205-208.

4. Roland, B. K.; Selby, H. D.; Carducci, M. D.; Zheng, Z. "Built to Order: Molecular Tinkertoys from The [Re₆(m3-Se)₈]²⁺ Clusters" J. Am. Chem. Soc. 2002,124, 3222-3223.

5. Selby, H. D.; Orto, P.; Zheng, Z. Cluster "Complexes as Ligands: A Supramolecular Approach to Porous Solids with Expanded Pores", in Submission, Angew. Chem. Int. Ed. Engl.

6. Zheng, Z.; Selby, H. D. "A Modular Crystal Engineering Approach to Coordination Polymers Supported by the Face-Capped [Re₆(m₃-Se)₈]²⁺ Clusters", in Submission, Polyhedron (special issue for ACS Symposium by Dunbar, Keller, and Stang).

7. Zheng, Z.; Selby, H. D.; Roland, R. K. "Hydrogen-Bonded Supramolecular Arrays of the Face-Capped [Re₆(m₃-Se)₈]²⁺ Clusters", to be submitted to Inorg.Chem.