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Over the last two decades, two basic synthetic strategies have emerged in the preparation of cage-shaped metal complexes: the directional-bonding approach, and the symmetry-interaction approach. Both these strategies use directional, rigid ligands, so that unwanted stoichiometries or geometries could be avoided. Our approach toward assemblies with large internal cavities with specific properties is based on: 1) increasing the size of the constituents of the cage to generate more internal space and 2) incorporating specific biding sites into the cage constituents to allow selective inclusion of the desired guest materials within the cage. These two conditions demand flexible ligands, because increasing the size of rigid ligands would create solubility problems and any specific binding site should possess a certain degree of flexibility if it is to accommodate a guest. Our concept is depicted in Figure 1: by grafting on a recognition site groups with donor properties, the cage precursors could be arranged in the proper orientation to form a cage by aligning them via pseudorotaxane formation.

Figure 1. Cartoon representation of supramolecular preorganization

of two cage precursors in the proper arrangement as to form cage-

like assemblies via pseudorotaxane formation: red circles – recognition

sites; blue spheres – donor sets for further complexation.

Initially we studied several systems (ligands and their metal complexes) predisposed to supramolecular interactions (hydrogen bonding and π-π stacking) to determine the best candidates for this process. We performed structural and statistical analysis on π-π stacking interactions on various ligands. Also, in the course of our investigations we developed new axles that could interact with various crown ethers to form anionic pseudorotaxanes with donor capabilities, and we studied the possibility to obtain neutral metal complexes starting upon their coordination to metal centers. In the course of our investigations, an interesting “hetero” [4]pseudorotaxane (see Figure 2), consisting of one axle having different recognition sites threaded by three crown ethers of different type, was isolated and its characterization and further use in coordination chemistry is currently ongoing.

Figure 2. Space filling representation of an NDI/NH₂⁺ based [4]pseudorotaxane.

These anionic pseudorotaxanes of the type NDI[CH₂CH₂OCH₂CH₂O(An)PS₂-NH₄]₂Ì15-DN38C10 (NDI= naphthalenediimide, An=anisole) and NDI[CH₂CH₂CH₂C(O)O-NH₄]₂Ì15-DN38C10 form, upon coordination to metal centers, neutral compounds, thus opening the way toward neutral Metal-Organic-Rotaxane-Frameworks. Our current research is focused on using the knowledge gained studying these systems to synthesize more complex (both cage-like and infinite 3-dimensional) architectures. The results made possible by the PRF grant were gathered into one published paper, a second one currently under consideration and two more to be submitted soon.

The broader impact of the PRF grant stems from the integration of research and education, which is one of the PI's highest focuses. From a teaching standpoint, the purpose of research is to introduce the student to the world of goal-oriented research with the primary aims of the student developing advanced laboratory techniques, a mature attitude toward experimental success and/or failure, and accumulating a large variety of skills for their subsequent scientific careers.

Results motivate students more than a flamboyant description of an interesting idea. As the students obtained results on their own, they discovered that doing research is exciting and not just “something that they have to do in order to graduate”. Using the grant funds, one undergraduate student received in 2008 summer support to work on the project funded by PRF and two students were supported during the summer of 2009. Their research achievements strengthen their chances to be accepted in good master and/or doctoral programs. In fact, one of this PI's former student who worked on this project and graduated in 2010 was accepted into the Chemistry Ph.D. program at Miami University, Ohio and one former undergraduate who received his BS in spring 2010 will continue to work with the PI during his masters studies, with plans for continuing his education by enrolling into a PhD program.

The funds from PRF also made possible the participation of undergraduate students to national and local conferences and symposia. For example, in August 2008, one undergraduate student presented his results at the 236th ACS National Meeting, Philadelphia, PA (the undergraduate poster session). Further, in April 2009, two undergraduate students presented their work at the Undergraduate Research Symposium, held at the University of Illinois at Urbana-Champaign. The students clearly benefited from these experiences as they developed their scientific communication skills and gained confidence in defending their research results in front of their peers.