Supramolecular Chemistry Involving Anions and Anion-pi Interactions

Abstract

Research in the Supramolecular Chemistry of Anions is a newer project in the Dunbar Group. The project has developed into a highly interdisciplinary endeavor, encompassing coordination chemistry, computational chemistry, and, biochemistry. Anion-π interactions, i.e., the noncovalent forces between electron-deficient aromatic systems and anions, have been relatively unexplored as compared to cation-pi interactions, primarily due to the counter-intuitive nature of aromatic rings being attracted to a negative charge. The vital role of anions in many key chemical and biological processes and the involvement of pi rings in molecular anion recognition and transport processes, however, indicate that anion-pi contacts may be prominent players in fields as diverse as medicine and environmental chemistry. Our tutorial review in Chemical Society Reviews presents a good overview of the aims and scope of the field.

Content

Coordination-driven self-assembly harnesses the structural versatility of metal ions and the directionality of metal-ligand interactions to promote the self-assembly of metallosupramolecular architectures, often endowed with unusual host-guest properties for applications such as sensing and catalysis. The outcome of reactions can be controlled by many factors including precursor concentration and the presence of exogenous molecules such as solvent, cations or anions that act as directing elements to selectively stabilize a desired assembly via non-covalent interactions during a delicately balanced thermodynamically controlled process. We are working towards gaining a deeper understanding of the role and strength of a newly recognized player in supramolecular chemistry, namely anion-π interactions with electropositive aromatic molecules. Our targets are cyclic metal complexes with bridging heterocyclic nitrogen ligands, specifically those with central tetrazine rings. Our results constitute excellent starting points for probing anion-π interactions in polygonal molecules that exhibit high stabilities in solution but which exhibit remarkable flexibility that allows them to be interconverted by addition of different anions. Reactions triggered by the capture or release of anions can be used to alter the physical and chemical properties of the compounds and to tune the ring size as evidenced by NMR spectroscopy and mass spectrometric studies.
Electrochemical data reveal rich, reversible redox chemistry involving the sequential removal of electrons from square and pentagon molecules with encapsulated guest anions that make close contacts with the central ring of the tetrazine ligands. Conversely, sequestration of the anion or addition of electrons destroys the cyclic unit due to the destabilizing effect that removing the anion template or adding charge has on the anion-π interactions. The lack of “empty” cavities in these metallacycles is taken to be a sign of the critical need for a caged anion, but comprehensive studies are required to support this contention. We plan to vary the metal ion from late 3d to earlier and 4d and 5d transition metals and to alter the electronic and steric effects of the tetrazine ligands. The collective data from solid-state, solution, gas phase and computational studies will be used to assess the strength of anion-pi forces and to assist in future experiments aimed at predicting structures. A second objective of the research is to capitalize on the stability of the pentagon molecules prepared in our labs, a virtually unexplored architecture for the design of large polyhedra. The use of our five-membered ring molecules as building blocks is possible because each metal vertex is equipped with two easily substituted solvent molecules. The groundwork for making large “superstructures” with exquisite control of symmetry has been laid by Stang and Fujita, but our approach is unique in that the faces of the polyhedra will be decorated with anion-templated metallacycles. A third aim is a molecular magnets application, namely we take advantage of the ease of reduction of tetrazine ligands to prepare the radical forms to act as bridges between paramagnetic metal ions. Both neutral and radical bridged dinuclear metal complexes are being prepared with a focus on mixed-valence combinations which offer high potential for strong electronic and magnetic coupling due to double exchange.


The overarching goals of this project are (a) to probe anion-π interactions in metal-containing assemblies with redox-active tetrazine-based ligands and (b) to take advantage of the findings to design new compounds with unusual structures and properties. Exploration of the solid-state, solution and gas phase properties are performed with an emphasis on correlating structures with properties. Computational studies are carried out to augment our understanding of the anion interactions. Characterization techniques include X-ray crystallography, solid-state and solution NMR spectroscopy whenever possible, mass spectrometry, electrochemistry, EPR, and electronic spectroscopy. In general the research is highly fundamental in that we are probing the nature of anion-π interactions which are still not well understood. We also seek to use these forces as tools in the synthesis of large functional molecules based on anion-π templated polygons and to capitalize on them to control the structures of supramolecular compounds by employing repulsive/attractive anion-pi contacts to trigger the opening/closing of polygons by reversible reduction/oxidation reactions. A final aim that fits well with the Dunbar group’s expertise in molecular magnetism is to use reduced tetrazine ligand complexes as platforms for paramagnetic molecules known as single molecule magnets.

Neutral- and Radical-Bridged Dinuclear, Trinuclear and Tetranuclear

Transition and Lanthanide Based Molecular Magnets

 

One aspect of the anion research is to employ one electron reduction of the bridging ligands to “turn off” the anion-pi interaction and to improve the magnetic properties.  Radical-bridged single molecule magnets have recently gained traction as a method for improving the magnetic properties of polynuclear SMMs. The unpaired electron on the radical organic bridging ligand can engage in direct exchange interactions with the metal center, leading to a high degree of magnetic coupling. Strong coupling interactions typically improve SMM behavior by separating the magnetic ground state from the excited states. In our lab, we explore this idea through the synthesis and characterization of neutral- and radical-bridged SMMs.

The main focus of this project is the synthesis of radical-bridged dinuclear complexes and their closed-shell neutral bridged analogs using a variety of underexplored bridging ligands that differ in their donor atoms (O vs. N) as well as their substituents in order to tune the degree of orbital overlap between the radical and metal ion and the electron donating ability of the radical. The hypothesis is that the radical bridged systems will exhibit superior magnetic coupling and enhanced properties. Dinuclear complexes offer the advantage of being more easily modeled than larger molecules so are establishing trends in properties in conjunction with modeling in order to guide research on metallacycles. Some dinuclear Single Molecule Magnets that have recently been prepared are shown below along with the thermal barrier for flipping the spins.

A one-electron reduction of the bridging ligands on these compounds is a top priority, as they have a high likelihood of producing exceptional SMMs. The bridging ligand abpy should also prove to be quite interesting as the LUMO of neutral abpy is a π* orbital located on the two bonded N atoms in the ligand, mimicking the high spin density of the the [N2]3- ligand. Thus we anticipate that this ligand will lead to large couplings for dilanthanide compounds and excellent SMM behavior.

Higher nuclearity structures were also isolated as shown below which is a very exciting new development in rare earth chemistry.  The supramolecular nature of the complexes makes them a fascinating target for tuning the interactions that dictate whether a dinuclear, trinuclear or tetranuclear compound is formed.

 

References

 

1. Campos-Fernández, C. S.; Clérac, R.; Dunbar, K. R., A One-Pot, High-Yield Synthesis of a Paramagnetic Nickel Square from       Divergent Precursors by Anion Template Assembly. Angew. Chem. Int. Ed. 1999, 38, 3477-3479.

2.  Schottel, B. L.; Bacsa, J.; Dunbar, K. R., Anion dependence of Ag(I) reactions with 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz):  isolation       of the molecular propeller compound [Ag2(bptz)3][AsF6]2.  Chem. Comm. 2005, 46-47.

3.  Campos-Fernández, C. S.; Schottel, B. L.; Chifotides, H. T.; Bera, J. K.; Bacsa, J.; Koomen, J. M.; Russell, D. H.; Dunbar, K. R., Anion      Template Effect on the Self-Assembly and Interconversion of Metallacyclophanes.  J. Am. Chem. Soc. 2005, 127, 12909-12923.

4. Schottel, B. L.; Chifotides, H. T.; Shatruk, M.; Chouai, A.; Pérez, L. M.; Bacsa, J.; Dunbar, K. R., Anion-p Interactions as Controlling     Elements in Self-Assembly Reactions of Ag(I) Complexes with p-Acidic Aromatic Rings.  J. Am. Chem. Soc. 2006, 128, 5895-5912.

5. Schottel, B. L.; Chifotides, H. T.; Dunbar, K. R., Anion-π Interactions: A Tutorial Review. Chem. Soc. Rev., 2008, 37, 68–83 (web     release September 13, 2007).

6. Giles, I. D.; Chifotides, H. T.; Shatruk, M.; Dunbar, K. R., Anion-Templated Self-Assembly of Highly Stable Fe(II) Pentagonal
    Metallacycles with Short Anion-p Contacts.
Chem. Commun., 2011, 47, 12604–12606.

7. Chifotides, H. T.; Dunbar, K. R., Anion-pi Interactions in Supramolecular Architectures. Acct. Chem. Res., 2013, 46, 894–906.

8. Alexandropoulos, D. I.; Dolinar, B. S.; Vignesh. K. R.; Dunbar, K. R., Putting a New Spin on Supramolecular Metallacycles: Co3 Triangle     and Co4 Square Bearing Tetrazine-Based Radicals as Bridges. J. Am. Chem. Soc., 2017, 139, 11040-11043.

9. Woods, T. J.; Stout, H. D.;  Dolinar, B. S.; Vignesh, K. R.; Ballesteros-Rivas, M. F.; Achim, C.; Dunbar, K. R., Strong Ferromagnetic     Exchange Coupling Mediated by a Tetrazine Radical in a Dinuclear Nickel Complex. Inorg. Chem., 2017, 20, 12094–12097.

10. Dolinar, B. S.; Alexandropoulos, D. I.; Vignesh, K. R.; James, T.; Dunbar, K. R., Lanthanide Triangles Supported by Radical Bridging       Ligands. J. Am. Chem. Soc., 2018, 140, 908–911.

11. Schulte, K. A.; Vignesh, K. R.; Dunbar, K. R., Effects of coordination sphere on unusually large zero field splitting and slow magnetic         relaxation in trigonally symmetric molecules. Chem.Sci., 2018, DOI: 10.1039/C8SC02820F.

12. Li, J.; Gómez-Coca, S.; Dolinar, B. S.; Yang, L.; Yu, F.; Kong, M.; Zhang, Y.; Song, Y.; Dunbar, K. R.; Hexagonal Bipyramidal Dy(III):       New Structural Archetype for Single Molecule Magnets. 2018, Inorg. Chem., in press.