Supramolecular Chemistry Involving Anions and Anion-pi Interactions
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.
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.
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.
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).
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.