Molecular magnetism is a field of science that is dedicated to the development of magnetic materials from molecular components. One of the eventual goals of this field is to develop high temperature single molecule magnets that would revolutionize magnetic data storage systems. One of the key aspects of molecular magnetism is the understanding of how to generate high spin molecules and to tailor the physical interactions between the unpaired electrons in those molecules. When I was in the Shultz group, research was focused on high spin organic molecules- that is we were investigating how to build molecular magnetic materials using organic molecules as the primary component. Our thinking was that the versatility offered in organic structures would enable a high degree of ability to tailor the magnetic properties of bulk materials made from these molecules by fine-tuning the molecular structure. To make a high spin organic molecule, you have to include a high spin coupling unit in the structure. A high spin coupling unit is one that is made up of a delocalized p-electron system such that there are a pair of degenerate molecular orbital that overlap on one or more atoms in the molecule. Basically, this means that two electrons are delocalized over the molecule, but since the orbitals they occupy are degenerate, each orbital gets one electron. These orbitals are called Signally Occupied Molecular Orbitals (SOMOs). In the lowest energy state of this kind of system, both of the electrons have the same spin and this situation is known as ferromagnetic coupling. The strength of the interaction between the electrons (J) is largely controlled by the extent of spatial overlap between the SOMOs. There are two molecular fragments that are able to accomplish this task, and they are known as the trimethylenemethane (TMM) (Figure 1a) and m-phenylene units (Figure 1b).

Figure 1. Structure and SOMOs of (a) TMM and (b) m-phenylene high spin coupling units.

You also have to provide protection for the high spin coupling unit since the electron density at the end of the ethylene portion of the molecule is very reactive and your biradical will rapidly decompose if this site is not blocked. So, here we come to the central question my research was focused on: Given that you are going to cram a whole bunch of stuff around this relatively small molecular fragment, how will that effect electron communication? In the TMM system, the case of strongest interaction will occur if all of the p orbitals are in the same plane and there tensional angles of the methylene groups is zero (Figure 2a). An increase in the tensional angle will decrease J (Figure 2b) until both methylene p orbitals are twisted 90 degrees out of the plane of the ethylene group where coupling will actually revert to a through space mechanisms and result in the electrons having opposite spins (known as antiferromagnetic coupling) and the molecule will have a S=0 ground state. In terms of developing an actual material, your goal is to have strong ferromagnetic coupling between electrons. However, having the ability to vary this interaction at the molecular level should ultimately allow the ability to tailor the bulk properties of a material it is used to make. With this in mind, we set out to explore how molecular structure effects spin coupling in TMM systems.

Figure 2. Expected effects of steric-induced twisting in TMM biradicals.

Before we continue, I really should note that I put this together from my memory of work and theory that I was involved with 8 years ago (at the time of writing). If you are interested in this stuff, you really should go and check out Dave’s web page.

To accomplish this project, I had to do synthesis. A lot of synthesis. This is really where I cut my teeth in the world of making molecules, and where I really learned to appreciate that to really explore molecular systems, you needed to know how to make them so you would never be limited to what is in a catalog. There were three types of radicals we wanted to study, and these included di-tert-butyl phenoxides, tert-butyl nitroxides, and semiquinones (Figure 3). We then wanted to put these onto several TMM based scaffolds, including adamantane, norbornane, biccyclo[3.3.1]nonane, and several others (Figure 4). This amounted to a pretty impressive list of molecules, and the first thing we had to do was to figure out how to make them.

Figure 3. Spin containing units used in this study

Figure 4. TMM protecting units used in this study

The first molecule we went after was the adamantane system (Figure5). To make these biradiacals, we started out with adamantane-2-ol which was converted to the corresponding carboxylic acid using H₂SO₄ and formic acid This was a really fun reaction since we typically ran the reaction at very dilute concentrations: 2 L of sulfuric acid for 2 g of adamantane-2-ol! This took a lot of ice to quench properly, and the reason that it had to be run very dilute was that, otherwise, the secondary carbocation would undergo an inter-molecular rearrangement to form the tertiary carbocation on another adamantine and then you would get the wrong product. After this, we esterified and reacted the ester with an excess of an aryl lithium reagent that would eventually become the spin containing unit. The final steps were the dehydration of the alcohol and deprotection of the radical precursors and finally radical generation.

Figure 5. Scheme showing the synthesis of adamantane based biradicals

Figure 6. Scheme showing the synthesis of norbornane based biradicals

7). The product of this reaction was reacted with sodium iodide in acetone (the Finkelstein Reaction!!!) and the product was cycleized by deprotonation using LiHDMS to produce the ketone. Now, we did mess around with trying the same methods to get our biradicals as with the adamantane and norbornane backbones, but we ran into all kinds of problems carbocation rearrangements, steric bulkiness and so on so we had to find a new way. I forget exactly how I stumbled onto the method we ended up with, but it was a beauty. The reaction, called the Barton-Kellog reaction, is used specifically to make sterically crowded alkenes. The reaction uses a diazoalkane and a thioketone as the reactants, and you initially start with the formation of a thioepoxide coupled with the release of N₂ (always look to loose the small molecule!) and then convert the thioepoxide into an alkene via a reaction with triphenyl phosphine. So, first we converted the backbone fragment ketones into diazoalkanes via reaction with hydrazine followed by oxidation with mercuric oxide. The spin containing units were made as ketones and converted to thioketones with phosphorous pentsulfide (Figure 8). The thioketones were then reacted with desired diazoalkanes to give the final products. And the world rejoiced. Yea! (respect to Monty Python and The Quest for the Holy Grail).

Figure 7. Production of the bicyclo[4.4.1] ketone and di-tert-butyl , bicyclo[3.3.1], and bicyclo[4.4.1] diazoalkanes.

Figure 8. Use of the Barton-Kellog reaction to finally give some of our more exotic biradicals.

Unfortunately, I no longer have any of the spectra we took in electronic format, so I wont be able to show any of it here. Of course, you can just go and get the papers anyway, so I’ll just kind of lay out what we found. The first molecule we made and studied was the adamantine bis(di-tert-butyl phenoxides) diradical. This molecule was our first step into studying twisting in the TMM system, and we hit pay dirt. When we started doing frozen solution Electron Paramagnetic Resonance (EPR) studies of this molecule, we found that sometimes it behaved like a S=1 system and sometimes like a S=0 system. After a lot of work, including one weekend when a friend and I stayed awake for 36 hours straight doing a photolysis experiment, we finally figured out that the times that there were differences were related to the solvent, and then we figured out that the two seemingly different systems were actually the same molecule but with the phenyl rings twisted at different angles. In one case, the phenyl-alkene torsion angle was relatively small and the molecule was S=1 (Figure 9a). When the molecule was S=0, one ring was twisted out of plane with the alkene, thus cutting off communication between the electrons (Figure 9a). Pretty wild since this type of behavior had never been seen before, and it landed me my first paper in JACS.

Figure 9. Adamantine bis(di-tert-butyl phenoxides) diradical in the proposed (a) S=1 and (b) S=0 conformations.

From this result, we worked out that the two hydrogens on the adamantane were partially responsible for this effect. That is, they generated some kind of steric interaction with the phenyl rings that interfered with the rings ability to remain in conjugation with the central alkene (Figure 10a). Well, we thought that this would be a pretty easy theory to test. If we just make different TMM blocking groups that shit these protons either away from or towards the phenyl rings, then we could alter the strength and nature of electron coupling in the TMM region. If the hydrongens were pulled back, then we assumed that the phenyl rings cold flatten out, yielding a S=1 system and strong coupling. On the other hand, pushing the hydrogens in should yield a S=0 system (Figure 10b).

Figure 10. Illustrations showing (a) proposed steric interactions between adamantine hydrogens and phenyl rings and (b) postulated effect of changing the framework of the TMM blocking group.

Long story short, what we expected to happen pretty much did. Like I pointed out earlier, the synthesis side of this was really the hard part. I spend three &*!#ing months trying to make that bicyclo[4.4.1] blocking group, it was an amazing challenge. But, in the end it worked. Even though I got the last spectrum and VT-EPR run done about 12 hours before I left town to head up to Mass to continue my education. I still remember that day- I had been at it for a week, and kept having all kinds of problems oxidizing the bastard to a biradical. I was living at Dave’s house at the time- I no longer had my apartment and all of my stuff was in storage up in Amherst. We had done a fair bit of partying, but I just couldn’t let that last molecule (the bicyclo[4.4.1] bisphenoxy radical) go. Then, I finally got it figured out and spend the entire night running the liquid helium cryostat and collecting data and BAM! It fit right in line with what we expected. I was so happy, I went right to Dave’s house and busted into this and his wife’s bedroom with the result. At 7:30 in the morning on Saturday. Well, in the end he didn’t mind and we had a few beers to celebrate before I packed my car and headed north. Unfortunately, I don’t think that this data has ever been published, but that’s life. Another thing, too. Perhaps I’ve become too cynical, but when I leave my current job (at UH), I sure as hell won’t be spending the last hours in Hawaii at my computer analyzing spreadsheets. But, I kind of miss that dedication of my younger self. I wonder if this happens to everyone?

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Papers I Co-authored as a result of this work

• “Preparation and Characterization of a Bis-semiquinone- A Bidentate Dianion Biradical.” Shultz, D. A.; Boal, A. K.; Driscoll, D. J.; Kitchin, J. R.; Tew, G. N. J. Org. Chem. 1995, 60, 3578-3579.
• “The Biradical, Bis(3,5-Di-tert-butyl-4-phenoxyl)methyleneadamantane, Exhibits Matrix-Dependent EPR Spectra Suggesting Rotomer Bisstability with Differential Exchange Coupling.” Shultz, D. A.; Boal, A. K.; Farmer, G. T. J. Am. Chem. Soc. 1997, 119, 3846-3847.
• “Effect of Aliphatic Amine Bases on the Aggregation of Alkali Metal Salts of 3,5-Di-tert¬-butyl semiquinone (3,5-DBSQ).” Shultz, D. A.; Boal, A. K.; Campbell, N. P. Inorg. Chem. 1998, 37, 1540-1543.
• “Synthesis of Bis(semiquinone)s and their Electrochemical and Electron Paramagnetic Resonance Spectral Characterization.” Shultz, D. A.; Boal, A. K.; Farmer, G. T. J. Org. Chem. 1998, 63, 9462-9469.
• “Structure-Property Relationships in Trimethylenemethane-Type Biradicals. 2. Synthesis and EPR Spectral Characterization of Dinitroxide Biradicals.” Shultz, D. A.; Boal, A. K.; Lee, H.; Farmer, G. T. J. Org. Chem. 1999, 64, 4386-4396.

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This page was created on 13 July 2006 and last updated on 13 July 2006