Martin Trent Lemaire wins one of the 5 IUPAC Prize for Young Chemists, for his Ph.D. thesis work entitled "Synthesis and Coordination Chemistry of Chelating Verdazyl Radicals."

Current address (at the time of application)

North Carolina State University
Raleigh, NC 27612, USA

E-mail: [email protected]

Academic degrees

Ph.D. Thesis

Title Synthesis and Coordination Chemistry of Chelating Verdazyl Radicals.
Adviser Dr. Robin G. Hicks
Thesis Committee Dr. David J. Berg, Department of Chemistry, University of Victoria; Dr. Thomas M. Fyles, Department of Chemistry, University of Victoria; Dr. J. T. Buckley, Department of Biochemistry and Microbiology, University of Victoria; Dr. Robert C. Thompson, Department of Chemistry, University of British Columbia.

Essay

Magnets serve an indispensable function in our technological-based society and are ubiquitous in all varieties of mechanical and electronic devices in science and industry. Traditional magnets are atom-based,
and are comprised of metals such as iron or nickel, the magnetism arising from the magnetic dipole moment that is a product of the presence of unpaired electrons. Recent synthetic efforts in chemistry have focused on the preparation of molecule-based magnetic materials that exhibit numerous desirable properties, including solubility, processibility, and synthetic tunability—features that are a direct result of their molecular nature and that are not shared by traditional atom-based magnets. The magnetic properties of these new materials may also be combined with other electronic or photonic characteristics of the molecule, and it is expected that these new combinations will generate novel properties. Such enthusiasm is embodied in the burgeoning fields of spintronics and nanotechnology. Researchers studying magnetic materials are very excited about the prospects for new nanoscale molecular materials as functional magnetic memory devices leading to dramatically enhanced data processing speeds and storage capacity in computers.

In order to rationally design a molecular magnetic material, strong communication between atoms or molecules with unpaired electrons (“spin”) is required. Specifically, the critical temperature (Tc) below
which the solid exhibits bulk magnetic behaviour is directly proportional to the strength (denoted by J, the exchange energy) of local spin-spin interactions. The challenge for the synthetic chemist is to chemically connect these spin-bearing building blocks so that they will interact as strongly as possible. A particular subset of building blocks that has been investigated in this regard is transition metal coordination complexes containing stable free radical ligands. This approach takes advantage of a strong direct magnetic interaction between the unpaired spin(s) on the paramagnetic transition metal centre, with the unpaired spin of the stable free radical ligand.

Using this metal-radical strategy, my research objectives included investigating the magnetic interactions in simple coordination complexes containing a radical of interest, and to develop structure- property relationships from these investigations. However, the variety of stable radical families is limited in scope. Extensive investigations have been made with nitroxide or semiquinone radicals, but only modest achievements have been obtained. There is much room for improvement, and the current paradigm is that new designs of radicals are needed in order to engineer enhanced new properties. Thus, my Ph.D. research focused on a different family of stable free radicals as ligands—the verdazyl radicals. No previous investigations had made use of verdazyls as ligands until the present study was undertaken, despite the system exhibiting a structure amenable to metal
coordination. Examples of selected verdazyl ligands that were prepared over the course of this work are shown in Figure 1.

These molecules contain a variety of aromatic heterocyclic substituents such as pyridine, bipyrimidine, and bipyridine, generating bidentate, bisbidentate, and tridentate ligands, respectively. Verdazyl radicals substituted in this manner are structural mimics of the well-recognized family of oligopyridine ligands. As an additional advantage, the rich and diverse history of transition metal coordination chemistry exhibited by oligopyridine ligands augured well for the potential coordinating
ability of verdazyl ligands.

In this research, using new synthetic protocols developed in the investigation, nine such verdazyl derivatives were prepared, and all were comprehensively characterized. These synthetic investigations were followed by the preparation of over twenty crystalline metal-ion complexes of this new family of ligands. An example of nickel(II) or manganese(II) complexes containing a verdazyl ligand (containing hexafluoroacetylacetonate ancillary ligands) is shown in Figure 2. The complexes depicted in Figure 2 represented the first ever verdazyl
complexes with paramagnetic transition metal centers.

A series of spectroscopic studies together with molecular structure determinations of the complexes and the parent radicals, clearly demonstrated that judiciously substituted verdazyl radicals were effective ligands and could bind to a wide variety of transition metal centres. The molecular structures of three representative complexes with the verdazyl ligands described in Figure 1 are depicted in Figure 3. Of note, verdazyl coordination complexes were, as expected, structurally analogous to similar metal-oligopyridine complexes.

The magnetic properties of these complexes were investigated using variable temperature magnetic susceptibility measurements, and very strong magnetic communication between the verdazyl and transition metal units were demonstrated for a number of systems. In particular, the nickel systems showed very marked effects. Magnetic coupling in nickel-verdazyl complexes was in all cases strongly ferromagnetic (all spins aligned parallel, generating a ground state with the highest possible spin multiplicity) with exchange coupling constants on the order of +200 cm^-1, making these complexes among the strongest metal-radical exchange coupled systems reported to date. Complexes containing other transition metal centers were also investigated, including manganese(II), cobalt(II), copper(I)/(II), and zinc(II) ions, as well as second row transition metals, including ruthenium(II) and palladium(II). As anticipated from the verdazyl-oligopyridine structural relationship, the magnetic properties of these complexes were in general easily rationalized on the basis of the combined electronic and molecular structural data obtained from the spectroscopic studies. Most importantly, through these investigations I was able to demonstrate a degree of rational structural control over the property of interest—magnetism—in metal-verdazyl complexes that is without precedent in coordination complexes containing other radical ligand families. The level of control over the structure and magnetic properties in metal-verdazyl coordination complexes cannot be as easily achieved with any other metal-radical systems currently under study.

From these studies, an entirely new set of building blocks was developed that clearly demonstrate the enormous potential offered by these units for magnetic materials. The metal-verdazyl structure property relationships gained through this study are invaluable for future rational synthetic investigations using new verdazyl ligands. The groundwork has been laid for the pursuit of unique polymeric arrays containing verdazyls and metals, and it is expected that bulk magnetism in these molecular based materials can soon be achieved.

 

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