This work expands the investigation into the behaviour of verdazyl radicals and N-alkylated tetrazines as ligands. These new ligands were coordinated to various metals as a means of exploring new properties in the metal-verdazyl and metal-tetrazine products. The synthesis of N,N’-diphenyl Kuhn and 6-oxo verdazyl radicals bearing a 2-pyridyl group at the C3 position was accomplished. Palladium(II) dichloride complexes of each of these radicals were prepared in order to study the differences in the structural, electronic, and electrochemical properties compared to corresponding complexes of the previously reported N,N’-dialkyl-6-oxoverdazyl ligands. The N,N’-diphenyl verdazyl ligands are structurally bulkier than their dialkyl counterparts resulting in increased interaction between the ligand and palladium as observed in the solid state. The radical complexes were investigated by EPR and shown to exhibit a small amount of spin density on the palladium atoms with most of the spin density remaining on the ligands. The UV-Visible spectra had a noticeable red-shift in the absorbance maxima of the complexes compared to the free ligands. The electrochemistry of the new palladium-verdazyl complexes showed that there was a positive increase to the reduction and oxidation potentials when compared to the free ligands. An N-benzyl tetrazine and its Ru(hfac)2 complex were synthesized from their corresponding radical species utilizing Mn2(CO)10 to photogenerate benzyl radicals. This method was found to give high yields of the tetrazine and its metal complex. Spectroscopic, structural, and electrochemical properties of the tetrazine and its Ru(hfac)2 complex are reported. These compounds were investigated in regards to the activation energy associated with the homolytic cleavage of the C-N bond in the inert solvent, tert-butylbenzene. The activation energy of C-N bond of the tetrazine was 155 kJmol-1 while its Ru(hfac)2 complex was 138 kJmol-1; this resulted in the rate of dissociation being a factor of ~40 greater for the Ru(hfac)2 complex at 393 K. This work presents the potential of coordination compounds in tuning the properties of molecules associated with the stable free radical polymerization process. / Graduate / 0488 / 0485 / cooper_johnston@hotmail.com

Préparation et étude de nouveaux dinitroxydes comme agents de polarisation en polarisation dynamique nucléaire (PDN) en phase solide. La principale limite de la RMN, en tant qu'outil de détection ou d'imagerie (IRM), est sa faible sensibilité qui résulte principalement de la faible différence d'énergie entre les états de spin nucléaire entre lesquels on observe la résonance. A l'équilibre thermique, la polarisation nucléaire PI (différence de population entre les états de spin) est très faible et le signal RMN, qui lui est proportionnel, sera peu intense. Le but de la Polarisation Dynamique Nucléaire (PDN) est d'augmenter l'intensité de signaux de RMN, en transférant vers des spins nucléaires de la polarisation de spin électronique PS, plus élevée que celle des spins nucléaires (PS/PI = 658 pour 1H). Depuis une quinzaine d'années, la PDN connaît un regain d'intérêt et un champ de développement exceptionnels. Cette renaissance de la PDN est surtout due aux importants travaux de fond du groupe de R. G. Griffin et ceux plus récents du groupe d'Ardenkjaer-Larsen. Ces travaux ont entre autre montré qu'avec des radicaux trityl ou des dinitroxydes, la PDN pouvait permettre d'atteindre de fortes augmentations du rapport signal sur bruit, en RMN en phases solide et liquide. Les propriétés de l'espèce paramagnétique à partir de laquelle se fait le transfert de polarisation spin électronique - spin nucléaire, jouent un rôle primordial dans l'efficacité de ce transfert. Au cours de notre travail, nous avons réalisé les synthèses de cinq nouveaux biradicaux de la famille des dinitroxides. / Preparation and study of new dinitroxydes as polarizing agents in dynamic nuclear polarization (DNP) in solid state. Nowadays, nuclear magnetic resonance (NMR) spectroscopy is one of the most important structure elucidation techniques in chemistry and biochemistry, NMR is also the underlying principle of magnetic resonance imaging (MRI). However, the use of NMR to investigate various materials or biological systems is still limited by its inherent low sensitivity. This arises from the relatively small size of the Zeeman interaction of the nuclear spins with an external magnetic field which leads to small Boltzmann polarizations (PI) and weak NMR signals. Dynamic Nuclear Polarization (DNP) is a prominent process to achieve a high non-equilibrium nuclear spin polarization by transferring to nuclear spins the higher electron spin polarization PS (PS/PI = 658 for 1H)) of unpaired electrons, belonging for example to stable free radicals. The past fifteen years has witnessed a renaissance in the use of DNP. This renewed interest is due to the outstanding work of the R. G. Griffin's group and the more recent work of the Ardenkjaer-Larsen's group. These authors have shown, among other, that with the use of trityl radicals or dinitroxides, PDN allowed to reach impressive signal enhancements for solid state and liquid NMR. The characteristics of the paramagnetic species used as polarizing agent play a pivotal role in the efficiency of a DNP process. In the course of our work we have performed the synthesis of five new dinitroxides, and through collaborations we tested their performance as polarizing agents for solid state PDN at 100 K, 9,4 T [263 GHz (RPE), 400 MHz (RMN)].

Significant advances in the design, synthesis, and chemistry of verdazyl radicals have been made, including (i) the systematic study of the electrochemistry of verdazyl radicals, (ii) the development of formazans as ancillary ligands en route to inorganic verdazyl radicals, and (iii) magnetostructural studies of verdazyl diradicals and copper (II) verdazyl complexes. The electrochemical properties of a family of verdazyl radicals were explored. Type I and type II verdazyl radicals were reversibly oxidized and reduced, and the potentials of such processes observed to be sensitive to substituent effects. The incorporation of electron-withdrawing substituents made verdazyl radicals harder to oxidize and easier to reduce, while the presence of electron-donating groups had the opposite effect. Type II verdazyls were harder to oxidize and less delocalized (based on relative cell potentials) than type I analogues. The difficulty in oxidation of type II verdazyls relates to the electron-withdrawing nature of the carbonyl functionality, while the decreased delocalization relates to twisting of the N-substituents. Twisting of the N-substituents was confirmed through the use of X-ray crystal structures, and DFT calculations were used to illustrate the decrease in delocalization of the unpaired electron associated with the twisting. The similarities of formazans to -diketiminate ligands prompted the study of their coordination chemistry. Boratatetrazines, the first main group complexes of formazans illustrated their ability to mimick beta-diketiminate ligands. Reduction of boratatetrazines with cobaltocene afforded highly reactive borataverdazyl radical anions isolobal to parent organic systems. The radicals were readily oxidized back to the boratatetrazine precursors limiting their characterization to the solid-state. Synthetic pathways to 3-substituted formazans allowed for the incorporation of bulky N-substituents, a feature of -diketiminates that has influenced their utility as catalysts. 3-Cyanoformazans were shown to exist as either the open or closed structure in solution and the solid-state, while 3-nitroformazans exist exclusively as the closed strcutre due to the presence of the relatively large nitro-substituent. A number of transition metal complexes of 3-substituted formazans were synthesized, and their X-ray crystal structures used to establish a correlation between steric bulk at the ligand and complex structure. When ortho-substituents are incorporated the N-aryl substituents twist relative to the formazan backbone, while relatively smaller N-aryl substituents remain relatively planar. Palladium hexafluoroacetylacetonate complexes of formazans were anticipated to have utility as precursors to palladaverdazyls due to their electron poor nature. However, although the complexes did allow for the structure property relationship of metal-formazan complexes to be further developed, palladaverdazyls were not realized. Comparison with boratatetrazines suggests the nature of the Pd-N bond may play a role in the instability of palladaverdazyls. The synthesis and characterization of verdazyl-based spin dimers was reported. The incorporation of iso-propyl N-substituents allowed for the first truly stable verdazyl diradicals to be isolated. Electrochemical, electronic, and magnetic properties of diradicals bridged by para- and meta-benzene were explored. Diradicals bridged by para-benzene were antiferromagnetically coupled while meta-benzene bridged diradicals were ferromagnetically coupled. Magnetostructural studies of copper (II) complexes of verdazyls were complicated by the coordinative flexibility of copper (II) ions and the presence of Jahn-Tellar distorted ligand fields. However, a correlation between structure and properties was established: axially bound verdazyl radicals were weakly ferromagnetically coupled to copper (II) ions, and equatorially bound verdazyl radicals were strongly antiferromagnetically coupled to copper (II).