The design, synthesis, and chemistry of stable verdazyl radicals and their precursors




Gilroy, Joseph Bryan

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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).



formazans, verdazyl, stable radicals, magnetochemistry, electrochemistry