Syntheses of Aminyl Diradicals and Nitroxide Tetra- and Octaradicals

This dissertation comprises of two main research projects: aminyl diradicals and calix[4]arene nitroxide tetra- and octaradicals. The design, multi-step synthesis and characterization, including NMR, EPR spectroscopy, magnetic measurements and X-ray crystallography of precursors and target compounds will be discussed.

Aminyl radicals, nitrogen-centered radicals, are typically short-lived and considered as reactive intermediates. Because of their unique magnetic properties, aminyls are attractive building blocks for high-spin di-and polyradicals. The challenge is in the design and synthesis of aminyls with increased stability. In this chapter, we have prepared triplet (S = 1) ground state aminyl diradicals, which are derivatives of aza-m-xylylene diradicals. Triplet (S = 1) ground state these aminyl diradicals are predicted to possess singlet-triplet energy gap, Δ_EST ≈ 10 – 14 kcal mol^–1. Kinetics experiments for these aminyl diradicals are carried out in solution of 2-methyltetrahydrofuran at room temperature with the half-life measured in minutes.

Nitroxides are one of the most studied stable radicals in chemistry and in other related fields because of their stability. To explore potential of nitroxides as paramagnetic contrast agents for magnetic resonance imaging (MRI), we designed and synthesized 1,3-alternate calix[4]arenes nitroxide tetraradical and octaradical. Attachment of nitroxides on a rigid, well-defined framework such as calix[4]arene locked in 1,3-alternate conformation allow for study of the exchange coupling through bond and through space. In this chapter, 1,3-alternate calix[4]arenes with phenylene spacers connecting mononitroxides and high-spin (S =1) dinitroxides on the upper rim providing calix[4]arene nitroxide tetraradicals and octaradicals, respectively, are synthesized and characterized by ¹H NMR and EPR spectroscopy. With the desire in biological applications, calix[4]arene nitroxide tetraradical and octaradical with hexaethylene glycol methyl ether (HEG) are also synthesized and characterized.