Nitrosoarenes: magnetic anisotropy, donor-acceptor bonding and host-guest chemistry

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Nitrosoarenes (Ar-N=O) were first reported by Baeyer in 1874 after successful synthesis of nitrosobenzene via a diphenyl mercury reaction with nitrosyl bromide. C-nitroso compounds (R-N=O), like nitrosobenzene, fall between hydroxylamines (R-NH-OH) and nitro compounds (R-NO2) on the amine oxidation scale making them common intermediates for many synthetic amine oxidation reactions. Nitrosobenzenes exhibit a number of unique properties, which stem from their structure. A small energy gap between the HOMO (n*) and LUMO (?*) orbitals of the N=O function allows for several unique chemical and spectroscopic properties of nitrosoarenes. Nitrosobenzene and its derivatives have a blue-green color in monomeric form and reversibly dimerize to a colorless form in the solid state, thus yielding a dynamic covalent bonding reaction for this class of compounds. One area of our studies focuses on the very large (2-3 ppm) magnetic anisotropy of the nitroso group, which is evaluated for a series of ortho-substituted nitrosoarenes. The nitroso (-N=O) functions of these molecules are sterically forced into a primary (majority) orientation in the molecule by the ortho substituent, thereby allowing the NMR analysis of the magnetic environment around an oriented -N=O group at room temperature. Multiple substrates have been synthesized, including oriented 1,3-(bis)nitroso compounds, which have double the amount of shielding of the “syn” proton. A series of NMR techniques is employed to prove the NMR signal assignments for these compounds including: 1H, 13C, NOE, COSY, HSQC, and HMBC NMR spectroscopy. Also studied is the electron donor-acceptor (DA) bonding ability of the N=O group of N,N-dimethyl-4-nitrosobenzene (DMANB) and N,N-diethyl-4-nitrosobenzene (DEANB), in the monomeric state, as they complex and co-crystallize with electron-poor alkenes such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and tetracyanoethylene (TCNE). Finally, encapsulation of N,N-dimethyl-4-nitrosobenzene (DMANB) and p-nitrosocumene (p-NC) by organic host molecules such as octa-acid (OA), cucurbit[7]uril (CB7), and cyclodextrins (?,?,?-cd) in aqueous solution is studied.

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