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Synthesis and Characterization of Free and Coordinated Nitroxide and Nitronyl Nitroxide Diradicals with a

Mikhail Zlokazov¹, Igor Zayakin¹, Galina Romanenko²

¹N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Ave. 47, Moscow 119991, Russian Federation.

Inorganic Chemistry

|March 16, 2026

Abstract:

A quantum-chemical investigation of nitroxide and nitronyl nitroxide diradicals with a 2,7-naphthalene coupling unit (bis-tert-butylnitroxide DR⁴, bis-nitronyl nitroxide DR⁶, and mixed tert-butylnitroxide/nitronyl nitroxide DR⁵) was carried out to predict conformation-dependent values of the intramolecular interactions. The coupling constants J are positive for symmetrical diradicals with tert-butyl-nitroxide or nitronyl nitroxide moieties (B3LYP/Def2-TZVPP: ∼440 and 60 K, respectively, for the most stable conformations), but it is negative for the mixed DR⁵ (∼-160 K). The triplet diradical DR⁴ was allowed to react with Mn(hfac)₂ to give a polymer complex [Mn(hfac)₂(DR⁴)]_n exhibiting non-linear field dependencies of magnetization at low temperatures due to short-range ordering within the polymer chains. Attempts to synthesize the mixed diradical DR⁵ revealed that the tert-butyl-nitroxide moiety is not stable and undergoes a previously unobserved degradation pathway to give the corresponding α-carbonyl nitrone. At the same time, diradical DR⁶ was successfully prepared by using the palladium-catalyzed reaction of 2,7-dibromonaphthalene with a (nitronyl nitroxide-2-ide)(triphenylphosphine)gold complex. Magnetic susceptibility studies proved that the diradical DR⁶ has the triplet ground state with the energy gap ΔE_ST = ∼24 and ∼10-18 K in frozen (glassy) solution and solid state, respectively. Surprisingly, naphthalene-2,7-diyl is comparable as a ferromagnetic coupler with m-phenylene, which provides the singlet-triplet energy gap of value ΔE_ST = ∼46 K.

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