Synthesis and Characterization of Free and Coordinated Nitroxide and Nitronyl Nitroxide Diradicals with a 2,7-Naphthalene Coupling Unit

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 ΔEST = ∼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 ΔEST = ∼46 K.