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-<i>tert</i>-butylnitroxide <b>DR</b>^<b>4</b>, bis-nitronyl nitroxide <b>DR⁶,</b> and mixed <i>tert</i>-butylnitroxide/nitronyl nitroxide <b>DR</b>^<b>5</b>) was carried out to predict conformation-dependent values of the intramolecular interactions. The coupling constants <i>J</i> are positive for symmetrical diradicals with <i>tert</i>-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 <b>DR</b>^<b>5</b> (∼-160 K). The triplet diradical <b>DR</b>^<b>4</b> was allowed to react with Mn(hfac)₂ to give a polymer complex [Mn(hfac)₂(<b>DR</b>^<b>4</b>)]_<i>n</i> 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 <b>DR</b>^<b>5</b> revealed that the <i>tert</i>-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 <b>DR</b>^<b>6</b> 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 <b>DR</b>^<b>6</b> has the triplet ground state with the energy gap Δ<i>E</i>_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 <i>m</i>-phenylene, which provides the singlet-triplet energy gap of value Δ<i>E</i>_ST = ∼46 K.