Generation and reactivity of N-pyrazolyl radicals in benzene solution

Direct Evidence for Neutral N-Pyrazolyl Radicals: Paddlewheel Dibismuthanes Bearing Pyrazolato Ligands with Very Short Bi–Bi Single Bonds

“…18 On the basis of this experimental fact, N-pyrazolyl radicals were, hence, postulated as intermediates, where the unpaired electron is delocalized over the two N atoms with a σ-type ground state (2σ). 18 However, no further evidence such as electron paramegnetic resonance (EPR) spectra for this radical species was provided. To date, the pyrazolyl radical structure with a σ (2σ) or π (2π) type is still a question of debate.…”

“…2.82 Å. 27 The planes of the pyrazolato in the face-to-face orientation were almost perpendicular to each other, with an average dihedral angle of close to 90°despite the average N−Bi−N angle being 84.27(7)°in 5α, 5β, and 5γ and 84.58 (18)°in 6, which are comparable to those found in the recently reported 1,2,4diazaphospholide dibismuthanes [{η 1 ,η 1 -3,5-R 2 dp} 2 (Bi−Bi)-{η 1 ,η 1 -3,5-R 2 dp} 2 ] [dp = 1,2,4-diazaphospholide; R = tBu (A), and Ph (B)]: 84.3(8)°for A and 84.35 (11)°for B (Tables 1 and S12 and S13). 16 The isolation of low-valent bismuth paddlewheel complexes 5 and 6 suggested that a redox reaction occurred.…”

“…Most importantly, neutral 3,5-substituted pyrazolyl radicals III were confirmed by EPR spectral analysis, which demonstrated that the unpaired electron of the pyrazolyl radicals is delocalized at the two N atoms with a σ-type ground state (2σ) instead of over the five-membered ring with a π-type ground state (2π). 18 ■ ASSOCIATED CONTENT * S Supporting Information…”

“…Although species 2 with possible σ (2σ) and π (2π) types has been theoretically postulated many times, , it has not been evidenced directly so far because of its high reactivity. Louw and co-workers reported that thermolysis of tert -butyl-1-pyrazolepercarboxylate and its 3-methyl derivative in a benzene solution at 140 °C leads to N -phenylated pyrazoles without the formation of isomeric C -phenyl derivatives . On the basis of this experimental fact, N -pyrazolyl radicals were, hence, postulated as intermediates, where the unpaired electron is delocalized over the two N atoms with a σ-type ground state (2σ) .…”

“…Louw and co-workers reported that thermolysis of tert -butyl-1-pyrazolepercarboxylate and its 3-methyl derivative in a benzene solution at 140 °C leads to N -phenylated pyrazoles without the formation of isomeric C -phenyl derivatives . On the basis of this experimental fact, N -pyrazolyl radicals were, hence, postulated as intermediates, where the unpaired electron is delocalized over the two N atoms with a σ-type ground state (2σ) . However, no further evidence such as electron paramegnetic resonance (EPR) spectra for this radical species was provided.…”