TY - JOUR
T1 - Conformationally constrained, stable, triplet ground state (S = 1) nitroxide diradicals. Antiferromagnetic chains of S = 1 diradicals
AU - Rajca, Andrzej
AU - Takahashi, Masahiro
AU - Pink, Maren
AU - Spagnol, Gaëlle
AU - Rajca, Suchada
PY - 2007/8/22
Y1 - 2007/8/22
N2 - Nitroxide diradicals, in which nitroxides are annelated to m-phenylene forming tricyclic benzo-bisoxazine-like structures, have been synthesized and characterized by X-ray crystallography, magnetic resonance (EPR and 1H NMR) spectroscopy, as well as magnetic studies in solution and in solid state. For the octamethyl derivative of benzobisoxazine nitroxide diradical, the conformationally constrained nitroxide moieties are coplanar with the m-phenylene, leading to large values of 2J (2J/k > 200 K in solution and 2J/k ≫ 300 K in the solid state). For the diradical, in which all ortho and para positions of the m-phenylene are sterically shielded, distortion of the nitroxide moieties from coplanarity is moderate, such that the singlet-triplet gaps remain large in both solution (2J/k > 200 K) and the solid state (2J/k ≈ 400-800 K), though an onset of thermal depopulation of the triplet ground state is detectable near room temperature. These diradicals have robust triplet ground states with strong ferromagnetic coupling and good stability at ambient conditions. Magnetic behavior of the nitroxide diradicals at low temperature is best fit to the model of one-dimensional S = 1 Heisenberg chains with intrachain antiferromagnetic coupling. The antiferromagnetic coupling between the S = 1 diradicals may be associated with the methyl nitroxide C-H⋯O contacts, including nonclassical hydrogen bonds. These unprecedented organic S = 1 antiferromagnetic chains are highly isotropic, compared to those of the extensively studied Ni(II)-based chains.
AB - Nitroxide diradicals, in which nitroxides are annelated to m-phenylene forming tricyclic benzo-bisoxazine-like structures, have been synthesized and characterized by X-ray crystallography, magnetic resonance (EPR and 1H NMR) spectroscopy, as well as magnetic studies in solution and in solid state. For the octamethyl derivative of benzobisoxazine nitroxide diradical, the conformationally constrained nitroxide moieties are coplanar with the m-phenylene, leading to large values of 2J (2J/k > 200 K in solution and 2J/k ≫ 300 K in the solid state). For the diradical, in which all ortho and para positions of the m-phenylene are sterically shielded, distortion of the nitroxide moieties from coplanarity is moderate, such that the singlet-triplet gaps remain large in both solution (2J/k > 200 K) and the solid state (2J/k ≈ 400-800 K), though an onset of thermal depopulation of the triplet ground state is detectable near room temperature. These diradicals have robust triplet ground states with strong ferromagnetic coupling and good stability at ambient conditions. Magnetic behavior of the nitroxide diradicals at low temperature is best fit to the model of one-dimensional S = 1 Heisenberg chains with intrachain antiferromagnetic coupling. The antiferromagnetic coupling between the S = 1 diradicals may be associated with the methyl nitroxide C-H⋯O contacts, including nonclassical hydrogen bonds. These unprecedented organic S = 1 antiferromagnetic chains are highly isotropic, compared to those of the extensively studied Ni(II)-based chains.
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U2 - 10.1021/ja0712017
DO - 10.1021/ja0712017
M3 - Article
C2 - 17655296
AN - SCOPUS:34548186153
SN - 0002-7863
VL - 129
SP - 10159
EP - 10170
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 33
ER -