10.1002/asia.201801615
Chemistry - An Asian Journal
FULL PAPER
(14902), 543 (903); Anal. Calcd. For C14H25N3O3: C, 59.34; H, 8.89; N,
14.83. Found: C, 59.59; H, 8.89; N, 14.83; ESR (powder): g = 2.0067;
cd/emu mol–1: –165 × 10–6 (pascal).
diradical system, 5 and 6, with rigid structures where two radicals
were linked by a single sp3-carbon. From the temperature
dependences of cpT values in the SQUID measurements, 5 and
6 were found to be in the singlet ground states with 2Jintra/kB = –
35.2 K and –13.6 K, respectively. Diradicals 3 and 4 were
previously investigated by Hideg and coworkers[9] and Tretyakov
and coworkers[8]; the latter group calculated in detail and propose
the triplet ground states. We have calculated the spin densities for
the diradical 3–6 in their ground states: broken symmetry singlet
states for 3 and 4, and triplet states for 5 and 6. The signs of the
small spin densities on the sp3 carbon atoms should be
highlighted. For these diradicals 3–6, the ground states could be
explained by the spin polarization model (Figure 8). The singlet
states for diradical 1 and 2-conformer-I could be explained by the
two competing opposite exchange interactions, antiferromagnetic
through-space interaction and ferromagnetic through-bond
interaction.
Synthesis of 2-(4',4',5',5'-tetramethylimidazoline-1'-oxyl-2'-yl)-2,5,5-
trimethylpyrrolidin-1-oxyl (6): An aqueous solution of NaNO2 (158 mg,
2.29 mmol, 2.2 eq.) was added to a CHCl3 solution of 5 (296 mg, 1.04
mmol, 1.0 eq.) with stirring at room temperature. Acetic acid (0.42 mL, 7.34
mmol, 7.0 eq.) was added dropwise to the solution. The mixture was stirred
at room temperature for 1 h. A saturated aqueous solution of NaHCO3 (12
mL) was added to this mixture. The organic layer was separated, and
washed with water and a saturated aqueous solution of NaCl. This solution
was dried over anhydrous Na2SO4 and filtrated. The filtrate was
concentrated under reduced pressure. The crude product was purified by
silica gel column chromatography using CH2Cl2–AcOEt (8:1 v/v) as eluent,
to afford diradical 6 as a brown solid (230 mg, 82%). The diradical 6 was
recrystallized from a hexane at 4 oC. 6: C14H25N3O2; MW 267.37; mp 85
oC (decomp.); TLC (silica) Rf 0.28 (8:1 CH2Cl2–AcOEt); MS (FAB+) m/z
267 [M+]; IR (KBr, cm–1): 2978, 2937, 1578, 1456, 1437, 1369, 1325, 1250,
1150, 872, 637, 540, 419; UV-Vis lmax/nm (e/M–1 cm–1) in CH2Cl2: 265
(8228), 381 (609); Anal. Calcd. For C14H25N3O2: C, 62.89; H, 9.42; N,
15.72. Found: C, 62.78; H, 9.55; N, 15.66; ESR (powder): g = 2.0071;
cd/emu mol–1: –160 × 10–6 (pascal).
Experimental Section
General methods: ESR spectra were recorded on a Bruker ELEXSYS
E500 spectrometer. 1H NMR spectra were recorded on JEOL JNM-LA400,
JNM-LA300, and Bruker AVANCE-III. EI-MS, FAB-MS, and APCI-MS
spectra were recorded on SHIMADZU GCMS-QP2010 Plus, JEOL JMS-
700T, and Bruker micrOTOF II-OCU. Infrared spectra were measured
using a SHIMADZU FTIR-8700 spectrometer. UV/VIS spectra were
measured using SHIMADZU UV-2550. Melting points were measured
using a Yanaco MP-J3 apparatus and were not corrected. X-ray data were
collected by a Rigaku Saturn CCD system with graphite monochromated
MoKa radiation. Magnetic susceptibility measurements were performed
using a Quantum Design SQUID magnetometer MPMS-XL. Silica gel 60
(100–200 mesh) and aluminium oxide 90 (70–230 mesh, neutral) were
used for column chromatography. The progress of reactions was
monitored using thin-layer chromatography (TLC silica gel 60 F254 or TLC
aluminium oxide 60 F254 neutral, if not otherwise specified). All
commercially available compounds were reagent grade and used without
further purification. Tetrahydrofuran (THF) was dried and distilled over
sodium–(benzophenone ketyl) under nitrogen. Dichloromethane (CH2Cl2)
Decomposition reaction of 5: Diradical 5 (17 mg, 0.0616 mmol) was
dissolved in dry CH2Cl2 under nitrogen. The solution was exposed to Xe
light (>300 nm) for 20 min, and then concentrated under reduced pressure.
The crude product was purified by silica gel column chromatography using
CH2Cl2–AcOEt (2:1 v/v) and EtOH, to afford diradical 6 (8 mg, 0.0299
mmol, 49%) and brown oil (7 mg) involved in compound 8 which were
identified by TLC and NMR.
Acknowledgements
This study was supported by Grant-in-Aid for Scientific Research
from JSPS KAKENHI (JP15H00956 for K.O., JP17K05790 for
M.K. and K.O., and JP26102005 for S.S.).
and n-hexane were dried and distilled over calcium hydride. 4,4,5,5-
Conflict of interest
[10a]
Tetramethylimidazoline-3-oxide-1-oxyl (7)
and 3,4-Dihydro-2,2,5-
[11]
trimethyl-2H-pyrrole-1-oxide (8)
were prepared according to the
The authors declare no conflict of interest.
reported procedure.
Synthesis of 2-(4',4',5',5'-tetramethylimidazoline-3'-oxide-1'-oxyl-2'-
yl)-2,5,5-trimethylpyrrolidin-1-oxyl (5): 4,4,5,5-Tetramethylimidazoline-
3-oxide-1-oxyl (7, 602 mg, 3.83 mmol, 1.0 eq.) was dissolved in dry THF
(8 mL) under nitrogen. After this solution was cooled at –78 oC, lithium
bis(trimethylsilyl)amide (LHMDS, 1.6 M THF solution, 2.4 mL, 3.84 mmol,
1.0 eq.) was added dropwise. This solution was stirred for 20 min at –78
oC and a dry THF (3 mL) solution of compound 8 (543 mg, 4.27 mmol, 1.1
eq.) was added dropwise. This mixture was stirred for 20 min at –78 oC.
To the cold reaction mixture, a saturated aqueous solution of NaHCO3 (6
mL) was added, and the cold mixture was poured into water. The organic
layer was separated, dried over anhydrous Na2SO4 and filtrated. The
filtrate was concentrated under reduced pressure. The crude product was
purified by silica gel column chromatography using hexane–Et2O (1:15 v/v)
as eluent, to afford diradical 5 as a purple solid (296 mg, 30%). The
diradical 5 was recrystallized from a CH2Cl2–hexane mixture at –30 oC. 5:
C14H25N3O3; MW 283.37; mp 135 oC (decomp.); TLC (silica) Rf 0.40 (1:15
hexane–Et2O); MS (FAB+) m/z 283 [M+]; IR (KBr, cm–1): 2984, 1456, 1404,
1373, 1167, 1142, 860, 542; UV-Vis lmax/nm (e/M–1 cm–1) in CH2Cl2: 326
Keywords: diradicals • nitroxides • electron spin resonance •
magnetic property
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