Preparation and characterization of new chiral pyrrolyl α-nitronyl nitroxide radicals in which the imidazolyl framework was directly bound to chiral center

Article abstract of DOI:10.1016/j.molstruc.2010.12.024

Two pairs of nitronyl nitroxide radicals with specific molecular structures in which the imidazole radical moiety was directly bound to their chiral center have been prepared in their enantiopure forms. In these specific molecular structures, chirality ma

Full text of DOI:10.1016/j.molstruc.2010.12.024

Journal of Molecular Structure 989 (2011) 10–19
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Preparation and characterization of new chiral pyrrolyl a-nitronyl nitroxide
radicals in which the imidazolyl framework was directly bound to chiral center
Xiang-yang Qin a,1, Gang Xiong b,1, Ya-guang Sun b, Mei-Yan Guo b, Yue Ma c, Qiao-feng Wang a, Peng Liu a,
Peng Gao a, Xiao-li Sun a,
⇑
a Department of Chemistry, School of Pharmacy, Fourth Military Medical University, Xi’an 710032, PR China
b Laboratory of Coordination Chemistry, Shenyang Institute of Chemical Technology, Shenyang 110142, PR China
c Department of Chemistry, Nankai University, Tianjin 300071, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two pairs of nitronyl nitroxide radicals with specific molecular structures in which the imidazole radical
moiety was directly bound to their chiral center have been prepared in their enantiopure forms. In these
specific molecular structures, chirality may directly affect the unpaired spin electronic cloud. The X-ray
crystal structures of them have been solved, which reveal that supermolecule helical chains are formed
Received 18 October 2010
Accepted 12 December 2010
Available online 14 January 2011
by intermolecular hydrogen bond interactions in the
cule structure shaped by means of intermolecular hydrogen bond interactions to link homochiral chains.
Circular dichroism spectroscopy of the two pairs of chiral -nitronyl nitroxides shows significant Cotton
effects between 200 and 400 nm. The magnetic properties of the two pairs of nitronyl nitroxide radicals
in the solid state were characterized by EPR spectroscopy and magnetic susceptibility measurements,
respectively. Magnetic susceptibility measurements of the solids reveal antiferromagnetic interactions
L-NNP crystal and the L-NNNBP is a 3D supermole-
Keywords:
Chirality
Imidazolidine
Magnetism
Nitroxide
a
in all cases. However, in the
L-NNNBP, the exchange interaction abruptly changes from untiferromagnetic
to a ferromagnetic interaction at 40.0 k.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
relatively few in the literature [7,11]. Besides, most of the spin den-
sity is equally shared between the four atoms of the two NO groups
and, the bridging sp2 carbon atom carries a significant negative
spin density [12] (Scheme 1). So an ideal chiral nitronyl nitroxide
should be featured by chiral center directly bound to position 2
of the imidazolyl ring. In this specific molecular structure, chirality
may directly affect the unpaired spin electronic cloud. So in this
paper, we describe the synthesis and structural, magnetic, and
optical characterization of chiral nitroxide. To our knowledge, the
molecular structure of NNP and NNNBP is reported for the first
time.
In 1984, Barron and Vrbancich gave a theoretically predicted
magnetochiral dichroism (MChD), which arises from an intramo-
lecular interaction between the magnetic and electric dipole tran-
sition moments in the excited state [1]. In 1997, Rikken and
Raupach observed the MChD effect of tris-(3-trifluoroacetyl-( )-
camphorato) europium(III) in the paramagnetic state [2]. Since
then, the magneto-optical phenomena have been a particularly
interesting and challenging area [3–5]. With the aim of observing
the strong magnetochiral effect of chiral molecular magnets, sev-
eral optically active nitroxide radicals and their metal complexes
were prepared [6]. Chiral nitroxides are well chosen as potential
precursors of chiral molecule-based magnets. However, there are
only few examples of chiral nitroxides [7–10]. According to the
view of Paul Rey, to observe the effects of chirality on structural,
magnetic and optical properties, one should try to introduce a chi-
ral center as close as possible to the oxyl group carrying most of
the unpaired spin [7]. Unfortunately, chiral nitronyl nitroxide rad-
icals with the chiral centers sitting very close to the oxyl group are
2. Experimental
2.1. Materials and methods
1H NMR and 13C NMR spectra were recorded on a Bruker AV-
300 NMR spectrometer (300 MHz) (Bruker, Karlsruhe, Germany),
with CDCl3 as solvent and TMS as internal reference. Optical rota-
tion values were measured at 20 °C on a PerkinElmer 343 polarim-
eter (PerkinElmer Bodenseewerk, Überlingen, Germany). High
resolution mass spectroscopy (HRMS) was carried out on a Varian
7.0T ESI-FTICR-MS (Varian, USA) instrument. IR was recorded on a
Bruker tensor 27 (Bruker Optics, German) instrument. Ultra-violet
⇑
Corresponding author. Tel.: +86 29 84774470; fax: +86 29 84776945.
E-mail address: xiaolis@fmmu.edu.cn (X.-l. Sun).
These authors are co-first authors.
1
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.12.024

X.-y. Qin et al. / Journal of Molecular Structure 989 (2011) 10–19
11
-O
ether (3:1) as eluant, giving a colorless oil product (4.3 g, 86.1%).
O
O
O
20
½aꢀD = –143.2 (c = 1.00, CH2Cl2); IR (KBr) cmꢁ1:1521, 1350 (
m
N+
N
N
N
N
2
2
2
NO2), 1618 (m amide bond), 3406 (m OH), 1596, 1438, 862 (m ben-
R
R
R
zene ring), 1107, 1049 (m
CAC ring of pyrrolidine); 1H NMR (CDCl3):
N+
8.28 (d, 2H, J = 8.1 Hz, AArH), 7.71 (d, 2H, J = 8.4 Hz, AArH), 4.52 (d,
2H, ACH2), 3.82(s, 1H, AOH), 3.45 (m, 2H, ACH2), 2.12 (m, 2H,
ACH2), 1.88 (m, 2H, ACH2); 13C NMR d (CDCl3): 169.47, 148.47,
142.65, 128.16, 123.72, 65.70, 61.35, 50.94, 28.1, 24.93. Anal. Calcd.
for C12H14N2O4: C, 57.59; H, 5.64; N, 11.19. Found: C, 57.62; H,
-O
O
Scheme 1. The distribution of the unpaired spin density.
5.72; N, 11.09%. D-configuration product was synthesized by the
same method. ½aꢀD = +143.4 (c = 1.00, CH2Cl2).
absorption spectra were recorded on a Jasco V-570 UV/vis/NIR
spectrophotometer (Jasco, Japan). The EPR spectra were obtained
using a Bruker ESP 300E spectrometer on solutions of radicals dis-
solved in dichloromethane. Circular dichroism spectra were re-
corded on a JASCO-715 spectrometer. Magnetic susceptibility
measurements were obtained with a Quantum Design SQUID
magnetometer.
20
2.3. Synthesis of
(compound 2)
L-N-p-nitrobenzoylpyrrolidinylformaldehyde
To a reaction mixture of L-N-p-nitrobenzoylpyrrolidinylmetha-
nol (3.77 g, 15 mmol) and trichloroisocyanuric acid (TCCA, 3.7 g,
16 mmol), CH2Cl2 (40 ml) was added, and the reaction mixture
was stirred and maintained at 0 °C for 5 min, followed by addition
of TEMPO (0.025 g, 0.16 mmol). Then the mixture was warmed to
room temperature and stirred for 20 min and filtered on Celite.
The precipitate was washed with CH2Cl2 (10 ml ꢂ 2). The com-
bined organic phases were washed with 40 ml of a saturated solu-
tion of Na2CO3 and then by 0.1 N HCl (40 ml) and brine (40 ml). The
organic phase was dried over anhydrous Na2SO4, and stripped of
solvent. The crude product was purified by column chromatogra-
phy on silica gel using ethyl acetate/petroleum ether/triethylamine
(2:1:0.1) as eluant, giving the product as colorless liquid (3.25 g,
All chemical reagents were purchased from Nanjing Tianzun
Zezhong Chemical Limited Company (Nanjing, China). CH2Cl2 was
distilled from CaH2. The other chemical reagents were used with-
out further purification.
2.2. Synthesis of
1)
L-N-p-nitrobenzoylpyrrolidinylmethanol (compound
p-nitrobenzoyl chloride (4.0 g, 22.0 mmol), Et3N (2.23 g, 3.1 ml)
were added to in dry CH2Cl2 (50 ml) with vigorous stirring in an ice
bath. To this mixture, a solution of L-prolinol (2.0 g, 20 mmol) in
dry CH2Cl2 (20 ml) was added dropwise over a period of 20 min
at 0 °C. The mixture was warmed to room temperature and stirred
for 2 h. Then the reaction mixture was treated with water and ex-
tracted with CH2Cl2. The organic layer was dried over anhydrous
Na2SO4 and concentrated. The crude product was purified by col-
umn chromatography on silica gel using ethyl acetate/petroleum
20
86.3%). ½aꢀD = –64.8 (c = 1.00, CH2Cl2); IR (KBr) cmꢁ1: 1521, 1350
(m NO2), 1633 (m amide bond), 1731 (m CHO), 1598, 1423, 864 (m
benzene ring), 1107, 1014 (m
CAC ring of pyrrolidine); 1H NMR
(CDCl3): 9.68 (1H, ACHO), 8.29 (d, 2H, J = 6.0 Hz, AArH), 7.75 (d,
2H, J = 9.0 Hz, AArH), 4.71 (m, 1H, ACH), 3.57 (t, 2H, ACH2), 2.24
(m, 2H, ACH2), 2.03 (m, 2H, ACH2). 13C NMR d (CDCl3): 198.73,
CH2OH
CHO
N
CH2OH
N
C
N
Et3N
TEMPO(0.01 mol equiv)
H
+
Cl
2
1
O
C
O
CH2Cl2, r.t.
TCCA, CH2Cl2, r.t.
C
O
NO2
NO2
NHOH
NHOH
CH3OH
r.t.
3
NO2
OH
O
N
N
N
C
N
N
N
O
NaIO4, 0 0C
CH2Cl2
OH
O
C
O
4
5
NO2
D- and L-NNNBP
NO2
Scheme 2. Synthetic route of chiral nitronyl nitroxides.

12
X.-y. Qin et al. / Journal of Molecular Structure 989 (2011) 10–19
CH2Cl2
167.82, 148.50, 141.74, 128.34, 123.55, 65.33, 49.9, 26.07, 25.1.
Anal. Calcd. for C12H12N2O4: C, 58.06; H, 4.87; N, 11.29%. Found:
UV (k
): 320 (ONCNO,
p
?
pꢃ
,
e
= 6.7 ꢂ 103 L/(mol cm)), 569.6
max
(n ? pꢃ). EPR (CH2Cl2): g factor, 2.0046; aN, 7.572. As anticipated,
C, 58.01; H, 4.92; N, 11.22.
sized by the same method. ½aꢀD = +64.6 (c = 1, CH2Cl2).
D
-configuration product was synthe-
nitronyl nitroxide radicals showed no NMR signal.
product was synthesized by the same method.
D-configuration
20
2.4. Synthesis of
L
-N-p-nitrobenzoylpyrrolidinyl(4,5-dihydro-4,4,5,5-
3. Results and discussion
tetramethyl-3-oxido-1H-imidazol-3-ium-1-oxyl-2-yl) (compound 5)
3.1. Synthesis
A
solution of
L-N-p-nitrobenzoylpyrrolidinylformaldehyde
(2.5 g, 10 mmol) and 2,3-bis (hydroxylamino)-2,3-dimethylbutane
(1.48 g, 10 mmol) in methanol (25 ml) was stirred at room temper-
ature for 6 h. After the reaction, the methanol was removed and the
residue was suspended in 200 ml CH2Cl2, aqueous NaIO4 (2.14 g,
10 mmol in 50 ml) was added dropwise over a period of 10 min
at 0 °C, and the mixture was stirred for a further 20 min at 0 °C.
The organic phase was separated, and the aqueous phase was ex-
tracted with CH2Cl2 (3 ꢂ 20 ml). The combined organic layers were
dried over anhydrous Na2SO4. The deep red solution was then
evaporated. The crude product was purified by column chromatog-
raphy on silica gel using absolute ether/ethyl acetate/acetone/hex-
ane (3:1:2:3) as eluent, giving a deep red solid product (2.7 g, 72.6
According to Ullman’s pioneering work [13], any aldehydes may
give rise to nitronyl nitroxides. So chiral aldehydes should be ob-
tained first. According to what has been reported, alcohol can be
oxidated to aldehyde with trichloroisocyanuric acid in the pres-
ence of catalytic TEMPO under mild conditions [14]. In the first
experiment, we found prolinol could not be directly oxidated to
aldehyde under the above conditions. However, with p-nitro-
benzoyl used as the N protecting group of prolinol, the oxidation
reaction can proceed mildly and rapidly, and the chiral aldehyde
2 was obtained. Followed by condensation of the resulting chiral
aldehyde 2 with 2,3-bis(hydroxylamino)-2,3-dimethylbutane 3 in
methanol solution at room temperature, 1,3-dihydroxyimidazoli-
dines 4 were rapidly obtained as stable white solids. It is possible
to isolate 1,3-dihydroxyimidazolidines. But, in our experiment,
the methanol was removed after the condensation reaction. The
%). MS (m/z): 376.17 (MH+); IR (KBr): 1371, 1139 (
cmꢁ1: 1525, 1350 (
NO2), 1623 ( amide bond), 1596, 1425, 865
benzene ring), 1174, 1139, 1016 ( CAC ring of pyrrolidine);
m NO), IR (KBr)
m
m
(m
m
Table 1
Crystal data and structure refinement for the L-NNP and L-NNNBP.
Compound
L-NNP
L-NNNBP
CCDC reference no
CCDC 758041
CCDC 774936
Empirical formula
C16H27N3O4
C36H48N8O11
Chemical formula weight (Mr)
Crystal system, space group
a, b, c (Å)
325.41
768.82
Monoclinic, P21
6.1016(12), 10.392(2), 14.488(3)
90, 101.312(3), 90
900.8(3)
2, 1.200
0.087
Monoclinic, P21
14.079(5), 10.210(3), 14.827(5)
90, 114.003(4), 90
1947.0(10)
2, 1.311
0.098
a
, b,
c (°)
V (Å3)
Z, calculated density (Mg m–3
)
)
Absorption coefficient (mm–1
Crystal size (mm)
0.36 ꢂ 0.28 ꢂ 0.16
0.37 ꢂ 0.28 ꢂ 0.15
Theta range for data collection
Limiting indices
Reflections collected/unique
Max. and min. transmission
Goodness-of-fit on F2
F(0 0 0)
2.43–25.10°
2.50–25.00°
ꢁ7 ( h ( 7, ꢁ6 ( k ( 12, ꢁ16 ( l ( 17
4530/2433 [R(int) = 0.0419]
0.9859 and 0.9698
0.959
ꢁ13 ( h ( 16, ꢁ11 ( k ( 12, ꢁ17 ( l ( 17
9705/3631 [R(int) = 0.0523]
0.9851 and 0.9650
0.981
352
816
Final R indices [I > 2sigma(I)]
R indices (all data)
R1 = 0.0469, wR2 = 0.1187
R1 = 0.0561, wR2 = 0.1234
R1 = 0.0750, wR2 = 0.2047
R1 = 0.1343, wR2 = 0.2536
Fig. 1. Views of the enantiomeric molecules of L-NNP in the crystal structure with all non-H atom labeling scheme and ellipsoids drawn at the 30% probability level and
definition of the angles APNN and TIM and their signs in the two diastereomeric conformations that pyrrolyl -nitronyl aminoxyl radicals can adopt.
a

X.-y. Qin et al. / Journal of Molecular Structure 989 (2011) 10–19
13
Table 2
Selected bond lengths (AÅ) and angle (°) for L-NNNP and L-NNNBP.
Table 2 (continued)
0
L-NNP
L
-NNP
C(19)AC(20)AC(23)
C(19)AC(20)AC(21)
C(22)AC(23)AC(20)
C(20)AC(23)AN(6)
C(20)AC(23)AC(24)
N(6)AC(25)AN(5)
N(5)AC(25)AC(26)
C(27)AC(26)AC(25)
C(26)AC(27)AC(28)
N(7)AC(29)AC(28)
O(8)AC(30)AC(31)
C(32)AC(31)AC(36)
C(36)AC(31)AC(30)
C(34)AC(33)AC(32)
C(33)AC(34)AN(8)
C(36)AC(35)AC(34)
117.6(10)
104.8(11)
117.0(9)
102.2(6)
114.2(9)
109.1(7)
122.3(8)
117.4(8)
105.5(8)
103.6(9)
119.8(8)
118.9(7)
117.3(7)
117.0(7)
118.4(7)
117.4(7)
N(5)AC(20)AC(21)
C(23)AC(20)AC(21)
C(22)AC(23)AN(6)
C(22)AC(23)AC(24)
N(6)AC(23)AC(24)
N(6)AC(25)AAC(26)
C(27)AC(26)AN(7)
N(7)AC(26)AC(25)
C(29)AC(28)AC(27)
O(8)AC(30)AN(7)
N(7)AC(30)AC(31)
C(32)AC(31)AC(30)
C(31)AC(32)AC(33)
C(33)AC(34)AC(35)
C(35)AC(34)AN(8)
C(35)AC(36)AC(31)
106.7(8)
115.6(10)
109.1(8)
107.7(9)
105.8(8)
128.4(8)
105.3(8)
110.5(7)
107.1(9)
121.0(8)
119.2(8)
123.7(7)
121.6(8)
123.7(8)
117.7(8)
121.3(7)
N(1)AO(2)
N(1)AC(11)
N(2)AC(10)
N(3)AC(5)
N(3)AC(9)
O(4)AC(5)
C(1)AC(4)
C(3)AC(4)
1.281(3)
1.509(4)
1.327(3)
1.361(4)
1.461(4)
1.348(4)
1.502(5)
1.520(5)
1.494(4)
1.523(4)
1.515(4)
N(1)AC(10)
N(2)AO(3)
N(2)AC(14)
N(3)AC(6)
O(1)AC(5)
O(4)AC(4)
C(2)AC(4)
C(6)AC(7)
1.339(4)
1.295(3)
1.496(4)
1.455(4)
1.212(4)
1.489(3)
1.500(5)
1.512(5)
1.513(5)
1.561(4)
1.522(4)
C(9)AC(10)
C(11)AC(12)
C(14)AC(15)
C(11)AC(13)
C(11)AC(14)
C(14)AC(16)
O(2)AN(1)AC(10)
C(10)AN(1)AC(11)
O(3)AN(2)AC(14)
C(5)AN(3)AC(6)
C(6)AN(3)AC(9)
O(4)AC(4)AC(2)
C(2)AC(4)AC(1)
C(2)AC(4)AC(3)
O(1)AC(5)AO(4)
O(4)AC(5)AN(3)
C(8)AC(7)AC(6)
N(3)AC(9)AC(10)
C(10)AC(9)AC(8)
N(2)AC(10)AC(9)
N(1)AC(11)AC(13)
C(13)AC(11)AC(12)
C(13)AC(11)AC(14)
N(2)AC(14)AC(15)
C(15)AC(14)AC(16)
C(15)AC(14)AC(11)
125.9(2)
O(2)AN(1)AC(11)
O(3)AN(2)AC(10)
C(10)AN(2)AC(14)
C(5)AN(3)AC(9)
C(5)AO(4)AC(4)
O(4)AC(4)AC(1)
O(4)AC(4)AC(3)
C(1)AC(4)AC(3)
O(1)AC(5)AN(3)
N(3)AC(6)AC(7)
C(7)AC(8)AC(9)
N(3)AC(9)AC(8)
N(2)AC(10)AN(1)
N(1)AC(10)AC(9)
N(1)AC(11)AC(12)
N(1)AC(11)AC(14)
C(12)AC(11)AC(14)
N(2)AC(14)AC(16)
N(2)AC(14)AC(11)
C(16)AC(14)AC(11)
122.0(3)
111.9(2)
121.16(19)
121.7(3)
112.4(2)
109.8(2)
111.8(3)
112.3(3)
127.1(3)
109.2(3)
103.7(3)
112.6(2)
112.4(2)
126.2(3)
110.2(2)
110.5(3)
115.0(3)
109.7(3)
109.8(2)
115.3(3)
125.6(3)
112.3(2)
125.3(3)
121.0(2)
102.3(2)
109.0(3)
111.2(3)
123.6(3)
102.9(3)
105.0(2)
103.2(3)
109.5(2)
124.2(2)
106.2(2)
100.0(2)
114.0(2)
105.7(3)
101.0(2)
114.5(2)
residue was suspended in dichlormethane and then treated with
aqueous NaIO4 to give an instantaneous red due to the formation
of nitronyl nitroxides 5 (Scheme 2).
The radicals of L- and D-NNP (L or D-tert-butyl 2-(4,5-dihydro-
4,4,5,5-tetramethyl-3-oxido-1H-imidazol-3-ium-1-oxyl-2-yl) pyr-
rolidine-1-carboxylate) were prepared by a previously published
method [15].
One of the key steps in the synthesis of nitronyl nitroxide rad-
icals is the oxidation of 1,3-dihydroxyimidazolidines. According
to a published method [16], when PbO2 was used as oxidant, it
was time-consuming, and it was difficult to observe the process
of reaction due to the color of PbO2. However, when NaIO4 was
used as oxidant, it was time-saving, and it was convenient to ob-
serve the process of the reaction despite the overoxidation product
easily obtained. So, aqueous NaIO4 would be the best choice to give
the corresponding chiral nitronyl nitroxide radicals, provided that
the amount of NaIO4 and the time of oxidation reaction were
strictly controlled and the reaction was carried out in an immisci-
ble solvent such as dichlormethane in which the product was
soluble.
L
-NNNBP
N(1)AO(1)
N(1)AC(2)
N(2)AC(7)
N(3)AC(12)
N(3)AC(11)
N(4)AO(4)
N(5)AO(7)
N(5)AC(20)
N(6)AC(25)
N(7)AC(30)
N(7)AC(29)
N(8)AO(10)
1.285(8)
1.504(10)
1.328(10)
1.355(10)
1.459(11)
1.187(11)
1.268(8)
1.475(11)
1.329(10)
1.332(11)
1.486(13)
1.215(10)
1.236(9)
125.3(6)
112.4(6)
122.4(6)
119.5(6)
111.5(6)
119.6(10)
124.6(7)
112.1(7)
122.2(7)
119.6(7)
111.4(7)
120.1(9)
110.2(7)
105.3(7)
99.7(6)
112.7(8)
101.9(6)
103.4(7)
109.2(7)
124.0(7)
103.7(7)
106.5(8)
103.9(8)
119.9(7)
121.4(7)
120.6(7)
120.1(7)
120.7(9)
119.5(8)
107.7(8)
N(1)AC(7)
N(2)AO(2)
N(2)AC(5)
N(3)AC(8)
N(4)AO(5)
N(4)AC(16)
N(5)AC(25)
N(6)AO(6)
N(6)AC(23)
N(7)AC(26)
N(8)AO(9)
N(8)AC(34)
1.339(10)
1.267(8)
1.495(10)
1.456(10)
1.186(11)
1.440(12)
1.339(10)
1.278(9)
1.504(11)
1.475(11)
1.181(10)
1.476(12)
1.240(10)
O(3)AC(12)
O(8)AC(30)
O(1)AN(1)AC(7)
C(7)AN(1)AC(2)
O(2)AN(2)AC(5)
C(12)AN(3)AC(8)
C(8)AN(3)AAC(11)
O(5)AN(4)AC(16)
O(7)AN(5)AC(25)
C(25)AN(5)AC(20)
O(6)AN(6)AAC(23)
C(30)AN(7)AC(26)
C(26)AAN(7)AC(29)
O(9)AN(8)AC(34)
C(3)AC(2)AAN(1)
N(1)AC(2)AC(1)
N(1)AC(2)AC(5)
C(4)AAC(5)AN(2)
N(2)AC(5)AC(2)
N(2)AC(5)AC(6)
N(2)AC(7)AN(1)
N(1)AC(7)AC(8)
N(3)AC(8)AC(9)
C(10)AC(9)AC(8)
C(10)AC(11)AN(3)
O(3)AC(12)AC(13)
C(18)AC(13)AC(14)
C(14)AC(13)AC(12)
C(14)AC(15)AC(16)
C(17)AC(16)AN(4)
C(16)AC(17)AC(18)
N(5)AC(20)AC(19)
O(1)AN(1)AC(2)
O(2)AN(2)AAC(7)
C(7)AN(2)AC(5)
C(12)AN(3)AC(11)
O(5)AN(4)AO(4)
O(4)AN(4)AC(16)
O(7)AN(5)AC(20)
O(6)AN(6)AC(25)
C(25)AN(6)AC(23)
C(30)AN(7)AC(29)
O(9)AN(8)AO(10)
O(10)AN(8)AC(34)
C(3)AC(2)AC(1)
C(3)AC(2)AC(5)
C(1)AAC(2)AC(5)
C(4)AC(5)AC(2)
C(4)AC(5)AC(6)
C(2)AAC(5)AC(6)
N(2)AC(7)AC(8)
N(3)AC(8)AC(7)
C(7)AC(8)AC(9)
C(9)AC(10)AC(11)
O(3)AC(12)AN(3)
N(3)AC(12)AC(13)
C(18)AC(13)AC(12)
C(15)AC(14)AAC(13)
C(17)AC(16)AC(15)
C(15)AC(16)AN(4)
C(13)AC(18)AC(17)
N(5)AC(20)AC(23)
122.0(6)
125.6(6)
111.4(6)
129.0(7)
122.1(10)
118.2(10)
123.2(7)
125.7(7)
111.9(7)
128.1(8)
124.6(9)
115.2(9)
112.4(9)
117.3(9)
110.6(8)
119.8(8)
107.3(8)
110.5(8)
126.8(7)
111.0(6)
112.9(7)
109.5(8)
120.8(7)
119.2(7)
117.7(7)
118.2(7)
120.5(8)
118.8(8)
120.2(8)
103.8(7)
3.2. Structural studies
The enantiopure radical L-NNP crystallizes in the Monoclinic
P21 space group (Table 1), in which the dihedral angle between
the C6N3C9C8 unit and C6C7C8 unit in pyrrolyl ring and the
N2C10N1C11 unit and the NCN unit in imidazolyl ring is 35.14°
and 21.56°, respectively. These suggest that the pyrrolyl ring and
imidazolyl ring are twisted. The R enantiomer in the crystals has
PM chirality and the S has MP, the torsion angles imidazolyl rings
(APNN) and the NCCN unit in the imidazolyl ring (TIM) being 82.5°
and ꢁ21.5°, respectively (Fig. 1). The substituent tert-butoxycar-
bonyl at the N-position and the C6N3C9C8 unit in pyrrolyl ring are
coplanar. The C11AC14 bond length is 1.561(4) Å, compared with
1.543 Å for Csp3ACsp3 bond in the structure correlation analysis of
cyclopentane. The distances between C11 and C14 and their neigh-
bouring N atoms are 1.509(4) and 1.496(4) Å, identical (within the
standard deviation) to the overall mean of all Csp3AN+ distances.
The distances in imidazole for the Csp3AN bond (N1AC10) and
the Csp2AN bond (C10@N2) are 1.339(4) and 1.327(3) Å, respec-
tively. This observation implies that little of the
is located in the CAN bond in the -nitronyl aminoxyl radicals. The
distances, angles and torsion angles on either ‘side’ of the a-nitro-
nyl aminoxyl unit are the same within the error involved in the
measurement, supporting the notion that the ONCNO group is fully
p-electron density
a

14
X.-y. Qin et al. / Journal of Molecular Structure 989 (2011) 10–19
Fig. 2. The helical chain via hydrogen bond in the crystal structure of L-NNP.
Fig. 3. The stacking chains via hydrogen bonded in the crystal structure of L-NNP.

X.-y. Qin et al. / Journal of Molecular Structure 989 (2011) 10–19
15
Table 3
Unusual in the molecule is the difference between the two NAO
bond lengths (1.2181(3) Å and 1.295(3) Å), an effect which is
caused by the formation of a strong intramolecular hydrogen bond
[Csp3AHꢄ ꢄ ꢄO] between a hydrogen at the 2-position of the pyridine
ring and the oxygen atom of the NO groups of the same molecule.
Moreover, the intermolecular hydrogen bond [Csp3AHꢄ ꢄ ꢄO] is
formed by the methyl hydrogen atom attached to the imidazolyl
ring of one molecule and the oxygen atom associated with the
longer NAO group of a neighbouring one, leads to the formation
of helical chains which run along the crystallographic a axis, as
shown in Fig. 2. These aggregated chains form highly corrugated
sheets that pile up along the a axis and stack in a parallel manner,
as shown in Fig. 3. All hydrogen bond interactions informations are
shown in Table 3.
Hydrogen bond distances and geometries found in crystals of L-NNP and L-NNNBP.
Radical H-bond
[Hꢄ ꢄ ꢄO]
[XAHAO]
distance (Å)
[XAHAO]
angle (°)
distance (Å)
C3AH3Cꢄ ꢄ ꢄO1
2.42
3.017(4)
120
L
L
-NNP
C9AH11ꢄ ꢄ ꢄO2
2.56
2.48
2.49
2.937(4)
3.045(5)
3.393(4)
102
117
158
C2AH2Aꢄ ꢄ ꢄO1
#1
C
16AH17Cꢄ ꢄ ꢄO3
NNNBP
O11AH11Cꢄ ꢄ ꢄO6
2.03
-
2.736(11)
139
O
11AH11Dꢄ ꢄ ꢄO1
2.06
2.53
2.52
2.57
2.54
2.57
2.55
2.38
2.854(12)
3.453(13)
3.393(14)
3.426(9)
154
161
152
154
165
119
133
157
C4AH4Cꢄ ꢄ ꢄO6#1
#2
C6AH6Aꢄ ꢄ ꢄO7
C
C
C
C
C
14AH14ꢄ ꢄ ꢄO3#3
#3
15AH15ꢄ ꢄ ꢄO1
3.445(9)
#3
29AH29Bꢄ ꢄ ꢄO4
3.164(14)
3.259(10)
3.254(10)
The enantiopure radical
P21 space group (Table 1), which includes two
and one water molecule. In -NNNBP monomer, the pyrrolyl ring is
twist, which is similar to the -NNP and the corresponding dihedral
L
-NNNBP crystallizes in the Monoclinic
#4
32AH32ꢄ ꢄ ꢄO8
33AH33ꢄ ꢄ ꢄO7
L
-NNNBP monomers
#4
L
For
L
-NNP: #1 1 ꢁ x, ꢁ1/2 + y, 2 ꢁ z; for -NNNBP: #1 2 ꢁ x, 1/2 + y, 1 ꢁ z; #2 x, y,
L
L
1 + z; #3 1 ꢁ x, ꢁ1/2 + y, 1 ꢁ z; #4 1 ꢁ x, ꢁ1/2 + y, ꢁz.
is 21.64°. Extraordinary, all the atoms of the imidrazolyl ring are al-
most coplanar. The dihedral angle between the phenyl ring and the
C9C10N3C11 unit in pyrrolyl ring (C7AC12) bridged by carbonyl
group is 54.81° (Fig. 4). The R enantiomer in the crystals has PM
conjugated [17], as depicted in Fig. 1. Selected structural parame-
ters were summarized in Table 2.
Fig. 4. Views of the enantiomeric molecules of L-NNNBP in the crystal structure with all non-H atom labeling scheme and ellipsoids drawn at the 30% probability level and
definition of the angles APNN and TIM and their signs in the two diastereomeric conformations that pyrrolyl -nitronyl aminoxyl radicals can adopt.
a
Fig. 5. The dimer linked by water molecule in the crystal structure of L-NNNBP.

16
X.-y. Qin et al. / Journal of Molecular Structure 989 (2011) 10–19
Fig. 6. The homochiral chains formed via hydrogen bond in the crystal structure of L-NNNBP.
Fig. 7. Tree-dimension supermolecule stacking via hydrogen bonded in the crystal structure of L-NNNBP.
chirality and the S has MP, the torsion angles APNN and TIM being
77°, 8.8° and 78°, 22°, respectively (Fig. 4). The C2AC5 and
C20AC23 bond length are 1.524(9) and 1.505(9) Å respectively,
compared with 1.543 Å for Csp3ACsp3 bond in the structure correla-
tion analysis of cyclopentane. The distances between C2, C5, C20
and C23 and their neighbouring N atoms are 1.493(47), 1.494(8),
1.464(8) and 1.487(8) Å, identical (within the standard deviation)
to the overall mean of all Csp3AN+ distances. The distances in imid-
azole for the Csp3AN bond (N1AC7), the Csp2AN bond (C7@N2), the
homochiral chains running to c axis, as shown in Fig. 6. And then,
a great deal of intermolecular and intramolecular hydrogen bonds
[CAHꢄ ꢄ ꢄO] between the methyl or benzene ring hydrogen atom at-
tached to NAO group of the imidazolyl ring of one molecule or
neighbouring molecule help to link the adjacent chains, which
leads to the formation of a 3D supermolecular structure running
along the crystallographic b axis, as depicted in Fig. 7. All hydrogen
bond informations are shown in Table 3.
C
sp3AN bond (N5AC25), the Csp2AN bond (C25@N6) are 1.338(7),
1.329(7), 1.330(7) and 1.323(7) Å, respectively. This observation
implies that little of the -electron density is located in the CAN
bond in the -nitronyl aminoxyl radicals. The distances, angles
and torsion angles on either ‘side’ of the -nitronyl aminoxyl unit
3.3. Circular dichroism
p
Circular dichroism (CD) is a very sensitive tool for the study of
chiral molecules [18]. The CD spectrum of the two pairs of chiral
compounds reported here reveal significant Cotton effects between
a
a
are the same within the error involved in the measurement, sup-
porting the notion that the ONCNO group is fully conjugated
[17], as depicted in Fig. 4. Selected structural parameters were
summarized in Table 2.
Unusual in the two monomers there is difference between the
two NAO bond lengths, the average value being 1.278 Å. Water
230 and 350 nm (Fig. 8). The chiral nitronyl nitroxides
and or -NNNBP dissolved in dichloromethane presented a strong
absorption peak at 320 nm, which was assigned to
pꢃ transitions
of the ONCNO, but present no significant optical activity in the vis-
ible part of the electromagnetic spectrum. This situation probably
reflects the lack of conjugation between the chiral center and the
ONCNO unit which is responsible for absorption in this area. The
D or L-NNP
D
L
p
–
molecule connect two neighbouring L-NNNBP monomers to form
a dimer by the formation of strong intermolecular hydrogen bond
[O11AHꢄ ꢄ ꢄO] between the hydrogen atom of water molecule and
the oxygen atom of the NO group in the imidazolyl ring, as de-
picted in Fig. 5. Further, dimer connects with its neighbors via
intermolecular hydrogen bonds [C6AH6Aꢄ ꢄ ꢄO7] to form a 1-D
chiral nitronyl nitroxides
D or L-NNP dissolved in dichloromethane
presented a weak positive one centred at 238 nm, which was as-
signed to
nitroxides
p–
pꢃ transitions of the ester bond. The chiral nitronyl
D
or -NNNBP dissolved in dichloromethane presented
L
a weak positive one centred at 242 nm, which was assigned to

X.-y. Qin et al. / Journal of Molecular Structure 989 (2011) 10–19
17
Fig. 8. (a) CD spectra of L-NNP and D-NNP in CH2Cl2; (b) CD spectra of L-NNNBP and D-NNNBP in CH2Cl2.
Fig. 9. (a) EPR spectra of L-NNP in CH2Cl2; (b) EPR spectra of of L-NNNBP in CH2Cl2.
p
–
pꢃ transitions of the phenyl chromophores. The mirror image
relationship was apparently displayed between the CD spectrum
of -configuration and that of -configuration of the two pairs of
chiral compounds (Fig. 8).
L
-NNP is 0.393 cm3 molꢁ1 K at 300 K, which is slightly larger than
the calculated value of 0.375 cm3 molꢁ1 K for a noncoupled free
radical (g = 2.0). Moreover, upon cooling, the value remains almost
unchanged down to 7.0 K where an abrupt decrease is clearly vis-
ible which may be due to a weak antiferromagnetic interaction
L
D
3.4. Electron paramagnetic resonance
within the crystals of L-NNP. A negative Weiss temperature was
obtained for
L
-NNP from the Curie–Weiss equation
v
= C/(T ꢁ h)
Due to its sensitivity and accuracy, EPR spectroscopy is the best
tool for the study of free radicals [13], as shown in Fig. 9. While di-
with C = 0.39 cm3 molꢁ1 K and h = ꢁ0.23 K, further indicating that
the crystals of
The
is slightly larger than the calculated value of 0.375 cm3 molꢁ1 K for
a noncoupled free radical (g = 2.0). With the same -NNP, upon
L-NNP have weak antiferromagnetic interaction.
lute solutions of both the
show the five line pattern with hyperfine splitting patterns typical
of the -nitronyl nitroxide radicals, similar to each other in the ra-
L-NNP and L-NNNBP in dichlormethane
v
mT value of D
-NNNBP is 0.398 cm3 molꢁ1 K at 300 K, which
a
L
tio 1:2:3:2:1, which is expected due to coupling with two identical
nitrogens (Fig. 9).
cooling, the value remains slow down to 8.0 K where an abrupt de-
crease is clearly visible which may be due to weak antiferromag-
netic interaction within the crystals of D-NNP. A negative Weiss
3.5. Magnetic properties of the solids
temperature was obtained for D-NNP from the Curie–Weiss equa-
tion
v
= C/(T ꢁ h) with C = 0.40 cm3 molꢁ1 K and h = ꢁ3.63 K, fur-
Magnetic susceptibility measurements of ground crystalline
samples of the radicals reveal that the exchange interactions in
all the materials are antiferromagnetic, but by varying degree.
ther indicating that the crystals of
D-NNP have weak
antiferromagnetic interaction. This finding implies that the antifer-
romagnetic interaction of -NNP is similar to that of -NNP.
D
L
The temperature dependence of
v
mT and vꢁm1 measured at 1000
mT value of
Oe for -NNP and -NNP are shown in Fig. 10. The v
L
D

18
X.-y. Qin et al. / Journal of Molecular Structure 989 (2011) 10–19
-1
Fig. 10. Temperature dependence of the product
v
mT and vm measured at 1000 Oe for L-NNP and D-NNP.
-1
Fig. 11. Temperature dependence of the product
vmT and vm measured at 1000 Oe for L-NNNBP and D-NNNBP.
The temperature dependence of
1000 Oe for -NNNBP and -NNNBP are shown in Fig. 11. The
mT value of
-NNNBP is 0.39 cm3 molꢁ1 K at 300 K, which is
v
mT and vꢁm1 measured at
is little difference between magnetic interactions of
D-NNNBP and
L
D
that of -NNNBP. Further experiments are necessary to confirm
L
v
L
whether the very weak ferromagnetic couplings in
exist.
L-NNNBP really
slightly larger than the calculated value of 0.375 cm3 molꢁ1 K for
a noncoupled free radical (g = 2.0). Moreover, upon cooling, the va-
lue remains almost unchanged down to 40.0 K where an abrupt in-
crease is clearly visible which may be due to weak ferromagnetic
4. Conclusions
interaction within the crystals of
L-NNNBP Under 40.0 K. The mag-
netic susceptibility of -NNNBP was measured twice in the differ-
ence laboratory, and the same result was obtained both times.
However, when the temperature was more than 40 K, a negative
L
In summary, two pairs of nitronyl nitroxide radicals with spe-
cific molecular structure in which the imidazole radical moiety
was directly bound to chiral center were prepared through a chiral
pool in their enantiopure forms. The chemical structures of the
radicals were characterized by UV–Vis, IR, HRMS, and EPR. The
CD spectrum of the two pairs of chiral compounds reported here
reveal significant Cotton effects between 230 and 350 nm. The
Weiss temperature was obtained for L-NNNBP from the Curie–
Weiss equation
v
m = C/(T ꢁ h) with C = 0.39 cm3 molꢁ1
K and
L-NNNBP have
h = ꢁ3.81 K, further indicating that the crystals of
weak antiferromagnetic interaction.
The
is slightly larger than the calculated value of 0.375 cm3 molꢁ1 K for
a noncoupled free radical (g = 2.0). With the same -NNNBP, upon
vmT value of
D
-NNNBP is 0.40 cm3 molꢁ1 K at 300 K, which
CD spectrum of
two pairs of chiral compounds present apparently mirror image
relationship. A helical chain in -NNP is took shape through mole-
L-configuration and that of D-configuration of the
L
L
cooling, the value gradually low down to 45.0 K where an abrupt
decrease is clearly visible which may be due to weak antiferromag-
cules connected together by the weak [Csp3AHꢄ ꢄ ꢄO] between the
methyl hydrogen atom attached to the imidazolyl ring of one
molecule and the oxygen atom of the ‘‘free’’ nitroxyl group of a
neighbouring one. The three dimensional structure within the
netic interaction within the crystals of
D-NNNBP. A negative
Weiss temperature was obtained for
D-NNNBP from the Curie–
Weiss equation
v
= C/(T ꢁ h) with C = 0.42 cm3 molꢁ1
K
and
crystal
chains in which the molecules are linked together similar to the at-
tended mode in -NNP. All of these chiral radicals present weak
L-NNNBP is characterized by the presence of homochiral
h = ꢁ19.80 K, further indicating that the crystals of
D-NNNBP have
weak antiferromagnetic interaction. This finding implies that there
L

X.-y. Qin et al. / Journal of Molecular Structure 989 (2011) 10–19
19
[4] P.-G. Lacroix, R. Clement, K. Nakatan, J. Zyss, I. Ledoux, Science 5147 (1994)
658–660.
[5] P. Kleindienst, GH. Wagnie‘re, Chem. Phys. Lett. 1 (1998) 89–97.
[6] (a) H. Kumagai, K. Inoue, Angew. Chem. Int. Ed. 1 (1999) 1601–1603;
(b) M. Minguet, DB. Amabilino, K. Wurst, J. Veciana, J. Chem. Soc. Perkin Trans.
2 (2001) 670–676;
antiferromagnetic interactions. But small difference exist among
antiferromagnetic interactions. In addition, in the -NNNBP, the
exchange interaction abruptly change from antiferromagnetic to
ferromagnetic interaction at 40.0 k.
L
(c) M. Minguet, D. Luneau, E. Lhotel, V. Villar, C. Paulsen, D.B. Amabilino,
Angew. Chem. Int. Ed. 4 (2004) 586–589.
[7] C. Hirel, J. Pécaut, S. Choua, P. Turek, DB. Amabilino, J. Veciana, Eur. J. Org.
Chem. 2 (2005) 348–359.
[8] M.-A. Tanaka, H. Kumagai, A. Markosyan, K. Inoue, Bull. Chem. Soc. Jpn. 1
(2007) 204–207.
[9] Y. Numata, K. Inoue, N. Baranov, M. Kurmoo, K. Kikuchi, J. Am. Chem. Soc. 32
(2007) 9902–9909.
[10] M. Minguet, DB. Amabilino, K. Wurst, J. Veciana, J. Solid State Chem. 2 (2001)
440–450.
[11] S. Shimono, R. Tamura, N. Ikuma, T. Takimoto, N. Kawame, O. Tamada, J. Org.
Chem. 2 (2004) 475–481.
[12] (a) A. Zheludev, V. Barone, M. Bonnet, B. Delley, A. Grand, E. Ressouche, J. Am.
Chem. Soc. 5 (1994) 2019–2027;
4.1. X-ray diffraction analyses
The compounds of L-NNP and L-NNNBP were crystallized from
ethyl acetate/petroleum/hexane (1:1:1) as deep red needles. The
X-ray crystal data were collected on a Diffractometer Bruker
SMART 1000CCD X with graphite-monochromated Mo Ka radiation
(k = 0.71073 Å). The structures were solved by direct methods and
refined on F2 by full-matrix least-squares technique by using
SHELXL-97 [19]. Anisotropic thermal parameters were applied to
all non-hydrogen atoms. The hydrogen atoms were located in a
difference Fourier map. The data have been deposited with the
Cambridge Crystallographic Data Centre. CCDC reference numbers
758041 and 774936.
(b) F. -M. Romero, R. Ziessel, M. Bonnet, Y. Pontillon, E. Ressouche, J. Schweizer,
J. Am. Chem. Soc. 7 (2002) 1298–1309.
[13] (a) E.-F. Ullman, J.-H. Osiecki, D.-G.-B. Boocock, R. Darcy, J. Am. Chem. Soc. 20
(1972) 7049–7059;
(b) J.-H. Osiecki, E.-F. Ullman, J. Am. Chem. Soc. 4 (1968) 1078–1079.
[14] L.-D. Luca, G. Giacomelli, A. Porcheddu, Org. Lett. 19 (2001) 3041–3043.
[15] X.-Y. Qin, G.-R. Ding, X.-W. Wang, J. Tan, G.-Z. Guo, X.-L. Sun, J. Chem. Res. – S 8
(2009) 511–514.
Acknowledgment
[16] M. Lamchen, T.-W. Mittag, J. Chem. Soc. C. (1966) 2300–2303.
[17] D.-B. Amabilino, J. Cirujeda, J. Veciana, Philos. Trans. Roy. Soc. Lond. A 357
(1999) 2873–2891.
This work was supported by the National Science Foundation of
China (Nos. 20672141 and 20802092).
[18] K. Nakanishi, N. Berova, W. Woody (Eds.), Circular Dichroism: Principles and
Applications, VCH, Weinheim, 1994.
[19] (a) G.-M. Sheldrick, SHELXL-97, University of Göttingen, Göttingen, Germany,
1997.;
References
[1] L.-D. Barron, J. Vrbancich, Mol. Phys. 3 (1984) 715–730.
[2] G.L.J.A. Rikken, E. Raupach, Nature 390 (1997) 493–494.
[3] O. Kahn, Magnetism: A Supramolecular Function, Kluwer Academic, Dordrecht,
1996.
(b) G.-M. Sheldrick, SHELXL 97, Program for Crystal Structure Refinement,
University of Göttingen, Göttingen, Germany, 1997.;
(c) G.-M. Sheldrick, Acta Cryst. A64 (2008) 112.