Syntheses, optical and intramolecular magnetic properties of mono- and di-radicals based on nitronyl-nitroxide and oxoverdazyl groups appended to 2,6-bispyrazolylpyridine cores

Article abstract of DOI:10.1039/c2ob06875c

This paper presents the synthesis of a series of nitronyl-nitroxide (NN), oxoverdazyl (OVZ) based mono-, and bi-radicals attached to 4-phenyl-2,6- bispyrazolylpyridine coupling unit, their optical, electron spin resonance (ESR) spectroscopic studies and computational analysis. The ESR studies revealed that the axial zero-field splitting (zfs) parameter of the NN biradical (D/hc = 0.00719 cm^-1) is larger than the OVZ biradical (D/hc = 0.00601 cm^-1). Additionally both biradicals displayed forbidden half-field transitions (ΔM_s = ±2; g_av ～ 4.01) at 170 K demonstrating their triplet nature. The cryogenic ESR measurements of the two biradicals showed a Curie magnetic behaviour of the ΔM_s = ±2 signal intensities (χ_EPR) down to 4.2 K. A detailed comparative analysis of the strength of hyperfine coupling, spin density distribution, zfs and the spin-spin exchange coupling (J) of both NN and OVZ based biradicals showed that the ground state spin multiplicity of both biradicals is probably triplet (S = 1) or it is nearly degenerate singlet-triplet states with J_NN ? J_OVZ. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ob06875c

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PAPER
Syntheses, optical and intramolecular magnetic properties of mono- and
di-radicals based on nitronyl-nitroxide and oxoverdazyl groups appended to
2,6-bispyrazolylpyridine cores†
Pramiti Hui, Khaja Md. Arif and Rajadurai Chandrasekar*
Received 8th November 2011, Accepted 6th January 2012
DOI: 10.1039/c2ob06875c
This paper presents the synthesis of a series of nitronyl-nitroxide (NN), oxoverdazyl (OVZ) based mono-,
and bi-radicals attached to 4-phenyl-2,6-bispyrazolylpyridine coupling unit, their optical, electron spin
resonance (ESR) spectroscopic studies and computational analysis. The ESR studies revealed that the
axial zero-field splitting (zfs) parameter of the NN biradical (|D/hc| = 0.00719 cm−1) is larger than the
OVZ biradical (|D/hc| = 0.00601 cm−1). Additionally both biradicals displayed forbidden half-field
transitions (ΔMs = 2; gav ∼ 4.01) at 170 K demonstrating their triplet nature. The cryogenic ESR
measurements of the two biradicals showed a Curie magnetic behaviour of the ΔMs = 2 signal intensities
(χEPR) down to 4.2 K. A detailed comparative analysis of the strength of hyperfine coupling, spin density
distribution, zfs and the spin–spin exchange coupling (J) of both NN and OVZ based biradicals showed
that the ground state spin multiplicity of both biradicals is probably triplet (S = 1) or it is nearly
degenerate singlet–triplet states with JNN ≫ JOVZ
.
The design and preparation of paramagnetic molecules is of in-
terest, due to their possible applications in various areas, such as
magneto-chemistry,1,2 bio-organic chemistry,3 and bio-inorganic
chemistry.4 Among stable radicals, nitroxides (NO), nitronyl-
nitroxide (NN) and oxoverdazyl (OVZ) have gained much
interest due to their stability, and their propensity to coordinate
with d and f block elements. These stable radical units (one or
more) attached to chelating ligand cores5,6 are very attractive
since the resultant paramagnetic or high-spin ligands might show
interesting magnetic properties (spin–spin exchange coupling
(J)1–6/or spin cross-over effect7) upon coordination with para-
magnetic transition metal ions. In biradical carrying ligands,
before metal coordination, understanding the parameters such as
zero-field splitting (zfs), spin density distribution, and the J
value is crucial. A clear understanding of these factors4d requires
a comparative ESR spectroscopy investigation of two different
classes of meta bridged biradicals8 connected to the same coup-
ling units or spacers. In this context, we designed and syn-
thesized a series of paramagnetic ligands carrying NN and OVZ
based mono and biradicals units (1–6) connected to 4-phenyl-
2,6-bispyrazolylpyridine (bpp) coupling unit (Chart 1). These
Chart 1 Nitronyl-nitroixde (NN) and oxoverdazyl (OVZ) based mono
and di-radicals decorated to 2,6-bispyrazolylpyrines (1–6).
School of Chemistry, University of Hyderabad, Prof. C. R. Rao Road,
Gachi Bowli, Hyderabad – 500 046, India. E-mail: rcsc@uohyd.ernet.
in, chandrasekar100@yahoo.com; Fax: +91(40)23134824; Tel: +91
(40)23134824
†Electronic supplementary information (ESI) available: General exper-
imental methods, synthesis, characterization data and spin density calcu-
lation. CCDC reference numbers 768145. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2ob06875c
radicals also include two m-phenylene bridging type NN and
OVZ based biradicals (3 and 6), leading to non-Kekulé struc-
tures for high-spin ground-state formation.8
In this paper we report the preparation of mono and biradicals,
(1–6) their optical and ESR spectroscopy investigations. Single
crystal X-ray structure of radical 1 is also presented. A detailed
This journal is © The Royal Society of Chemistry 2012
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analysis to unravel the intramolecular magnetic properties, i.e.
the interplay between spin polarization, dipolar coupling
strengths and the ground-state spin properties also will be
presented.
It is important to mention here that the synthesis of NN radi-
cals is not straightforward. The traditional Ullman condensation
route,9 has a series of limitations i.e. the reaction of an aryl or
alkyl aldehyde group with 2,3-dimethyl-2,3-bis-(hydroxylamino)
butane (BHA) followed by oxidation of the diamagnetic conden-
sation product to get NN radicals. Here the synthesis of BHA
from 2,3-dimethyl-2,3-bis-(nitro)butane is very sensitive to reac-
tion conditions, such as the quality and particle size of the zinc
dust used in the reduction reaction. In most cases the synthesis is
very delicate and often not reproducible. Alternatively, a decade
ago, Ovcharenko and Rey et al. have developed an elegant
“diamino route” for the synthesis of NN radicals by the conden-
sation reaction of 2,3-dimethyl-2,3-dinitrobutane with aryl-alde-
hydes bypassing the preparation of BHA.10 Based on this
alternative procedure we have synthesized 2,3-dimethyl-2,3-di-
aminobutane by reducing 2,3-dimethyl-2,3-dinitrobutane using
tin metal under acidic reflux conditions.
The main precursors for the syntheses (Scheme 1) of NN and
OVZ based mono and biradicals (1–6) are the mono- and di-
aldehyde functionalized bpp (7–9), respectively. We have pre-
pared compounds 7–9 under Suzuki cross-coupling conditions
by reacting 4′-iodo-2′,6′-dipyrazolyl-pyridines7e with the corre-
sponding boronic acid derivative of mono or dialdehydes in
modest yields of ca. 55%. The five-member imidazolidine ring
derivatives (10–12) were prepared by reacting the aldehyde pre-
cursor (7–9) with 2,3-dimethyl-2,3-diaminobutane at 60 °C
under an inert atmosphere in very good yields. The formation of
the five-member imidazolidine moiety is evident from the
1H-NMR chemical shifts of the proton attached to tertiary
carbon (N–CH–N) typically in the range of 5.27–5.30 ppm
(Fig. 1 for 12). Furthermore LC-MS analyses also confirmed the
formation of a five-member imidazolidine ring by displaying m/z
peaks at 414 for 10 and 11 and 541 for 12. Finally the introduc-
tion of a 1,3-diol in the five-member imidazolidine moiety was
effected by treatment of compounds (10–12) with m-CPBA fol-
lowed by oxidation with NaIO4 under phase transfer conditions
(DCM–water) to get blue coloured NN radicals (1–3) in decent
yields. The Rf values of radicals 1–3 in acetone–hexane (1 : 9)
are 0.17, 0.11, and 0.30, respectively. Radical 1 was crystallized
in toluene for single crystal X-ray analysis.
Scheme 1 Synthetic route to nitronyl-nitroixde (NN) and oxoverdazyl
(OVZ) based mono- and di-radicals attached to 2,6-bispyrazolylpyrines
(1–6). Reagents and conditions: (a) 3-formylphenyl-boronic acid (for 7),
4-formylphenyl-boronic acid (for 8), 3,5-di-formylphenyl-boronic acid
(for 9), Pd(PPh3)4–1,4-dioxane–Na2CO3/3d/70 °C; (b) 2,3-diamino-2,3-
dimethylbutane–CHCl3/2d/60 °C (c) 2,4-diisopropylcarbono-hydrazide
bishydrochloride–sodium acetate–EtOH/15 h/RT; (d) m-CPBA–NaIO4–
CH2Cl2–H2O/0 °C/1 h (e) benzoquinone–toluene/60 °C
The OVZ-based mono (4 and 5) and m-phenylene type diradi-
cals (6) were also prepared from 7–9. We employed Brook
et al.’s procedure to synthesize N,N′-diisopropyl substituted
OVZ radicals (4–6).2h It gives OVZ higher stability compared to
its N,N′-dimethyl substituted analogs. Reaction of 1 equiv of
2,4-diisopropylcarbono-hydrazide bis-hydrochloride with the
appropriate aldehyde precursors in reflux ethanol afforded the
values of radicals 4–6 in CH2Cl2–EtOAc (9 : 1) are 0.58, 0.64,
and 0.77, respectively.
1
mono (13 and 14) and bistetrazanes (15). The H-NMR spectra
The single crystal X-ray structure of 1 showed that the torsion
angles (Φ) between the phenyl ring and the NN radical are
27.60° (C9-C8-C18-N7), and 28.59° (C7-C8-C18-N6) (Fig. 2).
Furthermore the phenyl ring and the chelating unit 2,6-bispyra-
zolylpyridine are in near planar configuration (Φ = ca. 24°). The
UV-Vis absorption studies of NN radicals (1–3) in toluene
showed characteristic bands for the n–π* transition in the visible
confirmed the formation of tetrazane six-member rings in 13–15
by displaying characteristic chemical shifts in the range of
5.23–5.3 ppm (Fig. 1 for 15). The tetrazenes were finally oxi-
dized with benzoquinone under reflux condition to get brown-
red colour mono (4 and 5) and dioxoverdazyl (6) radicals. The
LC-MS analyses further established the formation of radicals by
exhibiting m/z peaks at 469 for 4 and 5, and 649 for 6. The Rf
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Fig. 3 UV-Vis spectra of NN and OVZ radicals (1–6) in toluene. Inset
shows the characteristic colour of the biradicals 3 and 6.
Fig. 1 Comparative 1H-NMR spectra of radical precursors 12 and 15
obtained from dialdehyde 9.
Fig. 2 ORTEP plot (50% probability ellipsoid) of NN radical 1.
range (λmax ∼ 600 nm) with very low extinction coefficient
together with the vibronic components (Fig. 3). The electronic
absorption spectra of OVZ radicals (4–6) displayed distinctive
features corresponding to verdazyl radicals. Radicals 4–6 dis-
played high energy bands at 360, 367 and 374 nm, respectively
together with broad low energy bands from 400–650 nm range.
X-band ESR spectroscopy studies of radicals 1–6 were carried
out in degassed toluene solution to understand the intramolecular
magnetic properties (Fig. 4 and 5; and ESI Fig. S1 and S2†).
The room temperature spectra of mono radicals (1 and 2) (c ∼
10−4 M) exhibited typical five-line pattern (ΔMs = 1; intensity
ratio of 1 : 2 : 3 : 2 : 1) of an NN radical due to the hyperfine
coupling (hfc) of the NN electron spin with two equivalent
Fig. 4 Isotropic ESR spectra of biradicals 3 and 6 in toluene (c ∼ 10−4
M) recorded at room temperature. The black and gray lines show the
experimental and simulated spectra, respectively.
nitrogen (14N) nuclei. Whereas the spectrum of biradical 3 (c ∼
10−4 M) showed characteristic nine-line pattern (ΔMs
=
1;
intensity ratio close to 1 : 4 : 10 : 16 : 19 : 16 : 10 : 4 : 1) due to
the hfc of the two electron spins with four equivalent nitrogen
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Fig. 5 Top: Electron spin resonance (X-band) frozen state ZFS spectra of biradicals 3 and 6 at 170 K are shown in black lines. The asterix (*) marks
and gray shaded regions denote the contribution of the monoradical impurity in 3 and 6. For comparison the monoradical spectra of 1 and 4 are
plotted in gray lines together with the spectra of 3 and 6, respectively. The inset show the forbidden ΔMs = 2 transitions. Bottom: Temperature depen-
dence (36–4.2 K) of the doubly integrated signal intensities (χEPR) and the peak-to-peak signal intensities (ΔBpp) of the ΔMs = 2 (Curie plot) tran-
sitions of 3 and 6.
(14N) nuclei. The experimental spectra of 1 and 2 were best
simulated11 with the nitrogen hfc (aN) values of 7.40 G and 7.41
G, respectively. For the case of biradical 3, due to the strong
spin–spin exchange coupling (2J) within the EPR limits, the
simulation perfectly fits by assuming four nitrogens with half the
monoradical hyperfine coupling with 2J ≫ 7 × 10−4 cm−1 (2J/
aN ≫ 1). The estimated isotropic g-values of radicals 1–3 are
2.0066, 2.0065 and 2.0071, respectively. The room temperature
spectra of both mono radicals (4 and 5) showed distinct nine-line
(ΔMs = 1 transitions) pattern of an OVZ radical. Additionally,
each of the nine lines is composed of a group of lines due to
further hfc with magnetically active nuclei of small spin den-
sities. Simulation of the experimental spectra of OVZ radicals (4
The triplet nature of biradical 3 was clearly apparent from its
axial zfs pattern (in toluene; c ∼ 10−4 M; 170 K) with an
unusual resolution of nitrogen hyperfine coupling (Fig. 5 top
left). The measured zfs parameter (2D) value of 3 is 15.4 mT
(|D/hc| = 0.00719 cm−1). By applying a point-dipole approxi-
mation, from the experimental D value, the estimated distance
between the two NN centres in 3 is ca. 7.12 Å. Interestingly, bi-
radical 6 also displayed an axial zfs pattern at 170 K with a 2D
value of 12.86 mT (|D/hc| = 0.00601 cm−1) proving its biradical
nature (Fig. 5 top right). From the D value, an intra-spin distance
of separation (r) of ca. 7.56 Å was estimated between the two
OVZ radical centres in 6. For comparison the corresponding
frozen state monoradical spectra of 1 and 4 are plotted together
with the zfs spectra of biradicals 3 and 6, respectively (see gray
lines in Fig. 5).
Furthermore the triplet nature of both NN and OVZ biradicals
(3 and 6) were clearly supported by the forbidden half-field tran-
sitions (ΔMs = 2; gav ∼ 4.01) recorded at 170 K (See Fig. 5; top
insets). Additionally, the temperature dependency of the ΔMs =
2 transitions signal intensities of 3 and 6 was followed down to
4.2 K. The measurements were carefully done at lower micro-
wave powers in order to ensure that the signals were not satu-
rated. The plot of the doubly integrated (DI) and of the peak-to-
peak (ΔBpp) signal intensities (EPR magnetic susceptibility;
and 5) showed the following hfc values: aN1 = 6.41 G, aN2
=
5.35 G and aH = 1.13 G for 4; and aN1 = 6.20 G, aN2 = 5.20 G
and aH = 1.13G for 5. Interestingly, the spectrum of biradical 6
only exhibited nine-line patterns with an intensity ratio analo-
gous to mono OVZ radical. To our surprise a few literatures
which reported the ground state spin multiplicity of OVZ biradi-
cals did not disclose their isotropic hfc pattern.2f,g In our case,
the obtained nine-line spectrum of biradical 6 demonstrated that
the spin–spin exchange coupling between the two radical centres
must be close to zero. From the simulation the estimated isotro-
pic g-values of radicals 4–6 are 2.0063, 2.0065 and 2.0071,
respectively.
χ
EPR) of ΔMs = 2 transitions as function of inverse temperature
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(Fig. 5) of 3 and 6 showed sharp increase in the χEPR value
while approaching cryogenic temperature. This behaviour clearly
suggested that the triplet state spin population increases with the
decrease of temperature by following the Curie pattern. More-
over this finding indicated that the ground-state spin multiplicity
is probably triplet (S = 1) or the triplet state is nearly degenerate
with the singlet state (S = 0) for biradicals (3 and 6). But it is
clear from the obtained hfc pattern that the J value of 3 is
definitely much higher than that of 6, hinting that the two NN
radical centres in biradical 3 are more strongly spin–spin
exchange coupled than the two OVZ radical centres in biradical
6.
Additionally, computational studies (DFT; B3LYP) were per-
formed to determine the spin density distribution pattern in bi-
radicals 3 and 6 (ESI Fig. S3†). The value of spin density (ρC)
distributions in NN nodal plane carbon (ρCNN = −0.2057) and
next phenyl coupler carbon (ρCPh = 0.0521) in biradical 3 are
larger than the OVZ nodal plane carbon (ρOCVZ = −0.129) and
next phenyl coupler carbon (ρCPh = 0.0340). But the comparison
of the torsion angles (Φ) between the phenyl ring and the radical
plane showed that they are more planar [Φ is ca. 15°]2h in biradi-
cal 6 than that of 3 [Φ = ca. 28°] (for 3 radical 1 in Fig. 2). This
data suggested that the electronic structure of the five-member
NN ring favours more spin density leakage to the coupling unit
compared to the six-member OVZ ring (more delocalization
inside the ring). This observation was further supported by the
different D values obtained for biradicals 3 and 6 in the low
temperature EPR measurements.
Experimental section
General methods
3-Formylphenyl-boronic acid, 4-formylphenyl-boronic acid, 3,5-
diformylphenyl-boronic acid, pyrazole and citrazinic acid were
obtained from Aldrich. Potassium, trifluoro acetic acid, diglyme,
and NaIO4 were purchased from Spectrochem Pvt. Ltd,
Mumbai. Dichloromethane, hexane, pet-ether, CHCl3, dioxane,
toluene and methanol solvents were obtained from Finar Chemi-
cals Limited, Ahmadabad, India. All solvents were used after
distillation. CO2Cl2, I2, LiOH, tetramethylammonium chloride,
POCl3, mCPBA, anhydrous MgSO4, K2CO3, and KI were
obtained from Avra Synthesis, Hyderabad, India. Deuterated sol-
vents CDCl3-d1, and DMSO-d6 were obtained from Aldrich and
1
Merck respectively. H and 13C NMR spectroscopic data were
recorded on a Bruker DPX 400 spectrometer with solvent proton
as internal standard (CDCl3-d1 = 7.26 ppm and DMSO-d6
=
2.50 ppm). Column chromatography was performed using
Merck silica gel (particle size 100–200 mesh). LC mass spec-
trometry was performed on Shimadzu LCMS-2010A mass spec-
trometer. IR spectra were recorded on JASCO FT/IR-5300.
Elemental analyses were recorded on a Thermo Finnigan Flash
EA 1112 analyzer. For thin-layer chromatography (TLC), silica
gel plates Merck 60 F254 were used and compounds were visu-
alized by irradiation with UV light. EPR spectra were recorded
on a Bruker-ER073 instrument equipped with an EMX microX
source for X band measurement using Xenon 1.1b.60 software
provided by the manufacturer. For the liquid helium temperature
measurements, a temperature controller supplied from Oxford
instruments (ITC 503S) was used.
4-(2,6-Di(pyrazol-1-yl)pyridin-4-yl)benzaldehyde
(8). In
Conclusions
100 mL flask, 1,4-dioxane (20 mL) and 2 M Na2CO3 (5 mL)
were taken together and N2 gas was bubbled through the solution
for 30 min. To this 4′-iodo-2′,6′-dipyrazolyl-pyridine (200 mg,
0.593 mmol), 4-formylphenylboronic acid (106 mg,
0.712 mmol), and Pd(PPh3)4 (43 mg, 0.031 mmol, 5%) were
added and heated to 70 °C for 3 d under nitrogen atmosphere.
The solvent was removed in vacuo and the remaining residue
was treated with water and extracted with CH2Cl2 (3 × 50 mL).
The combined organic layer was dried with MgSO4 and the
solvent was removed in vacuo to get a light greenish solid
residue. This coloured residue was washed with methanol to
remove the soluble impurities and to isolate a white colour
powder of 8. Yield: 100 mg (53%). 1H NMR (400 MHz, CDCl3-
d1, δppm) 10.10 (s, 1H, –CHO), 8.61 (d, 2H), 8.13 (d, 2H),
8.03–7.95 (m, 4H), 7.79 (s, 2H), 6.53 (m, 2H) ppm. 13C NMR
(100 MHz, CDCl3-d1, δppm) δ 191.6, 152.7, 150.8, 143.2, 142.6,
136.9, 130.4, 128.0, 127.3, 108.2, 107.5 ppm. LCMS analysis
(m/z): found 316.10; calcd 315.11. FT-IR (KBr): 2963, 2917,
28589, 1703 (–CHO), 1620, 1557, 1466, 1395, 1262, 1209,
1032, 937, 792 cm−1. Anal. calcd for C18H13N5O (315.11):
found C, 68.65; N, 22.45; H 4.09%; requires C, 68.56; N, 22.21;
H, 4.16%.
Recently, Eaton, Neese et al.4d have reported that at shorter spin
distances or in delocalized spin systems the use of point-dipole
approximation is inadequate (i.e., deviation from R−3 behaviour).
Most of the time the intra-radical distance derived from the
experimental zfs is inconsistent with the X-ray structure. More-
over the spin–spin contribution to the zfs parameter dictates the
zfs when S ≥ 1. In our present case, to study the relative differ-
ence in the D values, two types (NN and OVZ) of biradicals con-
nected to the same 4-pheny-2,6-bispyrazolylpyridine coupling
unit (3 and 6) were synthesized. The obtained difference in the
D and r (calculated via point-dipole approximation) values
between the two types of biradicals attached to the same unsatu-
rated molecular backbone (3 and 6) are 0.00118 cm−1 and ca.
0.44 Å, respectively. The significant difference observed in the
D values of 3 and 6 clearly confirmed the existing variations in
the orientation of the two spin carrying units, their spin density
distributions and the resulting dipolar coupling strengths. From
the computational studies it is clear that in 3 the spin density
wave is more delocalized into the whole molecular backbone
there by reducing the average r value between the two spins
(larger D value) compared to biradical 6 (smaller D value).
Additionally since the amount of spin density wave propagation
is also related to the spin–spin exchange interaction (J), the
obtained results demonstrated that the NN biradicals in 3 are
rather more strongly exchange coupled than the OVZ biradicals
3-(2,6-Di(pyrazol-1-yl)pyridin-4-yl)benzaldehyde (7). Similar
procedure for the synthesis of 8 was applied. 4′-Iodo-2′,6′-dipyr-
azolylpyridine (200 mg, 0.5932 mmol), 3-formylphenyl-boronic
acid (106 mg, 0.712 mmol), 2 M Na2CO3 (5 mL) and Pd(PPh3)4
in 6 i.e. JNN ≫ JOVZ
.
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(43 mg, 0.031 mmol, 5%). White colour powder of 7. Yield:
92 mg (51%) H NMR (400 MHz, CDCl3-d1, δppm): 10.13 (s,
(63%).1H-NMR (400 MHz, CDCl3-d1, δppm): 8.62 (s, 2H), 8.14
(s, 1H), 8.01 (s, 1H), 7.79–7.72 (m, 6H), 6.55 (s, 2H), 5.30 (s,
1H, N−CH−N), 4.30–4.12 (m, 2H), 1.33–1.13 (m, 12H).
13C-NMR (100 MHz, CDCl3-d1, δppm) δ: 154.4, 150.6, 142.3,
137.5, 129.2, 127.6, 127.2, 126.5, 125.5, 107.9, 107.4, 62.1,
31.9, 31.4, 31.0, 29.7, 29.4, 25.3, 23.6, 23.0, 22.7, 14.1. LCMS
analysis (m/z) found 472.00; calcd 471.56. FT-IR (KBr): 3435.5,
3288.9, 3165.5, 2999.6, 1701.4, 1641.6, 1566.3, 1413.9, 1049.4,
1012.7, 923.99, 810.2, 650.1, 623.1, 522.8, 464.9 cm−1. Anal.
calcd for C25H29N5O (471.25): found C, 63.58; N, 26.45; H,
6.25; requires, 63.68; H, 6.20; N, 26.73%.
1
1H, –CHO), 8.62–8.61 (d, 2H), 8.32 (t, 1H), 8.15 (s, 2H),
8.10–8.08 (m, 1H), 8.01 (m, 1H), 7.8 (d, 2H), 7.71–7.67 (t, 1H),
6.54–6.53 (q, 2H). 13CNMR (100 MHz, CDCl3-d1, δppm): 191.7,
152.7, 150.8, 142.6, 138.5, 137.2, 133.0, 130.5, 129.9, 128.6,
127.3, 108.2, 107.3. LCMS analysis (m/z): found 316.10; calcd
315.11. FT-IR (KBr): 3432, 2963, 1699 (–CHO), 1618, 1562,
1522, 1470, 1399, 1262, 1036, 949, 862, 799, 540 cm−1. Anal.
calcd for C18H13N5O (315.11): found C 68.65, N 22.45, H
4.09%; requires C 68.56, N 22.21, H 4.16%.
5-(2,6-Di(pyrazol-1-yl)pyridin-4-yl)isophthalaldehyde (9).
Similar procedure for the synthesis of 8 was applied. 4′-Iodo-
2′,6′-dipyrazolyl-pyridine (200 mg, 0.59 mmol), 3,5-diformyl-
phenylboronic acid (126 mg, 0.712 mmol) and Pd(PPh3)4
(80 mg, 0.031 mmol, 5%). White colour powder of 9. Yield:
6,6′-(5-(2,6-Di(pyrazol-1-yl)pyridin-4-yl)-1,3-phenylene)bis
(2,4-diisopropyl-1,2,4,5-tetrazinan-3 one) (15). Compound 15
also synthesized by same procedure. 2,4-Diisopropylcarbono-
hydrazide bishydrochloride (152 mg, 0.87 mmol), 5-(2,6-di
(pyrazol-1-yl)pyridin-4-yl)isophthalaldehyde (9) (100 mg,
0.343 mmol), and sodium acetate (72 mg, 0.87 mmol). Yield:
1
125 mg (58%). H-NMR (400 MHz, CDCl3-d1, δppm): 10.21 (s,
1
2H, –CHO), 8.62–8.56 (d, 4H), 8.48 (s, 1H), 8.18 (s, 2H), 7.81
(s, 2H), 6.55 (m, 2H). 13C-NMR (100 MHz, CDCl3-d1, δppm) δ:
190.4, 151.3, 151.0, 142.8, 139.8, 137.9, 133.1, 131.2, 127.3,
108.4, 107.2. LCMS analysis (m/z): found 344.0; calcd 343.11.
FT-IR (KBr): 3403, 2919, 1698(–CHO), 1616, 1464, 1397,
1262, 1020, 801 cm−1. Anal. calcd for C18H13N5O (541.82):
found C, 66.32; N 20.54, H 3.88%; requires C 68.47, N 20.40,
H 3.82%.
111 mg (59%). H-NMR (400 MHz, CDCl3-d1, δppm): 8.58 (s,
2H), 8.11 (s, 2H), 7.86 (s, 1H), 7.75 (m, 4H), 6.48 (s, 2H), 5.24
(s, 2H, N−CH−N), 4.88–4.13 (m, 4H), 1.23–1.05 (m, 24H).
13C-NMR (100 MHz, CDCl3-d1, δppm) δ: 154.0, 150.6, 149.9,
147.1, 145.8, 142.7, 136.5, 127.2, 107.9, 107.4, 63.6, 31.9,
31.4, 31.0, 29.7, 29.4, 25.3, 23.6, 23.0, 22.7, 14.2. LCMS analy-
sis (m/z): found 655.60; calcd 655.80. Anal. calcd for
C33H45N13O2 (655.80): found C, 60.58; N, 27.45; H, 6.85%;
requires, C, 60.44; H, 6.92; N, 27.77%; FT-IR (KBr): 3435.5,
3288.9, 2980.3, 2935.9, 2878.1, 1738.0, 1707.2, 1641.6, 1564.4,
1412.0, 1250, 1155.5, 1045.5, 1012.7, 920.1, 852.6, 788.9,
General procedure for tetrazanes
646.2, 530.5 cm−1
.
6-(3-(2,6-Di(pyrazol-1-yl)pyridin-4-yl)phenyl)-2,4-diisopro-
pyl-1,2,4,5-tetrazinan-3-one (13). In a typical procedure, 2,4-di-
isopropylcarbonohydrazide
bishydrochloride
(81
mg,
General procedure for 4,4,5,5-tetramethylimidazolidines
0.468 mmol) and 3-(2,6-di(pyrazol-1-yl)pyridin-4-yl)benz-
aldehyde (7) (100 mg, 0.312 mmol) were dissolved completely
in a minimum amount of ethanol (10 mL). To this clear solution
was added sodium acetate (51 mg, 0.624 mmol) and the solution
was allowed to stir at room temperature for 15 h (no noticeable
colour change was found). The turbid solution was filtered to
remove the sodium acetate and the filtrate was evaporated
in vacuo to get a yellowish-white residue. The residue was
recrystallized from heptane to get white colour tetrazane com-
2,6-Di(pyrazol-1-yl)-4-(4-(4,4,5,5-tetramethylimidazolidin-2-
yl)phenyl)pyridine (11). In a typical procedure, a solution of
4–2,6-di(pyrazol-1-yl)pyridin-4-yl-benzaldehyde (8) (200 mg,
0.634 mmol) in chloroform (10 mL) was added drop-wise
to a solution of 2,3-diamino-2,3-dimethylbutane (110 mg,
0.952 mmol) in chloroform (20 mL) in a round bottom flask.
The reaction mixture was heated at 60 °C for 2 d under argon to
get a light turbid solution. The TLC confirmed the complete con-
sumption of 8 (Rf = 0.55; 3 : 2 DCM–hexane). The solution was
directly dried with Na2SO4 and evaporated in vacuo to get com-
1
pound 13. Yield: 87 mg (60%). H-NMR (400 MHz, CDCl3-d1,
δppm): 8.62–8.61 (d, 2H), 8.14 (s, 2H), 8.01 (s, 1H), 7.79–7.72
(m, 4H), 7.51–7.49 (m, 1H), 6.52 (d, 2H), 5.29 (s, 1H, N−CH
−N), 4.88–4.16 (m, 2H), 1.33–1.13 (m, 12H). 13C-NMR
(100 MHz, CDCl3-d1, δppm) δ: 154.4, 150.9, 149.1, 146.5,
142.6, 132.3, 132.2, 131.5, 128.6, 128.5, 127.1, 108.1, 107.8,
63.0, 31.9, 31.4, 31.0, 29.7, 29.4, 25.3, 23.6, 23.0, 22.7, 14.1.
LCMS analysis (m/z): found 471.30; calcd 471.56. FT-IR (KBr):
2982, 1722, 1647, 1574, 1417, 1255, 1105, 1051, 771, 652, 623,
457 cm−1. Anal. calcd for C25H29N5O (471.25): found C 63.48,
N 26.59, H 6.25%; requires C, 63.68; H, 6.20; N, 26.73%.
1
pound 11 as a white powder. Yield: 172 mg (66%). H-NMR
(400 MHz, CDCl3-d1, δppm): 8.63 (s, 2H), 8.18 (s, 2H), 7.84,
7.82, 7.8, 7.73, 6.53 (s, 2H), 5.27 (s, 1H, N–CH–N), 1.25, 1.23,
1.13. 13C-NMR (100 MHz, CDCl3-d1, δppm): 153.9, 150.6,
145.8, 142.4, 136.5, 127.2, 108.0, 107.2, 73.0, 63.0, 31.0, 25.3,
23.6, 14.2. LCMS analysis (m/z): found, 414.25; calcd, 413.52.
FT-IR (KBr): 3335, 2978, 2919, 2855, 1615, 1541, 1460, 1391,
1273, 1171, 694 cm−1. Anal. Calcd for C18H13N5O (541.82):
found C 69.58, N 23.85, H 6.51. Required C, 69.71; H, 6.58; N,
23.71.
6-(4-(2,6-Di(pyrazol-1-yl)pyridin-4-yl)phenyl)-2,4-diisopro-
pyl-1,2,4,5-tetrazinan-3-one(14). Compound 14 was synthesized
by above tetrazane procedure. 2,4-Diisopropylcarbonohydrazide
bishydrochloride (81 mg, 0.468 mmol), 4-(2,6-di(pyrazol-1-yl)
pyridin-4-yl)benzaldehyde (8) (100 mg, 0.312 mmol), and
sodium acetate (51 mg, 0.624 mmol). Yield: 93 mg
2,6-Di(pyrazol-1-yl)-4-(3-(4,4,5,5-tetramethylimidazolidin-2-
yl)phenyl)pyridine(10). Compound 10 was synthesized by above
condensation procedure. 3-(2,6-Di(pyrazol-1-yl)pyridin-4-yl)
benzaldehyde (7) (200 mg, 0.634 mmol) and 2,3-diamino-2,3-
dimethylbutane (110 mg, 0.952 mmol) Yield: 163 mg (62%);
2444 | Org. Biomol. Chem., 2012, 10, 2439–2446
This journal is © The Royal Society of Chemistry 2012

View Article Online
1H-NMR (400 MHz, CDCl3-d1, δppm): 8.62 (s, 2H), 8.14 (s,
2H), 8.01 (s, 1H), 7.79–7.74 (m, 4H), 7.72 (s, 1H), 6.52 (s, 2H),
5.30 (s, 1H, N−CH−N), 1.34–1.14 (s, 12H). 13C-NMR
(100 MHz, CDCl3-d1, δppm): 154.4, 150.6, 145.2, 144.0, 142.3,
140.5, 139.0, 137.5, 129.2, 127.6, 127.2, 126.5, 125.5, 108.0,
107.5, 97.2, 87.3, 73.1, 63.0, 31.5, 31.0, 25.3, 24.0, 23.1, 14.1.
LCMS analysis (m/z): found, 413.65; calcd, 413.52. FT-IR
(KBr): 3335, 2978, 2919, 2855, 1615, 1541, 1460, 1391, 1273,
1171, 694 cm−1. Anal. Calcd for C18H13N5O (541.82): found C
69.58, N 23.85, H 6.51. Required C, 69.71; H, 6.58; N, 23.71.
cm−1). FT-IR (KBr): 3233, 2982, 2922, 1754, 1654, 1556, 1464,
1391 (N–O), 1262, 1221, 1088, 945, 874, 831, 749, 651, 517,
440 cm−1
.
4-(3,5-Bis(1-oxy-3oxo-4,4,5,5-tetramethylimidazolidin-2-yl)
phenyl)-2,6-di(1H-pyrazol-1-yl)pyridine (3). m-Chloroperben-
zoic acid (168 mg, 3.336 mmol), 4-(3,5-bis(4,4,5,5-tetramethyl-
imidazolidin-2-yl)phenyl)-2,6-di(1H-pyrazol-1-yl)pyridine (12)
(30 mg, 0.556 mmol) and NaIO4 (23 mg, 1.112 mmol) Yield:
4 mg (20%). Rf = 0.30 (10% acetone–hexane). EPR (in toluene
c ∼ 10−4 M, RT, ν = 9.13829 GHz, 0.998 mW power, 1 scan):
nine lines, giso = 2.0071. UV-Vis (in toluene c = 5 × 10−3 M):
λabs (ε) 592 nm (524 M−1 cm−1), 303 nm (2007 M−1 cm−1).
FT-IR (KBr): 2924, 2854, 1734, 1616, 1560, 1521, 1466, 1394,
1261, 1124, 1039, 952, 862, 788, 540 cm−1. EPR (in toluene
4-(3,5-Bis(4,4,5,5-tetramethylimidazolidin-2-yl)phenyl)-2,6-di
(1H-pyrazol-1-yl)pyridine (12). Compound (12) was synthesized
by above condensation procedure. 5-(2,6-Di(pyrazol-1-yl)
pyridin-4-yl)isophthalaldehyde (9) (200 mg, 0.583 mmol) and
2,3-diamino-2,3-dimethylbutane (192 mg, 1.748 mmol). Yield:
c ∼ 10−4 M, ν = 9.14572 GHz, 0.998 mW power, 1 scan): giso
=
2.0071, |D/hc| = 0.00719 cm−1, ΔMs = 2 transition gav ∼ 4.01.
Anal. calcd for C31H35N9O4 (597.67): N, 21.09; found C, 62.15;
H, 5.95; N, 21.32; requires C, 62.30; H, 5.90.
1
172 mg (55%). H-NMR (400 MHz, CDCl3-d1, δppm): 8.61 (s,
2H), 8.13 (s, 2H), 7.99 (s, 1H), 7.90 (s, 2H), 7.77 (s, 2H), 6.52
(s, 2H), 5.30 (s, 2H, N−CH−N), 1.30–1.16 (m, 24H). 13C-NMR
(100 MHz, CDCl3-d1, δppm): 154.5, 150.6, 142.3, 137.7, 127.2,
125.7, 125.1, 108.0, 73.4, 63.3, 25.4, 24.5, 23.6. LCMS analysis
(m/z): found, 540.8; calcd, 539.35. FT-IR (KBr): 3337, 2969,
2932, 2855, 1721, 1617, 1561, 1466, 1397, 1208, 1169, 1042,
General procedure for OVZ radicals
6-(3-(2,6-Di(pyrazol-1-yl)pyridin-4-yl)phenyl)-2,4-diisopropyl
-3-oxoverdazyl (4). In a typical procedure, 6-(3-(2,6-di(pyrazol-
1-yl)pyridin-4-yl)phenyl)-2,4-diisopropyl-1,2,4,5-tetrazinan-3-
one (13) (100 mg, 0.212 mmol) was heated with benzoquinone
(34 mg, 0.318 mmol) in 5 mL toluene at 40 °C for 30 min.
When TLC indicated that the starting material had disappeared
the resulting solution was cooled, filtered and evaporated.
Chromatography of the residue on silica gel using CH2Cl2–
EtOAc (9 : 1) gave a brown solid of 4. Yield: 25 mg (24%). Rf =
0.58 in CH2Cl2–EtOAc (9 : 1). LCMS analysis calcd (m/z):
468.53, found 468.4. UV-Vis (in toluene): λabs 360 and 374 nm.
FT-IR (KBr): 3243, 2984, 2932, 1734, 1634, 1516, 1464, 1373,
1262, 1221, 1088, 945, 874, 831, 760, 681, 527, 448 cm−1. EPR
(in toluene c ∼ 10−4 M, RT, ν = 9.14577 GHz, 0.998 mW
938, 862, 758 cm−1
.
General procedure for NN radicals
2,6-Di(pyrazol-1-yl)-4-(3-(1-oxy-3oxo-4,4,5,5-tetramethylimi-
dazolidin-2-yl)phenyl)pyridine (1). A solution of m-chloroper-
benzoic acid (84 mg, 0.145 mmol) in methylene chloride (5 mL)
was added drop-wise to a mixture containing 10 (20 mg,
0.048 mmol) and saturated solution of NaHCO3 (1 mL) in an ice
bath and stirred for one hour. A light blue tint was slowly devel-
oped indicating the formation of a nitronyl-nitroxide radical. To
this solution, NaIO4 (21 mg, 0.0968 mmol) in water (10 mL)
was added drop-wise and then stirred for 30 min to get a dark
blue colour solution, which indicated the complete oxidization
of the radical precursor. From the phase transfer solution, the
dark blue organic layer was separated, and the light blue
aqueous layer was twice extracted with dichloromethane (20 mL
× 2). The organic layers were combined and dried with MgSO4
and the solvent was evaporated in vacuo to get radical 1 as a
blue solid. Yield: 6 mg (30%). Rf = 0.17 (10% acetone–hexane).
EPR (in toluene; c ∼ 10−4 M, RT, ν = 9.13782 GHz, 0.998 mW
power, 1 scan): five lines, giso = 2.0066, aN = 7.41 G. UV-Vis
(in toluene c = 5 × 10−3 M): λabs (ε) 567 nm (1296 M−1 cm−1),
300 nm (ε = 6970 M−1 cm−1). FT-IR (KBr): 3253, 2982, 2931,
1734, 1654, 1526, 1464, 1390, 1265, 1224, 1188, 945, 834, 831,
power, 1 scan): nine lines, giso = 2.0063, aN1 = 6.41 G, aN2
=
5.35G and aH = 1.13G. Anal. calcd for C25H26N9O (468.53):
found C, 64.17; H, 5.48; N, 26.88; requires C, 64.09; H, 5.59;
N, 26.91.
6-(4-(2,6-Di(pyrazol-1-yl)pyridin-4-yl)phenyl)-2,4-diisopro-
pyl-3-oxoverdazyl (5). Compound (5) was synthesized by follow-
ing the above procedure. 6-(4-(2,6-Di(pyrazol-1-yl)pyridin-4-yl)
phenyl)-2,4-diisopropyl-1,2,4,5-tetrazinan-3-one (14) (100 mg,
0.212 mmol) and benzoquinone (34 mg, 0.318 mmol) Yield:
27 mg (26%). Rf = 0.64 in CH2Cl2–EtOAc (9 : 1). LCMS analy-
sis (m/z): calcd 468.53; found 469.05. UV-Vis (in toluene): λabs
400–650 nm. FT-IR (KBr): 3243, 2984, 2932, 1734, 1634,
1516, 1464, 1373, 1262, 1221, 1088, 945, 874, 831, 760, 681,
527, 448 cm−1. EPR (in toluene c ∼ 10−4 M, RT, ν = 9.14808
749, 680, 567, 443 cm−1
.
2,6-Di(pyrazol-1-yl)-4-(4-(1-oxy-3oxo-4,4,5,5-tetramethylimi-
dazolidin-2-yl)phenyl)pyridine (2). m-Chloroperbenzoic acid
(84 mg, 0.145 mmol), 2,6-di(pyrazol-1-yl)-4-(4-(4,4,5,5-tetra-
GHz, 0.998 mW power, 1 scan): nine lines, giso = 2.0063, aN1
=
6.20 G, aN2 = 5.20 G and aH = 1.13G. Anal. calcd for
C25H26N9O (468.53): found C 64.15, N 26.78, H 5.48; requires
C, 64.09; H, 5.59; N, 26.91.
methylimidazolidin-2-yl)phenyl)pyridine
(11)
(20
mg,
0.048 mmol) and NaIO4 (21 mg, 0.0968 mmol) Yield: 7 mg
(35%). Rf = 0.11 (10% acetone–hexane). EPR (in toluene c ∼
10−4 M, RT, ν = 9.14166 GHz, 0.998 mW power, 1 scan): five
lines, giso = 2.0067, aN = 7.40 G. UV-Vis (in toluene c = 5 ×
10−3 M): λabs (ε) 537 nm (667 M−1 cm−1), 314 nm (1482 M−1
6-(3,5-(2,6-Di(pyrazol-1-yl)pyridin-4-yl)phenyl)-2,4-diisopro-
pyl-3-oxoverdazyl (6). Compound (6) was synthesized by follow-
ing the above procedure. 6,6′-(5-(2,6-Di(pyrazol-1-yl)pyridin-4-
yl)-1,3-phenylene)bis(2,4-diisopropyl-1,2,4,5-tetrazinan-3 one)
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 2439–2446 | 2445

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4 (a) D. S. Marlin, E. Bill, T. Weyhermüller, E. Bothe and K. Wieghardt, J.
Am. Chem. Soc., 2005, 127, 6095–6108; (b) P. Ghosh, E. Bill,
T. Weyhermüller and K. Wieghardt, J. Am. Chem. Soc., 2003, 125, 3967–
3979; (c) C. Mukherjee, U. Pieper, E. Bothe, V. Bachler, E. Bill,
T. Weyhermüller and P. Chaudhuri, Inorg. Chem., 2008, 47, 8943–8956;
(d) C. Riplinger, J. P. Y. Kao, G. M. Rosen, V. Kathirvelu, G. R. Eaton,
S. S. Eaton, A. Kutateladze and F. Neese, J. Am. Chem. Soc., 2009, 131,
10092–10106.
5 (a) C. Rajadurai, A. Ivanova, V. Enkelmann and M. Baumgarten, J. Org.
Chem., 2003, 68, 9907; (b) C. Rajadurai, S. Ostrovsky, K. Falk,
V. Enkelmann, W. Haase and M. Baumgarten, Inorg. Chim. Acta, 2004,
357, 581; (c) C. Rajadurai, O. Fuhr, V. Enkelmann and M. Baumgarten,
J. Phys. Org. Chem., 2006, 19, 257–262; (d) C. Rajadurai, V. Enkelmann,
G. Zoppellaro and M. Baumgarten, J. Phys. Chem. B, 2007, 111, 4327;
(e) C. Rajadurai, V. Enkelmann, V. Ikorskii, V. I. Ovcharenko and
M. Baumgarten, Inorg. Chem., 2006, 45, 9664; (f) C. Rajadurai, K. Falk,
S. Ostrovsky, V. Enkelmann, W. Haase and M. Baumgarten, Inorg. Chim.
Acta, 2005, 358, 3391.
6 (a) G. Zoppellaro, A. Ivanova, V. Enkelmann, A. Geies and
M. Baumgarten, Polyhedron, 2003, 22, 2099–2110; (b) G. Zoppellaro,
V. Enkelmann, A. Geies and M. Baumgarten, Org. Lett., 2004, 6, 4929–
4932; (c) C. Stroh, P. Turek, P. Rabu and R. Ziessel, Inorg. Chem., 2001,
40, 5334; (d) C. Stroh and R. Ziessel, Tetrahedron Lett., 1999, 40, 4543–
4596; (e) F. M. Romero, D. Luneau and R. Ziessel, Chem. Commun.,
1998, 551; (f) D. Luneau, F. M. Romero and R. Ziessel, Inorg. Chem.,
1998, 37, 5078; (g) D. Shiomi, K. Ito, M. Nishizawa, K. Sato, T. Takui
and K. Itoh, Synth. Met., 1999, 103, 2271; (h) K. Osanai, A. Okazawa,
T. Nogami and T. Ishida, J. Am. Chem. Soc., 2006, 43, 14008–14009;
(i) R. G. Hicks, M. T. Lemaire, L. K. Thompson and T. M. Barclay,
J. Am. Chem. Soc., 2000, 122, 8077; ( j) T. M. Barclay, R. G. Hicks,
M. T. Lemaire and L. K. Thompson, Chem. Commun., 2000, 2141;
(k) S. Fokin, V. Ovcharenko, G. Romanenko and V. Ikorskii, Inorg.
Chem., 2004, 43, 969; (l) H. Oshio, T. Yaginuma and T. Ito, Inorg.Chem.,
1999, 38, 2750.
7 (a) P. Gutlich and H. A. Goodwin, Spin crossover in transition metal
compounds, in Topics in Current Chemistry, ed. Springer, Berlin, 2004,
Vol. 233; (b) N. Chandrasekhar and R. Chandrasekar, Dalton Trans.,
2010, 39, 9872–9878; (c) C. Rajadurai, Z. Qu, O. Fuhr, B. Gopalan,
R. Kruk, M. Ghafari and M. Ruben, Dalton Trans., 2007, 32, 3531;
(d) C. Rajadurai, O. Fuhr, R. Kruk, M. Ghafari, H. Hahn and M. Ruben,
Chem. Commun., 2007, 2636–2638; (e) C. Rajadurai, F. Schramm,
S. Brink, O. Fuhr, R. Kruk, M. Ghafari and M. Ruben, Inorg. Chem.,
2006, 45, 10019; (f) J. M. Holland, J. A. McAllister, Z. Lu, C. A. Kilner,
M. Thornton-Pett and M. A. Halcrow, Chem. Commun., 2001, 577.
8 (a) T. Ishida and H. Iwamura, J. Am. Chem. Soc., 1991, 113, 4238;
(b) F. Kanno, K. Inoue, N. Koga and H. Iwamura, J. Phys. Chem., 1997,
97, 13267; (c) K. Inoue and H. Iwamura, J. Am. Chem. Soc., 1994, 116,
3173; (d) K. Inoue and H. Iwamura, Adv. Mater., 1996, 8, 73;
(e) W. T. Borden, H. Iwamura and J. A. Berson, Acc. Chem. Res., 1994,
27, 109; (f) W. T. Borden and E. R. Davidson, J. Am. Chem. Soc., 1977,
99, 4587.
(15) (100 mg, 0.152 mmol) and benzoquinone (49 mg,
0.457 mmol) Yield: 21 mg (20%). Rf = 0.77 in CH2Cl2–EtOAc
(9 : 1). LCMS analysis (m/z): calcd 649.74; found 648.75.
UV-Vis (in toluene): λabs 360 and 480 nm. FT-IR (KBr): 3246,
2982, 2936, 1736, 1634, 1555, 1470, 1370, 1258, 1223, 1101,
949, 874, 831, 762, 679, 527, 448 cm−1. EPR (in toluene c ∼
10−4 M, ν = 9.14808 GHz, 0.998 mW power, 1 scan): giso
2.0071, |D/hc| = 0.00601 cm−1, ΔMs = 2 transition with gav
=
∼
4.01. Anal. calcd for C33H39N13O2 (649.75): found: C 61.22, N
27.86, H 6.16; requires C, 61.00; H, 6.05; N, 28.02.
Acknowledgements
We are grateful to DST (Fast Track Scheme No. SR/FTP/
CS-115/2007), New Delhi for financial support. P. H. thanks
CSIR for a SRF.
Notes and references
1 (a) H. Iwamura and N. Koga, Acc. Chem. Res., 1993, 26, 346–351;
(b) A. Rajca, Chem. Rev., 1994, 94, 871–893; (c) Magnetic Properties of
Organic Materials, ed. P. M. Lahti, Marcel Dekker, New York, 1999, pp.
1–713; (d) Molecular Magnetism, ed. K. Itoh and M. Kinoshita, Gordon
and Breach, Kodansha, 2000, pp. 1–337; (e) M. Baumgarten, High spin
molecules directed towards molecular magnets, in EPR of Free Radicals
in Solids, Trends in Methods and Application, ed. A. Lund and
M. Shiotani, Kluwer, 2003/2004, ch. 12, pp. 491–528.
2 (a) A. Rajca, J. Wongsriratanakul and S. Rajca, Science, 2001, 294,
1503–1505; (b) S. Rajca, A. Rajca, J. Wongsriratanakul, P. Butler and
S. Choi, J. Am. Chem. Soc., 2004, 126, 6972–6986; (c) A. Rajca,
J. Wongsriratanakul and S. Rajca, J. Am. Chem. Soc., 2004, 126, 6608–
6626; (d) A. Rajca, J. Wongsriratanakul, S. Rajca and R. L. Cerny,
Chem.–Eur. J., 2004, 10, 3144–3157; (e) E. Fukuzaki and H. Nishide,
J. Am. Chem. Soc., 2006, 128, 996–1001; (f) D. J. R. Brook and
G. T. Yee, J. Org. Chem., 2006, 71, 4889–4895; (g) J. B. Gilroy,
S. D. J. McKinnon, P. Kennepohl, M. S. Zsombor, M. J. Ferguson, L.
K. Thompson and R. G. Hicks, J. Org. Chem., 2007, 72, 8062–8069;
(h) E. C. Parè, D. J. R. Brook, A. Brieger, M. Badik and M. Schinke,
Org. Biomol. Chem., 2005, 3, 4258–4261; (i) P. Taylor and P. M. Lahti,
Chem. Commun., 2004, 2656; ( j) Md. E. Ali, S. Vyas and S. N. Datta, J.
Phys. Chem. A, 2005, 109, 6272–6278; (k) Md. E. Ali and S. N. Datta, J.
Phys. Chem. A, 2006, 110, 2776–2784.
3 (a) E. J. Hustedt and A. H. Beth, Annu. Rev. Biophys. Biomol. Struct.,
1999, 28, 129–153; (b) S. S. Eaton and G. R. Eaton, Biol. Magn. Reson.,
2000, 19, 1–27; (c) C. Altenbach, K.-J. Oh, R. Trabanino, K. Hideg and
W. Hubbell, Biochemistry, 2001, 40, 15471–15482; (d) P. Hanson,
G. Millhauser, F. Formaggio, M. Crisma and C. Toniolo, J. Am. Chem.
Soc., 1996, 118, 7618–7625; (e) D. Hilger, Ye. Polyhach, E. Padan,
H. Jung and G. Jeschke, Biophys. J., 2007, 93, 3675–3683;
(f) G. Jeschke, Biochim. Biophys. Acta, Bioenerg., 2005, 1707, 9–102.
9 J. H. Osiecki and E. F. Ullman, J. Am. Chem. Soc., 1968, 90, 1078.
10 C. Hirel, K. E. Vostrikova, J. Pècaut, V. I. Ovcharenko and P. Rey,
Chem.–Eur. J., 2001, 7, 2008.
11 WINEPR-SIMfonia programme was used for the simulation of the exper-
imental ESR spectra.
2446 | Org. Biomol. Chem., 2012, 10, 2439–2446
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