"Super hybrid tridentate ligands": 4-substituted-2-(1-butyl-1H-1, 2,3-triazol-4-yl)-6-(1H-pyrazol-1-yl)pyridine ligands coordinated to Fe(ii) ions display above room temperature spin transitions

Article abstract of DOI:10.1039/c0dt00373e

A series of novel "super hybrid tridentate ligands" based on (2-(1-butyl-1H-1,2,3-triazol-4-yl)-6-(1H-pyrazol-1-yl)pyridine (tpp) derivatives were synthesized. Their Fe(ii) complexes display around (_T = 287 K) and above room temperature (_T ? 375 K) spin transition temperatures.

Full text of DOI:10.1039/c0dt00373e

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“Super hybrid tridentate ligands”: 4-substituted-2-(1-butyl-1H-1,2,3-
triazol-4-yl)-6-(1H-pyrazol-1-yl)pyridine ligands coordinated to Fe(II) ions
display above room temperature spin transitions†
Naisa Chandrasekhar and Rajadurai Chandrasekar*
Received 27th April 2010, Accepted 27th July 2010
DOI: 10.1039/c0dt00373e
A series of novel “super hybrid tridentate ligands” based on (2-(1-butyl-1H-1,2,3-triazol-4-yl)-6-(1H-
pyrazol-1-yl)pyridine (tpp) derivatives were synthesized. Their Fe(II) complexes display around (T 1
=
2
287 K) and above room temperature (T 1 ꢀ 375 K) spin transition temperatures.
2
spin transition complexes,10 in order to fine tune the ligand
field strength of the btp unit, we have designed and prepared
a new class of mixed tridentate ligand system (tpp) with three
different coordination units by combining triazole, pyridine, and
pyrazole units. We have used pyrazole unit, since in most cases,
it forms switchable HS↔LS Fe(II) complexes due to its moderate
ligand-field strength (Scheme 1).1i–h,10,11
1. Introduction
A variety of iron(II) complexes are known to show reversible
spin transition (ST) between two different spin states (high-spin,
HS ↔ low-spin, LS) with respect to external stimulus such as
temperature, light irradiation and pressure.1–10a–d,11 Recently, there
has been a growing interest in obtaining ST complexes displaying
technologically appealing around and above room temperature ST
due to their possible applications in molecular electronic devices
and MRI contrast agents.9,10a–d Among spin transition compounds,
2,6-bispyrazolylpyridine (bpp) ligand derivatives coordinated to
Fe(II) ion have been attracting considerable interest.1h,i,10a–d,11
A
recent new member to the club of tridentate ligands is 2,6-
bis(1H-1,2,3-triazol-4-yl)pyridines (btp) synthesized using Click
chemistry.10g,12
In the literature a number of reports are available on the
syntheses of various tridentate ligands by combining various
heterocyclic rings (e.g. pyridine, bipyridine, pyrazole, triazole,
tetrazole, etc.).1–11 These heterocyclic rings are very fascinating due
to the tunable ligand properties they offer in a chelating ligand unit
(e.g. terpy, bpp, btp, etc.).
Scheme 1 Representation of the components used for the synthesis of
Fe(II) spin transition complexes (I–III).
Nevertheless, the preparation of novel hybrid tridentate lig-
ands also involves serious organic synthetic protocols. We10 and
other group11 have reported on the syntheses of several Fe(II)
complexes using various bpp ligand derivatives exhibiting diverse
ST temperatures. Recently, the groups of Flood and Hecht have
independently reported the synthesis of Fe(btp)2·X2 complex.12
In both reports the spin state of the Fe(II) ion was diamagnetic
(S = 0; low-spin) due to the strong ligand field strengths of
the btp units.12 Motivated by our previous successful results
on synthesizing several Fe(bpp)2·X2 (X = counter anion) type
In this article, we report a new synthetic protocol for the prepa-
ration of “super hybrid ligands” by connecting pyrazole, and tria-
zole units to the 2,6-position of the pyridine ring to form a novel 2-
(1-butyl-1H-1,2,3-triazol-4-yl)-6-(1H-pyrazol-1-yl)pyridine (tpp)
derivatives (L1–L3). We also report the preparation of their Fe(II)
compounds (I–IV), single crystal X-ray structures of I, II and
IV, variable temperature magnetic properties of I–III displaying
technologically attractive above room temperature ST and also the
powder XRD studies of the Fe(II) complexes.
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
2. Result and discussion
2.1 Synthesis and characterization
† Electronic supplementary information (ESI) available: Spectroscopic and
experimental data for 1-5, L1-L3 and metal complexes I, II and IV. CCDC
reference numbers 772647–772649. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt00373e
For the synthesis of tpp ligand derivatives (L1–L3), a multi-
step synthetic protocol from the low-priced citrazinic acid
was developed (Scheme 2). Conversion of citrazinic acid into
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2.2. Crystallographic studies
The single crystal X-ray diffraction crystallographic parameters,
and selected bond lengths are given in Table 1 and Table 2. The
crystallographic molecular structures of the octahedral Fe(II)N6
type complexes I, II and IV are displayed in Fig. 1 and 2. At
100 K, complex I displayed an orthorhombic Pbcn symmetry with
four complex molecules per unit cell. The asymmetric units in I
consist of the [Fe(L1)2] cation, two corresponding ClO4 anions and
one CH3CN solvate molecule. The average Fe–N bond distance
˚
(1.944 A) obtained at 100 K showed the LS state of the Fe(II)
cation. Interestingly, the Fe–N bond distances decreased in the
˚
following order: Fe(1)–N(4)pyrazole = 1.974(3) A > Fe(1)–N(1)triazole
=
˚
˚
1.952(3) A > Fe(1)–N(2)pyridine = 1.906(3) A. The variation in the
Fe–N bond lengths clearly indicated the metal ion biting ability
of the ring nitrogens in the following order: pyrazole > triazole
> pyridine. Complex II revealed a monoclinic P21/n symmetry.
The asymmetric units in II consist of the [Fe(L1)2] cation, two
corresponding BF4 anions and one CH3CN molecule. At 100 K,
˚
the average Fe–N bond distance (1.938 A) signified the LS state of
the Fe(II) cation. Moreover, in comparison to complex I, a notable
variation in the Fe–Npyrazole and Fe–Ntriazole bond lengths was found
with no change in the Fe–Npyridine distances at 100 K. The increasing
Scheme
2
Syntheses of ligands (L1, L2 and L3), Reagents and
(b) (i) K/diglyme/
conditions: (a) PBr3/Br2/P2O5/CH3OH/18
h
pyrazole/3 d/110 ◦C (ii) HCl/C2H5OH/Conc. H2SO4 (c) TMSA/
[Pd(PPh3)4]/CuI/THF/(C2H5)3N/4 h/RT (d) KF/Methanol/30 min/RT
(e) Butyl azide/Sodium ascorbate/Copper(II) sulfate/10 h/RT
(f ) LiOH/THF/HCl (2M) (g) C2O2Cl2/NaN3/TFA/K2CO3 (h)
NaNO2/Conc. HCl/aq. KI.
order of the Fe–N bond distances are as follows: Fe(1)–N(8)pyridine
=
=
=
˚
˚
1.905(3) A < Fe(1)–N(2)pyridine = 1.907(3) A, < Fe(1)–N(11)triazole
˚
˚
˚
1.947(3) A < Fe(1)–N(4)pyrazole = 1.947(3) A, Fe(1)–N(1)triazole
˚
1.959(3) A < Fe(1)–N(9)pyrazole = 1.967(3) A. Complex IV revealed
¯
a triclinic P1symmetry. The asymmetric units in IV consist of the
2,6-dibromo-isonicotinicacidmethylester 1 was carried out as
reported. The mononucleophilic substitution of the pyrazole unit
on 1 was successfully carried out to isolate ethyl-2-bromo-6-
(1H-pyrazol-1-yl)isonicotinate 2 in 74% yield. Transformation
of compound 2 into 3 was achieved via Sonogashira coupling
reaction conditions by using trimethylsilylacetylene (TMSA) in
(Et)3N/THF solvents using Pd(PPh3)4 catalyst to prepare com-
pound 3 in 78% yield. Deprotection of 3 in the presence of
KF/MeOH gave the Click precursor 4 in 99% yield. Formation of
triazole units on 4 was realized by employing Click reaction condi-
tions i.e., by using Cu(SO4)2 and sodium ascorbate in the presence
of butylazide. This Click reaction provided an ester derivative
of btp ligand L1 in a quantitative 99% yield. The saponification
reaction readily converted compound L1 into its carboxylic acid
derivative. Furthermore sequential conversion of the carboxylic
acid into acyl azide followed by a thermal Curtius rearrangement
and succeeding hydrolysis of the trifluoroacetamide provided the
amino derivative L2 (74% yield). From L2, the 4-iodo derivative L3
was synthesized by diazotization and reaction with KI in a modest
40% yield. The ligands were characterized by employing NMR
(1H and 13C), LC-MS, FTIR and elemental analysis techniques.
The mononuclear iron(II) complexes [FeII(L1)2](ClO4)2 (I) and
[FeII(L1)2](BF4)2 (II), were synthesized from L1 by using the
respective FeII salts in acetonitrile. Complexes [FeII(L2)2](ClO4)2
(III) and FeII(L3)2](ClO4)2 (IV) were prepared from ligands L2 and
L3, respectively by using FeII(ClO4)2. Single crystals suitable for
XRD studies were obtained in a quantitative yield by diffusing
diisopropylether into an acetonitrile solution of complexes under
N2 atmosphere at RT. After a week, wine-red color crystals of
complexes I–II and yellow color crystals of complex IV were
obtained.
Fig. 1 ORTEP view of low-spin complexes [FeII(L1)2](ClO4)2·CH3CN (I)
and [FeII(L1)2](BF4)2·CH3CN (II) (50% probability thermal ellipsoids) at
100 K. All hydrogen atoms and remaining counter ions are omitted for
clarity. The right side photographs show the low-spin state wine-red color
of the single crystals I and II at 100 K.
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Table 1 Crystallographic parameters of complexes I, II and IV
[FeII(L1)2](ClO4)2·CH3CN (I)
[FeII(L1)2](BF4)2·CH3CN (II)
[FeII(L2)2]·(ClO4)2 (IV)
Formula
C38H46N14O12Cl2Fe
1017.64
Wine red
100(2) K
C36H44N13O4B2F8Fe
952.30
Wine red
100(2) K
0.71073
C28H34Cl2FeN14O8
821.44
yellow
100(2) K
0.71073
Formula weight
Crystal colour
T/K
˚
Wavelength/A
0.71073
Crystal system,
Orthorhombic
Pbcn
21.3932(19)
13.5171(12)
15.5665(14)
90
90
90
4501.4(7)
4, 1.502
0.531
Monoclinic
P21/n
12.8665(11)
20.8850(18)
15.9996(14)
90
93.515(2)
90
4291.3(6)
4, 1.474
Triclinic
P1
11.9124(13)
12.0189(13)
14.8282(16)
66.581(2)
76.138(2)
62.629(2)
1725.8(3)
2, 1.581
¯
Space group
˚
a, (A)
˚
b, (A)
˚
c, (A)
a, (deg.)
b, (deg.)
g , (deg.)
3
˚
V, (A )
Z, rc. (mg m-3)
m (Mo, Ka) (mm-1)
F(000)
0.442
1960
0.663
848
2112
Crystal size/mm
theta range for the data collection (deg.)
Final R indices [I > 2s(I)]
R indices (all data)
GoF on F2
0.38 ¥ 0.16 ¥ 0.12
1.78 to 26.09
R1 = 0.0754, wR2 = 0.1397
R1 = 0.0855, wR2 = 0.1439
1.264
0.38 ¥ 0.18 ¥ 0.10
1.61 to 28.31
R1 = 0.0901, wR2 = 0.1906
R1 = 0.1114, wR2 = 0.1996
1.201
0.32 ¥ 0.22 ¥ 0.04
1.50 to 26.00
R1 = 0.0873, wR2 = 0.1783
R1 = 0.1241, wR2 = 0.1948
1.119
Table 2 Selected bond lengths of complexes I, II and IV
[Fe(L2)2] cation and two corresponding ClO4 anions. The
Fe–N bond lengths Fe(1)–N(8) = 2.122(4) A, Fe(1)–N(2) =
˚
˚
Bond distances (A)
Fe(1)–N(2)
[FeII(L1)2](ClO4)2·CH3CN (I)
˚
˚
˚
2.131(4) A, Fe(1)–N(1) = 2.173(4) A, Fe(1)–N(11) = 2.186(4) A,
1.906(3)
˚
˚
Fe(1)–N(9) = 2.202(5) A and Fe(1)–N(4) = 2.204(5) A clearly
showed the HS state of the iron(II) cation even at 100 K, which is
in line with the yellow colour of the crystals. Unfortunately, we
were unable to get single crystals of complex III suitable for X-ray
diffraction, but the wine-red color of the crystals clearly indicated
the LS state of the Fe(II) ion even at room temperature.
Fe(1)–N(4)
Fe(1)–N(1)
Bond distances (A)
Fe(1)–N(8)
Fe(1)–N(4)
Fe(1)–N(9)
Fe(1)–N(5)
Fe(1)–N(3)
Fe(1)–N(11)
1.974(3)
1.952(3)
[FeII(L1)2](BF4)2·CH3CN (II)
˚
1.905(3)
1.907(3)
1.967(3)
1.947(3)
1.959(3)
1.907(3)
˚
Bond distances (A)
[FeII(L2)2](ClO4)2(IV)
2.204(5)
2.3. Bulk magnetic properties
Fe(1)–N(9)
Fe(1)–N(4)
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(8)
Fe(1)–N(11)
2.131(4)
2.173(4)
2.186(4)
2.202(5)
The temperature dependent magnetic susceptibilities of powdered
polycrystalline samples of I–III were measured on a quantum
design vibrating sample magnetometer (VSM-SQUID) set-up in
the temperature range of 375–4 K at continuous heating (↑) and
cooling (↓) cycle with an applied DC magnetic field of 0.1 T
(Fig. 3). In order to improve the accuracy of the measurements,
many data points were collected at 0.1 K intervals. Compound
I revealed a reversible ST behavior with a wide hysteresis loop.
At 375 K the product of the molar magnetic susceptibility and
temperature (cT) of compound is 3.8 emu K mol-1, which is an
expected value for a HS iron(II) ion in the S = 2 state. The temper-
ature dependent magnetic behaviour showed the involvement of
two types of ST behaviour: (i) upon cooling the cT value of the HS
state abruptly decreased down to 2.5 emu K mol-1 at ca. 260 K;
(ii) followed by a steady but gradual decrease of the magnetic
moment reaching the minimum value of ca. 1.0 emu K mol-1
at 4 K displaying an incomplete ST. Calculation of the number
of fraction (f a) involved in the first steep transition showed that
ca. 45% of the molecules undergo abrupt ST behaviour with the
ST temperatures of 292 K for the heating (T 1 ↑) and 282 K for
2.122(4)
the cooling (T
↓) mode cycle with 10 K wide2thermal hysteresis
Fig. 2 ORTEP view of the high-spin complex [FeII(L2)2](ClO4)2 (IV) (50%
probability thermal ellipsoids) at 100 K. All hydrogen atoms and remaining
counter ions are omitted for clarity. The right hand photograph shows the
high-spin state orange color of the single crystal IV at 100 K.
1
2
loop. Importantly, the observed T 1 value of I is very close
2
to room temperature i.e. 287 K. The remaining fraction (f b
=
55%) of the molecules displayed a gradual and incomplete ST
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Fig. 3 The cT vs. T plot for the complexes I–III measured in the temperature range of 4–375 K in the (↓) cooling and (↑) heating mode cycle with an
applied magnetic field of 0.1 T.
behaviour without reaching the minimum magnetic moment value
corresponding to the LS state. In contrary, compound II showed
a near completion of the ST event even above room temperature.
At 375 K, the cT value of 0.8 emu K mol-1. Upon further cooling
the magnetic moment decreased gradually and reached a lowest
value of ca. 0.02 emu K mol-1 at 5 K indicating the completion
of the ST event. Complex III also displayed a similar ST trend
as complex II. The ST event was almost completed at room
temperature and below this temperature the cT value gradually
decreased down to 4 K to reach a minimum cT value of ca.
0.25 emu K mol-1. The cT value steeply increased above 350 K
and only reached the minimum value of ca. 1.5 emu K mol-1 at
375 K. This behaviour clearly demonstrated that the HS state
and the ST temperature is somewhere above 375 K for both II
and III. Furthermore the ST properties of complexes I and II
are independent of lattice-solvents, since the powdered crystalline
sample used for the magnetic studies showed no characteristic
bands for the lattice-acetonitrile molecules in the FTIR data (ESI,†
Figure S19 and S20).10c Additionally, the experimental powder
XRD data of powdered samples of I and II showed different
diffraction peak patterns in comparison to their calculated data
(obtained from their single crystal data). This further confirmed
the loss of lattice-acetonitrile molecules from the crystals during
the preparation of the powdered samples (ESI,† Figure S1
and S2).
Spectrochem Pvt. Ltd., Mumbai. Column chromatography was
performed using Merck silica gel (particle size 100–200 mesh).
THF, triethylamine, benzene, dichloromethane, hexane, petroleum
ether, CHCl3 and methanol solvents were obtained from Finar
Chemicals Limited, Ahmadabad, India. The solvents were used
as dried and distilled for reactions. All solvent were used after
distillation.
4.2 Instrumental methods
1H and 13C NMR spectroscopic data were recorded on a
Bruker DPX 400 spectrometer with solvent proton as internal
standard (CDCl3-d1 = 7.26). Deuterated solvent CDCl3-d1 was
obtained from Aldrich. LC mass spectrometry was performed
on a Shimadzu LCMS-2010A mass spectrometer. 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 visualized by irradiation with UV
light. Magnetic susceptibilities were measured in the temperature
range 375–4 K on a Quantum Design VSM-SQUID operating at
0.1 Tesla. Single Crystal-XRD data for complexes I, II and IV
were collected on a Bruker SMART CCD Diffractometer with
˚
Mo-Ka (l = 0.71073 A) radiation. Data reduction was performed
using Bruker SAINT software. Structures were solved and refined
using SHELXL-97 with anisotropic displacement parameters for
non-H atoms.
3. Conclusions
We have demonstrated the synthesis of a family of novel “super
hybrid tridentate ligand derivatives” containing pyridine, pyrazole
and triazole rings (L1–L3). These hybrid btp ligands were efficiently
used to prepare iron(II) complexes I–III exhibiting around and
above room temperature ST temperatures.
4.3 Syntheses of Ligands (L1–L3) and their Fe(II) complexes
Methyl-2,6-dibromoisonicotinate (1). Compound (1) was pre-
pared according to literature procedure.13
Ethyl-2-bromo-6-(1H-pyrazol-1-yl)isonicotinate (2). In a clean
and dry 250 mL three-neck flask, small pieces of K metal flakes
(0.9, 20 mmol, 1.1 eq) were dispersed completely in diglyme
(100 mL) by vigorously stirring the mixture for 1 h under N2
atmosphere. To this solution pyrazole (1.4 g, 20.0 mmol, 1.2 eq)
was added slowly to minimize the violent reaction that occurred
due to the formation of potassium salt of pyrazole. The mixture
was stirred for about 30 min at 60 ◦C until all the K was
reacted to pyrazole to afford a white turbid solution. Methyl-
2,6-dibromoisonicotinate 1 (5.2 g, 16.8 mmol, 1.0 eq) was added
into this reaction mixture and heated at 110 ◦C for 3 d under N2
atmosphere. The mixture was poured into water and neutralized
4. Experimental section
4.1 Materials
The materials citrazinic acid, [Pd(PPh3)4], triflouroacetic acid, 1-
bromo butane, Fe(ClO4)2·xH2O and Fe(BF4)3·6H2O were pur-
chased from Aldrich USA. Sodium azide, KI, K2CO3, ethanol,
NaNO2, sodium thiosulfate and diethylether were procured from
Merck, India. Oxalylchloride, PBr3, iodine, Cu(I)I, NaHCO3
and trimethylsilyl acetylene were obtained from Avra Snthesis,
Hyd and Acros USA respectively. Br2 was purchased from
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by conc. HCl (10 mL), the white precipitate that formed was
filtered and dried. (If precipitate forms upon pouring into water,
no neutralization is required).
Ethyl-2-(1-butyl-1H-1,2,3-triazol-4-yl)-6-(1H-pyrazol-1yl)iso-
nicotinate (L1). Compound 4 (1.0 g, 4.14 mol, 1.0 eq) was dis-
charged in a clean 100 mL one-neck round bottom flask containing
30 mL of ethanol–water (1 : 1). After 5 min butyl azide (0.6 mL,
4.98 mmol, 1.1 eq) and sodium ascorbate (0.1 g, 0.45 mmol,
10 mol%) were added. To this aqueous solution a freshly prepared
CuSO4 (0.05 g, 0.21 mmol, 5 mol%) was added, immediately a
sharp color change from pale-yellow to intense wine–red color
was observed, during the course of the reaction the intense of
color turned to pale–orange. The progress of the reaction was
monitored by TLC (silica, 1 : 19 ratio of methanol : DCM). The
crude reaction mixture was concentrated on a vacuum evaporator.
The obtained solid residue was redissolved in chloroform and
passed through a short celite plug and resultant solution was
concentrated. The obatined pale yellow residue was washed with
pet-ether to isolate a yellow solid of L1 in 99% yield (1.57 g).
Anal. Calcd for C17H20N6O2: C, 59.99; H, 5.92; N, 24.69. Found:
C, 59.96; H, 5.94; N, 24.65. LC–MS m/z calcd 383.6, found 383.5.
The white precipitate was dissolved in 50 mL of ethanol and
3 mL conc. H2SO4 and put under reflux for 4 h. The mixture
was poured into a 50 mL of water, extracted with chloroform
(2 ¥ 50 mL) and washed with brine solution (2 ¥ 10 mL). The
collected organic layers were combined and dried over MgSO4
and the chloroform was evaporated on a vacuum evaporator to
obtain a pale yellow residue, which upon washing with pet-ether
gave compound 2 as white powder in 74% yield (3.7 g). Anal.
Calcd. for C11H10N3O2Br: C, 44.62; H, 3.40; N, 14.19. Found: C,
44.59; H, 3.44; N, 14.16. LC–MS m/z calcd. 296.1, found 295.8.
1H NMR (400 MHz, CDCl3, 25 ◦C) d (ppm): 8.53 (d, 1H), 8.43 (s,
1H), 7.88 (d, 1H), 7.76 (s, 1H), 6.49–6.47(m, 1H), 4.43 (q, 2H, 3J =
7.04 Hz, 1¢¢), 1.42 (t, 3H, 3J = 7.18 Hz, 2¢¢). 13C NMR (100 MHz,
CDCl3, 25 ◦C) d (ppm): 163.2, 152.6, 143.03, 142.6, 140.4, 127.7,
124.6, 110.7, 108.5, 62.3, 14.1.
◦
1H NMR (400 MHz, CDCl3, 25 C) d (ppm): 8.56 (d, 1H), 8.55
Ethyl-2-(1H-pyrazol-1-yl)-6-((trimethylsilyl)ethynyl)isonicoti-
nate (3). Compound 2 (2.8 g, 9.5 mmol), CuI (0.050 g) and
[Pd(PPh3)4] (0.60 g, 0.05 mmol) were discharged in a clean and
dry 250 mL three-neck round bottom flask. The flask was put
under vacuum followed by flushing with argon gas for 2–3 times.
To this dry THF (15 mL) and triethylamine (35 mL) were added.
Trimethylsilylacetylene (TMSA) (2.0 mL, 14.2 mmol, 1.5 eq) was
added drop wise within 20 min. On addition of TMSA, the
yellow color turbid solution changed to deep brown color. The
progress of reaction was monitored by TLC (silica, 1 : 19 ratio
of ethylacetate: pet-ether). After disappearance of the starting
material the reaction mixture was filtered and washed with hexane
or THF. The filtrate was evaporated by rotary evaporator and the
crude reaction mixture was purified by column chromatography
(silica, 1 : 19 ratio of ethyl acetate: pet-ether) to get a pale yellow
crystalline solid of 3 in 78% yield (2.32 g). Anal. Calcd for
C16H19N3O2Si: C, 61.31; H, 6.11; N, 13.41. Found: C, 61.38; H,
(d, 1H), 8.38 (s, 1H), 8.14 (s, 1H), 7.74 (s, 1H), 6.45–6.44 (m, 1H),
3
4.49-4.43 (m, 4H, 1¢¢,1¢¢¢), 1.99 (q, 2H, J = 7.35 Hz, 2¢¢¢), 1.46–
3
1.40 (m, 5H, 2¢¢, 3¢¢¢), 1.00 (t, 3H, J = 7.33 Hz, 4¢¢¢), 13C NMR
(100 MHz, CDCl3, 25 ◦C) d (ppm): 164.4, 151.8, 151.8, 149.9,
141.2, 142.8, 141.7, 127.05, 122.4, 116.9, 110.9, 108.0, 62.0, 50.3,
32.2, 19.7, 14.3, 13.4
2-(1-butyl-1H-1,2,3-triazol-4-yl)-6-(1H-pyrazol-1-yl)isonicoti-
nicacid (5). Compound L1 (1.3 g, 3.4 mmol, 1.0 eq) was dissolved
in THF (25 mL) and a solution of LiOH (0.11 g, 4.4 mmol, 1.3
eq) in 75 mL of water was added to it under N2. After stirring
20 min at RT, THF was removed on a rotary evaporator. The
solution was cooled in an ice bath and 2 M HCl (pH = 2–3)
(0.56 ml, 6.24 mmol) was added drop-wise until a white precipitate
appeared. The mixture was then stirred for 1 h. The white powder
that was formed was collected by vacuum filtration and it was
dried to get compound 5 in 98% yield (1.3 g). Anal. Calcd for
C17H16N6O2: C, 57.68; H, 5.16; N, 26.91. Found: C, 57.71; H,
1
6.20; N, 13.45. LC–MS m/z calcd 313.1, found 313.8. H NMR
(400 MHz, CDCl3, 25 ◦C) d (ppm): 8.65 (d, 1H), 8.48 (d, 1H), 7.9
(s, 1H), 7.78 (s, 1H), 6.50–6.49 (m, 1H), 4.46 (q, 2H, 3J = 7.52 Hz,
5.20; N, 26.86. LC–MS m/z calcd 312.1, found 312.6. H NMR
1
(400 MHz, CDCl3, 25 ◦C) d (ppm): 8.69 (s, 1H), 8.26 (s, 1H), 8.02
(s, 1H), 7.82 (s, 1H), 6.53 (m, 1H), 4.49 (t, 2H, 3J = 7.02 Hz, 1¢¢¢),
2.01 (q, 2H, 3J = 7.46 Hz, 2¢¢¢), 1.45 (m, 2H, 3J = 7.89 Hz, 3¢¢¢), 1.02
(t,3H, 3J = 7.02 Hz, 4¢¢¢), 13C NMR (100 MHz, CDCl3, 25 ◦C) d
(ppm): 165.9, 149.3, 147.1, 142.6, 127.4, 117.8, 111.8, 108.1, 50.6,
32.2, 19.7, 13.5.
3
1¢¢), 1.45 (t, 3H, J = 7.45 Hz, 2¢¢), 0.33 (s, 9H, 1¢¢¢). 13C NMR
◦
(100 MHz, CDCl3, 25 C) d (ppm): 163.95, 152.1, 142.7, 142.6,
142.0, 127.7, 124.4, 111.8, 108.2, 102.6, 96.7, 62.1, 14.2, -0.37.
Ethyl-2-ethynyl-6-(1H-pyrazol-1-yl)isonicotinate (4). Com-
pound 3 (2.0 g, 6.4 mmol, 1.0 eq) was loaded in a 250 ml
two-neck round bottom containing degassed methanol. To this
solution, KF (0.75 g, 12.7 mmol, 2.0 eq) was added under N2
atmosphere. After 30 min, the completion of the reaction was
confirmed by TLC (silica, 1 : 9 ratios of ethyl acetate: pet-ether)
and reaction mixture was concentrated in vacuo. The obtained
solid compound was redissolved in chloroform and filtered. The
filtrate was evaporated to get desired compound (4) as white
powder > 99% yield (1.54 g). Anal. Calcd for C13H11N3O2: C,
64.72; H, 4.60; N, 17.42. Found: C, 64.68; H, 4.64; N, 17.39.
LC–MS m/z calcd 241.1, found 241.7. 1H NMR (400 MHz,
2-(1-butyl-1H-1,2,3-triazol-4-yl)-6-(1H-pyrazol-1-yl)pyridine-
4-amine (L2). Compound L1 (1.8 g, 5.77 mmol, 1.0 eq) was
dissolved in a mixture of THF (15 mL)/CH2Cl2 (45 mL) solvents.
To this (COCl)2 (0.84 g, 0.6 mL, 1.22 eq) was added slowly while
stirring under N2 to reduce gas evolution. After stirring 4 h at RT,
the solvent was removed. The residue was dissolved in dry acetone
(30 mL) and added to a solution of NaN3 (1.57 g, 24.14 mmol,
4.32 eq) in water (30 mL). The residue was extracted immediately
with ether (2 ¥ 75 mL) and dried over MgSO4 followed by solvent
evaporation under vacuum. The obtained solid was dissolved in
dry benzene (50 mL) and trifluoroacetic acid (TFA) (1.0 g, 0.7 mL,
8.65 mmol, 1.50 eq) was added to it. After stirring 16 h at reflux,
the mixture was cooled and the benzene was removed by vacuum
evaporation. The residue was dissolved in methanol (50 mL) and
K2CO3 (1.91 g, 14.4 mmol, 2.17 eq) was added to it. The mixture
◦
CDCl3, 25 C) d (ppm): 8.61 (d, 1H), 8.51 (s, 1H), 7.91 (s, 1H),
7.78 (s, 1H), 6.50-6.47(m, 1H), 4.44 (q, 2H, 3J = 7.20 Hz, 1¢¢), 3.26
3
(s, 1H, 1¢¢¢), 1.43 (t, 3H, J = 6.94 Hz, 2¢¢), 13C NMR (100 MHz,
◦
CDCl3, 25 C) d (ppm): 163.9, 152.3, 142.8, 141.3, 127.6, 126.3,
124.4, 112.4, 108.4, 81.8, 78.4, 62.3, 14.22.
9876 | Dalton Trans., 2010, 39, 9872–9878
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[FeII(L3)2](ClO4)2·CH3OH (III). In a 100 mL Schlenk tube, a
solution of L3 (100 mg, 0.26 mmol) and Fe(ClO4)2·6H2O (33 mg,
0.13 mmol) in methanol (25 mL) was heated at 80 ◦C for 6 h. The
reaction mixture was cooled to RT and 100 mL of diisopropyl
ether was added to the flask under a N2 atmosphere to yield wine–
red precipitate of the complex (III). Wine–red coloured crystals
were grown from diffusing the diisopropyl ether into a methanol
solution of the complex (III) under N2. Elemental analysis calcd
for, C29H33Cl2FeI2N12O9: C, 34.16; H, 3.12; N, 17.43%. Found: C,
34.25; H, 3.26; N, 17.36%. FT-IR (KBr) n in cm-1: 3511, 3117,
3081, 2961, 2866, 1614, 1548, 1474, 1408, 1254, 1213, 1088, 1011,
845, 752, 623.
3
4
was stirred vigorously for 8 h at RT of the methanol was removed
and water (100 mL) was added to it. The mixture was cooled in
an ice bath for 2 h to get compound as white precipitate which
upon filtration gave white powder of L2 in 74% yield (1.2 g). Anal.
Calcd for C14H17N7: C, 59.35; H, 6.05; N, 34.60. Found: C, 59.39;
H, 6.02; N, 34.58. LC–MS m/z calcd 283.2, found 283.4. 1H NMR
(400 MHz, CDCl3, 25 ◦C) d (ppm): 8.60 (d, 1H), 8.11(s, 1H), 7.73
(d, 1H), 7.39 (s, 1H), 7.19 (s, 1H), 6.45–6.44 (m, 1H), 4.53 (s,
2H, –NH2), 4.42 (t, 2H, J = 7.12 Hz, 1¢¢¢), 1.94 (q, 2H, J =
7.62 Hz, 2¢¢¢), 1.47–1.27(m, 2H, 3J = 7.72 Hz,3¢¢¢), 0.98 (t, 3H, 3J =
7.02 Hz,4¢¢¢).13C NMR (100 MHz, CDCl3, 25 ◦C) d (ppm): 155.6,
152.4, 149.3, 148.1, 141.5, 127.0, 122.0, 107.1, 103.9, 96.2, 50.2,
32.2, 19.7, 13.4.
3
3
[FeII(L2)2](ClO4)2 (IV). A similar procedure as for I was
applied using Fe(ClO4)2·6H2O (50 mg, 0.15 mmol). Crystallization
yielded yellow color crystals (Yield ca. 40 mg). FT-IR (KBr) n
in cm-1: 3462, 3352, 3225, 3127, 2963, 1636, 1526, 1493, 1408,
1277, 1215, 1088, 1007, 972, 768, 623
2-(1-butyl-1H-1,2,3-triazol-4-yl)-4-iodo-6-(1H-pyrazol-1-yl)-
pyridine (L3). Compound L2 (0.72 g, 2.54 mmol, 1.0 eq) was
suspended in conc. HCl (20 mL) and stirred for 24 h at RT. The
mixture was cooled in an ice bath and NaNO2 (0.35 g, 5.1 mmol,
2.0 eq) dissolved in a minimum amount of water (2 mL) was added
drop-wise. To this KI (1.05 g, 6.35 mmol, 2.5 eq) in water (5 mL)
was slowly added and the solution was stirred for 5 min. THF
(10 mL) was added into the reaction mixture and the solution was
neutralized by adding solid NaHCO3. The product was extracted
with diethylether (2 ¥ 50 mL) and washed with Na2S2O3 solution.
The collected organic layers were combined, dried over MgSO4
and the solvent was evaporated by vacuum evaporator to afford
a white color product L3 in 40% yield (0.4 g). Anal. Calcd for
C14H15N6I: C, 42.65; H, 3.84; N, 21.32. Found: C, 42.60; H, 3.86; N,
21.29. LC–MS m/z calcd. 394.2, found 394.1. 1H NMR (400 MHz,
Acknowledgements
We are grateful to DST New Delhi for financial support under
Fast Track Scheme for Young Scientist (Grant No. SR/FTP/CS-
115/2007). Our special thanks to Centre for Nanotechnology
(CNT), UoH for providing the PPMS and VSM-SQUID facility.
NC thanks CSIR New Delhi for a JRF.
References
◦
CDCl3, 25 C) d (ppm): 8.58 (d, 1H), 8.46(d, 1H), 8.36 (d, 1H),
1 For general reviews: (a) Spin Crossover in Transition Metal Com-
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3
3
1¢¢¢), 1.96 (q, 2H, J = 7.6 Hz, 2¢¢¢), 1.48–1.39 (m, 2H, J = 7.32
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◦
0.15 mmol) in acetonitrile (15 mL) was heated at 80 C for 6 h.
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ether was added to it under a N2 flow to yield a red–orange
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9878 | Dalton Trans., 2010, 39, 9872–9878
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The Royal Society of Chemistry 2010
©