Synthesis and characterisation of two high spin iron(II) complexes of 3,4-diphenyl-5-(2-pyridyl)-1,2,4-triazole

Article abstract of DOI:10.1080/10610278.2012.691608

The stepwise synthesis of the new ligand 4,5-diphenyl-3-pyridyl-1,2,4- triazole (phppt) from N-phenyl-pyridine-2-thiocarboxamide and benzohydrazide is reported. Two iron(II) complexes, [Fe(phppt)₂(SCN) ₂]·2MeOH and [Fe(phppt)₂

Full text of DOI:10.1080/10610278.2012.691608

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Synthesis and characterisation of two high spin iron(II)
complexes of 3,4-diphenyl-5-(2-pyridyl)-1,2,4-triazole
Reece G. Miller a , Guy N.L. Jameson a , Juan Olguín a & Sally Brooker a
a Department of Chemistry, and the MacDiarmid Institute for Advanced Materials and
Nanotechnology, University of Otago, PO Box 56, Dunedin, 9054, New Zealand
Published online: 14 Jun 2012.
To cite this article: Reece G. Miller , Guy N.L. Jameson , Juan Olgun & Sally Brooker (2012) Synthesis and characterisation of
two high spin iron(II) complexes of 3,4-diphenyl-5-(2-pyridyl)-1,2,4-triazole, Supramolecular Chemistry, 24:8, 547-552, DOI:
10.1080/10610278.2012.691608
To link to this article: http://dx.doi.org/10.1080/10610278.2012.691608
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Supramolecular Chemistry
Vol. 24, No. 8, August 2012, 547–552
Synthesis and characterisation of two high spin iron(II) complexes
of 3,4-diphenyl-5-(2-pyridyl)-1,2,4-triazole
´
Reece G. Miller, Guy N.L. Jameson, Juan Olguın and Sally Brooker*
Department of Chemistry, and the MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Otago, PO Box 56,
Dunedin 9054, New Zealand
(Received 19 March 2012; final version received 3 May 2012)
The stepwise synthesis of the new ligand 4,5-diphenyl-3-pyridyl-1,2,4-triazole (phppt) from N-phenyl-pyridine-2-
thiocarboxamide and benzohydrazide is reported. Two iron(II) complexes, ½FeðphpptÞ2ðSCNÞ2ꢀ·2MeOH and [Fe(phppt)2
(SeCN)2]·Et2O, have been prepared and structurally characterised at 90 K, showing that both complexes are high spin (HS).
¨
The Mossbauer spectrum of [Fe(phppt)2(SeCN)2]·1.5MeOH is consistent with the iron(II) ion being HS at 5.2 K.
¨
Keywords: triazole; iron(II); high spin; Mossbauer spectrum; structure determination
Introduction
it was found that the ethylation, step (ii), could be omitted
if a longer reflux time was used. However, in the case of
phppt, the latter method afforded only very low yields;
ethylation of (1) to give (2) was found to be more
successful. Analysis of the 1H NMR spectrum of crude (2)
indicated that the reaction did not proceed to completion.
Attempts to purify (2) were unsuccessful, in large part
because it is prone to decomposition. Purification proved
unnecessary as the crude material was used successfully in
the final step, (iii), to produce phppt which was readily
purified by recrystallisation from methanol.
Triazole-containing ligands have been shown previously to
give a ligand field strength which is often appropriate for
iron(II) spin crossover (1–4). This research group has
developed an efficient pathway for the synthesis of both 4-R-
3,5-dipyridyl-1,2,4-triazole (Rdpt) and 4-R-3-pyridyl-5-
phenyl-1,2,4-triazole (Rppt) ligands (5). We (6–10) and
others (11–19) have investigated the magnetic properties of
a range of iron(II) complexes of such ligands (3). Spin
crossover has been frequently observed, particularly in the
caseofthe [Fe(Rdpt)2(NCE)2] complexes(3, 6, 7, 9, 11–19).
Instead of the anticipated mononuclear complex, the
2:1 reaction of phdpt (Figure 1) with [Fe(py)4(SCN)2]
gave a trinuclear complex, [Fe(phdpt)4(SCN)6], in which
two of the Rdpt ligands are bis-bidentate (bridging) and
two are bidentate (end capping) (8). This trinuclear
complex was shown to be [HS-HS-HS], where HS ¼ high
spin, from 300 to 2 K. In order to restrict the number of
possible binding modes and to facilitate the formation of a
mononuclear [Fe(Rdpt)2(NCE)2] complex, we turned our
attention towards the synthesis of the diphenyl analogue of
phdpt, phppt (Figure 1). The synthesis and characterisation
of this new ligand and two iron(II) complexes of it are
described herein.
Synthesis of complexes
The 1:2 reactions, under argon at room temperature, of
[Fe(py)4(ECN)2] with E ¼ S or Se with phppt in
MeOH/CHCl3 (4:1) gave red solutions. Pale orange
block-shaped single crystals of [Fe(phppt)2(SCN)2]·2-
MeOH (3·2MeOH) and a few orange single crystals of
[Fe(phppt)2(SeCN)2]·Et2O (4·Et2O), suitable for X-ray
crystallography (see the next section), were grown from
these reaction solutions by vapour diffusion of diethyl
ether under an argon atmosphere. Pure samples of both
complexes were obtained by filtration under nitrogen
and drying the solids under a flow of nitrogen. Both the
crystal structure and elemental analysis data show that
3 is a methanol solvate, 3·2MeOH. However, in the case
of 4, the single crystals are shown by the X-ray
structure determination to be 4·Et2O, while microana-
lysis of the bulk sample, of orange microcrystals plus
the few single crystals, fits best as a methanol solvate
(between 1 and 2 MeOH; best fit is 1.5 MeOH, see
experimental).
Results and discussion
Synthesis of ligand
We have previously reported an effective synthetic route
to synthesise 3,4,5 tri-substituted triazoles (5). It was
extended to include the new triazole ligand phppt
(Scheme 1). In the synthesis of the related ligand phdpt,
*Corresponding author. Email: sbrooker@chemistry.otago.ac.nz
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2012 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2012.691608
http://www.tandfonline.com

548
R.G. Miller et al.
between the plane of the triazole ring and both of the
phenyl groups, at positions 3 and 4 (Table 2; 43.2, 48.68 vs
65.5, 64.98, respectively). As a result, the p overlap
between the phenyl groups and central triazole will be poor
and mesomeric effects will be limited. This is consistent
with the results observed for complexes of other Rdpt
ligands with aromatic substituents off the 4 position, and
may help to explain why these ligands appear to give
a relatively weak ligand field compared to other Rdpt
ligands.
There are two half occupancy methanol molecules
present in the asymmetric unit of 3·2MeOH, which
hydrogen bond to each other but do not make any significant
interactions with the iron complex. There is a single ether
molecule present in the asymmetric unit of 4·Et2O. It makes
no significant interactions with the complex.
Figure 1. General structure of Rdpt and Rppt ligands, as well as
the structure of 4-phenyl-3,5-di(2-pyridyl)-1,2,4-triazole (phdpt)
and 4,5-diphenyl-3-pyridyl-1,2,4-triazole (phppt).
¨
Mossbauer spectroscopy
X-ray crystal structures of complexes
¨
The Mossbauer spectrum of 4·1.5MeOH was recorded at
The crystal structure data for 3·2MeOH and 4·Et2O were
recorded at about 90 K; no change in colour was observed
on cooling the crystals to this temperature. The complexes
are isostructural (Figure 2). In both cases, half of the
structure is present in the asymmetric unit with the other
half being generated by a centre of inversion. The central
iron(II) is bound to two bidentate phppt ligands, and to two
ECN ligands orientated trans to each other. The bond
lengths and angles observed are in the ranges expected for
HS iron(II) (Table 1) (20).
5.2 K (Figure 3) and consists of a single sharp quadrupole
doublet. The isomer shift (d ¼ 1:15 mm s21) and large
quadrupole splitting (DEq ¼ 3:30 mm s21) are highly
characteristic of HS iron(II) (21). The parameters are
very similar to those previously observed for related
compounds (6, 8).
Conclusion
A new triazole-based ligand, 4,5-diphenyl-3-pyridyl-
1,2,4-triazole (phppt), and two iron(II) complexes of this
ligand, [Fe(phppt)2(SCN)2]·2MeOH and [Fe(phppt)2
(SeCN)2]·Et2O, have been successfully synthesised and
In both structures, the triazole ring is not far from
being coplanar with the pyridine ring at position 2 (Table 2;
25.5, 26.88); however, a significant rotation is observed
Scheme 1. Synthetic pathway used to produce the new ligand phppt. (i) Na2S·9H2O, S8; (ii) ethyl bromide, NaOEt, EtOH reflux and
(iii) BuOH reflux.

Supramolecular Chemistry
549
Figure 2. Perspective views of the complexes: (top) [Fe(phppt)2(SeCN)2]·Et2O (4·Et2O) and (bottom) [Fe(phppt)2(SCN)2]·2MeOH
(3·2MeOH). Hydrogen atoms and solvent molecules have been omitted for clarity.
characterised. The X-ray crystal structures of both
complexes clearly indicate that the iron(II) ions are HS
at 90 K. In addition, the low temperature (5.2 K),
involves the higher field anion NCSe, rather than NCS,
current efforts are focused on the synthesis of new ligands
which will impose a higher ligand field than phppt does, so
that new generations of iron(II) spin crossover complexes
can be accessed.
¨
Mossbauer spectrum of [Fe(phppt)2(SeCN)2]·1.5MeOH
shows that this complex remains HS. As this complex
˚
Table 1. Selected bond lengths [A] and angles [8] for [Fe(phppt)2(SCN)2]·2MeOH and [Fe(phppt)2(SeCN)2]·Et2O, at 89 and 91 K,
respectively. Symmetry transformations used to generate equivalent atoms A 2x þ 1, 2y, 2z þ 1.
[Fe(phppt)2(SeCN)2]·Et2O
[Fe(phppt)2(SCN)2]·2MeOH
Fe(1)-N(40)
Fe(1)-N(1)
Fe(1)-N(5)
N(40)-Fe(1)-N(1)
N(40)-Fe(1)-N(1A)
N(40)-Fe(1)-N(5A)
N(1)-Fe(1)-N(5A)
N(40)-Fe(1)-N(5)
N(1)-Fe(1)-N(5)
2.128(3)
2.178(2)
2.178(2)
83.60(10)
96.40(10)
90.93(10)
103.59(9)
89.07(10)
76.41(9)
2.126(2)
2.189(2)
2.186(2)
84.33(9)
95.67(9)
89.83(8)
103.60(8)
90.17(8)
76.40(8)

550
R.G. Miller et al.
transmission geometry with an applied field of 47 mT
Table 2. Angles [8] between mean plane of the triazole and the
specified rings for [Fe(phppt)2(SCN)2]·2MeOH and
[Fe(phppt)2(SeCN)2]·Et2O.
¨
parallel to the g-rays. The zero velocity of the Mossbauer
spectra refers to the centroid of the room temperature
spectrum of a 25 mm metallic iron foil. Analysis of the
spectra was carried out using the WMOSS program (SEE
Co., formerly WEB Research Co).
2-Pyridine
3-Phenyl
4-Phenyl
(3·2MeOH) Triazole
(4·Et2O) Triazole
26.8(2)
25.5(2)
43.2(1)
48.6(1)
65.5(1)
64.9(1)
Both complexations were carried out under argon
using standard Schlenk conditions. All solvents and
reagents were of reagent grade and used received unless
otherwise stated, with the exception of methanol which
was HPLC grade and ethanol which was distilled from
Mg/I2. [Fe(py)4(SCN)2] and [Fe(py)4(SeCN)2] were
prepared from [Fe(H2O)6](BF4)2 according to the litera-
ture (23). N-phenyl-pyridine-2-thiocarboxamide and ben-
zohydrazide were prepared as previously reported (5, 8).
Ethyl N-phenylpyridine-2-carboximidothiate
A solution of sodium ethanoate was prepared by
dissolving sodium (437 mg, 19 mmol) in distilled etha-
nol. The resulting colourless solution was stirred for
30 min, and then N-phenylpyridine-2-thiocarboxamide
(4.06 g, 19 mmol) and ethyl bromide (3.60 g, 33 mmol)
were added. The resulting orange solution was refluxed for
6 h during which time a white precipitate of NaBr formed.
The mixture was filtered, and then the solvent was
removed at reduced pressure to give a brown oil. This was
dissolved in DCM (150 mL) and washed with distilled
water (3 £ 150 mL), saturated NaCl (50 mL) and saturated
NaHCO3 (50 mL) aqueous solutions. The orange DCM
solution was then dried over MgSO4, and the solvent
was removed at reduced pressure to yield crude ethyl
N-phenylpyridine-2-carboximidiothiate as a brown oil
(4.55 g, 18.6 mmol, 98%). This crude material could be
used in subsequent reactions without further purification
and should be used promptly.
¨
Figure 3. The Mossbauer spectrum of [Fe(phppt)2(SeCN)2]·
1.5MeOH (4·1.5MeOH) recorded at 5.2 K. The spectrum was
recorded as a sucrose blend of the crystalline material.
Experimental
General
Elemental analyses (C, H, N and S) were carried out in the
Campbell Microanalytical Laboratory, University of
Otago, and have a standard error of ^0.3%. IR spectra
1
were recorded on a Bruker ATR-IR spectrometer. All H
NMR spectra were recorded at 298 K on a Varian 400 MHz
Inova NMR spectrometer using solvent residual peaks as a
reference (chloroform d ¼ 7.260 ppm (22)). The assign-
ments are based on the atom-labelling scheme used in the
X-ray crystal structures. ESI mass spectrometry was carried
Phppt
An orange solution of freshly prepared ethyl N-phenylpyr-
idine-2-carboximidothiate (2.0 g, 8.1 mmol) and benzohy-
drazide (1.35 g, 8.9 mmol) was refluxed in BuOH (50 mL)
for 36 h. The resulting orange solution was allowed to cool
to room temperature, resulting in the formation of some
white precipitate. It was further cooled in the freezer before
filtering off the solid and washing it with Et2O (5 mL). The
crude product (985 mg, 3.30 mmol, 41%) was recrystallised
from hot MeOH (40 mL). After cooling in the freezer, the
resulting white crystalline solid was isolated by vacuum
filtration, washed with MeOH (5 mL) and dried in vacuo
(795 mg, 2.66 mmol, 33% overall).
out on a Bruker micrOTOFQ mass spectrometer.
57
¨
Fe Mossbauer spectra were also recorded at the
University of Otago. Approximately, 10 mg of sample was
ground together with crystalline sucrose and placed in a
nylon sample holder (12.8-mm diameter, 1.6-mm thick-
¨
ness) with Kapton windows. Mossbauer spectra were
¨
measured on a Mossbauer spectrometer from Science
Engineering & Education Co. (SEE Co., Edina, MN, USA)
equipped with a closed cycle refrigerator system from
Janis Research Co., Wilmington, MA, USA and Sumitomo
Heavy Industries Ltd., Nishitokyo-City, Tokyo, Japan.
Data were collected in constant acceleration mode in
Found: C, 76.20; H, 4.74; N, 19.08 Calcd. for C19H14N4
C, 76.49; H, 4.73; N, 18.78%. 1H NMR (400 MHz, CDCl3)
8.33 (1H, ddd, JH3-H4 ¼ 5 Hz, JH3-H5 ¼ 2 Hz, JH3-H6
¼
1 Hz, pyH3), 8.11 (1H, dt, JH6-H5 ¼ 8 Hz, JH6-H4 ¼ JH6-

Supramolecular Chemistry
551
¼ 1 Hz, pyH6), 7.75 (1H, td, JH4-H3 ¼ JH4-H5 ¼ 8 Hz,
applied. The structures were solved by direct methods
(SHELXS-97) (24, 25) and refined against all F2 data
(SHELXL-97) (26, 27). All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were inserted at
calculated positions and rode on the atoms to which they
were attached with U(H) ¼ 1.2U(non-H). CCDC 869782
& 869783.
H3
JH4-H6 ¼ 2 Hz, pyH4), 7.20–7.46 (11H, m, pyH5,
2 £ phH2-6) ppm. ATR-IR n/cm21 ¼ 1582 (m), 1496 (m),
1469 (m), 1444 (m), 1425 (m), 801 (m), 771 (m), 715 (s),
601 (m). ESI-MS (pos.) m/z: [Na(phppt)]þ (expected,
321.1116; found, 321.1104).
[Fe(phppt)2(SCN)2]·2MeOH (3·2MeOH)
[Fe(phppt)2(SCN)2]·2MeOH
C42H36FeN10O2S2 (3·2MeOH), M ¼ 832.78 g mol21, tri-
Under argon, to a pale yellow solution of phppt (100 mg,
0.335 mmol) in 4:1 MeOH/CHCl3 (40 mL) was added a
pale yellow solution of [Fe(py)4(SCN)2] (78 mg,
0.16 mmol) in MeOH (5 mL) causing the solution to turn
deep red. After stirring for 1 h at room temperature, the
resulting red solution was subjected to Et2O vapour
diffusion (still under argon), resulting in the formation of
pale orange crystals suitable for X-ray diffraction. These
were filtered off and dried under a flow of N2 (30.8 mg,
0.040 mmol, 25%).
˚
˚
clinic, a ¼ 8.9230(17) A, b ¼ 9.5562(19) A,
˚
c ¼ 13.421(2) A, a ¼ 95.117(10)8, b ¼ 108.385(10)8,
3
˚
g ¼ 111.310(9)8, V ¼ 984.5(3) A , T ¼ 89(2) K, space
group P-1, Z ¼ 1, 14,321 reflections measured, 3832
unique (Rint ¼ 0.0553) which were used in all calculations.
Final wR2 ¼ 0.1123 (all data), R1 ¼ 0.0453 (I . 2s).
[Fe(phppt)2(SeCN)2]·Et2O
C44H38FeN10OSe2 (4·Et2O), M ¼ 936.61 g mol21, tricli-
Found: C, 60.42; H, 4.12; N, 16.82; S, 7.55 Calcd. for
C40H28N10S2Fe·2MeOH C, 60.58; H, 4.36; N, 17.20; S,
7.70% ATR-IR n/cm21 ¼ 2052 (s), 2028 (s), 1602 (w),
1495 (m), 1470 (s), 1435 (m), 1024 (w), 792 (s), 727 (m),
691 (s), 612 (w). ESI-MS (pos.) m/z: [Fe(phppt)2(SCN)]þ
(expected, 710.1538; found, 710.1463), [Na(phppt)2]þ
(expected, 619.2335; found, 619.2290), [Na(phppt)]þ
(expected, 321.1116; found, 321.1076).
˚
˚
nic, a ¼ 8.9147(3) A, b ¼ 10.0514(5) A, c ¼ 13.2605(5)
˚
A, a ¼ 73.230(2)8, b ¼ 70.767(2)8, g ¼ 67.756(2)8,
3
˚
V ¼ 1019.99(7) A , T ¼ 90(2) K, space group P-1, Z ¼ 1,
12,338 reflections measured, 4176 unique (Rint 0.0404)
which were used in calculations. Final wR2 ¼ 0.0858
(all data) and R1 ¼ 0.0428 (I . 2s).
[Fe(phppt)2(SeCN)2]·1.5MeOH (4·1.5MeOH)
Acknowledgements
Under argon, to a pale yellow solution of phppt (100mg,
0.335 mmol) in 4:1 MeOH/CHCl3 (40 mL) was added solid
[Fe(py)4(SeCN)2] (96.7 mg, 0.168 mmol), causing the
solution to turn deep red as the solid dissolved. After stirring
for 20min at room temperature, the resulting deep red
solution was subjected to Et2O vapour diffusion (still under
argon), resulting in the formation of a few orange crystals of
[Fe(phppt)2(SeCN)2]·Et2O suitable for X-ray diffraction and
orange microcrystals. These were collected by filtration and
dried under a flow of N2(g) to give [Fe(phppt)2(SeCN)2]·1.5-
MeOH (56 mg, 0.0634mmol, 38%).
We are grateful to the University of Otago, the Marsden Fund
(RSNZ) and the MacDiarmid Institute for Advanced Materials
and Nanotechnology for funding this research, including the
¨
purchase of the Mossbauer Spectrometer (MacDiarmid Institute).
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