Synthesis and structures of two mononuclear iron(ii) complexes derived from polypyridine ligands

Article abstract of DOI:10.1016/j.mencom.2020.01.033

Three polypyridine ligands such as tri(2-pyridyl)methane, (2,2’-bipyridin-6-yl)di(2-pyridyl)methane and 2,6-bis[di(2-pyridyl)methyl] pyridine as well as their new iron(II) mononuclear complexes have been obtained in a one-pot synthesis. Detailed structural analyses and magnetic susceptibility measurements confirm the expected six-coordinate octahedral geometry and the metric parameters are consistent with lowspin iron(II) in the complexes.

Full text of DOI:10.1016/j.mencom.2020.01.033

Mendeleev
Communications
Mendeleev Commun., 2020, 30, 100–102
Synthesis and structures of two mononuclear iron(ii) complexes
derived from polypyridine ligands
Hongqiao Wang,a,b Nan Wu,a Jing Zheng,a Chunyang Zheng*a,b and Dunjia Wanga
a Hubei Key Laboratory of Pollutant Analysis and Reuse Technology, Hubei Normal University,
Huangshi 435002, P. R. China. E-mail: zcy800204@163.com
b Department of Chemistry, The School of Art and Science of Hubei Normal University, Huangshi
435002, P. R. China
DOI: 10.1016/j.mencom.2020.01.033
Three polypyridine ligands such as tri(2-pyridyl)methane,
(2,2'-bipyridin-6-yl)di(2-pyridyl)methane and 2,6-bis[di(2-
pyridyl)methyl]pyridine as well as their new iron(ii) mono-
nuclear complexes have been obtained in a one-pot synthesis.
Detailed structural analyses and magnetic susceptibility
measurements confirm the expected six-coordinate octahedral
geometry and the metric parameters are consistent with low-
spin iron(ii) in the complexes.
Nꢀꢃꢁ
Nꢀ2ꢁ
Nꢀꢃꢁ
Nꢀ2ꢁ
Nꢀꢂꢁ
Nꢀꢃꢁ
Nꢀꢂꢁ
ꢇeꢀꢃꢁ
ꢇeꢀꢃꢁ
Nꢀꢄꢁ
Nꢀꢅꢁ
Nꢀꢂꢁ
Nꢀ2ꢁ
Nꢀꢆꢁ
loꢀꢁspin
Keywords: polypyridine, one-pot synthesis, metal complexes, low-spin.
Polypyridine compounds are excellent ligands for building up large
polymetallic compounds as they stabilize a single labile coordi-
nation site in transition metal coordination compounds. They attract
considerable attention due to their fascinating photo- and electro-
chemistry properties.1,2 The majority of ligands have N(2), N(3),
N(4) or N(5) donor sets, therefore their electronic properties may
be tuned by varying the combination of N donors as well as
substituents (electron-withdrawing vs. electron-donating).3–5 Of
considerable interest are their applications in metallo-supramolecular
chemistry and their presence in coordination compounds possessing
interesting magnetic properties which can be rationally designed,
predictably assembled and easily modified.1,2
to carbaninon A. Treatment of this carbanion with the appropriate
electrophiles enabled performing the next stage in the same vessel
(see Scheme 1, steps iv, vi, vii) to afford ligands Py4, Py3 and Py5.†
The crystal structure of Py5 was confirmed by X-ray crystallo-
graphy (Figure 1).‡
The synthesis of two metal coordination compounds 1 and 2
was carried out by reaction of 2 equiv. of Py3 and 1 equiv. of Py4
with Fe(ClO4)2·6H2O salt in DMF or MeCN under an N2
atmosphere.§ Interestingly, variation in reactant ratio [Py4/
Fe(ClO4)2·6H2O being 1:1, 2:1 or 2:1] resulted in the same
complex 2.
The molecular structures of compounds 1 and 2 were determined
by X-ray crystallography at 296 K and 100 K, respectively (see
Figure 1).‡ Complexes 1 and 2 crystallize in the monoclinic space
Among numerous polypyridine ligands, tri(2-pyridyl)methane
(Py3) and its derivatives have been synthesized and their transition
metal coordination compounds have been studied in terms of
tripodal acceptor ligands.6,7 In the recent year, we have studied
†
1
(2,2'-Bipyridin-6-yl)di(2-pyridyl)methane (Py4). H NMR (300 MHz,
CDC13) d: 8.59–8.65 (m, 3H), 8.26–8.30 (m, 2H), 7.62–7.80 (m, 5H),
7.36–7.40 (m, 3H), 7.15–7.19 (m, 2H), 6.12 (s, 1H). MS (ESI), m/z: 325.22
([M+H]+), 671.00 ([2M+Na]+).
Tri(2-pyridyl)methane (Py3). H NMR (300 MHz, CDC13) d: 8.57–8.60
(m, 3H), 7.59–7.65 (m, 3H), 7.33 (d, 3H), 7.15 (t, 3H), 6.00 (s, 1H). MS (ESI),
m/z: 248.15 ([M+H]+), 516.93 ([2M+Na]+).
some new cyano-bridged heterometallic molecular clusters with
reversible charge-transfer-induced spin transition (CTIST) or
spin crossover (SCO) behavior.8,9 We have reported two iron(ii)
tri(pyridyl)phosphine sulfide compounds lacking spin crossover
behavior.10 In order to design and produce SCO systems that can
be tuned at room temperature, we have synthesized some poly-
pyridine ligands to adjust the ligand-field strength on the Feii centres.
Herein, we report on the syntheses of tripodal tri(2-pyridyl)-
methane (Py3), new tetradendate (2,2'-bipyridin-6-yl)di(2-pyridyl)-
methane (Py4), and pentadendate 2,6-bis[di(2-pyridyl)methyl]pyridine
(Py5). Crystal structure study of two new iron(ii) mononuclear
coordination compounds, viz. [Fe(Py3)2][ClO4]2·2DMF 1 and
[Fe(Py4)(MeCN)2][ClO4]2·H2O 2, was also performed.
Ligands Py3, Py4 and Py5 were obtained by one-pot procedures
(Scheme 1). Di(2-pyridyl)methane was prepared in situ by treatment
of lithiated 2-methylpyridine (LiC6H6N) with 0.5 equiv. of 2-fluoro-
pyridine, which allowed us to avoid formation of side products.11,12
The role of the second equivalent of LiC6H6N was to cause depro-
tonation in initially formed di(2-pyridyl)methane leading finally
1
2,6-Bis[di(2-pyridyl)methyl]pyridine (Py5). 1H NMR (300 MHz, CDC13)
d: 8.53 (dd, 4H, J 4.8 Hz, J 0.8 Hz), 7.47–7.61 (m, 5H), 7.17–7.21 (m, 6H),
7.07–7.12 (m, 4H), 5.93 (s, 2H). MS (ESI), m/z: 416.33 ([M+H]+),
438.18 ([M+Na]+).
‡
Crystal data for (Py5). C27H21N5, M = 415.49, monoclinic, space group C2,
a = 16.031(3), b = 7.3086(15) and c = 12.132(4) Å, b = 130.676(2)°,
V = 1078.0(5) Å3, Z = 2, dcalc = 1.280 g cm–3, m(MoKa) = 0.078 mm–1,
T = 296(2) K, 7119 reflections measured, 3084 independent reflections
(Rint = 0.0454), final R1 = 0.0500 [I > 2s(I)], wR2 = 0.1361, GOF = 1.086.
Crystal data for 1. C38H40Cl2FeN8O10, M = 895.53, monoclinic, space
group P21/n, a = 12.4008(6), b = 12.8255(6) and c = 13.8889(7) Å,
b = 112.111(2)°, V = 2046.52(17) Å3, Z = 2, dcalc = 1.453 g cm–3,
m(MoKa) = 0.566 mm–1, T = 296(2) K, 22473 reflections measured,
6488 independent reflections (Rint = 0.0496), final R1 = 0.0599 [I > 2s(I)],
wR2 = 0.1516, GOF = 1.030.
© 2020 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2020, 30, 100–102
ꢀaꢁ
ꢀbꢁ
ꢀcꢁ
i
ii
Nꢀ2ꢁ
Nꢀꢃꢁ
ꢇeꢀꢃꢁ
Nꢀ2ꢁ
N
N
N
Nꢀ2ꢁ
Nꢀ2ꢁ
Nꢀꢂꢁ
Nꢀꢃꢁ
Nꢀꢃꢁ
Nꢀꢂꢁ
N
N
N
Nꢀꢂꢁ
ꢇeꢀꢃꢁ
Nꢀꢄꢁ
Nꢀꢅꢁ
Me
Me
I–
Nꢀꢂꢁ
Nꢀꢃꢁ
Nꢀꢆꢁ
Nꢀꢃꢁ
O
Nꢀ2ꢁ
N
iv
iii
Figure 1 Molecular structures of (a) Py5 , (b) complex 1 and (c) complex
2; anions and solvent molecules are omitted for clarity.
N
N
N
N
vi
N
Br
Py4
ꢀe
a
N
c
b
C
ꢁ
Cgꢃ
v
N
N
Li+
N
N
Cgꢂ
2.ꢈ2ꢃ ꢉ
ꢁꢄꢃ0ꢅ
N
Me
N
ꢇ.ꢆꢊꢃ ꢉ
A
ꢁꢄꢃ0ꢅ
Cgꢇ
2.ꢋꢃ0 ꢉ
Cgꢂ
ꢁꢄꢃꢆꢅ
Py3
2.ꢈ2ꢃ ꢉ
Cgꢃ
Cg2
ꢇ.ꢋꢇꢂ ꢉ
ꢇ.ꢊꢈ0 ꢉ
N
N
N
vii
ꢇ.ꢋꢇꢂ ꢉ
Cg2
N
N
Cgꢇ
Cgꢃ
ꢁꢄꢃꢆꢅ
2.ꢋꢃ0 ꢉ
Py5
Figure 2 Packing diagram of molecules 1 illustrating intermolecular
C–H···π and π–π interactions; anions, solvent molecules and part of hydrogen
atoms are omitted for clarity. Symbols Cg1, Cg2, Cg3 and Cg4 denote the
centroids of four pyridine rings [N(4),C(17)–C(21)], [N(3),C(12)–C(16)],
[N(1),C(1)–C(5)] and [N(2),C(7)–C(11)], respectively.
Scheme 1 Reagents and conditions: i, MeI; ii, K3Fe(CN)6; iii, Br2, PPh3;
iv, BuLi, THF, –78°C, then 2-fluoropyridine, –20°C, then reflux, 1 h; v, LiC6H6N;
vi, 2-fluoropyridine; vii, 2,6-difluoropyridine.
–
group P21/n and triclinic space group P1with two DMF molecules
in 1 and one H2O molecules in 2, respectively. Ligand Py3 acts as
tripodal one and the Feii atom lies on an inversion center and is
octahedrally coordinated by all the N atoms with expected six-
coordinate geometry. In complex 2, Feii ion is surrounded by six
nitrogen atoms from one Py4 ligand and two MeCN molecules.
The coordination Fe–N bond lengths (Table 1) in complex 1
(average 1.980 Å) and 2 (average 1.9532 Å) are similar to reported
average [Fe–N(py)6]2+ bond length in [FeL2]2+ based on 1,1,1-tri-
(2-pyridyl)ethane ligand.13 In addition, the distortion parameter
values S of geometries around Feii ions are 14.4° for 1 and 48.5°
for 2, respectively. Moreover, the Fe(1)–N(3) bond length (1.904 Å)
in complex 2 is obviously shorter than other Fe(1)–N(3) bondlengths
(from 1.941 to 1.986 Å), and the value S (48.5°) in complex 2 is
larger than that (14.4°) in 1. The above mentioned phenomenon
may be caused by the steric hindrance effect of the ligand Py4. In
general, high spin Feii ions tend to form more distorted octahedra
and thus have larger S value than their low spin counterparts.14
According to above structural analysis, we can infer that the spin
state of Feii ion in 1 and 2 is the LS state. The cMT values (see
Figure S1, Online Supplementary Materials) of 1 and 2 are
below 0.1 cm3 K mol–1 at 250 K and agree with the expected value
for LS Feii unit, which is also confirmed by the above crystallo-
graphic data and is in agreement with a low-spin iron(ii) in N(6)
surrounding. In complex 2, short intermolecular π–π interactions
are found between the coordinated pyridine ligands and the pyridine
ligands (3.861, 3.934 and 3.670 Å). In addition, in the crystal
structure of 2 there are weak C–H···π intermolecular hydrogen
bonds (2.721 and 2.910 Å), and the stability of the crystal structure
can be accounted by these interactions (Figure 2).
Crystal data for 2. C25H24Cl2FeN6O9, M = 679.25, triclinic, space group
–
P1, a= 10.9963(11), b= 11.8255(10) and c= 12.1227(12) Å, a= 77.3000(10)°,
b = 64.4770(10)°, g = 82.6570(10)°, V = 1386.8(2) Å3, Z = 2, dcalc
=
In summary, a tripodal ligand tri(2-pyridyl)methane (Py3) and
new tetradendate ligand (2,2'-bipyridin-6-yl)di(2-pyridyl)methane
(Py4) have been synthesized by a high-yielding one-pot procedure
= 1.627 cm–3, m(MoKa) = 0.802 mm–1, T = 100(2) K, 9766 reflections
measured, 5278 independent reflections (Rint = 0.0356), final R1 = 0.0406
[I > 2s(I)], wR2 = 0.0925, GOF = 1.029.
CCDC 1935066, 1578633 and 1578634 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk.
Table 1 Mean Feii–N bond lengths (Å) and distortion parameters (°) for
complexes 1 and 2.
§
Synthesis of [Fe(Py3)2][ClO4]2 ·2HC(O)NMe2 1. A solution of
Complex 1
Complex 2
Fe(ClO4)2·6H2O (0.036 g, 0.1 mmol) and Py3 (0.052 mg, 0.2 mmol) in
DMF (3 ml) was stirred at room temperature for 2 h under an argon
atmosphere. Then it was placed under the diffusion of diethyl ether vapor
slowly without disturbance. Dark-red blocks crystals suitable for X-ray
structure analysis were grown within about 7 days. Yield 0.060 g.
Synthesis of [Fe(Py4)(MeCN)2][ClO4]2 ·H2O 2. A solution of
Fe(ClO4)2·6H2O(0.036 g, 0.1 mmol) and Py4 (0.032 mg, 0.1 mmol) in MeCN
(3 ml) was stirred at room temperature for 2 h under an argon atmosphere.
Then it was placed under the slow diffusion of diethyl ether vapor without
disturbance. Dark-red blocks crystals suitable for X-ray structure analysis
were obtained within about 8 days. Yield 0.032 g.
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(1)–N(4)
Fe(1)–N(5)
Fe(1)–N(6)
Fe(1)–N(average)
1.9795(14)
1.9783(14)
1.9821(14)
1.9858(15)
1.9563(15)
1.9036(15)
1.9861(15)
1.9410(15)
1.9470(16)
1.9533(15)
48.5
1.9800(14)
14.4
a
SFe(1)
a The sum of the deviation from 90° of the 12 cis-angles of the FeN6 octahedron.
– 101 –

Mendeleev Commun., 2020, 30, 100–102
2 M. Nippe, R. S. Khnayzer, J. A. Panetier, D. Z. Zee, B. S. Olaiya, M. Head-
using inexpensive substrates. Based on these two polypyridine
ligands, two mononuclear iron(ii) complexes have been obtained. The
expected six-coordinate octahedral geometry has been confirmed
by structural analyses and magnetic susceptibility measurements,
and the metric parameters are consistent with low-spin iron(ii) in
1 and 2. The structural analysis of complex 2 has revealed that
the [Fe(Py4)]2+ entity possesses two accessible coordination
sites, which indicates that the mononuclear iron(ii) complex can
be obtained using the co-ligand NCX− (X = S, Se, or BH3). Further
studies on the fabrication of new iron(ii) metal coordination
compounds with the new Py4 ligand showing SCO behavior are
ongoing.
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This work was supported by the Science and Technology
Foundation for Creative Research Group of HBDE (T201810),
the National Undergraduate Training Program for Innovation and
Entrepreneurship (project no. 201813256003) and the Key Fund of
The School of Art and Science of Hubei Normal University
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Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2020.01.033.
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