Tridentate N?N?N iron(II) and cobalt(II) complexes of ion-paired structures: Synthesis, characterization and magnetism

Article abstract of DOI:10.1016/j.molstruc.2008.03.027

Two kinds of ion-paired iron and cobalt complexes ligated by 2-(benzimidazolyl)-6-(1-(arylimino)ethyl)pyridyl [iron (1a) and cobalt (1b)] or N-(pyridin-2-yl)methylene)-quinolin-8-amine (iron, 2a) were synthesized and well characterized by IR, EA and X-ray

Full text of DOI:10.1016/j.molstruc.2008.03.027

Journal of Molecular Structure 890 (2008) 95–100
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
ˆ ˆ
Tridentate N N N iron(II) and cobalt(II) complexes of ion-paired structures:
Synthesis, characterization and magnetism
Kefeng Wang a, Peng Hao a, Deqing Zhang b, Wen-Hua Sun a,
*
a Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, No. 2, Beiyijie,
Zhongguancun, Beijing 100080, China
b Key Laboratory of Organic Solids and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two kinds of ion-paired iron and cobalt complexes ligated by 2-(benzimidazolyl)-6-(1-(arylimi-
no)ethyl)pyridyl [iron (1a) and cobalt (1b)] or N-(pyridin-2-yl)methylene)-quinolin-8-amine (iron, 2a)
were synthesized and well characterized by IR, EA and X-ray diffraction analysis. They were confirmed
to be in the form of [ML2]2+[MCl4]2ꢀ or [ML2]2+2[MCl4]1ꢀ and the cations took octahedral coordinated
geometry. Structure research indicated that complex 1a was in low-spin state while complex 1b was
in high-spin state. Furthermore, their magnetic susceptibility measurements were performed and the
complexes showed interesting magnetic properties.
Received 26 December 2007
Received in revised form 18 March 2008
Accepted 20 March 2008
Available online 28 March 2008
Dedicated to Professor F. Albert Cotton in
honor of the publication of his 1600th
article.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Iron and cobalt complex
ˆ
ˆ
Tridentate N N N complex
Magnetism
1. Introduction
din-2-yl)methylene)-quinolin-8-amine [9]. In general, the trigonal
bipyramid geometry was observed for those complexes with equal
As the key part of coordination chemistry, rational design and
synthesis of novel ligand always receive much attention. In recent
years, rapid progress of ethylene reactivity was made in tridentate
molar ligand to metal core, and those complexes performed good to
high catalytic activities. However, the metal (iron and cobalt)
complexes bearing 6-difluoro-N-(1-(6-(1-methyl-1H-benzo-imida-
zol-2-yl)pyridin-2-yl)ethylidene)benzenamine (1, Scheme 1) and
iron complex ligated by N-((pyridin-2-yl)methylene)-quinolin-8-
amine (2, Scheme 1)didnotshow anyactivitytowardethylene. Single
X-raydiffractionanalysiswasperformedandtheformationoftheion-
pairedcomplexeswasconfirmed.Theion-pairedironcomplexeshave
been wanted in magnetic materials [10], which have been widely
investigated and indeed attracted much attention [11]. Herein, we re-
portthesynthesis, characterization ofthe titled ion-pairedcomplexes
and the study of their unique magnetic properties.
ˆ
ˆ
N N N complexes, which are of considerable current interest and
could be reflected by the recent articles [1]. It was found out that,
while complexes bearing ligands containing bulky substituents are
generally of trigonal bipyramid geometry and performed good cat-
alytic activities for polymerization or oligomerization of ethylene
[2], there is a tendency of the formation of ion-paired complexes
for steric unhindered ligand [3]. In such ion-paired complex, there
are two ligands coordinated with one metal core. It would be inter-
esting to research the various properties derived from different
structures, which were formed from same ligand framework along
with varying steric effect.
2. Experimental
In devising late-transition metal complexes as catalysts for
ethylene polymerization and oligomerization, our group recently
2.1. General considerations
ˆ
ˆ
developed series of tridentate N N N complexes containing
ligandssuchas2-imino-1,10-phenanthrolines[4], 2-(benzoimidazol-
yl)-6-iminopyridines [5], 2-quinoxalinyl-6-iminopyridines [6],
2-(benzimidazol-2-yl)-1,10-phenanthrolines [7], 2-methyl-2,4-
bis(6-iminopyridin-2-yl)- 1H-1,5-benzodiazepines [8] and N-((pyri-
All manipulations of air and/or moisture-sensitive compounds
were carried out under an atmosphere of nitrogen using standard
Schlenk techniques. Solvents were refluxed over an appropriate
drying agent, distilled and degassed before using. 1-(6-(1-
Methyl-1H-benzo[d]imidazol-2-yl)pyridin-2-yl)ethanone was pre-
pared by literature method [5]. Other reagents were purchased
from Aldrich, Acros or local supplier. Elemental analyses were
* Corresponding author. Tel.: +86 10 62557955; fax: +86 10 62618239.
E-mail address: whsun@iccas.ac.cn (W.-H. Sun).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.03.027

96
K. Wang et al. / Journal of Molecular Structure 890 (2008) 95–100
scribed as follows (Scheme 2). Ligand 1 (152 mg, 0.42 mmol) and
FeCl2ꢁ4H2O (79.5 mg, 0.4 mmol) were added to a Schlenk tube, fol-
lowed by the addition of freshly distilled ethanol (2 mL) with rapid
stirring at room temperature. The solution turned deep blue imme-
diately, and after 1 min blue power was obtained. The reaction
mixture was stirred for 10 h, and after the mixture stood for
30 min, the supernatant was removed. The precipitate was washed
with diethyl ether twice and dried in vacuum to furnish the pure
product as a blue powder in 79.5% yield. IR (KBr; cmꢀ1): 3395;
2362; 2342; 1591 (C@N); 1540; 1525; 1472; 1419; 1390; 1335;
1283; 1241; 1022; 1003; 782; 748. Anal. Calcd for
C42H32Cl4F4Fe2N8: C, 51.57; H, 3.30; N, 11.45. Found: C, 52.05; H,
3.81; N, 11.53.
F
N
1
N
N
2
N
N
N
F
N
Scheme 1. Ligands 1 and 2.
performed on a Flash EA 1112 microanalyzer. IR spectra were
recorded on a Perkin-Elmer System 2000 FT-IR spectrometer using
KBr disc in the range of 4000–400 cmꢀ1. The 1H and 13C NMR
spectra were recorded on a Bruker DMX-300 instrument with
TMS as the internal standard. Variable-temperature magnetic sus-
ceptibilities (in the temperature range of 5–300 K) were measured
on a Quantum Design MPMS-7 SQUID magnetometer in a field of
1 T. Diamagnetic corrections were made with Pascal’s constants
for all constituent atoms.
2.2.2.2. Synthesis of complexes complex 1b. Complex 1b was synthe-
sized by the reaction of CoCl2 with the corresponding ligand 1 in
ethanol (Scheme 2). Ligand 1 (152 mg, 0.42 mmol) and CoCl2
(51.6 mg, 0.4 mmol) were added to a Schlenk tube, followed by
the addition of freshly distilled ethanol (2 mL) with rapid stirring
at room temperature. The solution turned deep green immediately,
and after 1 min green power was obtained. The reaction mixture
was stirred for 10 h, and after the mixture stood for 30 min, the
supernatant was removed. The precipitate was washed with
diethyl ether twice and dried in vacuum to furnish the pure prod-
uct as a green powder in 72.7% yield. IR (KBr; cmꢀ1): 3450; 3093;
3057; 1594 (C@N); 1486; 1471; 1422; 1401; 1370; 1330; 1280;
1256; 1025; 1003; 781; 748. Anal. Calcd for C42H32Cl4Co2F4N8: C,
51.24; H, 3.28; N, 11.38. Found: C, 51.24; H, 3.72; N, 10.85.
2.2. Synthesis of ligand and complexes
2.2.1. Synthesis of ligand 6-difluoro-N-(1-(6-(1-methyl-1H-benzo[d]
imidazol-2-yl) pyridin-2-yl)ethylidene)benzenamine (1)
A solution of 2,6-difluoroaniline (106 mg, 0.875 mmol), 1-(6-(1-
methyl-1H-benzo [d] imidazol-2-yl) pyridin-2-yl) ethanone
(200 mg, 0.796 mmol), and a catalytic amount of p-toluenesulfonic
acid in toluene (25 mL) were refluxed for 24 h. After solvent evap-
oration, the crude product was purified by column chromatogra-
phy on silica gel with petroleum ether/ethyl acetate (3:1) as
eluent to afford the product as a white powder in 65.9% yield
(Scheme 2). Mp 177.0–178.0 °C. 1HNMR (300 MHz, CDCl3, TMS):
8.56–8.54 (d, 1H, J = 7.5 Hz, –Py), 8.43–8.41 (d, 1H, J = 7.5 Hz, –
Py), 8.00–7.95 (t, 1H, J = 7.8 Hz, –Py), 7.87–7.85 (d, 1H, J = 6.6 Hz,
–Ph), 7.47–7.45 (d, 1H, J = 7.8 Hz, –Ph), 7.37 (m, 2H, –Ph), 7.10–
6.96 (m 3H, –Ph), 4.35 (s, 3H, –CH3), 2.47 (s, 3H, –CH3). 13CNMR
(75.45 MHz, CDCl3, TMS): 172.07, 154.73, 151.38, 149.9, 149.57,
142.63, 137.66, 137.39, 126.36, 124.25, 123.60, 122.84, 122.08,
120.23, 111.91, 111.60, 109.99, 33.10, 18.06. ESI-MS: 363.1
(M+H+), 385.0 (M+Na+). IR (KBr; cmꢀ1): 3430; 1647 (C@N); 1582;
1470; 1434; 1388; 1364; 1329; 1276; 1238; 1215; 1153; 1116;
1071; 774; 743. Anal. Calcd for C21H16N4F2: C, 69.60; H, 4.45; N,
15.46. Found: C, 69.23; H, 4.55; N, 14.90.
2.2.2.3. Synthesis of complexes complex 2a. Complex 2a was pre-
pared by one-pot reaction as the reported procedure [9] (Scheme
3). A suspension of pyridine-2-aldehyde (2.00 mmol), 8-amino-
quinoline (2.00 mmol), and FeCl2ꢁ4H2O (2.00 mmol) in glacial ace-
tic acid (25 mL) was refluxed for 2 h. The precipitate was collected
by filtration and washed with diethyl ether (3ꢂ 5 mL). Then the
collected solid was dissolved in methanol, concentrated and pre-
cipitated with diethyl ether. After washing with diethyl ether the
collected solid was dried in vacuum. The desired complex 2a was
obtained as a green powder in 91.5% yield (0.56 g) IR (KBr):
3397, 3262, 1594 (C@N), 1532, 1503, 1460, 1428, 1390, 1296,
1252, 1214, 1069, 1028, 829, 766 cmꢀ1
. Anal. Calcd for
C30H22Cl8Fe3N6ꢁ2MeOH: C, 39.15; H, 3.08; N, 8.56. Found: C,
38.97; H, 3.23; N, 8.09%.
2.3. X-ray crystallography measurements
2.2.2. Synthesis of complexes
2.2.2.1. Synthesis of complexes complex 1a. The complex 1a were
synthesized by the reaction of FeCl2ꢁ4H2O with the corresponding
ligand 1 in ethanol. A typical synthetic procedure for 1a can be de-
Single-crystal X-ray study for complex 1b was carried out on a
Rigaku R-AXIS Rapid IP diffractometer with graphite-monochro-
mated Mo-Ka radiation (k = 0.71073 Å) at 293(2) K. Intensity data
for crystals of 1a and 2a were collected on a Bruker SMART 1000
CCD diffractometer with graphite monochromated Mo-Ka radia-
tion (k = 0.71073 Å). Cell parameters were obtained by global
refinement of the positions of all collected reflections. Intensities
NH2
F
F
p-TsOH
N
N
+
N
O
toluene
reflux
+
+
FeCl2·4H2O
N
N
NH2
O
F
N
N
MCl2
2-
L2M2+·MCl4
N
CH3COOH reflux
N
1a M=Fe
1b M=Co
-
L2Fe2+·2FeCl4
F
1
2a
Scheme 2. Synthesis of ligand 1 and complexes 1a and1b.
Scheme 3. Synthesis of complexes 2a.

K. Wang et al. / Journal of Molecular Structure 890 (2008) 95–100
97
Table 1
method was employed to synthesize its iron complex. The stoichi-
ometric amounts of the 8-amino-quinoline, pyridine-2-aldehyde
and the ferrous dichloride were mixed and refluxed in presence
of acetic acid for 2 h to afford the corresponding complex 2a
(Scheme 3). The resultant air-stable and green product were pre-
cipitated from the reaction solution and collected by filtration,
washed with diethyl ether to remove the acetic acid and dried in
vacuum. The complex was characterized with elemental analysis
and IR spectroscopy. In the IR spectra, the frequency of
1594 cmꢀ1 could be ascribed to the stretching vibration of C@N
bond [13c]. Different with 1a and 1b,ꢀelemental analysis result
supported the structure of L2Fe2þ ꢁ 2FeCl4 , perhaps due to the effect
Summary of crystallographic data for 1a, 1b and 2a
1aꢁ5H2O
667620
1bꢁH2O
667621
2a
CCDC No.
Formula
Fw
Cryst. syst.
Space group
a (Å)
b (Å)
c (Å)
a (deg)
b (deg)
c (deg)
V (Å3)
667622
C42H32Cl4F4Fe2N8O5 C42H32Cl4Co2F4N8O C30H22Cl8Fe3N6
1058.26
Triclinic
1000.42
Triclinic
917.69
Monoclinic
P2(1)/c
15.4132(4)
13.3718(3)
19.9764(5)
90
ꢀ
ꢀ
P1
P1
11.3600(1)
14.8427(2)
15.5477(2)
70.615(5)
81.300(8)
78.660(8)
2414.1(5)
2
10.752(2)
14.351(3)
17.534(4)
108.67(3)
107.71(3)
97.18(3)
2366.4(8)
2
90
90
of acetic acid, iron atom in the anion part was oxidized to be Fe3+
which was further confirmed by single-crystal X-ray analysis.
,
4117.18(17)
4
1.480
1.590
1832
Z
Dcalcd (g cmꢀ3
)
1.456
0.888
1072
1.404
0.983
1012
l [mmꢀ1
F(000)
]
3.2. Crystal structures
h Range [°]
No. of data collected 33975
No. of unique data
Goodness-of-fit
R
Rw
1.39–25.01
1.55–25.01
7818
7818
1.058
0.0818
0.2466
1.32–28.35
21515
9004
0.720
0.0763
0.2207
To confirm their structures, the single-crystal X-ray crystallog-
raphy was employed, and it was confirmed that these complexes
exhibit ion-paired structural feathers. However, there are five iso-
lated water molecular for 1a and one isolated water molecular for
1b. Because they are disordered in the single crystal, it is difficult to
find their hydrogen atoms from different Fourier synthesis. There-
fore, the hydrogen atoms were not given in the crystal data.
Single crystals of complexes 1a and 1b were individually ob-
tained by slow diffusion of diethyl ether into their methanol solu-
tions under nitrogen atmosphere. The molecular structures are
8487
0.961
0.0983
0.2774
were corrected for Lorentz and polarization effects and empirical
absorption. The structures were solved by direct methods and re-
fined by full-matrix least-squares on F2. All non-hydrogen atoms
were refined anisotropically. All hydrogen atoms were placed in
calculated positions. Structure solution and refinement were per-
formed by using the SHELXL-97 package [12]. Crystal data and pro-
cessing parameters for complexes 1a, 1b and 2a are summarized in
Table 1. CCDC-667620–667622 contain the supplementary crystal-
lographic data for this paper, these data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033).
3. Results and discussions
3.1. Synthesis and characterization of the complexes
The desired ligand 6-difluoro-N-(1-(6-(1-methyl-1H-benzo[d]imi-
dazol-2-yl) pyridine-2-yl)ethylidene)benzenamine (1) was easily
prepared in moderate yield (65%) through the condensation reac-
tions of 1-(6-(1-methyl-1H-benzoimidazol-2-yl) pyridin-2-yl) etha-
none and the 2,6-difluoroaniline using our previous method
(Scheme 2) [5a], and sufficiently characterized by elemental analysis,
1H and 13C NMR, IR, and ESI mass spectrometry.
The iron complex 1a was prepared by stirring an ethanol solu-
tion of 1 equiv of FeCl2ꢁ4H2O and the corresponding ligand 1 at
room temperature under nitrogen (Scheme 2). The obtained com-
plex was precipitated from the reaction solution and separated
as blue powders in good yield (79.5%) with good purity. Formation
of the complex was confirmed by elemental analysis and IR. In
comparison with the IR spectra of the free ligand (1647 cmꢀ1) for
C@N group, the stretching vibration in complex 1a was shifted to-
ward lower frequency of 1591 cmꢀ1 with the reduced intensity,
indicating an effective coordination between ligand and metal.
Similarly, its cobalt analogue 1b was conveniently prepared as
green powders in 72.7% yield (Scheme 2) with the lower frequency
of 1594 cmꢀ1 for C@N along with a decreased intensity.
Though much effort was made in the synthesis of the Schiff-
base compound, N-((pyridin-2-yl)methylene)-quinolin-8-amine,
pure product could not be obtained due to decomposition of those
particular Schiff-base compounds [9,13a]. Therefore, one-pot
Fig. 1. (a) Molecular structure of 1a. Hydrogen atoms, and solvent have been om-
itted for clarity. (b) Simplified structure.

98
K. Wang et al. / Journal of Molecular Structure 890 (2008) 95–100
shown in Figs. 1 and 2, respectively. Selected bond lengths and an-
gles are given in Table 2. The X-ray diffraction analysis revealed
that complex 1a existed as an ion pair comprised of [FeL2]2+ and
[FeCl4]2ꢀ (Fig. 1). In the cation part, the iron atom is coordinated
by six nitrogen atoms from two ligands, and its coordination geom-
etry at the iron center of [FeL2]2+ can be described as distorted
octahedron. The average coordinated Fe–N bond length is
1.946 Å, comparable with other low-spin Fe (II) complexes [10],
indicating the low-spin state of 1a at 293 K, which is in agreement
with the magnetic susceptibility measurement. Compared with the
iron analogues with bulkier substituents [5a], the Fe–N(imino)
bonds are much shorter by about 0.2 Å for 1a. While the N(3)–
C(13) distance is 1.279(1) Å, typically for C@N bond, the N(7)–
C(37) distance is 1.333(1) Å, a bit longer than normal distance.
The two ligands are almost vertical to each other, forming a dihe-
dral angle of 92.0°. The geometry at the iron center of [FeCl4]2ꢀ can
be described as a distorted tetrahedron with Fe(2)–Cl distances of
2.228(3)–2.267(3) Å and Cl–Fe(2)–Cl angles of 106.66(2)–
112.64(2) °. The distance between Fe(1) and Fe(2) is 8.221 Å.
Compared with 1a, the structure of complex 1b showed similar
coordination manner. The cobalt atom in the cation of complex 1b
is octahedral, with six coordination bonds to the nitrogen atoms of
two ligands. The bond lengths of two C@N bonds are 1.288(8) Å
(N(3)–C(13)) and 1.296(8) Å (N(7)–C(34)), respectively. However,
the Co–N bonds are longer than its iron analogue 1a (see Table 2
for comparison), and comparable with the cobalt analogues [5a],
indicating that 1b is in high-spin state, which is further confirmed
by the magnetic property. The Co(1)–N bond distances varied, indi-
cating that coordination geometry was not perfectly octahedral. The
two ligands are almost vertical to each other, forming a dihedral an-
gle of 98.2°. The geometry at the cobalt center of [CoCl4]2ꢀ can be de-
scribed as a distorted tetrahedron with Co(2)–Cl distances of
2.272(3)–2.282(3) Å and Cl–Co(2)–Cl angles of 108.22(1)–
110.68(1)°. The distance between Co(1) and Co(2) is 8.016 Å.
Table 2
Selected bond lengths (Å) and angles (deg) for 1a and 1b
1a (M@Fe)
1b (M@Co)
Bond lengths
M(1)–N(1)
M(1)–N(2)
M(1)–N(3)
M(1)–N(5)
M(1)–N(6)
M(1)–N(7)
N(3)–C(13)
N(7)–C(34)
1.966(7)
1.892(7)
1.993(7)
1.961(7)
1.883(7)
1.982(7)
1.279(1)
1.333(1)
2.100(5)
2.075(5)
2.209(5)
2.104(5)
2.054(5)
2.162(5)
1.288(8)
1.296(8)
Bond angles
N(2)–M(1)–N(1)
N(2)–M(1)–N(3)
N(1)–M(1)–N(3)
N(6)–M(1)–N(5)
N(6)–M(1)–N(7)
N(5)–M(1)–N(7)
79.7(3)
79.4(3)
158.9(3)
80.8(3)
79.2(3)
159.5(3)
76.8(2)
74.6(2)
151.3(2)
77.45(2)
74.88(2)
151.72(2)
were grown from a CH2Cl2 solution layered with pentane. The
molecular structure is shown in Fig. 3. Selected bond lengths and
angles are given in Table 3. The central iron atom was six coordi-
nated, and the geometry around the iron atom could be described
as a distorted octahedron in which N(3a), N(3b), N(1a) and N(1b)
atoms compose an equatorial plane while the other two nitrogen
atoms(N(2a) and N(2b)) occupy the axial position. The Fe(1)–N
bond lengths are in the range from 1.881 Å to 1.967 Å, comparable
with 1a and other low-spin [FeL2]2+ complexes [10], indicating the
low-spin character of Fe(1). However, there are two [FeCl4]ꢀ anion
part in the structure, suggesting the iron atoms in [FeCl4]ꢀ oxidize
to be Fe3+. In the structure of 2a, the Fe(1) atom locates 0.0127 Å
above the equatorial plane towards N(2a). The bond distances of
N(2a)–C(10a) and N(2b)–C(10b) are 1.271(1) Å and 1.295(1) Å,
respectively, displaying typical C@N double bond character. In li-
gand A and B, the quinolinyl ring plane and the pyridyl ring plane
are almost coplanar with a dihedral angel of 2.8° and 3.1°, respec-
tively. The two ligands are almost vertical to each other, with the
dihedral angel of 89.9°. The distances of Fe(1)–Fe(2) and Fe(1)–
Fe(3) are 6.602 Å and 10.383 Å, respectively.
Although there were some literatures about the analogue salts
of 2a [13], however, no structure was reported before. Crystals of
ferrous complex 2a suitable for X-ray structural determination
3.3. Magnetic research
In research of magnetic properties, temperature can exert great
influence. With the change of temperature, their spin crossover
Fig. 2. Molecular structure of 1b. Hydrogen atoms, and solvent have been omitted
Fig. 3. Molecular structure of 2a. Hydrogen atoms have been omitted for
for clarity.
clarity.

K. Wang et al. / Journal of Molecular Structure 890 (2008) 95–100
99
Table 3
Selected bond lengths (Å) and angles (deg) for 2a
Bond lengths
Fe(1)–N(1A)
Fe(1)–N(2A)
Fe(1)–N(3A)
N(2A)–C(10A)
1.951(7)
1.881(7)
1.955(6)
1.271(1)
Fe(1)–N(1B)
1.967(7)
1.904(7)
1.935(7)
1.295(1)
Fe(1)–N(2B)
Fe(1)–N(3B)
N(2B)–C(10B)
Bond angles
N(2A)–Fe(1)–N(1A)
N(2A)–Fe(1)–N(3A)
N(1A)–Fe(1)–N(3A)
83.1(3)
82.2(3)
165.3(3)
N(2B)–Fe(1)–N(1B)
N(2B)–Fe(1)–N(3B)
N(3B)–Fe(1)–N(1B)
83.1(3)
80.7(3)
163.8(3)
were observed for some octahedral coordination iron complexes
[10,11]. Temperature dependence of magnetic measurements for
complexes 1a, 1b and 2a were investigated in the temperature
range 5–300 K under a magnetic field of 1T and were shown in
Figs. 4–6, respectively. It was notably that all these complexes
composed of anion part and cation part, and contributions from
each part should be put into consideration.
For complex 1a, the xmT value in 300 K is 0.98 emu molꢀ1
K
indicating the iron(II) with low-spin state in the cation part, which
is in good agreement with the X-ray analysis result, and the anion
part [FeCl4]2ꢀ will be responsible for non-zero xmT values. With
lower temperature, the xmT value gradually decreased and in
60 K the value becomes 0.69 emu molꢀ1 K. The decrease of xmT va-
lue below 30 K is due to the zero-field splitting and/or intermolec-
ular antiferromagnetic interactions in the paramagnetic
impurities. Fitting the values to the Curie–Weiss law [1/xm = (Tꢀ
h)/C] gives Curie constant C = 0.98 emu molꢀ1 K and Weiss con-
stant h = ꢀ 20.05 K.
Fig. 5. Magnetic susceptibility of complex 1b.
For complex 1b, the xmT value in 300 K is 4.97 emu molꢀ1 K,
much higher than complex 1a, indicating that the cobalt complex
1b is in the high-spin state (S = 3/2) in accordance with other
high-spin cobalt complex [10o]. When the temperature decreased,
the xmT value for 1b decreased. However, the xmT value is still
higher than 3.00 emu molꢀ1 K even in 5 K, indicating that no
spin-crossover phenomenon took place. Fitting the values to the
Curie–Weiss law gives Curie constant C = 5.03 emu molꢀ1 K and
Weiss constant h = ꢀ 9.17 K.
For complex 2a, the xmT value at 300 K is 2.12 emu molꢀ1 K,
indicating that about two thirds of the molecules are in the high-
spin states due to the ratio of Fe(III) to Fe(II) in 2a, and the xmT
value is nearly constant when temperature goes down to 50 K,
Fig. 6. Magnetic susceptibility of complex 2a.
followed by sudden decrease below 50 K, which could be
attributed to the zero-field splitting and/or intermolecular
antiferromagnetic interactions in the paramagnetic impurities.
Fitting the values to the Curie–Weiss law gives Curie constant
C = 2.29 emu molꢀ1 K and Weiss constant h = ꢀ 4.56 K.
4. Conclusions
In general, by controlling the bulkiness of the substituent of 2-
(benzoimidazolyl)-6-iminopyridines and N-((pyridin-2-yl)methy-
lene)-quinolin-8-amine derivatives, the ion-paired complexes
were formed containing metal(II) ligated by two tridentate
ˆ
ˆ
(N N N) ligands and tetrachlorometallic anions. Observed the dif-
ferent spin states for iron complex (1a) and cobalt complex (1b),
the iron complex 1a showed a magnetic property changes within
single-crystal-to-single-crystal transformation. Therefore, metal
ˆ
ˆ
complexes containing tridentate N N N ligands would be excellent
candidates as molecular sensors with high potential.
Fig. 4. Magnetic susceptibility of complex 1a.

100
K. Wang et al. / Journal of Molecular Structure 890 (2008) 95–100
[7] (a) M. Zhang, S. Zhang, P. Hao, S. Jie, W.-H. Sun, P. Li, X. Lu, Eur. J. Inorg. Chem.
Acknowledgement
(2007) 3816–3826;
(b) M. Zhang, P. Hao, W. Zuo, S. Jie, W.-H. Sun, J. Organomet. Chem. 693 (2008)
483–491.
[8] (a) S. Zhang, I. Vystorop, Z. Tang, W.-H. Sun, Organometallics 26 (2007) 2456–
2460;
The project was supported by the National Natural Science
Foundation of China (Grant No. 20674089).
(b) S. Zhang, W.-H. Sun, X. Kuang, I. Vystorop, J. Yi, J. Organomet. Chem. 692
(2007) 5307–5316.
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