Syntheses, structure and properties of dinuclear Co complexes with bis(catecholate) ligands – Effect of a quinoline ring in the terminal group

Article abstract of DOI:10.1016/j.poly.2019.07.035

Two types of biscatechol, namely H₄L1 (5,5′-(buta-1,3-diyne-1,4-diyl)bis(3-t-butylcatechol)) and H₄L2 (5,5′-(ethyne-1,2-diyl)bis(3-t-butylcatechol)) were synthesized. In these ligands, two redox active catechol moieties are connected by one or two triple bonds. Also, tpa (tris(2-pyridylmethyl) amine), bpqa (bis(2-pyridylmethyl)(2-quinolylmethyl)amine) and pbqa ((2-pyridylmethyl)bis(2-quinolylmethyl)amine) were synthesized as terminal ligands of the tetracoordinated tripod type. In total, six dinuclear Co complexes were synthesized from these biscatechol and terminal ligands as follows: [Co₂(L1)(tpa)₂](BF₄)₂ (1), [Co₂(L1)(bpqa)₂](PF₆)₂ (2), [Co₂(L1)(pbqa)₂](PF₆)₂ (3), [Co₂(L2)(tpa)₂](BF₄)₂ (4), [Co₂(L2)(bpqa)₂](PF₆)₂ (5), [Co₂(L2)(pbqa)₂](PF₆)₂ (6). Of the six dinuclear Co complexes, complex 6, which was isolated as a mixed valent state Co^II(HS)-[SQ-Cat]-Co^III(LS) compound, showed an absorption intensity at around 703 nm (MLCT bands) that increased with increasing temperature in acetonitrile solution. In addition, an investigation of the magnetic properties of the complex 6 with SQUID showed that the χ_MT value gradually increased as the temperature increased from 150 to 380 K. This suggests that a transition from Co^III(LS) (S = 0) to Co^II(HS) (S = 3/2) accompanies the temperature rise. This means the steric hindrance and electronic effect of the quinolyl groups around the Co ion produce a coordination atmosphere weaker than that of pyridyl groups, with the result that the Co^III ions easily convert to Co^II ions.

Full text of DOI:10.1016/j.poly.2019.07.035

Polyhedron 171 (2019) 480–485
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Syntheses, structure and properties of dinuclear Co complexes with bis
(catecholate) ligands – Effect of a quinoline ring in the terminal group
a
Yusaku Suenaga a, , Takuto Mibu , Takashi Okubo a,b, Masahiko Maekawa b, Takayoshi Kuroda-Sowa a
⇑
a Department of Science, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
b Research Institute of Science and Technology, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Two types of biscatechol, namely H4L1 (5,50-(buta-1,3-diyne-1,4-diyl)bis(3-t-butylcatechol)) and H4L2
(5,50-(ethyne-1,2-diyl)bis(3-t-butylcatechol)) were synthesized. In these ligands, two redox active cate-
chol moieties are connected by one or two triple bonds. Also, tpa (tris(2-pyridylmethyl) amine), bpqa
(bis(2-pyridylmethyl)(2-quinolylmethyl)amine) and pbqa ((2-pyridylmethyl)bis(2-quinolylmethyl)
amine) were synthesized as terminal ligands of the tetracoordinated tripod type. In total, six dinuclear
Co complexes were synthesized from these biscatechol and terminal ligands as follows: [Co2(L1)(tpa)2]
(BF4)2 (1), [Co2(L1)(bpqa)2](PF6)2 (2), [Co2(L1)(pbqa)2](PF6)2 (3), [Co2(L2)(tpa)2](BF4)2 (4), [Co2(L2)
(bpqa)2](PF6)2 (5), [Co2(L2)(pbqa)2](PF6)2 (6). Of the six dinuclear Co complexes, complex 6, which was
isolated as a mixed valent state CoII(HS)-[SQ-Cat]-CoIII(LS) compound, showed an absorption intensity
at around 703 nm (MLCT bands) that increased with increasing temperature in acetonitrile solution. In
addition, an investigation of the magnetic properties of the complex 6 with SQUID showed that the
Article history:
Received 4 June 2019
Accepted 24 July 2019
Available online 1 August 2019
Keywords:
Dinuclear Co complex
Bis(catecholate) ligand
X-ray crystal structure
Magnetic property
Effect of ancillary ligand
v
MT value gradually increased as the temperature increased from 150 to 380 K. This suggests that a tran-
sition from CoIII(LS) (S = 0) to CoII(HS) (S = 3/2) accompanies the temperature rise. This means the steric
hindrance and electronic effect of the quinolyl groups around the Co ion produce a coordination atmo-
sphere weaker than that of pyridyl groups, with the result that the CoIII ions easily convert to CoII ions.
Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction
motion of the N,N0-ligand [6–8]. A. Dei and colleagues used the
tetracoordinated tripod tpa (tris(2-pyridylmethyl)amine) as a ter-
There has been great interest in studying the magnetic proper-
ties of molecular compounds. A particularly important subject in
the field is the preparation of tunable molecular magnetic materi-
als. Tunable materials are those in which the magnetic properties
can be switched by some external perturbation. This is an impor-
tant objective, because such tunable materials can be used for
future molecular memory and switching devices [1–5]. In com-
plexes composed of a redox active organic ligand and a transition
metal, electron transfers occur because the d orbital of the metal
minal ligand to introduce a methyl group at the 6-position and
focused on the change in the valence of the Co ions due to the dif-
ference in the ligand bulkiness [9–12]. The structure of a dinuclear
Co complex having 1,4-dihydroxy-2,5-benzoquinone as a ligand
and tpa as a terminal ligand was clarified by Sato et al. and it
showed that VT behavior is instigated by heat and also by light
[13,14]. A quinoline ring containing a tetradentate terminal ligand
is sterically bulky compared with pyridine and is expected to show
weak metal coordination. Therefore, terminal tripod ligands con-
taining quinolyl groups are expected to produce pseudo-state CoII
ion species.
We synthesized a biscatechol linked by a AC@NA bond in 2005
and published the properties of its dinuclear Co complex [15].
Since then, this biscatechol ligand has been synthesized by a differ-
ent method and its dinuclear Co complex has been successfully iso-
lated, but so far it has not been possible to synthesize a complex
exhibiting VT behavior. In this paper, as a result of a new synthesis
of biscatechols linked by triple bonds and research aimed at
clarifying the properties of their dinuclear Co complexes, we found
that when a part of the pyridine ring of tpa was substituted with a
ion and the p* orbital of the organic molecules are physically close
to each other, resulting in a change in the valence of the metal ion
complex, which affects the magnetic properties. Complexes con-
taining Co ions as the transition metal ion and catechols as the
redox active organic ligands have been well studied. Studies by
C. G. Pierpont and colleagues showed that some mononuclear Co
complexes coordinated to two 3,5-di-t-butylcatechol molecules,
undergo a VT (valence tautomerism) change via a simple bending
⇑
Corresponding author.
E-mail address: suenagay@chem.kindai.ac.jp (Y. Suenaga).
https://doi.org/10.1016/j.poly.2019.07.035
0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.

Y. Suenaga et al. / Polyhedron 171 (2019) 480–485
481
terminal quinoline ring, we obtained a novel dinuclear Co complex
having a reversible spin transition from the CoIII low spin state to
the CoII high spin state.
homogeneous, and it was stirred for a while. After that, a saturated
NaHCO3 aqueous solution (50 mL) was added. This solution was
extracted three times with chloroform (1 time: 50 mL), and the
organic layer was washed with brine (100 mL). After drying
the organic layer with Na2SO4, filtration and concentration gave
the crude product, which was purified by column chromatography
(silica gel: 250 mL, hexane: ethyl acetate = 5:1) to get H4L1 as a
dark brown solid (567 mg, 1.50 mmol). 1H NMR (400 MHz, CDCl3,
d, ppm): 1.37 (s, 18H), 5.40–6.00 (m, 4H), 6.86 (s, 2H), 7.07 (s,
2H). 13C NMR (400 MHz, CDCl3, d, ppm): 29.68, 34.87, 72.26,
81.91, 112.07, 116.57, 124.91, 136.89, 142.52, 145.39.
For H4L2, triethylamine (20 mL), tetrakis(triphenylphosphine)-
palladium(0) (485 mg, 0.420 mmol) and copper(I) iodide (28 mg,
0.145 mmol) were added to a 100 mL flask containing compound
5
(2.602 g, 7.26 mmol); a solution of compound 7 (4.399 g,
10.5 mmol) in triethylamine (30 mL) was then added and stirred,
followed by stirring overnight with heating at reflux (100 °C).
The reaction solution was concentrated and the obtained crude
was purified by column chromatography (silica gel: 250 ml, hex-
ane:ethyl acetate = 20:1 ? 10:1 ? 3:1) to obtain a yellow solid
compound (9) (1.302 g, 1.88 mmol). In a 100 ml flask, compound
9 (1.302 g, 1.88 mmol) was dissolved in chloroform (40 mL), 2 M
HCl (10 mL) was added, MeOH (40 mL) was added to make the
solution homogeneous, and the mixture was stirred for 5 hours.
After that, a saturated NaHCO3 aqueous solution (50 mL) was
added; this solution was extracted three times with chloroform
(1 time: 50 mL), and the organic layer was washed with brine
(100 mL). After drying the organic layer with Na2SO4, filtration
and concentration gave the crude product, which was purified by
column chromatography (silica gel: 250 mL, hexane:ethyl acet-
2. Experimental
Preparations were performed using the usual Schlenk tech-
niques. All solvents were dried and distilled by standard methods
before use. The standard chemicals were obtained from Wako
Chemical Co., Japan, and used without further purification. The
terminal ligands tpa (tris(2-pyridylmethyl) amine), bpqa (bis(2-
pyridylmethyl)(2-quinolyl methyl)amine) and pbqa ((2-pyridyl-
methyl)bis(2-quinolylmethyl)amine) were synthesized according
to the literature [16]. These organic chemicals were identified by
IR, 1H NMR and elemental analysis. The IR spectra were measured
as KBr disks on a JASCO FT/IR-8000 spectrometer and 1H NMR
spectra with a JEOL JMS-ESC 400 FT-NMR spectrometer at 23 °C.
UV–Vis spectra were recorded on a SHIMADZU UV-2450 spectrom-
eter and cyclic voltammetric measurements were made using an
ALS/CHI 660A instrument. Solutions of the complexes were pre-
pared in CH3CN containing (NBu4)(BF4) (ca. 0.1 M) as a supporting
electrolyte. Platinum wire working and counter electrodes were
used with an Ag/AgNO3 reference electrode. The Fc/Fc+ couple
ate = 5:1 ? 3:1 ? 1:1) to get H4L2 as
a dark brown solid
(184 mg, 0.520 mmol). 1H NMR (400 MHz, CDCl3, d, ppm): 1.35
(s, 18H), 5.35 (br, 2H), 5.98 (br, 2H), 6.88 (s, 2H), 7.06 (s, 2H). 13C
NMR (400 MHz, CDCl3, d, ppm): 29.36, 34.72, 87.59, 113.78,
115.73, 123.56, 136.63, 142.32, 144.02.
2.2. [Co2(L1)(tpa)2](BF4)2, complex 1
A solution of Co(BF4)2ꢀ6H2O (0.10 mmol) in dry methanol
(2 mL), Ar gas aerated for 5 min, was added to a stirred solution
of tpa (0.10 mmol) in dry MeOH solution (5 mL) and the resulting
solution was stirred for 30 min. A solution of Et3N (0.20 mmol) and
H4L1 (0.050 mmol) in Et2O (7 mL), Ar gas aerated for 15 min, was
added to the above methanol solution slowly. The solution was
allowed to stand at ꢁ13 °C for 3 days to obtain the desired complex
as a dark green precipitate. The yield obtained was 47%. IR (KBr,
cmꢁ1): 3420s, 3073m, 2955m, 2120w, 1635m, 1610s, 1572m,
1545w, 1482m, 1438m, 1415s, 1384m, 1349m, 1279m, 1249m,
1158m, 1083s, 1056s, 1037s, 992w, 905w, 885w, 836w, 811w,
771m, 730w, 699w, 667w, 642w, 612w, 582w, 532w, 522w,
504w; ESI-MS, [Co2(L1)(tpa)2]2+ m/z: 536.1614; Anal. Calc. for
appeared at +0.08 V (DE = 65 mV) versus Ag/AgNO3 with this
experimental arrangement and the ferrocene couple was used as
an internal reference. Magnetic susceptibility data were collected
on fresh microcrystalline samples on a Quantum Design MPMSXL7
SQUID magnetometer within the temperature range 5–380 K. EPR
spectra were obtained on a JEOL JES-FA200 spectrometer.
2.1. Synthesis of H4L1 and H4L2
C
60H58N8O4Co2B2F8ꢀ6H2O: C, 53.20; H, 5.21; N, 8.27%. Found: C,
52.56; H, 4.82; N, 7.51%.
The two biscatechols were synthesized according to Scheme S1
(described in the Supporting Information). For H4L1, compound 7
(3.281 g, 9.15 mmol), triethylamine (50 mL), dichlorobis(triph-
enylphosphine)palladium (401 mg, 0.572 mmol) and copper(I)
iodide (54 mg, 0.286 mmol) were added to a 100 mL flask under
an Ar atmosphere. The mixture solution was stirred and refluxed
(100 °C) overnight. After filtration, the reaction solution was con-
centrated and the obtained crude mixture was purified by column
chromatography (silica gel: 250 ml, hexane:ethyl acetate = 20:1 ?
2.3. [Co2(L1)(bpqa)2](PF6)2, complex 2
A solution of Co(OAc)2ꢀ4H2O (0.10 mmol) in dry MeOH solution
(2 mL), Ar gas aerated for 5 min, was added to bpqa (0.10 mmol) in
dry MeOH solution (5 mL) and the resulting solution was stirred
for 30 min. A solution of KPF6 (0.20 mmol), Et3N (0.20 mmol) and
H4L1 (0.050 mmol) in dry MeOH (7 mL), Ar gas aerated for
15 min, was added to above methanol solution. The resultant solu-
tion was allowed to stand at ꢁ13 °C for 3 days, resulting in a dark
green precipitate, which was filtered to obtain the desired com-
plex. The yield obtained was 18%. IR (KBr, cmꢁ1): 3435m, 3079m,
2956m, 2145w, 1609m, 1569m, 1541m, 1517m, 1435s, 1414s,
5:1) to get
a reddish yellow solid compound (8) (1.477 g,
2.07 mmol). Compound 8 (1.477 g, 2.06 mmol) was then dissolved
in chloroform (30 mL) in a 100 ml flask, 2 M HCl (10 mL) and
MeOH (30 mL) were added sequentially to make the solution

482
Y. Suenaga et al. / Polyhedron 171 (2019) 480–485
1369m, 1279s, 1249s, 1160m, 1128m, 1055w, 1037w, 994w,
970m, 846s, 767m, 669w, 558 m; ESI-MS, [Co2(L1)(bpqa)2](PF6)+
m/z: 1318.3697; Anal. Calc. for C68H62N8O4Co2P2F12ꢀCH3OH: C,
55.43; H, 4.45; N, 7.49%. Found: C, 55.96; H, 4.75; N, 7.00%.
and wR2 = 0.1439. All the full-occupancy non-hydrogen atoms
were refined anisotropically. The positions of all the hydrogen
atoms were determined from difference electron density maps
and included, but not refined. Atomic scattering factors and
anomalous dispersion terms were taken from the usual sources.
Computations were carried out using the CrystalStructure [20]
crystallographic software package, except for refinement which
was performed using SHELXL 2015 [21]. Selected bond lengths and
bond angles for (10) are listed in Table S2.
2.4. [Co2(L1)(pbqa)2](PF6)2, complex 3
Complex 3 was synthesized using almost the same procedure as
for complex 2. The resulting dark green precipitate was filtered to
obtain the desired complex. The yield obtained was 34%. IR (KBr,
cmꢁ1): 3404s, 3080m, 2957m, 2155w, 1619s, 1604s, 1568s,
1514s, 1461s, 1435s, 1415s, 1364s, 1342s, 1301s, 1252s, 1210m,
1158m, 1127m, 1053w, 1022w, 960w, 846s, 781m, 759m, 662w,
636w, 617w, 604, 558s, 526w, 512w; ESI-MS, [Co2(L1)(pbqa)2]
(PF6)+ m/z: 1417.3524; Anal. Calc. for C76H66N8O4Co2P2F12ꢀ5H2O:
C, 55.21; H, 4.63; N, 6.78%. Found: C, 55.66; H, 4.86; N, 7.47%.
Crystal
data
for
(10):
M = 1436.12,
crystal
size
0.4 ꢂ 0.2 ꢂ 0.2 mm, trigonal, space group R-3, a = 32.9851(11) Å,
b = 32.9851(11) Å, c = 16.2797(5) Å, V = 15339.5(11) Å3, Z = 9, F
(0 0 0) = 6714, Dcalc = 1.399 g cmꢁ1 = 0.635 mmꢁ1, 2hmax = 55.0°,
, l
reflection collected 9096, independent reflection 7047 (Rint = 0.0),
GOF (F2) = 1.028, R1 = 0.0496 [I > 2
r (I)], wR2 = 0.1439 (all data),
largest diff. peak and hole 1.032 and ꢁ1.271 e Åꢁ3
.
CCDC reference number 1919513 is available as a file in .cif
format.
2.5. [Co2(L2)(tpa)2](BF4)2, complex 4
Complex 4 was synthesized using almost the same procedure as
for complex 1. Complex 4 was isolated as a dark green precipitate.
The yield obtained was 55%. IR (KBr, cmꢁ1) 3423s, 3073m, 2953m,
2146w, 1610s, 1572w, 1541w, 1483m, 1439m, 1414s, 1385m,
1349m, 1281m, 1247m, 1210m, 1157m, 1123m, 1083s, 1056s,
1037s, 905w, 886w, 817w, 770m, 665w, 647w, 533w, 520w,
4. Results and discussion
4.1. Syntheses of the dinuclear Co complexes
Six dinuclear Co complexes were synthesized using biscatechols
(H4L1 and H4L2) and terminal ligands (tpa, bpqa and pbqa). They
are [Co2(L1)(tpa)2](BF4)2 (1), [Co2(L1)(bpqa)2](PF6)2 (2), [Co2(L1)
(pbqa)2](PF6)2 (3), [Co2(L2)(tpa)2](BF4)2 (4), [Co2(L2)(bpqa)2]
(PF6)2 (5) and [Co2(L2)(pbqa)2](PF6)2 (6). First, a methanol solution
of Co(BF4)2ꢀ6H2O and tpa were well mixed. After preparing the
methanol solution, which immediately changed from pink to green
for[Co(tpa)](BF4)3, a methanol solution containing H4L1 (or H4L2)
and triethylamine as a deprotonating agent was gently added to
the methanol solution so as not to disturb the interface. The solu-
tion was allowed to stand at ꢁ13 °C for 3 days. The precipitates
that formed were washed with cold methanol to obtain pale green
precipitates. When bpqa and pbqa, having quinoline rings, were
used in place of tpa as the terminal ligand, Co(OAc)2ꢀ4H2O was
used as a starting material instead of Co(BF4)2ꢀ6H2O. Lastly, KPF6
was added and anion exchange was carried out. The reason for this
is to prevent the formation of a red dinuclear CoII complex in which
Fꢁ ions are doubly-crosslinked due to dissociated Fꢁ ions accompa-
nying the hydrolysis of the BF4ꢁ ions [22,23]. From the elemental
analysis of the obtained compounds, they were found to be dinu-
clear Co complexes containing two counter anions. IR and ESI-MS
data are shown in the Supporting Information. For complex (1),
we replaced the BFꢁ4 ion by the ClOꢁ4 ion and succeeded in crystal-
lizing the complex (as 10) and determined its molecular structure.
Thus, its structure is described in the next section.
505w; ESI-MS, [Co2(L2)(tpa)2]2+ m/z: 524.1624; Anal. Calc. for C58
-
H
58N8O4Co2B2F8ꢀ5H2O: C, 53.07; H, 5.22; N, 8.54%. Found: C, 53.29;
H, 5.13; N, 8.40%.
2.6. [Co2(L2)(bpqa)2](PF6)2, complex 5
Complex 5 was synthesized using almost the same procedure as
for complex 2. The resulting dark green precipitate was filtered to
obtain the desired complex. The yield obtained was 17%. IR (KBr,
cmꢁ1): 3421s, 3079m, 2956m, 2868m, 2145s, 1609s, 1569s,
1541s, 1517s, 1481s, 1436s, 1414s, 1386s, 1359s, 1341s, 1287s,
1247s, 1209m, 1159m, 1129m, 1095m, 1056w, 1038w, 1002w,
924w, 845s, 768m, 739m, 721m, 668w, 644w, 558s, 528w,
504w; ESI-MS, [Co2(L2)(bpqa)2](PF6)+ m/z: 1293.3451; Anal. Calc.
for C66H62N8O4Co2P2F12ꢀ1.5CH3OH: C, 54.52; H, 4.61; N, 7.54%.
Found: C, 55.11; H, 4.72; N, 7.00%.
2.7. [Co2(L2)(pbqa)2](PF6)2, complex 6
Complex 6 was synthesized using almost the same procedure as
for complex 2. The resulting dark green precipitate was filtered to
obtain the desired complex. The yield obtained was 55%. IR (KBr,
cmꢁ1): 3424s, 3079m, 2957m, 2147m, 1619s, 1603, 1568s,
1515s, 1481s, 1435s, 1414s, 1388s, 1361s, 1340s, 1312s, 1247s,
1207m, 1179m, 1159m, 1146m, 1128m, 1098m, 1053w, 1036w,
1022w, 960w, 926w, 844s, 781m, 756m, 676w, 659w, 636w,
604w, 558s, 531w, 500w; ESI-MS, [Co2(L2)(pbqa)2](PF6)+ m/z:
1393.3567; Anal. Calc. for C74H66N8O4Co2P2F12ꢀ4H2O: C, 54.55; H,
4.70; N, 6.88%. Found: C, 54.72; H, 4.53; N, 6.41%.
4.2. Crystal structure of [Co2(L1)(tpa)2](ClO4)2 (10)
X-ray data collection was performed at 100 K to increase the
apparent low diffracting power of the crystals obtained. Even if
the final X-ray crystal structure determination is not of excellent
quality, it provides a strong enough framework to describe the dif-
ferent magnetic and electronic properties. The crystallographic
data and selected atomic distances and angles are presented in
Tables S1 and S2, respectively. The two cobalt ions are six-coordi-
nated in a cis-distorted pseudo-octahedral coordination, and each
of the tripodal ligands adopts a folded conformation around the
metal ion (Figs. 1, S12 and S13). The average CoAO and CoAN bond
lengths are 1.870 and 1.928 Å for both Co ions, values which are
consistent with a low-spin CoIII state. The metal-donor atoms
lengths strongly indicate that a CoIII-Cat (Cat = catechol) charge
distribution at 100 K. This suggestion is confirmed by analysis of
3. Crystallography
Data collection for [Co2(L1)(tpa)2](ClO4)2 (10) were performed
on a Rigaku/MSC Mercury CCD area detector coupled with a Rigaku
XtaLAB P200 diffractometer with graphite-monochromated Mo K
a
(0.71073 Å) radiation. The structure was solved by a direct method
[17] and expanded using Fourier techniques [18]. The final cycle of
full-matrix least-squares analysis [19] on F2 was based on 9096
observed reflections and 435 variable parameters, and converted
with unweighted and weighted agreement factors of R1 = 0.0496

Y. Suenaga et al. / Polyhedron 171 (2019) 480–485
483
temperature, while the absorption intensity at 350 nm decreased.
This behavior was reversible and represents electron transfer from
CoIII(LS)-Cat to CoII(HS)-SQ. Complex 6 was isolated as a mixed
valence-state CoII(HS)-[SQ-Cat]-CoIII(LS) with two counter anions
(PFꢁ6 ), from the results of its elemental analysis. From the spectral
changes brought about by temperature changes, the absorption
band at 350 nm can be assigned to the catecholate p–p* transition,
while the 703 nm band is due to a metal-to-ligand charge transfer
(MLCT) from the Co ion to the ligand. However, no spectral change
associated with changing temperature was observed for complexes
1–3, containing H4L1, or complexes 4 and 5, containing H4L2. Judging
from these results, the spectral changes in complex 6 due to temper-
ature variation in acetonitrile solution resulted from a change from
CoII(HS)-[SQ-Cat]-CoIII(LS) to CoII(HS)-[SQ-SQ]-CoII(HS). This is
thought to be due to the close spacing between the two Co units of
complex 6 and the fact that the Co ion tends to be in the +II high spin
state due to the bulkiness of the quinoline ring ligands.
Fig. 1. Crystal structure of complex 10.
the C(30)AC(25) (1.418 Å) and CAO (1.347, 1.345 Å) bond lengths
of the dioxolene ligand, which gives a strong indication that the
two catechol rings in complex 10 are in the same plane. The bond
distance of the C(36)AC(37) triple bond is 1.203 Å and the C
(27)AC(36)AC(37) angle is 176.5°. The packing structure is shown
in Fig. S13. Six molecules of the dinuclear Co complex are present
in a hexagon and form a columnar structure along the c axis direc-
tion. However, no special interactions were found between the Co
binuclear complex molecules.
4.4. Magnetic property
Changes in the magnetic susceptibility (vMT) due to tempera-
ture are shown in Fig. 4 for complex 4 and 5. Since the dinuclear
complexes using tpa and pbqa as a terminal ligand do not exhibit
magnetism, the atomic valence of Co is +III and can be judged to be
in a low spin state. In complex 5, the biscatechol molecules are par-
tially oxidized (L2ꢁ3Å:SQꢁÅ-Catꢁ2) and the
vMT values are almost
4.3. Electronic spectra
constant, with a value of 1.2 cm3 Kmolꢁ1. For [Co2(L2)(pbqa)2]
(PF6)2 (6), the magnetic susceptibility gradually increased from
2.0 cm3 Kmolꢁ1 to 2.74 cm3 Kmolꢁ1 between 50 and 380 K
(Fig. 5). From the magnetic susceptibility value at 150 K or lower,
assuming that complex 6 is CoII(HS)-[SQ-Cat]-CoIII(LS), the value
of the magnetic susceptibility obtained from the spin is 2.0 cm3
Kmolꢁ1. As the temperature rises, it is presumed that a change to
CoII(HS)-[SQ-SQ]-CoII(HS) occurs, since the susceptibility increases
from around 150 K, but at 380 K it is still not complete. The value of
the magnetic susceptibility remained at 2.74 (3.75 cm3 Kmolꢁ1 in
the theoretical formula) since the spin transfer was not complete.
EPR spectra have been measured for complex 6 in the solid state.
Complex 6 display a radical at around g = 2.00, weakly (Fig. S11).
With other bpqa complexes and dinuclear Co complexes using
H4L1 (complexes 4 and 5), such a spin transfer phenomenon could
not be confirmed (Fig. S10).
The electronic spectra of the dinuclear Co complexes, measured
in acetonitrile, are shown in Fig. 2. For [Co2(L1)(tpa)2](BF4)2 (1),
absorption bands appeared at 350 and 700 nm; the former is a
p–p* transition from the ligand and the latter is a ligand-to-metal
charge transfer (LMCT) between L1 and the Co ion. Based on SQUID
results, complex 1 can be regarded as CoIII(LS)-[Cat-Cat]-CoIII(LS),
where LS denotes a low spin state of the CoIII ion, with all six of
its d electrons occupying t2g orbitals. On the other hand, in
[Co2(L2)(pbqa)2](PF6)2 (6), in addition to the
p–p* transition
absorption band derived from the ligand at around 350 nm, a
prominent absorption band at 703 nm and another weak band at
500 nm were observed. In the dinuclear Co complexes using either
H4L1 or H4L2 ligands, the absorption intensity at 703 nm of the
complexes containing pbqa is larger than that for the tpa com-
plexes, and it is shifted to a longer wavelength. This indicates that
the ligand field strength is weakened by the bulkiness of the quino-
line ring.
5. Conclusion
The electronic spectra of complex 6 in acetonitrile solution
at ꢁ40 to +50 °C are shown in Fig. 3. Interestingly, the absorp-
tion intensities at 500 and 703 nm increased with increasing
A total of six dinuclear Co complexes were synthesized from
two types of biscatechol ligand, having different numbers of triple
Fig. 2. UV–Vis spectra of the dinuclear Co complexes in acetonitrile solution. (a) blue line, complex 1; red line, complex 2; green line, complex 3. (b) blue line, complex 4; red
line, complex 5; green line, complex 6. (Color online.)

484
Y. Suenaga et al. / Polyhedron 171 (2019) 480–485
Fig. 3. UV–Vis spectra at various temperature for complex 6. left, total spectra; right, magnification of each band.
substituted by one or two quinoline rings for the latter two ligands.
The compounds were identified from elemental analysis, mass
spectrometry and IR spectra.
The dinuclear Co complex obtained from the tetracoordinated
tripod ligand tpa has a valence number of +III and showed a broad
electronic absorption band in the vicinity of 700 nm, which is con-
sidered to be due to a LMCT. In the dinuclear Co complex [Co2(L2)
(pbqa)2](PF6)2 (6) the electronic absorption spectrum revealed that
the valence number of the Co ion is +II valence and the absorption
at 703 nm reversibly changed with temperature. Variable-temper-
ature magnetic susceptibility measurements show solely changes
from CoIII low spin to CoII high spin with increasing temperature.
The bulky quinolyl groups around the Co ion produce a coordina-
tion atmosphere weaker than for pyridyl groups, with the result
that the CoIII ions change to CoII ions easily.
Fig. 4. Magnetic susceptibility of complex 4, blue line, and complex 5, green line.
(Color online.)
Acknowledgements
bonds, and terminal ligands (tpa, bpqa and pbqa), in which the
pyridine ring of the tetracoordinated tripod ligand tpa is partially
The authors give appreciation to Prof. Y. Mikata of Nara
Women’s University for the syntheses of tpa, bpqa and pbqa. Also,
we thank Dr. K. Sugimoto of the Japan Synchrotron Research Insti-
tute (Spring 8) for refinement of the X-ray structure.
Appendix A. Supplementary data
CCDC 1919513 contains the supplementary crystallographic
data for [Co2(L1)(tpa)2](ClO4)2. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data to this article can
be found online at https://doi.org/10.1016/j.poly.2019.07.035.
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