New ternary ligands consisting of an N4 bridging ligand and two terpyridines, and their Co(ii) and Ni(ii) dinuclear complexes. Structure, redox properties, and reaction with acid

Article abstract of DOI:10.1039/c3dt33011g

Two ternary ligands consisting of two 2,2′:6′,2′′- terpyridines and one N4-quadridentate μ₂,η²- bridging ligand were synthesized. The N4 bridge is 1,4-bis(2-pyridyl)phthalazine in ligand 1, and 3,6-bis(2-pyridyl)pyridazine in ligand 2. Two Co(ii) dinuclear complexes [(1)Co₂(μ-OH)]³⁺ and [(2)Co ₂(μ-OH)]³⁺, and one Ni(ii) dinuclear complex [(1)Ni₂(μ-Cl)]³⁺ were obtained. In the crystal structures of [(1)Co₂(μ-OH)]³⁺ and [(1)Ni ₂(μ-Cl)]³⁺, two pyridine rings are twisted around the pyridine-phthalazine bonds to avoid steric repulsion between the hydrogen atoms. The pyridine rings also showed a significant tilt from the octahedral coordination plane, which causes the large positive shift of the first reduction potentials. Upon the addition of a proton, the cobalt dinuclear complexes can release one cobalt ion selectively, and the dinuclear complexes can be easily restored by the addition of a tertiary amine.

Full text of DOI:10.1039/c3dt33011g

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New ternary ligands consisting of an N4 bridging
ligand and two terpyridines, and their Co(II) and Ni(II)
dinuclear complexes. Structure, redox properties, and
reaction with acid†
Cite this: DOI: 10.1039/c3dt33011g
Hiroki Kon and Toshi Nagata*
Two ternary ligands consisting of two 2,2’:6’,2’’-terpyridines and one N4-quadridentate μ2,η2-bridging
ligand were synthesized. The N4 bridge is 1,4-bis(2-pyridyl)phthalazine in ligand 1, and 3,6-bis(2-pyridyl)-
pyridazine in ligand 2. Two Co(II) dinuclear complexes [(1)Co2(μ-OH)]3+ and [(2)Co2(μ-OH)]3+, and one
Ni(II) dinuclear complex [(1)Ni2(μ-Cl)]3+ were obtained. In the crystal structures of [(1)Co2(μ-OH)]3+ and
[(1)Ni2(μ-Cl)]3+, two pyridine rings are twisted around the pyridine–phthalazine bonds to avoid steric repul-
sion between the hydrogen atoms. The pyridine rings also showed a significant tilt from the octahedral
coordination plane, which causes the large positive shift of the first reduction potentials. Upon the
addition of a proton, the cobalt dinuclear complexes can release one cobalt ion selectively, and the
dinuclear complexes can be easily restored by the addition of a tertiary amine.
Received 16th December 2012,
Accepted 31st January 2013
DOI: 10.1039/c3dt33011g
www.rsc.org/dalton
Furthermore, we successfully utilized such cobalt complexes in
photoreactions using porphyrins as the photosensitizer.4
Introduction
Cobalt(II) and nickel(II) complexes have attracted much atten-
As the next step, we considered expansion of this “binary
tion as potential catalysts for various reductive reactions such ligand” concept to dinucleating ligands. Since we are particu-
as hydrogen evolution,1a–e,2a,b CO2 reduction,1f–i,2c reductive larly interested in redox catalysis with two proximal vacant
dehalogenation1j,2d,2e and others.1k,2f In designing such com- sites, we planned to use an N4-quadridentate μ2,η2-bridging
plexes, the 2,2′:6′,2′′-terpyridine (trpy) ligand is especially
useful1f because of its well-defined coordination geometry. In
order to utilize such complexes in catalytic redox reactions, we
need to introduce vacant coordination sites into the metal
center (Fig. 1). However, in the case of 3d metals, it is often
difficult to synthesize polypyridyl complexes with a vacant site,
because they tend to form homoleptic, coordination saturated
complexes through fast ligand exchange. In order to solve this
problem, we previously designed and synthesized binary
ligands consisting of terpyridine and bipyridine (bpy) (10 in
Fig. 2).3 By using these binary ligands, we prepared “hetero-
leptic” polypyridine cobalt(II) mononuclear complexes that are
stable even under repetitive electrochemical redox cycles.
Research Center for Molecular Scale Nanoscience, Institute for Molecular Science,
5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan. E-mail: toshi-n@ims.ac.jp;
Fax: +81-564-59-5531; Tel: +81-564-59-5531
†Electronic supplementary information (ESI) available: Synthetic procedures for
compounds 2, 5, 6b, 7b, and 8b; the X-ray structures of the dinickel complex
(Fig. S1 and S2, Table S1); supplementary electrochemical results (Fig. S2–S6,
S8); acid-dependent change of the ESI-MS (Fig. S7). CCDC 915668 and 915669.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt33011g
Fig. 1 Possible designs of dinuclear complexes with N4 bridging ligands: (a)
desired structure with two vacant sites pointing inwards, (b) reported structures
with two facing N4 ligands, (c) reported structures with one or two extra chelat-
ing ligands, and (d) the design in this work.
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Co2(μ-OH)]3+, and the former two complexes were character-
ized by X-ray crystallography. The cyclic voltammograms of the
dicobalt complexes revealed interesting changes in compari-
son with the mononuclear cobalt(II) complex. Furthermore, we
examined the reaction of these dinuclear complexes with acid,
and observed reversible formation of the mononuclear cobalt
complexes by acid-induced elimination of one cobalt ion.
Results and discussion
Design and syntheses
The structures of the new ternary ligands 1 and 2 are shown in
Fig. 2. Both ligands consist of an N4 bridge and two terpyri-
dines linked by a butylene linker. The N4 bridge can bind two
metal ions in close proximity and two terpyridines can fill the
coordination sites in meridional tridentate mode, leaving one
coordination site open for each ion. Furthermore, the butylene
linker has appropriate length and freedom for the N4 bridge
and terpyridines to adapt the octahedral geometry.
The synthetic routes of the ternary ligands 1 and 2 are
shown in Scheme 1. The stannylpyridine 5 was synthesized
from 2,5-dibromopyridine according to the procedure reported
in previous literature11 except that the protecting group of the
aldehyde was different. The protected dialdehyde 6 of the N4
Fig. 2 Chemical structures of the ternary ligands (1 and 2) prepared in this
study, and the binary ligand (10) from ref. 3.
motif, namely 1,4-bis(2-pyridyl)phthalazine or 3,6-bis-
(2-pyridyl)pyridazine (Fig. 1a). Such kinds of N4 ligands have
been extensively used for studies of dinuclear metal complexes
including cobalt and nickel complexes.5 In designing these
complexes, it is important to block the open coordination sites
selectively to avoid uncontrollable polymerization while retain-
ing the vacant sites for reactions. In preceding work, this was
achieved either by using two N4-bridging ligands in a face-to-
face manner (Fig. 1b) or by appending one or two extra chelat-
ing ligands at each end of the N4 ligands (Fig. 1c). However, in
both cases, at least two coordination sites are left open for
each metal ion, which could cause complicated results when
applied to catalytic reactions. On the other hand, our approach
with “binary ligands” will leave only one open site for each
metal ion (Fig. 1d), thereby providing better control of the
reactivity.
It is also notable that recent reports have revealed interest-
ing chemistry of dinuclear complexes including polypyridyl
ligands,6 especially ruthenium complexes.7 Although synthesis
of similar compounds with 3d metals would suffer from
ligand exchange, our “linked ligand” approach can overcome
such a problem. There are also other reports of dinuclear
cobalt and nickel complexes with intriguing catalytic proper-
ties.8,9 However there is only one known well-characterized
example including terpyridine, [[Co(trpy)(bpy)]2(μ-O2)](PF6)4.10
In this paper, we wish to report the syntheses of new
“ternary” ligands 1 and 2, which consist of two terpyridines
and one N4 bridge which is 1,4-bis(2-pyridyl)phthalazine in 1
and 3,6-bis(2-pyridyl)pyridazine in 2. They form stable di-
nuclear complexes [(1)Co2(μ-OH)]3+, [(1)Ni2(μ-Cl)]3+ and [(2)- Scheme 1 Syntheses of ligands 1 and 2, and their Co/Ni complexes.
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bridge was synthesized by the Stille coupling reaction of 5 and
The complex cation [(1)Co2(μ-OH)]3+ has an approximate Cs
1,4-dichlorophthalazine or 3,6-dichloropyridazine in the pres- symmetry. The cobalt ions are octahedral six-coordinate and
ence of catalytic amounts of CuI.7a,12 The acetal protecting are bridged by the phthalazine and one hydroxo ligand. The
group was removed using trifluoroacetic acid, and the dialde- bond distances of Co–N are 2.07–2.19 Å, which are in a similar
hyde 7 was oxidized by the reaction with NaClO2 to give the range as in the Co(II) mononuclear complex we reported pre-
dicarboxylic acid 8. Finally, the ternary ligand 1 or 2 was pre- viously (2.08–2.19 Å)3 and other phthalazine or pyridazine
pared by the condensation reaction with 8 and the amino ter- bridged Co(II) dinuclear complexes with acyclic ligands.5a,b,d
pyridine 913 using EDC·HCl.
Syntheses of the dinuclear cobalt(II) and nickel(II) com-
plexes [(1)Co2(μ-OH)]3+, [(1)Ni2(μ-Cl)]3+ and [(2)Co2(μ-OH)]3+ are
also shown in Scheme 1. The complexes [(1)Co2(μ-OH)]3+ and
[(2)Co2(μ-OH)]3+ were synthesized by the reaction of 1 or 2 and
Co(OAc)2·4H2O in EtOH. The nickel dinuclear complex [(1)-
Ni2(μ-Cl)]3+ was synthesized by the reaction of
1 and
NiCl2·6H2O. Interestingly, the nickel complex with the ternary
ligand 2 was not obtained, in spite of many trials under
various conditions and starting materials. Furthermore,
neither a μ-chloro dicobalt(II) complex nor a μ-hydroxo dinickel(II)
complex was obtained. Although we do not yet fully under-
stand the cause of this sharp selectivity, it is likely that one of
the reasons is the overcrowding caused by the close location of
the N4 ligand and two terpyridine ligands (see below).
Crystal structures
The structures of [(1)Co2(μ-OH)]3+ and [(1)Ni2(μ-Cl)]3+ were
examined by X-ray crystallography. Details of the crystallo-
graphic parameters and the structure refinements are listed in
Table 1. The structure of the complex cation [(1)Co2(μ-OH)]3+ is
shown in Fig. 3, and the selected bond distances and angles
are given in Table 2. The structure of [(1)Ni2(μ-Cl)]3+ is roughly
similar to the dicobalt complex, and is shown in the ESI
(Fig. S-1†).
Table 1 Crystallographic data for [(1)Co2(μ-OH)](PF6)3·5CH3COCH3 and [(1)-
Ni2(μ-Cl)](PF6)3·2CH3COCH3·C6H5CH3
[(1)Co2(μ-OH)](PF6)3·
5CH3COCH3
[(1)Ni2(μ-Cl)](PF6)3·
2CH3COCH3·C6H5CH3
Formula
Formula weight
Temperature/°C
Crystal system
C85H87N12O10Co2P3F18 C104H100N12O6Ni2ClP3F18
1989.46
−150
2201.74
−150
Monoclinic
C2/c
Monoclinic
P21/n
Space group
a/Å
b/Å
c/Å
α/°
19.817(3)
23.206(3)
19.702(3)
90
17.106(3)
27.459(4)
21.567(3)
90
β/°
93.073(2)
90
9047(2)
4
5.204
1.460
101.727(2)
90
9919(2)
4
5.510
1.474
γ/°
V/Å3
Z
μ/cm−1
dcalcd/g cm–3
No. of obsd reflns 69 421
38 445
(I > 2σ(I))
No. of unique
reflns (Rint
No. of parameters 1183
20 404 (0.0308)
11 155 (0.0613)
Fig. 3 Drawings of the X-ray structure of the complex cation [(1)Co2(μ-OH)]3+
:
)
(a) an ORTEP side view, (b) bottom view, (c) space-filling model. The dotted
arrows in (c) indicate the 3-hydrogens causing steric repulsion with the phthala-
zine peri hydrogens.
710
GOF
R1, wR2
1.044
0.0622, 0.1837
1.079
0.0625, 0.1273
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Table 2 Selected bond distances (Å) and angles (°) for [(1)Co2(μ-OH)]3+
from pyridine nitrogens to the metal ions, which results in the
decrease of the electron density on the metal ions and hence
the positive shift of the first reduction potential in comparison
with the mononuclear complex (see below).
Bond distances
Bond angles
Co1–O1 1.960(2) Co1–O–Co2 125.85(10) N1–Co1–N2 75.58(9)
The dinickel complex [(1)Ni2(μ-Cl)]3+ showed bond lengths
and angles commonly observed in other nickel(II) polypyridine
complexes.14 The structure was similar to that of [(1)-
Co2(μ-OH)]3+, however there was a characteristic difference in
the manner in which the pyridine rings were twisted. In the
cobalt complex, both the 3-hydrogens were on the same side
(the nearer side in Fig. 3) of the phthalazine plane, whereas in
the nickel complex, they were on opposite sides (Fig. S-1†).
These different conformations resulted in the longer Ni⋯Ni
distance (3.754 Å) in [(1)Ni2(μ-Cl)]3+ than the Co⋯Co distance
(3.484 Å) in [(1)Co2(μ-OH)]3+ (Fig. S-2†). This observation is
consistent with the different bridging anion; the nickel
complex needs to have longer Ni–Ni distance to accommodate
the larger chloride bridging anion. These M⋯M distances are
similar to those reported in the pyridazine-bridged Co2(μ-OH)
complex (3.477 Å)5b and the phthalazine-bridged Ni2(μ-Cl)
complex (3.678 Å).5g
Co1–N1 2.199(3) O–Co1–N1
Co1–N2 2.071(3) O–Co1–N2
Co1–N3 2.172(3) O–Co1–N3
Co1–N4 2.130(3) O–Co1–N4
Co1–N5 2.168(3) O–Co1–N5
102.10(9)
96.84(9)
94.34(9)
87.21(9)
161.08(9)
N1–Co1–N3 147.85(9)
N1–Co1–N4 98.04(9)
N1–Co1–N5 83.62(9)
N2–Co1–N3 75.16(9)
N2–Co1–N4 173.01(9)
N2–Co1–N5 102.06(9)
Cyclic voltammograms
The cyclic voltammograms (CVs) of [(1)Co2(μ-OH)]3+, [(1)-
Ni2(μ-Cl)]3+ and [(2)Co2(μ-OH)]3+ are shown in Fig. 5. In the CV
of [(1)Co2(μ-OH)]3+, reversible waves were observed at −1.02,
−1.43 and −1.81 V. The first two waves were confirmed as a
one electron process by controlled potential coulometry (ESI,
Fig. S-3†). By comparison with the voltammogram of ligand 1,
Fig. 4 Partial structures around the metal ion in (a) [(1)Co2(μ-OH)]3+, (b) [(10)-
CoCl]+ (ref. 3), and (c) the phthalazine-bridged diruthenium complex reported
by Thummel (ref. 7a).
The pyridine rings attached to phthalazine are not coplanar the most negative wave can be tentatively assigned to the
with the phthalazine ring to avoid the steric hindrance reduction of the ligand, and the more positive two waves to the
between the hydrogen atoms at the 3-positions of pyridine and redox couples of Co2(I,II)/Co2(II,II) and Co2(I,I)/Co2(I,II).
the peri (5,8-) positions of phthalazine. This situation is visual-
The potentials of the first two reduction waves were signifi-
ized in the space-filling drawing (Fig. 3c). Similar distortion cantly separated (0.41 V), which indicates that the electronic
was also observed in the dinuclear ruthenium complex by interaction between the two cobalt ions is relatively strong in
Thummel.7a In the present case, the pyridine rings connected comparison with other pyridazine/phthalazine-bridged di-
to the phthalazine ring are not only twisted from the phthala- cobalt complexes previously reported (0.14–0.29 V).1e,5b,d,f Such
zine plane but also significantly tilted from the octahedral strong interaction can be partly attributed to the hydroxo
coordination plane. Fig. 4 shows the partial structures around bridge; however, there is also a report of another μ-hydroxo-
one metal ion with coordination atoms and one pyridine ring μ2,η2-pyridazine Co2 complex with separation as small as
of the N4 bridge. For comparison, the corresponding partial 0.11 V.5b At present, we cannot give a totally satisfactory explana-
structures of the mononuclear complex and the phthalazine- tion for this exceptionally strong electronic coupling between
bridged dinuclear ruthenium complex reported by Thummel7a the Co centers, which should be a subject of future studies. The
are also shown. In the dinuclear complexes [(1)Co2(μ-OH)]3+, first reduction potential of [(1)Co2(μ-OH)]3+ was shifted to the
the dihedral angle between the pyridine ring and the Co–O1– positive (0.22 V) in comparison with that of the mononuclear
N2–N5 plane is 23.5°. This dihedral angle is much larger than Co(II) complex.3 This positive shift can be attributed to the
the small angle of 1.6° in the cobalt mononuclear complex or decrease in the electronic donation from the pyridine nitrogen,
8.6° in the phthalazine-bridged diruthenium complex. The which was caused by the tilted coordination of the pyridine
unusual tilting in the present complexes is likely to be caused (see above).
by the steric repulsion between the terpyridine ligand and the
Although the first two reduction waves were well separated
ortho (2-) hydrogen of the pyridine ring. Indeed, the estimated from the subsequent ones, it may be the case that the ligand is
distances between the planes of the central rings of the terpyri- not completely “innocent” (i.e. redox inactive), as suggested
dine and the ortho hydrogens are 2.781(2) and 2.772(2) Å in from the spectroelectrochemical results (ESI, Fig. S-4†). Con-
[(1)Co2(μ-OH)]3+, which are less than the sum of the van der trolled-potential electrolysis of ligand 1 at −2.10 V showed
Waals radii of hydrogen (1.20 Å) and carbon (1.70 Å). This characteristic peaks at 540 and 800 nm (ε = 14 000 and 11 000
tilted structure causes a decrease of the electronic donation M−1 cm−1 respectively). On the other hand, controlled-
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lengths for Ni(II)/Ni(III) (typically 0.03–0.05 Å)15 compared with
those for Co(II)/Co(III) (ca. 0.2 Å).3
The CV of [(2)Co2(μ-OH)]3+ also showed a similar voltammo-
gram with [(1)Co2(μ-OH)]3+, except that the reduction poten-
tials are shifted to the negative by about 0.2 V. Although we
can only speculate about the structure of [(2)Co2(μ-OH)]3+, it is
highly possible that the N4 bridge in [(2)Co2(μ-OH)]3+ is closer
to coplanar than in [(1)Co2(μ-OH)]3+ because of the lack of
steric repulsion by the peri hydrogens. Consequently, the pyri-
dine rings would be coplanar with the coordination plane,
resulting in “normal” reduction potential.
Reaction with acid
Peters and co-workers examined the reaction of some dicobalt
complexes (of type b in Fig. 1) with a proton source and
revealed that the reduction of protons occurred.1e During our
examination of redox reactions of these dinuclear complexes
in the presence of various additives, we found that the reac-
tion of [(1)Co2(μ-OH)]3+ with H+ showed interesting behavior.
Fig. 6 shows the change of the CVs of [(1)Co2(μ-OH)]3+ when
CF3SO3H was added. Initially, the CV of [(1)Co2(μ-OH)]3+
shows the redox couples at −1.03 V and −1.42 V as shown in
Fig. 6a. After the addition of 1 eq. of CF3SO3H to the solution,
a new peak at −1.20 V appeared with a decrease of the peaks
Fig. 5 Cyclic voltammograms (negative side) for (a) ligand 1, (b) [(1)-
Co2(μ-OH)]3+, (c) [(1)Ni2(μ-Cl)]3+, (d) ligand 2, and (e) [(2)Co2(μ-OH)]3+ in DMF
with 0.1 M Bu4NClO4.
potential electrolysis of the dicobalt complex at −1.20 V caused
the appearance of two peaks at 480 and 710 nm (ε = 8000 and
5000 M−1 cm−1 respectively). The spectrum is quite different
from the reduced ligand, which is consistent with the metal-
centered reduction. However, even after electrolysis of the di-
cobalt complex at −2.10 V, the characteristic peak at 800 nm did
not appear. This result suggests that there is significant inter-
action between the metal center and the ligand π system in the
reduced state.
In the positive region, an irreversible wave was observed at
1.47 V (ESI, Fig. S-6†). This corresponds to the oxidation of the
cobalt ion, and is shifted to the positive direction by more
than 1 V in comparison with that of the mononuclear cobalt
complex.3 This large positive shift cannot be fully explained by
the decrease of the electron donation; one additional cause
may be the steric overcrowding provided by two terpyridine
groups, which would be even more severe for the more
compact Co(III) state.
The CV of [(1)Ni2(μ-Cl)]3+ showed a similar behavior with
the dicobalt complex. The positive shift of the oxidation poten-
tial was smaller than the dicobalt case (ca. 0.2 V). Such a differ-
Fig. 6 The changes of the CVs of [(1)Co2(μ-OH)]3+ with the addition of
CF3SO3H (a)
0
eq., (b) 1.0 eq., (c) 2.0 eq. and (d) after the addition of
ence can be reasonably explained by smaller changes in bond 5 eq. iPr2EtN.
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(Fig. S-8†), which indicates that the same reaction occurred in
[(2)Co2(μ-OH)]3+. On the other hand, the CV of [(1)Ni2(μ-Cl)]3+
was unchanged upon the addition of CF3SO3H (Fig. S-9†),
which indicates that the nature of the bridging anion is impor-
tant for the reaction with CF3SO3H. That is, the hydroxo-
bridged dinuclear complex releases the second metal ion more
easily upon protonation than the chloride-bridged complex.
Conclusions
We synthesized ternary ligands consisting of two terpyridines
and one N4 bridge which is either 1,4-bis(2-pyridyl)phthala-
zine or 3,6-bis(2-pyridyl)pyridazine. These ligands exclusively
formed cobalt(II) and nickel(II) dinuclear complexes with OH
or Cl as the bridging anion respectively. The structures of the
phthalazine-bridged complexes revealed characteristic defor-
mation caused by the steric hindrance within the organic
ligands. The electrochemistry of these dinuclear complexes
was qualitatively similar to the reported compounds with the
pyridazine/phthalazine bridge; however, some of the redox
potentials were significantly different from the reported
values, which is (at least partly) caused by the presence of the
nearby terpyridine groups. The cobalt dinuclear complexes
reacted with a proton and released one cobalt ion selectively,
and the dinuclear complexes were restored by the addition of a
small excess of amine. This easy restoration is derived from
the effect of the partial structure (like the binary ligands)
which can readily pick up the free cobalt ions. These character-
istic reactions can be useful to design and synthesize more
intriguing compounds such as heterodinuclear metal
complexes.
Fig. 7 Two possible structures formed from [(1)Co2(μ-OH)]3+ by the addition of
CF3SO3H.
at −1.03 V and −1.42 V (Fig. 6b). When one more equiv. of
CF3SO3H was added, the original redox couples completely
disappeared and the peak at −1.20 V increased with the
appearance of a new peak at −0.24 V (Fig. 6c). When 5 eq. of
N,N-diisopropylethylamine (iPr2EtN) was added to the solu-
tion, the original redox waves were restored as shown in
Fig. 6d.
We performed the reaction tracking by ESI-MS spectra (ESI,
Fig. S-7†). In the initial state, only one main peak was observed
at m/z = 421.1, which was assigned to the trication [1 + 2Co +
OH]3+. After the addition of 10 eq. of CF3SO3H, a new peak
appeared at m/z = 427.1, which was assigned to [1 + 2Co + OH
+ H2O]3+. After the addition of 30 eq. of CF3SO3H, another new
peak at m/z = 396.1 became most prominent, which was
assigned to [1 + Co + H+]3+. There are two possible structures
for this formula, one is “Co(trpy)2” and another is “Co(trpy)-
(bpy)” (Fig. 7). For the “Co(trpy)2” type to be produced, at least
one ligand substitution reaction must take place. On the other
hand, the “Co(trpy)(bpy)” type can be produced by simply
releasing one cobalt ion. We therefore propose that the latter
one is more likely. The CV observations also support this
speculation. The voltammetric waves at −1.20 and −0.24 V
were very similar to those of the mononuclear CoII(trpy)(bpy)
Experimental section
General
Reagents were purchased from Wako, Nacalai, and Aldrich,
and used as received unless otherwise noted. The syntheses of
compounds 2, 5, 6b, 7b and 8b are described in the ESI.†
1H and 13C NMR spectra were measured at room temperature
with JEOL LA400 and LA500 spectrometers; tetramethylsilane
was used as the internal reference. ESI-MS spectra were
recorded on a Waters Micromass LCT.
Syntheses
1,4-Bis[5-(1,3-dioxane-2-yl)-2-pyridyl]phthalazine
(6a). A
complex which we reported previously.3 Furthermore, the CV mixture of 5 (1.07 g, 2.35 mmol), 1,4-dichlorophthalazine
almost turned back to the initial shape after the addition of (196 mg, 0.98 mmol), Pd(PPh3)4 (24.3 mg, 0.021 mmol) and
excess iPr2EtN, and the ESI-MS spectrum also showed one CuI (8.1 mg, 0.043 mmol) in DMF (10 ml) was heated to reflux
major peak at m/z = 421.1, which is the same as in the initial for 24 h under N2. Dichloromethane and sat. NaHCO3 aq. were
state. This observation is also consistent with the “Co(trpy)- added, and the mixture was extracted with CH2Cl2. The
(bpy)” type, because this structure can easily restore the origi- organic phase was dried over Na2SO4, and evaporated. The
nal dinuclear complex by simply picking up one Co(II) ion.
product was purified by column chromatography (alumina,
The other cobalt dinuclear complex [(2)Co2(μ-OH)]3+ also CH2Cl2–MeOH = 100 : 0 – 99 : 1). The residual solid was recrys-
showed a similar change of the CV after addition of CF3SO3H tallized from CH2Cl2–hexane and the pale brown solid was
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obtained by filtration. Yield: 325 mg (0.711 mmol, 72%). 1H powder. Yield: 277 mg (0.246 mmol, 88%). Elemental analysis
4
NMR (400 MHz, CDCl3, 25 °C, TMS): δ = 8.92 (d, J(H,H) = 1.6 calcd (%) for 1·2H2O (C70H60N12O6): C 72.15, H 5.19, N 14.42;
3
4
Hz, 2H; py-6-H), 8.79 (dd, J(H,H) = 6.4 Hz, J(H,H) = 3.2 Hz, found: C 72.41, H 5.09, N 14.44. 1H NMR (400 MHz, CDCl3,
2H; phthalazine-6,7-H), 8.28 (d, J(H,H) = 8.0 Hz, 2H; py-3-H), 25 °C, TMS): δ = 8.95 (d, 4J(H,H) = 2.4 Hz, 2H; py-2-H),
3
3
4
8.08 (dd, J(H,H) = 8.0 Hz, J(H,H) = 2.0 Hz, 2H; py-4-H), 7.88 8.67–8.69 (m, 6H; phthalazine-6,7-H, trpy-3′-H, trpy-5′-H),
(dd, 3J(H,H) = 6.4 Hz, 4J(H,H) = 3.4 Hz, 1H; phthalazine-5,8-H), 8.60–8.62 (m, 8H; trpy-3-H, trpy-6-H, trpy-3′′-H, trpy-6′′-H), 8.13
3
3
4
5.71 (s, 2H; dioxane-2-H), 4.32–4.37 (m, 4H; dioxane-4,6-H), (d, J(H,H) = 8.0 Hz, 2H; py-5-H), 7.89 (dd, J(H,H) = 8.0 Hz, J
4.05–4.11 (m, 4H; dioxane-4,6-H), 2.25–2.34 ppm (m, 2H; (H,H) = 2.2 Hz, 2H; py-4-H), 7.84 (dd, J(H,H) = 6.4 Hz, J(H,H)
3
4
3
4
dioxane-5-H).
= 3.2 Hz, 2H; phthalazine-5,8-H), 7.77 (td, J(H,H) = 7.6 Hz, J
2,2′-(1,4-Phthalazinediyl)dipyridine-5,5′-dicarbaldehyde (7a). (H,H) = 2.0 Hz, 4H; trpy-4-H, trpy-4′′-H), 7.53 (dd, 3J(H,H) = 7.6
4
3
4
Compound 6a (258 mg, 0.565 mmol) was dissolved in CHCl3 Hz, J(H,H) = 2.0 Hz, 2H; Ar-6-H), 7.40 (td, J(H,H) = 8.0 Hz, J
(6 ml), and H2O (6 ml) and trifluoroacetic acid (4 ml) were (H,H) = 1.6 Hz, 2H; Ar-5-H), 7.25 (overlapping with CHCl3, 4H;
added and stirred at 80 °C. After 8 h, the reaction mixture was trpy-5-H, trpy-5′′-H), 7.04–7.09 (m, 4H; Ar-3-H, Ar-4-H), 6.94 (t,
neutralized by the addition of the sat. NaHCO3 aq. and 3J(H,H) = 5.6 Hz, 2H; amide-H), 4.16 (t, J(H,H) = 4.8 Hz, 4H;
3
extracted with CH2Cl2. The organic phase was dried over –NHCH2CH2CH2CH2O–), 3.55 (q, 3J(H,H)
=
6.0 Hz, 4H;
1
Na2SO4, and evaporated. The H NMR of the reaction mixture –NHCH2CH2CH2CH2O–),
1.92–1.97
ppm (m, 8H;
was examined, and if the deprotection of the cyclic acetal was –NHCH2CH2CH2CH2O–). 13C NMR (100 MHz, CDCl3, 25 °C,
incomplete, this operation was repeated once again. The TMS): δ = 165.6 (–NHCO–), 157.7 (Ar-2-C), 156.4 (trpy-2′-C, trpy-
residual solid was recrystallized from CH2Cl2–hexane and the 6′-C), 156.2 (trpy-2-C, trpy-2′′-C), 155.9 (py-6-C), 155.1 (py-2-C),
pale yellow solid was obtained by filtration. Yield: 156 mg 148.9 (trpy-6-C, trpy-6′′-C), 148.5 (Pht-1-C, Pht-4-C), 147.3 (trpy-
1
(0.458 mmol, 81%). H NMR (400 MHz, CDCl3, 25 °C, TMS): δ 4′-C), 137.0 (trpy-4-C, trpy-4′′-C), 135.4 (py-3-C), 132.4 (Pht-6-C,
4
= 10.29 (s, 2H; –CHO), 9.31 (d, J(H,H) = 2.0 Hz, 2H; py-2-H), Pht-7-C), 130.8 (Ar-4-C), 130.3 (Ar-6-C), 130.2 (py-4-C), 128.2
3
4
8.93 (dd, J(H,H) = 6.4 Hz, J(H,H) = 3.2 Hz, 2H; phthalazine- (Pht-5-C, Pht-8-C), 126.9 (Ar-1-C), 125.9 (Pht-4a-C, Pht-8a-C),
3
3
6,7-H), 8.55 (d, J(H,H) = 8.0 Hz, 2H; py-5-H), 8.46 (dd, J(H,H) 125.1 (py-5-C), 123.8 (trpy-5-C, trpy-5′′-C), 121.9 (trpy-3-C, trpy-
= 8.0 Hz, 4J(H,H) = 2.2 Hz, 2H; py-4-H), 8.00 ppm (dd, 3J(H,H) = 3′′-C), 121.5 (trpy-3′-C, trpy-5′-C), 121.1 (Ar-5-C), 112.2 (Ar-3-C),
6.4 Hz, 4J(H,H) = 3.2 Hz, 1H; phthalazine-5,8-H). 13C NMR 67.6 (–NHCH2CH2CH2CH2O–), 39.7 (–NHCH2CH2CH2CH2O–),
(100 MHz, CDCl3, 25 °C, TMS): δ = 190.4 (–CHO), 173.1 (py-6- 26.7
(–NHCH2CH2CH2CH2O–),
26.4
ppm
(–NHCH2-
C), 156.1 (Pht-1-C, Pht-4-C), 150.7 (py-2-C), 137.2 (py-4-C), CH2CH2CH2O–).
132.9 (Pht-6-C, Pht-7-C), 131.1 (py-3-C), 126.9 (py-5-C), 126.2
(Pht-5-C, Pht-8-C), 126.0 (Pht-4a-C, Pht-8a-C).
Compound [(1)Co2(μ-OH)]3+. Compound
0.0177 mmol) was suspended in EtOH (4 ml) and the solution
1
(20.0 mg,
2,2′-(1,4-Phthalazinediyl)dipyridine-5,5′-dicarboxylic
acid was stirred at room temperature. To the suspension, an EtOH
(8a). Compound 7a (365 mg, 1.07 mmol), NaClO2 (80%, solution (4 ml) of Co(OAc)2·4H2O (14.7 mg, 0.0590 mmol) was
373 mg, 3.31 mmol) and NaH2PO4·H2O (509 mg, 3.26 mmol) added and stirred. After a few minutes, the solution changed
were suspended in a mixture of CH3CN (30 ml)–H2O (9 ml)– to a red-brown homogeneous solution. After 8 h, a saturated
H2O2 (35%, 2 ml), and the mixture was stirred for 16 h at room NH4PF6 aqueous solution (3 ml) was added to the reaction
temperature. The reaction mixture was poured into 10% HCl mixture, a brown precipitate was formed and collected by fil-
aq. (100 ml) and stirred for 10 min. The white powder was iso- tration. Yield: 27.6 mg (0.162 mmol, 92%). The orange-brown
lated by suction filtration, washed with H2O, and dried in crystals were obtained from the diffusion of acetone–toluene
vacuo at 80 °C. Yield: 315 mg (0.846 mmol, 79%). 1H NMR mixed solvent. Elemental analysis calcd (%) for [(1)Co2(μ-OH)]-
(400 MHz, [D6]DMSO, 25 °C): δ = 9.32 (s, 2H; py-2-H), 8.65 (dd, (PF6)3·4H2O (C70H65N12O9Co2F18P3): C 47.47, H 3.70, N 9.49;
3J(H,H) = 6.4 Hz, 4J(H,H) = 3.4 Hz, 2H; phthalazine-6,7-H), 8.57 found: C 47.75, H 3.73, N 9.37.
(dd, 3J(H,H) = 8.0 Hz, 4J(H,H) = 2.0 Hz, 2H; py-4-H), 8.32 (d,
Compound [(1)Ni2(μ-Cl)]3+. This compound was synthesized
3
3J(H,H) = 8.0 Hz, 2H; py-5-H), 8.08 ppm (dd, J(H,H) = 6.4 Hz, by the same procedure as for [(1)Co2(μ-OH)]3+ by using 1
4J(H,H) = 3.2 Hz, 1H; phthalazine-5,8-H).
(25.0 mg, 0.0221 mmol) and NiCl2·6H2O (24.5 mg,
Compound 1. Compounds 8a (104 mg, 0.279 mmol), 913 0.103 mmol). Yield: 32.0 mg (0.019 mmol, 86%). The pale
(307 mg, 0.772 mmol), N,N-dimethyl-4-aminopyridine (DMAP, yellow-brown crystals were obtained from the diffusion of
173 mg, 1.42 mmol), 1-hydroxybenzotriazole monohydrate acetone–toluene mixed solvent. Elemental analysis calcd (%)
(HOBt·H2O, 190 mg, 1.41 mmol) and 1-ethyl-3-(3-dimethyl- for [(1)Ni2(μ-Cl)](PF6)3·4H2O (C70H64N12O8Ni2ClF18P3): C 46.99,
aminopropyl)carbodiimide hydrochloride (EDC·HCl, 269 mg, H 3.61, N 9.40; found: C 46.70, H 3.58, N 9.39.
1.41 mmol) were dissolved in CH2Cl2 (8 ml) and stirred at
Compound [(2)Co2(μ-OH)]3+. This compound was syn-
room temperature. After 16 h, the reaction mixture was thesized by the same procedure as for [(1)Co2(μ-OH)]3+ by
extracted with saturated NaHCO3 aq. and the organic phase using 2 (18.4 mg, 0.017 mmol) and Co(OAc)2·4H2O (14.3 mg,
was dried over Na2SO4, and evaporated. The residual oily com- 0.057 mmol). Yield: 26.6 mg (0.016 mmol, 94%). Elemental
pound was purified by column chromatography (alumina, analysis
calcd
(%)
for
[(2)Co2(μ-OH)](PF6)3·6H2O
CH2Cl2–MeOH = 100 : 0 – 97 : 3). Finally, the product was (C66H67N12O11Co2F18P3): C 45.12, H 3.84, N 9.57; found: C
recrystallized from CH2Cl2–hexane and obtained as a white 45.05, H 3.63, N 9.36.
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Dalton Transactions
X-ray diffraction studies
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D. K. Kennepohl, B. Moubaraki, K. S. Murray,
J. D. Cashion, G. B. Jameson and S. Brooker, Chem.–Eur. J.,
2003, 9, 3772; (e) S. Brooker, P. G. Plieger, B. Moubaraki
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Suitable single crystals of [(1)Co2(μ-OH)]3+ and [(1)Ni2(μ-Cl)]3+
were obtained from the diffusion of acetone–toluene mixed
solvent. The measurements were performed at −150 °C. Data
collection was made on a Mercury CCD area detector coupled
with a Rigaku/MSC diffractometer with graphite monochro-
mated Mo Kα radiation (λ = 0.71070 Å) by use of the Crystal-
Clear software.16 Calculations were carried out on
CrystalStructure17 and ShelXle18 program packages. The struc-
tures were solved by direct methods and expanded by Fourier
and difference Fourier techniques. The crystals contained
acetone and toluene molecules as crystal solvents. Details of
crystal parameters and structure refinement are given in
Table 1.
Electrochemical methods
Cyclic voltammograms were measured with an ALS/CHI Model
660 voltammetric analyzer at a scan rate of 100 mV s−1. The
working and counter electrodes were a platinum disk and a
platinum wire, respectively. The sample solutions (ca. 0.001 M)
in DMF with 0.1 M n-Bu4NClO4 were deoxygenated with a
stream of nitrogen gas. All values of redox potentials are
reported with reference to Fc/Fc+ (Fc = ferrocene); in practice,
an Ag/Ag+ electrode was used as a reference (−0.32 V vs. Fc/
Fc+). Spectroelectrochemistry was performed with an Ocean
Optics DH-2000-BAL light source and a USB2000-UV-Vis spec-
trometer, in a quartz cell (path length 1 mm) with a platinum
mesh electrode.
Acknowledgements
The authors thank Prof. Koji Tanaka (IMS) for his permission
to use the X-ray diffractometer and the ESI-MS spectrometer,
Mr Seiji Makita for performing elemental analysis, and the
Instrument Center of IMS for use of the NMR spectrometer.
This work was financially supported by IMS, a Grant-in-aid
for Scientific Research (C) (no. 22550133) from the Japan
Society for Promotion of Science (JSPS), a Grant-in-aid for
Scientific Research on Innovative Areas (no. 21200057) from
the Ministry of Education, Culture, Sports, Science and Tech-
nology (MEXT), and the Nanotechnology Support Project by
MEXT.
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