Linear tetranickel string complexes with mixed supported ligands and mixed-valence units [Ni2]3+: Synthesis, crystal structure, and magnetic studies

Article abstract of DOI:10.1002/ejic.201000145

The synthesis, crystal structures, and magnetic properties of linear tetranickel string complexes supported by mixed 2-(a-pyridylamino)-1,8- naphthyridine (Hpyany) and N-(p-tolylsulfonyl)dipyridyldiamine (H ₂tsdpda) ligands are reported. In comparing the crystal structure of [Ni₄(pyany)₂(tsdpda)₂Cl] (1) with that of [Ni₄(pyany)₂(tsdpda)₂Cl(H₂O)] (PF₆) (2), the one-electron-reduced compound 1 displays shorter Ni(3)-Ni(4) (ca. 2.28 A) and longer Ni(3)-N (ca. 2.02 A) bond lengths. Similar trends have also been observed for axial NCS^--substituted derivatives [Ni₄(pyany)₂(tsdpda)₂(NCS)] (3) and [Ni₄(pyany)₂(tsdpda)₂(NCS)₂] (4). These structural variations indicate the formation, of a mixed-valence [Ni ₂]³⁺ unit and a three-electron, two-center Ni(4)-Ni(3) a bond. Magnetic measurements of 2 and 4 show that both terminal Ni(1) and Ni(4) ions are in the high-spin states (S = 1) and are antiferromagnetically coupled. The one-electron-reduced complexes 1 and 3, however, exhibit a delocalized mixed-valence [Ni₂J³⁺ unit (S = 3/2), which is antiferromagnetically coupled with the terminal high-spin Ni^II ion.

Full text of DOI:10.1002/ejic.201000145

FULL PAPER
DOI: 10.1002/ejic.201000145
Linear Tetranickel String Complexes with Mixed Supported Ligands and
Mixed-Valence Units [Ni2]3+: Synthesis, Crystal Structure, and Magnetic
Studies
Che-Wei Yeh,[a] Isiah Po-Chun Liu,[a,b][‡] Rui-Ren Wang,[a] Chen-Yu Yeh,[c]
Gene-Hsiang Lee,[a] and Shie-Ming Peng*[a,b]
Keywords: Molecular wire / Mixed-valent compounds / Magnetic properties / Nickel / Ligand effects
The synthesis, crystal structures, and magnetic properties of
linear tetranickel string complexes supported by mixed 2-(α-
pyridylamino)-1,8-naphthyridine (Hpyany) and N-(p-tolyl-
sulfonyl)dipyridyldiamine (H2tsdpda) ligands are reported. In
comparing the crystal structure of [Ni4(pyany)2(tsdpda)2Cl]
(1) with that of [Ni4(pyany)2(tsdpda)2Cl(H2O)](PF6) (2), the
one-electron-reduced compound 1 displays shorter Ni(3)–
Ni(4) (ca. 2.28 Å) and longer Ni(3)–N (ca. 2.02 Å) bond
lengths. Similar trends have also been observed for axial
NCS–-substituted derivatives [Ni4(pyany)2(tsdpda)2(NCS)]
(3) and [Ni4(pyany)2(tsdpda)2(NCS)2] (4). These structural
variations indicate the formation of a mixed-valence [Ni2]3+
unit and a three-electron, two-center Ni(4)–Ni(3) σ bond.
Magnetic measurements of 2 and 4 show that both terminal
Ni(1) and Ni(4) ions are in the high-spin states (S = 1) and
are antiferromagnetically coupled. The one-electron-re-
duced complexes 1 and 3, however, exhibit a delocalized
mixed-valence [Ni2]3+ unit (S = 3/2), which is antiferromag-
netically coupled with the terminal high-spin NiII ion.
method to fine-tune the properties of metal string
complexes.[2d]
Introduction
Metal string complexes mimic the electric wires in our
macroscopic world down to the atomic scale, and are ex-
pected to serve as molecular wires in the fabrication of fu-
ture nanoelectronic devices.[1] Because of this potential ap-
plication, numerous metal string complexes have been syn-
thesized and characterized in the past two decades.[2] The
general approach to synthesize the metal string complexes
is to utilize oligo-α-pyridylamido ligands. These ligands
contain numerous amido and pyridino groups, which can
stabilize the cationic 1D linear transition-metal backbone.
Recently, oligo-(α-pyridylamido) ligands have been
modified by insertion of naphthyridino and anilino groups.
Interestingly, the resulting metal string complexes exhibit
novel physical properties. For instance, the bis(naphthyrid-
ylamido) (bna–) ligand is less anionic than the tripyridyldi-
amido (tpda2–) ligand. The resulting pentanickel string
complex [Ni5(bna)4(NCS)2]·(NCS)2 has a reduced metal
framework and greater electronic mobility, which shows
about 40% conductance enhancement relative to that of the
[Ni5(tpda)4(NCS)2] complex.[3] This result suggests that the
modification of supporting organic ligands is a suitable
In addition to the crystal structures, the physical proper-
ties of metal string complexes may also be modulated by
utilizing mixed ligands. In our previous report, trinickel
string complexes with various mixed ligands exhibited vari-
ous spin states because of the different interactions between
the axial ligands and metal ions.[4] It suggested that a novel
metal string complex may be synthesized by carefully
designing its surrounding ligands, since the coordinated
ability and steric hindrance of these ligands are quite dif-
ferent, and these can modulate the physical properties of
the linear metal framework.
Based on the above ideas, new tetranickel string complexes
with mixed 2-(α-pyridylamino)-1,8-naphthyridine (Hpyany)
and N-(p-tolylsulfonyl)dipyridyldiamine (H2tsdpda) ligands
(Scheme 1) have been synthesized and isolated. The crystal
structures, and magnetic properties of these tetranickel string
complexes are reported and discussed in this work.
[a] Department of Chemistry, National Taiwan University,
Taipei, Taiwan
Scheme 1. Hpyany and H2tsdpda ligands.
[b] Institute of Chemistry, Academia Sinica,
Taipei, Taiwan
For clarity, the tetranickel string complexes presented in
this paper are as follows: [Ni4(pyany)2(tsdpda)2Cl] (1), [Ni4-
(pyany)2(tsdpda)2Cl(H2O)](PF6) (2), [Ni4(pyany)2(tsdpda)2-
(NCS)] (3), and [Ni4(pyany)2(tsdpda)2(NCS)2] (4).
[c] Department of Chemistry, National Chung Hsing University,
Taichung, Taiwan
[‡] Current address: Institut für Anorganische Chemie,
Universität Bonn, Bonn, Germany
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Scheme 2. Synthesis of complex 1 [dba = dibenzylideneaceton; dppp = 1,3-bis(diphenylphosphanyl)propane].
Scheme 3. Synthesis of complexes 2, 3, and 4.
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Tetranickel String Complexes
Results and Discussion
Synthesis and Characterization
The overall synthetic routes to the Hpyany and H2tsdpda
ligands, and then to complexes 1–4 are summarized in
Schemes 2 and 3. The Hpyany ligand was prepared by reac-
tion of 2-chloro-1,8-naphthyridine and 2-aminopyridine
with palladium catalyst. The H2tsdpda ligand was prepared
by reaction of dipyridyldiamine (H3dpda) with p-tolu-
enesulfonyl chloride (pTsCl) in pyridine. Treatment of Hpy-
any and H2tsdpda with NiCl2 in the presence of tBuOK
generated compound 1. The one-electron-oxidized com-
pound 2 was obtained by treating 1 with [FeCp2][PF6] (Cp
= cyclopentadienyl) in CH2Cl2. The axial ligand exchange
of compound 2 to generate compound 4 was achieved by
stirring 2 with an excess amount of NaNCS in CH2Cl2. The
one-electron-reduced compound 3 was obtained by treat-
ment of 4 with N2H4 in CH2Cl2 or by reaction of Hpyany
and H2tsdpda with Ni(OAc)2 in naphthalene heated at re-
flux followed by ligand exchange.
X-ray Analysis
The crystallographic data for 1–4 are listed in Table 1.
The labeled ORTEP plots excluding solvent molecules and
selected bond lengths for all complexes are reported in Fig-
ures 1, 2Ͻyigr2 pos="x11"Ͼ, 3, 4, and 5. The core struc-
Figure 1. Molecular structure of complex 1. Ellipsoids are drawn
at 30% probability levels. Hydrogen atoms and interstitial solvent
molecules have been omitted for clarity.
tures of 1–4 reveal a tetranickel framework, which are heli- are of a low-spin nickel(II) electronic configuration (S = 0)
cally wrapped by two pyany– and two tsdpda2– ligands. judging by their short Ni–N bond lengths (ca. 1.90 Å).[6]
These ligands adopt a (2,2)-trans arrangement, which The axial ligands at the terminal NiII ions of 2 and 4 are
–
causes these compounds to exhibit an approximate C2 sym- different. The large PF6 ion of 2 does not bond to the
metry. Crystallographically, compounds 1–4 all exhibit one Ni(4) ion due to the steric interaction with the two tolylsul-
–
independent metal string in an asymmetric unit.
fonyl groups. Instead of the PF6 ion, a neutral H2O mole-
The tetranickel cores of 2 and 4 are [Ni4]8+, in which the cule occupies the axial position. Hydrogen bonds were ob-
coordination environment of the terminal Ni(1) and Ni(4) served between the hydrogen atoms of H2O and oxygen
atoms is square pyramidal, whereas that of Ni(2) and Ni(3) atom of two tolylsulfonyl groups. The small NCS– ligand,
is square planar. The outmost Ni(1)–N and Ni(4)–N bond however, does not show steric interaction with the tolylsul-
lengths are around 2.10 Å, thereby suggesting a high-spin fonyl groups. Compound 4 has two axial NCS– ligands and
nickel(II) (S = 1) character.[5] The inner Ni(2) and Ni(3) is suitable for conductivity measurement.[3]
Table 1. X-ray crystallographic data for 1, 2, 3, and 4.
1·3.5CH2Cl2·C6H14
2·2CH2Cl2·Et2O
3·4CH2Cl2·0.5EtOH
4·3.5CH2Cl2
Formula
Mr
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C69.5H67Cl8N16Ni4O4S2 C66H62Cl5F6N16Ni4O6PS2
C66H57Cl8N17Ni4O4.5S3
1806.95
C65.5H53Cl7N18Ni4O4S4
1783.51
monoclinic
C2/c
39.7482(9)
16.0945(5)
25.8662(6)
90
118.1843(14)
90
1815.41
triclinic
P1
1796.5
triclinic
triclinic
¯
¯
¯
P1
P1
15.0592(5)
15.9191(6)
17.8018(6)
81.483(2)
74.805(2)
68.839(2)
3833.9(2)
2
14.3336(2)
14.8998(2)
18.7954(3)
75.8719(8)
87.4940(7)
68.2975(9)
3611.92(9)
2
14.9080(4)
15.6760(4)
17.6095(5)
81.8419(17)
75.5677(18)
70.4780(18)
3748.57(17)
2
α [°]
β [°]
γ [°]
V [Å3]
Z
14585.3(7)
8
T [K]
150(2)
1.573
150(2)
1.652
0.0759/0.2297
0.1179/0.2614
150(2)
1.601
0.0738/0.1981
0.1141/0.2330
293(2)
1.624
0.0711/0.1936
0.1380/0.2300
ρcalcd. [gcm–3]
R1[a]/wR2[b] [IϾ2σ(I)] 0.0755/0.2061
R1[a]/wR2[b] (all data)
0.1039/0.2331
½
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = {Σ[w(F2o – Fc2)2]/Σ[w(F2o)2]} , in which w = 1/σ2(F2o) + (aP)2 + bP, P = (F2o + 2F2c)/3.
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Figure 3. Molecular structure of complex 3. Ellipsoids are drawn
at 30% probability levels. Hydrogen atoms and interstitial solvent
molecules have been omitted for clarity.
Ni(3)–N bond length (ca. 2.00 Å) than 2 and 4. These struc-
tural variations indicate that an extra electron occupies the
Figure 2. Molecular structure of complex 2. Ellipsoids are drawn
at 30% probability levels. Hydrogen atoms and interstitial solvent dx2
molecules have been omitted for clarity.
– y2
orbital of Ni(3), which results in the formation of a
three-electron, two-center Ni(4)–Ni(3) σ bond and the S =
3/2 mixed-valence [Ni2]3+ unit.[3,7] The Ni(1)–N and Ni(2)–
The one-electron-reduced compounds 1 and 3 possess a N bond lengths of 1 and 3 are around 2.09 and around
[Ni4]7+ core. Compounds 1 and 3 display a significantly 1.90 Å, respectively. Considering these Ni–N bond lengths
shorter Ni(4)–Ni(3) bond length (ca. 2.28 Å) and a longer and the coordination spheres of Ni(1) (square pyramidal)
Figure 4. Molecular structure of complex 4. Ellipsoids are drawn at 30% probability levels. Hydrogen atoms and interstitial solvent
molecules have been omitted for clarity.
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Tetranickel String Complexes
Scheme 4. Coupling scheme for complexes 1 (X = Cl) and 3 (X =
NCS).
Magnetism
Magnetic susceptibility measurements for compounds 1–
4 were made on polycrystalline samples in the temperature
range of 4–300 K (Figure 6). The χMT product values
(cm3 mol–1 K) of compounds 2 and 4 at 300 K are 2.03 and
1.81, respectively. These values are similar to that obtained
from complexes with two uncoupled high-spin NiII (S = 1)
ions (ca. 2).[3,6] The χMT values of 2 and 4 decrease grad-
ually with decreasing temperature, which clearly indicates
that an antiferromagnetic coupling is operating. To study
the coupling constant of 2 and 4, an isotropic Heisenberg–
Dirac–van Vleck (HDvV) Hamiltonian – see Equation (1) –
with S1 = S2 = 1 was used.[8]
H = –JS1S2 + gβSH
(1)
Figure 5. Selected interatomic distances [Å] observed for complexes
1–4.
Figure 6. Plot of χMT versus T for complexes 1–4. The solid line
represents the best theoretical fit.
and Ni(2) (square planar), the Ni(1) and Ni(2) ions were
assigned to high-spin (S = 1) and low-spin (S = 0) nickel(II)
ions, respectively.[5,6]
The best-fit parameters obtained were J = –70.7 cm–1
and g = 2.04 for 2; J = –68.8 cm–1 and g = 1.98 for 4.
In our previous reports, the mixed-valence (MV) [Ni2]3+ The coupling constant J is in agreement with the moderate
unit (Scheme 4) was found to be stabilized by four less an- antiferromagnetic interaction.
ionic naphthyridyl groups.[3,7] The crystal structural analy-
According to the X-ray structural analysis and our pre-
sis of 1 and 3, however, suggests that the [Ni2]3+ unit can vious studies, the spin centers of 1 and 3 can be assigned to
3
be stabilized by only two instead of four naphthyridyl S =
/ and S = 1 (Scheme 4). The χMT product values
2
groups, which might allow us to modulate the physical (cm3 mol–1 K) of compounds 1 and 3 at 300 K are 1.81 and
properties of metal string complexes by modifying the other 1.83, respectively, which are significantly lower than the
3
two ligands.
theoretical expected value for two uncoupled S = /2 and S
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cooling the mixture to 70 °C, hexane was added and the resulting
precipitate was filtered out. The solid (yield 314 mg, 67%) was ex-
tracted with CH2Cl2 and layered with hexane. After one week,
deep green crystals were obtained; yield 23.4 mg, 5%. MS
= 1 magnetic centers (ca. 2.88), thus indicating a relative
strong antiferromagnetic interaction.[9] Upon cooling, the
χMT product values of 1 and 3 decrease rapidly to reach a
value of 0.46 and 0.28, respectively, at 4 K. This pattern
suggests the presence of an antiferromagnetic coupling be-
(MALDI): m/z
=
1390 [Ni4(pyany)2(tsdpda)2Cl
+
H]+.
C125H106Cl4N32Ni8O10S4 (2956.01): calcd. C 50.79, H 3.61, N
15.16; found C 50.46, H 3.60, N 15.28.
3
tween S = /2 and S = 1 centers and a S = ½ ground state.
By means of the appropriate HDvV Hamiltonian [see
[Ni4(pyany)2(tsdpda)2Cl(H2O)](PF6) (2): The complex 1 (20.0 mg,
0.0144 mmol) was dissolved in CH2Cl2 (50 mL) and treated with
[FeCp2][PF6] (5.30 mg, 0.0170 mmol). The resulting solution was
stirred for 2 h and dried under vacuum. The powder was extracted
with CH2Cl2/diethyl ether (1:1) to get rid of the FeCp2. The solvent
was removed under vacuum. The residue was dissolved in CH2Cl2.
Brown crystals were obtained by slow diffusion of diethyl ether into
this solution; yield 16.2 mg, 70%. MS (FAB): m/z = 1389 [Ni4-
(pyany)2(tsdpda)2Cl]+. C121H98Cl4F12N32Ni8O10P2S4 (3189.82):
calcd. C 45.56, H 3.10, N 14.05; found C 45.67, H 3.15, N 13.98.
3
Equation (1), S1 = /2, S2 = 1], the experimental suscep-
tibility curve could be well reproduced with the set of pa-
rameter J = –121 cm–1 and g = 2.12 for 1; J = –89.3 cm–1
and g = 1.97 for 3, which shows a relative stronger antifer-
romagnetic interaction.
Conclusion
Four linear tetranickel string complexes supported by
mixed pyany– and tsdpda2– ligands were synthesized and
studied. An X-ray crystal structural analysis and the mag-
netic properties of compounds 1 and 3 indicate the forma-
[Ni4(pyany)2(tsdpda)2(NCS)] (3): A mixture of Hpyany (150 mg,
0.675 mmol), H2tsdpda (230 mg, 0.675 mmol), naphthalene (30 g),
and Ni(OAc)2·4H2O (400 mg, 1.61 mmol) was placed in an Erlen-
meyer flask. After stirring the mixture at 200 °C for 18 h, the mix-
ture was cooled to 150 °C and NaNCS (270 mg, 3.33 mmol) was
added. The solution then was stirred for an additional 2 h. After
cooling the mixture to 70 °C, hexane was added and the resulting
precipitate was filtered out. The solid (yield 353 mg, 74%) was ex-
tracted with CH2Cl2 and layered with hexane. After one week, deep
3
tion of a mixed-valence S = /2 [Ni2]3+ unit. It was found
that the [Ni2]3+ unit can be stabilized by two naphthyridyl-
containing ligands and the remaining two surrounding li-
gands can be modified. It is noteworthy that even-num-
bered metal strings exhibit structural and magnetic behav-
ior that is different from odd-numbered ones.[10] The physi- green crystals were obtained; yield 47.7 mg, 10%. MS (FAB): m/z
= 1413 [Ni4(pyany)2(tsdpda)2NCS + H]+. C63H50Cl4N17Ni4O4S3
cal properties of even-numbered metal string complexes,
(1581.96): calcd. C 47.83, H 3.19, N 15.05; found C 47.52, H 3.35,
could thus be tentatively fine-tuned by utilizing various sets
N 15.96.
of mixed ligands, which introduces a new approach to the
development of future metal string complexes.
[Ni4(pyany)2(tsdpda)2(NCS)2]
(4):
Complex
2
(20.0 mg,
0.0124 mmol) was dissolved in CH2Cl2 (50 mL) and treated with
NaNCS (10.0 mg, 0.123 mmol). The resulting solution was stirred
for a week and dried under vacuum. The solid was extracted with
CH2Cl2 and layered with hexane. After one week, deep brown crys-
tals were obtained; yield 12.8 mg, 70%. MS (FAB): m/z = 1413
[Ni4(pyany)2(tsdpda)2NCS + H]+. C64H50Cl4N18Ni4O4S3 (1640.05):
calcd. C 46.87, H 3.07, N 15.37; found C 46.51, H 3.41, N 15.70.
Experimental Section
Materials: All reagents and solvents were purchased from commer-
cial sources and were used as received unless otherwise noted. The
precursor 2-chloro-1,8-naphthyridine and the ligand N-(p-tolylsul-
fonyl)dipyridyldiamine (H2tsdpda) were prepared according to the
literature procedures.[3,10]
Physical Measurements: FAB mass spectra were recorded with a
JEOL HX-110 HF double-focusing spectrometer operating in the
positive-ion detection mode. The MALDI spectra were performed
with a MALDI-TOF mass spectrometer Voyager DE-STR. 1H
NMR spectra were recorded in DMSO with a Bruker AMX
400 MHz spectrometer. Magnetic susceptibility data were collected
with a Quantum external magnetic field 3000 G instrument.
2-(α-Pyridylamino)-1,8-naphthyridine (Hpyany): 2-Chloro-1,8-
naphthyridine (4.95 g, 30.2 mmol), tBuOK (4.37 g, 39.0 mmol),
[Pd2(dba)3] (0.820 mg, 0.895 mmol), and dppp (0.740 mg,
1.80 mmol) were placed in a flame-dried flask under argon. The
mixture was stirred and heated at reflux in toluene (150 mL) for
72 h. The solvent was removed under reduced pressure. The mix-
ture was purified by column chromatography over silica gel (Hpy-
any/silica gel = 0.9 wt.%) with CH2Cl2/acetone (1:4), then the yel-
low powder of the Hpyany was obtained. Yield: 60%. 1H NMR
(400 MHz, [D]DMSO): δ = 10.3 (s, 1 H), 8.81 (dd, J = 4.0, 4.8 Hz,
1 H), 8.58 (d, J = 8.8 Hz, 1 H), 8.29 (dd, J = 3.6, 4.8 Hz, 1 H),
8.19 (t, J = 19.2 Hz, 1 H), 8.18 (t, J = 16.8 Hz, 1 H), 7.80 (tt, J =
15.6, 16.0 Hz, 1 H), 7.60 (d, J = 9.2 Hz, 1 H), 7.33 (dd, J = 7.6,
8 Hz, 1 H), 6.99 (tt, J = 12.0, 12.4 Hz, 1 H) ppm. MS (FAB): m/z
= 223.1 [C13N4H10 + H]+. C13H10N4 (222.25): calcd. C 70.26, H
4.54, N 25.21; found C 70.27, H 4.66, N 25.55.
X-ray Structure Determinations: Crystallographic data were col-
lected at 150(1) K with a NONIUS Kappa CCD diffractometer
with graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å).
Cell parameters were retrieved and refined with DENZO-SMN
software on all observed reflections. Data reduction was performed
with the DENZO-SMN software.[11] An empirical absorption was
based on the symmetry-equivalent reflection, and absorption cor-
rections were applied with the SORTAV program.[12] All the struc-
tures were solved and refined with the SHELX-97 programs.[5] The
hydrogen atoms were included in calculated positions and refined
with a riding mode.
[Ni4(pyany)2(tsdpda)2Cl] (1):
A mixture of Hpyany (150 mg,
0.675 mmol), H2tsdpda (230 mg, 0.675 mmol), naphthalene (30 g),
and NiCl2 (230 mg, 1.77 mmol) was placed in an Erlenmeyer flask.
After stirring the mixture at 200 °C for 12 h, tBuOK (240 mg,
2.14 mmol) in tBuOH (3 mL) was added dropwise. The solution
then turned dark green and was stirred for an additional 8 h. After
CCDC-763548 (for 1), -763680 (for 2), -763682 (for 3), and -763681
(for 4) contain the supplementary crystallographic data for this pa-
per. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
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Tetranickel String Complexes
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Acknowledgments
[3]
The authors would like to acknowledge financial support from the
National Science Council and the Ministry of Education of Tai-
wan.
[4]
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Received: February 5, 2010
Published Online: June 2, 2010
Eur. J. Inorg. Chem. 2010, 3153–3159
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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