Novel linear hexanuclear cobalt string complexes (Co6 12+) and one-electron reduction products (Co6 11+) supported by four bpyany2- ligands

Article abstract of DOI:10.1039/b515311e

The new ligand, 2,7-bis(α-pyridylamino)-1,8-naphthyridine (H ₂bpyany), was synthesized by the reaction of 2,7-dichloro-1,8- naphthyridine with 2-aminopyridine in the presence of t-BuOK under palladium(0)-catalyzed conditions. The preparation and characterization of novel hexacobalt string complexes, [Co₆(₆-bpyany) ₄(NCS)₂](PF₆)_n (n = 1 (1); n = 2 (2)) and [Co₆(₆-bpyany)₄(OTf) ₂](OTf)_n (n = 2 (3); n = 1 (4)) are presented. The crystal structures for compounds 1-4 have been determined by X-ray crystallography. Compounds 1 and 4 have the Co₆¹¹⁺ configurations and are air-stable. Compounds 2 and 3 with Co₆¹²⁺ configurations are structurally similar to 1 and 4, respectively. The electrochemistry of 1 displays four redox couples at E_1/2 = -0.55, +0.38, +0.91, and +1.18 V (vs. Ag/AgCl). The magnetic data show that compounds 1 and 4 are in a spin state of S = 1/2, and 2 and 3 in a spin state of S = 1. The results of the EHMO calculations on compounds 1 and 2 are in agreement with their magnetic measurements. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b515311e

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www.rsc.org/dalton | Dalton Transactions
Novel linear hexanuclear cobalt string complexes (Co612+) and one-electron
reduction products (Co611+) supported by four bpyany2− ligands
Chih-Hsien Chien,a Jung-Che Chang,b Chen-Yu Yeh,*b Gene-Hsiang Lee,a Jim-Min Fanga and
Shie-Ming Peng*a,c
Received 28th October 2005, Accepted 27th January 2006
First published as an Advance Article on the web 7th February 2006
DOI: 10.1039/b515311e
The new ligand, 2,7-bis(a-pyridylamino)-1,8-naphthyridine (H2bpyany), was synthesized by the
reaction of 2,7-dichloro-1,8-naphthyridine with 2-aminopyridine in the presence of t-BuOK under
palladium(0)-catalyzed conditions. The preparation and characterization of novel hexacobalt string
complexes, [Co6(l6-bpyany)4(NCS)2](PF6)n (n = 1 (1); n = 2 (2)) and [Co6(l6-bpyany)4(OTf)2](OTf)n
(n = 2 (3); n = 1 (4)) are presented. The crystal structures for compounds 1–4 have been determined by
11+
X-ray crystallography. Compounds 1 and 4 have the Co6 configurations and are air-stable.
12+
Compounds 2 and 3 with Co6 configurations are structurally similar to 1 and 4, respectively. The
electrochemistry of 1 displays four redox couples at E1/2 = −0.55, +0.38, +0.91, and +1.18 V (vs.
Ag/AgCl). The magnetic data show that compounds 1 and 4 are in a spin state of S = 1/2, and 2 and 3
in a spin state of S = 1. The results of the EHMO calculations on compounds 1 and 2 are in agreement
with their magnetic measurements.
ligands with naphthyridyl units. For example, the ligand H2tpda
(tripyridyldiamine) can be modified by substitution of the central
pyridyl group with a naphthyridyl group to form the new ligand
2,7-bis(a-pyridylamino)-1,8-naphthyridine (H2bpyany).
Introduction
Owing to their fascinating bond nature, the metal–metal multiple
bonds in dinuclear metal complexes have been an interesting
and vital research topic.1–4 Recently, various linear multinuclear
metal complexes and infinite one-dimensional metal complexes
have been synthesized and investigated due to their promising
applications as nanoscale electronic devices.5–15 During the past
decade, our seminal and systematic approach on the all-syn oligo-
(a-pyridyl)amido ligands coordinated metal ions has successfully
In this paper, we report the synthesis of the novel ligand
opened up a new chapter, extending this territory from dinuclear
H2bpyany and its hexanuclear cobalt string complexes. The crystal
to linear oligo-nuclear metal complexes.16–20 An STM study of
structures, magnetic properties, electrochemistry and qualitative
one-dimensional metal string complexes revealed the potential
molecular orbital calculations of these hexanuclear cobalt string
applications as molecular wires and molecular switches.21 A similar
complexes are discussed in this work.
concept of extended metal atom chain (EMAC) has been proposed
For clarity, the complexes described in this paper are listed
as follows: [Co6(l6-bpyany)4(NCS)2](PF6) (1), [Co6(l6-bpyany)4-
(NCS)2](PF6)2 (2), [Co6(l6-bpyany)4(OTf)2](OTf)2 (3), [Co6(l6-
bpyany)4(OTf)2](OTf) (4).
by Cotton.22–25 On the basis of the electrochemistry of the multin-
uclear metal complexes with oligo-(a-pyridyl)amido ligands, these
complexes are stable in various oxidation states. Indeed, numerous
oxidized forms of these complexes have been synthesized and
structurally characterized.18,25 However, the reduced forms of these
complexes are generally unstable to be isolated and characterized.
Results and discussion
A variety of di- or multinuclear metal complexes coordinated
with 1,8-naphthyridine (ny) or its derivatives have been synthesized
Syntheses
The preparation of the novel ligand, H2bpyany, is similar to that
of oligo-a-pyridylamino ligands we have previously reported.19
The new ligand can be readily synthesized in 66% yield by the
palladium-catalyzed cross coupling reaction of 2,7-dichloro-1,8-
naphthyridine28 with 2-aminopyridine in the presence of t-BuOK
(Scheme 1).
and their structures, chemical, and physical properties have
been extensively investigated.26,27 The dinuclear nickel compound
[Ni2(ny)4Br2](BPh4) is a mixed-valence complex with average
charges of +1.5 on each nickel ion.27 In order to obtain the reduced
forms of linear multimetal complexes, one of our approaches is to
modify the ligands by replacing some of the pyridyl groups in the
The preparation of the oligopyridylamine-based metal string
complexes was carried out in refluxing naphthalene. Under
modified conditions, the hexanuclear cobalt string complexes can
be obtained in high yields. The mixture of H2bpyany ligand with
t-BuOK in the melting naphthalene and DMF co-solvent were
aDepartment of Chemistry, National Taiwan University, Taipei, 106, Taiwan.
E-mail: smpeng@ntu.edu.tw; Fax: 886-2-83693765; Tel: 886-2-23638305
bDepartment of Chemistry, National Chung Hsing University, Taichung,
402, Taiwan
◦
cInstitute of Chemistry, Academia Sinica, Taipei, 115, Taiwan
heated at ca. 130–140 C to form presumably the corresponding
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Scheme 1 The synthetic routes of H2bpyany and its hexanuclear cobalt complexes: (i) Pd2(dba)3, dppp and t-BuOK in refluxing toluene at 120 ◦C;
(ii) (1). t-BuOK in melting naphthalene/DMF. (2). CoCl2 in DMF (3). NaSCN in CH2Cl2/MeOH (4). KPF6 in CH2Cl2/MeOH; (iii) [Cp2Fe](PF6) in
CH2Cl2/MeOH; (iv) (1). NaOTf in CH2Cl2/MeOH (2). 3 eq. AgOTf in CH2Cl2/MeOH; (v) (1). N2H4·H2O in CH2Cl2. (2). Anhydrous MgSO4(s).
potassium salt K2bpyany. Addition of CoCl2 in DMF followed by
KPF6 afforded the desired product [Co6(l6-bpyany)4(NCS)2]-
(PF6) (1). One-electron oxidation product [Co6(l6-bpyany)4-
(NCS)2](PF6)2 (2) was prepared by reacting compound 1 with
[Cp2Fe](PF6) in a mixture of CH2Cl2 and MeOH. The axial ligand
exchange was accomplished by reacting compound 2 with excess
NaOTf and 3 eq. AgOTf to form compound 3. The reaction of
compound 3 with hydrazine gave compound 4. These complexes 1–
4 have been characterized by various spectrometry. The IR active
C≡N stretching vibrations of compound 1 and 2 were observed at
2058 and 2060 cm−1, respectively.
system with space group P41212 residing on a special position of 2-
fold symmetry perpendicular to the hexacobalt axis. Fig. 1–4 show
Crystal structures
The crystal data for compounds 1–4 are listed in Table 1.
Compounds 1, 3 and 4 crystallize in the space groups Pna21,
C2/c and P21/n, respectively, with the hexacobalt chains locating
at general positions. Compound 2 crystallizes in the tetragonal
Fig. 1 ORTEP drawing of 1. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms are omitted for clarity.
Table 1 Crystal data for compounds 1–4
1·6CH2Cl2·H2O
2·4CH2Cl2
3·6CH2Cl2
4·5CH2Cl2·O(C2H5)2·0.75H2O
Formula
C80H62Cl12Co6F6N26OPS2
2391.63
150(1)
NONIUS, Kappa CCD
C78H56Cl8Co6F12N26P2S2
2348.73
150(1)
NONIUS, Kappa CCD
0.71073
C82H60Cl12Co6F12N24O12S4
2708.76
150(1)
NONIUS, Kappa CCD
0.71073
C84H69.50Cl10Co6F9N24O10.75S3
2562.40
150(1)
NONIUS, Kappa CCD
0.71073
Formula weight
Temperature/K
Diffractometer
˚
Wavelength/A
0.71073
Crystal system
Space group
Orthorhombic
Pna21
27.3324(4)
12.9324(2)
25.6423(4)
90
90
90
9063.9(2)
4
1.566
0.45 × 0.35 × 0.15
1.49–27.50
60692
18376 (Rint = 0.0517)
0.0722, 0.1943
0.0937, 0.2117
1.041
Tetragonal
P41212
17.3930(2)
17.3930(2)
28.0060(5)
90
90
90
8472.3(2)
4
1.578
0.18 × 0.15 × 0.12
1.38–25.00
46542
7470 (Rint = 0.0761)
0.0502, 0.1229
0.0778, 0.1369
1.019
Monoclinic
C2/c
48.8644(4)
16.9122(1)
24.1983(2)
90
96.5410(4)
90
19867.4(3)
8
Monoclinic
P21/n
12.9988(2)
28.3349(5)
26.0264(3)
90
93.2970(8)
90
9570.2(2)
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
V/A
Z
3
˚
4
Absorption coefficient/mm−1
Crystal size/mm
1.481
1.452
0.40 × 0.40 × 0.30
1.27–27.50
91642
22806 (Rint = 0.0493)
0.0560, 0.1552
0.0896, 0.1731
1.075
0.25 × 0.20 × 0.15
1.06–25.00
100270
16872 (Rint = 0.0672)
0.0891, 0.2599
0.1200, 0.2924
1.029
h range for data collection/◦
Reflection collected
Independent reflections
R1, wR2 (I > 2r(I))a
R1, wR2 (all data)a
GOF
ꢀ
ꢀ
ꢀ
ꢀ
2
a R1 = ||Fo| − |Fc||/ |Fo|; wR2 = [ w(Fo − Fc2)2/ w(Fo2)2]1/2; w = 1/r2(Fo2) + (aP)2 + bP, where P = [max(0 or Fo2) + 2(Fc2)]/3.
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◦
˚
Table 2 Average bond distances (A) and torsion angles ( ) for compounds
1–4a
1
2
3
4
Co(1)–Co(2)
Co(2)–Co(3)
Co(3)–Co(4)
Co(1)–N(1)
Co(2)–N(2)
Co(3)–N(3)
Co(1)–Xb
N(1)–C(1)
N(1)–C(5)
N(2)–C(5)
N(2)–C(6)
N(3)–C(6)
N(3)–C(10)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(6)–C(7)
C(7)–C(8)
C(8)–C(9)
C(9)–C(10)
Co(1) · · · Co(6)
u(1)c
2.313(1)
2.273(1)
2.256(1)
1.981(7)
1.917(6)
1.932(6)
2.070(7)
1.353(11)
1.366(10)
1.370(9)
1.359(10)
1.354(9)
1.375(10)
1.359(13)
1.387(14)
1.367(12)
1.409(11)
1.425(11)
1.366(12)
1.405(12)
1.408(11)
2.313(1)
2.255(1)
2.245(2)
1.984(5)
1.910(5)
1.936(5)
2.034(6)
1.354(8)
1.361(8)
1.374(8)
1.375(8)
1.358(8)
1.365(8)
1.368(10)
1.387(10)
1.357(10)
1.407(9)
1.414(9)
1.363(10)
1.414(9)
1.403(9)
2.283(1)
2.243(1)
2.267(1)
1.970(3)
1.911(3)
1.934(3)
2.223(3)
1.353(5)
1.362(5)
1.380(5)
1.362(5)
1.358(5)
1.364(5)
1.368(6)
1.388(7)
1.373(6)
1.400(6)
1.421(6)
1.358(6)
1.409(6)
1.407(5)
2.284(1)
2.249(1)
2.251(1)
1.972(7)
1.911(7)
1.935(6)
2.244(5)
1.354(10)
1.360(10)
1.378(10)
1.363(10)
1.358(10)
1.371(10)
1.366(12)
1.393(13)
1.372(12)
1.405(12)
1.425(11)
1.359(12)
1.413(12)
1.401(11)
Fig. 2 ORTEP drawing of 2. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms are omitted for clarity.
11.429(1)
21.05
16.23
11.381(2)
19.88
17.58
11.318(1)
22.18
16.66
11.317(1)
21.87
16.74
u(2)c
u(3)c
14.20
15.86
14.37
15.18
a D4 symmetry is used to calculate average bond◦distances and torsion
angles. b X = axial ligand. c Torsion angle u(k) ( ) is defined as N(k)–
Co(k)–Co(k + 1)–N(k + 1), k = 1, 2, or 3.
Fig. 3 ORTEP drawing of 3. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms are omitted for clarity.
helically wrapped by four bpyany2− ligands with smaller average
torsion angle φ(3) Nny–Co(3)–Co(4)–Nny of 14.20–15.86◦ relative
to the average Npy–Co(n)–Co(n + 1)–Namido angle of 22.5◦ for the
pentacobalt string complex [Co5(tpda)4(NSC)2] (tpda = dianion
of tripyridyldiamine).17,18b The average Co–Co distances of 1 from
˚
outer to inner are 2.313(1), 2.273(1) and 2.256(1) A, respectively.
Similar to those of 1, the average Co–Co distances of 2 are
˚
2.313(1), 2.255(1) and 2.245(2) A, respectively. The average Co–
˚
˚
Naxial distances are 2.070(7) A for 1 and 2.034(6) A for 2. The
average Co–N distances of 1 are 1.980(7), 1.917(6), and 1.932(6)
˚
A, comparable to those of 2. It should be noted that addition of
one electron to compound 2 does not show significant structural
changes. When weak axial ligands (OTf−) are coordinated to Co6
chain, no obvious structural changes are observed for compounds
3 and 4. For example, the respective Co–Co distances of 3 are
Fig. 4 ORTEP drawing of 4. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms are omitted for clarity.
˚
the ORTEP drawings of compounds 1–4, respectively. The crystal
structures of these compounds have much in common except that
the axial ligands or the oxidation states are different. Compounds
1–4 all have roughly D4 symmetry without considering axial
2.283(1), 2.243(1) and 2.267(1) A whereas those of 4 are 2.284(1),
˚
2.249(1) and 2.251(1) A. In all these compounds, the outer Co–
Co distances, in general, are slightly longer than the inner three
Co–Co distances due to square-pyramidal environment for Co(1)
and Co(6). On the basis of structural analysis of compounds 1–4,
the reduction of compounds 2 and 3 occurs on the delocalized
hexacobalt chain.
˚
ligands. Therefore, average bond distances (A) and torsion angles
(◦) for compounds 1–4 are calculated in terms of D4 symmetry, as
shown in Table 2. In all these complexes, the hexacobalt chains are
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Resonance analysis
(vs. Ag/AgCl). All electrochemical reactions undergo one-electron
transfer, which is confirmed by spectroelectrochemistry.29 Fig. 6
illustrates spectral changes of compound 1 at applied potentials
from +0.20 to +0.46 V in CH2Cl2 containing 0.1 M TBAP. The
peaks at 298, 412, 629 and 798 nm decrease in their absorption
intensity whereas those at 273, 572 and 745 nm increase with
clear isosbestic points at 235, 290, 330, 350, 485, 580, 705 and
845 nm. The resulting spectrum is analogous to that of complex
2 obtained by chemical method, indicating that the oxidation
occurred predominantly at the hexacobalt core. The second 1-
e− oxidation was continuously performed at applied potentials
from +0.46 to +0.95 V as shown in Fig. 7. The peaks at 273,
335, 412 and 572 nm decrease in their intensity and those at 700
and 880 nm increase when the applied potential is increased from
+0.46 to +0.95 V. The presence of clear isosbestic points at 270, 320
and 620 indicates the formation of only one product nm during
the process of electrolysis. The spectral changes of the third 1-e−
oxidation process of 1 were depicted in Fig. 8. The peaks at 270
To examine the qualitative electronic effects of the deprotonated
bpyany2− ligand, the charge distributions and p bond orders were
analyzed by the resonance structures. The negative charges are
assumed to resonate only on the nitrogen atoms. As illustrated in
Scheme 2, the bpyany2− ligand has a C2 axis passing through the
C(9)–C(10) bond and the negative charges symmetrically reside
on the nitrogen atoms. The charge distributions from outer to
inner nitrogens are −15/67, −30/67 and −22/67, respectively.
The amido position has larger negative density than the nitrogens
on pyridyl and naphthyridyl groups, indicating that the ligand field
of the amido nitrogens is stronger than those on the pyridyl and
naphthyridyl groups. Thus, the Co(2)–N(2) distances (1.910–1.917
˚
A) are the shortest among the Co(k)–N(k) bond distances (k =
1, 2 and 3, respectively). On the contrary, the outer nitrogen of
the pyridyl groups have the least charge distribution (−15/67).
˚
Hence, the Co(1)–N(1) distances (1.970–1.984 A) are the longest.
The C–C and C–N p bond orders are also shown in Scheme 2. It is
well-known that bond order is inversely proportional to the bond
distance. The C(7)–C(8) bond has a p character of 46/67 which is
the largest among all C–C bonds. In compounds 1–4, the respective
C(7)–C(8) bond distances of 1.366(12), 1.363(10), 1.358(6) and
1.359(12) are shorter as compared to other C–C bond lengths in
the ligands. As expected for the smallest p bond character (21/67)
of the C(6)–C(7) and C(8)–C(9) bond, longer bond distances are
observed in the structures. The C–N bond orders and distances
can also be analyzed by such a method. For instance, the N(2)–
C(5) bond is of the smallest bond order (15/67), consistent with
the longer C–N bond distances of 1.370(9), 1.374(8), 1.380(5) and
˚
1.378(10) A for compounds 1–4, respectively.
Fig. 6 Electronic absorption spectral changes for the first oxidation
of compound 1 in CH2Cl2 containing 0.1 M TBAP at various applied
potentials from +0.20 to +0.46 V.
Scheme 2 Charge distributions on nitrogen atoms and p bond orders on
C–C and C–N bonds of the bpyany2− ligand.
Electrochemistry
Similar to the tri- and pentanuclear cobalt complexes,18b,25e com-
pound 1 displays rich redox chemistry as evinced by its cyclic
voltammogram (Fig. 5). Compound 1 exhibits four reversible
redox couples at E1/2 = −0.55, +0.38, +0.91 and +1.18 V
Fig. 7 Electronic absorption spectral changes for the first oxidation
of compound 1 in CH2Cl2 containing 0.1 M TBAP at various applied
potentials from +0.46 to +0.95 V.
Fig. 5 The cyclic voltammogram of compound 1 in CH2Cl2 containing
0.1 M TBAP with scan rate = 100 mV s−1
.
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configuration.30 In the case of our hexacobalt complexes, the d
electrons are considered to be delocalized over the Co6 chain. If
the energy gap between HOMO and LUMO is small enough, one
of the paired electrons in HOMO would fill in LUMO, resulting in
a spin state of S = 1 (e.g. compounds 2 and 3). Magnetic moments
for compounds 3 and 4 are of 3.30 and 2.25 B.M., respectively,
at 300 K, suggesting compounds 3 and 4 being in S = 1, 1/2
configurations, respectively.
EHMO calculation
To further study the electronic properties for the Co6 species, the
extended Hu¨ckel molecular orbital (EHMO) method using the free
software CACAO31 was performed on the dication of the com-
pound 2. The crystal data of the dication of 2 were used as the
input file. The default values for Co, C, N, H and S are used for the
calculations without considering the solvents and counter anions,
PF6−. The z axis direction passes through the hexacobalt chain.
The qualitative molecular orbital diagram is shown in Fig. 10.
Fig. 8 Electronic absorption spectral changes for the first oxidation
of compound 1 in CH2Cl2 containing 0.1 M TBAP at various applied
potentials from +0.95 to +1.3 V.
and 412 nm decrease whereas those at 634 nm increase in intensity
with isosbestic points at 250, 365 and 385 nm. All the oxidation
processes are reversible at the time scale of spectroelectrochemistry
(about 2 h). However, attempts to isolate and crystallize the two-
and three-electron oxidation products were unsuccessful because
of poor quality and stability of the crystals.
Magnetic properties
The plots of effective magnetic moments (leff, B.M.) vs. tempera-
ture (K) for compounds 1 and 2 are shown in Fig. 9. Compound
11+
1 with Co6 configuration has effective magnetic moments of
2.06 B.M. at 300 K, which can be considered as a spin state of
S = 1/2, although it is slightly higher than the spin-only leff of
12+
1.73 B.M. Compound 2 with Co6 configuration is expected to
be diamagnetic and has a S = 0 ground state if the linear com-
binations of the six d7 cobalt ions (each S = 1/2) are considered
by simple MO treatments. However, compound 2 has a magnetic
moment of 2.79 B.M. at 300 K, indicating a spin state of S =
1. The well-known compound [Co2(triza)4] (triza = anion of di-
p-tolytrizenate) was reported to be diamagnetic with r2p4d2d*2p*4
Fig. 10 The qualitative molecular orbital diagram of the linear six
cobalt(II), CoII–CoII–CoII–CoII–CoII–CoII system for dication of com-
pound 2.
Fig. 9 Temperature-dependent effective magnetic moments for com-
pounds 1 and 2.
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The r bonds are formed mainly from cobalt dz2 orbitals, and d p
bonds are mostly contributed by cobalt dxz or dyz atomic orbitals.
occurs at E1/2 = +0.54 V vs. Ag/AgCl (saturated). The working
electrode was polished with 0.03 lm alumina on Buehler felt
pads and was subjected to ultrasound for 1 min prior to each
experiment. The reproducibility of individual potential values was
within 5 mV. Optical thin layer electrochemical (OTTLE) spectra
were accomplished with the use of a 1 mm UV cell, a 100 mesh
platinum gauze as working electrode, a platinum wire as auxiliary
electrode, and a Ag/AgCl (saturated) reference electrode.
2
2
The weak d bonds are constructed from cobalt dx or dxy atomic
-y
2
2
orbitals. The M–L rbonds predominantly formed from dx or dxy
-y
atomic orbitals are not shown in Fig. 10 because they offer weak or
no metal–metal bondings. The MO calculations suggest that there
are two unpaired electrons filled in the r*(4) and d*(6) orbitals. The
small energy difference of 0.084 eV between these SOMOs (r*(4)
and d*(6)) results in paramagnetism rather than diamagnetism
for compound 2. The average Co–Co bond order is 0.6 on the
Syntheses
12+
basis of MO calculations. When reduction is performed on Co6
compound, the extra electron would fill into r*(4) orbitals to
afford the complex with a spin state of S = 1/2 and the one
unpaired electron occupies in the d*(6) orbital. The calculations
also reveal the sum of Co–Co bond orders for compounds 1 and 2
are of 2.5 and 3.0, respectively. The separations for Co(1) · · · Co(6)
2,7-Bis(a-pyridylamino)-1,8-naphthyridine (H2bpyany). To a
flame-dried flask under argon were placed 2,7-dichloro-1,8-
naphthyridine (1.98 g, 10 mmol),28 2-aminopyridine (2.16 g,
23 mmol), t-BuOK (2.81 g, 25 mmol), Pd2(dba)3 (183 mg,
0.20 mmol), and dppp (165 mg, 0.40 mmol). The mixture was
stirred and refluxd in toluene (100 mL) for 48 h and turned to
deep brown. The solvent was removed under reduced pressure.
The mixture was poured into water and the precipitate was filtered
to obtain the solid. Column chromatography was performed on
silica gel using ethyl acetate as the eluent affording the bright
yellow product, H2bpyany. Yield: 2.09 g, 66%. 1H NMR (400 MHz;
DMSO-d6): d 10.16 (2 H, s), 8.29 (2H, m), 8.26 (2H, J = 8.4 Hz,
d), 8.02 (2 H, J = 8.4 Hz, d), 7.77 (2 H, m), 7.64 (2H, J =
8.4 Hz, d), 6.96 (2H, m); 13C NMR (100 MHz; DMSO-d6): d
155.5, 154.6, 153.7, 147.3, 137.5, 137.2, 116.5, 113.2, 112.4, 110.6;
IR (KBr) m/cm−1 = 3260, 3189 (NH), 1619, 1582, 1522, 1481,
1458, 1435, 1404, 1379, 1348, 1323, 1304 (py); UV/Vis (CH2Cl2)
kmax/nm (e/104 dm3 mol−1 cm−1) = 262 (4.47), 363 (3.84), 381
(5.62), 414 (0.524), 440 (0.116); MS(FAB) m/z 315 ([M + H]+);
EA (%) H2bpyany: calcd. C 68.78, H 4.49, N 26.74; found C
68.53, H 4.52, N 26.90.
˚
are of 11.429(1) and 11.381(2) A, inversely proportional to the
corresponding bond orders for 1 and 2.
Conclusion
We have synthesized the novel ligand, H2bpyany, containing naph-
thyridyl group and its corresponding hexacobalt complexes. On
the basis of magnetic measurements, the one-electron reduction
species 1 and 4 have a spin state of S = 1/2 whereas compounds 2
and 3 (Co612+) are in a spin state of S = 1. The EHMO calculations
are consistent with the magnetic measurements for compound 1
and 2. Attempts to synthesize other types of hexanuclear metal
complexes are underway in this laboratory.
Experimental
Materials
[Co6(l6-bpyany)4(NCS)2](PF6) (1). The mixture of H2bpyany
(251 mg, 0.80 mmol), naphthalene (25 g), and DMF (5 mL)
placed in an Erlenmeyer flask was heated at ca. 100 ◦C and
then t-BuOK (224 mg, 2.0 mmol) was slowly added. The color
immediately changed to dark brown. After heating the solution
at about 130–140 ◦C for 15 h followed by 15 min at refluxing
(ca. 200 ◦C), a clear orange solution was formed, and then
a solution of anhydrous CoCl2 (182 mg, 1.4 mmol) in DMF
(5 mL) was added dropwise. After refluxing for another 30 min,
NaSCN (32 mg, 0.40 mmol) was added slowly. The color changed
from dark brown to dark green. The mixture was refluxed for
further 3 h, and then concentrated to the amount of ca. 5 mL,
followed by cooling to about 70 ◦C. The residue was treated with
hexane to precipitate the metal complexes. The precipitates were
collected by suction filtration, and rinsed with hexane to remove
the residual naphthalene and DMF. The solid was extracted
with CH2Cl2 (50 mL) and filtered. After the CH2Cl2 solution
was treated with excess KPF6 in MeOH (25 mL) and stirred
overnight, the solvent was removed under vacuum. The residual
solid was extracted with CH2Cl2 (50 mL) and filtered. Dark purple
crystals were obtained by slow diffusion of n-pentane vapor into
the filtrate. Yield 50 mg, 30%. IR (KBr) m/cm−1: 2058 (C≡N),
1711, 1608, 1585, 1556, 1508, 1467, 1436, 1363, 1340(py), 849,
839(P–F); UV/Vis (CH2Cl2) kmax/nm (e/104 dm3 mol−1 cm−1) =
306 (6.28), 415 (11.2), 628 (2.55), 799 (1.38); MS(FAB) m/z 1718
([Co6(l6-bpyany)4(NCS)2]+), 1660 ([Co6(l6-bpyany)4(NCS)]+); EA
All reagents and solvents were purchased from commercial sources
and were used as received unless otherwise noted. CH2Cl2 used for
electrochemistry was dried over CaH2 and freshly distilled prior
to use. Tetra-n-butylammonium perchlorate (TBAP) was recrys-
tallized twice from ethyl acetate and dried under vacuum. The
precursor, 2,7-dichloro-1,8-naphthyridine was prepared according
to the literature.28
Physical measurements
Absorption spectra were performed on a Varian Carry 50 spec-
trophotometer. IR spectra were obtained from a Nicolet Fourier-
Transform in the range 500–4000 cm−1. FAB mass spectra were
taken on a JEOL HX-110 HF double-focusing spectrometer
operating in the positive ion detection mode. Magnetic sus-
ceptibility was collected by a Quantum external magnetic field
10 000 G. Electrochemistry was carried out on a CH Instruments,
(Model 750A) using CH2Cl2 solvent with 0.1 M TBAP and
1 mM analytes. Cyclic voltammetry was recorded with a home-
made three-electrode cell equipped with a BAS glassy carbon
(0.07 cm2) disk as the working electrode, a platinum wire as
the auxiliary electrode, and a home-made Ag/AgCl (saturated)
reference electrode. The reference electrode is separated from the
bulk solution by a double junction filled with electrolyte solution.
Potentials are reported vs. Ag/AgCl (saturated) and referenced
to the ferrocene–ferrocenium ([Cp2Fe]/[Cp2Fe]+) couple which
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(%) [Co6(l6-bpyany)4(NCS)2](PF6)·CH2Cl2: calcd. C 46.22, H
absorption corrections were applied with the SORTAV program.33
All the structures were solved by using the SHELXS-9734 and
refined with SHELXL-9735 by full-matrix least squares on F2
values. Hydrogen atoms were fixed at calculated positions and
refined using a riding mode.
CCDC reference numbers 283614–283617.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b515311e
2.59, N 18.69; found C 46.74, H 2.73, N 18.95.
[Co6(l6-bpyany)4(NCS)2](PF6)2 (2). To a solution of com-
pound 1 (93 mg, 0.05 mmol) in a mixture of CH2Cl2 (25 mL)
and MeOH (1 mL) was added [Cp2Fe](PF6) (23 mg, 0.07 mmol)
under argon atmosphere. The resulting solution turned to dark
brown immediately. After stirring for 30 min, the solvent was
removed under vacuum. The residual solid was rinsed with
ethyl acetate to remove excess [Cp2Fe](PF6) and the byproduct
[Cp2Fe]. The remaining solid was dissolved in a mixture of
CH2Cl2 (50 mL) and MeOH (2 mL) and then filtered. Deep
brown crystals were obtained by slow diffusion of ether into
the filtrate. Yield: 73 mg, 73%. IR (KBr) m/cm−1: 2060 (C≡N),
1709, 1610, 1602, 1554, 1508, 1468, 1437, 1365, 1346(py), 843
(P–F); UV/Vis (CH2Cl2) kmax/nm (e/104 dm3 mol−1 cm−1) = 276
(9.28), 412 (11.49), 572 (2.02), 757 (0.94); MS(FAB) m/z 1718
([Co6(l6-bpyany)4(NCS)2]+), 1660 ([Co6(l6-bpyany)4(NCS)]+); EA
(%) [Co6(l6-bpyany)4(NCS)2](PF6)2·2CH2Cl2: calcd. C 41.89, H
2.41, N 16.71; found C 42.06, H 2.51, N 16.88.
Acknowledgements
The authors are grateful to the National Science Council of Taiwan
and the Ministry of Education of Taiwan for financial support of
this work.
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©