Coordination Chemistry of an Unsymmetrical Naphthyridine-Based Tetradentate Ligand toward Various Transition-Metal Ions

Article abstract of DOI:10.1002/ejic.201600163

An unsymmetrical ligand, 2-(2-pyridinyl)-7-(pyrazol-1-yl)-1,8-naphthyridine (L₅) was prepared for the construction of a series of dinuclear complexes. Treatment of L₅with [Ru₂(μ-OAc)₄Cl] followed by anion metathesis afforded [(L₅)(μ-OAc)₃Ru₂](PF₆) (3). Reaction of L₅with 2 equiv. of Ni(OAc)₂provided [Ni₄(L₅)₂(μ-OH)₄(CF₃COO)₂](CF₃COO)₂(5). Reaction of [Re₂(CO)₈(CH₃CN)₂] with L₅in a refluxing chlorobenzene solution gave a mixture of dirhenium (6) and monorhenium (7) complexes. The monocobalt complex 8 was obtained from complexation of L₅with CoCl₂. These new complexes were characterized by elemental analysis and spectroscopic techniques. The structures of complexes 3, 5 and 8 were further confirmed by X-ray crystallography. Nickel complex 5 was evaluated as a catalyst for reduction reactions involving the conversion of ester functionalities into their corresponding alcohols.

Full text of DOI:10.1002/ejic.201600163

DOI: 10.1002/ejic.201600163
Full Paper
Tetradentate Ligands
Coordination Chemistry of an Unsymmetrical Naphthyridine-
Based Tetradentate Ligand toward Various Transition-Metal Ions
Bing-Chen Tsai,[a] Yi-Hung Liu,[a] Shie-Ming Peng,[a] and Shiuh-Tzung Liu*[a]
Abstract: An unsymmetrical ligand, 2-(2-pyridinyl)-7-(pyrazol- complexes. The monocobalt complex 8 was obtained from
1-yl)-1,8-naphthyridine (L5) was prepared for the construction complexation of L5 with CoCl2. These new complexes were
of a series of dinuclear complexes. Treatment of L5 with [Ru2(μ- characterized by elemental analysis and spectroscopic tech-
OAc)4Cl] followed by anion metathesis afforded [(L5)(μ- niques. The structures of complexes 3, 5 and 8 were further
OAc)3Ru2](PF6) (3). Reaction of L5 with 2 equiv. of Ni(OAc)2 pro- confirmed by X-ray crystallography. Nickel complex 5 was
vided [Ni4(L5)2(μ-OH)4(CF3COO)2](CF3COO)2 (5). Reaction of evaluated as a catalyst for reduction reactions involving the
[Re2(CO)8(CH3CN)2] with L5 in a refluxing chlorobenzene solu- conversion of ester functionalities into their corresponding alco-
tion gave a mixture of dirhenium (6) and monorhenium (7) hols.
Introduction
Whereas dinuclear metal complexes have, for several years,
been an attractive topic in coordination chemistry and catalysis
circles, the number of recent investigations into these species
have increased dramatically.[1] Compared to monometallic spe-
cies, dimetallic complexes can exhibit possible synergistic ef-
fects by virtue of interactions between the metal centers; such
effects might lead to improvements in reactivity and/or select-
ivity.[1] In order to have an efficient interaction between metal
ions for catalysis, Feringa et al. proposed that the optimum dis-
tance between the two metal centers is between 3.5 and
6.0 Å.[1d] Accordingly, the introduction of ligands able to accom-
modate metal ions in close proximity to each other has received
much attention. Among various ligands designed for dinuclear
Figure 1. Selected 2,7-disubstituted naphthyridine-based ligands.
systems, 2,7-disubstituted 1,8-naphthyridine-based ligands (L1–
L4) (Figure 1) have been extensively employed in the construc-
The reduction of esters to their corresponding alcohols is a
tion of dimetallic complexes containing two metal ions within
practical approach to generating desired starting materials in
a short distance.[2–16] Indeed, ligand L1 has been shown to form
organic synthesis. The reduction of ester functionalities using
stable dirhodium [Rh–Rh 2.405(2) Å],[13] diruthenium [Ru–Ru
sodium borohydride is relatively difficult due to the low reactiv-
2.273(4) Å][4] and dicopper [Cu–Cu 3.094 Å] complexes.[13c,16j] In
ity of the reducing agent.[17] However, the reducing power of
addition, the metal ions are separated by 3.22 Å in the dinickel
NaBH4 can be enhanced with the assistance of transition metal
complex based on ligand L2,[16i] 2.9534(2) Å in the dicopper
complexes. Among various metal salts, nickel complexes are
complex based on ligand L3, and 2.449(1)–2.7328(5) Å in the
known to be good catalysts in this regard.[18] In this work, we
dicopper complexes based on ligand L4.[15] As a continuation
also report the effective reduction of esters using a newly pre-
of our efforts to develop new dimetallic complexes, we report
pared dinickel complex as a catalyst, and we endeavor to un-
here the preparation of unsymmetrical naphthyridine-based li-
derstand possible synergistic effects unique to the dimetallic
gand L5 (Figure 1) and its complexation with various metal ions.
system in question.
[a] Department of Chemistry, National Taiwan University,
Results and Discussion
1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
E-mail: stliu@ntu.edu.tw
http://www.ch.ntu.edu.tw/
ORCID(s) from the author(s) for this article is/are available on the WWW
under http://dx.doi.org/10.1002/ejic.201600163.
Preparation and Characterization of L5
The desired ligand, 2-(2-pyridinyl)-7-(pyrazol-1-yl)-1,8-naphthyr-
idine (L5), was prepared by double substitution reactions of 2,7-
Eur. J. Inorg. Chem. 2016, 2783–2790
2783
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
dichloro-1,8-naphthyridine (1) (Scheme 1). First, the introduc- ration of 3 was unequivocally confirmed by X-ray crystallo-
tion of the pyridinyl group was achieved by a Stille coupling graphic analysis. The ORTEP plot of 3 is shown in Figure 3, and
reaction of 1 with 2-(tributylstannyl)pyridine to give 2 in 34 % some selected bond lengths and angles of 3 and [Ru2(L1)(μ-
isolated yield. Subsequently, thermal reaction of 2 with excess OAc)3](PF6) (4),[14] for comparison, are summarized in Table 1.
pyrazole rendered L5 in 75 % yield. Ligand L5 was characterized
by NMR spectroscopy and mass spectrometry. ESI-HR mass
spectra of L5 showed a peak at m/z = 274.1093, which is con-
sistent with the molecular formula C16H11N5 + 1 [M + H]. Fig-
1
ure 2 shows the H NMR spectra of L1, L2 and L5 for the pur-
1
poses of comparison. The H NMR signals for H-6, H-7, and H-8
appear in the range of δ = 8.29–8.35 ppm as a complicated
multiplet, showing the unsymmetrical nature of the molecule.
The signals representative of H-10 and H-11 appear at δ = 6.53
and 8.97 ppm, respectively, which are similar to those observed
Scheme 2. Preparation of diruthenium complex 3.
for L2 (δ = 6.65 and 8.86 ppm).
Scheme 1. Synthesis of L5. (i) (2-pyridinyl)SnBu3, PdCl2(PPh3)2, toluene, reflux;
(ii) pyrazole, 110 °C.
Figure 3. ORTEP plot of complex 3 (30 % probability level).
Table 1. Selected bond length and bond angles.
Complex 3
[(L1)Ru2(μ-OAc)3] (4)[a]
Ru(1)–Ru(2)
Ru(1)–N(5)
Ru(1)–N(3)
Ru(2)–N(2)
2.2764(4)
2.300(4)
2.061(3)
2.029(3)
2.239(3)
76.71(15)
168.28(10)
176.28(10)
178.49(13)
74.41(15)
163.74(11)
175.86(11)
178.27(13)
2.28(2)
2.24(2)
2.05(2)
2.00(2)
2.21(2)
78.1(6)
167.2(4)
175.6(7)
178.7(6)
77.1(6)
167.7(5)
176.0(7)
179.8(9)
Ru(2)–N(1)
N(1)–Ru(2)–N(2)
N(1)–Ru(2)–Ru(1)
O(1)–Ru(2)–O(5)
N(2)–Ru(2)–O(3)
N(3)–Ru(1)–N(5)
N(5)–Ru(1)–Ru(2)
O(2)–Ru(1)–O(6)
N(3)–Ru(1)–O(4)
Figure 2. Comparison of the 1H NMR spectra of L1, L2 and L5.
[a] Ref.[14]
Diruthenium Complex 3
The X-ray structure of 3 shows two ruthenium ions bridged
Dark purple complex 3 was prepared by reaction of [Ru2(μ- by L5 and three acetate ligands resulting in what looks like a
OAc)4Cl] with L5 in methanol followed by anion exchange paddlewheel structure, which is similar to that of 4, and is in
(Scheme 2). Complex 3 is air-stable both in the solid state and line with expectations. The geometry around the ruthenium
in solution. It is slightly soluble in organic solvents such as centers can be described as a distorted octahedron. The dis-
CHCl3, CH2Cl2 and diethyl ether, and much more soluble in acet- tance of Ru(1)–Ru(2) in 3 is 2.2763(4) Å, which is essentially
one and acetonitrile. ESI-MS analysis of 3 revealed a peak at identical to that found in 4, indicative of typical metal–metal
m/z = 653.9510 for the ion [Ru2(L5)(μ-OAc)3]+, consistent with bonding. The 2-pyridinyl and 1-pyrazole groups occupy the ax-
the formula of the cationic portion of 3. The molecular configu- ial sites trans to the Ru–Ru bond. The distance for the Ru(1)–
Eur. J. Inorg. Chem. 2016, 2783–2790
www.eurjic.org
2784
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
N(5)pyrazolyl linkage is slightly longer than that of the Ru(1)–N(5) selected bond lengths and angles are listed in Table 2. The
bond. Other than this difference, all other bond lengths and cation consists of two (μ-OH)(μ-CF3COO)-bridged dimers of
angles in 3 are comparable with those observed in 4.
[Ni2(μ-OH)2(CF3COO)]+. Each ligand L5 coordinates two structur-
ally similar six-coordinate NiII centers bridged by two hydroxy
ligands. The assignment of the formal μ-hydroxy bridges was
based on the required charge balance for the chemical compo-
sition. Each of the metal centers achieves a distorted-octahedral
configuration with the two nickel atoms separated by 2.992 Å.
The bridging angles of Ni(1)–O(1)–Ni(2) and Ni(1)–O(2)–Ni(2) are
90.11(10)° and 95.18(10)°, respectively. The bond lengths for all
Ni–O linkages are not equal, but remain in the normal range.
The bite angle of N(2)–Ni(1)–N(1) is similar to that of the N(3)–
Ni(2)–N(5) angle, indicating no significant effect of the nitrogen
donors from pyridinyl or pyrazol-1-yl groups on this parameter.
Dinickel Complex 5
Under reflux conditions, reaction of L5 with 2 equiv. of Ni(OAc)2
in methanol/CF3COOH (3:1) provided dinickel complex 5
(Scheme 3). Complex
5 exists in a dimeric form as
[Ni2(L5)(CF3COO)(μ-OH)2(H2O)](CF3COO) with the trifluoroacet-
ate and hydroxy groups as the bridging ligands. ESI-MS analysis
of
5 showed a peak at m/z = 727.9282 for the ion
[Ni2(L5)(CF3COO)3]+, indicative of the dinickel framework of the
structure. The UV/Vis absorption spectrum of 5 in methanol
exhibits bands at 357 and 372 nm (log ε = 4.19 and
4.25 M
–1 cm–1, respectively) attributable to charge transfer tran-
Table 2. Selected bond lengths [Å] and bond angles [°].
sitions. In addition, complex 5 shows a weak d–d transition at
659 nm, suggesting octahedral coordination around the Ni cen-
ter.[19]
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–O(2A)
N(2)–Ni(1)–N(1)
O(2)–Ni(1)–N(1)
O(2A)–Ni(1)–N(2)
Ni(1)–O(1)–Ni(2)
2.049(3)
2.131(3)
2.018(2)
77.5(1)
179.2(1)
174.8(1)
90.1(1)
Ni(2)–N(3)
Ni(2)–N(5)
2.122(3)
2.054(3)
2.069(3)
77.3(1)
173.3(1)
166.7(1)
95.2(1)
Ni(2)–O(3)(water)
N(3)–Ni(2)–N(5)
N(5)–Ni(2)–O(1)
O(4)–Ni(2)–N(3)
Ni(1)–O(2)–Ni(2)
Dirhenium Complex 6
Reaction of [Re2(CO)8(CH3CN)2] with an equimolar amount of L5
in a sealed tube with chlorobenzene as the solvent provided a
mixture of air-stable di- and monorhenium complexes 6 and 7
in 61 % and 39 % isolated yields, respectively (based on ligand)
(Scheme 4). Both complexes 6 and 7 were isolated by chroma-
tography. Conversion of mononuclear species 7 into 6 can be
achieved by reaction of 7 with Re2(CO)10. This observation is
similar to those made during thermal reactions of L1 with
carbonylrhenium compounds presented in our previous
works.[16g]
Scheme 3. Preparation of dinickel complex 5.
The crystal structure of 5 contains a discrete [Ni4(L5)2(μ-
OH)4(CF3COO)2(H2O)2]2+ cation and two trifluoroacetate anions.
An ORTEP plot of the cationic portion of 5 is shown in Figure 4;
Scheme 4. Preparation of the rhenium complexes.
Both complexes 6 and 7 were characterized by spectroscopic
methods. Carbonyl absorptions at 2013 cm–1 (sharp) and
1887 cm–1 (broad) in the IR spectrum of 7 are consistent with
the existence of an Re(CO)3 fragment, which is similar to the
analogue [(L1)Re(CO)3Cl]. Coordination of a putative Re(CO)3Br
Figure 4. ORTEP plot of complex 5 (30 % probability level).
Eur. J. Inorg. Chem. 2016, 2783–2790
www.eurjic.org
2785
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
fragment to the pyridinyl moiety in ligand L5, and not the pyr- the detailed structural features of 8 were confirmed by means
1
1
azolyl, is confirmed by H NMR spectroscopic analysis. Both H of X-ray crystallography. The coordination motif for complex 8
NMR spectra of L5 and 7 are shown in Figure 5 for the purposes is depicted in Figure 7. The complex is a bis(chloride)-bridged
of comparison. The signal for the C-1 proton shifts downfield CoII dimer in which two metal ions are related to each other
significantly in complex 7; this is not the case for the C-11 pro- through an inversion center which bisects Co(1) and Co(1A) and
ton, clearly illustrating the coordination mode of the “bipyr- also Cl(1) and Cl(1A). The geometry of each cobalt ion is de-
idine” moiety to the metal center. For complex 6, the disappear- scribed as a distorted trigonal-bipyramidal structure, and the
1
ance of the C-8 proton resonance in the H NMR spectrum of coordination environment comprises two nitrogen donors from
6 (Figure 6) suggests that metalation occurs at this position to L5, two terminal chloride ions, and a bridging chloride ion. The
yield the dinuclear species. The ESI-HR mass spectrum of 6 in axial sites are occupied by the naphthyridine nitrogen atoms
an acetonitrile matrix shows a peak at m/z = 880.9976, consist- [N(2)] and the bridging chloride ion [Cl(1A)]. Thus, the distance
ent with the formula [M
– Cl +
CH3CN]+ (calcd. for for the Co(1)–N(2) linkage [2.133(5) Å] is slightly longer than
C25H13N6O7Re2 880.9961). These observations clearly illustrate that of the Co(1)–N(1) bond [2.097(5) Å].
the structures of both complexes.
Figure 5. Comparison of the 1H NMR spectra for L5 and 7.
Figure 7. ORTEP plot of cobalt complex 8 (30 % probability level). Selected
bond lengths [Å] and angles[°]: Co(1)–N(1) 2.097(5), Co(1)–N(2) 2.133(5),
Co(1)–Cl(1) 2.339(2), Co(1)–Cl(2) 2.290(2); N(1)–Co(1)–N(2) 76.6(2), N(1)–Co(1)–
Cl(2) 126.9(2), N(1)–Co(1)–Cl(1) 116.1(1), Cl(2)–Co(1)–Cl(1) 116.86(7).
Reduction of Esters Catalyzed by Dinickel Complex 5
Among these newly prepared complexes, we investigated the
ability of dinickel complex 5 to catalyze the reduction of esters
to their corresponding alcohols; such reductions constitute a
useful approach to desired precursors in organic synthesis.
In this study, the reduction of methyl benzoate to benzyl
alcohol was chosen as the model reaction to establish optimal
catalytic conditions. In a typical experiment, a mixture of
PhCO2Me, NaBH4 and nickel complex was heated to a specific
temperature in either THF or diethyl ether. Crude products were
Figure 6. 1H NMR spectrum of 6.
1
analyzed by H NMR spectroscopy, and results are summarized
in Table 3. The reduction proceeded smoothly to provide the
benzyl alcohol when using 5 as the catalyst. By screening vari-
ous parameters it was ultimately determined that ideal condi-
tions involved the use of ZnCl2 as an additive with THF as the
Cobalt Complex 8
Reaction of L5 with 2 equiv. of CoCl2 in anhydrous methanol solvent at a reaction temperature of 45 °C (Table 3, Entry 6).
under reflux conditions gave cobalt complex 8 as a bright green Presumably, ZnCl2 plays some role in the reduction by virtue of
solid. ESI-HRMS analysis of the complex revealed two major its Lewis acid character.[20] Evaluation of various nickel com-
peaks at m/z = 367.0056 and 768.9769, corresponding to the plexes as catalysts for this reduction revealed the clear overall
formulas of [L5 + CoCl]+ and [8 – Cl]+, respectively. However, superiority of complex 5 (Table 3, Entries 10–14).
Eur. J. Inorg. Chem. 2016, 2783–2790
www.eurjic.org
2786
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Table 3. Reduction of PhCOOMe catalysed by 5 and related Ni complexes.[a]
Entry
Cat. (mol-%)
Additive
Solvent
Temp [°C]
Yield [%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
5 (1.5)
5 (1.5)
5 (1.5)
5 (1.5)
5 (1.5)
5 (1.5)
5 (0.75)
5 (1.5)
–
–
THF
THF
THF
THF
THF
THF
THF
Et2O
EtOH
THF
THF
THF
THF
THF
r.t.
65
65
65
65
45
45
40
65
45
45
45
45
45
10
54
92
90
85
95
59
52
17
38
83
48
52
53
ZnCl2
ZnBr2
LiCl
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
5 (1.5)
NiCl2 (3)
9 (1.5)[c]
10 (3)[c]
11 (3)[c]
10 + 11 (1.5 each)
[a] Reaction conditions: A mixture of methyl benzoate (0.3 mmol), additive (0.3 mmol), Ni complex and NaBH4 (0.6 mmol) in solvent (1 mL) was stirred for
12 h. [b] NMR-based yields. [c] Complexes 9–11 are shown in Scheme 5.
All the nickel complexes tested appear to exert catalytic ac- tries 14–16). Despite this apparently high level of activity, these
tivities in this reduction, although significant differences were dimetallic reducing conditions failed to reduce the amide func-
noted. Both complexes 5 and 9 (Scheme 5) are dinuclear sys- tional group (Table 4, Entry 18).
tems, and showed higher efficiency relative to their mononu-
Table 4. Reduction of esters as catalysed by complex 5.[a]
clear analogues. This observation suggests that some coopera-
tive effect between adjacent metal centers within the dimetallic
systems, which is absent in mononuclear systems, may benefit
such reductions.
Scheme 5. Nickel complexes as catalysts for the reduction of PhCOOMe.
According to proposals by Jones and James,[21] the coopera-
tivity index (α) for a dimetallic system is formulated as α = (Ao
– Ap)/Aave, where Ao is the observed activity of the dimetallic
complex, Ap is the predicted activity of the dimetallic complex,
AP = A1 + A2 where A1 is the measured activity of the monome-
tallic complex that closely mimics the first center and A2 is the
measured activity of the second center, and finally, Aave = (A1
+ A2)/2. Taking the performance of the mixture of 10 and 11
(Scheme 5) as AP, the cooperativity index (α) of 5 in this catalysis
is 1.53, which is slightly larger than unity. This result does, in
fact, suggest that 5 benefits from a slight cooperativity effect
during the course of benzoate reduction.
With optimized reaction conditions now in hand, we ex-
plored the generality of this catalysis with other esters including
lactones (Table 4). All benzoates with various substituents were
found to undergo reduction to afford the respective benzyl al-
cohols in good to excellent yields (Table 4, Entries 1–7). This
catalytic system also readily effected reduction of various lact-
ones leading to production of the corresponding diols (Table 4,
Entries 9–13). It was also noticed that α,ꢀ-unsaturated esters
were fully reduced to the related alkyl alcohols (Table 4, En-
[a] Reaction conditions: A mixture of ester (0.3 mmol), ZnCl2 (0.3 mmol), 5
(4.5 × 10–3 mmol) and NaBH4 (0.6 mmol) in THF (1 mL) was stirred at 45 °C
for 12 h. [b] Isolated yields.
Eur. J. Inorg. Chem. 2016, 2783–2790
www.eurjic.org
2787
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
ESI-HRMS: calcd. for C22H20N5O6Ru2 [M – PF6]+ 653.9501, found
653.9515. C22H20F6N5O6PRu2 (797.53): calcd. C 33.13, H 2.53, N 8.78;
found C 33.06, H 2.07, N 8.59.
Conclusions
We have prepared a set of dinuclear complexes based on an
unsymmetric ligand, 2-(2-pyridinyl)-7-(pyrazol-1-yl)-1,8-naph-
thyridine (L5). Both ruthenium and nickel complexes are dinu-
clear systems with short metal–metal distances, Ru–Ru
(2.2763 Å) in 3 and Ni–Ni (2.992 Å) in 5. The distance between
nickel atoms in 5 is significantly shorter than that found in 9.
For 6, two metal ions are not seated in the “pocket” of L5.
Rather, in this case, one of the metal centers underwent o-
metallation at the naphthyridine ring. From the observed for-
mation of complexes 7 and 8, we established that the chelation
ability of the bipyridine moiety in L5 is superior to that of the
pyridine–pyrazole moiety. In addition, nickel species 5 is an ef-
fective catalyst for the reduction of esters and lactones. Other
transition metal complexes containing L5 were also prepared,
but their precise structures not yet elucidated. Studies of the
coordination chemistry of 5 with other metal ions are currently
underway.
Dinickel Complex 5: A mixture of L5 (28 mg, 0.10 mmol) and
Ni(OAc)2·4H2O (58 mg, 0.23 mmol) in a mixed solvent MeOH (3 mL)/
CF3COOH (1 mL) was heated in an oil bath at 100 °C for 18 h. Upon
cooling, slow addition of diethyl ether to the reaction mixture gave
a green precipitate (43 mg, 64 %) as the desired complex. UV/Vis
(MeOH): λmax (log ε) = 659 (0.39), 372 (4.25), 357 (3.19), 262 (4.27),
249 (4.25), 212 (4.18
M ) nm. Conductivity (MeOH):
–1 cm–1
89.1 Ω–1 cm mol–1. C42H31F15N10Ni4O16 (M + CF3COOH) (1565.53):
calcd. C 34.75, H 2.15, N 9.65; found C 34.74, H 2.15, N 9.94.
Rhenium Complexes 6 and 7: A mixture of L5 (20 mg, 0.07 mmol)
and Re2(CO)8(CH3CN)2 (50 mg, 0.07 mmol) in chlorobenzene (2 mL)
was loaded in a sealed tube. The mixture was heated at 200 °C for
4 h. After the reaction, the chlorobenzene was removed under re-
duced pressure, and the residue was chromatographed on silica gel
by elution with ethyl acetate/acetone (5:1). Two bands on the col-
umn were separated and collected: red solid as complex 7 (16 mg,
39 %) and dark red solid as complex 6 (38 mg, 61 %). Complex 7:
IR (KBr): ν = 1887 (br.), 2013 cm–1 1H NMR (400 MHz, CDCl3): δ =
.
˜
9.26 (d, J = 5.6 Hz, 1 H), 9.09 (d, J = 2.8 Hz, 1 H), 8.58 (d, J = 8.8 Hz,
1 H), 8.49 (d, J = 8.4 Hz,1 H), 8.38 (d, J = 8.8 Hz, 1 H), 8.37 (d, J =
8 Hz, 1 H), 8.26 (d, J = 8.4 Hz, 1 H), 8.12 (t, J = 8 Hz, 1 H), 7.85 (s, 1
H), 7.59 (t, J = 8 Hz, 1 H), 6.61 (dd, J = 2.8, 2 Hz,1 H) ppm. ESI-HRMS:
calcd. for C19H11ClN5NaO3Re [M + Na]+ 602.0006, found 602.0008.
C19H11ClN5O3Re (578.98): calcd. C 39.41, H 1.91, N 12.10; found C
Experimental Section
General: Chemicals and solvents were of analytical grade and used
without further purification. NMR spectra were recorded with a
400 MHz spectrometer. Chemical shifts are given in ppm relative
to Me4Si for 1H and 13C NMR spectra. Compound 1 was prepared
according to a previously reported method.[16b]
39.03, H 1.66, N 11.86. Complex 6: IR (KBr): ν = 1899–2016 (br.),
˜
2097 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 9.32 (d, J = 2 Hz, 1 H),
2-Chloro-7-(pyridin-2-yl)-1,8-naphthyridine (2): A mixture of 2,7-
dichloro-1,8-naphthyridine (1) (1.0 g, 5.0 mmol), 2-(tributylstann-
yl)pyridine (2.8 g, 7.5 mmol) and PdCl2(PPh3)2 (178 mg, 0.25 mmol)
in pre-dried toluene (60 mL) was heated at reflux temperature un-
der an inert gas for 48 h. After removal of the toluene, the residue
was washed with hexane to remove the unreacted 2-(tributyl-
stannyl)pyridine. The residue was chromatographed on silica gel
with elution of dichloromethane/ethyl acetate (5:1, v/v) to give 1 as
9.27 (d, J = 4.8 Hz, 1 H), 8.78 (s, 1 H), 8.43 (d, J = 8.4 Hz, 1 H), 8.38
(d, J = 8.4 Hz, 1 H), 8.28 (d, J = 8.8 Hz,1 H), 8.13 (t, J = 8 Hz, 1 H),
7.97 (s, 1 H), 7.60 (t, J = 6 Hz, 1 H), 6.71 (s,1 H) ppm. ESI-HRMS: calcd.
for C25H13N6O7Re2 [M – Cl + CH3CN]+ 880.9933, found 882.9976.
C23H10ClN5O7Re2 (876.21): calcd. C 31.53, H 1.15, N 7.99; found C
31.33, H 1.24, N 7.63.
Cobalt Complex 8: A mixture of L5 (30 mg, 0.11 mmol) and CoCl2
(29 mg, 0.23 mmol) in anhydrous methanol (2 mL) was heated at
reflux under nitrogen for 24 h. After cooling, addition of diethyl
ether to the reaction mixture gave complex 8 as a bright green
solid (33 mg, 75 %). UV/Vis (MeOH): λmax (log ε) = 416 (2.48), 371
1
a yellow solid (400 mg, 34 %). H NMR (400 MHz, CDCl3): δ = 8.81
(d, J = 8 Hz, 1 H), 8.74 (d, J = 8 Hz, 1 H), 8.7–8.72 (m, 1 H), 8.29 (d,
J = 8 Hz, 1 H), 8.14 (d, J = 8 Hz, 1 H), 7.86 (t, J = 8 Hz, 1 H), 7.48 (d,
J = 8 Hz, 1 H), 7.36–7.39 (m, 1 H) ppm. This compound was used
for the next step of synthesis without further characterization.
(4.49), 356 (4.10), 260 (4.21), 249 (4.21), 212 (4.13 M
–1 cm–1) nm. ESI-
HRMS: calcd. for C16H11ClCoN5 [1/2M – Cl]+ 367.0035, found
367.0055. C32H22Cl4Co2N10 (806.27): calcd. C 47.67, H 2.75, N 17.37;
found C 47.29, H 2.55, N 17.00.
2-(Pyrazol-1-yl)-7-(pyridin-2-yl)-1,8-naphthyridine (L5): A mix-
ture of 1 (48.1 mg, 0.2 mmol) and pyrazole (40.5 mg, 0.6 mmol) was
heated in a sealed tube at 110 °C for 24 h. The resulting mixture
was washed with water to remove excess pyrazole, and the residue
was passed through a filtration column to give L5 as a light yellow
solid (41.6 mg, 75 %). 1H NMR (400 MHz, CDCl3): δ = 8.97 (d, J =
2.6 Hz, 1 H), 8.77 (d, J = 8 Hz, 1 H), 8.72 (d, J = 8 Hz, 1 H), 8.66 (d,
J = 8 Hz, 1 H), 8.29–8.35 (m, 3 H), 7.88 (t, J = 8 Hz, 1 H), 7.8 (d, J =
0.96 Hz, 1 H), 7.34–7.39 (m, 1 H), 6.53–6.54 (m, 1 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 159.8, 155.0, 154.3, 152.9, 149.0, 142.8, 139.4,
137.5, 136.8, 128.0, 124.5, 122.5, 121.4, 119.3, 113.4, 108.6 ppm. ESI-
HRMS: calcd. for C16H12N5 [M + H]+ 274.1093, found 274.1092.
Catalytic Reduction of Ester Substrates: A mixture of ester
(0.3 mmol), ZnCl2 (41 mg, 0.3 mmol), Ni complex 5 (6 mg,
4.5 × 10–3 mmol) and NaBH4 (23 mg, 0.6 mmol) in THF (1 mL) was
stirred at 45 °C for 12 h. After completion of the reaction, 1
M HCl
(2 mL) was added and the mixture extracted with diethyl ether (2 ×
5 mL). The organic extracts were combined and dried with MgSO4.
The desired product was purified by chromatography with hexane/
EtOAc as eluent. All reduced products are known compounds, and
their spectral analyses are in agreement with the reported data.
Diruthenium Complex 3: A mixture of L5 (24.6 mg, 0.09 mmol) and
Ru2(OAc)4Cl (48.3 mg, 0.10 mmol) in methanol (3 mL) was stirred at
room temperature for 20 h. KPF6 (18.8 mg, 0.10 mmol) was added
to the mixture with stirring for another 2 h. After removal of the
solvent, the mixture was dissolved in dichloromethane (10 mL) and
washed with water (2 × 3 mL). The organic portion was dried and
concentrated. Upon crystallization in methanol/diethyl ether, the
desired compound was obtained as a purple solid (45.8 mg, 63 %).
Benzyl Alcohol: Yield 29.3 mg, 90 %. 1H NMR (400 MHz, CDCl3):
δ = 7.35–7.38 (m, 5 H), 4.59 (s, 2 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 140.7, 128.2, 127.2, 126.8, 64.5 ppm.
4-Methoxybenzyl Alcohol: Yield 36.5 mg, 88 %. 1H NMR (400 MHz,
CDCl3): δ = 7.24 (d, J = 8.4 Hz, 2 H), 6.86 (d, J = 8.4 Hz, 2 H), 4.53
(s, 2 H), 3.78 (s, 3 H), 2.63 (br., 1 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 158.9, 133.0, 128.5, 113.7, 64.6, 55.1 ppm.
Eur. J. Inorg. Chem. 2016, 2783–2790
www.eurjic.org
2788
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
4-Chlorobenzyl Alcohol: Yield 39.4 mg, 92 %. 1H NMR (400 MHz,
Crystal Data for 8: C32H22Cl4Co2N10, Fw = 806.26, monoclinic,
CDCl3): δ = 7.29 (d, J = 8.4 Hz, 2 H), 7.22 (d, J = 8.4 Hz, 2 H), 4.56 P21/n, a = 9.5111(7) Å, b = 17.3587(8) Å, c = 10.4103(8) Å, ꢀ =
(s, 2 H), 2.82 (br., 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 139.1,
133.2, 128.5, 128.3, 64.2 ppm.
110.918(8)°, V = 1605.47(19) Å3, Z = 2, Dcalcd. = 1.668 Mg/m3,
F(000) = 812, crystal size: 0.20 × 0.15 × 0.10 mm, θ = 3.28–24.99°,
5831 reflections collected, 2799 reflections [R(int) = 0.0445], final R
indices [I > 2σ(I)]: R1 = 0.0633, wR2 = 0.1641, for all data R1 = 0.1023,
wR2 = 0.1899, goodness-of-fit on F2 = 1.026.
4-Bromobenzyl Alcohol: Yield 51.2 mg, 91 %. 1H NMR (400 MHz,
CDCl3): δ = 7.47 (d, J = 8.8 Hz, 2 H), 7.24 (d, J = 8.8 Hz, 2 H), 4.64
(s, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 139.7, 131.6, 128.6,
121.4, 64.5 ppm.
Acknowledgments
We thank the Ministry of Science and Technology, Taiwan, for
financial support (NSC103-2113-M-002-002-MY3).
2-(Hydroxymethyl)pyridine: Yield 29.1 mg, 89 %. 1H NMR
(400 MHz, CDCl3): δ = 8.51 (d, J = 4.8 Hz, 1 H), 7.64–7.68 (m, 1 H),
7.27 (d, J = 8 Hz, 1 H), 7.15–7.19 (m, 1 H), 4.74 (s, 1 H), 3.95 (br., 1
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 159.3, 148.4, 136.7, 122.3,
120.6, 64.2 ppm.
Keywords: Dimetallic complexes · Nickel · Ruthenium ·
Rhenium · Naphthyridine · Reduction
1
Benzene-1,2-dimethanol: Yield 35.2 mg, 85 %. H NMR (400 MHz,
CDCl3): δ = 7.26 (m, 4 H), 4.53 (s, 4 H), 4.42 (br., 2 H) ppm. 13C
NMR(100 MHz, CDCl3): δ = 139.0, 129.3, 128.2, 63.3 ppm.
[1] Recent reviews: a) M. Boiocchi, L. Fabbrizzi, Chem. Soc. Rev. 2014, 43,
1835–1847; b) J.-P. Launay, Coord. Chem. Rev. 2013, 257, 1544–1554; c)
Y. Sunatsuki, R. Kawamoto, K. Fujita, H. Maruyama, T. Suzuki, H. Ishida,
M. Kojima, S. Iijima, N. Matsumoto, Coord. Chem. Rev. 2010, 254, 1871–
1881; d) E. K. van den Beuken, B. L. Feringa, Tetrahedron 1998, 54, 12985–
13011.
3-(o-Hydroxyphenyl)-1-propanol: Yield 35.9 mg, 74 %. 1H NMR
(400 MHz, CDCl3): δ = 7.08–7.12 (m, 2 H), 6.84–6.90 (m, 2 H), 3.65
(t, J = 6.4 Hz, 2 H), 2.78 (t, J = 7.2 Hz, 2 H), 1.85–1.92 (m, 2 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 154.1, 130.5, 127.5, 127.3, 120.6,
115.6, 60.9, 32.2, 25.4 ppm.
[2] a) M. Sarkar, H. Doucet, J. K. Bera, Chem. Commun. 2013, 49, 9764–9766;
b) P. Daw, T. Ghatak, H. Doucet, J. K. Bera, Organometallics 2013, 32,
4306–4313; c) A. Sinha, M. Majumdar, M. Sarkar, T. Ghatak, J. K. Bera,
Organometallics 2013, 32, 340–349; d) S. M. W. Rahaman, D. Das, N. Sad-
hukhan, A. Sinha, J. K. Bera, Inorg. Chim. Acta 2011, 374, 320–326; e) J. K.
Bera, N. Sadhukhan, M. Majumdar, Eur. J. Inorg. Chem. 2009, 4023–4038;
f) S. K. Patra, J. K. Bera, Organometallics 2007, 26, 2598–2603; g) S. K.
Patra, J. K. Bera, Organometallics 2006, 25, 6054–6060; h) J. K. Bera, E. J.
Schelter, S. K. Patra, J. Bacsa, K. R. Dunbar, Dalton Trans. 2006, 4011–
4019; i) S. K. Patra, N. Sadhukhan, J. K. Bera, Inorg. Chem. 2006, 45, 4007–
4015.
[3] G.-M. Lin, M. Sigrist, E.-C. Horng, C.-h. Chen, C.-Y. Yeh, G.-H. Lee, S.-M.
Peng, Z. Anorg. Allg. Chem. 2015, 641, 2258–2265.
[4] a) W.-F. Fu, L.-F. Jia, W.-H. Mu, X. Gan, J.-B. Zhang, P.-H. Liu, Q.-Y. Cao, G.-
J. Zhang, L. Quan, X.-J. Lv, Q.-Q. Xu, Inorg. Chem. 2010, 49, 4524–4533;
b) Y. Chen, J.-S. Chen, X. Gan, W.-F. Fu, Inorg. Chim. Acta 2009, 362, 2492–
2498.
[5] a) T. C. Davenport, T. D. Tilley, Dalton Trans. 2015, 44, 12244–12255; b)
T. C. Davenport, T. D. Tilley, Angew. Chem. Int. Ed. 2011, 50, 12205–12208;
Angew. Chem. 2011, 123, 12413–12416.
[6] a) H. Araki, K. Tsuge, Y. Sasaki, S. Ishizaka, N. Kitamura, Inorg. Chem. 2007,
46, 10032–10034.
[7] X.-J. Su, M. Gao, L. Jiao, R.-Z. Liao, P. E. M. Siegbahn, J.-P. Cheng, M.-T.
Zhang, Angew. Chem. Int. Ed. 2015, 54, 4909–4914; Angew. Chem. 2015,
127, 4991–4996.
[8] L. A. Wilkinson, L. McNeill, P. A. Scattergood, N. J. Patmore, Inorg. Chem.
2013, 52, 9683–9691.
2-Phenylethanol: Yield 33.0 mg, 90 %. 1H NMR (400 MHz, CDCl3):
δ = 7.30–7.34 (m, 2 H), 7.23–7.26 (m, 3 H), 3.84 (t, J = 6.4 Hz, 2 H),
2.86 (t, J = 6.4 Hz, 2 H), 1.88 (br., 1 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 138.5, 129.0, 128.5, 126.4, 63.5, 39.1 ppm.
1,4-Butanediol: Yield 21.9 mg, 81 %. 1H NMR (400 MHz, CDCl3): δ =
3.63 (t, J = 6 Hz, 4 H), 3.08 (br., 1 H), 1.61–1.67 (m, 4 H) ppm. 13C
NMR(100 MHz, CDCl3): δ = 62.5, 29.8 ppm.
1,5-Pentanediol: Yield 24.7 mg, 79 %. 1H NMR (400 MHz, CDCl3):
δ = 3.61 (t, J = 6.4 Hz, 4 H) 2.22 (br., 1 H), 1.53–1.60 (m, 4 H),
1.42–1.44 (m, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 62.5, 32.2,
21.9 ppm.
1,6-Hexanediol: Yield 26.9 mg, 76 %. 1H NMR (400 MHz, CDCl3):
δ = 3.61 (t, J = 6 Hz, 4 H), 1.94 (br., 2 H), 1.53–1.57 (m, 4 H), 1.34–1.38
(m, 4 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 62.7, 32.5, 25.4 ppm.
Crystallography: Crystals suitable for X-ray determination were ob-
tained for 3, 5·(CF3COOH)2(CH3CN)2 and 8 by recrystallization. Cell
parameters were determined using a Siemens SMART CCD diffrac-
tometer. The structures were solved using the SHELXS-97 pro-
gram[22] and refined using the SHELXL-97 program[23] by full-matrix
least squares on the F2 values. CCDC 1453770 (for 3), 1453771 (for
5), and 1453772 (for 8) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre.
[9] J. D. Aguirre, D. A. Lutterman, A. M. Angeles-Boza, K. R. Dunbar, C. Turro,
Inorg. Chem. 2007, 46, 7494–7502.
Crystal Data for 3: C24H23F6N6O6PRu2, Fw = 838.59, monoclinic,
P21/c, a = 12.7015(2) Å, b = 31.9155(5) Å, c = 8.19920(10) Å, ꢀ =
104.634(2)°, V = 3215.92(8) Å3, Z = 4, Dcalcd. = 1.732 Mg/m3, F(000) =
1656, crystal size: 0.25 × 0.15 × 0.10 mm, θ = 2.97–27.49°, 35142
reflections collected, 7266 reflections [R(int) = 0.0294], final R indi-
ces [I > 2σ(I)]: R1 = 0.0361, wR2 = 0.1174, for all data R1 = 0.0432,
wR2 = 0.1220, goodness-of-fit on F2 = 1.153.
[10] T. Tanase, H. Takenaka, E. Goto, J. Organomet. Chem. 2007, 692, 175–183.
[11] M. Basato, A. Biffis, G. Martinati, C. Tubaro, C. Graiff, A. Tiripicchio, L. A.
Aronica, A. M. Caporusso, J. Organomet. Chem. 2006, 691, 3464–3471.
[12] T. Suzuki, Inorg. Chim. Acta 2006, 359, 2431–2438.
[13] a) W. R. Tikkanen, E. Binamira-Soriaga, W. C. Kaska, P. C. Ford, Inorg. Chem.
1984, 23, 141–146; b) W. R. Tikkanen, E. Binamira-Soriaga, W. C. Kaska,
P. C. Ford, Inorg. Chem. 1983, 22, 1147–1148; c) W. R. Tikkanen, C. Krue-
ger, K. D. Bomben, W. L. Jolly, W. C. Kaska, P. C. Ford, Inorg. Chem. 1984,
23, 3633–3638.
Crystal Data for 5: C24H19F9N6Ni2O9, Fw = 823.87, triclinic, P1¯, a =
10.4810(3) Å, b = 12.7454(3) Å, c = 13.0431(4) Å, α = 77.674(3)°, ꢀ =
[14] E. Binamira-Soriaga, N. L. Keder, W. C. Kaska, Inorg. Chem. 1990, 29, 3167–
3171.
66.605(3)°, γ = 81.187(2)°, V = 1557.70(7) Å3, Z = 2, Dcalcd.
=
[15] C. He, S. J. Lippard, Inorg. Chem. 2000, 39, 5225–5231.
[16] a) C.-H. Lee, C.-L. Wu, S.-A. Hua, Y.-H. Liu, S.-M. Peng, S.-T. Liu, Eur. J. Inorg.
Chem. 2015, 1417–1423; b) W.-H. Tang, Y.-H. Liu, S.-M. Peng, S.-T. Liu, J.
Organomet. Chem. 2015, 775, 94–100; c) C.-Y. Huang, K.-Y. Kuan, Y.-H. Liu,
S.-M. Peng, S.-T. Liu, Organometallics 2014, 33, 2831–2836; d) B.-S. Liao,
1.757 Mg/m3, F(000) = 828, crystal size: 0.25 × 0.20 × 0.10 mm, θ =
2.98–27.49°, 14362 reflections collected, 6911 reflections [R(int) =
0.0285], final R indices [I > 2σ(I)]: R1 = 0.0499, wR2 = 0.1422, for all
data R1 = 0.0695, wR2 = 0.1629, goodness-of-fit on F2 = 1.093.
Eur. J. Inorg. Chem. 2016, 2783–2790
www.eurjic.org
2789
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Y.-H. Liu, S.-M. Peng, K. R. Reddy, S.-H. Liu, P.-T. Chou, S.-T. Liu, Dalton
Trans. 2014, 43, 3557–3562; e) M.-U. Hung, B.-S. Liao, Y.-H. Liu, S.-M. Peng,
S.-T. Liu, Appl. Organomet. Chem. 2014, 28, 661–665; f) T.-C. Su, Y.-H. Liu,
S.-M. Peng, S.-T. Liu, Eur. J. Inorg. Chem. 2013, 2362–2367; g) Y.-S. Lan, B.-
S. Liao, Y.-H. Liu, S.-M. Peng, S.-T. Liu, Eur. J. Org. Chem. 2013, 5160–5164;
h) T.-P. Cheng, B.-S. Liao, Y.-H. Liu, S.-M. Peng, S.-T. Liu, Dalton Trans. 2012,
41, 3468–3473; i) B.-S. Liao, Y.-H. Liu, S.-M. Peng, S.-T. Liu, Dalton Trans.
2012, 41, 1158–1164; j) Y.-H. Chang, Z.-Y. Liu, Y.-H. Liu, S.-M. Peng, J.-T.
Chen, S.-T. Liu, Dalton Trans. 2011, 40, 489–494.
[19] L. K. Das, A. Biswas, J. S. Kinyon, N. S. Dalal, H. Zhou, A. Ghosh, Inorg.
Chem. 2013, 52, 11744–11757.
[20] T. Yamakawa, M. Masaki, H. Nohira, Bull. Chem. Soc. Jpn. 2004, 77, 549–
552.
[21] N. D. Jones, B. R. James, Adv. Synth. Catal. 2002, 344, 1126–1134.
[22] For information regarding SHELXS-90, see: G. M. Sheldrick, Acta Crystal-
logr., Sect. A: Found. Crystallogr. 1990, 46, 467.
[23] G. M. Sheldrick, SHELXL-97, University of Göttingen, Göttingen, Germany,
1997.
[17]
S. T. Lin, J. A. Roth, J. Org. Chem. 1979, 44, 309–310.
Received: February 18, 2016
Published Online: April 26, 2016
[18] J. M. Khurana, P. Sharma, Bull. Chem. Soc. Jpn. 2004, 77, 549–552.
Eur. J. Inorg. Chem. 2016, 2783–2790
www.eurjic.org
2790
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim