Practical synthesis of new β-diketone-connected bipyridine and its conversion to pyrazole-centered bipyridine ligand

Article abstract of DOI:10.1080/00397910701487408

Two new polydentate ligands, 1-(6-hydroxymethyl-2-pyridyl)-3-(2-pyridyl)- propane-1,3-dione (6) and 1,3-bis(6-hydroxymethyl-2-pyridyl)-propane-1, 3-dione (8), have been synthesized starting from 2,6-pyridinedicarboxylic acid by conventional esterification, reduction, and condensation reactions. They were further converted to two new polydentate ligands, 3-(6-hydroxymethyl-2-pyridyl)- 5-(2-pyridyl)-1H-pyrazole (7) and 3,5-bis(6-hydroxymethyl-2-pyridyl)-1H-pyrazole (9), by reaction with hydrazine hydrate. These four compounds were characterized by proton nuclear magnetic resonance (1H NMR), electrospray ionization-mass spectrum (ESI-MS), infrared spectrum (IR), and elemental analyses. The structure of 9 was also determined by X-ray diffraction. Copyright Taylor & Francis Group, LLC.

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Practical Synthesis of New
β‐Diketone‐Connected Bipyridine and Its
Conversion to Pyrazole‐Centered Bipyridine
Ligand
Wei Zhang a , Jianhui Liu a , Hongjun Zhu a , Weiming Gao a & Licheng Sun a
b
a State Key Laboratory of Fine Chemicals, Dalian University of Technology,
Dalian, China
b Department of Chemistry, Organic Chemistry, Royal Institute of Technology
(KTH), Stockholm, Sweden
Version of record first published: 05 Oct 2007
To cite this article: Wei Zhang, Jianhui Liu, Hongjun Zhu, Weiming Gao & Licheng Sun (2007): Practical
Synthesis of New β‐Diketone‐Connected Bipyridine and Its Conversion to Pyrazole‐Centered Bipyridine Ligand,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
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Synthetic Communicationsw, 37: 3393–3402, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701487408
Practical Synthesis of New b-Diketone-
Connected Bipyridine and Its Conversion
to Pyrazole-Centered Bipyridine Ligand
Wei Zhang, Jianhui Liu, Hongjun Zhu, and Weiming Gao
State Key Laboratory of Fine Chemicals, Dalian University
of Technology, Dalian, China
Licheng Sun
State Key Laboratory of Fine Chemicals, Dalian University
of Technology, Dalian, China and Department of Chemistry, Organic
Chemistry, Royal Institute of Technology (KTH), Stockholm, Sweden
Abstract: Two new polydentate ligands, 1-(6-hydroxymethyl-2-pyridyl)-3-(2-
pyridyl)-propane-1,3-dione (6) and 1,3-bis(6-hydroxymethyl-2-pyridyl)-propane-1,
3-dione (8), have been synthesized starting from 2,6-pyridinedicarboxylic acid by
conventional esterification, reduction, and condensation reactions. They were further
converted to two new polydentate ligands, 3-(6-hydroxymethyl-2-pyridyl)-5-(2-
pyridyl)-1H-pyrazole (7) and 3,5-bis(6-hydroxymethyl-2-pyridyl)-1H-pyrazole (9),
by reaction with hydrazine hydrate. These four compounds were characterized
by proton nuclear magnetic resonance (1H NMR), electrospray ionization-mass
spectrum (ESI-MS), infrared spectrum (IR), and elemental analyses. The structure of
9 was also determined by X-ray diffraction.
Keywords: crystal structure, polydentate ligands, pyrazole derivatives, synthesis
INTRODUCTION
Many dimetallic Ru and Mn oxo complexes have been synthesized to mimic
the structure and/or function of the oxygen-evolving complex (OEC) in
Received December 8, 2006
Address correspondence to Licheng Sun, State Key Laboratory of Fine Chemicals,
Dalian University of Technology, Dalian 116012, China. E-mail: lichengs@kth.se
3393

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W. Zhang et al.
Figure 1. The structures of compounds Hbpp, 7, 9, and the labelling scheme for
compound 7.
photosystem II (PSII).[1] Unfortunately, the existence of fragile bridges
between the two metal atoms makes the complexes easy to decompose, so
they lose their reactivities very soon.[2] Recently, some researchers have
proved that ligands with multiple coordination sites can not only facilitate
dimer formation but also increase greatly their activities.[3] The 3,5-bis-(2-
pyridyl)-1H-pyrazole ligand (Hbpp, Fig. 1) proves to be one of the candidates
that can stabilize a dimeric structure through strong chelating ability. On one
hand, its bent geometry allows for a halide or small molecule to bridge both
metals, which can then cooperatively influence the subsequent reactivity.
On the other hand, deprotonating the ligand can lower the overall positive
charge on the complex and make dimetallic complexes more stable.[4]
As part of our research work for the study of polynuclear Ru or Mn
complexes, we focused our attention on ligands with multicoordination
centers to synthesize homo- or/and hetero-polynuclear complexes. We
noticed that few derivatives of Hbpp have been reported, since it was first
synthesized in 1969.[5] Therefore, we synthesized 3-(6-hydroxymethyl
-2-pyridyl)-5-(2-pyridyl)-1H-pyrazole (7) and 3,5-bis(6-hydroxymethyl-2-
pyridyl)-1H-pyrazole (9) based on the structure of Hbpp. Compounds 1-(6-
hydroxymethyl-2-pyridyl)-3-(2-pyridyl)-propane-1,3-dione (6) and 1,3-bis
(6-hydroxymethyl-2-pyridyl)-propane-1,3-dione (8) obtained in the synthetic
process are other two polynucleating ligands that have not been reported.
These four compounds can be used as effective ligands for homo-or heteropoly-
nuclear complexes. In addition, they can also be applied to prepare new
ligands possessing more coordinate sites through reactions with the active
hydroxyl group.
RESULTS AND DISCUSSION
The synthesis started with a modified esterification of the commercially
available pyridyl-2,6-dicarboxylic acid (1) in refluxing absolute methanol in
acid media, affording methyl pyridyl-2,6-dicarboxylate (2) in good yield.[6]
It was regioselectively reduced to methyl 2-(hydroxymethyl) picolinate
(3) by NaBH4 (0.6 eq.) in absolute methanol upon refluxing.[7] We found
that ester exchange occurred easily if EtOH was used to give the ethyl
ester. It can be further converted to b-ketoester by a ester condensation

b-Diketone-Connected Bipyridine
3395
Scheme 1.
between compound 3 and the freshly distilled EtOAc in the present of
NaOC2H5 upon refluxing, similar to the procedure from 6-methyl-2-picolinate
to 2-acetyl-6-methylpyridine.[8] The b-ketoester can be easily converted to 4
by hydrolysis with excess of H2SO4 (20%) and then evolve to carbon dioxygen
after refluxing for 2 h. The two steps from pyridyl ester 3 to pyridyl keto 4
were carried out in one pot in a consecutive manner without isolation of the
medium product. Compared to the existing three-step procedure in the litera-
ture with no details of its preparation or characterization, our method seems
more easier and more efficient (Scheme 1).[9]
With 4 in hand, we then turned our attention to our target 1,3-dicarbonyl
compounds and pyrazole derivatives. Claisen condensation reaction is a
common method to give 1,3-dicarbonyl compounds. The first new b-diketone
compound,
1-(6-hydroxymethyl-2-pyridyl)-3-(2-pyridyl)-propane-1,3-dione
(6), was synthesized following a Claisen condensation of 3 with 5. In the
presence of NaOC2H5, the sodium salt of the b-diketone was obtained.
Further treatment of it with acetic acid yielded the corresponding
1
b-diketone 6 in 46% yield. From its H NMR spectrum, we can conclude
that in chloroform the b-diketone is in equilibrium with the corresponding
keto-enols, in an approximate ratio of 20% b-diketone and 80% keto-enol.
This 1,3-dicarbonyl compound exists dominantly in the enol form, which is
stabilized by a very favourable intramolecular hydrogen bond in a six-
1
membered ring. The H NMR spectrum is symmetrical because the proton
very rapidly hops from one oxygen to the other, making the two halves of
Scheme 2.

3396
W. Zhang et al.
Scheme 3.
the ring and the two sp2 carbon baring oxygen the same. By reaction of
compound 6 with hydrazine hydrate in refluxing ethanol, the expected
Hbpp-derived ligand, 3-(6-hydroxymethyl-2-pyridyl)-5-(2-pyridyl)-1H-
pyrazole (7), was obtained in high yield (Scheme 2). For this unsymmetrical
pyrazole, the two tautomerism is not the same, which was confirmed by its 13
NMR spectrum where 25 peaks were observed.
C
Another new b-diketone compound, 1,3-bis(6-hydroxymethyl-2-
pyridyl)-propane-1,3-dione (8), was similarly synthesized following a
Claisen condensation of 3 with 4 in 46% yield. In chloroform, this
compound exists in an approximate ratio of 30% of b-diketone and 70% of
keto-enol. By reaction with hydrazine hydrate, it was converted to pyrazole
(9) in 60% yield (Scheme 3). Although the two identical tautomers
1
of pyrazole were expected, the H NMR and 13C NMR spectrum indicate
that this molecule is actually unsymmetrical. Its structure was further
confirmed by X-ray analysis. Figure 2 shows the geometry of the molecule
with the atomic numbering scheme. The molecule is essentially planar with
the dihedral angle between each pyridyl ring and the central pyrazole
pentagon being about 68. The pyridine rings show a trans geometry; that is,
one pyridyl N atom is located at the opposite side of the second pyridyl N
atom. Figure 3 shows a view of the molecular packing in the crystal. The
Figure 2. Molecular structure of compound 9.

b-Diketone-Connected Bipyridine
3397
Figure 3. Unit-cell packing diagram of compound 9.
molecules are arranged in a regular sandwich form. In the same layer, the N
(2) atom of pyrazole ring forms strong, intermolecular hydrogen bonds with
the pyrazole proton on atom N (3) of the neighboring molecule, a distance
˚
of 2.946 A. Furthermore, one oxygen atom from one molecule interacted
with two hydrogen atoms of two hydroxyls from the other two molecules
˚
respectively and formed two hydrogen bonds, whose lengths are 2.703 A
˚
and 2.676 A. In other words, the combination of these two intermolecular
H-bonding interactions makes the molecules pack more tightly.
In summary, the four novel polynuleating ligands (6–9), can be easily
synthesized from a cheaper and commercially available 2,6-pyridinedicar-
boxylic acid and 2-acetylpyridine. Additionally, a new convenient method
to prepare 1-(6-hydroxymethyl-2-pyridyl)ethanone (4) seems better than the
method reported. To the best of our knowledge, it is the first time the active
group was induced to Hbpp, which can be used to extend the structure of
Hbpp to much bigger ligands. Extension of this methodology may lead to a
series of new Hbpp derivatives. This will provide more transition-metal
complexes bearing these new ligands as a model for the study to mimic the
structure and function of the oxygen-evolving complex (OEC) in PSII.
EXPERIMENTAL
The NMR spectra were recorded on a Varian Inova spectrometer at 400 MHz.
Chemical shifts were given in d units relative to the tetramethylsilane (TMS)
signal as an internal reference. IR were recorded on a Nexus spectrometer.
ESI-MS spectra were obtained on HP1100LCMS spectrometer. Melting
points were measured on X-4 apparatus and are not corrected. Elemental
analyses were carried out on a Carlo Erba MOD-1106 elemental analyzer.
X-ray diffraction was measured on a Smart Apex II diffractometer. 2,6-Pyri-
dinedicarboxylic acid and 2-acetylpyridine were chemical grade. Methanol,

3398
W. Zhang et al.
ethyl acetate, and toluene were dried and purified by the standard purification
procedures.
Crystal Structure Determinations
X-ray crystallography for compound 9: empirical formula C15H14N4O2,
˚
M ¼ 282.30, T ¼ 273(2)K, triclinic, space group P-1 with a ¼ 4.553(2) A,
˚
˚
b ¼ 10.529(5) A, c ¼ 13.986(7) A, a ¼ 92.748(7)8, b ¼ 97.259(7)8 and
3
˚
˚
g ¼ 99.127(8)8, V ¼ 655.1(6) A , Z ¼ 2, l(Mo-Ka) ¼ 0.71073 A,
m ¼ 0.099 mm21
,
Dc ¼ 1.431 g/cm3, F(000) ¼ 296,2.94 , u ,25.01,
R ¼ 0.0825, vR ¼ 0.2017, S ¼ 0.946. Largest diff. peak and hole: 0.354
and 20.341eA23. Crystallographic data for the structures of
˚
23
˚
eA
compound 9 have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication number CCDC 624684. A copy of the
data can be obtained, free of charge, by contacting the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K. (e-mail:
data_request@ccdc.cam.ac.uk, fax: þ44 1223 336033).
Methyl Pyridyl-2,6-dicarboxylate (2)
A 500-mL, round-bottom flask equipped with a reflux condenser and addition
funnel was charged with pyridyl diacid 1 (60 g, 0.36 mol) and dried methanol
(250 mL). H2SO4 (98%, 20 mL) was slowly added over 1 h through the
funnel. The resulting solution was then refluxed for 1 day, allowed to cool
to room temperature, and then poured into ice water (200 mL). The solution
was neutralized with Na2CO3, filtered, and extracted with several portions
of CH2Cl2. The solvent was removed by rotary evaporation to give a white
1
product (55 g, 79%): ESI-MS (m/z): 196.1([M þ H]þ); H NMR (CDCl3):
d
8.33 (d, 3J3,4 ¼ 3J5,4 ¼ 7.6 Hz, 2H, pyridyl H3, H5), 8.04
3
(t, J4,3 ¼ 3J4,5 ¼ 7.6 Hz, 1H, pyridyl H4), 4.04 (s, 6H, -OCH3).
Methyl 2-(Hydroxymethyl)picolinate (3)
Pyridine 2 (20 g, 0.1 mol) was refluxed with sodium borohydride (2.34 g,
0.06 mol) in 200 mL of dry methanol for 5 h. After being cooled, the
solution was concentrated to ca. 50 mL, and the same amount of water was
added. The solution was further concentrated to a final volume of 50 mL
and extracted with several portions of CH2Cl2 (50 mL), and then the solvent
was removed by rotary evaporation. The residue was purified by column
chromatography on silica gel with eluent of CH2Cl2/MeOH (40:1, v/v) to
give a white solid (8.25 g, 48%): ESI-MS(m/z): 168.1 ([M þ H]þ); 1H
3
NMR (CDCl3):
d
8.05 (d, J3,4 ¼ 8.0 Hz, 1H, pyridyl H3), 7.88

b-Diketone-Connected Bipyridine
3399
(t, 3J4,3 ¼ 8.0 Hz, 3J4,5 ¼ 7.6 Hz, 1H, pyridyl H4), 7.54 (d, 3J5,4 ¼ 7.6 Hz, 1H,
pyridyl H5), 4.88 (s, 2H, -CH2OH), 4.02 (s, 3H, -OCH3), 3.14 (s, br, 1H, -OH).
1-(6-Hydroxymethyl-2-pyridyl)-ethanone (4)
Dry toluene (80 mL) and freshly distilled EtOAc (6 mL) were added to the
freshly prepared EtONa (1.23 g, 18 mmol). After stirring for 10 min,
compound 3 (2 g, 12 mmol) was added, and then the mixture was refluxed
for 5 h to afford a deep red solution. The solvent was evaporated in vacuum
to leave the crude sodium enolate. The residue was heated with 80 mL of
excess H2SO4 (20%) for 2 h. The cooled mixture was poured onto ice, neutral-
ized to pH 8 with Na2CO3, filtered, and extracted with several portions of
CH2Cl2. The extract was dried over anhydrous Na2SO4. The solvent was
eliminated by rotary evaporation to give a yellow oil. Further purification
was performed by column chromatography on silica gel with eluent of
CH2Cl2/MeOH (40 : 1, v/v) and finally gave a light yellow oil (0.83 g,
46%). ESI-MS(m/z): 152.1 ([M þ H]þ); 1H NMR (CDCl3): d 7.97
3
3
(d, J3,4 ¼ 7.6 Hz, 1H, pyridyl H3), 7.86 (t, J4,3 ¼ 3J4,5 ¼ 7.6 Hz, 1H,
3
pyridyl H4), 7.45 (d, J5,4 ¼ 7.6 Hz, 1H, pyridyl H5), 4.85 (s, 2H, -CH2OH),
3.6 (s, br, 1H, -OH), 2.75 (s, 3H, -OCH3).
1-(6-Hydroxymethyl-2-pyridyl)-3-(2-pyridyl)-propane-1,3-dione (6)
Freshly prepared sodium ethoxide (0.62 g, 9 mmol) was added to the dry
toluene (15 mL) solution of compound 3 (1 g, 6 mmol), and then toluene
(5 mL) of compound 5 (1.08 g, 8.5 mmol) was added very slowly. During
the addition, care should be taken to ensure that the temperature is kept
between 45 and 608C. During this period, the mixture changed from yellow
to dark red. After stirring for 1h, an orange precipitate appeared, and the
reaction was stirred for another 30 min at room temperature. The solvent
was removed by rotary evaporation. The solid was slowly poured with
constant and vigorous stirring into a mixture containing acetic acid (8 mL),
water (8 mL), and ice (25 g). A pale white solid was formed, and it was
filtered and purified by column chromatography on silica gel with eluent of
CH2Cl2/MeOH (40 : 1, v/v) to give a pale yellow product (0.7 g, 46%).
TOF MS EIþ: calcd. 256.0848; measured: 256.0841; IR (KBr): 3292, 2831,
1613, 1578, 1560, 1456, 1289, 1094, 1077, 991, 788 cm21
;
1H NMR
3
(CDCl3): d 16.00 (s, br, 1H, HC55C-OH), 8.73 (d, J12,11 ¼ 4.8 Hz, 1H,
3
pyridyl H12), 8.16 [s, 1H, HC55C(O)], 8.13 (d, J9,10 ¼ 6.4 Hz, 1H, pyridyl
3
H9), 8.05 (d, J3,4 ¼ 7.6 Hz, 1H, pyridyl H3), 7.89–7.82 (m, 2H, pyridyl H4,
3
H10), 7.47–7.43 (m, 1H, pyridyl H11), 7.37 (d, J5,4 ¼ 7.6 Hz, 1H, pyridyl
H5), 4.85 [s, H2C(C55O)2], 4.84 (s, 2H, -CH2OH); Anal. calcd. for
C14H12N2O3: C, 65.62; H, 4.72; N, 10.93. Found: C, 65.53; H, 4.76; N, 10.82.

3400
W. Zhang et al.
3-(6-Hydroxymethyl-2-pyridyl)-5-(2-pyridyl)-1H-pyrazole (7)
Compound 6 (510 mg, 2 mmol) and hydrazine hydrate (80%, 0.5 mL) were
dissolved in dry ethanol (30 mL) and refluxed for 3 h. After removing the
solvent, the residue was purified by column chromatography on silica gel
with eluent of CH2Cl2/MeOH (10 : 1, v/v) to give a white solid (451 mg,
90%): mp 187–1898C; TOF MS EI þ : calcd. 252.1011; measured:
252.1013; IR (KBr): 3275, 2875, 1599, 1579, 1449, 1417, 1308, 1176,
1
1157, 1092, 1061, 1001, 965, 786 cm21; H NMR (DMSO): d 13.73 (s, 1H,
-NH), 8.63 (s, 1H, pyridyl H12), 8.0 (d, J ¼ 6.8 Hz, 1H, pyridyl H9), 7.88
(s, br, 3H, pyridyl H3, H4, H10,), 7.45–7.41 (m, 2H, pyridyl H5, pyrazole
H4), 7.36 (s, 1H, pyridyl H11), 5.46 (t, J ¼ 5.6 Hz, 1H, -OH), 4.65
(d, J ¼ 5.6 Hz, 2H, -CH2OH); 13C NMR (DMSO): d 161.6, 152.2, 152.1,
151.1, 149.4, 149.3, 148.0, 146.7, 143.2, 143.1, 137.9, 137.5, 137.3, 136.8,
123.1, 122.7, 120.3, 119.4, 119.2, 118.9, 118.3, 117.4, 102.1, 64.4, 64.1.
Anal. calcd. for C14H12N4O: C, 66.65; H, 4.79; N, 22.21. Found: C, 66.51;
H, 4.93; N, 22.08.
1,3-Bis(6-hydroxymethyl-2-pyridyl)-propane-1,3-dione (8)
The procedure is similar to compound 6 except for using compound 4 instead
of compound 5. The crude product was purified by column chromatography on
silica gel with eluent of CH2Cl2/MeOH (10:1, v/v) to give a pale yellow solid
in 46% yield. ESI-MS (m/z): 287.1 ([M þ H]þ); IR (KBr): 3411, 2924, 2854,
1
1618, 1578, 1448, 1307, 1051, 992, 798 cm21; H NMR (CDCl3): d 8.09
3
(d, J3,4 ¼ 7.6 Hz, 2H, pyridyl H3), 8.08 [s, 1H, CH55C(O)], 7.90
3
3
(t, J4,3 ¼ 3J4,5 ¼ 8.0 Hz, 2H, pyridyl H4), 7.47 (d, J5,4 ¼ 7.6 Hz, 2H,
pyridyl H5), 4.89 (s, 4H, -CH2OH), 4.84 (s, H2C(C55O)2); Anal. calcd. for
.
C15H14N2O4 0.5CH3OH: C, 61.58; H, 5.33; N, 9.27. Found: C, 61.78; H,
5.20; N, 9.34.
3,5-Bis(6-hydroxymethyl-2-pyridyl)-1H-pyrazole (9)
The procedure is similar to that for compound 7. The crude product
was purified by column chromatography on silica gel with eluent of
CH2Cl2/MeOH (5:1, v/v) to give a white solid in 58% yield, mp 195–
1978C; ESI-MS(m/z): 283.1 ([M þ H]þ); IR (KBr): 3258, 2931, 1599,
1578, 1454, 1427, 1306, 1183, 1157, 1070, 1043, 1001, 798 cm21 1H
;
NMR (DMSO): d 13.64 (s, 1H, -NH), 7.90–7.81 (m, 4H, pyridyl H4,H3),
7.43 (s, 1H, pyrazole H4), 7.41 (d, 2H, J ¼ 1.6 Hz pyridyl H5), 5.46
(t, J ¼ 5.6 Hz, 1H, -CH2OH), 5.44 (t, J ¼ 5.6 Hz, 1H, -CH2OH), 4.66
(d, J ¼ 6.0 Hz, 2H, -CH2OH), 4.63 (d, J ¼ 6.0 Hz, 2H, -CH2OH);
13C NMR (CDCl3): d 161.6, 152.2, 151.1, 146.8, 143.1, 137.9, 137.3, 119.4,

b-Diketone-Connected Bipyridine
3401
118.9, 118.2, 117.4, 102.1, 64.4, 64.1; Anal. calcd. for C15H14N4O2: C, 63.82;
H, 5.00; N, 19.85. Found: C, 63.36; H, 5.11; N, 19.46.
ACKNOWLEDGMENTS
The authors are grateful to the Natural Science Foundation of China (No.
20128005 and 20672017), the Swedish Energy Agency, and the Swedish
Research Council for financial support.
REFERENCES
1. (a) Ru¨ttinger, W.; Dismukes, G. C. Synthetic water-oxidation catalysts for artificial
photosynthetic water oxidation. Chem. Rev. 1997, 97, 1–24; (b) Yagi, M.;
Kaneko, M. Molecular catalysts for water oxidation. Chem. Rev. 2001, 101,
21–35; (c) Mukhopadhyay, S.; Mandal, S. K.; Bhaduri, S.; Armstrong, W. H.
Manganese clusters with relevance to photosystem II. Chem. Rev. 2004, 104,
3981–4026.
2. (a) Gersten, S. W.; Samuels, G. J.; Meyer, T. J. Catalytic oxidation of water by an
oxo-bridged ruthenium dimmer. J. Am. Chem. Soc. 1982, 104, 4029–4030;
(b) Roecker, L.; Kutner, W.; Gilbert, J. A.; Simmons, M.; Murray, R. W.;
Meyer, T. J. Instability of the oxidation catalysts [(bpy)2(py)Ru(O)]2þ and
[(trpy)(phen)Ru(O)I2þ in basic solution. Inorg. Chem. 1985, 24, 3784–3791.
´
3. (a) Sens, C.; Romero, I.; Rodryguez, M.; Llobet, A.; Parella, T.; Benet-
Buchholz, J. A new Ru complex capable of catalytically oxidizing water to
molecular dioxygen. J. Am. Chem. Soc. 2004, 126, 7798–7799; (b) Zong, R.;
Thummel, R. P. A new family of Ru complexes for water oxidation. J. Am.
Chem. Soc. 2005, 127, 12802–12803.
´
´
4. (a) Pons, J.; Lopea, X.; Casabo, J. Copper complexes with a pyrazole derivative
ligand: Crystal structure of tetrakisf[3,5-bis(pyridin-2-yl)-pyrazolate-(aqua)cop-
per(II)]nitrate monohydrateg. Inorg. Chim. Acta 1992, 195, 61–66; (b) Pons, J.;
´
Sanchez, F. J.; Labarta, A. Mixed bridged, dinuclear copper(II) complexes with
dinucleating, pyrazole derived ligands. Inorg. Chim. Acta 1993, 208, 167–171;
(c) Ni-iya, K.; Fuyuhiro, A.; Yagi, T.; Nasu, S.; Kuzushita, K.; Morimoto, S.;
Kaizaki, S. Synthesis, stereochemistry, and spin transition of heterometal
dinuclear Fe(II)-Cr(II) complex bridged by 3,5-bis(pyridin-2-yl)-pyrazolate.
Bull. Chem. Soc. Jpn. 2001, 74, 1891–1897; (d) Catalano, V. J.; Craig, T. J. Mono-
metallic and dimetallic ruthenium(II)-terpyridine complexes employing the
tetradentate ligands dipyridylpyrazolyl, dipyridyloxadiazole, and their dimethyl
´
derivatives. Inorg. Chem. 2003, 42, 321–334; (e) Sens, C.; Rodryguez, M.;
Romero, I.; Llobet, A. Synthesis, structure, and spectroscopic, photochemical,
redox, and catalytic properties of ruthenium(II) isomeric complexes containing
dimethyl sulfoxide, chloro, and the dinucleating bis(2-pyridyl)pyrazole ligands.
Inorg. Chem. 2003, 42, 2040–2048.
5. Ball, P. W.; Blake, A. B. Magnetic properties of polynuclear complex, Part 1:
Superexchange in some binuclear Nickel(II) complexes. J. Chem. Soc. A 1969,
1415–1422.

3402
W. Zhang et al.
6. Hull, K. G.; Visnick, M.; Tautz, W.; Sheffron, A. Synthesis of Ro 25–8210 via
an enantioselective oxazaborolidine-catalyzed reduction. Tetrahedron 1997, 53,
12405–12414.
7. Fife, T. H.; Przystas, T. J. Effects of divalent metal ions on hydrolysis of esters of
2-(hydroxymethyl)picolinic acid; Metal ion catalysis of the carboxyl, hydroxide
ion, and water reactions. J. Am. Chem. Soc. 1982, 104, 2251–2257.
8. Amin, H. B.; Taylor, R. Electrophilic aromatic reactivities via pyrolysis of
1-arylethyl esters, part 19: Substituent effects in pyridine. J. Chem. Soc., Perkin
Trans. 2 1979, 5, 624–627.
9. Katsura, Y.; Inoue, Y.; Tomishi, T.; Ishikawa, H.; Takasugi, H. Studies on
antiulcer drug, 7 : 2-Guanidino-4-pyridylthiazoles as histamine H2-receptor
antagonists with potent gastroprotective effects against nonsteroidal antiinflam-
matory drug-induced injury. J. Med. Chem. 1994, 37, 57–66.