Preparation and characterization of 4-Amino-2-Anilinopyridine and its chlorodiruthenium(III,II) complex

Article abstract of DOI:10.1080/00958972.2014.957199

A new bridging agent, 4-Amino-2-Anilinopyridine (aap), was synthesized and used as an equatorial ligand in the preparation of a diruthenium complex (4,0) Ru₂(aap)₄Cl. Both the ligand and the diruthenium complex were characterized by thermal analysis, MALDI-TOF mass spectrometry, IR and NMR (¹H and ¹³C) spectroscopy, and X-ray crystallography. The structural analysis revealed that the complex existed as a (4,0) isomer, in which the amino group on the pyridyl moiety was not involved in chemical bonding.

Full text of DOI:10.1080/00958972.2014.957199

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Preparation and characterization of
4-amino-2-anilinopyridine and its
chlorodiruthenium(III,II) complex
Naga V. Naidua, Hadi Armanb, Yuanjian Dengc & Xin Weic
a Department of Environmental and Interdisciplinary Sciences,
Texas Southern University, Houston, TX, USA
b Department of Chemistry, University of Texas at San Antonio,
San Antonio, TX, USA
c Department of Chemistry, Texas Southern University, Houston,
TX, USA
Accepted author version posted online: 28 Aug 2014.Published
online: 23 Sep 2014.
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To cite this article: Naga V. Naidu, Hadi Arman, Yuanjian Deng & Xin Wei (2014) Preparation and
characterization of 4-amino-2-anilinopyridine and its chlorodiruthenium(III,II) complex, Journal of
Coordination Chemistry, 67:18, 3006-3017, DOI: 10.1080/00958972.2014.957199
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 18, 3006–3017, http://dx.doi.org/10.1080/00958972.2014.957199
Preparation and characterization of 4-amino-2-
anilinopyridine and its chlorodiruthenium(III,II) complex
NAGA V. NAIDU†, HADI ARMAN‡, YUANJIAN DENG*§ and XIN WEI*§
†Department of Environmental and Interdisciplinary Sciences, Texas Southern University, Houston,
TX, USA
‡Department of Chemistry, University of Texas at San Antonio, San Antonio, TX, USA
§Department of Chemistry, Texas Southern University, Houston, TX, USA
(Received 19 March 2014; accepted 21 July 2014)
A new bridging agent, 4-amino-2-anilinopyridine (aap), was synthesized and used as an equatorial
ligand in the preparation of a diruthenium complex (4,0) Ru2(aap)4Cl. Both the ligand and the diru-
thenium complex were characterized by thermal analysis, MALDI-TOF mass spectrometry, IR and
NMR (1H and 13C) spectroscopy, and X-ray crystallography. The structural analysis revealed that
the complex existed as a (4,0) isomer, in which the amino group on the pyridyl moiety was not
involved in chemical bonding.
Keywords: 4-Amino-2-anilinopyridine; Diruthenium complex; X-ray; Thermal analysis; NMR
*Corresponding authors. Email: deng_yj@tsu.edu (Y. Deng); wei_xi@tsu.edu (X. Wei)
© 2014 Taylor & Francis

Studies on a diruthenium(III,II) complex
3007
1. Introduction
Diruthenium complexes have been widely studied due to their catalytic properties [1],
potential anticancer activity [2], and rich redox chemistry [3–8]. One group of mixed-
valent diruthenium complexes with a general formula Ru2(Le)4(La)x (where Le is an
equatorial chelating agent, La is an axial ligand, and x = 1 or 2) has been extensively
investigated [5–13]. A paddle wheel like structure of the diruthenium complex is shown
in chart 1. Common equatorial ligands include acetate [9], anilinopyridinate (ap) and
substituted-ap anions [10], N,N′-dimethylbenzamidinate (dmba) [11], N,N′-diphenylform-
amidinate (dpf) [12], and 2,2′-dipyridylamide (dpa) [13]. Studies have shown that axial
ligands have greater impact on the molecular structures and redox properties of the diru-
thenium complexes, whereas variations of the equatorial ligands have more subtle effects
on the redox properties [10–13]. Most of the studies on diruthenium complexes that
contain ap or substituted-ap bridging ligands have focused on the influence of aniline
substitution on the structure and chemical properties of the complexes. Since ap is an
unsymmetrical ligand, diruthenium complexes with four identical ap ligands may exist in
up to four different isomeric forms, which are commonly designated as the (4,0), (3,1),
(2,2)-cis, and (2,2)-trans conformations depending on whether the pyridyl nitrogen (Np)
and the aniline nitrogen (Na) of different ap ligands are on the same plane or not.
Recently a new diruthenium complex containing the pyridyl substitution with a methyl
group on the para position has been reported [14]. This complex was determined to be
a (3,1) isomer. We are intrigued by the fact that methyl is a weak electron-donating
group and what would happen if the pyridyl substitution involves a stronger electron-
donating group such as an amino group. Also, adding an amino group to the pyridyl
moiety further expands the functionality of the complex. It is thus anticipated that fine
tuning the molecular structure and subsequently the physiochemical properties would
become possible. Indeed, replacing the methyl group attached to the pyridyl moiety with
an amino group, resulted in the formation of a (4,0) isomer rather than the (3,1) isomer,
as reported [14].
Chart 1. Schematic representation of a paddle like structure of diruthenium complexes.

3008
N.V. Naidu et al.
2. Experimental
2.1. Materials and measurements
ACS grade aniline, 4-amino-2-chloropyridine (acp), lithium chloride, ruthenium(III) chlo-
ride trihydrate, isopropanol, acetonitrile, trifluoroacetic acid, methanol, and dichloromethane
were purchased from Sigma-Aldrich Chemical Co. Deuterated DMSO-d6 was supplied by
Cambridge Isotope Laboratories, Inc. Sinapinic acid used as the matrix in MALDI-MS anal-
ysis was purchased from ProteoChem Inc. All the chemicals were used as received. The
precursor Ru2(CH3COO)4Cl was synthesized according to the reported method [15].
Melting point was determined by differential scanning calorimetry (DSC) using a Q2000
DSC (TA Instruments) at a heating rate of 20 °C min−1 with N2 purge (flow rate:
50 mL min−1). Approximately, 10 mg samples were contained in crimped aluminum DSC
pans and lids. Thermogravimetric analysis (TGA) was performed on a Q5000 IR TGA (TA
Instruments) at a heating rate of 20 °C min−1 in air (air flow rate: 25 mL min−1). In a
platinum TGA pan, 5–10 mg samples were placed. Mass spectra were obtained with Bruker
Daltonics Microflex MALDI-TOF mass spectrometer equipped with a nitrogen laser
(λ = 337 nm). The matrix was prepared by adding a solution of sinapinic acid and trifluoro-
acetic acid into a mixture of acetonitrile, methanol, and deionized water. Then the analyte
was dissolved in the matrix, spotted on the target plate, and dried under a stream of nitro-
1
gen. The MALDI-TOF was validated using acp as the standard prior to sample analysis. H
and 13C NMR spectra were obtained on a JEOL 400 MHz spectrometer. The NMR experi-
ments on the aap ligand were conducted in deuterated dimethylsulfoxide (DMSO-d6), on
which signals were locked. The NMR spectra of the starting materials (acp and aniline)
1
were also collected for the purpose of validation. Chemical shifts for H and 13C NMR
spectra were referenced to the instrument’s locked values for DMSO-d6 at 2.50 and
39.52 ppm, respectively [16].
2.2. Synthesis
2.2.1. Synthesis of 4-amino-2-anilinopyridine (aap). Acp (0.500 g, 2.89 mmol) was dis-
solved in excess aniline (12.3 mL, 101 mmol) in a dried 50 mL round bottom flask equipped
with a condenser. The mixture was refluxed overnight under N2. After the mixture was
cooled to room temperature, 20 mL of 10 wt. % NaOH was added to the flask, and the
mixture was stirred for 15 min to neutralize the produced HCl. The content was then trans-
ferred to a 250 mL round-bottom flask containing 125 mL water. The water and most of the
unreacted aniline were removed using a rotovap at 60 °C. The crude product was transferred
to a beaker and stirred with hot water, followed by decanting the aqueous phase that con-
tained NaOH, NaCl, or unreacted aniline. The hot water treatment was repeated a few more
times until a pale brown solid was formed and later determined to be the aap product. The
yield was approximately 83% (0.44 g). The ligand was soluble in isopropanol, methanol, and
CH2Cl2, but insoluble in water. Crystals suitable for X-ray diffraction analysis were obtained
by slow evaporation of the aap solution of methanol : water (9 : 1 v/v) at room temperature.
Anal. Calcd for C11H11N3 (%): C, 71.32; H, 5.99; N, 22.69. Found: C, 71.48; H, 6.04; N,
22.68. IR bands (cm−1): 3400m, 3330s, 3140m, 1600s, 1526m, 1442s, 1303sp, 1230w,
1
974sp, 802s, 747sp, 689sp. H NMR (DMSO-d6): δ 8.45 (s, 1H), 7.66 (d, 1H, J = 6.0 Hz),
7.57 (dd, 2H, J1 = 8.0 Hz, J2 = 1.4 Hz), 7.18 (t, 2H, J = 7.5 Hz), 6.78 (t, 1H J = 7.3 Hz),

Studies on a diruthenium(III,II) complex
3009
6.01 (dd, 1H, J1 = 5.7 Hz, J2 = 1.8 Hz), 5.97 (d, 1H, J = 1.8 Hz), 5.71 (s, 2H); 13C NMR
(DMSO-d6): δ 156.8, 155.2, 147.3, 142.6, 128.4, 119.4, 117.8, 103.2, 92.5.
2.2.2. Synthesis of (4,0) Ru2(aap)4Cl. Aap (1.56 g, 8.44 mmol) was dissolved in 20 mL
of isopropanol. To this solution, 0.500 g of Ru2(CH3COO)4Cl (1.05 mmol) was added and
the mixture was refluxed overnight under N2. After cooling to room temperature, the mix-
ture was filtered. The solid was washed with three portions of 20 mL of isopropanol and
dried in vacuo. The impure greenish solid was then dissolved in acetone and filtered again.
The filtrate was collected and left for evaporation at room temperature. Dark green solid
was recovered as the title complex, with 72% (0.74 g) yield. The complex was further puri-
fied by silica gel column chromatography using acetone/hexane (7 : 3 v/v) as eluant; only a
single green band was observed. Slow evaporation of the complex solution in acetone/tolu-
ene media yielded a green crystal suitable for X-ray analysis. The complex was soluble in
DMSO and dichloromethane. IR bands (cm−1): 3300w, 1605s, 1480s, 1435sp, 1245sp,
1196sp, 985s, 877s, 696sp.
2.3. X-ray crystallography
Single crystals of C11H11N3 were selected and mounted on a Rigaku Saturn724+ (2 × 2 bin
mode) diffractometer. The crystal was kept at 295 K during data collection. Using Olex2
[17], the structure was solved with the SHELXS [18] structure solution program using
direct methods and refined with the SHELXL [19] refinement package using least squares
minimization. Two molecules were seen in the asymmetric unit with one molecule of the
two disordered between two positions. The disordered molecules were modeled with
restrained distances and thermal parameters.
A green prism of [2(C44H40ClN12Ru2)]·2C3H6O·C7H8 having approximate dimensions
of 0.20 × 0.12 × 0.10 mm was mounted on a MicroMount. All measurements were
made on a Rigaku AFC12/Saturn724 CCD fitted with Mo Kα radiation at −175 2 °C,
using the ω scan technique to a maximum 2θ value of 55.0°. Cell constants and an ori-
entation matrix for data collection obtained from a least-squares refinement using 9523
reflections in the range of 4.0° < 2θ < 80.2° corresponded to a primitive triclinic cell.
For Z = 1 and FW = 2157.23, the calculated density is 1.503 g/cm3. Based on a statisti-
cal analysis of intensity distribution, and the successful solution and refinement of the
structure, the space group was determined to be P-1. Of the 14,313 reflections that were
collected, 8356 were unique (Rint = 0.08). The data were corrected for Lorentz and
polarization effects.
Structure solution and refinement: The data-set was corrected for absorption based on
multiple scans [20] and reduced using standard methods [21]. The structure was solved by
direct methods with SHELXS-97 [18] and refined by a full-matrix least-squares procedure
on F2 using SHELXL-97 [19] with anisotropic displacement parameters for non-hydrogen
atoms and a weighting scheme of the form w = 1/[σ2(Fo ) + aP2 + bP], where P =
2
2
2
(Fo + 2Fc )/3. All hydrogens were placed on calculated positions, with an isotropic
displacement parameter amounting to 1.2–1.5 times the value of the equivalent isotropic
displacement parameter of the respective carrier atom. Pertinent crystallographic data for
the complex are given in table 1.

3010
N.V. Naidu et al.
Table 1. Crystallographic data and structure refinement for aap and [2(C44H40ClN12Ru2)]·2C3H6O·C7H8.
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal color, shape
Crystal dimensions (mm)
Crystal system
Space group
Unit cell dimensions
a (Å)
C11H11N3
185.23
295.15
C101H100N24O2Cl2Ru4
2157.23
−175 °C
0.71075
0.71075
White, needle
0.16 × 0.11 × 0.1
Monoclinic
Cc
Green, prism
0.20 × 0.12 × 0.10
Triclinic
P-1
5.934(4)
10.652(5)
b (Å)
23.689(15)
12.916(5)
c (Å)
13.716(9)
19.373(8)
α (°)
90
72.97(3)
β (°)
99.146
82.49(3)
γ (°)
90
1904(2)
69.346(19)
2383.6(18)
V (Å3)
Z
8
1
DCalcd (g cm−3
F (0 0 0)
)
1.293
784
0.081
1.503
1100
0.742
μ (Mo Kα, cm−1
)
θ Range for data collection (°)
Reflections collected/unique
Max. and min. transmission
Refinement method
1.719–25.349
7899/3469 (Rint =0.0662)
0.992 and 0.987
Full-matrix least-squares on F2
3469/285/380
1.008
4.7–50.04
14,313/8356 (Rint =0.077)
1.000 and 0.514
Full-matrix least-squares on F2
8356/619/0
1.143
[I > 2σ(I)]
Data/restraints/parameters
Goodness-of-fit on F2
Final R indicesa
[I > 2σ(I)]
R1 =0.0658, wR2 =0.1383
R1 =0.1359, wR2 =0.1629
0.128 and −0.135
R1 =0.066, wR2 =0.1603
R1 =0.0750, wR2 =0.1647
1.93 and −1.83
R indices (all data)
Largest difference peak and hole (e Å−3
)
aR1 = Σ||Fo| − |Fc||/Σ|Fo|, wR2 =[Σ(w(Fo2 – Fc2)2)/Σ w(Fo2)2]1/2
.
3. Results and discussion
3.1. Synthesis
The synthetic routes from the starting materials, aniline, and acp, to aap, and finally to
Ru2(aap)4Cl are outlined in chart 2.
To obtain pure aap, the aniline must be in exceedingly excessive amount. Even at a 10 : 1
aniline-to-acp molar ratio, the resulting products were a mixture of the desired aap and its
derivatives that contained various numbers of amino pyridine moieties, as evidenced by the
MALDI-MS spectrum [see figure 1(a)]. Under the current experimental conditions, a 35 : 1
aniline-to-acp molar ratio was required to obtain pure aap [figure 1(b)]. Thus, careful
control of the aniline-to-acp molar ratio is needed to obtain aap.
Although toluene has been used as a solvent in preparing similar complexes [14], it is
not suitable for the present work because neither the aap nor the Ru2(CH3COO)4Cl precur-
sor is soluble in this solvent. As a result, this heterogeneous reaction becomes lengthy and
requires high temperature. Moreover, the complex product may not dissolve in toluene; it
would be a challenge to separate and purify the final product using chromatographic
method. In this work, isopropanol was found to be an ideal solvent, in which both the aap
and the precursor could be dissolved, while the complex product would simply precipitate.
Therefore, when isopropanol was used as the solvent, the synthesis became a homogeneous
reaction, which could be accomplished at lower temperatures and in much shorter time. The

Studies on a diruthenium(III,II) complex
3011
NH2
NH2
NH2
Reflux
overnight
Ru2(CH3COO)4Cl
+
+
HCl
N
Cl
N
N
H
aniline
4-amino-2-chloropyridine (acp)
4-amino-2-anilinopyridine (aap)
Reflux in
isopropanol
overnight
NH2
NH2
N
N
N
N
Cl
Ru
Ru
N
N
N
N
NH2
NH2
Ru2(aap)4Cl
Chart 2. Synthetic routes to aap and Ru2(aap)4Cl.
diruthenium product could be conveniently collected through filtration. Although four iso-
mers are possible, only the (4,0) isomer was obtained in the present study. Kadish et al.
reported a diruthenium complex that contained the modified ap ligand involving pyridyl
substitution with methyl group at the para position, which yielded a diruthenium complex
with (3,1) isomeric form [14].
3.2. Thermal analysis
The new aap ligand has a m.p. of 96.0 °C, 3 °C higher than that of acp. But no m.p. of the
complex could be detected before it underwent thermal degradation, due to its high
molecular mass. Figure 2 exhibits the TGA thermograms of aap and Ru2(aap)4Cl. Unlike
Ru2(CH3COO)4Cl, which lost all the ligands in a very narrow temperature range of
310–340 °C (not shown), Ru2(aap)4Cl underwent a multiple-step degradation from 258 to

3012
N.V. Naidu et al.
Figure 1. MALDI-MS spectra of products obtained by mixing aniline with acp in molar ratios of (a) 10 : 1,
(b) 35 : 1, and (c) Ru2(aap)4Cl.
Figure 2. TGA thermograms of (a) aap and (b) Ru2(aap)4Cl.
366 °C, losing all the ligands except chlorine. Further heating to 1000 °C left the residue of
ruthenium metal with a composition of 20.0%, which was in agreement with that calculated
(20.7%) for Ru2(aap)4Cl, proving the successful synthesis of the complex.

Studies on a diruthenium(III,II) complex
3.3. Spectroscopic characterization
3013
In order to determine the optimized aniline-to-acp molar ratio for the synthesis of aap, the
MALDI-TOF mass spectrometry was employed. When the aniline-to-acp molar ratio was
10 to 1, the reaction between the two starting materials became very complex. As can be
seen from figure 1(a), the mass difference between the base peak for aap (m/z 184.8 Da)
and other peaks was simply an integral multiple of the mass of aminopyridine moiety
[C5H3N2]+ (m/z 91.5 Da). However, at much higher aniline-to-acp molar ratio (35 : 1), the
MALDI spectrum for the reaction system showed only one peak at m/z 184.8 Da with little
fragmentation [figure 1(b)], indicating the formation of aap ligand. Figure 1(c) presents the
mass spectrum of the diruthenium complex Ru2(aap)4Cl. MALDI analysis yielded two
groups of peaks with highest abundance in each group at m/z 933.8 and 973.5 Da, respec-
tively. Each group shows a pattern typical of isotopic ruthenium distribution, being similar
to other mono- or diruthenium complexes [22]. The two peaks might be attributed to frag-
ments [M – Cl – 3H2]+ and [M – H]+, respectively, although the assignments looked
uncommon. It is the latter mass and the isotopic pattern which provides an important evi-
dence to confirm the 4 : 2 ligand-to-metal molar ratio in the formula.
The aap ligand exhibits several NH stretches in the 3150–3400 cm−1 region. Upon
coordination to ruthenium, these NH vibrations appear in the same region, suggesting
that the NH2 group on the pyridyl moiety remains uncoordinated. A comparison of the
1H NMR spectra of aap and the starting materials further reveals the formation of the
1
aap ligand. In general, H NMR frequencies of the aniline moiety in aap shifted down-
field relative to pure aniline. In contrary, the proton signals of the pyridine moiety,
except the amino protons, shifted upfield after its conjugation with the aniline that is a
molecule carrying relatively higher electron density. Replacement of one proton on the
NH2 group of aniline by pyridine resulted in the largest downfield shift of the NH pro-
ton from 4.99 to 8.55 ppm, indicating that the electron density on nitrogen was largely
shared by the pyridine. However, no systematic shifts were observed in their 13C NMR
spectra. Carbons attached to the amino groups in the aniline and pyridine, respectively,
shifted in opposite directions, one from 148.6 to 142.6 ppm and the other from 150.8 to
155.2 ppm, further substantiating the mutual influence due to the formation of a larger
conjugated π system.
3.4. X-ray analysis
The stoichiometry of the diruthenium complex was also supported by single-crystal X-ray
diffraction. ORTEP diagrams of aap and Ru2(aap)4Cl with the atom numbering are shown
in figure 3(a) and (b), respectively. Selected bond lengths, bond angles, and torsion angles
for aap and Ru2(aap)4Cl are summarized in table 2. Similar to other diruthenium com-
plexes, the geometries around Ru1 and Ru2 were octahedral and square pyramidal, respec-
tively. As revealed by the structure, the diruthenium complex is a (4,0) isomer, where Ru1
is coordinated with four pyridyl nitrogens (Np) and one chloride, and Ru2 is coordinated
with four aniline nitrogens (Na). The Ru–Ru bond length is 2.2901(13) Å, which is almost
the same as reported for similar diruthenium complexes with other substituted-ap ligands
[23, 24]. The Ru–Cl bond distance is 2.4947(19) Å, comparable to that in the methyl-
substituted complex (2.4770(13) Å) [14]. All these values agree well with previously
reported diruthenium complexes with four identical substituted-ap bridging ligands [10].

3014
N.V. Naidu et al.
Figure 3. ORTEP representations of (a) aap and (b) Ru2(aap)4Cl (H atoms omitted for clarity). Ellipsoids are at
50% probability.
Table 2. Selected bond lengths, bond angles, and torsion
angles for aap and Ru2(aap)4Cl.
Ru2(aap)4Cl
aap
Bond lengths (Å)
Ru1–Ru2
Ru1–Cl1
Ru1–N7
Ru1–N1
Ru1–N10
Ru1–N4
Ru2–N3
Ru2–N9
Ru2–N6
Ru2–N12
N1–C5
2.2901(13)
2.4947(19)
2.092(5)
2.097(5)
2.113(5)
2.118(5)
2.045(5)
2.049(5)
2.052(5)
2.062(5)
1.385(7)
1.332(8)
1.437(8)
1.31(2)
1.42(3)
1.41(3)
N3–C5
N3–C6
Bond angles (°)
Ru2–Ru1–Cl
N1–Ru1–N7
N1–Ru1–N10
N1–Ru1–N4
N7–Ru1–N4
N1–Ru1–Ru2
C1–N1–C5
179.39(3)
91.1(2)
86.5(2)
176.4(2)
88.0(2)
88.17(14)
117.4(5)
116.5(5)
119.8(5)
112.7(18)
117.2(2)
128.8(15)
N1–C5–N3
C5–N3–C6
Torsion angles (°)
Cl1–Ru1–Ru2–N3
N1–Ru1–Ru2–N3
N7–Ru1–Ru2–N3
N1–C5–N3–C6
62(4)
−17.4(2)
108.5(2)
164.4(6)
2.7(8)

Studies on a diruthenium(III,II) complex
3015
Upon complexation, the bond lengths involving pyridyl nitrogen N1–C1 and N1–C5 of
aap were lengthened (see table 2). However, the bond lengths involving the aniline nitro-
gen N3–C5 and N3–C6 were shortened. As a result, the bond angle of C1–N1–C5 of the
pyridyl group increased from 112.7(18)° to 117.4(5)°, while the bond angle of C5–N3–C6
of the aniline moiety decreased from 128.8(18)° to 119.8(5)°. Concomitant changes in
bond lengths and angles are due to formation of the five-member chelate ring. The bond
angle of Ru2–Ru1–Cl [179.39(4)°] is closer to linear than that in the methyl-substituted
complex (177.64(4)°) [14]. Other bond angles, such as Np–Ru1–Cl, Ru2–Ru1–Np or Ru2–
Ru1–Na, are in agreement with the expected geometry around Ru1 and Ru2, respectively.
It seems that the stronger electron-donating power of the amino group on the pyridine
results in adoption of a (4,0) configuration rather than the (3,1) fashion. Intermolecular
hydrogen bonds are also observed in the crystal structure. Although the hydrogen bond
between N11–H and Cl1 (aminopyridyl, HÁ Á ÁCl, 2.76 Å; N–HÁ Á ÁCl 157°) is rather weak, it
is this hydrogen bond that causes the two diruthenium complexes to interact in a head-on
fashion, i.e. their chloro ligands pointing to each other (see figure 4). The solvated acetone
molecules then bring the two diruthenium complexes together through the formation of
stronger hydrogen bonds between N8–H and O1 (aminopyridyl, HÁ Á ÁO, 2.34 Å; N–HÁ Á ÁO
140°) and between N2–H and O1′ (aminopyridyl, HÁ Á ÁO, 2.45 Å; N–HÁ Á ÁO 137°). The
uncoordinated aap is basically planar (torsional angle 2.7°). However, when aap is
coordinated to ruthenium, the ligand becomes twisted by approximately 15° from planarity
(torsional angle 165°).
Figure 4. Packing diagram of the Ru2(aap)4Cl complex, viewed down the a-axis.

3016
N.V. Naidu et al.
4. Conclusion
In this paper, we present the synthesis and characterization of a new bridging ligand,
4-amino-2-anilinopyridine (aap) and its diruthenium complex (4,0) Ru2(aap)4Cl. The aap
was obtained by refluxing a mixture of aniline/4-amino-2-chloropyridine with a 35 : 1 molar
ratio. The (4,0) Ru2(aap)4Cl was prepared in isopropanol by reacting aap with Ru2(CH3-
COO)4Cl (8 : 1 molar ratio) under N2. Both compounds were characterized by thermal anal-
ysis, MALDI-TOF mass spectrometry, IR and NMR (1H and 13C) spectroscopy, and X-ray
crystallography. The structural analysis revealed that the complex exists as a (4,0) isomer,
in which the amino group on the pyridyl moiety was not involved in chemical bonding.
Supplementary material
Crystallographic data for the aap ligand and its diruthenium complex have been deposited
with the Cambridge Crystallographic Data Center, CCDC No. 965916 and 965917, respec-
tively. Copies of the data can be obtained, free of charge, on application to The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44 1223/336 033 or E-mail:
deposit@ccdc.cam.ac.uk).
Acknowledgements
We would like to thank Dr. Tuan D. Phan for his participation in discussions at the early
stage of this research. We are grateful to Dr. Lee M. Daniels (Rigaku Americas Corp., 9009
New Trails Drive, The Woodlands, Texas, USA) for taking the initial X-ray structure data
for the aap ligand and Dr. Joseph H. Reibenspies (Department of Chemistry, X-ray Diffrac-
tion Laboratory, Texas A&M University, College Station, Texas, USA) for refining the data.
X.W. would like to acknowledge the grant funding support from NSF-CREST Program.
N.V.N. is grateful to the Title III fellowship program from Texas Southern University for
financial assistance.
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