1-(2′-Pyridylazo)-2-naphtholate complexes of ruthenium: Synthesis, characterization, and DNA binding properties

Article abstract of DOI:10.1016/j.poly.2008.05.023

Reaction of 1-(2′-pyridylazo)-2-naphthol (Hpan) with [Ru(dmso)₄Cl₂] (dmso = dimethylsulfoxide), [Ru(trpy)Cl₃] (trpy = 2,2′,2″-terpyridine), [Ru(bpy)Cl₃] (bpy = 2,2′-bipyridine) and [Ru(PPh₃)₃Cl₂] in refluxing ethanol in the presence of a base (NEt₃) affords, respectively, the [Ru(pan)₂], [Ru(trpy)(pan)]⁺ (isolated as perchlorate salt), [Ru(bpy)(pan)Cl] and [Ru(PPh₃)₂(pan)Cl] complexes. Structures of these four complexes have been determined by X-ray crystallography. In each of these complexes, the pan ligand is coordinated to the metal center as a monoanionic tridentate N,N,O-donor. Reaction of the [Ru(bpy)(pan)Cl] complex with pyridine (py) and 4-picoline (pic) in the presence of silver ion has yielded the [Ru(bpy)(pan)(py)]⁺ and [Ru(bpy)(pan)(pic)]⁺ complexes (isolated as perchlorate salts), respectively. All the complexes are diamagnetic (low-spin d⁶, S = 0) and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a Ru(II)-Ru(III) oxidation on the positive side of SCE. Except in the [Ru(pan)₂] complex, a second oxidative response has been observed in the other five complexes. Reductions of the coordinated ligands have also been observed on the negative side of SCE. The [Ru(trpy)(pan)]ClO₄, [Ru(bpy)(pan)(py)]ClO₄ and [Ru(bpy)(pan)(pic)]ClO₄ complexes have been observed to bind to DNA, but they have not been able to cleave super-coiled DNA on UV irradiation.

Full text of DOI:10.1016/j.poly.2008.05.023

Polyhedron 27 (2008) 2943–2951
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
1-(20-Pyridylazo)-2-naphtholate complexes of ruthenium: Synthesis,
characterization, and DNA binding properties
Semanti Basu a, Sarmistha Halder a, Indrani Pal a, Saheli Samanta b, Parimal Karmakar b, Michael G.B. Drew c,
Samaresh Bhattacharya a,
*
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Raja S. C. Mullick Road, Kolkata, West Bengal 700 032, India
b Department of Life Science and Biotechnology, Jadavpur University, Kolkata 700 032, India
c Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Reaction of 1-(20-pyridylazo)-2-naphthol (Hpan) with [Ru(dmso)4Cl2] (dmso = dimethylsulfoxide),
[Ru(trpy)Cl3] (trpy = 2,20,200-terpyridine), [Ru(bpy)Cl3] (bpy = 2,20-bipyridine) and [Ru(PPh3)3Cl2] in
refluxing ethanol in the presence of a base (NEt3) affords, respectively, the [Ru(pan)2], [Ru(trpy)(pan)]+
(isolated as perchlorate salt), [Ru(bpy)(pan)Cl] and [Ru(PPh3)2(pan)Cl] complexes. Structures of these
four complexes have been determined by X-ray crystallography. In each of these complexes, the pan
ligand is coordinated to the metal center as a monoanionic tridentate N,N,O-donor. Reaction of the [Ru(b-
py)(pan)Cl] complex with pyridine (py) and 4-picoline (pic) in the presence of silver ion has yielded the
[Ru(bpy)(pan)(py)]+ and [Ru(bpy)(pan)(pic)]+ complexes (isolated as perchlorate salts), respectively. All
the complexes are diamagnetic (low-spin d6, S = 0) and show characteristic 1H NMR signals and intense
MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a Ru(II)–Ru(III)
oxidation on the positive side of SCE. Except in the [Ru(pan)2] complex, a second oxidative response
has been observed in the other five complexes. Reductions of the coordinated ligands have also been
observed on the negative side of SCE. The [Ru(trpy)(pan)]ClO4, [Ru(bpy)(pan)(py)]ClO4 and [Ru(bpy)
(pan)(pic)]ClO4 complexes have been observed to bind to DNA, but they have not been able to cleave
super-coiled DNA on UV irradiation.
Article history:
Received 18 January 2008
Accepted 22 May 2008
Available online 1 July 2008
Keywords:
1-(20-Pyridylazo)-2-naphthol
Ruthenium complexes
DNA binding properties
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
as well as the azo-nitrogen are soft in nature and hence stabilize
ruthenium(II) [4], while the phenolate-oxygen is hard in nature
The chemistry of ruthenium has been attracting considerable
current interest [1], largely because of the fascinating photochem-
ical, photophysical and redox properties exhibited by complexes
of this metal. As all these properties are primarily directed by
the coordination environment around the metal center, complex-
ation of ruthenium by ligands of selected types is of significant
importance, and the present study has originated from our inter-
est in this area [2]. Herein we have selected 1-(20-pyridylazo)-2-
naphthol (1) as the principal ligand, which has been abbreviated
as Hpan, where H stands for the potentially dissociable phenolic
proton. This ligand is known to bind to metal centers, via dissoci-
ation of the acidic proton, as a tridentate N,N,O-donor forming
two adjacent five-membered chelate rings (2) [3]. It is interesting
to note that out of the three donor atoms, the pyridine-nitrogen
and is a recognized stabilizer of higher oxidation states of ruthe-
nium [5]. Therefore coordination of ruthenium by ligand (1) is ex-
pected to impart interesting redox properties in the resulting
complexes. The main objective of the present study has been to
synthesize a series of ruthenium complexes of this selected ligand
(1), and study their spectral and electrochemical properties. It
may be relevant to mention here that though chemistry of com-
plexes of ligand (1) with many other metals has been studied
well [3], that with ruthenium appears to have received only mar-
ginal attention [6]. In the present work, reactions of ligand (1)
have been carried out with four different ruthenium starting
materials, which have afforded the homoleptic and three hetero-
leptic complexes. Two more mixed-ligand complexes have also
been derived from one of the three heteroleptic complexes by fur-
ther reaction. An account of the chemistry of all these complexes
is presented in this paper, with special reference to their synthe-
sis, structure and, spectral, electrochemical and DNA binding
properties.
* Corresponding author. Tel.: +91 33 2414 6223; fax: +91 33 2414 6584.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.05.023

2944
S. Basu et al. / Polyhedron 27 (2008) 2943–2951
NaClO4 (0.5 mL) was added, whereby a brown precipitate is ob-
tained, which was collected by filtration, washed with cold water,
and dried in vacuo over P4O10. Yield: 100 mg (65%). Anal. Calc. for
N
N
N
N
M
N
OH
N
O
C
30H21N6O5ClRu: C, 52.82; H, 3.08; N, 12.32. Found: C, 52.94; H,
1
3.14; N, 12.25%. H NMR in CDCl3, d ppm: 6.50–6.72 (2H*); 6.77
(t, 1H, J = 6.1); 7.2–8.0 (10H*); 8.08 (d, 2H, J = 5.5); 8.31 (t, 1H,
J = 8.0); 8.47 (d, 2H, J = 8.0); 8.72 (d, 2H, J = 8.0); 10.14 (d, 1H,
J = 8.6).
2.2.3. [Ru(bpy)(pan)Cl]
Hpan (70 mg, 0.28 mmol) was dissolved in ethanol (30 mL) and
to it was added triethylamine (50 mg, 0.50 mmol). Then [Ru(b-
py)Cl3] (100 mg, 0.28 mmol) was added to the mixture and it
was refluxed for 6 h to afford a brown solution. The solvent was
then evaporated and the solid mass obtained was purified by pre-
parative thin layer chromatography on a silica plate using 1:5 ace-
tonitrile–benzene as the eluant. A brown band separated, which
was extracted with acetonitrile and evaporation of the extract
afforded [Ru(bpy)(pan)Cl] as a green crystalline solid. Yield:
104 mg (70%). Anal. Calc. for C25H18N5OClRu: C, 55.49; H, 3.33; N,
12.95. Found: C, 55.42; H, 3.39; N, 13.02%. 1H NMR in CDCl3, d
ppm: 6.65 (t, 1H, J = 6.4); 6.78 (d, 1H, J = 4.8); 6.85 (d, 1H,
J = 9.2); 7.00 (t, 1H, J = 6.5); 7.26–7.41 (2H*); 7.40 (d, 1H, J = 9.2);
7.53 (t, 1H, J = 7.5); 7.61–7.65 (2H*); 7.70 (d, 1H, J = 8.4); 7.80 (t,
1H, J = 6.6); 8.00 (t, 2H, J = 6.9); 8.30 (d, 1H, J = 8.1); 8.47 (d, 1H,
J = 5.7); 10.07 (d, 1H, J = 8.5); 10.46 (d, 1H, J = 5.7).
2. Experimental
2.1. Materials
Commercial ruthenium trichloride, was obtained from Arora
Matthey, Kolkata, India, was converted to RuCl3 ꢀ 3H2O by repeated
evaporation with concentrated hydrochloric acid. AgNO3 and
dimethylsulfoxide were purchased from Merck, India. Triethyl-
amine, 2,20-bipyridine, 2,20,200-terpyridine, and 1-(20-pyridylazo)-
2-naphthol (Hpan) were purchased from Loba Chemie, Mumbai,
India. Pyridine and 4-picoline were purchased from Fluka Chemie.
[Ru(dmso)4Cl2], [Ru(trpy)Cl3], [Ru(bpy)Cl3] and [Ru(PPh3)3Cl2]
were synthesized by following reported procedures [7]. Calf thy-
mus (CT) DNA and Tris buffer were procured from Sigma Chemical
Company. The dry powder of CT DNA was dissolved in 10 mM Tris
buffered saline, pH 7.2 (TBS), and dialyzed overnight against the
same buffer so that the A260/A280 of the dialyzed solution was
greater than 1.90. The DNA concentrations were adjusted accord-
2.2.4. [Ru(bpy)(pan)(py)]ClO4
To a solution of [Ru(bpy)(pan)Cl] (100 mg, 0.19 mmol) in etha-
nol (30 mL) was added AgNO3 (31 mg, 0.19 mmol). The mixture
was warmed and stirred for 30 min, and the deposited AgCl was
separated by filtration. To the filtrate was added pyridine (15 mg,
0.19 mmol). The resulting solution was heated at reflux for 4 h. It
was then concentrated to about 10 mL, and a saturated aqueous
solution of NaClO4 (0.5 mL) was added, whereby a reddish-brown
precipitate is obtained, which was collected by filtration, washed
with water, and dried in vacuo over P4O10. It was further purified
by preparative thin layer chromatography on a silica plate. Using
1:5 acetonitrile–benzene as the eluant, a reddish-brown band sep-
arated, which was extracted with acetonitrile. Evaporation of this
extract gave [Ru(bpy)(pan)(py)]ClO4 as a crystalline solid. Yield:
76 mg (60%). Anal. Calc. C30H23N6O5ClRu: C, 52.66; H, 3.36; N,
12.29. Found: C, 51.97; H, 3.41; N, 12.34. 1H NMR in CDCl3, d
ppm: 6.81 (d, 2H, J = 9.2); 7.09 (t, 1H, J = 6.3); 7.19–7.26 (3H*);
7.40 (t, 3H*); 7.51 (d, 1H, J = 9.3); 7.60–7.70 (3H*); 7.72–7.81
(4H*); 8.18 (t, 1H, J = 7.7); 8.25–8.41 (3H*), 8.57 (d, 1H, J = 8.0);
9.82 (d, 1H, J = 8.4).
ing to its absorbance at 260 nm using e
260 = 6.6 mMꢁ1 cmꢁ1. SC
pUC18 DNA and agarose were purchased from Bangalore Genei
Pvt. Ltd., Bangalore, India. Purification of dichloromethane, aceto-
nitrile and preparation of tetrabutylammonium perchlorate (TBAP)
for electrochemical work were performed as reported in the liter-
ature [8]. All other chemicals and solvents were reagent grade
commercial materials and were used as received.
2.2. Synthesis
2.2.1. [Ru(pan)2]
Hpan (130 mg, 0.52 mmol) was dissolved in ethanol (50 mL)
and triethylamine (55 mg, 0.54 mmol) was added to it. Then
[Ru(dmso)4Cl2] (100 mg, 0.21 mmol) was added and the mixture
was refluxed for 24 h to afford a brownish-violet solution. The sol-
vent was then evaporated and the solid mass obtained was purified
by preparative thin layer chromatography on a silica plate using
1:10 acetonitrile–benzene as the eluant. A violet band separated,
which was extracted with 1:1 acetonitrile–dichloromethane solu-
tion and evaporation of the extract afforded [Ru(pan)2] as a violet
crystalline solid. Yield: 49 mg (40%). Anal. Calc. for C30H20N6O2Ru:
C, 60.29; H, 3.35; N, 14.07. Found: C, 60.20; H, 3.39; N, 13.95%. 1H
NMR in CDCl3, d ppm1: 6.61 (t, 2H, J = 6.2); 6.85 (d, 2H, J = 9.2); 7.32
(d, 2H, J = 5.3); 7.39 (t, 4H*); 7.49 (d, 2H, J = 9.2); 7.64–7.73 (6H*);
10.07 (d, 2H, J = 8.5).
2.2.5. [Ru(bpy)(pan)(4-pic)]ClO4
This complex was synthesized by following the same above pro-
cedure using 4-picoline instead of pyridine. Yield: 77 mg 60%. Anal.
Calc. for C31H25N6O5ClRu: C, 53.32; H, 3.58; N, 12.04. Found: C,
53.23; H, 3.49; N, 12.10%. 1H NMR in CDCl3: d ppm: 2.38 (s, 3H,
CH3); 6.82 (d, 1H, J = 9.2); 7.10 (t, 1H, J = 6.2); 7.15–7.20 (4H*);
7.37 (t, 1H, J = 7.5); 7.51 (d, 1H, J = 9.2); 7.55–7.85 (6H*); 8.00–
8.12 (2H*); 8.18 (t, 1H, J = 7.7); 8.29–8.24 (3H*), 8.56 (d, 1H,
J = 8.1); 9.83 (d, 1H, J = 8.4).
2.2.2. [Ru(trpy)(pan)]ClO4
Hpan (57 mg, 0.23 mmol) was dissolved in ethanol (50 mL) and
triethylamine (23 mg, 0.23 mmol) was added to it. Then [Ru(tr-
py)Cl3] (100 mg, 0.23 mmol) was added and the mixture was re-
fluxed for 5 h to afford a brownish-red solution. It was then
concentrated to about 10 mL, and a saturated aqueous solution of
2.2.6. [Ru(PPh3)2(pan)Cl]
Hpan (26 mg, 0.10 mmol) was dissolved in ethanol (30 mL) and
to it was added triethylamine (10 mg, 0.10 mmol). Then
[Ru(PPh3)3Cl2] (100 mg, 0.10 mmol) was added to the solution
and it was refluxed for 4 h. The [Ru(PPh3)2(pan)Cl] complex sepa-
rated as a brown precipitate, which was collected by filtration,
washed with hexane and dried in air. Yield: 70 mg (74%). Anal. Calc.
for C51H40N3OP2ClRu: C, 67.36; H, 4.40; N, 4.62. Found: C, 67.25; H,
1
Chemical shifts are given in ppm and multiplicity of the signals along with the
associated coupling constants (J in Hz) are given in parentheses. Overlapping signals
are marked with an asterisk.

S. Basu et al. / Polyhedron 27 (2008) 2943–2951
2945
1
4.43; N, 4.66%. H NMR in CDCl3, d ppm: 6.29–6.39 (2H*); 6.7–7.5
Crystal data for C30H21N6O5ClRu, M = 680.03, size 0.05 ꢂ 0.10 ꢂ
0.20 mm3, monoclinic, space group C2/c, a = 30.611(4) Å,
(36H*); 7.74 (d, 1H, J = 5.4); 9.20 (d, 1H, J = 8.5). 31P NMR in CDCl3,
d ppm: 24.14.
b = 11.6611(14) Å, c = 16.491(2) Å,
U = 5352.5(13) Å3, Z = 8, Dcalc = 1.688 g cmꢁ3, F(000) = 2736, k =
0.71073 Å, T = 150 K,
= 0.741 mmꢁ1, 18729 reflections collected,
a = c = 90°, b = 114.595(13)°,
2.3. Physical measurements
l
7821 unique (Rint = 0.109). Final goodness-of-fit = 0.615, R1 =
Microanalyses (C, H, N) were performed using a Heraeus Carlo
Erba 1108 elemental analyzer. Magnetic susceptibilities were mea-
sured using a PAR 155 vibrating sample magnetometer fitted with
a Walker Scientific L75FBAL magnet. 1H NMR spectra were re-
corded in CDCl3 solution on a Bruker Avance DPX 300 NMR spec-
trometer using TMS as the internal standard. ESR spectra were
recorded with a JEOL JES-FA200 X-band spectrometer fitted with
a quartz Dewar for measurements at 77 K (liquid dinitrogen). All
ESR spectra were calibrated with an aid of DPPH (g = 2.0037). IR
spectra were obtained on a Shimadzu FTIR-8300 spectrometer
with samples prepared as KBR pellets. Electronic spectra were re-
corded on JASCO V-570 and Shimadzu 2401 spectrophotometers.
Electrochemical measurements were made using a CH Instruments
model 600A electrochemical analyzer. A platinum disc working
electrode, a platinum wire auxiliary electrode and an aqueous sat-
urated calomel reference electrode (SCE) were used in the cyclic
voltammetry experiments. All electrochemical experiments were
performed under a dinitrogen atmosphere. All electrochemical
data were collected at 298 K and are uncorrected for junction po-
0.0486, wR2 = 0.0790, R indices based on 2125 reflections with
[I > 2r(I)].
Crystal data for C25H18N5OClRu, M = 540.96, size 0.10 ꢂ 0.10 ꢂ
0.40 mm3, orthorhombic, space group P212121, a = 10.2654(9) Å,
b = 13.2256(12) Å,
2132.5(3) Å3, Z = 4, Dcalc = 1.685 g cmꢁ3
0.71073 Å, T = 100 K,
= 0.890 mmꢁ1, 14079 reflections collected,
5257 unique (Rint = 0.045). Final goodness-of-fit = 1.03, R1 = 0.039,
wR2 = 0.0967, R indices based on 5257 reflections with [I > 2 (I)].
c = 15.7074(14) Å,
a
= b =
c
= 90°,
U =
,
F(000) = 1088, k =
l
r
Crystal data for C52H42N3OP2Cl3Ru, M = 994.25, size 0.10 ꢂ
0.20 ꢂ 0.30 mm3, monoclinic, spacegroup Cm, a = 18.325(3) Å,
b = 15.2504(12) Å, c = 9.6397(8) Å,
a
=
c
= 90°, b = 122.099(5)°,
F(000) = 1016, k =
= 0.632 mmꢁ1, 7207 reflections collected,
U = 2282.1(5) Å3, Z = 4, Dcalc = 1.447 g cmꢁ3
0.71073 Å, T = 150 K,
,
l
4280 unique (Rint = 0.040). Final goodness-of-fit = 1.007, R1 =
0.0384, wR2 = 0.0845, R indices based on 3596 reflections with
[I > 2r(I)].
3. Results and discussion
tential. Fluorescence studies were performed with
a Hitachi
F4500 spectrofluorimeter. Electrophoresis experiments were car-
ried out on a BIORAD electrophoretic system using TBS. The DNA
binding studies were carried out as follows: (i) Fluorescence stud-
ies were performed with the complex and DNA dissolved sepa-
rately in TBS, and the samples were excited at 480 nm. (ii) For
the DNA–agarose gel studies, SC pUC18 DNA was incubated in
the presence of different concentrations of the complexes. These
solutions were monitored on agarose gel. The DNA was visualized
under UV light.
3.1. Syntheses and crystal structures
The primary objective of the present study, as already men-
tioned above, has been to synthesize a series of ruthenium com-
plexes containing 1-(20-pyridylazo)-2-naphthol (Hpan, 1), either
as the sole ligand or as one of the ligands. For preparing the homo-
leptic ruthenium complex of Hpan, [Ru(dmso)4Cl2] has been se-
lected as the starting material because of its demonstrated ability
to undergo displacement of all the six monodentate ligands by che-
lating ligands [2j,2n]. Reaction of Hpan with [Ru(dmso)4Cl2] in
refluxing ethanol in the presence of triethylamine has indeed affor-
ded the expected bis-complex, viz. [Ru(pan)2], in a decent yield
(Scheme 1). In order to authenticate coordination mode of ligand
(1) in this complex, its structure has been determined by X-ray
crystallography. The structure is shown in Fig. 1 and selected bond
parameters are given in Table 1.
2.4. X-ray crystallography
Single crystals of [Ru(pan)2] and [Ru(PPh3)2(pan)Cl] were ob-
tained by slow diffusion of acetonitrile into dichloromethane solu-
tions of the complexes, followed by evaporation of the resulting
solution. Single crystals of [Ru(trpy)(pan)]ClO4 and [Ru(bpy)-
(pan)Cl] were obtained by slow evaporation of a solution of the
complex in 1:1 dichloromethane–ethanol and acetonitrile, respec-
tively. Selected crystal data and a summary of the data collection
parameters appear below, the full details are provided in Table
S1. Data on the [Ru(bpy)(pan)Cl] crystal were collected on a Enraf
Nonius CAD-4 diffractometer and those for the other three crystals
were collected on an Oxford Diffraction X-Calibur CCD system
The structure shows that in this complex 1-(20-pyridylazo)-2-
naphthol is coordinated to the metal center, via dissociation of
the phenolic proton, as a monoanionic N,N,O-donor (2, M = Ru),
[Ru(PPh3)2(pan)Cl]
[Ru(pan)2]
using graphite monochromated Mo K
a radiation (k = 0.71073 Å).
For these three crystals, X-ray data reduction was carried out using
the CRYSALIS program [9c]. The structures were solved by direct
methods using the SHELXS-97 program [9a]. In the structure of Ru-
(pan)2, the data were very weak and all atoms except for Ru were
refined isotropically. In the other structures, all non-hydrogen
atoms were refined anisotropically. Empirical absorption correc-
tions were carried out using the ABSPACK program [9b]. The struc-
tures were refined on F2 using the SHELXL-97 program [9a].
[Ru(PPh3)3Cl2]
EtOH, NEt3
Reflux
[Ru(dmso)4Cl2]
EtOH, NEt3
Reflux
N
N
N
OH
Crystal data for C30H20N6O2Ru, M = 597.59, size 0.05 ꢂ 0.05 ꢂ
0.30 mm3, triclinic, space group P1, a = 8.774(9) Å, b =
[Ru(bpy)Cl3]
EtOH, NEt3
Reflux
[Ru(trpy)Cl3]
EtOH, NEt3
Reflux
ꢀ
12.685(12) Å, c = 12.883(13) Å,
a
= 62.69(10)°, b = 75.07(9)°,
c
= 82.15(8)°, U = 1231(2) Å3, Z = 2, Dcalc = 1.612 g cmꢁ3, F(000) =
604, k = 0.71073 Å, T = 150 K,
l
= 0.679 mmꢁ1, 7248 reflections col-
lected, 5936 unique (Rint = 0.225). Final goodness-of-fit = 0.414,
R1 = 0.0701, wR2 = 0.0878, R indices based on 630 reflections with
[Ru(bpy)(pan)Cl]
[Ru(trpy)(pan)]ClO4
[I > 2r(I)].
Scheme 1.

2946
S. Basu et al. / Polyhedron 27 (2008) 2943–2951
because of the strain imposed by the two adjacent five-membered
chelate rings. The phenolic C–O distance is normal [2n], while the
average N–N distance is notably longer than that in uncoordinated
azo ligands [10] and the observed elongation is attributable to the
metal-to-p
*(azo) back-bonding [11]. The absence of any solvent of
crystallization in the crystal lattice of [Ru(pan)2] indicates the pos-
sible existence of non-covalent interactions between the individual
complex molecules. To sort this out, packing pattern in the lattice
has been examined, which shows that hydrogen-bonding interac-
tions of two types, viz. C–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀ
p interactions, are active
in the lattice (Fig. 2). The phenolate-oxygen of each coordinated
pan of any complex molecule each coordinated pan of any complex
molecule is hydrogen-bonded to two aryl hydrogens of two other
pan ligands belonging to two different complex molecules. The d-
hydrogen in the azo-linked phenyl ring of the naphthol fragment
of each pan is involved in a C–Hꢀ ꢀ ꢀ
p interaction with the p-cloud
Fig. 1. View of the [Ru(pan)2] complex (ellipsoids at 25% probability, hydrogens not
over the other ring of the naphthol fragment of another pan of a
second neighboring complex molecule. Both of these hydrogen-
bonding interactions are extended throughout the entire lattice,
and they appear to be responsible for holding the crystal together.
As 1-(20-pyridylazo)-2-naphthol (1) serves as a N,N,O-donor and
forms a planar chelate (2), in the Ru(pan) fragment only three coor-
dination sites on ruthenium remain accessible. To occupy these
three available sites either a planar tridentate ligand, or a combina-
tion of a bidentate and a monodentate ligand, or three monoden-
tate ligands can be utilized to obtain the mixed-ligand
complexes. Accordingly three different ruthenium starting materi-
als, viz. [Ru(trpy)Cl3] (trpy = 2,20,200-terpyridine), [Ru(bpy)Cl3]
(bpy = 2,20-bipyridine) and [Ru(PPh3)3Cl2], have been chosen. Reac-
tions of ligand (1) with these three starting materials in refluxing
ethanol in the presence of triethylamine have afforded the targeted
mixed-ligand complexes, viz. [Ru(trpy)(pan)]+ (isolated as the per-
chlorate salt), [Ru(bpy)(pan)Cl] and [Ru(PPh3)2(pan)Cl], respec-
tively, in good yields (Scheme 1). It is interesting to note here
that during synthetic reactions of Hpan with [Ru(trpy)Cl3] and
[Ru(bpy)Cl3], ruthenium undergoes a one-electron reduction, and
either the solvent (ethanol) or the base (NEt3) might have served
as the reducing agent. Structures of these three mixed-ligand com-
plexes have also been determined by X-ray crystallography. Struc-
ture of the [Ru(trpy)(pan)]ClO4 complex (Fig. 3) shows that in it the
pan ligand is coordinated to ruthenium as before. The remaining
three coordination sites on ruthenium are occupied by the three
terpyridine-nitrogens in the usual manner. The N5O coordination
sphere around ruthenium is similarly distorted from ideal octahe-
dral geometry as in [Ru(pan)2], because like the 1-(20-pyridylazo)-
2-naphtholate anion (pan), terpyridine also forms two adjacent
five-membered chelate rings. Bond parameters (Table 1) in the
Ru(trpy) fragment are found to be normal, as observed in other
structurally characterized complexes of ruthenium having this
fragment [12]. Bond lengths in the Ru(pan) fragment are compara-
ble to those observed in the [Ru(pan)2] complex. An examination of
the packing pattern in the lattice of the [Ru(trpy)(pan)]ClO4 com-
plex shows the existence of three types of hydrogen-bonding inter-
included for clarity).
Table 1
Selected bond lengths (Å) and bond angles (°) for [Ru(pan)2], [Ru(trpy)(pan)]ClO4,
[Ru(bpy)(pan)Cl] and [Ru(PPh3)2(pan)Cl] complexes
[Ru(pan)2]
Bond distances (Å)
Ru(1)–N(3)
Ru(1)–N(5)
Ru(1)–N(41)
Ru(1)–N(91)
1.973(10)
1.965(10)
1.985(11)
2.068(10)
Ru(1)–O(6)
Ru(1)–O(30)
N(3)–N(4)
2.053(9)
2.105(8)
1.427(13)
1.349(11)
N(5)–N(52)
Bond angles (°)
N(3)–Ru(1)–N(5)
N(41)–Ru(1)–O(30)
N(91)–Ru(1)–O(6)
N(3)–Ru(1)–N(41)
177.8(5)
160.0(3)
157.4(3)
81.7(4)
N(3)–Ru(1)–O(30)
N(5)–Ru(1)–N(91)
N(5)–Ru(1)–O(6)
78.2(4)
77.7(4)
79.8(4)
[Ru(trpy)(pan)]ClO4
Bond lengths (Å)
Ru(1)–N(11)
Ru(1)–N(22)
Ru(1)–N(28)
Ru(1)–N(31)
2.049(4)
1.998(4)
2.058(4)
2.032(4)
Ru(1)–N(38)
Ru(1)–O(52)
N(37)–N(38)
1.919(4)
2.077(3)
1.337(4)
Bond angles (°)
N(11)–Ru(1)–N(28)
N(22)–Ru(1)–N(38)
N(31)–Ru(1)–O(52)
N(11)–Ru(1)–N(22)
156.43(16)
177.82(16)
158.65(15)
78.09(16)
N(22)–Ru(1)–N(28)
N(31)–Ru(1)–N(38)
N(38)–Ru(1)–O(52)
78.55(16)
77.49(16)
81.18(15)
[Ru(bpy)(pan)Cl
Bond lengths (Å)
Ru–N(1)
Ru–N(3)
Ru–N(4)
Ru–N(5)
1.910(3)
2.039(3)
2.042(3)
2.093(3)
Ru–O
Ru–Cl
O–C(1)
N(1)–N(2)
2.102(2)
2.3830(10)
1.289(4)
1.316(4)
Bond angles (°)
N(1)–Ru–N(5)
N(3)–Ru–O
174.92(12)
160.94(10)
172.45(9)
N(1)–Ru–N(3)
N(4)–Ru–N(5)
N(1)–Ru–O
79.23(12)
78.03(12)
81.72(10)
N(4)–Ru–Cl
[Ru(PPh3)2(pan)Cl]
actions, viz. C–Hꢀ ꢀ ꢀO, C–Hꢀ ꢀ ꢀN and C–Hꢀ ꢀ ꢀ
p interactions (Fig. S1).
Bond lengths (Å)
Ru(1)–N(41)
Ru(1)–N(44)
Ru(1)–O(55)
Ru(1)–P(2)
2.058(4)
1.916(5)
2.130(4)
2.3873(9)
Ru(1)–Cl(1)
O(55)–C(54)
N(43)–N(44)
2.4786(14)
1.291(6)
1.327(6)
Two perchlorate-oxygens are hydrogen-bonded to pyridyl-hydro-
gens of the terpyridine. One pyridyl-hydrogen of the terpyridine
is hydrogen-bonded to an azo-nitrogen of the pan ligand of another
complex molecule. The
pan is involved in a C–Hꢀ ꢀ ꢀ
pyridine ring of pan of another complex molecule.
c
-hydrogen of the naphthol fragment of
Bond angles (°)
p
interaction with the -cloud of the
p
N(41)–Ru(1)–O(55)
N(44)–Ru(1)–Cl(1)
P(2)–Ru(1)–P(2a)
159.50(14)
173.99(15)
166.74(4)
N(41)–Ru(1)–N(44)
N(44)–Ru(1)–O(55)
79.5(2)
80.02(17)
Structure of the [Ru(bpy)(pan)Cl] complex (Fig. 4) shows that
the pan ligand is coordinated in the expected N,N,O-mode, while
the remaining three sites are occupied by a 2,20-bipyridine, coordi-
nated in the usual N,N-fashion, and a chloride. The observed Ru–Cl
length (Table 1) is normal, and so are the bond distances in the
Ru(bpy) fragment [13]. The octahedral N4OCl coordination sphere
with N–Ru–N and N–Ru–O bite angles of ꢃ79°. In this complex
ruthenium is thus sitting in a N4O2 coordination sphere, which is
significantly distorted from ideal octahedral geometry, primarily

S. Basu et al. / Polyhedron 27 (2008) 2943–2951
2947
Fig. 2. Packing diagram of [Ru(pan)2] complex.
in this complex is much less distorted compared to the structures
of the [Ru(pan)2] and [Ru(trpy)(pan)]+ complexes, because of the
flexibility associated with a bidentate and a monodentate ligands
replacing a tridentate ligand forming two adjacent five-membered
chelate rings. An examination of the packing pattern in the lattice
of the [Ru(bpy)(pan)Cl] complex shows the existence of three types
of non-covalent interactions, viz. C–Hꢀ ꢀ ꢀO, C–Hꢀ ꢀ ꢀCl and C–Hꢀ ꢀ ꢀ
p
interactions (Fig. S2). The phenolate oxygen of pan is hydrogen-
bonded to a pyridyl hydrogen of bpy belonging to a neighboring
complex molecule. The chloride is hydrogen-bonded to a pyridyl
C–H of pan of an adjacent complex molecule. Besides these hydro-
gen-bonding interactions there are intermolecular C–Hꢀ ꢀ ꢀ
actions between two pyridyl C–H’s of the bipyridine ligand and
naphthyl interaction between a
p inter-
p
-cloud. There is also a C–Hꢀ ꢀ ꢀ
p
pyridyl C–H of pan ligand of one molecule with a pyridyl
p cloud
of another.
The structure of the [Ru(PPh3)2(pan)Cl] complex (Fig. 5) has
mirror symmetry and shows the pan ligand and the chloride shar-
ing an equatorial plane with the metal at the center, where the
chloride is obviously trans to the coordinated azo nitrogen, and
the two triphenylphosphines occupy the remaining two axial posi-
tions and hence they are mutually trans. Compared to the struc-
tures of the previous complexes, the N2OP2Cl coordination sphere
in the present complex is far less distorted, which is attributable
to the freedom in disposition of three monodentate ligands. The
Ru–P lengths (Table 1) are normal [2i], but the Ru–Cl distance is
notably longer than that observed in the [Ru(bpy)(pan)Cl] complex
and the observed elongation is attributable to stronger trans effect
of the azo-nitrogen than that of the bpy-nitrogen. In the crystal lat-
tice of this complex, there exists one molecule of dichloromethane
per molecule of the complex. To find out the non-covalent interac-
tion(s) active in the lattice, particularly between the solvent mole-
cule and the complex molecule, packing pattern in the lattice has
been examined (Fig. S3), which reveals that each complex mole-
cule is linked to four surrounding complex molecules through
Fig. 3. View of the [Ru(trpy)(pan)]+ complex (ellipsoids at 25% probability,
hydrogens not included for clarity).
C–Hꢀ ꢀ ꢀ
p
interactions. The phenyl C–H of triphenylphosphine is in-
cloud on both the
volved in non-covalent interaction with the
p
pyridyl and naphthyl ring of pan. Each coordinated chloride is en-
gaged in C–Hꢀ ꢀ ꢀCl hydrogen bonding with the hydrogens of the
nearest dichloromethane molecule.
Fig. 4. View of the [Ru(bpy)(pan)Cl] complex (ellipsoids at 25% probability,
hydrogens not included for clarity).

2948
S. Basu et al. / Polyhedron 27 (2008) 2943–2951
Table 2
Electronic spectral and cyclic voltammetric data
Compound
Electronic spectral dataa kmax, nm
, Mꢁ1 cmꢁ1
Cyclic
(
e
)
voltammetric
datab
[Ru(pan)2]
606(11240), 566c(9370),
548c(9240), 470(9710), 408(8040),
368(10390), 302(13020)
548(16730), 480(24060),
456c(16070), 376c10710),
318(32980), 282(28680),
272(29200), 228(47040)
610c(13140), 584c(12570),
504(36050), 476(22170),
444c(12360), 368(22290),
296(48210)
550(5560), 480(18770),
456(9610)c, 368(12430),
307(14780)c, 269(41870)
595c(11250), 572(12800),
555c(11900), 492(31500),
465(20500), 389c(16600),
346(19200), 293(43900)
1.62d, 0.70e(80)f,
ꢁ0.94e(89)f
[Ru(trpy)(pan)]ClO4
1.66d, 0.86e(69)f,
ꢁ0.86e(90)f,
ꢁ1.56e(60)f
[Ru(bpy)(pan)Cl]
1.59d, 0.44e(78)f,
ꢁ0.88e(90)f,
ꢁ1.58g
[Ru(PPh3)2(pan)Cl]
1.54d, 0.47e(67)f,
ꢁ1.00e(96)f
[Ru(bpy)(pan)(py)]ClO4
1.69d, 0.82e(70)f,
ꢁ0.92e(80)f,
ꢁ1.52g
[Ru(bpy)(pan)(pic)]ClO4 594c(7800), 576c(8400),
492(22100), 464(14000),
1.64d, 0.79e(71)f,
ꢁ0.94e(83)f,
ꢁ1.57g
388c(11200), 342(12500),
292(29600)
Fig. 5. View of the [Ru(PPh3)2(pan)Cl] complex (ellipsoids at 25% probability,
hydrogens not included for clarity).
a
In dichloromethane.
Solvent, 1:9 dichloromethane–acetonitrile; supporting electrolyte, TBAP; ref-
b
The coordinated chloride in the [Ru(bpy)(pan)Cl] complex has
been found to undergo facile displacement by other monodentate
ligands under relatively mild condition. This has been manifested
in the reactions of [Ru(bpy)(pan)Cl] with pyridine (py) and 4-pico-
line (pic) carried out in ethanolic medium in the presence of silver
ion, for easy removal of the coordinated chloride, which have
yielded the desired mixed-ligand complexes, viz. [Ru(bpy)(pan)-
(py)]+ and [Ru(bpy)(pan)(pic)]+, isolated as perchlorate salts in
good yields. Composition of these two complexes has been verified
by their microanalytical data. Both of these complexes are assumed
to have a similar structure as the precursor [Ru(bpy)(pan)Cl] com-
plex, with the chloride replaced by pyridine or 4-picoline.
erence electrode, SCE; scan rate, 50 mV sꢁ1
.
c
Shoulder.
Anodic peak-potential (Epa) value.
E1/2 = 0.5(Epa + Epc), where Epc = cathodic peak-potential.
d
e
f
D
Ep = (Epa ꢁ Epc) in mV.
g
Epc value.
complexes. In the spectra of the [Ru(trpy)(pan)]ClO4, [Ru(bpy)-
(pan)(py)]ClO4 and [Ru(bpy)(pan)(pic)]ClO4 complexes two intense
bands are observed near 1080 and 620 cmꢁ1, which are attributed
to the perchlorate ion. Infrared spectrum of [Ru(PPh3)2(pan)Cl]
shows sharp bands at 744, 698 and 519 cmꢁ1 due to the coordi-
nated PPh3 ligands.
3.2. Spectral properties
All the complexes are found to be soluble in ethanol, methanol,
acetonitrile, dichloromethane and chloroform, producing reddish-
brown solutions. Electronic spectra of the complexes have been re-
corded in dichloromethane solution. Each complex shows several
intense absorptions in the visible and ultraviolet region (Table 2).
The absorptions in the ultraviolet region are believed to be due
to transitions within the ligand orbitals and those in the visible re-
gion are likely to be due to allowed metal-to-ligand charge-trans-
fer transitions. To have a better understanding of the nature of
transitions in the visible region, qualitative EHMO calculations
have been performed on computer-generated models2 of all the
complexes [14]. Composition of some selected molecular orbitals is
given in Table S2 and partial MO diagram of the [Ru(pan)2] complex
is shown in Fig. 6. The top three filled orbitals, viz. the highest occu-
pied molecular orbital (HOMO) and the next two filled orbitals
(HOMOꢁ1 and HOMOꢁ2), are close in energy and have major contri-
butions from the ruthenium t2-orbitals and relatively less contribu-
tions from the coordinated ligands. The lowest unoccupied
molecular orbital (LUMO), though has some contribution from the
metal center, is localized mostly on the pan ligand and is concen-
trated heavily on the azo (-N@N-) fragment. The next couple of va-
cant orbitals (LUMO + 1, LUMO + 2, etc.) are delocalized over either
pan or the co-ligand. The lowest energy absorption in the visible re-
gion may therefore be assigned to a transition between the strongly
All the six complexes, viz. [Ru(pan)2], [Ru(trpy)(pan)]ClO4,
[Ru(bpy)(pan)Cl], [Ru(PPh3)2(pan)Cl], [Ru(bpy)(pan)(py)]ClO4 and
[Ru(bpy)(pan)(pic)]ClO4, are diamagnetic, which corresponds to
the bivalent state of ruthenium (low-spin d6, S = 0) in them. 1H
NMR spectra of these complexes have been recorded in CDCl3 solu-
tion and a spectrum has been deposited as supplementary material
(Fig. S4). 1H NMR spectrum of the [Ru(pan)2] complex shows all the
expected signals within 6.5–10.1 ppm, of which the most deshield-
ed doublet at 10.07 ppm is assignable to the proton nearest to the
pyridine-nitrogen. Spectra of the other three complexes are com-
plex in nature due to overlap of signals arising from both pan
and other organic ligand(s) in the similar region. However, in all
these three spectra the isolated doublet near 10.0 ppm, which is
diagnostic of coordinated pan, is clearly observed. 1H NMR spec-
trum of the [Ru(bpy)(pan)(py)]ClO4 complex shows most of the ex-
pected signals clearly, though few signals could not be distinctly
identified due to their overlap with other signals. The methyl signal
from the 4-picoline fragment of the [Ru(bpy)(pan)(pic)]ClO4 com-
plex is observed at 2.38 ppm, while rest of its 1H NMR spectrum
is very similar to that of the [Ru(bpy)(pan)(py)]ClO4 complex.
Infrared spectra of the complexes show many bands of different
intensities within 400–4000 cmꢁ1. Assignment of each individual
band to a specific vibration has not been attempted. However,
strong bands observed at 1606, 1498, 1199, 1134, 758 cmꢁ1 in
the spectrum of the [Ru(pan)2] complex are assignable to the coor-
dinated pan ligand. Such bands are also observed in the other five
2
In the [Ru(PPh3)2(pan)Cl] complex phenyl rings of the triphenylphosphines are
replaced by hydrogen.

S. Basu et al. / Polyhedron 27 (2008) 2943–2951
2949
Fig. 6. Partial molecular orbital diagram of [Ru(pan)2]: (a) interaction diagram and (b) highest occupied and lowest unoccupied molecular orbitals.
delocalized HOMO and LUMO with mixed metal-to-ligand charge-
transfer (MLCT) and intra-ligand charge-transfer (ILCT) transition.
(pan)Cl]+ complex g\ = 2.21 and g = 1.99). A selected spectrum is
shown in Fig. 7. The observed anisotropic nature of the spectra con-
k
firms the +3 oxidation state of ruthenium in the oxidized complexes,
which in turn supports assignment of the first oxidative response to
Ru(II)–Ru(III) oxidation to be correct. The second oxidative response
is irreversible in nature and is tentatively assigned to oxidation of
the coordinated pan ligand. A reductive response, quasi-reversible
in nature, is displayed by all the complexes near ꢁ0.9 V versus
SCE, and in view of the composition of the LUMO this reduction is
assigned to reduction of the azo (–N@N–) fragment of the
coordinated pan ligand. A second irreversible reductive response
is observed in the [Ru(trpy)(pan)]ClO4, [Ru(bpy)(pan)Cl],
3.3. Electrochemical properties
Electrochemical properties of all the complexes have been stud-
ied by cyclic voltammetry in 1:9 dichloromethane–acetonitrile3
solution (0.1 M TBAP). The voltammetric data are given in Table
2. All the complexes show two oxidative responses on the positive
side of SCE. In view of the composition of the HOMO, the first oxi-
dation is tentatively assigned to the Ru(II)–Ru(III) oxidation. This
oxidation is reversible in nature, characterized by a peak-to-peak
separation (DEp) of 67–80 mV, which remains unchanged upon
changing the scan rate, and the anodic peak-current (ipa) is almost
equal to the cathodic peak-current (ipc) as expected for a reversible
electron-transfer process. Reversibility of the Ru(II)–Ru(III) oxida-
tion indicates that the one-electron oxidized species might be sta-
ble on a time scale much longer than the cyclic voltammetric time
scale. To investigate this, as well as to verify assignment of this oxi-
dative response to Ru(II)–Ru(III) oxidation, all the six complexes
have been coulometrically oxidized at an appropriate potential4
in 1:9 dichloromethane–acetonitrile solution (0.1 M TBAP). The oxi-
dations have been smooth and quantitative for each complex. How-
ever, the oxidized solutions are found to stable only for the
[Ru(bpy)(pan)Cl] and [Ru(PPh3)2(pan)Cl] complexes, which have
much less Ru(II)–Ru(III) oxidation potential. For the rest four com-
plexes, color of the solution containing the oxidized species changes
rapidly indicating fast decomposition. ESR spectra of solutions con-
taining the [Ru(bpy)(pan)Cl]+ and [Ru(PPh3)2(pan)Cl]+ complexes
have been recorded at 77 K. Both the complexes show an axial ESR
spectrum with two distinct signals (g\ and g ; for the [Ru(bpy)-
k
(pan)Cl]+ complex g\ = 2.28 and g = 1.90; for the [Ru(PPh3)2
k
200
250
300
350
400
3
A
little dichloromethane was necessary to take the complex into solution.
Addition of large excess of acetonitrile was necessary to record the redox responses in
proper shape.
Field (mT)
4
Coulometric oxidation has been carried out at a potential 200 mV higher than the
Fig. 7. ESR spectrum of the [RuIII(bpy)(pan)Cl]+ complex in 1:9 dicloromethane–
anodic peak-potential (Epa).
acetonitrile solution at 77 K.

2950
S. Basu et al. / Polyhedron 27 (2008) 2943–2951
Fig. 9. Results of the gel electrophoresis experiment for [Ru(trpy)(pan)]ClO4. Lane
1, SC pUC18 DNA control without irradiation. Lane 2, DNA control with irradiation
at 254 nm. Lanes 3–7, DNA + complex with [DNA]/[complex] ratio of 1, 20, 40, 60,
70, respectively with irradiation at 254 nm. The complex concentration was 30
for Lanes 3–7.
lM
and reaches saturation at [DNA]:[complex] = 70:1 (Fig. 8a). Such
increase in intensity of absorption upon interaction with DNA is,
though relatively less common, precedent in the literature [16].
In order to explore the interaction one step further, detailed spec-
trofluorimetric experiments with complex and DNA has also been
carried out. The complex shows emission peaks at 515 nm and
554 nm when excited at 480 nm. With the increasing amount of
DNA the fluorescence intensity increases for both the peaks (Fig.
8b), and reaches its optimum value in the same ratio (i.e.
[DNA]:[complex] = 70:1). Absorption and fluorescence spectro-
scopic studies in Tris–HCl buffer (pH 7.2) solution in the presence
and absence of DNA show that the other two complexes, viz. [Ru-
(bpy)(pan)(py)]ClO4 and [Ru(bpy)(pan)(pic)]ClO4, also interact
with DNA.
In order to examine whether the above three complexes can
also bring about cleavage of DNA upon UV irradiation, a property
usually exhibited by such polypyridine complexes [15], interaction
of these complexes with a super-coiled DNA, viz. SC pUC18 DNA,
has been studied by gel electrophoresis. In these experiments SC
pUC18 DNA was incubated in the presence of different concentra-
tions of the complexes, followed by irradiation at 254 nm for dif-
ferent time durations. The samples were then analyzed by
agarose gel electrophoresis. After the run the DNA was stained
with EtBr and visualized by UV transilluminator. A representative
picture of EtBr stained DNA is shown in Fig. 9. The extreme left line
is for SC pUC18 DNA and in the following five lanes are the bands
for increasing DNA complex ratio (i.e. [DNA]/[complex] = 1, 20, 40,
60 and 70. This shows that the complex does not induce any obser-
vable conformation changes in SC DNA confirming that they can-
not cleave DNA upon UV irradiation. Moreover we could not see
any retardation of the SC pUC18 DNA. This is perhaps due to the
dissociation of the complex from DNA during the run of gel
electrophoresis.
Fig. 8. (a) Absorption spectrum of the [Ru(trpy)(pan)]ClO4 in Tris–HCl/NaCl buffer
(10 mM, pH 7.2) solution in the absence (---) and presence (—) of CT DNA
([DNA]:[complex] = 20:1, 40:1, 50:1, 60:1, 65:1, 70:1; the complex concentration
was 30 lM). (b) Fluorescence spectrum of the [Ru(trpy)(pan)]ClO4 in Tris–HCl/NaCl
buffer (10 mM, pH 7.2) solution in the absence (---) and presence (—) of CT DNA
(kexcitation = 480 nm, [DNA]:[complex] = 40:1, 50:1, 60:1, 65:1, 70:1; the complex
concentration was 30 lM).
[Ru(bpy)(pan)(py)]ClO4 and [Ru(bpy)(pan)(pic)]ClO4 complexes
around ꢁ1.6 V versus SCE, which is assigned to reduction of the
coordinated polypyridine (trpy or bpy) ligand [13].
3.4. DNA binding properties
As cationic transition metal complexes containing polypyridine
ligands are known to display DNA interaction properties [15], we
have also explored such possibility in the three such cationic com-
plexes, viz. [Ru(trpy)(pan)]ClO4, [Ru(bpy)(pan)(py)]ClO4 and [Ru-
(bpy)(pan)(pic)]ClO4. These three complexes are fairly soluble in
water, another property welcome in DNA binding agents. Interac-
tion of the [Ru(trpy)(pan)]ClO4 complex with DNA has been mon-
itored initially by absorption and fluorescence spectroscopic
studies in Tris–HCl buffer (pH 7.2) solution. In the absorption spec-
troscopic studies, the maximum at 480 nm, displayed by the com-
plex only, is found to increase in intensity upon addition of DNA.
The intensity increases with increasing ratio of [DNA]:[complex]
4. Conclusions
The present study shows that 1-(20-pyridylazo)-2-naphthol
(Hpan) can bind strongly to ruthenium as a monoanionic N,N,O-
donor affording stable complexes. The mixed-ligand [Ru(bpy)-
(pan)Cl] complex exhibits interesting reactivity via dissociation
of the Ru–Cl bond and such reactions with different bridging and

S. Basu et al. / Polyhedron 27 (2008) 2943–2951
2951
6113;
terminal ligands are currently under exploration. The [Ru(trpy)-
(pan)]ClO4, [Ru(bpy)(pan)(py)]ClO4 and [Ru(bpy)(pan)(pic)]ClO4
complexes can effectively bind to DNA without causing damage
to the DNA double helix.
(c) A.K. Das, A. Rueda, L.R. Falvello, S.M. Peng, S. Bhattacharya, Inorg. Chem. 38
(1999) 4365;
(d) A.K. Das, S.M. Peng, S. Bhattacharya, J. Chem. Soc., Dalton Trans. (2000)
181;
(e) F. Basuli, S.M. Peng, S. Bhattacharya, Inorg. Chem. 39 (2000) 1120;
(f) F. Basuli, S.M. Peng, S. Bhattacharya, Inorg. Chem. 40 (2001) 1126;
(g) I. Pal, F. Basuli, T.C.W. Mak, S. Bhattacharya, Angew. Chem., Int. Ed. 40
(2001) 2923;
Acknowledgements
(h) K. Majumder, R.J. Butcher, S. Bhattacharya, Inorg. Chem. 41 (2002)
4605;
(i) R. Acharyya, S.M. Peng, G.H. Lee, S. Bhattacharya, Inorg. Chem. 42 (2003)
7378;
(j) A. Das, S.M. Peng, G.H. Lee, S. Bhattacharya, New J. Chem. 28 (2004)
712;
(k) S. Nag, P. Gupta, R.J. Butcher, S. Bhattacharya, Inorg. Chem. 43 (2004)
4814;
(l) P.K. Sinha, L.R. Falvello, S. Bhattacharya, Indian J. Chem. 43A (2004)
1846;
(m) P. Gupta, S. Dutta, F. Basuli, S.M. Peng, G.H. Lee, S. Bhattacharya, Inorg.
Chem. 45 (2006) 460;
(n) S. Halder, R. Acharyya, S.M. Peng, G.H. Lee, M.G.B. Drew, S. Bhattacharya,
Inorg. Chem. 45 (2006) 9654;
The authors thank the reviewers for their critical comments,
which have been useful in revising the manuscript. They also thank
Prof. P.K. Bharadwaj, Department of Chemistry, Indian Institute of
Technology, Kanpur, for his help with the X-ray diffraction data
collection on the crystal of [Ru(bpy)(pan)Cl]. Thanks are also due
to Prof. S. Pal of University of Hyderabad (for recording the esr
spectra) and, Prof. K.P. Das and Mr. P. Dey of Bose Institute (for
their help in recording the fluorescence spectra). Professor Michael
G.B. Drew thanks the Engineering and Physical Sciences Research
Council, United Kingdom and the University of Reading, for finan-
cial help. S. Halder thanks the University Grants Commission, New
Delhi [Grant No. 10–2(5)2004(1)-E.U.II] for her fellowship.
(o) S. Nag, R.J. Butcher, S. Bhattacharya, Eur. J. Inorg. Chem. (2007) 1251.
[3] (a) C.D. Hubbard, A.D. Pacheco, J. Inorg. Nucl. Chem. 39 (1977) 1373;
(b) M. Katsura, I. Taira, I. Tasuku, F. Masatoshi, Bull. Chem. Soc. Jpn. 51 (1978)
1743;
Appendix A. Supplementary data
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CCDC 644619 and 644622 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@ccdc.cam.ac.uk. Full details of data collection are provided
in Table S1and composition of selected molecular orbitals in Table
S2. Packing diagrams showing non-covalent interactions in com-
plexes [Ru(trpy)(pan)]ClO4 (Fig. S1), [Ru(bpy)(pan)Cl] (Fig. S2)
and [Ru(PPh3)2(pan)Cl] (Fig. S3), and 1H NMR spectrum of [Ru
(pan)2] (Fig. S4) have been deposited. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.poly.2008.05.023.
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