-
Indian Journal of Che mistry Vol. 40A, December 200 I , pp.
1288-1294
Synthesis, spectroscopic, luminescence, electrochemical and
antibacterial studies of ruthenium(II) polypyridyl complexes
containing 3-hydroxyflavones as
co-ligand
Lall an Mishra· & Ashok K. Singh .
Fac ulty of Sc ience, Che mistry Department , Banaras Hindu Uni
versity, Varanasi 221 005 , India
Received 27 October 2000; revised 30 January 2001
Some mononuclear Ru(ll ) polypyridy l (2,2'-bipyridy l/1,
10-phenanthro line) compl exes contammg 3-hydroxy-4'-2 4
substituted Oavones (L H and L H) as co-ligands have been sy
nthesised and characteri sed on the bas is o f the ir elemental I I
I
analyses, FAB mass and spectral (IR , UV/visible, H NMR, and H-
H COS Y NMR) data . Luminescence, e lectroche mi cal and anti
bacteri al properties of the ligands and their Ru(ll ) compl exes
have also been d iscussed.
3-Hydroxy fl avone (fl avonol) deri vat ives have been used as
important yell ow dyes since ancient time. For example, weld; a
yellow dye deri ved from the seeds and leaves of Raseda lutelo L,
commonly known as dye's rocket, is used in the production of
Lincoln green' . Another fl avone deri vati ve, quercetin has also
been reported to have been used as dye but it degrades in li ght.
Both type of dyes have been reported2 to show wide sp·ectrum
antiviral and antitumor activities also. It has been observed that
these fl avones interfe re with the initi al steps of the sy
nthesis of viral RNA. Although the molecul ar mechani sm of such
activities is not understood completely, yet there is a probability
th at they inhibit the fo rmati on of minus strand RN A of poliov
irus by interacting with one of the proteins involved in the
binding of the virus-repli cation complex to ves icu lar membrane
where virus rep lication takes place. Thus, the li mited attention
paid to chelates of fl avanols and their bioacti viti es, prompted
us to attach some flava nol deri vati ves with ruthenium(H)
polypyridy ls, as polypyridyls of Ru(JI ) are objects of interest
in view of their long-li ved charge-transfer excited state
properties bes ides their signi fica nt redox behaviour3.
Materials and Methods Analytical grade solvents,
4-benzyloxybenzaldehyde,
4-chlorobenzaldehyde, 2'-hydroxyacetophenone and RuC1 3.3H20
were purchased fro m Aldrich and used as such, whereas
cis-[Ru(bpy)2 CI 2].2H20 (bpy = 2,2'-bipy ridine) and [Ru(phenh
CI2] (phen = I, I 0-phen-anthroline) were prepared using reported
method
4•
Progress of the reactions was monitored by thin layer
chromatographic technique. All the complexes were prepared under
nitrogen atmosphere. UV /Vi s spectra of the complexes were
recorded at room temperature using a Varian Cary 2390
spectrophotometer. However, their 1 H NMR spectra were recorded on
Jeol FX 90Q FT NMR spectrometer. FAB-mass and elemental analyses
data were obtained from CDRI Lucknow, India. Electrochemi cal studi
es of the complexes were carri ed out on a Electrochemi cal
Interface SI 1287 potentiostat. The lumi nescence data were
obtained from the University of Sevill a, Spain.
Synthesis of ligands The li gands were synthes ised usin g a
reported2
method (Scheme-l ). 2'-Hydroxy-acetophenone (0.02 mol, 2.72g)
and 4-benzy loxybenzaldehyde (0.02 mol, 4.24 g) were di ssolved
together in ethanol (200 cm3) under stirring to whi ch aqueous NaOH
(50%, 12cm3) was added dropwise . After stirri ng the reaction mi
xture fo r 96 h at roo m temperature, it was diluted with water and
then acidified with HCI (1 0%). The precipitate thus obtained was
filtered off and crystalli zed from eth anol and dried in vacuo.
2'-Hydroxychalcone (L 1 H) thus obtained (330 mg) was di ssolved in
MeOH (l 5 cm3) under sti rri ng and to thi s warm H20 2 (30%; I 0
cm
3) was added over a period of
15 min. The solution was diluted wi th H20 fo llowed by
acidification by HCI ( I 0%). The CH2Cl2 layer, after drying on
Na2S04, was evaporated and the residue obtai ned (eH) was
crystallised from methanol; m.p. 168°C. 2'-Hydroxyacetophenone and
4-chlorobenzaldehyde were also condensed similarl y. The
corresponding chalcone (L3H) isolated was then
-
MISHRA eta/.: Ru(ll ) POLYPYRIDYL COMPLEXES CONTAINING 3-H
YDROXYFLAYONES 1289
+ 0 . EtOH OH POCH,$ CH, H NaOH (50%)
(~ = p~nyl)
Synthesis of ligands
Scheme I
H
LH
l NaOH(lO%) H,o, (30%)
OCH2~
rAIOH }-10;-cl ~L3H-
l NaOH(lO%) H,0,(30%) rf)(0)-(0)-cl ~0;-
cyclised into 3-hydroxyflavone (L 4H) (Scheme I). These products
were characterised by comparing their IR, 1H NMR and 13C NMR with
earlier reported5 data.
Synthesis of complexes Ruthenium (II) complexes (M 1 and M5)
were
prepared by the following general procedure. An ethanolic
solution (5 cm3) of cis-[Ru(bpyh Ch].2H20 (1 mmol, 520 mg)
containing ethanolic solution (5 cm3) of respective ligands (L2H
and L 4H) , (1 mmol, 344 mg and 272 mg respectively) and a few
drops of Et3N were refluxed together for 14 h. After cooling at
room temperature, respective solutions were concentrated in vacuo.
To the concentrated solutions, saturated aqueous solution of NH4PF6
was added and the corresponding solids thus obtained were washed
successively with H20 , EtOH and then Et20 , dried in vacuo and
purified by column chromatographic technique using neutral alumina
as column support and CH3CN aqueous KN03-water (7 : 1:0.5 v/v) as
eluents6. Eluates thus obtained were concentrated and corresponding
residues were then dissolved in acetone and reprecipitated by add
iti on of saturated aqueous sol uti on of NH4PF6. The crystalline
solids thus obtained were fi ltered and washed successively with
H20 , EtOH and Et20 and then dried in vacuo. The complexes of the
ligands L
2H and L
4H with
[Ru(phenh C1 2l (M2 and M6) were also prepared and
purified using simjlar procedures as above. The analytical data
along with other physical
properties of the ligands and their complexes are given in Table
I whereas a representative FAB-rriass spectrum of complex M
6 is shown in Fig. 1. The
complex M4
was prepared by direct debenzoylation2
of the complex M2
(1 mmol) in CH3COOH (10 cm3
)
and cone. HCI (5 cm3) for 6 h on a water-bath. After cooling at
room temperature, solution was diluted with water (25 cm3) when a
precipitate was obtained, which was filtered and washed
successively with excess H20, EtOH and Et20 and was crystallized
from EtOH. However, the complex M
3 was isolated
and purified using the method reported for M 1
condensing the debenzoylated2 ligand eH (L2,H) and cis-[Ru(bpyh
CI2] .2H20.
Results and Discussion The complexes were found to be thermally
stab le
and soluble in acetone, acetonitrile, DMF and DMSO. The molar
conductances of the complexes recorded in CH3CN solution (10.
3 M) were found to be consistent with the reported values7 .
IR Spectra of chalcone (L1H and L
3H) gave
characteristic peaks at 3600, 1640 and 980 em· ' which are
assigned to v( -OH), v(C=O) and v(CH=CH) respectively in view of
earlier report5 . IR spectra of eH and L 4H also showed similar
peaks as above except that the v(CH=CH) was absent and a new peak
was observed at 3450 em· ' ass igned as v(OH). These evidences
support the cyclisation of L 1H, L3H into corresponding L 2H and L
4H products. However, in the IR spectra of the complexes, a peak
observed at -1620 cm- 1 in the spectra of the free ligands.
For further support of the cyclisation, NMR ( 1 H and 13C)
spectra of L 1H and L3H recorded in DMSO-d6 were compared with the
corresponding NMR spectra for eH and L 4H. From the peak positions
shown in Table 2, it is evident that HC=CH protons exhibit a
doublet at 8 7.01-7.06 ppm in the spectrum of L 1H and L3H. Major
peaks observed at 8 193.61, 163.55, 148.24 and 136.08 ppm in the
13C NMR spectrum of L 1 H are ass igned as C=O, C-OH (C-~) and
(C-a) carbons, respectively which are shifted to 8 173.3, 155 .3 1,
145 .29 and 137.76 ppm, respectively in the 13C NMR spectrum of
L
2H. Similary, peaks
observed at 8 193.85, 164.03, 144.29 and 136.08 ppm in the 13C
NMR spectrum of L3H ass igned as above also shifted to
8173.81,155.76,144.17 and 138.94 respectively in the spectrum of
L
4H. Thus, the
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1290 INDIAN J CHEM, SEC A, DECEMBER 2001
Table I-Physical and analytical data of Ru (II) complexes
Complexes FAB-mass data Yield % Found (Calc.) (colour)
I 2 [M- PF; ] 757 (756) 48 .3 M [RuL (bpy)2]PF6;
(Dark brown) 2 2
[M-2 PF6- ] 805 (804) 57. 1 M [RuL (phen)2](PF6h .2H20
3 2' (Light brown)
M [RuL H(bpyh](PF6) 2.7H20 [M] 957 (957) 50.0 (Yelowish)
M4
[RuL2 'H(phenh]2PF6.EtOH [M] I 005 (I 005) 48.0 (Reddish
yellow)
5 4 + 50.5 M [RuL (bpy)2] PF6.4EtOH [M- PF6- ] 685 (684)
(Yellowish) 6 4 [M] 878 (878) 49.8 M [RuL (phen)2] PF6.EtOH
(B lack)
100 \S4
90 l\S
80
70 1!6
60
50 120
'0 640 28! 7ll
30 791
20 467
10 171
0 100 200 300 ,00 500 600 700 800 900
Fig. 1-FAB-mass spectrum of the complex [RuL \Phen)2](PF6)
cyclisation of L 1 and L3 into corresponding compounds L2H and L
4H is supported by their 1H and 13C NMR spectra.
Calc. (Found), % .c H
56.0 (55 .9) 3.4(3.4)
48.4 (48.8) 3 . .1 (2.7)
38.4 (38.3) 3.6 (3.6)
53.9 (54.3) 3.7 (3 .5)
50:8 (50.9) 4.7 (4.8)
53.3 (53.2) 3.5 (3.3)
.Qlll N (ohm·1 cm2 rno1" 1)
6.2 (6."2 ) 260
5.3 (4.9) 270
5.5 (5.1 ) 230
5.8(6.1 ) 260
5.5 (5.5 ) 150
5.3 (6.0) 147
/-
In the complexes M 1, M
5 and M
6, the deprotonation
of the -OH proton of 3-hydroxy group in eH-L 4H is supported by
the number of counter-anions present in the molecular formula
obtained on the basis of elemental analyses. This is further
supported by the disappearance of -OH proton in their 1H NMR
spectra. However, in the spectra of the complexes M
2,M3,M
4, OH proton did not deprotonate upon
coordination . Furthermore, 1 H NMR spectra of the complexes
M
3 and M
4 did not show any peak at -8
5.2 ppm as assigned for> OCH2 protons in the spectra of the
complexes M
1 and M
2• However, additional
peak due to -OH proton at 8 9.6 ppm and 8 9.5 ppm appeared in
the spectra of M3 and M4 complexes respectively. In the spectra of
the complexes peaks observed at 8 7 .1 2. - 7.42 ppm were assigned
to four protons of aromatic group attached to -OCH2(Ph) (M
1
and M2
) along wi th four protons of aromatic group attached to
chromone pati (Pht) at 8 8.0-8.6 ppm (Fig. 1 ).
Fig. 2- 1H- 1H COSY NMR spccti"U m of complex [RuL \bpy)2]PF6 in
DMSO-c/6 at room t emp~ra t re.
However, five protons of aromatic group attached to -OCH2(Ph
-
MISHRA et al.: Ru(ll) POL YPYRIDYL COMPLEXES CONTAINING
3-HYDROXYFLA VONES 1291
Table 2- 1H NMR data of ligands and their Ru(ll) complexes
Compound 8 ppm
L1H 5.19 (2H, s, -OCH2), 7.0 (2H, d, Ph) 7.06 (2H, d, CH=CH),
7.42 (5H, m, Ph Ph-OCH2 > PhOH.
Thus, based on the elemental analysis and spectroscopic data,
structures for the complexes are proposed as shown in Structure
I.
-
1292 IND IAN J CHEM, SEC A, DECEMBER 200 1
Table 4 - Electrochemical data* of li gands and their Ru (II )
complexes
Compound Oxidation peaks, £ 112/V Reduction peaks, £ 112/V
0.4, 0.95 , 1.0, 1.36
0.18, 1.0
0.07 ,0.5, 1.08, 1.5 0.6, 1.05, 1.05, 1.3 1 0.36,0.54, 1.03, 1.5
0.32,0.64,0.8 1' 1.32 0.1 8,0.54,0.86, 1.12 0.18,0.54,0.9, 1.46
- 0.54,- 1.06,- 1.65 - 1.5,- 1.0
- 1.95,- 1.45,- 1.62 - 0.93,- 1.30, - 1.45, - 1.8 1 - 1.70, -
1.54, -0.45 -0.7 1, -1.45,- 1.92 - 0.78,- 1.33,- 1.9 -0.76, -1.25,
-1.54, -0.9
*Obtain in acetonitrile solution (I o·3 M) containing 0.1 M
tetrabutylammonium perchlorate (TBAP) at room temperature using
Ag/AgCI as a reference electrode. Scan rate was 100 mY/s.
Electrochemistry The electrochemical behaviour of the free
ligands
and their metal complexes were studied by cyclic voltammetry in
CH3CN (10.
3M) containing TBAP as supporting electrolyte (warning
:perchlorate salts are explosive , use of small amounts is
recommended) graphite disc as working electrode, platinum wire as
an auxiliary electrode and Ag/ Ag + as reference electrode. The
cyclic voltammogram of free fl avone L2H showed three oxidation
peaks at 0.4, 1.0, 1.36 V with very weak conesponding reductions.
However, in the cyclic voltammogram of its metal complexes (Mt
& M2) three oxidation peaks were also observed at 0.5,
(1.08-1.05) and 1.34 V with concomitant increase in the current
intensity of the peaks at 1.08-1.05 V indicating that the ox
idation Ru(II)- Ru(III) also occurs in the same potential range. In
the complexes M
5 and M
6 stronger oxidati on peaks were
observed as compared to those obtained for I ?
complexes M and M- e,xcept that the peaks are diminished in the
case of M
5 and M
6 (Fig. 3)
com plexes with concomitant en hancement in peak intensity at
0.54 V. This indicates that in these complexes Ru (II) -7 Ru(HI) ox
idation occurs at 0.54 V. Additionally , it was quite interesti ng
to note that in the cyclicvoltammogram of the complexes M3 and
M
4
Ru(U) -7 Ru(III) oxidati on occurred at (1.03-0.80) V. Thus the
oxidation of Ru (II) -7 Ru(III) is found to be tuned with the
functiona lity present at the para position of terminal aromati c
group and oxidation potential is found to decrease with the
decreasing electron donor power of the groups attached, viz. ,
PhOCH2 ~ PhOH > PhCI. Reduction of ligands L
2H and L 4H was found to occur at 0.6, 0.3, 1.06 to 1.0 V
(broad) which in complexes containing polypyridy l
40
20
0
-
MISHRA et al.: Ru(II) POLYPYRIDYL COMPLEXES CONTAINING
3-HYDROXYFLAVONES 1293
Table 5-Luminescence data* of Ru (II) complexes
Compound Amax (em) !Rei# (em) Rel (nm)
Mt 412,530 0.01, 0.15 0.004, 0.006 , 416,530 0.01, 0.25
0.0007,0.01 M-
MJ 413,473,529 0.06, 0.06,0.19 0.003,0.003, 0.008
M4 411,530 O.Q7, 0.20 0.003,0.008 Ms M6
*Recorded in acetonitrile at 25°C. Solutions were 10·6 M (Acx
=440 nm), #Relative to standard [Ru(bpy)3f+ (-)No luminescence was
observed.
.--,200 :J
.Q
£ Vl c
$>! c
0
450 500 550
Wavelength {nm)
600
Fi g. 4-Luminesccnce spectrum of the complex !RuL\ bpyh l PF6 in
CH3CN ( 10-
6M) at room temperature.
region at -450 nm for ruthenium complexes 10 is quite weak and
the emissions are mainly originating from the ligand part. Since
the redox orbitals of both free ligands and Ru(Il) complexes are
found to li e at similar energies as is ev ident from their
oxidation potential data , it was found difficult to make clear
distinction on the origin of the emissions. Excited state life time
measurement of all the compounds in CH3CN (10'
3 M) was made using a single photon counting FL 900 CD Edinburgh
Analytical Instrument at the Physical Chemistry Department,
University of Sevilla, Spain and the experimental data were
processed by the deconvolution method and was found to be < Ins
in a ll compounds. Since measuring limit of the instrument was upto
I ns level, so we could not measure below thi s region.
Thus redox and emission behaviour of the free ligands and their
Ru(ll) complexes were found quite similar indicating that energy
gaps between LUMO of
Table 6-Antibacterial activity of the ligands and their Ru (II)
complexes agai nst E. Coli in DMSO*
Compound (%)inhibition
L2H 17.95
L4H 17.30
Mt 37.52 Mz 17.30 MJ 40.78 M4 20.00 Ms 37.52 M6 75.04
*Concentration of the solution was 10 J1M.
free ligands and HOMO of the metal in complexes are very
low.
Antibacterial activity Antibacterial activity of the free
ligands and their
complexes was evaluated in DMSO at 10 pM concentration and the
data shown in Table 6 are expressed as the (%)inhibition in growth
of bacteria against the control (free DMSO) in which growth was
considered to be I 00%. The activity data were obtained by
subtracting any inhibition shown by the solvent (DMSO). The data
show that activity in all cases could be related to the presence of
groups at 4-position of terminal phenyl ring (Fig. I) and is found
to be most significant in case there is a ch loro group attached at
position 4-of the ring. However, in other cases where OCH2Ph or OH
groups are at 4-position activity remain s almost the same. Another
feature for the variation of activity could be related with the te
rminal ligand attached to ruthenium in the complexes. Activity is
hi gher when it is I , I 0-phenanthroline ring whereas it is lower
in case of 2,2'-bipyridine ring as terminal li gand. This findin g
is consistent with the reported observation3· 11 • In order to
explore the molecular mechanism of the antibacterial behaviour, we
allowed the most act ive compound (M
6) to interact with plasmid DNA
PBR322 at different compound-nuc leotide binding ratios (D/N).
Since, DNA is thought to be most common site for interaction in
view of an earlier report 12. Similar electrophoretic mobility 13
of the DNA in the presence and absence of compound clearly
indicates that the re is no binding of the compound with the
tertiary structure of the D A. Thus antibacterial property of the
complexes foll ows a different mechanism; probably it is in
consonance with the theoretical density calculation made by Mishra
et al. 14.
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1294 INDIAN J CHEM, SEC A, DECEMBER 2001
Acknowledgement One of the authors (LM) acknowledges UGC,
New
Delhi for financial assistance and Dr. A K Tripathi , Biotech. ,
BHU, Varanasi for his help in evaluating antibacterial
activity.
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