Mononuclear Ru(II) carbonyl complexes containing quadridentate schiff bases as N2O2 donors: Synthesis, spectral, catalytic properties, and antibacterial activity

Article abstract of DOI:10.1080/15533174.2011.555874

Reactions of Ru₃(CO)₁₂ with novel tetradentate Schiff bases ligands H₂L^1-5 in benzene under reduced pressure afforded the formation of new Ru(II) complexes of the type [Ru(CO) ₂(L^1-5)] wher

Full text of DOI:10.1080/15533174.2011.555874

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Mononuclear Ru(II) Carbonyl Complexes Containing
Quadridentate Schiff Bases as N2O2 Donors:
Synthesis, Spectral, Catalytic Properties, and
Antibacterial Activity
Ayman A. Abdel Aziz a
a Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt
Published online: 22 Apr 2011.
To cite this article: Ayman A. Abdel Aziz (2011) Mononuclear Ru(II) Carbonyl Complexes Containing Quadridentate Schiff
Bases as N2O2 Donors: Synthesis, Spectral, Catalytic Properties, and Antibacterial Activity, Synthesis and Reactivity in
Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:4, 384-393
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:384–393, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.555874
Mononuclear Ru(II) Carbonyl Complexes Containing
Quadridentate Schiff Bases as N2O2 Donors: Synthesis,
Spectral, Catalytic Properties, and Antibacterial Activity
Ayman A. Abdel Aziz
Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt
of their biological activity including anti-tumor, antibacterial,
fungicidal, and anti-carcinogenic properties.[15,16] However, lit-
erature survey reveals that little work has been done on bi-
ological studies of Schiff base complexes of ruthenium.[17−19]
Multidentate ligands are extensively used in coordination chem-
istry since they can be applied in the construction of new
frameworks with interesting properties.[20] Among these lig-
ands, tetradentate Schiff bases are well known to coordinate
with various metal ions forming stable compounds, and some
of these complexes are recognized as oxygen carriers.[21] Fur-
thermore, tetradentate Schiff base complexes are increasingly
important for designing metal complexes related to synthetic
and natural oxygen carriers.[22] Carbonyl derivatives of tran-
sition metals, on the other hand, are useful intermediates in
the synthesis of important coordination compounds[23−26] and
have applications in catalysis of important reactions such as
epoxidation, carbonylation, hydrogenation, and hydroformyla-
tion reactions.[27−30] Some ruthenium(II) Schiff base complexes
Ru(PPh3)2L), where L is a tetradentate ligand derived from
condensation of salicylaldehyde with 1,2-diaminobenzene, 2,3-
diaminotoluene, 1,2-diaminocyclohexane, were reported and
tested for catalytic oxidation of primary and secondary alcohols
in the presence of N-methylmorpholine-N-oxide co-oxidant.[31]
Photo substitution reactivities of tetetradentate Schiff base–
ruthenium(II) complexes containing phosphorus or sulfur donor
atoms were reported.[32] In a previous work, the interaction of
the tetradentate Schiff base N,N\-bis(salicylidene)4,5-dichloro-
1,2-phenylenediamine with M(CO)6 (M = Cr and Mo) un-
der different conditions afforded the dicarbonyl precursors
[Cr(CO)2(H2L)], [Mo(CO)2(L)], the oxo complex [Cr(O)(L)],
and the dioxo complex [Mo(O2)(L)].[33]
Reactions of Ru3(CO)12with novel tetradentate Schiff bases
ligands H2L1−5 in benzene under reduced pressure afforded the
formation of new Ru(II) complexes of the type [Ru(CO)2(L1−5)]
where L = anion of N,N/bis(salicylaldehyde)4,5-dimethyl-1,
2-phenylenediamine (saldmophn), N,N/bis(salicylaldehyde)4,5-
dichloro-1,2-phenylenediamine (saldcophn), N,N/bis(salicylalde-
hyde)2,3-diaminonaphthalene (saldnap), N,N/bis(o-vanillin)4,
5-dimethy-l,2-phenylenediamine (ovaldmophn), and N,N/bis(o-
vanillin)4,5-dichloro-1,2-phenylenediamine (ovaldcophn), respec-
tively. All the newly synthesized complexes were investigated
using elemental analysis, FT-IR, UV-Vis, mass spectrometry,
and magnetic measurements. An octahedral structure has been
tentatively proposed for the new complexes. All the new complexes
have been found to be efficient catalysts for oxidation of primary
and secondary alcohols into corresponding carbonyl compounds
in presence of N-methylmorpholine-N-oxide (NMO) as the source
of oxygen. In view of the biological activity, the ligands showed
moderate bactericidal activities, where upon chelation with
ruthenium, the bactericidal activities increased and the results
have been compared with some known antibiotics.
Keywords bactericidal activities, catalytic activity, characterization,
N2O2 donors, ruthenium, synthesis
INTRODUCTION
Organometallic complexes containing Schiff bases have
played an important role in coordination chemistry of transition
metals, mainly due to their stability, ease of preparation, struc-
tural variability, and variety of applications.[1–10] Schiff bases
are a class of chelators capable of forming coordinate bonds
with many of metal ions through both azomethine groups and
phenolic groups, or via its azomethine or phenolic groups.[11−14]
Chemotherapeutic Schiff bases and their complexes receive sig-
nificant interest and attention of inorganic biochemists because
Herein, the synthesis and characterization of a series of novel
coordination compounds of ruthenium(II) Schiff base using
simple and inexpensive tetradentate Schiff base ligands derived
from N,N\-bridged tetradentate ligands involving N2O2 donor
atoms were described. Catalytic activity of the new synthesized
complexes for oxidation of a number of primary and secondary
alcohols in the presence of N-methylmorpholine-N-oxide
(NMO) were investigated. The reported Schiff base ligands
and their complexes were tested for antibacterial activities.
Received 8 September 2010; accepted 31 October 2010.
Address correspondence to Ayman A. Abdel Aziz, Chemistry De-
partment, Faculty of Science, Ain Shams University, Cairo, Egypt.
E-mail: aymanaziz31@gmail.com
384

MONONUCLEAR Ru(II) CARBONYL COMPLEXES
385
TABLE 1
Elemental analyses and some physical properties of the Schiff bases ligands (H2L1−5) and the reported Ru(II) complexes
(Calculated) Found (%)
Mass
Spectrometry
ꢀm
(ꢁ−1 cm2
Yield
(%)
Compounds
M.Wt Color
C
H
N
Cl
—
Ru
—
m/z
mol−1
)
H2L1 C22H20N2O2
63
70
79
89
88
91
77
79
84
87
344.41 Yellow (76.72) (5.85) (8.13)
345 [P+]
—
76.21 5.72 7.98
385.25 Yellow (62.35) (3.66) (7.27) (18.40)
H2L2 C20H14N2O2Cl2
H2L3 C24H18N2O2
H2L4 C24H24N2O4
H2L5 C22H18N2O4Cl2
[Ru(CO)2(L1)](1)
[Ru(CO)2(L2)] (2)
Ru(CO)2(L3)] (3)
[Ru(CO)2(L4)] (4)
[Ru(CO)2(L5)] (5)
—
—
—
—
386 [P+]
367 [P+]
405 [P+]
446 [P+]
—
—
62.19
366.42 Yellow (78.71) (4.95) (7.64)
78.40 4.84 7.58
404.46 Orange (71.27) (5.98) (6.92)
71.01 5.77 6.78
445.30 Yellow (59.34) (4.07) (6.29) (15.92)
3.60
7.25
18.38
—
—
—
—
59.21
(57.71) (3.63) (5.61)
57.49 3.49 5.47
3.82
6.11
15.75
—
—
499.48 Red
540.26 Red
521.43 Red
559.47 Red
600.31 Red
(20.22) 444 [P -2CO]+
19.99
13.9
14.6
10.5
15.8
17.3
(48.91) (2.23) (5.18) (13.12) (18.69) 485 [P -2CO]+
48.77
(59.89) (3.09) (5.37)
59.24 2.99 5.14
(55.81) (3.96) (5.00)
54.99 3.74 4.93
2.12
5.09
13.01
—
18.53
(19.37) 466 [P -2CO]+
19.22
—
(18.05) 504 [P -2CO]+
17.95
(48.02) (2.68) (4.66) (11.81) (16.82) 545 [P -2CO]+
47.98
2.45
4.43
11.69
16.51
Instruments
EXPERIMENTAL
Materials
4,5-dimethyl-1,2-phenylendiammine, 4,5-dichloro-1,2- ph-
enlendiammine 2,3-diaminonaphthalene, salicylaldehyde, 3-
methoy-2-hydroxy-benzaldehyde (o-vanillin), benzyl alcohol,
cinnamyl alcohol, cyclohexanol, butan2-ol, and Ru3(CO)12were
supplied from Aldrich. All solvents were of analytical grade.
IR measurements (KBr Pellets) were carried out on a
Unicam-Mattson 1000 FT-IR. Elemental analyses were carried
out on a Perkin-Elmer 2400 CHN elemental analyzer. Mass
spectra of the solid complexes were performed on a Shimadzu
GC MS-QP 5050A spectrometer. 1H NMR measurements were
performed on JEOL-270 MHz NMR DMSO-d6 as a solvent
and TMS as an internal reference. Molar conductivities of
TABLE 2
The infrared (cm−1) and UV-Vis spectral data of the Schiff bases ligands (H2L1−5) and the reported Ru(II) complexes
Compound
ν (OH)
ν (C=N) ν (C-O)
ν (C≡O)
ν (Ru-O) ν (Ru-N) UV-Vis λmax(nm)(DMSO)
H2L1
3428 (br) 1617 (s) 1278 (m)
3444 (br) 1616 (s) 1279 (m)
3418 (br) 1612 (s) 1277 (m)
3389 (br) 1610 (s) 1276 (m)
3394 (br) 1608 (s) 1274 (m)
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
237a, 328b
283a, 375b
H2L2
H2L3
271a, 343b
H2L4
243a, 273b
H2L5
253a, 303b
[Ru(CO)2(2L1)] (1)
[Ru(CO)2(L2)] (2)
Ru(CO)2(L3)] (3)
[Ru(CO)2(L4)] (4)
[Ru(CO)2(L5)] (5)
—
—
—
—
—
1599 (s) 1378 (m) 2012 (s) 1935 (s) 462 (w)
1603 (s) 1310 (m) 2049 (s) 1978 (s) 437 (w)
1600 (s) 1382 (m) 2017 (s) 1934 (s) 471 (w)
1600 (s) 1320 (m) 2037 (s) 1943 (s) 465 (w)
1598 (s) 1300 (m) 2036 (s) 1956 (s) 468 (w)
497 (w)
467 (w)
506 (w)
500 (w)
478 (w)
240a, 345b, 386c, 521d
274a, 380b, 402c, 573d
265a, 357b, 401c, 533d
227a, 285b, 389c, 532d
272a, 323b, 395c, 555d
1
∗s, strong; m, medium; w, weak; br, broad; c, 1A1g → T1g; d MLCT.

386
A. A. ABDEL AZIZ
ether 40–60, and then recrystallized from hot methanol. The
isolated complexes were dried in vacuo for a few hours. The
purity of the isolated compounds was checked by TLC.
CHO
OH
N
N
NH2
H2N
H3C
+
HO
OH
Determination of Ruthenium Content by Extraction
Spectrophotometry
H2L1
H2L2
H2L3
(saldmophn)
(saldcophn)
(saldnap)
=
H3C
Cl
The percentages of ruthenium in the new synthe-
sized complexes were determined by using 1-nitroso-2-
naphthol reagent[36] by adopting spectrophotometric extraction
technique.[37] A known amount of ruthenium complex was taken
in a platinum crucible; 5 mL of conc. nitric acid was added and
gently heated to get a solid substance. The procedure was re-
peated 3–4 times until complete oxidation was achieved. Then
so formed ruthenium tetroxide was dissolved in 100 ml of dou-
ble distilled water. Aliquots of ruthenium tetroxide ranging from
1 to 5 mL were taken in a series of 50 mL separating funnels.
2 mL each of 1-nitroso-2-naphthol (0.1%) and ascorbic acid
(0.1%) were added. The aqueous phase was made up to 15 mL
with double distilled water. Then, 10 mL of carbon tetrachloride
was added and shaken for few min. The ruthenium-1-nitroso-
2-naphthol complexes formed were extracted into the carbon
tetrachloride layers. Absorbances of these layers were mea-
sured at 645 nm using carbon tetrachloride as blank solution.
The amount of ruthenium was deduced from the calibration
curve.
Cl
CHO
OH
N
N
NH2
H2N
H3C
+
HO
OH
OCH3
OCH3
OCH3
H2L4
(ovaldmophn)
(ovaldcophn)
=
H3C
Cl
H2L5
Cl
SCH. 1.
Catalytic Oxidation of Alcohol
complexes were measured by using Jenway 4010 conductiv-
ity meter. Magnetic moments were determined on a Sherwood
Scientific magnetic moment balance (Model N0: MK1) at room
temperature (25◦C) using Hg[Co(SCN)4] as a calibrant: diamag-
netic corrections were calculated from Pascal’s constants.[34]
Catalytic oxidation of butan-2-ol, benzyl alcohol, cinnamyl
alcohol, and cyclohexanol to corresponding aldehydes and ke-
tones, respectively, by the synthesized ruthenium-Schiff bases
complexes as catalysts were carried out in the presence of NMO
as co-oxidant. To 20 mL CH2Cl2 containing alcohol (1 mmol)
and NMO (3 mmol), Ru(II) complex (0.01 mmol) was added.
The solution was heated under reflux with stirring for 3 h in
Syntheses of the Ligands
The ligands H2L1 and H2L2 were synthesized as described
in literature,[33,35] H2L3,5 were synthesized by a similar way
according to the synthetic route shown in Scheme 1.
To a hot methanolic solution containing 10 mmol of the
amine, 20 mmol of aldehyde was added drop wise. The yel-
low solution obtained was refluxed for 2 h. After cooling, the
precipitate Schiff base was collected by filtration and recrys-
tallized from methanol. The ligands were dried in a desiccator
over anhydrous CaCl2 under vacuum. The resultant product was
washed with ethanol, and the purity of the ligands was checked
by TLC.
˚
the presence of 4 A molecular sieves (0.5 g). The mixture was
filtered and the filtrate was evaporated to dryness and extracted
with diethyl ether. The ether extracts were evaporated to give
aldehydes/ketones that were isolated and quantified. Butan-2-
al, benzaldehyde, cinammaldehyde, and cyclohexanone were
quantified as their 2,4-dinitrophenyl hydrazone derivative.[38]
The catalytic activities of the reported complexes were deter-
mined from yield % and the turn over (T.O.) conversion of
substrate, as follows:
Wt. of oxidized substrate (product)
Yield (%) =
T.O. =
× 100 [1]
Wt. of substartae
mmoles of product
mmoles of catalyst
Synthesis of Ru(II) Carbonyl Complexes
[2]
All the new complexes were prepared by the following gen-
eral procedure. A mixture of Ru3(CO)12 (0.16 mmol) and Schiff
base (0.48 mmol) in ca. 20 mL benzene was heated under re-
duced pressure in sealed tube for 1 h. The reaction mixture was
cooled and the solvent was removed on a vacuum line. The red
Antibacterial Studies
The ligands and their complexes have been tested for the
residue formed was washed for several times by hot petroleum in vitro growth inhibitory activity against four bacterial strains

MONONUCLEAR Ru(II) CARBONYL COMPLEXES
387
TABLE 3
The 1H NMR spectral data (δ, ppm) of the Schiff bases ligands (H2L1−5) and the reported Ru(II) complexes
H3C-attached to
H3CO-attached to
Compound
OH(phenolic)
CH=N
Aromatic protons
diamine benzene ring
salicyaldhyde ring
H2L1
13.12 (s)
12.57 (s)
12.93 (s)
12.99 (s)
12.49 (s)
—
8.96 (s)
9.01 (s)
9.09 (s)
8.43 (s)
8.55 (s)
8.97 (s)
8.24 (s)
8.92 (s)
8.37 (s)
8.49 (s)
6.95-7.69 (m)
6.98-7.85 (m)
6.99-8.00 (m)
6.88-7.04 (m)
6.92-7.78 (m)
6.47-7.70 (m)
6.49-7.48 (m)
6.87-8.02 (m)
6.48-7.10 (m)
6.42-7.58 (m)
2.33 (s)
—
—
—
—
2.31
—
—
2.35
—
—
—
—
3.65
3.84
—
—
—
3.66
3.83
H2L2
H2L3
H2L4
H2L5
[Ru(CO)2(L1) ] (1)
[Ru(CO)2(L2) ] (1)
[Ru(CO)2(L3) ] (3)
[Ru(CO)2(L4) ] (4)
[Ru(CO)2(L5) ] (5)
—
—
—
—
s, singlet; m, multiplet.
(2 Gram positive and 2 Gram negative) involved in dental caries IR Spectra
because bacteria can achieve resistance to antibiotics through
biochemical and morphological modifications.[39] Gram +ve
were Staphylococcus aureus and Bacillus cereus, and Gram -ve
were Escherichia coli and Pseudomonas aeruginosa. The bac-
terial stock cultures were obtained from Microbiology Depart-
ment, Faculty of Science, Ain Shams University, Cairo, Egypt.
The compounds tested were dissolved in DMSO at a final con-
centration of 25, 50, and 100 µg/ml. Antibacterial activity was
measured using a well diffusion method[40] Petri plates contain-
ing 20 mL of Mueller Hinton agar medium were inoculated with
a 24 hour culture of the bacterial strains. Wells (6 mm diameter)
were punched in the agar and filled with microorganisms. The
well was then filled with the test solution using a micropipette,
and the plates were incubated at 37◦C for 24 h for bacteria.
During this period, the test solutions diffused and the growth
of the inoculated microorganisms was affected. The effective-
ness of an antimicrobial agent was assessed by measuring the
zones of inhibition around the well. The diameter of the zone
is measured to the nearest millimeter (mm). Triplicates of each
plate have been done. The average of three replicates for each
extract, antibiotic, and combination were calculated. Both chlo-
ramphenicol and streptomycin was used as standard antibiotics.
Tentative assignments of the important IR bands have been
made, are summarized in IR data of the Schiff base ligands,
and their metal complexes listed in Table 2. In general, the
ligands exhibited very similar IR features. In order to study the
binding of Schiff base ligands to the metal in the complexes,
the IR spectrum of the free Schiff base ligands were compared
with that of the metal complexes. The discussion is confined to
the most important vibrations of the 4000–400 cm−1 region in
relation to structure.
Schiff bases are capable of forming coordinate bonds with
many metal ions through both azomethine group and pheno-
lic group or via its azomethine or phenolic groups.[42] All the
Schiff bases showed a broad band of medium intensity in the
3418–3444 cm−1 region characteristic of ν(O–H) of the phe-
–1
nolic group. The strong band in the region 1612–1617 cm
,
characteristic of the azomethine group present in the free lig-
ands, has been shifted to lower wave numbers in the range
1599–1603 cm−1 in accordance with the coordination of the
azomethine function to the metal ion for all the complexes,[43,44]
and this lowering of the wave number may be attributed to
the decrease in electron density on the nitrogen atom of the
azomethine group. A medium band corresponding to phenolic
oxygen ν(C–O) was observed in the region 1277–1279 cm−1
.
On complexation, this band is shifted to higher frequency in
the range 1310–1382 cm−1 for Ru(II) complexes. This was
further supported by the disappearance of the νOH in all the
complexes. The co-ordination of the azomethine nitrogen and
phenolic oxygen were further supported by the appearance of
two non-ligand bands at 437–471 cm−1 and 467–506 cm−1
due to ν(Ru–O)[42] and ν(Ru–N)[45−48], respectively. The IR
spectra of the complexes (1–5) exhibited two terminal car-
bonyl stretching bands in the range (2049–1834), which are
attributed to symmetric and asymmetric frequencies of two CO
groups.[49] The number of carbonyl bonds provides important
clues to the environment of the metal center.[50] The presence
RESULTS AND DISCUSSION
Physical characteristics, elemental analysis, and mass spec-
tral of the free ligands and its complexes are given in
Table 1. The analytical data are in good agreement with the pro-
posed molecular formula. Magnetic measurements were carried
out on a Sherwood Scientific magnetic balance according to the
Gouy method. The new Ru(II) complexes 1–5 were found to
have diamagnetic characteristics to the low spin d6 Ru(II) cen-
ters in these complexes. The molar conductivity (ꢀm) of 1×10−3
M solutions of Ru(II) complexes (1–5) in DMSO indicate that
they are non-electrolyte.[41]

388
A. A. ABDEL AZIZ
FIG. 1. IR spectrum of Schiff base ligand (H2L2).
FIG. 2. IR spectrum of Ru(II) carbonyl complex (2).

MONONUCLEAR Ru(II) CARBONYL COMPLEXES
389
TABLE 4
A comparison of the spectra of the free Schiff bases with the
spectra of the corresponding complexes shows that the chemical
shift observed for the OH protons in the ligands was not observed
in any of the complexes. The absence of –OH signals, indicates
deprotonation of the hydroxy group of the Schiff bases and
confirms the bonding of oxygen to the metal ions (C O Ru).
The presence of a sharp singlet for the –C(H)=N proton clearly
indicates that the magnetic environment is equivalent for all
such protons, suggesting the presence of a planar ligand in these
complexes.[57]
In the complexes, the position of azomethine signal was
downfield in comparison with that of the free ligands, sug-
gesting deshielding of the azomethine proton due to its co-
ordination to ruthenium through the azomethine nitrogen. Multi-
plets are observed around δ 6.8–8.1 ppm; all the complexes have
been assigned to aromatic protons of Schiff base ligands.[58,59]
The multiplets of aromatic protons appeared within the range
7.95–8.00, ppm and they were slightly affected by chelation.
In ligand H2L1 and its complexes, the proton peak of –CH3
at 2.33 ppm was not affected by chelation. One sharp singlet
appeared at δ 3.65–3.84 ppm, which indicates the presence of
methoxy proton in the spectra of the ligands H2L4−5, and in their
corresponding ruthenium complexes no significant change was
observed.[60]
Catalytic oxidation of alcohols by Ru(II)-Schiff bases
complexes in the presence of NMO
Complex
1
Substrate
Producta Yield(%)b Turnoverc
Butan-2-ol
Benzyl alcohol
Cinnamyl alcohol
Cyclohexanol
Butan-2-ol
Benzyl alcohol
Cinnamyl alcohol
Cyclohexanol
Butan-2-ol
Benzyl alcohol
Cinnamyl alcohol
Cyclohexanol
Butan-2-ol
Benzyl alcohol
Cinnamyl alcohol
Cyclohexanol
Butan-2-ol
A
A
A
K
A
A
A
K
A
A
A
K
A
A
A
K
A
A
A
K
90
71
83
36
87
72
82
35
86
71
80
33
91
77
85
32
90
74
82
32
92
75
88
36
89
79
91
35
88
73
82
33
93
77
86
32
92
76
85
32
2
3
4
5
Benzyl alcohol
Cinnamyl alcohol
Cyclohexanol
A = corresponding aldehydes; K = corresponding ketone.
aIsolated as 2,4-dinitrophenyl hydarzone derivative.
bYield based on substrate.
Electronic Spectra
The electronic spectra of the Schiff bases and their ruthe-
nium complexes were recorded in DMSO solution in the region
200–800 nm, and the spectral data are listed in Table 2. All
the ruthenium complexes (1–5) are diamagnetic, indicating the
presence of ruthenium in the +2 oxidation state. The electronic
spectra of the complexes in DMSO showed four bands in the
227–555 nm region. Upon complexation, bathochromic shifts
were exerted in the π–π* electronic transitions. On the other
hand, the n–π* electronic transitions exhibited hypsochromic
cmmoles of product per mmoles of catalyst.
of two CO bands in the IR spectrum suggested that they were
bound to the metal in cis-position.[49] Also, the ν CO modes
in complexes moves to lower wave numbers when compared
with the starting carbonyl complexes.[51,52] A representative
FT-IR spectra of H2L2 and its Ru(II) complex is shown in
Figures 1 and 2.
shifts. The ground state of Ru(II) is A1g, arising from the t26g
1
configuration in an octahedral environment. The excited states
corresponding to the t52ge1g configuration are T1g, T2g, T1g,
3
3
1
1H NMR Spectra
1
A survey of literature revealed that the NMR spectroscopy and T2g. Hence, four bands corresponding to the transitions:
3
1
3
1
1
1
1
has been proved useful in establishing the nature and struc- 1A1g → T1g, A1g → T2g, A1g → T1g and A1g → T2g
ture of many Schiff bases as well as their complexes in solu- are possible in order of increasing energy. The bands in the
tions. The 1H NMR spectra of all the complexes were recorded 521–555 nm region have been assigned to the spin allowed
1
to confirm the binding of Schiff bases to the ruthenium ion 1A1g → T1g transition, and the bands around 386–402 nm have
(Table 3). The 1H NMR spectrum of the parent Schiff bases lig- been assigned to charge transfer transitions arising from the
ands H2L1−5 showed singlet signal 12.57–13.12 ppm range, excitation of electron from the metal t2g level to the empty
which was attributed to two phenolic OH protons. The molecular orbitals derived from π* level of the ligands.[61,62]
OH signals disappear with addition of the D2O.[53] The lig- The high intensity bands around 227–274 nm and 285–380
ands also showed one singlet in the 8.96–9.09 ppm range, nm have been designated as n–π* and π–π* transitions,
which are attributed to the azomethine (–CH=N–) protons, respectively, for the electrons localized on the azomethine group
respectively.[54,55] The 1H NMR spectra of the ligands re- of the Schiff bases. The pattern of the electronic spectra for
vealed a multiplet at 6.95–8.00 ppm corresponding to aromatic all the complexes indicate the presence of an octahedral envi-
protons.[56] Representative H NMR spectrum of H2L3 and its ronment around Ru(II) ion similar to that of other ruthenium
1
Ru(II) complex is shown in Figures 3 and 4.
octahedral complexes.[61,63−65] A selected UV-Vis spectra of

390
A. A. ABDEL AZIZ
TABLE 5
Antibacterial activities of the Schiff base ligands (H2L1−5) and the reported Ru(II) complexes
Diameter of inhibition zone (mm)a
S. aureus (%)
B. cereus (%)
E. coli (%)
P. aeruginosa (%)
25 50 100
25
50
100
25
50
100
25
50
100
Compound
H2L1
µg/ml µg/ml
µg/ml
µg/ml µg/ml µg/ml µg/ml µg/ml µg/ml µg/ml µg/ml µg/ml
6
0.7 0.5
7
8
0 .5
4
0.2
5
7
0.2
0.7
6
9
0.3
0.5 10 0.7 13 1.0 18 0.3
0.5 10 0.5 13 0.7
6
0.3
8
0.7 10 0.3
4
9
9
0.5
6
0.5 10 0.3
[Ru(CO)2(L1)] (1) 12 0.8 13 0.7 15 0.7 11 0.3
0.5 10 0.6 13 1.0
0.6 10 0.6 12 1.1
H2L2
8
0.7
8
0.3 11 0.7
8
0.5 11 1.1 14 1.0
8
[Ru(CO)2(L2)] (2) 13 1.1 16 0.9 18 1.2 15 0.5 18 1.4 20 1.0 12 0.9 16 1.3 19 1.0 15 0.9 15 1.0 19 0.5
H2L3
[Ru(CO)2(L3)] (3) 10 1.0 13 0,9 15 1.0 10 0.7 12 1.1 15 0.8 11 0.8 15 1.0 18 1.1 10 0.7 11 1.0 14 1.2
7
1.0
9
0.3 11 1.0
7
0.5 11 0.5 13 0.8
6
0.4
7
0.7 10 0.5
6
0.6
7
0.5 11 1.0
H2L4
[Ru(CO)2(L4)] (4) 11 1.2 12 1.0 15 1.3 12 0.5 16 1.2 20 1.2
7
0.5
9
0.4 10 0.8
6
0.2
8
0.6 11 0.7
5
9
6
0.5
1.0 14 1.1 17 0.9
0.5 0.4 11 0.5
7
0.4 11 0.6
6
9
8
0.5
0.4 11 1.0 16 1.2
0.5 0.9 11 1.1
7
0.5
9
0.5
H2L5
9
0.9 11 1.0 14 1.1
9
0.3 10 0.7 12 1.0
8
8
[Ru(CO)2(L5)] (5) 13 0.7 16 1.3 18 1.0 17 1.0 18 1.0 22 1.4 11 0.7 13 0.4 17 0.4 14 0.7 12 0.5 15 1.1
Chloramphenicol 22 1.0 23 1.0 27 1.0 21 1.0 24 1.0 27 1.0 17 0.8 21 0.9 24 1.0 18 0.7 21 1.0 23 1.0
Streptomycin
21 1.0 24 1.0 28 1.0 21 1.0 24 1.0 26 1.0 18 0.8 21 0.9 25 1.0 19 0.6 20 0.5 24 1.0
aInhibition zone diameter (mm) standard deviation.
FIG. 3. 1H NMR spectrum of Schiff base ligand H2L3.

MONONUCLEAR Ru(II) CARBONYL COMPLEXES
391
FIG. 4. 1H NMR spectrum Ru(II) carbonyl complex (3).
H2L4 ligand and its corresponding Ru(II) complex is shown in nium metal may have +2 formal oxidation state (d6 electronic
Figure 5.
configuration).
In view of these spectral studies, along with elemental
analysis data, the tentative structure proposed for the com-
plexes (1–5) is shown in Scheme 2, suggested that ruthe-
Catalytic Activity
Catalytic oxidation of alcohols is an important industrial
process due to the wide variety of products synthesized by
this route. Hence, studies on synthesis and catalytic activity
of metal complexes derived from Salen-type ligands with N and
O donor atoms type ligands have gained greater momentum.[66]
Ru-based oxidation catalysis is a powerful and extremely ver-
satile synthetic tool to afford selectively oxygenate products
both in homogeneous and in heterogeneous conversions.[67]
O
CO
N
Ru
CO
N
O
FIG. 5. Absorption spectra of 1 × 10−5 M of Schiff base ligand (a) H2L4 and
(b) its Ru(II) carbonyl complex.
SCH. 2. General structure of Ru(II) Schiff base complexes.

392
A. A. ABDEL AZIZ
OH
O
Ru(CO)2L1-5 (0.01 mmol), NMO (3 mmol)
reflux 3 h, CH2Cl2
+
H2O + NM
R\
R\
R
R
R = alkyl, aryl; R\ alkyl, aryl, H
(1 mmol)
SCH. 3. Ru(II) Schiff base complexes/NMO catalyzed oxidation of alcohol.
Particularly, the use of ruthenium complexes to catalyze ox- an important factor that controls the antimicrobial activity. On
idation of alcohols by oxygen atom donors has been well chelation, the polarity of the metal ion will be reduced to a
documented.[68,69]
greater extent due to the overlap of the ligand orbital and par-
Catalytic oxidation of butan-2-ol, benzyl alcohol, cyclohex- tial sharing of positive charge of metal ion with donor groups.
anol, cinnamyl alcohol, and cyclohexanol by the synthesized Further, it increases the delocalization of π-electrons over the
ruthenium(II)-complexes catalysts was carried out in CH2Cl2 in whole chelate ring and increases the liphophilicity. This in-
the presence of NMO as co-oxidant (Scheme 3). Butan-2-al was creased lipophilicity enhances the penetration of the complexes
formed from butan-2-ol, benzaldehyde was formed from benzyl into lipid membrane and blocking the metal binding sites on
alcohol, cinammaldehyde was formed from cinnamyl alcohol, enzymes of microorganism. These complexes also disturb the
and cyclohexanol was converted into cyclohexanone after 3 h of respiration process of the cell, and thus block the synthesis of
reflux with stirring. In no case was there any detectable oxidation proteins that restrict the further growth of the organism. Fur-
of alcohols in the absence of the ruthenium complexes. All the thermore, the mode of action of the complexes may involve
synthesized complexes were found to catalyze the oxidation of formation of hydrogen bond through azomethine (–CH=N )
alcohols to aldehydes, and secondary alcohols to ketones, but the group with the active centers of cell constituents resulting in
yields % and turn over were found to vary with the different cat- interference with the normal cell process.[74] The different com-
alysts used, as indicated in Table 4. The relatively higher product plexes exhibited microbial activity with small variations against
yield obtained for the oxidation of benzyl alcohol than for cy- the bacterial species, and this difference in activity could be
clohexanol was due to the fact that the α-CH moiety of benzyl attributed to the impermeability of the cell of the microbes or
alcohol is more acidic than that of cyclohexanol.[70] Results of differences in the ribosomes of the microbial cells,[75] which, in
the present investigations suggest that the complexes were able the case of Gram +ve, is single layered, and in the case of Gram
to react efficiently with NMO to yield a high valent ruthenium- –ve, is a multilayered structure.[76] The increase in activity of
oxo species[71] capable of oxygen atom transfer to alcohols.
Ru(II) complexes, with the increase in their concentration, was
attributed to the effect of metal ions on the normal cell process.
Although all Ru(II) complexes were found to have potential
antibacterial activity against the bacterial microbes, they could
not reach the effectiveness of the conventional bacterial control
chloramphenicol and streptomycin.
Antibacterial Activity
Bacteria can achieve resistance to antibiotics through bio-
chemical and morphological modifications. The biological ac-
tivity of the parent Schiff bases H2L1−5 and their Ru(II) com-
plexes (1–5) have been screened in vitro to access their in-
hibitory activity against four species of bacteria S. aureus, B.
cereus, E. coli, and P. aeruginosa. The test solutions were pre-
pared in DMSO, and the obtained results are summarized in
CONCLUSION
A series of novel mononuclear Ru(II) carbonyl Schiff base
Table 5. The remarkable activity of the Schiff base ligands may complexes have been synthesized by the reaction of Ru3(CO)12
be arise from the hydroxyl groups, which may play an impor- with novel quadridentate Schiff bases prepared by the con-
tant role in the antibacterial activity, as well as the presence of densation of salicylaldehyde or o-vanillin with 4,5-dimethyl-
two imine groups, which imports in elucidating the mechanism 1,2-phenylendiamine, 4,5-dichloro-1,2-phenylendiamine, and
of transformation reaction in biological systems. The results 2,3-diaminonaphathalene. An octahedral structure has been pro-
showed that Ru(II) complexes possess more cytotoxicity than posed for the new complexes. The newly synthesized complexes
the free Schiff base ligands under identical experimental con- were found to act as efficient catalysts for the oxidation of alco-
ditions. This would suggest that the chelation could facilitate hols to their corresponding carbonyl compounds by using NMO
the ability of a complex to cross a cell membrane[72] and can as the co-oxidant. The new complexes gave a higher yield of
be explained on the basis of Overtone’s and Tweedy’s chelation oxidation products, so those complexes can be used as low cost,
theory.[73] According to Overtone’s concept of cell permeability, reliable, and highly efficient catalysts for oxidation of alcohols.
the lipid membrane that surrounds the cell favors the passage The in vitro antibacterial activity of the new Ru(II) complexes
of only rapid soluble materials due to which liposolubility has were studied against four different microbes, and it was found

MONONUCLEAR Ru(II) CARBONYL COMPLEXES
393
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