Synthesis, characterization and catalytic activities of ruthenium complexes containing triphenylphosphine/triphenylarsine and tetradentate Schiff bases

Article abstract of DOI:10.1016/S1387-7003(03)00021-2

Ruthenium(II) complexes of the type [Ru(CO)(B)(L)] (B = AsPh₃, pyridine, piperidine or morpholine; L = dianion of tetradentate Schiff bases) have been synthesized and characterized by physico-chemical methods. These complexes are found to be ef

Full text of DOI:10.1016/S1387-7003(03)00021-2

ARTICLE IN PRESS
Inorganic Chemistry Communications 6 (2003) 486–490
www.elsevier.com/locate/inoche
Synthesis, characterization and catalytic activities of
ruthenium complexes containing
triphenylphosphine/triphenylarsine and tetradentate Schiff bases q
*
R. Karvembu, S. Hemalatha, R. Prabhakaran, K. Natarajan
Department of Chemistry, Bharathiar University, Coimbatore 641 046, India
Received 24 October 2002; accepted 3 January 2003
Abstract
Ruthenium(II) complexes of the type [Ru(CO)(B)(L)] (B ¼ AsPh3, pyridine, piperidine or morpholine; L ¼ dianion of tetrad-
entate Schiff bases) have been synthesized and characterized by physico-chemical methods. These complexes are found to be effective
catalysts in the oxidation of primary and secondary alcohols using N-methylmorpholine-N-oxide as oxidant. The catalytic activity of
these triphenylarsine complexes have been compared with that of triphenylphosphine complexes and with similar ruthenium(III)
complexes. The formation of high valent Runþ2@O species as catalytic intermediate is proposed for the catalytic processes.
Ó 2003 Published by Elsevier Science B.V.
Keywords: Ruthenium(II) triphenylarsine complexes; Tetradentate Schiff bases; Catalytic oxidation study; Intermediate ruthenium-oxo species
1. Introduction
rived from ‘‘salen’’ type ligands have gained greater
momentum [8]. Besides, the catalytic activity of ruthe-
nium complexes with tertiary phosphine or tertiary ar-
sine ligand is well documented [9].
Catalytic oxidation of alcohols is an important in-
dustrial process due to the wide variety of products
synthesized by this route. Three important natural en-
zymes used for oxidation reactions are cytochrome P-
450, peroxidases and catalases. All these enzymes have
iron(III) porphyrin as the central unit. Hence, several
investigations have been made on the reactions of syn-
thetic metalloporphyrins to understand the mechanism
of action of porphyrin containing enzymes [1]. Though
metalloporphyrins catalyse oxidation reactions, the
catalytic yield is not satisfactory to have any commercial
viability. Moreover, it is not easy to synthesize metal-
loporphyrins and this led scientists to lookfor other li-
gands to make novel complexes to be employed as
catalysts in oxidation reactions [2–7]. Hence, studies on
synthesis and catalytic activity of metal complexes de-
We have previously reported the catalytic oxidation
properties of several ruthenium complexes containing
tertiary phosphine or tertiary arsine and b-diketones
[10,11]. In the present work, we report the synthesis,
spectral characterization and catalytic activity of some
ruthenium(III) tetradentate Schiff base complexes with
triphenylarsine. We have also compared the catalytic
oxidation properties of several ruthenium(II) and ru-
thenium(III) complexes which have similar structure.
The general structure of the Schiff base ligands used in
this study is given in Fig. 1.
2. Experimental
2.1. Materials
q Presented in the 35th International Conference on Coordination
Chemistry, 21–26 July, 2002; University of Heidelberg, Germany.
* Corresponding author. Tel.: +91-422-242-2222; fax: +91-422-242-
2387.
The compound ½RuHClðCOÞðAsPh3Þ3Š, and Schiff
base ligands were prepared according to literature
methods [12,13]. The complexes of the type
E-mail address: k_natraj6@yahoo.com (K. Natarajan).
1387-7003/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V.
doi:10.1016/S1387-7003(03)00021-2

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487
were evaporated to give corresponding aldehyde which
was then quantified as 2,4-dinitrophenylhydrazone de-
rivative [15].
3. Results and discussion
Fig. 1. Structure of Schiff base ligands.
The tetradentate Schiff bases ðH2LÞ on reacting with
ruthenium(II) complexes, ½RuHClðCOÞðAsPh3Þ2ðBފ
yielded new complexes of the type [Ru(CO)(B)(L)]
(where B ¼ pyridine (py), piperidine (pip) or morpholine
(mor); L ¼ dianionic tetradentate Schiff base).
½RuHClðCOÞðAsPh3Þ2ðBފ, (B ¼ pyridine, piperidine,
morpholine) have been prepared by the modified pro-
cedure [14]. Instrumental details for elemental analysis,
IR, UV–Vis and NMR spectra are the same as described
earlier [10].
½RuHClðCOÞðAsPh3Þ ðBފ þ H2L
2
! ½RuðCOÞðBÞðLފ þ 2AsPh3 þ H2 þ HCl
2.2. Synthesis of complexes of the type [Ru(CO)(B)
(L)] (B ¼ AsPh3, py, pip or mor; L ¼ dianion of tetrad-
entate Schiff base)
The analytical data obtained for the compounds do
confirm the molecular formula suggested for the new
complexes. The analytical data and the spectral data of
the complexes are given in Tables 1–3, respectively. In
all the reactions, the Schiff bases replace two tripheny-
larsines, a chloride and a hydride from the starting
complexes leading to the new hexa coordinated com-
plexes. The Schiff bases in all these complexes behave as
dibasic tetradentate ligands. One interesting aspect is
that in all the new complexes, the heterocyclic nitrogen
bases (pyridine, piperidine or morpholine) remain intact
without being replaced. The reason for non-replacement
of the coordinated bases is due to the fact that the Ru–
As bond is more labile compared to Ru–N bond because
of better r donating ability of nitrogen [14]. The same
observation has been made for triphenylphosphine
complexes of ruthenium [11].
To
a
solution of ½RuHClðCOÞðAsPh3Þ2ðBފ
(B ¼ AsPh3, py, pip or mor) (0.1 g, 0.092–0.117 mmol)
in benzene (25 cm3), the Schiff base (0.027–0.038 g,
0.092–0.117 mmol) was added and the mixture was he-
ated under reflux for 6 h. The resulting solution was
concentrated to ca. 3 cm3 and cooled. A small quantity
of petroleum ether (60–80 °C) was added to the con-
centrated solution and the precipitated complex was
filtered off, washed with petroleum ether and recrystal-
lized from CH2Cl2/petroleum ether (60–80 °C) mixture
and dried in vacuo.
2.3. Oxidation of benzyl alcohol and cyclohexanol
The free Schiff bases show a very strong absorption
around 1620–1600 cmꢀ1 in their IR spectra which is
characteristic of the azomethine (C@N) group. In the IR
spectra of the new complexes, the absorption due to
C@N is observed at a lower region (1610–1580 cmꢀ1),
indicating the coordination of nitrogen atom of the Schiff
base to ruthenium [16]. A strong band observed around
To a solution of alcohol (1 mmol) in dichloromethane
(20 cm3) was added, N-methylmorpholine-N-oxide
(NMO) (3 mmol) and the ruthenium complex (0.01
mmol). The solution was stirred for 3 h at room tem-
perature, then the mixture was evaporated to dryness
and extracted with petroleum ether (60–80 °C)
ð2 Â 25 cm3Þ. The combined petroleum ether extracts
Table 1
Analytical data of data of ruthenium(II) Schiff base complexes
Complexes
Molecular formula Color
Yield (%) M.p (°C)
Elemental analyses found (Calc.) (%)
Carbon
Hydrogen
Nitrogen
½RuðCOÞðAsPh3ÞðL1ފ
½RuðCOÞðpyÞðL1ފ
C39H29N2O3AsRu
C26H19N3O3Ru
C26H25N3O3Ru
C26H23N3O4Ru
C37H33N2O3AsRu
C24H23N3O3Ru
C24H29N3O3Ru
C23H27N3O4Ru
C39H37N2O3AsRu
C26H27N3O3Ru
C26H33N3O3Ru
C25H31N3O4Ru
Reddish orange
Brown
67
60
58
144
69
62.01 (62.4)
59.81 (59.77)
58.71 (59.08)
56.39 (56.60)
60.57 (60.91)
57.08 (57.36)
56.41 (56.68)
53.89 (54.11)
61.50 (61.82)
58.56 (58.86)
57.89 (58.19)
55.38 (55.75)
3.67 (3.89)
3.37 (3.67)
4.87 (4.77)
4.02 (4.37)
4.22 (4.56)
4.27 (4.61)
5.41 (5.75)
5.02 (5.33)
4.52 (4.92)
5.02 (5.13)
6.02 (6.20)
5.42 (5.80)
3.50 (3.74)
7.81 (8.04)
7.59 (7.95)
7.59 (7.92)
3.54 (3.84)
7.97 (8.36)
8.39 (8.26)
7.84 (8.23)
3.50 (3.70)
7.60 (7.92)
7.54 (7.83)
7.45 (7.80)
½RuðCOÞðpipÞðL1ފ
½RuðCOÞðmorÞðL1ފ
½RuðCOÞðAsPh3ÞðL2ފ
½RuðCOÞðpyÞðL2ފ
Reddish brown
Brickred
78
64
80
Yellowish brown
Yellow
69
60
62
61
72
58
62
66
127
109
84
½RuðCOÞðpipÞðL2ފ
½RuðCOÞðmorÞðL2ފ
½RuðCOÞðAsPh3ÞðL3ފ
½RuðCOÞðAsPh3ÞðL3ފ
½RuðCOÞðAsPh3ÞðL3ފ
½RuðCOÞðAsPh3ÞðL3ފ
Yellowish green
Pale brown
Yellowish brown
Yellowish green
Brown
87
115
99
95
98
Yellowish green

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Table 2
IR and electronic spectral data of ruthenium(II) Schiff base complexes
Compound
Molecular formula
mC@N
mCAO
mCBO
Bands for coor-
dinated nitrogen
bases
kmax
½RuðCOÞðAsPh3ÞðL1ފ
½RuðCOÞðpyÞðL1ފ
½RuðCOÞðpipÞðL1ފ
½RuðCOÞðmorÞðL1ފ
½RuðCOÞðAsPh3ÞðL2ފ
½RuðCOÞðpyÞðL2ފ
½RuðCOÞðpipÞðL2ފ
½RuðCOÞðmorÞðL2ފ
½RuðCOÞðAsPh3ÞðL3ފ
½RuðCOÞðpyÞðL3ފ
½RuðCOÞðpipÞðL3ފ
½RuðCOÞðmorÞðL3ފ
C39H29N2O3AsRu
C26H19N3O3Ru
C26H25N3O3Ru
C26H23N3O4Ru
C37H33N2O3AsRu
C24H23N3O3Ru
C24H29N3O3Ru
C23H27N3O4Ru
C39H37N2O3AsRu
C26H27N3O3Ru
C26H33N3O3Ru
C25H31N3O4Ru
1580
1600
1605
1605
1590
1605
1600
1600
1597
1590
1610
1597
1310
1310
1320
1310
1300
1310
1320
1320
1300
1310
1310
1310
1940
1950
1930
1930
1950
1955
1927
1932
1942
1947
1922
1924
–
236,261,330
232,250,340
232,261,333
232,373
1070
1088
1080
–
232,261,344
232,247,348
232,340
1070
1070
1090
–
236,337
232,254,348
232,243,340
236,337
1060
1080
1090
236,337
Note. m in cmꢀ1 ; kmax in nm.
other bands around 265–230 nm have been assigned to
charge transfer transitions.
Table 3
1H NMR data of ruthenium(II) complexes
1H NMR spectra of the complexes ½RuðCOÞðAsPh3Þ
ðL1ފ, ½RuðCOÞðmorÞðL1ފ and ½RuðCOÞðpipÞðL2ފ have
been recorded to confirm the binding of Schiff bases to
the ruthenium ion and to find out the presence of
triphenylarsine or heterocyclic nitrogen bases. The
spectra of all the three complexes showed a singlet in the
region 8.64–8.82 ppm which has been assigned to
azomethine proton. The spectra of ½RuðCOÞðmorÞðL1ފ
and ½RuðCOÞðpipÞðL2ފ showed a singlet at 8.87 and
9.12 ppm, respectively, which have been assigned due to
NH proton of piperidine and morpholine. This signal is
not found in ½RuðCOÞðAsPh3ÞðL1ފ. This confirms the
presence of coordinated nitrogen base in these com-
plexes. Further, the signal for methylene protons ap-
pears as a broad singlet in the region 3.15–3.95 ppm in
½RuðCOÞðpipÞðL2ފ. The spectra of ½RuðCOÞðmorÞðL1ފ
showed two signals for methylene protons, one at 3.23
ppm which is due to methylene protons nearer to oxygen
and another signal at 3.95 ppm which is due to methy-
lene protons nearer to nitrogen. The multiplet observed
around 7.02–7.85 ppm in all the three complexes have
been assigned to the protons of phenyl group of the
triphenylarsine and ligands.
Complexes
1H NMR (ppm)
½RuðCOÞðAsPh3ÞðL1ފ
7.02–7.85 [m, aromatic]
8.82 [s, CH]
½RuðCOÞðmorÞðL1ފ
3.32 [s, CH2ð1Þ]
3.95 [s, CH2ð2Þ]
7.25–7.77 [m, aromatic]
8.64 [s, CH]
8.87 [s, NH]
½RuðCOÞðpipÞðL2ފ
Note. d in ppm.
3.15 [s, CH2]
7.26–7.78 [m, aromatic]
8.71 [s, CH]
9.12 [s, NH]
1280–1270 cmꢀ1 in the free Schiff bases has been assigned
to phenolic C–O stretching. On complexation, this band
has been shifted to higher frequency (1320–1300 cmꢀ1
)
showing the other coordination is through phenolic ox-
ygen atom [17]. This is further supported by the disap-
pearance of the broad band at 2980–2900 cmꢀ1 due to O–
H in the complexes. For all the new complexes, the IR
spectra showed a strong band in the region 1955–1920
cmꢀ1 due to terminally coordinated carbonyl group. For
the complexes, [Ru(CO)(B)(L)] (where B ¼ py, pip or
mor; L ¼ dianion of tetradentate Schiff base), the IR
spectra showed a medium intensity band in the region
1090–1070 cmꢀ1 which is characteristic of coordinated
nitrogen bases [14]. Characteristic bands for tripheny-
larsine are also present in the expected region.
On the basis of elemental analyses, IR, electronic and
1H NMR spectral data, the following octahedral struc-
ture has been proposed for the new ruthenium(II)
complexes.
The electronic spectra of the complexes were recorded
in dichloromethane. The electronic spectra of the com-
plexes showed two to three bands in the region 375–230
nm. The bands appearing in the region 375–330 nm have
1
been assigned to the spin allowed transition 1A1g ! T1g.
This assignment is in conformity with the assignments
made for similar other octahedral complexes [18]. The

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489
3.1. Catalytic activity
heterocyclic bases to ruthenium ion, as compared to
triphenylarsine. The formation of the active species is
therefore less favored in the former case than in the
latter, resulting in lower yield [11]. The triphenylphos-
phine ruthenium(II) complexes possess greater catalytic
activity than that of the triphenylarsine complexes. The
same observation has been made recently [21].
Ruthenium(II) complexes exhibit better catalytic ac-
tivity than similar ruthenium(III) complexes, attributed
to more lability of Ru(II)–P bond compared to Ru(III)–
P bond. Besides, the catalytic efficiency of the complexes
derived from o-hydroxyacetophenone and diamines is
lower than the one derived from salicylaldehyde and
diamines. The essential difference between these two
complexes is that the hydrogen atom of the aldehyde
group of salicylaldehyde is replaced by a methyl group
in the 2-hydroxyacetophenone complex. Hence, it seems
that the presence of the electron-donating methyl group
enhances the catalytic activity of the acetophenone
complex over the salicylaldehyde complex. This is in
agreement with the earlier observation [22]. However,
there is also a report in which the catalytic activity is
increased by electron donating substituent [23].
The oxidation of benzyl alcohol and cyclohexanol was
carried out with new ruthenium(II) complexes in the
presence of NMO as oxidant and dichloromethane as
solvent. The data of catalytic oxidation is given in Table
4. The catalytic efficiency was compared with the
previously reported [16,19] similar triphenylphosphine
ruthenium(II) and ruthenium(III) complexes. Benzalde-
hyde was formed from benzyl alcohol and cyclohexanol
was converted into cyclohexanone after stirring for
about 3 h, which was then quantified as its 2,4-dini-
trophenylhydrazone derivatives. In no case was there any
detectable oxidation of alcohols in the presence of NMO
alone and without the ruthenium complex.
The relatively higher product yield obtained for oxi-
dation of benzyl alcohol compared to cyclohexanol is
due to the fact that a-CH unit of benzyl alcohol is more
acidic than cyclohexanol [20]. Complexes that do not
contain heterocyclic nitrogen bases exhibit better cata-
lytic activity than the complexes with heterocyclic ni-
trogen bases. This result is in accord with our
observation, taking into account the stronger binding of
Table 4
Catalytic oxidation data of alcohols by ruthenium(II) Schiff base complexes in the presence of N-methylmorpholine-N-oxide
Complexes
Substrate
Product
Yield (%)
Turnover
½RuðCOÞðAsPh3ÞðL1ފ
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
A
K
A
K
A
K
A
K
A
K
A
K
A
K
A
K
A
K
A
K
A
K
A
K
A
K
A
K
A
K
A
K
63.6
47.2
54.5
37.8
45.3
37.8
45.3
28.3
81.0
66.3
54.5
37.8
27.0
28.3
62.7
56.7
45.3
28.3
18.3
9.40
27.0
19.0
18.3
9.40
90.7
75.1
81.0
75.1
45.3
28.3
27.0
9.40
66
47
57
38
47
38
47
28
85
66
57
38
28
28
66
57
47
28
19
9
½RuðCOÞðpyÞðL1ފ
½RuðCOÞðpipÞðL1ފ
½RuðCOÞðmorÞðL1ފ
½RuðCOÞðAsPh3ÞðL2ފ
½RuðCOÞðpyÞðL2ފ
½RuðCOÞðpipÞðL2ފ
½RuðCOÞðmorÞðL2ފ
½RuðCOÞðAsPh3ÞðL3ފ
½RuðCOÞðpyÞðL3ފ
½RuðCOÞðpipÞðL3ފ
½RuðCOÞðmorÞðL3ފ
½RuðCOÞðPPh3ÞðL1ފ
½RuðCOÞðPPh3ÞðL2ފ
½RuðClÞðPPh3ÞðL1ފ
½RuðClÞðAsPh3ÞðL1ފ
28
19
19
9
94
75
85
75
47
28
28
9

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