Ruthenium(III) complexes of dipicolinic acid with PPh3/AsPh3 as co-ligand: Synthesis and structural characterization

Article abstract of DOI:10.1016/j.poly.2006.01.023

New hexa-coordinated Ru(III) complexes of the type [Ru(dipic)(EPh₃)₂X] have been synthesized by reacting 2,6-pyridine dicarboxylic acid (dipicolinic acid, H₂dipic) with the appropriate starting complexes [RuX₃(EPh₃)₃] (where X = Cl, Br; E = P, As). The ligand behaves as tridentate dibasic chelate. Dipicolinic acid which was expected to form a bridge between two metal centers, formed only mononuclear complexes, irrespective of the metal to ligand ratio. All the complexes have been characterized by analytical and spectroscopic (IR, electronic and EPR) data. Single-crystal X-ray analysis of the complex [Ru(dipic)(PPh₃)₂Cl] revealed that the coordination environment around ruthenium metal consists of an NO₂P₂Cl octahedron with (dipic) occupying equatorial plane. Electrochemical behavior of the complexes was studied using cyclic voltammetry. The new complexes were found to catalyze the oxidation of alcohols to aldehydes using N-methyl morpholine-N-oxide as co-oxidant.

Full text of DOI:10.1016/j.poly.2006.01.023

Polyhedron 25 (2006) 2223–2228
www.elsevier.com/locate/poly
Ruthenium(III) complexes of dipicolinic acid with PPh3/AsPh3 as
co-ligand: Synthesis and structural characterization
*
Duraiswamy Sukanya, Rathinasabapathi Prabhakaran, Karuppannan Natarajan
Department of Chemistry, Bharathiar University, Maruthamalai Road, Coimbatore 641 046, Tamilnadu, India
Received 4 November 2005; accepted 18 January 2006
Available online 20 March 2006
Abstract
New hexa-coordinated Ru(III) complexes of the type [Ru(dipic)(EPh3)2X] have been synthesized by reacting 2,6-pyridine dicarboxylic
acid (dipicolinic acid, H2dipic) with the appropriate starting complexes [RuX3(EPh3)3] (where X = Cl, Br; E = P, As). The ligand behaves
as tridentate dibasic chelate. Dipicolinic acid which was expected to form a bridge between two metal centers, formed only mononuclear
complexes, irrespective of the metal to ligand ratio. All the complexes have been characterized by analytical and spectroscopic (IR, elec-
tronic and EPR) data. Single-crystal X-ray analysis of the complex [Ru(dipic)(PPh3)2Cl] revealed that the coordination environment
around ruthenium metal consists of an NO2P2Cl octahedron with (dipic) occupying equatorial plane. Electrochemical behavior of the
complexes was studied using cyclic voltammetry. The new complexes were found to catalyze the oxidation of alcohols to aldehydes using
N-methyl morpholine-N-oxide as co-oxidant.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium(III) complexes; Dipicolinic acid; Crystal structure; EPR; Electrochemistry; Catalytic activity
1. Introduction
report the synthesis, characterization (XRD), spectroscopic
properties and redox behavior of Ru(III) compounds con-
taining PPh3/AsPh3 and dipicolinic acid. The new com-
plexes were tested for their catalytic activity in the
oxidation of alcohols.
Ruthenium chemistry of triphenylphosphine and triphe-
nylarsine has been enriched with various ligand types [1–4]
and their role as potential catalysts for many organic syn-
theses is well documented [5,6]. Work on dicarboxylic acids
as ligands for ruthenium is meagre [7–10]. Among the
dicarboxylic acids, dipicolinic acid is known for its various
ligating modes [11–15] and application in analytical chem-
istry [16,17], corrosion inhibition [18], decontamination of
nuclear reactors [19] and diverse biological activity
[20–22]. Moreover, dipicolinate complexes have been used
as electron carriers in some model biological systems [23],
as specific molecular tools in DNA cleavage [24] and as
NO scavengers [9]. The paucity of ruthenium complexes
containing both triphenylphosphine/arsine and dicarbox-
ylic acid for use as catalyst precursor in organic synthesis
has initiated the synthesis of such compounds. Herein, we
2. Experimental
2.1. Materials
RuCl3 Æ 3H2O and 2,6-pyridine dicarboxylic acid were
purchased from Himedia and used without further purifi-
cation. Solvents were distilled following the literature pro-
cedures [25]. The starting complexes [RuCl3(PPh3)3] [26],
[RuCl3(AsPh3)3] [27], [RuBr3(AsPh3)3] [28], [RuBr3(PPh3)2
(MeOH)] [29] were prepared as reported earlier.
2.2. Physical measurements
*
Infrared spectra of the ligand and the complexes
have been recorded on a Nicolet Avatar model FT-IR
Corresponding author. Tel.: +91 422 2424655; fax: +91 422 2422387.
E-mail address: k_natraj6@yahoo.com (K. Natarajan).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.01.023

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D. Sukanya et al. / Polyhedron 25 (2006) 2223–2228
spectrophotometer using KBr discs in the range of 4000–
400 cmꢀ1. Electronic spectra were recorded on a Systronics
119 UV–Vis spectrophotometer using methanol as solvent.
Elemental analyses were performed with a Vario EL III
Elementar analyzer. Room temperature EPR spectra were
recorded using an E-112 Varian model instrument. Cyclic
voltammetric experiments were carried out using BAS
CV-27 electrochemical analyzer with a glassy carbon work-
ing electrode. A platinum wire and silver–silverchloride
electrode were used as counter- and reference electrodes,
respectively. Melting points were recorded with a Lab
India apparatus.
Calc. for RuC43H33NO4BrAs2: C, 59.32; H, 3.82; N, 1.60.
Found: C, 59.27; H, 3.94; N, 1.58%. IR: 1673, 1350,
1434, 1077, 692 cmꢀ1. UV: kmax: 268(4706), 315(5652),
416(10184), 534(590) nm(dm3/mol/l); m.p. >270 ꢁC.
[Ru(dipic)(PPh3)2Br]
(4)
was
prepared
from
[RuBr3(PPh3)2(MeOH)] (0.0897 g, 0.1 mmol) and dipicoli-
nic acid (0.018 g, 0.1 mmol) as orange brown crystals.
Yield: 75%. Anal. Calc. for RuC43H33NO4BrP2: C, 53.82;
H, 3.46; N, 1.45. Found: C, 53.79; H, 3.63; N, 1.39%. IR:
1666, 1347, 1440, 1080, 690 cmꢀ1. UV: kmax: 268(4344),
416(9180) nm(dm3/mol/l); m.p. >240 ꢁC.
2.5. Catalytic oxidation
2.3. Crystallography
To a solution of the alcohol (1 mmol) in CH2Cl2 (20 ml),
N-methyl morpholine-N-oxide (3 mmol) and ruthenium
complex (0.01 mmol) were added. The solution was heated
under reflux for 3 h. The mixture was evaporated to dry-
ness and extracted with petroleum ether (60–80 ꢁC). The
combined petroleum ether mixture was filtered and evapo-
rated to give the corresponding aldehyde, which was then
quantified as its 2,4-dinitrophenyl hydrazone derivative
[31].
Single crystals of [Ru(dipic)(PPh3)2Cl] suitable for X-ray
diffraction studies were obtained from slow evaporation of
a solution of the complex in benzene/ethanol mixture.
Intensity data were collected on a Nonius MACH 3 four
circle diffractometer equipped with graphite monochro-
˚
mated Mo Ka radiation (k = 0.7107 A). The structure
was solved by the direct method. All non-hydrogen atoms
were refined anisotropically by full-matrix least-squares,
with a ridging model for the hydrogen atoms, using the
SHELXTL/PC package [30].
3. Results and discussion
2.4. Synthesis of [Ru(dipic)(EPh3)2X] (X = Cl, Br;
E = P, As)
New hexa-coordinated Ru(III) complexes of the type
[Ru(dipic)(EPh3)2X] have been prepared by reacting 2,6-
pyridine dicarboxylic acid (dipicolinic acid, H2dipic) with
[RuCl3(PPh3)3], [RuCl3(AsPh3)3], [RuBr3(AsPh3)3] and
[RuBr3(PPh3)2(MeOH)] in a 1:1 mole ratio. The ligand
replaces one EPh3 and two X atoms, (E = As or P; X =
Cl or Br), from the starting complex to yield [Ru(dipic)-
(EPh3)2X] (Scheme 1). Dipicolinic acid acts as a dibasic
tridentate ligand in all the complexes. The analytical data
conform to the stoichiometry of the ruthenium(III) com-
plexes as [Ru(dipic)(EPh3)2X].
All the new ruthenium(III) complexes were prepared by
the following general procedure. A benzene (20 ml) solu-
tion of [RuX3(EPh3)3] (where E = P, As; X = Cl, Br)
(0.1 mmol) was added to a refluxing solution of dipicoli-
nic acid (0.1 mmol) in ethanol (20 ml). The mixture was
heated under reflux for 6 h. The solution was filtered
while hot, reduced to half of its volume and left for slow
evaporation. The crystalline product that separated out
was filtered off, washed with ethanol and dried under vac-
uum. The product was recrystallized from benzene/
ethanol mixture.
3.1. Infrared spectroscopy
[Ru(dipic)(PPh3)2Cl]
(1)
was
prepared
from
The IR spectrum of the free ligand (H2dipic) shows a
strong band around 1700 cmꢀ1 assignable to m(C@O) of
COOH moiety [7]. In all the complexes, the band due
to mas(COOꢀ) was observed in the region 1675–
1610 cmꢀ1 and that due to ms(COOꢀ) in the region
1352–1344 cmꢀ1. A large difference of 327–307 cmꢀ1
between mas and ms vibrations indicates a monodentate
coordination of the carboxylic group in all the complexes
[32]. A broad band observed in the region 3340–
2500 cmꢀ1 region due to m(O–H) of the carboxyl group
in the free ligand disappeared in all the complexes indicat-
ing the deprotonation and subsequent coordination
through oxygen donor atom. The absence of any peak
around 1700 cmꢀ1 reveals that both COOH groups are
involved in coordination. All the characteristic peaks
due to PPh3/AsPh3 were observed in the usual regions
[31].
[RuCl3(PPh3)3] (0.0994 g, 0.1 mmol) and dipicolinic acid
(0.018 g, 0.1 mmol) as reddish brown crystals. Yield:
75%. Anal. Calc. for RuC43H33NO4ClP2: C, 62.51; H,
4.02; N, 1.69. Found: C, 62.04; H, 4.51; N, 1.74%. IR:
1659, 1352, 1431, 1091, 697 cmꢀ1. UV: kmax: 268(4236),
405(9219) nm(dm3/mol/l); m.p. >230 ꢁC.
[Ru(dipic)(AsPh3)2Cl] (2) was prepared
from
[RuCl3(AsPh3)3] (0.112 g, 0.1 mmol) and dipicolinic acid
(0.018 g, 0.1 mmol) as shining red crystals. Yield: 80%.
Anal. Calc. for RuC43H33NO4ClAs2: C, 56.44; H, 3.63;
N, 1.53. Found: C, 56.17; H, 3.82; N, 1.60%. IR: 1671,
1344, 1434, 1077, 692 cmꢀ1
. UV: kmax: 268(4767),
315(6296), 530(964) nm(dm3/mol/l); m.p. >290 ꢁC.
[Ru(dipic)(AsPh3)2Br] (3) was prepared from
[RuBr3(AsPh3)3] (0.125 g, 0.1 mmol) and dipicolinic acid
(0.018 g, 0.1 mmol) as brown crystals. Yield: 85%. Anal.

D. Sukanya et al. / Polyhedron 25 (2006) 2223–2228
2225
O
C
O
EPh3
EPh3
C
X
X
EPh3
HO
O
X
N
Ethanol/Benzene
Reflux, 6h
N
Ru
+
Ru
C
O
X
C
O
EPh3
O
EPh3
HO
E = As, P
X = Cl,Br
Scheme 1.
3.2. EPR spectroscopy
reported Ru(III) complexes [35]. A representative case is
depicted in Fig. 1.
The EPR spectra of the powdered samples were
recorded at room temperature at X-band frequencies.
The nature of the spectra revealed the absence of any
hyperfine splitting due to interaction with any other nuclei
present in the complexes. Complexes 1 and 4 showed two
lines with two different ‘g’ values (gx = gy ¼ gz) indicating
magnetic anisotropy and suggestive of tetragonal distor-
tion in octahedral geometry [33]. However, complexes 2
and 3 showed three lines with three different ‘g’ values
(gx ¼ gy ¼ gz) indicating rhombic distortion [34]. The ‘g’
values are in the range 2.12–2.21. The nature of spectra
obtained is in good agreement with that of the previously
3.3. Electrochemistry
The redox behavior of the complexes was studied using
cyclic voltammetry at a glassy carbon working electrode at
a scan rate of 100 mV sꢀ1. [NBu4]BF4 (0.1 M) was used as
supporting electrolyte in acetonitrile solution of 0.001 M of
complex. The cyclic voltammetric data are given in Table 1
and a representative cyclic voltammogram is displayed in
Fig. 2. The E1/2 of the oxidation process was in the range
from 0.71 to 0.79 V and reduction process was in the range
from ꢀ0.73 to ꢀ0.80 V. The oxidation and reduction waves
Fig. 1. EPR spectrum of 3.
Table 1
Cyclic voltammetric dataa of new Ru(III) complexes
Complex
Ru(III)–Ru(IV)
Ru(III)–Ru(II)
Epc (V)
Epa (V)
Ef (V)
DEp (mV)
Epc (V)
Epa (V)
Ef (V)
DEp (mV)
1
2
3
4
a
0.60
0.65
0.72
0.69
0.84
0.77
0.84
0.88
0.72
0.71
0.78
0.79
236
120
127
192
ꢀ0.87
ꢀ0.84
ꢀ0.87
ꢀ0.84
ꢀ0.72
ꢀ0.62
ꢀ0.73
ꢀ0.71
ꢀ0.79
ꢀ0.73
ꢀ0.80
ꢀ0.78
152
224
141
137
Supporting electrolyte: [NBu4]ClO4 (0.05 M); complex concentration: 0.01 M; scan rate: 100 mV sꢀ1; all the potentials are referenced to Ag/AgCl.

2226
D. Sukanya et al. / Polyhedron 25 (2006) 2223–2228
1.0E-04
8.0E-05
6.0E-05
4.0E-05
2.0E-05
0.0E+00
-2.0E-05
-4.0E-05
-6.0E-05
SA1a
1800
1300
800
300
-200
-700
-1200
E(mV)
Fig. 2. Cyclic voltammogram of 1.
are due to the metal centered Ru(III) ! Ru(IV) and
Ru(III) ! Ru(II) processes, respectively. Any redox behav-
ior of the ligand in the range 0.71 to ꢀ0.80 V is ruled out
because dipicolinic acid showed quasi-reversible and irre-
versible processes only around ꢀ1.58 V in its complexes
[36]. Moreover, potential difference between the two suc-
cessive oxidation processes is ꢁ1.5 V which agrees well
with the average potential difference between the redox
3.4. Crystal structure of [Ru(dipic)(PPh3)2Cl]
The ORTEP diagram of [Ru(dipic)(PPh3)2Cl] is shown
in Fig. 3. The crystal data and refinement parameters are
summarized in Table 2. Selected bond lengths and bond
Table 2
Crystal data and structure refinement parameters for [Ru(dipic)(PPh3)2Cl]
processes of the ruthenium center (RuII/III–RuIII/IV
)
Empirical formula
Formula weight
Temperature (K)
C43H33ClNO4P2Ru
826.16
293(2)
(ꢁ1.0–1.5 V) observed for other mononuclear complexes
[37]. All the complexes exhibit quasi-reversible oxidative
couples with peak to peak separations (DEp) of 120–
236 mV [38]. Replacement of PPh3 by AsPh3 showed no
much variation in the redox potential. It has been observed
from the electrochemical data that the present ligand sys-
tem is ideally suitable for stabilizing the higher oxidation
state of the ruthenium ion [39–41].
˚
Wavelength (A)
0.71073
Crystal system, space group
Unit cell dimensions
monoclinic, C2/c
˚
a (A)
34.882(14)
9.998(4)
29.132(12)
90
121.32(3)
90
8679(6)
8, 1.265
0.535
3368
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
Z, calculated density (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
Crystal size (mm)
h Range for data collection (ꢁ)
Limiting indices
0.22 · 0.19 · 0.14
2.15–24.98
0 6 h 6 41,
ꢀ1 6 k 6 11,
ꢀ34 6 l 6 29
8653/7584 (0.0171)
99.4
Reflections collected/unique (Rint
Completeness to h = 24.98 (%)
Absorption correction
)
w-scan
Maximum and minimum transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
0.9998 and 0.9647
full-matrix least-squares on F2
7584/0/469
1.057
Final R indices [I > 2r(I)]
R1 = 0.0327,
wR2 = 0.0913
R1 = 0.0517,
wR2 = 0.1055
0.530 and ꢀ0.603
R indices (all data)
Largest difference in
peak and hole (e Aꢀ3
)
Fig. 3. ORTEP diagram [Ru(dipic)(PPh3)2Cl] (1).

D. Sukanya et al. / Polyhedron 25 (2006) 2223–2228
2227
Table 4
angles are given in Table 3. The N(1)–Ru(1)–Cl(1) bond
Catalytic activity of new Ru(III) complexes
angle is 178.95(9)ꢁ showing that Cl atom lies trans to ring
nitrogen. The bite angles around Ru(III) are N(1)–
Complex
Substrate
Yielda (%)
Turnover numberb
Ru(1)–O(4) = 77.42(10)ꢁ;
N(1)–Ru(1)–O(3) = 77.28(9)ꢁ;
1
benzyl alcohol
cinnamyl alcohol
56
60
57
59
O(4)–Ru(1)–Cl(1) = 103.60(7)ꢁ and O(3)–Ru(1)–Cl(1) =
101.70(6)ꢁ, summing up the in-plane angle to be exactly
360ꢁ. This shows the high planarity of the Cl and O, N, O
donor atoms of dipicolinic acid. This is further supported
by the other cis angles mentioned in Table 3. Thus, the acid
occupies the equatorial plane around the Ru(III) octahe-
dron, along with Cl. The bond angle P(1)–Ru(1)–
P(2) = 175.62(3)ꢁ shows that the two PPh3 groups are trans
to each other occupying the axial positions. The two Ru–P
bonds are slightly bent away from the dipicolinic acid
towards the Cl atom which is evident from the P(1)–
Ru(1)–Cl(1) bond angle is 88.84ꢁ which is smaller than
P(1)–Ru(1)–N(1) = 91.41ꢁ and P(2)–Ru(1)–N(1) = 92.51ꢁ
[42]. The Ru–P, Ru–O, Ru–N and Ru–Cl bond lengths
found in the complex agree well with that reported for sim-
ilar ruthenium complexes [3,7,43,44].
2
3
benzyl alcohol
cinnamyl alcohol
36
60
37
59
benzyl alcohol
cinnamyl alcohol
30
67
30
66
4
benzyl alcohol
cinnamyl alcohol
39
62
40
61
a
Yield based on substrate.
Moles of product per mole of catalyst.
b
is due to the fact that a-CH unit of cinnamyl alcohol is
more acidic than benzyl alcohol [45]. The replacement of
PPh3 by AsPh3 group in the complexes did not result in
much. On comparing the catalytic activity of these com-
plexes with that of the other reported ruthenium(III) com-
plexes, it is found that theses complexes exhibit
comparatively lower activity [31,46]. This may be due to
the stronger chelation of the ligand which may hinder the
formation of catalytically active species.
3.5. Catalytic activity
The oxidation of benzyl alcohol and cinnamyl alcohol to
benzaldehyde and cinnamaldehyde was carried out with
the new ruthenium(III) complexes as catalysts in the pres-
ence of N-methyl morpholine-N-oxide as co-oxidant in
CH2Cl2. After 3 h of reflux the aldehydes formed were
quantified as their 2,4-dinitrophenyl hydrazone derivatives
[25]. The results obtained are given in Table 4. In cinnamyl
alcohol, only the alcoholic group gets oxidized selectively
without affecting the double bond. The relatively higher
yield of cinnamaldehyde when compared to benzaldehyde
4. Conclusion
Dipicolinic acid can coordinate to the metal in a number
of fashions. In spite of this variable ligating ability, it acts
as tridentate dibasic chelating ligand forming only mono-
nuclear complex irrespective of the metal-to-ligand ratio.
Acknowledgement
UGC-CSIR, New Delhi, India, is acknowledged for
their financial assistance.
Table 3
˚
Selected bond lengths (A) and bond angles (ꢁ) for [Ru(dipic)(PPh3)2Cl]
Bond lengths
Appendix A. Supplementary data
Ru(1)–Cl(1)
Ru(1)–O(4)
Ru(1)–N(1)
Ru(1)–P(1)
Ru(1)–O(3)
Ru(1)–P(2)
2.3340(12)
2.050(2)
1.996(3)
2.3945(12)
2.053(2)
2.4052(12)
Crystallographic data for [Ru(dipic)(PPh3)2Cl] (1) have
been deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 288561. Copies of this information
may be obtained free of charge from the Director, CCDC,
12, Union Road, Cambridge, CB2 1EZ, UK (fax: +44 1223
336033; e-mail: deposit@ccdc.cam.ac.uk or http://www.
ccdc.cam.ac.uk). Supplementary data associated with this
article can be found, in the online version, at
doi:10.1016/j.poly.2006.01.023.
Bond angles
N(1)–Ru(1)–O(4)
O(3)–Ru(1)–P(1)
N(1)–Ru(1)–O(3)
Cl(1)–Ru(1)–P(1)
O(4)–Ru(1)–O(3)
N(1)–Ru(1)–P(2)
N(1)–Ru(1)–Cl(1)
O(4)–Ru(1)–P(2)
O(4)–Ru(1)–Cl(1)
O(3)–Ru(1)–P(2)
O(3)–Ru(1)–Cl(1)
Cl(1)–Ru(1)–P(2)
N(1)–Ru(1)–P(1)
P(1)–Ru(1)–P(2)
O(4)–Ru(1)–P(1)
77.42(10)
90.81(8)
77.28(9)
88.84(3)
154.70(8)
92.57(8)
178.95(9)
89.36(8)
103.60(7)
91.85(8)
101.70(6)
87.22(3)
91.41(8)
175.62(3)
89.72(8)
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