Ruthenium(II) complexes with 2-(benzylimino-methyl)-4-R-phenol containing the trans(PPh3),cis(CO,Cl)-{Ru(PPh3)2(CO)Cl}+ unit

Article abstract of DOI:10.1016/j.jorganchem.2009.11.020

Reactions of [Ru(PPh₃)₃Cl₂] with 2-(benzylimino-methyl)-4-R-phenol (H^RL, R = H, Cl, Br and OMe) in boiling methanol in presence of triethylamine afford ruthenium(II) complexes of general formula [Ru(^RL)(PPh₃)₂(CO)Cl] in 57-64% yield. Microanalysis, spectroscopic (infrared, electronic and NMR) and cyclic voltammetric measurements have been used for the characterization of the complexes. Crystal structures of two representative complexes have been determined by X-ray crystallography. The carbonyl, the chloride, the N,O-donor ^RL^- and the two mutually trans PPh₃ molecules assemble a distorted octahedral CClNOP₂ coordination sphere around the metal centre in each complex. The complexes display the Ru(II) → Ru(III) oxidation in the potential range 0.62-1.16 V (vs. Ag/AgCl).

Full text of DOI:10.1016/j.jorganchem.2009.11.020

Journal of Organometallic Chemistry 695 (2010) 630–633
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Note
Ruthenium(II) complexes with 2-(benzylimino-methyl)-4-R-phenol containing
the trans(PPh3),cis(CO,Cl)-{Ru(PPh3)2(CO)Cl}+ unit
*
Raji Raveendran, Samudranil Pal
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e i n f o
a b s t r a c t
Reactions of [Ru(PPh3)3Cl2] with 2-(benzylimino-methyl)-4-R-phenol (HRL, R = H, Cl, Br and OMe) in boil-
ing methanol in presence of triethylamine afford ruthenium(II) complexes of general formula
[Ru(RL)(PPh3)2(CO)Cl] in 57–64% yield. Microanalysis, spectroscopic (infrared, electronic and NMR) and
cyclic voltammetric measurements have been used for the characterization of the complexes. Crystal
structures of two representative complexes have been determined by X-ray crystallography. The car-
bonyl, the chloride, the N,O-donor RLꢁ and the two mutually trans PPh3 molecules assemble a distorted
octahedral CClNOP2 coordination sphere around the metal centre in each complex. The complexes display
the Ru(II) ? Ru(III) oxidation in the potential range 0.62–1.16 V (vs. Ag/AgCl).
Article history:
Received 29 October 2009
Accepted 16 November 2009
Available online 22 November 2009
Keywords:
Ruthenium(II)
Schiff base
Carbonyl
Redox properties
Crystal structures
Ó 2009 Elsevier B.V. All rights reserved.
C–Hꢀ ꢀ ꢀp interaction
1. Introduction
2. Experimental
In majority of the organometallic complexes of ruthenium, the
oxidation state of the metal centre is +2 [1–4]. Among the few
known ruthenium(III) complexes containing the M–C bond [5–
15], the cyclo-metallated species are particularly scarce [12–
15]. Recently we have demonstrated that the Schiff bases (H2L)
derived from various acid hydrazides and benzaldehyde or aceto-
phenone are very efficient in producing cyclo-metallated ruthe-
nium(III) complexes [16,17]. Reaction of H2L, [Ru(PPh3)3Cl2] and
N(C2H5)3 (1:1:2 mole ratio) in methanol under aerobic condition
leads to the facile formation of trans-[Ru(L)(PPh3)2Cl] where L2ꢁ
acts as C,N,O-donor. In our attempts to synthesize similar
ortho-metallated ruthenium(III) species we have treated a mix-
ture of 2-(benzylimino-methyl)-4-R-phenol (HRL, R = H, Cl, Br
and OMe) and N(C2H5)3 with [Ru(PPh3)3Cl2] in methanol in pres-
ence of air. However, instead of the ortho-metallated ruthe-
nium(III) complexes bivalent ruthenium complexes of molecular
formula [Ru(RL)(PPh3)2(CO)Cl] have been obtained. Herein we de-
scribe the synthesis, characterization and the redox properties of
these complexes. X-ray structures of two representative com-
plexes are also reported.
2.1. Materials
The Schiff bases (HRL) were prepared in 70–75% yields by con-
densation reactions of equimolar amounts of benzylamine and
the corresponding 5-R-salicylaldehyde in methanol [14].
[Ru(PPh3)3Cl2] was prepared by following a reported procedure
[18]. All other chemicals and solvents were of analytical grade
available commercially and were used without further
purification.
2.2. Physical measurements
A Thermo Finnigon Flash EA1112 series elemental analyzer
was used to collect the microanalytical (C, H, N) data. Room tem-
perature (298 K) magnetic susceptibilities were measured using a
Sherwood Scientific balance. Solution electrical conductivities
were measured with a Digisun DI-909 conductivity meter. Infra-
red spectra were collected on a Nicolet 5700 FT-IR spectropho-
tometer. A Cary 100 Bio UV/vis spectrophotometer was used to
record the electronic spectra. The 1H (Si(CH3)4 as internal stan-
dard) NMR spectra were collected with the help of a Bruker
400 MHz NMR spectrometer. Cyclic voltammetric experiments
with dichloromethane solutions of the complexes containing tet-
rabutylammonium perchlorate (TBAP) as the supporting electro-
lyte were performed with the help of a CH-Instruments model
* Corresponding author. Tel.: +91 40 2313 4829; fax: +91 40 2301 2460.
E-mail address: spsc@uohyd.ernet.in (S. Pal).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.11.020

R. Raveendran, S. Pal / Journal of Organometallic Chemistry 695 (2010) 630–633
631
Table 1
620A electrochemical analyzer. The three electrode measure-
ments were carried out at 298 K under dinitrogen atmosphere
with a platinum disk working electrode, a platinum wire auxiliary
electrode and an Ag/AgCl reference electrode. Under identical
condition the Fc+/Fc couple was observed at 0.65 V. The potentials
reported in this work are uncorrected for junction contributions.
Crystal data for [Ru(ClL)(PPh3)2(CO)Cl]ꢀCH3CN (2ꢀCH3CN) and [Ru(BrL)(PPh3)2(-
CO)Cl]ꢀCH3CN (3ꢀCH3CN).
Complex
2ꢀCH3CN
3ꢀCH3CN
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
RuC53H44N2O2Cl2P2 RuC53H44N2O2ClBrP2
974.87
Monoclinic
C2/c
1019.27
Monoclinic
C2/c
2.3. Synthesis of [Ru(HL)(PPh3)2(CO)Cl] (1)
20.877(4)
12.466(3)
35.548(7)
90.822(4)
9251(3)
8
21.031(3)
12.4573(16)
35.479(4)
91.647(2)
9291(2)
8
b (Å)
c (Å)
Solid [Ru(PPh3)3Cl2] (195 mg, 0.20 mmol) was added to a meth-
anol solution (30 ml) of HHL (43 mg, 0.20 mmol) and N(C2H5)3
(0.06 ml, 44 mg, 0.43 mmol) and the mixture was boiled under re-
flux for 5 h. The solid obtained after cooling to room temperature
was collected by filtration, washed thoroughly with n-hexane
and dried in air. This solid was dissolved in minimum amount of
dichloromethane and transferred to a neutral aluminium oxide col-
umn. The first moving light yellow band was eluted with dichloro-
methane and discarded. The second brownish yellow band
containing the complex was eluted with dichloromethane contain-
ing 5% acetone. Recrystallization of the complex was performed
from dichloromethane–hexane (1:1) mixture. Yield: 105 mg
(58%). Anal. Calc. for RuC51H42NO2P2Cl: C, 68.11; H, 4.71; N, 1.56.
b (°)
V (Å3)
Z
q
l
(g cmꢁ3
(mmꢁ1
)
1.400
1.457
)
0.567
1.367
Reflections collected
Reflections unique
Reflections [I P 2
Parameters
R1, wR2 [I P 2
51842
52189
10590
10594
r
(I)]
8582
7605
560
560
r(I)]
0.0708, 0.1361
0.0925, 0.1440
1.072
0.0489, 0.1114
0.0764, 0.1215
1.041
R1, wR2 [all data]
Goodness-of-fit on F2
Largest difference in peak and
1.072, ꢁ0.752
0.904, ꢁ0.467
hole (e Åꢁ3
)
Found: C, 67.84; H, 4.53; N, 1.38%. UV–vis in CH2Cl2 (k (nm) (
e
(Mꢁ1 cmꢁ1))): 418 (4190), 358 (8650), 294 sh (11800). Epa (V):
0.68.
in 1:2:1 mole ratio under aerobic condition result into the forma-
tion of ruthenium(II) complexes [Ru(RL)(PPh3)2(CO)Cl] (1 (R = H), 2
(R = Cl), 3 (R = Br) and 4 (R = OMe)) in 58–64% yields. The elemental
(C, H, N) analysis data for the complexes are consistent with the
above general formula. The solvent methanol used during the
synthesis is in all likelihood the source of the carbonyl in these
complexes [16]. The diamagnetic nature of 1–4 confirms the +2
oxidation state and the low-spin character of the metal centre in
each complex. All the complexes behave as non-electrolyte in solu-
tion. It may be noted that in boiling ethanol the reaction of HRL
(R = H) with [Ru(PPh3)3Cl2] in 2:1 mole ratio is known to afford
trans-[RuII(HL)2(PPh3)2] and [RuIII(HL)(HL0)(PPh3)] where HLꢁ is
The other three complexes having the same general formula
[Ru(RL)(PPh3)2(CO)Cl] (2 (R = Cl), 3 (R = Br) and 4 (R = OMe)) were
synthesized from [Ru(PPh3)3Cl2], the corresponding Schiff base
(HRL) and N(C2H5)3 in 60–64% yields by following procedures very
similar to that described above for 1 (R = H). Anal. Calc. for
RuC51H41NO2P2Cl2 (2): C, 65.60; H, 4.43; N, 1.50. Found: C, 65.49;
H, 4.37; N, 1.36%. UV–vis in CH2Cl2 (k (nm) (e
(Mꢁ1 cmꢁ1))): 424
(2200), 348 (8150), 295 sh (12300). Epa (V): 0.81. Anal. Calc. for
RuC51H41NO2P2ClBr (3): C, 62.62; H, 4.22; N, 1.43. Found: C,
62.28; H, 4.01; N, 1.22%. UV–vis in CH2Cl2 (k (nm) (
416 (3100), 340 (6960), 270 sh (38600). E1/2 = (Epa + Epc)/2 (V)
Ep = Epa ꢁ Epc (mV)): 1.16 (110). Anal. Calc. for RuC52H44NO3P2Cl
(4): C, 67.20; H, 4.77; N, 1.51. Found: C, 66.97; H, 4.61; N, 1.40%.
UV–vis in CH2Cl2 (k (nm) (
(Mꢁ1 cmꢁ1))): 434 (2200), 353
e
(Mꢁ1 cmꢁ1))):
H
2ꢁ
N,O-donor and L0 is C,N,O-donor [14].
Infrared spectra of 1–4 in KBr display a strong band in the range
1914–1919 cmꢁ1. This band is attributed to the metal coordinated
carbonyl group [24]. The origin of the medium intensity band ob-
served within 1624–1630 cmꢁ1 is most likely due to the C@N
stretch of the ligand (RLꢁ) [25–30]. In all the spectra three strong
(D
e
(6350), 298 sh (10100). Epa (V): 0.62.
2.4. X-ray crystallography
Complexes 2 and 3 crystallize as 2ꢀCH3CN and 3ꢀCH3CN in the
space group C2/c from acetonitrile solution. For both crystals, unit
cell parameters and the intensity data at 298 K were obtained on a
Bruker-Nonius SMART APEX CCD single crystal diffractometer,
equipped with a graphite monochromator and a Mo Ka fine-focus
sealed tube (k = 0.71073 Å). The SMART software was used for data
acquisition and the SAINT-PLUS software was used for data extraction
[19]. The data were corrected for absorption with the help of SADABS
program [20]. Both structures were solved by direct methods and
refined on F2 by full-matrix least-squares procedures. In each case,
the asymmetric unit contains one complex molecule and one ace-
tonitrile molecule. All non-hydrogen atoms were refined aniso-
tropically. The hydrogen atoms were included in the structure
factor calculation at idealized positions by using a riding model.
The SHELX-97 programs [21] were used for structure solution and
refinement. The ORTEX6a [22] and PLATON packages [23] were used
for molecular graphics. Selected crystal data are listed in Table 1.
3. Results and discussion
Fig. 1. Cyclic voltammogram (scan rate 50 mV sꢁ1) of [Ru(BrL)(PPh3)2(CO)Cl] (3) in
dichloromethane (0.1 M TBAP) at 298 K.
In boiling methanol, reactions of 2-(benzylimino-methyl)-4-R-
phenol (HRL, R = H, Cl, Br and OMe), N(C2H5)3 and [Ru(PPh3)3Cl2]

632
R. Raveendran, S. Pal / Journal of Organometallic Chemistry 695 (2010) 630–633
bands are observed at ꢂ745, ꢂ696 and ꢂ515 cmꢁ1. Such bands are
common for complexes of the trans-{Ru(PPh3)2} moiety
[14,16,17,27–30].
Electronic spectra of 1–4 were collected using their dichloro-
methane solutions. The spectral profiles of the four complexes
are very similar. They display three strong absorptions in the
Table 2
Selected bond lengths (Å) and bond angles (°) for [Ru(ClL)(PPh3)2(CO)Cl]ꢀCH3CN
(2ꢀCH3CN) and [Ru(BrL)(PPh3)2(CO)Cl]ꢀCH3CN (3ꢀCH3CN).
Parameter
2ꢀCH3CN
3ꢀCH3CN
Ru–O(1)
Ru–N(1)
Ru–Cl(1)
Ru–C(15)
Ru–P(1)
Ru–P(2)
C(15)–O(2)
O(1)–Ru–N(1)
O(1)–Ru–Cl(1)
O(1)–Ru–C(15)
O(1)–Ru–P(1)
O(1)–Ru–P(2)
N(1)–Ru–Cl(1
N(1)–Ru–C(15)
N(1)–Ru–P(1)
N(1)–Ru–P(2)
Cl(1)–Ru–C(15)
Cl(1)–Ru–P(1)
Cl(1)–Ru–P(2)
C(15)–Ru–P(1)
C(15)–Ru–P(2)
P(1)–Ru–P(2)
Ru–C(15)–O(2)
2.067(3)
2.083(3)
2.068(2)
2.073(3)
2.4329(9)
1.840(3)
2.4180(9)
2.4005(9)
1.146(4)
88.78(9)
87.95(6)
176.91(12)
86.50(6)
87.94(6)
176.25(7)
94.19(13)
92.80(7)
91.70(7)
89.11(11)
85.17(3)
90.02(3)
94.22(10)
91.09(10)
172.77(3)
178.3(3)
2.4400(12)
1.828(4)
2.4039(11)
2.4186(11)
1.153(5)
89.17(12)
87.95(8)
176.51(15)
87.71(8)
86.72(8)
176.56(9)
94.22(16)
91.83(9)
92.89(9)
88.68(14)
89.94(3)
85.06(4)
C49
C36
C50
C51
C48
C47
C45
C35
C34
C37
C46
C44
C13
C38
C43
C42
C39
Cl2
C4
P2
C40
C14
C41
C12
C5
C7
C6
C3
N1
91.38(13)
93.89(13)
172.65(4)
178.7(4)
C11
C9
C1
O1
C2
C8
C10
Ru
C15
Cl1
O2
C17
C33
P1
C16
C21
C18
C32
C29
C31
C30
wavelength ranges 416–434, 340–358 and 270–298 nm due to me-
tal-to-ligand charge transfer and ligand centred transitions.
Proton NMR spectra of 1–4 in CDCl3 display the methylene pro-
tons of RLꢁ as a singlet within 4.34–4.40 ppm. A broad weak signal
observed in the range 8.38–8.45 is perhaps associated with the me-
tal coordinated azomethine proton. The aromatic protons appear
as complex multiplets within 7.0–7.9 ppm. The methyl protons of
the methoxy substituent on the ligand in 4 resonate as a singlet
at 3.53 ppm.
The cyclic voltammograms of 1–4 display a metal centred oxi-
dation in the potential range 0.62–1.16 V. Except for 3, the oxida-
tion is irreversible for the remaining three complexes. In the case
of 3 also the cathodic peak current is smaller than the anodic peak
current (Fig. 1). The trend in the potential values indicates that the
oxidation of the metal centre becomes more difficult as the substi-
tuent on RLꢁ becomes more electron withdrawing. Thus the poten-
tial is highest for R = Br and it is lowest for R = OMe.
Molecular structures of complexes 2 and 3 determined by X-
ray crystallography are shown in Fig. 2. Selected bond parameters
for both complexes are listed in Table 2. The metal centre is in
distorted octahedral CClNOP2 coordination sphere in each com-
plex. The N,O-donor RLꢁ, the carbonyl C-atom and the chloride
form a CClNO square plane around the metal centre and the
two PPh3 molecules occupy the axial positions. The cis and trans
bond angles are in the ranges 85.06(4)–94.22(16)° and
172.65(4)–176.91(12)°, respectively. The Ru–O(phenolate), the
Ru–N(imine), the Ru–Cl and the trans Ru–P bond lengths are com-
parable with the bond lengths observed in ruthenium(II) com-
plexes having the same coordinating atoms [27,31–37]. The Ru–
C and the C–O bond lengths and the Ru–C–O bond angle are with-
in the range reported for carbonyl coordinated ruthenium(II) spe-
cies [35–39]. In each of the two molecules, one ortho-C–H of the
benzyl phenyl ring is directed towards the centroid (Cg) of a phe-
nyl ring of one of the PPh3 ligands (Fig. 2). In 2, the Hꢀ ꢀ ꢀCg and
the Cꢀ ꢀ ꢀCg distances and the C–Hꢀ ꢀ ꢀCg angle are 2.80 and
3.695(7) Å and 163°, respectively. The corresponding values for
3 are 2.79 and 3.698(6) Å and 165°, respectively. These parame-
C28
C27
C22
C19
C20
C26
C25
C23
C24
(a)
C25
C26
C27
C20
C24
C23
C30
C21
C19
C22
C31
C13
C29
C16
Br
C18
C32
C8
C28
P1
C5
C14
C33
C17
C3
C12
C11
C7
C4
N1
C9
C6
C1
O1
C2
C10
Ru
C15
C47
O2
Cl1
C35
P2
C48
C34
C39
C36
C49
C46
C41
C40
C51
C50
C37
C42
C43
C38
C45
C44
(b)
Fig. 2. Molecular structures of (a) [Ru(ClL)(PPh3)2(CO)Cl] (2) and (b)
[Ru(BrL)(PPh3)2(CO)Cl] (3) with the atom labeling schemes. All non-hydrogen atoms
are represented by their 30% probability thermal ellipsoids. Except for the benzyl
group phenyl ring of RLꢁ, all other hydrogen atoms are omitted for clarity.
ters clearly indicate an intramolecular C–Hꢀ ꢀ ꢀ
p interaction in both
2 and 3 [40].

R. Raveendran, S. Pal / Journal of Organometallic Chemistry 695 (2010) 630–633
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4. Conclusion
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complexes trans(PPh3),cis(CO,Cl)-[Ru(RL)(PPh3)2(CO)Cl] (HRL, 2-
(benzylimino-methyl)-4-R-phenol (R = H, Cl, Br and OMe)) are de-
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in 1:2:1 mole ratio under reflux condition afford these complexes
in good yield. Originally these reactions were performed in search
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is expected to act as C,N,O-donor ligand due to ortho-metallation
at the benzyl group phenyl ring. Molecular structures of
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p interaction
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Acknowledgements
Financial assistance received from the Department of Science
and Technology (DST) (Grant No. SR/S1/IC-10/2007) is gratefully
acknowledged. Dr. R. Raveendran thanks the Council of Scientific
and Industrial research for a Senior Research Fellowship. X-ray
crystal structures were determined at the National Single Crystal
Diffractometer Facility, School of Chemistry, University of Hydera-
bad (established by the DST). We thank the University Grants Com-
mission, New Delhi for the facilities provided under the UPE and
CAS programs.
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