Ruthenium-mediated reduction of oximes to imines. Synthesis, characterization and redox properties of imine complexes of ruthenium

Article abstract of DOI:10.1039/a907021d

Reaction of three oxime ligands (oximes of salicylaldehyde (HL¹-O), 2-hydroxyacetophenone (HL²-O) and 2-hydroxynaphthylaldehyde (HL³-O); where H stands for the phenolic proton and O for the oxime oxygen) with [Ru(PPh₃)₃Cl₂] in a 1:1 molar ratio brings about reduction of the oximes to imines and affords complexes of the form [Ru(PPh₃)₂(L)Cl₂], where L stands for the deprotonated imine ligand which is coordinated as a N,O-donor forming a six-membered chelate ring. The structure of the [Ru(PPh₃)₂(L²)Cl₂] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is composed of NOP₂Cl₂ with the two PPh₃ ligands in mutually trans and the two chlorides in mutually cis positions. The [Ru(PPh₃)₂(L)Cl₂] complexes are one-electron paramagnetic (low-spin d⁵, S= 1/2) and show rhombic EPR spectra in 1:1 dichloromethane-toluene solution at 77 K. In dichloromethane solution the [Ru(PPh₃)₂(L)Cl₂] complexes show several intense LMCT transitions in the visible region, together with a weak ligand field transition near 1700 nm. Cyclic voltammetry on the [Ru(PPh₃)₂(L)Cl₂] complexes shows a ruthenium(III)-ruthenium(II) reduction near -0.4 V vs. SCE and a ruthenium(III)-ruthenium(IV) oxidation in the range 0.88-1.15 V vs. SCE. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a907021d

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Ruthenium-mediated reduction of oximes to imines. Synthesis,
characterization and redox properties of imine complexes of
ruthenium
Anjan Kumar Das,a Shie-Ming Pengb and Samaresh Bhattacharya*a
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University,
Calcutta 700 032, India
b Department of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China
Received 31st August 1999, Accepted 15th November 1999
Reaction of three oxime ligands (oximes of salicylaldehyde (HL1–O), 2-hydroxyacetophenone (HL2–O) and
2-hydroxynaphthylaldehyde (HL3–O); where H stands for the phenolic proton and O for the oxime oxygen) with
[Ru(PPh3)3Cl2] in a 1:1 molar ratio brings about reduction of the oximes to imines and affords complexes of the
form [Ru(PPh3)2(L)Cl2], where L stands for the deprotonated imine ligand which is coordinated as a N,O-donor
forming a six-membered chelate ring. The structure of the [Ru(PPh3)2(L2)Cl2] complex has been solved by X-ray
crystallography. The coordination sphere around ruthenium is composed of NOP2Cl2 with the two PPh3 ligands
in mutually trans and the two chlorides in mutually cis positions. The [Ru(PPh3)2(L)Cl2] complexes are one-electron
paramagnetic (low-spin d5, S = 1/2) and show rhombic EPR spectra in 1:1 dichloromethane–toluene solution at
77 K. In dichloromethane solution the [Ru(PPh3)2(L)Cl2] complexes show several intense LMCT transitions
in the visible region, together with a weak ligand field transition near 1700 nm. Cyclic voltammetry on the
[Ru(PPh3)2(L)Cl2] complexes shows a ruthenium()–ruthenium() reduction near Ϫ0.4 V vs. SCE and a
ruthenium()–ruthenium() oxidation in the range 0.88–1.15 V vs. SCE.
method.2 The oximes were prepared by reacting equimolar
Introduction
amounts of the respective aldehydes and hydroxylamine, fol-
lowing a reported procedure.3 Purification of dichloromethane
and preparation of tetrabutylammonium perchlorate (TBAP)
for electrochemical work was carried out as reported in the
literature.4
Metal-promoted chemical transformation of organic molecules
has been of significant current interest.1 Herein we wish to dis-
close an example of ruthenium-mediated reduction of oximes
to imines (eqn. 1). It may be noted here that such reduction of
Preparations
[Ru(PPh3)2(L1)Cl2]. [Ru(PPh3)3Cl2] (100 mg, 0.10 mmol) was
refluxed with salicylaldoxime (16 mg, 0.11 mmol) in ethanol
(50 cm3) for 2 h. A green microcrystalline precipitate of [Ru-
(PPh3)2(L1)Cl2] started to separate out during the reflux. After
cooling the solution to room temperature, the precipitate
was collected by filtration, washed with ethanol and dried in
air. Recrystallization from 1:4 dichloromethane–hexane gave
[Ru(PPh3)2(L1)Cl2] as a green crystalline solid in 72% yield.
Anal. Calc. for C43H35NOCl2P2Ru: C, 63.23; H, 4.41; N, 1.72.
Found: C, 63.30; H, 4.45; N, 1.73%.
oximes to imines appears to be unusual. The ligands used in the
present study are oximes of salicylaldehyde, 2-hydroxyaceto-
phenone and 2-hydroxynaphthaldehyde. All these ligands are
abbreviated in general as HL–O, where H stands for the dissoci-
able phenolic proton and O for the oxime oxygen. Individual
abbreviations are shown with structure 1. The ruthenium com-
[Ru(PPh3)2(L2)Cl2]. This complex was prepared by following
the same procedure as above, using the oxime of 2-hydroxy-
acetophenone (HL2–O) instead of salicylaldoxime. The yield
was 70%. Anal. Calc. for C44H38NOCl2P2Ru: C, 63.61; H, 4.58;
N, 1.69. Found: C, 63.66; H, 4.61; N, 1.70%.
[Ru(PPh3)2(L3)Cl2]. This complex was prepared by following
the same synthetic procedure as for [Ru(PPh3)2(L1)Cl2], using
the oxime of 2-hydroxynaphthaldehyde (HL3–O) instead of
salicylaldoxime. The yield was 70%. Anal. Calc. for C47H37-
NOCl2P2Ru: C, 64.8; H, 4.25; N, 1.60. Found: C, 64.24; H, 4.68;
N, 1.69%.
plex utilized for bringing about reduction of these oximes was
[Ru(PPh3)3Cl2]. Reaction of the oximes with [Ru(PPh3)3Cl2]
afforded a group of complexes of the type [Ru(PPh3)2(L)Cl2],
where L stands for the deprotonated imine ligand. The chem-
istry of these complexes is described here with special reference
to synthesis, characterization and redox properties.
Physical measurements
Microanalyses (C, H, N) were performed using a Perkin-Elmer
240C elemental analyzer. IR spectra were obtained on a Perkin-
Elmer 783 spectrometer with samples prepared as KBr pellets.
Electronic spectra were recorded on a Shimadzu UV 240
spectrophotometer. Magnetic susceptibilities were measured
Experimental
Materials
[Ru(PPh3)3Cl2] was synthesized by following a literature
J. Chem. Soc., Dalton Trans., 2000, 181–184
This journal is © The Royal Society of Chemistry 2000
181

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Table 1 Crystallographic data for [Ru(PPh3)2(L2)Cl2]
Empirical formula
fw
C44H38NOP2Cl2Ru
830.7
Space group
a/Å
b/Å
c/Å
Monoclinic, P21/n
9.471(3)
23.981(3)
17.290(5)
97.23(3)
3895.8(17)
4
β/Њ
V/Å3
Z
Crystal size/mm
0.50 × 0.50 × 0.50
25
T/ЊC
µ/cmϪ1
Rf
Rw
GOF
6.471
0.026a
0.028b
2.75c
¹
2

²
b
2
a Rf = Σ Fo| Ϫ |Fc /Σ|Fo|.
Rw = [Σw(|Fo| Ϫ |Fc|) /Σw(Fo) ] . c GOF =
¹
2

²
[Σw(|Fo| Ϫ |Fc|) /(M Ϫ N)] , where M is the number of reflections and
N is the number of parameters refined.
Table 2 Selected bond distances and bond angles for [Ru(PPh3)2-
(L2)Cl2]
Fig. 1 View of the [Ru(PPh3)2(L2)Cl2] molecule.
Bond distances/Å
Bond angles/Њ
and selected bond parameters are listed in Table 2. The oxime
of 2-hydroxyacetophenone has lost the oxime oxygen and the
resulting imine ligand is coordinated to ruthenium as a biden-
tate N,O-donor ligand, forming a six-membered chelate ring
with a bite angle of 86.36Њ. The two PPh3 ligands are mutually
trans, as is usually observed in complexes of ruthenium() con-
taining the Ru(PPh3)2 moiety,6 and the two chloride ligands
occupy mutually cis positions. The NOP2Cl2 coordination
sphere around ruthenium is distorted octahedral in nature. The
Ru–N, Ru–O, Ru–P and Ru–Cl bond lengths are all quite
unremarkable, as are the phenolic C–O and imine C–N dis-
tances.7 In view of the observed similarity in spectral and
electrochemical properties (vide infra), the other two [Ru-
(PPh3)2(L)Cl2] complexes are assumed to have a similar
structure. The mechanism of this reaction is not yet clear. How-
ever, an oxygen from the oxime ligand is probably transferred
to a PPh3,, dissociated from [Ru(PPh3)3Cl2]. Indirect evidence
for this oxo-transfer comes from detection of OPPh3 in the
residue of the synthetic reactions (after isolation of [Ru-
(PPh3)2(L)Cl2]), identified by its characteristic infrared spec-
trum (νP–O observed at 1185 cmϪ1). Triphenylphosphine is well
known to act as an oxygen-scavenger in many oxo-transfer reac-
tions.8 The role of [Ru(PPh3)3Cl2] in this reaction is not yet
clear, but that it does not act as a mere supplier of PPh3 is clear
from the fact that a mixture of PPh3 and the oxime is unable to
bring about an oxo-transfer reaction. Prior coordination of the
oxime ligand to ruthenium, followed by oxo-transfer from the
metal-bound ligand appears probable.
Infrared spectra of the [Ru(PPh3)2(L)Cl2] complexes show
strong vibrations near 520, 695 and 740 cmϪ1, which are attrib-
uted to the Ru(PPh3)2 fragment.9 A sharp peak near 3300 cmϪ1
is consistent with the presence of an N–H bond10 in the
coordinated imine fragment of the phenolate ligands. Two
ν(Ru–Cl) stretches are observed near 330 and 320 cmϪ1 due to
the cis-RuCl2 fragment.11 The [Ru(PPh3)2(L)Cl2] complexes are
soluble in polar organic solvents, such as dichloromethane,
chloroform, acetonitrile, etc., producing intense green solu-
tions. The electronic spectra of these complexes have been
recorded in dichloromethane solution. Spectral data are pre-
sented in Table 3 and a representative UV-vis spectrum is
shown in Fig. 2. Each complex shows a few very intense
absorptions in the ultraviolet region, several intense absorp-
tions in the visible region and a weak absorption in the near-
IR region. The absorptions in the ultraviolet region are
attributable to transitions occurring within the ligand
orbitals. The intense absorptions observed in the visible
region may be assigned to ligand-to-metal charge-transfer
Ru–Cl1
Ru–Cl2
Ru–P1
Ru–P2
Ru–O1
Ru–N1
O1–C1
N1–C7
2.3696(10)
P1–Ru–P2
178.319(21)
174.53(6)
172.34(5)
98.92(4)
2.3718(9)
2.4242(7)
2.4176(8)
1.9687(15)
2.0225(19)
1.319(3)
Cl2–Ru–N1
Cl1–Ru–O1
Cl1–Ru–Cl2
O1–Ru–N1
86.36(7)
1.298(3)
using a PAR 155 vibrating sample magnetometer. EPR spectra
were recorded on a Varian Model 109C E-line X-band spec-
trometer fitted with a quartz Dewar for measurements at 77 K
(liquid dinitrogen). All spectra were calibrated against the spec-
trum of DPPH (g = 2.0037). Electrochemical measurements
were made using a PAR model 273 potentiostat. A platinum
disc or graphite working electrode, a platinum wire auxiliary
electrode and an aqueous saturated calomel reference electrode
(SCE) were used in a three-electrode configuration. An RE
0089 X-Y recorder was used to trace the voltammograms.
Electrochemical measurements were made under a dinitrogen
atmosphere. All electrochemical data were collected at 298 K
and are uncorrected for junction potentials.
Crystallography
Single crystals of [Ru(PPh3)2(L2)Cl2] were grown by slow diffu-
sion of hexane into a dichloromethane solution of the complex.
Selected crystal data and data collection parameters are given
in Table 1. Data were collected on an Enraf-Nonius CAD-4
diffractometer using graphite-monochromated Mo-Kα radi-
ation (λ = 0.71073 Å). Three standard reflections measured
every 3600 s of X-ray exposure showed no significant intensity
variation over the course of data collection. X-Ray data reduc-
tion and structure solution and refinement were carried out
using the NRCVAX package.5
CCDC reference number 186/1739.
See http://www.rsc.org/suppdata/dt/a9/a907021d/ for crystal-
lographic files in .cif format.
Results and discussion
Reaction of the oximes (HL–O) with [Ru(PPh3)3Cl2] proceeded
smoothly in refluxing ethanol to afford imine complexes of the
type [Ru(PPh3)2(L)Cl2] in good yields. Formation of the imine
complexes has been authenticated by structural characteriz-
ation of [Ru(PPh3)2(L2)Cl2]. The structure is shown in Fig. 1
182
J. Chem. Soc., Dalton Trans., 2000, 181–184

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Table 3 Electronic spectral and cyclic voltammetric data
Cyclic voltammetric dataa,b
E1/2/V (∆Ep/mV)
Electronic spectral dataa
λmax/nm (ε/MϪ1 cmϪ1
Compound
µeff/µB
)
RuIII–RuIV
RuIII–RuII
[Ru(PPh3)2(L1)Cl2]
[Ru(PPh3)2(L2)Cl2]
[Ru(PPh3)2(L3)Cl2]
1.86
1.91
1.93
1745 (21), 685 (2200), 360c (9300), 298 (26100)
1670 (28), 672 (1800), 350c (8100), 297 (23200)
1740 (34), 710 (2300), 367c (10000), 300 (26600)
1.15 (70)
0.88 (70)
0.90 (70)
Ϫ0.38d
Ϫ0.41d
Ϫ0.36d
a In dichloromethane solution. b Supporting electrolyte TBAP; reference electrode SCE; E1/2 = 0.5 (Epa ϩ Epc), where Epa and Epc are anodic and
cathodic peak potentials, respectively; ∆Ep = Epa Ϫ Epc; scan rate 50 mV sϪ1 c Shoulder. d Epc value.
.
Table 4 EPR g-valuesa and derived parametersb of the [Ru(PPh3)2(L)Cl2] complexes
Compound
g1
g2
g3
∆/λ
V/λ
∆E1/λ
∆E2/λ
[Ru(PPh3)2(L1)Cl2]
[Ru(PPh3)2(L2)Cl2]
[Ru(PPh3)2(L3)Cl2]
2.489
2.443
2.443
2.162
2.183
2.266
1.815
1.841
1.831
4.8060
4.9915
4.6053
3.4051
3.0288
1.7442
3.2603
3.6083
3.8219
6.7362
6.7317
5.7561
a In 1:1 dichloromethane–toluene solution at 77 K. b Spin–orbit coupling constant (λ) for complexed ruthenium() is ca. 1000 cmϪ1
.
Fig. 4 Cyclic voltammograms of a 1.2 × 10Ϫ3 M solution of [Ru-
(PPh3)2(L2)Cl2] in dichloromethane (0.1 M TBAP) at a scan rate of
Fig. 2 Electronic spectrum of [Ru(PPh3)2(L1)Cl2] in dichloromethane
solution.
50 mV sϪ1
.
in terms of the gross molecular symmetry of these complexes,
containing the three non-equivalent P–Ru–P, O–Ru–Cl and
N–Ru–Cl axes. The rhombic distortion can be thought of as a
combination of axial distortion (∆, which splits t2 into a and
e) and rhombic distortion (V, which splits e). The splitting
pattern is illustrated in Fig. 3. Spin–orbit coupling causes
further changes in the energy gaps. Thus two electronic transi-
tions (transition energies ∆E1 and ∆E2; ∆E1 < ∆E2) are possible
within these three levels. All these energy parameters have been
computed (Table 4) using the observed g-values, the g-tensor
theory of low-spin d5 complexes and a reported method.12 The
axial distortion is observed to be much stronger than the rhom-
bic. The ∆E1 transition falls in the infrared region (3200–3800
cmϪ1) and could not be detected. The ∆E2 transition, which
is expected to occur near 6000 cmϪ1 (ca. 1667 nm), is indeed
displayed by all three [Ru(PPh3)2(L)Cl2] complexes as a weak
absorption near the predicted energies (Tables 3 and 4). The
EPR data analysis thus shows that the [Ru(PPh3)2(L)Cl2] com-
plexes are significantly distorted from ideal octahedral geom-
etry, as observed in the crystal structure of [Ru(PPh3)2(L2)Cl2].
The electrochemical properties of the [Ru(PPh3)2(L)Cl2]
complexes have been studied in dichloromethane solution (0.1
M TBAP) by cyclic voltammetry. Voltammetric data are given
in Table 3 and a representative voltammogram is displayed in
Fig. 4. All three complexes show one oxidative response on the
positive side of SCE and one reductive response on the negative
side. The oxidation is assigned to ruthenium()–ruthenium()
oxidation. This oxidation is quasi-reversible in nature, charac-
terized by a peak-to-peak separation (∆Ep) of 70 mV, and the
cathodic peak current (ipc) is lower than the anodic peak current
Fig. 3 EPR spectrum of [Ru(PPh3)2(L3)Cl2] in 1:1 dichloromethane–
toluene solution at 77 K.
transitions. The origin of the weak absorption in the near-IR
region is discussed below.
Magnetic susceptibility measurements show that the [Ru-
(PPh3)2(L)Cl2] complexes are one-electron paramagnetic (Table
3), which corresponds to the trivalent state of ruthenium (low-
spin d5, S = ¹) in these complexes. Electron paramagnetic
¯
²
resonance (EPR) spectra of the [Ru(PPh3)2(L)Cl2] complexes,
recorded in 1:1 dichloromethane–toluene solution at 77 K,
show rhombic spectra with three distinct signals (g1, g2 and g3,
in decreasing order of magnitude). A representative spectrum is
shown in Fig. 3 and the spectral data are presented in Table 4.
The observed rhombicity of the EPR spectra is understandable
J. Chem. Soc., Dalton Trans., 2000, 181–184
183

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2 T. A. Stephenson and G. Wilkinson, J. Inorg. Nucl. Chem., 1966, 28,
(ipa). The reductive response is irreversible and is assigned to
ruthenium()–ruthenium() reduction. The one-electron
nature of these responses has been confirmed by comparing
their current heights with the standard ferrocene/ferrocenium
couple under identical experimental conditions.
945.
3 A. I. Vogel, A Text Book of Practical Organic Chemistry, ELBS,
London, 1971, pp. 957–958.
4 (a) D. T. Sawyer and J. L. Roberts, Jr., Experimental Electro-
chemistry for Chemists, Wiley, New York, 1974, pp. 167–215; (b) M.
Walter and L. Ramaley, Anal. Chem., 1973, 45, 165.
5 E. J. Gabe, Y. Le Page, J. P. Charland, F. L. Lee and P. S. White,
J. Appl. Crystallogr., 1989, 22, 384.
Conclusion
6 S. Chattopadhyay, N. Bag, G. K. Lahiri and A. Chakravorty,
J. Chem. Soc., Dalton Trans., 1990, 3389; S. Bhattacharya and
C. G. Pierpont, Inorg. Chem., 1991, 30, 1511; M. Menon, A.
Pramanik, N. Bag and A. Chakravorty, J. Chem. Soc., Dalton
Trans., 1995, 1417.
7 K. Sui, S. M. Peng and S. Bhattacharya, Polyhedron, 1999, 18, 631;
F. Basuli, A. K. Das, G. Mostafa, S. M. Peng and S. Bhattacharya,
submitted for publication.
The present study shows an interesting oxo-transfer reaction
mediated by [Ru(PPh3)3Cl2]. The applicability of [Ru(PPh3)3Cl2]
as a mediator to bring about oxo-transfer reactions from vari-
ous oxygen-containing ligands is currently under investigation.
The reactivity of the cis-RuCl2 fragment of the [Ru(PPh3)3(L)-
Cl2] complexes is also being explored.
8 B. A. Moyer, B. K. Sipe and T. J. Meyer, Inorg. Chem., 1981, 20,
1475; M. E. Marmoin and K. J. Takeuchi, J. Am. Chem. Soc., 1988,
110, 1472; C. M. Che and K. Y. Yong, J. Chem. Soc., Dalton Trans.,
1989, 2065.
9 S. Bhattacharya and C. G. Pierpont, Inorg. Chem., 1991, 30, 2906;
N. C. Pramanik and S. Bhattacharya, Trans. Met. Chem., 1999, 24,
95.
10 J. R. Dyer, Application of Absorption Spectroscopy of Organic
Compounds, Prentice-Hall of India Pvt. Ltd., New Delhi, 1989, 37.
11 S. Choudhury, M. Kakoti, A. K. Dev and S. Goswami, Polyhedron,
1992, 11, 3183.
12 B. Bleany and M. C. M. O’Brien, Proc. Phys. Soc., London, Sect. B,
1956, 69, 1216; J. S. Griffith, The Theory of Transition Metal Ions,
Cambridge University Press, London, 1961, p. 364; S. Bhattacharya
and A. Chakravorty, Proc. Indian Acad. Sci., Chem. Sci., 1985, 95,
159.
Acknowledgements
Financial assistance received from the Council of Scientific and
Industrial Research, New Delhi [Grant No. 01(1408)/96/
EMR-II] is gratefully acknowledged. Thanks are also due to
the Third World Academy of Sciences for financial support
enabling the purchase of an electrochemical cell system.
References
1 C. Pearson and A. L. Beauchamp, Inorg. Chem., 1998, 37, 1242;
T. Hashimoto, A. Endo, N. Nagao, G. P. Sato, K. Natrajan and
K. Shimizu, Inorg. Chem., 1998, 37, 5211; P. Paul, B. Tyagi, A. K.
Balikakhiya, M. M. Bhadbhade, E. Suresh and G. Rama-
chandraiah, Inorg. Chem., 1998, 37, 5733; J. Y. Lu, B. R. Cabrera,
R. J. Wang and J. Li, Inorg. Chem., 1998, 37, 4480; E. V. Rybak-
Akimova, A. Y. Nazarenko and S. S. Silchenko, Inorg. Chem., 1999,
38, 2974.
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J. Chem. Soc., Dalton Trans., 2000, 181–184