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

D

A

LTO

N

FULL PAPER

J. Chem. Soc., Dalton Trans., 2000, 181–184 181 This journal is © The Royal Society of Chemistry 2000

Ruthenium-mediated reduction of oximes to imines. Synthesis,

characterization and redox properties of imine complexes of

ruthenium

Anjan Kumar Das,

_{Shie-Ming Peng}

_{and Samaresh Bhattacharya *}

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 (HL}2_{–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 ⫺0.4 V vs. SCE and a

ruthenium()–ruthenium() oxidation in the range 0.88–1.15 V vs. SCE.

Introduction

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

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-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.

Experimental

[Ru(PPh3)3Cl2] was synthesized by following a literature

method.2 _{The oximes were prepared by reacting equimolar}

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

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%.

[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(L

3_)Cl

2]. 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%.

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

182 J. Chem. Soc., Dalton Trans., 2000, 181–184

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 di

ffu-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 solureduc-tion 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 Table 1 Crystallographic data for [Ru(PPh3)2(L

2_)Cl 2] Empirical formula fw Space group a/Å b/Å c/Å β/⬚ V/Å3 Z Crystal size/mm T/⬚C µ/cm⫺1 Rf Rw GOF C44H38NOP2Cl2Ru 830.7 Monoclinic, P21/n 9.471(3) 23.981(3) 17.290(5) 97.23(3) 3895.8(17) 4 0.50 × 0.50 × 0.50 25 6.471 0.026a 0.028b 2.75c a_R

f= Σ Fo|⫺ |Fc /Σ|Fo|. bRw= [Σw(|Fo|⫺ |Fc|)2/Σw(Fo)2]¹². cGOF=

[Σw(|Fo|⫺ |Fc|)2_{/(M⫺ N)]¹², where M is the number of reflections and} N is the number of parameters re_fined.

Table 2 Selected bond distances and bond angles for [Ru(PPh3)2 -(L2_)Cl

2]

Bond distances/Å Bond angles/⬚

Ru–Cl1 Ru–Cl2 Ru–P1 Ru–P2 Ru–O1 Ru–N1 O1–C1 N1–C7 2.3696(10) 2.3718(9) 2.4242(7) 2.4176(8) 1.9687(15) 2.0225(19) 1.319(3) 1.298(3) P1–Ru–P2 Cl2–Ru–N1 Cl1–Ru–O1 Cl1–Ru–Cl2 O1–Ru–N1 178.319(21) 174.53(6) 172.34(5) 98.92(4) 86.36(7)

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 86.36⬚. 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 cm⫺1). Triphenylphosphine is well

known to act as an oxygen-scavenger in many oxo-transfer reac-tions.8 _{The role of [Ru(PPh}

3)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 cm⫺1, which are attrib-uted to the Ru(PPh3)2 fragment.9 A sharp peak near 3300 cm⫺1

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 cm⫺1 _{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

Fig. 1 View of the [Ru(PPh3)2(L 2_)Cl

J. Chem. Soc., Dalton Trans., 2000, 181–184 183 Table 3 Electronic spectral and cyclic voltammetric data

Electronic spectral dataa

Cyclic voltammetric dataa,b

E1/2/V (∆Ep/mV)

Compound µeff/µB λmax/nm (ε/M⫺1 cm⫺1) Ru

III_–RuIV _RuIII_–RuII

[Ru(PPh3)2(L1)Cl2] [Ru(PPh3)2(L 2_)Cl 2] [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) ⫺0.38d ⫺0.41d ⫺0.36d a_{In dichloromethane solution.} b_{Supporting electrolyte TBAP; reference electrode SCE; E}

1/2= 0.5 (Epa⫹ Epc), where Epa and Epc are anodic and cathodic peak potentials, respectively; ∆Ep= Epa⫺ Epc; scan rate 50 mV s⫺1. c_Shoulder. d_E

pc value.

Table 4 EPR g-valuesa _{and derived parameters}b _{of the [Ru(PPh}

3)2(L)Cl2] complexes

Compound g1 g2 g3 ∆/λ V/λ ∆E1/λ ∆E2/λ

[Ru(PPh3)2(L1)Cl2] [Ru(PPh3)2(L 2_)Cl 2] [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 cm}⫺1_.

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¯_²) in these complexes. Electron paramagnetic 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 Fig. 2 Electronic spectrum of [Ru(PPh3)2(L

1_)Cl

2] in dichloromethane solution.

Fig. 3 EPR spectrum of [Ru(PPh3)2(L3)Cl2] in 1 : 1 dichloromethane– toluene solution at 77 K.

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

cm⫺1) and could not be detected. The ∆E2 transition, which

is expected to occur near 6000 cm⫺1 (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(L

2_)Cl 2].

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. 4 Cyclic voltammograms of a 1.2 × 10⫺3 M solution of [Ru-(PPh3)2(L2)Cl2] in dichloromethane (0.1 M TBAP) at a scan rate of 50 mV s⫺1.

184 J. Chem. Soc., Dalton Trans., 2000, 181–184

(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.

Conclusion

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.

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.

2 T. A. Stephenson and G. Wilkinson, J. Inorg. Nucl. Chem., 1966, 28, 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.

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.

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.