Indian Journal of Chemistry Yol. 44A, April 2005, pp. 705-709

Synthesis, spectra, redox property and catalytic activity of ruthenium(III) Schiff base complexes

G Venkatachalam, S Maheswaran & R Ramesh*

School of Chemistry ~harathidasan_U..!1 iversity ,_Tiruchirappalli 620 024:":ndia

Received 30 September 2004; revised 8 February 2005

A series of ruthenium(llI) complexes of the type [RuX2(EPh3ML)] (where E= P or As, X= C I or Br; L = bidentate Schiff bases) have been sy nthesized and characterized on the basis of analytical (elemental analysis, magnetic susceptibility) and spectral methods (Ff-IR, UY-vis, EPR). EPR spectra of the powdered samples exhibit three lines with different 'g' values indicating a rhombic distortion in these complexes. Cyclic voltammogram of the complexes displays one reversible oxidation (RuIV/Ru lll

) and two reversible reduction peaks (RUII/Ru", Ru"/Ru l) with L'iEp=60-80 mY suggesting one electron

transfer process. These complexes effectively catalyze the oxidation of primary alcohols to their corresponding aldehydes up to 91 .5% in the presence of N-methylmorpholine-N-ox ide (NMO) as co-oxidant. The formation of high-valent Ru n

+2=O

species as catalytic intermediate is proposed for the catalytic processes.

IPC Code: Int. Cl.7 C07F15/00; BOIJ23/46

The use of ruthenium complexes to catalyze oxidation of alcohols by oxygen atom donors has been well documented l

-3

• The accessibility of ruthenium higher oxidation states4

.5 converts them into excellent candidates as catalyst for redox reactions. Particularly, metal complexes of ruthenium are demonstrated to be useful laboratory and industrial homogeneous catalysts in the epoxidation of alkenes and oxidation of alcohols using iodosylbenzene, sodium hypochlorite, hydrogen peroxide and N-

h I h I· N . d I 6-9 met y morp 0 me- -OXI e as oxygen sources' . Further, the oxidation of organic substrates mediated by high valent ruthenium-oxo species evokes much interest in the modelling of cytochrome P-4501O. Sharpless and co-workers II carried out an yield oriented study of oxidation of cholestanol, geraniol , etc., catalyzed by ruthenium complexes in the presence of N-methylmorpholine-N-oxide (NMO) and N,N-dimethylaniline-N-oxide. The catalytic activities of ruthenium complexes containing tertiary phosphine or tertiary arsine li gands are also well established 12. 13.

In continuation of our systematic investigation on synthesis and characterization of ruthenium chelates using simple and inexpensive Schiff base ligands I 4

-16

,

we describe here synthesis, spectral and electrochemical behavior of ruthenium(JU) Schiff base complexes and the study of catalytic oxidation of primary alcohols by such complexes in the presence of N-methylmorpholine-N-oxide as co-catalyst. The

following Schiff bases (Structure I) have been used to prepare the new ruthenium(III) complexes.

~:" R= - 'CH3 (HL1)

= - C6H11 (HL2)

H ~ =-C5H.N (HL3;

Structure I

Materials and Methods RuCI 3.3H20 was purchased from Loba-Chemie and

was used without further purification. All the reagents used were of analytical grade or chemically pure grade. Solvents were purified and dried according to the standard procedures. The analyses of carbon, hydrogen and nitrogen were performed in Carlo-Erba l106-model 240 Perkin-Elmer analyzer at Central Drug Research Institute, Lucknow, India. The percentage of ruthenium and halides were determined by the reported methods I7

, 18. IR spectra of the li gands and complexes were recorded in KBr pellets with a Perkin-Elmer 597 spectrophotometer in the range 4000-200 cm- I

. Electronic spectra of the complexes were recorded in CHCb solution with a Cary 300 Bio UV-vis Varian spectrophotometer in the range 800-200 nm. EPR spectra of the powdered samples at room temperature were recorded with a JEOL-FA 200 EPR spectrophotometer at X-band frequencies and DPPH was used as a field maker (g=2.0036).

706 INDIAN J CHEM, SEC A, APRIL 2005

Table 1- Analytical data of Ru(lII) Schiff base complexes

Complex Colour M. wt. M.P Found (Calcd.) (%) (0C) C H N Ru CI/Br

I [RuCI2(PPh)h(Ll)] Green 830.60 107 63.61 (63.62) 4.61 (4.63) 1.61 (1.65) 12.85 (12.16) 8.42 (8.53) 2 [RuCI2(AsPh3h(Ll)] Green 918.60 126 57.52 (57.60) 4.16 (4.20) 152 (1.49) 10.82 (11.00) 7.28(7.71)

3 [RuBr2(AsPh3h(Ll)] Green 1007.50 135 52.45 (52.50) 3.80 (3.70) 1.39 (1.40) 10.43 (10.03) 15.70 (15 .86) 4 [RuBr2(PPh)h(Ll)] Green 919.50 122 57.46 (57.42) 4.16 (4.10) 1.52 (1 .50) 1l.21 (10.99) 17.03 (17 .37) 5 [RuCliPPh3h(L2)] Green 898.73 144 65.48 (65.60) 5.15 (5.21) 1.56 (1.52) 11.64 (11.24) 8.12 (7 .88) 6 [RuCI2(AsPh3h(L2)] Green 986.73 148 59.64 (60.00) 4.69 (4.62) 1.41 (1.39) 10.01 (10.24) 7.26 (7.18) 7 [RuBr2(AsPh}h(L2)] Green 1075.63 125 54.71 (54.68) 4.31 (4.45) 1.30 (1.25) 9.58 (9.39) 14.88 (14.55)

8 [RuBr2( PPh3h(L2)] Green 987.63 139 59.58 (59.70) 4.69 (4.70) 1.41 (1.35) 10.01 (10.23) 16.43 (16.18) 9 [RuCI2(PPh3h(L3)] Green 893.67 120 64.50 (64.60) 4.40 (4.50) 3.13 (3 .20) 11.73 (11.30) 8.18 (7.93)

10 [RuCI2(AsPh3h(L3)] Green 989.67 171 58.72 (58.80) 4.00 (3.90) 2.85 (2.90) 10.82 (10.29) 7.33 (7.22)

II [RuBr2(AsPh3h(L3)] Green 1070.57 138 53.85 (53.92) 3.67 (3.70) 2.61 (2.60) 9.66 (9.44) 15.61 (14.92)

12 [RuBr2(PPh)h(L3)] Green 982.57 143 58.66 (58.70) 4.00 (4.10) 2.85 (2.78) 10.03 (10.28) 16.82 (16.26)

Magnetic susceptibilities were recorded on EG and G model 155 vibrating sample magnetometer. The electrochemical data were collected with a BAS model CY-27 electrochemical analyzer and the experiments were carried out in acetonitrile solution using glassy carbon working electrode and all the potentials were referenced to Agi AgCl electrode. Melting points were recorded with a Boeties micro heating table and were uncorrected. The starting complexes [RuCh(PPh3)3] 19, [RuCb(AsPh3h]20, [RuBr3(AsPh3h]21 and [RuBr3(PPh3h(CH30H)]22 were prepared by the reported methods .

Preparation of Schiff base ligands

The monobasic bidentate Schiff bases were prepared by mixIng equimolar amounts of salicylaldehyde (0.0025 mol) with an appropriate amine such as methylamine, cyclohexylamine and 2-aminopyridine (0.0025 mol), both dissolved in methanol (50 cm3)23. The reaction mixture was refl uxed for an hour and then cooled. The soli( product so separated was filtered, washed and dried. Yield: 80-90 %.

Preparation of ruthenium(III) Schiff base complexes [RuXiEPh))z(L)] (where E = P or As, X = CI or Br; L = bidentate Schiff base anion)

A solution of [RuX3(EPh3h ] (where E = P, X = CI; E = As, X = CI or Br) and [RuBr3(PPh3h(CH30H)] (0 .124-0. 157 g; 0.125 mmol) and the respective Schiff base (0.0 16-0.025 g; 0.125 mmol ) in benzene (20 cm") was heated under reflux for 5 h. The resulting solu tion was concentrated to ca. 3 cm3 and a small quantity of petroleum ether (60-80°C) was added. The

precipitated complexes were removed by filtration, washed with petroleum ether (60-80°C) and recrystallized from CH2Cl2/petroieum ether (60-80°C) and dried in vacuum. Yield: 58-82%.

Catalytic oxidation

Catalytic oxidation of primary alcohols to corresponding aldehydes by ruthenium(IlI) complexes were studied in the presence of NMO as co-oxidant. A typical reaction using the complex as a catalyst and benzyl alcohol , cinnamyl alcohol and Il-butanol as substrates at 1: 100 molar ratio is described as follows. A solution of ruthenium complex (0 .0 1 mmol) in 20 cm3 CH2CI2 was added to the solution of substrate ( I mmol) and NMO (3 mmol). The solution mixture was refluxed for 3 h and the solvent was then evaporated from the mother liquor under reduced pressure. The solid residue was then extracted with pet. ether (60-

80°C) (20 cm\ the e ther extracts evaporated to g ive corresponding aldehyde which was then quantified as 2,4-di ni tropheny Ihydrazone deri vati ve24.

Results and Discussion The new hexa-coordinated ru thenium(Irl )

complexes having the general molecu lar formula [RuX2(EPh3h(L)] have been obtained from the reaction of [RuX3(EPh3h] or [RuBr3(PPh:,h(CH30H)] with various Schiff bases in \: 1 molar ratio in benzene as shown in Scheme J. The analytical data (Table I) are in good agreement with mol ecul ar formula arrived for all the complexes . All the complexes are green in color and are quite stable in air.

VENKATACHALAM et al.: CATALYTIC ACTIVITY OF RUTHENIUM(III) SCHIFF BASE COMPLEXES 707

[RuX3(EPh3)3] or

[RuBr3(PPh3)2(CH30H)] + Schiff base

where E = P , X = CI; E = As, X = CI or Br

Benzene Reflux ,.

5h ~. 0,,----r~x

Ru

-N/ I "x I EPh3

H R

where E = P or As, X = CI or Br;

Scheme I

IR spectra exhibit a strong band at ca. 1615-1625 cm·1 characteristic of azomethine group (>C=N) in the free Schiff bases. In the spectra of all the complexes, a shift to lower frequency of this band (ca. 1590-1601 cm·l) indicates that the azomethine nitrogen of the ligand is one of the coordinating atoms 14, 15,25. A band of medium intensity at ca. 3300-3400 cm·1 in the free ligand due to Y(OH) di sappeared on complexation with metal ion indicating the deprotonation of phenolic proton prior to coordination. This is further supported by the increase in the absorption frequency of phenolic C-O band from 1260-1280 cm·1 in the free li gands to 1304-1316 cm·1 in the ruthenium complexes I4. 16.25.26. This indicates that other coordination site in the Schiff base is phenolic oxygen. The Y(Ru.CI) and Y(R u.Br) bands were observed around 330 cm·1 region and the other characteristic bands near 520, 695 and 740 cm·1 are due to PPh3/ AsPh3 fragments 111 the spectra of the complexes27 .

The electronic spectra of all the complexes in CHCI, showed two well-resolved transitions in the region 705-330 nm. The ground state of ruthenium(llI) (tz/ configurati on) is zTzg and the first excited doublet levels in the order of increasing energy are zT1g and 2 Azg ,both arisi ng from the tz/ e/ configuration. In most of the ruthenium(III) complexes, the electronic spectra show only charge transfer bands28.29. Since in a d5 system and especially in ruthen ium(lIl ) which has relatively high ox idi zing properties, the charge transfer bands of the type L7[y~ fZg are prominent in the low-energy region whi ch obscure the weaker d-d transitions. However, the extinction coefficient fo r the bands in the 705-608 nm regions are found to be very low as compared to that of charge transfer bands. Hence, the band around 705-608 nm have been ass igned to ZTZ8 ~2A2g transition which is in conformity with assignments made for similar octahedral ruthenium(lll) complexes26.29.3o. The olher band in the region 448-330 nm has been ass igned to charge transfer trans ition. The pattern of

the electronic spectra of all complexes indicates an octahedral environment around the ruthenium (III) ion26,29.3o.

The magnetic moments of the complexes, VI Z.,

[RuCh(PPh3h(Ll )], [RuCh(AsPh3h(L2)] and [RuBrlCAsPh3h(L3)] have been measured at room temperature and the values obtained are 1.86, 1.90 and l.92 B.M ., respectively. These values correspond to one unpaired electron suggesting a low spin t2/ configuration for the ruthenium(JII) ion in an octahedral environment.

The solid state EPR spectra of powdered samples were recorded at room temperature. All the complexes exhibit three lines with three different "g" values indicating magnetic ani sotropy in these systems. The average "g" values are in the range 2.02-2.23 and these values fit very well with the values obtained for similar ruthenium(llI ) complexes27.JJ.J2 . The presence of three different "g" values is an indication of rhombic distortion in these complexes.

Electrochemistry As the complexes are soluble in acetonitrile, the

redox behavior of some of the complexes have been examined at a glassy carbon electrode using cycl ic vol tammetry and the data are presented in Table 2. All the ligands are inactive at the poten ti al range concerned; the redox waves are ass igned to metal centered only . Cyclic voltammogram of all the complexes (1 x 10.1 M) exhibit a reversible ox idation and two reversi ble reduction peaks at the scan rate of 100 mY S·I. A representati ve cyclic vo ltammogram of [RuCI2(As Ph3h(L I] is shown in Fig. I. The oxidation and the reduction of each complex are characteri zed by well-defined waves wi th Er values in the range 0.41 to 0.65 V and - 0.17 to - 0.79 V versus Ag/ AgCl electrode. The redox processes with peak-to-peak separati on (t:..Ep) values for ox idation (RUIII/Ru IV) and reduction couples (Ruili/Ru ll , Rull/Ru l) are fo und to be 60-80 mY suggestive of a si ngle step one electron

f 15 27 '10 H .. I h h trans er process . '--. . ence, It IS C ear t at t e

708 INDIAN J CIiEM, SEC A, APRIL 2005

Table 2 - Electrochemical data of Ru(lII) Schiff base complexes

Complex RUlii/Ruiv RUII/Ru" Ru"/Rul

Ep, (V) Epc(V) EII2(V) ~Ep(mV) Ep, (V) Epc(V) EII2 (V) ~Ep(mV) Ep, (V) Epc(V) EII2 (V) ~Ep(mV)

I 0.54 0.47 0.50 70 -0.24 -0.3 1 -0.28 70 -0.47 -0.53 -0.50 60

2 0.53 0.61 0.57 80 -0.23 -0.29 -0.26 60 -0.75 -0.83 -0.79 80

5 0.61 0.69 0.65 80 -0.26 -0.33 -0.30 70 -0.49 -0.57 -0.53 80

6 0.53 0.61 0.57 80 -0.23 -0.29 -0.26 60 -0.75 -0.83 -0.79 80

II 0.49 0.43 0.46 60 -0.14 -0.20 -0.17 60 - 0.53 -0.60 -0.57 70

12 0.45 0.38 0.41 70 -0. 19 -0.26 -0.23 70 -0.61 -0.69 -0.65 80

Supporting electrolyte [NBu41CI04 (0.05 M); concentration of the complex 0.001 M; all the potentials referenced to Ag/AgCI £1" = 0.5 (£p.+£pc), where £p" and £pc are anodic and cathodic peak potentials respectively; scan rate 100 mVs·1

Table 3 - Catalytic oxidation data of alcohols by ruthenium(III) complexes in the presence of NMO

Complex Substrate Product Yield· (%)

Benzyl alcohol A 85.3 n-Butanol A 53.9 Cinnamyl alcohol A 87.3

6 Benzyl alcohol A 80.5 II -B utanol A 49.8 Cinnamyl alcohol A 80.6

II Benzyl alcohol A 81.6 II -B utanol A 38. 1 Cinnamyl alcohol A 91.5

12 Benzyl alcohol A 79.8 II-Butanol A 49.2 Cinnamyl alcohol A 85.8

A = Corresponding aldehydes "Yield based on substrate; alcohol (I mlllol); NMO (3 Illlllol); catalyst (0.0 I Illlllol)

7·000

<{

::t..+000

-9 '000

1200 400 -400 E(V)

-1200

Fig. I - Cyclic voitalllmogram of [RuCI1(AsPh,h(L1]

present ligand system is ideally suitable for stabilizing the higher oxidation state of ruthenium ion.

On the basis of elemental analyses, JR , electronic and EPR spectral data, an octahedral structure

(Scheme J) has been tentatively proposed for all the ruthenium(III) complexes.

Catalytic activity Some of the present ruthenium(III) complexes have

been used for catalytic oxidation of alcohols in the presence of NMO as co-oxidant and results are summarized in Table 3. The oxidations were performed in dichloromethane, from which water was removed using powdered molecular sieves. All complexes oxidize primary alcohols to thei r corresponding aldehydes with high yield. The aldehydes formed after 3 h by reflux were quantifi ed as their 2,4-dinitrophenylhydrazone derivatives and there was no detectable oxidation in the absence of ruthenium complexes. The relatively higher yield for oxidation of cinnamyl alcohol than benzyl alcohol or n-butanol is due to the presence of a-C H unit in cinnamyl alcohol, which is more acidic than benzyl alcohol or n-butanol. Unsaturated alcohol like cinnamyl alcohol is selectively oxidized at the alcohol group with high yield without competing double bond. This is an important characteristic of ruthenium/NMO system. The catalytic efficiency of these ruthenium(III) Schiff base complexes in the case of oxidation of benzyl alcohol, cinnamyl alcohol and n-butanol is more when compared to the earli er repore.33,34 on si milar ruthenium complexes. Further, the catalytic yield by these complexes in the presence of NMO (up to 91.5%) is found to be more than the other catalytic systems containing t-BuOOH or H20~

as co-oxidants35.36. A high valent oxo r etal complex is usually suggested to be the reactive in termediate in the oxidation process. Hence, it is relevant from the cyclic voltammetric data that the oxidation effected by catalysts is likely to occur via ruthenium hi gher oxidation states, which is easily accessible through chemical ox idation with co-oxidants like NMO, t-BuOOH, PhIO, etc3

.12

.

VENKATACHALAM et al.: CATALYTIC ACTIVITY OF RUTHENIUM(I1I) SCHIFF BASE COMPLEXES 709

Acknowledgement Financial assistance received from the Council of

Scientific and Industrial Research (CSIR), New Delhi [Grant No. 01 (l72S)/02/EMR-II] is gratefully acknowledged. One of the authors (GV) thanks CSIR for the award of Senior Research Fellowship (SRF).

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