Polyethylene carboxylate-bound triruthenium clusters as alcohol oxidation catalysts

Reactive Polymers, 12 (1990) 291-295 Elsevier Science Publishers B.V., Amsterdam

291

POLYETHYLENE CARBOXYLATE-BOUND TRIRUTHENIUM CLUSTERS AS ALCOHOL OXIDATION CATALYSTS D A V I D E. BERGBREITER * and D A V I D R. T R E A D W E L L

Department of Chemistry, Texas A&M University, College Station, TX 77843 (U.S.A.) (Received October 14, 1989; accepted in revised form January 22, 1990

We have found that addition of polyethylene oligomers containing a terminal carboxylic acid group to solutions of [(CH3CH2CO2)oRu30(H20)3 ] +CH3CH2CO 2 allows us to recover quantitatively and reuse this ruthenium cluster as a catalyst for oxidation of primary and secondary alcohols with oxygen.

INTRODUCTION

Oxidation catalysts are of interest especially when the overall oxidizing agent is oxygen [1]. While m a n y variations of insoluble polymers have been described which are useful as recyclable stoichiometric oxidants, there are fewer examples of such systems being used for oxidation catalysts [2,3]. Here we describe some of our initial work on soluble polymer-bound metal cluster catalysts in which carboxylate groups on the terminus of a relatively oxidation-insensitive polyethylene oligomer have been used to prepare recyclable alcohol oxidation catalysts. These catalysts, which are based on known low molecular weight triruthenium carboxylate clusters [4], oxidize primary and secondary alcohols with oxygen as the oxidant [5].

* To whom correspondence should be addressed. 0923-1137/90/$03.50

Our prior work on polyethylene-bound catalysts has emphasized the utility of the polyethylene-like solubility of a terminally functionalized ethylene oligomer to which a catalyst has been attached either covalently via c a r b o n - c a r b o n or c a r b o n - h e t e r o a t o m obonds (e.g. a crown ether or other phase transfer catalyst) or via coordination to a polyethylene-bound phosphine or phosphite ligand [6]. In this work, the known insolubility of polyethylene at 25 ° C was coupled with the solubility of this linear polymer at 100 ° C to afford an oxidation catalyst whose solubility was quantitatively switched on and off by heating and cooling respectively. By using linear ethylene oligomers with M n of ca. 2000, this solubility property of polyethylene was coupled to an end group. The modest size of these oligomers further aided the catalysis by facilitating formation of solutions with sufficient molar concentrations of catalyst. Here we describe extensions of this chemistry which

© 1990 Elsevier Science Publishers B.V.

292

also take advantage of the comparative oxidation-insensitivity of an alkane-like ethylene oligomer.

RESULTS AND D I S C U S S I O N We have successfully used polyethylenebound ruthenium clusters as homogeneous catalysts for oxidation of primary and secondary alcohols (eqn. 1). In the case of the HO .~ R

0 .~

(PE-CO2)6Ru30(H20)~PE-CO2 , R'

02, 1oo°c

R

R'

(11 R = alkyl or aryl;

R' = alkyl, aryl or H

primary alcohols, the oxidation stopped at the aldehyde stage. Representative examples of these oxidations are listed in Table 1. These oxidation catalysts are somewhat less active than their precursors [5] apparently because

TABLE 1 Alcohol oxidation using 02 and polyethylene-bound Ru 3 clusters a Alcohol

Substrate/ catalyst

Turnovers ( d - 1)

Time (h)

1-Dodecanol 1-Dodecanol 1-Dodecanol 1-Dodecanol 1-Dodecanol Cyclohexanol Geraniol

265 624 886 886 886 116 136

25 26 b 16 ~ 13 d 20 ~ 8 30 f

120 62 69 40 88 42 17

a The oxidation reaction was carried out at 100 ° C in toluene or (preferably) chlorobenzene using magnetic stirring for agitation and 1 atm of 02. b A Vibromixer was used in place of the magnetic stirrer, c This reaction was carried out in an autoclave under 40 psi of 02. d This reaction was carried out in an autoclave using recycled catalyst• e This reaction was carried out in an autoclave using twice recycled catalyst, f This reaction used octane as the solvent and also produced 6% of product in which the double bond has isomerized.

of catalyst decomposition. Nonetheless, the catalysts can still be recycled several times. Some initial oxidations were carried out in toluene solution. Interestingly, in these cases, there was a significant amount of oxidation of the toluene solvent presumably to form benzoic acid. Switching to chlorobenzene avoided this problem without affecting catalyst activity. An unresolved problem with these polyethylene-bound ruthenium clusters is their thermal decomposition. This thermal instability, which was similar to that previously noted by Drago for a lower molecular weight cluster [5], was confirmed by UV-visible studies of solutions of these polyethylenebound catalysts. The active oxidation catalyst containing a propionate ligand had a )~maxof 686 nm while the active catalyst containing a polyethylene carboxylate ligand had its )kma x at a slightly lower wavelength (?,max=658 nm), possibly reflecting a different polarity for the cluster containing the large hydrocarbon ligands. The thermally deactivated polyethylene cluster had a )kma x of 640 rim. This thermal lability, which was also noted by Drago [5] is the principal limit to catalyst recyclability in the case of this cluster catalyst. The polyethylene-bound cluster 1 used in eqn. (1) was prepared from the heptacarboxylate triruthenium cluster, [(CH3CH 2 CO2 ~6Ru 30(H20) 3] + CH3CH2CO2 prepared earlier by Spencer and Wilkinson [7] and used previously as an alcohol oxidation catalyst by Bilgrien et al. [5]. Exchange of this propionate-ligated ruthenium cluster was accomplished using the carboxylated ethylene oligomer [8] and the exchange reactions shown in eqn. (2). In this reaction, the dark green solution of the propionate cluster was added PE-CO 2 + HI (CH3CH 2CO2)6Ru 30(HzO)3] + •CH3CHECO 2 [ (PE-CO2H)a(CH3CH2CO2)5Ru 30(H20)3] + (2a) • CH3CH2CO2 + CH3CH2CO2H

(2b)

293

(2c) (2d) (2e) [ (PE-CO 2H)5(CH 3CH2COz ),Ru 30(H20)3] + •CH3CH2CO 2 + PE-CO2H [ (PE-CO 2H)6Ru 30(H20)3] + CHaCH2CO 2 + CH 3CH 2CO2 H

(2f)

to a suspension of an equivalent amount of carboxyl-terminated ethylene oligomer. Heating this solution of 1 and the suspension of the carboxylated oligomer to 100°C in toluene produced a dark green solution. To minimize decomposition of the cluster, this solution was re-cooled to 25°C within 10 minutes from when a solution had formed. As expected, the polyethylene carboxylate entrapped most of the ruthenium cluster. Our prior studies of the solubility of terminally-functionalized polyethylene oligomers have shown that such oligomers precipitate quantitatively from solution. We therefore expected that any cluster containing at least one oligomeric carboxylate ligand would likewise precipitate. We also expected that clusters containing no polyethylene carboxylate ligands would not be entrapped in virgin polyethylene and we demonstrated that this was indeed the case in control experiments in which the propionate cluster was not entrapped in virgin polyethylene to any detectable extent. If we assume that a carboxyl group on the end of an ethylene oligomer (typical M n of 2000) is chemically equivalent to the carboxyl group of propionic acid, each step in the series of exchange equilibria shown in eqn. (2) should be adiabatic. Entropy considerations should therefore produce a series of equilibria constants with the Keq being 6/1, 5/2, 4/3, 3/4, 2/5 and 1/6 *. Solving these six equilibria and three additional equations * We are grateful to a referee who pointed out the importance of entropy considerations in these equilibria.

expressing the concentrations of the various possible complexes in terms of the initial concentrations of propionate, polyethylene carboxylate and ruthenium allowed us to calculate that the addition of 7 tool of P E CO2H/mol of cluster would produce an equilibrium mixture in which only 1/64 of the original cluster would remain. Assuming that last equilibrium involving simple exchange of the counter ion carboxylate (eqn. 3) was both adiabatic and isoentropic reduced this amount by an additional factor of 2. This allows us to predict that only 1/128th of the original propionate cluster would remain in solution if all clusters containing at least one polyethylene carboxylate were to precipitate• Qualitatively, this was the case since the filtrate was colorless after a deeply colored solution containing [ ( P E - CO 2H) 6Ru 30 ( H 20)3 ] + CH 3CH 2C02 + PE-CO2H

[ (PE-CO2H)6Ru 30(H20)3 ] + PE_CO2 + CH3CHRCO2H

(3)

a 1/1 (mol/mol) ratio of polyethylene carboxylic acid and the propionate cluster was cooled to precipitate the polyethylene oligomer. To further establish that ruthenium recovery was high, we redissolved the precipitate from this experiment and precipitated in again. Inductively coupled plasma spectroscopic analysis of the filtrate from this second dissolution precipitation showed that < 2% of the initial ruthenium remained in solution. This high recovery of ruthenium cluster is precedented by our other work with polyethylene-bound phosphine ligands. Specifically, we had noted previously that stoichiometric amounts of polyethylene diphenylphosphine and tetrakis(triphenylphosphine)palladium(0) when codissolved in toluene and cooled also produced a colorless suspension of a polyethylene-entrapped catalyst [6,9]. In this prior case, the chemical nonequivalence of this polyethylenediaryl phosphine versus a triarylphosphine ligand complicated the ques-

294 tion and was a plausible explanation for our results. Nonetheless, the overall outcome was otherwise much the same as what was noted with these anionic ligands.

EXPERIMENTAL SECTION

General procedures Standard procedures were used for handling all air-sensitive materials [10]. UV-visible spectra were recorded on a PE 552 spectrophotometer. IR spectra were recorded using an IBM Model 40S FTIR spectrometer. N M R spectra were determined using a Varian XL-200 FT N M R spectrometer. The progress of oxidation reactions was monitored using GC on a 25-m megabore capillary column. Solvents and reagents were obtained from Aldrich. All solvents were purified by distillation prior to use.

Bu(CH26H2)n602 H This was prepared using a literature procedure [8]. While the molecular weight for different preparations varied, N M R analysis of the methyl ester (prepared using methanol and acid) showed that typical M n values were in the range 1800-2000.

Ru30(R 602) 6(H20)7 RCOf This was prepared using a modification of the original synthesis [11]. Propionic acid (5 mL) was added to ethanol and the sodium salt was prepared by addition of a stoichiometric amount of sodium metal. Then 0.2 g of hydrated ruthenium chloride (0.77 mmol) was added. The dark red-black solution was heated to 100°C and it turned green. After further heating for 1 h, the solution was cooled to room temperature and product purified by chromatography using a 1.5-cm × 20-cm Sephadex LH column. Two bands eluted and the

first green band was collected. The product had a ~max of 686 nm in toluene. The polyethylene-substituted complex was prepared by substitution of polyethylene carboxylic acid for propionic acid at 100 °C in toluene. The resultant solution was cooled to 25 °C to precipitate the polymer-bound product which was washed with 2-propanol. The resulting green powder was used immediately in oxidation reactions and was characterized by UV-visible spectroscopy and ICP analysis: ~kmax of 658 nm in toluene, 1.09% ruthenium after digestion of the polymer with acid [9].

Typical oxidation procedure In a typical reaction, 0.282 g of the polyethylene-bound oxidant prepared above was suspended in 50 mL of chlorobenzene. Cyclohexanol (0.305 g, 3 mmol) and hexadecane (an internal standard) were added. The mixture was then heated to 100 °C where it became homogeneous. Oxygen was introduced with a gas inlet tube. The oxidations were carried out for varying amounts of time as noted in Table 1.

CONCLUSION In summary, carboxylate groups at the terminus of an ethylene oligomer can be useful as ligands for a transition metal cluster which acts as an oxidation catalyst. The carboxylated ethylene oligomer both provides for recovery of the catalyst and is more successful than expected in complexing these ruthenium clusters and im removing them from solution in the presence of a stoichiometric amount of a propionate ligand.

ACKNOWLEDGMENTS Support of this research by the Texas Advanced Technology Research Program and

295 the R o b e r t A. W e l c h F o u n d a t i o n is g r a t e f u l l y acknowledged.

REFERENCES 1 J.J. Spivey, Complete catalytic oxidation of volatile organics, Ind. Eng. Chem. Res., 26 (1987) 21652180; C.L. Bailey and R.S. Drago, Utilization of 02 for the specific oxidation of organic substrates with cobalt(II) catalysts, Coord. Chem. Rev., 79 (1987) 321-332. 2 R.T. Taylor, Polymer-bound oxidizing agents in: W.T. Ford (Ed.), Polymeric Reagents and Catalysts, ACS Symposium Series, 308 (1986) 132-154. 3 (i) C.U. Pittman, Jr., Polymer-supported catalysts, in: G. Wilkinson (Ed.), Comprehensive Organometallic Chemistry, Vol. 8, Pergamon, Oxford, 1982, p. 553-661. (ii) F.R. Hartley, Supported Metal Complexes. A New Generation of Catalysts, D. Reidel, Dordrecht, 1985. 4 S.A. Fouda, B.C.Y. Hui and G.L. Rempel, Hydrogen reduction of ~-3-oxo-triruthenium(III) acetate in dimethylformamide, Inorg. Chem., 17 (1978) 32133220.

5 C. Bilgrien, S. Davis and R.S. Drago, The selective oxidation of primary alcohols to aldehydes by oxygen employing a trinuclear ruthenium carboxylate catalyst, J. Am. Chem. Soc., 109 (1987) 3786-3787. 6 D.E. Bergbreiter in: D.E. Bergbreiter and C.R. Martin (Eds.), Functional Polymers, Plenum Press, New York, NY, 1989. 7 A. Spencer and G. Wilkinson, Reactions of /~-3oxotriruthenium carboxylates with ~r-acid ligands, J. Chem. Soc., Dalton, (1974) 786-792. 8 D.E. Bergbreiter, J.R. Blanton, R. Chandran, M.D. Hein, K.-J. Huang, D.R. Treadwell and S.A. Walker, Anionic Synthesis of Terminally Functionalized Ethylene Oligomers, J. Polym. Sic., Polym. Chem., 27, (1989) 425-4226. 9 D.E. Bergbreiter and D.A. Weatherford, Polyethylene-bound soluble recoverable palladium(0) catalysts, J. Org. Chem., 54 (1989) 2726-2730. 10 D. Shriver and M.A. Drezdzon, The Manipulation of Air-Sensitive Compounds, 2nd edn., Wiley, New York, NY, 1986. 11 A. Spencer and G. Wilkinson, ~t3-Oxo-triruthenium carboxylate complexes, J. Chem. Soc., Dalton, (1972) 1570-1577.