Flow Reactors for Preparative Chemistry in Supercritical Fluid Solution

Organometallics 1995, 14, 3876-3885

3876

Flow Reactors for Preparative Chemistry in Supercritical Fluid Solution: “Solvent-Free”Synthesis and Isolation of Cr(CO)&!2&) and ( q‘-c5H5)MdC0)2(q 2-H2) James A. Banister, Peter D. Lee, and Martyn Poliakoffk Department of Chemistry, University of Nottingham, Nottingham, England NG7 2RD Received May 24, 1995@ We describe the use of supercritical flow reactors as a new but relatively straightforward approach to carrying out reaction chemistry in high-pressure fluids (e350 bar). Two photochemical flow reactors are described in detail, one reactor for the synthesis of Cr(C015(C2H4) from the reaction of Cr(CO)s with supercritical CZH4 (SCCZH~), the other reactor for the generation of CpMn(C0)2(r2-Hdfrom CpMn(C0)s and HZin supercritical COZ(scCOZ). Both compounds are isolated by rapid expansion of the supercritical solution. This is the first time t h a t either compound has been isolated as a solid, and both are found t o be not nearly a s labile as had been anticipated. Indeed, CpMn(C0)2(q2-Hz)is one of the simplest dihydrogen compounds so far to have been isolated, yet it is one of the more robust compounds, taking 2 h t o react with moderately high pressures of CO or C2H4. Other reactions involving scC2Hs a s the fluid or N2 a s the reactant (e.g., to form CpMn(C0)2N2) are described briefly. So far, the reactions have been carried out on a modest scale, ca. 20-40 mg per h, but this is more a limitation of the photochemistry rather than of the reactors themselves. All reactions are carried out without the use of any conventional organic solvents.

Introduction There has been an upsurge of interest in the use of supercritical fluids as media for reaction chemistry.’I2 This interest has been stimulated in part by the possibility of using these fluids, particularly supercritical C02 (scCOa),as environmentally acceptable substitutes for organic solvents. Equally important has been the promise of chemical resuls which would be difficult or even impossible to achieve by more conventional routes. Successful examples have included controlling the product distribution in the dimerization of isophorthe synthesis4 of the triddinitrogen) complex, CpRe(Nd3, and the reaction5a of hex-3-yne with SCCOZ. Nevertheless, whatever the attractions of supercritical fluids, the prospect of handling these high-pressure fluids has been quite discouraging for nonspecialist chemists. This paper describes how such reactions can be carried out relatively simply in miniature flow reactors. E-mail: [email protected].

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e-Abstract published in Advance ACS Abstracts, July 1, 1995.

(1)For an excellent general introduction to supercritical fluids, see: McHugh, M. A,; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice, 2nd ed.; Butterworth-Heinemann: Boston, 1994. (2)For a recent review of reactions in supercritical fluids, see: Savage, P. E.; Gopaian, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. A . Inst. Chem. Eng. J . , in press. ( 3 )Hrnjez, B. J.; Mehta, A. J.; Fox, M. A.; Johnston, K. P. J . Am. Chem. Soc. 1989,111,2662. (4)( a ) Howdle, S. M.; Grebenik, P.; Perutz, R. N.; Poliakoff, M. J . Chem. Soc., Chem. Commun. 1989,1517. (b) Howdle, S. M.; Healy, M. A,; Poliakoff, M. J . Am. Chem. SOC.1990,112,4804. ( 5 )( a ) Reetz, M. T.; Konen, W.; Strack, T. Chimia 1993,47, 493. (b) In a recent lecture, it was suggested that this reaction might have occurred a s a multiphase reaction (Le., between COn and catalyst, both dissolved in liquid hexyne) and not in SCCOZ; Dinjus, E. COSTDechema Workshop, Lahnstein, Germany, April 1995.

0276-7333/95/2314-3876$09.00/0

Although synthetic chemists rarely use the term “batch processing,’’ most laboratory-scale synthetic chemistry involves batch reactions in flasks, Schlenk tubes, or similar containers. Given a suitable autoclave, a supercritical reaction can also be carried out as a batch process, but there are a number of problems not normally encountered in conventional solvents. For example, dissolving any solute in a fluid alters the critical temperature of that fluid, and, in general, each solute has a different effect.’ The critical temperature is important because properties such as solvent power, dielectric constant, etc. are most easily tuned close to the critical point. Thus, in many experiments, it is crucial to maintain the reaction temperature close to critical. However, this can be difficult with a closed batch reactor because the critical temperature of the reaction mixture may change significantly as reactants are converted into products. More prosaic, but equally important, is the fact that small autoclaves can hold only small amounts of material, while large autoclaves filled with high-pressure fluids have considerable safety constraints associated with them. Many of these difficulties are avoided by use of a flow reactor, in which the reaction is carried out as a continuous process and a significant throughput is achieved even though the total volume of the highpressure system is small6 Scale-up of the reactor is less important than in a batch process because, within limits, a larger quantity of product can be obtained merely by running the flow reactor for a longer time. Furthermore, reaction conditions are much easier t o control than in a batch reactor because the composition of the fluid a t any particular point is constant as a 16) Tundo, P Continuous Flow Methods in Organic Synthesis, Ellis Horwood Chichester, U K , 1991

0 1995 A m c r ~ p ~Cbem7cdl n Sii~

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Flow Reactors for Supercritical Chemistry function of time. Most crucially, spectroscopic monitoring of the fluid can be used for optimization of the reaction conditions in real time.7 In this paper, we give two examples of how such a reactor can be realized in practice. We describe the isolation of two organometallic compounds, Cr(C0)5(C2H4)and CpMn(C0)2(q2-H2), both of which were previously believed to be too labile for isolation. Cr(C0)5(C2H4)is structurally the simplest d6 metalalkenekarbonyl compound but has long been considered to be highly labile. Although W(C0)5(C2H4)has been known for many years,8 Cr(C0)5(C2H4)was first identified (by IR spectroscopy) only quite recently, during the photolysis of Cr(CO)6 in cryogenic liquid Xe s o l ~ t i o n . ~ Formation of cis-, and trans-Cr(C0)4(CzH4)2was also observed during this reaction, and shortly afterward Grevels and co-workers succeeded in isolating the trans isomer following the low-temperature photolysis of Cr(COk and C2H4 in hydrocarbon solution.1° Although Grevels et al. had also managed to isolate the closely related mono-olefin complex, Cr(C0)5(cis-~yclohexene),~~ Cr(C0)5(C2H4) defied attempts to be isolated from alkane solution. Photochemical experiments in the gas phase confirmed that Cr(C0)5(C2H4)was also thermally labile under these conditions.12 Indeed, Weitz and coworkers have exploited this lability13 to obtain an estimate of the (C0)5Cr-(C2H4) bond dissociation energy, 103 & 10 k J mol-l. We recently published a preliminary report14 of how Cr(C0)5(C2H4)can be isolated from supercritical C2H4 ( s c C ~ Hat ~ )room temperature, and here we describe the procedure in full detail. CpMn(C0)2H2 was first detected by Leong and Cooper15 at the end of a multistep route involving the dianion [CpMn(C0)2I2-. However, like Cr(C0)5(C2H4), CpMn(C0)2H2 could not be isolated from the solvent (THF) in which it was prepared. Subsequently, the compound was generated in supercritical Xe (scXe) s o l ~ t i o n by ~ ~photolysis J~ of CpMn(C0)a in the presence of H2. The wavenumbers of its Y(C-0) bands and the ease with which the coordinated Ha was displaced by N2 led to the reformulation of the compound as the “nonclassical” dihydrogen compound17 CpMn(C0)2(q2-H2). Isolation of pure CpMn(C0)2(q2-H2)is an important goal because structurally it is considerably simpler than most of the known dihydrogen compounds. Thus, both Cr(C0)5(C2H4)and CpMn(C0)2(q2-H2)are examples of a wider class of transition metal complexes with ligands (7) For reviews of vibrational spectroscopy in supercritical solution, see: ( a ) Buback, M. Angew. Chem., Int. Ed. Engl. 1991,30,641. (b) Poliakoff, M.; Kazarian, S. G.; Howdle, S. M. Angew. Chem., Int. Ed. Engl., in press. (8) Stolz, I. W.; Dobson, G. R.; Sheline, R. K. Inorg. Chem. 1963,2, 1264. (9) Gregory, M. F.; Jackson, S. A,; Poliakoff, M.; Turner, J. J., J . Chem. Soc., Chem. Commun. 1986,1175. (10) Grevels, F.-W.; Jacke, J.; Ozkar, S. J . Am. Chem. Soc. 1987, 109,7536. (11)Grevels, F.-W.; Skibbe, V. J . Chem. Soc., Chem. Commun. 1984, 681. (12) Weiller, B. H.; Grant, E. R. J . A m . Chem. SOC.1987,109,1252. (13)Wells, J. R.; House, P. G.; Weitz, E. J . Phys. Chem. 1994,98, 8343. (14) Banister, J. A,; Howdle, S. M.; Poliakoff, M. J . Chem. SOC., Chem. Commun. 1993,1814. (15)Leong, V. S.; Cooper, N. J. Organometallics 1988,7, 2080. (16)Howdle, S. M.; Poliakoff, M. J . Chem. Soc., Chem. Commun. 1989,1099, (17) For excellent reviews on dihydrogen complexes see: Jessop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992, 121, 155. On hydrides: Transition Metal Hydrides; Ed. Dedieu, A,, Ed.; VCH Publishers, Inc.: New York, 1992.

Organometallics, Vol. 14, No. 8, 1995 3877 (i.e., C2H4 and H2) weakly bound to the metal. Such compounds are often quite simple to generate photochemically in solution, but they are extremely dif‘ficult to isolate as solids. The problems are both practical and psychological. Removal of the solvent usually leads to removal of the labile ligand as well, and the belief that such compounds are unstable often discourages further attempts at isolation. In the following sections, we explain how these compounds can be generated in supercritical solution and how both of them can be isolated as solid compounds for the first time. First, we outline the general principles of our approach and describe the key components that are required. Then, we show how the method works in practice and describe very briefly some of the reactions, which can be obtained after CpMn(C0)2(q2H2) has been isolated. Finally, the Experimental Section lists the precise experimental parameters, pressure, flow rates, etc., needed for the preparations. It is important, even at this stage, to stress that the method is potentially far more general than the specific photochemical reactions described in this paper. A broadly similar approach could be applied to a wide range of thermal and photochemical reactions in both inorganic and organic chemistry.

Results Principles of the Flow Reactors. Preliminary IR experiments in a miniature static cell with 300 nm, to remove the short-wavelength output of the lamp, which otherwise caused unacceptable decomposition of the product. IR Spectroscopy. All IR spectra were recorded on a Nicolet Model 205 FTIR interferometer, which permits almost unlimited repetitive scanning. Spectra were obtained with 2 or 4 cm-' resolution. When needed, spectra were processed further on a Nicolet Model 620 Data Station. Materials. All materials were used without further purification; CpMn(C0h and Cr(CO)6 (99%) from Aldrich; C02

Banister et al. (SFC grade) from BOC; H2, N2, C2H4 (99.8%), and C2H6 (3.5 Grade) from Air Products. Preparation of Cr(CO)s(C&). The apparatus is set up as in Figure 5 but without the collection vessel between the back pressure regulator, BPR, and the bubble flow meter, M. The extraction vessel, R, is then filled with Cr(CO)6 ('100 mg), and the valves on either side are closed. C2H4 is flowed at low pressure (150 psi) through the bypass valve, B, to flush residual air from the system. The other valves are then opened so that the reservoir, R, is also flushed with C2H4 (at these pressures Cr(C0)e is virtually insoluble in C2Hd One valve is now closed and the BPR is set to an increased pressure, 1220 psi (i.e., supercritical). The control valve is adjusted t o give a flow rate of ca. 200 mL of gas per min, measured at atmospheric pressure with the flow meter, M. For C2H4, this flow rate corresponds t o a usage of ca. 0.5 mol h-l or 14 g h-l. A background FTIR spectrum is recorded (i.e., scC2H4 without Cr(CO)6). B is closed, and the valves are opened so that S C C ~flows H ~ through R; Cr(CO)6now dissolves into the fluid. The concentration of Cr(CO)6 is checked via FTIR, see Figure 6a, and the pressure is adjusted, if necessary, to obtain the required concentration of Cr(COh. The valves on R are closed, and B is reopened to flush Cr(CO)6out of the system. The gas flow is then stopped briefly, and the collection vessel (see Figure 4) is connected between the BPR and M. The gas flow is restarted, and any air is flushed from the collection vessel. B is closed, and the values are opened so that scC2H4 again flows through R. The UV lamp is then switched on, the extent of the UV photolysis is monitored via FTIR and, again, the conditions are adjusted if required to improve the overall conversion. Photolysis continues, and crude Cr(C0)5(C2H4)is collected, typically, a t 40 mg h-l. The reactor is not truly continuous in operation because the stock of Cr(CO)6 will eventually run out. The longest continuous operation has been ca. 2 h, but there is no reason why it should not be run longer because very little deposit builds up on the UV window. The collection vessel is then detached from the BPR, and product is removed in a glove box. (If Cr(CO)dC2H4) is collected in the glass vessel, see Figure 3, the crude product can be worked up on a conventional Schlenk line.) Preparationof CpMn(C0)z(q2-H2). The reactor is set up as in Figure 7. The sapphire window, W, is removed from the variable volume cell, R, and ca. 130 mg of solid CpMn(CO)3is loaded into the cell which is then sealed (by replacing the window) and purged with H2. With 200 psi of Hz still left in the cell, it was filled with COZ,and the resulting solution of CpMn(C0)s in HdC02 is stirred by using a magnet to agitate a magnetic flea inside the cell. The back-pressure regulator, BPR, is then set at 2500 psi, the COz pump, PP, at ca. 3050 psi, and the H2 compressor, PC, a t ca. 2950 psi. The dosage unit, DU, is set to switch once every 0.70 s. The flow of fluid is then allowed to stabilize, and the BPR is adjusted to pulse regularly. A background IR spectrum is recorded, and the FTIR interferometer is started scanning repetitively (ca. 1 spectrum per min). The syringe pump, SP, is now used to pressurize the variable-volume cell behind the piston. The pressure output from the pump continues to rise until the forward pressure on the piston exceeds the friction from the O-rings, driving the piston forward and the solution of CpMn(co)~ into the flowing stream of COfi2. Once the pressure output from the Brownlee pump is stabilized, the flow rate is limited by the controlling software to a preset value (normally 100 pL min-'1. The UV lamp is turned on as soon as the v(C-0) bands of CpMn(C0)s are detected in the FTIR spectrum. With care, the system can then be adjusted to give almost complete photolysis of CpMn(CO)3with CpMn(C0)n(v2-H2)as the sole product, detectable in by FTIR. CpMn(C0)z(v2-Hz),P, collects in the glass collection vessel attached t o the BPR. The efficiency of collection is improved by cooling the vessel with ice. The reactor is then run until the photolysis is seen to be less efficient,39and the bands of unreacted CpMn-

Flow Reactors for Supercritical Chemistry (C0)s appear in the spectrum. Typically this occurs after 6090 min. Yields of crude CpMn(CO)z(q2-H2)are 15-30 mg.

Conclusions In this paper, we have described two flow reactors that can be used to isolate new compounds. Both of the compounds, Cr(C0)5(C2H4)and CpMn(C0)2(v2-H2),were previously thought to be too labile for isolation as solids. Both have transpired to be surprisingly robust. Indeed, it is quite possible that non-supercritical routes will be found to these compounds, now that their stability is apparent. What these flow reactors do provide is quite a general approach to the synthesis of particular classes of compounds. Other C2H4 compounds and other Hz compounds should be accessible by similar procedures. Modifications of the reactor will lead to a whole range of new reactions. Such modifications are straightforward because the design is inherently flexible. The reactors are built up of simple components, most of which are commercially available or easily machined. Other types of reactor can (39) At the end of several runs, the cell was found to contain a pure white powder, sufficiently paramagnetic to be moved by a magnet. This powder was insoluble in organic solvents but soluble with effervescence in hydrochloric acid. It was amorphous by X-ray diffraction, decomposed to a brownish black solid on heating, and gave a DRIFTS spectrum consistent with [CO#-. This suggests that the solid was MnC03. Precisely how this carbonate is formed is not known, but presumably it originates from traces of water present in the Con, probably via transient formation of CpMn(CO)z(OHZ). MnC03 was also formed during the reaction of CpMn(CO)3with Nz in SCCOZbut not in the reaction of CpMn(C0)s and Hz in scCzH6. Similarly, photolysis of (toluene)Cr(CO)3in H2/scC02 gave rise to a blue/green precipitate which decomposed at 258 "C (as measured by DSC), the same decomposition temperature as that of an authentic sample of Cr2(CO&. For further discussion of the formation of carbonato complexes, see: Alvarez, R.; Carmona, E.; Galindo, A.; Gutierrez, E.; Marin, J. M.; Monge, A,; Poveda, M. L.; Ruiz, C.; Savariault, J. M. Organometallics 1989,8, 2430 and references therein. (40) Robertson, D. G. Unpublished data, 1995. (41)Note Added in Proof. For a very recent molecular modeling calculation on Cr(C0)5(CZH4)and related species, see: White, D. P.; Brown, T. L. Inorg. Chem. 1995,34, 2718.

Organometallics, Vol. 14,No. 8, 1995 3885 be developed merely by rearranging the basic components, much in the manner of a child's construction toy. Obvious extensions include the option of recycling the supercritical solvent or the replacement of the photochemical irradiation cell with a heated tube so that thermal reactions can be carried out. Flow reactors have considerable advantages for safety, because the total volume under pressure is small and also because the fluid is free to expand thereby reducing the chances of an unanticipated build up of pressure. Of course, there are limitations t o such a design. For example, in thermal reactions, the chemistry will take place under conditions of constant pressure rather than of constant volume which are found in a sealed autoclave. The precise consequences of this difference have yet to be explored. The scale of the reactions is still modest but the small size of the reactor combined with on-line spectroscopic monitoring means that even small quantities of compounds can be manipulated with a surprising degree of precision. Ultimately, the most appealing feature of the technique may be the possibility of carrying out synthetic chemistry in the total absence of conventional organic solvents.

Acknowledgment. We thank Mr. D. G. Robertson for obtaining the FT-Raman data on the Cr(CO)dC2H4 system. We are grateful t o Dr. S. M. Howdle and Dr. P. Mountford for their help and guidance. We are pleased to acknowledge our fruitful collaboration with Dr. G. A. Lawless and Dr. M. P. Waugh via an EU COST program. We thank the EPSRC Clean Technology Unit for Grants No. GFUH95464 and No. GWJ95065 and for a Fellowship t o M.P. We also thank the Royal Society, the Royal Academy of Engineering, BP Chemicals, and Zeneca Ltd. for support. We thank Dr. A. Banister, Dr. S. M. Critchley, Mr. J. G. Gamble, Dr. M. W. George, Mr. M. Guyler, Dr. M. Jobling, Mr. T. P. Lynch, Dr. K.H. Pickel, Mr. K. Stanley, Professor J. J. Turner, and Dr. R. J. Watt for their help and advice. OM9503813