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Anal. Chem. 1991, 63, 2073-2075 (16) Slnha, M. P.; Friedlander, S. K. J . cdldd Interface Scl. 1986, 772, 573-582. (17) Kaufmann, R. L. In Physical and Chemical Chamcterizatkm of IndvvMuel Akbome ParMbs; Spurny, K. R., Ed.; Ellis Horwood, Ltd.: Chlchester, U.K.. 1986; Chapter 13. (18) Wbser, P.; Wurster, A. In Physical and Chemical Chamctedzatbn of IndlbMual Akborne Pa&bs; Spurny, K. R., Ed.; Ellis Horwood, Ltd.: Chlchester, U.K., 1986; Chapter 14. (19) De Waele, J. K. E.; Adams, F. C. In Physical and Chemical Chamctedzatbn of Indivklual Akttme Particles; Spumy, K. R., Ed.; Ellis Horwood, Ltd.: Chlchester, U.K., 1986 Chapter 15. (20) Slnha, M. P. Rev. Scl. Instrum. 1984, 55, 886-891. (21) Marljnlssen, J.; Scarlett. 6.;Verheluen, P. J . Aerosol Sci. 1988. 79. 1307-13 10. (22) Spengler, 6.;Karas. M.; Bahr. U.; Hllbnkamp, F. J . Phys. Chem. 1987, 97, 8502-6506. (23) Karas, M.; Bachmann, D.;Hlllenkamp, F. Anal. Chem. 1985, 57, 2935-2939. (24) Wleser, P.; Wurster, R.; Seller, H. Atmos. Envlron. 1980, 74, 485-494. (25) Van Orleken, R.; Adams, F. I n Chemisw of MuNphase Atmospheric Systems; Jaeschke. W., Ed.; NATO AS1 Series Vol. 06; SprlngerVerlag: Berlin. 1986; p 81. (26) Klnsel, G. R.; Johnston, M. V. Int. J . Mass Spectrom. Ion Processes 1989. 97. 157-176. (27) Klnsel, (3. R.: Mowry, C. D.; McKeown. P. J.; Johnston, M. V. Int. J . Mass Spectrom. Ion Process8s 1991, 704, 35-44. (28) Opsal. R. 6.;Owens, K. G.; Rellly, J. P. Anal. Chem. 1985, 57, 1884- 1889.
(29) See, for example: Vertes, A.; Juhasz, P.; Ani. P.: Czltrovszky, A. Int. J . Maw Spectrom. IOn P r w 1986, ~ 83, 45-70. (30) Press, W. H.; Peukolsky. S. A. Compvt. Phys. 1990, 669-672.
To whom correspondence may be addressed.
P. J. McKeown M. V. Johnston* Department of Chemistry and Biochemistry University of Delaware Newark, Delaware 19716
D. M. Murphy* Aeronomy Laboratory NOAA/ERL Boulder, Colorado 80303 RECEIVED for review April 4,1991. Accepted June 24,1991. This research was supported in part by a grant to M.V.J. from the National Science Foundation (CHE9096266). P.J.M. acknowledges a research fellowship from the National Oceanic and Atmospheric Administration.
TECHNICAL NOTES Design and Evaluation of an Electrochemical Cell for the Study of Organometallic Complexes at Increased Gas Pressures James E. Anderson* and Eileen T. Maher Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167
INTRODUCTION Determination of the electron-transfer properties of organometallic compounds by electrochemical and spectroelectrochemical methods is an area of current interest (1-6). These studies often involve complexes containing gas-phase ligands such as carbon monoxide (CO) (1,7,8) or may examine the reactions of electrogenerated species with gases such as CO, (9, IO). The range and type of electrochemical experiments for systems that involve gas-phase species would be enhanced by an ability to perform measurements at increased pressures. For example, the concentration of a gas-phase reactant could be increased to allow observation of a specific chemical reaction. If the concentration of the gas could be systematically varied by controlling the pressure, titration experiments commonly used to examine ligand loss or addition coupled with electron transfer (11-13) could be easily performed. However, conventional electrochemical cells are not designed to allow measurements at pressures significantly greater than 1 atm (14, 15). Electrochemical cells have been designed to work at high pressures and in general are used to perform measurements in near-critical and supercritical fluids (16-18). These conditions typically require both very high pressures and temperatures such as 240 bar 400 K (18))and consequently, the cell is made from alumina and/or stainless steel components. These metal components are potentially reactive toward several different classes of organometallic compounds, and consequently, decomposition of the compound as well as degradation of the cell could occur. In addition, the extreme air sensitivity of some complexes would prohibit their use with the high-pressure cells that are described in the literature. 0003-2700/9 110363-2073$02.50/0
In this paper, we report the design and evaluation of an electrochemical cell that can be conveniently used at increased gas pressures for highly reactive organometallic complexes. With the exception of the platinum electrodes, the cell is made entirely from glass and pressures up to 125 psi above ambient pressure (Pa of approximately 9.5 atm) can be obtained. We will demonstrate the use of this cell by examining the electron-transfer properties of osmocene as a function of CO pressure.
EXPERIMENTAL SECTION (A) Apparatus. Figure 1 shows the design and the placement of the electrodes in the electrochemical cell. The cell walls are thick glass capable of withstanding pressures up to 200 psi, and the four inlets are ACE glass joints (Part 5027-20) designed for high-pressure applications. Three of the inlets are for the electrodes while the fourth is for the delivery of the reactant gas. The electrodes are platinum wire, sealed in glass, connected to copper wire via solder or Hg. The electrodes have a rim in the glass wall to mechanically prevent the electrode from slipping from the cell, while air tight seals are obtained via the ACE glass joints. The electrode separation is minimized by the shape of the cell and careful electrode placement. In our case, the working and reference electrodes are approximately 1.0 cm apart. The working electrode is a platinum disk,approximate area of 0.0020 cm2,while the auxiliary and reference electrodes are platinum wire. The Pt wire pseudoreference electrode could be easily replaced with a bridge (Pt wire sealed in glass) to accommodate a standard reference electrode. In addition to the high-pressure cell and the gas supply tank with regulator, our experimental setup has a Matheson pressure gauge (Part63-5622) and a pressure release valve. The pressure gauge is accurate to 0.5 psi. Electrochemical experiments were performed with a BAS-100A. All potentials are reported vs the Pt wire pseudoreference electrode 0 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991 A.
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I
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exterior of the cell should be taped to minimize fragmentation of the glass, and proper safety equipment should be worn by all personnel in the laboratory. In addition, the potential toxicity of the reactant gas must be considered, and appropriate methods to trap or ventilate the gas in the event of an explosion must be planned. RESULTS AND DISCUSSION
U B. TOP VIEW OF CELL
GAS INLET *REFERENCE
AUXILi AQY
WORKING
Flgure 1. Diagram of cell showing (A) side view and electrode placement and (B) inlets for the electrodes and the gas connection.
and have a precision of 0.005 V. The ferrocene/ferrocenium couple was used as an internal standard. (B) Reagents. Ferrocene [(CbH&Fe] was purchased from Aldrich, osmocene [(C,Hb)20s]was obtained from the laboratory of Dr L. B. Kool, and both were used without further purification. Carbon monoxide (CO) was purchased (grade 2.3) from Wesco. The acetonitrile (CH3CN),dichloromethane (CH2C12),and tetrahydrofuran (THF) used for electroanalysis were purchased (Aldrich)as spectroscopic grade, purified and dried by standard methods (19, 20), stored over calcium hydride (CH3CN),phosphorus pentoxide (CH2C12),or sodium/benzophenone (THF) under an inert atmosphere, and distilled just prior to use. Tetrabutylammonium tetrafluoroborate [TBA(BF,)] was purchased from Aldrich, doubly recrystallized from ethanol, and dried in a vacuum oven at 50 "C. ( C ) Procedure. The concentration of supporting electrolyte was 0.2 M unless otherwise stated. All solid and solution transfers were carried out by standard Schlenk methodologies (21). If the analyte is not air sensitive or is mildly air sensitive, the following procedure is used. The cell is degassed for approximately 20 min with either argon or nitrogen prior to addition of the solvent. The solvent, which has been previously degassed, is transferred to the cell and is then additionally degassed again for approximately 10 min. After recording the background in the electrochemical experiment, the complex of interest is added to the cell under a flow of inert gas. The electrochemical response is measured, and then the reactant gas is bubbled through the cell for approximately 20 min to replace the inert gas. The electrochemical response is measured under these conditions,after which the cell is pressurized with the reactant gas. If the analyte is reactive to air as a solid species, it is first loaded into the cell in a drybox. The solvent, which has been previously degassed, is added to the cell and degassed again, and the electrochemical response in the absence of the reactant gas is obtained. The reactant gas is then bubbled through the cell, the electrochemical response is determined, and the cell is then ready to be pressurized. The electrochemical background of the solvent and supporting electrolyte is obtained in a separate experiment by normal techniques. Safety Considerations. Operation of this cell at high pressures presenb the possibility of severe injury if proper safety procedures are not followed. While the heavy wall glass can withstand sig nificantly greater pressures, we feel that 125 psi should be the upper limit of operation since the construction of the cell may compromise the strength of the glass. In addition, operation at 125 psi provides toleration for any fluctuation in pressure that may occur. When in operation, the cell should be enclosed in a hood capable of containing fragmented glass in case of erplosion due to failure of the cell or the electrode connections. The
(A) Evaluation of the Cell. The relatively small area of the electrode as well as the close electrode placement minimizes any iR effects that are encountered in nonaqueous solvents. Hence, the cyclic voltammetric response obtained for ferrocene in CH&N with TBA(BF4) as the supporting electrolyte yields the expected diffusion-controlled reversible one-electron response. The oxidation of ferrocene is located at Ellz = 0.37 V vs the Pt wire reference electrode and is characterized by a value of Upof 78 mV at 100 mV/s. This does not change significantly over s m rates of 10-1OOO mV/s. In addition, the ratio of peak current to the square root of scan rate (iP/v1t2) is constant, and the ratio of the peak currents is unity. Similar results are obtained with both methylene chloride and T H F solvent systems. No variation in the response for ferrocene was observed by cyclic voltammetry when either CO or argon was used to pressurize the cell up to 125 psi above ambient pressure. Hence, no significant pressure effects on the electrochemical response due to the cell are observed. (B) Osmocene Electrochemistry as a Function of CO Pressure. Unlike ferrocene, the oxidation of osmocene is not a reversible one-electron process (I,22,23) but rather has two one-electron-transfer steps a t platinum. The one-electronoxidation product has been shown (24) to be the metalmetal-bonded dimer [(Cp)20s]2+.The dimer is a relatively insoluble green solid with most counterions but is soluble as the PF, salt. [(C~),OS]~(PF& is reactive toward strong donor ligands, and isolation of [(C~),OS(NCCH,)]~+ is possible after dissolving in acetonitrile. The coordinated acetonitrile can be replaced with other ligands (LX7such as bromide and chloride to form complexes of the general form [(Cp)zOs(L)](x+2)-(24). These properties suggested that the oxidation of osmocene in acetonitrile in the presence of a reactant gas, such as carbon monoxide, would be an excellent candidate to further demonstrate the applicability of our cell. Figure 2a is the cyclic voltammetric response obtained for a solution of ferrocene and osmocene in acetonitrileTBA(BF4) from -0.10 to 1.7 V at a scan rate of 100 mV/s with an argon atmosphere. The ferrocene wave is located at Ellz = 0.21 V (labeled Fc in Figure 2a) and is characterized with a Upof 136 mV. The large value for Upand the change in E1/2 are due to precipitation of the osmocene dimer on the electrode surface. Evidence for this surface effect is that upon multiple scans the current rapidly decreases for all of the waves. In addition, bulk electrolysis of osmocene in acetonitrile-TBA(BF,) results in the formation of a green coating on the working electrode that passivates the electrode surface. The electrode surface could be cleaned by either application of a negative potential (-1.5 V) or by removing and polishing the working electrode. There are two oxidation waves at Epa= 0.59 (wave 1,Figure 2a) and 1.44 V (wave 2, Figure 2a) that are due to osmocene. Wave 1 is characterized by a value of Ep- Eplzof 84 mV and a constant value of ip/u1/2.In addition, the value of Epafor wave 1 shifts to positive potentials upon an increase in scan rate. Wave 1 is characterized as a reversible one-electron oxidation process followed by a chemical reaction (25, 26). Similar behavior is found for osmocene in the absence of the ferrocene internal standard. The value of E, for wave 1is found to shift in a positive direction as the CO pressure is increased. Figure 2b dem-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
Fc
1
2
P-
f
2uR
L
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[CO]. Additional errors are introduced in assuming Henry's law behavior for this system. Since reactivity between osmocenium ion and CO is demonstrated by a shift in E,, as a function of CO concentration, the principle advantage of this cell design is clearly demonstrated. The product of the chemical reaction is tentatively assigned and will naturally need to be verified by additional methods and techniques. The cell is relatively easy to construct and operate safely. Consequently, a new "tool" is now availble that can be used to study the reactions of gas-phase species in conjunction with electron transfer.
ACKNOWLEDGMENT
E [VOLT)
In addition to the gift of osmocene, several helpful discussions with L. B. Kool are acknowledged.
LITERATURE CITED For examples see ref 1-6: Connelly. N. G.;Geiger, W. E. A&. Organomet. Chem. 1984, 23, l. Oeiger, W. E.; Saker, A.; Edwln. J. J. Am. Chem. Soc. 1990, 172,
00
71 13.
D
w"
960
m
----. --.
1000. 112. 458. .
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Anderson, J. E.; Gregory, T. P.; McAndrews. C. M.; Kool, L. E. Organometallics 1990, 9 , 1702. Anderson, J. E.; Maher, E. T. Organometallics 1991, 10, 1248. Mevs. J. M.; Geiger, W. E. J. Am. Chem. Soc. 1989, 1 1 1 , 1922. Swistak, C.; Cornillon, J.-L.; Anderson, J. E.; Kadish, K. M. Urganometallics 1987, 6 , 2146. Bollnger, C. M.; Story, N.; Sullivan, 6. P.; Meyer, T. J. Inovg. Chem. 1988, 27, 4582. Pugh, J. R.; Bruce, M. I?. M.; Sullivan, E. P.; Meyer, T. J. I w g . Chem. 1991, 30, 86. Boyd, D. C.; Rodman, G. S.;Mann, K. R. J. Am. Chem. Soc. 1988, 108, 1779. Kadish, K. M.; Bottomley. L. A.; Cheng, J. S. J. Am. Chem. SOC. 1078, 100, 2731. Titration experiments with gas-phase ligands can be performed with gas flow controllers andlor by the use of gas blends. For example, see ref 8. Hawkridge. F. M. I n Laboratuy Techniques in E k t r ~ a ~ ) L t lChem~al istry; Kissinger. P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1984;Chapter 12. Electrochemical Cell &sign; White, R. E., Ed.; Plenum Press: New York, 1984. Flarsheim, W. M.; Tsou, Y.4.; Trachtenberg, I.; Johnston, K. P.; Bard, A. J. J. Phys. Chem. 1988, 90, 3857. Crooks, R. M.; Bard, A. J. J. Phys. Chem. 1087, 91, 1274. Flarsheim, W. M.; Bard, A. J.; Johnston, K. P. J. Phys. Chem. 1989. 93, 4234. Riddick, J. A.; Bunger, W. 6.; Sakano. T. K. Organic soh.ents, 4th 4.; InterscienceWiley: New York, 1986. Kadlsh, K. M.: Anderson, J. E. Pure Appl. Chem. 1087, 59(5)707. Shriver, D. F.; Drezdzon, M. A. The Manipoletbn of A t Sensltive Compounds. 2nd ed.; Wiley: New York 1986. Kuwana, T.; Bublltz, D. E.; Hoh, G. J. Am. Chem. Soc.1960, 82,
'
9401 920
-
Bond, A. M.; Broomhead. J. A.: HollenkamD. A. F. Inora. Chem. 1088. 27, 978. Lehman, R. E.; Bockman, T. M.; Kochi, J. K. J. Am. Chem. Soc.
0
1
1.0
B
1.2
1.4
1.6
1.8
2.0
2.2
-lOg[CO]
Figwe 2. (a, top) Cyclic voltammetric response of an acetonitrile solution containing ferrocene and OSmOcene with 0.03 M TBA(BF,) at a scan rate of 100 mV/s. The waves due to ferrocene are marked Fc, and the waves due to osmocene are numbered. (b, bottom) Plot of E , for wave 1 vs -log [CO]. The data are fit by a straight llne with a slope of 94 mV/log [CO] with a correlation c o " l e n t ( r 2 )of 0.87. The x axis points were generated by taking the negative log of the concentration of CO, in moles per liter. This is tacitly assuming an activity coeffictent of unity for CO.
onstrates this response by plotting the peak potential as a function of -log [CO], where the concentration of CO was approximated by the use of Henry's law. The Henry's law constant was determined for CO in CH&N to be 4.105 X 10-4 by correlating log X,to the solvent polarizability for 10 different solvents (27). The value of EMfor each point in Figure 2b is the average of approximately 10 cyclic voltammetric scans at a given pressure. The values measured for E,, at a given pressure were typically within 5 mV of the average. The working electrode was cleaned after each scan by application of a negative potential. The slope of the line is 94 mV/log [CO], which clearly indicates formation of a CO adduct. Based on the slope of the line and a value of 1for n,one CO is added upon oxidation. Hence, we are tentatively assigning (28) the product of the oxidation in the presence of CO as [(Cp)20s(CO)]2+.Although there is some error in the method used to determine the value of X 2 for our calculations, it will not significantly effect the slope of the line describing the shift of Epas a function of log
5811. Denisovich, L. I.; Zakurln, N. V.; Bezrukova, A. A.; Gubin, S. P. J. Organomet. Chem. 1974, 81, 207. Droege, M. W.; Harman, W. D.; Taube, H. Inorg. Chem. 1887. 26,
1309. Our data are in qualitative agreement with that presented in the literature. See refs 22 and 23 for a complete discussion. Nicholson, R. S.; Shah 1. Anal. Chem. 1064, 36, 706. Wllhelm, E.; Battino, R. Chem. Rev. 1973, 73, 1. Further characterization of this species and related compounds will be presented in a future publication.
RECEIVED for review March 19,1991. Accepted May 24,1991. Acknowledgment is made by J.E.A. to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
