In the Laboratory
An Electrochemical Experiment to Monitor the Isomerization of trans- to cis-[RuCl2(dppb)(phen)] An Undergraduate Cyclic Voltammetry Experiment for Inorganic Chemistry Salete L. Queiroz, Márcio P. de Araujo, and Alzir A. Batista* Departamento de Química, Universidade Federal de São Carlos, CP 676, CEP 13565-905, São Carlos, SP, Brazil; *
[email protected] Kenneth S. MacFarlane and Brian R. James* Department of Chemistry, The University of British Columbia, Vancouver, BC V6T 1Z1, Canada; *
[email protected] Electrochemical analyses have developed substantially in the last few decades and have found many applications in chemical research, including routine use in analytical chemistry. It is thus essential that students become more aware of this topic, and there is an increasing desire to include more information about electrochemical measurements in the undergraduate curriculum (1). Of techniques available for studying electrode processes, potential sweep methods are probably the most widely used, particularly by non-electrochemists. These consist of applying a continuously time-varying potential at the working electrode, which results in oxidation or reduction reactions of electroactive species in solution. Cyclic voltammetry (CV) appears to be the most popular electrochemical technique for characterization of inorganic compounds. The experiment described here is well suited to students who have completed a lecture series on basic coordination chemistry and associated electrochemical techniques and who have an understanding of linear sweep voltammetry at solid electrodes (2). Extensive coverage of CV theory is not necessary, as students can easily identify oxidation and reduction waves for reversible electrochemical couples and gain an appreciation for the speed and simplicity with which this information is obtained (1). Students will also be introduced to the evaluation of electrochemical and thermodynamic stability of geometric isomers of coordination complexes in solution. The experiment thus (i) serves as a good introduction to the use of electrochemical techniques, (ii) gives students experience in electrochemical principles and in measuring current and potential, (iii) gives students the opportunity to calculate half-wave potentials from the experimental oxidation and reduction waves, and (iv) illustrates the use of CV to monitor an isomerization process involving an inorganic coordination compound. The experiment is easily completed in one laboratory period of 4 h. Experimental Procedure General Methods All materials are commercially available and were used as received. The [RuCl2(PPh3)3] complex and the ligands, dppb (1,4-bis(diphenylphosphino)butane) and phen (1,10phenanthroline monohydrate), were obtained from Aldrich. The synthetic reactions have to be carried out under an inert atmosphere (N2 or Ar), whereas this is not essential (but is recommended) for the CV measurements.
Preparation of [RuCl2(dppb)(PPh3)] (1) The preparation of 1 from [RuCl2(PPh3)3], adapted from literature procedures (3), is described in detail in the accompanying article (4 ). Preparation of trans-[RuCl2(dppb)(phen)] (2) For the electrochemical experiment, better cyclic voltammograms are obtained if an isolated solid sample of trans-2 is used. In our previous article (4), an in situ sample of trans-2 was particularly convenient for 31P{1H} monitoring of the trans → cis isomerization reaction. The preparation of the well-characterized trans-2 from the precursor 1 is adapted from a literature procedure (5). Although trans-2 is air-stable in the solid state, 1 and trans-2 are somewhat air-sensitive in solution and an inert atmosphere technique is required. Further, the synthesis is carried out in the dark by wrapping the reaction flask in aluminum foil. Phen (110 mg, 0.61 mmol) is dissolved in deoxygenated toluene (8 mL) and 1 (0.097 g, 0.11 mmol) is added. The solution is stirred at room temperature for 20 min and then reduced in volume to ~2 mL. Deoxygenated hexane (5 mL) is added to precipitate a purple solid. This is filtered in air, washed with ethanol (5 mL) and ether (5 mL), and dried under vacuum for ~20 min. Yield: 69 mg (80%).
Figure 1. Cyclic voltammograms of cis- and trans-[RuCl2(dppb)(phen)] (2) in CH2Cl2, initially 1.0 mM in trans-2 and 0.1 M in [Bu4N][PF6]. Scan rate, 100 mV s ᎑1. ––– , trans-2, present initially; ᎑ ᎑ ᎑, trans-2 and cis -2, present after 15 min; ..., cis-2, present after 60 min.
JChemEd.chem.wisc.edu • Vol. 78 No. 1 January 2001 • Journal of Chemical Education
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In the Laboratory
CV Measurements For the electrochemistry experiment, a standard threeelectrode cell is configured: the reference electrode is Ag/AgCl, the working electrode is a platinum foil, and auxiliary one is a platinum wire. The electrochemistry cell is provided with a gas bubbler to maintain an inert atmosphere for the deoxygenated CH2Cl2 solution. Five milliliters of solution, 1.0 mM in trans-2 and 0.1 M in [Bu4N][PF6], is typically used for the CV measurements. The cell should be maintained in the dark by covering with aluminum foil, to avoid photochemical isomerization of trans-2 to the cis form. The first CV (Fig. 1) shows the electrochemical process for trans-2 (Eanode = 0.52 V; Ecathode = 0.42 V; the formal reduction potential E1/2 = (Eanode + Ecathode)/2 = 0.47 V vs Ag/AgCl). After this CV is obtained the cell is left in ambient laboratory light to allow isomerization to cis-2. After about 15 min, a second electrochemical process is clearly observed because of the presence of a mixture of trans- and cis-2 isomers. After further exposure to light for about an hour, depending on local conditions, the solution is seen to contain only cis-2, E1/2 = 0.66 V (Fig. 1).
isomer is close to unity, the data are consistent with an electrochemically reversible process:
Hazards
Literature Cited
There are no significant hazards associated with the experimental procedures. Discussion The experiment nicely demonstrates how cyclic voltammetry may be used to detect and monitor a trans → cis geometric isomerization for an octahedral Ru(II) complex. The system provides a good example of a kinetic trans product being converted to the more thermodynamically stable cis product via a photochemical or thermochemical nonreversible pathway. The scan rates in the CV can be varied from 100 to 500 mV s᎑1 to show that the E1/2 values are independent of the scan rate; and, as the current ratio, ianode/icathode, for each
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[RuCl2(dppb)(phen)]+ + e → ← [RuCl2(dppb)(phen)] Ferrocene (at equimolar concentrations) may be added as an internal standard (for Fc+/Fc, E1/2 = 0.48 V vs Ag/AgCl). In this case it is found that the ratio of the i value for either isomer to that for ferrocene is close to unity; this demonstrates that the electrochemical processes involve one electron. In theory, the Eanode – Ecathode values for a one-electron process should be ~59 mV, but experimental values are typically much higher (6 ), as seen here. The reduction potential for the cis isomer is ~0.20 V higher than that of the trans isomer, a well-established phenomenon in Ru(III)/Ru(II) systems (5). Acknowledgments We thank CNPq, CAPES, and FAPESP in Brazil and NSERC in Canada for financial support.
1. Wheeler J. F.; Wheeler S. K.; Wright L. L. J. Chem. Educ. 1997, 74, 72. 2. Mocellin, E.; Russel, R.; Ravera, M. J. Chem. Educ. 1998, 75, 773 and refs therein. 3. Jung, C. W.; Garrou, P. E.; Hoffman, P. R.; Caulton, K. G. Inorg. Chem. 1984, 23, 726. MacFarlane, K. S.; Joshi, A. M.; Rettig, S. J.; James, B. R. Inorg. Chem. 1996, 35, 7304. 4. Queiroz, S. L.; de Araujo, M. P.; Batista, A. A.; MacFarlane, K. S.; James, B. R. J. Chem. Educ. 2001, 78, 87–89. 5. Queiroz, S. L.; Batista, A. A.; MacFarlane, K. S.; Oliva, G.; Gambardella, M. T. do P.; Santos, R. H. A.; Rettig, S. J.; James, B. R. Inorg. Chim. Acta 1998, 267, 209. 6. Bard, A. J.; Faulkner, L. L. Electrochemical Methods; Wiley: New York, 1980.
Journal of Chemical Education • Vol. 78 No. 1 January 2001 • JChemEd.chem.wisc.edu
