The Modern Student laboratory cene. The numerical results from each scan rate are analyzed and tabulated to yield the following information. scan rate lP peak potentiah peak currents peak separation the number of electrons associated with the proeess (as calculated from the Randles-Sevick equation (11)) .student plots of i, vs. vm. This exercise leads to a discussion of topics such as rate of electron transfer, the characteristics of a ureversible" system, and the Nernst equation. ElectrochemicalStudy After studying the redox chemistry of f e m e n e , the students turn to a n electrochemical studv of the irreversible Fe(II)/Fe(I) process of [CpFe(tol)lt ( k g . 2a). The cyclic voltammogram of [CpFe(tol)lt provides a basis for discussion of several topics. the influence of Ligands on metal-based redox reactions radical lifetime versus the time scale of the experiment graphical and numerical features of an irreversiblecouple For example, when a Cp-ligand of ferrocene is replaced with a toluene group to form [CpFe(tol)l+,the potential of the Fe(III)/Fe(II) couple shifts to positive values beyond the solvent window, and thus is not observed. When compared to FeCpz, this represents a shiR of a t least 1.6 V to a more positive potential due to the electronic difference between toluene and Cp-. The Fe(II)/Fe(I) reductive couple of [CpFe(tol)l+( E , , = -1.355 V) exhibits a similar shiR relative to FeCp, and is now observed within the solvent window. The Fe(II)/Fe(I) reduction of [CpFe(tol)ltbecomes reversible a t 4 0 "C (12). This exoeriment mav be carried out (as time oermits). bv" placing the electrochemical cell in an acetonitrile-dry-ice bath and recording the cyclic voltammogram. Observation of this phenomenon facilitates a discussion of the relationship between the lifetime of an intermediate species and temperature. The reduction of other [CpFe(arene)lt com~ l e x e sleads to the formation of more-stable Fe(1) radical species (13). The Photolysis Reactions Formation of[FeCp8 and Iron Film Photolysis of [CpFe(tol)li in neat CH&N solution or in the Dresence of added nucleo~hilesresults in the reactions b 2c are examples of the outlked in Figure 1.~ i g u r e h and cyclic voltammograms obtained as [CpFe(tol)l+is photochemically converted to products in CHsCN/TBAH. As the ~hotolvsisconsumes ICoFe(tol)l+. a decrease in - . current for t h ~ s~rrevcrslblereduction 1s ubserved Concomltant with the loss of reductlvc current for ICoFe(tnl11. ~. is the appearance of a quasireversible one-electron oxidation (+0.520 V; oxidation of FeCpz to [FeCp21+)and a n irregularly shaped, irreversible two-electron reduction (-1.020 V; reduction of solvated Fe(I1) to Fe(0)) due to the newly formed photoproducts. The new two-electron reductive wave represents reduction of solvated Fe(I1) to a passivatingiron metal film that deposits on the electrode surface. One must remove this film after each scan in order to obtain high-quality data. .
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Figure 2. (a)Cyclic voltammogram obtained from 10.0 mL of 5 mM [CpFe(tol)]+ in 0.1 M CHBCNfrBAHat a scan rate of 0.200 Vls. (b) Cyclicvoltammogram ofthe solution from Figure Zamllected after 15 min of photolysis. (c)Cyclic voltammogram of the solution from Figure 2a collected after 40 min of photolysis. (d) Photolyzed solution from Figure 2c after adding 14.0 mg 1,lO-phenanthroline. (For example, the fdm can be removed by wiping the electrode with a n acetone-soaked tissue.) Failure to completely remove this film will result in broad, featureless waves that are very difficult to interpret. Formation of [~e(~hen)d'+ Phenanthroline (14 me) is added to the solution at the end of the photolysis res&ing in formation of [Fe(phen)JZ+ from the solvated Fe(I1) (Fie. 2d). The cvclic voltammogram now shows a new quasireverslble one-electron oxidative couple at +1.255 V due to the FetII~tnFe(I1IJoxidation of the deep-red complex [Fe(phen)dh. The cathodic pro-
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The Modern Student laboratory Photochemical and Electrochemical Studies of an Organometallic Sandwich Compound An Inorganic Chemistry Experiment David C. Boyd
University of St. Thomas, St. Paul, MN 55105 Brian J. Johnson
St. John's University, Collegeville. MN 56321 Kent R. ~ a n n '
University of Minnesota, Minneapolis, MN 55455 The field of organometallic chemistry has developed rapidly since the synthesis and characterization of f e m e n e in the early 1950's. In particular, cyclic n-donor complexes of the transition metals constitute an important class of organometallic molecules. This experiment gives the student an opportunity to synthesize and study a representative member of this family, the iron(I1)sandwich compound [CpFe(tal)lPF, containing cyclopentadienyl and toluene ligands, where Cp is qS-C5H5and to1 is $-toluene. This compound undergoes photochemical disproportionation (I)in neat acetonitrile solution to produce ferrocene (FeCpz) solvated iron(I1)as [F~(cH~cN)& free toluene
Further, the experiment is an example of an "integrated" laboratory exercise. A compound is synthesized and then studied by several complementary techniques. The data is then analyzed in order to draw mechanistic conclusions about the reactions under investigation. The open-ended nature of the experiment allows it to be carried out in two 4-h periods, or more if desired. Conceivably the experiment could serve as the framework for an entire term of lab work in which the students formulate a mini research project. Theory
The Reactions
In 1970, Nesmeyanov and coworkers reported that the photolysis of an acetonitrile solution of [CpFe(arene)l+produced solvated Fe(I1) and ferrocene in equimolar quantities (1).A detailed investigation of this reaction revealed that a purple photointermediate [CpFe(CH3CN)31+could Because all of the iron-containing species are electrochembe characterized at -40 O C (3).Upon warming, the purple icallv active. the course of the reaction may be monitored species decomposed to form the final products. by cyclic voltammetry. If added ligands such as -phosphines, phosphites, or isocyanides were present, substituted complexes of the type [CpFe(CH3CN)3..(L),I+ were isolated (I, 20°c, L [c~F~(L)~(cH~cN)]+ 4). Chelating ligands such a s warm to ~o'c,L phenanthroline (phen) react with the 20°c photogenerated [Fe(CH3CN)61Z+ or [Fe(phen)J2+ [CpFe(tol)]+ [CpFe(CH3CN)d++ toulene [CpFe(CH3CN)31+to form the deep-red hv warm to 20°c. L complex [Fe(phen)312+,but they do not 20 O c phen read with ferrocene. hv These reactions are summarized in Figure 1. Electrochemistrv. .. in -particular cv1/2FeCp2+ l/2[Fe(phen)d2+ clic voltammetry, is a very useful tool &I I the characterization of these complexes. In this experiment, students examine Figure 1. Summary of reactions of [CpFe(tol)lt some of the photochemical reactions shown in Figure 1. Electrochemistry is An Integrated and Open-Ended Project used to identify the reaction products and to formulate a mechanism for the photolyses. Despite the widespread application of photochemistry and electrochemistry in research, the disciplines have reIntroductory Literature ceived relatively little attention in the undergraduate inorganic laboratory (2). This experiment is designed to fill Before beginning the experiment, the students become that gap. familiar with the principles underlying the reactions and concepts to be be used. Introductory material describing 'Author to whom corresoondence should be addressed
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The Modern Student lclborcltory basic concepts of photochemistry and electrochemistry are included in the student lab manual and reviewed in the prelab lectures. Several suitable reviews appropriate for this introduction have been published previously (5-11). Experimental Reagents
The synthesis and electrochemical measurements described in this paper are relatively insensitive to chemical made or oriein. With the excention of the anhvdrous aluH n u m chlozde (which should be fresh) and thlelectrolyte (which was dried as described below). ..no reaeent drvine or purification is required.
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Electrochemical Measurements
Electrochemical measurements were carried out under an inert atmosnhere with a Bioanalvtical Svstems CV-1B and UM)on soiutions Laving a n iron (SJU) or CV-~~'(usT comnlex concentration of 5 x lo3 M. A three-electrode configuration was used in all experiments.
'glassy-carbon electrode platinum-wire auxiliary electrode aqueousAgCVAg reference electrode containing 1.0 M KC1 The solvent/sunnortine electrolvte svstem consisted of a hexafluorophos0.1 M solution of'tktra-nTbutyla-o& hate (TBAH) (Southwestern Analvtical or Aldrich) in &e&ly distilled reagent-grade aceto&trile. The electrolyte solution was dried over 80-200-mesh activated alumina before use. All potentials a m reported versus AgCVAg. The [FeCpzTFeCp, couple was observed at +0.520 V in the experiGnta1 system used to generate the figures. Photochemical Experiments Caution:Despite the Pyrex filtering it is harmful to look at Hg radiation. The lamp should be screened from direct view.
Photochemical experiments were carried out with the Pyrex-filtered output of a 150-W Hg street lamp modified on site. However, any source of visible light will suffice. Preparation of [CpFe(tol)]PFe Caution:Aluminum chloride is corrasive and moisture-sen-
sitive.
Synthesis [CpFe(tol)lPF6was prepared by a modification of the method of Nesmeyanov e t al. ( I ) . A 3-necked 250-mL round-bottom flask equipped with a magnetic stir bar, nitrogen inlet, and reflux condenser connected to a mineral oil bubbler is assembled and then purged with nitrogen for 5 min. With the nitrogen off, 2.5 g of ferrocene, 0.5 g aluminum powder, 5.0 g aluminum chloride, 50 mL toluene, and 0.25 mL of water are added. The mixture is refluxed for 2 h with stirring under a slow stream of nitrogen. After refluxing, the dark-brown solution is cooled and may be stored in the dark for up to a week with no ill effects. Workup The migture is then hydrolyzed by pouring it, with stirring, into 100 mL of a water-ice mixture in a fume hood. A316
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Any resulting chunks should be broken up with a glass rod. As much solid a s possible is transferred from the roundbottom flask to the-hydrolysis flask. The resulting mixture is then transferred to a separatory funnel. In the funnel, the yellow or yellow-green aqueous layer containing the product is separated from the organic layer. Ascorbic acid (0.5 g) is added to the mixture to reduce any ferricinium ion (lFeCpz+l)to ferrocene. The aqueous layer is extracted with three 25-mL portions of hexane to remove ferrocene and then slowly suction-fdtered to remove the remaining aluminum metal. Asolutiou of 2 g W F 6 in 10 mL of water is added gradually to the filtrate with stirring to form a yellow precipitate of crude [CpFe(tol)lPF6. If the mother liquor is still quite yellow, more W F 6 may be added. Stir for about 10 min, suction-filter, and allow to air-dry. The yield of crude product is about 75%. ElectrochemicalAnalysis The product is purified before carrying out electrochemical measuremenis by allowing a CH& solution to pass through a short (10-cm) column of F-20 alumina. The column is wranned with aluminum foil to avoid nroloneed exposure of t6; solution to room light. The vol-e oceluted solution is reduced to about 5 mL. and the vellow microcrystalline product is precipitated kith diethi1 ether. Analytical Data
Results and Discussion This experiment frequently occupies two or three periods of an advanced inorganic laboratory course. The first period is devoted to preparation and purification of the sandwich compound, while the electrochemistry and photochemistry experiments require one or two lab periods as scheduling permits. F e m n e is used a s a "warm UD"compound d u r i w the laboratory session devoted to ele&oche&stry in ord'kr to familiarize the student with the equipment. The photochemical experiments are carried out in one or two subsequent laboratory sessions. If three lab periods are not available. the instructor can c a m out one or more ~hotolysis expekments and then give k e data to the students. In this wav. the students mav draw mechanistic conclusions about tKe system even iftlme precludes collection of a full data set. The initial electrochemical investigation of ferrocene serves three purposes.
The exercise familiarizes the student with the instrumentation and techniques used in electzochemical studies. *The student gains experience with one of the products iFeCpz)produced in the photolysis of [CpFe(tol)l+. .The student then collects data for numerical analysis and discussion. Scan-Rate Study
The students investigate the basic principles of cyclic voltammetry and the influence of scan rate on a cyclic voltamrnograrn by carrying out a scan rate study on ferro(Continuedon page A318)
When phenanthmline is added before photolysis, cyclic voltammetry shows that no thermal reaction occurs. Subsequent photolysis of [CpFe(tol)l+in the presence of phen results exclusively in formation of [Fe(phen)312t(Fig.3) as indicated by the oxidation and reduction processes obsewed a t +1.2255 V and -1.40 V, respectively. This result indicates that the ~ r e c e d i mechanism n~ aoolies onlv in the absence of ligandsother t h i n solvent. (This photolysis will not run to completion in this wncentration regime due to an inner filter effect imposed by the strongly absorbing l F e ( ~ h e n ) ~compound. l~+ The students should be told this so they will not have to wait for all IC~Fe(tol)l+ to be consumed.) ~ t u i e n t sare able to rationalize this result by arguing that ~henanthrolinetram the ~botointermediatebefore the disproportionation t#;fermceeneand solvated Fe(I1) occurs. Thus the left-hand side of Firmre - 1 has been 'discovered".
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Flexibility in the Project
Figure 3. Cyclic voltammogram obtained from 10.0 mL of 5 mM [CpFe(tol)]+ in 0.1 M CH3CNlTBAHwllected after 60 min of photolysis in the presence of 30 mg of 1,lo-phenanthroline at a scan rate of 0.200 v/s.
This laboratory is sufliciently open-ended that students can pursue "mini research projects" simply by augmenting or otherwise modifving the analvsis . -~rocedure. For example, an-emphasis on synthetic chemistry may be achieved using different arenes as the solvent in the synthesis portion of the experiment to yield the correcesses between -1.00 V and -1.500 V are due to reduction compounds. Then students can sponding of the phen moities in lFe(phed312+to coordinated radical - [CpFe(arene)l+ . ail prepare and characterize &eir own wmpound or series anions. The peak current for the oxidation of [Fe(phen)312+ of compounds. is approximately equal to that for the oxidation of FeCp2 leading to the conclusion that FeCp2 and [ ~ e ( p h e n ) ~ l ~ ' Alternatively, students can prepare compounds of the formula (and therefore solvated Fe(I1)) are produced in equal pmportions. [CpFe(CH3CN)L2It Identifying the Products by photolyzing in the presence of the appropriate phosphite ligand, where L is P(OPb)3or P(OMe)3. The initial study of FeCp,will allow the student to idenAdvanced techniaues such as m s t a l erowth bv slow soltify this compound as a photoproduct, while solvated Fe(I1) vent diffusion are readily added tdthe Gocedure Layering may be identified by derivatization with phenanthroline. a concentrated C H X L solution of the arene comolex with When phenanthroline is added, [ F e ( ~ h e n ) ~may l ~ +be idenether is an effective c&stal-growth wmbination, but other tified from the characteristic red color encountered in precombinations work as well. vious lab work, or by examining the reaction stoichiometry and appling the 18-electron rule. Literature Cied Nevertheless, the students are able to postulate the 1.Nesmeyanov,A N.; Vi%au,N.A.; Shiloutseva,L. S.Dokl.Akod NouhSSSR 1910, 190,857. mechanism below or some variation of it by using some 2. Please see the exeellmt %ate of the Art' seriea on Inorganic Photoehemistry lo J combination of the following strategies and facts. ChPm Ed=. 1889,60.785-887. 3. Gill, T. P ; Mann, K R. I n o g ChPm 1 ~ , 2 2 , 1 9 8 6 1 9 9 1 . experimental observations 4. la) Gill, T. P ; Mann. K R. lnorg Chom. 1980,19,30073110. (b) Gill, T P;Mann, K R. J . Organomrt Chem. 1881,216.6S71. the 18-electronrule 5. Adamsan, A. W ;Fleischer, P. D . Conqta ofInagonir Photoehemistry;Wiley: New the analysis of possible ligands in the system York. 1975. .the assumption that the anionic Cp- ligand is mare tightly 6. Wrighton. M. S., E d . A d ~ d c e in s Chemistry &ties, 168: he.Chem. Sar.: WashingtonD.C., 1978. bound than a neutral toluene ligand The Mechanism
7. Porter. G. B. J Chem Educ lS88,60,78S790. 8. Faulkner, L. R. J. Chem. Educ 1883.60,26>261. 9. Evan% D. H.; O'Connell. K. M.; Peteram,R. A.; Kelly, M. J. J Chom E h . 1885.MI,
290-293.
1889.60.691-702. P. T: H e i n e m , W R. J. Chem. E d v r 1988,60,102706. 12.Boyd.D.C.: Bohling,D.A.:Mann,K.R.J Am. Chem. Soe IS&, 107,1611-1M4 13.Hamon.J.R:Astlue, D.:Miehand,P J.Am. Chem.Sar. 1881,103.7Sb766. 10. Mabbatt, G . A J Chem Educ
11. Kiss-,
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