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
-
7 +
t
(Continued on page A3161
Volume 69 Number 12 December 1992
A315
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.
-
.-
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
Journal of Chemical Education
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)
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.
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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.
.
A318
Journal of Chemical Education
&
E IUOLT I
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-
-
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".
..
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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