A Modern Iterative Approach to a Classical Organometallic Laboratory Experiment

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Clifford E. Myers, M. Stanley Whittingham and Wayne E. Jones, Junior. 1

State University of New York in Binghamton, Binghamton, New York 13902

(Funded by the National Technology Foundation)

(Submitted to T. Chemical Education)

Since ferrocene is credited with the fast acceleration of recent organotransition metal chemistry (1, 2) as well as the cyclopentadienyl group is extensively used as being a stabilizing ligand, it is only installing that the synthesis of ferrocene be designed into a sophisticated undergraduate inorganic laboratory. In our four credit rating course, the scholars work in pairs and have the opportunity to select half a dozen experiments via a total of nine. Three of these trials must be selected from the part of materials chemistry and the subject areas include the synthesis of anhydrous CrCl3, a high temperature superconductor, the ZSM-5 zeolite as well as the lithium intercalation of WO3. Three wet experiments are also selected. These include the activity of W(CO)4, metal complexes of DMSO, a tris(bipyridyl)ruthenium complex, ferrocene, and the acetylation of ferrocene. If ferrocene is chosen, it must be required for conjunction with the acetylation of ferrocene and these labs make up a pair of the three wet labs which might be done by students. Each research laboratory incorporates an open ended issue that the college student may solution with the aid of catalogue research or CAChe molecular modeling computer software with the Task Leader extension. This iterative approach forms confidence inside the students capacity to explore the unknown and reinforces the standard idea of the scientific method.

The ferrocene synthesis continues to be an extremely effective and well-liked selection. The scholars enjoy the diverse technical expertise acquired within this experiment. They are techniques which a student might not be introduced to again as an undergraduate including the use of air-less glassware whilst working on a vacuum line, cyclic voltammetry, bulk electrolysis, thin-layer and line chromatography. In addition , the compounds are seen as standard strategies such as melting point determination, IR and UV-Vis spectroscopies.

Trials

Preparation of Ferrocene

Ferrocene is produced with a customization of the prep reported by Jolly (3). The yield inside the reported synthesis was 93% (3). Cyclopentadiene undergoes a 4+2 cycloaddition to form dicyclopentadiene. For this reason, cyclopentadiene is usually filtered before employ. Dicyclopentadiene boils at 170C and cyclopentadiene boils by 42. a few C. For efficiency, the dicyclopentadiene dimer is thermally cracked using a fractional work apparatus ahead of time by the teaching assistant. While this is usually done on the day of the experiment, we certainly have found that cyclopentadiene can be stored with out significant dimerization in a foil covered box in a freezer for several days. At the beginning of the lab period, the scholars grind KOH in a mortar and quickly transfer it to a tared vial. KOH is hygroscopic and should always be ground in small servings (2 g). A nitrogen glove tote is a beneficial investment for this step in the process. In addition to protecting the students from the corrosive KOH, this ensures that the KOH can be dry. The FeCl2. 4H20 will also enter into solution better if it is carefully ground. It really is then put in a tared vial.

The pre-weighed KOH (15 g) is put in a 100 mL (14/20) three-neck round bottom flask equipped with a magnetic mixing bar.

1, 2-Dimethoxyethane (30 mL) is added with stirring to the KOH. One part of the neck of the guitar is stoppered and the other is linked to a vacuum series through a gas adapter. Even though the mixture is usually slowly stirred and the flask is being cleared with a stream of nitrogen, the cyclopentadiene (2. 75 mL) is definitely added. The resulting answer is increased colored. The main neck can then be fitted with a pressure equalizing dropping channel (25 mL) with its stopcock open. In a second 1 neck round bottom flask that is when you have a nasal septum, FeCl2. 4H20 (3. 25 g) and DMSO (12. 5 mL) are stirred under a nitrogen atmosphere to dissolve the FeCl2. 4H20.

After about a few minutes, the stopcock is sealed and the FeCl2 solution is usually added to the pressure equalling dropping direct. The reaction mixture in the three-neck flask is usually stirred vigorously and the purging with nitrogen is continued. After about 10 minutes, the stopper is put on the dropping funnel, the nitrogen flow is decreased and drop-by-drop addition with the FeCl2 solution is commenced. The rate of addition is adjusted so the entire remedy is added in 30 minutes. Then the dropping funnel stopcock is shut down and vigorous stirring in the dark green option is continued for an additional half an hour. Finally, the nitrogen flow is ended and the mixture is included with a mixture of 6M HCl (45 mL) and crushed ice (50 g). Some of the resulting slurry may be used to rinse your reaction flask to maximize the item yield. The slurry is stirred for about 15 minutes and the orange medications is collected on a Buchner or Hirsch funnel and washed with four 5-mL portions of water. The moist sound is spread out on a significant watch a glass and dried out in the air. The compound is then purified through sublimation within a large glass petri dish that is positioned on a warm hot menu (100 C). Care is utilized to avoid charring the ferrocene. The purified ferrocene is then characterized by shedding point perseverance, UV-Vis and IR spectroscopies, and cyclic voltammetry. Our company is incorporating a bulk electrolysis to generate the ferrocenium cation.

Prep of Acetylferrocene

Acetylferrocene is synthesized under mild circumstances with a adjustment of the method reported by Bozak (4). The scholars are supplied with ferrocene during the second laboratory period so that the acetylation of ferrocene may take place concurrently while using purification of ferrocene. This encourages learners to develop multi-tasking skills.

A mixture of ferrocene (1. five g) and acetic anhydride (5 mL) is prepared in a small Erlenmeyer flask. For this mixture, 85% H3PO4 (1 mL) is added dropwise with constant stirring. This kind of addition is usually exothermic which is accompanied by a difference in color. Following addition of the phosphoric acidity, the Erlenmeyer flask is usually fitted with a CaCl2 blow drying tube. The dark green remedy is then heated up in a beaker of drinking water on a hot plate for ten a few minutes (50 C). During this time, the answer becomes flower colored. The mixture can then be poured above ice (20 g) to a large beaker that will cater to the gas (CO2) created during the NaHCO3 neutralization. Drinking water is used to rinse the reaction flask and maximize the product deliver. When the glaciers has melted, small volumes of sodium bicarbonate will be added till gas development stops. The pH might be tested with pH daily news to ensure that neutrality is achieved. This is accompanied by cooling the resulting fruit solution within an ice bath for thirty minutes during which time a brown medicine forms. This precipitate is usually collected by simply suction filtration using a rough fritted channel. The dark brown solid is then washed with distilled drinking water to remove harmful particles until it is pale fruit in color. It is after that dried in air to get 15 minutes.

Thin coating chromatography can be used to improve the conditions intended for column chromatography of acetylferrocene. TLC dishes (silica gel) are provided to get student employ. Alternatively, microscopic lense slides can also be used as TLC plates by utilizing a slurry that includes silica solution (40 g) and chloroform (100 mL). A small amount of the crude acetylferrocene, which is a mono- and diacetylferrocene/ferrocene mixture, is definitely dissolved in a vial in toluene (2-3 drops). A bit of ferrocene is also dissolved in a separate vial in toluene. A range is penciled on each slip approximately one particular cm above the bottom of the TLC plate. The plates are spotted utilizing a fine capillary applicator roughly on the pen line. Every plate is going to contain two spots, you are ferrocene and one is primitive acetylferrocene. The spots are allowed to air dry and after that a second location is utilized at the same location to obtain a focused area of mixture. The id of the spot is mentioned with a pencil mark. The plates will be individually placed with the spotted end in the solvent in five producing chambers. The chambers retain the following solutions: petroleum azure, toluene, ethyl ether, ethyl acetate and a mixture of 10% ethyl acetate and 90% petroleum azure. The pen mark must be above the solvent level. The solvent containers are protected while the discs are growing. The china are removed when the solvent front offers traveled around 3/4 of the distance with the plate. The plates are air dried. The TLC discs may be designed in an iodine chamber. This will result in brownish spots that may be marked and identified so the plates might be included in a laboratory statement. The solutions that provide maximum separation of the two elements are selected as line chromatography solutions. For instance, ferrocene may elute with toluene while the acetylferrocene remains within the column and is also then eluted with a toluene/ethyl acetate mix. The color from the spots is helpful to detect the individual rings that elute from the steering column. The primitive acetylferrocene can be dissolved inside the solution that may be selected to elute the first aspect.

The column is assembled simply by placing a tiny piece of cup wool into the bottom of the column (50 mL buret). The cup wool is then covered which has a small amount of fine sand and the buret is filled with the solvent that was decided to dissolve the crude combination. A powdered funnel is employed to slowly and gradually fill the column with dry silica gel to a height of approximately 30 centimeter. The line is never in order to dry. At the same time, the line may be made by the traditional slurry method. A small amount of silica gel may be included in the crude acetylferrocene way to make a slurry that may be then included in the top in the column and covered with a small amount of sand. The two alternatives (or mixtures) are after that used to purify the elementary acetylferrocene. The ferrocene band is removed and the solvent is taken out of the acetylferrocene band by simply rotary evaporation. It may after that be recrystallized from chloroform. The acetylferrocene is characterized by melting level determination, IRGI and UV-Vis spectroscopies, and cyclic voltammetry.

Conversation

The trial and error procedure for the synthesis of ferrocene supplied above was adopted following several failed attempts to include newer microscale techniques that utilize ethylene glycol (5) as the solvent rather than 1, 2-dimethoxyethane. When ethylene glycol utilized, an extremely viscous reaction mix resulted that was not capable of being stirred effectively in the micro-glassware. Our success rate with the revised preparing is 100%.

Each of our advanced undergrad inorganic lab is educated in the term format with two three-hour weekly classes. The students discover how to multi-task to complete their laboratory responsibilities efficiently. We have provided the following suggested format (Table 1) to complete the synthesis and portrayal of ferrocene and acetylferrocene in two and a half weeks. This formatting is not provided to the students. They are innovative and they are required to post their own schedules before beginning job. The formatting allows course instructors and teaching assistants to flexibility inside the method of making certain the students make use of their period efficiently.

Table 1 . Suggested Period Management Schedule Day Program

1 Synthesis of Cp2Fe, educating assistant to supply cyclopentadiene

2 Sublimation of Cp2Fe, students are given Cp2Fe to perform the acetylation

several Thin coating and line chromatography of acetylferrocene accompanied by rotary evaporation, begin portrayal of Cp2Fe (melting point, UV-Vis, IR)

some Characterization of acetylferrocene (melting point, UV-Vis, IR), Disparition modeling

5 End characterization including cyclic voltammetry and volume electrolysis

Crude ferrocene and acetylferrocene were synthesized in 51-79% and 27-58% yield correspondingly. An fresh melting stage range of 169-171 C was obtained to get ferrocene. The reported shedding point selection is 173-174 C (3). For acetylferrocene, the fresh melting level range was 80-83 C as compared with all the reported variety of 81-83 C (7). Infrared spectroscopy was performed by the students about ferrocene and acetylferrocene both as a KBr pellet and since a Nujol mull about NaCl discs. The infrared spectra had been comparable to these reported to get ferrocene (3) and acetylferrocene (8). The key difference involving the spectra of ferrocene and acetylferrocene is of course seen a carbonyl stretch in 1736 cm-1 that is present in the acetylferrocene and lacking in the ferrocene. Some learners also noticed a peak at 893 cm-1 that is attributed to the monoacetylferrocene diamond ring. They did not really observe highs that could be related to the one particular, 2-diacetylferrocene complex at 917 cm-1 or possibly a doublet as a result of 1, 3-diacetylferrocene complex in 922 and 905 cm-1 (8). The experimental UV-Vis spectra of ferrocene and acetylferrocene were obtained in acetonitrile and Beers rules was used to calculate the molar absorptivity. The ULTRAVIOLET spectrum to get ferrocene shows maxima by 330 nm (2 = 52) and 440 nm (2 = 90), and a rising short-wavelength consumption at 225 nm (2 = 5051). This is similar with the reported spectrum in ethanol (3). The AS WELL AS spectrum pertaining to acetylferrocene reveals maxima at 219 nm (2 sama dengan 2 . a couple of x 104), 266 nm (2 sama dengan 5268) and 320 nm (2 sama dengan 1124). Apart from the worked out molar absorptivity of the peak at 219 nm, this is certainly comparable together with the reported spectrum in 95% ethanol (8). The students also observed peaks assigned to ferrocene in their acetylferrocene selections.

The electrochemistry element of this clinical was the new that most students were exposed to cyclic voltammetry and the volume electrolysis strategy. An Amel System 5000 Potentiostat utilized for all measurements. For cyclic voltammetry, the electrochemical cell was a 90 mL beaker equipped with a Ag/AgCl reference point electrode (student prepared), a BAS (West Lafayette, IN) platinum-disk operating electrode (2 mm diameter) and a large (1 cm2) platinum banner counter electrode. After having verified a set background of tetrabutylammonium hexafluorophosphate (0. 01 M) assisting electrolyte in acetonitrile in the range zero. 0 to 1. 0 Sixth is v vs . Ag/AgCl, cyclic voltammograms of ferrocene and acetylferrocene (approximately a few. 2 by 10-3 M) were attained at check rates of 100 five-hundred mV/sec. An average cyclic voltammogram of ferrocene showed a reversible oxidation for E1/2 sama dengan +0. thirty five V or Ag/AgCl with Ep/2 sama dengan 0. 057V. A typical cyclic voltammogram of acetylferrocene likewise showed a reversible oxidation in E1/2 = +0. 54.99 V or Ag/AgCl with Ep/2 sama dengan 0. 044V. Small highs for ferrocene were also obvious in the acetylferrocene cyclic voltammogram. These answers are comparable to the reported Electronic of acetylferrocene at +0. 27 V vs . the ferrocene/ferrocenium couple (6).

A second fresh electrochemical aspect that was recently introduced into this kind of laboratory is a bulk electrolysis of ferrocene to ferrocenium. The electrochemical cell was a 100 milliliters beaker equipped with an Ag/AgCl reference electrode (student prepared), a DÉBAUCHÉ (West Lafayette, IN) reticulated vitreous carbon (RVC) functioning electrode and an extremely large platinum banner counter electrode. After having verified a set background of tetrabutylammonium hexafluorophosphate (0. 01 M) assisting electrolyte in acetonitrile in the range 0. 0 to 1. 0 Versus vs . Ag/AgCl, the bulk electrolysis of ferrocene (approximately 7. 5 by 10-4 M) was obtained on a number of occasions. As expected, a new peak in the UV-Vis was observed at 620 nm as well as the solution changed color by orange to blue. Sadly to date, these kinds of experimental circumstances are not reproducible.

Like a supplement to their standard chemical substance characterization, learners used cache memory molecular building program to build a ferrocene molecule in both the eclipsed and staggered conformations and also to remove an electron to acquire information about the ferrocenium cation. The results on this modeling had been then discussed in relation to their very own experimental findings.

When the students have got synthesized and derivatized ferrocene, they have an experimental background for a comparison of the unsubstituted ferrocene compared to acetylated ferrocene. They also have a understanding of the actual R groups that are chemically practical. This is particularly meaningful in the event the student features completed organic and natural chemistry and is able to relate the familiar benzene substituents with the ferrocene molecule. We now have found that if a scholar proceeds throughout the iterative problem before understanding the acetylation test, they design strange, marvelous and not practical molecules with the aid of the CAChe system. It must be anxious that molecular modeling is merely a tool. The input is influenced to a large level by the comprehension of the agent which may be enhanced with advice from the trainer.

A natural progression in the completion of the 2 syntheses is a introduction of the iterative problem. Students will be asked to design a ferrocene with particular properties such as a different shaded ferrocene. This question is answered using CAChe modeling where electronic spectra from the gas period ferrocene as well as the substituted ferrocene may be generated by ZINDO (Zerners Intermediate Neglect of Differential Overlap). A more thorough iterative task involves equally library function and molecular modeling. The scholars are asked to find the planning of a replaced ferrocene inside the library. They could also design and style a synthesis and what is synthesis with the aid of library sources. They then style the complicated and foresee its spectroscopic characteristics based on what they are able to calculate from your molecular unit and their understanding of general substance trends.

Since the college students became familiar with cyclic voltammetry, one craze of interest requires the ionization potential with the substituted ferrocenes. One student project engaged a comparison of several noted substituted ferrocenes (6) and their gas stage models (Figure 1 and Table 2). The gas phase versions were applied since the anticipated solvent dependence has not been noticed using the Disparition system as a result of initial restrictions with project leader. The first calculated ionization potentials had been adjusted by subtracting 7. 647 eV. This sets the ferrocene/ferrocenium couple for zero ones own customary (6). These principles and a least pieces regression plan were in that case plotted. In general, a downward trend in the least squares regression is observed with the more readily reduced ferrocenes containing electron withdrawing substituents having confident ionization potentials. Conversely, a lot more easily oxidized ferrocenes with electron giving substituents are calculated with negative ionization potentials. Deviations from experimental data might be accounted for since the student was comparing gas phase ferrocene models and acetonitrile ferrocene electrochemistry (6).

Fig. 1 . Student CAChe Job

Desk 2 Pupil CAChe Job

Summary

The incorporation of an iterative question in to each of our advanced inorganic undergrad laboratories has allowed students to plumb the depths with their chemical understanding and to get new tools that improve their use of the scientific method. The students enjoy the high success rate of the ferrocene/acetylferrocene lab. They also appreciate the possibility to acquire fresh synthetic techniques such as the make use of Schlenk tactics. In addition , the use of novel instrumental analysis just like electrochemistry is beneficial to their total undergraduate education. They manage to thrive within the diverse direct exposure and the possibility to stretch themselves. This allows these to become pumped up about chemistry and like the try things out that they are performing, they come complete circle and view chemistry in a new light being a useful, valuable tool. Digging in the iterative question to a classical lab can for that reason provide an further richness to the traditional rainy chemistry.

Acknowledgments

Research supported by NSF under Scholarships # DUE-9452023 and DUE-9452131.

Books Cited

1 ) Kauffman, George B. J. Chem. Educ. 1983, 70, 185.

2 . Kealy, T. J., Pauson, P. T. Nature 1951, 168, 1039.

3. Jolly, W. D., The Synthesis and Characterization of Inorganic Compounds, Prentice-Hall: New Jersey, 70.

4. Bozak, R. At the. J. Chem. Educ. 1966, 43, 73.

5. Szafran, Z., Pike, R. Meters., Singh, M. M., Microscale Inorganic Chemistry, Wiley: New york city, 1991.

six. Geiger, Bill E. T. Organomet. Chem. 1990, twenty two, 142.

six. Wade, Leroy G. M. Chem. Educ. 1978, fifty five, 208.

almost eight. Rosenblum, Myron, Chemistry of the Iron Group Metallocenes: Ferrocene, Ruthenocene, Osmocene Part A single, Interscience Publishers: New York, 1965.

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