In the Laboratory
Synthesis and Characterization of Transition-Metal Complexes Containing 1,1´-Bis(diphenylphosphino)ferrocene Chip Nataro* and Stephanie M. Fosbenner Department of Chemistry, Lafayette College, Easton, PA 18042; *
[email protected] Ferrocene, C10H10, is among the most widely used organotransition-metal compounds.1 Ferrocene has been employed in a variety of applications, but is most commonly thought of as a standard in electrochemical studies (1). An additional characteristic of ferrocene that is of particular importance is the ease of preparing ferrocene derivatives. Substitution on the cyclopentadienyl rings of ferrocene can be carried out using an assortment of reagents to give products with varying substitution patterns. One such derivative is 1,1′-bis(diphenylphosphino)ferrocene (dppf ). The primary use of dppf is as a ligand in catalytic applications (2–4). The phosphorus atoms in dppf can bond to another transition-metal center bringing the iron of dppf into close proximity of another metal center. The proximity of two metal centers is likely a factor in some of the unique reactivity observed for complexes containing dppf (2–4). Although commercially available, a previous report (5) has outlined an excellent synthesis of dppf that provides students the opportunity to handle organolithium reagents2 under an inert atmosphere,
Fe(C5H5)2 + 2BuLi
TMEDA hexanes
Fe(C5H4Li)2 2 PPh2Cl
(1)
Fe(C5H4PPh2)2 where TMEDA is tetramethylethylenediamine. Upon purification of dppf, students are directed to characterize the product by 1H NMR. As an additional study students can prepare NiCl2(dppf ) and consider the possible geometries this complex can adopt. The students are left to mull over the consequences of these different structures without actually performing characterization of NiCl2(dppf ). They could be provided with the proper structure or conduct a search of the literature, but both of these options deprive the students of an opportunity to examine data they obtain and to determine the structure on their own. This experiment expands upon the previously published laboratory exercise (5). Students prepare different metal complexes with a general formula of MCl2(dppf ). The fourcoordinate complexes are either tetrahedral or square planar. The geometry affects the electron configuration of the metal, in particular the number of unpaired electrons, and in turn the magnetic properties of the complex.3 Students characterize the complex they prepare by 1H and 31P{1H} NMR and UV–vis spectroscopy and use the information to determine the geometry of the complex. Students also investigate the electrochemical properties of their complex. 1412
The goals of this experiment are
• To prepare a metal complex of the general formula MCl2(dppf ).
• To characterize the products using UV–vis spectroscopy.
• To characterize their complex using a variety of NMR techniques.
• To use cyclic voltammetry (CV) to study the oxidative electrochemistry of their complex.
• To present students with potential projects for further study.
Complex Synthesis The dppf can either be prepared in a previous laboratory period (5) or purchased. Each student (or group) is responsible for the synthesis of one of the five complexes with a formula MCl2(dppf ) (M = Co, Ni, Pd, Pt, or Zn). Although higher yields can be obtained by performing the syntheses under an inert atmosphere, it is not required. The Co, Ni, and Zn complexes are prepared using a similar method (6). The metal dichloride (CoCl2⋅6H2O, NiCl2⋅4H2O, or ZnCl2) is dissolved in a minimal quantity of a 2:1 mixture of 2-propanol and methanol. This is added to a solution of dppf in warm 2-propanol. The reaction is refluxed for two hours and then cooled to room temperature. The product precipitates and is collected by filtration. Washing the solid with a minimal quantity of methanol followed by drying yields the product in sufficient purity for further studies. The Pd and Pt complexes are prepared from cisMCl2(nitrile)2 (nitrile = acetonitrile or benzonitrile) (6). A solution of the cis-MCl2(nitrile)2 in toluene is added to a solution of dppf in toluene. The reaction is stirred for 12 hours during which time the product precipitates. The product is collected by filtration, washed with a minimal quantity of toluene, and dried. The purity is adequate for further studies. Hazards Toluene, 2-propanol, and ferrocene are flammable and harmful. Ethanol is flammable. Chloroform-d is a cancer suspect agent, so appropriate precautions should be used. Dichloromethane, tetrabutylammonium hexafluorophsophate, bis(acetonitrile)dichloroplatinum, and PdCl2(dppf ) are harmful. CoCl2⋅6H2O and NiCl2⋅4H2O are toxic and harmful for the environment. Bis(acetonitrile)dichloropalladium and NiCl2(dppf ) are toxic. Zinc chloride is corrosive and harmful for the environment. While there are no known precautions for dppf, CoCl2(dppf ), PtCl2(dppf ), and ZnCl2(dppf ), appropriate
Journal of Chemical Education • Vol. 86 No. 12 December 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
care should be used. The synthesis should be performed in a wellventilated hood away from any possible ignition sources. UV–Vis Spectroscopy All of the compounds should be characterized by UV–vis spectroscopy. Most of the compounds are significantly different in color from dppf, so students can determine that they have formed the product by visual inspection. The UV–vis spectrum of each compound should be determined using dichloromethane as the solvent.
Table 1. 31P{1H} NMR Data and Number of Unpaired Electrons 31P
Compound dppf
–16.8
0
CoCl2(dppf)
—
3
NiCl2(dppf)
—
2
PdCl2(dppf)
34.0
0
PtCl2(dppf)
13.1b
0
ZnCl2(dppf)
The compounds are characterized by NMR spectroscopy. The Co and Ni complexes are paramagnetic, and the Pd, Pt, and Zn complexes are diamagnetic. Having reviewed the orbital splitting patterns for tetrahedral and square-planar geometries, the students with the Co complexes should be aware that their compounds are paramagnetic regardless of the geometry. These students perform the Evans method with their sample,4 so they are required to prepare a sample with a known concentration and insert a sealed capillary of CDCl3 into their sample (7–11). These students still attempt to obtain a 31P{1H} NMR spectrum, but it quickly becomes evident that they will not obtain any useful information from this experiment. After obtaining the 1H NMR spectrum, the students determine the number of unpaired electrons (Table 1) using the Evans method calculations (8). They should obtain a magnetic moment of approximately 4.5 μB (5). The magnetic moment and the UV–vis spectrum provide sufficient information for the students to determine that the compound is tetrahedral (12). Students can confirm the tetrahedral geometry from the X-ray structure (13). The students who prepared the Zn complex should realize that their compound is diamagnetic regardless of the geometry; therefore, they do not need to know the concentration of their sample. For comparison, students can either obtain or be given the 1H and 31P{1H} NMR spectra of dppf. There is little difference in the chemical shifts observed in the 1H NMR spectra of the free dppf and the dppf coordinated to Zn. However, the difference in the chemical shifts in the 31P{1H} NMR spectra for the free and coordinated dppf, while relatively small (Table 1), is large enough to confirm formation of the complex. The NMR results are not enough for the students to determine the structure of ZnCl2(dppf ). However, as the two other 3d metal complexes synthesized in this lab are tetrahedral, it would be reasonable for the students to suggest that ZnCl2(dppf ) is also tetrahedral (6, 14). Students who prepare the Ni, Pd, or Pt complexes conclude that if their compound is tetrahedral it will be paramagnetic but if it is square planar it will be diamagnetic. How the students go about preparing their sample is left to the discretion of the instructor. If time and instrument access are not an issue, it is better to have the students assume their complex is either paramagnetic or diamagnetic. If they assume it is paramagnetic, they accurately determine the concentration of their sample and insert a sealed capillary of CDCl3, neither of which will affect the spectra if their complex is diamagnetic. If they assume their complex is diamagnetic, the 31P{1H} NMR spectrum quickly confirms or disproves their hypothesis. If time or instrument access are a concern, it would be best to inform the students that
aIn
–21.3
CH2Cl2. Data from ref 6.
b 1J P–Pt
0
= 3770 Hz.
Detector Response
NMR Spectroscopy
Unpaired Electrons
NMR Shift (ppm)a
25
20
15
10
5
0
Chemical Shift (ppm) Figure 1.
31P{1H}
NMR Spectrum of PtCl2(dppf).
the Ni complex is tetrahedral and therefore paramagnetic. The magnetic moment for the Ni complex should be approximately 3.7 μB (6). While all of these complexes provide students with the opportunity to perform NMR on nuclei other than 1H and 13C, it is the Pt complex that offers the most valuable experience. Because there is a single signal for the two equivalent phosphorus atoms in PtCl2(dppf ), this compound serves as an excellent starting point for exploring multinuclear NMR. Students seem to have a difficult time grasping the idea that there can be coupling between different nuclei, for example, phosphorus– fluorine. This is most likely an unfortunate and unintended consequence of focusing on proton–proton coupling in organic chemistry. In PtCl2(dppf ) the student is forced to confront the coupling between phosphorus and platinum. Initially students will view the 31P NMR spectrum of PtCl2(dppf ) as three singlets (Figure 1). Upon closer inspection they may question why the two smaller “singlets” are the same height and equally spaced around the central peak. At this point they can be introduced to the impact of NMR active nuclei that are not 100% abundant on coupling patterns (15).
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 12 December 2009 • Journal of Chemical Education
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In the Laboratory
NMR and electrochemistry. Compared to the dppf analogues, the 31P NMR spectra for the dippf complexes should occur downfield while the potentials at which oxidation occur should be less positive (14).
Reversible Oxidation − Red
−e
+e−
Ox
Chemically Irreversible Oxidation Red
−e−
Ox
Z
Chemically Reversible Oxidation − Red
−e
+e−
Ox
Z
Scheme I. Mechanism involved in electrochemical reactions.
Cyclic Voltammetry The potential at which oxidation of dppf occurs is approximately 0.18 V versus FcH0/+ (6). The oxidation is complicated by a follow-up reaction (6, 16). A detailed examination of this process, which is called an EC mechanism (16, 17), is likely beyond the scope of an inorganic laboratory; however, a brief explanation is warranted. For any cyclic voltammetry (CV) in which anodic electrochemistry is performed, the reduced form of the analyte (Red) is oxidized (Ox) (Scheme I) during the forward sweep. For a chemically and electrochemically reversible oxidation, Ox is stable on the CV time scale so the ratio of the cathodic and anodic peak currents will be approximately one. However, if Ox is unstable on the CV time scale, there is a competition between Ox decomposing to form Z and Ox being reduced back to Red. If the rate of formation of Z is greater than the rate of electron transfer, there will be no cathodic wave in the CV (i.e., a chemically irreversible system). If the rates of Z formation and electron transfer are similar, a cathodic wave will be observed; however, the ratio of the peak currents will be less than one (i.e., a chemically reversible system). Upon coordination the potential at which dppf oxidation occurs is approximately 0.40 V more positive than that of free dppf (6). Oxidation of the Pd and Pt complexes each exhibit a single, chemically and electrochemically reversible wave. Oxidation of the Co, Ni, and Zn complexes is either chemically irreversible or chemically reversible (6). A detailed analysis (6) of the difference is not essential, although students should be able to conclude that the complexes that display reversible electrochemistry are square planar while the more complicated electrochemistry is associated with the tetrahedral complexes. Additional Studies If this laboratory is used in a more advanced course,5 students can fully examine and assign the electronic transitions for their compounds in the UV–vis spectrum. Although slightly more air-sensitive than dppf, the isopropyl analogue, 1,1´-bis(diisopropylphosphino)ferrocene (dippf ), is commercially available. If a freshly purchased bottle is used, the syntheses described for dppf can also be carried out with dippf. The yields tend to be slightly lower although still more than sufficient for
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Conclusion This lab provides students with the opportunity to synthesize and characterize compounds and examine the oxidative electrochemistry of the compounds they prepare. Crystal (ligand) field theory, multinuclear NMR, and cyclic voltammetry are all introduced in this exercise. Acknowledgments We thank the donors to the Petroleum Research Fund administered by the American Chemical Society for partial support of this work and the Kresge Foundation for the purchase of the Jeol NMR. CN wishes to thank the reviewers for the insights and suggestions for improving this lab. Notes 1. Searching the structure of ferrocene using SciFinder Scholar returned 9821 references on April 10, 2009. 2. The synthesis of dppf should only be attempted in an advanced lab course because of the use of organolithium reagents. 3. Lafayette College only offers inorganic lab with our secondyear inorganic course. Crystal field theory and ligand field theory are topics addressed in the fourth-year inorganic course, so students performing this lab are not familiar with these concepts prior to this lab. 4. The magnetic susceptibility of these compounds can also be measured using a magnetic susceptibility balance. 5. Electronic spectra are not covered in the second-year inorganic course associated with this lab. For a more advanced lab, obtaining and interpreting the visible spectra is a very useful addition to this lab exercise.
Literature Cited 1. Geiger, W. E. Organometallics 2008, 26, 5738–5765. 2. Chien, S. W.; Hor, T. S. A. The Coordination and Homogeneous Catalytic Chemistry of 1,1´-Bis(diphenylphosphino)ferrocene and Its Chalcogenide Derivatives. In Ferrocenes: Ligands, Materials and Biomolecules, Štěpnička, P., Ed.; John Wiley and Sons, Ltd.: West Sussex, UK, 2008; pp 33–116. 3. Gan, K.-S.; Hor, T. S. A. 1,1´-Bis(diphenylphosphino)ferrocene. Coordination Chemistry, Organic Syntheses and Catalysis. In Ferrocene, Togni, A., Hayashi, T., Eds.; VCH: New York, 1995; pp 3–104. 4. Martinak, S. L.; Sites, L. A.; Kolb, S. J.; Bocage, K. M.; McNamara, W. R.; Rheingold, A. L.; Golen, J. A.; Nataro, C. J. Organomet. Chem. 2006, 691, 3627–3632; and references therein. 5. Butler, I. R. The Use of Organolithium Reagents in the Preparation of Ferrocene Derivatives. In Inorganic Experiments, 2nd ed., Woollins, J. D., Ed.; Wiley-VCH: Weinheim, Germany, 2003; pp 88–94. 6. Corain, B.; Longato, B.; Favero, G.; Ajò, D.; Pilloni, G.; Russo, U.; Kreissl, F. R. Inorg. Chim. Acta 1989, 157, 259–266. 7. Evans, D. F. J. Chem. Soc. 1959, 2003–2005.
Journal of Chemical Education • Vol. 86 No. 12 December 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory 8. Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. Synthesis and Technique in Inorganic Chemistry, 3rd ed.; University Science Books: Sausalito, CA, 1999; pp 117–130. 9. Ostfeld, D.; Cohen, I. A. J. Chem. Educ. 1972, 49, 829. 10. Sur, S. K. J. Magn. Reson. 1989, 82, 169–173. 11. Schubert, E. M. J. Chem. Educ. 1992, 69, 62. 12. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Oxford, 2001; pp 1129–1133. 13. Park, T.-J.; Huh, S.; Kim, Y.; Jun, M.-J. Acta Crystallogr. Sect. C: Cryst. Struct. Commun. 1999, 55, 848–850. 14. Although the structure of ZnCl2(dppf ) has not been determined, the structure of the closely related ZnCl2(1,1´-bis(diisopropylphosphino)ferrocene) has been determined to be tetrahedral. Ong, J. H. L.; Nataro, C.; Golen, J. A.; Rheingold, A. L. Organometallics 2003, 22, 5027–5032. 15. Nataro, C.; McNamara, W. R.; Maddox, A. F. When Nuclei Cannot Give 100%. In Modern NMR Spectroscopy in Educa-
tion; Rovnyak, D., Stockland, R., Eds.; ACS Symposium Series 969; American Chemical Society: Washington, DC, 2007; pp 246–275. 16. Mabbott, G. A. J. Chem. Educ. 1983, 60, 697–702. 17. Van Benschoten, J. J.; Lewis, J. Y.; Heineman, W. R.; Roston, D. A.; Kissinger, P. T. J. Chem. Educ. 1983, 60, 772–776.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Dec/abs1412.html Abstract and keywords Full text (PDF)
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Student handouts including prelab questions
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