Laboratory Experiment pubs.acs.org/jchemeduc
Synthesis and Migratory-Insertion Reactivity of CpMo(CO)3(CH3): Small-Scale Organometallic Preparations Utilizing Modern Glovebox Techniques Matthew T. Whited* and Gretchen E. Hofmeister Department of Chemistry, Carleton College, Northfield, Minnesota 5505, United States S Supporting Information *
ABSTRACT: Experiments are described for the reliable small-scale glovebox preparation of CpMo(CO)3(CH3) and acetyl derivatives thereof through phosphine-induced migratory insertion. The robust syntheses introduce students to a variety of organometallic reaction mechanisms and glovebox techniques, and they are easily carried out during three laboratory meetings. Furthermore, the series of complexes offers several interesting case studies for symmetry analysis of IR and NMR spectra.
KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Group Theory/Symmetry, IR Spectroscopy, NMR Spectroscopy, Organometallics, Synthesis
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Herein, an adaptation of Winter’s CpMo experiments to a series of reliable, small-scale (50−200 mg) preparations is reported that is performed entirely in a glovebox. Lithium triethylborohydride is used as a reductant,13 which avoids mercury and the potential for generating highly toxic organomercury compounds. Because students prepare CpMo(CO)3(CH3) in an inert-atmosphere glovebox, purification of this air-sensitive intermediate by sublimation is unnecessary. The experiments may be easily completed over three 4-h laboratory meetings, and IR and NMR spectra of the reactants, intermediates, and final products provide useful case studies for the role of symmetry in determining organometallic structures. Major pedagogical goals for this set of laboratory experiments are · To introduce students to basic processes for performing small-scale preparations in an inert-atmosphere glovebox · To help students apply knowledge of fundamental organometallic transformations to predict products in a laboratory setting · To help students apply symmetry and group theory considerations to interpretation of the IR and NMR spectra and propose geometries for each reactant or product that are consistent with these spectra and relevant primary literature.
he preparation, characterization, and analysis of organometallic complexes have become indispensable tools for synthetic chemists, as demonstrated by three recent Nobel Prizes for work in the field of organometallic catalysis.1−3 Most organometallic compounds are air-sensitive, and students are often introduced to Schlenk techniques for handling them in the upper-division undergraduate laboratory.4−7 Because such synthesis and analysis are routinely performed on small scales using modern glovebox techniques, a robust set of laboratory experiments is needed to expose advanced undergraduate students to a variety of organometallic reaction classes using these techniques. In this article, a series of experiments based on cyclopentadienyl-supported molybdenum (CpMo) complexes are presented. To the best of our knowledge, only two experiments that use glovebox techniques to prepare airsensitive products have been published in this Journal.8,9 Several years ago, a series of laboratory experiments from Caltech10,11 was adapted using the CpFe(CO)2 (“Fp”) fragment to introduce students to several organometallic reactions and the importance of symmetry considerations in the characterization of the resultant molecules. However, students were continually frustrated by low yields, and it was desirable to move away from the use of sodium/mercury amalgam required for reducing the Fp dimer. Winter has described a less-sensitive relative of the Fp experiment using the CpMo framework, but his procedures also call for the use of sodium/mercury amalgam as a reductant and sublimation of an important intermediate.12 © 2014 American Chemical Society and Division of Chemical Education, Inc.
Published: May 8, 2014 1050
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EXPERIMENTAL PROCEDURES
and students should assume that all products are toxic. Students and instructors should consult the MSDS for each chemical prior to laboratory work, and waste should be collected and disposed of according to safety regulations.
Synthesis of Mo(II) Methyl and Acetyl Complexes
Starting with commercially available [CpMo(CO)3]2, phosphine-supported Mo(II) acetyl complexes of the general form trans-CpMo(PR3)(CO)2(COCH3) can be prepared in three steps (Scheme 1, see Supporting Information for full details).
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RESULTS AND DISCUSSION This set of experiments has been run successfully with more than 60 students over a period of four years in the “Laboratory in Advanced Inorganic Chemistry” course at Carleton College. Yields for all three steps are routinely high (>50%) and excellent spectroscopic purities are obtained by nearly all students (see Supporting Information for representative student spectra of all compounds). The preparations reported have been optimized for small-scale preparations in an inertatmosphere glovebox, and syntheses are routinely performed in 20 mL disposable scintillation vials. The glovebox is equipped with a −35 °C freezer, purge valve, and vacuum inlet for performing filtrations and removing solvent, but the freezer is not necessary for the procedure reported. Use of vacuum in the glovebox is an important tool in this set of preparations, and most gloveboxes can easily be retrofitted to provide a vacuum inlet connected to an external pump and solvent trap if one is not available. In the Supporting Information, suggestions are provided for vacuum setup in a glovebox to facilitate student use in curricular laboratories. The phosphine-induced migratory-insertion reaction is quite sensitive to solvent, and the reaction is best performed in acetonitrile. In diethyl ether, a much poorer donor than acetonitrile, the reaction proceeded exceptionally slowly (ca. 50% conversion over the period of 1 week as opposed to full conversion in less than 1 day in acetonitrile). The importance of solvent can be explained by the equilibrium that likely precedes phosphine association (Scheme 2). As with related
Scheme 1. General Synthetic Scheme for Preparation of Mo(II) Methyl and Acetyl Complexes
Following a related procedure by Gladysz and co-workers,13 lithium triethylborohydride solution (“Super Hydride”) is used to reduce [CpMo(CO)3]2. The reaction occurs quickly at ambient temperature, with concomitant evolution of hydrogen gas (Scheme 1). The Mo(0) anion is intercepted with methyl iodide, via an SN2-type oxidative addition, to form CpMo(CO)3(CH3),14 which is then isolated as a pure yellow solid in reasonable to good yields (typically 60−80%) following extraction into pentane and filtration through activated alumina.12 The final acetyl complex is prepared by phosphine-induced migratory insertion of a carbonyl ligand into the Mo−CH3 bond. Students most commonly run this reaction using triphenylphosphine, but several student groups (ca. 10 students total) have used methyldiphenylphosphine successfully (quantitative yield by 31P NMR and >60% recovered yields). As an indication of the generality of the synthesis, several student groups have used different phosphines in parallel independentstudy style experiments.15,16
Scheme 2. Coordinating Solvents Favor Migratory Insertion by Raising the Concentration of the Acetyl Intermediate Available for Trapping by Phosphine
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HAZARDS Care should be taken to avoid exposure to all chemicals used in this laboratory, and performing the syntheses in the glovebox should minimize student exposure. The tetrahydrofuran, acetonitrile, pentane, and deuterated NMR solvents utilized are flammable and harmful. Special care should be taken with iodomethane, which is toxic and volatile, to avoid contamination of the glovebox and exposure of researchers during removal of materials from the glovebox. As a result, it is recommended that waste containing iodomethane be kept separate and sealed prior to removal from the glovebox, then immediately transferred to a fume hood for disposal. Lithium triethylborohydride solution is extremely reactive with moisture and should be handled only in an inert atmosphere and quenched with 2-propanol/toluene, if necessary. Triethylborane, which is formed as a byproduct of the lithium triethylborohydride reduction, is flammable, corrosive, toxic, and volatile. It is removed under vacuum during work-up of CpMo(CO)3(CH3), so it is recommended that the solvent trap be immediately moved to a fume hood once syntheses are complete. Hazards for organometallic products are unknown,
reactions, migratory insertion of a ligand leaves a vacant coordination site.17 Although the uninserted methyl species is thermodynamically favored, the equilibrium can be shifted by solvent coordination. The solvated migratory insertion product may react with a phosphine ligand, trapping the acetyl complex. In the absence of a strongly coordinating solvent, the concentration of Mo(II) acetyl is exceptionally low, so the reaction proceeds slowly. Assessment of student gains from this set of experiments has been performed in both formative and summative manners. The laboratory was performed after students had been briefly introduced to classes of organometallic reactions. Though student instructions provided some guidance (see Supporting Information), these were by no means exhaustive. This allowed 1051
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the instructors to prompt students to write balanced equations or propose mechanisms to emphasize the link between textbook reactions and the experiments they were performing. For example, student instructions provided a general scheme for the reduction of [CpMo(CO)3]2 to [CpMo(CO)3]− using LiEt3BH (Scheme 1), omitting the H2 and Et3B byproducts. In the laboratory, students were asked what the reductant was and what the byproduct of [CpMo(CO)3]− formation was. This enabled students to predict that gas formation should be observed, and students routinely expressed amazement when they saw their predictions confirmed by a vigorously bubbling solution during the reduction. Student knowledge of glovebox theory and operation was also routinely assessed during the laboratory, as students were asked why they were performing steps in a particular way (e.g., performing three pump−backfill cycles on all materials brought into the glovebox antechamber) to strengthen their confidence in using the equipment. The experiments ran for a period of 3 weeks, during which time students were given increasing independence. Following the three-week series of preparations, students prepared lab reports with journal-style experimental sections and symmetry analysis of their products using NMR and IR spectral data. The experimental sections were used to assess student understanding of experimental procedures performed, as well as operation of the glovebox, while spectral analysis and geometry assignments provided the opportunity for students to apply standard principles of symmetry and group theory to a real chemical problem.18 Students supported their proposed structures using related reports from the primary literature and by consideration of plausible structures that were inconsistent with spectral data. The series of complexes presented in this set of experiments provided a useful context for students to use symmetry and group-theory arguments in the interpretation of spectral data. Students began by analyzing the carbonyl region of the infrared spectrum of the dimeric [CpMo(CO)3]2 starting material (Figure S1). There are three obvious CO stretches, all >1900 cm−1 in dichloromethane solution, indicating that there are no bridging carbonyl ligands. The fact that there are only three CO stretches for six carbonyl ligands implied a high degree of symmetry, and students ultimately concluded that the complex was likely to be centrosymmetric but did not contain a 3-fold rotational axis. The proposed structure corresponded with the reported single-crystal X-ray structure of the dimer,19 which has C2h symmetry. Other likely isomers with C2v or C2 symmetry would exhibit 5 or 6 IR-active CO stretching modes, respectively.20,21 A search of relevant primary literature via SciFinder22 or the Cambridge Crystallographic Database23 allowed students to predict that the methyl complex CpMo(CO)3(CH3) should adopt a four-legged piano-stool geometry with two equivalent carbonyl ligands. This geometry was supported by 13C NMR spectroscopy, wherein two Mo−CO resonances (δ 240.0 and 226.9 ppm in CDCl3) were observed. On the basis of a grouptheory analysis of the Cs structure, students predicted the presence of three IR-active stretches (2A′ + A″) for the methyl complex, but in fact only two bands were observed (Figure 1). The lower-energy band was broadened and slightly split, consistent with overlapping A′ and A″ modes.24 When analyzing the data for CpMo(CO)3(CH3), students were confronted with seemingly contradictory spectral data and
Figure 1. Representative student infrared spectrum of CpMo(CO)3(CH3) in CH2Cl2, with overlapping A′ and A″ CO stretching modes labeled.
concluded that they were unable to resolve all of the IR-active modes predicted by group theory. The structure of the migratory-insertion product, transCpMo(PPh3)(CO)2(COCH3), may also be elucidated by NMR. Two isomers, in which the CO ligands are cis and trans, can be envisioned. Although both should exhibit IR-active symmetric and antisymmetric stretching modes, the trans product should have only one M−CO resonance by 13C NMR, whereas the cis product would have two. The 13C resonances for the carbonyl and acetyl carbons are also doublets due to two-bond coupling to 31P. In most cases (>90%), students were able to assign all resonances in the 1H, 13C, and 31P NMR spectra, frequently utilizing the primary literature and reference spectra collected for the free phosphines. In nearly all cases students proposed the correct final structure and provided multiple literature examples in support of their claims.
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CONCLUSIONS A reliable synthesis of phosphine-supported CpMo(II) acetyl complexes has been presented, introducing students to smallscale glovebox manipulations involving several fundamental organometallic transformations: chemical reduction, oxidative addition, and migratory insertion. These experiments provided a valuable opportunity for undergraduate students to perform and analyze organometallic transformations in an environment and on a scale that was relevant for practicing organometallic chemists. The procedures were completed during three 4-h laboratory periods, and the products were well-suited for analysis by infrared and multinuclear NMR spectroscopy.
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ASSOCIATED CONTENT
S Supporting Information *
Handouts and instructions for students, full synthetic procedures, sample student spectra for all products, implementation notes, and recommendations for vacuum setup in a glovebox. This material is available via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 1052
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Notes
(18) Harris, D. C.; Bertolucci, M. D. Symmetry and Spectroscopy. An Introduction to Vibrational and Electronic Spectroscopy; Dover Publications, Inc.: New York, 1978. (19) Adams, R. D.; Collins, D. M.; Cotton, F. A. Molecular Structures and Barriers to Internal Rotations in Bis(η 5 cyclopentadienyl)hexacarbonylditungsten and its Molybdenum Analog. Inorg. Chem. 1974, 13 (5), 1086−1090. (20) Cotton has noted that [CpMo(CO)3]2 exists as a mixture of rotamers in polar solution, so the highest-energy CO stretch in its IR spectrum (CH2Cl2 solvent) is probably due to the totally symmetric stretching mode of the minor cis rotamer and the third predicted stretch of the C2h isomer is actually buried under the slightly broadened stretch at 1912 cm−1.19 (21) Adams, R. D.; Cotton, F. A. Structural and Dynamic Properties of Dicyclopentadienylhexacarbonyldimolybdenum in Various Solvents. Inorg. Chim. Acta 1973, 7 (1), 153−156. (22) SciFinder Scholar. Chemical Abstracts Service: Columbus, OH, 2014. (23) Allen, F. H. The Cambridge Structural Database: A Quarter of a Million Crystal Structures and Rising. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58 (3), 380−388. (24) King, R. B.; Houk, L. W. Carbonyl Stretching Frequencies and Force Constants in Cyclopentadienylmolybdenum Tricarbonyl Derivatives. Can. J. Chem. 1969, 47 (16), 2959−2964.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from the Department of Chemistry at Carleton College, as well as additional funding through a grant to Carleton College from the Howard Hughes Medical Institute. Marion Cass is also acknowledged for helpful discussions and input in these procedures. We are grateful to the numerous students who have helped test and refine these procedures.
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REFERENCES
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