Straightforward Preparation Method for Complexes Bearing a

Laboratory Experiment pubs.acs.org/jchemeduc

Straightforward Preparation Method for Complexes Bearing a Bidentate N‑Heterocyclic Carbene To Introduce Undergraduate Students to Research Methodology Alberto Fernández,* Margarita López-Torres, Jesús J. Fernández, Digna Vázquez-García, and Ismael Marcos Departamento de Química Fundamental, Facultade de Ciencias e Centro de Investigacións Científicas Avanzadas (CICA), Universidade da Coruña, 15071 A Coruña, Spain S Supporting Information *

ABSTRACT: A laboratory experiment for students in advanced inorganic chemistry is described. In this experiment, students prepare two metal complexes with a potentially bidentate-carbene ligand. The complexes are synthesized by reaction of a bisimidazolium salt with silver(I) oxide or palladium(II) acetate. Silver and palladium complexes are binuclear and mononuclear species, respectively, in which the carbene ligand behaves in a different coordination mode. The reaction conditions are straightforward and the complexes air-stable species, even though the palladium compound must be prepared using air-free techniques. Students must also assign the signals in the 1H and 13C-{1H} NMR spectra. The complexes showed a fluxional behavior in solution as can be deduced from their 1H NMR spectra. The students are given the 3D structures of the complexes as an aid to understand this dynamic behavior. The students should also use the FAB−mass spectrum of the silver complex and the concept of isotopic pattern in order to discriminate between a mononuclear and a binuclear structure. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Hands-On Learning/Manipulation, Inorganic Chemistry, Microscale Lab, Organometallics, NMR Spectroscopy, Synthesis

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occasions, the use of advanced equipment, for example, glove boxes, is necessary. This is the reason why preparation of such species in an undergraduate laboratory is often avoided. The discovery of the more stable NHCs8 and their metal complexes opened the door to the design of new synthetic experiments that could be carried out by relatively inexperienced students using ordinary laboratory equipment. For instance, the preparation of silver carbenes does not require, in most of the cases, the use of an inert atmosphere. In other cases, as in the preparation of the palladium complex herein synthesized, the use of an inert atmosphere is needed, but the syntheses withstand small amounts of moisture or oxygen with reasonable yields. On the other hand, complexes with NHCs are frequently prepared using imidazolium salts, which are stable under benchtop conditions, as carbene precursors. These salts are easily deprotonated by treating them with an auxiliary base prior to the reaction with the metal compound, and in some cases, even the aid of the external base is unnecessary if a basic metal salt is used, simplifying the synthesis.

ince their discovery, the metal complexes derived from carbene ligands have garnered a growing interest among the organometallic community due to their numerous applications, mainly as catalysts.1,2 In addition, from the teaching perspective, the complexation of a disubstituted carbon atom has interesting bonding implications.3 Carbenes were initially classified according to the nature of the substituents at the carbenic carbon in Schrock and Fischer carbenes, classification that is also supported by the distinct properties of their metal complexes. On the other hand, the development of the more stable N-heterocyclic carbenes (NHCs), which can be regarded as a subclass of Fischer carbenes, and their metal complexes was a milestone in organometallic chemistry.4 They show numerous applications as catalysts,5 some of which led Robert H. Grubbs, Yves Chauvin, and Richard R. Schrock to be awarded the Nobel Prize in 2005.6 However, catalysis is not the only application of these species since their uses in other fields, such as medicinal chemistry, have an enormous potential.7 Thus, their study in any organometallic course is welljustified. Nevertheless, most carbenes are highly reactive and unstable species. Consequently, the preparation of these species and their complexes requires synthetic talent, and in many © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: July 15, 2016 Revised: July 7, 2017

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DOI: 10.1021/acs.jchemed.6b00523 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Scheme 1. Preparation of 1,1′-Dimethyl-3,3′-methylenediimidazolium Dibromide (1)

Scheme 2. Preparation of Bis(1,1′-dimethyl-3,3′-methylenediimidazolin-2,2′-diylidene) Disilver (2)

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Excellent examples of metal complexes with NHC ligands have previously been reported in this Journal.9 These lab experiments describe the preparation of complexes bearing monodentate NHCs as ligands. However, metal complexes bearing chelating carbenes are receiving increasing attention due to the additional stability given by the chelate effect that confers in them interesting applications in catalysis and synthesis.10 These complexes are often prepared by transmetalation reactions using Ag(I)−NHC complexes as starting materials.11 However, little attention has been paid to the study of the silver(I) complexes themselves, in spite of offering numerous pedagogical advantages. For example, Ag(I)−NHCs are prepared with reasonably inexpensive silver salts [usually silver(I) oxide] and using a simple procedure which does not require air-free conditions. Herein, we present a new laboratory experiment designed to be carried out by upper-division undergraduate students in a bachelor’s degree in chemistry. In this experiment we describe the preparation of silver(I) and palladium(II) complexes with a bidentate-carbene ligand. The experiment highlights the different bonding modes of the carbene, which acts as a chelating ligand in the palladium complex but as a bridging ligand in the silver compound, due to the different geometrical requirements of the metal atom. The synthesis is performed by reacting directly the imidazolium precursor with the basic metal salt (silver oxide or palladium acetate) capable of deprotonation of the imidazolium without the aid of any auxiliary base. The material and equipment required for the experiment are of common use in an undergraduate laboratory. A number of important concepts are taught in this laboratory, inclusive of the synthetic procedures used in the preparation of the complexes, e.g., the laboratory setup required for a multistep synthesis, the manipulation of air-sensitive compounds, and vacuum filtration. Moreover, students should characterize the imidazolium salt, as well as the metal complexes, using the traditional techniques, such us NMR spectroscopy and mass spectrometry. Additionally, the availability of molecular structures based on single crystal Xray diffraction data for these complexes allows the student to be initiated in this technique. Since an intermediate level of organometallic chemistry is needed, the experiments depicted here are designed for third/fourth year undergraduates.

EXPERIMENTAL DESCRIPTION

The experiment is designed to simulate the daily routine followed in a research laboratory. Accordingly, a specific synthetic problem (the preparation of carbene complexes) is presented to the student who must first perform a literature search in order to find an adequate synthetic route for the complexes. This route should also include the isolation and characterization procedures. The students are given prelab questions and instructions 1 week in advance of the laboratory sessions. They include the use of a scientific database and questions related to the synthetic and theoretical issues. Once this task is completed, a brief one-on-one discussion between instructor and student is conducted to gauge the level of preparation attained by the student. After this, if the prelab work is complete, the student is given a detailed experimental procedure; if this is not the case, the student must re-elaborate it by addressing detailed recommendations, before being given the experimental procedure. The experimental work is carried out in four lab sessions with a duration of 4 h each. After the experiment is finished, students used proton and carbon NMR spectroscopy and mass spectrometry for the characterization of the obtained compounds; then, all the results are given to the instructor. The experiment was introduced to a fourth year undergraduate laboratory course in a bachelor’s degree in chemistry dedicated to the synthesis and characterization of organometallic species. The 20−25 students who are regularly enrolled in this course are split up in three teams and one instructor assigned to each of these teams. The students work individually under the supervision of the assigned instructor. All the students succeeded in the experiment with reasonable yields. In general, they found the synthetic procedure easy, although the preparation of complex 3 required the use of simple inert atmosphere techniques. Some difficulties may be found in the preparation of this complex (see Supporting Information). Largely, the students found the methodology encouraging. They were also in agreement with the idea that this methodology improved their problem-solving ability. Even though the students found this methodology more timeconsuming and demanding, they preferred it to the recipebased experiments to which they were more accustomed. B




Scheme 3. Preparation of 1,1′-Dimethyl-3,3′-methylene-4-diimidazolin-2,2′-diylidene Palladium(II) Dibromide (3)

Figure 1. Molecular structure for complex 3. Color code: Pd, pink; Br, brown; N, blue; C, gray; H, white (diastereotopic methylene protons in orange and green).

time and temperature and the solvents used in the final purification (Scheme 3). The reaction must be carried out in air-free conditions, by reaction of imidazolium salt 1 and Pd(OAc)2 in dry dimethyl sulfoxide. Freshly opened good quality dry DMSO can be directly used. After degassing the solution, the reaction mixture was heated first at 50 °C and then at 190 °C. The solvent was then removed by vacuum distillation and the mixture heated at 85 °C. After solvent removal, the solid was washed with THF and Et2O. Frequent errors made by the students are the inadequate degas of the initial mixture and poor handling of the vacuum line during the final distillation of dimethyl sulfoxide. See the Supporting Information section for more details about the synthetic methods.

Synthesis

According to the intended learning outcomes (see Supporting Information), students are expected to design and carry out the synthesis of a carbene precursor and its metal derivatives. Therefore, students will apply techniques frequently used in the laboratory. According to this, the experimental procedure involves three different components with increasing level of difficulty. The imidazolium salt 1 was easily prepared by direct reaction between N-methyl imidazole and dibromomethane (Scheme 1), using a slight modification of a previously published method.12 In particular, the starting reagents were refluxed in the absence of solvent. The white solid resulting after cooling to room temperature was triturated with diethyl ether and washed with acetone and diethyl ether. The imidazolium salt is hygroscopic and must be stored in a vacuum desiccator. The reaction is simple, and good yields (85−90%) are expected. The method used was a slight modification of a previously published one (Scheme 2).13 The imidazolium bromide, 1, was stirred with an excess of silver(I) oxide in the absence of light using water as solvent. The silver metal precipitated was filtered using Celite, and complex 2 precipitated by addition of ammonium hexafluorophosphate and was filtered and washed with water. Since no inert atmosphere manipulations are necessary, yields are good. In general, the cause of poor yields is careless manipulations during the final filtration. The method used was a modification to one previously described.14 The modifications consisted of a different reaction

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HAZARDS All reagents should be handled in a well-ventilated hood, with students wearing gloves, safety goggles, and lab coats. Acetone, tetrahydrofuran, and diethyl ether are highly flammable; Nmethyimidazole, dibromomethane, and diethyl ether are harmful substances; acetone and tetrahydrofuran may cause eye irritation; palladium(II) acetate, ammonium hexafluorophosphate, and silver(I) oxide may cause eye or skin damage. Tetrahydrofuran is suspected of causing cancer. Palladium(II) acetate is expensive; however, in this microscale experiment the amounts of reagents used are affordable; nevertheless, if necessary, palladium(II) acetate can be regenerated easily following the method described by Granell et al.15 C




Figure 2. Molecular structure for complex 2. Color code: Ag, pink; C, gray; N, blue. H, white (diastereotopic methylene protons in orange and green).

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RESULTS AND DISCUSSION

signals for the two ligands, due to the symmetric nature of the complex. Mass spectrometry may help in this situation because the m/z value and its isotopic pattern are different for the mononuclear and binuclear formulations. For this reason, the student must calculate the mass and the relative intensities of the four most intense peaks expected for the molecular ion (after loss of one PF6− counterion) and compare these data with the experimental spectrum.

Characterization

According to the intended proposed learning outcomes (see Supporting Information), characterization is an important part of this experiment. In particular, students are expected to choose and apply some of the typical techniques used in organometallic chemistry, such as NMR spectroscopy and mass spectrometry. In addition, fluxionality is a frequent problem in this field of chemistry and NMR analysis is often used to detect and explain such behavior. Consequently, characterization was mainly based upon NMR spectroscopy. The resonances in the 1H NMR spectra are easily assigned with the help of the instructor (see Supporting Information). The most noticeable characteristic in the 1H NMR spectra of complexes 2 and 3 is the absence of the resonance corresponding to the carbenic hydrogens H2 (see labeling in Scheme 2) consequent of metalation. The most noticeable characteristic in the 13C-{1H} NMR spectra of these complexes is the high-frequency shift of the resonance corresponding to the carbenic carbon, as compared to the imidazolium salt, due to formation of the M−C bond. However, the signals for these quaternary carbons are of low intensity and, in the case of silver complex 2, difficult to assign. Help from the instructor may be necessary. Particular attention requires the signal assigned to the CH2 protons, which appeared as a singlet in the spectrum of palladium complex 3 and as a broad signal in the spectrum of 2. However, according to the single crystal X-ray diffraction structures13,16 of these complexes (see Figures 1 and 2) the two protons in the methylene groups are diastereotopic and should appear in the spectra as two different signals. These apparently contradictory findings may be attributed to the fluxional behavior of the complexes, which interchange rapidly the position of the methylene protons in solution. In the case of complex 2 the signal is almost indistinguishable from the baseline, indicating that the coalescence happens at room temperature. In a postlab question, the students are asked to explain the apparent contradiction between solution behavior and solid state structure with the help of the excellent contribution of Herrmann et al.12 In this work, the authors studied the fluxional behavior of a variety of complexes bearing bidentate imidazolium salts, including complex 3. On the other hand, silver complex 2 is a binuclear molecule with two bidentate-carbene ligands bridging the two metal centers. However, the NMR spectra showed only one set of

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CONCLUSIONS A convenient, straightforward preparation of air-stable complexes with bidentate carbenes is described. The experiment uses equipment typically available in undergraduate laboratories. The students have to identify the products using the standard spectroscopic techniques. This experiment provides an experimental entry point into the study of organometallic complexes with bidentate carbenes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00523. Detailed experimental procedures; instructions for the students and the instructor; 1 H, 13C-{1H} NMR, and mass spectra of compounds 1, 2 and 3; and X-ray diffraction structures (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Alberto Fernández: 0000-0003-2504-6016 Jesús J. Fernández: 0000-0003-4938-0342 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Xunta de Galicia, Spain, for financial support (Grant 10 PXIB 103 226, PRGRC2014/042). REFERENCES

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Straightforward Preparation Method for Complexes Bearing a

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