LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
A One-Pot Self-Assembly Reaction To Prepare a Supramolecular Palladium(II) Cyclometalated Complex: An Undergraduate Organometallic Laboratory Experiment Alberto Fernandez,*,# Margarita Lopez-Torres,# Jesus J. Fernandez,# Digna Vazquez-García,# and Jose M. Vila*,† #
Departamento de Química Fundamental, Universidade da Coru~na, Alejandro de la Sota N° 1, 15071 A Coru~na, Spain Departamento de Química Inorganica, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain
†
bS Supporting Information ABSTRACT: A laboratory experiment for students in advanced inorganic chemistry is described. Students prepare palladium(II) cyclometalated complexes. A terdentate [C,N,O] Schiff base ligand is doubly deprotonated upon reaction with palladium(II) acetate in a self-assembly process to give a palladacycle with a characteristic tetranuclear structure. This complex reacts with triphenylphosphine in a ligand-substitution process, cleaving of the tetranuclear structure. Students assign the resonances in the 1H, 13C-{1H}, and 31P-{1H} NMR spectra of the prepared compounds, whose 3D structure is given to understand the relative spatial disposition of the cyclometalated fragments. Mass spectra are interpreted taking into account the characteristic isotopic pattern originated by the tetranuclear [Pd4L4]+ molecular ion. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Coordination Compounds, Mass Spectrometry, NMR Spectroscopy, Organometallics, Synthesis
Scheme 1. The Cyclometallation Reaction
O
ne of the routes for the synthesis of supramolecular moieties is the self-assembly process where building blocks form an organized structure or pattern as a consequence of specific local interactions among the components, without external direction. In the classic sense, molecular units self-organize into ordered structures by weak noncovalent interactions, but in a broad definition, the assembly may be established through stronger interactions such as, for example, coordinate dative bonds.1 Furthermore, cyclometalated complexes may be defined as a class of organometallic compounds in which an organic ligand is bonded to a metal center through a σ metal carbon bond and a typical dative bond between a donor atom and the metal.2 The compounds may be considered to be midway between the classic organotransition-metal complexes and coordination compounds, and consequently, they display the characteristic reactivity typical of both types of species, for example, ligand substitution reactions, insertion reactions into the metal carbon bond, or catalytic activity toward the Suzuki or Heck reactions.3 They are usually synthesized through the classical cyclometalation reaction, described by Trofimenko.4 This is the direct reaction between the organic ligand and a transitionmetal salt that yields the cyclometalated complex after C H bond activation (Scheme 1). In the laboratory experiment reported herein, the student prepares a cyclometalated complex by reaction between the Schiff base ligand (Scheme 2) Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
2,3,4-(MeO)3 C 6 H 2 C(H)dN[2-OHC 6 H 4 ] and Pd(OAc)2 (Scheme 3 and Figure 1). 5 Because of the basicity of the acetate ions, the ligand is doubly deprotonated and coordinates to the palladium atom through a carbon atom, a CdN nitrogen, and a phenolate oxygen; subsequently, the Pd ligand units self-assemble into tetrameric moieties through bridging phenolate oxygen atoms (Figure 1). Thus, cyclometalation and self-assembly both take place in a onepot process. Cleavage of the tetranuclear structure is also carried out using a thermodynamically favorable ligand substitution reaction (Scheme 4 and Figure 2). Laboratory projects to synthesize cyclometalated complexes related to bidentate [C,N] ligands to give dinuclear compounds
Published: October 13, 2011 156
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Journal of Chemical Education
LABORATORY EXPERIMENT
Scheme 2. Preparation of Ligand 1
Scheme 3. Preparation of the Tetranuclear Complex 2
Figure 2. Structure of [Pd{2,3,4-(MeO)3C6HC(H)dN[2-(O)C6H4]} (PPh3)], 3.
for purification of the complexes. The availability of molecular structures based on single crystal X-ray diffraction data for these complexes allows the student to be initiated in this technique. An intermediate level of coordination and organometallic chemistry knowledge is needed, and because both are normally taught in advanced inorganic and organic courses, the experiments depicted here are designed for second- or third-year undergraduates, or for those students who have taken at least a basic inorganic course as well as an initial laboratory in inorganic chemistry covering the basic synthetic and characterization techniques.
Figure 1. Structure of the tetramer 2, [Pd{2,3,4-(MeO)3C6HC(H)dN [2-(O)C6H4)]}]4: gray, carbon; red, oxygen; blue, nitrogen; yellow, palladium.
Scheme 4. Preparation of 3
have been previously described.6 However, in the present case, the synthesis of a cyclometalated compound derived from a terdentate [C,N,O] ligand poses an example of the utility of selfassembly reactions to prepare supramolecular moieties, and therefore, new concepts must be used by the student. In addition, the synthetic procedure is different. A number of important concepts are taught in this laboratory, including the synthetic procedures used in the preparation of the complexes and the study of their reactivity, for example, the laboratory setups required for a multistep synthesis, the manipulation of air sensitive compounds, vacuum filtration, recrystallization, or column chromatography in the final stages
’ EXPERIMENT OVERVIEW The experiment is designed to simulate the daily routine followed in a research laboratory; consequently, a specific synthetic problem is presented to the student who must first perform a literature search in order to find an adequate synthetic route for the complexes, which should also include the isolation and characterization procedures. The students are given prelab questions and instructions one week in advance of the laboratory sessions. They include the use of a scientific database and questions related to the synthesis 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 interaction, the student is given a detailed experimental procedure. The experimental work is carried out in four lab sessions of between one and four hours each. After the experiment is finished, students used proton and carbon NMR spectroscopy and mass spectrometry for the characterization of the final products; all the results then are given to the instructor. ’ EXPERIMENTAL DETAILS The experimental procedure can be divided into three parts. Preparation of 2,3,4-(MeO)3C6H2C(H)dN[2-(OH)C6H4] (1)
The Schiff base ligand was prepared by a condensation reaction between 2-aminophenol and 2,3,4-trimethoxybenzaldehyde (Scheme 2) using chloroform as solvent and refluxing the solution for 24 h in a Dean Stark apparatus (see Supporting Information). The ligand was isolated as a brown solid by evaporating the solvent in a rotatory evaporator and air-dried. 157
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respectively. The HCdN proton resonance, 8.17 ppm, shows a smaller low-field shift than in the parent cyclometalated complex 2, in agreement with opening of the polynuclear structure. Surprisingly, the H5 resonance, 5.51 ppm, shows a greater shift than in 2 due to shielding by the phosphine phenyl rings; this shielding also affects the C4 MeO proton signal, which is also shifted to lower frequency upon phosphine coordination to the metal. The visualization of the 3D structure of the complex can also be of great help to the student in this case. The mass FAB (fast-atom bombardment) spectra of 2 and 3 showed the clusters of peaks corresponding to the molecular ions. In the case of complex 2, the characteristic pattern of a tetranuclear complex was found, whereas the spectra of 3 showed the expected pattern for a mononuclear ion. The student must understand the concept of isotopic pattern and calculate the relative intensities of the four most intense peaks corresponding to the molecular ions and compare them with the experimental values (alternatively the complete set of peaks corresponding to the molecular ion may be simulated and compared).
Preparation of [Pd{2,3,4-(MeO)3C6HC(H)dN[2-(O)C6H4)]}]4 (2)
Ligand 1 and palladium(II) acetate, in stoichiometric proportions, were suspended in degassed toluene and sealed under nitrogen, in a 25 mL Schlenk tube (Scheme 3). The mixture was heated with magnetic stirring at 60 °C for approximately 20 h, after which the flask was opened, the solution filtered, the solvent removed in a rotary evaporator, and the resulting red oil purified by column chromatography in silica gel. The final product was eluted as a red solid after elution with dichloromethane/ethanol (0.4%) and solvent removal. Preparation of [Pd{2,3,4-(MeO)3C 6HC(H)dN[2-(O)C 6 H4]} (PPh3 )] (3)
The tetranuclear complex was reacted with triphenylphosphine in acetone in a 25 mL Erlenmeyer flask for 1 h at room temperature with magnetic stirring (Scheme 4). Then, the solvent was removed in a rotary evaporator and the resulting solid recrystallized from acetone/hexane. The final product was separated as violet crystals after vacuum filtration.
’ HAZARDS All reagents should be handled in a well-ventilated hood, with students wearing gloves, safety goggles, and lab coats. Toluene, n-hexane, ethanol, and acetone are highly flammable; dichloromethane and chloroform are toxic and irritant; 2,3,4,-trimethoxybenzaldehyde, palladium(II) acetate, and triphenylphosphine are irritants; 2-aminophenol is toxic and irritant. The starting material, palladium(II) acetate, is expensive; however, in this microscale experiment, the amounts of reagents used are affordable. If necessary, palladium acetate can be regenerated easily following the method described by Granell et al.6 ’ CHARACTERIZATION The resonances in the 1H NMR spectrum of Schiff base 1 can be assigned with the help of the instructor (see Supporting Information). In the 1H NMR spectrum of the tetranuclear complex 2, the resonance at 7.90 ppm corresponding to H6 (see labeling in Scheme 3) is absent as a consequence of metalation; thus, the H5 resonance is assigned as a singlet (5.67 ppm) instead of the doublet signal (6.78 ppm) in the spectrum of ligand 1. One noticeable characteristic of the NMR spectrum is the high-field shift observed in the signals corresponding to the HCdN (8.98 ppm) and H5 (6.78 ppm) proton resonances; the low δ values are due to the tetranuclear structure of the complex, which puts the HCdN and H5 protons in the shielding zone of the phenyl rings of a neighboring metalated moiety. This effect is also observed in the resonance corresponding to the MeO group at C4, which appears at 2.92 ppm. To visualize the spatial disposition of the metalated ligands, the student may use an interactive 3D visualization program (most are freely available for the academic community). The coordinates of the atoms ate given in PDB files in the Supporting Information. The most noticeable characteristics in the 13C-{1H} spectrum are the high-frequency shifts of the C6, CdN, and C1 resonances, as compared to the free ligands, due to formation of the metalated ring. The resonance assigned to the C O carbon was shifted to higher frequency ca. 15 ppm consequent upon Pd O bond formation. The most noticeable feature in the 1H NMR spectrum of complex 3 is the coupling of the H5 and HCdN resonances to the 31P nucleus, appearing as doublets; J(PH) 3.9 and 10.2 Hz,
’ CONCLUSION A convenient preparation for an air-stable self-assembled tetranuclear cyclometalated complex is described. The experiment uses equipment typically available in undergraduate laboratories. The students can identify the products using the standard spectroscopic techniques. This experiment provides an experimental entry point into the study of both coordination and organometallic complexes as well as the self-assembly process. ’ ASSOCIATED CONTENT
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Supporting Information Prelab student material including questions; student synthesis procedure; notes for the instructor including chemicals and equipment needed, hazards, and answers to student questions; a schematic of the Dean Stark apparatus; NMR and mass spectra; PDB files. This material is available via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES (1) (a) Fujita, M.; Tominaga, M.; Hori, A.; Therrien, B. Acc. Chem. Res. 2005, 38, 369–378. (b) Swiegers, G. F.; Malefetse, T. J. J. Inclusion Phenom. Macrocyclic Chem. 2001, 2, 253–264.(c) Haiduc, I.; Edelmann, F. Supramolecular Organometallic Chemistry; Wiley-VCH: Weinheim, Germany, 1999. (2) (a) Omae, I. Coord. Chem. Rev. 1988, 83, 137. (b) Dupont, J.; Consorti, C. S.; Spencer, J. Chem. Rev. 2005, 105, 2527–2571.(c) Dupont, J.; Pfeffer, M. Palladacycles; Wiley-VCH: Weinheim, Germany, 2008. (3) Omae, I. J. Organomet. Chem. 2007, 692, 2608–2632. (4) Trofimenko, S. Inorg. Chem. 1973, 12, 1215. (5) Fernandez, A.; Vazquez-García, D.; Fernandez, J. J.; LopezTorres, M.; Suarez, A.; Castro-Juiz, S.; Vila, J. M. New J. Chem. 2002, 26, 398–404. (6) (a) Arnaiz, F. J. J. Chem. Educ. 1996, 73, A126. (b) Albert., J.; Cadena, M.; Granell, J. J. Chem. Educ. 2003, 80, 801–802.
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