Formation of an Oxidant-Sensible Pd(II) Coordination Compound and

Laboratory Experiment pubs.acs.org/jchemeduc

Formation of an Oxidant-Sensible Pd(II) Coordination Compound and Its 1H NMR Specific Characterization: A Preparative and Analytical Challenge in Current Coordination Chemistry Maria L. Abraham and Iris M. Oppel* Institute of Inorganic Chemistry, RWTH Aachen University, 52074 Aachen, Germany S Supporting Information *

ABSTRACT: A three-part experiment that leads to the synthesis of palladium(II) complex starting from a C3-symmetric triaminoguanidinium-based ligand is presented. In the first part, the preparation of tris-benzylidenetriaminoguanidinium chloride ([H6Br3L]Cl) by an acidic catalyzed 3-fold imine formation reaction of 5-bromo-2-hydroxybenzaldehyde and triaminoguanidinium chloride is described. Starting from the second part, the reaction procedures are performed under inert gas atmosphere. The conversion of PdCl2 with acetonitrile to give [Pd(MeCN)4]Cl2 as a precursor is performed at 80 °C. The third part of the experiment is a three-step procedure that begins with deprotonation of [H6Br3L]Cl, followed by transfer of [Pd(MeCN)4]Cl2 to the reaction flask, where the chelation of Pd(II)-precursor leads to the in situ species [Pd(MeCN)H3Br3L]. Addition of PPh3 as the final reaction step yields [Pd(PPh3)H3Br3L]. This sequence of experiments provides an excellent example of ligand and transition metal preparation and its subsequent complexation to form an oxidant-sensitive organometallic species. Undergraduate students get the opportunity to learn Schlenk techniques and to investigate 1H NMR spectra of organic and organometallic compounds starting from relatively simple to more advanced spectra. The distinct result of the chemical shift of the 4JP−H-affected doublet mirrors the preparative success of the student and demonstrates the utility of 1H NMR experiments. KEYWORDS: Second-Year Undergraduate, Interdisciplinary/Multidisciplinary, Analytical Chemistry, Inorganic Chemistry, Inquiry-Based/Discovery Learning, Organometallics, Oxidation/Reduction, NMR Spectroscopy

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step, equimolar conversion of 1 with Pd(PEt3)2Cl2 occurred under inert conditions, yielding into [(Pd(PEt3)H3Br3L]. In the second step, H2O2 was added stoichiometrically to oxidize the coordination compound under 1,2,4-triazole formation of [Pd(PEt3)H2Br3L′]. Each coordination compound can be characterized by either Single Crystal Diffraction or 1H NMR spectroscopy. Characteristic for each species is the 4JH−Pcoupling between the P atom and the imine H atom, Him, at the coordination pocket splitting this signal into a doublet of 15.4 Hz. The three-step experiment represented here (Scheme 1) includes the synthesis of 1, the formation of [Pd(MeCN)4]Cl2 2, their reaction together resulting an intermediate species [Pd(MeCN)H3Br3L], and the final coordination of PPh3 resulting in [Pd(PPh3)H3Br3L] 3, the target molecule. In the undesired case of air contact, the intramolecular 1,2,4-triazole formation occurs so that [Pd(PPh3)H2Br3L′] 4 is formed. 1H NMR analyses of 1 and 3 (eventually 4) allows students to gain experience in this characterization technique and to deepen their experience. We suggest performing NMR experiments on a 300 or 400 MHz device in order to evaluate the ligand signals during the reaction. These experiments have been successfully performed by second-year bachelor students. No specialized

uclear Magnetic Resonance (NMR) Spectroscopy is a powerful tool for the investigation of organometallic and coordination compounds. Investigation by 1H NMR allows one to get immediate insight into reaction processes via product formation. The preparation of air instable compounds is an omnipresent pathway for chemical synthesis. Handling chemicals under inert condition for the first time is a challenge every student has to overcome. The formation of air sensitive compounds often deals with potential exothermically formed side products when oxygen or water contact takes place. Therefore, a harmless but O2-sensitive coordination reaction with a distinct result correlated to the oxidized product seems to be an ideal experiment for undergraduate research studies. The coordination chemistry of tris-benzylidenetriaminoguanidinium ([H6L]+)-ligands has been widely investigated, so that discrete coordination compounds such as single-1 and doublewalled tetrahedra,2 but also an octahedron3 and a trigonal bipyramid4 are reported. Tris-(5-bromo-2-hydroxy)benzylidene triaminoguanidinium chloride [H6Br3L]Cl 1 (Scheme 1) is predestinated for NMR analyses due to its C3-symmetric backbone and its unique substitution pattern resulting in only six chemical different H atoms. Recently, we published an intramolecular oxidant induced 1,2,4-triazole formation, monitoring the cyclization of the triaminoguanidinum backbone by 1H NMR studies.5 In the first © 2014 American Chemical Society and Division of Chemical Education, Inc.

Published: October 24, 2014 2174

dx.doi.org/10.1021/ed500447f | J. Chem. Educ. 2014, 91, 2174−2177

Journal of Chemical Education

Laboratory Experiment

Scheme 1. Synthesis Overview To Achieve Target Compound [Pd(PPh3)H3Br3L] 3a

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Starting from triaminoguanidinium chloride and 5-bromo-2-hydroxybenzaldehyde, preparation of tris-(5-bromo-2-hydroxy)benzylidene triamininoguanidinium chloride [H6Br3L]Cl 1 is achieved. Refluxing PdCl2 in acetonitrile leads to formation of [Pd(MeCN)4]Cl2 2, which is thermodynamically stable as [Pd(MeCN)2Cl2] 2b. Deprotonation of 1 by Na2CO3 is followed by conversion with freshly prepared 2 at 70−80 °C to give the in situ [Pd(MeCN)H3Br3L]. In case of proper inert gas atmosphere, addition of PPh3 yields [Pd(PPh3)H3Br3L] 3, while oxygen contained atmosphere results in [Pd(PPh3)H2Br3L′] 4.

Scheme 2. Preparation of Tris-(5-bromo-2-hydroxy)benzylidene Triamininoguanidinium Chloride [H6Br3L]Cl 1a

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The C3-symmetric ligand carries six chemically different H atoms (OH, NH, Him, H3, H4 and H6).

Drying under reduced pressure is important for further product handling. A light yellow product is analyzed by 1H NMR in DMSO-d6. The 1H NMR spectrum shows the presence of six signals corresponding to the C3-symmetry of the ligand. It should be pointed out to students that, because of the aromatic substitution pattern, the aromatic signals are fully characterizable as 3JH−H-coupling of 8.4 Hz between H3 and H4 and 4 JH−H-coupling of 2.4 Hz between H4 and H6 occurs. Under inert conditions, the synthesis of tetrakis(acetonitrile)palladium chloride 2 is necessary as the palladium−chloride bonds in PdCl2 have to be broken first. Using (freshly dried) acetonitrile directly as solvent, the color change of the red suspension, to first an orange and then a red solution, indicates the reaction progress. It is important to keep the solution at high temperature as the formation of the thermodynamically favored [Pd(MeCN)2Cl2] 2b occurs at room temperature. In this case, an orange solid precipitates,

techniques or equipment are required apart from access to a dual inert-gas vacuum manifold, Schlenk-ware, a vacuum pump, and freshly dried acetonitrile, pentane, and oxygen-free tetrahydrofuran (THF).

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EXPERIMENT OVERVIEW Each step of this three-step synthesis (Scheme 1) is of educational interest. The organic reaction, Schiff base coupling to give tris-(5-bromo-2-hydroxy)benzylidene triamininoguanidinium chloride [H6Br3L]Cl 1, is a one-pot synthesis from the 3-fold condensation of triaminoguanidinium chloride and 5bromo-2-hydroxy-benzaldehyde, and therefore, it can be carried out in air (Scheme 2). This reaction is a good example of Le Chatelier’s principle as 1 becomes insoluble and precipitates from the reaction solvent, shifting the chemical equilibrium to the side of product formation.6 The yellow product 1 is collected by filtration and purified by washing with cold Et2O. 2175




which can be turned back into 2 by energy input via refluxing7 (Scheme 3).

symmetry by Pd(II)-coordination in one of the three binding pockets, each of the hydrogen atoms of the molecules create its own signal. In consequence, each of the six ligand signals of 1 occurs three times with respect to the aromatic rings I−III. Declaring the benzylidene unit of the coordination pocket ring I, for the adjacent NCH-Atom, we observe the multiplicity change from a singlet to a doublet with a characteristic 4JH-Pcoupling of 15.4 Hz at a chemical shift of 8.0 ppm. In the undesired case of air contact, the intramolecular 1,2,4-triazole formation occurred during the reaction procedure, so that [Pd(PPh3)H2Br3L′] 4 is formed (Scheme 1). In that case, the 1 H NMR spectrum usually reveals the signal sets for each compound 3 and 4. As both compounds carry the NCHAtom at ring I, two characteristically 15.4 Hz-doublets between 8.00 and 8.25 ppm are present. The ratio between these signals corresponds to the ratio of the guanidinium (nonoxidized) compound 3 and the 1,2,4-triazole compound 4, attributed to the downfield doublet (all spectra are depicted in the Supporting Information).

Scheme 3. Preparation of [Pd(MeCN)4]Cl2 2 and Temperature Dependent Side Reaction Forming [Pd(MeCN)2Cl2] 2b

Under inert conditions, the synthesis of [Pd(PPh3)H3Br3L] 3, can be performed as a three-step one-pot reaction (Scheme 4). 1 and Na2CO3 are placed in the final reaction Schlenk flask and dissolved in acetonitrile. Heating up to reflux, deprotonation of the C3-symmetric ligand takes place, visible by a deep yellow color, which can be attributed to the delocalized πsystem. With a cannula, the entire solution of 2 is transferred into the Schlenk flask containing the yellow ligand solution. It should be pointed out that this transfer under inert conditions is the crucial step. First, both solutions have to be handled at elevated temperatures; second, the cannula has to be warm during the procedure to prevent formation of 2b in the tube, and third, two septa are pierced by the cannula, creating small voids in the system. The reaction step can be handled without problems (see Supporting Information for glassware scheme) by isolating the cannula with alumina foil and using a slight overpressure of inert gas. During the transfer of 2 to the ligand solution, a color change from yellow to orange and red occurs. The coordination of Pd(II) takes place rapidly in one of the three times chelating coordination pockets. As Pd(II) favors square planar coordination, the fourth position stays occupied by one acetonitrile molecule of the precursor. 8 The intermediate coordination compound [Pd(MeCN)H3Br3L] can be used directly without further purification. The final reaction step is the addition of PPh3 resulting in the dark red target coordination compound of [Pd(PPh3)H3Br3L] 3. Purification of 3 is performed in three steps: solvent removal under reduced pressure, followed by washing with dry pentane, redissolving in dry THF and transfer via cannula, equipped with a filter tip, into a new Schlenk flask. After solvent removal, product formation is confirmed by 1H NMR immediately. The 1 H NMR spectrum shows a nonsymmetric set of H-signal. As the former C3-symmetry of the ligand is reduced to C1-

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HAZARDS Pentane and diethyl ether are extremely flammable and are harmful in case of ingestion. Acetonitrile and THF are readily flammable. Diethyl ether and THF can form explosive peroxides. THF is irritating to the eyes and to the respiratory system. The compounds 3 and 4 have not been fully tested for toxicity and, therefore, should be handled with care.

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SUMMARY OF PROCEDURE The reactions are appropriate for an advanced undergraduate inorganic laboratory class or for a graduate class. All the reagents are relatively inexpensive. The reaction sequence requires four laboratory sessions of approximately 1 day (7 h) each to be completed. We recommend the first day to be devoted to the synthesis of tris(5-bromo-2-hydroxy)benzylidene triamininoguanidinium chloride [H6Br3L]Cl 1. In the mean time, the students dry acetonitrile, any other solvents they need, and prepare the glassware. For the synthesis of 2 and 3, we recommend two consecutive days for laboratory work. In principle, the reaction can be done in one laboratory session of 7 h. Although 2 can be stored in form of 2b at room temperature under inert conditions, a direct conversion is preferred. Once all of the reactants for the synthesis of compound 3 are added, the final reaction mixture should not

Scheme 4. Preparation of [Pd(PPh3)H3Br3L] 3a

a

Coordination of Pd(II) results in an asymmetry compound, in which each hydrogen atom gets its own 1H NMR signal. For Him of ring I, a distinct doublet at δ = 7.99 ppm (CD2Cl2) or δ = 8.07 ppm (THF-d8) occurs, due to 4JH-P-coupling of 15.2 Hz, identifying 3. 2176




rest for longer times. Solvent removal and storage of the powder under inert condition is preferred as the oxidation of 3 to 4 occurs only slowly in the solid state. Thin layer chromatography of PPh3 and the reaction mixture can be used to monitor the free PPh3 and the product formation. The experiments represented here are instructive as they link organic and organometallic chemistry. They give students insight into modern coordination chemistry in combination with 1H NMR spectroscopy. Students discover the coherence of molecular symmetry and 1H NMR signals, multiplicity patterns of organic functional groups, and, by reaction monitoring, they reflect their own preparative skills.

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ASSOCIATED CONTENT

S Supporting Information *

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H NMR Spectra of compounds 1, 3, and 4. A student handout containing detailed experimental procedures. CCDC 10063591006360 contain the supplementary crystallographic data for this paper. This material is available via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank T. Gossen (IAC, RWTH Aachen University) for NMR measurements and C. M. Merkens (IAC, RWTH Aachen University) for crystal data collection.

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REFERENCES

(1) (a) Müller, I. M.; Robson, R.; Separovic, F. A Metallosupramolecular Capsule with the Topology of the Tetrahedron, 33, Assembled from Four Guanidine-Based Ligands and Twelve Cadmium Centers. Angew. Chem., Int. Ed. 2001, 40, 4385−4386. (b) Müller, I. M.; Möller, D.; Schalley, C. A. Rational Design of Tightly Closed Coordination Tetrahedra that are Stable in the Solid State, in Solution, and in the Gas Phase. Angew. Chem., Int. Ed. 2005, 44, 480−484. (2) Oppel, I. M.; Föcker, K. Rational Design of a Double-Walled Tetrahedron Containing Two Different C3-Symmetric Ligands. Angew. Chem., Int. Ed. 2008, 47, 402−405. (3) Müller, I. M.; Spillmann, S.; Franck, H.; Pietschnig, R. Rational Design of the First Closed Coordination Capsule with Octahedral Outer Shape. Chem.Eur. J. 2004, 10, 2207−2213. (4) Müller, I. M.; Möller, D. Rational Design of a Coordination Cage with a Trigonal-Bipyramidal Shape Constructed from 33 Building Units. Angew. Chem., Int. Ed. 2005, 44, 2969−2973. (5) Abraham, M. L.; Schulze, A. C.; Korthaus, A.; Oppel, I. M. Oxidant-Induced Intramolecular Triazole Formation. Dalton Trans. 2013, 42, 16066−16072. (6) Müller, I. M.; Möller, D. A New Ligand for the Formation of Triangular Building Blocks in Supramolecular Chemistry. Eur. J. Inorg. Chem. 2005, 257−263. (7) Synthesis adopted by Brauer, G.; Handbuch der Präparativen Anorganischen Chemie, 3rd ed.; Enke: Stuttgart, Germany, 1975; Vol. 3, ISBN 3-432-02328-6. (8) Green, M. L. H.; Parkin, P. Application of the Covalent Bond Classification Method for the Teaching of Inorganic Chemistry. J. Chem. Educ. 2014, 91 (6), 807−816.

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Formation of an Oxidant-Sensible Pd(II) Coordination Compound and

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