In the Laboratory
The Mechanochemical Reaction of Palladium(II) Chloride with a Bidentate Phosphine David E. Berry,* Philippa Carrie, Kelli L. Fawkes, Bruce Rebner, and Yao (Shirley) Xing Department of Chemistry, University of Victoria, Victoria, British Columbia, Canada, V8W 3V6 *
[email protected] The technique of grinding two solids together to effect a chemical change has been known for many years (1) and indeed the simple production of lead(II) iodide in a mortar and pestle has been described in this Journal (2). Renewed interest in the solid-state reactions of phosphorus chemistry (3, 4) caught our attention; the “green advantage” of no solvent is inviting. Somewhat surprisingly, it has been shown that the grinding process may be even more efficient than the solvent-based approach (5). The formation of [PtCl2(PPh3)2] by ball-milling techniques (6) has a particularly useful precedent for a teaching experiment. The price of the precious metals keeps their use to a modest level in the undergraduate laboratory, in spite of their wide range of utility in demonstrating modern inorganic chemistry. The traditional route for M = Pd or Pt is to first prepare an intermediate such as [MCl2(NCR)2] (R = methyl or phenyl), followed by a substitution with 2 equiv of phosphine (7). Although these two steps individually generate good yields, inexperienced hands in a teaching lab will inevitably decrease the overall yield and, thus, invites a more efficient process. We began an undergraduate research project to prepare [MCl2{P(CH2)nP}] complexes, where P(CH2)nP represents a bidentate phosphine. Quick success was noted by grinding the metal chloride salt with a stoichiometric amount of the solid phosphine. However, it is clearly not practical to grind by hand for 30 min or longer using a mortar and pestle. The commercial ball mills described in the literature (3-6) were beyond our budget, so we developed a grinding mechanism from an overhead stirrer (Figure 1). Around the time of this project, an intriguing development in the topic of pincer ligands appeared in the literature (8). Such ligands tend to be tridentate, often contributing as much as five electrons to the metal center. Several reviews on transition-metal pincer-type complexes have been written and are collated in the cited literature of a recent article (9). The chemistry of interest is given in Scheme 1. In the case of short backbone P(CH2)nP ligands, the cis monomer [PdCl2{P(CH2)nP}] is the usual structure, even though the trans isomer is preferred for unidentate phosphines. For longer backbones, the trans dinuclear complex [PdCl2{P(CH2)nP}]2 results (10). In this experiment, P(CH2)nP is the linear bidentate phosphine Ph2P(CH2)5PPh2 (11). When the palladium dimer (I) is heated in dimethylformamide (DMF) under a nitrogen atmosphere, it gives the symmetrically deprotonated pincer complex (II). The pincer complex is known to be a catalyst for Suzuki-Miyaura coupling. Combining a mechanochemical production of [PdCl2{Ph2P(CH2)5PPh2}]2 with the deprotonation shown in Scheme 1 allows the undergraduate to look at cyclometalated chemistry without
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resorting to the more commonly used bulky ligands containing tertbutyl groups (10). But2P(CH2)nPBut2 are reported to be air sensitive liquids (12). The Milling Process A stoichiometric amount of PdCl2 was ground with a stoichiometric amount of solid 1,5-bis(diphenylphosphino)pentane. The milling process can be successfully achieved by using an overhead stirrer driving a four-winged propeller. The blades are angled such that they ride up and over 3/16 in. steel ball bearings grinding the powder against the side of a 50 mL round-bottomed flask. Although the stirring is inclined to stop if a ball-bearing gets wedged under the blade, even the action of flicking the ball bearings vigorously around the flask all night provides reasonable milling. We have had no structural failures caused by the ball bearings breaking the glassware, but we have had problems with “walking” apparatus when not effectively clamped to a rigid framework. Extraction of the finely ground mixture into dichloromethane gives yields in the 20-45% range for most students, with the lower number likely reflecting the efficiency of precipitation by the addition of hexanes. Since we recently acquired access to a 31P NMR probe, we now require the students to routinely check this diagnostic
Figure 1. The overhead stirrer set up for grinding.
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 5 May 2010 10.1021/ed800161a Published on Web 03/10/2010
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Journal of Chemical Education
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In the Laboratory
Scheme 1. The Cyclometalation Reaction to Produce the Pincer Complex
parameter of the milled product. Our development work showed similar results to those reported by Shaw and co-workers (10). Two peaks consistently appear around 16.5 ppm and 26.6 ppm in a ratio of ∼3:1 by 31P NMR (all 31P NMR shifts quoted in the present article are referenced against external 85% H3PO4), which Shaw speculates as the mixture of trans (as the dominant component) and cis isomers of [PdCl2{Ph2P(CH2)5PPh2}]2. We are not aware of a single-crystal X-ray structure of this particular compound, but the But analogue does have the trans form as shown on the left-hand side of Scheme 1.
Acknowledgment
The Deprotonation This step follows the literature fairly closely (8). Using the dimeric palladium product from the milling process, the students typically set up a reflux under nitrogen with a recirculating coldwater bath as coolant for the condenser. The solvent, dimethylformamide, is used from the bottle with no deoxygenation procedures taken. The reflux is typically run for a period of 1624 h with no observable variation in the results. After cooling, the solvent is removed by distillation to a liquid nitrogen trap in vacuo and the residue is recrystallized from dichloromethane and hexanes. A full NMR characterization is required, with definitive data coming from each of 1H, 31P, and 13C 135DEPT NMR spectra. The results of a crystal structure determination can be viewed in a diagram in the literature or on screen using the Mercury software (13). This helps the student relate the symmetry to the interpretation of the NMR data as reported in the literature (8). Hazards 1,5-Bis(diphenylphosphino)pentane is an irritant to the eyes, respiratory system, and skin. Dichloromethane is both toxic and a possible carcinogen. It is harmful by inhalation, contact with skin, and if swallowed. It is also a neurological hazard. Dimethylformamide is both toxic and a possible carcinogen. It is harmful by inhalation, contact with skin, and if swallowed. Hexanes are flammable and harmful by inhalation or swallowing. They are also irritating to the skin. Palladium(II) chloride is corrosive and is irritating to the eyes, respiratory system, and skin. Learning Outcomes We have used this experiment in a third-year course, which is the final inorganic lab course for the chemistry-major students and a required course for honors students. The experiment can be completed in two three-hour lab sessions, with some out-ofclass time needed to take down the overnight reactions. The number of the students succeeding in the synthesis has been high.
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In the most recent term, all students had evidence of the pincer compound being the major product by 31P NMR, with ∼25% having a spectrum completely free of phosphorus-containing contaminants. For an experiment of this length, we typically require a short data report with tabulated results and assignments. A 20 min one-on-one interview follows, in which the instructor will initiate discussion with appropriate questions. Examples of these questions might include: Why is the product of milling thought to be a dimer? What evidence is there for this? How do you know which carbon was deprotonated in the final step? Which spectrum is your best indicator of success? One unexpected learning opportunity has been found in assigning the 1H and 13C NMR spectra of the free phosphine, Ph2P(CH2)5PPh2. The backbone chain exhibits the three environments that one might expect from a symmetrical arrangement. However, the assignment of peaks is not straightforward and indeed contradicts what the novice might assume to be “rules”. The assignment can be completed unequivocally using COSY and HETCOR spectra that can either be run or supplied for the student (available in the supporting information).
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Vol. 87 No. 5 May 2010
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We thank the Work-Study program funded by University of Victoria for supporting P.C. (2004-2005) and Y.S.X. (2005-2006). Literature Cited 1. Beyer, M. K.; Clausen-Schaumann, H. Chem. Rev. 2003, 105, 2921–2948. 2. de Vos, W.; Verdonk, A. H. J. Chem. Educ. 1983, 62, 238–240. 3. Balema, V. P.; Wiench, J. W.; Pruski, M.; Pecharsky, V. K. J. Am. Chem. Soc. 2002, 124, 6244–6245. 4. Balema, V. P.; Wiench, J. W.; Pruski, M.; Pecharsky, V. K. Chem. Commun. 2002, 724–725. 5. Antesberger, J.; Cave, G. W. V.; Ferrarelli, M. C.; Heaven, M. W.; Raston, C. L.; Atwood, J. L. Chem. Commun. 2005, 892–894. 6. Balema, V. P.; Wiench, J. W.; Pruski, M.; Pecharsky, V. K. Chem. Commun. 2002, 1606–1607. 7. Kharasch, M. S.; Seyler, R. C.; Mayo, F. R. J. Am. Chem. Soc. 1938, 60, 882–884. 8. Neo, K. E.; Neo, Y. C.; Chien, S. W.; Tan, G. K.; Wilkins, A. L.; Henderson, W.; Hor, T. S. A. Dalton 2004, 2281; Erratum, 2005, No. 22, 3702. 9. Korshin, E. E.; Leitus, G.; Shimon, L. J. W.; Konstantinovski, L.; Milstein, D. Inorg. Chem. 2008, 47, 7177–7189. 10. Al-Salem, N. A.; Empsall, H. D.; Markham, R.; Shaw, B. L.; Weeks, B. J. Chem. Soc., Dalton Trans. 1979, 1972–1982. 11. Sacconi, L.; Gelsomini, J. Inorg. Chem. 1968, 7, 291–294. 12. Pryde, A.; Shaw, B. L.; Weeks, B. J. Chem. Soc., Dalton Trans. 1976, 322–326. 13. Cambridge Crystallographic Data Centre Home Page. http:// www.ccdc.cam.ac.uk/ (accessed Feb 2010).
Supporting Information Available Student handout; 1H COSY and HETCOR spectra of Ph2P(CH2)5PPh2; detailed information for an instructor to adopt this experiment. This material is available via the Internet at http://pubs. acs.org.
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r 2010 American Chemical Society and Division of Chemical Education, Inc.
