In the Laboratory edited by
Advanced Chemistry Classroom and Laboratory
Joseph J. BelBruno
Ruthenium(II)–dppm Coordination Chemistry
Dartmouth College Hanover, NH 03755
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An Advanced Inorganic Miniproject Simon J. Higgins Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, UK;
[email protected] Traditionally, B.Sc.(Hons) courses in chemistry in UK universities have included a research project. The introduction of the M.Chem. degree and the greater diversity in educational backgrounds of students entering chemistry courses have meant that this now usually forms the practical component of Year 4, and Year 3 often includes a mix of traditional laboratory courses and miniprojects of various kinds (1). At Liverpool, all students take a techniques-based practical course in the first semester and are given a choice of synthesis- or measurementbased miniprojects in the second semester. Students choosing inorganic synthesis work do projects that are initially carefully guided but are designed to be open ended. The aims of these miniprojects are to teach the students new synthetic and characterization techniques relevant to modern inorganic chemistry research; to give a flavor of the dilemmas and challenges of open-ended research; and to introduce the use of the library and database information in research. A team work element is included. Other miniprojects include (i) the effects of phosphine ligands on Co-catalyzed hydroformylation, (ii) the use of nickel–phosphine complexes in aryl–aryl coupling reactions, (iii) the coordination chemistry of fluoride with Ni(II) and Cu(II), and (iv) the scope of a Ni(II)templated macrocycle synthesis (2). A key operational detail is that, although some of the procedures involve inert atmospheres, few of the products are significantly air sensitive as solids, enabling the miniprojects to be conducted in any undergraduate laboratory without the need for glove-boxes or double-manifold vacuum lines. Ruthenium(II)–diphosphine complexes [RuCl2(P–P)2] are of importance in homogeneous catalysis, and they are starting materials for the preparation of interesting organometallic complexes (e.g. conjugated organometallic systems [3], dihydrogen complexes [4 ]). Ligands like Ph2PCH2PPh2 have an extensive coordination chemistry, since they are capable of chelating, binding through only one P atom to a metal, or bridging two metal atoms (5). This experiment introduces students to both these important areas of modern coordination chemistry. There is considerable detail in the handouts describing the early experiments but less detail for the later experiments, because the students are expected to familiarize themselves with relevant literature and should accumulate some experience with the necessary techniques. The course is assessed on laboratory performance (50%), a written report including a literature survey and discussion (35%), and an oral presentation (15%). The latter two components are prepared after the conclusion of the laboratory work. The laboratory performance is assessed partly continuously (by periodic inspection of laboratory notebooks and samples and discussion with the student) and partly by outcome.
Experiment Operation The academic year 1999–2000 saw the third year of operation of this course. Approximately 18–24 students choose inorganic chemistry experiments, and this project has been operated with 4–6 students, working in groups of 2 or 3 to prepare starting materials. They spend, on average, 7.5 hours per week in the laboratory for 9 weeks and a further 1.5 hours per week on library work. First, they prepare and characterize the diphosphine Ph2PCH2PPh2 (dppm; 1), by Li cleavage of PPh3 in thf: PPh3 + 2Li → LiPh + LiPPh2 2,2–Dimethylchloropropane is then added to remove the stronger base LiPh: LiPh + Me3CCl → Me2C=CH2 + LiCl + PhH Finally, half an equivalent of CH2Cl2 is added: 2LiPPh2 + CH2Cl2 → Ph2PCH2PPh2 + 2LiCl The labile Ru(II) complex [RuCl2(PPh3)3] (2) is prepared from RuCl3⭈3.5H2O and excess PPh3 in MeOH; this preparation is based on a method in Inorganic Syntheses (6 ). This provides good experience of the handling of air-sensitive compounds. The cost of RuCl3⭈3.5H2O is not prohibitively high. Next, the students individually prepare [RuCl2(dppm)2] (3) by reaction of the labile 2 with 1 in CH2Cl2: [RuCl2(PPh3)3 ] + 2Ph2PCH2PPh2 → [RuCl2(Ph2PCH2PPh2)2 ] + 3PPh3 Students characterize complex 3 by submitting it for C and H microanalyses (done in-house) and obtaining 31P{1H} and 1H NMR spectra (the latter hands-on). The data obtained, together with some background knowledge of Ru(II) chemistry, should enable the students to deduce the formula and geometry of 3. Complex 3 is then heated in refluxing 1,2-dichloroethane and the product is also characterized by 31P{1H} and 1H NMR spectroscopy. If 31P{1H} NMR spectroscopy is not available, electronic spectroscopy (7 ) can provide information in support of deductions made using 1H NMR spectroscopy. 13C NMR spectroscopy is of little value because the complexes are not very soluble, and coupling to 31P makes the ligand methylene resonances complex multiplets, rendering a good signal-to-noise ratio even harder to obtain. All the 6coordinate Ru(II) complexes are air stable. Each student is then given a different extension to the miniproject, for which the results are not always known in advance (the supplemental material contains three interesting examples for which the outcome is knownW). For example, there has recently been revived interest in the coordination
JChemEd.chem.wisc.edu • Vol. 78 No. 5 May 2001 • Journal of Chemical Education
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In the Laboratory
chemistry of stibine ligands (8), and one extension concerns making Ph2SbCH2SbPh2 (dpsm; 4) and a Ru(II) complex 5 (9). This gives insight into interesting trends in the coordination chemistry of the series dppm, Ph2AsCH2AsPh2 (dpam), and dpsm. Another extension concerns the synthesis and properties of the corresponding bromide trans-[RuBr2(dppm)2] (6), from RuBr3⭈nH2O and dppm. A third possibility is to substitute another diphosphine for dppm; examples tried so far include dppe (Ph2PCH2CH2PPh2), available from the miniproject on Ni(II)-catalyzed C–C coupling reactions, which gives [RuCl2(dppe)2] (7). Hazards Other than the normal risks associated with handling chemicals, there are no unusual hazards in this experiment. Particular safety points are outlined in detail in the online lab documentation.W In our implementation, students are expected to assess the hazards of their chemicals and procedures before commencing any experiment, using safety data available on CD-ROM, and they must have these assessments checked and signed by an instructor before starting work.
contrast to dppm, which chelates. Students are prompted to look up the behavior of the corresponding diarsine Ph2AsCH2AsPh2 (dpam), which forms trans-[RuCl2(dpam– As,As′ )(dpam–As)2] under similar conditions (13). Refluxing RuBr3⭈nH2O with dppm in water–ethanol gives brown trans-[RuBr2(dppm)2] (6) in moderate yield. On refluxing 6 in 1,2-dichloroethane, an equilibrium mixture of cis and trans isomers is obtained. The students found that trans-[RuCl2(dppe)2] (7) was the only product when 2 is treated with dppe in CH2Cl2. This is recovered unchanged upon prolonged reflux in either 1,2-dichloroethane or the higher-boiling chlorobenzene (a 3:1 mixture of the cis and trans isomers of 7 was reportedly obtained on treating [RuCl2(dmso)4] with dppe in refluxing toluene [14 ]). W
Supplemental Material
The complete description of this experiment and supplemental materials are available in this issue of JCE Online. Literature Cited
Results Dppm (1) is readily prepared in yields of 65–75% from inexpensive starting materials. A related experiment, the synthesis of dppe from NaPPh2 in liquid ammonia, has been published (10). The experiment works best with small teams because some of the manipulations require at least two people. The crude diphosphine may be filtered and washed in air, but care should be taken not to pull too much air through the filter cake, and the product should be dried under vacuum as soon as possible. Complex 2 should be obtained in almost quantitative yield. Complex trans-3 forms from 2 in excellent yield; this mild ligand substitution reaction works better than the published methods (11, 12). The 31P{1H} NMR spectrum of 3 shows a singlet, and the ligand methylene region in the 1H NMR spectrum shows one (multiplet) resonance, confirming trans geometry. On heating for 16 h in refluxing 1,2-dichloroethane, trans-3 is converted to cis-3. The 31P{1H} NMR spectrum of cis-3 shows a pair of triplets and the 1H NMR spectrum shows an AB pattern for the chemically inequivalent PCH2P protons; the additional couplings to P broaden the resonances. Detailed NMR data are included in the supplemental material.W In the extensions, the distibine dpsm (4) reacts with RuCl3⭈3.5H2O to give trans-[RuCl2(η1–dpsm)4] (5), in which each distibine binds through only one antimony atom—in
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1. North, M. J. Chem. Educ. 1998, 75, 630. 2. Cameron, J. H. J. Chem. Educ. 1995, 72, 1033. 3. Zhu, Y. B.; Clot, O.; Wolf, M. O.; Yap, G. P. A. J. Am. Chem. Soc. 1998, 120, 1812. 4. Fong, T. P.; Forde, C. E.; Lough, A. J.; Morris, R. H.; Rigo, P.; Rocchini, E.; Stephan, T. J. Chem. Soc., Dalton Trans. 1999, 4475. 5. Chaudret, B.; Delavaux, B.; Poilblanc, R. Coord. Chem. Rev. 1988, 86, 191. 6. Hallman, P. S.; Stephenson, T. A.; Wilkinson, G. Inorg. Synth. 1970, 12, 237. 7. Klassen, D. M.; Crosby, G. A. J. Mol. Spectrosc. 1968, 25, 398. 8. Hill, A. M.; Levason, W.; Webster, M.; Albers, I. Organometallics 1997, 16, 5641. 9. Evans, T.; Genge, A. R. J.; Hill, A. M.; Holmes, N. J.; Levason, W.; Webster, M. J. Chem. Soc., Dalton Trans. 2000, 655. 10. Lucas, C. R.; Walsh, K. A. J. Chem. Educ. 1987, 64, 265. 11. Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1982, 21, 1037. 12. Chakravarty, A. R.; Cotton, F. A.; Schwotzer, W. Inorg. Chim. Acta 1984, 84, 179. 13. Mague, J. T.; Mitchener, J. P. Inorg. Chem. 1972, 11, 2714. 14. Bautista, M. T.; Cappellani, E. P.; Drouin, S. D.; Morris, R. H.; Schweitzer, C. T.; Sella, A.; Zubkowski, J. J. Am. Chem. Soc. 1991, 113, 4876.
Journal of Chemical Education • Vol. 78 No. 5 May 2001 • JChemEd.chem.wisc.edu
