In the Laboratory
Micropreparation of [RuH2{P(C6H5)3}4]: A Transition Metal Hydride Compound
W
Donald E. Linn, Jr. Department of Chemistry, Indiana University–Purdue University Fort Wayne, Fort Wayne IN 46805-1499
The M–H bond plays a prominent role in organometallic chemistry because the metal hydrogen bond can undergo a variety of insertion reactions such as olefin isomerization, hydrogenation, hydrosilation, and hydroformylation (1). This inorganic laboratory experiment is designed to introduce inert atmosphere inorganic synthesis and to be accessible to upperlevel undergraduate students. Emphasis is placed on spectral and chemical properties. The title compound is chosen to demonstrate some aspects of transition metal hydride chemistry (2). This preparation makes use of borohydride, BH 4᎑, to replace the chloride ligands of dichlorotris(triphenylphosphine)ruthenium(II), [RuCl2(PPh3)3] (Ph = C6H5). This starting material is prepared from ruthenium trichloride trihydrate and triphenylphosphine, PPh3: RuCl3ⴢ3H 2O + 3PPh3 + CH3OH → [RuCl2(PPh 3) 3] + 3H2O + HCHO + HCl
(1)
This compound is not air-sensitive, requires no special handling, and can easily be prepared in a 3-hour laboratory. Preparation of the hydride is done in a well-ventilated hood using argon or hydrogen as a purge gas. Syringe and cannula techniques are appropriate to this reaction. A 50-mL three-neck flask with a stir bar for each student and several shared cannulas and syringe needles are required in this step. The hydrogenation is summarized as follows. MeOH
[RuCl2(PPh3)3] + 2BH4᎑ + PPh3 → toluene
[RuH2(PPh3) 4] + 2B(OCH3) 3 + 3H2
(2)
The dihydride, [RuH2(PPh3)4], can be prepared in a second 3-hour laboratory period. Spectroscopic measurements and additional reaction exercises are performed in a third laboratory period. Dihydrido-(η2-dihydrogen)tris(triphenylphosphine)-ruthenium(II) is prepared by adding hydrogen: H2
[RuCl 2(PPh 3)3] + 2BH4᎑ + 6CH3OH → MeOH/toluene
[RuH2(H2)(PPh 3)3] + 2B(OCH3) 3 + 2Cl᎑ + 6H2
(3)
Each of these compounds can be readily obtained by mixing toluene solutions of [RuH2(PPh 3)4] or [RuH2(H2)(PPh 3) 3] with the appropriate ligand and characterized by IR or NMR spectroscopy. The total time to prepare an NMR solution on a vacuum Schlenk line can vary depending on the number of students, but 20 minutes for an NMR tube vacuum fill cycle is a reasonable estimate. The tetrakis(phosphine) derivative, materials for this article are available on JCE Online at http://jchemed.chem.wisc.edu/Journal/issues/1999/ Jan/abs70.html. W Supplementary
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as a dry solid, can tolerate limited exposure to the air. The above “tetrahydride”, [RuH2(H2)(PPh 3) 3], can be converted into a variety of dihydrides simply by losing H2 from the coordination sphere. Here two of the “hydride” ligands are more accurately illustrated to show a dihydrogen ligand displacement reaction: H Ph3P
H Ru
Ph3P
L PPh3
H
+L
Ph3P
PPh3
Ru
Ph3P
H
+ H2
(4)
H
H
L = CO, N 2, PPh3
Thus, it can be shown dramatically that the usual assumption of substitution-inert behavior in low-spin d6 classical coordination complexes does not apply to these organoruthenium complexes. This helps introduce also the high trans influence of the hydride ligand. Preparation of Dichlorotris(triphenylphosphine)ruthenium(II), [RuCl2(PPh3)3] (3 ) A 250-mL round-bottom flask is charged with 125 mL of methanol and fitted with a reflux condenser and nitrogen bubbler. The methanol had been refluxed under argon for 1 hour and cooled to room temperature.1 The ruthenium trichloride trihydrate, RuCl2ⴢ3H2O (1.0 g, 3.8 mmol) is added under argon along with the triphenylphosphine, PPh3 (7.0 g, 26.5 mmol). This mixture is refluxed for 3 hours and then allowed to cool to room temperature. Filtration by means of a 30-mL glass fritted funnel (10–20 µm) yields shiny, black, air-stable crystals, which are washed twice with 20 mL of ether to yield 3.4 g (100%) of [RuCl 2(PPh3)3]. 31P{1H} NMR (C D ): δ 43.2 (br s). (The chemical shift stan6 6 dard is external 85% H3PO4, which is δ 0. All spectra were collected at 27 °C in either C6D6 or C6H6/C6D6.) Preparation of Dihydrido tetrakis(triphenylphosphine)ruthenium(II), [RuH2(PPh3)4] A 1:1 methanol/toluene solvent mixture (40 mL) is prepared in a 50-mL three-neck round-bottom flask equipped with a heating mantle, stir bar, reflux condenser, and mineral oil bubbler. This mixture is degassed by refluxing for 1 hour under a flow of argon. (Alternatively, to save time, the solvents can be purified by distilling under argon before use, as was done above for methanol.1) The flask is cooled to room temperature and the above [RuCl2(PPh 3 )3] (0.20 g, 0.20 mmol), triphenylphosphine (1.2 g, 4.6 mmol), and sodium borohydride
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
In the Laboratory
(0.16 g, 4.2 mmol) are added successively under a stream of argon, with stirring, to obtain a light yellow solution, which turns to a deep canary yellow mixture. The mixture is stirred 1 hour at room temperature to yield a voluminous precipitate. The product is isolated by either filtration or by cannula decantation with washes of degassed methanol (2 × 20 mL) and diethyl ether (3× 20 mL) under argon to obtain 0.1–0.2 g (50–90%) of [RuH2(PPh3)4]. The decanting operation can be accomplished most economically by a filter cannula (see Fig. 1A) (4). Anhydrous diethyl ether is transferred with a stream of argon. Students transfer dry and degassed C6D6, which the instructor has purified,2 by vacuum transfer to valved NMR tubes (see Fig. 1B). 31P{1H} NMR (C6D6, Ar) shows a mixture of “free” PPh3 at δ ᎑5.5, [RuH2(PPh 3) 3] at δ 57, and [RuH2(PPh 3) 4] at δ 48.8 (2P, triplet, P trans to P, J = 14 Hz) and 40.6 ppm (2P, P trans to H, J = 14 Hz) (see Fig. 2A). Selective decoupling of the aromatic region at δ 7.2 (decoupling only aryl protons) yields two different multiplet patterns (hydride coupling to a cis phosphorus is generally less than half of that to a trans phosphorus [see Fig. 2B]) (1b). The preparation of Nujol mulls does require any special precautions. IR data shows νRuH(Nujol) = 2080 cm᎑1.
(A)
inert gas vacuum
evacuated flask
(B)
O-ring joints Schlenk tube Valved NMR tube
Discussion Typical student yields of [RuCl2 (PPh 3) 3], dichlorotris(triphenylphosphine)ruthenium(II), are ≈100% and of the yellow product, [RuH 2 (PPh 3) 4], or dihydridotetrakis(triphenylphosphine)ruthenium(II), 50–90%. The 31P NMR experiment is particularly telling with regard to the experimentalist’s technique because exposure to air causes darkening of solutions and the appearance of a triphenylphosphine oxide peak at δ ~23 ppm in the 31 P NMR. Further, nitrogen causes the formation of [RuH2(N2)(PPh3) 3]. NMR solutions are prepared most effectively in a dry, degassed solvent using NMR tubes sealed with valves.3 Performing the same experiment without exogenous phosphine under hydrogen produces [RuH2(H2)(PPh 3 )3] in similar yields [1H NMR: δ ᎑7.05 (br s,Ru–H)]. Slow bubbling of nitrogen or carbon monoxide (CAUTION: Use a well-ventilated hood) through these solutions in an NMR tube for 5 min gives solutions of light tan [RuH2(N 2)(PPh 3)3] and colorless [RuH2(CO)(PPh3)3] as confirmed by 31P{1H} NMR. Spectral data in the hydride region also confirm the structures: 31P{1H} NMR: [RuH2(N2)(PPh 3) 3]: δ 56.0 (d, 2P, JPP = 15.7 Hz, P trans to P); 43.4 (t, 1P, P trans to H); [RuH2(CO)(PPh 3) 3]: δ 61.8 (d, 2P, JPP = 17.5 Hz, P trans to P); 47.5 (t, 1P, P trans to H). The infrared spectra show ν N≡N(Nujol) = 2150 cm᎑1, and ν C≡O(Nujol) = 1940 cm᎑1. These species also show doublets at 1950–1910 cm᎑1 for ν RuH(Nujol). Acknowledgments I wish to acknowledge the students in CHM 343 for their cooperation in developing this experiment. William Fordyce, Dow Chemical Company, suggested the modification of the original Inorganic Synthesis preparation of [RuCl 2(PPh3)3]. James Whitcraft designed the line art. Notes 1. Solvent stills (3 L) with solvent reservoirs (500 mL) can be set up in a hood to facilitate this laboratory. Here methanol is distilled from
Figure 1. (A) Modification of the filtration cannula designed by M. L. H. Green, in which a stainless steel cannula is attached to a Luer hub to tubing adapter. The tubing end of the adapter is covered by neatly wiring a firm, folded piece of filter paper. Here is shown in a solution transfer apparatus with this type filter cannula, which is used to isolate the yellow solid hydride compound, [RuH2(PPh3)4]. (B) A vacuum Schlenk line (has both vacuum and inert atmosphere manifolds) equipped with O -ring connections is shown. To transfer the NMR solvent to a valved NMR tube here the Schlenk tube containing the solvent (on right) and inert atmosphere manifold are under solvent vapor. The NMR tube containing 4–5 mg of compound is evacuated and then cooled with liquid nitrogen. The NMR tube is then closed to vacuum and the stopcock to the manifold is opened. Approximately 0.8 mL of solvent is transferred, the valve on the NMR tube is closed, and the tube is removed from the system.
magnesium methoxide under argon and is transferred readily within 10 minutes to each student’s apparatus using an 18-gauge cannula. Toluene can be purified by distillation from sodium metal. Diethyl ether (pint container of anhydrous grade sealed with a 10-mm septum) is transferred under argon by cannula. 2. Preparation of the NMR solvent (C6H6) is best done by the instructor by stirring with sodium benzophenone ketyl at 65 °C overnight, freeze-pump-thaw degassing, and transferring on a vacuum line. 3. Several recent innovations have made it more easy to study the NMR of air-free solutions (see “Valves, Tubes and Air Sensitive Samples in NMR” at www.wilmad.com/html/nf/NMR006.html ). The only modification to the tubes with threaded valves is the addition of O-ring joints for connection to a vacuum line (see Fig. 1B).
Literature Cited 1. (a) Shriver, D. H.; Atkins, P.; Langford, C. H. Inorganic Chemistry, 2nd ed.; Freeman: New York, 1994; Chapters 9 and 16. (b) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals; Wiley: New York, 1988; Chapters 3 and 10.
JChemEd.chem.wisc.edu • Vol. 76 No. 1 January 1999 • Journal of Chemical Education
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In the Laboratory 2. Szafram, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic Chemistry; Wiley: New York, 1991; Chapter 9. 3. Hallman, P. S.; Stephenson, T. A.; Wilkinson, G. Inorg. Synth. 1970, 12, 238. Knoth, W. H. Inorg. Synth. 1974, 15, 31. Harris, R. O.; Hota, N. K.; Sadavoy, L.; Yuen, J. M. C. J. Organomet. Chem. 1973, 54, 259. 4. Shriver, D. F.; Dredzon, M. A. The Manipulation of Air-Sensitive Compounds; Wiley: New York, 1986; Chapter 1.
(A) H [RuH2(PPh3)3]
H
Ph3PA
Ru
PAPh3
Ph3PB PBPh3 PBPh3
60
50
40
30
PPh3
20
10
0
-10
δ
(B)
48
48
49
39.8
40.8
49
39.8
40.8
δ
(ppm)
δ
Figure 2. (A) 31 P{1H} NMR spectrum of 5 mM [RuH2(PPh3)4] in benzene at 27 °C. Solutions were prepared using NMR tubes which are sealed with valves (Conditions: spectral frequency = 80.9843 MHz; spectral width = 20 kHz; acquisition time = 1.17 s; tip angle = 30°; repetition time = 3 s; data points = 48 k; waltz-16 decoupling at 0.8 W; number of transients = 360; time of experiment = 22 min.) (B) Top: Expanded view of resonances due to [RuH2(PPh3)4]. Bottom: As above, except off-resonance decoupled with continuous irradiation of aryl protons (δ 7.2); number of transients = 1200; time of experiment = 83 min. The peak at d 40.8 ppm is assigned to the phosphorus atoms mutually trans to phosphine ligands (PA) and the peak at d 41.8 corresponds to the phosphorus atoms trans to hydride ligands (PB). The 1H and 31P NMR are performed simultaneously. The off-resonance decoupling experiment is an optional exercise. Another optional assignment is a 1H NOESY experiment to demonstrate the reaction [RuH2(PPh3)4] = [RuH 2(PPh3)3] + PPh3.
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