In the Laboratory
Ruthenium Complexes with h1-Organic Ligands An Advanced Undergraduate Inorganic/Organometallic Chemistry Experiment Marie P. Cifuentes, Fiona M. Roxburgh, and Mark G. Humphrey* Australian National University, Canberra, ACT 0200, Australia
Organometallic chemistry courses typically introduce students to a wide variety of both monohapto and polyhapto ligands. Many experiments illustrating the reactivity of polyhapto organic ligands exist (e.g., 1–7), of which the majority focus on embellishment of the stable ferrocene unit. In contrast, experiments containing transformations of important monohapto organic ligands are comparatively scarce. We present below an experiment involving interconversions of several monohapto organic ligands that are often met in an advanced undergraduate organometallic chemistry course. The reactions to be carried out are summarized in Scheme I, the synthetic aspects of which can be comfortably accommodated in about 8 h of laboratory time. The experiment can be performed utilizing a vacuum-line double-manifold system, but can also be carried out successfully using a simple nitrogen manifold. The latter conditions are described below. +
Ru Ph3P
OMe
HC CCO2Me
Cl
MeOH, ∆ , 2 h
PPh3
Ru Ph3P
C
PPh3
CH2CO2Me
NaOMe MeOH RT, 5 min
OMe Ru Ph3P
CHCl3, ∆, 45 min
C C
O
H
Ph3P
OMe Ru
C
PPh3
CHCO2Me
C OMe
Scheme I
Experimental Procedure
General Considerations We recommend carrying out all reactions and manipulations in an efficient fume hood if possible. All reactions should be carried out in nitrogen-purged flasks with nitrogensaturated solvents. Chloroform (AR grade, used as received) and methanol (freshly distilled from magnesium/iodine prior to the laboratory session) are distributed to students when needed. RuCl(PPh 3)2( η5-C5H5) (8, 9) can be prepared in large (13 g) quantities before the laboratory class, stored under air for months if necessary, and distributed to students as required. Methyl propiolate can be obtained by published *Corresponding author. Email:
[email protected].
procedures (esterification of commercially available [Aldrich] propiolic acid) (10). It is distilled as an (approximately) 1:2 mixture with methanol (the exact ratio can be rapidly ascertained by 1H NMR) and can then be nitrogen-saturated and distributed by laboratory staff as needed. The sodium methoxide solution can be prepared by students, or prepared by laboratory staff to more efficiently utilize the students’ laboratory time. Its preparation is straightforward: sodium metal (0.2 g) is cut under petrol (40–60 °C boiling range) and then added to dry methanol (20 mL); the resultant solution of sodium methoxide is deoxygenated thoroughly by bubbling N2 through it vigorously for 10 min. Although ruthenium is a comparatively inexpensive platinum group metal, its use in an undergraduate class can become costly. Consequently, it is recommended that all residues be collected and the ruthenium recycled by laboratory staff following established procedures (11).
Preparation of [Ru{C(OMe)CH2CO2Me}(PPh3)2(η5C5H5)]PF6 ( 12) RuCl(PPh3 )2(η5 -C5H 5) + HC≡CCO2Me + MeOH + NH4PF6 → [Ru{C(OMe)CH2 CO2 Me}(PPh3) 2(η5 -C5 H5 )]PF6 + NH4 Cl
A three-necked 500-mL round-bottomed flask is equipped with a nitrogen supply ( T-piece to an oil bubbler) above a water-cooled condenser and magnetic stirrer. Place RuCl(PPh 3) 2( η5-C 5H5) (500 mg, 0.69 mmol), NH4PF6 (337 mg, 2.07 mmol), and methanol (150 mL) into the roundbottomed flask, using the methanol to wash the solids into the flask. A screw-cap adapter (“thermometer jack”) containing a Pasteur pipet connected to a N 2 supply is inserted into one of the other necks of the flask, and N2 is bubbled through the solution for 10 min to thoroughly deoxygenate it. The screw-cap adapter is then replaced with a stopper. Methyl propiolate (ca. 280 mg, excess, as a mixture with deoxygenated MeOH as above) is added against a positive pressure of N2. The mixture is heated to reflux and maintained at this temperature for 2 h, during which time there is a slight but perceptible darkening in color. It is then allowed to cool to room temperature, affording a dark orange solution. The mixture is transferred to a suitable one-necked flask and the solvent is removed using a rotary evaporator. At this stage the residue can, if necessary owing to time constraints, be stored under N 2. It is dissolved in a small amount of dichloromethane and the solution is filtered (to remove NH4Cl); the solvent is then removed on the rotary evaporator. The residue is triturated with diethyl ether (to remove unreacted methyl propiolate) to afford the carbene complex [Ru{C(OMe)CH2CO2Me}(PPh3)2( η5-C5H5)]PF6 as a mustard-yellow powder (yields typically 70%).
JChemEd.chem.wisc.edu • Vol. 76 No. 3 March 1999 • Journal of Chemical Education
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In the Laboratory
Preparation of Ru{C(OMe)=CHCO2Me}(PPh3) 2(η5C5H5) ( 13 ) [Ru{C(OMe)CH2CO2 Me}(PPh3 )2 (η5-C 5H 5)]PF6 + MeO{ → Ru{C(OMe)=CHCO2 Me}(PPh3)2 (η5 -C5 H5) + MeOH
A two-necked 100-mL round-bottomed flask is equipped with a nitrogen supply (as above, but without the condenser) and magnetic stirrer. Methanol (30 mL) is added to this flask and is deoxygenated by purging with N2 for 10 min. The carbene complex [Ru{C(OMe)CH2CO2 Me}(PPh 3) 2( η5 C5H 5)]PF6 (300 mg, 0.315 mmol), prepared as above, is added to the round-bottomed flask against a positive flow of N2. This is quickly followed by the deoxygenated sodium methoxide solution (see “General Considerations”). The carbene complex dissolves and a fine yellow precipitate develops over a few minutes; after stirring for 5 min, this precipitate is collected by filtration to give a bright-yellow microcrystalline solid of the σ-vinyl complex Ru{C(OMe)=CHCO2Me}(PPh3)2(η5-C5H5) A typical yield is 60%. |––––––––––––––––––––––––––|
Preparation of Ru{C(OMe)=CHC(O)OMe}(PPh3)(η5C 5 H5) ( 13 )
bottomed flask, and the solvent is removed on a rotary evaporator. At this stage the sample can, if necessary owing to time constraints, be stored under N2. The residue is triturated with the minimum quantity of diethyl ether (ca. 2–3 mL) for 10 min (to remove PPh3), to afford a yellow-orange powder of the cyclic vinyl complex |–––––––––––––––––––––––––|
Ru{C(OMe)=CHC(O)OMe}(PPh3 )(η5 -C5 H5 )
(NOTE: It may be necessary to triturate for longer periods.) The product is washed with the minimum amount of cold petrol (60–80 °C boiling range). Care needs to be taken with this washing step because the product is slightly soluble in petrol. Yields are typically 60%. Results The products can be identified from their 1H NMR (CDCl3 or d6-acetone) and IR spectra (dichloromethane solvent or KBr disc) (which the students are asked to obtain) and their 13C NMR and mass spectra (which are supplied). Some important data are listed in Table 1. Students will observe the importance of IR and NMR spectroscopy in the characterization of organometallic compounds and obtain evidence for
Ru{C(OMe)=CHCO2 Me}(PPh3)2 (η5 -C5 H5)] → –––––––––––––––––––––––––|
|
Ru{C(OMe)=CHC(O)OMe}(PPh3 )(η5 -C5 H5 ) + PPh3
Ru{C(OMe)=CHCO2Me}(PPh3)2(η5-C5H5) (the σ-vinyl complex) (100 mg, 0.124 mmol), prepared as above, is dissolved in chloroform (20 mL) in a two-necked 50-mL round-bottomed flask equipped with a nitrogen supply and a water-cooled condenser, as above. The resultant solution is thoroughly deoxygenated. It is then heated to reflux and maintained at this temperature for 45 min, during which time the color of the solution deepens to yellow-orange. The flask is then cooled, the solution is transferred to a one-necked round-
Unusual chemical shifts of carbons formally multiply bonded to metals. The carbene carbon resonates at 298 ppm, and the metal-bound carbons of related vinylidene complexes (see Scheme II) resonate around 360 ppm, both of which are substantially downfield of the normal organic region. Utility of the isotope distribution in the mass spectra in confirming product identity for many metal complexes. If the students have not been exposed to the variety of ionization techniques currently available, a short discussion contrasting the more common soft ionization techniques such as fast atom bombardment (FAB) with the classical electron impact (EI) ionization may be useful. Reduction of ester carbonyl stretching frequency on O-coordination, on proceeding from the σ-vinyl complex to the chelating cyclic vinyl complex.
Table 1. Important Spectroscopic Data for the [RuX(PPh3)m(η η-C5H5)]n+ Complexes X
m
n
IR Data/cm{1 a
C(OMe)CH2CO2Me
2
1
C(OMe)=CHCO2Me
2
C(OMe)=CHC(O)OMe 1
a d
402
NMR Data
Mass Spectroscopy Data c
δ(1H)/ppm b
δ(13C)/ppm b
ν(CO) 1735 ν(PF) 847
C5H5 4.84 CH2 4.53 RuCOCH3 3.74 C(O)OCH3 3.41
CH2 53.1 OCH3 62.0, 62.1 C5H5 92.1 C=O 165.2 RuC 298.3 d
807 (M+)
0
ν(CO) 1692
Major isomer OCH3 55.0, 58.3 C5H5 85.9 =CH 102.2 C=O 171.0 Minor isomer OCH3 49.7, 62.7 C5H5 87.4 =CH 103.4 C=O 181.9
807 ([M+H]+)
0
ν(CO) 1550
Major isomer =CH 5.38 C5H5 4.34 RuCOCH3 3.67 C(O)OCH3 2.45 Minor isomer =CH 5.76 C5H5 4.23 RuCOCH3 3.52 C(O)OCH3 3.08 =CH 5.05 C5H5 4.37 RuCOCH3 3.59 C(O)OCH3 3.28
C5H5 92.2
In CH 2Cl2 solution. b In CDCl3/SiMe4 solution. c Fast atom bombardment, 3-O2NC 6H4CH2OH matrix. JC- P 16 Hz.
Journal of Chemical Education • Vol. 76 No. 3 March 1999 • JChemEd.chem.wisc.edu
In the Laboratory Z and E isomers of the vinyl complex, characterized by pairs of resonances in the 1H NMR spectrum. N OTE : Some reaction of the σ-vinyl complex to form the cyclic vinyl complex occurs on acquiring the 13C NMR spectrum at ambient temperature.
affords a mixture of Z and E isomers in the present case, but can be directed to one stereochemistry by varying the nature of the co-ligands or acetylene precursor (13). + H+
Discussion
[M]
C
The last point can be understood by “pushing electrons” to (cation) and from (neutral complex) the 18-electron metal, as can the metal-promoted facile alkene isomerization (which occurs on converting the 3:1 isomer ratio of the vinyl complex to the all-Z geometry of the cyclic complex). The carbene and σ-vinyl complexes prepared as above belong to a set of important interconvertible η1-organic ligands (see Scheme II). Reaction of a terminal acetylene with RuCl(PPh3)2(η 5-C5H 5) proceeds by loss of chloride and coordination of a vinylidene ligand (formed by a 1,2-hydrogen shift at the acetylene). In the present case, the vinylidene complex reacts quickly with methanol, by way of a formal addition across the C=C bond. Note that the vinylidene complex also reacts with O 2, by means of a cycloaddition across the C=C bond to afford [Ru(CO)(PPh3)2( η5-C 5H5)]+, easily identifiable from its IR (ν (CO) 1980 cm{1) and 1H NMR spectra (δ(C5H 5) 4.99 ppm). This provides a useful check on how carefully students have conducted the work under an inert atmosphere. For other terminal acetylenes (e.g., phenylacetylene), it is possible to isolate the intermediate vinylidene complex, which may be deprotonated to the corresponding acetylide complex (9). Deprotonation of the carbene complex
H [M]+
R - H+
Several important aspects of organometallic and inorganic chemistry are illustrated in this experiment, around which suitable exercises may be constructed: The EAN rule. All complexes prepared here possess 18 valence electron configurations. “Tailoring” coordination mode and reactivity by choice of ligands. The steric bulk of the cyclopentadienyl and phosphine co-ligands favors η1 -vinylidene rather than η2alkyne coordination and ensures that nucleophilic and electrophilic reactivity are ligand centered rather than at the metal. The stabilization of reactive organic species (examples: vinylidene, carbene) utilizing transition metal centers. These ligands have minimal independent existence (e.g., the lifetime of free vinylidene is 10{10 s). The significance of a transition metal in “directing” reactivity to a specific carbon of the organic ligand. For the present series of complexes, the neutral complexes add electrophiles at the carbon β to the metal, the cationic complexes add nucleophiles at the carbon α to the metal, and the cationic complexes react with bases by proton abstraction from the β carbon.
C
η1-acetylide
C
C
η1-vinylidene
R
MeOH
MeO
+ H+ C
MeO
CHR
C - H+
[M] η1-vinyl
CH2R
+
[M]
η1-carbene
Scheme II Interconversion of some η1-organic ligands
The syntheses and some reactivity (phosphine substitution, hydride formation, reactions with xanthates or tin(II) chloride) of RuCl(PPh3) 2(η5-C5H5) have been reported in this Journal (8). It is clearly possible to link these complementary experiments. Alternatively, extension of the chemistry described above to the less reactive phenylacetylene, so as to permit isolation and interconversion of representative η1-vinylidene and η1acetylide complexes, would also be useful (9). Literature Cited 1. 2. 3. 4.
Gokel, G. W.; Ugi, I. K. J. Chem. Educ. 1972, 49, 294. Lombardo, A.; Bieber, T. I. J. Chem. Educ. 1983, 60, 1080. Gilbert, J. C.; Monti, S. A. J. Chem. Educ. 1973, 50, 369. Pinhas, A. R.; Kevill, D. N.; D’Souza, M. J. J. Chem. Educ. 1992, 69, 1034. 5. Newirth, T. L.; Srouji, N. J. Chem. Educ. 1995, 72, 454. 6. Davis, J.; Vaughan, D. H.; Cardosi, M. F. J. Chem. Educ. 1995, 72, 266. 7. Barnett, K. W. J. Chem. Educ. 1974, 51, 422. 8. Ballester, L.; Gutiérrez, A.; Perpiñán, M. F. J. Chem. Educ 1989, 68, 777. 9. Bruce, M. I.; Hameister, C.; Swincer, A. G.; Wallis, R. C. Inorg. Synth. 1982, 21, 78. 10. Extension of the procedure described in Vogel, A. I. Practical Organic Chemistry, 4th ed.; Longman: London, 1984; p 501. 11. Cenini, S.; Mantovani, A.; Fusi, A.; Keubler, M. Gazz. Chim. Ital. 1975, 105, 267. 12. Adapted from Bruce, M. I.; Swincer, A. G. Aust. J. Chem. 1980, 33, 1471. 13. Adapted from Bruce, M. I.; Duffy, D. N.; Humphrey, M. G.; Swincer, A. G. J. Organomet. Chem. 1985, 282, 383.
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