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42. ACETYLIDE COMPLEXES OF RUTHENIUM

Submitted by FREDERICK R. LEMKE*yChecked by TRACEY A. HITT,z JENNIFER L. STEELEz, and

GREGORY S. GIROLAMIz

Acetylenes can react with metal complexes in a number of ways. Internalacetylenes usually coordinate to metal centers in a η2-fashion, in which theCºC triple bond donates electrons to the metal center, and both carbon atomsformmetal-carbon bonds. Terminal acetylenes can also coordinate to metal centersin this fashion, but can react in other ways as well. In particular, because theacetylenic hydrogen atom is relatively acidic for a hydrocarbon, the C-H bond ofterminal acetylenes can also add oxidatively to metal centers to form metalacetylide complexes. In some cases, however, a third kind of reaction is seen,in which the terminal acetylene rearranges to a vinylidene complex, M�C�CHR.An example of this rearrangement is the reaction of the ruthenium cyclopenta-dienyl complex (η5-C5H5)RuCl(PPh3)2 with phenylacetylene to give [(η5-C5H5)Ru(�C�CHPh)(PPh3)2]�.

Closely related to this vinylidene complex is the ruthenium acetylide compound(η5-C5H5)Ru(CºCPh)(PPh3)2, which was originally prepared by treatment of thechloro complex (η5-C5H5)RuCl(PPh3)2 with copper phenylacetylide, CuCºCPh.1

Later, the acetylide complex was prepared by accident during attempts to purify thevinylidene complex [(η5-C5H5)Ru(�C�CHPh)(PPh3)2]� by column chromatog-raphy on alumina.2 The alumina acted as a base and deprotonated the vinylideneligand, and subsequently it was found that the vinylidene complexes could easilybe deprotonated by addition of many different kinds of bases such as sodiummethoxide, methyllithium, or sodium bicarbonate. The deprotonation of themonosubstituted vinylidene complexes to the corresponding acetylide complexesproceeds in high yield, and consequently this reaction sequence is the syntheticmethod used in the following preparation of the acetylide complexes (Scheme 1).The experimental procedure is a scaled-down and simplified version of a prepara-tion previously published in Inorganic Syntheses.3

The vinylidene/acetylido interconversions carried out in this experiment aresimilar to those seen for other metal complexes4,5 and are relevant to the known*Science and Math Division, Mott Community College, 1401 East Court Street, Flint, MI 48503.yDeceased December 20, 2013.zSchool of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801.

Inorganic Syntheses, Volume 36, First Edition. Edited by Gregory S. Girolami andAlfred P. Sattelberger 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.

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ability of (η5-C5H5)RuCl(PPh3)2 to catalyze the dimerization of phenylacetylene toa mixture of Z- and E-1,4-diphenyl-1-buten-3-yne.6 The chemistry of rutheniumcyclopentadienyl complexes has been reviewed.7

A. (CYCLOPENTADIENYL)BIS(TRIPHENYLPHOSPHINE)-CHLORORUTHENIUM(II)

RuCl3 ? 3H2O � C5H6 � 52 PPh3 ® �η5-C5H5�RuCl�PPh3�2

� 12 O�PPh3 � 2HCl � 5

2 H2O

Procedure3

The reaction is carried out in a 500mL two-necked round-bottomed flask equippedwith a magnetic stir bar, dropping funnel, and a reflux condenser topped with anitrogen bypass. The apparatus is purged with nitrogen and charged with triphe-nylphosphine (8.4 g, 32mmol) and degassed ethanol (300mL).* The triphenyl-phosphine is dissolved in the ethanol by heating to reflux. Hydrated rutheniumtrichloride (2.1 g, 8mmol) is dissolved in boiling ethanol (40mL), and thenallowed to cool. Freshly distilledy cyclopentadiene (4mL, 50mmol) is addedto the ruthenium trichloride solution, and the mixture is transferred to the droppingfunnel. The dark brown solution is then added to the refluxing triphenylphosphinesolution over 10min. The reaction mixture is dark brown in color, which after 1 hlightens to a dark red-orange with an orange precipitate.z The solution is now safeto expose to air. The solution is cooled to room temperature to complete theprecipitation. The solid is collected by filtration, washed with ethanol (4� 10mL)and hexanes (4� 10mL), and dried in vacuum. Yield: 5.14 g (89%).

*The checkers note that the ratio of 150mL of ethanol per gram of hydrated RuCl3 is important to get a goodyield in the final precipitation step: for example, the yield drops considerably if 200mLg�1 is used.yThe checkers cracked the dicyclopentadiene to cyclopentadiene byheating it in aflask immersed in a siliconeoil bath kept at 200°C and distilling through a 10 in. tall Vigreux column at atmospheric pressure.zThe checkers did not see the orange precipitate until after the solution is allowed to cool. They allowedthe solution to stand overnight before filtering it.

Ru

Ph3P

Ph3P

Cl Ru

Ph3P

Ph3P

CCPhHC

MeOHCHPh

+

Cl– Ru

Ph3P

Ph3P

CMeOH

CPhNaOMe

Scheme 1. Reaction sequence to form the ruthenium phenylacetylide complex.

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Anal. Calcd. for C41H35ClP2Ru: C, 67.7; H, 4.8. Found: C, 67.8; H 5.3.

Properties

(η5-C5H5)Ru(PPh3)2Cl is an orange solid that is stable to air and moisture. It issparingly soluble in saturated hydrocarbons and alcohols, but is soluble in aromatichydrocarbons, chlorocarbons, acetonitrile, and acetone. 1H NMR (CDCl3): δ 7.36(br, JHH� 7.5Hz,o-CH), 7.23 (t, JHH� 7.5Hz,p-CH), 7.14 (t, JHH� 7.5Hz,m-CH),4.08 (s, C5H5). 31P{1H} NMR (CDCl3): δ 38.6 (s). Melting point: 130–133°C.

Characterization

Record the yield. Characterize the product by IR and 1H NMR spectroscopies andmelting point.

B. (CYCLOPENTADIENYL)BIS(TRIPHENYLPHOSPHINE)-(PHENYLACETYLIDO)RUTHENIUM(II)

�η5-C5H5�RuCl�PPh3�2 � HCºCPh® ��η5-C5H5�Ru��C�CHPh��PPh3�2�Cl��η5-C5H5�Ru��C�CHPh��PPh3�2�Cl � NaOCH3 ®

�η5-C5H5�Ru�CºCPh��PPh3�2 � NaCl � HOCH3

Procedure3

(η5-C5H5)RuCl(PPh3)2 (1.45 g, 2mmol) is suspended in methanol (100mL) in a200mL two-necked flask equipped with a reflux condenser and a nitrogen bypass.Phenylacetylene (0.33mL, 3mmol) is added dropwise to the suspension, which iskept strictly under nitrogen, and then the mixture is heated to reflux temperature for30min. The (η5-C5H5)RuCl(PPh3)2 gradually dissolves to form a dark red solu-tion. After the solution has cooled, sodium metal (0.2 g, 9mmol) is added in smallpieces whereupon a yellow crystalline precipitate forms as the sodium dissolves.The solid is filtered from the mother liquor, washed with methanol (4 � 3 mL) andhexanes (4� 3mL), and dried in vacuum. Yield: 1.37 g (86%).

Anal. Calcd. for C49H40P2Ru: C, 74.3; H, 5.1; P, 7.8. Found: C, 73.7; H, 5.4; P, 7.6.

Properties

(η5-C5H5)Ru(CºCPh)(PPh3)2 is a yellow solid that is only mildly sensitive to airand moisture. It can be recrystallized from CH2Cl2/MeOH. IR: 2065 cm�1 (sharp,CºC stretch). 1H NMR (CDCl3): δ 4.30 (s, C5H5), 7.06 (m,m-CH and p-CH), 7.52

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(br, o-CH). 31P{1H} NMR (CDCl3): δ 51.0 (s). 13C{1H} NMR (CDCl3): δ 85.2(s, C5H5), 123.5 (s, CPh), 127.3–140.1 (Ph).

*Mass spectrum:m/e 792 (parent ion).Melting point: 202–205°C (dec.).

Many related compounds with other acetylenes can be made, including(η5-C5H5)Ru(CºCR)(PPh3)2, where R� p-tolyl, n-pentyl, n-hexyl, and so on.

Characterization

Record the yield. Characterize the product by IR and 1H NMR spectroscopies andmelting point.

Questions

1. Cyclopentadiene, which is a reagent in the preparation of (η5-C5H5)RuCl-(PPh3)2, is not commercially available. Why not, and how is it obtained forthis experiment?

2. Propose a mechanism for the reaction of (η5-C5H5)RuCl(PPh3)2 withHCºCPh to give the vinylidene compound [(η5-C5H5)Ru(�C�CHPh)-(PPh3)2]

�.3. In the IR spectrum, the CºC stretching frequencies of phenylacetylene and

(η5-C5H5)Ru(CºCPh)(PPh3)2 are 2111 and 2068 cm�1, respectively.Explain the difference in terms of the bonding between Ru and the acetylidegroup. It may be of interest to know that the CºC stretch in lithiumphenylacetylide, LiCºCPh, appears at 2030 cm�1.

4. What would be the product of adding 1 equiv of HCl to (η5-C5H5)Ru-(CºCPh)(PPh3)2?

5. Which compound should be easier to oxidize by one electron, (η5-C5H5)-RuCl(PPh3)2 or (η5-C5H5)Ru(CºCPh)(PPh3)2?

Notes for Instructors

Cyclopentadiene is prepared by the thermal cracking of its Diels–Alder dimer,dicyclopentadiene (3a,4,7,7a-tetrahydro-4,7-methanoindene). It can be preparedby placing dicyclopentadiene (50mL) into a 500mL two-necked round-bottomedflask fitted with a 15 cm Vigreux column topped by a water-cooled distillationhead. The flask should have at least 10 times the volume of dicyclopentadiene, soas to contain the large amounts of foam generated during heating, which can easilytravel up the Vigreux column and contaminate the desired cyclopentadiene

*Although the 13C NMR resonance for the ruthenium-bound carbon of the phenylacetylide ligand hasnot been reported, in similar compounds this resonance appears near δ 230.

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product. A thermometer is placed in the thermometer well of the distillation head,and the distillation head is also attached to a 100mL receiving flask. Thedistillation is conducted under nitrogen with a heating bath at 130°C. The cyclo-pentadiene distills over at a head temperature of 40°C. Cyclopentadiene is stablefor several hours at room temperature, but it should be used quickly becauseeventually it redimerizes. Dimerized cyclopentadiene can be saved and reused inthe cracking apparatus.

As a special project or extension of this experiment, the students could isolateand characterize the intermediate vinylidene complex, either by working up thereaction of (η5-C5H5)RuCl(PPh3)2 with phenylacetylene without adding sodiummethoxide or by protonation of the isolated phenylacetylide complex withNH4PF6.

3

Another special project would be to employ different para-substituted phenyl-acetylenes (HCºCC6H4X, where X�H, F, Me, OMe) and evaluate how theπ-bonding between the ruthenium fragment and the acetylide depends on theelectronic properties of the para-substituent.8

References

1. O. M. Abu Salah and M. I. Bruce, J. Chem. Soc., Dalton Trans. 2311 (1975).2. M. I. Bruce and R. C. Wallis, Aust. J. Chem. 32, 1471 (1979).3. M. I. Bruce, C. Hameister, A. G. Swincer, and R. C. Wallis, Inorg. Synth. 21, 78 (1982).4. A. Davison and J. P. Selegue, J. Am. Chem. Soc. 100, 7763 (1978).5. A. Davison and J. P. Selegue, J. Am. Chem. Soc. 102, 2455 (1980).6. R. U. Kirss, R. D. Ernst, and A. M. Arif, J. Organomet. Chem. 689, 419 (2004).7. M. O. Albers, D. J. Robinson, and E. Singleton, Coord. Chem. Rev. 79, 1 (1987).8. F. Paul, B. G. Ellis, M. I. Bruce, L. Toupet, T. Roisnel, K. Costuas, J. F. Halet, C. Lapinte,

Organometallics 25, 649 (2006).

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