Microwave Synthesis of Benchmark Organo-Iron Complexes

Nov 6, 2009 - Sean M. Garringer, Andrew J. Hesse, John R. Magers, Kristopher R. Pugh,. Stacy A. O'Reilly, and Anne M. Wilson*. Clowes Department of ...
1 downloads 0 Views 747KB Size
Download PDF
Organometallics 2009, 28, 6841–6844 DOI: 10.1021/om900821c

6841

Microwave Synthesis of Benchmark Organo-Iron Complexes Sean M. Garringer, Andrew J. Hesse, John R. Magers, Kristopher R. Pugh, Stacy A. O’Reilly, and Anne M. Wilson* Clowes Department of Chemistry, Butler University, 4600 Sunset Avenue, Indianapolis, Indiana 46208 Received September 21, 2009 Summary: Microwave-assisted reaction techniques have been applied to the formation of a variety of organo-iron species. The species synthesized include ferrocene and acetylferrocene, piano stool complexes such as CpFe(CO)2I, CpFe(PPh3)(CO)I, and CpFe(PPh3)(CO)(COMe), and bisphosphine iron complexes. The use of microwave-assisted reactions has decreased reaction times while maintaining or improving yields as compared to traditional methods.

Ever since the first reports of ferrocene,1 organo-iron complexes have become staples of classical organometallic synthesis. The ease of formation, commercial availability, relative cost, and synthetic utility of many organo-iron reagents make these ubiquitous compounds available to inorganic and organic chemists alike.2 However, the formation of many of these fundamental compounds requires long reaction times and high heat. These reaction conditions are ideal conditions to exploit microwave-assisted synthesis. Since microwave synthesis was introduced in the literature, there has been an explosion of applications to organic synthesis, inorganic synthesis, and organometallic synthesis.3 Of these, applications to the synthesis of transition metal complexes have been relatively underexploited.4 Reaction sequences requiring a long time at elevated temperatures are ideal candidates for the application of microwave heating. When evaluating organo-iron compounds as targets for microwave synthesis, three classes of compounds were ideal candidates: sandwich complexes; cyclopentadienyl carbonyl

complexes; and phosphine tricarbonyl complexes. Several compounds from each of these classes were chosen for this study. Classical preparations of the sandwich compounds ferrocene5 and acetyl ferrocene6 are known in the literature. Cyclopentadienyl iron carbonyl dimers [CpFe(CO)2]2 (FpFp, Cp = C5H5) and [Cp*Fe(CO)2]27 (Fp*-Fp*, Cp* = C5(CH3)5) are common starting materials for other iron carbonyl reagents. Piano stool compounds CpFe(CO)2I,8 CpFe(PPh3)(CO)I,9 and CpFe(PPh3)(CO)(COMe)10 are also well-known compounds. Lastly, the phosphine carbonyl complexes,11 especially bis-phosphine complexes of iron,12 have also been reported, characterized, and utilized as reactive intermediates in the chemical literature. Many of these classical preparations require refluxing solvent conditions for substantial amounts of time, some even requiring days, in order to make these compounds in yields ranging from poor to excellent. However, we sought to reduce the time required for the synthesis of these complexes to minutes or hours rather than days, as well as improve the yields for some of the poor-yielding processes.

Experimental Section General Procedures. All reagents were purchased from chemical suppliers (Aldrich Chemical Co.) and used without further purification. Infrared spectra were taken with a Nicolet/Thermo Electron Avatar 370 FTIR spectrometer. Nuclear magnetic

*Corresponding author. E-mail: [email protected]. (1) (a) Kealy, T. J.; Pauson, P. J. Nature 1951, 168, 1039. (b) Miller, S. A.; Tebboth, J. A.; Tremaine, J. F. J. Chem. Soc. 1952, 632. (2) Pearson, A. J. Iron Compounds in Organic Synthesis; Academic Press, Inc.: San Diego, 1994. (3) Recent review: Kappe, C. O. Angew. Chem., Int. Ed. 2004, 43, 6250. (4) (a) Baghurst, D. R.; Mingos, D. M. P.; Watson, M. J. J. Organomet. Chem. 1989, 368, C43. (b) Baghurst, D. R.; Mingos, D. M. P. J. Organomet. Chem. 1990, 384, C57. (c) VanAtta, S. L; Duclos, B. A.; Green, D. B. Organometallics 2000, 19, 2397. (d) Ardon, M.; Hayes, P. D.; Hogarth, G. J. Chem. Educ. 2002, 79, 1249. (e) Whittaker, A. G.; Mingos, D. M. P. J. Chem. Soc., Dalton Trans. 2002, 3967. (f) Ardon, M.; Hogarth, G.; Oscroft, D. T. W. J. Organomet. Chem. 2004, 689, 2429. (g) Cooke, J. Chem. Educ. 2008, 13, 353. (5) (a) Jolly, W. L. Inorg. Synth. 1968, 11, 120. (b) Jolly, W. L. The Synthesis and Characterization of Inorganic Compounds; Prentice Hall: Toronto, 1970; pp 484-488. (6) (a) Weinmayr, V. J. Am. Chem. Soc. 1955, 77, 3009. (b) Hauser, C. R.; Lindsay, J. K. J. Org. Chem. 1957, 22, 482. (c) Graham, P. J.; Lindsey, R. V.; Parshall, G. W.; Peterson, M. L.; Whitman, G. M. J. Am. Chem. Soc. 1957, 79, 3416. (d) Bozak, R. E. J. Chem. Educ. 1966, 43, 73. (e) Mohrig, J. R.; Hammond, C. N.; Morrill, T. C.; Neckers, D. C. Experimental Organic Chemistry, A Balanced Approach: Macroscale and Microscale; W. H. Freeman and Company: New York, 1997; pp 220-226.

(7) (a) Piper, T. S.; Wilkinson, G. J. Inorg Nucl. Chem. 1956, 3, 104. (b) King, R. B.; Stone, F. G. A. Inorg. Synth. 1963, 7, 99. (c) King, R. B. Organometallic Synthesis; Academic Press: New York, 1965; pp 151-152. (d) Li, H.-J.; Turnbull, M. M. J. Organomet. Chem. 1991, 419, 245. (8) (a) Treichel, P. M.; Shubkin, R. L.; Barnett, K. W.; Reichard, D. Inorg. Chem. 1966, 7, 1177. (b) Brown, D. A.; Lyons, H. J.; Manning, A. R.; Rowley, J. M. Inorg. Chim. Acta 1969, 3, 346. (9) Bibler, J. P.; Wojcicki, A. Inorg. Chem. 1966, 5, 889. (10) (a) Cotton, F. A.; Parish, R. V. J. Chem. Soc. 1960, 1440. (b) Clifford, A. F.; Mukherjee, A. K. Inorg. Chem. 1963, 2, 151. (c) Clark-Lewis, J. W.; Singh, R. P. J. Chem. Soc. 1964, 2825. (11) (a) Manuel, T. A. Inorg. Chem. 1963, 2, 854. (b) Ittel, S. D.; Tolman, C. A.; Krusic, P. J.; English, A. D.; Jesson, J. P. Inorg. Chem. 1978, 17, 3432. (c) Gallup, D. L.; Morse, J. G. Inorg. Chem. 1978, 17, 3438. (d) Keiter, R. L.; Keiter, E. A.; Hecker, K. H.; Boecker, C. A. Organometallics 1988, 7, 2466. (e) Brunet, J. J.; Kindela, F. B.; Neibecker, D. J. Organomet. Chem. 1989, 368, 209. (f) Therien, M. J.; Trogler, W. C. Inorg. Synth. 1990, 28, 173. (g) Keiter, R. L.; Keiter, E. A.; Boecker, C. A.; Miller, D. R. Synth. React. Inorg. Met. Org. Chem. 1991, 21, 473. (h) Keiter, R. L.; Keiter, E. A.; Boecker, C. A.; Miller, D. R. J. Organomet. Chem. 1991, 21 (3), 473. (i) Luo, L.; Nolan, S. P. Inorg. Chem. 1993, 32, 2410. (j) Li, C.; Nolan, S. P. Organomet. 1995, 14 (3), 1327. (k) Keiter, R. L.; Keiter, E. A.; Boecker, C. A.; Miller, D. R.; Hecker, K. H. Inorg. Synth. 1997, 31, 210. (l) Li, T.; Lough, A. J.; Morris, R. H. Chem.;Eur. J. 2007, 13, 3796. (12) (a) Akhtar, M.; Ellis, P. D.; MacDiarmid, A. G.; Odom, J. D. Inorg. Chem. 1972, 11 (12), 2917. (b) Langford, G. R.; Akhtar, M.; Ellis, P. D.; MacDiarmid, A. G.; Odom, J. D. Inorg. Chem. 1975, 14 (12), 2937. (c) Sowa, J. R., Jr.; Zanotti, V.; Facchin, G.; Angelici, R. J. J. Am. Chem. Soc. 1992, 114, 160.

r 2009 American Chemical Society

Published on Web 11/06/2009

pubs.acs.org/Organometallics

6842

Organometallics, Vol. 28, No. 23, 2009

resonance spectroscopy was performed on a Bruker Avance 250 MHz NMR spectrometer in CDCl3 (Aldrich) unless otherwise noted. Microwave reactions were performed using a CEM Discover LabMate single-mode microwave instrument (CEM Corporation, Matthews, NC). Microwave reactions were performed in specially designed Pyrex tubes equipped with a stir bar and sealed with a Teflon/silicon septum. Solvents (THF and acetonitrile) were purified through an MBraun solvent purification system (MB SPS, alumina). DMF and DMSO were purchased in SureSeal bottles (Aldrich Chemical Co.). Absolute ethanol was purchased from Pharmco Products, Inc. and degassed for 0.5 h prior to use. Chromatography was performed using a Chromatotron radial chromatograph utilizing a 4 mm silica gel plate. All compounds reported were spectroscopically identical to literature values (FTIR, 1H NMR, or both) as characterized in the literature. A list of compounds with CAS numbers, yields (literature references and those reported here), and references for spectroscopic data can be found in the Supporting Information. Ferrocene. To a 10 mL reaction vial equipped with a magnetic stirrer was added KOtBu (0.42 g, 3.73 mmol). The vial was sealed with a septum cap, evacuated, and flushed with nitrogen. Diglyme (5 mL) and dicyclopentadiene (0.25 mL, 1.87 mmol) were added to the reaction vial. The vessel was heated in a microwave reactor (200 W, 10 min, 185 °C, max T = 200 °C, max P = 140 psi). The contents of the vial were shaken and syringed into a Schlenk flask containing a magnetic stirrer and FeCl2 3 4H2O (0.25 g, 1.24 mmol) in 3 mL of DMSO. The microwave vial was rinsed with an additional 5 mL aliquot of DMSO and added to the Schlenk flask. The resulting solution was allowed to stir for 5 min. The solution was then acidified with 1.0 M HCl (4 mL), placed in a separatory funnel, and extracted with 2  15 mL of hexanes. The hexanes solutions were combined, washed with 2  15 mL of water, dried over MgSO4, and filtered. The solution was then concentrated in vacuo to give 0.20 g (86%) of ferrocene. Acetylferrocene. To a 10 mL reaction vial equipped with a magnetic stirrer was added ferrocene (0.20 g, 0.80 mmol), acetic anhydride (0.74 mL, 8.0 mmol, 10 equiv), and one drop of concentrated phosphoric acid as a catalyst. The vial was sealed with a septum cap and allowed to purge under nitrogen for 10 min. The vial was then heated in a microwave reactor (300 W, 5 min, 125 °C, max T = 134 °C, max P = 25 psi). The dark red, oily solid was diluted with methylene chloride (35 mL), washed with water (25 mL), washed with aqueous NaHCO3 (25 mL), dried over MgSO4, and concentrated to give a dark red-orange solid. Chromatography (4:1 hexanes/ethyl acetate) gives 0.17 g (75%) of acetyl ferrocene. [Cp*Fe(CO)2]2. To a 10 mL reaction vial sealed with a septum cap, purged with nitrogen, and equipped with a magnetic stir bar were added Fe(CO)5 (0.05 mL, 0.37 mmol), Cp*-H (0.06 mL, 0.37 mmol), and 1.0 mL of DMF. The vessel was heated in a microwave reactor (200 W, 10 min, 150 °C, max T = 185 °C, max P = 118 psi). The dark purple solution was diluted with CH2Cl2 (10 mL), washed with water (2  10 mL), and dried over MgSO4. Removal of solvent gave 81 mg (88%) of the red-purple crystals. CpFe(CO)2I. To a 10 mL reaction vial equipped with a magnetic stirrer were added [CpFe(CO)2]2 (0.23 g, 0.66 mmol) and iodine (0.17 g, 0.66 mmol). The vial was sealed with a septum cap and allowed to purge under nitrogen for 10 min. THF (5 mL) was added, and the vessel was heated in a microwave reactor (150 W, 10 min, 90 °C, max T = 96 °C, max P = 30 psi). The solvent was evaporated, the resulting solid was triturated with hexanes and filtered, and the residue was washed with hexanes and dried in air to give 0.17 g (90%) of red-brown crystals. CpFe(CO)(PPh3)I. To a 10 mL reaction vial equipped with a magnetic stirrer were added CpFe(CO)2I (0.20 g, 0.70 mmol) and PPh3 (0.17 g, 0.70 mmol). The vial was sealed with a septum

Garringer et al. cap and purged under nitrogen for 10 min. To this was added THF (5 mL), and the vessel was heated in a microwave reactor (150 W, 20 min, 90 °C, max T = 95 °C, max P = 30 psi). Upon cooling to room temperature, the solution was concentrated by half and diluted with hexanes in order to fully precipitate the green crystals out of the reaction mixture. The product was filtered, and the residue was washed with hexanes and dried in air to give 0.29 g (76%) of green crystals. Direct Formation of CpFe(CO)(PPh3)I from [CpFe(CO)2]2. To a 10 mL reaction vial equipped with a magnetic stirrer was added [CpFe(CO)2]2 (0.23 g, 0.66 mmol) and iodine (0.17 g, 0.66 mmol). The vial was sealed with a septum cap and allowed to purge under nitrogen for 10 min. THF (5 mL) was added, and the vessel was heated in a microwave reactor (150 W, 10 min, 90 °C, max T = 99 °C, max P = 36 psi). Upon cooling to room temperature, the reaction vial was uncapped, PPh3 (0.34 g, 1.32 mmol) was added, the septum cap was replaced, and the vial was purged again with nitrogen. The reaction vessel was heated again in the microwave reactor (150 W, 10 min, 90 °C, max T = 96 °C, max P = 32 psi). Upon cooling to room temperature, the solution was concentrated by half and diluted with hexanes in order to fully precipitate the green crystals out of the reaction mixture. The product was filtered, and the residue was washed with hexanes and dried in air to give 0.55 g (77%) of green crystals. CpFe(CO)(PPh3)(COMe). To a 10 mL reaction vial was added PPh3 (0.20 g, 0.80 mmol) and CpFe(CO)2Me7a,13 (0.15 g, 0.80 mmol). A septum cap was added, and the vial was purged with nitrogen for 10 min. Acetonitrile (5 mL) was added, and the reaction vial was heated in the microwave reactor (300 W, 60 min, 110 °C, max T=118 °C, max P=40 psi). By IR, there was no starting material present in solution (starting material FTIR: 2003, 1942 cm-1). Solvent was removed in vacuo to give 0.31 g (86%) as an orange solid. (Ph3P)2Fe(CO)3. To a 10 mL reaction vial was added PPh3 (0.29 g, 1.1 mmol). A septum cap was added, and the vial was purged with nitrogen. To the vial was added 3.0 mL of a 0.167 M ethanolic solution of KHFe(CO)411d,e,h,14 (0.50 mmol), and the reaction vial was heated in the microwave reactor (150 W, 5 min, 100 °C, max T = 112 °C, max P = 50 psi). The reaction vial was cooled to room temperature to fully precipitate the yellow product. The product was collected by suction filtration, washed with water (10 mL) and ethanol (10 mL), and dried, giving 0.20 g (60%) of yellow crystals. [(p-CH3C6H4)3P]2Fe(CO)3. This compound was prepared as (Ph3P)2Fe(CO)3 using P(p-CH3C6H4)3 (0.33 g, 1.1 mmol). The product was isolated as 0.25 g (67%) of pale yellow crystals. (Ph2PCH2CH2PPh2)Fe(CO)3. This compound was prepared as (Ph3P)2Fe(CO)3 using Ph2PCH2CH2PPh2 (0.22 g, 0.55 mmol). The product was isolated as 0.16 g (60%) of moderately air sensitive, yellow-orange crystals. (Ph2PCH2CH2CH2PPh2)Fe(CO)3. This compound was prepared as (Ph3P)2Fe(CO)3 using Ph2PCH2CH2CH2PPh2 (0.23 g, 0.55 mmol). The product was isolated as 0.11 g (44%) of moderately air sensitive, yellow-orange crystals.

Results and Discussion The traditional synthesis of ferrocene, a common undergraduate experiment, is a straightforward albeit messy reaction sequence.5 However, the multistep reaction path (13) (a) Reger, D. L.; Fauth, D. J.; Dukes, M. D. Synth. React. Inorg. Metal-Org. Chem. 1977, 7, 151. (b) Li, H.; Turnbull, M. M. J. Organomet. Chem. 1991, 419, 245. (14) (a) Takegami, Y.; Watanabe, Y.; Masada, H.; Kanaya, I. Bull. Chem. Soc. Jpn. 1967, 40, 1456. (b) Wada, F.; Matsuda, T. J. Organomet. Chem. 1973, 61, 365. (c) Brunet, J.-J.; Taillefer, M. J. Organomet. Chem. 1988, 348, C5. (d) Brunet, J.-J.; Taillefer, M. J. Organomet. Chem. 1989, 361, C1.

Note

Organometallics, Vol. 28, No. 23, 2009 Scheme 1. Microwave Synthesis of Ferrocene

requires a retrocycloaddition (cracking) of the dicyclopentadiene dimer, followed by synthesis, isolation, and sublimation of the ferrocene. While a good pedagogical experience for students, synthesis in this manner is neither expedient nor applicable to other metallocene derivatives. As ferrocene itself is reasonably priced, it is most often purchased rather than synthesized. However, a new methodology utilizing the microwave for the generation of cyclopentadienyl anion and the subsequent synthesis of ferrocene could be applied to more expensive metallocene derivatives such as vanadocene, chromocene, manganocene, ruthenocene, nickelocene, and others. The exploration of synthesis of ferrocene in the microwave began by utilizing freshly cracked cyclopentadiene (Cp-H) and the classical conditions (KOH as a base and FeCl2 3 4H2O as the source of iron) in the microwave. It was found that ferrocene was formed in DMSO as a solvent after several solvents were tested (DME, DMF, THF, toluene, water). Since DMSO has a high boiling point, we suspected that the dicyclopentadiene dimer would engage in retrocycloaddition at the reaction temperature and form cyclopentadiene in situ. Optimal reaction conditions were achieved at 175 °C for 45 min, resulting in a 20% conversion of dimer to monomer. A one-pot synthesis in DMSO of ferrocene from dicyclopentadiene, FeCl2 3 4H2O, and KOH gave ferrocene in a 40-70% isolated yield.15 However, this one-pot synthesis was inconsistent, and we found that this synthesis sometimes resulted in the microwave vial exceeding the pressure limit of our microwave reactor. Given the explosion potential, another pathway to ferrocene was explored. In order to drive the retrocycloaddition to completion, base was added to the dicyclopentadiene in the presence of diglyme. Potassium tert-butoxide was found to be the optimal base for these reactions. Upon only 10 min of microwave heating at 185 °C, complete conversion to the cyclopentadienyl anion had been achieved. In addition, the maximum pressure was 140 psi, well within the tolerances of the microwave reactor, providing a much safer generation of the cyclopentadienyl anion. This anion mixture was added directly to a solution of FeCl2 3 4H2O in DMSO. The conversion to ferrocene was complete in 5 min, resulting in a consistent 86% yield over these two simple steps (Scheme 1). We envision the synthesis of the cylcopentadienyl anion to be applicable to other organometallic systems. In order to demonstrate the utility and for the sake of completeness, the acylation of ferrocene was examined in a microwave reactor. This acylation reaction is so straightforward that it is routinely incorporated into organic laboratory manuals.6e A recent microwave experiment was reported using Nafion sheets as the proton source under solvent-free conditions.16 As would be expected, the acylation takes place in a very straightforward manner in 5 min in the microwave (Scheme 2). This reaction gives a 75% yield of the desired (15) Hesse, A. Nontraditional Synthesis of Organometallic Compounds and Baylis-Hillman Products; B.S. Thesis, Butler University, Indianapolis, IN; May, 2007; http://digitalcommons.butler.edu/ugtheses/52/. (16) Birdwhistell, K. R.; Nguyen, A.; Ramos, E. J.; Kobelja, R. J. Chem. Educ. 2008, 85 (2), 261.

6843

Scheme 2. Microwave Acylation of Ferrocene

Scheme 3. Microwave Synthesis of Fp*-Fp*

Scheme 4. Synthesis of CpFe(PPh3)(CO)I

product with small amounts of ferrocene starting material (less than 20%), with only trace amounts of the bis-acetylated product seen by NMR. The significant reduction of bisacetylation seen in this reaction pathway is an improvement on the methodology. We are unable to make a comparison to the Nafion-catalyzed pathway, as only the acetyl ferrocene yields (40-60%) were reported. The dicarbonyl cyclopentadienyl iron dimer (Fp-Fp) is a ubiquitous starting material in iron chemistry. However, derivatives such as Fp*-Fp* is over 10 times more expensive than Fp-Fp. The synthesis of Fp*-Fp* has been reported in the literature7 by refluxing iron pentacarbonyl in highboiling solvents such as octane (bp 127), 2,2,5-trimethylhexane (bp 125), or xylenes (bp 140) for 24-48 h. Successful formation of the Fp*-Fp* in similar (88%) yields from iron pentacarbonyl and the pentamethylcyclopentadiene can be achieved in 10 min in the microwave utilizing DMF as a solvent (a comparable yield is also obtained in 4 h utilizing toluene as a solvent). This pathway does generate a significant amount of gas, but does not exceed the pressure tolerances of our microwave reactor (Scheme 3). Microwave heating has also been utilized to synthesize a variety of piano stool (CpFeL3) complexes. While not a challenging reaction, the synthesis of CpFe(CO)2I from [CpFe(CO)2]2 and I2 in the microwave was achieved in a 90% isolated yield. This compound has been reported to be useful for living polymerization reactions with Ti(OiPr)4.17 The iodide was converted to CpFe(PPh3)(CO)I in a similar manner in a 76% isolated yield. Liberation of carbon monoxide does not seem to retard the reaction even though the microwave vessel is a sealed system. Yields are comparable to traditional synthetic methods where the carbon monoxide is vented from the system. We were successfully able to utilize this intermediate in a two-step, one-pot procedure to give CpFe(PPh3)(CO)I in an overall yield of 77% from [CpFe(CO)2]2 (Scheme 4). Another piano stool complex was examined, specifically a derivative of CpFe(CO)2Me. The migration of the methyl group to give CpFe(PPh3)(CO)(COMe) upon exposure to PPh3 has been hastened from a 48 h reflux to a 30 min microwave reaction (Scheme 5). The migration of the methyl (17) Kotani, Y.; Kamigaito, M.; Sawamoto, M. Macromolecules 1999, 32, 6877.

6844

Organometallics, Vol. 28, No. 23, 2009

Garringer et al. Scheme 6. Microwave Synthesis of Bisphosphine Iron Carbonyls

Scheme 5. Microwave Alkyl Migration

Table 1. Bisphosphine Iron Tricarbonyl Complexes phosphine

product

yield

PPh3 P(p-C6H4CH3)3 Ph2PCH2CH2PPh2 Ph2PCH2CH2CH2PPh2

Fe(CO)3(PPh3)2 Fe(CO)3[P(p-C6H4CH3)3]2 Fe(CO)3(Ph2PCH2CH2PPh2) Fe(CO)3(Ph2PCH2CH2CH2PPh2)

60% 67% 60% 40%

group, generating the acyl complex, provides access into asymmetric systems at iron.18 Iron phosphine tricarbonyl complexes are also common complexes. The formation of these complexes is usually achieved by refluxing the phosphine for a period of hours to days with either Fe(CO)5 via photochemical methods11c or Fe2(CO)9 via thermal heating, with the latter being preferred due to decreased reaction times and lability of the iron-iron bond.11,12a Other labile ligands work as well.12d Unfortunately, direct heating of Fe(CO)5 in the presence of phosphines with or without solvent over a variety of reaction times did not produce a significant amount of phosphine tricarbonyl product. There have been recent reports utilizing KHFe(CO)4 as a more reactive source of “Fe(CO)3” for the formation of phosphine tricarbonyl complexes. Even with the use of this more reactive reagent, the formation of the complexes still required reflux for 2-12 h. Utilizing microwave heating, the phosphine complexes were formed in 5 min in 40-67% yields (see Table 1 and Scheme 6). This methodology is well suited to the formation of several phosphine complexes. However, the corresponding phosphine tricarbonyl com(18) (a) Davies, S. G. Pure Appl. Chem. 1988, 60, 13. (b) Davies, S. D. Aldrichim. Acta 1990, 23, 31. (c) Case-Green, S. C.; Costello, J. F.; Davies, S. G.; Heaton, N.; Hedgecock, C. J. R.; Prime, J. C. J. Chem. Soc., Chem. Commun. 1993, 1621. (19) For a review of the reactivity of KHFe(CO)4, please see: Brunet, J. J. Chem. Rev. 1990, 90, 1041.

plexes of rigid bidentate phosphines (þ/- BINAP, 1,2-bis(diphenylphosphino)benzene, and 1,2-bis(diphenylphosphino)ethylene) were not formed under these conditions. It could be that the rigidity of these phosphines requires higher temperatures than the maximum of 110 °C of ethanol. In addition, utilizing KHFe(CO)4 for the formation of other iron ligand tricarbonyl complexes (benzylidene acetone and dienes) has proven unsuccessful using microwave heating, likely due to the known reactivity of KHFe(CO)4 with alkenes and enones.19 The microwave reactor can be an important tool for the synthesis of organometallic complexes. For several iron-containing building blocks, such as sandwich compounds, piano stool compounds, and phosphine tricarbonyl complexes, the synthesis of these key compounds can be completed in minutes rather than days. In addition, increased yields were observed utilizing microwave heating for many of these compounds. Microwaves are successfully able to effect reaction transformations for organo-iron systems, regardless of liberation of carbon monoxide, and inexpensive iron reagents seem to work extremely well in these synthetic pathways.

Acknowledgment. We thank Holcomb Academic Grants, Butler Summer Institute, and the Lilly Endowment for gracious support of undergraduate research at Butler University. Supporting Information Available: Data table including CAS numbers for all synthesized compounds, key references for NMR or IR data used for comparison, literature percent yield, and experimental percent yield. This material is available free via the Internet at http://pubs.acs.org.