In the Laboratory
Rotor-shaped Cyclopentadienyltetraphenylcyclobutadienecobalt
W
An Advanced Inorganic Experiment Darren K. MacFarland* and Rebecca Gorodetzer Department of Chemistry, Gettysburg College, Gettysburg, PA 17325; *
[email protected] Students, like their professors, often are intrigued with interesting molecular structures. Organometallic complex syntheses in advanced inorganic or organic courses usually begin with the synthesis of ferrocene, a historically interesting and air-stable compound (1, 2). We report a synthetic experiment of an alternative compound that has a more interesting structure and the same air stability that makes ferrocene desirable: cyclopentadienyltetraphenylcyclobutadienecobalt (1). Synthetic Objectives Our objective was to find a synthesis that involved some inert-atmosphere techniques (Schlenk or glovebox) typically taught in an advanced inorganic course but generated an airstable product that would allow purification and characterization to take place outside of an inert atmosphere. In this preparation, the starting material is air-sensitive CpCo(CO)2 and the reaction is performed under an inert atmosphere. The product is robust; purification (chromatography and recrystallization) is performed on the benchtop. In addition to meeting our synthetic criteria, cobalt complex 1 is also structurally interesting. Complex 1 contains two different carbocyclic ligands. Students find the stability of the cyclobutadiene ring surprising, given its antiaromaticity when not bound to a metal. As in ferrocene a cyclopentadienyl ligand is introduced as a six electron ligand. Best of all, crystal structures (3) show that the four phenyl groups on the cyclobutadiene ring are all tilted, appearing as a molecular fan or rotor (Figure 1). It is this feature that most captivates student interest.
and reaction times tend to suffer (3), and if the acetylene is asymmetric, isomers are formed. After the reaction reaches reflux, its progress may be monitored by observation of CO coming through the bubbler. This gives students a physical method for following the progress of the reaction. We found that the rate of bubbling slows from 1–2 seconds to 10 seconds per bubble over 2 hours. Once synthesized the product is air-stable. Purification is straightforward. The reaction mixture is poured onto dry silica, then eluted with toluene. A single recrystallization, from dichloromethane:heptane yields the product. Hazards CpCo(CO)2 is toxic and should be treated accordingly. Gloves must be worn as it is toxic from contact with skin. A reviewer suggested that latex gloves are inappropriate; the MSDS indicates the use of rubber gloves. CO produced during the reaction poses a hazard but can be controlled by running the reaction in a hood.
Co
Synthetic Overview We have modified literature procedures to make the synthesis more lab-friendly. Literature procedures reflux the mixture for 22–24 hours (3–5). We have shortened this time to 2 hours. Observed yields range from 22–38%, down from literature yields of 64–75%, but at this scale students still recover around 300–500 mg of product, more than enough for analysis. Yields may be improved by overnight reflux if this is convenient. Benzene (3, 4) has been replaced by the more benign toluene as eluent. All reagents and solvents are commercially available, though the more adventurous instructor may save some money by synthesizing the CpCo(CO)2 from Co2(CO)8 (3, 4). The overall reaction is shown in Figure 2. Diphenylacetylene may be replaced with other acetylenes (3); we have used phenylnitrophenylacetylene synthesized in a different course. The downside is that yields
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Figure 1. Cyclopentadienyltetraphenylcyclobutadienecobalt (1), showing the rotor conformation of the phenyl rings.
Ph OC
CO
Ph
Co
+
Ph
Ph
Ph Co
2
+
2 CO
Ph
Figure 2. Synthetic scheme for cyclopentadienyltetraphenylcyclobutadienecobalt (1).
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In the Laboratory
Analysis Analysis of the product can be done using either melting point (the product melts at 262 ⬚C and the major impurity melts at 342 ⬚C) or IR spectroscopy (3, 4). After recrystallization our melting points were between 257–262 ⬚C. If an NMR spectrometer is available, proton and carbon NMR spectroscopy can be performed in CDCl3 (IR and NMR spectra are provided in the Supplemental MaterialW). Broad resonances at δ 7.45, 7.2, and 6.8 ppm in the 1H NMR are caused by the asymmetry of the phenyl rings; without the rotor structure, a sharper spectrum would be expected. The key features in the 13C NMR are the signals at δ 83.2 and 74.8 ppm that represent the cyclopentadienyl ring and the cyclobutadiene ring respectively. In the IR spectrum absorbances at 1595 and 1497 cm᎑1 are observed; perhaps as important is the absence of carbonyl stretches, indicating that none of the possible cyclopentadienone product is present. Discussion In addition to the interesting structure, the reaction pathway is of some pedagogical interest as two different products can be formed from the starting materials. Reaction of CpCo(CO)2 and diphenylacetylene with heat gives product 1. However, when the same reactants are irradiated rather
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than heated CO inserts into the cyclization reaction of the bound acetylenes to give a cyclopentadieneone complex (6). Last, formation of the cyclobutadiene ligand is a cycloaddition reaction; most ligands are added directly as opposed to being synthesized on the metal, adding another level of interest to the project. W
Supplemental Material
Instructions for the students and notes for the instructor, including the NMR and IR spectra of the complex, are available in this issue of JCE Online. Literature Cited 1. Williamson, K. L. Macroscale and Microscale Organic Experiments; 4th ed.; Houghton Mifflin: Boston, 2003. 2. Inorganic Experiments; Woolins, J. D., Ed.; VCH: Weinheim, Germany, 1994. 3. Harrison, R. M.; Brotin, T.; Noll, B. C.; Michl, J. Organometallics 1997, 16, 3401–3412. 4. Rausch, M. D.; Genetti, R. A. J. Org. Chem. 1970, 35, 3888– 3897. 5. Rausch, M. D.; Genetti, R. A. J. Am. Chem. Soc. 1967, 89, 5502–5503. 6. Sheats, J. E.; Hlatky, G. J. Chem. Educ. 1983, 60, 1015–1016.
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