In the Laboratory
Simple Synthesis and Use of a Nickel Alkene Isomerization Catalyst An Advanced Lab in Inorganic/Organometallic Chemistry1 Kurt R. Birdwhistell* and Joann Lanza Chemistry Department, Loyola University, 6363 St. Charles Ave, New Orleans, LA 70118 The following laboratory experiment was designed for use in an upper-level inorganic or organic synthesis or unified lab course. This experiment in transition metal organometallic catalysis accomplishes the following objectives: (i) introduces students to important concepts in organometallic/inorganic chemistry; (ii) simple synthesis and characterization of a precatalyst without inert atmosphere techniques; (iii) a homogeneous catalytic reaction, which is not a hydrogenation, that can be done in three hours and (iv) a catalytic reaction that can be monitored quantitatively as a function of time by gas chromatography. This lab, based on a series of articles by Tolman (1a–d), has students synthesize and characterize Ni[P(OEt) 3 ]4 and use it as a precatalyst for the homogeneous catalytic isomerization of alkenes. Catalytic alkene isomerization has a number of commercial applications. Of current interest is the hydrogenation and isomerization of methylene interrupted diene fatty acids to cis-monoene fatty acids for use as cooking oils (2). Many heterogeneous catalysts can affect this transformation, but these catalysts produce some trans fatty acids as well. Studies have shown that trans fatty acids raise cholesterol levels in the blood. The selectivity of homogeneous catalysts pays off to provide essentially pure cis fatty acid (3). Finally, catalyzed alkene isomerization is also a component of the Shell SHOP process, which produces linear biodegradable C11–C15 fatty alcohols for detergents (4). Materials and Methods The heptenes, triethylphosphite, and diethylamine were used as obtained from Aldrich Chemical Co. Use a freshly opened bottle of triethylphosphite or distill it before use. The 1-heptene used was 97% pure. The nickel(II) chloride and methanol were reagent grade. CAUTION : The following reaction needs to be done in a hood owing to the volatility of diethylamine and triethylphosphite.
Synthesis of Tetrakistriethylphosphitenickel(0), Ni[P(OEt)3]4 (5) A 100-mL beaker is charged with NiCl2 · 6 H 2O (2.6 g, 11 mmol) and 50 mL of methanol. The mixture is stirred for 5 min to dissolve the nickel chloride, resulting in an emerald green solution. This solution is cooled in an ice bath and 9.4 mL (9.1 g, 55 mmol) of triethylphosphite is added over a 2-min period, producing a blood-red solution. After stirring 5 min, diethylamine (2.5 mL, 1.8 g, 24 mmol) is added dropwise via a syringe. Upon addition of half the diethylamine, a white precipitate of Ni[P(OEt)3 ]4 forms. The addition is continued slowly until the deep-red solution just *Corresponding author.
begins to fade. (NOTE: Addition of too much diethylamine will result in a green nickel(II) contaminant. It is better to sacrifice yield and get a pure product by stopping the addition of diethylamine early.) The mixture is left stirring in an ice bath for 10 min after addition of diethylamine. The white precipitate is isolated by filtration of the solution on a 30-mL medium glass fritted funnel and washed with 3 × 10 mL of ice cold methanol. The solid is dried in vacuo in a vacuum desiccator. The white solid has a melting point of 109 °C in a sealed capillary (lit mp 108 °C) (6). The yield is ca. 1.0 g or 13%. The nickel complex can be characterized by its far-IR spectrum in the region 1600–300 cm{1 (7). Ni[P(OEt)3]4 can be handled in the air for 30 min without decomposition, but should be stored in vacuo or under nitrogen away from light for longer periods.
Catalytic Isomerization of 1-Heptene A 100-mL 3-neck flask is fitted with a nitrogen inlet, septum, stir bar, and oil bubbler. The flask is charged with Ni[P(OEt) 3]4 (71 mg, 0.098 mmol) and flushed with nitrogen for several minutes before addition of 20.00 mL of diethyl ether via pipet, resulting in a colorless solution. This solution is sparged with nitrogen for several minutes. Next 1-heptene (0.20 mL, 1.4 mmol) is added via syringe. An initial GC is taken at this time. This chromatogram will represent time zero. The reaction mixture is cooled in an icewater bath and 1.00 mL of 0 °C 0.1 M H2SO4 in methanol is added via pipet. Addition of H 2SO4 initiates the isomerization. The nitrogen flow is decreased to prevent evaporation of heptene. The initial molar concentrations of reactants are: [Ni[P(OEt)3]4] = 4.6 × 10{3 ; [heptene] = 0.067, [H2 SO4] = 4.7 × 10{3. At nickel concentrations of 0.003 M or less one must be careful to exclude oxygen. At these low concentrations the catalyst can be poisoned by oxygen, preventing the complete isomerization of heptenes (1d). One-milliliter aliquots of the reaction mixture are taken via syringe at 4.5, 10, 17, and 29 min. The samples are syringed into test tubes and the tubes are shaken immediately in the air. This procedure quenches the reaction aliquot by poisoning the catalyst with oxygen. One microliter of each sample is injected into a GC with a flame ionization detector. A plot of mole percent heptenes vs. time is constructed from the GC data (see Fig. 1). Chromatography The gas chromatography was run on a Hewlett Packard 5890 with the following parameters: injector temperature, 150 °C; detector FID temperature, 250 °C; isothermal oven, 40 °C; run time, 3.00 min; capillary column, crosslinked methylsilicone, HP-1, 12 m × 0.2 mm × 0.33 µm. Retention times (min) were 1-heptene (1.35); 2-heptene (1.57); 3-heptene (1.47). Cis and trans isomers were not separated using these GC parameters.
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In the Laboratory
Preparation of the Nickel Precatalyst The Ni[P(OEt)3 ]4 complex is prepared in reasonable yield by the method of Meier and Basolo (5). The balanced reaction is shown below. NiCl2 · 6 H2 O + 5 P(OEt) 3 + 2 HNEt 2 Ni[P(OEt)3 ]4 + 2 (H2NEt2 )Cl + (EtO)3P(O) + 5 H2 O The green nickel(II) octahedral complex is reduced by the triethylphosphite to a colorless tetrahedral tetrakisphosphite nickel(0) complex. This reaction illustrates the relationship between geometry, d electron count, and electronic spectra in inorganic transition metal chemistry (a green octahedral Ni(II) converted to a colorless tetrahedral Ni(0) complex).
Catalytic Isomerization of 1-Heptene The Ni[P(OEt)3 ]4 complex is used as a precatalyst for the catalytic isomerization of 1-heptene to an equilibrium mixture of isomers:
As shown in Figure 1 the equilibrium product distribution is 1-heptene : 2-heptene : 3-heptene = 1:20:78. This ratio represents the equilibrium distribution of products based on the difference in the free energies of formation of the isomers. Catalysts do not affect the equilibrium constant, but provide a mechanism to reach equilibrium more rapidly (8). The isomerization of 1-heptene is initiated by adding H2 SO4 to an ether solution of 1-heptene and Ni[P(OEt) 3]4, producing HNi[P(OEt)3]4+ in situ (1): Ni[P(OEt)3 ]4 + H2SO 4
HNi[P(OEt)3]4+ + HSO4 1
{
This protonation provides another illustration of the relationship between structure and electronic spectra as we convert from a 4-coordinate colorless Ni(0) complex to a fivecoordinate pale-yellow Ni(II) hydride complex. This is probably the first exposure students have to a nonhydridic inorganic hydride. The hydrides used in organic chemistry such as LiAlH4 or NaBH4 are hydridic. In contrast, the nickel hydride (1) produced by protonation is a weak acid (pKa = 1.7) (1a). Many transition metal hydrides are weak acids (9). The five-coordinate structure of the nickel hydride 1 can be confirmed by 1H-NMR and 31P-NMR. The 1H-NMR displays a quintet at {14.3 ppm (J PH = 26.5 Hz) for the hydride proton and signals at 1.27 ppm (methyl), 3.87 ppm (methylene). A singlet is observed at {135 ppm in the 31P-NMR. The NMR signals are independent of counterion, consistent with a five-coordinate structure (1a). Although the isomerization begins immediately upon formation of nickel hydride, 1, there is good evidence to indicate the nickel hydride complex is a catalyst precursor in the isomerization reaction (1d). Experimental evidence suggests that HNi[P(OEt)3]4+ dissociates one triethylphosphite ligand to produce the active catalyst. Ligand dissociation forming an unsaturated metal complex is a common pathway for catalyst initiation. The isomerization of 1-heptene at 0 °C can be followed by plotting the GC data as area% (or mol %) of the heptene isomers vs. time (Fig. 1). As shown in Figure 1, a short (5min) initiation period is observed before the reaction reaches the maximum isomerization rate. The initiation pe-
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riod is particularly evident at low acid concentrations (< 0.01 M). After protonation the catalyst rapidly isomerizes 1-heptene at 0 °C to form 2-heptene, then 3-heptene. One way to express catalyst efficiency is to determine the turnover frequency of the catalyst.
Catalyst Turnover Frequency (TOF) Turnover frequency is simply the ratio of moles of product to moles of catalyst over time. This is a common way to express catalyst efficiency and also impresses on the students the 1:1 stoichiometry between reactants and catalyst. The turnover frequency (TOF) equation for the reaction of A going to product P catalyzed by catalyst B is defined (10):
TOF =
{ d[A] / dt d[P] / dt or [B] [B]
In this reaction several isomerizations occur in rapid sequence, so it is difficult to obtain an accurate TOF. An estimate of the TOF for this reaction can be obtained by looking at the formation of 2-heptene and 3-heptene in Figure 1. For example, the moles of 1-heptene, 2-heptene, and 3heptene can be calculated at 4.5 and 10 min. Time (h)
1-Heptene (mmol)
2-Heptene (mmol)
3-Heptene (mmol)
0.07
1.4
0.05
0.01
0.17
0.84
0.24
0.34
TOF† = [(0.19 mmol 2-heptene) + 2(0.33 mmol 3-heptene)] ÷ (0.10 mmol catalyst) ÷ 0.1 h = 80 h{1 of 3-heptene are multiplied by 2 because 3-heptene is pro duced from two complete catalytic cycles.
†mmol
Mechanism of Catalytic Isomerization of 1-Heptene This catalytic reaction illustrates a number of important organometallic and inorganic reaction types and concepts. Figure 2 shows a simplified scheme for the catalytic isomerization of terminal alkenes. See the article by Tolman (1d) for a detailed catalytic scheme and an in-depth discussion. Catalysis is initiated via a ligand dissociation reaction that opens a coordination site on the metal. Many transition metal catalysts initiate catalysis in this manner. The
mol % Heptenes
Results and Discussion
Figure 1. Plot of mol % heptenes vs. time obtained from GC data for the isomerization of 1-heptene by HNi[P(OEt)3] 4+ at 0 °C in diethyl ether.
Journal of Chemical Education • Vol. 74 No. 5 May 1997
In the Laboratory dents (i) do a control reaction using all reagents except the nickel complex; (ii) determine the effect of added ligand (triethylphosphite) on isomerization rate; (iii) determine the formation constant for the nickel hydride by UV-vis spectroscopy; (iv) use GC/MS to determine the amount of product deuteration that occurs by running the reaction in CH3 OD; or (v) determine the decomposition rate of the nickel hydride. These sorts of experiments illustrate to students the variety of information that can be used to help sort out a mechanism. Tolman uses all of these data to help define and refine the mechanism of the isomerization reaction (1a–1d). This lab allows students to synthesize and characterize a precursor complex without inert atmosphere techniques. Students can conveniently convert their precursor complex into a catalytic hydride complex and readily assess the catalytic efficiency of their catalyst by using GC to monitor the rate of 1-heptene isomerization. This experiment combines synthetic and physical techniques to give students a broad introduction to synthetic and mechanistic organometallic chemistry in a relatively short time. Figure 2. Simplified catalytic scheme for the isomerization of terminal alkenes by HNi[P(OEt)3 ]4+ .
catalytic cycle is a good point around which to discuss ideas of electron counting in organometallic transition metal chemistry. Organometallic catalytic systems often are found to cycle between 16- and 18-electron complexes (11). In this case the 18-electron complex 1 dissociates a phosphite ligand to make the 16-electron catalyst 2. The next step is alkene association, producing the 18-electron complex 3. Catalysis continues with an alkene insertion into a metal hydride bond to form a secondary metal alkyl, 4 (16-electron). The alkene insertion reaction can occur in two different directions and only the reaction shown (Markovnikov addition, in organic-chemical terminology) will produce an isomerization. The nickel alkyl 4 undergoes β-hydride elimination to form an 18-electron alkene hydride, 5. β-Hydride elimination is a common reaction pathway for transition metal alkyls containing β-hydrogens (9). At this point the alkene may dissociate, resulting in an isomerized internal alkene and regeneration of the catalyst, 2; or the alkene hydride complex 5 can continue through another catalytic cycle, resulting in further isomerization. Summary This experiment could be expanded by having the stu-
Note 1. Presented at the 208th American Chemical Society Meeting in the Division of Chemical Education, Washington, DC, Aug. 25, 1994 (paper #242).
Literature Cited 1. (a) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 4217; (b) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 6777; (c) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 6785; (d) Tolman, C. A. J. Am. Chem. Soc. 1972, 94, 2994. 2. Adler, T. Sci. News 1994, 145, 296–297. 3. Tucker, J. R.; Riley, D. P. J. Organomet. Chem. 1985, 279, 49–62. 4. Reuben, B.; Wittcoff, H. J. Chem. Educ. 1988, 65, 605–607. 5. Meier, M.; Basolo, F. Inorg. Synth. 1990, 28, 104–105. 6. Vinal, R. S.; Reynolds, L. T. Inorg. Chem. 1964, 3, 1062–1063. 7. Myers, V. G.; Basolo, F.; Nakamoto, K. Inorg. Chem. 1969, 8, 1204–1206. 8. Atkins, P. W. Physical Chemistry, 5th ed.; Freeman: New York, 1994; p 282. 9. Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science: Mill Valley, CA, 1987; pp 91, 386. 10. Shriver, D. E.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry; Freeman: New York, 1994; p 711. 11. Miessler, G. L.; Tarr, D. A. Inorganic Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1991; p 486.
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