In the Laboratory
Improved Synthesis of Geodken’s Macrocycle through the Synthesis of the Dichloride Salt
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J. H. Niewahner,* Keith A. Walters, and Ashley Wagner Department of Chemistry, Northern Kentucky University, Highland Heights, KY 41099; *
[email protected] In a continuing effort to improve our third-year inorganic chemistry lab course that requires first-semester organic lecture and lab, we sought the introduction of additional metal complexation reactions. Based on previous reports (1, 2), we chose to have the students prepare the copper and molybdenum complexes of Geodken’s macrocycle, 1, and acquire both the emission and absorption spectra. This experiment (and lab time restraints) required the instructor to prepare the macrocycle in advance for the students (Scheme I). Earlier reports presented the synthesis of the macrocycle in four steps utilizing a nickel template complex, 2, with overall yields of 40% (1) and 22% (2). We were unsatisfied with this yield and, following some experimentation, adapted the synthesis to eliminate one step and increase the overall yield of 1 to 65%. This increase is largely due to the optimization of the first step (synthesis of 2), which improved from 40% (1) to 88%. Our adapted synthesis also involves the synthesis of the dihydrochloride salt of the macrocycle, H2C22H22N4⭈2HCl, 3. Following the preparation of 1, students can proceed with metal complexation as described previously (2). This experiment provides a more efficient method for the production of Geodken’s macrocycle and provides an opportunity to teach students the effect of solvent on the composition of the collected product. In this report we present synthetic and characterization details for the preparation of 1–3, and in the accompanying Supplemental MaterialW we provide a complete lab handout for students to prepare these compounds, should the instructor wish them to prepare both the macrocycle and the subsequent transition-metal complexes (1, 2).
titative analysis was performed by Galbraith Laboratories. Spectral data are in excellent agreement (when available) to those published (1, 2) unless otherwise noted.
Required Materials and Hazards The following reagents are needed to complete this experiment, with significant hazards also listed (3): 1,2-phenylenediamine (toxic), nickel(II) acetate tetrahydrate (toxic), acetylacetone (flammable, toxic), anhydrous n-butanol (flammable, harmful), methanol (flammable, toxic), and hydrogen chloride gas (highly toxic). The 1,2-phenylenediamine was obtained from Aldrich and recrystallized from ethanol before use. The nickel(II) acetate tetrahydrate, acetylacetone, and n-butanol were used as supplied from Aldrich. Methanol (HPLC grade) was obtained from Fisher and used as supplied. Hydrogen chloride gas was obtained from Linde Gas Products. Experimental and Results All syntheses were carried out under normal atmospheric conditions in a fume hood. Infrared spectra were taken with a Nicolet Nexus 470 FT-IR using KBr pellets. NMR spectra were acquired using a JEOL Eclipse 500 MHz NMR. Quan-
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Scheme I. Synthesis of Geodken’s macrocycle.
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In the Laboratory
Preparation of NiC22H22N4 (2)
1H
In a typical preparation for 2, 17.85 g of 1,2-phenylenediamine (0.1650 mole, 4 eq), 17 mL of 2,4-pentanedione (0.17 mole, 4 eq), 10.20 g Ni(C2H3O2)2⭈4H2O (0.04100 mole, 1 eq), and 250 mL of n-butanol were added to a 500mL round-bottom flask equipped with a stirbar. The flask was attached to a condenser, and the mixture was refluxed while stirring for 3 hours using a heating mantle and magnetic stirrer. The mixture was then cooled to room temperature, and 50 mL of methanol was added. The solution was then cooled to about ᎑5 ⬚C in a salt兾ice bath for about 15 minutes, during which time a dark purple–blue solid precipitated from solution. The precipitate was collected on a Büchner funnel, washed with 30 mL of cold methanol, and air dried, producing 14.38 g of product 2 (87.42% yield). As previously reported (2), yields decreased with decreasing reaction time. For each hour of decreased reaction time, we observed a decrease of between 5–10%. However, even with the shorter reaction times yields are better than those initially reported (40% for a three-hour reaction) (2). IR (KBr): 1549 (arom C⫽C), 1464 (arom C⫽C), 1397 (arom CN), 1271 (arom CN), 1040 (o-disubs arom CH), 744 (o-disubs arom CH) cm᎑1. 1H
NMR (500 MHz, C6D6, δ): 1.75 (s, 12H, methyl), 4.67 (s, 2H, ⫽CH), 6.53 (m, 8H, arom). M᎑1cm᎑1),
UV–vis (CHCl3), λ nm: 285 (33,931 394 (39,291 M᎑1cm᎑1), 428 (sh, 12,499 M᎑1cm᎑1), 588 (6,766 M᎑1cm᎑1).
While the absorption data are comparable to one previous report (2), the absorptivities are higher than those in another report (1). This disparity may be due to vacuum sublimation of 2 conducted in the earlier report (1), which may have removed high absorbing impurities that are possibly present in our samples.
Preparation of H2C22H22N4⭈2HCl (3) A mixture of 5.004 g of 2 (0.01247 moles) in 100 mL of methanol was added to a 250-mL Erlenmeyer flask. A stream of HCl gas was bubbled into the reaction mixture using a Pasteur pipet attached to a hose from the HCl cylinder. The mixture changed color from green–blue to green then brown, and the flask was very hot. The gas was bubbled through the solution for about 20 minutes until the solution began to drop in temperature and some white precipitate began to form. The mixture was then cooled in a 3 ⬚C ice bath for 15 minutes, during which time more white precipitate formed. The precipitate was collected on a Büchner funnel, washed with cold methanol, and air dried. A yield of 4.797 g of 3 was obtained (92.13% yield). Duplicate gravimetric analysis of chloride in 3 by precipitation of silver chloride resulted in mole ratios of HCl:macrocycle of 2.041 and 1.987. IR (KBr): 3430 (2⬚ arom NH), 3140 (CH), 2927 (CH3), 2817 (CH3), 1570 (arom C⫽C), 1311 (arom CN), 1046 (o-disubs arom CH), 762 (o-disubs arom CH) cm᎑1.
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NMR (500 MHz, D2O, δ): 2.54 (s, 12H, methyl), 4.68 (s, 2H, ⫽CH), 7.32 (m, 8H, arom).1 Quantitative Analysis, Calcd for H2C22H22N4·2HCl: 16.99 Cl, 63.30 C, 6.29 H, 13.43 N; found 18.14 Cl, 63.45 C, 6.34 H, 13.35 N. All values are given as percentages.
Preparation of H2C22H22N4 (1) In a 50-mL beaker, 2.046 g of 3 (0.004902 moles) was added along with enough methanol (5–10 mL) to form a slurry. Triethylamine was added dropwise with stirring until a the mixture had a pH of 9, during which time a bright yellow precipitate formed. The precipitate was then collected on a Büchner funnel, washed with cold methanol, and air dried to yield 1.349 g (79.89% yield). IR (KBr): 3450 (br) (2⬚ arom NH), 1616 (arom C⫽C), 1555 (arom C⫽C), 1363 (arom CN), 1183 (arom CN), 1040 (o-disubs arom CH), 741 (o-disubs arom CH) cm᎑1. 1H
NMR (500 MHz, CDCl3, δ): 2.13 (s, 12H, methyl), 4.87 (s, 2H, ⫽CH), 6.99 (s, 8H, arom), 12.6 (s, 2H, NH). UV–vis (CHCl3) λ nm: 284 (22,263 M᎑1cm᎑1), 346 (37,489 M᎑1cm᎑1).
Again, these absorptivity values are higher than those previously reported (1), presumably for the same reason described previously. Discussion of Results In the synthesis of 2, previous reports (1, 2) use a (2:2:1) stoichiometric ratio of 1,2-diaminobenzene, 2,4pentanedione, and nickel(II) acetate tetrahydrate. However, if the ratio is adjusted to (4:4:1) as in the current report, the yield greatly improves. This increased yield can easily be explained on the basis of Le Châtlier’s principle. Increasing the molar ratio of reactants as described above clearly forces the equilibrium to the product side of the reaction, thereby increasing the yield. A more interesting result was the unexpected solvent effect upon the addition of HCl to product 2. Previously reported methods (2) to produce 1 first converted nickel complex 2 to a tetrachloronickelate(II) salt ([H4C22H22N4][NiCl4]) by using ethanol as the solvent. Following this precipitation, the tetrachloronickelate salt is converted to the hexafluorophosphate salt of the free macrocycle ([H4C22H22N4][PF6]2). This protocol was necessary to prevent nickel(II) reinsertion. However, if methanol is used instead of ethanol, a solution of the dihydrochloride salt (H2C22H22N4⭈2HCl) is instead formed. This salt eventually precipitates when a large quantity is produced or the solution begins to cool. Interestingly, the reinsertion problem mentioned above is not encountered here. It is likely that the more polar methanol solvates [NiCl4]2− along with H+, leaving the free macrocycle available for incorporation of the HCl(g). Conversely, in the less polar ethanol solvent as soon
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In the Laboratory
as the [NiCl4]2− is formed association with [H4C22H22N4]2+ occurs and [H4C22H22N4][NiCl4] salt precipitates. The final isolation of free macrocycle 1 proceeds as previously reported (2) with a considerably higher yield. At this point, our students have a macrocycle that can be complexed with various transition metals, including copper, molybdenum, and tungsten (2, 4).
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Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Notes 1. The amine hydrogens are presumably in rapid exchange with the acid protons and, therefore, do not appear.
Conclusion In this article, previously reported syntheses for Geodken’s macrocycle were refined to produce a higher product yield with fewer steps. Furthermore, the synthesis gives instructors the opportunity to discuss how a slight change in the polarity of a solvent can have quite a radical change on the resulting product. This experiment has become one of the most popular in our lab, with opportunities for students to explore coordination chemistry and study the products using several spectroscopic techniques.
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Literature Cited 1. Goedken, V. L.; Weiss, M. C.; Place, D.; Dabrowiak, J. Inorg. Synth. 1980, 20, 115–119. 2. Chipperfield, John R.; Woodward, Simon. J. Chem. Educ. 1994, 71, 75–77. 3. Sigma–Aldrich MSDS Database. http://www.sigmaaldrich.com/ catalog/search/AdvancedSearchPage (accessed Nov 2006). 4. Cotton, F. Albert; Czuchajowska, Joanna. Polyhedron 1990, 9, 1217–1220.
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