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Anhydrous chromous acetate revisited – A very simple synthetic route
Inorganica Chimica Acta 391 (2012) 179–181
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Note
Anhydrous chromous acetate revisited – A very simple synthetic route Ophir Levy, Benny Bogoslavsky, Avi Bino ⇑ Institute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, 91904 Jerusalem, Israel
a r t i c l e
i n f o
Article history: Received 5 March 2012 Received in revised form 19 April 2012 Accepted 20 April 2012 Available online 28 April 2012
a b s t r a c t We report the synthesis of crystalline anhydrous dichromium(II) tetraacetate by a one-pot reaction in which chromium metal reacts in the open with a mixture that comprises acetic acid, acetic anhydride and hydrobromic acid. This new route is much superior to all previous multi-step synthetic procedures that involve strict anaerobic conditions or expensive and flammable starting materials. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Cr(II) complexes Chromium Chromium(II) acetate Metal–metal bond Synthetic methods X-ray diffraction
1. Introduction Chromous acetate, ‘‘Cr2(O2CCH3)4’’, is a common starting material for a variety of dinuclear Cr(II) complexes [1]. It also serves as a reagent in several types of organic reactions such as the abstraction of halides from alkyl halides [2] and in commercial polymerization processes [3]. The first synthetic procedure of the di-aqua complex was reported in 1884 [4], although the exact nature of the compound was not known at that time. The molecular structure of the di-aqua complex, [Cr2(O2CCH3)4(H2O)2], was determined crystallographically in 1953 and 1971 [5]. The diamagnetic nature of the compound, along with the relatively short Cr–Cr distance, supports the view that a quadruple metal-to-metal bond exists in this dinuclear complex. Due to the extreme sensitivity of chromous acetate towards oxidation, mainly by air oxygen, strict anaerobic conditions have been applied during its preparation. Most synthetic routes leading to the formation of the brick-red compound start with the production of the bright blue aqua ion [Cr(H2O)6]2+, either by dissolving metallic chromium in dilute mineral acids or by reducing Cr(III) salts with zinc amalgam. The addition of an acetate salt or acetic acid to these solutions forms a heavy reddish precipitate of [Cr2(O2CCH3)4(H2O)2] [6,7]. The air-sensitive product is filtered, washed and kept under inert atmosphere. The preparation of chromous acetate has been used for many years to test the syn-
thetic skills of undergraduate students due to its extreme air sensitivity. Anhydrous chromous acetate, [Cr2(O2CCH3)4], depicted in Fig. 1, is prepared by heating the di-aqua derivative under vacuum for extended periods of time [7]. This procedure yields an amorphous, somewhat contaminated and extremely reactive substance that spontaneously ignites upon contact with oxygen [8]. A few small crystals of [Cr2(O2CCH3)4] were obtained by sublimation of the anhydrous material in a sealed tube at 230 °C. An X-ray singlecrystal analysis revealed that instead of the two axially bonded water ligands that exist in the di-aqua complex, the anhydrous complex features two axial Cr O interactions with oxygen atoms from neighboring bridging acetate ligands [9]. An alternative route to the production of anhydrous chromous acetate involves the use of the rather expensive and highly flammable chromocene, Cr(C5H5)2, as a starting material. Beneš et al. reported that the direct reaction of chromocene with acetic acid or acetic anhydride leads to the formation of the anhydrous form in high yields [10]. Here, we report that crystalline anhydrous chromous acetate can be prepared in relatively high yields and pure form by using a low-cost one-pot reaction in the open without applying the commonly used oxygen exclusion techniques.
2. Experimental 2.1. Materials
⇑ Corresponding author. E-mail addresses: [email protected] (O. Levy), [email protected] (B. Bogoslavsky), [email protected] (A. Bino). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.04.029
Chromium powder (325 mesh) P99% and iodine P99.999% (Aldrich); acetic anhydride P99% (Bio-Lab); acetic acid P99.7%,
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O. Levy et al. / Inorganica Chimica Acta 391 (2012) 179–181
(3.07 g, 9.0 mmol, 62.2%). Anal. Calc. for the anhydrous compound, Cr2C8H12O8: C, 28.25; H, 3.56. Found: C, 27.98; H, 3.36%. Unlike the amorphous material, the crystalline powder is stable in the open for several hours, but slowly oxidized by air. 2.3. Powder X-ray diffraction Powder X-ray diffraction patterns were measured using a Philips PW1820 diffractometer. 3. Results and discussion The powder X-ray diffraction pattern of the solid, prepared as described above, along with the simulated pattern generated by using the literature crystallographic parameters of [Cr2(O2CCH3)4] [9], are shown in Fig. 2. It can be seen that the two patterns are practically identical. The fact that [Cr2(O2CCH3)4] can be prepared by such a straightforward manner using relatively inexpensive and simple starting materials and under aerobic conditions renders this procedure superior to all traditional ones. Here are some interesting facts associated with this unusual reaction. Elemental chromium does not react with a 4:1 acetic acid/anhydride mixture without the addition of other substances. We believe that the oxidation of chromium metal in the presence of a strong acid is accomplished by H+ ions according to: Cr0 + 2H+ ? Cr+2 + H2. The chromous ions then react with acetic acid according to: 2Cr+2 + 4CH3COOH ? Cr2(O2CCH3)4 + 4H+. We found that the use of a bromide salt such as KBr, instead of HBr, is also effective, probably because the interaction of the salt with acetic acid produces small amounts of HBr that dissolves the metal. The resulting chromous ions and acetic acid produce the dinuclear complex and H+ ions. This will keep the reaction running until all metal is consumed. Hydrochloric acid, elemental bromine or iodine may also be used instead of hydrobromic acid. The free halogens probably oxidize chromium to some extent forming HX that serves as the main oxidant as described above. It is noteworthy that water from the aqueous solutions of HBr or HCl is fully absorbed by the acetic anhydride to form acetic acid and the environment remains anhydrous throughout the reaction and formation of [Cr2(O2CCH3)4]. Ion-exchange chromatography of the filtrate after dilution with water revealed that the rest of the chromium in this reaction ends
Fig. 1. Structure of [Cr2(O2CCH3)4] [9]. Green, Cr; red, O; black, C. Hydrogen atoms are omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
hydrobromic acid P47% and hydrochloric acid 37.0–38.0% (Merck); potassium bromide 99.5% (Riedel-de Haën); bromine >99.0% (Fluka). All reagents and solvents were used without further purification. 2.2. Preparation of [Cr2(O2CCH3)4] In a typical preparation, 1.51 g (29.0 mmol) of chromium powder (325 mesh) was placed in a 100 mL round-bottom flask. A 50 mL solution of acetic anhydride and acetic acid (1:4, vol. ratio) was added, followed by the addition of hydrobromic acid (47%) (1 mL, 8.7 mmol). The flask was equipped with a stirring bar and a regular condenser, and the mixture was stirred and refluxed for 1.5 h in the open. During the course of the reaction, the color of the solution turned brown, and brown precipitate accumulated at the bottom of the flask. After 1.5 h, the mixture was allowed to cool down for 20 min. Then, the condenser was replaced by a glass stopper and the flask was cooled in an ice bath for 30 min. The precipitate was filtered and washed with acetone and dried under vacuum. Brownish red crystalline powder was obtained
10
15
20
25
30
35
40
45
50
55
2θ (°) Fig. 2. Diffractograms of a [Cr2(O2CCH3)4] powder (in blue) and a powder diffraction simulation from single crystal (in red) [9]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
O. Levy et al. / Inorganica Chimica Acta 391 (2012) 179–181
up as Cr(III) in the trinuclear complex, [Cr3O(O2CCH3)6(H2O)3]+. This species is the result of air oxidation of dissolved chromous acetate in the reaction mixture. However, when air is excluded during the reaction, the yield of the product is very similar. In conclusion, the simplicity and relative low cost of reagents makes the above procedure ideal for student laboratories and a source for anhydrous chromous acetate as a starting material for other mono- or dinuclear Cr(II) species or a reducing agent in preparative organic chemistry. References [1] F.A. Cotton, R.A. Walton, Multiple Bonds between Metal Atoms, Wiley, 1982.
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[2] L.A. Paquette (Editor-in-chief), Encyclopedia of Reagents for Organic Synthesis, Wiley, England, 1995. [3] M. Lee, H. Nakamura, Y. Minoura, J. Polym. Sci., Part A: Polym. Chem. 14 (1976) 961. [4] (a) E. Peligot, C.R. Hebd. Seances Acad. Sci. 19 (1844) 609; (b) E. Peligot, Ann. Chim. Phys. 12 (1844) 528. [5] (a) J.N. van Niekerk, F.R.L. Schoening, J.F. de Wet, Acta Crystallogr. 6 (1953) 501; (b) F.A. Cotton, B.G. DeBoer, M.D. LaPrade, J.R. Pipal, D.A. Ucko, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 27 (1971) 1664. [6] J.C. Reeve, J. Chem. Educ. 62 (1985) 444. [7] L.R. Ocone, B.P. Block, Inorg. Synth. 8 (1966) 125. [8] R.J. Lewis Sr., Hazardous Chemicals Desk Reference, sixth ed., Wiley, New Jersey, 2008. [9] F.A. Cotton, C.E. Rice, G.W. Rice, J. Am. Chem. Soc. 99 (1977) 4704. [10] L. Beneš, J. Kalousová, J. Votinsky´, J. Organomet. Chem. 290 (1985) 147.
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Note
Anhydrous chromous acetate revisited – A very simple synthetic route Ophir Levy, Benny Bogoslavsky, Avi Bino ⇑ Institute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, 91904 Jerusalem, Israel
a r t i c l e
i n f o
Article history: Received 5 March 2012 Received in revised form 19 April 2012 Accepted 20 April 2012 Available online 28 April 2012
a b s t r a c t We report the synthesis of crystalline anhydrous dichromium(II) tetraacetate by a one-pot reaction in which chromium metal reacts in the open with a mixture that comprises acetic acid, acetic anhydride and hydrobromic acid. This new route is much superior to all previous multi-step synthetic procedures that involve strict anaerobic conditions or expensive and flammable starting materials. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Cr(II) complexes Chromium Chromium(II) acetate Metal–metal bond Synthetic methods X-ray diffraction
1. Introduction Chromous acetate, ‘‘Cr2(O2CCH3)4’’, is a common starting material for a variety of dinuclear Cr(II) complexes [1]. It also serves as a reagent in several types of organic reactions such as the abstraction of halides from alkyl halides [2] and in commercial polymerization processes [3]. The first synthetic procedure of the di-aqua complex was reported in 1884 [4], although the exact nature of the compound was not known at that time. The molecular structure of the di-aqua complex, [Cr2(O2CCH3)4(H2O)2], was determined crystallographically in 1953 and 1971 [5]. The diamagnetic nature of the compound, along with the relatively short Cr–Cr distance, supports the view that a quadruple metal-to-metal bond exists in this dinuclear complex. Due to the extreme sensitivity of chromous acetate towards oxidation, mainly by air oxygen, strict anaerobic conditions have been applied during its preparation. Most synthetic routes leading to the formation of the brick-red compound start with the production of the bright blue aqua ion [Cr(H2O)6]2+, either by dissolving metallic chromium in dilute mineral acids or by reducing Cr(III) salts with zinc amalgam. The addition of an acetate salt or acetic acid to these solutions forms a heavy reddish precipitate of [Cr2(O2CCH3)4(H2O)2] [6,7]. The air-sensitive product is filtered, washed and kept under inert atmosphere. The preparation of chromous acetate has been used for many years to test the syn-
thetic skills of undergraduate students due to its extreme air sensitivity. Anhydrous chromous acetate, [Cr2(O2CCH3)4], depicted in Fig. 1, is prepared by heating the di-aqua derivative under vacuum for extended periods of time [7]. This procedure yields an amorphous, somewhat contaminated and extremely reactive substance that spontaneously ignites upon contact with oxygen [8]. A few small crystals of [Cr2(O2CCH3)4] were obtained by sublimation of the anhydrous material in a sealed tube at 230 °C. An X-ray singlecrystal analysis revealed that instead of the two axially bonded water ligands that exist in the di-aqua complex, the anhydrous complex features two axial Cr O interactions with oxygen atoms from neighboring bridging acetate ligands [9]. An alternative route to the production of anhydrous chromous acetate involves the use of the rather expensive and highly flammable chromocene, Cr(C5H5)2, as a starting material. Beneš et al. reported that the direct reaction of chromocene with acetic acid or acetic anhydride leads to the formation of the anhydrous form in high yields [10]. Here, we report that crystalline anhydrous chromous acetate can be prepared in relatively high yields and pure form by using a low-cost one-pot reaction in the open without applying the commonly used oxygen exclusion techniques.
2. Experimental 2.1. Materials
⇑ Corresponding author. E-mail addresses: [email protected] (O. Levy), [email protected] (B. Bogoslavsky), [email protected] (A. Bino). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.04.029
Chromium powder (325 mesh) P99% and iodine P99.999% (Aldrich); acetic anhydride P99% (Bio-Lab); acetic acid P99.7%,
180
O. Levy et al. / Inorganica Chimica Acta 391 (2012) 179–181
(3.07 g, 9.0 mmol, 62.2%). Anal. Calc. for the anhydrous compound, Cr2C8H12O8: C, 28.25; H, 3.56. Found: C, 27.98; H, 3.36%. Unlike the amorphous material, the crystalline powder is stable in the open for several hours, but slowly oxidized by air. 2.3. Powder X-ray diffraction Powder X-ray diffraction patterns were measured using a Philips PW1820 diffractometer. 3. Results and discussion The powder X-ray diffraction pattern of the solid, prepared as described above, along with the simulated pattern generated by using the literature crystallographic parameters of [Cr2(O2CCH3)4] [9], are shown in Fig. 2. It can be seen that the two patterns are practically identical. The fact that [Cr2(O2CCH3)4] can be prepared by such a straightforward manner using relatively inexpensive and simple starting materials and under aerobic conditions renders this procedure superior to all traditional ones. Here are some interesting facts associated with this unusual reaction. Elemental chromium does not react with a 4:1 acetic acid/anhydride mixture without the addition of other substances. We believe that the oxidation of chromium metal in the presence of a strong acid is accomplished by H+ ions according to: Cr0 + 2H+ ? Cr+2 + H2. The chromous ions then react with acetic acid according to: 2Cr+2 + 4CH3COOH ? Cr2(O2CCH3)4 + 4H+. We found that the use of a bromide salt such as KBr, instead of HBr, is also effective, probably because the interaction of the salt with acetic acid produces small amounts of HBr that dissolves the metal. The resulting chromous ions and acetic acid produce the dinuclear complex and H+ ions. This will keep the reaction running until all metal is consumed. Hydrochloric acid, elemental bromine or iodine may also be used instead of hydrobromic acid. The free halogens probably oxidize chromium to some extent forming HX that serves as the main oxidant as described above. It is noteworthy that water from the aqueous solutions of HBr or HCl is fully absorbed by the acetic anhydride to form acetic acid and the environment remains anhydrous throughout the reaction and formation of [Cr2(O2CCH3)4]. Ion-exchange chromatography of the filtrate after dilution with water revealed that the rest of the chromium in this reaction ends
Fig. 1. Structure of [Cr2(O2CCH3)4] [9]. Green, Cr; red, O; black, C. Hydrogen atoms are omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
hydrobromic acid P47% and hydrochloric acid 37.0–38.0% (Merck); potassium bromide 99.5% (Riedel-de Haën); bromine >99.0% (Fluka). All reagents and solvents were used without further purification. 2.2. Preparation of [Cr2(O2CCH3)4] In a typical preparation, 1.51 g (29.0 mmol) of chromium powder (325 mesh) was placed in a 100 mL round-bottom flask. A 50 mL solution of acetic anhydride and acetic acid (1:4, vol. ratio) was added, followed by the addition of hydrobromic acid (47%) (1 mL, 8.7 mmol). The flask was equipped with a stirring bar and a regular condenser, and the mixture was stirred and refluxed for 1.5 h in the open. During the course of the reaction, the color of the solution turned brown, and brown precipitate accumulated at the bottom of the flask. After 1.5 h, the mixture was allowed to cool down for 20 min. Then, the condenser was replaced by a glass stopper and the flask was cooled in an ice bath for 30 min. The precipitate was filtered and washed with acetone and dried under vacuum. Brownish red crystalline powder was obtained
10
15
20
25
30
35
40
45
50
55
2θ (°) Fig. 2. Diffractograms of a [Cr2(O2CCH3)4] powder (in blue) and a powder diffraction simulation from single crystal (in red) [9]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
O. Levy et al. / Inorganica Chimica Acta 391 (2012) 179–181
up as Cr(III) in the trinuclear complex, [Cr3O(O2CCH3)6(H2O)3]+. This species is the result of air oxidation of dissolved chromous acetate in the reaction mixture. However, when air is excluded during the reaction, the yield of the product is very similar. In conclusion, the simplicity and relative low cost of reagents makes the above procedure ideal for student laboratories and a source for anhydrous chromous acetate as a starting material for other mono- or dinuclear Cr(II) species or a reducing agent in preparative organic chemistry. References [1] F.A. Cotton, R.A. Walton, Multiple Bonds between Metal Atoms, Wiley, 1982.
181
[2] L.A. Paquette (Editor-in-chief), Encyclopedia of Reagents for Organic Synthesis, Wiley, England, 1995. [3] M. Lee, H. Nakamura, Y. Minoura, J. Polym. Sci., Part A: Polym. Chem. 14 (1976) 961. [4] (a) E. Peligot, C.R. Hebd. Seances Acad. Sci. 19 (1844) 609; (b) E. Peligot, Ann. Chim. Phys. 12 (1844) 528. [5] (a) J.N. van Niekerk, F.R.L. Schoening, J.F. de Wet, Acta Crystallogr. 6 (1953) 501; (b) F.A. Cotton, B.G. DeBoer, M.D. LaPrade, J.R. Pipal, D.A. Ucko, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 27 (1971) 1664. [6] J.C. Reeve, J. Chem. Educ. 62 (1985) 444. [7] L.R. Ocone, B.P. Block, Inorg. Synth. 8 (1966) 125. [8] R.J. Lewis Sr., Hazardous Chemicals Desk Reference, sixth ed., Wiley, New Jersey, 2008. [9] F.A. Cotton, C.E. Rice, G.W. Rice, J. Am. Chem. Soc. 99 (1977) 4704. [10] L. Beneš, J. Kalousová, J. Votinsky´, J. Organomet. Chem. 290 (1985) 147.