Chemistry

Academician A. N. NESMEYANOV, N. S. KOCHETKOVA, R. B. MATERIKOVA

DESTRUCTION OF CYCLOPENTADIENYL COMPOUNDS OF METALS BY BROMINE AND POTASSIUM HYPOBROMITE

The aim of the present work is to study the reaction of cleavage of the ring–metal bond in various cyclopentadienylmetal compounds under comparable conditions, with isolation of both the inorganic and the organic part of the molecule. Until now, cleavage of the ring–metal bond in cyclopentadienyl derivatives of metals has not been widely studied. Data are available only for ferrocene and its derivatives (¹–⁴), and also for cobalticinium (⁵), in the latter case the action of hydrochloric acid completely destroying the organic part of the molecule. See also the work of Grignard and Courtot (⁶, ⁷) on C₅H₅MgBr. We succeeded, by bromination under mild conditions, in carrying out the cleavage of (C₅H₅)₂Fe, pentaethanodiferrocene (⁸, ⁹), (C₅H₅)₂Hg, and C₅H₅Tl with preservation of the organic part of the molecule. Dicyclopentadienylmercury is thermally unstable at temperatures above −15°. Therefore the reaction with bromine was carried out at −17 to −20° for all the compounds studied. Under these conditions only C₅H₅Br₃ is formed, and the endpoint of the reaction with Br₂ becomes very sharp.

Pentaethanodiferrocene, besides C₅H₅Br₃, also forms a bromide corresponding to the central part of the molecule. The latter evolves HBr at room temperature during the course of the experimental work-up. The structure of the bromide obtained will be studied further. Analysis and molecular weight give the gross formula C₂₀H₂₁Br₇. In the products of the bromination reaction of ferrocene (I) and pentaethanodiferrocene (II), along with the bromides C₅H₅Br₃ and C₂₀H₂₁Br₇, salts of the cations I and II, insoluble in the reaction mixture, were also found (cf. the work of Riemschneider (¹⁰)). These salts are not destroyed by bromine under the experimental conditions; by reduction of them I and II were recovered, and the yield of bromide was calculated on the ferrocene or pentaethanodiferrocene that had reacted, respectively. For I, II, and C₅H₅Tl the yield of C₅H₅Br₃ is about 50% of theory. In the case of bromination of (C₅H₅)₂Hg the yield of C₅H₅Br₃ is close to quantitative.

We were also the first to carry out the destruction of cyclopentadienyl compounds of Fe, Hg, and Tl by a solution of potassium hypobromite. In this case cleavage of the ring–metal bond and bromination of the organic part of the molecule to hexabromocyclopentadiene C₅Br₆ occur. On the action of potassium hypobromite on cyclopentadiene under comparable conditions, see (¹¹) and (¹²). The reaction temperature was maintained at about 0°. The yields of C₅Br₆ are close to the yield described for cyclopentadiene.

In the reaction products of (C₅H₅)₂Fe, along with C₅Br₆, C₅HBr₅ was also found. Small amounts of this bromide are also formed in the destruction of C₅H₅Tl. In contrast to potassium hypobromite, potassium hypoiodite does not react with ferrocene under analogous conditions. It is known that the ferrocene system is preserved upon oxidation of acyl derivatives of ferrocene with iodine in pyridine on heating (¹³–¹⁵), and also upon the action of iodine on ferrocene and its derivatives (¹⁶).

Reaction of (C₅H₅)₂Hg with Br₂. To a solution of 0.33 g of (C₅H₅)₂Hg in 20 ml of n-heptane and 3 ml of benzene, cooled to −20°, 4 ml of a heptane solution of bromine containing 0.157 g of bromine per 1 ml of solution was added dropwise, the temperature of the bath being maintained within −17–20°. Absorption of bromine was instantaneous. At the same time, a cream-colored finely crystalline precipitate forms on the walls of the flask (further addition of bromine gives an unchanged [[unclear: continuation cut off at page bottom]]).

disappearing coloration in the solution). The resulting colorless solution was decanted, the precipitate was washed twice with n-heptane (10 ml each), which was combined with the main solution. Weight of the air-dry precipitate of HgBr₂: 0.35 g. The heptane was evaporated. Weight of the crystalline cream-colored residue: 0.57 g. It rapidly darkens in the light (!). It has the characteristic odor of unsaturated bromides. M.p. 47–55°. After chromatography on SiO₂ (\(d = 12\) mm, \(h = 300\) mm, from a heptane : benzene mixture 8 : 2, then heptane : benzene 1 : 1), protected from light, 0.41 g of transparent, colorless, strongly light-refracting crystals with m.p. 63–64° was obtained.

Found, %: C 19.83, 19.86; H 1.71, 1.67; Br 78.65, 78.30
\( \mathrm{C_5H_5Br_3} \). Calculated, %: C 19.69; H 1.65; Br 78.50

The yield of tribromocyclopentene was 93%. At the experimental temperature of −2–5°, 6 ml of bromine solution was absorbed, and after chromatography on SiO₂ (\(d = 12\) mm, \(h = 300\) mm, from heptane), isomers of pentabromocyclopentane were obtained, m.p. 96–101° and 105–107°; yield 49%.

Reaction of \( \mathrm{C_5H_5Tl} \) with \( \mathrm{Br_2} \). To a suspension of 1.64 g of freshly sublimed \( \mathrm{H_5C_5Tl} \) in 20 ml of n-heptane and 3 ml of benzene, cooled to −20°, with efficient stirring and at a temperature of −17–20°, 6 ml of a heptane solution containing 0.157 g of bromine per 1 ml of solution was slowly added dropwise, at such a rate that the solution became decolorized before addition of the next portion of bromine. After this, absorption of bromine practically ceased. The resulting solution was decanted; the precipitate in the flask was washed with heptane and benzene, which were combined with the main solution. Removal of the solvents protected from light gave 0.4 g of bromide. The precipitate from the flask was washed with ether; evaporation of the ether gave 0.18 g of a brown resinous nonflammable residue containing Tl. The residue after ether treatment weighed 1.75 g. Sublimation returned 0.64 g of \( \mathrm{C_5H_5Tl} \). Chromatography of the bromide on SiO₂ (\(d = 12\) mm, \(h = 300\) mm, from a heptane : benzene mixture 8 : 2, then heptane : benzene 1 : 1), protected from light, gave 0.25 g of \( \mathrm{C_5H_5Br_3} \) with m.p. 57–59°; yield 35%; on repeated chromatography, a bromide with m.p. 62.5–64° was obtained.

Found, %: C 19.52, 19.82; H 1.62, 1.69; Br 78.16
\( \mathrm{C_5H_5Br_3} \). Calculated, %: C 19.69; H 1.65; Br 78.50

At a temperature of +20°, pentabromocyclopentane was obtained, m.p. 103–104° (from alcohol), giving no melting-point depression with an authentic sample; yield 51%.

Reaction of \( \mathrm{(C_5H_5)_2Fe} \) with \( \mathrm{Br_2} \). To a solution of 0.49 g of \( \mathrm{(C_5H_5)_2Fe} \) in 20 ml of n-heptane and 3 ml of benzene, cooled to −20°, a heptane solution of bromine containing 0.157 g of bromine per 1 ml was added dropwise at the rate of bromine absorption; immediately, formation of a ferricinium salt precipitate began, at first blue-green, then changing to khaki color. In total, over 1 h at −17–20°, 10.5 ml of solution was added, after which absorption of bromine ceased. The precipitate was filtered off protected from moisture and washed on the filter with abs. benzene until the bromine color disappeared in the wash solution, while the color of the precipitate changed back to the original one and the volume of the precipitate noticeably decreased.

The organic solutions were evaporated to dryness protected from light. Weight of the residue: 0.27 g. Chromatography of the bromide on SiO₂ (\(d = 12\) mm, \(h = 300\) mm, from a heptane : benzene mixture 8 : 2, then heptane : benzene 1 : 1) gave 0.14 g of \( \mathrm{C_5H_5Br_3} \) with m.p. 58–61°. A mixed sample with an authentic sample gave no melting-point depression; a mixed sample with \( \mathrm{C_5H_5Br_5} \) gave a depression (12°). The precipitate obtained from the reaction was dissolved in dilute aqueous methanol, washed with benzene and ether; evaporation of the ether–benzene layer gave 0.02 g of resin containing no iron. The washed aqueous solution was reduced with Na₂SO₃; \( \mathrm{(C_5H_5)_2Fe} \) was extracted with benzene; the benzene solution was dried with Na₂SO₄ and evaporated. 0.27 g of \( \mathrm{(C_5H_5)_2Fe} \) was recovered. The yield of bromide based on reacted ferrocene was 43%.

Reaction of pentaethynodiferrocene with bromine. 0.6 g of pentaethynodiferrocene was dissolved in 3 ml of benzene, and then with stir-

20 ml of n-heptane was added with stirring. The resulting suspension was cooled to −20° with stirring. At −17–20° over 1 h 15 min, 14 ml of a heptane solution containing 0.157 g of bromine per 1 ml was added. Standing for 20 min does not lead to disappearance of the excess bromine in the solution. The precipitate of salts was filtered off with protection from moisture and washed repeatedly on the filter with heptane, then 3 times with a heptane + benzene mixture 8 : 2 and once with ether. The resulting solutions must be kept on solid CO₂. The residue was completely dissolved in 150 ml of methanol. The resulting dark-brown solution was rapidly diluted to 1.5 l with water and, after addition of 50 g of Na₂SO₄, extracted 3 times with small portions of benzene and 5 times with ether. Then the blue-green aqueous solution was reduced with sodium sulfite and extracted with benzene and ether; evaporation of the latter after drying gave 0.30 g of the starting pentaethanoferrocene, purified by reprecipitation from ether into methanol. All the remaining organic solutions were combined and the solvents removed without heating, and when room temperature is reached, evolution of HBr is observed. The resulting yellow residue was exhaustively treated with 70% aqueous methanol. The resulting solution was evaporated, and the residue was extracted with ether; after removal of the ether, the weight of crystalline C₅H₅Br₃ was 0.22 g. After purification by chromatography (SiO₂, $d = 12$ mm, $h = 300$ mm, from a heptane : benzene mixture 8 : 2, then heptane : benzene 1 : 1), m.p. 60–62°. There is no depression of the melting point with authentic C₅H₅Br₃.

The residue after treatment with 70% methanol was treated with abs. methanol. Evaporation of the solution gave 0.24 g of a light-brown powder, which was washed with heptane; after recrystallization from abs. ethanol, m.p. 65–70° (with decomposition). A sample of the bromide solution (heptane : benzene 1 : 1) obtained in this way was checked for absence of pentabromocyclopentane and tribromocyclopentane by thin-layer chromatography (SiO₂, exposure time 10 min, layer thickness 1.5 mm, development with metallic iodine).

Found, %: C 26.67, 26.94; H 2.42, 2.62; Br 69.49, 69.43

Mol. wt. 760, 746 (benzene, cryoscopy). Gross formula C₂₀H₂₁Br₇. Yield of C₅H₅Br₃ based on pentaethanoferrocene reacted: 61% of theory.

Reaction of (C₅H₅)₂Hg with KOBr. A solution of 0.34 g of (C₅H₅)₂Hg in 20 ml of benzene was added to a reagent cooled to +2°, prepared from 20 ml of H₂O, 4.5 g of KOH, and 0.65 ml of Br₂. At 0–2° the reaction mixture was kept for 2 h with efficient mechanical stirring. 0.2 g of HgO was filtered off. The benzene layer was separated, and the aqueous layer was washed twice with benzene. The combined benzene solution was dried over Na₂SO₄.

Evaporation of the benzene gave 0.68 g of yellow crystals; after chromatography (SiO₂, $d = 12$ mm, $h = 100$ mm, from n-heptane), 0.60 g of bromide with m.p. 70–72° was obtained. Repeated chromatography gave C₅Br₆ with m.p. 86–86.5°.

Found, %: C 11.22, 11.05; Br 88.99, 89.09
C₅Br₆. Calculated, %: C 11.20; Br 88.80

There is no depression of the melting point with authentic C₅Br₆ (¹²). Yield 61%.

Reaction of C₅H₅Tl with KOBr. 18 g of KOH was dissolved in 80 ml of H₂O. The solution was cooled to +2°. With stirring, 2.5 ml of Br₂ was added so that the temperature of the reaction mixture was no higher than +3—+4°. The solution was cooled to +2°. In one portion, a suspension of 1.8 g of C₅H₅Tl in 15 ml of benzene was added. The reaction mixture was kept for 40 min at +2° with efficient stirring, during which a dark-brown powder of Tl oxide formed. The precipitate was filtered off, washed with benzene until the odor of C₅Br₆ disappeared, dried in air, and sublimed at 90–110°/1 mm Hg. On sublimation, C₅H₅Tl was not found. Weight 1.06 g. The benzene solution was separated from the aqueous layer; the latter was washed 3 times with benzene, which was combined with the main solution. The benzene layer was dried over Na₂SO₄ and evaporated. The residue consisted of dark-yellow crystals with a characteristic odor. Weight 1.95 g, m.p. 67–80°. By chromatography (SiO₂, $d = 12$ mm, $h = 300$ mm, from n-heptane), 1.32 g of bright-yellow

plates with m.p. 81–83°; repeated chromatography raises the m.p. to 86–87°. There is no depression of the melting point with an authentic sample of \(\mathrm{C_5Br_6}\). Yield 55%.

Reaction of \((\mathrm{C_5H_5})_2\mathrm{Fe}\) with KOBr. To a solution of 18 g of KOH in 80 ml of \(\mathrm{H_2O}\) at \(+2\)—\(+4^\circ\) was added 2.5 ml (0.098 M) of \(\mathrm{Br_2}\), and then at \(+2^\circ\) a solution of 0.32 g of \((\mathrm{C_5H_5})_2\mathrm{Fe}\) in 20 ml of benzene was added. The mixture was kept at \(+2\)—\(+15^\circ\) with efficient stirring for 1.5 h; during the first 30 min no visible changes occurred; then the mixture darkened and a dark-brown precipitate separated; the increase in precipitate ceased 1 h–1 h 15 min after the beginning of the experiment. The precipitate was filtered off, washed with benzene, and dried in air. It does not burn, dissolves in HCl, and contains \(\mathrm{Fe^{3+}}\). The benzene solution was separated from the aqueous layer; the latter was washed three times with benzene, which was combined with the main solution.

The combined benzene solution was dried with \(\mathrm{Na_2SO_4}\) and evaporated. The weight of the crystalline yellow-orange residue was 2.70 g.

Chromatography of three portions of 0.9 g each (\(\mathrm{SiO_2}\), \(d = 12\) mm, \(h = 300\) mm, from \(n\)-heptane) gave 0.05 g of \((\mathrm{C_5H_5})_2\mathrm{Fe}\); 0.26 g of \(\mathrm{C_5Br_6}\), m.p. 64–75°; and 0.80 g of a mixture of \(\mathrm{C_5Br_6}\) and \(\mathrm{C_5HBr_5}\). The dark-violet chromatographic columns were washed with benzene, methanol, water, and then with a concentrated aqueous solution of \(\mathrm{Na_2SO_3}\) acidified with \(\mathrm{H_2SO_4}\). The reduced columns acquired an amber-yellow color. Ferrocene was extracted from the adsorbent with ether. On evaporation of the ether, the weight of \((\mathrm{C_5H_5})_2\mathrm{Fe}\) was 0.13 g. The mixture of \(\mathrm{C_5Br_6}\) and \(\mathrm{C_5HBr_5}\) was separated by chromatography on a two-stage column (\(\mathrm{SiO_2}\), \(d_1 = 25\) mm, \(d_2 = 12\) mm, \(h_{\mathrm{общ}} = 600\) mm, from \(n\)-heptane), giving 0.50 g of \(\mathrm{C_5Br_6}\), m.p. 85–86°, and 0.15 g of light-yellow soft crystals, m.p. 100–102°. See \((^{12})\). The depression of the melting point of the mixture with \(\mathrm{C_5Br_6}\) is about 50°.

Found, %: Br 87.32
\(\mathrm{C_5HBr_5}\). Calculated, %: Br 86.75

Thus, 60% of \((\mathrm{C_5H_5})_2\mathrm{Fe}\) was recovered. The total yield of bromides, based on the ferrocene that reacted, was 71%.

Attempted destruction of \((\mathrm{C_5H_5})_2\mathrm{Fe}\) with a solution of KOJ. To a solution of 18 g of KOH in 80 ml of \(\mathrm{H_2O}\) at \(+2\)—\(+4^\circ\) was added 12.5 g (0.098 M) of \(\mathrm{J_2}\), the salt formed partly precipitating. Then, at 2°, a solution of 0.32 g of \((\mathrm{C_5H_5})_2\mathrm{Fe}\) in 20 ml of benzene was added. The mixture was kept at \(+2\)—\(+15^\circ\) with efficient stirring for 1.5 h, with no changes occurring. The benzene solution was separated, the aqueous layer was acidified, and iodine was extracted with chloroform. Inorganic iron was not detected. The benzene solution was washed with water, with a dilute solution of \(\mathrm{Na_2S_2O_3}\), again with water, dried with \(\mathrm{Na_2SO_4}\), and evaporated. The Beilstein test for halogen was negative. The weight of recovered ferrocene was 0.32 g. M.p. 170–172°. It gives no depression of the melting point with authentic \((\mathrm{C_5H_5})_2\mathrm{Fe}\). After chromatography of 0.25 g of substance (\(\mathrm{Al_2O_3}\), \(d = 12\) mm, \(h = 150\) mm, from \(n\)-heptane), the weight was 0.22 g. M.p. 171.5–172.5°.

Institute of Organoelement Compounds
Academy of Sciences of the USSR

Received
31 V 1962

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