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A method for synthesizing polyynes in solution
Carbon 43 (2005) 2792–2800 www.elsevier.com/locate/carbon
A method for synthesizing polyynes in solution Franco Cataldo
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Soc. Lupi arl, Chemical Research Institute, Via Casilina 1626/A, 00133 Rome, Italy Received 13 March 2005; accepted 23 May 2005 Available online 14 July 2005
Abstract A new simple method for synthesizing polyynes (or oligoynes as called in earlier literature) in solution is reported. The method involves the hydrolysis of calcium carbide in a solution of ammonium chloride and cuprous and cupric chloride followed by acid hydrolysis of the resulting product. The polyynes are released and are scavenged with a hydrocarbon solvent like heptane. The polyynes in solution are produced in relatively high concentration (up to 102 M), without soot and PAHs secondary products and these are clear advantages over the polyynes synthesis with the submerged electric carbon arc technique. The mechanism of polyynes formation is explained. At relatively high concentration the polyynes are unstable in solution and give a brown precipitate resembling cork. It is a product of polyaddition reactions between the polyynes chains. The polyynes are stable in solution at relatively high dilution. In acidic solutions, in the presence of copper ions, the polyynes are altered into other products. The FT-IR spectra and the stability of copper polyynides are presented and discussed. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbyne; Synthesis; Chromatography; Chemical structure
1. Introduction Polyynes can be considered as low molecular weight end-capped oligomeric model compounds for the hypothetical one-dimensional carbon allotrope carbyne. In the early literature very short chain polyynes are referred to as oligoynes; in the present paper we will call them polyynes. A milestone in the synthesis of polyynes is represented by the step synthesis proposed by Walton and co-workers in 1972 [1]. The mentioned synthetic approach allowed an excellent control of the chemical structure and length of the polyyne chains but was relatively complex. Recently, a number of papers appeared in the literature on the synthesis and on the structure
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of end-capped polyynes, cyclic polyynes and ene-ynes, demonstrating that polyynes and related molecules are the object of intensive studies and great interest [2–7]. Cyanopolyynes were easily prepared by arcing graphite electrodes in a cyanogen atmosphere [8,9] or, more simply, by arcing graphite electrodes in liquid nitrogen [10]. Polyynes in hydrocarbon solutions can be prepared by focusing a laser beam into a suspension of graphite particles or even fullerene particles in a solvent [11,12]. The submerged carbon arc technique in hydrocarbons or other solvents was proved to be an extremely simple and versatile technique for the synthesis of polyynes in solution [13–18]. The submerged electric arc synthesis has permitted an easy access to the polyynes solutions and the first chemical properties of these unusual molecules have been explored in terms of photolysis, ozonolysis and oxidation resistance to air [19]. Particularly remarkable in this context is the work of Heymann [20] who has explored
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quantitatively the thermal stability in solution of C8H2 obtained by prolonged submerged carbon arc and purified by preparative HPLC. One of the drawbacks of the submerged electric arc synthesis is the fact that polyynes are obtained at high dilution (105–106 M) and special techniques are needed in order to reach higher concentration [13]. Another drawback of the submerged carbon arc synthesis is the formation of polycyclic aromatic hydrocarbons (PAHs) and soot as secondary products together with polyynes, although the concentration of PAHs remains at least 1–2 orders of magnitude lower than the polyynes concentration [21,22]. Again special techniques are needed to purify polyynes produced from the carbon arc [13], including preparative liquid chromatography [20]. In the present work, we will show that polyynes in solution can be prepared at high concentration, without soot and without PAHs secondary products, starting from acetylene produced in situ from the hydrolysis of calcium carbide and coupled in situ by the presence of suitable copper ions.
2. Experimental 2.1. Materials and equipment Calcium carbide technical grade was from Aldrich. All other solvents and reagents were obtained from Fluka. The electronic absorption spectra were recorded on a Shimadzu UV160A spectrophotometer while the HPLC analysis was conducted on an Agilent Technologies liquid chromatograph model 1100 equipped with a diode-array detector. The column was a Zorbax Elipse XDB C-8. The analysis was conducted under isocratic conditions using a mobile phase composed of CH3CN/ water 80:20 vol/vol. The crude heptane solution of polyynes was injected directly into the column. Under these circumstances the polyynes concentration was too high and required a dilution. Therefore an assay of the polyynes solution recovered was diluted at least 15 times with pure heptane and 25 ll were injected. The diodes of the detector were set at 202, 225, 250, 295 and 350 nm. Polyynes were identified on the basis of their retention times and on the basis of their characteristic electronic absorption spectra. For quantitative analysis the molar extinction coefficients of polyynes known from literature [1] were used. For further details on the separation and identification of polyynes see previous works [13–19,21,22]. FT-IR spectra were obtained on a Nicolet IR300 spectrometer from Thermo-Electron Corp. The spectra were recorded in transmittance with the solid embedded in KBr pellets or as a film on ZnSe crystal in reflectance.
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2.2. Standard procedure for the polyynes synthesis with Cu+/Cu2+ solutions NH4Cl (7.5 g) was dissolved in 200 ml of distilled water in a tick-walled conical flask with glass stopper equipped with a valve. CuCl2 Æ 2H2O (10.3 g) and CuCl (5.2 g) were added to the ammonium chloride solution and stirred to ensure the dissolution (there may remain undissolved matter). Heptane (50 ml) is added to the solution followed by calcium carbide (CaC2, 3.5 g) in small portions. After the addition of CaC2 the flask fitted with the stopper is hand-shaken for a few seconds and then the excess of acetylene is released by opening the valve. The operation is repeated several times until all calcium carbide is consumed. When the first portion of CaC2 is added the mixture turns into a violet colour but on further CaC2 addition it becomes dark-brown. The reaction mixture is again shaken for about 2 min and 35 ml of conc. hydrochloric acid are added to cause the hydrolysis of the acetylides. Shaking is prolonged for 2 min and then the crude mixture is filtered (using a standard paper filter) with the aid of an aspirator. A dark precipitate is collected on the filter paper. The filtrate consists of a green aqueous phase and the heptane layer containing the dissolved polyynes. The heptane solution is recovered with the aid of a separatory funnel. An assay of the polyynes heptane solution (0.1 ml), diluted 25 times with pure heptane displays the following electronic absorption spectrum (in nm) 200 (s), 207 (sh), 216 (ms), 227 (s), 240 (m), 253 (m), 260 (w), 276 (mw), 297 (w). Similar results as just described can be obtained by changing the ratio of Cu+/Cu2+ or even by using only Cu+ or Cu2+ solutions. 2.3. Polyynes synthesis with Cu2+ solution NH4Cl (5.8 g) was dissolved in 150 ml of water and then CuCl2 Æ 2H2O (12.6 g) was added and dissolved. A light blue homogeneous solution was obtained; heptane (50 ml) was added to the solution. Then CaC2 (7.5 g) was added to the solution in small portions as described in Section 2.2. The reaction mixture turns black with abundant dark precipitate. Concentrated hydrochloric acid (35 ml) was then added and the mixture was strongly hand-shaken. Filtration with a aspirator permits to separate the precipitate from the water solution and the hexane solution. The polyynes solution in hexane was analyzed after dilution by electronic spectroscopy, it displayed the same spectrum reported in Section 2.2. 2.4. Polyynes synthesis with Cu+ solution CuCl (7.1 g) was dissolved together with 10.0 g of NH2OH Æ HCl (hydroxylamine hydrochloride) in 80 ml of conc. ammonia and 50 ml of water. Acetylene was
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passed into the solution owing to abundant precipitation of red-brown dicopper acetylide (Cu2C2). The precipitate aqueous suspension was divided into two parts having equal volumes. The first part was treated with 40 ml of heptane and then a 16% aqueous solution of HCl was added gradually. After each addition a sample of hexane solution was checked spectrophotometrically. A total of 75 ml of HCl were used for a complete hydrolysis of the acetylide. The organic layer was recovered simply by decantation from the aqueous phase. Both the spectrophotometric analysis and the HPLC analysis revealed the presence of polyynes in the organic solvent. The other portion of Cu2C2 was treated with 7.3 g of CuCl2 Æ 2H2O, under stirring for 30 min at room temperature. Heptane (40 ml) was added and the hydrolysis was conducted with the addition of 90 ml of 16% aqueous solution of HCl. The hydrolysis products remained trapped in the solvent and consisted of a mixture of polyynes; they were determined both spectrophotometrically and by HPLC. Their concentration was about two orders of magnitude larger than in the previous case. The organic layer was recovered by filtration and separation from the aqueous phase with the aid of a separatory funnel. 2.5. Polyynes addition products upon prolonged contact with Cu+/Cu2+ catalyst A polyynes solution in heptane was prepared following a procedure analogous to that described in Section 2.2. The addition of 10.0 g of CaC2 in small portions to an aqueous solution prepared by dissolving 10.0 g of NH4Cl, 16.0 g of CuCl2 Æ 2H2O and 6.0 g of CuCl in 150 ml of water covered with a layer of 50 ml of heptane. After the mentioned steps the solution was treated with 50 ml of conc. HCl. The reaction mixture was left for 7 days in a closed conical flask. The filtration of the mixture with the aid of an aspirator afforded a clear-brown aqueous solution and a yellow heptane layer other than a black insoluble inorganic precipitate on the paper filter. The organic layer was separated with the aid of a separatory funnel. The HPLC analysis revealed a plethora of new products none of them was recognized because of the lack of standard reference compounds. Polyynes were still present: C8H2 for instance was definitely identified. The most important compounds with their HPLC retention times and spectra are reported (excluded the polyynes): Rt = 1.590 min (k in nm): 209, 228, 248, 254, 260, 268, 278, 283, 300, Rt = 1.617 min (k in nm): 208, 220, 242, 267, 283, 298, Rt = 1.790 min (k in nm): 218, 224, 232, 255, 268, 286, 302,
Rt = 2.176 min (k in nm): 212, 220, 245, 258, 270, 286, 309, Rt = 2.483 min (k in nm): 205, 232, 255, 270, 288, 308, 328, Rt = 2.537 min (k in nm): 200, 228, 240, 255, 270, 288, 305, 326, Rt = 2.603 min (k in nm): 228, 240, 257, 272, 290, 309, Rt = 2.830 min (k in nm): 215, 242, 252, 264, 278, Rt = 3.150 min (k in nm): 220, 255, 278, 296, 309, 332, Rt = 3.416 min (k in nm): 205, 232, 242, 255, 275, 290, 310, Rt = 3.616 min (k in nm): 210, 215, 228, 238, 250, 262, 298, 303, 322, 345, 372, Rt = 3.910 min (k in nm): 209, 260, 275, 300, 320, 342, 363, Rt = 4.230 min (k in nm): 240, 278, 295, 315, 330, 346, 375.
2.6. Polyynes hydrogenation The hydrogenation of the polyynes prepared as described in Section 2.2 was achieved by reacting the heptane solution with Zn dust and 15% hydrochloric acid following the same procedure already described in detail for the polyynes produced with the carbon arc [10,23]. The hydrogenated ene-ynes are no more precipitable as acetylides. In fact an heptane solution of ene-ynes prepared from the acetylides treated with an ammonia/NH2OH solution of CuCl does not yield any precipitate. Therefore, the hydrogenated polyynes appear to be completely free from terminal acetylene groups. 2.7. Polyynes stability in concentrated solutions The concentrated solutions of polyynes obtained according to the procedures described in Sections 2.2– 2.4 on standing, even in closed flasks separate a brown material which can be recovered by decantation of the solution. It is insoluble in all common solvents and its FT-IR spectrum in KBr is as follows (absorption peaks in cm1): 3294 (s), 2956 (sh), 2929 (m), 2859 (mw), 2180 (m) 2100 (mw), 1773 (sh), 1714 (sh), 1617 (s), 1384 (ms), 1213 (s), 1122 (s), 978 (sh), 877 (w), 757 (w), 662 (mw).
3. Results and discussion The polyynes synthesis with the electric arc was proven to be an extremely useful and versatile way to produce polyynes in solution. That synthesis has many implications both in the field of astrochemistry [24] and in the combustion chemistry and soot formation [25]. Furthermore, the importance of polyynes and their potentiality in many different fields of chemistry has been reviewed [13].
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In the present paper, we show the simplest way to produce the polyynes in solution even without the use of the electric carbon arc. The synthetic approach presented here for the polyynes is a development of our earlier observation that dicopper acetylide (Cu2C2) and dicopper diacetylide (Cu2C4) left to age in humid air undergo a slow coupling reaction [26–28]. When the aged copper compounds are hydrolyzed with hydrochloric acid, they release polyynes into an extracting hydrocarbon medium [26–28] and a carbonaceous residue containing carbynoid structures [29–31]. The drawback of the described synthetic approach involved the handling of almost dry and more or less aged dicopper acetylide and dicopper diacetylide both proven to be explosive [26–28]. Instead, the synthetic approach described here is completely safe because the acetylides are formed in situ. When dispersed in aqueous solution the acetylides are completely safe and not explosive. They are oxidized and hydrolyzed always in water solution in a completely safe procedure which is in contrast with our old approach [26–28] which required the isolation of the acetylides. 3.1. Evidence of polyynes formation from oxidized dicopper acetylide When calcium carbide (CaC2) is hydrolyzed in water it releases pure acetylene together with some secondary products like vinylacetylene, divinylacetylene and phosphine. In any case acetylene is largely the main hydrolytic product while other secondary products occur mainly as trace impurities. To verify this, we have hydrolyzed 5 g of CaC2 in 100 ml of 9% aqueous solution of NH4Cl covered with 50 ml layer of heptane. The acetylene released and other impurities caused a saturation of the heptane solvent. The spectrum of an acetylene-saturated solution in heptane is shown in Fig. 1A. It consists of a broad band showing features at 220, 226, 240 and 261 nm. Despite the high concentration of acetylene in the mentioned solvent the relative intensity of the absorption band appears low. This is in line with literature data [32] according to which acetylene displays its most intense electronic transition at very short wavelengths well beyond the range of commercial spectrophotometers. In fact the most intense transition occurs at 152 nm followed by another moderate transition at 182 nm [32]. Only a weak transition is quoted to appear at about 220 nm [32]. Thus, the spectrum of Fig. 1A is showing this transition probably overlapping with the electronic transition of other impurities which may give a detectable contribution to the spectrum of Fig. 1A because of their much higher molar extinction. They are produced only in very small, almost trace quantities. The HPLC analysis of the heptane solution of acetylene shows essentially two components, one with a reten-
Fig. 1. Electronic absorption spectra. (A) Acetylene and other impurities produced by the hydrolysis of CaC2 in water and dissolved in heptane; no polyynes are present as revealed by HPLC analysis. (B1–B3) Polyynes produced by the acid hydrolysis of dicopper acetylide in presence of Cu2+ ions. B1 is the spectrum in heptane at the beginning of the hydrolysis and B3 at the end of the hydrolysis. (C) Ene-ynes derived from the hydrogenation of polyynes with Zn dust and HCl.
tion time Rt = 1.353 min whose spectrum shows a maximum at 220 nm with a shoulder at 225 nm and another one eluting at Rt = 1.673 min having a maximum at 237 nm and a shoulder at 260 nm. The summation of the spectra of these two compounds closely resembles the crude electronic spectrum shown in Fig. 1A. As reported in Section 2.4, when a Cu2C2 dispersion in water is treated with a gradual addition of HCl, it undergoes a partial hydrolysis and the products are transferred into an heptane layer over the aqueous phase. Fig. 1B1 shows the spectrum of the products collected in this early hydrolysis stage. After further HCl addition, the spectrum changes into that shown in Fig. 1B2 and when an excess of HCl has been added to ensure the complete hydrolysis of the acetylide, then the spectrum of Fig. 1B3 is obtained. The sequence of spectra in Fig. 1B1–B3 shows the gradual evolution and accumulation of polyynes into the heptane solution. The spectrum in Fig. 1B3 is fully analogous to the spectra of polyynes mixtures obtained by arcing graphite electrodes into hydrocarbon solvents [13–18]. In particular the band at about 200 nm is due to C6H2, the bands at about 205, 216 and 227 are due to C8H2 and the bands at 240 and 253 are essentially due to C10H2 [17].
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Cu+ solution and the resulting Cu2C2 was subsequently oxidatively coupled by the action of air or Cu2+ ions. The one-step route proposed in Section 2.2 involved instead the direct hydrolysis of CaC2 into a ammonium chloride solution of Cu+/Cu2+ chlorides. The hydrolysis of CaC2 causes the in situ formation of Cu2C2 which it is immediately oxidized by the Cu2+ ions present in solution. Therefore, the subsequent acid hydrolysis inevitably causes the release of the polyynes, the coupling products, into the heptane layer. The procedure is completely safe (no handling of Cu2C2), advantageous because it permits in one shot and in a few minutes to produce polyynes solutions having concentration up to 102 M, and which do not contain any secondary products as it happens when polyynes are produced by arcing. The cleanliness of the resulting solution is shown by the HPLC chromatogram reported in Fig. 2. Only five peaks are present. Among them two peaks having retention times Rt = 1.3 min and Rt = 1.67 min have already been discussed in Section 3.1. Both have been detected in the hydrolysis of CaC2 in the absence of Cu ions. The first peak is due to acetylene while the other peak is another CaC2 hydrolysis product. The polyyne C6H2 has an Rt = 1.5 min, polyyne C8H2 eluates at Rt = 1.9 min and finally C10H2 appears at Rt = 2.7. All three polyynes have been definitely identified on the basis of their retention time and their characteristic electronic absorption spectra. Using the molar extinction coefficients available in Ref. [1], it was possible to calculate the relative concentrations of the polyynes produced by the new chemical route. The data are reported in Table 1 and show that C6H2 was quite always dominant although C8H2 was also abundant. However these results are in contrast with the relative concentration of the polyynes produced with the submerged electric carbon arc or by laser ablation of graphite particles in solution. In fact in these cases the dominant polyyne was always C8H2 and also very long polyyne chains were detected [13]. The chemical route proposed here not only produces more C6H2
The mentioned assignment has been corroborated by the HPLC analysis of the solution. As shown in Section 2.4, the polyynes are formed directly from Cu2C2 after acid hydrolysis. Since no polyynes are formed from the hydrolysis of CaC2, it is demonstrated that the Cu2C2 intermediate plays a role in the coupling reaction. More precisely, in the described circumstances, the coupling reaction occurred because of the oxidation by the atmospheric oxygen since no other oxidants were specifically added before the hydrolysis. However, as described in the second part of Section 2.4, when Cu2C2 is deliberately oxidized with the addition of CuCl2, then a larger amount of polyynes are formed as testified by the fact that the polyynes concentration was 102–103 M about two orders of magnitude larger than the concentration obtained by Cu2C2 hydrolysis oxidized in air. Indeed, oxygen gas bubbling should be another way to increase the polyyne yield. Section 2.5 has shown that a prolonged contact of the polyynes solution with the copper catalyst in presence of aqueous HCl has deleterious effects for the polyynes content. After one week of contact, the HPLC analysis shows the presence of a plethora of different products while the polyynes are still detectable, but are no more the dominant species. Because of the lack of standard reference spectra, it appears impossible to establish with certainty the chemical structure of the resulting mixture of products. What is certain is that the electronic absorption spectra reported in Section 2.5 strongly suggest the presence of ene-yne moieties. 3.2. The simplest procedure to prepare polyynes solution Once having clarified the mechanism of polyynes formation and the necessity to avoid prolonged contact of the polyynes with the copper catalyst, we have developed a one-step synthesis of polyynes without the need to pass through the Cu2C2 isolation (even in water suspension). In fact, in Section 3.1 the polyynes were obtained after two steps: first acetylene was passed into a
mAU 1750 1500 1250 1000 750 500 250 0 0
1
2
3
4
5
6
7
min
Fig. 2. HPLC chromatogram detected at 225 nm by the diode-array detector (see Section 3.2). The main components are C6H2, C8H2 (peak at Rt 1.9 min) and C10H2 (peak at Rt 2.75 min).
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Table 1 Synthesis of polyynes: a comparison with carbon arc synthesis (yield in mol%) Electric arc
Data from HPLC-DAD analysis This work Section 2.2 from CaC2
This work Section 2.2 from CaC2
This work Section 2.2 from CaC2
This work Section 2.3 from CaC2
This work Section 2.4 from CaC2
Cu species Molar ratio
None None
Cu(I)/Cu(II) 1.15:1
Cu(I)/Cu(II) 1:1
Cu(I)/Cu(II) 0.80:1
Cu(II) 1
Cu(I) 1
Polyyne C6H2 C8H2 C10H2 C12H2 C14H2 C16H2
20.3 61.2 14.8 2.9 0.6 0.2
56.8 35.8 6.6 0.7 0.1 0.0
48.7 49.1 2.2 Traces 0.0 0.0
67.4 31.2 1.2 0.2 0.0 0.0
80.9 18.7 0.4 0.0 0.0 0.0
83.3 15.6 1.1 0.0 0.0 0.0
85.5
53.4
97.7
56.1
28.7
Total conc. (mM)
0.01
Note: The concentration of the polyynes was calculated using the molar extinction coefficients taken from Ref. [1].
rather than C8H2 but also the longer chains are present, but only at very low concentration, in trace amount or in undetectable amounts as summarized well in Table 1. From Table 1 emerges also another important result. The distribution of products depends on the type of Cu ions present in solution. If only Cu2+ ions (Section 2.3) or only Cu+ ions are present (Section 2.4), then the coupling reaction does not lead to very long chains and C6H2 is by far the dominant specie. If instead the synthesis is conducted with the simultaneous presence of Cu+ and Cu2+ ions (Section 2.2), then the amount of C8H2 becomes quite high and may equal the amount of C6H2 (Table 1). Of course, many parameters affect the distribution of products, like the molar ratio Cu+/ Cu2+, the reaction time, the hydrolysis conditions and so on. Cu2+ ions do not form acetylides but the in situ formation of acetylene from CaC2 may cause a partial reduction of the Cu2+ ions to Cu+ ions, so that in situ formation of Cu2C2 in the solution can be observed which then is easily oxidized by the excess Cu2+ ions (Section 2.3). The formation of C6H2 in large excess also in this case (Table 1) may be explained by the fact that small acetylide amounts are formed so that there is not too much availability for the coupling reaction. 3.3. Polyynes hydrogenation Polyynes can be easily hydrogenated to ene-ynes by the action of Zn dust and aqueous HCl [13,15,23]. That is also the case for the polyynes produced by the synthesis of this paper. Fig. 1C it reports the electronic absorption spectrum of the crude hydrogenated mixture. The spectrum is the same as that of the product derived from the hydrogenation of the polyynes produced with a submerged electric arc [23]. The HPLC analysis of the hydrogenated mixture reveals three main components. The first one eluting at Rt = 1.66 min and having an
absorption maximum at 220 nm and a shoulder at 260 nm has been assigned to hydrogenated C6 on the basis of its retention time. This compound is followed by a second one having an Rt = 1.94 min whose retention time and spectrum correspond to that of hydrogenated C8 already synthesized [23]. In fact the absorption maxima were found at 210, 248, 258 and 268 nm in agreement with previous results [23]. The third important component was found at Rt = 2.3 min with a spectrum having the following maxima: 190, 225, 265, 277, 290 and 303 nm. In terms of retention time and also in terms of electronic absorption spectrum this component corresponds to hydrogenated C8 (another isomer), as already detected and discussed previously [23]. A minor component was found at Rt = 2.86 min with a spectrum corresponding to that of a hydrogenated C10: 228, 255, 265, 278, 303, 320 and 334 nm, again in agreement with previous results [23]. It is interesting that the hydrogenated ene-ynes are no longer able to form insoluble acetylide salts with Cu+ solutions. In fact if the hydrogenated solution containing ene-ynes derivatives is shaken with the Ilosvays reagent (Cu+ dissolved in ammonia/hydroxylamine aqueous solution) no precipitate can be detected. This experimental fact has the important implication that of all the hydrogenated polyynes, the resulting ene-ynes do not have anymore terminal acetylene groups. 3.4. Dicopper polyynides The polyynes produced either with the submerged carbon arc or with the chemical route described in this paper, react easily with CuCl solutions dissolved in NH3/NH2OH aqueous solution to give copper-coloured precipitates: the copper polyynides (by analogy with copper acetylides). When polyynes are produced through the carbon arc, prolonged arcing is needed in
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order to be able to collect the copper polyynides. This technique has been proposed as an industrial way to purify the polyynes from the PAHs secondary products [13]. When polyynes are produced from CaC2 hydrolysis described here, it is possible to precipitate quantitatively the polyynes as polyynides leaving the heptane solution virtually free from any detectable polyynes. The HPLC analysis confirms the complete absence of polyynes and permits the detection of low levels of unknown impurities or by-products. The copper polyynides are copper-coloured when freshly produced and can be collected by filtration. On standing they become black powders or flakes in an oxidation process which parallels the behaviour of dicopper acetylide and diacetylide [26–28]. They are not as sensitive (explosive) to shocks as the parent acetylide and the diacetylide, even when dry, but when heated they deflagrate. Thus, they remain dangerous materials to handle and utmost care must be exercised when handling these products working always with a few mg at a time. Fig. 3 shows the FT-IR spectrum of freshly prepared polyynides (embedded in KBr). The spectrum displays relatively intense acetylenic band at 2106 cm1 followed by two other bands at lower wavenumbers in absorption regions which suggest the presence of the allenic moieties: 1968 and 1823 cm1. It is possible that the complexation of these molecules with copper ions causes a certain triple bond conjugation and rearrangement along the carbon chain, as already hypothesized in previous works [26,27]. When the copper polyynides are hydrolyzed with HCl to remove the copper ions, the resulting free chains have
the structure of linear polyynes without any allenic or cumulenic moiety. This is based on the HPLC analysis of the resulting polyynes solutions using a diode-array detector. Another feature of the copper polyynides is their sensitivity to ageing. After ageing in air for a day or more, when they are treated with HCl are no more able to release back all the polyynes which have been complexed. Instead they hydrolyze only partially but large fractions of them remains as dark insoluble matter resembling carbonaceous matter, but containing a high level of copper. We have called this matter Cu–carbon [30], and have shown by solid state 13C NMR that it is characterized by a relatively high level of sp-hybridized carbon atoms mixed with sp2 and sp3 and stabilized with complexed copper. 3.5. Stability of polyynes solutions in hydrocarbons The stability of polyynes in diluted solution was a matter of surprise [19,20]. It has been proven that the polyynes remain unchanged for months in dilute solutions kept in closed flasks at room temperature in diffuse daylight or in the dark. The polyynes distribution and their concentration was monitored for months in methanol, ethanol and hexane or heptane. No significant changes were recorded, when the polyynes concentration were in the range of 106–105 M. Instead, cyanopolyynes and dicyanopolyynes are much less stable in solution at room temperature and can survive only 1– 2 days [13]. Polyynes are slowly photolyzed in solution by the UV light [19].
0.15
1378
0.14
609
0.09
1968
0.08 0.07 0.06
1823
2106
0.05 0.04
836
Absorbance
0.10
474
1207
1593
0.11
1461
0.13 0.12
0.03 0.02 0.01 2500
2000
1500
1000
500
Wavenumbers (cm-1)
Fig. 3. FT-IR spectrum (KBr) of dicopper polyynides, the insoluble Cu(I) salts of polyynes.
F. Cataldo / Carbon 43 (2005) 2792–2800
978
1122
0.18
1213
0.20
1384
1679
0.22
1620
1720
0.24
2799
662
0.14 0.12
757
Absorbance
0.16
0.10
2176
0.08 0.06
2100
0.04 0.02
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Fig. 4. FT-IR spectrum (KBr) of the cork-like precipitate spontaneously formed from relatively high concentration polyynes solutions (102 M in heptane) on standing in closed flasks for 24 h or less.
The new synthetic approach proposed here, allows to prepare polyynes at much higher concentrations than before (see Table 1). These solutions are unstable and during 1 day storage start to give a light brown precipitate on the walls and at the bottom of the flask. Heymann in his quantitative study on the thermal stability of the polyynes [20] has established that the polyynes solutions become unstable when their concentrations will be >103 M. Our results confirm Heymanns observation. The light brown precipitate can be separated quite easily from the mother solution simply by decantation. Its colour and even its consistency resembles that of cork, especially if produced from the most concentrated solutions. It is natural at this point to recall a material described in the earlier literature [33] called ‘‘cuprene’’ and derived for instance by the polycondensation of acetylene over warm copper surfaces in presence of air. Also that material was described as similar to cork in terms of consistency and colour. The FT-IR spectrum of the precipitate obtained in the present study is shown in Fig. 4. Particularly remarkable are the residual acetylenic bands at 3294 and 2180 and 2100 cm1 strongly suggesting that it is a three-dimensional polymer derived from the crosslinking reaction of the polyynes species. Furthermore, bands of oxygen groups can be observed at 1773, 1714 (carbonyl groups) and 1122 cm1 (C–O bending) but it is unclear at present whether they were formed during the crosslinking reaction or subsequently upon exposure to air during isolation and handling. Another remarkable feature of the ‘‘concentrated’’ polyynes solutions is the fact that they separate the brown precipitate until a much lower polyynes concen-
tration is reached. Then the residual polyynes in solution are stable. It appears that above a certain concentration edge a self-polymerization reaction occurs and below that threshold the polyynes are unable to react with each other at least at room temperature.
4. Conclusions The hydrolysis of calcium carbide in an ammonium chloride solution of copper ions followed by treatment with HCl ensures a very simple and effective method to produce polyynes in solution, provided that an adequate hydrocarbon solvent like heptane is added to dissolve the polyynes. The new method is safe, simple and economic and offers improvements over the submerged electric carbon arc. The advantages of the new method can be enumerated as follows: no electric equipment is needed; the polyynes are formed without soot and PAHs secondary products produced in the electric arc synthesis; the polyynes solutions can be obtained easily at relatively high concentration, of the order of 102 M, while the polyynes concentration is 3–4 orders of magnitude lower with the electric carbon arc, unless special techniques are adopted [13]. The mechanism of polyynes formation has been clarified. It involves the in situ oxidative coupling reaction of the dicopper acetylide under the action of Cu2+ ions or other oxidizing agents. The resulting polyynes acetylides are hydrolyzed with HCl which can be scavenged with an appropriate solvent.
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The relatively concentrated polyynes solution prepared with the new route has permitted the exploration of new chemical aspects of the polyynes behaviour. The polyynes solutions at concentration >103 M are unstable and separate a light brown polymer which resembles cork and which is similar to ‘‘cuprene’’. The FT-IR spectrum of this material confirms that the polymer is a polyaddition product of the polyynes, although there is evidence for oxidation. Perhaps, the polyaddition product is not stable in air or the polyaddition reaction is initiated by oxygen. Copper polyynides have been isolated in copious quantity by precipitation of polyynes from their solutions by addition of Cu+ ions; the polyynides thermal behaviour has been explored and the FT-IR spectrum is reported. Polyynes are altered by prolonged contact with an acidic aqueous solution containing copper ions.
References [1] Eastmond R, Johnson TR, Walton DRM. Silylation as a protective method for terminal alkynes in oxidative couplings. A general synthesis of the parent polyynes H(C„C)nH (n = 4– 10, 12). Tetrahedron 1972;28:4601–16. [2] Eisler S, Slepkov AD, Elliott E, Luu T, McDonald R, Hegmann FA, et al. Polyynes as a model for carbyne: synthesis, physical properties and nonlinear optical response. J Am Chem Soc 2005;127:2666–76. [3] Diederich F, Stang PJ, Tykwinski RR. Acetylene chemistry. Chemistry, biology and materials science. Weinheim: WileyVCH; 2004. [4] Szafert S, Gladysz JA. Carbon in one dimension: structural analysis of higher conjugated polyynes. Chem Rev 2003;103: 4175–205. [5] Gibtner Th, Hampel F, Gisselbrecht JP, Hirsch A. End-cap stabilized oligoynes: model compounds for the linear sp carbon allotrope carbyne. Chem Eur J 2002;8:408–31. [6] Hlavaty J, Kavan L, Kubista J. Carbonaceous material from endcapped carbines. Carbon 2002;40:345–9. [7] Siemsen P, Livingston RC, Diederich F. Acetylenic coupling: a powerful tool in molecular construction. Angew Chem Int Ed 2000;39:2632–57. [8] Gro¨sser T, Hirsch A. Dicyanopolyynes: formation of new rodshaped molecules in a carbon plasma. Angew Chem Int Ed Engl 1993;32:1340–2. [9] Schermann G, Gro¨sser T, Hampel H, Hirsch A. Dicyanopolyynes: a homologous series of end-capped linear sp carbon. Chem Eur J 1997;3:1105–15. [10] Cataldo F. Polyynes: a new class of carbon allotropes. About the formation of dicyanopolyynes from an electric arc between graphite electrodes in liquid nitrogen. Polyhedron 2004;23: 1889–96. [11] Tsuji M, Tsuji T, Kuboyama S, Yoon SH, Korai Y, Tsujimoto T, et al. Formation of hydrogen-capped polyynes by laser ablation of graphite particles suspended in solution. Chem Phys Lett 2002;355:101–8.
[12] Tsuji M, Kuboyama S, Matsuzaki T, Tsuji T. Formation of hydrogen-capped polyynes by laser ablation of C60 particles suspended in solution. Carbon 2003;41:2141–8. [13] Cataldo F, editor. Polyynes: synthesis, properties and applications. Boca-Raton/New York: CRC Press/M. Dekker; 2005. [14] Cataldo F. Simple generation and detection of polyynes in an arc discharge between graphite electrodes in various solvents. Carbon 2003;41:2671–2. [15] Cataldo F. Synthesis of polyynes in a submerged electric arc in organic solvents. Carbon 2004;42:129–42. [16] Cataldo F. Submerged electric arc between graphite electrodes: a one-pot tool for the synthesis of long chain polyynes in solution. Tetrahedron Lett 2004;45:141–4. [17] Cataldo F. Polyynes production in a solvent-submerged electric arc between graphite electrodes. Part 1: synthesis and spectroscopy. Fullerenes Nanotubes Carbon Nanostruct 2004;12:603–17. [18] Cataldo F. Polyynes production in a solvent-submerged electric arc between graphite electrodes. Part 2: analysis by liquid chromatography. Fullerenes Nanotubes Carbon Nanostruct 2004;12:619–31. [19] Cataldo F. Polyynes production in a solvent-submerged electric arc between graphite electrodes. Part 3: chemical reactivity and stability toward air, ozone and light. Fullerenes Nanotubes Carbon Nanostruct 2004;12:633–46. [20] Heymann D. Thermolysis of the polyyne C8H2 in hexane and methanol: experiments and theoretical study. Carbon, in press, doi:10.1016/j.carbon.2005.03.038. [21] Cataldo F. Synthesis of polyynes with electric arc part 5: analysis of secondary products. Fullerenes Nanotubes Carbon Nanostruct 2005;13:21–30. [22] Cataldo F. Polyynes and cyanopolyynes synthesis from the submerged electric arc: about the role played by the electrodes and solvents in polyynes formation. Tetrahedron 2005;60: 4265–74. [23] Cataldo F. Synthesis of polyynes with electric arc part 4: hydrogenation to ene-ynes. Fullerenes Nanotubes Carbon Nanostruct 2004;12:765–79. [24] Cataldo F. Cyanopolyynes: carbon chain formation in a carbon arc mimicking the formation of carbon chains in the circumstellar medium. Int J Astrobiol 2004;3:237–46. [25] Cataldo F. Soot and other products formation from the submerged carbon arc in halogenated solvents. Fullerenes Nanotubes Carbon Nanostruct 2005;13(4), in press. [26] Cataldo F. Structural relationships between dicopper diacetylide and dicopper acetylide. Eur J Solid State Inorg Chem 1998;35: 281–91. [27] Cataldo F. From dicopper diacetylide to carbyne. Eur J Solid State Inorg Chem 1998;35:293–304. [28] Cataldo F. From dicopper acatylide to carbyne. Polym Int 1999; 48:15–22. [29] Cataldo F. Spectroscopical characterization of carbonaceous matter prepared through Glaser coupling reaction route. Carbon 1999;37:161–3. [30] Cataldo F, Capitani D. Preparation and characterization of carbonaceous matter rich in diamond-like carbon and carbyne. Mater Chem Phys 1999;59:225–31. [31] Cataldo F. Raman scattering investigation of carbynoid and diamond-like carbon. Fullerene Sci Technol 2001;9:153–60. [32] Mason SF. Molecular electronic absorption spectra. Quarterly Rev Roy Chem Soc 1961;15:287–371. [33] Rutledge TF. Acetylenes and allenes. New York: Reinhold Publishing Corp.; 1969.
A method for synthesizing polyynes in solution Franco Cataldo
*
Soc. Lupi arl, Chemical Research Institute, Via Casilina 1626/A, 00133 Rome, Italy Received 13 March 2005; accepted 23 May 2005 Available online 14 July 2005
Abstract A new simple method for synthesizing polyynes (or oligoynes as called in earlier literature) in solution is reported. The method involves the hydrolysis of calcium carbide in a solution of ammonium chloride and cuprous and cupric chloride followed by acid hydrolysis of the resulting product. The polyynes are released and are scavenged with a hydrocarbon solvent like heptane. The polyynes in solution are produced in relatively high concentration (up to 102 M), without soot and PAHs secondary products and these are clear advantages over the polyynes synthesis with the submerged electric carbon arc technique. The mechanism of polyynes formation is explained. At relatively high concentration the polyynes are unstable in solution and give a brown precipitate resembling cork. It is a product of polyaddition reactions between the polyynes chains. The polyynes are stable in solution at relatively high dilution. In acidic solutions, in the presence of copper ions, the polyynes are altered into other products. The FT-IR spectra and the stability of copper polyynides are presented and discussed. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbyne; Synthesis; Chromatography; Chemical structure
1. Introduction Polyynes can be considered as low molecular weight end-capped oligomeric model compounds for the hypothetical one-dimensional carbon allotrope carbyne. In the early literature very short chain polyynes are referred to as oligoynes; in the present paper we will call them polyynes. A milestone in the synthesis of polyynes is represented by the step synthesis proposed by Walton and co-workers in 1972 [1]. The mentioned synthetic approach allowed an excellent control of the chemical structure and length of the polyyne chains but was relatively complex. Recently, a number of papers appeared in the literature on the synthesis and on the structure
*
Tel./fax: +39 062050800. E-mail address: cdcata@flashnet.it
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.05.024
of end-capped polyynes, cyclic polyynes and ene-ynes, demonstrating that polyynes and related molecules are the object of intensive studies and great interest [2–7]. Cyanopolyynes were easily prepared by arcing graphite electrodes in a cyanogen atmosphere [8,9] or, more simply, by arcing graphite electrodes in liquid nitrogen [10]. Polyynes in hydrocarbon solutions can be prepared by focusing a laser beam into a suspension of graphite particles or even fullerene particles in a solvent [11,12]. The submerged carbon arc technique in hydrocarbons or other solvents was proved to be an extremely simple and versatile technique for the synthesis of polyynes in solution [13–18]. The submerged electric arc synthesis has permitted an easy access to the polyynes solutions and the first chemical properties of these unusual molecules have been explored in terms of photolysis, ozonolysis and oxidation resistance to air [19]. Particularly remarkable in this context is the work of Heymann [20] who has explored
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quantitatively the thermal stability in solution of C8H2 obtained by prolonged submerged carbon arc and purified by preparative HPLC. One of the drawbacks of the submerged electric arc synthesis is the fact that polyynes are obtained at high dilution (105–106 M) and special techniques are needed in order to reach higher concentration [13]. Another drawback of the submerged carbon arc synthesis is the formation of polycyclic aromatic hydrocarbons (PAHs) and soot as secondary products together with polyynes, although the concentration of PAHs remains at least 1–2 orders of magnitude lower than the polyynes concentration [21,22]. Again special techniques are needed to purify polyynes produced from the carbon arc [13], including preparative liquid chromatography [20]. In the present work, we will show that polyynes in solution can be prepared at high concentration, without soot and without PAHs secondary products, starting from acetylene produced in situ from the hydrolysis of calcium carbide and coupled in situ by the presence of suitable copper ions.
2. Experimental 2.1. Materials and equipment Calcium carbide technical grade was from Aldrich. All other solvents and reagents were obtained from Fluka. The electronic absorption spectra were recorded on a Shimadzu UV160A spectrophotometer while the HPLC analysis was conducted on an Agilent Technologies liquid chromatograph model 1100 equipped with a diode-array detector. The column was a Zorbax Elipse XDB C-8. The analysis was conducted under isocratic conditions using a mobile phase composed of CH3CN/ water 80:20 vol/vol. The crude heptane solution of polyynes was injected directly into the column. Under these circumstances the polyynes concentration was too high and required a dilution. Therefore an assay of the polyynes solution recovered was diluted at least 15 times with pure heptane and 25 ll were injected. The diodes of the detector were set at 202, 225, 250, 295 and 350 nm. Polyynes were identified on the basis of their retention times and on the basis of their characteristic electronic absorption spectra. For quantitative analysis the molar extinction coefficients of polyynes known from literature [1] were used. For further details on the separation and identification of polyynes see previous works [13–19,21,22]. FT-IR spectra were obtained on a Nicolet IR300 spectrometer from Thermo-Electron Corp. The spectra were recorded in transmittance with the solid embedded in KBr pellets or as a film on ZnSe crystal in reflectance.
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2.2. Standard procedure for the polyynes synthesis with Cu+/Cu2+ solutions NH4Cl (7.5 g) was dissolved in 200 ml of distilled water in a tick-walled conical flask with glass stopper equipped with a valve. CuCl2 Æ 2H2O (10.3 g) and CuCl (5.2 g) were added to the ammonium chloride solution and stirred to ensure the dissolution (there may remain undissolved matter). Heptane (50 ml) is added to the solution followed by calcium carbide (CaC2, 3.5 g) in small portions. After the addition of CaC2 the flask fitted with the stopper is hand-shaken for a few seconds and then the excess of acetylene is released by opening the valve. The operation is repeated several times until all calcium carbide is consumed. When the first portion of CaC2 is added the mixture turns into a violet colour but on further CaC2 addition it becomes dark-brown. The reaction mixture is again shaken for about 2 min and 35 ml of conc. hydrochloric acid are added to cause the hydrolysis of the acetylides. Shaking is prolonged for 2 min and then the crude mixture is filtered (using a standard paper filter) with the aid of an aspirator. A dark precipitate is collected on the filter paper. The filtrate consists of a green aqueous phase and the heptane layer containing the dissolved polyynes. The heptane solution is recovered with the aid of a separatory funnel. An assay of the polyynes heptane solution (0.1 ml), diluted 25 times with pure heptane displays the following electronic absorption spectrum (in nm) 200 (s), 207 (sh), 216 (ms), 227 (s), 240 (m), 253 (m), 260 (w), 276 (mw), 297 (w). Similar results as just described can be obtained by changing the ratio of Cu+/Cu2+ or even by using only Cu+ or Cu2+ solutions. 2.3. Polyynes synthesis with Cu2+ solution NH4Cl (5.8 g) was dissolved in 150 ml of water and then CuCl2 Æ 2H2O (12.6 g) was added and dissolved. A light blue homogeneous solution was obtained; heptane (50 ml) was added to the solution. Then CaC2 (7.5 g) was added to the solution in small portions as described in Section 2.2. The reaction mixture turns black with abundant dark precipitate. Concentrated hydrochloric acid (35 ml) was then added and the mixture was strongly hand-shaken. Filtration with a aspirator permits to separate the precipitate from the water solution and the hexane solution. The polyynes solution in hexane was analyzed after dilution by electronic spectroscopy, it displayed the same spectrum reported in Section 2.2. 2.4. Polyynes synthesis with Cu+ solution CuCl (7.1 g) was dissolved together with 10.0 g of NH2OH Æ HCl (hydroxylamine hydrochloride) in 80 ml of conc. ammonia and 50 ml of water. Acetylene was
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passed into the solution owing to abundant precipitation of red-brown dicopper acetylide (Cu2C2). The precipitate aqueous suspension was divided into two parts having equal volumes. The first part was treated with 40 ml of heptane and then a 16% aqueous solution of HCl was added gradually. After each addition a sample of hexane solution was checked spectrophotometrically. A total of 75 ml of HCl were used for a complete hydrolysis of the acetylide. The organic layer was recovered simply by decantation from the aqueous phase. Both the spectrophotometric analysis and the HPLC analysis revealed the presence of polyynes in the organic solvent. The other portion of Cu2C2 was treated with 7.3 g of CuCl2 Æ 2H2O, under stirring for 30 min at room temperature. Heptane (40 ml) was added and the hydrolysis was conducted with the addition of 90 ml of 16% aqueous solution of HCl. The hydrolysis products remained trapped in the solvent and consisted of a mixture of polyynes; they were determined both spectrophotometrically and by HPLC. Their concentration was about two orders of magnitude larger than in the previous case. The organic layer was recovered by filtration and separation from the aqueous phase with the aid of a separatory funnel. 2.5. Polyynes addition products upon prolonged contact with Cu+/Cu2+ catalyst A polyynes solution in heptane was prepared following a procedure analogous to that described in Section 2.2. The addition of 10.0 g of CaC2 in small portions to an aqueous solution prepared by dissolving 10.0 g of NH4Cl, 16.0 g of CuCl2 Æ 2H2O and 6.0 g of CuCl in 150 ml of water covered with a layer of 50 ml of heptane. After the mentioned steps the solution was treated with 50 ml of conc. HCl. The reaction mixture was left for 7 days in a closed conical flask. The filtration of the mixture with the aid of an aspirator afforded a clear-brown aqueous solution and a yellow heptane layer other than a black insoluble inorganic precipitate on the paper filter. The organic layer was separated with the aid of a separatory funnel. The HPLC analysis revealed a plethora of new products none of them was recognized because of the lack of standard reference compounds. Polyynes were still present: C8H2 for instance was definitely identified. The most important compounds with their HPLC retention times and spectra are reported (excluded the polyynes): Rt = 1.590 min (k in nm): 209, 228, 248, 254, 260, 268, 278, 283, 300, Rt = 1.617 min (k in nm): 208, 220, 242, 267, 283, 298, Rt = 1.790 min (k in nm): 218, 224, 232, 255, 268, 286, 302,
Rt = 2.176 min (k in nm): 212, 220, 245, 258, 270, 286, 309, Rt = 2.483 min (k in nm): 205, 232, 255, 270, 288, 308, 328, Rt = 2.537 min (k in nm): 200, 228, 240, 255, 270, 288, 305, 326, Rt = 2.603 min (k in nm): 228, 240, 257, 272, 290, 309, Rt = 2.830 min (k in nm): 215, 242, 252, 264, 278, Rt = 3.150 min (k in nm): 220, 255, 278, 296, 309, 332, Rt = 3.416 min (k in nm): 205, 232, 242, 255, 275, 290, 310, Rt = 3.616 min (k in nm): 210, 215, 228, 238, 250, 262, 298, 303, 322, 345, 372, Rt = 3.910 min (k in nm): 209, 260, 275, 300, 320, 342, 363, Rt = 4.230 min (k in nm): 240, 278, 295, 315, 330, 346, 375.
2.6. Polyynes hydrogenation The hydrogenation of the polyynes prepared as described in Section 2.2 was achieved by reacting the heptane solution with Zn dust and 15% hydrochloric acid following the same procedure already described in detail for the polyynes produced with the carbon arc [10,23]. The hydrogenated ene-ynes are no more precipitable as acetylides. In fact an heptane solution of ene-ynes prepared from the acetylides treated with an ammonia/NH2OH solution of CuCl does not yield any precipitate. Therefore, the hydrogenated polyynes appear to be completely free from terminal acetylene groups. 2.7. Polyynes stability in concentrated solutions The concentrated solutions of polyynes obtained according to the procedures described in Sections 2.2– 2.4 on standing, even in closed flasks separate a brown material which can be recovered by decantation of the solution. It is insoluble in all common solvents and its FT-IR spectrum in KBr is as follows (absorption peaks in cm1): 3294 (s), 2956 (sh), 2929 (m), 2859 (mw), 2180 (m) 2100 (mw), 1773 (sh), 1714 (sh), 1617 (s), 1384 (ms), 1213 (s), 1122 (s), 978 (sh), 877 (w), 757 (w), 662 (mw).
3. Results and discussion The polyynes synthesis with the electric arc was proven to be an extremely useful and versatile way to produce polyynes in solution. That synthesis has many implications both in the field of astrochemistry [24] and in the combustion chemistry and soot formation [25]. Furthermore, the importance of polyynes and their potentiality in many different fields of chemistry has been reviewed [13].
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In the present paper, we show the simplest way to produce the polyynes in solution even without the use of the electric carbon arc. The synthetic approach presented here for the polyynes is a development of our earlier observation that dicopper acetylide (Cu2C2) and dicopper diacetylide (Cu2C4) left to age in humid air undergo a slow coupling reaction [26–28]. When the aged copper compounds are hydrolyzed with hydrochloric acid, they release polyynes into an extracting hydrocarbon medium [26–28] and a carbonaceous residue containing carbynoid structures [29–31]. The drawback of the described synthetic approach involved the handling of almost dry and more or less aged dicopper acetylide and dicopper diacetylide both proven to be explosive [26–28]. Instead, the synthetic approach described here is completely safe because the acetylides are formed in situ. When dispersed in aqueous solution the acetylides are completely safe and not explosive. They are oxidized and hydrolyzed always in water solution in a completely safe procedure which is in contrast with our old approach [26–28] which required the isolation of the acetylides. 3.1. Evidence of polyynes formation from oxidized dicopper acetylide When calcium carbide (CaC2) is hydrolyzed in water it releases pure acetylene together with some secondary products like vinylacetylene, divinylacetylene and phosphine. In any case acetylene is largely the main hydrolytic product while other secondary products occur mainly as trace impurities. To verify this, we have hydrolyzed 5 g of CaC2 in 100 ml of 9% aqueous solution of NH4Cl covered with 50 ml layer of heptane. The acetylene released and other impurities caused a saturation of the heptane solvent. The spectrum of an acetylene-saturated solution in heptane is shown in Fig. 1A. It consists of a broad band showing features at 220, 226, 240 and 261 nm. Despite the high concentration of acetylene in the mentioned solvent the relative intensity of the absorption band appears low. This is in line with literature data [32] according to which acetylene displays its most intense electronic transition at very short wavelengths well beyond the range of commercial spectrophotometers. In fact the most intense transition occurs at 152 nm followed by another moderate transition at 182 nm [32]. Only a weak transition is quoted to appear at about 220 nm [32]. Thus, the spectrum of Fig. 1A is showing this transition probably overlapping with the electronic transition of other impurities which may give a detectable contribution to the spectrum of Fig. 1A because of their much higher molar extinction. They are produced only in very small, almost trace quantities. The HPLC analysis of the heptane solution of acetylene shows essentially two components, one with a reten-
Fig. 1. Electronic absorption spectra. (A) Acetylene and other impurities produced by the hydrolysis of CaC2 in water and dissolved in heptane; no polyynes are present as revealed by HPLC analysis. (B1–B3) Polyynes produced by the acid hydrolysis of dicopper acetylide in presence of Cu2+ ions. B1 is the spectrum in heptane at the beginning of the hydrolysis and B3 at the end of the hydrolysis. (C) Ene-ynes derived from the hydrogenation of polyynes with Zn dust and HCl.
tion time Rt = 1.353 min whose spectrum shows a maximum at 220 nm with a shoulder at 225 nm and another one eluting at Rt = 1.673 min having a maximum at 237 nm and a shoulder at 260 nm. The summation of the spectra of these two compounds closely resembles the crude electronic spectrum shown in Fig. 1A. As reported in Section 2.4, when a Cu2C2 dispersion in water is treated with a gradual addition of HCl, it undergoes a partial hydrolysis and the products are transferred into an heptane layer over the aqueous phase. Fig. 1B1 shows the spectrum of the products collected in this early hydrolysis stage. After further HCl addition, the spectrum changes into that shown in Fig. 1B2 and when an excess of HCl has been added to ensure the complete hydrolysis of the acetylide, then the spectrum of Fig. 1B3 is obtained. The sequence of spectra in Fig. 1B1–B3 shows the gradual evolution and accumulation of polyynes into the heptane solution. The spectrum in Fig. 1B3 is fully analogous to the spectra of polyynes mixtures obtained by arcing graphite electrodes into hydrocarbon solvents [13–18]. In particular the band at about 200 nm is due to C6H2, the bands at about 205, 216 and 227 are due to C8H2 and the bands at 240 and 253 are essentially due to C10H2 [17].
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Cu+ solution and the resulting Cu2C2 was subsequently oxidatively coupled by the action of air or Cu2+ ions. The one-step route proposed in Section 2.2 involved instead the direct hydrolysis of CaC2 into a ammonium chloride solution of Cu+/Cu2+ chlorides. The hydrolysis of CaC2 causes the in situ formation of Cu2C2 which it is immediately oxidized by the Cu2+ ions present in solution. Therefore, the subsequent acid hydrolysis inevitably causes the release of the polyynes, the coupling products, into the heptane layer. The procedure is completely safe (no handling of Cu2C2), advantageous because it permits in one shot and in a few minutes to produce polyynes solutions having concentration up to 102 M, and which do not contain any secondary products as it happens when polyynes are produced by arcing. The cleanliness of the resulting solution is shown by the HPLC chromatogram reported in Fig. 2. Only five peaks are present. Among them two peaks having retention times Rt = 1.3 min and Rt = 1.67 min have already been discussed in Section 3.1. Both have been detected in the hydrolysis of CaC2 in the absence of Cu ions. The first peak is due to acetylene while the other peak is another CaC2 hydrolysis product. The polyyne C6H2 has an Rt = 1.5 min, polyyne C8H2 eluates at Rt = 1.9 min and finally C10H2 appears at Rt = 2.7. All three polyynes have been definitely identified on the basis of their retention time and their characteristic electronic absorption spectra. Using the molar extinction coefficients available in Ref. [1], it was possible to calculate the relative concentrations of the polyynes produced by the new chemical route. The data are reported in Table 1 and show that C6H2 was quite always dominant although C8H2 was also abundant. However these results are in contrast with the relative concentration of the polyynes produced with the submerged electric carbon arc or by laser ablation of graphite particles in solution. In fact in these cases the dominant polyyne was always C8H2 and also very long polyyne chains were detected [13]. The chemical route proposed here not only produces more C6H2
The mentioned assignment has been corroborated by the HPLC analysis of the solution. As shown in Section 2.4, the polyynes are formed directly from Cu2C2 after acid hydrolysis. Since no polyynes are formed from the hydrolysis of CaC2, it is demonstrated that the Cu2C2 intermediate plays a role in the coupling reaction. More precisely, in the described circumstances, the coupling reaction occurred because of the oxidation by the atmospheric oxygen since no other oxidants were specifically added before the hydrolysis. However, as described in the second part of Section 2.4, when Cu2C2 is deliberately oxidized with the addition of CuCl2, then a larger amount of polyynes are formed as testified by the fact that the polyynes concentration was 102–103 M about two orders of magnitude larger than the concentration obtained by Cu2C2 hydrolysis oxidized in air. Indeed, oxygen gas bubbling should be another way to increase the polyyne yield. Section 2.5 has shown that a prolonged contact of the polyynes solution with the copper catalyst in presence of aqueous HCl has deleterious effects for the polyynes content. After one week of contact, the HPLC analysis shows the presence of a plethora of different products while the polyynes are still detectable, but are no more the dominant species. Because of the lack of standard reference spectra, it appears impossible to establish with certainty the chemical structure of the resulting mixture of products. What is certain is that the electronic absorption spectra reported in Section 2.5 strongly suggest the presence of ene-yne moieties. 3.2. The simplest procedure to prepare polyynes solution Once having clarified the mechanism of polyynes formation and the necessity to avoid prolonged contact of the polyynes with the copper catalyst, we have developed a one-step synthesis of polyynes without the need to pass through the Cu2C2 isolation (even in water suspension). In fact, in Section 3.1 the polyynes were obtained after two steps: first acetylene was passed into a
mAU 1750 1500 1250 1000 750 500 250 0 0
1
2
3
4
5
6
7
min
Fig. 2. HPLC chromatogram detected at 225 nm by the diode-array detector (see Section 3.2). The main components are C6H2, C8H2 (peak at Rt 1.9 min) and C10H2 (peak at Rt 2.75 min).
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Table 1 Synthesis of polyynes: a comparison with carbon arc synthesis (yield in mol%) Electric arc
Data from HPLC-DAD analysis This work Section 2.2 from CaC2
This work Section 2.2 from CaC2
This work Section 2.2 from CaC2
This work Section 2.3 from CaC2
This work Section 2.4 from CaC2
Cu species Molar ratio
None None
Cu(I)/Cu(II) 1.15:1
Cu(I)/Cu(II) 1:1
Cu(I)/Cu(II) 0.80:1
Cu(II) 1
Cu(I) 1
Polyyne C6H2 C8H2 C10H2 C12H2 C14H2 C16H2
20.3 61.2 14.8 2.9 0.6 0.2
56.8 35.8 6.6 0.7 0.1 0.0
48.7 49.1 2.2 Traces 0.0 0.0
67.4 31.2 1.2 0.2 0.0 0.0
80.9 18.7 0.4 0.0 0.0 0.0
83.3 15.6 1.1 0.0 0.0 0.0
85.5
53.4
97.7
56.1
28.7
Total conc. (mM)
0.01
Note: The concentration of the polyynes was calculated using the molar extinction coefficients taken from Ref. [1].
rather than C8H2 but also the longer chains are present, but only at very low concentration, in trace amount or in undetectable amounts as summarized well in Table 1. From Table 1 emerges also another important result. The distribution of products depends on the type of Cu ions present in solution. If only Cu2+ ions (Section 2.3) or only Cu+ ions are present (Section 2.4), then the coupling reaction does not lead to very long chains and C6H2 is by far the dominant specie. If instead the synthesis is conducted with the simultaneous presence of Cu+ and Cu2+ ions (Section 2.2), then the amount of C8H2 becomes quite high and may equal the amount of C6H2 (Table 1). Of course, many parameters affect the distribution of products, like the molar ratio Cu+/ Cu2+, the reaction time, the hydrolysis conditions and so on. Cu2+ ions do not form acetylides but the in situ formation of acetylene from CaC2 may cause a partial reduction of the Cu2+ ions to Cu+ ions, so that in situ formation of Cu2C2 in the solution can be observed which then is easily oxidized by the excess Cu2+ ions (Section 2.3). The formation of C6H2 in large excess also in this case (Table 1) may be explained by the fact that small acetylide amounts are formed so that there is not too much availability for the coupling reaction. 3.3. Polyynes hydrogenation Polyynes can be easily hydrogenated to ene-ynes by the action of Zn dust and aqueous HCl [13,15,23]. That is also the case for the polyynes produced by the synthesis of this paper. Fig. 1C it reports the electronic absorption spectrum of the crude hydrogenated mixture. The spectrum is the same as that of the product derived from the hydrogenation of the polyynes produced with a submerged electric arc [23]. The HPLC analysis of the hydrogenated mixture reveals three main components. The first one eluting at Rt = 1.66 min and having an
absorption maximum at 220 nm and a shoulder at 260 nm has been assigned to hydrogenated C6 on the basis of its retention time. This compound is followed by a second one having an Rt = 1.94 min whose retention time and spectrum correspond to that of hydrogenated C8 already synthesized [23]. In fact the absorption maxima were found at 210, 248, 258 and 268 nm in agreement with previous results [23]. The third important component was found at Rt = 2.3 min with a spectrum having the following maxima: 190, 225, 265, 277, 290 and 303 nm. In terms of retention time and also in terms of electronic absorption spectrum this component corresponds to hydrogenated C8 (another isomer), as already detected and discussed previously [23]. A minor component was found at Rt = 2.86 min with a spectrum corresponding to that of a hydrogenated C10: 228, 255, 265, 278, 303, 320 and 334 nm, again in agreement with previous results [23]. It is interesting that the hydrogenated ene-ynes are no longer able to form insoluble acetylide salts with Cu+ solutions. In fact if the hydrogenated solution containing ene-ynes derivatives is shaken with the Ilosvays reagent (Cu+ dissolved in ammonia/hydroxylamine aqueous solution) no precipitate can be detected. This experimental fact has the important implication that of all the hydrogenated polyynes, the resulting ene-ynes do not have anymore terminal acetylene groups. 3.4. Dicopper polyynides The polyynes produced either with the submerged carbon arc or with the chemical route described in this paper, react easily with CuCl solutions dissolved in NH3/NH2OH aqueous solution to give copper-coloured precipitates: the copper polyynides (by analogy with copper acetylides). When polyynes are produced through the carbon arc, prolonged arcing is needed in
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order to be able to collect the copper polyynides. This technique has been proposed as an industrial way to purify the polyynes from the PAHs secondary products [13]. When polyynes are produced from CaC2 hydrolysis described here, it is possible to precipitate quantitatively the polyynes as polyynides leaving the heptane solution virtually free from any detectable polyynes. The HPLC analysis confirms the complete absence of polyynes and permits the detection of low levels of unknown impurities or by-products. The copper polyynides are copper-coloured when freshly produced and can be collected by filtration. On standing they become black powders or flakes in an oxidation process which parallels the behaviour of dicopper acetylide and diacetylide [26–28]. They are not as sensitive (explosive) to shocks as the parent acetylide and the diacetylide, even when dry, but when heated they deflagrate. Thus, they remain dangerous materials to handle and utmost care must be exercised when handling these products working always with a few mg at a time. Fig. 3 shows the FT-IR spectrum of freshly prepared polyynides (embedded in KBr). The spectrum displays relatively intense acetylenic band at 2106 cm1 followed by two other bands at lower wavenumbers in absorption regions which suggest the presence of the allenic moieties: 1968 and 1823 cm1. It is possible that the complexation of these molecules with copper ions causes a certain triple bond conjugation and rearrangement along the carbon chain, as already hypothesized in previous works [26,27]. When the copper polyynides are hydrolyzed with HCl to remove the copper ions, the resulting free chains have
the structure of linear polyynes without any allenic or cumulenic moiety. This is based on the HPLC analysis of the resulting polyynes solutions using a diode-array detector. Another feature of the copper polyynides is their sensitivity to ageing. After ageing in air for a day or more, when they are treated with HCl are no more able to release back all the polyynes which have been complexed. Instead they hydrolyze only partially but large fractions of them remains as dark insoluble matter resembling carbonaceous matter, but containing a high level of copper. We have called this matter Cu–carbon [30], and have shown by solid state 13C NMR that it is characterized by a relatively high level of sp-hybridized carbon atoms mixed with sp2 and sp3 and stabilized with complexed copper. 3.5. Stability of polyynes solutions in hydrocarbons The stability of polyynes in diluted solution was a matter of surprise [19,20]. It has been proven that the polyynes remain unchanged for months in dilute solutions kept in closed flasks at room temperature in diffuse daylight or in the dark. The polyynes distribution and their concentration was monitored for months in methanol, ethanol and hexane or heptane. No significant changes were recorded, when the polyynes concentration were in the range of 106–105 M. Instead, cyanopolyynes and dicyanopolyynes are much less stable in solution at room temperature and can survive only 1– 2 days [13]. Polyynes are slowly photolyzed in solution by the UV light [19].
0.15
1378
0.14
609
0.09
1968
0.08 0.07 0.06
1823
2106
0.05 0.04
836
Absorbance
0.10
474
1207
1593
0.11
1461
0.13 0.12
0.03 0.02 0.01 2500
2000
1500
1000
500
Wavenumbers (cm-1)
Fig. 3. FT-IR spectrum (KBr) of dicopper polyynides, the insoluble Cu(I) salts of polyynes.
F. Cataldo / Carbon 43 (2005) 2792–2800
978
1122
0.18
1213
0.20
1384
1679
0.22
1620
1720
0.24
2799
662
0.14 0.12
757
Absorbance
0.16
0.10
2176
0.08 0.06
2100
0.04 0.02
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Fig. 4. FT-IR spectrum (KBr) of the cork-like precipitate spontaneously formed from relatively high concentration polyynes solutions (102 M in heptane) on standing in closed flasks for 24 h or less.
The new synthetic approach proposed here, allows to prepare polyynes at much higher concentrations than before (see Table 1). These solutions are unstable and during 1 day storage start to give a light brown precipitate on the walls and at the bottom of the flask. Heymann in his quantitative study on the thermal stability of the polyynes [20] has established that the polyynes solutions become unstable when their concentrations will be >103 M. Our results confirm Heymanns observation. The light brown precipitate can be separated quite easily from the mother solution simply by decantation. Its colour and even its consistency resembles that of cork, especially if produced from the most concentrated solutions. It is natural at this point to recall a material described in the earlier literature [33] called ‘‘cuprene’’ and derived for instance by the polycondensation of acetylene over warm copper surfaces in presence of air. Also that material was described as similar to cork in terms of consistency and colour. The FT-IR spectrum of the precipitate obtained in the present study is shown in Fig. 4. Particularly remarkable are the residual acetylenic bands at 3294 and 2180 and 2100 cm1 strongly suggesting that it is a three-dimensional polymer derived from the crosslinking reaction of the polyynes species. Furthermore, bands of oxygen groups can be observed at 1773, 1714 (carbonyl groups) and 1122 cm1 (C–O bending) but it is unclear at present whether they were formed during the crosslinking reaction or subsequently upon exposure to air during isolation and handling. Another remarkable feature of the ‘‘concentrated’’ polyynes solutions is the fact that they separate the brown precipitate until a much lower polyynes concen-
tration is reached. Then the residual polyynes in solution are stable. It appears that above a certain concentration edge a self-polymerization reaction occurs and below that threshold the polyynes are unable to react with each other at least at room temperature.
4. Conclusions The hydrolysis of calcium carbide in an ammonium chloride solution of copper ions followed by treatment with HCl ensures a very simple and effective method to produce polyynes in solution, provided that an adequate hydrocarbon solvent like heptane is added to dissolve the polyynes. The new method is safe, simple and economic and offers improvements over the submerged electric carbon arc. The advantages of the new method can be enumerated as follows: no electric equipment is needed; the polyynes are formed without soot and PAHs secondary products produced in the electric arc synthesis; the polyynes solutions can be obtained easily at relatively high concentration, of the order of 102 M, while the polyynes concentration is 3–4 orders of magnitude lower with the electric carbon arc, unless special techniques are adopted [13]. The mechanism of polyynes formation has been clarified. It involves the in situ oxidative coupling reaction of the dicopper acetylide under the action of Cu2+ ions or other oxidizing agents. The resulting polyynes acetylides are hydrolyzed with HCl which can be scavenged with an appropriate solvent.
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The relatively concentrated polyynes solution prepared with the new route has permitted the exploration of new chemical aspects of the polyynes behaviour. The polyynes solutions at concentration >103 M are unstable and separate a light brown polymer which resembles cork and which is similar to ‘‘cuprene’’. The FT-IR spectrum of this material confirms that the polymer is a polyaddition product of the polyynes, although there is evidence for oxidation. Perhaps, the polyaddition product is not stable in air or the polyaddition reaction is initiated by oxygen. Copper polyynides have been isolated in copious quantity by precipitation of polyynes from their solutions by addition of Cu+ ions; the polyynides thermal behaviour has been explored and the FT-IR spectrum is reported. Polyynes are altered by prolonged contact with an acidic aqueous solution containing copper ions.
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