摘要:

Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) Identification of matrix-isolated methoxyl radicals by recording EPR spectra under photolysis Irina V. Malakhova* and Vladimir K. Ermolaev G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation. Fax: +7 3832 34 1878; e-mail root@reverse.nsk.su For the first time gas phase radicals have been observed, using the matrix isolation method, during methanol oxidation on a Pt/SiO2 catalyst.A method of identifying the radicals generated is suggested, which utilises the differing resistance of MeO· and MeO2 · radicals to photolysis using the full light of a high pressure mercury lamp. It is necessary to identify gas phase radicals and to measure their concentration when studying the mechanisms of gas phase reactions. Spectral analysis is widely used for this purpose.A method suggested by Panfilov1 and developed by Nalbandyan et al.2 is based on the isolation of the radicals at 77 K in the reaction product matrix, whose EPR spectra are then recorded. The method has already been widely applied in studies of the mechanisms of the gas phase oxidation of various hydrocarbons.3 Peroxide radical (RO2 · ) can be easily isolated in the product matrix.Its low chemical activity allows RO2 · transportation from the reaction zone to the zone of product isolation without significant concentration loss.4 It is, a priori, assumed that the EPR spectrum allows identification of the isolated radical.However, when the reaction produces several types of radicals with similar EPR spectral parameters, for example, when the spectrum is a superposition of the MeO· and MeO2 · spectra, it is difficult to analyse the matrix spectrum. It is well known that these radicals (main intermediates in CH4 and MeOH oxidation) have different reactivities, and initiate different reaction routes.Thus, to understand the reaction mechanism it is important to estimate the relative radical concentrations. As with radicals HO· and HO2 · the EPR spectra of MeO· and MeO2 · are expected to be similar as regards the axial anisotropy of the g-tensor and large value of Dg = g|| – g^ and gav = 2.014–2.015. Spectral superposition shows the additional broadening of components near g|| and g^, which diminishes the difference between the spectral parameters and thus complicates the estimation of the relative radical concentrations.In the present paper we suggest a new method of identifying the nature and concentrations of matrix isolated radicals (MeO2 · or MeO·). The method is based on the different behaviour of the radicals under photolysis. Earlier, based on the results of refs. 7 and 8, we have shown9 that photoconversion MeO2 · proceeds mainly through reaction (1). The concentration of HCO· is ca. 1/3 of the initial concentration of MeO2 · . The same result was achieved in the photolysis of MeO2 · in an argon matrix.10 The EPR spectrum of HCO· shows an asymmetric doublet gav = 2.001, the splitting value being ca. 130 G. These factors allow ready identification of the HCO· radical, even when spectra from several type of radicals are superimposed.9 We have isolated radicals desorbed from the catalyst surface during methanol oxidation by molecular oxygen (pressure 1.33 Pa, methanol–oxygen ratio 1:1 and reaction temperature 820–920 K) when both MeO2 · and MeO· are formed.The experimental set up is described in detail elsewhere.11 Absolute methanol, twice de-gassed by crystallization and pumping, passes from the evaporator into the reactor through a capillary.Methanol vapour pressure is regulated by a cooling mixture of CaCl2 and ice. Pre-dried oxygen is accurately metered from a vessel. Catalyst SiP(II), which is a mechanical mixture of silica and platinum black (2 wt%), is pressed in tablets under 50 atm and put over the reactor grid as a one-grain layer.The radicals, together with other reaction products, are frozen out in the side arm of a Dewar vessel installed in the EPR resonator and connected to the flow vacuum system. Radical photolysis in the reaction product matrix occurs under the full light of a 500 W high pressure mercury lamp (DRsh-500, Russia). Figure 1 presents the EPR spectra of matrix isolated radicals before (1) and after (2) photolysis.The spectrum shows an asymmetric line typical of paramagnetic species with an axial anisotropy of g-tensor, whose g^ component splits into a quadruplet. Curve 3 in Figure 1 presents a model spectrum simulated using the program of Shubin.12 The model spectral parameters are equal to the experimental ones: g|| = 2.03, g^ = 2.008, A^ = 8.7 G.The individual component width 7 G and A|| = 1.02 G were assumed to be those determined for HO· radicals.13,14 The Me group was assumed to rotate around the C–O bond. Apparently, the model spectrum is in good agreement with the experimentally obtained spectrum after photolysis. Similar spectra were described,15 in which radical isolation was used to study methane oxidation on SiO2 and Al2O3 at 900 K, and in methanol oxidation over a catalyst in the total oxidation of hydrocarbons.9,11 According to refs. 16 and 17 at a low pressure of reagents at 770–870 K molecular oxygen completely dissociates on platinum, and oxygen atoms react with methanol via reaction MeO2 · HCO· + diamagnetic products (1) g|| g^ 5 mT HCO 1 2 3 Figure 1 EPR spectra of matrix isolated radical before (1) and after (2) photolysis; EPR spectrum (3), simulated from the experimental parameters.Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83-128) (2). The maximum reaction rate is attained when the methanol– oxygen ratio in the reaction mixture is 1:1. The reaction route producing methoxyl radicals (2)17 also produces hydroxyl radicals (HO·) in the same amount, which were recorded by laser fluorescence over platinum, the effective activation energy being 20 kcal mol–1.In our experiments we failed to record HO· radicals, which is most likely due to either the rapid recombination of HO· on the walls during transportation to the isolation region or strong adsorption of HO· on the SiO2 surface.The effective activation energy for MeO· formation was determined to be 7 kcal mol–1. Such a difference in the activation energies may be explained if we assume that radical desorption limits radical generation. Taking into account the fact that our catalyst is a mechanical mixture of silica and platinum black, we expect that methoxyl radicals form on platinum, as for HO· radicals, and then spill over to the Pt/SiO2 interface, where desorption is easier with respect to energy.This idea is confirmed indirectly by the effect of catalyst training on the rate of radical generation. Thus, 1 h catalyst treatment by oxygen at Ttr = 940 K and P(O2) = 15 Torr increases the rate of radical generation by 2–2.5 times. Photolysis produces HCO· radical lines in the EPR spectra. Their contribution to the spectrum intensity is 6%, and the relative intensity of isolated radicals decreases by 20% (see Figure 2).Therefore, we believe that ca. 20% of methylperoxide radicals, formed in secondary reaction MeO· + O· ® MeO2 · , convert to HCO· under light exposure, while the photolysis of methoxyl radical does not proceed under our experimental conditions.Therefore, we may employ the high resistance of MeO· radicals to photolysis, using the full light of a high pressure mercury lamp, as a factor which allows us to distinguish methoxyl and methylperoxide radicals during methanol oxidation using EPR spectroscopy and matrix isolation techniques. The present study was supported by the Russian Foundation for Basic Research (grant no. 95-03-08915a). References 1 V. N. Panfilov, Kinet. Katal., 1964, 5, 40 [Kinet. Catal. (Engl. Transl.), 1964, 5, 49]. 2 A. B. Nalbandyan and A. A. Mashtayan, Elementarnye protsessy v medlennykh gazofaznykh reaktsiyakh (Elemental processes in slow gas phase reactions), Acad. Sci. Armen. SSR, Erevan, 1975, p. 259 (in Russian). 3 A. B. Nalbandyan and I. A. Vardanyan, Sovremennoe sostoyanie problemy gazofaznogo okisleniya organicheskikh soedinenii (Modern problems in gas phase oxidation of organic compounds), Acad.Sci. Armen. SSR, Erevan, 1986, p. 227 ( in Russian). 4 E. G. Garibyan, A. A. Muradyan and T. A. Garibyan, Armen. Khim. Zh., 1978, 31, 466 (in Russian). 5 S. L. Boyd, R. J. Boyd and L. R. C. Barclay, J. Am. Chem. Soc., 1990, 112, 5724. 6 K. U.Ingold and J. R. Morton, J. Am. Chem. Soc., 1964, 86, 3400. 7 M. Ya. Melnikov, L. N. Baider and N. V. Fok, Khim. Vys. Energ., 1977, 11, 338 [High Energy Chem. (Engl. Transl.), 1977, 11, 370]. 8 S. N. Pak, V. K. Yermolaev and Z. R. Ismagilov, React. Kinet. Catal. Lett., 1986, 32, 429. 9 V. K. Ermolaev, S. N. Pak, L. G. Krishtopa, Z. R. Ismagilov and K. I. Zamaraev, Khim. Fiz., 1988, 7, 1141 (in Russian). 10 D. Bhattacharya and J. E. Willard, J. Phys. Chem., 1982, 86, 962. 11 Z. R. Ismagilov and S. N. Pak, Catal. Lett., 1992, 15, 353. 12 A. A. Shubin and G. M. Zhidomirov, Zh. Strukt. Khim., 1989, 30 (3), 67 [J. Struct. Chem. (Engl. Transl.), 1989, 30, 414]. 13 K. J. Kayama, J. Chem. Phys., 1963, 39, 1507. 14 H. C. Box and E. E. Budzinski, J. Chem. Phys., 1970, 53, 1059. 15 L. A. Nersesyan, I. A. Vardanyan, E. M. Kegeyan, L. Ya. Margolis and A. B. Nalbandyan, Dokl. Akad. Nauk SSSR, 1975, 220, 605 [Dokl. Chem.(Engl. Transl.), 1975, 220, 100]. 16 B. A. Sexton, R. D. Rendulic and A. E. Hughes, Surf. Sci., 1982, 121, 181. 17 M. P. Zum Mallen and L. D. Schmidt, J. Catal., 1996, 161, 230. MeOH + O· MeO· + HO· (2) 20 15 10 5 0 20 40 60 80 (a) (b) 1015N t/min Figure 2 Matrix isolated radical concentration versus photolysis time: (a) total EPR absorbance, (b) HCO· radical formation. Received: Moscow, 3rd December 1997 Cambridge, 12th January 1998; Com. 7/08973B

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年代:1998

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Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) Lipase-mediated transformation of D-xylose and D-galactose into less common L-aldoses Galina D. Gamalevich,a Boris N. Morozov,b Alexey L. Vlasyukb and Edward P. Serebryakov*a a N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow, Russian Federation. Fax: +7 095 135 5328 b The Higher Chemical College, Russian Academy of Sciences, 125820 Moscow, Russian Federation Xylitol and dulcitol have been converted into L-xylose and L-fucose, respectively, by employing lipase-mediated enantioselective deacylation or acylation of their cyclic acetal derivatives as the key step.As a part of our studies on the enantioselective hydrolysis of various racemic esters catalysed by porcine pancreatic lipase (PPL), the hydrolysis of 1-O-acetyl-2,4:3,5-di-O-methylidene- DL-xylitol was shown to afford 2,4:3,5-di-O-methylidene- L-xylitol of ca. 100% enantiomeric purity.1 In principle, this transformation might be used as a short cut to the ‘unnatural’ L-xylose from its abundant D-counterpart; however, the difficulty of removing the O-methylidene protecting groups was an obstacle to achieving this.On the other hand, PPL-catalysed hydrolysis of the corresponding di-O-benzylidene derivative was practically devoid of enantioselectivity.1 Now we report the preparation of L-xylose (L-1) from xylitol 2 using PPL-catalysed hydrolysis of 1-O-acetyl-2,4:3,5-di-Oethylidene- DL-xylitol (rac-3a) as the key step. Acetate rac-3a was obtained from 2 in two conventional steps via racemic alcohol rac-3 (Scheme 1).In contrast with its di-O-methylidene analogue, at 45% conversion rac-3a gave not levorotatory, but dextrorotatory alcohol D-3 {[a]D 22 +3.6°, (c 1.0, H2O)}.† Using Pfitzner–Moffatt oxidation, the latter was converted into aldehyde D-4 {mp 164 °C (sublimed), [a]D 22 +13.4°, (c 1.0, H2O)} which was deprotected to afford practically enantiopure D-xylose {[a]D 22 +73.9° (15 min) ® +18.8° (2 h), (c 1.0, H2O)}.The enantiomeric purity of intermediate D-3 was confirmed by the 19F and 1H NMR spectra of its (S)-MTPA ester. In order to obtain the required enantiopure alcohol L-3, enzymatic hydrolysis of rac-3a was extended to 55% conversion. Column chromatography on SiO2 or, better, partitioning of the products between the aqueous phase and CHCl3 followed by filtration of the concentrated organic layer through a pad of SiO2 using hexane–AcOEt (2:1, v/v) as the eluent afforded acetate L-3a as a microcrystalline solid {mp 55–56 °C (from petroleum ether, –60 °C), [a]D 22 +7.30°, (c 1.0, CHCl3)}.This was saponified to give enantiopure L-3 {[a]D 22 –3.5°, (c 1.0, H2O)}† whose 1H and 13C NMR spectra contained only signals attributable to the diequatorial diastereoisomer, while its (S)-MTPA ester displayed no signals attributable to the D-counterpart in its 19F and 1H NMR spectra.Oxidation of L-3 resulted in 2,4:3,5-di-O-ethylidene-L-xylose (L-4) with mp 162–164 °C (sublimed) and [a]D 22 –13.4°, (c 1.0, H2O). Lit.,2 mp 152–160 °C, [a]D 20 –13.2° (H2O). Acid-catalysed hydrolysis of diacetal L-4 gave the target L-xylose with mp 145–146 °C (from EtOH) and [a]D 22 –76.3° (15 min) ® –18.4° (2 h) (c 1.0, H2O).Lit.,3 mp 144 °C, [a]D 20 –79.3° ® –18.6° (H2O). Allowing for 55% conversion of rac-3a at the enzymatic kinetic resolution step, the material yield of L-1 from 2 is about 16% over six steps of the synthesis. This is comparable with earlier syntheses of L-1 from other sugars.3,4 Since meso-pentaol 2 is obtained directly from D-1, this work represents a formal synthesis of L-1 from D-1 based on controlled asymmetrization of a meso precursor (‘meso-trick’).An alternative PPL-mediated approach from 2 to L-1, † Although our specimens of D-3 and L-3 had mp 139–140 °C (from AcOEt), whereas earlier2 mp 164–165 °C was reported for L-3, the [a]D values of both specimens almost coincided with those reported in ref. 2, and the NMR spectra of their (S)-MTPA esters were in good agreement with the assigned structures. involving the transformation of 2 into 1-O-acetyl-2,3:4,5-di- O-isopropylidene-DL-xylitol (rac-5a) via the corresponding alcohol (rac-5)5 and enzymatic hydrolysis of rac-5a to 35–45% conversion, proved to be inefficient.In this case the e.e. of the resulting alcohol L-5 {mp 34–35 °C (from hexane, –60 °C), [a]D 22 +3.25° (c 2.0, EtOH)} was only 26–28%. Lit. (for L-5),6 mp 33–35 °C (from hexane, –60 °C), [a]D 18 +12.5° (c 2.0, EtOH). When the unconverted fraction of the acetate (i.e., mainly D-5a) was again hydrolysed in the presence of PPL to 38% conversion, and residual D-5a was saponified, the specimen of alcohol D-5 thus obtained {mp 33–35 °C (from hexane, –60 °C), [a]D 22 –3.6° (c 2.0, EtOH)} had about 29% e.e.Another example of lipase-mediated ‘meso-trick’ strategy in carbohydrate chemistry is represented by the formal synthesis CH2OH OH HO CH2OH i 70±5% O O O O Me CH2OR Me H H O O O O Me ROCH2 Me H H (1:1) rac-3 R = H rac-3a R = Ac ii 90% iii C = 45% C = 55% R S D-3 (2S,4R) yield 86% ca. 100% e.e. L-3a (2R,4S) yield ca. 100% ca. 100% e.e. L-3 (2R,4S) ca. 100% e.e. v 98% iv 70% iv 71% O O O O Me CHO Me H H R O O O O Me CHO Me H H S D-4 ca. 100% e.e. L-4 ca. 100% e.e. vi 80% vi 73% O OH HO D-1 ca. 100% e.e. O HO L-1 ca. 100% e.e. 2 OH OH HO HO OH OH Scheme 1 Reagents and conditions: i, MeCHO (3 equiv.)/conc. HCl, 50 °C, 3 h; ii, Ac2O–DMAP/Py, 20 °C; iii, H2O (pH 7)/PPL, 20 °C, 19–25 h; iv, dicyclohexylcarbodiimide (DCC) (3 equiv.)–H3PO4 (0.5 equiv.)–DMSO, 20±2 °C, 16 h; v, KOH (1 equiv.)/MeOH, 20 °C, 3 h; vi, H2O–Me2CO (1:2, v/v)–H2SO4 (cat.), 60 °C, 7 h.Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83-128) of L-fucose 6 from D-galactose, where dulcitol 7 was used as the starting material.At the beginning [Scheme 2(A)], 7 was converted into a mixture of two isomeric diacetonides, where the major component had a symmetric meso structure 8 and the minor one was racemic (rac-9º9 + ent-9, 1:1). The required meso diol 8 was isolated from its mixture with rac-9 either by fractional crystallisation (cf. ref. 7) or by converting this mixture into the corresponding diacetates 8a and rac-9a, isolating the poorly soluble 8a by recrystallisation, and saponifying it back to 8.Originally, it was planned to asymmetrize 8a by PPL-catalysed hydrolysis. However, even at rather low substrate-to-enzyme ratios (8a:PPL = 1:2, w/w) and long exposures (72–120 h) no conversion of 8a was detected. As an alternative, controlled acylation of diol 8 using vinyl acetate and the lipase from Candida rugosa (= C.cylindracea, CCL, Fluka, 2U mg–1) as the catalyst was undertaken [Scheme 2(B)]. At optimal exposures (19–23 h) the yield of levorotatory monoacetate 10 {mp 71–72 °C (from Et2O–hexane), [a]D 22 –9.02° (c 1.0, CHCl3)} amounted to 40–43%, the recovery of diol 8 and the yield of diacetate 8a being 47% and 8%, respectively. Longer exposures increased the yield of 10 up to 73%, but at the expense of e.e.([a]D 22 –6.4° after 44 h). Mesylation of 10 and subsequent treatment of mesylate 11 with NaI led to the wax-like iodide 12 (mp 45–47 °C) which was cleanly hydrogenolysed (with concomitant deacylation) over skeletal Ni in the presence of K2CO3 in MeOH to give the known8 2,3:4,5-di-O-isopropylidene-L-fucitol 13 with mp 59–59.5 °C (from hexane) and [a]D 22 +11.63° (c 1.0, EtOH).Finally, alcohol 13 was oxidised into the corresponding oxo-diketal 14, and the latter was immediately hydrolysed to give the target sugar 6 {mp 138–140 °C (from EtOH), [a]D 22 –110° (15 min) ® –74.7° (4 h) (c 0.95, H2O)}. Lit.,9 {mp 137–139 °C (from EtOH), [a]D –75°(24 h) (c 0.95, H2O)}. Taking into account the content of 8 in the starting mixture of isomeric diols, the yield of L-fucose from 7 was ca. 6% over nine steps. This is comparable with earlier syntheses of 6 from other sugar derivatives.9,10 Our results confirm the usefulness of lipases in carbohydrate synthesis (cf. reviews 11 and 12). We are indebted to Dr. A. V. Ignatenko and Dr.M. I. Struchkova (N. D. Zelinsky Institute of Organic Chemistry, Moscow) for recording most of the NMR spectra.One of us (E. P. S.) is grateful to Mr. R. C. Cook, Cottenham, Cambs., UK) for a timely and generous gift of xylitol. This work was supported by grant no. 96-03-33396, awarded by the Russian Foundation for Basic Research, and by grant no. 3-110 from the National Research Program ‘Engineering Enzymology’. References 1 G. D. Gamalevich and E.P. Serebryakov, Mendeleev Commun., 1996, 221. 2 R. K. Ness and H. G. Fletcher, J. Am. Chem. Soc., 1952, 74, 5341. 3 L. von Vargha, Ber., 1935, 68, 18. 4 (a) T. A. Reichstein, A. Gruessner and R. Oppenauer, Helv. Chim. Acta, 1933, 16, 1019; (b) J. Honeyman, J. Chem. Soc., 1946, 990; (c) E. Dimant and M. Banay, J. Org. Chem., 1960, 25, 475; (d) D. C. Hudson and H. Weigel, J.Chem. Soc., 1961, 1546; (e) G. Wulff and A. Hansen, Carbohyd. Res., 1987, 164, 123. Two more recent syntheses of L-1, based on consecutive use of aldolase, phosphatase and L-iditol isomerase (C. W. Borisenko, A. Spatelstein and G. M. Whitesides, J. Am. Chem. Soc., 1989, 111, 9275) or on a D-galactose oxidase–catalase tandem (R. L. Root, J. B. Durrwachter and C.-H. Wong, J.Am. Chem. Soc., 1985, 107, 2997) give much better yields, but require serious involvement with enzyme chemistry. 5 R. M. Hann, A. T. Ness and C. S. Hudson, J. Am. Chem. Soc., 1944, 66, 73. 6 (a) E. J. Bourne, G. P. McSweeny and L. F. Wiggins, J. Chem. Soc., 1952, 3113; (b) T. Okuda, S. Saito, K. Watanabe and H. Isono, Carbohydr. Res., 1978, 67, 117. 7 R. M. Hann, W. J. Maclay and C.S. Hudson, J. Am. Chem. Soc., 1939, 61, 2432. 8 A. T. Ness, R. M. Hann and C. S. Hudson, J. Am. Chem. Soc., 1942, 64, 982. 9 M. Dejter-Juszynski and H. M. Flowers, Carbohydr. Res., 1973, 28, 144. 10 (a) S. Akiya and S. Suzuki, Yakugaku Zasshi, 1954, 74, 1296; (b) A. Tanimura, Eisei Shikenjo Hokoku, 1959, 77, 123 (C.A., 1961, 55, 12306g); (c) T. Chiba and S. Tejima, Chem. Pharm.Bull., 1979, 27, 2838; (d) J. Defaye, A. Gadelle and C. C. Wong, Carbohyd. Res., 1981, 94, 131; (e) S. Takahashi and H. Kurihara, J. Chem. Soc., Perkin Trans. 1, 1997, 607. A recent, highly efficient three-step, one-pot synthesis involves expensive substrates and the use of an aldolase, dephosphorilase and fucose isomerase [C.-H. Wong, PCT Int. Appl. WO 97 15683 (C.A., 1997b, 126, 342533)]. 11 F. Theil, Chem. Rev., 1995, 95, 2203. 12 C.-H. Wong and G. M. Whitesides, in Enzymes in Synthetic Organic Chemistry (Tetrahedron Organic Chemistry Series, 12), Elsevier/Redwood Books, Trowbridge (UK), 1995, pp. 70–108. O O Me Me H H O O OR Me Me O O Me Me H H O O OH Me Me S R D-5a R = Ac D-5 R = H L-5 CH2OH OH HO O O Me Me OR O O RO H H Me Me O O Me Me H OR RO H O O Me Me i 82.5% (1:1) 8 R = H 8a R = Ac (9 + ent-9) R = H (9a + ent-9a) R = Ac 8:rac-9 = ca. 5:3 8 ii, 76% iii, 89% 8a A B 8 O O Me Me OAc O O HO H H Me Me iv 43% v 87% O O Me Me OAc O O X H H Me Me 10 11,12 11 X = OMs 12 X = I vi 92.5% C 12 vii 98% O O Me Me R O O Me H H Me Me 48%, from 13 via 14 ix O OH HO HO Me OH 6 13 R = CH2OH 14 R = CHO viii 7 CH2OH OH HO S R Scheme 2 Reagents and conditions: i, Me2CO–conc. H2SO4 (cat.), 20 °C, 1 h; ii, Ac2O/Py; iii, KOH/MeOH, 20 °C, 90 min; iv, H2C=CHOAc (3 equiv.)–CCL (1 equiv., w/w)/Et2O, 22±2 °C; v, MsCl (1.1 equiv.)/Py– CHCl3, 20 °C, 48 h; vi, NaI/Me2CO, 60 °C, 48 h; vii, H2–Ni (cat.)–K2CO3/ MeOH, 20 °C, 1 atm, 3 h; viii, DCC (3 equiv.)–H3PO4 (cat.)–DMSO, 20 °C, 18 h; ix, AcOH–H2O (6:4, v/v), 100 °C, 2 h. Received: Moscow, 14th April 1998 Cambridge, 11th May 1998; Com. 8/03088J

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年代:1998

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Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) trans-1,3-Dihydroxy-1,3-dimethyl-1,3-disilacyclobutane Konstantin A. Lyssenko, Tatyana V. Astapova, Mikhail Yu. Antipin and Natalia N. Makarova* A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 117813 Moscow, Russian Federation. Fax: +7 095 135 5085; e-mail: mishan@xray.ineos.ac.ru Hydrolysis of 1,3-dimethyl-1,3-dichloro-1,3-disilacyclohexane has made it possible to isolate for the first time 1,3-dimethyl- 1,3-dihydroxy-1,3-disilacyclobutane, with subsequent establishment of its crystalline structure by X-ray diffraction. In recent years, various reviews1,2 have summarised the use of organosilanes with different reactive groups in the preparation of monomeric organosilanes and in the possible formation of intra- and intermolecular hydrogen bonds in crystals of monomeric organosilicon compounds: silanols with one to four Si–OH groups, silandiols,3–6 and linear7–11 and cyclic12–14 dihydroxyorganosiloxanes. Dihydroxyorganocyclosiloxanes form cyclolinear12 and lamellar14 associates through intermolecular hydrogen bonding.The stereoisomerism of 2,8-dihydroxydecamethylcyclohexasiloxane determines the formation of either dimeric molecules for its cis-isomer (at the expense of both intra- and intermolecular hydrogen bonds) or lamellar associates for its trans-isomer.14 Crystals of cis-2,6-dihydroxy- 2,6-dimethyl-4,4,8,8-tetraphenyltetrasiloxane are assembled into cyclolinear chains as a result of intermolecular hydrogen bonding.12 Investigation of the properties of cyclolinear polyorganosiloxanes by X-ray diffraction and atomic microscopy techniques has revealed that the molecular structure of polymers with cyclohexasiloxane chains has a layered packing over a broad temperature range, whereas the cyclolinear polymers with cyclotetrasiloxane chains are characterised by a different type of molecular structure in their chains.15 These findings show that the molecular structure of the polymeric chain is apparently capable of inheriting the conformation and molecular structure of the initial dihydroxyorganocyclosiloxanes, assembled with the aid of intermolecular hydrogen bonds into associates of different types.The relevant literature does not contain any information on the crystalline structures of dihydroxydisilacycloalkane associates.In order to see whether the inheritance of the conformation of dihydroxycyclocarbosilanes does take place in the cyclolinear polyorganocarbosiloxanes, we synthesised 1,3-dihydroxy- 1,3-dimethyl-1,3-disilacyclobutane (compound 1) by neutral hydrolysis of 1,3-dichloro-1,3-dimethyl-1,3-disilacyclobutane, prepared as in ref. 16 The structure of compound 1 was confirmed by IR, 1H and 29Si NMR spectroscopy and elemental analysis.† The distinctive features of the crystalline and molecular structures of 1,3-dihydroxy-1,3-dimethyl-1,3-disilacyclobutane were investigated by X-ray diffraction in order to establish the type of hydrogen bonds created in the formation of associates, to clarify the structural peculiarities of the strained disilacycloalkanes and to compare such data with the molecular structure of a cyclolinear carbosiloxane polymer formed from compound 1.The results of X-ray diffraction studies have demonstrated that compound 1 crystallises in the form of two independent molecules in different crystallographic centres (Figure 1). The basic geometric characteristics of the molecules are much alike and have the values expected for this class of compounds.17,18 The molecules of compound 1 include an inversion centre lying in the middle of the long Si–Si axis.The four-membered rings are planar. The silicon atoms are characterised by a slightly distorted tetrahedral coordination with the endocyclic angle decreasing to 91.7(2)°; this magnitude is similar to the angle in 1,3-diphenyl-1,3-dimethyl-1,3-disilacyclobutane (92.9°).Analysis of the crystal structure of compound 1 shows that the two independent molecules perform different functions in the formation of hydrogen-bonded associates. Thus, the hydrogen bonds formed by the H(1) and H(2) atoms of molecule A [O(1)–O(2) 2.787(4) Å, H(1)–O(1) 1.94 Å, O(1)– H(1)–O(2) 166.7°] connect molecules into a chain of zig zag type (~A···B~) nearly parallel to the direction [ 0] in the crystal.At the same time, the hydrogen atoms of molecule B [H(2) and H(2A)] take part in the formation of hydrogenbonded helixes (~B···A'~) [O(2)–O(1') (3/2 – x, –1/2 + y, 3/2 – z) 2.784(4) Å, H(1)–O(1) 1.94 Å, O(1)–H(1)–O(2) 162.5°] which are oriented approximately normally to the above-mentioned chain.This results in the formation of a tube 2.46 Å in diameter. The schemes illustrating the formation of such tubes are presented in Figure 2. † Procedure for preparing compound 1. A solution of 1,3-dimethyl- 1,3-dichloro-1,3-disilacyclobutane (2.1 g, 0.011 mol) in 22 ml of diethyl ether was slowly (over 60 min) added to a mixture of aniline (2.21 g, 0.0238 mol), water (0.41 g, 0.0227 mol) and 50 ml of diethyl ether, maintained at 0–2 °C with continuous stirring.Cooling was then stopped, the reaction mixture was stirred for another hour and the precipitate formed was filtered off. The ether solution was washed three times with water, dried over Na2SO4 and solvent was distilled off in vacuo. After recrystallisation (×3) from hexane–benzene–ether (1:1:2), crystals were isolated in a 18.9% yield.Characteristics of compound 1: [colourless crystals (from hexane)] mp 81–82 °C. 1H NMR (400 MHz, 25 °C, [2H6]acetone + CCl4) d: 0.21, 0.27 (CH3), 0.28, 0.34 (CH2), 4.78, 4.85 (OH); 29Si NMR d: 2.22, 5.04. IR (KBr, n/cm–1, solvent [2H6]acetone + CCl4, C = 0.005mol dm–3) 827, 861 (Si–C), 1047 (SiO), 1252, 1245 (Si–C), 3200–3400 (OH), 3670 (free OH).Crystallographic data for compound 1 at 25 °C: monoclinic crystals (C4H12O4Si2), space group P21/n, a = 9.640(2) Å, b = 7.352(2) Å, c = 11.297(2) Å, b = 99.62(3)°, V = 789.3(3) Å3, Z = 4 (two independent molecules in the crystal symmetry centres), dcalc = 1.248 g cm–3, m(MoKa) = 3.74 cm–1, F(000) = 320, M = 148.32. Intensities of 1761 reflections were measured with a P3/PC Siemens four-circle diffractometer (MoKa-radiation, q/2q-scan, 2qmax £ 52°); of these, 1600 independent reflections were used for computations and refinements. The structure was solved by direct methods and refined using the leastsquares technique in the anisotropic–isotropic (H atoms) approximation on F2 to wR2 = 0.1845 and GOF = 1.033 for all independent reflections [R1 = 0.0690 on F for all 1293 observed independent reflections with I > 2s(I)].All computations were performed with an IBM PC/AT using the program SHELXTL PLUS 5.0. Atomic coordinates, thermal parameters, bond lengths and bond angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). For details see ‘Notice to Authors’, Mendeleev Commun., 1998, Issue 1.Any request to CCDC should include all literature citation and the reference number 1135/25. Figure 1 General view of compound 1 illustrating the formation of ~A···B~ chains. Mean bond lengths (Å) for molecule A in 1: Si(1)–O(1) 1.648(3), Si(1)–C(1) 1.875(4), Si(1)–C(2) 1.860(4); bond angles (°): O(1)–Si(1)–C(2) 106.2(2), C(2)–Si(1)–C(1) 115.4(3), O(1)–Si(1)–C(1) 114.6, C(1)–Si(1)–C(1A) 91.2(2), Si(1)–C(1)–Si(1A) 88.2(2).Si(2A) C(3) H(2) Si(2) O(2) C(4) H(1) O(1) Si(1) C(2) C(1) Si(1A) A B 11Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83-128) This work was supported by the Russian Foundation for Basic Research (grant nos. 97-03-3378 and 96-03-32645). References 1 P. D. Lickiss, Adv. Inorg. Chem., 1995, 42, 147. 2 P. D.Lickiss, Tailor-Made Silicon–Oxygen Compounds from Molecules to Materials, ed. R. Corriu, Vieweg, Wiesbaden, 1996, ch. 4, p. 47. 3 N. H. Buttrus, C. Eaborn, P. B. Hitchcock and P. D. Lickiss, J. Organomet. Chem., 1986, 302, 159. 4 N. H. Buttrus, C. Eaborn, P. B. Hitchcock and A. K. Saxena, J. Organomet. Chem., 1985, 248, 291. 5 L. Parkanyi and G. Bocelli, Cryst. Struct. Commun., 1978, 7, 335. 6 J. D. Bunning, J. E. Lydon, C. Eaborn, P. H. Jackson, J. W. Goodby and G. W. Gray, J. Chem. Soc., Faraday Trans., 1982, 713. 7 A. P. Polishchuk, T. V. Timofeeva, M. Yu. Antipin, N. N. Makarova, N. A. Golovina, Yu. T. Struchkov and O. D. Lavrentovich, Metalloorg. Khim., 1991, 4, 147 [Organomet. Chem. (Engl. Transl.), 1991, 4, 77]. 8 A. P. Polishchuk, M. Yu. Antipin, T. V. Timofeeva, N.N. Makarova, N. A. Golovina and Yu. T. Struchkov, Kristallografiya, 1991, 36, 92 (Crystallography USSR, 1991, 36, 50). 9 A. P. Polishchuk, N. N. Makarova, M. Yu. Antipin, T. V. Timofeeva, M. A. Kravers and Yu. T. Struchkov, Kristallografiya, 1990, 35, 446 (Crystallography USSR, 1990, 35, 258). 10 A. P. Polishchuk, T. V. Timofeeva, N. N. Makarova, M. Yu. Antipin and Yu.T. Struchkov, Liq. Cryst., 1991, 9, 433. 11 W. Glegg, Acta Crystallogr., Sect. C, 1983, 39, 901. 12 V. E. Shklover, I. L. Dubchak, Yu. T. Struchkov, V. P. Mileshkevich, G. A. Nikolaev and Yu. V. Zygankov, Zh. Strukt. Khim., 1981, 22 (2), 99 [J. Struct. Chem. (Engl. Transl.), 1981, 22, 225]. 13 I. L. Dubchak, V. E. Shklover, Yu. T. Struchkov, E. S. Hynku and A. A. Zhdanov, Zh. Strukt.Khim., 1981, 22 (5), 156 [J. Struct. Chem. (Engl. Transl.), 1981, 22, 770]. 14 N. G. Furmanova, V. I. Andrianov and N. N. Makarova, Zh. Strukt. Khim., 1987, 28 (2), 113 [J. Struct. Chem. (Engl. Transl.), 1987, 28, 557]. 15 Yu. K. Godovsky, N. N. Makarova and E. V. Matukhina, Polymer Preprints, 1998, 39, 485. 16 W. A. Krier, J. Am. Chem. Soc., 1964, 86, 1601. 17 F. H. Allen, O. Kennard, D. G. Watso, L. Brammer, A. G. Orpen and R. Taylor, J. Chem. Soc., Perkin Trans. 2, 1987, S1. 18 K. Haykawa, H. Tachikawa, T. Suzuki, N. Choi and M. Murakami, Tetrahedron Lett., 1995, 36, 3181. Si(1) O(1) H(1) O(2) H(2) Si(2) Si(1') O(1') H(1') O(2') H(2') Si(2') 90° Figure 2 A packing scheme illustrating the formation of ~A···B~···~A''···B''~ chains and tubes in the crystal structure of compound 1. The methyl groups are omitted for clarity. Received: Moscow, 19th January 1998 Cambridge, 12th March 1998; Com. 8/00704G

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年代:1998

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Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) The determination of the molecular structure of 2-fluoro-3,5-di-tert-butyl-1,3,2-oxazaphospholene by means of electron diffraction and ab initio calculations Victor A. Naumov,*a Marwan Dakkuori,*b Rida N. Ziatdinovaa and Heinz Oberhammer*c a A. E. Arbuzov Institute of Organic and Physical Chemistry, Russian Academy of Sciences, 420088 Kazan, Russian Federation.E-mail: naumov@glass.ksu.ras.ru b Abteilung für Elektrochemie, University of Ulm, 89069 Ulm, Germany. Fax: +49 731 502 5409; e-mail: marwan.dakkuori@chemie.uni-ulm.de c Institut für Physikalische und Theoretische Chemie, University of Tuebingen, 72076 Tuebingen, Germany. Fax: +49 7071 29 6910; e-mail: heinz.oberhammer@uni-tuebingen.de The molecular structure of gaseous 2-fluoro-3,5-di-tert-butyl-1,3,2-oxazaphospholene has been determined by electron diffraction and ab initio calculations.The structural analysis has shown that the diheterophospholene ring possesses a P-envelope conformation with axial orientation of the P–F bond which is very long, 1.641(11) Å. It is well known1 that the P–Cl bond length depends strongly on the nature of the substituents at phosphorus.In compounds of the type 1–4: the P–Cl bond length changes from 2.04 Å (type 1) to 2.18 Å (type 4). It is interesting to note that P–Cl bonds in compounds of type 4 are considerably longer than the sum of the covalent radii of P and Cl (2.09 Å). One can expect that similar changes in P–F bond lengths occur in fluorine-containing compounds of phosphorus.With this aim we studied the structure of 2-fluoro- 3,5-di-tert-butyl-1,3,2-oxazaphospholene in the gas phase by electron diffraction. Diffraction photographs were recorded at 40 °C on Kazan EMR-100M ED-instruments at three camera distances which cover the s-ranges 3.00 < s < 10.50, 4.00 < s < 15.25 and 11.00 < s < 27.50 Å–1. Refinements of the structure were carried out by a least-squares method based on molecular intensities.The geometric structures were also optimized by ab initio calculations at the HF/6-31G** level.2 Vibrational amplitudes lij and perpendicular amplitudes Kij were derived from the theoretical force field (HF/3-21G*)2 and used in the experimental analysis. For describing the geometry of the five-membered ring, the bond lengths P–O, P–N, C=C and N–C(4), the bond angles O–P–N, C=C–N, and P–N–C(4), and the torsional angle C(4)–N–P–O were chosen as independent parameters (Figure 1).Planarity was assumed for the N–C=C–O moiety. This assumption is justified by the ab initio calculation which predicts this dihedral angle to be 0.4°. The substituents and their orientation were described by the P–F, N–C(7), C(5)–C(8), C–C(Me) and C–H bond lengths, the O–P–F, N–P–F, P–N–C(7), C(4)–N–C(7), C=C–C(8), N–C(7)–C(Me) and C(5)–C(8)–C(Me) bond angles and by the C(9)–C(7)–N–P and C(21)–C–C=C dihedral angles.Because of large correlations between some of the geometric parameters the following assumptions were made during the structure refinement: (1) C3v symmetry was assumed for the methyl and tert-butyl groups and all C–C bond lengths and C(Me)–C–C(Me) bond angles in the two groups were set as equal.These assumptions are justified by the ab initio calculations which predict deviations to be less than 0.007 Å and 0.8°. (2) The differences between bond lengths, d(PX), d(NC), d(CC), and between bond angles �(XNC), �(XPF) were constrained to the ab initio values (see Table 1).The bond length differences were applied to the ra structure. The refined bond lengths were converted to ra distances. PCl3 Cl2PR Cl–P O N O Cl–P Cl–P O N N and 1 2 3 4 aUncertainty values in parentheses are 3s values. bMean values for similar parameters. cd(PX) = r(P–O) – r(P–N). dDependent parameter. ed(NC) = = r[N–C(4)] – r[N–C(7)].fd(CC) = r[C(5)–C(8)] – r[C–C(Me)]. g�(XNC) = �[P–N–C(7)] – �(C–N–C). h�(XPF) = �(O–P–F) – �(N–P–F). Table 1 Geometric parameters for 2-fluoro-3,5-di-tert-butyl-1,3,2-oxazaphospholene as obtained from electron diffraction analysis and ab initio calculations. Bond lengths/Å Bond angles/° Parameters Experimental ab initio Parameters Experimental ab initio ra ra O–P–N 92.7(11) 91.2 P–F 1.641(11)a 1.638 1.610 P–N–C(4) 107.5(12) 108.7 P–O 1.645 (9) 1.644 1.628 C=C–N 112.4(16) 112.9 P–N 1.706 1.705 1.689 C=C–Od 113.0(18) 111.2 d(PX)c –0.061(20) –0.061 P–O–Cd 112.1(20) 112.8 C–Od 1.365(9) 1.362 1.387 P–N–C(7) 125.1 (19) 127.1 C=C 1.344(ass.) 1.342 1.318 C–N–C 119.9 121.9 N–C(4) 1.435 (9) 1.431 1.414 �(XNC)g 5.2 5.2 N–C(7) 1.494 1.490 1.473 O–P–F 102.5 (20) 98.8 d(NC)e –0.059 –0.059 N–P–F 105.1 101.4 C–C(Me) 1.542 (3) 1.535 1.535b �(XPF)h –2.6(11) –2.6 C(5)–C(8) 1.513 1.506 1.506 C=C–C 131.4(42) 132.9 d(CC)f –0.029 –0.029 N(C5)–C–C(Me) 110.6 (11) 109.5b C–H 1.095(4) 1.079 1.085 =C–C–C(Me) 110.6 109.5b C(Me)–C(7,8)–C(Me)d 108.4(11) 109.5b R-factor: 3.44 Torsional angles/° C(4)–N–P–O 13.2(54) 15.5 C(9)–C(7)–N–P 144.7(60) 139.5 C(Me)–C–C=C 126.1(98) 120.0Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83-128) With these assumptions the least-squares refinement resulted in a C=C bond [1.344(29) Å] and a C–O bond [1.366(24) Å] with very large error limits. The C=C bond length correlates strongly with other ring bonds and angles.This value is in good agreement with the experimental C=C bond length in methyl vinyl ether, 1.343(6) Å.3 On the other hand, HF/6-31G** values for C=C double bond lengths are known to be too short.On the basis of the experimental C=C bond length in methyl vinyl ether and HF/6-31G** values for this compound (1.320 Å) and for the phospholene (1.318 Å), an ra value of 1.341 Å is estimated in the oxazaphospholene. Therefore the C=C distance was constrained to 1.344 Å (Table 1).The structure analysis showed that the diheterophospholene ring possesses a P-envelope conformation with axial orientation of the P–F bond. The tert-butyl group bonded to N is pseudoequatorial. The sum of angles at the N atom is 354.2(15)° in good agreement with the ab initio result of 357.7°. As expected from trends in P–Cl bond lengths, the P–F bond in oxazaphospholene [1.641(12) Å] is very long.It is about 0.08 Å longer than that in PF3 [1.565(1) Å].4 Intermediate values have been reported for compounds of the type F2PX, e.g. F2PBut [1.589(4) Å],5 F2PNH2 [1.587(4) Å]6 and F2POCH3 [1.591(10) Å].7 A very long P–F bond has been observed in FPBu2 t 1.619(7) Å.5 The variations in the P–F bond lengths, however, are considerably smaller than those for the P–Cl bond lengths.The C–O and C–N bonds in the ring are shorter than normal single bonds and this indicates conjugation in the O–C=C–N part of the ring. The P–O and P–N bonds in the ring, however, are longer than or equal to those in non-cyclic compounds such as P(OCMe)3 [r(P–O) = 1.620(2) Å],1 F2POMe [r(P–O) = = 1.56(2) Å], P(NMe2)3 [r(P–N) = 1.70(1) Å],1 ClP(NMe2)2 [r(P–N) = 1.730(5) Å]1 and F2PNH2 [r(P–N) = 1.650(4) Å].In cyclic compounds such as 2-chloro-3-methyl-1,3,2-oxazaphospholane or in 1,3,4,2-oxadiazaphospholene P–O and P–N bonds are 1.62–1.63 and 1.70 Å respectively.1 We are grateful for financial supn Foundation for Basic Research (grant no. 96-03-00008G) and to the Deutsche Forschungsgemeinschaft which made this cooperative research possible. We are grateful to Dr.Yu. V. Balitzkii for providing us with the sample of oxazaphospholene. References 1 V. A. Naumov and L. V. Vilkov, Molekulyarnye struktury phosphororganicheskikh soedinenii (Molecular structures of organophosphorus compounds), Nauka, Moscow, 1986, p. 319 (in Russian). 2 (a) M. J. Frisch, G.W. Trucks, H. B. Schlegel, P.M.W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D.J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, and J. A. Pople, Gaussian-94, Revision D.4, Gaussian, Inc., Pittsburgh PA, 1995. (b) SPARTAN version 5.0, Wavefunction, Inc., 18401 Von Karman Avenue, Suite 370, Irvine, CA 92612, USA. 3 Van den Enden and H. J. Geise, J. Mol. Struct., 1983, 97, 139. 4 Y. Kawashima and A. P. Cox, J. Mol. Struct., 1977, 65, 318. 5 H. Oberhammer, R. Schmutzler and O. Stelzer, Inorg. Chem., 1978, 17, 1254. 6 H. Britton, J. E. Smoth, P. L. Lee, K. Cohn and R. H. Schwendeman, J. Am. Chem. Soc., 1971, 93, 6772. 7 E. G. Godding, C. E. Jones and R. H. Schwendeman, Inorg. Chem., 1974, 13, 178. Figure 1 Atomic numbering of 2-fluoro-3,5-di-tert-butyl-1,3,2-oxazaphospholene. C(25) C(8) C(21) C(29) C(5) C(4) O(3) N(2) P(1) F(6) C(7) C(13) C(9) C(17) Received: Cambridge, 27th January 1998 Moscow, 14th April 1998; Com. 8/00718G

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年代:1998

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Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) Cluster models in the quantum-chemical analysis of the coordination of imidazoline nitroxides on the surface of silica gel Nikolai D. Chuvylkin,*a Andrei M. Tokmachev,b Aleksandr V. Fionovb and Elena V. Luninab a N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow, Russian Federation. Fax: +7 095 135 5328 b Department of Chemistry, M.V. Lomonosov Moscow State University, 119899 Moscow, Russian Federation. Fax: +7 095 939 4575 A ‘minimal’ cluster quantum-chemical model of paired acid sites on the surface of SiO2 is presented; the model provides adequate explanation of the radiospectroscopic and thermochemical data available on the two-centre and one-centre adsorption of nitroxide probes on the silica surface.The method of paramagnetic surface complexes (PSC) of probe molecules with active solid state centres has been used successfully to study the structures and properties of oxide surfaces. EPR spectroscopy enables a number of physicochemical characteristics of the PSC formed to be determined reliably, and coordination phenomena which are observed on oxides1–3 to be better understood, based on the data obtained by making use of adequate quantum chemical methods.During the past few years interest in the use of stable imidazoline and imidazolidine nitroxides such as 2,2,4,5,5-pentamethyl- 3-imidazolin-N-oxyl 1 and 2-phenyl-2,4,5,5-tetramethyl- 3-imidazolin-N-oxyl 2 as the paramagnetic probes has increased markedly.This is due to such ‘bifunctional’ radicals possessing two alternate electron-donating centres that usually compete in PSC formation. This is why the structural–chemical information content of EPR spectra is so extensive.4–6 The opportunity for the simultaneous coordination of a ‘bifunctional’ nitroxide radical at both its electron-donating centres allows the relative arrangement of surface acid sites on oxides to be studied at the molecular level.In particular, EPR spectroscopy has been applied to investigations of the coordination of imidazoline and imidazolidine nitroxides on the silica surface with differing degrees of dehydroxylation. In this case the recorded characteristics of the EPR spectra were qualitatively interpreted by making use of an assumption suggesting that the two electron-donating centres in the nitroxide radical were involved in donor–acceptor binding with the paired neighbouring acid sites of silica gel.5,6 It is widely known that silanol groups are typical adsorption centres on the silica surface.Thus, it is quite natural to assume that imidazoline radicals chiefly interact with just such acid sites on SiO2.Under similar conditions the experimentally measured5 rotational mobility of these radicals on silica gel was substantially less than that of the TEMPO radical 3 containing merely a single electron-donating group. Based on these experimental data it was concluded that two-centre adsorption of imidazoline radicals on silica was achieved. This conclusion has been confirmed by the results of studies on the orientation motion of coordinated nitroxides by an electron spin echo method.7 When the temperature rises the rotational mobility of imidazoline radicals increases, and the shapes of the transformed EPR spectra, as well as the correlation times tc, point to the fact that the hydrogen bonds between the oxygen atoms of the radical NO groups and the hydrogen atoms of the silica surface acid sites are broken.This phenomenon is reversible and there is consequently a dynamic equilibrium between fast- and slowrotating adsorption forms of imidazoline radicals. Temperature dependences of the proportions of these adsorption forms allow the thermodynamic characteristics of the process to be established. In particular, the standard enthalpies of the transition (DH0 ~ 7–9 kcal mol–1) are sufficiently close to the heats of formation of the hydrogen bonds between the silica acid sites and the typical donor molecules, while the entropy increments DS0 are ca. 20 cal mol–1 K–1, providing evidence for a pronounced change in the number of degrees of freedom in the course of the coordination transformation.5 In view of the insufficient completeness and certain ambiguity or ‘indirectness’ of the structural–chemical information obtained in the above-mentioned radiospectroscopic investigations, we attempted quite naturally to verify the adequacy of the concepts established in so doing with the help of quantum-chemical calculations using the PSC model.This work is devoted to the quantum-chemical analysis of EPR data on the coordination of imidazoline nitroxides such as 2,2,4,5,5-pentamethyl-3-imidazolin- N-oxyl 1 and 2-phenyl-2,4,5,5-tetramethyl-3-imidazolin-N-oxyl 2 on the surface acid sites of the partially dehydroxilated silica gel.N N O N N O N O 1 2 3 1 2 34 5 1 2 34 5 Si O O O O H H O Si Si O O O H O O H O O Si O Si O Si O O O O O O H O H Si O Si O Si O Si O O O O O O O O O H O H CC I CC II CC III CC IV N N X O Si O O HO OH H H N N X O Si O O HO OH H H 1 X = Me 2 X = Ph Figure 1 Cluster one-centre PSC of imidazoline nitroxides with model acid site Si(OH)4.q g gMendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83-128) As in the preceding work3,8 the calculations of structural, magnetic resonance, electrostatic and energy quantities were performed by a semi-empirical MNDO method and there was demonstrated that this method was capable of representing these properties of the model cluster PSC no worse, and in some respects even better, than widely-distributed ab initio calculation procedures.As was shown,3,8 the best agreement between the calculated and experimental radiospectroscopic and energy quantities can be achieved with a fixed geometry of the model cluster with the structural parameters determined for the oxide by crystallographic methods.Thus in the present work the structurally rigid geometric parameters were assumed to be ‘frozen’ (completely unchanging). The covalent clusters as models for the surface acid sites on SiO2 were constructed from regular truncated tetrahedrons with an intracluster interatomic distance r(Si–O) equal to 1.62 Å.Free valencies on the boundaries of these clusters were saturated with hydrogen atoms. The structural parameters of the coordinated imidazoline nitroxides and OH groups directly in contact with them, as well as the Si–O–Si angles between the ‘coupled’ silicon-oxygen tetrahedrons, were subjected to variations.It means that the SiO4 tetrahedra had fixed geometries but the bond angle between the tetrahedra was altered as the nitroxide radicals were introduced into the system. The Hartree– Fock value of the proportionality coefficient between the spin density rs N of the valence s-atomic orbital and the isotropic hyperfine coupling constant aN iso was used for the nitrogen atom nucleus.The results of the calculations on free radicals 1 and 2 as well as their complexes (Figure 1) with coordination via the O or N(3) atom are summarized in Table 1. It is seen from the analysis of these data that all kinds of coordination features are insensitive to the presence of a large volume substituent at position 2 of the imidazoline ring. Under both methods of coordination linear hydrogen bonds appear, while the radical as a whole is nonlinearly coordinated.The hydrogen bond N···H–O–Si is somewhat longer than the O···H–O–Si bond. Coordination via the O atom inverts (in a way peculiar to free nitroxides) the ratio of spin densities rO:rN(1) of the O and N(1) atoms, and such an inversion has been observed experimentally. In contrast, with adsorption via the N(3) atom all structural and magnetic resonance parameters of the NO group in the PSC formed are practically identical to those peculiar to the free radicals 1 and 2.This is due to the relative insensitivity of the NO group to coordination via the distant N atom. According to the data in Table 1 the positive charge QR acquired by the imidazoline nitroxide is independent of the electron-donating site [O or N(3)] involved in coordination with the model cluster acid site, i.e.Si(OH)4. However, the electric dipole moments |m| of differing PSC are distinguished in these cases by almost 2 D. The calculated complexation energies DEc provide evidence for the considerably greater stability (by ~7 kcal mol–1) of one-centre adsorption forms in which imidazoline radicals are coordinated not via the NO group but via their second electron-donating position.The temperature dependences5 of the shapes of the corresponding EPR spectra with their anisotropic splittings also point to these findings without ambiguity. The values of the complexation energies are indicative of the existence of strong hydrogen bonds. In a quantum-chemical analysis of the experimentally investigated two-centre coordination of radicals 1 and 2 on the surface of partially dehydroxylated silica gel we have used four-model cluster constructions (CC) containing geminal (CC I) and vicinal (CC II) silanol groups as well as those separated by one (CC III) and two (CC IV) silicon–oxygen tetrahedra.Applied to the optimization specified above the start geometry of each PSC studied was found to suggest the presence of two hydrogen bonds N···H–O–Si and N–O···H–O–Si, each with bond lengths of ca. 1.9 Å and 1.8 Å, respectively. As our calculations by the MNDO method demonstrated, such two-centre coordination of imidazoline radicals 1 and 2 to the geminal silanol groups in CC I is energy-disfavoured (there is no minimum on the potential energy surface) and this is accounted for by purely geometric factors. Indeed, the spacing of the O and N(3) atoms in the radicals makes up around 3.5 Å, i.e.exceeds even twice the length of the Si–O bond. Thus, structural reasons for the most energy-favourable linear configurations of two hydrogen bonds are missing for these PSC. The vicinal silanol groups of silica gel (CC II) are separated in space to a sufficient degree, and conclusions about their ability to form a two-centre nitroxyl PSC can be drawn merely on the basis of energetic criteria.Analogous calculations performed by us in this case also rule out the possibility of two-centre coordination of imidazoline radicals, since their assumed two-point adsorption forms convert in the course of full geometry optimization into a single-point one (g = 180°) with bonding via the N(3) atom.Unless the lengths of both hydrogen bonds do not vary in these PSC but instead are taken as equal to the corresponding values in Table 1, such ‘forced’ two-centre coordination of radicals 1 and 2 at the vicinal silanol groups is energy-unfavourable by 10 kcal mol–1 because of artificially induced structural strains arising due to the requirement of partial optimization of radical geometry. Further insertion of one (CC III) or two (CC IV) silicon– oxygen tetrahedra between the vicinal silanol groups moves the surface acid sites apart rather significantly.The structural, magnetic resonance, electrostatic and energy quantities calculated by the MNDO method which characterize the simultaneous coordination bonding of imidazoline radicals 1 and 2 with both acid sites of the model cluster Si3O10H8 (Figure 2) and of its analogue Si4O13H10 are shown in Table 2.Comparing the data Table 1 Structural, magnetic resonance, electrostatic and energy characteristics of free imidazoline radicals and their PSC with the Si(OH)4 model cluster acid sites. Bond type Radical r(H···X)/Å q/° g/° j/° aN iso/G rN(1) rO QR |m|/D DEc / kcal mol–1 — 1 — — — 17 19 0.40 0.59 0.00 2.5 — 2 — — — 18 20 0.41 0.59 0.00 2.4 — H···O 1 1.79 19 180 11 22 0.57 0.44 0.25 5.6 –9.1 2 1.83 22 180 12 22 0.56 0.46 0.25 5.4 –8.9 H···N(3) 1 1.92 — 180 16 19 0.41 0.58 0.25 3.7 –16.1 2 1.95 — 180 16 20 0.41 0.59 0.26 3.7 –15.9 Table 2 Structural, magnetic resonance, electrostatic and energy characteristics of coordination of the imidazoline radicals with two acid sites of the model clusters Si3O10H8 and Si4O13H10.Cluster Radical r(H···O)/Å r[H···N(3)]/Å q/° g/° g'/° j/° aN iso/G rN(1) rO QR |m|/D DEc / kcal mol–1 Si3O10H8 1 1.82 1.93 58 179 176 12 22 0.54 0.46 0.27 6.8 –24.3 2 1.86 1.96 60 178 178 14 22 0.55 0.44 0.28 6.5 –23.7 Si4O13H10 1 1.83 1.93 55 180 177 13 22 0.55 0.45 0.27 7.2 –24.4 2 1.86 1.97 57 179 178 14 22 0.55 0.43 0.27 7.0 –23.9Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) in Tables 1 and 2 it is easy to see that both hydrogen bonds H···O and H···N(3) in the two-centre PSC (Table 2) are somewhat longer than those in the corresponding one-centre PSC (Table 1), and in the case of the Si3O10H8 cluster they assume slightly non-linear configurations (g and g' are not equal to 180°).This finding shows that there are small structural strains in such an adsorption form. At the same time, the degrees of pyramidality (j) of the N(1) atom in the radicals 1 and 2 for single-point and two-point adsorption are very similar. From a comparison of Tables 1 and 2 it is obvious that the isotropic hyperfine coupling constants aN(1) iso and spin densities rN(1) and rO of the N(1) and O atoms in the two-centre PSC (Table 2) are much the same as those in the one-centre PSC with binding through the O atom (Table 1).This is representative of the fact that the coordination of the imidazoline radicals via the N(3) atom yields little information on their structural, radiospectroscopic and electrostatic parameters. It is important to note that even though the ratios of spin densities rN(1) :rO are inverted (Table 2) in comparison with those peculiar to the free radicals 1 and 2 (Table 1) the portions of spin transfer from O to N(1) are, however, smaller than in the case of one-centre coordination via the O atom.This gives evidence for the fact that the hydrogen bonds N–O···H–O–Si in the two-centre PSC are weaker than those in the one-centre PSC.Of particular interest is the fact that in the presence of two hydrogen bonds (Table 2) and in the absence of one of them (Table 1) the radical fragments in the model PSC have very similar positive charges QR. According to the calculated energies of complexation DEc (Tables 1 and 2) the two-point adsorption of imidazoline radicals on the surface acid sites of silica gel must be much more favourable than the single-point one.The differences in the DEc values are an indication of the preference of the radicals 1 and 2 to be simultaneously coordinated via both their electron-donating sites rather than via a single N(3) atom. This difference is ca. 8 kcal mol–1 and is in good agreement with the experimentally established5 enthalpies of transformation of slow-rotating imidazoline adsorption forms into fast-rotating ones. The insensitivity of the calculated radiospectroscopic and energy properties of PSC to the presence of a fairly voluminous phenyl substituent in the imidazoline ring also has experimental confirmation.6 The analysis of the Si4O13H10 cluster structure shows that under two-point adsorption the cluster assumes a curved shape and the distance between the two silanol groups involved in the complexing is much the same as that in the short-cut analogue Si3O10H8.The calculated characteristics of the coordination of these acid sites with radicals 1 and 2 (see Table 2) provide convincing evidence for the resemblance in geometries and electronic structures of paired acid sites in the ‘minimal’ (Si3O10H8) and extended (Si4O13H10) cluster quantum-chemical models.Thus, the results of quantum-chemical analysis performed reveal that imidazoline nitroxides are capable of forming both one-centre and two-centre PSC with the differing acid sites of partially dehydroxylated silica gel.To judge by the calculated energies of complexing DEc, these radicals explicitly prefer (in the case of single-point adsorption) to be coordinated by such acid sites via the electron-donating position N(3) of the five-membered ring rather than via the NO group. One must emphasize that Si3O10H8 and Si4O13H10 clusters allow an adequate quantum-chemical interpretation of the whole range of accumulated radiospectroscopic and thermochemical data on the adsorption of paramagnetic imidazoline derivatives on the surface of silica gel to be proposed.The Si3O10H8 cluster is the smallest to be investigated with these aims in view, but enlargement of this cluster reveals that the insertion of extra silicon–oxygen tetrahedra does not invoke significant changes in the calculated adsorption characteristics.Less complex cluster constructions which contain only vicinal or, more often, geminal silanol groups are unsuitable for such calculations because the two-centre coordination of imidazoline radicals to the groups indicated is energy-inconsistent due to pronounced structural strains. We are very grateful to the Russian Foundation for Basic Research for financial support (grant no. 95-03-08196a). References 1 E. V. Lunina, G. L. Markaryan, O. O. Parenago and A. V. Fionov, Coll. Surf., 1993, 72, 333. 2 E. V. Lunina, in Kataliz (Catalysis), eds. O. A. Petrii and V. V. Lunin, Moscow State University, Moscow, 1987, p. 287 (in Russian). 3 N. D. Chuvylkin, A. M. Tokmachev, A. V. Fionov and E. V. Lunina, Izv. Akad. Nauk, Ser. Khim., 1997, 1743 (Russ. Chem. Bull., 1997, 46, 1649). 4 L. B. Volodarskii, I. A. Grigor’ev, S. A. Dikanov, V. A. Resnikov and G. I. Schukin, Imidazolinovye nitroksil’nye radikaly (Imidazoline nitroxides), Nauka, Sibirskoe otdelenie, Novosibirsk, 1988, p. 216 (in Russian). 5 G. L. Markaryan and E. V. Lunina, Zh. Fiz. Khim., 1992, 66, 2480 (Russ. J. Phys. Chem., 1992, 66, 2316). 6 G. L. Markaryan and E. V. Lunina, Zh. Fiz. Khim., 1996, 70, 1670 (Russ. J. Phys. Chem., 1996, 70, 1553). 7 A. N. Kudryashov, S. A. Dzuba, R. I. Samoilova, G. L. Markaryan, E. V. Lunina and Yu. D. Tsvetkov, J. Magn. Reson., 1993, 105, 204. 8 N. D. Chuvylkin, A. M. Tokmachev, A. V. Fionov and E. V. Lunina, Mendeleev Commun., 1997, 15. N N X O 1 X = Me 2 X = Ph Si O Si O Si O O HO OH H H HO OH OH OH Figure 2 Cluster two-centre PSC of imidazoline nitroxides with model acid site Si3O10H8. g' g Received: Moscow, 18th March 1998 Cambridge, 1st May 1998; Com. 8/02396D

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年代:1998

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Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) Microwave-assisted nitric acid digestion of organic matrices Irina V. Kubrakova,*a Andrei A. Formanovsky,b Tamara F. Kudinovaa and Nicolai M. Kuz’mina a V. I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 117975 Moscow, Russian Federation. Fax: +7 095 938 2054; e-mail: elkor@geokhi.msk.ru b M.M. Shemyakin–Yu. A. Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, 117871 Moscow, Russian Federation. Fax: +7 095 330 5592; e-mail: synorg@ibch.siobc.rus.ru Activation energy values, calculated for microwave-assisted oxidation of organic substances from direct pressure and temperature measurements, have confirmed the identity of the oxidation decomposition mechanisms operating under thermal and microwave heating. The effects taking place when using microwave radiation for the acceleration of chemical and physical processes are so significant that chemists who have applied these microwave systems for the dissolution of samples in the routine analysis of various objects1–4 speak of a ‘microwave revolution’ in sample preparation.5 The theoretical aspects of microwaveassisted decomposition have not been considered since the paper of Kingston and Jassie,6 in which thermodynamic equations were used in the calculation of heating parameters based on direct temperature measurements.The first steps in the detailed investigation of microwaveassisted oxidation of organic matrices with nitric acid were made in several papers,7–11 in which the similarity of the main oxidation products under microwave and thermal heating was demonstrated.The similarity of the oxidation mechanisms can therefore be assumed. However, the oxidation kinetics are quite different, and the oxidation time at the same temperature is much longer for conventional heating than for microwave heating.11 The possibility of comparing the qualitative characteristics of oxidation, e.g.activation energy (Ea), seems to be important for an understanding of microwave effects. An attempt to solve this complicated experimental task has only been made once, by Pratt.12 The work presented here is devoted to the development of a method for the estimation of Ea for the microwaveassisted oxidation of organic substances, directly from actual measurements of temperature and pressure in the system.The rate of the chemical process depends on temperature according to the Arrhenius equation: where k is the rate constant, A is the frequency factor, Ea is the activation energy, R is the gas constant and T is the absolute temperature. For the evaluation of Ea for different processes a method using continuous temperature rise was proposed13,14 (the socalled ‘dynamic method’).The method is based on the fact that for a first-order reaction in the absence of side processes, the rate constant is proportional to the quantity of substance formed; under these conditions the use of a logarithmic relationship between a parameter which is proportional to the quantity of substance formed and reciprocal temperature is appropriate.Modern, commercially available microwave digestion systems, supplied with a sensor for direct temperature measurements and fitted with a pressure controller, seem to be ideal for carrying out such experiments. These can therefore be used in the investigation of processes for which a pressure change in the system takes place, in part, as a result of the formation of gaseous products.† The basis for the use of a dynamic method in the evaluation of Ea when studying the oxidation of organic substances lies in the pressure change in a closed vessel.In the initial stage of the reaction this change is proportional to the rate of formation of oxidation products (mainly CO2, because NO2 is soluble at high pressures), i.e.to the decomposition rate. Thus, simultaneous measurement of the pressure in a closed vessel and the temperature of the reaction mixture (condensed phase) makes it possible to calculate the activation energy directly from the experimental data.‡ The absence of any effect due to substance quantity, as well as the shape of the curve obtained, confirm the first-order rate constant.It has also been shown that the side effects connected with the partial solubility of gaseous reaction products and with vaporisation of water are negligible. To verify this possibility, we calculated Ea for individual compounds (amino acids, saccharose), chosen according to the composition of the most important natural substances † Equipment. Experiments were carried out using a laboratory microwave digestion system MDS-2000 (2450 MHz, 600 W) (CEM Corp., USA) in closed vessels LDV (pressure control up to 15 atm) and HDV (40 atm).The temperature of the reaction mixture was measured using a fibre optic probe in the range 20–200 °C. Investigation technique. For oxidation, 0.1–0.4 g of sample was placed in a vessel of volume 100 ml, and 3–4 ml of concentrated HNO3 (56%) and 0–1 ml of H2O were added.After connection of the pressure and temperature sensors to the vessel, the parameters were measured every 5–15 s during 10–20 min. To estimate Ea, experimental data on pressures at elevated temperatures in the form of a logarithmic relationship between pressure and reciprocal temperature were used. Ea was calculated from the slope of a straight line and obtained using a least-squares method.‡ This approach can also be more strongly argued. aData obtained by Pratt.12 Table 1 Ea values of organic substances, determined from experiments under microwave heating. Class of organic substances Individual substances Activation energy/ kJ mol–1 Hydrocarbons Saccharose Galactose 73.3 73.5 Amino acids Lysine Phenylalanine Tryptophan 24.0 50.0 (50.0a) 32.0 (29.8a) k = Aexp(–Ea /RT) 1 2 150 100 50 10 5 1 2 3 t/min T/°C P/atm Figure 1 Temperature (1) and pressure (2) in a closed vessel versus time during the microwave-assisted oxidation of tryptophan with nitric acid.Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83-128) (proteins and hydrocarbons). The values obtained lie within a conventional range for Ea, characteristic of such reactions (40–120 kJ mol–1), and these correlate with the strength of the chemical bonds in the compounds. The most important feature is the good coincidence of the Ea values obtained in our work, using the dynamic method for phenylalanine and tryptophan, with the results obtained by Pratt12 using the electrochemical detection of amino acid decomposition products at various temperatures (static method).Figure 1 shows the temperature and pressure changes in the system with time during the oxidation of tryptophan with nitric acid; in Figure 2 an Arrhenius plot is presented. Data calculated for several compounds are given in Table 1. The data obtained allow us to conclude that: i, fast and simple determination of Ea from microwave heating of reaction mixtures directly from current measurements is possible; ii, the Ea values coincide for oxidation under both traditional and microwave heating.According to the Arrhenius equation, the rate constant can increase in two cases: due to decreasing activation energy, i.e. if the mechanism is changed; or due to increasing factor A, which depends on the frequency and efficiency of collisions.The second reason, closely connected with the mechanism of microwave heating, can be assumed to be the major factor causing a significant acceleration of chemical processes in a microwave field. References 1 H. Matusiewicz and R. E. Sturgeon, Progr. Analyt. Spectroscopy, 1989, 21. 2 M. Bettinelli, U. Baroni and N.Pastorelli, Anal. Chim. Acta, 1989, 159. 3 R. Chakraborty, A. K. Das, M. L. Cervera and M. de la Guardia, Fresenius’ J. Anal. Chem., 1996, 99. 4 I. Kubrakova, Spectrochim. Acta, 1997,1469. 5 S. S. Borman, in Introduction to Microwave Sample Preparation, eds. H. M. Kingston and L. B. Jassie, ACS, Washington, 1989, ch. XVII. 6 H. M. Kingston and L. B. Jassie, Anal. Chem., 1986, 2534. 7 K. W. Pratt, H. M. Kingston, W. A. MacCrehan and W. F. Koch, Anal. Chem., 1988, 2024. 8 S. S. Que Hee and J. R. Boyle, Anal. Chem., 1988, 1033. 9 H. Matusiewicz and R. E. Sturgeon, Fresenius’ J. Anal. Chem., 1994, 428. 10 L. B. Jassie, Microwave Dissolution: Development and Application of a New Sample Preparation Technique, The American University, Washington DC, 1989, p. 236. 11 A. Krushevska, R. M. Barnes, C. J. Amarasiriwaradena, H. Foner and L. Martines, J. Anal. At. Spectrom., 1992, 851. 12 K. W. Pratt, 41st Meeting of the International Society of Electrochemistry, J. Heyrovsky Centennial Congress on Polarography, Prague, Czechoslovakia, 1990, Tu-119. 13 M. Bacci, M. Bini, A. Checcucci, A. Ignesti, L. Millanta, N. Rubino and R. Vanni, J. Chem. Soc., Faraday Trans., 1981, 1503. 14 R. E. Sturgeon, C. L. Chakrabarti and C. H. Langford, Anal. Chem., 1976, 1792. 4 2 2.4 2.6 2.8 ln P 1000/T Figure 2 Arrhenius plot for the microwave-assisted oxidation of tryptophan with nitric acid. Received: Moscow, 12th February 1998 Cambridge, 1st May 1998; Com. 8/01636D

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Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) An investigation of potassium -oxobis[pentachlororuthenium(IV)] in hydrochloric acid solution under microwave irradiation Aleksandr A. Nesterov, Aleksandr V. Bashilov,* Valentin K. Runov and Nikolai M. Kuz’min Department of Chemistry, M. V. Lomonosov Moscow State University, 119899 Moscow, Russian Federation. Fax: +7 095 939 4675; e-mail: kuzmin@analyt.chem.msu.ru A study of the ruthenium(IV) and (III) species in HCl solution under microwave irradiation shows a decrease in hydrolysis time, depolymerisation and reduction of ruthenium(IV) to ruthenium(III) by an order of magnitude, as compared with conventional heating of solutions in a boiling water bath.A knowledge of the forms of existence of platinum group metals in solution is fundamental to any elaboration of their photometric, luminescence, chromatographic or electrochemical properties.In this study, the parameter tested depends on the initial form of the metal in solution. The main difficulties in an investigation of the hydrolysis, depolymerisation, redox- and other processes involving the platinum-group metals arise from their low transformation rates, rather than the variety of their forms.We supposed that such processes would be intensified in a microwave field as with the well-known solid matrix decomposition reaction.1 In the present work, ruthenium(IV) and (III) states in HCl solution were studied as a model. A freshly-prepared 8×10–5 M solution of potassium m-oxobis[ pentachlororuthenium(IV)] K4[Ru2OCl10] in 6 M HCl was studied.Irradiation of the solution (V = 10 ml) was carried out in a MLS-1200 MEGA microwave unit from Milestone (Italy) in fluoropolycarbon autoclaves (CHCl = 0.6, 2, 4, 6, 8, 10 M). The process was controlled by UV-VIS molecular absorption spectroscopy. The percentage recovery of the ruthenium complexes was calculated by well-known approaches to the analysis of multicomponent mixtures.2 This is based on a comparison of the absorption characteristics of irradiated solutions and literature data on the absorption spectra of previuosly identified states.The data for ruthenium(IV) and (III) states in HCl solution under microwave irradiation, involving heating of the solutions with a boiling water bath (98 °C) and at room temperature (20 °C), are summarized in Table 1.Only the existence of forms of ruthenium with a percentage > 15% are presented. The areas of predominance of certain forms (� 90%), and therefore methods and CHCl, are marked. It was shown that hydrolysis time, depolymerisation and reduction of ruthenium(IV) decreased to several minutes as compared with hours (heating the reaction mixture with a boiling water bath), or even months and years (20 °C).Moreover, an increase in the power and/or time of microwave irradiation results in an increase in the rates of the processes studied (depolymerisation at a higher degree) and their proceeding to completion. To intensify the processes in a microwave field, an increase in irradiation power with a simultaneous increase in irradiation time seems reasonable. However, in closed working systems, autoclaves are automatically exposed (to provide safe functioning) which results in wastage of part of the sample. We proposed a new microwave irradiation procedure that increases microwave irradiation efficiency in closed systems, which we term ‘multistep irradiation’. This consists of conventional irradiation of a solution at the maximum possible power of the unit and time (i.e., conditions of maximum recovery of the products), at which autoclave exposure does not take place.The autoclaves are then cooled and repeatedly irradiated under extreme conditions. This increases the recovery of the products and decreases the reaction time. The proposed procedure allows an increase in the recovery of [RuCl6]3– (10 M HCl) and the ‘cationic form’ of ruthenium(IV) (0.6 M HCl) from 70% to 95% and from 60% to 98%, respectively, at maximum possible power (1000 W) and duration of experiment (5 min) using a (5+5+5) min mode.A change in microwave irradiation parameters and HCl concentration allowed us to obtain solutions with predominant forms, such as [RuCl6]2– (95%) and [(RuOH)2(OH)2]4+ (98%).This is impossible when heating initial solutions with a boiling water bath. Hydrolysis of [Ru2OCl10]4– (20 °C, 0.6 M HCl) without microwave irradiation and with heat removal with flowing cold water and liquid nitrogen was studied. The composition of solutions after 1 h is: [Ru2OCl10]4– (80%), [Ru2O(H2O)2Cl8]2– (20%); [Ru2O(H2O)2Cl8]2– (80%), [Ru2O2(H2O)2Cl6]2– (20%) and [Ru2O2(H2O)2Cl6]2– (90%), [Ru2O(H2O)2Cl8]2– (10%). Based on the difference in recovery of the most hydrolysed form, [Ru2O2(H2O)2Cl6]2– (i.e., 90% in the case of heat removal with nitrogen; 20% in the case of heat removal with water; 0% without irradiation), we conclude that microwave irradiation directly affects (i.e.via non-thermal effects) the process under investigation.Thus, hydrolysis, depolymerisation and reduction of ruthenium( IV) were accelerated dozens of times under microwave irradiation, as compared with traditional convection heating of m Table 1 Proposed states of ruthenium(IV) and (III) in hydrochloric acid solution with respect to CHCl, power of microwave irradiation, temperature and time. Initial form K4[Ru2OCl10], CRu = 8×10–5 M.Conditions Concentration of HCl/M 0.6 2 4 6 8 10 5 min, 150 W [Ru2O(H2O)2Cl8]2– 80% [Ru2O2(H2O)2Cl6]2– 20% [Ru2OCl10]4– 95% [Ru2OCl10]4– 95% [Ru2OCl10]4– 95% [Ru2OCl10]4– 95% [Ru2OCl10]4– 95% 5 min, 500 W [Ru2O2(H2O)2Cl6]2– 85% [Ru2O(H2O)2Cl8]2– 65% [Ru2O2(H2O)2Cl6]2– 35% [Ru2O(H2O)2Cl8]2– 90%[Ru2OCl10]4– 80% [RuCl6]2– 20% [Ru2OCl10]4– 70% [RuCl6]2– 30% [RuCl6]3– 35% [RuCl6]2– 35% [Ru2OCl10]4– 30% 5 min, 1000 W Cationic form of ruthenium(IV) 60% [Ru2O2(H2O)2Cl6]2– 40% [Ru2O(H2O)2Cl8]2– 40% [Ru2O2(H2O)2Cl6]2– 40% [RuCl6]2– 75% [Ru(H2O)Cl5]2– 25% [RuCl6]2– 90% [RuCl6]2– 45% [RuCl6]3– 35% [Ru2OCl10]4– 20% [RuCl6]3– 70% [RuCl6]2– 20% 1 month, without irradiation, 20 °C [Ru2O2(H2O)2Cl6]2– 100%[Ru2O(H2O)2Cl8]2– 50% [Ru2O2(H2O)2Cl6]2– 50% [Ru2O(H2O)2Cl8]2– 95%[Ru2OCl10]4– 100% [Ru2OCl10]4– 95%[Ru2OCl10]4– 95% 9 h, without irradiation, 98 °C [RuCln(H2O)6-n](3-n)+ (n = 1, 2) 60% Cationic form of ruthenium(IV) 20% [Ru2O(H2O)2Cl8]2– 50% [Ru2O2(H2O)2Cl6]2– 50% [Ru2O(H2O)2Cl8]2– 75% [Ru(H2O)Cl5]2– 25% [RuCl6]3– 35% [RuCl6]2– 25% [Ru2OCl10]4– 25% [RuCl6]3– 65% [RuCl6]2– 18% [Ru2OCl10]4– 17% [RuCl6]3– 90%Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83-128) solutions, though the transformation principles may be different in some cases. It is likely that the role of non-thermal effects is significant. An appropriate selection of the conditions can provide rapid synthesis of the predominant forms. This work was carried out with the financial support of the Russian Foundation for Basic Research (grant nos. 96-03-33555a and 96-15-97306). We would like to thank Academician Yu. A. Zolotov for helpful discussion of this material. References 1 Introduction to Microwave Sample Preparation: Theory and Practice, eds. H. M. Kingston and L. B. Jassie, American Chemical Society, Washington, DC, 1988. 2 I. A. Bershtein and Yu. L. Kaminsky, Spektrofotometricheskii analiz v organicheskoi khimii (Spectrophotometric analysis in organic chemistry), Khimiya, Leningrad, 1986 (in Russian). Received: Moscow, 25th December 1997 Cambridge, 6th April 1998; Com. 8/0017

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年代:1998

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Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) Mild template synthesis of nickel(II) and copper(II) chelates with an (N,N,S,S)-tetradentate ligand in metal hexacyanoferrate(II)-immobilised matrix systems Oleg V. Mikhailov* and Albina I. Khamitova Kazan State Technological University, 420015 Kazan, Russian Federation. Fax: +7 8432 76 5403; e-mail: omikh@cnit.ksu.ras.ru Mild template synthesis of polydentate coordination compounds of nickel(II) and copper(II) with 2,8-dithio-3,7-diaza-5-oxanonane- 1,9-dithioamide in gelatine-immobilised matrices has been carried out for the first time.It is known that the processes of template synthesis, which afford the possibility of constructing coordination compounds of d-elements with polydentate ligands from simpler fragments (so-called ‘ligand synthons’), mostly occur only under rather drastic conditions in solution and in the solid phase.However, it might be believed that specific conditions formed upon complexation in 3d metal-containing gelatine-immobilised matrices (GIM)1–7 would enable at least certain template processes to occur under mild conditions, primarily at room temperature. We report here that we have been able to perform such syntheses involving Ni2[Fe(CN)6]–GIM, Cu2[Fe(CN)6]–GIM and such ligand synthons as dithiooxamide H2N–C(=S)– C(=S)–NH2 and formaldehyde CH2O to give coordination compounds of nickel(II) and copper(II) with a new polydentate ligand, viz., 2,8-dithio-3,7-diaza-5-oxanonane-1,9-dithioamide.† The resulting compounds colour the polymeric bulk of the GIM, which becomes brown (Ni) or greenish-brown (Cu).The UV-VIS spectra of these compounds contain only a shoulder due to the intense charge transfer band, whose maximum is in the UV region. It should be noted specifically that at other concentrations of the reagents or in the absence of formaldehyde, a violet compound (lmax = 580 nm) with spectral characteristics similar to those of the NiL(OH2)2 complex1,3,5,8 is formed in the system in the case of nickel(II). In the case of copper(II), a dark-blue compound with spectral characteristics similar to those of the known [Cu(HL)2]n chelate2,9 (where H2L is dithiooxamide) is formed.It is apparent that both dithiooxamide and formaldehyde participate in the complexation that occurs under these specific conditions. Decomposition of the polymeric binder of GIM by enzymes according to the known procedure5,9 allowed us to isolate dark-brown compounds of molecular formula MC6S4N4OH8 according to elemental analysis [for nickel(II), found (%): Ni, 17.5; C, 21.3; S, 37.5; N, 16.5; O, 4.8; H, 2.4; calc.(%): Ni, 17.32; C, 21.25; S, 37.83; N, 16.52; O, 4.72; H, 2.36; for copper(II): found (%): Cu, 18.5; C, 20.9; S, 37.3; N, 16.1; O, 4.8; H, 2.4; calc.(%): Cu, 18.45; C, 20.96; S, 37.21; N, 16.34; O, 4.69; H, 2.35]. These compounds are almost insoluble in water, ethanol, acetone, chloroform, benzene and tetrachloromethane; and poorly soluble in dimethylformamide, dimethyl sulfoxide and hexamethylenephosphotriamide. The UV-VIS spectra of their solutions in dimethyl sulfoxide are almost identical to those of their source polymeric matrices indicating that the immobilised compound is the same as that isolated from GIM in the case of both Ni and Cu.The DTA data indicate that both compounds are very heat-resistant and do not undergo pyrolysis even at 600 °C. The IR spectra of both compounds have a wide n(NH) band in the 3400–3500 cm–1 region typical of NH groups uncoordinated to a metal ion.Hence, at least a portion of the N † This synthesis occurs on contact of M2[Fe(CN)6]–GIM (M = Ni, Cu) with alkaline solutions (pH > 10) containing dithiooxamide and formaldehyde. The concentration of M2[Fe(CN)6] in the matrix was 0.1 to 0.2 mol dm–3. The concentration of dithiooxamide in the solution was 3.0×10–3–5.0×10–1 mol dm–3, and the molar ratio of dithiooxamide to formaldehyde was 0.5–2.0.The duration of the process was 10–12 min at 18–20 °C. atoms in these compounds are not bound to nickel(II) or copper(II). In addition, the IR spectra of the compounds under study contain n(C=S) [680 cm–1 (Ni), 650 cm–1 (Cu)] (usually recorded at 705–570 cm–1) and n(C=N) bands (1640 cm–1) (usually observed at 1690–1625 cm–1)10,11 indicating the presence of C=S and C=N groups, respectively.Unfortunately, the IR spectra obtained in the region < 1000 cm–1, where n(M–S) and n(M–N) (M = Ni, Cu) frequencies should be observed,10 do not allow us to reliably assign the bands they contain to the stretching vibrations indicated above. It should be especially noted that two medium-intensity peaks at ca. 2940 and 2865 cm–1 (Ni) and at ca. 2910 and 2860 cm–1 (Cu) belonging to n(CH2) (according to the literature data,10,11 these bands lie within the 2945–2915 and 2870–2845 cm–1 ranges, respectively) and a band due to the stretching vibrations of the bridging C–O–C group at 1120–1100 cm–1 (usually observed in the 1200–1100 cm–1 range)10,11 is also present in these spectra.These bands are absent in the IR spectra of dithiooxamide and in any of the coordination compounds of nickel(II) and copper(II) with this ligand known to date.4 Thus, one may conclude that both of the brown compounds isolated contain H2C–O–CH2 structural groups. Since such groups are not present either in dithiooxamide [and in any of its chelates with nickel(II) and copper(II)] or in formaldehyde, it is quite evident that the formation of coordination compounds of nickel(II) and copper(II) with some new ligand, which is ‘assembled’ from dithiooxamide and formaldehyde fragments, occurs during complexation in the MII–dithiooxamide–formaldehyde systems (M = Ni, Cu).It is noteworthy that the UV-VIS absorption spectra of aqueous solutions of dithiooxamide of any concentration in the 400–700 nm region at pH > 10 do not change even on addition of significant amounts of formaldehyde for at least 2 days, and no indication of a chemical reaction between these compounds is observed.Therefore, we have no doubt that the reaction between the reagents mentioned above does not take place at all in the absence of a metal ion. A similar phenomenon is possible only in template synthesis12–14 where dithiooxamide and formaldehyde act as ligand synthons.The analysis of kinetic curves of complexation, D = f(cF, cL, t) [where D is the optical density of the metal–chelate GIM corresponding to the concentration of hexacyanoferrate(II) of metal(II) in the matrix (cF), dithiooxamide in solution (cL) and the duration of the process t], according to the reported procedure,15 provides clear evidence that the addition of exactly two dithiooxamide molecules and two formaldehyde molecules per MII ion takes place in the course of the process, in complete agreement with route (2).On this basis, the template synthesis in the systems under study can be presented by Scheme 1. The compound synthesised by us, (2,8-dithio-3,7-diaza-5-oxanonandithioamido- 1,9)nickel(II), like all known4 complexes of nickel(II) with dithiooxamide, is diamagnetic and does not give an EPR signal.This indicates a coordination number of 4 and a planar coordination of the ligand donor centres to nickel(II), which is also in complete agreement with its structure according to Scheme 1. Such a conclusion agrees completely with its UV-VIS spectrum and its brown colour typical of coplanar nickel(II) chelates.The structurally similar copper(II)Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83-128) compound is paramagnetic (meff = 1.92 mB) and gives an EPR signal with g|| = 2.21 and g^ = 2.05, which is also typical of planar coordination of ligand donor centres to copper(II) and the N2S2 composition of these centres.This agrees with the structure of the copper complex according to Scheme 1. In this connection, it should be specially mentioned that that process occurs only in GIM, and we failed to obtain these coordination compounds in the reaction of Ni[Fe(CN)6] or Cu2[Fe(CN)6] [as nickel(II) or copper(II)] with dithiooxamide and formaldehyde in solution or in the solid phase at room temperature. This fact indicates the specific role of gelatine-immobilised matrix systems in template synthesis via Scheme 1.Unfortunately, we have not yet been able to carry out X-ray diffraction studies of the metal-polydentate compounds synthesised by us, since the method of their isolation from GIM affords extremely small crystals hardly usable for these studies at the contemporary technical level.For this reason, we leave the question of their steric structure open, and additional studies are required to clarify this aspect. This work was financially supported by the Russian Foundation for Basic Research (grant no. 96-03-32112). References 1 O. V. Mikhailov and G. K. Budnikov, Bull. Chem. Soc. Jpn., 1989, 89, 4016. 2 O.V.Mikhailov, Monatsh. Chem., 1990, 121, 601. 3 O.V.Mikhailov, Monatsh. Chem., 1991, 122, 595. 4 O.V.Mikhailov, Usp. Khim., 1995, 64, 704 (Russ. Chem. Rev., 1995, 64, 657). 5 O.V.Mikhailov, Transition Metal Chem., 1996, 21, 363. 6 O.V.Mikhailov, Indian J. Chem., 1991, 30A, 252. 7 O.V.Mikhailov, Zh. Neorg. Khim., 1992, 37, 362 (Russ. J. Inorg. Chem., 1992, 37, 172). 8 N.Nakamoto, Infrakrasnye spektry neorganicheskikh i koordinatsionnykh soedinenii (Infrared Spectra of Inorganic and Coordination Compounds), Mir, Moscow, 1991 (in Russian). 9 L. A. Kazitsyna and N. V. Kupletskaya, Primenenie UF-, IK-, YaMR i mass spektroskopii v organicheskoi khimii (The Employment of UV-, IR-, NMR- and Mass Spectroscopy in the Organic Chemistry), Moscow University Publishers, Moscow, 1979 (in Russian). 10 N. V. Herbeleu and F. K. Zhovmir, Zh. Neorg. Khim., 1982, 27, 547, (J. Inorg. Chem. USSR, 1982, 27, 309). 11 N. V. Herbeleu and V. B. Arion, Templatnyi sintez makrotsiklicheskikh soedinenii (Template Synthesis of Macrocyclic Compounds), Stiinta, Kishinev, 1990 (in Russian). 12 Sh. Mahammad, S. P. Varkey and F. Farha, Polyhedron, 1994, 14, 2319. 13 O. V. Mikhailov, Zh. Koord. Khim., 1992, 18, 1173 (Russ. J. Coord. Chem., 1992, 18, 1008). H2N C S C S NH2 S HN NH S H2C O CH2 NH S NH S M M2 [Fe(CN)6] + 4 + 4CH2O + 4OH– + [Fe(CN)6]4– + 6H2O M = Ni, Cu Scheme 1 2 Received: Moscow, 22nd October 1997 Cambridge, 8th April 1998; Com. 7/07976A

ISSN:0959-9436     

出版商:RSC    

年代:1998

数据来源: RSC

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Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) A new phenomenon involving the formation of liquid mobile metal–carbon particles in the low-temperature catalytic graphitisation of amorphous carbon by metallic Fe, Co and Ni Oleg P. Krivoruchko* and Vladimir I. Zaikovskii G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation.Fax: +7 3832 34 3056; e-mail: opkriv@catalysis.nsk.su The reaction of Fe, Co and Ni with amorphous carbon and its catalytic graphitisation at relatively low temperatures (600–700 °C) in vacuo is accompanied by the formation of liquid mobile metal–carbon (M–C) particles; the mechanism by which these particles are formed and maintained in the liquid state is suggested.The transition of amorphous carbon into graphite occurs at 2500–3500 °C.1,2 Graphitisation accelerates and occurs at substantially lower temperatures in contact with graphitisation catalysts such as Fe, Co and Ni.1–6 Depending on the state of these catalysts, graphitisation mechanisms are classified as solid-phase3–5 and liquid-phase ones (at temperatures above 1500 °C).1,2 In the present work, we discovered and studied for the first time the formation of liquid mobile M–C particles at relatively low temperatures (600–670 °C) in the reaction of Fe, Co and Ni with amorphous carbon† accompanied by catalytic graphitisation of the latter.The experiments were carried out in situ in the column of a JEM-100CX electron microscope equipped with a video system. The residual pressure of the gases was 10–4–10–5 Torr.The chemical and phase transformations of catalyst precursors and the formation of liquid intermediate M–C compounds and graphite is described by Schemes (1)–(3) for the Fe–C, Ni–C and Co–C individual systems: (where d1 and d2 indicate possible changes in the oxide stoichiometry). Figure 1 shows a typical electron microscopic picture of Fe–C particles and graphite tracks formed on movement of liquid particles through the amorphous carbon film.Transition mode of the formation of M–C particles in the liquid state. Heating of metal hydroxides in the chamber of the electron microscope results in the processes shown in Schemes (1) and (2). Reaching the Tcr temperature and reduction of the oxides to the metals [Scheme (3)] are critical for the transition of catalyst particles to the liquid state. The Tcr values are 640, 600 and 670 °C, respectively, for the Fe–C, Ni–C and † Amorphous carbon films were 200–300 Å thick and were obtained by thermal atomisation of spectrally-pure carbon rods in vacuo (10–4 Torr).One-phase crystalline hydroxides, namely, a-FeO(OH), Co(OH)2 and Ni(OH)2, with specific surface areas of 82, 19 and 75 m2 g–1, respectively, were used as catalyst precursors. The a-FeO(OH) sample consisted of needle-shaped crystals with characteristic sizes l×h = 2500×200 Å; Co(OH)2 consisted of thin plane crystals 0.2–1.0 mm wide and 300–500 Å thick, and Ni(OH)2 consisted of hexagonal thin crystals (50–200 Å).The carbon films were coated with the hydroxides from suspensions in ethanol by means of ultrasonic treatment and then dried.The carbon films were attached to molybdenum meshes and placed in the heating block of an electron microscope. Co–C systems. A prerequisite for this transition is contact of the oxide particles with the amorphous carbon film. Simultaneously with reduction of the oxides to metals, absorption of carbon by the metals occurs.At the same time, the particles rapidly (unresolvable on the real time scale) sink into the carbon film bulk. The existence of the particles in the liquid state is confirmed by the continuous change in their shapes, the disappearance of point reflexes on micro-electronograms and the absence of a contrast typical of crystals in the TEM images.For example, Figure 2(a) shows how strongly the contrast of a particle image decreases at the instant of its transition to the liquid state (0–0.15 s). The state of the M–C particles formed upon interaction of metal oxides with amorphous carbon depends on their size, i.e. either totally liquid particles or those containing both crystalline and liquid parts are formed. Sizes 100–500 Å.The formation of the liquid state over the total bulk of Fe–C, Ni–C and Co–C particles is characteristic of these sizes. Liquid M–C particles of these sizes are formed almost simultaneously. Flow of the particles through the carbon film starts [Figures 1 and 2(a)]. Sizes 500–1500 Å. Initially, only the outer parts of the particles in contact with the amorphous carbon pass into the liquid state.The particle cores are immobile and remain in the solid crystalline state. The thin liquid M–C layer that has formed flows away from the particle core. The following possibilities may result: 1) The thin liquid M–C layer is drawn back into the parent particle, leaving an empty space in the film due to the absorption of carbon. Consequently, the absorbed carbon is distributed throughout the particle bulk.As a result of repeated acts of carbon absorption, the concentration of carbon in the particle becomes sufficient for the transition of the whole bulk to the liquid state. Subsequently, liquid M–C particles start moving through the carbon film [Figure 2(b), 0.90–1.05 s]. {(Fe–C)liq, (Co–C)liq, (Ni–C)liq + graphite}/Cam Fe3O4, CoO1 – d1, NiO1 – d2/Cam a-FeO(OH), Co(OH)2, Ni(OH)2/Cam 250–400 °C a-Fe2O3, CoO, NiO/Cam (1) (2) (3) a-Fe2O3, CoO, NiO/Cam 400–550 °C Fe3O4, CoO1 – d1, NiO1 – d2/Cam Fes, Cos, Nis/Cam 600–670 °C Tcr (rapid transition) Figure 1 Electron microscopic picture of Fe–C particles and graphite tracks in amorphous carbon film formed after in situ heating to 640 °C followed by cooling the sample to 20 °C inside the electron microscope. 2000 ÅMendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83-128) 2) The thin liquid layer separates from the parent particle to form a liquid M–C particle 100–500 Å in size, which then moves independently [Figure 2(b), 26.10–53.70 s]. Sizes 1500–5000 Å. Such large particles are most typical of cobalt- and nickel-containing samples. Only the outer parts of the particles, which are in contact with amorphous carbon, pass into the liquid M–C state due to their interaction with carbon.The particle cores remain solid and immobile [Figure 2(c)]. According to micro-diffraction data obtained in situ, the structure of the metal particles corresponds to the metal. Quasi-steady state mode for the formation and movement of liquid M–C particles. As soon as they have formed, liquid Fe–C, Ni–C and Co–C particles start moving in random directions through the amorphous carbon film, transforming the amorphous carbon into graphite tracks.This unusual liquid state is realised and maintained only under dynamic particle movement conditions. M–C particles do not react with the graphite phase. Therefore, when no amorphous carbon remains, the M–C particles stop and dissolved carbon rapidly settles out as graphite layers covering the surface.The movement of individual M–C particles, 100–1000 Å large and not interacting with each other, through the amorphous carbon film is quasiperiodic and is characterised by accelerations and decelerations. This results in the formation of a track in the form of a graphite pipe separated by walls (Figure 3).The transition to the liquid state requires the absorption of such an amount of carbon that its volume is about the same as that of the metal particle. It follows that the limiting concentration of dissolved carbon in liquid M–C particles at 600–700 °C is close to 50 atom%. Hence, the liquid particles observed constitute an anomalously supersaturated solution of carbon in the metal.Liquid M–C particles with initial sizes from 100 to 1000 Å gradually change their sizes to 200–400 Å after a series of fusions and break-ups as they flow through the carbon film [Figure 2(d)], displaying self-organising properties with respect to their morphology and size. The quasi-steady state graphitisation rates of liquid Fe–C, Ni–C and Co–C particles moving in the film at the rates of 10–4, 10–6 and 3×10–6 cm s–1 are 1.6×10–5, 1.6×10–7 and 4.8×10–7 mol cm–2 s–1, respectively.These rates are 2–4 orders of magnitude higher than the rates of steady-state diffusion through solid Ni or Fe.7 Transition mode of formation of solid particles from liquid M–C particles. Decreasing the temperature to 30–40 °C below the critical value results in the transition of liquid M–C particles to the solid state.After short stops (tens of seconds) followed by returning the temperature above the critical value, the solid particles pass into the liquid mobile state once more. After longer stops (1–30 min), metal carbides (FeC, Co3C) and face-centred cubic Ni are formed. It is known that carbides of metals of the iron group are stable up to 400–550 °C.8–10 At higher temperatures, the carbides decompose to give eventually the metal and graphite.The stability of carbides decreases in the order Fe3C > Co3C > Ni3C, which explains our results on the phase composition of solid particles. The key question is the reason for this considerable decrease in the melting points of solid particles (by 500–900 °C) in Figure 2 Fragments of the video record.The arrows indicate the particles and the directions of their movements: (a) formation of a liquid Fe–C particle: 0 s, solid Fe3O4 particle (A); 0.15 s, liquid Fe–C particle immediately after formation; 0.30–1.35 s, flowing of the liquid particle in the carbon film; (b) evolution of a Ni–C particle of ca. 1500 Å size: 0 s, slowly moving Ni–C particle (A) and a graphite track (B) behind it; 0.90–1.05 s, formation of a thin liquid layer (C) and rapid drawing it back into the parent particle; 26.10–53.70 s, formation and separation of a thin liquid layer (D) with formation of a separate liquid Ni–C particle; (c) evolution of a Co–C particle of ca. 5000 Å size: 0 s, immobile Co–C particle containing a crystalline core (A) and liquefied thin layers (B) on the edges; 1.05–14.25 s, drawing fragments of the thin liquid layer into the parent particle; (d) repeated division of a Co–C liquid particle: 0–12.00 s, initial division of a horseshoe-shaped particle; 14.25–16.00 s, subsequent division of the fragments that formed. 300 Å A B C D (a) (b) (c) (d) 1000 Å 1000 Å 1000 Å A 0 s 0 s 0.15 s 0.90 s 0.30 s 0.60 s 1.35 s 1.05 s 26.10 s 52.50 s 53.70 s 0 s 2.40 s 1.05 s 1.35 s 9.90 s 14.25 s 12.00 s 16.00 s 14.25 s 8.40 s 0 s A BMendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) comparison with those of the usual metals or their eutectics with carbon, which remain in the solid state up to 1100 °C.3 At 700 °C, the solubility of carbon in these metals is 0.2–0.4 atom%.2,7 Hence, the formation of unusual liquid M–C particles, anomalously supersaturated with carbon, at relatively low temperatures (600–700 °C) cannot be explained in terms of metal–carbon interaction in equilibrium systems.In addition, this phenomenon cannot be a consequence of the dimensional effect of melting point decrease, which is observed only for metal and M–C particles smaller than 100 Å.Thus, on heating of ‘metal particle–amorphous carbon’ systems in vacuo to 600–700 °C, an efficient interaction mechanism is realised, which involves the activation of the C–C bond, abstraction of carbon atoms and dissolution of atomic carbon in the metal with formation of a liquid M–C state. When a limiting concentration (up to 50 atom%) of dissolved carbon is reached, the structure of solid metal particles becomes unstable.The ‘amorphous carbon–mobile liquid metal–carbon particles– graphite’ systems studied by us are open and substantially nonequilibrium physicochemical systems with self-organisation. The liquid M–C particles are catalytic intermediates of lowtemperature graphitisation of amorphous carbon.This study was financially supported by the Russian Foundation for Basic Research (grant no. 96-03-33890). References 1 A. Oya and H. Marsh, J. Mater. Sci., 1982, 17, 309. 2 V. B. Fedorov, M. Kh. Shorshov and D. K. Khakimova, Uglerod i ego vzaimodeistvie s metallami (Carbon and its Interaction with Metals), Metallurgiya, Moscow, 1978 (in Russian). 3 F. J. Derbyshire, A. E. B. Presland and D. L. Trimm, Carbon, 1975, 13, 111. 4 R. Lamber, W. Jaeger and G. Schulz-Ekloff, Surf. Sci., 1988, 197, 402. 5 R. Anton, O. Reetz and F. Schmidt, J. Catal., 1994, 149, 474. 6 O. P. Krivoruchko, V. I. Zaikovskii and K. I. Zamaraev, Dokl. Akad. Nauk, 1993, 329, 744 (in Russian). 7 J. J. Lander, H. E. Kern and A. L. Beach, J. Appl. Phys., 1952, 23, 1305. 8 H. C. Ecksrom and W. A. Adcock, J. Am. Chem. Soc., 1950, 72, 1042. 9 S. Nagakura and S. Oketani, Trans. Iron Steel Inst. Jpn., 1968, 8, 265. 10 M. P. Manning, J. E. Garmirian and R. C. Reld, Ind. Eng. Chem. Process. Des. Dev., 1982, 21, 404. 500 Å Figure 3 Graphite track formed after flowing a liquid Fe–C particle through an amorphous carbon film. Quasi-periodic graphite walls are observed about every 200–300 Å. Received: Moscow, 3rd March 1998 Cambridge, 16th April 1998; Com. 8/02177E

ISSN:0959-9436     

出版商:RSC    

年代:1998

数据来源: RSC

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Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83–128) Catalytic and structural properties of ultradispersed silver powder, prepared by a wire explosion technique Bair S. Bal’zhinimaev,*a Vladimir I. Zaikovskii,a Larisa G. Pinaeva,a Anatolii V. Romanenkoa and Gennadii V. Ivanovb a G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation.Fax: +7 3832 34 3766; e-mail: iccct@catalysis.nsk.su b Institute of Petroleum Chemistry, Siberian Branch of the Russian Academy of Sciences, 634055 Tomsk, Russian Federation. Fax: +7 3822 25 8457; e-mail: canc@ihn.tomsk.su Ultradispersed silver powder obtained by electric explosion revealed 100% selectivity in the catalytic epoxidation of ethylene with molecular oxygen.Interest in the study of the catalytic properties of energy-saturated ultradispersed metals obtained by electric explosion of wires in gaseous media has increased in recent years.1 They have been found to be more active and selective than ordinary metal powders in several catalytic reactions.2 Unfortunately, the literature does not provide data on the structure of ultradispersed powders (UDP), which would make it possible to explain their unusual catalytic properties.In this work, we have studied the structure of silver UDP exploded in an argon atmosphere, to explain its extremely high selectivity in ethylene epoxidation. For comparison, similar studies were carried out with ordinary catalysts (Ag powder, Ag/C) prepared by equilibrium procedures. The catalytic activity was measured in a flow-circulation set-up described elsewhere.3 In comparison with ordinary Ag powder, the activity of UDP decreases more rapidly with time, whereas the activity of the Ag/C catalyst remains almost unchanged (Figure 1).Under nearly steady state conditions, the activity of the samples follows the order: ordinary powder >> >> Ag/C > UDP.The selectivity changes in the opposite order, i.e. the ultradispersed powder shows the highest selectivity in epoxidation (90%), while the ordinary silver powder has the lowest selectivity (40%). Preliminary treatment of UDP and Ag/C samples with oxygen at enhanced temperatures (230–260 °C) decreases their catalytic activity but sharply increases their selectivity (Figure 1).However, the selectivity of the ordinary powder remains practically unchanged. Almost 100% selectivity with respect to ethylene oxide is achieved in the case of the ultradispersed powder. This is surprising for the gas-phase production of ethylene oxide on solid catalysts, since such high selectivities have previously been observed only in homogeneous epoxidation of olefins by peroxide compounds. According to HREM data (JEM-2010 microscope), a fresh sample of silver UDP consists of metallic globules 50–200 nm in size (mean size 100 nm) linked in chains.Silver particles contain a large number of extended defects of a polysynthetic twin type along the (111) planes of the face-centred silver lattice, which are repeated each 1–10 nm.The selected-area electron diffraction pattern of these particles contains reflexes of twins and elongated reflexes along (111), indicating the presence of both twins and many (over-equilibrium) stacking faults. In fact, a high-resolution picture (Figure 2) clearly displays planes with distance d111 = 2.35 Å, and a modulation frequency of their image intensity up to the lattice spacing.Evidently, the exit of frequently alternating defects on the surface leads to a strong roughening of the regular structure of the faces, e.g. many atomic-size steps form. Treatment of silver UDP in oxidative or reaction mixtures results in the complete annealing of stacking faults, while twin defects are retained. In this respect, silver UDP behaves similarly to Ag/a-Al2O3 catalysts.4 The significant difference between silver UDP and Ag/C catalysts and ordinary silver powder and Ag/a-Al2O3 is that when a silver surface is treated with O2 or the reaction mixture, specific particles of ordered structure up to 10 nm in size appear on the surface (Figure 3).These particles are localised only at sites of exit of extended defects onto the surface (twinning boundaries), whereas they are not detected on regular areas.Direct resolution of the lattice gives interplanar distances of 3.3 and 2.7 Å, which are close to the parameters of the silver oxide (Ag2O) cubic lattice. A three-dimensional silver oxide phase can hardly form from the metal due to the thermodynamics at these O2 pressures and temperatures. Therefore, it is unlikely that these unusual (a) (b) 1.0 0.8 0.6 0.4 0.2 0 2 4 6 100 75 50 25 Time/h Selectivity (%) REtO/molecules m–2 s–1 6 4 2 0 R/10 Figure 1 Time dependences of epoxidation rate (a) and selectivity with respect to ethylene oxide (b) at 230 °C on different samples: , ordinary Ag powder; , Ag UDP; , Ag UDP preoxidized at 260 °C for 2 h; , Ag/ C; , Ag/C preoxidised at 240 °C for 2 h.Feed gas: C2H4 (2%), O2 (7%), He (91%).Time/h 25 Å B A B A A Figure 2 High-resolution electron microscopic image of twins and stacking faults at (111) planes in a silver UDP crystal. Angles and roughnesses on the surface show the exits of extended defects: twinning boundaries (A) and stacking faults (B).Mendeleev Communications Electronic Version, Issue 3, 1998 (pp. 83-128) particles are Ag2O ionic crystals.As follows from ex situ HREM, they disappear on sample storage in air and reappear after treatment of the sample with oxidative or reaction mixtures at T = 230–260 °C. Finally, the XPS study did not reveal photoelectron lines of the surface Ag2O oxide at a depth up to 10 nm on etching with He+ ions. Both in the starting UDP samples and in those treated with oxidative or reaction mixtures, only the lines of metallic Ag and of the so-called ‘covalent’ (electrophilic) oxygen5 were observed.It should be noted that analogous oxide-like structures formed, though in much smaller amounts, on the Ag/C catalyst. However, such structures have not been observed on the ordinary silver powder and on Ag/a-Al2O3. As we found previously, the epoxidation rate is proportional to the concentration of p-complexes of C2H4 with Ag+ cations,6 or, which is the same, to the concentration of chemisorbed (nucleophilic) O2– oxygen ions.Moreover, electrophilic oxygen atoms localised on defect surface areas show epoxidating ability,5–7 rather than nucleophilic oxygen atoms, which are reactive only in deep oxidation reactions. Since chemisorbed oxygen (O2–) is slightly submerged under the surface,7 the chemisorption of O2 molecules increases the micro-strains of the silver lattice due to an increase in the Ag–Ag interatomic distances in the oxide layer.Obviously, these micro-strains will relax much more easily in the case of more defective crystals. Considering that treatment with O2 or the reaction mixture leads to the complete annealing of the over-equilibrium stacking faults, the relaxation process should be accompanied by the transfer of many silver and oxygen atoms.It may be assumed that during rearrangement and rationalisation of the Ag surface layers, oxygen chemisorbed on the defects is accumulated in the vicinity of the more stable extended defects (exits of twin boundaries on the surface) thus eventually producing small oxide-like particles and causing the complete disappearance of the oxide layer.It therefore becomes clear why the formation of oxide-like structures is accompanied by a monotonous decrease in the reaction rate along with an increase in the selectivity towards ethylene oxide (Figure 1). Most probably, epoxidation acts occur in the vicinity of the boundary between the metal and oxide-like particles, where Ag+ cations and electrophilic oxygen atoms are localised. They are necessary for the activation of C2H4 and epoxidation acts, respectively. In our opinion, the absence of substantial coverage by nucleophilic oxygen causes high selectivities.The reaction energy in the case of ordinary Ag powder is clearly insufficient to produce oxide-like structures.Therefore, reaction evidently occurs on randomly appearing centres, whose immediate vicinity contains nucleophilic and electrophilic oxygen atoms. As a consequence, the activity of the ordinary Ag powder is much higher than that of UDP, while, on the contrary, the selectivity is much lower. This study was financially supported by the Russian Foundation for Basic Research (grant no. 97-03-32544a). References 1 O. V. Salova, N. N. Michalenko and V. M. Gryaznov, Zh. Fiz. Khim., 1992, 66, 2062 [Russ. J. Phys. Chem. (Engl. Transl.), 1992, 66, 1097]. 2 G. V. Ivanov, N. A. Yavorovskii, Yu. A. Kotov, V. I. Davydovich and G. A. Mel’nikova, Dokl. Akad. Nauk SSSR, 1984, 275, 873 (in Russian). 3 S. N. Goncharova, A. V. Khasin, S. N. Filimonova and D. A. Bulushev, Kinet. Katal., 1991, 32, 852 [Kinet. Catal. (Engl. Transl.), 1991, 32, 768]. 4 S. V. Tsybulya, G. N. Kryukova, S. N. Goncharova, A. N. Shmakov and B. S. Bal’zhinimaev, J. Catal., 1995, 154, 194. 5 V. I. Bukhtiyarov, A. I. Boronin, I. P. Prosvirin and V. I. Savchenko, J. Catal., 1994, 150, 262. 6 S. N. Goncharova, E. A. Paukshtis and B. S. Bal’zhinimaev, Appl. Catalysis A: General, 1995, 126, 67. 7 V. I. Bukhtiyarov, A. I. Boronin, I. P. Prosvirin and V. I. Savchenko, J. Catal., 1994, 150, 268. 8 D. A. Bulushev and B. S. Bal’zhinimaev, Kinet. Katal., 1996, 37, 149 [Kinet. Catal. (Engl. Transl.), 1996, 37, 140]. 50 Å Figure 3 High-resolution electron microscopic image of an oxide-like particle located on the surface of a silver crystal. Received: Moscow, 3rd March 1998 Cambridge, 25th March 1998; Com. 8/02179A

ISSN:0959-9436     

出版商:RSC    

年代:1998

数据来源: RSC