Molybdenum-catalysed oxidation of cyclohexene with hydrogen peroxide in the presence of alcohols and X-ray structures of octamolybdate [C(4)mim](4)[Mo8O26] and tetraperoxodimolybdate [Htmpy](2)[{MoO(O-2)(2)}(2)(mu-O)] complexes

1 REVISED Molybdenum-catalysed oxidation of cyclohexene with hydrogen peroxide in the presence of alcohols and X-ray structures of octamolybdate [C4mim]4[Mo8O26] and tetraperoxodimolybdate [Htmpy]2[{MoO(O2)2}2(-O)] complexes Carlos J. Carrasco,a Francisco Montilla,* a Eleuterio Álvarez,b Matthew Herbert,a and Agustín Galindo* a a Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, Aptdo 1203, 41071 Sevilla, Spain. b Instituto de Investigaciones Químicas, CSIC-Universidad de Sevilla, Avda. Américo Vespucio 49, 41092 Sevilla, Spain. ___________________________________________________________________________ ABSTRACT The direct formation of a -alcoxyalcohol from the corresponding olefin was investigated through the study of the oxidation of cyclohexene with hydrogen peroxide, as test reaction, catalysed by commercially available MoO3 in the presence of alcohols. The formation of the corresponding -alcoxycyclohexanol was favourable in the order Me > Et > iPr > tBu, reaching approximately no yield for tBuOH. In this reaction, the lack of selectivity was due to the simultaneous formation of cyclohexane-1,2-diol by epoxide hydrolysis, a reaction that it is competitive with respect to the epoxide ring-opening reaction by the alcohol. In order to decrease the cyclohexane-1,2-diol yield, by preventing the hydrolysis reaction, several strategies were analysed and discussed. In particular, 2-methoxycyclohexanol was obtained with high yields and 99 % selectivity by using as catalyst the [C4mim]4[Mo8O26] complex (C4mim = 1-butyl-3-methylimidazolium). The structure of the latter octamolybdate and also that of the tetraperoxodimolybdate [Htmpy]2[{MoO(O2)2}2(-O)] (tmpy = 2,4,6-trimethylpyridine) complex were determined by X-ray crystallography. The latter complex shows a 2-oxygen bridging atom and two 2-2-1-peroxo bridging ligands. This overall coordination assembly is unprecedented for binuclear molybdenum complexes. Keywords: Oxidation; Molybdenum; Hydrogen peroxide; -alcoxyalcohol; X-ray ___________________________________________________________________________ * Corresponding author. Fax: +34 954 557153. E-mail addresses: [email protected], m[email protected] 2 1. Introduction Epoxides are versatile intermediates in organic synthesis [ 1 ], that may react with a variety of nucleophiles to afford 1,2-bifunctional compounds through ring-opening reactions [ 2 ]. Usually, the oxygen-containing nucleophiles behave as poor nucleophilic reagents and, consequently, the reactions require the presence of a catalyst [2]. In particular, the epoxide ring-opening reaction with alcohols (Scheme 1) can be achieved with various catalysts [ 3 ], and the search of such a processes carried out in an enantioselective way is a challenging research [2, 4 ]. INSERT Scheme 1 The formation of the final -alcoxyalcohol in a single step, namely starting from the corresponding olefin by using an appropriate oxidant and the alcohol, is highly desirable (Scheme 2). Catalysts required for this transformation should be efficient in the olefin epoxidation and, as well, in the epoxide ring-opening reaction by the alcohol. The first descriptions of this reaction were early reported [ 5 ] and, at the present, this transformation can be performed by several catalysts in homogeneous [ 6 ] and heterogeneous conditions [ 7 ]. Concerning the catalysed epoxidation the use of benign oxidants, such as oxygen or hydrogen peroxide, is advantageous in terms of environmental considerations [ 8 ]. INSERT Scheme 2 Forming part of our recent investigations into catalysed oxidations in non-conventional media [ 9 ], and taking into consideration the acquired knowledge in oxodiperoxomolibdenum complexes and their activity in olefin epoxidation [ 10 , 11 ], we planned to explore the oxodiperoxomolibdenum catalysed olefin oxidation with hydrogen peroxide in the presence of alcohols (Scheme 2). Oxo-molybdenum(VI) precursors are well-known as catalysts for olefin epoxidation reactions [ 12 ] and, in particular oxodiperoxomolibdenum(VI) complexes have been proved to behave as active catalysts when using hydrogen peroxide as oxidant [10,11, 13 , 14 ]. Additionally, oxo-molybdenum(VI) complex MoO2Cl2 is an efficient catalyst in ring-opening reactions of epoxides [ 15 ]. Thus, the choice of oxodiperoxomolibdenum(VI) species for the direct formation of -alcoxyalcohol from the corresponding olefin seems quite reasonable. Here, we report the results concerning 3 the study of Mo-catalysed oxidation of cyclohexene with hydrogen peroxide in the presence of alcohols. Additionally, during our studies, the octamolybdate [C4mim]4[Mo8O26] (C4mim = 1-butyl-3-methylimidazolium) and tetraperoxodimolybdate [Htmpy]2[{MoO(O2)2}2(-O)] (tmpy = 2,4,6-trimethylpyridine) complexes were isolated and their X-ray structures are also discussed. 2. Results and discussion 2.1. Mo-catalysed oxidation of cyclohexene with hydrogen peroxide in the presence of alcohols In order to investigate the direct formation of a -alcoxyalcohol from the corresponding olefin, we selected as test reaction the oxidation of cyclohexene with hydrogen peroxide catalysed by commercially available MoO3 in the presence of alcohols (Scheme 3). Reactions were on a 1 mmol scale employing a solution of MoO3 (2.5 % mmol) in hydrogen peroxide, prepared as described elsewhere [10c]. For the purpose of simplicity the solution is referred to in this work simply as aqueous [Mo(O)(O2)2(H2O)n]. The reaction temperature was fixed at 60 ºC, as in other oxidation reactions catalysed by MoO3 studied for us [10a,b], and, in general, the reaction mixture was analysed after 18 h. After this time, products were extracted with Et2O (4 x 5 mL) and the yields of the reaction products cyclohexene epoxide, cyclohexane-1,2-diol and 2-alcoxycyclohexanol were calculated by gas chromatography (50 l of n-octane as internal standard). Table 1 displays a selection of the results obtained with methanol. Other experimental details are included in the corresponding table footnotes. INSERT Scheme 3 and Table 1 The inspection of Table 1 reveals that the catalyst precursor [Mo(O)(O2)2(H2O)n] was effective in the ring-opening reaction of the epoxide because the cyclohexene epoxide yield was very low (in most of the cases is ≤ 5 %). In fact, the reaction in acetone with added water gave almost exclusively the cyclohexane-1,2-diol, although the reaction in such a solvent was slow (entry 1). When methanol was employed as solvent, the reaction was faster with a conversion of 65 % at 18 h (entry 2) and with 87 % selectivity to 2-methoxycyclohexanol. 4 The conversion was complete with longer reaction times (entry 3). With the intention of diminishing the cyclohexane-1,2-diol yield, formed by simple epoxide hydrolysis, several strategies were analysed. For example, the methanol–hydrogen peroxide mixture was previously dried over Na2SO4 before the reaction (entry 4) or the aqueous hydrogen peroxide was substituted by the non-aqueous urea-hydrogen peroxide adduct, UHP, (entry 5). In both cases, the selectivity to the 2-methoxycyclohexanol did not increase (entry 4) or even decrease (entry 5). An easier method of inhibiting hydrolysis is to employ coordinating base species which block access of the epoxide to the acidic metal centre [ 16 ], thus inhibiting the hydrolysis mechanism [ 17 ]. Additionally these bases are also found to accelerate the rate of the epoxidation step, improving the efficiency of the catalysis [11, 18 ]. For instance, we demonstrated the efficiency of polydimethylsiloxane-terminated pyridine in the selective epoxidation of cyclohexene in chloroform [10c]. Several N-donor additives were considered (entries 6-11) and for the pyrazol-derived ligands different substituents (electron withdrawing and donating groups) were analysed. In general, conversions and selectivity to the 2-methoxycyclohexanol were of the same order of magnitude and were similar to the essay carried out without added N-donor additive. Thus, the complete inhibition of simple epoxide hydrolysis was not achieved by means of this procedure. The best results concerning the selectivity to the 2-methoxycyclohexanol was got by raising the methanol volume until 5 ml (entry 12). A complete selectivity to this product was observed, although an important drawback was the deceleration of the reaction (34 % conversion at 18 h). This result evidenced that selectivity to the 2-methoxycyclohexanol was controlled by the ratio MeOH:H2O in the solvent mixture. With these precedents, we decided to investigate other alcohols under the same experimental conditions. Table 2 collects these results. An examination of the results obtained for all the studied alcohols reveals that the formation of the corresponding -alcoxycyclohexanol was favourable in the order Me > Et > iPr > tBu. For the iPrand tBualcohols (entries 5 and 6, respectively) the selectivity to the -alcoxycyclohexanol decreases drastically, reaching approximately 0 for the tBuOH. This trend had been previously observed in the reaction catalysed by tungstic acid [5b] and, also for this catalyst, the -alcoxyalcohol yield with tBuOH was zero. This fact is not surprising because the tBu group hinder the nucleophilic attack to the epoxide, precluding the 2-tbutoxycyclohexanol formation. Concerning EtOH, the reaction was slower than that with MeOH giving 79 % of conversion after 66 h. In these conditions, the selectivity to 2-ethoxycyclohexanol was 60 %. We also 5 analysed with EtOH the possibility of decreasing the cyclohexane-1,2-diol yield by employing the same strategies used for MeOH. Again, drying the ethanol–hydrogen peroxide mixture (entry 2 of Table 2) either by using UHP (entry 3) or by adding a N-donor additive (entry 4) did not produce a significant selectivity improvement to the 2-ethoxycyclohexanol. INSERT Table 2 The catalytic features of polyoxometalates are well-known [ 19 ] and, in particular, polyoxomolybdates have been reported as catalyst in several oxidation reactions [ 20 ]. For this, we also investigated the activity of the certain polyoxomolybdates [C4mim]4[Mo8O16], [Bu4N]2[Mo6O19] and [Htmpy]2[{MoO(O2)2}2(-O)] as catalysts of our selected test reaction in methanol (entries 1, 3 and 4 of Table 3). In general, conversions and selectivity to the 2-methoxycyclohexanol were of the same order of magnitude and were similar to those observed with [Mo(O)(O2)2(H2O)n] as catalyst. It is worth to point out that, as showed in entry 2 of Table 3 for [C4mim]4[Mo8O16] catalyst, the increase in both the catalyst loading and the methanol ratio in the solvent mixture afforded the maximum selectivity to 2-methoxycyclohexanol, 99 % after 18 h. This result is better than that reported for tungstic acid, for which a maximum yield of 70 % was obtained [5b], or for ruthenium complexes [6b], and also better than those observed for other non-metal catalysed systems [5a,6a]. INSERT Table 3 2.2 Synthesis and X-ray structures of [C4mim]4[Mo8O26] and [Htmpy]2[{MoO(O2)2}2(  -O)] The addition of [C4mim]Cl onto an aqueous solution of octamolybdate, prepared according the details included in Experimental, affords a white solid that was isolated and characterised as [C4mim]4[Mo8O26] complex. Recrystallization from acetonitrile gave crystals that were used for an X-ray characterization. Fig. 1 shows the structure of [C4mim]4[Mo8O26] and Table 4 collects selected structural parameters. INSERT Fig. 1 and Table 4 6 Crystal packing of [C4mim]4[Mo8O26] complex is built up from the assembly of discrete centrosymmetric octamolybdate [Mo8O26]4− units and 1-butyl-3-methylimidazolium cations [C4mim]+ as charge-compensating ions (Fig. 1). The anion displays the typical structure of a -octamolybdate composed by 8 octahedra with 14 terminal oxygen atoms and 12 bridging oxygen atoms (6 μ2, 4 μ3 and 2 μ5) [ 21 ]. The Mo=O bond distances are within the 1.69-1.72 Å range, in agreement with those observed for other compounds containing the -octamolybdate anion [ 22 ]. The longest Mo-O bond lengths are found in the bridging oxygen atom coordinated in a μ5-fashion (for instance, 2.4934(18) Å for Mo(5)-O(18)). Other structural parameters are typical for this anion and require no further comments. During our studies of the Mo-catalysed olefin epoxidation [10], we investigated the influence of added N-donor ligands in the activity of the oxodiperoxo catalyst species [11]. In one of the essays we analyse the influence of 2,4,6-trimethylpyridine (tmpy) as a N-donor with poor coordinating properties and, during the corresponding working up of the reaction of MoO3 in hydrogen peroxide with tmpy, we isolate crystals of the new complex [Htmpy]2[{MoO(O2)2}2(-O)]. The preparative synthesis of this compound was optimised and the corresponding details are included in Experimental. NMR spectra are not informative because only signals due to the 2,4,6-trimethylpyridinium are observed. Fig. 2 displays the structure of this binuclear complex characterised by X-ray crystallography, while Table 5 shows selected structural parameters. INSERT Fig. 2 and Table 5 Crystals of compound [Htmpy]2[{MoO(O2)2}2(-O)] are composed of 2,4,6-trimethylpyridinium cations and binuclear [{MoO(O2)2}2(-O)]2anions, containing an oxygen atom and two peroxo ligands as bridging connectors between the two molybdenum atoms. The tetraperoxodimolybdate dianion has been structurally characterised in other complexes [ 23 ] and, in particular, Łasocha and co-workers have reported several X-ray structures of this type of compounds [ 24 ]. The Mo=O bond lengths are similar to other tetraperoxodimolybdate compounds (1.676(12) and 1.694(12) Å) [23,24], while the Mo-O distances are within the range 1.92 – 1.97 Å, with the exception of Mo(1)-O(7) and Mo(2)- O(5) (2.613(12) and 2.584(12) Å, respectively) for the oxygen atoms of the bridging peroxide groups. The O-O distances of peroxide ligands are close to 1.48 Å. The unique features of [Htmpy]2[{MoO(O2)2}2(-O)] with respect to other related complexes is the absence of 7 additional water ligands completing the coordination sphere of molybdenum atoms, as occurred also in the Cs2[{MoO(O2)2}2(-O)] complex [24a], and the simultaneous presence of the 2 oxygen bridging atom and two bridging 2-2-1 peroxo bridging ligands. In general, the oxodiperoxomolybdenum(VI) moiety is characterised by the presence of an apical oxo group and two equatorial peroxo ligands giving a trigonal bipyramid with two vacant coordination positions. One of them in the [{MoO(O2)2}2(-O)]2anion is the oxygen bridging atom, cis with respect the oxo group, while the other vacant position per molybdenum atom, which is trans with respect the oxo group, is generally occupied by coordinated water molecules or other additional bridging ligands [23,24]. In the Cs2[{MoO(O2)2}2(-O)] complex [24a], this vacant position, trans with respect the oxo group, is completed by weak intermolecular interactions with the peroxo ligands of a neighbour tetraperoxodimolybdate anion. In our complex [Htmpy]2[{MoO(O2)2}2(-O)] the trans positions with respect the oxo group are saturated by the bridging peroxo groups. The overall coordination is sketched in Scheme 4. This coordination assembly is unprecedented for molybdenum complexes, although related tungsten complexes with this structural feature are known [ 25 ]. The crystal packing of [Htmpy]2[{MoO(O2)2}2(-O)] (see Fig. 3) is composed of arrays of [{MoO(O2)2}2(-O)]2anions that are interconnected by bifurcated hydrogen-bonds with the N-hydrogen atoms of the 2,4,6-trimethylpyridinium cations. The N(1)...O(2)#3 and N(2)...O(10)#1 distances are 2.848(18) and 2.840(18) Å, respectively (where the symmetry transformations used to generate equivalent atoms are #1: -x+1,-y,-z and #3 -x+1,-y+1,-z+1). INSERT Scheme 4 and Fig. 3 3. Conclusions The oxidation of cyclohexene with hydrogen peroxide, as test reaction, in the presence of alcohols and catalysed by aqueous [Mo(O)(O2)2(H2O)n] was investigated. The direct formation of the corresponding -alcoxyalcohol from cyclohexene was completely effective for methanol with very good conversions and selectivity to 2-methoxycyclohexanol. The selectivity decrease was due to the simultaneous formation of cyclohexane-1,2-diol by epoxide hydrolysis, a reaction that it is competitive with respect to the epoxide ring-opening 8 reaction by the alcohol. Several strategies were studied in order to decrease the cyclohexane-1,2-diol yield, being the ratio MeOH:H2O in the solvent mixture of decisive importance. In particular, 2-methoxycyclohexanol was obtained with high yields and 99 % selectivity by using as catalyst the [C4mim]4[Mo8O26] compound. The X-ray structure of the latter complex was determined by X-ray crystallography. Other alcohols gave lower conversions and for the tert-butylalcohol the 2-tert-butoxycyclohexanol formation was not observed because the tBu group hinders the nucleophilic attack to the epoxide. Additionally, the unprecedented coordination mode 2-2-1for the bridging peroxo ligands in molybdenum complexes was X-ray evidenced in the tetraperoxodimolybdate [Htmpy]2[{MoO(O2)2}2(-O)] complex. 4. Experimental 4.1. General Chemicals were obtained from commercial sources and used as supplied. Tetrabutylammonium hexamolybdate [ 26 ] and [C4min]Cl [ 27 ] were synthesised according to the literature procedure. Infrared spectra were recorded on a Perkin-Elmer Model 883 spectrophotometer (Nujol emulsion in NaCl or KBr plates or in pressed KBr pellets). NMR spectra were recorded using a Bruker AMX-300 spectrometer with 13C{1H} and 1H shifts referenced to the residual solvent signals. All data are reported in ppm downfield from Si(CH3)4. The gas chromatograms (GC) were obtained using a Varian Chromatogram CP3800 with nitrogen as the carrier gas. The chromatogram used a Varian automatic injector, model CP-8410, flame ionisation detector (FID), and a Varian column, model CP-8741. Elemental analyses (C, H, N) were conducted by the Centro de Investigaciones, Tecnología e Innovación (CITIUS) of the University of Sevilla on an Elemental LECO CHNS 93 analyser. 4.2. General procedure for catalytic reaction. The reactor (a 50 mL vial equipped with a Young valve and containing a stirrer flea) was charged with 0.5 M aqueous [Mo(O)(O)2(H2O)n] (50 µL, 0.025 mmol), the base additive as specified, the solvent (2 mL), the oxidant (30 % aqueous H2O2, 1.5 mmol) and the 9 cyclohexene substrate (1 mmol), in the aforementioned order. The reactor was sealed and heated at 60 ºC, maintaining constant stirring in a thermostatted oil bath for the duration of the reaction. Upon completion the reactor was immediately cooled to 0º C and the products extracted with diethyl ether (3 x 4 mL). The resulting solution was dried (MgSO4) and analysed by GC (50 l of n-octane as internal standard). 4.3. Synthesis of [C4mim]4[Mo8O26] A solution of ammonium heptamolybdate (1.994 g; 1.61 mmol) in water (25 ml) was acidified to pH ≈ 4 with HCl (2 M). Then, the addition of a solution of [C4min]Cl (1 g, 5.65 mmol) dissolved in the minimum amount of water give rise to the precipitation of [C4mim]4[Mo8O26] as a white solid in good yield (2.296 g, 95 % yield). Recrystallization from acetonitrile or dimethylsulfoxide at -20 ºC affords well formed crystals of the compound. The product was washed with ethanol followed by diethyl ether and dried in vacuum. IR (KBr, cm-1): 411, 450, 475, 522, 554, 623, 661, 714, 842, 913, 939, 1021, 1107, 1163, 1250, 1132, 1436, 1463, 1568, 1612, 1718, 2872, 2958, 3068, 3139. 1H NMR (d6-dmso, 300 Hz):  = 0.90 (t, J = 7 Hz, 3H, CH2CH2CH2CH3), 1.27 (sx, J = 7 Hz, 2H, CH2CH2CH2CH3), 1.77 (qn, J = 7 Hz, 2H, CH2CH2CH2CH3), 3.88 (s, 3H, NCH3), 4.19 (t, J = 7 Hz, 2H, CH2CH2CH2CH3), 7.68 (s, 1H, CH), 7.75 (s, 1H, CH), 9.13 (s, 1H, CH). 13C{1H} NMR (d6-dmso, 75.47 Hz):  = 13.8 (s, CH2CH2CH2CH3), 19.3 (s, CH2CH2CH2CH3), 32.0 (s, CH2CH2CH2CH3), 36.2 (s, NCH3), 49.0 (s, CH2CH2CH2CH3), 122.7 (s, CH), 124.1 (s, CH), 137.1 (s, CH). Elemental analysis calculated for Mo8C32H60O26N8: C, 22.08; H, 3.47; N, 6.44. Experimental: C, 20.71; H, 3.45; N, 6.42 %. 4.4. Synthesis of [Htmpy]2[{MoO(O2)2}2(  -O)] Hydrogen peroxide (40 ml, aqueous 30%) was added to solid MoO3 (1.44g, 10 mmol) and the mixture was stirred at 60 ºC until complete dissolution of solid. The yellow solution was left to cool at room temperature and, then, 2,4,6-trimethylpyridine (1.32 ml, 10 mmol) was added dropwise. This gave an orange solution which on stirring for a further hour returned to a yellow solution. The pH of the solution was between 4.0-4.5. The solution was 16 Table 1 Molybdenum-catalysed oxidation of cyclohexene with hydrogen peroxide in the presence of methanola Entry Solvent Oxidant Additive Conversion (%) Selectivity to epoxide (%) Selectivity to cyclohexane-1,2-diol (%) Selectivity to 2-methoxycyclohexanol (%) 1 Acetone H2O2 (30 %) H2O b 17 5 95 0 2 MeOH H2O2 (30 %) - 65 1 12 87 3 MeOH c H2O2 (30 %) - 100 1 22 77 4 MeOH d H2O2 (30 %) - 79 1 14 85 5 MeOH c UHP - 60 11 14 75 6 MeOH H2O2 (30 %) py 39 3 9 88 7 MeOH H2O2 (30 %) pz 89 1 14 85 8 MeOH H2O2 (30 %) 3-Mepz 88 0 21 79 9 MeOH H2O2 (30 %) 3,5-Me2pz e 75 1 12 87 10 MeOH H2O2 (30 %) 4-Brpz e 77 1 15 84 11 MeOH H2O2 (30 %) 3,5-(CF3)2pz e 71 1 15 84 12 MeOH f H2O2 (30 %) - 34 0 0 100 a Reaction conditions: aqueous [Mo(O)(O2)2(H2O)n] 0.025 mmol, cyclohexene 1.0 mmol, solvent 2.0 mL, oxidant 1.5 mmol. T = 60 ºC, t = 18 h. After extraction with Et2O (4 x 5 mL), the yields and selectivity were calculated by GC (50 l of n-octane as internal standard). b 10 mmol of added H2O. c Reaction time = 66 h. d An excess of anhydrous Na2SO4 was added. e 0.1 mmol of additive. f 5 ml of MeOH were used. (Abbreviations: py = pyridine, pz = pyrazole, 3-Mepz = 3-methylpyrazole, 3,5-Me2pz = 3,5dimethylpyrazole, 4-Brpz = 4-Bromopyrazole, 3,5-(CF3)2pz = 3,5-bis(trifluoromethyl)pyrazole). 17 Table 2 Molybdenum-catalysed oxidation of cyclohexene with hydrogen peroxide in the presence of several alcoholsa Entry Solvent Oxidant Conversion (%) Selectivity to epoxide (%) Selectivity to cyclohexane-1,2-diol (%) Selectivity to 2-alcoxycyclohexanol (%) 1 EtOH b H2O2 79 2 38 60 2 EtOH c H2O2 63 1 34 65 3 EtOH UHP 35 15 26 59 4 EtOH d H2O2 70 2 37 61 5 iPrOH H2O2 70 1 66 33 6 tBuOH H2O2 55 1 99 0 a Reaction conditions: aqueous [Mo(O)(O2)2(H2O)n] 0.025 mmol, cyclohexene 1.0 mmol, solvent 2.0 mL, 30 % aqueous hydrogen peroxide 1.5 mmol. T = 60 ºC, t = 18 h. After extraction with Et2O (4 x 5 mL), the yields and selectivity were calculated by GC (50 l of n-octane as internal standard). b Reaction time = 66 h. c An excess of anhydrous Na2SO4 was added. d 2 eq of 3,5-Me2pz were added. 18 Table 3 Molybdenum-catalysed oxidation of cyclohexene with hydrogen peroxide in the presence of methanola Entry Solvent Catalyst Oxidant Conversion (%) Selectivity to epoxide (%) Selectivity to cyclohexane-1,2-diol (%) Selectivity to 2-methoxy cyclohexanol (%) 1 MeOH [C4min]4[Mo8O26] H2O2 55 3 12 85 2b MeOH [C4min]4[Mo8O26] H2O2 99 1 0 99 3 MeOH [Bu4N]2[Mo6O19] H2O2 67 1 13 86 4 MeOH [Htmpy]2[{MoO(O2)2}2(-O)] H2O2 54 0 10 90 a Reaction conditions: [Mo] 0.025 mmol, cyclohexene 1.0 mmol, methanol 2.0 mL, 30 % aqueous hydrogen peroxide 1.5 mmol. T = 60 ºC, t = 18 h. After extraction with Et2O (4 x 5 mL), the yields and selectivity were calculated by GC (50 l of n-octane as internal standard). b [Mo] 0.136 mmol, methanol 5.0 mL. 19 Table 4 Selected bond lengths [Å] and angles [°] for [C4mim]4[Mo8O26] ___________________________________________________________________________________ Mo(1)-O(1) 1.7585(18) Mo(1)-O(2) 1.9515(16) Mo(1)-O(3) 1.6928(19) Mo(1)-O(4) 2.1308(17) Mo(1)-O(4)#1 2.3922(17) Mo(1)-O(5) 1.9420(16) Mo(2)-O(2)#1 2.3540(18) Mo(2)-O(4) 2.2971(16) Mo(2)-O(5) 1.9989(18) Mo(2)-O(6) 1.705(2) Mo(2)-O(7) 1.7115(18) Mo(2)-O(13)#1 1.8972(18) Mo(3)-O(2)#1 2.0061(17) Mo(3)-O(4)#1 2.3684(16) Mo(3)-O(5) 2.3429(18) Mo(3)-O(8) 1.6995(18) Mo(3)-O(9) 1.698(2) Mo(3)-O(10) 1.8944(18) Mo(4)-O(1) 2.2612(19) Mo(4)-O(4)#1 2.4694(17) Mo(4)-O(10) 1.9125(17) Mo(4)-O(11) 1.710(2) Mo(4)-O(12) 1.703(2) Mo(4)-O(13) 1.9298(17) Mo(5)-O(14) 1.9192(17) Mo(5)-O(15) 1.7138(18) Mo(5)-O(16) 1.707(2) Mo(5)-O(17) 1.9073(18) Mo(5)-O(18) 2.4934(18) Mo(5)-O(19) 2.2730(18) Mo(6)-O(18) 2.3785(16) Mo(6)-O(18)#2 2.1565(17) Mo(6)-O(19) 1.7555(19) Mo(6)-O(20) 1.9443(17) Mo(6)-O(21) 1.6921(18) Mo(6)-O(24)#2 1.9517(16) Mo(7)-O(17)#2 1.9129(19) Mo(7)-O(18)#2 2.3207(16) Mo(7)-O(20) 1.9907(18) Mo(7)-O(22) 1.7006(19) Mo(7)-O(23) 1.7006(18) Mo(7)-O(24) 2.3696(17) Mo(8)-O(14) 1.9085(19) Mo(8)-O(18) 2.2848(16) Mo(8)-O(20) 2.3677(18) Mo(8)-O(24) 2.0009(17) Mo(8)-O(25) 1.7072(18) Mo(8)-O(26) 1.7005(19) O(3)-Mo(1)-O(1) 103.76(9) O(3)-Mo(1)-O(5) 103.17(8) O(1)-Mo(1)-O(5) 96.35(8) O(3)-Mo(1)-O(2) 100.16(8) O(1)-Mo(1)-O(2) 96.12(8) O(5)-Mo(1)-O(2) 150.02(7) O(3)-Mo(1)-O(4) 100.60(8) O(1)-Mo(1)-O(4) 155.64(8) O(5)-Mo(1)-O(4) 78.51(7) O(2)-Mo(1)-O(4) 78.91(7) O(3)-Mo(1)-O(4)#1 176.30(7) O(1)-Mo(1)-O(4)#1 79.62(7) O(5)-Mo(1)-O(4)#1 77.76(6) O(2)-Mo(1)-O(4)#1 77.84(7) O(4)-Mo(1)-O(4)#1 76.02(7) O(6)-Mo(2)-O(7) 104.69(10) O(6)-Mo(2)-O(13)#1 102.20(9) O(7)-Mo(2)-O(13)#1 100.11(8) O(6)-Mo(2)-O(5) 96.58(8) O(7)-Mo(2)-O(5) 100.57(8) O(13)#1-Mo(2)-O(5) 147.41(7) O(6)-Mo(2)-O(4) 95.36(8) O(7)-Mo(2)-O(4) 159.69(8) O(13)#1-Mo(2)-O(4) 78.39(7) O(5)-Mo(2)-O(4) 73.49(6) O(6)-Mo(2)-O(2)#1 164.79(8) O(7)-Mo(2)-O(2)#1 87.20(8) O(13)#1-Mo(2)-O(2)#1 84.64(7) O(5)-Mo(2)-O(2)#1 71.53(7) O(4)-Mo(2)-O(2)#1 72.49(6) O(9)-Mo(3)-O(8) 105.11(10) O(9)-Mo(3)-O(10) 101.41(9) O(8)-Mo(3)-O(10) 102.37(8) O(9)-Mo(3)-O(2)#1 96.65(8) O(8)-Mo(3)-O(2)#1 100.70(8) O(10)-Mo(3)-O(2)#1 145.70(7) O(9)-Mo(3)-O(5) 163.77(7) O(8)-Mo(3)-O(5) 88.46(8) O(10)-Mo(3)-O(5) 83.91(7) O(2)#1-Mo(3)-O(5) 71.66(7) O(9)-Mo(3)-O(4)#1 94.81(8) O(8)-Mo(3)-O(4)#1 159.64(8) O(10)-Mo(3)-O(4)#1 77.27(6) O(2)#1-Mo(3)-O(4)#1 72.33(6) O(5)-Mo(3)-O(4)#1 71.21(6) O(12)-Mo(4)-O(11) 104.58(11) O(12)-Mo(4)-O(10) 102.73(8) O(11)-Mo(4)-O(10) 98.87(9) O(12)-Mo(4)-O(13) 102.71(8) O(11)-Mo(4)-O(13) 97.67(9) O(10)-Mo(4)-O(13) 144.91(8) O(12)-Mo(4)-O(1) 91.20(9) O(11)-Mo(4)-O(1) 164.19(9) O(10)-Mo(4)-O(1) 78.22(7) O(13)-Mo(4)-O(1) 77.49(7) O(12)-Mo(4)-O(4)#1 160.70(9) O(11)-Mo(4)-O(4)#1 94.71(8) O(10)-Mo(4)-O(4)#1 74.43(6) O(13)-Mo(4)-O(4)#1 73.54(6) O(1)-Mo(4)-O(4)#1 69.50(6) O(16)-Mo(5)-O(15) 104.59(10) O(16)-Mo(5)-O(17) 103.27(9) 20 O(15)-Mo(5)-O(17) 98.10(8) O(16)-Mo(5)-O(14) 103.74(9) O(15)-Mo(5)-O(14) 98.05(8) O(17)-Mo(5)-O(14) 143.71(8) O(16)-Mo(5)-O(19) 90.11(8) O(15)-Mo(5)-O(19) 165.30(8) O(17)-Mo(5)-O(19) 78.02(7) O(14)-Mo(5)-O(19) 78.16(7) O(16)-Mo(5)-O(18) 160.06(8) O(15)-Mo(5)-O(18) 95.35(8) O(17)-Mo(5)-O(18) 73.52(7) O(14)-Mo(5)-O(18) 72.79(7) O(19)-Mo(5)-O(18) 69.95(6) O(21)-Mo(6)-O(19) 104.23(9) O(21)-Mo(6)-O(20) 101.45(8) O(19)-Mo(6)-O(20) 96.85(8) O(21)-Mo(6)-O(24)#2 101.00(8) O(19)-Mo(6)-O(24)#2 97.07(8) O(20)-Mo(6)-O(24)#2 149.70(7) O(21)-Mo(6)-O(18)#2 98.88(8) O(19)-Mo(6)-O(18)#2 156.89(7) O(20)-Mo(6)-O(18)#2 78.40(7) O(24)#2-Mo(6)-O(18)#2 78.25(7) O(21)-Mo(6)-O(18) 174.32(8) O(19)-Mo(6)-O(18) 81.45(7) O(20)-Mo(6)-O(18) 77.53(6) O(24)#2-Mo(6)-O(18) 78.11(6) O(18)#2-Mo(6)-O(18) 75.44(7) O(23)-Mo(7)-O(22) 105.37(9) O(23)-Mo(7)-O(17)#2 99.83(9) O(22)-Mo(7)-O(17)#2 101.34(9) O(23)-Mo(7)-O(20) 100.83(9) O(22)-Mo(7)-O(20) 98.96(9) O(17)#2-Mo(7)-O(20) 145.89(7) O(23)-Mo(7)-O(18)#2 160.14(8) O(22)-Mo(7)-O(18)#2 94.40(8) O(17)#2-Mo(7)-O(18)#2 77.72(7) O(20)-Mo(7)-O(18)#2 73.64(6) O(23)-Mo(7)-O(24) 88.40(8) O(22)-Mo(7)-O(24) 164.83(8) O(17)#2-Mo(7)-O(24) 82.10(7) O(20)-Mo(7)-O(24) 71.69(7) O(18)#2-Mo(7)-O(24) 71.73(6) O(26)-Mo(8)-O(25) 104.57(9) O(26)-Mo(8)-O(14) 102.29(9) O(25)-Mo(8)-O(14) 100.59(9) O(26)-Mo(8)-O(24) 96.98(9) O(25)-Mo(8)-O(24) 99.46(8) O(14)-Mo(8)-O(24) 147.56(7) O(26)-Mo(8)-O(18) 95.36(7) O(25)-Mo(8)-O(18) 159.77(8) O(14)-Mo(8)-O(18) 78.13(7) O(24)-Mo(8)-O(18) 74.27(6) O(26)-Mo(8)-O(20) 164.47(8) O(25)-Mo(8)-O(20) 88.00(8) O(14)-Mo(8)-O(20) 83.91(7) O(24)-Mo(8)-O(20) 71.57(7) O(18)-Mo(8)-O(20) 71.77(6) ___________________________________________________________________________________ Symmetry transformations used to generate equivalent atoms: #1 -x+1,-y+2,-z #2 -x+2,-y+1,-z+1. 21 Table 5 Selected bond lengths [Å] and angles [°] for [Htmpy]2[{MoO(O2)2}2(-O)] ___________________________________________________________________________________ Mo(1)-O(1) 1.676(12) Mo(1)-O(2) 1.966(12) Mo(1)-O(3) 1.960(15) Mo(1)-O(4) 1.963(12) Mo(1)-O(5) 1.917(13) Mo(1)-O(6) 1.949(10) Mo(2)-O(6) 1.927(10) Mo(2)-O(7) 1.970(12) Mo(2)-O(8) 1.966(11) Mo(2)-O(9) 1.913(14) Mo(2)-O(10) 1.972(14) Mo(2)-O(11) 1.694(12) O(2)-O(3) 1.495(17) O(4)-O(5) 1.475(17) O(7)-O(8) 1.465(17) O(9)-O(10) 1.494(17) O(1)-Mo(1)-O(5) 104.6(6) O(1)-Mo(1)-O(6) 102.9(6) O(5)-Mo(1)-O(6) 82.2(5) O(1)-Mo(1)-O(3) 105.1(6) O(5)-Mo(1)-O(3) 149.9(6) O(6)-Mo(1)-O(3) 87.0(5) O(1)-Mo(1)-O(4) 107.7(6) O(5)-Mo(1)-O(4) 44.7(5) O(6)-Mo(1)-O(4) 123.4(5) O(3)-Mo(1)-O(4) 127.1(6) O(1)-Mo(1)-O(2) 103.7(6) O(5)-Mo(1)-O(2) 129.9(5) O(6)-Mo(1)-O(2) 129.5(5) O(3)-Mo(1)-O(2) 44.8(5) O(4)-Mo(1)-O(2) 87.4(5) O(1)-Mo(1)-Mo(2) 131.5(4) O(5)-Mo(1)-Mo(2) 56.6(3) O(6)-Mo(1)-Mo(2) 37.1(3) O(3)-Mo(1)-Mo(2) 99.2(4) O(4)-Mo(1)-Mo(2) 88.8(4) O(2)-Mo(1)-Mo(2) 122.8(4) O(11)-Mo(2)-O(9) 104.7(7) O(11)-Mo(2)-O(6) 103.0(6) O(9)-Mo(2)-O(6) 85.9(5) O(11)-Mo(2)-O(8) 108.1(6) O(9)-Mo(2)-O(8) 127.7(6) O(6)-Mo(2)-O(8) 123.5(5) O(11)-Mo(2)-O(7) 105.3(6) O(9)-Mo(2)-O(7) 149.7(6) O(6)-Mo(2)-O(7) 83.3(5) O(8)-Mo(2)-O(7) 43.7(5) O(11)-Mo(2)-O(10) 103.1(6) O(9)-Mo(2)-O(10) 45.2(5) O(6)-Mo(2)-O(10) 129.0(5) O(8)-Mo(2)-O(10) 87.9(6) O(7)-Mo(2)-O(10) 129.5(5) O(11)-Mo(2)-Mo(1) 132.4(5) O(9)-Mo(2)-Mo(1) 98.3(4) O(6)-Mo(2)-Mo(1) 37.6(3) O(8)-Mo(2)-Mo(1) 88.5(3) O(7)-Mo(2)-Mo(1) 57.4(3) O(10)-Mo(2)-Mo(1) 122.3(4) O(3)-O(2)-Mo(1) 67.4(7) O(2)-O(3)-Mo(1) 67.8(7) O(5)-O(4)-Mo(1) 66.0(7) O(4)-O(5)-Mo(1) 69.3(7) Mo(2)-O(6)-Mo(1) 105.3(5) O(8)-O(7)-Mo(2) 68.0(7) O(7)-O(8)-Mo(2) 68.3(6) O(10)-O(9)-Mo(2) 69.5(8) O(9)-O(10)-Mo(2) 65.3(7) ___________________________________________________________________________________ 22 Table 6 Summary of crystallographic data and structure refinement results for [C4mim]4[Mo8O26] and [Htmpy]2[{MoO(O2)2}2(-O)]. [C4mim]4[Mo8O26] [Htmpy]2[{MoO(O2)2}2(-O)] formula C32H60Mo8N8O26 C16H24Mo2N2O11 fw 1740.40 612.25 crystal system Triclinic Triclinic space group P 1 P 1 a, Å 10.8826(4) 8.653(3) b, Å 12.5912(5) 9.350(4) c, Å 19.6589(8) 13.886(5) α, deg. 84.6960(10) 90.639(8) β, deg. 84.2860(10) 90.689(9) γ, deg. 84.5940(10) 97.866(8) V, Å3 2659.06(18) 1112.7(7) Z, F(000) 2, 1704 2, 608 Dcalc, Mg·m-3 2.174 1.821 μ, mm-1 1.911 1.185 θmax, deg 25.0 30.77 no. reflns collected 67371 27302 no. reflns used 14924 6818 no. of param. 712 288 R1(F) [F 2>2σ(F 2)][a] 0.0270 0.0750 wR2(F2)[b](all data) 0.0648 0.2223 S[c] (all data) 1.042 1.068 [a] R1(F) = ∑(Fo-Fc)/∑Fo for the observed reflections [F 2>2σ(F 2)]. [b] wR2(F2) = {∑ [w(Fo2Fc2)2]/ ∑ w(Fo2)2}1/2. [c] S = {∑ [w(Fo2Fc2)2]/(n-p)}1/2; (n = number of reflections, p = number of parameters) 23 References [ 1 ] (a) Aziridines and Epoxides in Organic Synthesis, ed. A. K. Yudin, Wiley-VCH, Weinheim, 2006, Ch. 6. (b) G. 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