DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, 3191–3194 3191 Reactions of oxo- and peroxo-molybdenum complexes with B(C6F5)3: crystal structures of cis-[MoO{OB(C6F5)3}(Á2-ONEt2)2] and cis-[MoO{OB(C6F5)3}{Á2-PhN(O)C(O)Ph}2] Linda H. Doerrer,a Jane R. Galsworthy,a Malcolm L. H. Green,a Michael A. Leech b and Matthias Müller b a Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR b Chemical Crystallography Laboratory, 9 Parks Road, Oxford, UK OX1 3PD Received 27th April 1998, Accepted 28th July 1998 The oxometal complexes [MoO2(h2-O-NR2)2], R = Et or CH2Ph, reacted with the strong Lewis acid B(C6F5)3 at their oxo functionality to give cis-[MoO{OB(C6F5)3}(h2-ONR2)2], (R = Et 1 or CH2Ph 2).Reaction of the peroxo complex [MoO(O2){h2-PhN(O)C(O)Ph}2] with the same Lewis acid led initially to the formation of [MoO(O2){B(C6F5)3}{h2- PhN(O)C(O)Ph}2] 3, which decomposes to form [MoO{OB(C6F5)3}{h2-PhN(O)C(O)Ph}2] 4. Compounds 1 and 4 have been characterised by X-ray crystallography.Introduction Alkyl peroxide transition metal complexes, A, play a central role as reactive intermediates in selective oxidation reactions which employ alkyl hydrogenperoxide as the oxygen source.1 Reaction is postulated as proceeding according to Scheme 1 via formation of an h1-co-ordinated O–OR ligand. Co-ordination of the organic substrate to the metal centre followed by oxygen transfer results in oxidation of the organic molecule.Sheldon and Van Doorn2c proposed that the main function of the metal catalyst in the co-ordinated peroxide complex was to act as a Lewis acid and remove electron density from the peroxidic oxygen. However, to date, the synthesis of well defined, soluble and reactive molybdenum alkyl peroxide complexes remains an unattainable goal. N,N-Dialkylhydroxylamino complexes containing an h2-ONR2 bound ligand, B, are structurally and electronically closely related to metal alkyl peroxide complexes and have been examined as well defined compounds for catalytic activity.3 Unfortunately these species do not exhibit similar reactivities to those of their alkyl peroxide analogues and are ineVective oxidation catalysts.4 The lack of reactivity has been attributed to strong complexation of the dialkylhydroxylamino ligand to the metal centre.In certain catalytic reactions oxometal complexes may be activated by addition of a Lewis acid co-catalyst which is thought to co-ordinate to the M]] O functionality thereby increasing the electrophilicity of the metal centre.5 We have recently reported the synthesis of several oxometal complexes containing a stable, approximately linear, M]] O–B(C6F5)3 moiety.6 As part of our continuing studies of the reactivity of B(C6F5)3 we describe the reactions of this Lewis acid with oxomolybdenum complexes containing ancillary dialkylhydroxylamino or peroxo ligands.Results and discussion Treatment of cis-[MoO2(h2-ONEt2)2] with 1 equivalent of B(C6F5)3 in toluene under ambient conditions yields cis- M O O R M O N R R A B [MoO{OB(C6F5)3}(h2-ONEt2)2] 1, which can be isolated as colourless crystals.The 11B NMR spectrum of compound 1 exhibits a signal at d 2.4, typical of a four-co-ordinate boron species and shifted upfield from that of B(C6F5)3 (d 59). A 1H NMR spectrum reveals two triplet signals, assignable to the methyl groups, and four multiplet resonances, due to the methylene protons, clearly indicating that co-ordination has occurred at a single Mo]] O unit.The methylene protons are also diastereotopic in the parent complex. An IR spectrum of complex 1 exhibits an absorption at 1008 cm21, tentatively ascribed to the n(N–O) stretching vibration, but assignment of the characteristic Mo]] O stretches is hampered by strong absorption of the fluorinated aryl rings in this region. Full characterising data for compound 1 is detailed in Table 1. The solid state structure of compound 1 has been determined by X-ray crystallography; selected bond angles and distances are reported in Table 2.The structure determination of complex 1 (Fig. 1) reveals that a molecule of B(C6F5)3 is bound to one Mo]] O unit. The B–O bond length [1.510(2) Å] is typical of those found within this family of compounds and the Mo]] O–B unit deviates slightly from linearity [170.09(8)8].6 The molybdenum centre displays pseudo-tetrahedral geometry with terminal oxo units occupying two of the vertices and the midpoint of the N–O bond of each h2-ONEt2 ligand occupying the remaining two.The presence of a Mo]] OB(C6F5)3 moiety and a non-co-ordinated Mo]] O unit within compound 1 allows us to assess the electron withdrawing capabilities of the boron Lewis acid. A significant lengthening of the Mo]] O bond of approximately 0.1 Å is observed upon co-ordination to the Lewis acid; Mo–O(1) 1.808(1), Mo–O(2) 1.6823(1) Å. For comparison the Mo]] O bond distances in the parent complex are 1.714(2) and 1.713(2) Å.6 The benzyl compound, [MoO2{h2-ON(CH2Ph)2}2], was synthesized from [MoO2(acac)2] (acac = h2-C5H7O2). The former complex has been previously reported as synthesized from Na2MoO4?2H2O and N,N-dibenzylhydroxylamine.3e Colourless Scheme 1 Proposed mechanism for alkene oxidation using metal alkyl peroxide complexes.M O O R M O R O A alkene +3192 J. Chem. Soc., Dalton Trans., 1998, 3191–3194 Table 1 Analytical and spectroscopic data for compounds 1, 2, 3 and 4 Complex a 1 [MoO{OB(C6F5)3}(h2-ONEt2)2] C, 38.4 (38.2); H, 2.6 (2.5); B, 1.3 (1.3); N, 3.2 (3.4) Spectroscopic data b IR: 3100–2850w, 1646vs, 1517vs, 1469vs, 1386vs, 1373vs, 1284vs, 1108m, 1096vs, 1008s, 978vs, 940m, 911vs, 791m, 772m, 762m, 748m, 729m, 689m, 676m, 668m, 656m, 628m, 588m, 576m 1H: 2.98 (2 H, m, J 7, CH2), 2.74 (2 H, m, J 7, CH2), 2.66 (2 H, m, J 7, CH2), 2.50 (2 H, m, J 7, CH2), 0.74 (6 H, t, J 7, CH3), 0.48 (6 H, t, J 7, CH3) 13C: 148.3 (d, J 240, C6F5), 140.1 (d, J 240, C6F5), 137.5 (d.J 250, C6F5), 51.7 (s, CH2), 50.6 (s, CH2), 10.4 (s, CH3), 9.4 (s, CH3) 11B: 2.4 (br) 2 [MoO{OB(C6F5)3}{h2-ON(CH2- Ph)2}2] C, 51.6 (51.9); H, 2.5 (2.6); B, 1.1 (1.0); N, 2.7 (2.6) IR: 3100–2820w, 1640m, 1512s, 1488m, 1467vs, 1393m, 1375m, 1354m, 1284m, 1096s, 1015m, 977vs, 949vs, 919s, 904s, 853m, 790m, 773m, 767m, 747m, 699m, 683m, 677m, 668m, 662m, 628m, 613m, 599m 1H: 7.41–6.92 (5 H, m, C6H5), 4.24–3.99 (2 H, m, CH2) 13C: 148.1 (d, J 233, C6F5), 140.2 (d, J 230, C6F5), 137.4 (d, J 236, C6F5), 131.8 (s, C6H5), 131.2 (s, C6H5), 130.7 (s, C6H5), 130.2 (s, C6H5), 130.0 (s, C6H5), 128.8 (s, C6H5), 61.2 (s, CH2), 60.1 (s, CH2) 11B: 2.7 (br) 3 [MoO(O2){B(C6F5)3}{h2-PhN- (O)C(O)Ph}2] C, 48.30 (48.89); H, 2.04 (1.85); B, 0.99 (1.02); N, 2.58 (2.59) 1H:c 7.24–6.65 (m, C6H5) 11B: c 3.8 (br) 4 [MoO{OB(C6F5)3}{h2-PhN(O)- C(O)Ph}2]?0.5C5H12 C, 50.66 (50.73); H, 2.42 (2.36); B, 1.09 (1.00); N, 2.48 (2.55) IR: 3120–2840w, 1900–1790w, 1613w, 1596w, 1493vs, 1453vs, 1447vs, 1351s, 1325m, 1237m, 1203m, 1069m, 1026m, 1014m, 920s, 904vs, 845m, 804m, 757s, 746m, 737m, 700s, 620m, 606s, 595m, 515m, 508m 1H: 7.52–7.17 (m, C6H5) 13C: 148.1 (d, J 237, C6F5), 139.8 (d, J 244, C6F5), 137.0 (d, J 247, C6F5), 133.7 (s, C6H5), 133.4 (s, C6H5), 131.3 (s, C6H5), 130.1 (s, C6H5), 129.9 (s, C6H5), 129.6 (s, C6H5), 129.0 (s, C6H5), 128.5 (s, C6H5), 126.9 (s, C6H5), 126.0 (s, C6H5), 118 [s(br), BC] 11B: 3.3 (br) a Analytical data given as found (calculated) in %, IR data (cm21) determined for KBr discs.b The NMR data (CDCl3, 298 K), unless otherwise stated, given as: chemical shift (d) [relative intensity, multiplicity (J in Hz), assignment]. c In C6D6. crystals of [MoO2{h2-ON(CH2Ph)2}2] were obtained, the single crystal structure of which confirmed the cis-oxo geometry of the starting material but the data were not of suYcient quality to be published. Upon reaction of [MoO2{h2-ON(CH2Ph)2}2] with 1 equivalent of B(C6F5)3 cis-[MoO{OB(C6F5)3}{h2-ON(CH2Ph)2}2] 2 was obtained as colourless, diamond shaped crystals. Compound 2 was fully characterised by spectroscopic techniques and elemental analysis (Table 1) and displays similar features to those of its ethyl analogue, 1.A general structural feature of cis-[MoO2(h2-ONR2)2] complexes is a relatively large O]] Mo]] O bond angle,3a,4 compared to related cis-[MoO2L2] complexes.This suggests that such dialkylhydroxylamino- complexes might be sterically capable of Fig. 1 View of the structure of [MoO{OB(C6F5)3}(h2-ONEt2)2] 1. Fluorine atoms omitted for clarity. binding two molecules of B(C6F5)3. However, reaction of [MoO2{h2-ON(CH2Ph)2}2] with 2 equivalents of the Lewis acid yielded only compound 2, even using prolonged reaction times. We then investigated the reaction of B(C6F5)3 with the peroxomolybdenum complex [MoO(O2){h2-PhN(O)C(O)Ph}2]. This compound has been shown eVectively to oxidise primary and secondary alcohols to the corresponding carbonyl compounds and to be capable of stereospecifically epoxidising allylic alcohols.7 Treatment of [MoO(O2){h2-PhN(O)C(O)Ph}2] with 1 equivalent of B(C6F5)3, in hexanes, gave a red-orange precipitate, 3.Elemental analysis data for 3 is consistent with the empirical formula [MoO(O2){B(C6F5)3}{h2-PhN(O)C(O)- Ph}2]. The compound exhibits a signal at d 3.8 in its 11B NMR spectrum indicative of a four-co-ordinate boron atom whilst the 1H NMR spectrum shows several resonances in the phenyl region, shifted from those of the starting complex.These data suggest that the Lewis acid is bound to either the oxo or peroxo functionality (Fig. 2) and that the organic ligands have Fig. 2 Possible structures of [MoO(O2){B(C6F5)3}{h2-PhN(O)C(O)- Ph}2] 3. Mo O O O O O O O N N Ph Ph Ph Ph Mo O O O O O O O N N Ph Ph Ph Ph B(C6F5)3 B(C6F5)3 Table 2 Selected bond distances (Å) and angles (8) for compound 1 Mo(1)–O(1) Mo(1)–O(2) Mo(1)–O(3) Mo(1)–O(4) Mo(1)–N(3) Mo(1)–N(4) O(1)–B(1) O(3)–N(3) O(4)–N(4) 1.808(1) 1.683(1) 1.936(1) 1.938(1) 2.140(1) 2.140(1) 1.510(2) 1.423(2) 1.425(2) O(1)–Mo(1)–O(2) O(1)–Mo(1)–O(3) O(1)–Mo(1)–O(4) O(2)–Mo(1)–O(3) O(2)–Mo(1)–O(4) O(3)–Mo(1)–N(3) O(4)–Mo(1)–N(4) B(1)–O(1)–Mo(1) 122.94(5) 114.32(4) 113.89(5) 107.84(5) 108.02(5) 40.47(4) 40.54(4) 170.09(8)J. Chem.Soc., Dalton Trans., 1998, 3191–3194 3193 retained their integrity. The interaction of a related oxoperoxomolybdenum complex, [MoO(O2)2L2], L = 2-(1-octylpyrazol- 3-yl)pyridine, with Brønsted and Lewis acids has been described.8 The NMR spectroscopic studies (17O and 1H) indicate that trifluoroacetic anhydride selectively attacks the peroxo ligand in the pyrazolylpyridine complex to form a mixture of isomers of the type [MoO(OR)42x(OR9)xL2] [R = C(O)CF3, R9 = OC(O)CF3, x = 0–4].In contrast, electrophilic attack of chlorotrimethylsilane occurs preferentially, but not selectively, at the peroxo ligands.8 These studies suggest that the peroxo ligands may provide the most likely site within [MoO(O2){h2- PhN(O)C(O)Ph}2] for electrophilic attack by B(C6F5)3.However, any stretches due to the Mo]] O and Mo(O2) units in the IR spectrum of 3 are masked by those of the Lewis acid and so, without structural determination or 17O labelling studies, the exact location of the B(C6F5)3 moiety cannot be confidently predicted.In order to determine the bonding mode of the Lewis acid moiety attempts were made to crystallise compound 3 from toluene solutions. However it decomposes under these conditions to give small orange crystals of [MoO{OB(C6F5)3}{h2- PhN(O)C(O)Ph}2] 4. Compound 4 can be independently synthesized in good yield by reaction of [MoO2{h2-PhN(O)- C(O)Ph}2] with B(C6F5)3 and has been fully characterised by IR and NMR spectroscopies, elemental analyses (Table 1) and a crystal structure determination.The NMR spectroscopic data for compound 4 are significantly diVerent from those of 3. The structure of compound 4 is shown in Fig. 3 and signifi- cant bond angles and distances are detailed in Table 3. The structure determination reveals the presence of a cis- MoO{OB(C6F5)3} unit with similar features to those described for compound 1; B(1)–O(1) 1.508(3), Mo(1)–O(1) 1.775(3), Mo(1)–O(2) 1.674(3) Å, and Mo(1)–O(1)–B(1) 169.07(18)8.It has been observed that the oxoperoxo-complex [Mo- O(O2){h2-PhN(O)C(O)Ph}2] will undergo gradual conversion into [MoO2{h2-PhN(O)C(O)Ph}2] and so the formation of 4 from 3 is unsurprising. In conclusion, we have demonstrated that oxo- and perhaps peroxo-functionalities in molybdenum complexes are suf- ficiently nucleophilic to form a dative interaction with B(C6F5)3. In the case of dioxomolybdenum complexes containing ancillary h2-ONR2 ligands, attack of the Lewis acid occurs preferentially at the oxometal unit and no evidence for reaction at the h2-ONR2 ligand is observed.Whilst the analogous reaction of Fig. 3 View of the structure of [MoO{OB(C6F5)3}{h2-PhN(O)C(O)- Ph}2] 4. Fluorine and hydrogen atoms omitted for clarity. B(C6F5)3 with a peroxooxomolybdenum complex may initially occur at the peroxo ligand it is followed by decomposition on standing to yield a B(C6F5)3 substituted dioxomolybdenum species. The Mo]] OB(C6F5)3 unit is reasonably stable to air and two complexes containing this motif have been crystallographically characterised.Experimental Fourier-transform 1H and 11B NMR spectra were recorded on a Bruker WM 300 spectrometer at 300 and 96 MHz respectively, 13C NMR spectra on a Bruker WM 300 spectrometer at 75.5 MHz or Varian Unity 500 spectrometer at 125.7 MHz: 1H and 13C shifts are reported with respect to d 0 for SiMe4, 11B with respect to d 0 for BF3?OEt2; all downfield shifts are positive.Infrared spectra were recorded on either a Mattson ‘Polaris’ Fourier-transform, Perkin-Elmer FT 1710 spectrophotometer, or Perkin-Elmer 457 grating spectrometers. Microanalyses were obtained from the microanalytical department of this department. All reactions were carried out under nitrogen using standard Schlenk techniques. Solvents were dried over suitable reagents and freshly distilled under N2 before use. The compounds HONEt2, HON(CH2Ph)2, [MoO2(acac)2] were used as received (Aldrich); [MoO2(h2-ONEt2)2],4 [MoO2{h2-PhN(O)C(O)Ph}2],7 [MoO(O2){h2-PhN(O)C(O)Ph}2] 7 and B(C6F5)3 9 were prepared as previously described.Preparations cis-[MoO{OB(C6F5)3}(Á2-ONEt2)2] 1. White [MoO2(h2- ONEt2)2] (0.304 g, 1 mmol) was partially dissolved in toluene (20 cm3) and a toluene solution (20 cm3) of B(C6F5)3 (0.512 g, 1 mmol) added. The mixture was stirred for 4 h during which time a yellow solution formed. After removal of solvent in vacuo the residue was washed with pentane and the desired product then extracted with toluene.This solution was concentrated and cooled to 220 8C resulting in the formation of colourless crystals of compound 1. Yield: 0.63 g, 77%. Alternative preparation of cis-[MoO2{Á2-ON(CH2Ph)2}2]. Orange [MoO2(acac)2] (1.22 g, 3.74 mmol) was suspended in CH2Cl2 and HON(CH2Ph)2 (1.22 g, 3.74 mmol) dissolved in CH2Cl2 (50 cm3) added. Ethanol (50 cm3) was added and the reaction mixture stirred for 1 h until an oV-white precipitate had formed.The solvent was removed under vacuum and the residue washed with Et2O to remove any unchanged hydroxylamine. The residue was extracted with CH2Cl2. Concentration and cooling to 220 8C resulted in the formation of colourless crystals. Yield: 0.57 g, 74%. cis-[MoO{OB(C6F5)3}{Á2-ON(CH2Ph)2}2] 2. White [MoO2- {h2-ON(CH2Ph)2}2] (0.552 g, 1 mmol) was suspended in CH2Cl2 (20 cm3) and a CH2Cl2 solution (20 cm3) of B(C6F5)3 (512 mg, 1 mmol) added. Over about 15 min the solid dissolved and a very pale yellow solution formed.After stirring for 1.5 h in total the solvent was removed in vacuo and the residue washed with hexane. The residue was extracted with toluene and the solution filtered oV. Both the hexane and toluene filtrates were separately concentrated and cooled to 220 8C leading to the formation of colourless crystals. Combined yield: 0.92 g, 86%. Table 3 Selected bond distances (Å) and angles (8) for compound 4 Mo(1)–O(1) Mo(1)–O(2) Mo(1)–O(3) Mo(1)–O(4) Mo(1)–O(5) Mo(1)–O(6) O(1)–B(1) 1.775(3) 1.674(3) 1.982(2) 1.979(2) 2.088(3) 2.164(3) 1.508(3) O(1)–Mo(1)–O(2) O(1)–Mo(1)–O(3) O(1)–Mo(1)–O(4) O(2)–Mo(1)–O(3) O(2)–Mo(1)–O(4) O(2)–Mo(1)–O(5) B(1)–O(1)–Mo(1) 103.68(13) 86.22(11) 105.95(11) 106.33(12) 87.47(12) 91.06(13) 169.07(18)3194 J.Chem. Soc., Dalton Trans., 1998, 3191–3194 [MoO(O2){B(C6F5)3}{Á2-PhN(O)C(O)Ph}2] 3. Yellow [Mo- O(O2){h2-PhN(O)C(O)Ph}2] (0.568 g, 1 mmol) was suspended in hexane (20 cm3) and a hexane solution (20 cm3) of B(C6F5)3 (512 mg, 1 mmol) added.There was an immediate change to red-orange and the reaction stirred for 30 min. The pale yellow filtrate was removed and the red-orange precipitate washed with hexane (3 × 10 cm3) and then dried in vacuo. cis-[MoO{OB(C6F5)3}{Á2-PhN(O)C(O)Ph}2] 4. OV-white [MoO2{h2-PhN(O)C(O)Ph}2] (0.552 g, 1 mmol) was suspended in toluene (20 cm3) and a toluene solution (20 cm3) of B(C6F5)3 (512 mg, 1 mmol) added.There was an immediate change to orange and after 1 h all the solid had dissolved. The solvent was removed in vacuo and the residue washed with hexane. The residue was extracted with toluene and the filtrate concentrated and layered with pentane leading to the formation of orange microcrystals. Yield: 0.87 g, 82%. Crystal structure determination of compounds 1 and 4 Crystals of compound 1 were grown from toluene solution at 253 K and of 4 from toluene layered with pentane at 298 K.In each case a crystal from the mother-liquid was immersed in highly viscous perfluoropolyether to exclude oxygen and prevent solvent loss. It was mounted on a glass fibre and plunged into a cold (150 K) nitrogen stream. Crystal data. Compound 1, C26H20BF15MoN2O4?0.5C7H8, M = 816.21 1 46.04, triclinic, space group P1� , a = 10.764(1), b = 12.107(1), c = 12.563(1) Å, a = 86.673(2), b = 85.919(2), g = 86.480(2)8, V = 1627.6 Å3, Z = 2, Dc = 1.76 g cm23, m = 5.138 cm21, colourless, crystal dimensions 0.23 × 0.31 × 0.18 mm.Compound 4, C44H20BF15MoN2O6?0.5C6H12, M = 1094.45, triclinic, space group P1� , a = 10.2840(8), b = 12.4090(8), c = 18.598(2) Å, a = 102.500(5), b = 98.190(4), g = 106.397(4)8, V = 2169.74 Å3, Z = 2, Dc = 1.68 g cm23, m = 4.14 cm21, yellow block, crystal dimensions 0.25 × 0.25 × 0.10 mm. Data collection and processing. The data for compounds 1 and 4 were collected at 150 and 100 K respectively on an Enraf- Nonius DIP2000 image plate diVractometer with graphitemonochromated Mo-Ka radiation (l = 0.71069 Å).For compound 1 19362 reflections were measured (1 < q < 268, 213 < h < 13, 215 < k < 15, 215 < l < 15). 6304 Unique reflections were obtained giving 5950 reflections with I > 3s(I). For compound 4 4580 reflections were measured (2 < q < 258, 0<h<12, 213 < k < 13, 221 < l < 20). 4580 Unique reflections were obtained giving 4201 reflections with I > 3s(I).The images were processed with the DENZO and SCALEPACK programs.10 Corrections for Lorentz-polarisation eVects were performed but not for absorption. Structure solution and refinement. The crystal structures were solved by direct methods and refined by the full-matrix leastsquares method. Compound 1 crystallised with toluene in a 1 : 0.5 ratio. The toluene molecules are disordered at the crystallographic inversion centre with a translation of about 1.4 Å along their molecular twofold axis. All non-hydrogen atoms of 1 were refined with anisotropic displacement parameters. All hydrogen atoms of the molybdenum compound could be located in Fourier-diVerence maps and were refined isotropically.The hydrogen atoms of the disordered toluene were added geometrically and included in the final refinement with fixed positional and thermal parameters. For compound 1, 548 refined parameters and 5960 observations resulted in an observation/refined parameter ratio of 10.9 : 1.Corrections for secondary extinction were applied and refinement completed using a Chebyshev weighting scheme 11 with parameters 1.67, 0.875, 1.28. Refinement on F converged at R = 0.025, R9 = 0.031 and goodness of fit = 1.07. A final Fourier-diVerence synthesis showed minimum and maximum residual electron densities of 20.46 and 0.37 e Å23. Compound 4 was crystallised from toluene solution layered with pentane. One molecule of pentane is incorporated in the unit cell with the central carbon atom, C(101), lying on the centre of inversion such that one half of a pentane molecule is in the asymmetric unit. Hydrogen atoms were generated geometrically and allowed to ride on the corresponding carbon atoms.For 4, 646 refined parameters and 4201 observations resulted in an observation/refined parameter ratio of 6.50 : 1. Corrections for secondary extinction were applied and refinement completed using a Chebyshev weighting scheme11 with parameters 1.30, 0.078, 0.968.Refinement on F converged at R = 0.0465, R9 = 0.0455 and goodness of fit = 1.1318. A final Fourier-diVerence synthesis showed minimum and maximum residual electron densities of 20.62 and 0.75 e Å23. All crystallographic calculations were carried out using the CRYSTALS program package.12 Neutral atom scattering factors were taken from ref. 13. CCDC reference number 186/1105. See http://www.rsc.org/suppdata/dt/1998/3191/ for crystallographic files in .cif format.Acknowledgements We thank the University of Oxford for a Violette and Samuel Glasstone Fellowship (J. R. G.), the Deutsche Gemeinschaft Forschung (M. M.), St. John’s College, Oxford (L. H. D.) and the EPSRC for support of this work. References 1 For reviews concerning transition-metal catalysis of epoxidation reactions see: J. E. Lyons, in Aspects of Homogeneous Catalysis, ed. R. Ugo, Reidel, Dordrecht, Boston, 1977, vol. 3, ch. 1 and refs. therein; R.A. Sheldon and J. K. Kochi, Metal Catalyzed Oxidations of Organic Compounds, Academic Press, New York, 1981. 2 (a) S. Chan-Cheng, J. W. Reed and E. S. Gould, Inorg. Chem., 1973, 12, 337; (b) R. A. Sheldon, Recl. Trav. Chim. Pays-Bas, 1973, 92, 253, 367; (c) R. A. Sheldon and J. A. Van Doorn, J. Catal., 1973, 31, 427; (d) M. N. Sheng and J. G. Zajacek, Adv. Chem. Ser., 1968, 76, 418; (e) M. N. Sheng and J. G. Zajacek, J. Org. 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