Article pubs.acs.org/IC
Molybdenum(VI) Dioxo Complexes Employing Schiff Base Ligands with an Intramolecular Donor for Highly Selective Olefin Epoxidation Martina E. Judmaier, Christof Holzer, Manuel Volpe, and Nadia C. Mösch-Zanetti* Institut für Chemie, Karl-Franzens-Universität Graz, Stremayrgasse 16, 8010 Graz, Austria S Supporting Information *
ABSTRACT: Reaction of [MoO2(η2-tBu2pz)2] with Schiff base ligands HLX (X = 1−5) gave molybdenum(VI) dioxo complexes of the type cis-[MoO2(LX)2] as yellow to light brown solids in moderate to good yields. All ligands coordinate via its phenolic O atom and the imine N atom in a bidentate manner to the metal center. The third donor atom (R2 = OMe or NMe2) in the side chain in complexes 1−4 is not involved in coordination and remains pendant. This was confirmed by X-ray diffraction analyses of complexes 1 and 3. Complexes 1, 3, and 5 exist as a mixture of two isomers in solution, whereas complexes 2 and 4 with sterically less demanding substituents on the aromatics only show one isomer in solution. All complexes are active catalysts in the epoxidation of various internal and terminal alkenes, and epoxides in moderate to good yields with high selectivities are obtained. In the challenging epoxidation of styrene, complexes 1 and 2 prove to be very active and selective. The selectivity seems to be influenced by the pendant donor arm, as complex 5 without additional donor in the side chain is less selective. Experiments prove that the addition of n-butyl methyl ether as intermolecular donor per se has no influence on the selectivity. The basic conditions induced by the NMe2 groups in complexes 3 and 4 lead to lower activity.
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INTRODUCTION
worthwhile to develop further homogeneous catalysts for selective epoxidation of styrene and other olefins. We have an ongoing interest in oxygen atom transfer (OAT) chemistry mediated by high valent molybdenum compounds.17−20 Only recently we started to investigate the catalytic behavior in epoxidation reactions.13 These molybdenum dioxo complexes contain a bidentate phenol based ligand with an adjacent pyrazole substituent. We found them to be highly reactive epoxidation catalysts for various substrates but unselective for styrene leading to several ring-opened products. The high reactivity prompted us to develop molybdenum dioxo compounds with phenol based ligands having a higher denticity to influence the selectivity. From rhenium based epoxidation catalysis with MTO it was shown that the addition of Lewis bases (e.g., pyridines and pyrazoles) modulates the Lewis acidity of the rhenium center and thereby increases the selectivity.21 We adopt this concept to molybdenum chemistry by introducing an intramolecular donor in the ligand design. We focused on Schiff base ligands as they are known to be effective ligands in oxidation catalysis.4 Furthermore, synthetic procedures for Schiff base ligands are widely published and are easy to modify. Here, we report the preparation of a set of new molybdenum(VI) dioxo complexes with bidentate phenol imine ligands equipped with a
Alkene epoxidation is one of the main routes for the production of epoxides, which are of high importance in both synthetic organic chemistry and chemical technology.1 Among them, styrene oxide represents one of the most interesting compounds as it is used for the manufacture of important commercial products (e.g., epoxy resins, cosmetics, surface coatings, sweeteners, perfume, drugs, etc.).2 To overcome the limitations of traditional processes using stoichiometric amounts of peracids, research has focused on the development of new synthetic methods. Metal catalyzed alkene to epoxide conversion in the presence of softer oxidants, such as H2O2, alkyl hydroperoxides, or air, has attracted considerable interest and led to the development of highly active catalysts.3−5 Molybdenum complexes with various types of ligands have been tested in the epoxidation of alkenes and among them, cis-[MoO2L2] complexes by Schiff base ligands prove to be active catalysts.6−11 Most of these complexes show high catalytic activity in the epoxidation of internal alkenes such as cyclooctene and cyclohexene. The epoxidation of terminal alkenes like styrene is more challenging because of favored ring-opening reactions of the epoxide,12,13 leading usually to relatively low conversions and poor selectivity. Only a very limited number of highly selective molybdenum(VI) catalysts employing other ligands in the epoxidation of styrene were reported,14−16 a prominent example being molybdenum(VI) dioxo half sandwich complexes with cyclopentadienyl derivatives.15 Thus, it is still © 2012 American Chemical Society
Received: July 6, 2012 Published: August 29, 2012 9956
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remains pendant. The formation of the complex is indicated by a color change from light yellow to orange (for 1, 3, and 5) or brown (for 2 and 4) of the solution. The unusual starting material [MoO2(η2-tBu2pz)2] offers the advantage of a very easy workup procedure, as the formed side product, bis-tert-butylpyrazole, together with residual ligand traces can be easily separated from the complexes via extraction with pentane or heptane. The pure compounds are isolated as yellow to light brown solids. Complexes 1−5 can be also prepared by more conventional methods using either [MoO2Cl2(dme)]28 or commercially available [MoO2(acac)2] as starting materials (Scheme 2). Both reaction pathways would be favorable as they involve fewer synthetic steps, but the obtained yields and purities were significantly lower. Furthermore, we were not able to isolate complexes 2 and 4 using [MoO2(acac)2]. All molybdenum complexes 1−5 are well soluble in common organic solvents such as toluene, tetrahydrofuran (THF), chloroform, and methanol at room temperature, but much less so in aliphatic hydrocarbons like pentane or heptane. The compounds are stable in the absence of moist air and can be stored under inert conditions for several weeks (complexes 3 and 4) to months (complexes 1, 2, and 5). In general complexes 1 and 2 with a methoxy group in the pendant arm prove to be more stable in solution than their NMe2 based counterparts 3 and 4. The latter compounds tend to decompose after a few days in solution. We suspect that the amine group leads to more basic conditions in comparison to the ether functionality and therefore traces of water may have a more pronounced effect, thus preferring the formation of polymeric molybdenum compounds. Complexes 1−5 were characterized by NMR and IR spectroscopy, mass spectrometry, and elemental analyses. Crystals suitable for X-ray diffraction analyses were obtained in the case of complexes 1 and 3. 1 H NMR spectra of the free ligands HL1 to HL5 show a broad resonance between 13.34 and 14.00 ppm for the aromatic OH proton. Disappearance of this signal indicates a coordination of the phenolic O atom to the metal center. The sharp signal of the imine proton in the ligand (8.14 to 8.39 ppm) is shifted to higher field and appears between 8.09 and 8.38 ppm upon complexation. Mass spectrometry as well as elemental analyses confirmed the formulation of complexes 1−5 as [MoO2(LX)2]. In principle, the design of the ligand would allow the formation of several isomers in solution, with respect to the ligand trans to the terminal oxygen ligands. Both ligands can coordinate via the phenolic O atom and the imine N atom either in a symmetric way (N,N and O,O isomer) or in an asymmetric way (N,O isomer) as shown in Figure 2. For complexes 2 and 4 only one isomer could be observed in solution. NMR spectra show one set of sharp resonances for just one type of ligand, indicating a symmetric coordination of the ligand to the metal center. Formation of the N,N isomer (Figure 2) is probably favored over the O,O isomer because the steric clash of the substituents on the aromatic rings is avoided. On the other hand, 1H NMR spectra of complexes 1, 3, and 5 show together with a set of sharp resonances, a set of broad resonances for another coordination environment. Low temperature NMR measurements of complexes 1 and 3 resolve the broad signals and indicate the formation of a symmetric (major isomer) and an asymmetric isomer (minor isomer) in solution in a 4:1 (complex 1) and a 2:1 (complex 3) ratio. Figure 3 shows a comparison of the aromatic region of complex 3 at 25 °C and −35 °C. The formation of a symmetric isomer S (N,N or O,O isomer) is indicated by the existence of one set of
pendant donor functionality and their highly selective catalytic epoxidation behavior toward various olefins.
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RESULTS AND DISCUSSION Synthesis of the Ligands. Syntheses of ligands HL1 to HL5 follow a single step procedure as described for ligand HL3 in the literature.22 Condensation of aromatic aldehydes with the appropriate secondary amines in methanol at room temperature results in the formation of the Schiff base ligands as yellow viscous oils (HL1−HL4) or solid (HL5) in quantitative yields (see Figure 1). All ligands were characterized by common
Figure 1. Ligands HL1−HL5 employed in this study. The ligands HL3 and HL5 were previously described in the literature.22,23
spectroscopic techniques and used without further purification. The proton of the imine group (Ar−CHN) at the aromatic ring is indicated by a single peak in the region between 8.14 and 8.39 ppm in the 1 H NMR spectra. The resonance of the corresponding C atom (Ar-CHN) is found between 166 and 168 ppm in the 13C NMR spectra. IR measurements show a strong absorption between 1631 cm−1 and 1634 cm−1 attributed to the stretching vibration of the νCN group, which is in good agreement with the literature.22,23 Mass spectrometry confirmed the formation of the expected ligands. Ligands HL3 and HL5 were previously described in the literature and have been used for the syntheses of several metal complexes.22−27 Synthesis of Molybdenum(VI) Dioxo Complexes. Molybdenum(VI) dioxo complexes [MoO2(LX)2] (X = 1−5) are readily accessible by reaction of [MoO2(η2-tBu2pz)2]17 with 2 equiv of the ligand in dry toluene at room temperature. The pyrazolate ligands are easily displaced by two Schiff base ligands HLX (X = 1−5), leading to disubstituted complexes in moderate to good yields (Scheme 1). All ligands coordinate via the Scheme 1. Synthetic Procedure for the Preparation of Disubstituted cis-[MoO2(LX)2] Complexes 1−5
phenolic O atom and the imine N atom to the metal center, the third donor atom (R2 = OMe or NMe2) in complexes 1−4 9957
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Scheme 2
mode of the cis-[MoO2]2+ fragment.14,29,30 The absence of a broad band around 3250 cm−1 in the spectra of the molybdenum complexes compared to the Schiff base ligands indicates the coordination of the phenolic oxygen atom after deprotonation. The characteristic stretching frequencies of the νCN band in the free ligand (1631−1634 cm−1) are shifted to lower wave numbers upon coordination of the imine nitrogen to the metal center and appears at 1625−1628 cm−1.7,8,29 Molecular Structure in the Solid State. Single crystals suitable for X-ray diffraction analysis of complex 1 and 3 were obtained from concentrated solutions in methanol at room temperature. Complexes 1 and 3 crystallized in the monoclinic space group Cc (1) and C2 (3) in the form of light yellow parallelepipeds (1) or yellow tablets (3). Molecular views of both compounds are shown in Figure 4. Selected bond lengths and angles are given in Table 1 and crystallographic data in Table 2. The X-ray structures of complexes 1 and 3 are very similar and confirm the formation of the symmetric N,N isomer (Figure 2). Both compounds exhibit a six-coordinate Mo atom in a distorted octahedral geometry. The metal center is ligated by two terminal oxygen atoms and two Schiff base ligands, each of them coordinating via the phenolic oxygen atom and the imine N atom to the metal center. The third donor atom in the side chain (R2 = OMe in 1 and R2 = NMe2 in 3) is not involved in coordination and hence the arm remains pendant. The molybdenum oxo groups show the expected mutual cis configuration and are located trans to the imine N atoms. All Mo−O bond lengths [1.9556(12) Å and 1.9611(11) Å for 1 and 1.942(4) Å for 3] as well as all MoO bond lengths [1.7013(11) Å and 1.7059(11) Å for 1 and 1.714(4) Å for 3] are in the expected range of cis[MoO2]2+ complexes. The Mo−N bonds [2.3303(12) Å and 2.37454(12) Å for 1 and 2.334(5) Å for 3] are somewhat longer because of the influence of the trans MoO ligand.7,14,29
Figure 2. Possible symmetric and asymmetric isomers in solution.
resonances for both coordinated ligands. The broad signals resolve into two sets of resonances of equal intensity at low temperature (e.g., two signals for the imine groups at 8.09 and 8.36 ppm) and can thus be assigned to the asymmetric isomer AS (N,O isomer). Molecular structures determined by X-ray diffraction analyses of complexes 1 and 3 reveal exclusively the symmetric N,N isomer. In several attempts we dissolved such single crystals in benzene-d6. Their 1H NMR spectra show always the two isomers in solution in the same ratio as in the bulk material, pointing to a dynamic equilibrium in solution. The three compounds 1, 3, and 5 that are found in the two isomeric forms (N,N and N,O isomer) in solution represent derivatives with two sterically demanding tBu groups in o- and p-positions of the phenolic ring. From a steric point of view 1, 3, and 5 are expected to form exclusively the N,N isomer. On the other hand, from an electronic point of view, the O,O isomer with the more electronegative phenolic oxygen coordinated trans to the metal oxo bond is predicted to be favored. We exclusively find such a bonding situation in [MoO2(L)2] complexes where L represents ß-ketiminates.19,20 For this reason, we attribute the occurrence of two isomers in complexes 1, 3, and 5 to the higher electron-donating capacity of the tBu groups, rendering the phenolic oxygen more nucleophilic so, at least partially, overruling the steric hindrance. The IR spectra of all [MoO2(LX)2] complexes 1−5 exhibit two strong νMoO bands in the region 900−904 cm−1 and 914−928 cm−1, characteristic for the symmetric and asymmetric stretching
Figure 3. 1H NMR spectra of complex 3 in chloroform-d at 25 °C (top) and −35 °C (bottom). The asterisk (*) denotes residual free ligand. S corresponds to the symmetric isomer (N,N or O,O isomer), and AS corresponds to the asymmetric N,O isomer shown in Figure 2. 9958
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Figure 4. Molecular structure and atom labeling scheme for complexes 1 (top) and 3 (bottom). Thermal ellipsoids have been drawn at 50% probability level. Hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) of Complexes 1 and 3 Complex 1
a
Mo1O1 Mo1O2
1.7013(11) 1.7059(11)
Mo1O3 Mo1O4
1.9556(12) 1.9611(11)
Mo1N1 Mo1N2
2.3303(12) 2.3754(12)
O1−Mo1O2 O1−Mo1O3 O1−Mo1O4 O2−Mo1O3 O2−Mo1O4
106.15(6) 93.77(5) 97.62(5) 98.35(5) 95.98(5)
158.44(4) 86.58(5) 166.75(5) 84.17(5) 78.33(5)
O1−Mo1N2 O2−Mo1N2 O3−Mo1N2 O4−Mo1N2 N2−Mo1N1
164.36(5) 89.00(5) 79.90(5) 84.35(5) 78.60(4)
Mo1O1
1.714(4)
O3−Mo1O4 O1−Mo1N1 O2−Mo1N1 O3−Mo1N1 O4−Mo1N1 Complex 3a Mo1O2
1.942(4)
Mo1N1
2.334(5)
O1−Mo1O1′ O1−Mo1O2 O1−Mo1O2′
107.0(3) 94.96(19) 96.18(19)
O2−Mo1O2′ O1−Mo1N1′ O1−Mo1N1
161.2(2) 90.1(2) 162.8(2)
O2−Mo1N1 O2−Mo1N1′ N1−Mo1N1′
80.23(16) 84.70(16) 73.1(3)
Symmetry equivalent atoms are generated by the symmetry operator 1 − x, y, 2 − z.
Epoxidation of Alkenes. Complexes 1−5 have been tested as catalysts in the epoxidation of several internal and
terminal alkenes using tert-butyl hydroperoxide (TBHP, 5.5 M in decane) as oxygen source. Optimal reaction conditions regarding 9959
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Table 2. Crystallographic Data and Structure Refinement for Complexes 1 and 3 1 empirical formula formula weight crystal description crystal size (mm) crystal system, space group unit cell dimensions
volume (Å3) Z, calculated density (g cm−3) F(000) linear absorption coefficient μ (mm−1) absorption correction temperature wavelength (MoKα) θ range for data collection limiting indices
reflections collected/unique completeness to θ max. refinement method data/restraints/parameters goodness-of-fit on F2 final R1,a wR2b [I > 2σ(I)] R indices (all data) largest diff. peak and hole (e Å−3) CCDC deposition no. a
Table 3. Epoxidation of Cyclooctene Catalyzed by 1: Effect of Cosolvent and Temperature conversion (%)
3
MoO6N2C36H56 708.77 parallelepiped, light yellow 0.70 × 0.31 × 0.25 monoclinic, Cc a = 31.098(3) Å b = 10.1937(8) Å c = 11.9756(9) Å α = 90° β = 107.044(2)° γ = 90° 3629.5(5) 4, 1.297 1504.0 0.406
MoO4N4C38H62 734.86 tablet, yellow 0.69 × 0.15 × 0.07 monoclinic, C2 a = 19.606(2) Å b = 6.8112(7) Å c = 15.1333(16) Å α = 90° β = 98.025(4)° γ = 90° 2001.1(4) 2, 1.223 788.0 0.368
multiscan 100(2) K 0.71073 Å 2.11 to 30.00° −42 ≤ h ≤ 43 −14 ≤ k ≤ 14 −16 ≤ l ≤ 16 36836/9485 [R(int) = 0.0233] 0.999 full matrix leastsquares on F2 9485/2/421 1.034 R1 = 0.0221 wR2 = 0.0576 R1 = 0.0225 wR2 = 0.0579 1.033 and −0.576 837295
multiscan 100(2) K 0.71073 Å 2.10 to 26.09° −24 ≤ h ≤ 0 −8 ≤ k ≤ 0 −18 ≤ l ≤ 18 58693/2133 [R(int) = 0.0967] 0.983 full matrix leastsquares on F2 2133/1/211 1.085 R1 = 0.0478 wR2 = 0.1288 R2 = 0.0496 wR2 = 0.1305 2.254 and −1.458 837296
cosolventa
35 °C
CH2Cl2 CHCl3 C2H4Cl2 heptane toluene TBHP/decaneb TBHP/H2Ob diethylether tBuOH or MeOH
80
50 °C
75 °C
93 88 96 97
57 63 95
