Investigation of Molybdenum Tetracarbonyl Complexes As Precursors

Organometallics 2010, 29, 883–892 DOI: 10.1021/om900948r

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Investigation of Molybdenum Tetracarbonyl Complexes As Precursors to MoVI Catalysts for the Epoxidation of Olefins Tatiana R. Amarante, Patrı´ cia Neves, Ana C. Coelho, Sandra Gago, Anabela A. Valente, Filipe A. Almeida Paz, Martyn Pillinger, and Isabel S. Gonc-alves* Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal Received October 29, 2009

The one-dimensional organic-inorganic hybrid material [MoO3(bipy)] (3) (bipy = 2,20 -bipyridine) is obtained rapidly and in quantitative yield by the reaction of the complex cis-[Mo(CO)4(bipy)] (1) with excess tert-butylhydroperoxide (TBHP) in n-decane/dichloromethane at room temperature. A similar oxidative decarbonylation of the complex cis-[Mo(CO)4(di-t-Bu-bipy)] (2) (di-t-Bu-bipy = 4,40 -di-tert-butyl-2,20 -bipyridine) leads to the isolation of the polynuclear complex [Mo8O24(di-t-Bubipy)4] (4). The structure of 4, as the CH2Cl2 solvate, was determined by X-ray crystallography. The unit cell contains two crystallographically independent octameric windmill-type complexes, formulated as [Mo8O24(di-t-Bu-bipy)4], both of which contain a central cubane-type Mo4(μ3-O)4 core. Four peripheral [MoO2(di-t-Bu-bipy)]2þ units cap the long edges of the Mo4 tetrahedron of the central cubane. The close packing of these complexes via weak offset π-π contacts involving the organic ligands leads to a structure having large channels (occupied by solvent molecules) running in various directions of the unit cell. Compounds 3 and 4 can be used as the basis for active catalytic systems for the liquid-phase epoxidation of cis-cyclooctene with TBHP as the oxidant, giving the corresponding epoxide as the only product. Notably higher activities, with no change in selectivity, are possible by using microwave-assisted heating instead of conventional oil bath heating and/or by increasing the reaction temperature from 55 °C to 75 °C. The excellent stability of these MoVI catalytic systems was confirmed by carrying out six consecutive reaction runs at 75 °C under microwave-assisted heating. The stable parent carbonyls (1 and 2) can be used as catalyst precursors since they are transformed into 3 and 4 under the operating catalytic conditions.

Transition metal carbonyl complexes play an essential role in modern organometallic chemistry and are used extensively as catalysts or precatalysts for a wide range of organic transformations.1 Molybdenum-catalyzed reactions are economically attractive because it is a relatively inexpensive element.2 In the Arco-Lyondell process for the epoxidation of propene, Mo(CO)6 is used as a precursor, being oxidized in situ by the oxidant tert-butylhydroperoxide (TBHP) to a cis-dioxo MoVI complex.3 In recent years considerable research has been carried out on the use of cyclopentadienyl molybdenum carbonyls of the type Cp0 Mo(CO)3X [Cp0 = η5-C5R5 (R = H, CH3, CH2Ph); X = Cl, alkyl] as

precursors to efficient Cp0 MoO2X catalysts for the liquidphase epoxidation of olefins.1f,4 The tricarbonyls are available effortlessly from Mo(CO)6. Furthermore, they serve as excellent precursors because they can be stored for long periods, especially when compared with the more sensitive oxo complexes that are formed by in situ oxidative decarbonylation. Tetracarbonyl complexes of the type cis-[Mo(CO)4(L)n], where L is a monodentate or bidentate organic donor ligand, are another family of potentially interesting precatalysts for olefin epoxidation. These compounds are usually prepared directly from Mo(CO)6 by thermal or photochemical activation5 or by treating cis-[Mo(CO)4(pip)2] (pip = piperidine) with a refluxing solution of the ligand L in an organic

*To whom correspondence should be addressed. Tel: þ351 234 378190. Fax: þ351 234 370084. E-mail: [email protected]. (1) (a) van Leeuwen, P. W. N. M. Homogeneous Catalysis: Understanding the Art; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2004. (b) Torrent, M.; Sola, M.; Frenking, G. Chem. Rev. 2000, 100, 439– 493. (c) Xu, Q. Coord. Chem. Rev. 2002, 231, 83–108. (d) Ellis, J. E. Organometallics 2003, 22, 3322–3338. (e) Dyson, P. J. Coord. Chem. Rev. 2004, 248, 2443–2458. (f) Freund, C.; Abrantes, M.; K€uhn, F. E. J. Organomet. Chem. 2006, 691, 3718–3729. (g) Esswein, A. J.; Nocera, D. G. Chem. Rev. 2007, 107, 4022–4047. (h) Hebrard, F.; Kalck, P. Chem. Rev. 2009, 109, 4272–4282. (2) Kimmich, B. F. M.; Fagan, P. J.; Hauptman, E.; Marshall, W. J.; Bullock, R. M. Organometallics 2005, 24, 6220–6229. (3) Bregeault, J.-M. J. Chem. Soc., Dalton Trans. 2003, 3289–3302.

(4) (a) Abrantes, M.; Santos, A. M.; Mink, J.; K€ uhn, F. E.; Rom~ao, C. C. Organometallics 2003, 22, 2112–2118. (b) Zhao, J.; Santos, A. M.; Herdtweck, E.; K€uhn, F. E. J. Mol. Catal. A: Chem. 2004, 222, 265–271. (c) Valente, A. A.; Seixas, J. D.; Gonc-alves, I. S.; Abrantes, M.; Pillinger, M.; Rom~ao, C. C. Catal. Lett. 2005, 101, 127–130. (d) Capape, A.; Raith, A.; K€uhn, F. E. Adv. Synth. Catal. 2009, 351, 66–70. (e) Al-Ajlouni, A. M.; Veljanovski, D.; Capape, A.; Zhao, J.; Herdtweck, E.; Calhorda, M. J.; K€uhn, F. E. Organometallics 2009, 28, 639–645. (f) Abrantes, M.; Paz, F. A. A.; Valente, A. A.; Pereira, C. C. L.; Gago, S.; Rodrigues, A. E.; Klinowski, J.; Pillinger, M.; Gonc-alves, I. S. J. Organomet. Chem. 2009, 694, 1826–1833. (5) (a) Manuta, D. M.; Lees, A. J. Inorg. Chem. 1986, 25, 1354–1359. Braga, S. S.; Rodrigues, S. S.; Pereira, C. C. L.; Gonc-alves, (b) Petrovski, Z.; I. S.; Pillinger, M.; Freire, C.; Rom~ao, C. C. New J. Chem. 2005, 29, 347– 354.

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Introduction

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Organometallics, Vol. 29, No. 4, 2010

solvent.6 Conventional thermal heating of reaction mixtures containing Mo(CO)6 are handicapped by the sublimation and deposition of the metal hexacarbonyl on the reflux condenser, and this deposit must be mechanically returned to the reaction flask.7 Sublimation of Mo(CO)6 is not a problem when the reactions are carried out using microwaveassisted heating, either in an open reflux system (with a modified conventional MW oven)8 or in sealed Teflon vessels (with a commercial MW oven).9 Microwave-assisted synthesis (MAS) of molybdenum tetracarbonyl complexes frequently leads to a reduction in reaction times and an increase in yields over those typically observed using conventional thermal heating. The rate enhancement results from the coupling of solvents with high dielectric loss tangents with the microwave’s irradiation, resulting in superheating, thereby leading directly to reaction acceleration. In the present work, MW-assisted heating has been applied for the synthesis of two molybdenum tetracarbonyl complexes bearing 2,20 -bipyridine ligands and for olefin epoxidation reactions carried out using the tetracarbonyl complexes as precatalysts. For comparison, the catalytic reactions were also carried out using conventional oil bath (OB) heating. With the aim of obtaining a better understanding of the Mo species formed under the reaction conditions used, the reaction of the complexes with an excess of the oxidant was performed separately and led to the isolation of a one-dimensional organic-inorganic hybrid material in the case of 2,20 bipyridine (bipy) and a cubane-based complex in the case of 4,40 -di-tert-butyl-2,20 -bipyridine (di-t-Bu-bipy). The singlecrystal X-ray structure of the latter complex is described.

Results and Discussion Synthesis and Characterization. The complexes [Mo(CO)4(bipy)] (1) and [Mo(CO)4(di-t-Bu-bipy)] (2) were obtained in excellent yields by the microwave-accelerated reaction of Mo(CO)6 with one equivalent of the organic ligand in toluene at 110 °C (Scheme 1). Oxidative decarbonylation of 1 and 2 was carried out by the dropwise addition of 5-6 M TBHP (10 equiv) in n-decane to a suspension of each complex in CH2Cl2. Upon stirring at room temperature, the mixtures turned from red to yellow or very pale yellow within 20 min. In the case of 1 a white solid precipitated, which was filtered, washed several times with diethyl ether, and vacuumdried. Characterization of the white solid by elemental analysis, FT-IR and Raman spectroscopy, and X-ray powder diffraction (XRPD) showed it to be the known organicinorganic hybrid compound [MoO3(bipy)] (3), which was first reported by Zubieta and co-workers (Scheme 1).10 Thus, the IR spectrum does not contain the four active ν(CtO) normal modes exhibited by 1 in the 1810-2010 cm-1 range. In addition to bands due to bipy, strong bands at 622, 882, and (6) (a) Darensbourg, D. J.; Kump, R. L. Inorg. Chem. 1978, 17, 2680– 2682. (b) Braunstein, P.; Taquet, J.-p.; Siri, O.; Welter, R. Angew. Chem., Int. Ed. 2004, 43, 5922–5925. (7) Nicholls, B.; Whiting, M. C. J. Chem. Soc. 1959, 551–556. (8) (a) Ardon, M.; Hayes, P. D.; Hogarth, G. J. Chem. Educ. 2002, 79, 1249–1251. (b) Ardon, M.; Hogarth, G.; Oscroft, D. T. W. J. Organomet. Chem. 2004, 689, 2429–2435. (9) (a) VanAtta, S. L.; Duclos, B. A.; Green, D. B. Organometallics 2000, 19, 2397–2399. (b) Coelho, A. C.; Almeida Paz, F. A.; Klinowski, J.; Pillinger, M.; Gonc-alves, I. S. Molecules 2006, 11, 940–952. (c) Braga, S. S.; Coelho, A. C.; Gonc-alves, I. S.; Almeida Paz, F. A. Acta Crystallogr., Sect. E: Struct. Rep. Online 2007, E63, m780–m782. (10) Zapf, P. J.; Haushalter, R. C.; Zubieta, J. Chem. Mater. 1997, 9, 2019–2024.

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Figure 1. View of the One-Dimensional Chains of [MoO 3(bipy)] (3).10 Scheme 1. Synthesis of Compounds 1-4

914 cm-1 are observed and are assigned to Mo-O vibrations. The corresponding tungsten analogue, [WO3(bipy)], exhibits a similar set of bands at 617, 886, and 934 cm-1.11 The Raman spectrum of 3 shows strong bands at 916, 1023, 1319, and 1598, together with medium and weak bands at 770, 873, 1060, 1160, 1268, 1494, and 1566 cm-1, in agreement with those exhibited by the tungsten compound. The observed XRPD pattern of 3 is in excellent agreement with the simulated pattern calculated from the cell parameters and atomic coordinates reported in ref 10 for the molybdenum compound (Figure S1 in the Supporting Information), indicating that the bulk product is monophasic and isostructural with the compound reported by Zubieta and coworkers. The structure consists of one-dimensional chains of corner-sharing distorted {MoO4N2} octahedra (Figure 1). To the best of our knowledge, the synthesis of the molybdenum and tungsten compounds has only previously been accomplished by the hydrothermal treatment (at 160 °C for 2-4 days in Teflon-lined acid digestion bombs) of mixtures containing MoO3 or H2WO4, bipy, and water. In the case of [MoO3(bipy)], the hydrothermal method suffers from a low yield (10%) and the concomitant formation of the 3:2 hybrid [Mo3O9(bipy)2]. The isolation of microcrystalline 3 in quantitative yield by the oxidative decarbonylation of [Mo(CO)4(bipy)] (1) at room temperature is therefore remarkable. The oxidative decarbonylation of 2 with TBHP gave a yellow solution. Manganese dioxide was added to destroy any excess TBHP, the mixture was filtered, and a white solid was isolated after the solution was concentrated and mixed with diethyl ether. As described below, a single-crystal X-ray diffraction analysis identified the product as [Mo8O24(di-t-Bubipy)4] (4) with a structure containing a central Mo4(μ3-O)4 cubane. In addition to bands due to di-t-Bu-bipy, the IR (11) Twu, J.; Fang, T.-H.; Hsu, C.-F.; Yu, Y.-Y.; Wang, G.-J.; Tang, C.-W.; Chen, K.-H.; Lii, K.-H. J. Mater. Chem. 1998, 8, 2181–2184.

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Figure 2. Schematic representations of the [Mo8O24(di-t-Bu-bipy)4] molecular unit present in compound 4 (top and side views) formed by the Mo1 and Mo2 metal centers. The two MoVI coordination environments are depicted in the top right, with atoms composing the asymmetric unit being represented as thermal ellipsoids drawn at the 50% probability level and hydrogen atoms as spheres with arbitrary radii. The atomic labeling scheme is provided for all atoms composing the MoVI coordination spheres. Selected bond lengths and angles are given in Tables 2 and 3. Symmetry transformations used to generate equivalent atoms: (i) 1/4-y, 1/4þx, 1/4-z; (ii) -1/4þy, 1/4-x, 1/4-z.

formula fw temp/K cryst syst space group a = b/A˚ c/A˚ R = β = γ/deg volume/A˚3 Z Dc/g cm-3 μ(Mo KR)/mm-1 cryst size/mm crystal type θ range index ranges

C82H116Cl20Mo8N8O24 3074.35 150(2) tetragonal I41/a 27.5771(6) 31.4506(8) 90 23918.1(10) 8 1.708 1.322 0.24 0.18 0.16 colorless prism 3.55 to 29.13 -36 e h e 36, -37 e k e 37, -43 e l e 43 523 071 16 062 (Rint = 0.0675) to θ = 29.13°, 99.8% 16 062/652 R1 = 0.0865, wR2 = 0.1833 R1 = 0.1724, wR2 = 0.2563 m = 0.0530, n = 1240.2795 3.571 and -2.112

reflns collected indep reflns data completeness data/params final R indices [I > 2σ(I)]a,b final R indices (all data)a,b weighting schemec largest diff peak and hole/e A˚-3 P P P P a R1 = Fo|-|Fc / |Fo|. bwR2 = { w[(Fo2 - Fc2)]/ [w(Fo2)2]}1/2. c w = 1/[σ2(Fo2) þ (mP)2 þ nP] where P = (Fo2 þ 2Fc2)/3. )

(12) Nishikawa, K.; Kido, K.; Yoshida, J.; Nishioka, T.; Kinoshita, I.; Breedlove, B. K.; Hayashi, Y.; Uehara, A.; Isobe, K. Appl. Organomet. Chem. 2003, 17, 446–448. (13) S€ uss-Fink, G.; Plasseraud, L.; Ferrand, V.; Stanislas, S.; Neels, A.; Stoeckli-Evans, H.; Henry, M.; Laurenczy, G.; Roulet, R. Polyhedron 1998, 17, 2817–2827. (14) (a) Allen, F. H. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 380–388. (b) Allen, F. H.; Motherwell, W. D. S. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 407–422.

Table 1. Crystal and Structure Refinement Data for [Mo8O24(di-t-Bu-bipy)4] 3 10CH2Cl2 (4)

)

spectrum of 4 exhibits a pair of strong bands at 902 and 939 cm-1 (cf. 901 and 935 cm-1 in the Raman spectrum), due to ν(ModO) vibrations, and several overlapping bands between 700 and 850 cm-1, assigned to stretching vibrations of the Mo-O (bridging) bonds. X-ray Crystallography. Single crystals of 4 were obtained by recrystallization from CH2Cl2/diethyl ether. The compound crystallized as a dichloromethane solvate, [Mo8O24(di-t-Bu-bipy)4] 3 10CH2Cl2 (Table 1). The crystal structure contains two crystallographically independent octameric windmill-type complexes (Figures 2 and 3). Both complexes contain a central cubane-type Mo4(μ3-O)4 core in which each Mo atom is further bonded to three oxygen atoms, resulting in the oxide cluster [Mo4(μ3-O)4O12]. Oxide clusters of this type are known to appear either as triple-cubane or windmilltype isomers.12,13 A search in the literature and in version 5.30 (November 2008 with four updates) of the Cambridge Structural Database14 for similar structural motifs indicates that 4 is, to the best of our knowledge, unprecedented. Even though windmill-type isomers structurally similar to those of 4 have been reported for a handful of organometallic and

transition metal complexes, none of them contain only one type of metal center, as is the case for 4. Examples of structurally similar organometallic complexes are those

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Figure 3. Schematic representations of the [Mo8O24(di-t-Bu-bipy)4] molecular unit present in compound 4 (top and side views) formed by the Mo3 and Mo4 metal centers. The two MoVI coordination environments are depicted in the top right, with atoms composing the asymmetric unit being represented as thermal ellipsoids drawn at the 50% probability level and hydrogen atoms as spheres with arbitrary radii. The atomic labeling scheme is provided for all atoms composing the MoVI coordination spheres. Selected bond lengths and angles are given in Tables 2 and 3. Symmetry transformations used to generate equivalent atoms: (iv) 3/4-y, 3/4þx, 3/4-z; (v) -3/4þy, 3/4-x, 3/4-z.

reported by the groups of Isobe, [(Cp*Rh)4W4O16],12 and S€ uss-Fink, [(η6-p-MeC6H4iPr)4Ru4Mo4O16],13 while the relevant transition metal complexes originate only from the groups of Michelsen and Glerup and contain a mixture of MVI and CrIII centers, {[WO4Cr(cyclam)]4}(ClO4)4 3 3H2O15 and {[MoO4Cr(bispictn)]4}(ClO4)4 3 3H2O,16 where cyclam = 1,4,8,11-tetraazacyclotetradecane and bispictn = N,N0 -bis(2-bipyridylmethyl)-1,3-propanediammine. The center of gravity of each [Mo8O24(di-t-Bu-bipy)4] octameric windmill-type complex is located at the 4-fold inversion axis of space group I41/a. Thus, only one-fourth of each complex belongs to the asymmetric unit of 4, and the octameric complexes exhibit 4-fold inversion symmetry around their centers of gravity. Mo1 and Mo3 compose the central [Mo4(μ3-O)4]8þ cubanes of the two complexes, with the μ3-bridging oxygen atoms being O2 and O9. The intracubane Mo 3 3 3 Mo distances vary between 3.1233(8) and 3.4342(8) A˚ (both belonging to the octameric complex represented in Figure 2) and are in good agreement with those found in related compounds containing MoVI centers with nonbonding Mo 3 3 3 Mo interactions.13,16,17 For comparison, MoV cubane complexes present Mo-Mo distances of about 2.6 A˚, corresponding to a Mo-Mo single bond.18 As expected, Mo1 and Mo3 possess highly distorted {MoO6} octahedral coordination geometries, with the Mo 3 3 3 O bonds being distributed into three distinct families (Table 2): ModO interactions with bond lengths ranging from 1.695(6) (15) Glerup, J.; Hazell, A.; Michelsen, K.; Weihe, H. Acta Chem. Scand. 1994, 48, 618–627. (16) Glerup, J.; Hazell, A.; Michelsen, K. Acta Chem. Scand. 1991, 45, 1025–1031. (17) Hill, L. M. R.; Abrahams, B. F.; Young, C. G. Chem.;Eur. J. 2008, 14, 2805–2810. (18) Jimtaisong, A.; Feng, L.; Sreehari, S.; Bayse, C. A.; Luck, R. L. J. Clust. Sci. 2008, 19, 181-195, and references therein.

Table 2. Selected Bond Distances (A˚) for the Molybdenum Coordination Environments Present in [Mo8O24(di-t-Bu-bipy)4] 3 10CH2Cl2 (4)a Mo1-O1 Mo1-O2 Mo1-O2i Mo1-O2ii Mo1-O3 Mo1-O4 Mo2-O1i Mo2-O4 Mo2-O5 Mo2-O6 Mo2-N1 Mo2-N2

1.960(6) 2.310(6) 1.915(6) 2.152(6) 1.698(6) 1.777(6) 1.837(6) 2.163(6) 1.690(8) 1.720(7) 2.322(10) 2.246(8)

Mo3-O7 Mo3-O8 Mo3-O9 Mo3-O9iv Mo3-O9v Mo3-O10 Mo4-O8v Mo4-O10 Mo4-O11 Mo4-O12 Mo4-N3 Mo4-N4

1.695(6) 1.778(5) 2.329(6) 1.898(5) 2.147(5) 1.960(6) 2.175(6) 1.840(6) 1.705(6) 1.723(6) 2.280(7) 2.239(7)

a Symmetry transformation used to generate equivalent atoms: (i) 1/4-y, 1/4þx, 1/4-z; (ii) -1/4þy, 1/4-x, 1/4-z; (iv) 3/4-y, 3/4þx, 3 /4-z; (v) -3/4þy, 3/4-x, 3/4-z.

to 1.778(5) A˚, Mo-Ob interactions found in the 1.898(5)1.960(6) A˚ range (Ob = double or triple bridging oxygen atom cis to both ModO), and Mo-Oc interactions found in the 2.147(5)-2.329(6) A˚ range (Oc = triple bridging oxygen atom trans to ModO). The markedly distinct bond distances registered for the ModO interactions arise from the bridges with the peripheral Mo2 and Mo4 metal centers, which weaken the Mo1dO4 and Mo3dO8 interactions, ultimately resulting in a lengthening of the bonds. The cis and trans internal O-Mo-O octahedral angles of Mo1 and Mo3 are in the 72.4(2)-105.1(3)° and 153.0(2)-170.4(3)° ranges, respectively (Table 3), further pointing to the highly distorted nature of their coordination polyhedra. Mo2 and Mo4 compose two peripheral [MoO2(di-t-Bubipy)]2þ bridging entities, which cap the long edges of the Mo4 tetrahedron of the central [Mo4(μ3-O)4]8þ cubane cores (Figures 2 and 3). The coordination polyhedra of these metal

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Table 3. Selected Bond Angles (deg) for the Molybdenum Coordination Environments Present in [Mo8O24(di-t-Bubipy)4] 3 10CH2Cl2 (4)a O1-Mo1-O2 O1-Mo1-O2ii O2i -Mo1-O1 O2i -Mo1-O2 O2i -Mo1-O2ii O2ii -Mo1-O2 O3-Mo1-O1 O3-Mo1-O2 O3-Mo1-O2i O3-Mo1-O2ii O3-Mo1-O4 O4-Mo1-O1 O4-Mo1-O2 O4-Mo1-O2i O4-Mo1-O2ii O7-Mo3-O8 O7-Mo3-O9 O7-Mo3-O9iv O7-Mo3-O9v O7-Mo3-O10 O8-Mo3-O9 O8-Mo3-O9iv O8-Mo3-O9v O8-Mo3-O10 O9iv -Mo3-O9 O9v -Mo3-O9 O9iv -Mo3-O9v O9iv -Mo3-O10 O10-Mo3-O9 O10-Mo3-O9v

81.3(2) 84.0(2) 154.3(3) 77.3(3) 76.3(3) 73.0(2) 97.5(3) 170.4(3) 101.2(3) 97.5(3) 105.1(3) 94.4(3) 84.5(3) 97.5(3) 157.4(3) 105.1(3) 169.4(2) 100.8(2) 97.1(2) 97.4(3) 85.5(2) 98.8(2) 157.8(2) 95.5(2) 76.8(2) 72.4(2) 76.0(2) 153.0(2) 81.7(2) 82.1(2)

O1i -Mo2-O4 O1i -Mo2-N1 O1i -Mo2-N2 O4-Mo2-N1 O4-Mo2-N2 O5-Mo2-O1i O5-Mo2-O4 O5-Mo2-O6 O5-Mo2-N1 O5-Mo2-N2 O6-Mo2-O1i O6-Mo2-O4 O6-Mo2-N1 O6-Mo2-N2 N2-Mo2-N1 O8v -Mo4-N3 O8v -Mo4-N4 O10-Mo4-O8v O10-Mo4-N3 O10-Mo4-N4 O11-Mo4-O8v O11-Mo4-O10 O11-Mo4-O12 O11-Mo4-N3 O11-Mo4-N4 O12-Mo4-O8v O12-Mo4-O10 O12-Mo4-N3 O12-Mo4-N4 N4-Mo4-N3

84.1(2) 90.9(3) 155.8(3) 70.6(3) 75.9(3) 107.2(3) 93.7(3) 105.3(4) 154.9(3) 87.7(3) 102.2(3) 156.9(4) 87.0(4) 91.6(3) 69.9(3) 74.0(2) 79.9(2) 84.8(2) 89.9(3) 157.7(3) 159.1(3) 102.0(3) 105.2(3) 86.2(3) 87.4(3) 90.9(3) 108.7(3) 155.2(3) 87.8(3) 70.4(3)

a Symmetry transformation used to generate equivalent atoms: (i) 1/4-y, 1/4þx, 1/4-z; (ii) -1/4þy, 1/4-x, 1/4-z; (iv) 3/4-y, 3/4þx, 3/4-z; (v) -3/4þy, 3/4-x, 3/4-z.

centers are composed of two terminal oxo groups [Mo-O = 1.690(8)-1.723(6) A˚], two oxygen atoms establishing the connections to the cubane cores [Mo-O = 1.837(6)-1.840(6) and 2.163(6)-2.175(6) A˚], and two N atoms belonging to the N, N-chelated di-t-Bu-bipy groups [Mo-N = 2.239(7)-2.322(10) A˚], ultimately describing distorted {MoN2O4} octahedra. Indeed, as observed for the other metal centers, the cis [69.9(3)108.7(3)°] and trans [154.9(3)-159.1(3)°] internal O-Mo-O octahedral angles clearly reveal the distortion of the polyhedra (Table 3). However, the Mo2 and Mo4 polyhedra seem to be significantly less distorted than those of Mo1 and Mo3, since the trans octahedral angle amplitude (Δ) for the latter is 17.4°, while that for Mo2 and Mo4 is about 4 times smaller, ca. 4.2°. The Mo 3 3 3 Mo distances between peripheral and cubane MoVI centers are in the 3.666(1)-3.7157(12) A˚ range, while those separating only peripheral centers are 6.1770(8) A˚ (Figure 2) and 6.2332(12) A˚ (Figure 3). The two [Mo8O24(di-tBu-bipy)4] octameric windmilltype complexes can be envisaged as conformational isomers. Indeed, as described in the previous paragraphs, while the connections between chemical entities to form these complexes are very similar, some geometrical features differ. This can be inferred from Figure 4, which depicts an overlay of the two octameric complexes. While the geometries of the central [Mo4(μ3-O)4]8þ cubane cores are practically identical, the location and the relative tilting of the peripheral [MoO2(di-t-Bu-bipy)]2þ bridging entities are different in the two complexes. For example, the “kink” angles of O4 and O10 for the Mo1/Mo2 and Mo3/Mo4 pairs are 140.9(3)° and 149.4(3)°, respectively (Figures 2 and 3). Another striking difference concerns the prominent 39.7° dihedral angle subtended between the two average

Figure 4. Structural overlay of the two crystallographically independent [Mo8O24(di-t-Bu-bipy)4] octamers present in 4. Hydrogen atoms and t-Bu substituent groups have been omitted for clarity. Overlay criterion: the four MoVI metal centers composing the cubane-type [Mo4(μ3-O)4]8þ core, with final rms of 0.019. The schematic representation was created using Mercury.19

planes containing the N,N-chelated aromatic rings. In addition, for each crystallographically independent dit-Bu-bipy moiety, the mutual dihedral angles between the substituted pyridyl rings also differ. While for the Mo1/ Mo2 octamer it is ca. 14.0°, in the other moiety the rings are almost coplanar, with the analogous angle being only ca. 4.5°. This structural feature, which seems to be at the origin of the presence of the two octameric isomers, may be rationalized by taking into consideration the interactions between neighboring complexes. As represented in Figure 5, the anchoring between adjacent windmill complexes is promoted by weak offset π-π contacts involving di-t-Bu-bipy moieties. Because of the intrinsic geometries of the complexes and the presence of the bulky tert-butyl substituents, one can assume that the effectiveness of these offset π-π contacts is greatly diminished. The overall distortions depicted in Figure 4 seem, thus, to be a crystallographic attempt to boost these interactions. The close packing of the two crystallographically distinct windmill-type complexes leads to a structure having large channels running in various directions of the unit cell, with the most prominent being parallel to the c-axis (Figure 6). CH2Cl2 solvent molecules were located inside these channels. Catalytic Epoxidation Using Conventional Oil Bath Heating. The catalytic performances of compounds 1-4 were investigated in the liquid-phase epoxidation of cis-cyclooctene (Cy8) with TBHP (5-6 M in decane) at 55 °C using an oil bath (OB) for heating and 1,2-dichloroethane as a cosolvent. For all experiments the epoxide was the only reaction product. When the tetracarbonyl complexes 1 and 2 were (19) Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M.; Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 389–397.

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Figure 5. Schematic representation of the offset π-π interactions between neighboring crystallographically distinct [Mo8O24(di-t-Bu-bipy)4] molecular units in compound 4.

Amarante et al.

Figure 6. Crystal packing of [Mo8O24(di-t-Bu-bipy)4] 3 10CH2Cl2 (4) viewed in perspective along the (a) [001] and (b) [010] directions of the unit cell edge. For clarity the two crystallographically independent [Mo8O24(di-t-Bu-bipy)4] octamers are represented with two distinct colors and all hydrogen atoms have been omitted. CH2Cl2 solvent molecules are represented in mixed ball-and-stick and space-filling mode in order to emphasize their location inside the channels that are formed by the close packing of the individual octamers.

used as precatalysts, the reaction of Cy8 was relatively slow, especially for 1 (Figures 7 and 8). Conversions at 6/24 h were 17/46% for 1 and 52/82% for 2, whereas in the absence of a catalyst only 4% conversion was reached at 24 h. Under the reaction conditions used, complex 2 is completely soluble, while 1 is poorly soluble. This difference must be related with the presence of the tert-butyl substituents on the pyridine rings in the case of 2.20 In an attempt to assess the catalytic contribution of the solubilized fraction for 1, after 1 h reaction the solution was filtered off at the reaction temperature of 55 °C through a 0.2 μm PVDF w/GMF Whatman membrane and left to react for a further 4 h. In this timer interval the Cy8 conversions rose 11% without filtration and 5% with filtration, suggesting that the catalytic reaction is partially homogeneous (if not mostly homogeneous, since the amount of dissolved active species may increase after 1 h). Hence, solubility differences may partially account for the different reaction rates observed. On the other hand, the latter may also be due to differences in the nature of the active oxidizing species (see discussion below). In parallel with that observed for complexes 1 and 2, 3 is apparently mostly insoluble, whereas 4 is completely soluble in the reaction medium. A filtration step was carried out for 3 after a reaction time of 1 h, as described above for 1. Between 1 and 5 h the Cy8 conversions rose 28% without filtration and 11% with filtration, suggesting that the catalytic

Figure 7. Cyclooctene conversion as a function of time for 1 [OB-55 °C (b); MW-55 °C (2 run 1, 9 run 2)] and 3 [OB-55 °C (O); MW-55 °C (Δ run 1, 0 run 2); MW-75 °C (] run 1, run 2, * run 3, þ run 4, ; run 5, ] (dashed line) run 6)].

(20) Daniel, T.; Suzuki, N.; Tanaka, K.; Nakamura, A. J. Organomet. Chem. 1995, 505, 109–117.

reaction is partially homogeneous. A similar result was obtained when, instead of filtration, the solution was separated

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Figure 8. Cyclooctene conversion as a function of time for 2 [OB-55 °C (b); MW-55 °C (2 run 1, 9 run 2)] and 4 [OB-55 °C (O); MW-55 °C (Δ); MW-75 °C (] run 1, run 2, * run 3, þ run 4, ; run 5, ] (dashed line) run 6)].

by centrifugation of the reaction mixture (cooled to room temperature) at 14 000 rpm for 10 min. At the time of writing the precise nature of the dissolved Mo species are unknown, but are presumably monomeric or oligomeric complexes derived from the one-dimensional chains present in 3. The activity of the catalytic system formed using 3 is either comparable to or somewhat lower than that typically reported for monomeric [MoO2Cl2(L)] complexes (L = 4,40 dialkyl-2,20 -bipyridine) used as catalysts in the same reaction, under similar conditions.21 Conversions for reaction times longer than 30 min were higher for 3 and 4 than for the respective precursors 1 and 2 (Figures 7 and 8; for all experiments the number of moles of molybdenum in the reaction medium was constant). To get an insight into the nature of the metal species present in the reaction medium in the case of 1 and 3, the solids were separated from the reaction mixtures by filtration after 24 h, washed with n-hexane, and air-dried at room temperature to give samples designated as 1-cat and 3-cat. The ATR FT-IR spectra of 1-cat and 3-cat are similar to that of 3, indicating that 1 undergoes oxidative decarbonylation in situ to give 3 (Figure 9). Attempts to isolate metal-containing species from the reaction carried out using complex 2 as the (pre)catalyst were unsuccessful, although it seems reasonable to assume that the tetracarbonyl complex converts into 4 since the latter was actually prepared by the reaction of complex 2 with TBHP. After the catalytic run with 4 a small amount of solid was precipitated by addition of pentane. The FT-IR spectrum of this solid was identical to that for freshly prepared 4, which suggests that the complex retains its structural integrity under the catalytic reaction conditions. According to the literature, the reaction of TBHP with [MoO2Cl2(L)] complexes, where L is an aromatic N-heterocyclic ligand such as 2,20 -bipyridine or 2,20 -bipyrimidine, (21) Al-Ajlouni, A.; Valente, A. A.; Nunes, C. D.; Pillinger, M.; Santos, A. M.; Zhao, J.; Rom~ao, C. C.; Gonc-alves, I. S.; K€ uhn, F. E. Eur. J. Inorg. Chem. 2005, 1716–1723.

Figure 9. ATR FT-IR spectra of 1 and 3 before (1, 3) and after (1-cat, 3-cat) the catalytic epoxidation reactions carried out using the OB and MW methods at 55 or 75 °C.

leads to the formation of MoVI η1-alkylperoxo complexes.21-23 Subsequently, the olefin attacks the Mo-bound peroxo oxygen atom and the transfer of the oxygen atom to the olefin gives the epoxide, with tert-butyl alcohol formed as a byproduct. The in situ oxidation of complexes 1 and 2 into active oxidizing species is expected to involve more elementary steps (i.e., oxidative decarbonylation with formation of oxomolybdenum complexes) than those when starting directly from 3 and 4, which may partly explain the faster overall epoxidation reaction in the presence of the latter compounds. For complexes 1 and 2, the reaction of Cy8 with TBHP at 30 °C was very sluggish, giving 6% and 9% conversion at 24 h, respectively. When aqueous H2O2 was used instead of TBHP, at 55 °C, the reaction of Cy8 in the presence of 4 gave 6% conversion after 24 h. Catalytic Epoxidation Using Microwave-Assisted Heating. For the four compounds under investigation, an outstanding increase in reaction rate was observed when MW heating was used instead of OB heating. For the set point of 55 °C, conversions at 6 h using the OB/MW methods were 17%/ 71%, 52%/84%, 48%/95%, and 65%/97% for 1, 2, 3, and 4, respectively (Figures 7 and 8). These different reaction rates can be attributed to the faster heating achieved with the MW method. Second runs were performed for complexes 1 and 2 by recharging the microreactor with oxidant and olefin in amounts equal to those used in the first run (Figures 7 and 8). For each complex, the reaction is faster in the second run Herdtweck, E.; Prazeres, (22) K€ uhn, F. E.; Groarke, M.; Bencze, E.; A.; Santos, A. M.; Calhorda, M. J.; Rom~ao, C. C.; Gonc-alves, I. S.; Lopes, A. D.; Pillinger, M. Chem.;Eur. J. 2002, 8, 2370–2383. (23) Al-Ajlouni, A. M.; G€ unyar, A.; Zhou, M.-D.; Baxter, P. N. W.; K€ uhn, F. E. Eur. J. Inorg. Chem. 2009, 1019–1026.

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compared with the first one, and the kinetic curves of the second runs for 1 and 2 are roughly coincident with those of the first runs for 3 and 4, respectively. These results are consistent with the above discussion regarding the in situ oxidation of 1 into 3 and 2 into 4. Turnover frequencies (TOFs) for the MW method (calculated at 10 min reaction) were 26, 30, 68, and 88 mol molMo-1 h-1 for 1-4. The TOF values for 1 and 2 are quite low compared to cyclopentadienyl molybdenum carbonyls of the type Cp0 Mo(CO)3X [Cp0 = η5-C5R5 (R = H, CH3, CH2Ph); X = Cl, alkyl] used as precursors to Cp0 MoO2X catalysts for the liquid-phase epoxidation of olefins.1f,4 However, the cyclopentadienyl complexes tend to give inactive complexes after one or two catalytic runs, whereas 1 and 2 lead to quite stable compounds of type 3 and 4 (as further evidenced below). For complex 4 the metal centers in the [Mo4(μ3-O)4]8þ core and those of the peripheral [MoO2(di-t-Bu-bipy)]2þ groups may possess different intrinsic catalytic activities (or some may be inactive), and therefore the TOF calculated on an Mo basis and the effective catalytic activity may be different; the TOF is presented as a “net value” for comparing catalytic activities. For the catalytic reaction using 3, the solid was separated from the reaction medium as described above for 3-cat and the OB method, giving a sample designated as 3-catMW. The ATR FT-IR spectra of 3-cat and 3-catMW are similar (and similar to that of 3), suggesting that the heating method does not affect the nature of the metal species (Figure 9). A second reaction run of 6 h was carried out for 3 as described above for 1 and 2. The kinetic curves for the two runs are roughly coincident, giving ca. 96% epoxide yield at 6 h (Figure 7). Hence, it seems that the catalytic system formed using 3 is fairly stable under the reaction conditions used. Increasing the reaction temperature from 55 to 75 °C using the MW method led to an increase in TOF from 68 to 518 mol molMo-1 h-1 for 3 and from 88 to 472 mol molMo-1 h-1 for 4, and 100% conversion was reached within 1 h for both compounds (Figures 7 and 8). In the absence of a catalyst, only 7% conversion was reached after 6 h. The ATR FT-IR spectrum of the solid recovered from the reaction carried out in the presence of 3 at 75 °C is similar to that of 3, indicating that the latter is fairly stable at this reaction temperature (Figure 9). Six consecutive catalytic runs were performed for 3 and 4 at 75 °C using the MW method. For each system, the reaction rate decreased slightly from the first to the second run, but then remained roughly constant for the remaining runs (Figures 7 and 8). These results indicate that the catalytic activity of these systems can be considerably improved by using the MW method and increasing the reaction temperature, without affecting catalyst stability.

Concluding Remarks We have shown that tetracarbonyl complexes of the type [Mo(CO)4(L)], which are available effortlessly and in high yields by the microwave-accelerated reaction of Mo(CO)6 with the bidentate ligand (L), are well worth investigating as precursors to oxomolybdenum(VI) compounds for catalytic applications. Changing the nature of the ligand strongly influences the course of the oxidative decarbonylation reaction with tert-butylhydroperoxide. With L = bipy, the one-dimensional organic-inorganic hybrid material [MoO3(bipy)] is obtained in quantitative yield. This represents a

Amarante et al.

dramatic improvement on the previously reported hydrothermal method of preparation, paving the way to the catalytic application of this class of compounds. With L = di-t-Bu-bipy, the complex [Mo8O24(di-t-Bu-bipy)4] is obtained with a unique solid-state structure comprising two crystallographically distinct windmill-type isomers. As generally found with oxomolybdenum(VI) complexes bearing bipyridine ligands, e.g., the monomeric complex [MoO2Cl2(bipy)], the catalytic systems formed using [MoO3(bipy)] and [Mo8O24(di-t-Bu-bipy)4] exhibit high selectivity and moderate activity for the epoxidation of ciscyclooctene (used as a model substrate), although the catalytic activity can be considerably improved by using microwave-assisted heating and higher reaction temperature, without affecting catalyst stability. Still, it is known that with complexes like [MoO2Cl2(L)] or [Mo2O4(μ2-O)Cl2(L)n] bipyridine ligands are not the best choice, and better catalytic results can be obtained with other bidentate ligands such as substituted 1,4-diazabutadienes and pyrazolylpyridines. Ongoing work in our laboratory is therefore concerned with examining other examples of [Mo(CO)4(L)] complexes as precursors to oxomolybdenum(VI) catalysts.

Experimental Section Materials and Methods. Microanalyses for CHN were performed at the University of Aveiro. Transmission IR spectra were measured on a Mattson 7000 FT-IR spectrometer. Attenuated total reflectance (ATR) FT-IR spectra were measured on the same instrument equipped with a Specac Golden Gate Mk II ATR accessory having a diamond top-plate and KRS-5 focusing lenses. FT-Raman spectra were recorded on a Bruker RFS 100 spectrometer with a Nd:YAG coherent laser (λ = 1064 nm). The microwave-assisted syntheses and catalytic reactions were carried out in a Discover S-Class (CEM Corporation, USA) microwave oven, at 2.45 GHz, under stirring and simultaneous cooling with compressed air (20 psi) to prevent bulk overheating. A vertical focused IR sensor was used for temperature measurement. Anhydrous solvents (dichloromethane, toluene, and n-hexane), Mo(CO)6, 5-6 M tert-butylhydroperoxide (TBHP) in n-decane, 2,20 -bipyridine (bipy), and 4,40 -di-tert-butyl-2,20 -bipyridine (dit-Bu-bipy) were obtained from Aldrich and used as received. Preparation of [Mo(CO)4(bipy)] (1) and [Mo(CO)4(di-t-Bubipy)] (2). Glass vessels with a capacity of 35 mL were loaded with toluene (20 mL), Mo(CO)6 (0.50 g, 1.90 mmol), and ligand (1.90 mmol), and the mixture was heated in the microwave oven at 110 °C for 30 min. The reaction temperature was reached and maintained using a dynamic control mode in which the power (max. 150 W) was automatically adjusted on the basis of the temperature feedback. After cooling to room temperature, the mixture was transferred to a Schlenk tube, and the resultant dark yellow solid was washed with n-hexane (30 mL) and pentane (10 mL) and finally vacuum-dried. Yields were 0.62 g (90%) for 1 and 0.82 g (92%) for 2. Satisfactory elemental analyses were obtained for both compounds, and FT-IR and NMR spectra were in agreement with published data.5a,9c,24 [MoO3(bipy)] (3). A solution of 5-6 M TBHP in n-decane (9.9 mmol) was added dropwise to a magnetically stirred suspension of [Mo(CO)4(bipy)] (1) (0.36 g, 0.99 mmol) in dichloromethane (20 mL). A white solid precipitated, and the solution turned very pale yellow. After stirring the mixture for 4 h at room temperature, the solid was filtered, washed several times with diethyl ether, and vacuum-dried. Yield: 0.29 g, 98%. Anal. Calcd for C10H8MoN2O3 (300.12): C, 40.02; H, 2.69; N, 9.33. Found: C, 39.70, H, 2.77; N, 9.50. IR (KBr, cm-1): ν = 3078w, (24) (a) Stiddard, M. H. B. J. Chem. Soc. 1962, 4712–4715. (b) Connor, J. A.; Overton, C. J. Chem. Soc., Dalton Trans. 1982, 2397–2402.

Article 1597s, 1576w, 1494m 1474s, 1443vs, 1314m, 1246w, 1161m, 1109w, 1059w, 1023s, 914vs (νModO), 882vs (νModO), 760vs, 735w, 622vs (νMo-O-Mo), 442 m, 419w, 400w, 354w, 319m, 279 m. Raman (cm-1): ν = 3079, 1598, 1566, 1494, 1429, 1319, 1268, 1160, 1060, 1023, 916, 873, 770, 653, 633, 442, 360, 319, 258, 236, 192. [Mo8O24(di-t-Bu-bipy)4] (4). A solution of 5-6 M TBHP in n-decane (6.3 mmol) was added dropwise to a magnetically stirred suspension of [Mo(CO)4(di-t-Bu-bipy)] (2) (0.30 g, 0.63 mmol) in dichloromethane (20 mL), and the mixture was stirred at room temperature for 4 h. The initially dark red solution turned yellow during the first 20 min of reaction. After 4 h manganese dioxide was added, the pale yellow solution was filtered off and concentrated under vacuum, and diethyl ether was added to precipitate a white solid. The solid was washed three times with diethyl ether, recrystallized from CH2Cl2/diethyl ether, and vacuum-dried at 40 °C for 45 min. Yield: 0.12 g, 68%. Anal. Calcd for C72H96Mo8N8O24 (2225.09): C, 38.86; H, 4.35; N, 5.03. Found: C, 38.82; H, 4.62; N, 5.45. IR (KBr, cm-1): ν = 2964s, 2907w, 2870w, 1615vs, 1548s, 1480m, 1464sh, 1411vs, 1366m, 1304m, 1252s, 1202m, 1122m, 1080m, 1031m, 939s, 902vs, 848s, 800sh, 773s, 708s, 606s, 550m, 412m, 377m. Raman: (cm-1): ν = 3078, 2969, 2907, 1612, 1546, 1491, 1415, 1368, 1315, 1253, 1202, 1124, 1031, 935, 901, 853, 802, 719, 551, 367, 203. Catalysis. The liquid-phase catalytic epoxidation reactions were carried out under air and autogenous pressure in closed borosilicate (nearly microwave transparent) reaction vessels (5 mL capacity) equipped with magnetic stirrers, using two different heating methods: conventional with the aid of a thermostated (55 °C) oil bath (denoted OB) or microwave-assisted heating (denoted MW) at 55 or 75 °C. Typically, the reaction vessel was loaded with metal complex corresponding to 0.018 mmol of molybdenum, 1.8 mmol of olefin, 2.75 mmol of oxidant (5-6 M TBHP in decane), and 2 mL of 1,2-dichloroethane (DCE). The MW experiments were carried out using a fixed power control (FPC) operation mode, which allows setting the temperature and MW power input: the power is constant until the temperature set point is reached, and then the MW oven switches to a feedback loop to modulate the applied power in order to maintain a constant temperature. The MW power input was optimized to reach the desired temperature in 30-40 s, without temperature overshooting (60 W for 55 °C and 185 W for 75 °C). Zero time was taken as either the instant the FPC operation mode was initiated or the instant the reaction vessel was immersed in the OB. The course of the reactions was monitored using a Varian 3900 GC equipped with a capillary column (SPB-5, 20 m 0.25 mm) and a flame ionization detector. For quantification of the products, undecane was used as an internal standard added after the reaction. The reaction products were identified by GCMS (Trace GC 2000 Series (Thermo Quest CE Instruments)DSQ II (Thermo Scientific)) using He as the carrier gas. Sampling was performed using a single reactor: for the MW method the MW irradiation was interrupted for sampling, which took ca. 45 s from the instant the MW irradiation was interrupted to the instant the reaction temperature set point (55 °C) was reached, causing a decrease in temperature of less than 10 °C. Single-Crystal X-ray Diffraction. Single crystals of [Mo8O24(di-t-Bu-bipy)4] 3 10CH2Cl2 (4) were harvested from the crystallization vial and immediately immersed in highly viscous FOMBLIN Y perfluoropolyether vacuum oil (LVAC 140/13, purchased from Sigma-Aldrich) to avoid degradation due to evaporation of the solvent. A suitable single crystal was mounted on a Hampton Research CryoLoop6 with the help of a Stemi 2000 stereomicroscope equipped with Carl Zeiss lenses. Data were collected on a Bruker X8 Kappa APEX II CCD area-detector diffractometer (Mo KR graphite-monochromated


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radiation, λ = 0.71073 A˚) controlled by the APEX2 software package25 and equipped with an Oxford Cryosystems Series 700 cryostream monitored remotely using the software interface Cryopad.26 Images were processed using SAINTþ,27 and data were corrected for absorption by the multiscan semiempirical method implemented in SADABS.28 The structure was solved using the Patterson synthesis algorithm implemented in SHELXS-97,29,30 which allowed the immediate location of the molybdenum atoms. All remaining nonhydrogen atoms were located from difference Fourier maps calculated from successive full-matrix least-squares refinement cycles on F2 using SHELXL-97.29,31 All non-hydrogen atoms were successfully refined using anisotropic displacement parameters. A total of five crystallographically independent CH2Cl2 solvent molecules were directly located from difference Fourier maps and were included in the final structural model with full site occupancy. PLATON32 estimated the presence of a residual solventaccessible volume of about 593 A˚3 (ca. 2.5% of the total unit cell volume), which may contain additional solvent molecules. All attempts to model these chemical moieties were unsuccessful and led to unstable structural refinements. The oxidation states of the molybdenum centers were further investigated using PLATON, with the valences of the Mo-O and Mo-N interactions being calculated from the measured bond distances following the theoretical models of Brese and O’Keeffe,33 and Brown and Altermatt.34 The sums of the bond valences at each atomic position are as follows (considering a þ6 oxidation state for each metal center): Mo1 þ5.88; Mo2 þ6.12; Mo3 þ5.92; Mo4 þ6.07. These results clearly suggest that the oxidation state of all crystallographically independent metal centers is indeed þ6. Hydrogen atoms bonded to carbon were located at their idealized positions using appropriate HFIX instructions in SHELXL: 43 for the aromatic and 137 for the terminal -CH3 groups belonging to the di-t-Bu-bipy ligand; 23 for the -CH2group associated with the solvent molecules. All of these atoms were included in subsequent refinement cycles in riding-motion approximation with isotropic thermal displacement parameters (Uiso) fixed at 1.2 (aromatic and -CH2-) or 1.5 (-CH3) times Ueq of the attached carbon atom. The last difference Fourier map synthesis showed the highest peak (3.571 e A˚-3) and deepest hole (-2.112 e A˚-3) located at 0.78 and 0.48 A˚ from Mo2 and Cl9, respectively. Crystal data and selected bond lengths and angles are shown in Tables 1-3. Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication No. CCDC-750392. Copies of the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (þ44) 1223-336-033; or e-mail: [email protected].

(25) APEX2 Data Collection Software, version 2.1-RC13; Bruker AXS: Delft, The Netherlands, 2006. (26) Cryopad, Remote monitoring and control, version 1.451; Oxford Cryosystems: Oxford, U.K., 2006. (27) SAINTþ Data Integration Engine, version 7.23a; Bruker AXS: Madison, WI, 2005. (28) Sheldrick, G. M. SADABS, version 2.01; Bruker AXS: Madison, WI, 1998. (29) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112–122. (30) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure Solution; University of G€ottingen: G€ottingen, Germany, 1997. (31) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refinement; University of G€ottingen: G€ottingen, Germany, 1997. (32) (a) Spek, A. L. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, 46, C34. (b) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13. (33) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47, 192–197. (34) Brown, I. D.; Altermatt, D. Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 244–247.

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Acknowledgment. We are grateful to the Fundac-~ao para a Ci^encia e a Tecnologia (FCT, Portugal), POCI 2010, OE and FEDER for funding through project no. PTDC/QUI/ 71198/2006. T.R.A., P.N., and S.G. thank the FCT for grants. We are also grateful to the FCT for the financial support toward the purchase of the single-crystal diffractometer.

Amarante et al. Supporting Information Available: A figure that compares the X-ray powder diffraction pattern of 3 with a simulated pattern calculated from the published crystal structure data for [MoO3(bipy]; full details of the crystal structure determination of 4 (CCDC 750392) in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

Investigation of Molybdenum Tetracarbonyl Complexes As Precursors

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