Synthesis and X-ray structure of a complex containing two (eta;3-allyl)MoIIunits bridged by MoVIO42ndash;and exhibiting an unusual type of aggregation

摘要

Mo Cl N OC OC N N N = bipy Mo O N OC OC N + ndash;AgCl Na2MoO4 H2O ndash;NaBF4 ndash;Me2CO Me2CO AgBF4 Mo N CO OC N Mo O O O Mo O N OC CO N 2 1 BF4 ndash; Synthesis and X-ray structure of a complex containing two (h3-allyl)MoII units bridged by MoVIO4 22 and exhibiting an unusual type of aggregation Cornelia Borgmann, Christian Limberg* and L�aszl�o Zsolnai Universit�at Heidelberg, Anorganisch-Chemisches Institut, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany. E-mail: Limberg@sun0.urz.uni-heidelberg.de Received (in Basel, Switzerland) 3rd July 1998, Accepted 26th October 1998 Treatment of the Mo(h3-C3H4Me)(bipy)(CO)2(Me2CO)+ cation with an aqueous MoO4 22 solution enabled the first structural characterisation of an (h3-allyl)MoO complex, which aggregates to a dimer showing MoNOhellip;Hndash;C contacts in the solid state.Organotransition-metal chalcogenide complexes are noteworthy in representing a link between solid, more or less ionic, metal chalcogenides and low-valent molecular organometallic systems.1 Despite the potential importance of organomolybdenum oxo systems containing molybdenum in the highest oxidation states +5 and +6 as catalytic intermediates in industrial processes,2 there are known as yet few model complexes of this sort.One example of heterogeneous catalysis where such species might play a significant role entails the oxidation of propene to acrolein using MoO3/Bi2O3 as the catalyst and this has gained considerable technical importance.Nontheless, the reaction mechanism has remained for the most part speculative. The results of their experiments on isotopic enrichment lead Grasselli and Burrington to suggest the intermediate formation of symmetric p-allyl complexes (or the chemisorption of delocalised allyl radicals) to Mo centres of the catalystrsquo;s surface.3 Furthermore recent investigations suggest that in heterogeneous oxidation catalyses where Mo-oxides are employed the oxygen atoms found in the organic oxidation products have their origin in previously bridging positions.4 So far there are no compounds existent in the literature which could be regarded as functional models for the oxo molybdenum p-allyl surface intermediates under discussion. Only few allyl molybdenum compounds containing ligands with oxygen donor functions of any kind have been synthesised and characterised structurally,5 and none of those bear the terminal or bridging O22 ligands characteristic to the catalyst.We have shown that complexes with (h3-allyl)Mo units in oxygen-rich coordination spheres containing RO2 ligands (R = Me, H) can be obtained if cationic complexes with labile ligands are employed as starting materials.6 Making use of the same principles we have now achieved for the first time the synthesis and structural characterisation of an allylndash;Mo complex containing an O22 link to another Mo centre. The interface reaction of a Mo(h3-C3H4Me)(bipy)(CO)2- (Me2CO)BF4 1 solution in acetonemdash;prepared by treatment of Mo(h3-C3H4Me)(bipy)(CO)2Cl with AgBF4 as established for the corresponding h3-C3H5 complex7mdash;with a solution of Na2MoO4 in water leads to the precipitation of a crystalline maroon solid insoluble in all common organic solvents.However, its elemental analysis suggested the composition {Mo(h3-C3H4Me)(bipy)(CO)2}2(m-MoO4) 2dagger; which had been aimed at (Scheme 1). An X-ray diffraction analysisDagger; of a single crystal yielded the structure shown in Fig. 1, which clearly proves the presence of an Ondash;MoO2ndash;O moiety bridging two Mo(h3-C3H4Me)- (bipy)(CO)2 units. One of the latter Mo(3/4) was disordered through a rotation by 0.5deg; around the Mo(1)ndash;O(2) axis and in Fig. 1 for clarity only the Mo(4) fragment is shown. The Mo(3/ 4)ndash;O(2) and Mo(2)ndash;O(1) distances 2.092(3) and 2.107(3) Aring; lie within a range characteristic for bonds of the molybdate unit to other Mo centres (2.02ndash;2.15 Aring;)8 indicating a real bonding situation rather than a weak coordination of the MoO4 22 unit.The Mo(1)ndash;O(1/2) bridging distances 1.795(3) and 1.797(2) Aring; are also typical (1.75ndash;1.85 Aring;) and the angles found within the central unit of 2 are all very close to the perfect tetrahedral angle. However, if just the solid state structure of the individual molecule is considered it does seem surprising that the two organometallic fragments coordinated to the MoO4 22 unit are not oriented symmetrically with respect to the latter: Mo(1/2/4) and O(1/2) are almost in a plane intersecting the C(13)ndash;Mo(2)ndash; C(15) angle while the corresponding C(55)ndash;Mo(4)ndash;C(53) angle of the other subunit escapes intersection by a rotation of this fragment by ca. 90deg; around the Mo(4)ndash;O(2) bond. An explanation can be found if the structure as a whole is considered: As obvious from Fig. 2 the molecules can pack very efficiently through an internal rotation as described and this Scheme 1 Fig. 1 Structural representation of {Mo(h3-C3H4Me)(bipy)(CO)2}2(m- MoO4) 2.Selected bond lengths (Aring;) and angles (deg;): Mo(1)ndash;O(3) 1.731(3), Mo(1)ndash;O(4) 1.739(3), Mo(1)ndash;O(1) 1.795(3), Mo(1)ndash;O(2) 1.797(3), Mo(2)ndash; O(1) 2.107(3), Mo(3/4)ndash;O(2) 2.092(4); Mo(1)ndash;O(1)ndash;Mo(2) 156.8(2), Mo(1)ndash;O(2)ndash;Mo(3/4) 150.8(2), O(3)ndash;Mo(1)ndash;Mo(4) 107.6(2), OCndash;Mo(2)ndash; CO 78.7(2), OCndash;Mo(3/4)ndash;CO 76.2(5); averaged distances and angles: Mo(2)ndash;Cmeso 2.243(4), Mo(2)ndash;Cterm 2.323(5), Mondash;N 2.254(9), Mondash;CO 1.97(1), OCndash;Mondash;Ntrans 168.9(5), OCndash;Mondash;Ncis 104.1(5).Chem. Commun., 1998, 2729ndash;2730 2729leads to symmetric dimeric aggregates. These show very short MoNOhellip;C distances suggesting that there are intermolecular contacts between the protons of the bipy ligands and the MoNO groups (compare Fig. 2 where the calculated H positions are shown; the MoNOhellip;H distances were calculated to lie between 2.39 and 2.77 Aring;). Of course the short Cndash;Hhellip;ONMo distances neither have to be indicative of attractive interactions,9 nor is the presence of the corresponding contacts necessary to explain the structural features observed, so that attention now focused on the MoNO bonds. The MoNO bond lengths in d0 molybdenum dioxo complexes usually fall in a narrow range and are not strongly effected by the nature of the other ligands at the metal.10 It has been shown, though, that classical H-bridging to a HNEt4 + cation8c can selectively lengthen one of two MoNO bonds in a MoO2 unit by ca. 0.03 Aring;.The MoNO distances observed in 2 1.731(3) and 1.739(3) Aring; are certainly located at the lsquo;tailrsquo; of the d(MoNO) distribution and have to be described as long, which may support the idea of attractive interactions.Complex 2 is EPR silent and magnetic measurements showed it to be diamagnetic at room temperature, which suggests the presence of a hitherto unknown MoIIndash;Ondash;MoVIndash;Ondash;MoII unit. Few compounds possessing two oxo bridged Mo centres differing in their oxidation states by two are known. An organometallic representative involving a bridging MoO4 22 unit, too, is the complex (CpMe 2MoIV)2(m-MoVIO4)2.8a In 2 the difference in oxidation states amounts to four, which is unusual and had to find an explanation in the redox potentials of the two subunits.Indeedmdash;probably due to the strongly paccepting ligandsmdash;the MoII(h3-C3H4Me)(bipy)(CO)2- (Me2CO)+ cation showed a reversible oxidation wave at a potential as high as 0.88 V (vs. SCE), which cannot be reached by molybdate11 and this fact allowed the isolation of 2.The molybdate complex reported here contains p-allylndash;Mo fragments which are bonded via lsquo;purersquo; oxygen bridges to MoVI centres also providing MoVINO groups in close proximity. It therefore already meets some of the requirements to serve as a structural model complex for surface intermediates during molybdenum oxide catalysed propene oxidation. However, other important features like the high oxidation states of all metal centres are still missing so that 2 is not suitable ulate the properties of these intermediates. Methallyl radicals are released at 193 deg;C but no oxidation or allyl shift occurs, probably due to the large intramolecular separation of the coordinated allyl ligands from the oxo groups.C. L. is grateful to the Deutsche Forschungsgemeinschaft for a scholarship and C. B. acknowledges financial support through the Landesgraduiertenf�orderung Baden-W�urttemberg. We also wish to thank Professor Dr G.Huttner for his generous support. Notes and references dagger; 0.389 g (2 mmol) of AgBF4 in 15 ml acetone were added to a suspension of 0.798 g (2 mmol) of Mo(h3-C3H4Me)(bipy)(CO)2Cl in 30 ml of acetone via cannula. After 20 min of stirring the AgCl precipitated was removed by filtering and the filtrate overlayered by a solution of 0.492 g (2 mmol) Na2MoO4middot;H2O in a mixture of 10 ml of deoxygenated water and 15 ml of acetone. At the interface crystals suitable for X-ray diffraction grew within 24 h.If the solution is stirred 2 precipitates immediately and almost quantitatively. The precipitate is washed with water and thf and dried in vacuo. Yield: 0.84 g (0.9 mmol, 95). Anal. Calc. for C32H30Mo3N4O8: C, 43.35; H, 3.41; N, 6.35. Found: C, 43.50; H, 3.56; N 6.14. Characteristic bands in the IR spectrum (KBr/cm21): 1934s n(CO), 1851s n(CO), 914w, 876s, 838vs (br), 805s (sh), 734m all n(Mondash;O) and n(MoNO). Dagger; Crystal structure data for 2middot;0.5 Me2CO: C33.5H33N4Mo3O8.5, Mr = 915.46, triclinic, space group P�1, Z = 2, a = 10.700(2), b = 13.361(3), c = 13.654(3) Aring;, a = 71.98(3), b = 83.48(3), g = 69.02(3)deg;, V = 1733.20 Aring;3, 3.4 2q 54deg;, Mo-Ka radiation, l = 0.71073 Aring;, w-scan, T = 200 K, m = 1.126 mm21, Dc = 1.754 g cm23, measured 15768, independent 6904, and observed reflections 4892, criterion: I 2s(I), structure solved by direct methods (program: SHELXS-97), refined versus F2 (program: SHELXL-97) with anisotropic temperature factors for all non-hydrogen atoms, 519 refined parameters with R = 0.039, residual electron density (max./min.): 0.698/20.837 e Aring;23.CCDC 182/1069. See http:// www.rsc.org/suppdata/cc/1998/2729/ for crystallographic data in .cif format. 1 F. Bottomley and L. Sutin, Adv. Organomet. Chem., 1988, 28, 339. 2 J. Sundermeyer, Angew. Chem., 1993, 105, 1195; Angew. Chem., Int. Ed. Engl., 1993, 32, 1144. 3 R. K. Grasselli and J. D. Burrington, Adv. Catal., 1981, 30, 133; G.W. Keulks, L. D. Krenzke and T. M. Notermann, Adv. Catal., 1978, 27, 183; R. K. Grasselli and J. D. Burrington, Ind. Engl. Chem. Prod. Res. Dev., 1984, 23, 394. 4 T. Ono, N. Ogata and Y. Mijaryo, J. Catal., 1996, 161, 78; T. Ono, H. Numata and N. Ogata, J. Mol. Catal., 1996, 105, 31. 5 M. G. B. Drew and G. F. Griffin, Acta Crystallogr., Sect. B, 1979, 35, 3036; M. S. Kralik, J. P. Hutchinson and R. D. Ernst, J. Am. Chem. Soc., 1985, 107, 8296; F. Dawans, J. Dewailly, J.Meunier-Piret and P. Piret, J. Organomet. Chem., 1974, 76, 53; S. J. Rettig, A. Storr and J. Trotter, Can. J. Chem., 1988, 66, 97; V. S. Joshi, V. K. Kale, K. M. Sathe, A. Sarkar, S. S. Tavale and C. G. Suresh, Organometallics, 1991, 10, 2898; J. W. Faller, J. T. Nguyen, W. Ellis and M. R. Mazzieri, Organometallics, 1993, 12, 1434; N. J. Christensen, P. Legzdins, J. Trotter and V. C. Yee, Organometallics, 1991, 10, 4021; K. R. Breakell, S. J. Rettig, A. Storr and J. Trotter, Can.J. Chem., 1979, 57, 139; D. Mohr, H. Wienand and M. L. Ziegler, J. Organomet. Chem., 1977, 134, 281; S. K. Chowdhury, V. S. Joshi, A. G. Samuel, V. G. Puranik, S. S. Tavale and A. Sarker, Organometallics, 1994, 13, 4092; W. Kl�aui, A. M�uller, W. Eberspach, R. Boese and I. Goldberg, J. Am. Chem. Soc., 1987, 109, 164; these references include ligands R2O, RO2, OH2, Pndash;O2, Sndash;O2 but not RCO22. 6 C. Borgmann, C. Limberg, H. Pritzkow, L. Zsolnai and E. Kaifer, J. Organomet. Chem., 1998, accepted. 7 P. Powell, J. Organomet. Chem., 1977, 129, 175. 8 (a) K. Prout and J.-C. Daran, Acta Crystallogr., Sect. B, 1978, 34, 3586; (b) G. Schoettel, J. Kress, J. Fischer and J. A. Osborn, J. Chem. Soc., Chem. Commun., 1988, 914; (c) D. Attanasio, V. Fares and P. Imperatori, J. Chem. Soc., Chem. Commun., 1986, 1476; (d) T. C. Hsieh and J. Zubieta, Inorg. Chem., 1985, 24, 1287; (e) T. C. Hsieh, S. N. Shaikh and J. Zubieta, Inorg. Chem., 1987, 26, 4079. 9 D. Braga, F. Grepioni, E. Tagliavini, J. J. Novoa and F. Mota, New. J. Chem., 1998, 755. 10 J. M. Mayer, Inorg. Chem., 1988, 27, 3899. 11 Gmelin Handbook of Inorganic Chemistry, Springer Verlag, Heidelberg, 8th edn., 1988, pp. 135. Communication 8/05154B Fig. 2 Structural arrangement of the molecules of 2 in the unit cell. Selected bond lengths (Aring;): O(3)ndash;C(9A) 3.180, O(3)ndash;C(8A) 3.279, O(4)ndash;C(5A) 3.213, O(4)ndash;C(8A) 3.421. 2730 Chem. Commun., 1998, 2729