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Mononuclear tungsten(VI) complexes with bis(phenolato) ligands: syntheses, characterisations and activity in ROMP reaction
Polyhedron 21 (2002) 1017 /1022 www.elsevier.com/locate/poly
Mononuclear tungsten(VI) complexes with bis(phenolato) ligands: syntheses, characterisations and activity in ROMP reaction Ari Lehtonen a,*, Reijo Sillanpa¨a¨ b a
b
Department of Chemistry, University of Turku, FIN-20014 Turku, Finland Department of Chemistry, University of Jyva¨skyla¨, PO Box 35, FIN-40351 Jyva¨skyla¨, Finland Received 26 November 2001; accepted 4 February 2002
Abstract The reaction of bulky ligand precursor 2,2?-methylenebis(4-methyl-6-tert -butylphenol) (H2mbp) or 2,2?-ethylidenebis(4,6-di-tert butylphenol) (H2ebp) with trisdiolatotungsten(VI) complex [W(eg)3] 1 (eg /ethanediolate dianion) provides heteroleptic complexes [W(mbp)(eg)2] 2 or [W(ebp)(eg)2] 3, respectively. Sterically less hindered 2,2?-dihydroxy-1,1?-dinaphtylmethane (H2dinap) forms heteroleptic disubstituted complex [W(dinap)2(eg)] 4. The X-ray crystal structure determinations confirmed that the isolated compounds are made of monomeric tris(diolato)tungsten(VI) molecules in which the central tungsten atom is bonded to six oxygen atoms forming a distorted octahedral coordination sphere around the metal ion. Complexes 2 and 3 catalyse the ring opening metathesis reaction of norbornene when activated by Et2AlCl. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: X-ray; Mononuclear tungsten(VI) complexes; Ligand; Heteroleptic
1. Introduction Chelating oxygen donor ligands, e.g. binaphthols and biphenols have confirmed to be useful accessories in organic syntheses with early transition metals [1]. The hard Lewis-base character of the phenoxide group makes these compounds ideal for attaching to the metal ions having high oxidation states (hard Lewis-acids). Sterically hindered auxiliary groups on the bis(phenolato) ligands allow the degree of shielding to be tuned without significantly altering the electronic properties of the ligands. Furthermore, the influence of the number of atoms between the phenoxide oxygen atoms in such ligands to the coordination geometry of metal can be also modified. 1,1?-coupled biphenolates and binaphtholates of W(VI), often with other ligands, have been used successfully as catalyst precursors in olefin metathesis
* Corresponding author. Tel.: /358-2-333-6733; fax: /358-2-3336700. E-mail address: [email protected] (A. Lehtonen).
reactions [2]. However, reports on tungsten complexes with bridged bis(phenolate)s are scarce. Heppert’s group has reported preparation and spectral characterisation of tungsten(VI) complexes [W( /X)Cl2(mmp)] and [W( /X)Cl2(ebp)] [X /O, NPh-2,6-Me2; H2mmp /2,2?methylenebis(4,6-dimethylphenol); H2ebp /2,2?-ethylidenebis(4,6-di-tert -butylphenol)] [3]. Nakamura’s group has studied complexes of sulfur-bridged bis(phenol), and has characterised several crystal structures, e.g. [W(h 2EtC /CEt)(tbp)Cl2] (H2tbp/2,2?-thiobis(4,6-dimethylphenol) [4]. In the context of a broader program aimed at developing transition metal catalysts for olefin metathesis reaction, our laboratory has recently become interested in the chemical manipulation of tungsten aryloxide complexes [2b,5]. Our long-term goal is to prepare polymer-based tungsten(VI) phenoxides and use them as recoverable catalyst precursors. To gain a greater degree of understanding this chemistry, we have now prepared three new monomeric tungsten(VI) complexes with bis(phenolato) ligands and used them as catalyst precursors for ring opening metathesis polymerisation of norbornene.
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 0 8 9 7 - 5
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2. Results and discussion 2.1. Synthesis and spectroscopy of the complexes The addition of 1 equiv. of the ligand precursors 2,2?methylenebis(4-methyl-6-tert -butylphenol) (H2mbp) or H2ebp to the refluxing toluene solution of [W(eg)3] 1 (H2eg/1,2-ethanediol) leads to the formation of the heteroleptic, monomeric compounds 2 and 3, respectively (Scheme 1). The red solid products can be purified by crystallisation or by column chromatography to obtain air stable complexes in high yields. They are soluble in chlorinated hydrocarbons, ethers and aromatic solvents but only slightly soluble in hexane. Bulky tert -butyl groups in ortho -positions of phenoxide moieties seems to prevent further substitution of eg ligands as the use of excess bis(phenol) do not result in complexes of type [W(biphenolate)2(eg)]. However, 2 equiv. of sterically less hindered 2,2?-dihydroxy-1,1?dinaphtylmethane (H2dinap) reacts with 1 equiv. of 1 in toluene at reflux temperature to form heteroleptic complex [W(dinap)2(eg)] (4) (Scheme 2). This intense red coloured compound is slightly soluble in toluene and chlorinated solvents and it is stable enough to be purified by column chromatography. Unfortunately, 4 was not soluble enough to be used in our catalyst tests (see below). Earlier studies on transition metal complexes of 2,2?methylenebisphenoxides have shown that the puckered eight-membered ring formed by chelation to metal centre undergoes slow flipping on the NMR timescale [6 /8]. Therefore, the methylene protons in the chelate backbone are typically diastereotopic at ambient temperature.[8] The 1H NMR spectra of complexes 2 and 3 show typical resonances for aromatic protons and alkyl substituents of ligands. In the aliphatic region of the spectra of these complexes are seen two multiplets for ethanediolate groups, which implies that there are two different environments for these methylene protons. The diastereotopic methylene protons in the chelate ring of 2 are separated by 2 ppm showing doublets at 3.3 and 5.4 ppm. Also the methine proton in the chelate ring of 3 shows doublets at 4.4 and 5.8, respectively. 1 H NMR spectrum of compound 4 is not very informative. Methylene protons in the bis(naphtolate)
Scheme 2. Activation and regeneration of catalyst precursors.
rings showed three multiplets at 4.8 /5.9 ppm. A sharp singlet at 5.7 ppm was assigned as a signal for ethanediolato protons, as similar signal for that ligand in comparable W(eg)(OAr)4 complex, [W(calix)(eg)] (calix /calix[4]arene), is found at 5.83 [9].
2.2. Crystal structures Compounds 2/4 form monomeric molecules in which six oxygen donors surround the central tungsten atoms (Figs. 1 /3). The distorted octahedral WO6 units are generally similar, but not identical in all these compounds, as seen from the bonding parameters presented
Fig. 1. Molecular structure of [W(mb)(eg)2] (2). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids have been drawn at 30% probability level.
Scheme 1. Preparation of complexes. (i) H2biphe /PhMe, reflux; (ii) 2 H2dinap /PhMe, reflux.
A. Lehtonen, R. Sillanpa¨a¨ / Polyhedron 21 (2002) 1017 /1022
1019
Table 1 ˚ ) and angles (8) for 2 /4 Selected bond lengths (A
Fig. 2. Molecular structure of [W(ebp)(eg)2] (3). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids have been drawn at 30% probability level.
b
Bond lengths W/O(1) W/O(2) W/O(3) W/O(4) W/O(5) W/O(6) C(1) /O(1) C(n)/O(2) c
1.8970(17) 1.8848(16) 1.9324(18) 1.9049(18) 1.9069(16) 1.9143(17) 1.373(3) 1.368(3)
1.891(6) 1.899(5) 1.923(11) 1.899(6) 1.925(7) 1.888(7) 1.361(12) 1.365(10)
1.930(8) 1.908(8) 1.894(7) 1.891(8) 1.879(8) 1.925(8) 1.397(13) 1.362(12)
1.911(4) 1.942(4) 1.904(4) 1.903(4) 1.902(5) 1.889(4) 1.366(7) 1.362(7)
Bond angles O(1)/W/O(2) O(1)/W/O(3) O(3)/W/O(4) O(2)/W/O(5) O(5)/W/O(6) O(4)/W/O(6) C(1) /O(1)/W C(n)/O(2)/W C(n)/O(3)/W C(n)/O(4)/W C(n)/O(5)/W C(n)/O(6)/W
87.35(7) 165.66(7) 77.86(8) 162.52(7) 78.72(8) 160.23(8) 148.82(15) 151.18(14) 119.10(17) 120.23(17) 119.79(17) 118.03(17)
88.7(3) 168.0(3) 78.3(3) 167.5(3) 79.5(3) 159.6(3) 152.9(6) 158.7(6) 119.1(7) 117.2(6) 117.9(7) 118.3(7)
84.6(3) 164.7(4) 78.0(3) 162.3(4) 78.4(4) 166.4(3) 130.1(7) 127.1(7) 121.0(8) 120.7(8) 121.3(8) 118.1(8)
84.7(2) 162.9(2) 84.9(2) 164.6(2) 78.4(2) 161.3(2) 141.3(4) 128.6(4) 131.3(4) 138.7(4) 121.0(4) 121.4(4)
b c
in Table 1. If we have [W(biphenolate)(eg)2] type compound, where biphenolate is a bidentate phenolate dianion (or two monodentate phenolate anions), the shape and rigidity of biphenolate ligand modify the actual octahedral coordination sphere of the W(VI) ion. Comparing the structural parameters of 2 and 3 shows that these have fairly similar overall structure, so the substituents in the main body of the ligand modify the structure of the complexes only slightly. To see the influence of different ligand bodies on the complex structure, we should compare the structures formed by 2,2?-methylenebis(phenolato) ligand and binaphtholato ligand, thus also bonding parameters of [W(bino)(eg)2]
W(bino)(eg)2
3
a
Fig. 3. Molecular structure of [W(dinap)2(eg)] (4). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids have been drawn at 30% probability level.
a
2
4
Values around W(1). Binaftolatobis(ethanediolato)tungsten(VI) in ref. [5b]. C(n) is a carbon atom bonded to a host oxygen atom.
have included in Table 1 [5b]. Consequently, we can see large changes in bonding parameters of these bis(phenolato) ligands to W(VI) ion. For ex. W /O(1) bond lengths, as well as O(1) /W /O(2) and C(1) /O(1)/W bond angles are quite different in 2 and [W(bino)(eg)2]. Normally O(phenolato) is bonded more tightly (shorter bond distances) to W(VI) center than O(alcoholato). However, we can see now that a rigid ligand bonds less effectively to W(VI) center. This phenomenon is also seen in the bonding parameters of 4, where two bis(naphtolato) ligands are attached to W(VI): one of these ligands is quite loosely bonded to W(VI) center. Finally, the bonding parameters of aliphatic diolato groups are quite similar in compounds studied now as in earlier reported tungsten(VI) diolato complexes with simple phenoxides or binaphtholates, e.g. [W(eg)2(OPhR2-2,6)2] [5a] and [W(bino)(eg)2] [5b]. 2.3. Polymerisation studies We have earlier studied ROMP processes employing complexes of type [W(diol)2(OAr)2] and found that the catalytic activities depends on the bulkiness of alkyl substituents at the phenoxide groups [2b]. In this study we used sterically hindered complexes 2 and 3 as precatalysts in ROMP of norbornene and dicyclopentadiene, using a Et2AlCl cocatalyst. Both tungsten precatalysts showed moderate activity for polymerisation of norbornene at 0 8C as the yields of polymers after 15
1020
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min reaction were 38% for 2 and 42% for 3, respectively. Though, they were inactive for the polymerisation of dicyclopentadiene. At 80 8C both precatalysts produced polynorbornene and polydicyclopentadiene in practically quantitative yields. However, the high yields of polydicyclopentadiene were not necessarily due to the increased activity of the catalysts in elevated temperature, but it is possibly simply due to the thermally induced cross-linking reaction [10]. The nature of the active species is unknown, but we can suppose the traditional model for precatalyst activation-dialkylation followed by a-H elimination to produce the reactive Schrock-type alkylidene [11]. Nevertheless, active catalysts can be transformed into compounds 2 or 3, respectively, by adding excess H2eg, thus these precatalysts can be recovered from reaction mixtures in nearly quantitative yields (Scheme 2).
3. Conclusion Bulky methylene-bridged bis(phenol)s react with trisdiolatotungsten(VI) complex [W(eg)3] (eg /ethanediolate dianion) providing heteroleptic complexes [W(biphenolate)(eg)2], where bis(phenol) /2,2?-CH2(4Me-6-t BuPhOH)2 or 2,2?-CHMe(4,6-t Bu2PhOH)2. Sterically less hindered 2,2?-dihydroxy-1,1?-dinaphtylmethane forms disubstituted complex [W(biphenolate)2(eg)]. In solid state these isolated compounds consist of monomeric tris(diolato)tungsten(VI) molecules in which the central tungsten atom is bonded to six oxygen atoms forming a distorted octahedral coordination sphere around the metal ion. Compounds of the type [W(biphenolate)(eg)2] can be activated by Et2AlCl to catalyse ring opening polymerisation metathesis of norbornene.
4. Experimental Chemicals were from commercial origins and were used without subsequent purification. Toluene used in polymerisation studies was distilled over sodium under a nitrogen atmosphere. Monomer solutions in toluene ˚ molecular sieves. 2,2?-dihydroxywere stored over 4 A 1,1?-dinaphthylmethane was prepared as described earlier [12]. NMR spectra were recorded on Bruker AM200 spectrometer. Elemental analyses were obtained using a Perkin/Elmer CHNS-Analyzer 2400. Analytical samples were dried in vacuo at 40 8C for 2 h prior to elemental and spectral analyses. 4.1. Preparation of complexes [W (eg )2(mbp )] (2) 1.09 g of 1 (3.00 mmol) was heated with 1.02 g of H2mbp (3.00 mmol) in 60 ml of toluene at
reflux to form a red solution. The H2eg that formed was removed by azeotropic distillation with toluene during 4 h. The solvent was evaporated and orange solid residue was column chromatographed (silica /CH2Cl2) to give complex 2 as orange/red solid, which was crystallised from 10 ml of acetonitrile to obtain intense red prisms with the formula 2 ×/CH3CN in yield of 82% (1.68 g). Samples were dried in vacuo at 40 8C for 6 h before elemental and spectral analyses to remove any solvent of crystallisation. For 2: dH (CDCl3, standard SiMe4): 7.07 (s, 1H, ArH), 6.97 (s, 2H, ArH), 6.92 (s, 1H, ArH), 5.72 (br, s, 2H, /OCH2 /), 5.53 (br, s, 2H, /OCH2 /), 5.38 (d, 1H, /CH2 /), 5.35 (br, s, 2H, /OCH2 /), 5.17 (br, s,2H, /OCH2 /), 3.28 (s, 1H, /CH2 /), 2.34 s, (s, 3H, p-CH3), 2.28 (s, 3H, p-CH3), 1.41 (s, 9H, C(CH3)3), 1.34 (s, 9H, C(CH3)3). Found: C, 50.2; H, 5.8. C27H38O6W requires C, 50.5; H, 6.0%. [W (ebp )(eg )2] (3) 1.09 g of 1 (3.00 mmol) was heated with 1.32 g of H2ebp (3.00 mmol) in 60 ml of toluene at reflux to form a red solution and the H2eg that formed was removed by azeotropic distillation with toluene during 4 h. The solvent was evaporated and an orange solid residue was crystallised from 40 ml of hexane to obtain complex 3 as orange-red microcrystals in yield of 90% (2.05 g). Crystals of 3 for X-ray structure analysis were grown from acetonitrile. For 3: dH (CDCl3, standard SiMe4): 7.24 (1H, ArH), 7.17 (m, 3H, ArH), 5.8 /5.1 (9 H, several multiplets, /OCH2 / and /CHCH3), 1.65 (d, 3H, /CHCH3), 1.32 (q, 36H, C(CH3)3). Found: C, 55.4; H, 6.9. C34H52O6W requires C, 55.1; H, 7.1%. [W (dinap )2(eg )] (4) 120 mg of 1 (0.33 mmol) was heated with 200 mg of H2dinap (0.67 mmol) in 60 ml of toluene at reflux for 4 h to form a red solution. The solution was then concentrated to 15 ml and allowed to stand at r.t. overnight to obtain complex 4 as red powder in 69% yield (190 mg). Crystals of 4×/CH2Cl2 for X-ray structure analysis were grown from CH2Cl2 by slow evaporation. dH (CDCl3, standard SiMe4): 8.80 / 7.20 (several multiplets, 24H, ArH), 5.90 /4,80 (several multiplets, 4H, /CH2 /), 5.73 (s, 4H, /OCH2 /). Found: C, 62.6; H, 4.0. C44H32O6W requires C, 62.9; H, 3.8%. 4.2. Polymerisation studies Polymerisations were performed under a nitrogen atmosphere applying the protocol reported by Barnes et al. [2]. Our experiments were carried out employing a 1:4.5:50 tungsten precatalyst: activator:monomer ratio. The precatalyst (2 /105 mol in 10 ml of toluene) was treated with 0.05 ml of Et2AlCl solution (1.8 M in toluene, 9/105 mol). The red reaction mixture was stirred for 5 min and 1 ml of 1 M olefinic monomer (1.0 mmol) in toluene was added. After 15 min stirring the solution was treated with 10 ml of methanol to precipitate polymer as a white solid. Polymers were
A. Lehtonen, R. Sillanpa¨a¨ / Polyhedron 21 (2002) 1017 /1022
swelled in toluene, re-precipitated by methanol and dried in vacuo at 40 8C for 8 h.
4.3. Single crystal X-ray diffraction Crystal data for compounds 2 /4, along with other experimental details, are summarised in Table 2. The crystallographic data were collected for 2 and 3 at 173 K on a Nonius Kappa CCD area-detector diffractometer using graphite monochromatised Mo Ka radiation (l/ ˚ ). Lattice parameters were determined from 0.71073 A ten images recorded with 18 8 scans and subsequently refined on all data. The data collection was performed using 8 and v scans with 18 steps using an exposure time of 5 s per frame for 2 and 2 s per frame for 3. The crystal-to-detector distances were 30 and 40 mm for 2 and 3, respectively. The data were processed using DENZO-SMN v0.93.0 [13]. MULABS [14] absorption correction was applied for the data of both compounds, but in final calculations the corrected data were used only for 3, because corrected data for 2 give higher R -values than the uncorrected one. Single-crystal data collection for 4 was performed at ambient temperature on a Rigaku AFC5S diffractometer using graphite monochromatised Mo Ka radia˚ ). Data reduction and subsequent tion (l /0.71069 A calculations were performed with TEXSAN for Windows [15]. The data were corrected for Lorenz and polarisation effects, for absorption (PSI-scans [16]) and for decay. Table 2 Crystal data and experimental details of the structure determination of 2 /4
Formula Mr Crystal system Space group (no.) ˚) a (A ˚) b (A ˚) c (A a (8) b (8) d (8) ˚ 3) U (A Z Dcalc (g cm 3) m (Mo Ka) (cm 1) Observed reflections Rint Parameters R1 wR2
2
3
4
C29H41NO6W 683.48 triclinic P/1¯ (2) 9.1940(1) 10.1566(1) 16.3405(2) 77.035(1) 82.012(1) 86.927(1) 1472.15(3) 2 1.542 39.63 6681 0.0159 332 0.022 (0.020) a 0.047 (0.046)
C34H52 O6W 740.61 orthorhombic Pca 21 (29) 15.8055(2) 16.5843(2) 26.5014(3) 90 90 90 6946.63(14) 8 1.416 33.65 14714 0.0600 736 0.084 (0.048) 0.104 (0.093)
C45H34Cl2O6W 925.47 triclinic P/1¯ (2) 11.289(3) 17.342(3) 10.924(2) 95.566(18) 117.161(14) 76.898(17) 1853.2(6) 2 1.658 33.12 6534 0.0368 487 0.081 (0.041) 0.076 (0.068)
R1 /SjjFoj/jFcjj/SjFoj, wR2 /{S[w (Fo2/Fc 2)2]/S[w (Fo2)2]}1/2 and w /1/[s 2(Fo2)/(aP )2/bP )], where P/(2Fc 2/Fo2)/3. a Values in parentheses for reflections with I /2.0s (I ).
1021
The structures were solved by direct methods using the SIR-92 program [17] and full-matrix least-squares refinements on F 2 were performed using the SHELXL-97 program [18]. For both all heavy atoms were refined anisotropically. The CH hydrogen atoms were included at the fixed distances with fixed displacement parameters from their host atoms. Compound 3 was refined as a racemic twin (Flack’s parameter is 0.65(1). Figures were drawn with ORTEP-3 for Windows [19].
5. Supplementary data Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 174185 /174187. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: /44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).
References [1] For example, (a) A. Van der Linden, C.J. Schaverien, N. Meijboom, C. Ganter, A.G. Orpen, J. Am. Chem. Soc. 177 (1995) 3008. (b) M. Chavarot, J. Byrne, P.Y. Chavant, J. Pardillos-Guindet, Y. Valle´e, Tetrahedron Asymm. 9 (1998) 3889. (c) J.B. Alexander, R.R. Schrock, W.M. Davis, K.C. Hultzsch, A.H. Hoveyda, J.H. Houser, Organometallics 19 (2000) 3700. (d) T. Chen, S.W. Hon, S.S. Weng, Synlett (1999) 816. [2] (a) D.L. Barnes, N.W. Eilerts, J.A. Heppert, W.H. Huang, M.D. Morton, Polyhedron 13 (1994) 1267; (b) A. Lehtonen, R. Sillanpa¨a¨, Inorg. Chem. Commun. 4 (2001) 108. [3] M.D. Morton, J.A. Heppert, S.D. Dietz, W.H. Huang, D.A. Ellis, T.A. Grant, N.W. Eilerts, D.L. Barnes, F. Takusagawa, D. VanderVelde, J. Am. Chem. Soc. 115 (1993) 7916. [4] Y. Nakayama, H. Saito, N. Ueyama, A. Nakamura, Organometallics 18 (1999) 3149. [5] (a) A. Lehtonen, R. Sillanpa¨a¨, Polyhedron 18 (1999) 175; (b) A. Lehtonen, R. Sillanpa¨a¨, Polyhedron 19 (2000) 2579; (c) A. Lehtonen, R. Sillanpa¨a¨, Inorg. Chem. 43 (2001) 6047. [6] J. Okuda, S. Fokken, H.C. Kang, W. Massa, Chem. Ber. 128 (1995) 221. [7] M.H. Chisholm, J.H. Huang, J.C. Huffman, W.E. Streib, D. Tiedtke, Polyhedron 16 (1997) 2941. [8] D.R. Mulford, P.E. Fanwick, I.P. Rothwell, Polyhedron 19 (2000) 35. [9] B. Xu, Y.J. Miao, T.M. Swager, J. Org. Chem. 63 (1998) 8561. [10] T.A. Davidson, K.B. Wagener, J. Mol. Cat. A: Chem. 133 (1998) 67. [11] K.J. Ivin, J.C. Mol, Olefin Metathesis and Metathesis Polymerization, Academic Press, London, 1997. [12] L.E. Mirci, Rom. Pat., 105571, 1992. [13] Z. Otwinowski, W. Minor, in: C.W. Carter, Jr., R.M. Sweet (Eds.), Methods in Enzymology, Macromolecular Crystallography, Part A, Academic Press, New York, 1997, pp. 307 /326.
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[14] R.H. Blessing, Acta Crystallogr., Sect. A 51 (1995) 33 /38. [15] TEXSAN for Windows. Structure Analysis Software. Molecular Structure Corporation, Texas 77381, USA, 1997. [16] A.C. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr., Sect. A 24 (1968) 351.
[17] A. Altomare, G. Cascarano, C. Giacovazzo, A. Gualiardi, M.C. Burla, G. Polidori, M. Camalli, J. Appl. Cryst. 27 (1994) 435. [18] G.M. Sheldrick, SHELXL-97, In: Program for Crystal Structure Refinement, University of Go¨ttingen, Germany, 1997. [19] L.J. Farrugia, J. Appl. Cryst. 30 (1997) 565.
Mononuclear tungsten(VI) complexes with bis(phenolato) ligands: syntheses, characterisations and activity in ROMP reaction Ari Lehtonen a,*, Reijo Sillanpa¨a¨ b a
b
Department of Chemistry, University of Turku, FIN-20014 Turku, Finland Department of Chemistry, University of Jyva¨skyla¨, PO Box 35, FIN-40351 Jyva¨skyla¨, Finland Received 26 November 2001; accepted 4 February 2002
Abstract The reaction of bulky ligand precursor 2,2?-methylenebis(4-methyl-6-tert -butylphenol) (H2mbp) or 2,2?-ethylidenebis(4,6-di-tert butylphenol) (H2ebp) with trisdiolatotungsten(VI) complex [W(eg)3] 1 (eg /ethanediolate dianion) provides heteroleptic complexes [W(mbp)(eg)2] 2 or [W(ebp)(eg)2] 3, respectively. Sterically less hindered 2,2?-dihydroxy-1,1?-dinaphtylmethane (H2dinap) forms heteroleptic disubstituted complex [W(dinap)2(eg)] 4. The X-ray crystal structure determinations confirmed that the isolated compounds are made of monomeric tris(diolato)tungsten(VI) molecules in which the central tungsten atom is bonded to six oxygen atoms forming a distorted octahedral coordination sphere around the metal ion. Complexes 2 and 3 catalyse the ring opening metathesis reaction of norbornene when activated by Et2AlCl. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: X-ray; Mononuclear tungsten(VI) complexes; Ligand; Heteroleptic
1. Introduction Chelating oxygen donor ligands, e.g. binaphthols and biphenols have confirmed to be useful accessories in organic syntheses with early transition metals [1]. The hard Lewis-base character of the phenoxide group makes these compounds ideal for attaching to the metal ions having high oxidation states (hard Lewis-acids). Sterically hindered auxiliary groups on the bis(phenolato) ligands allow the degree of shielding to be tuned without significantly altering the electronic properties of the ligands. Furthermore, the influence of the number of atoms between the phenoxide oxygen atoms in such ligands to the coordination geometry of metal can be also modified. 1,1?-coupled biphenolates and binaphtholates of W(VI), often with other ligands, have been used successfully as catalyst precursors in olefin metathesis
* Corresponding author. Tel.: /358-2-333-6733; fax: /358-2-3336700. E-mail address: [email protected] (A. Lehtonen).
reactions [2]. However, reports on tungsten complexes with bridged bis(phenolate)s are scarce. Heppert’s group has reported preparation and spectral characterisation of tungsten(VI) complexes [W( /X)Cl2(mmp)] and [W( /X)Cl2(ebp)] [X /O, NPh-2,6-Me2; H2mmp /2,2?methylenebis(4,6-dimethylphenol); H2ebp /2,2?-ethylidenebis(4,6-di-tert -butylphenol)] [3]. Nakamura’s group has studied complexes of sulfur-bridged bis(phenol), and has characterised several crystal structures, e.g. [W(h 2EtC /CEt)(tbp)Cl2] (H2tbp/2,2?-thiobis(4,6-dimethylphenol) [4]. In the context of a broader program aimed at developing transition metal catalysts for olefin metathesis reaction, our laboratory has recently become interested in the chemical manipulation of tungsten aryloxide complexes [2b,5]. Our long-term goal is to prepare polymer-based tungsten(VI) phenoxides and use them as recoverable catalyst precursors. To gain a greater degree of understanding this chemistry, we have now prepared three new monomeric tungsten(VI) complexes with bis(phenolato) ligands and used them as catalyst precursors for ring opening metathesis polymerisation of norbornene.
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 0 8 9 7 - 5
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2. Results and discussion 2.1. Synthesis and spectroscopy of the complexes The addition of 1 equiv. of the ligand precursors 2,2?methylenebis(4-methyl-6-tert -butylphenol) (H2mbp) or H2ebp to the refluxing toluene solution of [W(eg)3] 1 (H2eg/1,2-ethanediol) leads to the formation of the heteroleptic, monomeric compounds 2 and 3, respectively (Scheme 1). The red solid products can be purified by crystallisation or by column chromatography to obtain air stable complexes in high yields. They are soluble in chlorinated hydrocarbons, ethers and aromatic solvents but only slightly soluble in hexane. Bulky tert -butyl groups in ortho -positions of phenoxide moieties seems to prevent further substitution of eg ligands as the use of excess bis(phenol) do not result in complexes of type [W(biphenolate)2(eg)]. However, 2 equiv. of sterically less hindered 2,2?-dihydroxy-1,1?dinaphtylmethane (H2dinap) reacts with 1 equiv. of 1 in toluene at reflux temperature to form heteroleptic complex [W(dinap)2(eg)] (4) (Scheme 2). This intense red coloured compound is slightly soluble in toluene and chlorinated solvents and it is stable enough to be purified by column chromatography. Unfortunately, 4 was not soluble enough to be used in our catalyst tests (see below). Earlier studies on transition metal complexes of 2,2?methylenebisphenoxides have shown that the puckered eight-membered ring formed by chelation to metal centre undergoes slow flipping on the NMR timescale [6 /8]. Therefore, the methylene protons in the chelate backbone are typically diastereotopic at ambient temperature.[8] The 1H NMR spectra of complexes 2 and 3 show typical resonances for aromatic protons and alkyl substituents of ligands. In the aliphatic region of the spectra of these complexes are seen two multiplets for ethanediolate groups, which implies that there are two different environments for these methylene protons. The diastereotopic methylene protons in the chelate ring of 2 are separated by 2 ppm showing doublets at 3.3 and 5.4 ppm. Also the methine proton in the chelate ring of 3 shows doublets at 4.4 and 5.8, respectively. 1 H NMR spectrum of compound 4 is not very informative. Methylene protons in the bis(naphtolate)
Scheme 2. Activation and regeneration of catalyst precursors.
rings showed three multiplets at 4.8 /5.9 ppm. A sharp singlet at 5.7 ppm was assigned as a signal for ethanediolato protons, as similar signal for that ligand in comparable W(eg)(OAr)4 complex, [W(calix)(eg)] (calix /calix[4]arene), is found at 5.83 [9].
2.2. Crystal structures Compounds 2/4 form monomeric molecules in which six oxygen donors surround the central tungsten atoms (Figs. 1 /3). The distorted octahedral WO6 units are generally similar, but not identical in all these compounds, as seen from the bonding parameters presented
Fig. 1. Molecular structure of [W(mb)(eg)2] (2). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids have been drawn at 30% probability level.
Scheme 1. Preparation of complexes. (i) H2biphe /PhMe, reflux; (ii) 2 H2dinap /PhMe, reflux.
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1019
Table 1 ˚ ) and angles (8) for 2 /4 Selected bond lengths (A
Fig. 2. Molecular structure of [W(ebp)(eg)2] (3). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids have been drawn at 30% probability level.
b
Bond lengths W/O(1) W/O(2) W/O(3) W/O(4) W/O(5) W/O(6) C(1) /O(1) C(n)/O(2) c
1.8970(17) 1.8848(16) 1.9324(18) 1.9049(18) 1.9069(16) 1.9143(17) 1.373(3) 1.368(3)
1.891(6) 1.899(5) 1.923(11) 1.899(6) 1.925(7) 1.888(7) 1.361(12) 1.365(10)
1.930(8) 1.908(8) 1.894(7) 1.891(8) 1.879(8) 1.925(8) 1.397(13) 1.362(12)
1.911(4) 1.942(4) 1.904(4) 1.903(4) 1.902(5) 1.889(4) 1.366(7) 1.362(7)
Bond angles O(1)/W/O(2) O(1)/W/O(3) O(3)/W/O(4) O(2)/W/O(5) O(5)/W/O(6) O(4)/W/O(6) C(1) /O(1)/W C(n)/O(2)/W C(n)/O(3)/W C(n)/O(4)/W C(n)/O(5)/W C(n)/O(6)/W
87.35(7) 165.66(7) 77.86(8) 162.52(7) 78.72(8) 160.23(8) 148.82(15) 151.18(14) 119.10(17) 120.23(17) 119.79(17) 118.03(17)
88.7(3) 168.0(3) 78.3(3) 167.5(3) 79.5(3) 159.6(3) 152.9(6) 158.7(6) 119.1(7) 117.2(6) 117.9(7) 118.3(7)
84.6(3) 164.7(4) 78.0(3) 162.3(4) 78.4(4) 166.4(3) 130.1(7) 127.1(7) 121.0(8) 120.7(8) 121.3(8) 118.1(8)
84.7(2) 162.9(2) 84.9(2) 164.6(2) 78.4(2) 161.3(2) 141.3(4) 128.6(4) 131.3(4) 138.7(4) 121.0(4) 121.4(4)
b c
in Table 1. If we have [W(biphenolate)(eg)2] type compound, where biphenolate is a bidentate phenolate dianion (or two monodentate phenolate anions), the shape and rigidity of biphenolate ligand modify the actual octahedral coordination sphere of the W(VI) ion. Comparing the structural parameters of 2 and 3 shows that these have fairly similar overall structure, so the substituents in the main body of the ligand modify the structure of the complexes only slightly. To see the influence of different ligand bodies on the complex structure, we should compare the structures formed by 2,2?-methylenebis(phenolato) ligand and binaphtholato ligand, thus also bonding parameters of [W(bino)(eg)2]
W(bino)(eg)2
3
a
Fig. 3. Molecular structure of [W(dinap)2(eg)] (4). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids have been drawn at 30% probability level.
a
2
4
Values around W(1). Binaftolatobis(ethanediolato)tungsten(VI) in ref. [5b]. C(n) is a carbon atom bonded to a host oxygen atom.
have included in Table 1 [5b]. Consequently, we can see large changes in bonding parameters of these bis(phenolato) ligands to W(VI) ion. For ex. W /O(1) bond lengths, as well as O(1) /W /O(2) and C(1) /O(1)/W bond angles are quite different in 2 and [W(bino)(eg)2]. Normally O(phenolato) is bonded more tightly (shorter bond distances) to W(VI) center than O(alcoholato). However, we can see now that a rigid ligand bonds less effectively to W(VI) center. This phenomenon is also seen in the bonding parameters of 4, where two bis(naphtolato) ligands are attached to W(VI): one of these ligands is quite loosely bonded to W(VI) center. Finally, the bonding parameters of aliphatic diolato groups are quite similar in compounds studied now as in earlier reported tungsten(VI) diolato complexes with simple phenoxides or binaphtholates, e.g. [W(eg)2(OPhR2-2,6)2] [5a] and [W(bino)(eg)2] [5b]. 2.3. Polymerisation studies We have earlier studied ROMP processes employing complexes of type [W(diol)2(OAr)2] and found that the catalytic activities depends on the bulkiness of alkyl substituents at the phenoxide groups [2b]. In this study we used sterically hindered complexes 2 and 3 as precatalysts in ROMP of norbornene and dicyclopentadiene, using a Et2AlCl cocatalyst. Both tungsten precatalysts showed moderate activity for polymerisation of norbornene at 0 8C as the yields of polymers after 15
1020
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min reaction were 38% for 2 and 42% for 3, respectively. Though, they were inactive for the polymerisation of dicyclopentadiene. At 80 8C both precatalysts produced polynorbornene and polydicyclopentadiene in practically quantitative yields. However, the high yields of polydicyclopentadiene were not necessarily due to the increased activity of the catalysts in elevated temperature, but it is possibly simply due to the thermally induced cross-linking reaction [10]. The nature of the active species is unknown, but we can suppose the traditional model for precatalyst activation-dialkylation followed by a-H elimination to produce the reactive Schrock-type alkylidene [11]. Nevertheless, active catalysts can be transformed into compounds 2 or 3, respectively, by adding excess H2eg, thus these precatalysts can be recovered from reaction mixtures in nearly quantitative yields (Scheme 2).
3. Conclusion Bulky methylene-bridged bis(phenol)s react with trisdiolatotungsten(VI) complex [W(eg)3] (eg /ethanediolate dianion) providing heteroleptic complexes [W(biphenolate)(eg)2], where bis(phenol) /2,2?-CH2(4Me-6-t BuPhOH)2 or 2,2?-CHMe(4,6-t Bu2PhOH)2. Sterically less hindered 2,2?-dihydroxy-1,1?-dinaphtylmethane forms disubstituted complex [W(biphenolate)2(eg)]. In solid state these isolated compounds consist of monomeric tris(diolato)tungsten(VI) molecules in which the central tungsten atom is bonded to six oxygen atoms forming a distorted octahedral coordination sphere around the metal ion. Compounds of the type [W(biphenolate)(eg)2] can be activated by Et2AlCl to catalyse ring opening polymerisation metathesis of norbornene.
4. Experimental Chemicals were from commercial origins and were used without subsequent purification. Toluene used in polymerisation studies was distilled over sodium under a nitrogen atmosphere. Monomer solutions in toluene ˚ molecular sieves. 2,2?-dihydroxywere stored over 4 A 1,1?-dinaphthylmethane was prepared as described earlier [12]. NMR spectra were recorded on Bruker AM200 spectrometer. Elemental analyses were obtained using a Perkin/Elmer CHNS-Analyzer 2400. Analytical samples were dried in vacuo at 40 8C for 2 h prior to elemental and spectral analyses. 4.1. Preparation of complexes [W (eg )2(mbp )] (2) 1.09 g of 1 (3.00 mmol) was heated with 1.02 g of H2mbp (3.00 mmol) in 60 ml of toluene at
reflux to form a red solution. The H2eg that formed was removed by azeotropic distillation with toluene during 4 h. The solvent was evaporated and orange solid residue was column chromatographed (silica /CH2Cl2) to give complex 2 as orange/red solid, which was crystallised from 10 ml of acetonitrile to obtain intense red prisms with the formula 2 ×/CH3CN in yield of 82% (1.68 g). Samples were dried in vacuo at 40 8C for 6 h before elemental and spectral analyses to remove any solvent of crystallisation. For 2: dH (CDCl3, standard SiMe4): 7.07 (s, 1H, ArH), 6.97 (s, 2H, ArH), 6.92 (s, 1H, ArH), 5.72 (br, s, 2H, /OCH2 /), 5.53 (br, s, 2H, /OCH2 /), 5.38 (d, 1H, /CH2 /), 5.35 (br, s, 2H, /OCH2 /), 5.17 (br, s,2H, /OCH2 /), 3.28 (s, 1H, /CH2 /), 2.34 s, (s, 3H, p-CH3), 2.28 (s, 3H, p-CH3), 1.41 (s, 9H, C(CH3)3), 1.34 (s, 9H, C(CH3)3). Found: C, 50.2; H, 5.8. C27H38O6W requires C, 50.5; H, 6.0%. [W (ebp )(eg )2] (3) 1.09 g of 1 (3.00 mmol) was heated with 1.32 g of H2ebp (3.00 mmol) in 60 ml of toluene at reflux to form a red solution and the H2eg that formed was removed by azeotropic distillation with toluene during 4 h. The solvent was evaporated and an orange solid residue was crystallised from 40 ml of hexane to obtain complex 3 as orange-red microcrystals in yield of 90% (2.05 g). Crystals of 3 for X-ray structure analysis were grown from acetonitrile. For 3: dH (CDCl3, standard SiMe4): 7.24 (1H, ArH), 7.17 (m, 3H, ArH), 5.8 /5.1 (9 H, several multiplets, /OCH2 / and /CHCH3), 1.65 (d, 3H, /CHCH3), 1.32 (q, 36H, C(CH3)3). Found: C, 55.4; H, 6.9. C34H52O6W requires C, 55.1; H, 7.1%. [W (dinap )2(eg )] (4) 120 mg of 1 (0.33 mmol) was heated with 200 mg of H2dinap (0.67 mmol) in 60 ml of toluene at reflux for 4 h to form a red solution. The solution was then concentrated to 15 ml and allowed to stand at r.t. overnight to obtain complex 4 as red powder in 69% yield (190 mg). Crystals of 4×/CH2Cl2 for X-ray structure analysis were grown from CH2Cl2 by slow evaporation. dH (CDCl3, standard SiMe4): 8.80 / 7.20 (several multiplets, 24H, ArH), 5.90 /4,80 (several multiplets, 4H, /CH2 /), 5.73 (s, 4H, /OCH2 /). Found: C, 62.6; H, 4.0. C44H32O6W requires C, 62.9; H, 3.8%. 4.2. Polymerisation studies Polymerisations were performed under a nitrogen atmosphere applying the protocol reported by Barnes et al. [2]. Our experiments were carried out employing a 1:4.5:50 tungsten precatalyst: activator:monomer ratio. The precatalyst (2 /105 mol in 10 ml of toluene) was treated with 0.05 ml of Et2AlCl solution (1.8 M in toluene, 9/105 mol). The red reaction mixture was stirred for 5 min and 1 ml of 1 M olefinic monomer (1.0 mmol) in toluene was added. After 15 min stirring the solution was treated with 10 ml of methanol to precipitate polymer as a white solid. Polymers were
A. Lehtonen, R. Sillanpa¨a¨ / Polyhedron 21 (2002) 1017 /1022
swelled in toluene, re-precipitated by methanol and dried in vacuo at 40 8C for 8 h.
4.3. Single crystal X-ray diffraction Crystal data for compounds 2 /4, along with other experimental details, are summarised in Table 2. The crystallographic data were collected for 2 and 3 at 173 K on a Nonius Kappa CCD area-detector diffractometer using graphite monochromatised Mo Ka radiation (l/ ˚ ). Lattice parameters were determined from 0.71073 A ten images recorded with 18 8 scans and subsequently refined on all data. The data collection was performed using 8 and v scans with 18 steps using an exposure time of 5 s per frame for 2 and 2 s per frame for 3. The crystal-to-detector distances were 30 and 40 mm for 2 and 3, respectively. The data were processed using DENZO-SMN v0.93.0 [13]. MULABS [14] absorption correction was applied for the data of both compounds, but in final calculations the corrected data were used only for 3, because corrected data for 2 give higher R -values than the uncorrected one. Single-crystal data collection for 4 was performed at ambient temperature on a Rigaku AFC5S diffractometer using graphite monochromatised Mo Ka radia˚ ). Data reduction and subsequent tion (l /0.71069 A calculations were performed with TEXSAN for Windows [15]. The data were corrected for Lorenz and polarisation effects, for absorption (PSI-scans [16]) and for decay. Table 2 Crystal data and experimental details of the structure determination of 2 /4
Formula Mr Crystal system Space group (no.) ˚) a (A ˚) b (A ˚) c (A a (8) b (8) d (8) ˚ 3) U (A Z Dcalc (g cm 3) m (Mo Ka) (cm 1) Observed reflections Rint Parameters R1 wR2
2
3
4
C29H41NO6W 683.48 triclinic P/1¯ (2) 9.1940(1) 10.1566(1) 16.3405(2) 77.035(1) 82.012(1) 86.927(1) 1472.15(3) 2 1.542 39.63 6681 0.0159 332 0.022 (0.020) a 0.047 (0.046)
C34H52 O6W 740.61 orthorhombic Pca 21 (29) 15.8055(2) 16.5843(2) 26.5014(3) 90 90 90 6946.63(14) 8 1.416 33.65 14714 0.0600 736 0.084 (0.048) 0.104 (0.093)
C45H34Cl2O6W 925.47 triclinic P/1¯ (2) 11.289(3) 17.342(3) 10.924(2) 95.566(18) 117.161(14) 76.898(17) 1853.2(6) 2 1.658 33.12 6534 0.0368 487 0.081 (0.041) 0.076 (0.068)
R1 /SjjFoj/jFcjj/SjFoj, wR2 /{S[w (Fo2/Fc 2)2]/S[w (Fo2)2]}1/2 and w /1/[s 2(Fo2)/(aP )2/bP )], where P/(2Fc 2/Fo2)/3. a Values in parentheses for reflections with I /2.0s (I ).
1021
The structures were solved by direct methods using the SIR-92 program [17] and full-matrix least-squares refinements on F 2 were performed using the SHELXL-97 program [18]. For both all heavy atoms were refined anisotropically. The CH hydrogen atoms were included at the fixed distances with fixed displacement parameters from their host atoms. Compound 3 was refined as a racemic twin (Flack’s parameter is 0.65(1). Figures were drawn with ORTEP-3 for Windows [19].
5. Supplementary data Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 174185 /174187. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: /44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).
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[14] R.H. Blessing, Acta Crystallogr., Sect. A 51 (1995) 33 /38. [15] TEXSAN for Windows. Structure Analysis Software. Molecular Structure Corporation, Texas 77381, USA, 1997. [16] A.C. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr., Sect. A 24 (1968) 351.
[17] A. Altomare, G. Cascarano, C. Giacovazzo, A. Gualiardi, M.C. Burla, G. Polidori, M. Camalli, J. Appl. Cryst. 27 (1994) 435. [18] G.M. Sheldrick, SHELXL-97, In: Program for Crystal Structure Refinement, University of Go¨ttingen, Germany, 1997. [19] L.J. Farrugia, J. Appl. Cryst. 30 (1997) 565.