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Ferrocene-based Ligands in Ruthenium Alkylidene Chemistry

Ian R. Butler a, Simon J. Coles b, Michael B. Hursthouseb, Dilwyn J. Roberts a, Naho Fujimoto a

a Department of Chemistry, The University of Wales, Bangor, Bangor, Gwynedd, U.K. Fax: 44-(0)1248 370528; Tel:44 1248 382390; E-mail: i.r.butler@bangor.ac.uk

b Department of Chemistry, The University of Southampton, Highfield, Southamptons, U.K. . Fax: 44 02380 596723; Tel:44 02380 596722; E-mail:s.j.coles@soton.ac.uk

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The synthesis and spectroscopic characterisation of ferrocene-ligand based ruthenium alkylidene complexes is reported as air stable solids which are highly active in norbornene polymerisation.

Metal alkyidene complexes have become crucially important in mainstream organic synthesis both in polymerisation and selective olefin metathesis, most notably as a consequence of the pioneering work of Grubbs1 and Schrock2. In the low oxidation state alkylidene research, the design and synthesis of ligands has been crucial to the development of more efficient catalysts. The pertinent ligand chemistry has been fine-tuned to give more stable and active catalysts3; the optimum ligands of choice currently are the substituted imidazolin-2-ylidenes ligands4 in ruthenium-based complexes however there remains considerable scope for examining alternatives.

We have a longstanding interest in the synthesis and use of ferrocene-based ligands5 both in catalysis and in material design. Given the successful application of ferrocene-based ligands in other areas of catalysis, the facile tuning of their steric and electronic properties, and the wide range of readilly available ligands it was of interest to explore the chemistry of metal alkylidenes using ferrocene-based phosphines ligands.

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Scheme 1 The Synthesis of Complexes 2a-2d.

NMR spectroscopy has been used extensively in product characterisation and where appropriate new complexes have been isolated and fully characterised. The alpha alkylidine proton resonance in ruthenium alkylidene complexes is particularly diognostic in NMR studies and thus constant refererence to these resonances will be made as an aid to product characterisation. The synthetic route chosen was the direct preparation of the new complexes starting with the commercially available Grubbs catalyst, scheme 1. This route was chosen because of its simplicity although, of course, the cheaper direct synthesis from dichloro-tris-(triphenylphosphine)ruthenium clearly would be applicable if scale up was required. The initial strategy was to use some basic ferrocene ligands beginning with bis-(diisopropylphophino)ferrocene, dipppf, 2b6. The direct reaction of a slight excess of dipppf with Grubbs catalyst 1a resulted in the formation of a geen-yellow solution, which on solvent removal and washing with petrol left a green-buff coloured solid. This material was characterised by NMR and mass spectrometry and thus was identified as the metal alklydene complex, 3b. The alpha proton on the alkylidene ligand was observed at 17.03 ppm as a triplet JP-H = 17.7 Hz which is shifted upfield from that of the starting compound from by approx 3 ppm. Clearly the use of a bidentate ligand changes the overall geometry which is reflected in the observation of the phosphorus-proton coupling in this case in contrast to complex 1a where no coupling is observed.

X

Fig. 1. Molecular structure of (3a) with hydrogens and two molecules of dichloromethane omitted for clarity.

The related ligands 2a, 2c, and 2d were then used in the synthesis and similar results were observed in each case. The alpha protons were observed respectively at 19.09, 17.20, and 17.83 ppm in the product complexes 3a, 3c, and 3d. A direct correlation is observed in the case of 3a-3d between ligand basicity and chemical shift. In the case of the reaction bis-diphenylphosphinoferrocene, 3a, dppf, partial precipitation was observed from the toluene solution immediately on warming the reaction mixture. The buff coloured precipitate was isolated and washed with petroleum ether before being recrystallised from a mixture of dichloromethane and petrol (b.p. 40-60o) to give dark green yellow crystalline material which was characterised both by NMR spectroscopy and single crystal X-ray diffraction‡. The ORTEP of this molecule is shown in figure 1, which shows the the geometry of these compounds which concurs with the data obtained from the NMR results. Each of these complexes 3a-3d was used in the test polymerisation of norbornene (10% solution in dichloromethane) and were observed to cause full gelation within seconds indicating comparable catayltic activity to that of Grubbs complex.

In olefin metathesis work the use of a alkylidene tethered ligands are required in the design of an efficient catalyst thus it was decided to use a difunctional vinyl-phosphine ligand. The ligand of choice initially was compound 5. Control reactions were first performed as follows : vinylferrocene and the monodentate diphenylphosphinoferrocene were reacted independently with a solution of Grubbs catalyst.

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Scheme II Ferrocenyl-substituted alkylidene ligands.

The stepwise addition of vinylferrocene to Grubbs complex results in the formation of the ferrocene substituted alkylidene compex 1b (alpha alkylidene proton resonance at +19.03ppm) with the diplacement of styrene as monitored by NMR. The addition of vinylferrocene to the complex 3a and 3b similarly results in styrene diplacement to form new complexes with the alpha alkyidene proton observed at XXX and +19.40 ppm, and respectiveley for 4a and 4b, scheme 2. The reaction of diphenylphosphinoferrocene with Grubbs complex run as the other control experiment indicated that phosphine substitution was extremely slow (only a very weak resonance (singlet) for a new alpha alkylidene resonance at 21.7 ppm in very low concentration was observed.). It is thus expected that the alkylidene metathesis will occur first in tethered ligands.

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Scheme III Ligand metathesis reaction.

The mixed vinylphosphine ligand 5 was prepared using conventional synthetic methodology and this was reacted with Grubbs complex initaily under ambient conditions. It was evident that a rapid reaction took place even at ambient temperature with the observation of the alpha alkylidene protons at 20.28 ppm as a doublet, however the starting complex was present even when a three fold excess of the ligand was added therefore an equilibrium exists. The phosphorus NMR spectrum indicated the presence of several products in solution in addition to the expected product 6. When this reaction was carried out under identical conditions to those used for the syntheses of complexes 3a-d a green yellow powder, which was observed to be highly active in norbornene polymerisation could indeed be isolated athough attempts at recrystallisation of this material failed. The subsequent work was carried out on the reaction of a ferrocene-based trisphosphine ligand to investigate whether the use of a tridentate phosphine of this type would diplace the alkylidene ligand in addition to the cyclohexylphosphine ligands..The ligand chosen for this study was bis-(1'-diphenylphosphinoferrocenyl)phenylphosphine, trifer, 7.7

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At 25o in CDCl3 in an NMR experiment the reaction is very slow with the observation of new small ferrocene resonaces at and a weak alpha alklylidene proton resonance at +20.6 ppm after 1h. After standing for 30h. it is evident that a new complex is present however the reaction is clearly not a clean one as interpeted from the NMR data. The reaction of at 80o of trifer with Grubbs catalyst leads to the rapid removal of the alkilydene ligand in addition to the phosphines however again the reaction is not a clean one Attempts to crystallise the product led to the formation of a green-yellow powder with dark microcrystals which desolvated on moderate drying. The phosphorus NMR of the powder obtained from the reaction of excess trifer with Grubbs complex indicated that a ruthenium ferrocenyl phosphine complex had formed (resonance observed at +26.55 ppm and +127.94 resonances) however there was also evidence for a pendant phosphine ( -18 ppm).

The latter behaviour is similar to that recently observed by us in related reactionsof ruthenium trifer complexex.8 In conclusion it is evident that these new ferrocene-based complexes will be useful materials to investigate in olefin metathesis and polymerisation reactions.

Notes and references

† Dichlorobis(diisopropylphosphineferrocene)benzyldeneruthenium (1b)

Yield = 67.94mg, 82.22%, Melting Point = Decomposes without melting

1H NMR (CDCl3) ( = 1.12-2.58 dd, 12H), 4.39 (2H, Cp), 4.47 (4H, 2 overlapping resonances, Cp), 4.68 (2H of Cp ring), 7.68 (t, meta-protons on phenyl ring, 3JH-H = 6.89Hz), 8.60 (s, agostic phenyl proton i.e ortho proton), 17.03 (t, carbene proton Ru =CH, JC-P = 17.72Hz) {H}31P NMR (CDCl3) ; ( = 57.66 (s, PiPr2 group), Low Resolution Fast Atom Bombardment MS: Calculated for C29H42FeRuP2Cl2 M+ = 680.416; found 460, 645[-35, loss of Cl], 609 [-35, loss of the other Cl]

‡ Crystallographic data: C43H38Cl6FeP2Ru, Monoclinic, space group P21/c, a = 13.9569(2), b = 14.6122(2), c = 20.0025(4)Å, ( = 98.6830(10)(, U = 4032.58(11)Å3, Dc = 1.625Mg m-3, Z = 4, T = 120(2) K, orange block, 0.18 x 0.14 x 0.06mm3. Data collection was carried out using an Enraf Nonius KappaCCD area detector and SHELXS-97 and SHELXL-97 programs were used for structure solution and refinement. 33883 reflections collected, 9187 independent [R(int) = 0.0525], giving R1 = 0.0385 for observed unique reflections [F2 > 2σ(F2)] and wR2 = 0.0947 for all data. The max. and min. residual electron densities on the final difference Fourier map were 1.126 and –1.262eÅ-3, respectively.

† Footnotes should appear here. These might include comments relevant to but not central to the matter under discussion, limited experimental and spectral data, and crystallographic data.

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2. (a) R.R. Schrock, Tetrahedron   1999,  55,  8141. (b) J. H. Oskam, R. R. Schrock, J.Amer.Chem. Soc.   1993, 115, 11831.(c) R. R. Schrock, J. S. Murdzek, G. C. Bazan, J. Robbins, M. DiMare, M. O'Regan, J. Amer.Chem. Soc.   1990,  112, 3875. (d) R. Toreki, R. R. Schrock, . J.Amer.Chem.Soc.   1990,  112, 2448. (e) C. J.Schaverien, J.C. Dewan, R.R. Schrock, J.Amer.Chem.Soc.   1986, 108. 2771.(f) S.M. Rocklage, J.D.Fellmann, G.A. Rupprecht, L.W. Messerle,R.R. Schrock, J.Amer.Chem.Soc.   1981,  103, 1440. (g) D.R. Cefalo, A.F. Kiely, M. Wuchrer, J. Y. Jamieson, R.R.Schrock,A.H. Hoveyda, J.Amer.Chem.Soc.   2001, 123,  3139 (h) J.H. Wengrovius,R.R. Schrock, M.R. Churchill, J.R. Missert,W.J. Youngs, J.Amer.Chem.Soc.   1980,  102, 4515. (i) G. S. Weatherhead, J. H. Houser, J.G. Ford, J. Y. Jamieson, R.R. Schrock, A.H. Hoveyda, H. Tetrahedron Lett.   2000, 41, 9553.(j) S. L..Aeilts, D.R. Cefalo, P. J. Bonitatebus, J. H. Houser, A. H. Hoveyda, R..R. Schrock, Angew.Chem.Int.Ed.   2001,  40, 1452.

4. (a) M. Scholl, T. M. Trnka, J. P Morgan, , R. H. Grubbs, Tetrahedron Lett. 1999, 40, 2247. (b) T Weskamp,.; F. J Kohl,.; W. A Herrmann, J. Organomet. Chem. 1999, 582, 362-365. (c) T Weskamp,.; F. J Kohl,.; W Hieringer,.; D Gleich,.; W. A. Herrmann, Angew. Chem., Int. Ed. 1999, 38, 2416. (d) L Ackermann, A Fürstner.; T Weskamp,.; F. J.; Kohl, W. A Herrmann, Tetrahedron Lett. 1999, 40, 4787-4790. (e) J Huang.; E. D Stevens,.; S. P Nolan, J. L Petersen. J. Am. Chem. Soc. 1999, 121, 2674. (f) M. Scholl, S. Ding, S.;C. W. Lee, R. H. Grubbs, R. H.., Org. Lett. 1999, 1, 953-956.

5. (a) I.R. Butler, M.G.B. Drew, C.H. Greenwell, E. Lewis, M. Plath, S. Mussig, J. Szewczyk, Inorg. Chem. Commun.,1999, 2, 576. (b) I. R..Butler, S Mussig, M Plath, Inorg. Chem. Commun, 1999, 2, 424. (c) A.L. Boyes, I.R.Butler, S.C. Quayle, Tetrahedron Lett. 1998, 39, 7763. (d) I.R. Butler, W.R. Cullen, S.J. Rettig, ASC White, J. Organomet. Chem. 1995, 492,157. (e) I. R. Butler, M Kalaji, M. Hursthouse, A..I. Karaulov, KMLA Malik J.. Chem. Soc.-Chem. Commun. 1995, 459

I.R. Butler, Polyhedron, 1992, 11, 3117

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7. (a) Butler, IR Davies, RL Synthesis, 1996, 1350. (b) I.R.Butler, S. J. Coles, M. Fontani, M.B. Hursthouse, E. Lewis, KLMA Malik, M. Meunier, and P. Zanello, J. Organomet. Chem. 2001, 637, 538.

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