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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: [email protected] b Department of Chemistry, The University of Southampton, Highfield, Southamptons, U.K. . Fax: 44 02380 596723; Tel:44 02380 596722; E-mail:[email protected]

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The synthesis and spectroscopic characterisation of phosphorus-proton coupling in this case in contrast to complex ferrocene-ligand based ruthenium alkylidene complexes is 1a where no coupling is observed. reported as air stable solids which are highly active in The related ligands 2a, 2c, and 2d were then used in the norbornene polymerisation. synthesis and similar results were observed in each case. The alpha protons were observed respectively at 19.09, 17.20, and Metal alkyidene complexes have become crucially important in 17.83 ppm in the product complexes 3a, 3c, and 3d. A direct mainstream organic synthesis both in polymerisation and correlation is observed in the case of 3a-3d between ligand selective olefin metathesis, most notably as a consequence of the basicity and chemical shift. In the case of the reaction bis- pioneering work of Grubbs1 and Schrock2. In the low oxidation diphenylphosphinoferrocene, 3a, dppf, partial precipitation was state alkylidene research, the design and synthesis of ligands has observed from the toluene solution immediately on warming the been crucial to the development of more efficient catalysts. The reaction mixture. The buff coloured precipitate was isolated and pertinent ligand chemistry has been fine-tuned to give more washed with petroleum ether before being recrystallised from a stable and active catalysts3; the optimum ligands of choice mixture of dichloromethane and petrol (b.p. 40-60o) to give dark currently are the substituted imidazolin-2-ylidenes ligands4 in green yellow crystalline material which was characterised both ruthenium-based complexes however there remains by NMR spectroscopy and single crystal X-ray diffraction‡. The considerable scope for examining alternatives. ORTEP of this molecule is shown in figure 1, which shows the We have a longstanding interest in the synthesis and use of the geometry of these compounds which concurs with the data ferrocene-based ligands5 both in catalysis and in material design. obtained from the NMR results. Each of these complexes 3a-3d Given the successful application of ferrocene-based ligands in was used in the test polymerisation of norbornene (10% solution other areas of catalysis, the facile tuning of their steric and in dichloromethane) and were observed to cause full gelation electronic properties, and the wide range of readilly available within seconds indicating comparable catayltic activity to that of ligands it was of interest to explore the chemistry of metal Grubbs complex. alkylidenes using ferrocene-based phosphines ligands. In olefin metathesis work the use of a alkylidene tethered ligands NMR spectroscopy has been used extensively in product are required in the design of an efficient catalyst thus it was characterisation and where appropriate new complexes have decided to use a difunctional vinyl-phosphine ligand. The ligand been isolated and fully characterised. The alpha alkylidine of choice initially was compound 5. Control reactions were first proton resonance in ruthenium alkylidene complexes is performed as follows : vinylferrocene and the monodentate particularly diognostic in NMR studies and thus constant refererence to these resonances will be made as an aid to product X characterisation. The synthetic route chosen was the direct Fig. 1. Molecular structure of (3a) with hydrogens and two molecules preparation of the new complexes starting with the commercially of dichloromethane omitted for clarity. available Grubbs catalyst, scheme 1. This route was chosen diphenylphosphinoferrocene were reacted independently with a because of its simplicity although, of course, the cheaper direct solution of Grubbs catalyst. synthesis from dichloro-tris-(triphenylphosphine)ruthenium The stepwise addition of vinylferrocene to Grubbs complex clearly would be applicable if scale up was required. The initial results in the formation of the ferrocene substituted alkylidene strategy was to use some basic ferrocene ligands beginning with compex 1b (alpha alkylidene proton resonance at +19.03ppm) bis-(diisopropylphophino)ferrocene, dipppf, 2b6. The direct 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. Scheme 1 The Synthesis of Complexes 2a-2d. 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 Scheme II Ferrocenyl-substituted alkylidene ligands. geometry which is reflected in the observation of the 1 CREATED USING THE RSC CHEMCOMM TEMPLATE - SEE HTTP://WWW.RSC.ORG/IS/JOURNALS/TEMPLATES/TEMPLATES.HTM FOR DETAILS

The mixed vinylphosphine ligand 5 was prepared using 33883 reflections collected, 9187 independent [R(int) = 0.0525], giving 2 2 conventional synthetic methodology and this was reacted with R1 = 0.0385 for observed unique reflections [F > 2(F )] 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.

Scheme III Ligand metathesis reaction. Grubbs complex initaily under ambient conditions. It was 1. selected references (a) M. Trnka and R.H. Grubbs Acc. Chem. Res., evident that a rapid reaction took place even at ambient 2001, 34, 18. and references therein (b) B. R. Maughon,; R. H. Grubbs, temperature with the observation of the alpha alkylidene protons Macromolecules 1997, 30, 3459. (c) Z. Wu, S. T. Nguyen,. R. H.. at 20.28 ppm as a doublet, however the starting complex was Grubbs,; , J. W Ziller.. J. Am. Chem. Soc. 1995, 117, 5503. (d) Cucullu, present even when a three fold excess of the ligand was added M. E.; Li, C.; Nolan, S. P.; Nguyen, S. T.; Grubbs, R. H.. organometallics 1998, 17, 5565. (e) M. Ulman, R.H. Grubbs, Organometallics 1998, 17, 2484. (f) M. Ulman, T.R. Belderrain,R.H. Grubbs, Tetrahedron Lett. 2000, 41, 4689. (g) D. .J. O'Leary, H. E. Blackwell,R.A. Washenfelder, K. Miura, R.H. Grubbs, Tetrahedron Lett. 1999, 40, 1091. 2. (a) R.R. Schrock, Tetrahedron 1999, 55, 8141. (b) J. H. Oskam, R. X R. Schrock, J.Amer.Chem. Soc. 1993, 115, 11831.(c) R. R. Schrock, J. therefore an equilibrium exists. The phosphorus NMR spectrum S. Murdzek, G. C. Bazan, J. Robbins, M. DiMare, M. O'Regan, J. indicated the presence of several products in solution in addition Amer.Chem. Soc. 1990, 112, 3875. (d) R. Toreki, R. R. Schrock, . to the expected product 6. When this reaction was carried out J.Amer.Chem.Soc. 1990, 112, 2448. (e) C. J.Schaverien, J.C. Dewan, under identical conditions to those used for the syntheses of R.R. Schrock, J.Amer.Chem.Soc. 1986, 108. 2771.(f) S.M. Rocklage, complexes 3a-d a green yellow powder, which was observed to 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. be highly active in norbornene polymerisation could indeed be Wuchrer, J. Y. Jamieson, R.R.Schrock,A.H. Hoveyda, isolated athough attempts at recrystallisation of this material J.Amer.Chem.Soc. 2001, 123, 3139 (h) J.H. Wengrovius,R.R. failed. The subsequent work was carried out on the reaction of a Schrock, M.R. Churchill, J.R. Missert,W.J. Youngs, J.Amer.Chem.Soc. ferrocene-based trisphosphine ligand to investigate whether the 1980, 102, 4515. (i) G. S. Weatherhead, J. H. Houser, J.G. Ford, J. Y. use of a tridentate phosphine of this type would diplace the Jamieson, R.R. Schrock, A.H. Hoveyda, H. Tetrahedron Lett. alkylidene ligand in addition to the cyclohexylphosphine 2000, 41, 9553.(j) S. L..Aeilts, D.R. Cefalo, P. J. Bonitatebus, J. H. ligands..The ligand chosen for this study was bis-(1'- Houser, A. H. Hoveyda, R..R. Schrock, Angew.Chem.Int.Ed. 2001, diphenylphosphinoferrocenyl)phenylphosphine, trifer, 7.7 40, 1452. o At 25 in CDCl3 in an NMR experiment the reaction is very slow 4. (a) M. Scholl, T. M. Trnka, J. P Morgan, , R. H. Grubbs, Tetrahedron with the observation of new small ferrocene resonaces at and a Lett. 1999, 40, 2247. (b) T Weskamp,.; F. J Kohl,.; W. A Herrmann, J. weak alpha alklylidene proton resonance at +20.6 ppm after 1h. Organomet. Chem. 1999, 582, 362-365. (c) T Weskamp,.; F. J Kohl,.; W After standing for 30h. it is evident that a new complex is Hieringer,.; D Gleich,.; W. A. Herrmann, Angew. Chem., Int. Ed. 1999, present however the reaction is clearly not a clean one as 38, 2416. (d) L Ackermann, A Fürstner.; T Weskamp,.; F. J.; Kohl, W. interpeted from the NMR data. The reaction of at 80 o of trifer A Herrmann, Tetrahedron Lett. 1999, 40, 4787-4790. (e) J Huang.; E. D with Grubbs catalyst leads to the rapid removal of the alkilydene Stevens,.; S. P Nolan, J. L Petersen. J. Am. Chem. Soc. 1999, 121, 2674. ligand in addition to the phosphines however again the reaction (f) M. Scholl, S. Ding, S.;C. W. Lee, R. H. Grubbs, R. H.., Org. Lett. is not a clean one Attempts to crystallise the product led to the 1999, 1, 953-956. formation of a green-yellow powder with dark microcrystals 5. (a) I.R. Butler, M.G.B. Drew, C.H. Greenwell, E. Lewis, M. Plath, S. which desolvated on moderate drying. The phosphorus NMR of Mussig, J. Szewczyk, Inorg. Chem. Commun.,1999, 2, 576. (b) I. the powder obtained from the reaction of excess trifer with R..Butler, S Mussig, M Plath, Inorg. Chem. Commun, 1999, 2, 424. (c) Grubbs complex indicated that a ruthenium ferrocenyl phosphine A.L. Boyes, I.R.Butler, S.C. Quayle, Tetrahedron Lett. 1998, 39, 7763. complex had formed (resonance observed at +26.55 ppm and (d) I.R. Butler, W.R. Cullen, S.J. Rettig, ASC White, J. Organomet. +127.94 resonances) however there was also evidence for a Chem. 1995, 492,157. (e) I. R. Butler, M Kalaji, M. Hursthouse, A..I. pendant phosphine ( -18 ppm). Karaulov, KMLA Malik J.. Chem. Soc.-Chem. Commun. 1995, 459 I.R. Butler, Polyhedron, 1992, 11, 3117 The latter behaviour is similar to that recently observed by us in 8 related reactionsof ruthenium trifer complexex. In conclusion it 6. Butler, IR Cullen WR Kim, TJ, Synth. React. Inorganic Met.-Org. is evident that these new ferrocene-based complexes will be Chem. 1985, 15, 109. useful materials to investigate in olefin metathesis and polymerisation reactions. 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. Notes and references Meunier, and P. Zanello, J. Organomet. Chem. 2001, 637, 538.

† Dichlorobis(diisopropylphosphineferrocene)benzyldeneruthenium (1b) Yield = 67.94mg, 82.22%, Melting Point = Decomposes without melting 1 H 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 3 on phenyl ring, JH-H = 6.89Hz), 8.60 (s, agostic phenyl proton i.e ortho 31 proton), 17.03 (t, carbene proton Ru =CH, JC-P = 17.72Hz) {H} P NMR i (CDCl3) ;  = 57.66 (s, P Pr2 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,  = 3 -3 98.6830(10), U = 4032.58(11)Å , Dc = 1.625Mg m , 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.

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