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Organometallics 2009, 28, 4632–4635 DOI: 10.1021/om9004737
Structures of Lithium Ferrocenylenecuprates and Their Oxidative Coupling Reactions Roberta Bomparola, Robert P. Davies,* Tracey Gray, and Andrew J. P. White Department of Chemistry, Imperial College London, South Kensington, London, U.K. SW7 2AZ Received June 4, 2009 Summary: Lithium ferrocenylenecuprate has been prepared from the treatment of ferrocene with butyllithium and copper(I) mesityl. Depending upon the choice of base (nBuLi/tmeda or tBuLi), one of two different cuprate complexes is obtained, both of which have been characterized using X-ray crystallography to reveal unique trimetallic clusters. In addition, lithium ferrocenylenecuprate is shown to undergo oxidative coupling to give poly-1,10 -ferrocenylene. Metalated ferrocenes have important applications in the preparation of ferrocene-containing polymers, pharmaceuticals, and ligands for catalysis.1,2 Lithioferrocene (FcLi) and 1,10 -dilithioferrocene (1,10 -FcLi2), in particular, have proven to be key intermediates in many reactions, and they can be prepared from the direct reaction of ferrocene with excess tert-butyllithium in thf or 2 equiv of n-butyllithium in the presence of tetramethylethylenediamine (tmeda), respectively.3-5 Mono- and dilithioferrocene readily undergo transmetalation or nucleophilic substitution reactions to give a wide range of ferrocene-containing products.1,2 In addition, recent work by Mulvey and co-workers has shown that direct multimetalation of ferrocene with Li-Mg,6 NaMg,6-8 Li-Zn,9 and Li-Mn10 mixed-metal systems is also possible. Our research interests concern the structures and reactivity of lithium organocuprates of general formula R2CuLi (R = *To whom correspondence should be addressed. E-mail:
[email protected]. Tel: þ44 207 5945754. (1) Stepnicka, P. Ferrocenes: Ligands, Materials and Biomolecules; Wiley: Hoboken, NJ, 2008. (2) Long, N. J. Metallocenes: An Introduction to Sandwich Complexes; Blackwell Science: Oxford, U.K., Malden, MA, 1998. (3) Butler, I. R.; Cullen, W. R.; Ni, J.; Rettig, S. J. Organometallics 1985, 4, 2196–2201. (4) Bishop, J. J.; Davison, A.; Katcher, M. L.; Lichtenb, Dw; Merrill, R. E.; Smart, J. C. J. Organomet. Chem. 1971, 27, 241–249. (5) Rausch, M. D.; Moser, G. A.; Meade, C. F. J. Organomet. Chem. 1973, 51, 1–11. (6) Henderson, K. W.; Kennedy, A. R.; Mulvey, R. E.; O0 Hara, C. T.; Rowlings, R. B. Chem. Commun. 2001, 1678–1679. (7) Clegg, W.; Henderson, K. W.; Kennedy, A. R.; Mulvey, R. E.; O0 Hara, C. T.; Rowlings, R. B.; Tooke, D. M. Angew. Chem., Int. Ed. 2001, 40, 3902–3905. (8) Andrikopoulos, P. C.; Armstrong, D. R.; Clegg, W.; Gilfillan, C. J.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E.; O0 Hara, C. T.; Parkinson, J. A.; Tooke, D. M. J. Am. Chem. Soc. 2004, 126, 11612–11620. (9) Barley, H. R. L.; Clegg, W.; Dale, S. H.; Hevia, E.; Honeyman, G. W.; Kennedy, A. R.; Mulvey, R. E. Angew. Chem., Int. Ed. 2005, 44, 6018–6021. (10) Garcia-Alvarez, J.; Kennedy, A. R.; Klett, J.; Mulvey, R. E. Angew. Chem., Int. Ed. 2007, 46, 1105–1108. (11) Bomparola, R.; Davies, R. P.; Hornauer, S.; White, A. J. Angew. Chem., Int. Ed. 2008, 47, 5812–5815. (12) Davies, R. P.; Hornauer, S. Chem. Commun. 2007, 304–306. (13) Davies, R. P.; Hornauer, S.; Hitchcock, P. B. Angew. Chem., Int. Ed. 2007, 46, 5191–5194. pubs.acs.org/Organometallics
Published on Web 07/30/2009
organo group),11-15 and this has now led us to examine the formation of novel cuprates incorporating ferrocene-based R groups. Although lithium ferrocenylenecuprates, (1,10 Fc)CuLi, have previously been proposed as intermediates in the synthesis of poly-1,10 -ferrocenylenes via oxidative coupling,16,17 as far as we are aware there are currently no structural or spectroscopic data available on these postulated species. In addition, very few ferrocenyl(ene)copper complexes have been structurally characterized at all, with the exception of the pioneering [1,2-Fc(CH2NMe2)Cu]4 tetramer by Nesmeyanov18a and some more recent work by J€ akle and co-workers on heteroleptic ferrocene-pentafluorobenzene copper(I) compounds, which were shown to exhibit unique intramolecular Cu 3 3 3 Fe interactions.18b The 1,1-ferrocenylenecuprate complex [(1,10 -Fc)6Cu4Li6][Li(thf)4]2 (1) was prepared from the in situ reaction of dilithioferrocene, (1,10 -Fc)Li2(tmeda)2/3,3 with CuIMes (Mes = 2,4,6-Me3C6H2) in hexane/thf.19 CuIMes was employed as the organocopper starting material, due to its high thermal stability and its previously reported successful application in the preparation of homoamido- and heteroamidocuprates.11-14 A small batch of highly air and moisture sensitive dark orange hexagonal crystals of 1 were obtained on storage of the reaction mixture at room temperature for 3 days. These were characterized by X-ray crystallography to reveal an ion-separated complex containing two thf-solvated lithium cations and a unique trimetallic [(1,10 -Fc)6Cu4Li6]2dianion (Figure 1). This is, as far as we are aware, the first structurally characterized example of a lithium organocuprate containing a dianionic ligand. It is apparent from the structure of 1 that complete reaction of the dilithioferrocene with CuI to form the (14) Davies, R. P.; Hornauer, S. Eur. J. Inorg. Chem. 2005, 51–54. (15) Bomparola, R.; Davies, R. P.; Hornauer, S.; White, A. J. P. Dalton Trans. 2009, 1104–1106. (16) Neuse, E. W.; Bednarik, L. Transition Met. Chem. 1979, 4, 87–94. (17) Neuse, E. W.; Bednarik, L. Transition Met. Chem. 1979, 4, 104– 108. (19) Synthesis of 1: 5 mmol of ferrocene (0.93 g, 5 mmol), previously purified via sublimation, was suspended in 15 mL of dry hexane. A 1.6 M solution of n-BuLi in hexane/cyclohexane (6.5 mL, 10.4 mmol) was added dropwise, followed by addition of tmeda (0.78 mL, 5.2 mmol). The reaction mixture was stirred at room temperature and under nitrogen for 16 h, to give (1,10 -Fc)Li2(tmeda)2/3, which was used without further purification. A solution of Cu5Mes5 3 C6H5CH3 (1.00 g, 1 mmol), prepared according to literature procedures (Tsuda, T.; Yazawa, T.; Watanabe, K.; Fujii, T.; Saegusa, T. J. Org. Chem. 1981, 46, 192-194), in toluene (12 mL) was added to the solution of (1,10 -Fc)Li2 to yield an orange precipitate. The precipitate was dissolved by addition of 5 mL of THF and heating to 60 °C to give a dark brown solution, from which a small quantity of orange-brown crystals of 1 were obtained after 3 days at room temperature. We were unable to obtain 1 in pure form due to contamination from mesityl-containing byproducts (see text for full discussion). r 2009 American Chemical Society
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Figure 1. (a) Molecular structure of the [(1,10 -Fc)6Cu4Li6]2anion in 1. (b) Cross-section of the [(1,10 -Fc)6Cu4Li6]2- cluster, with only selected atoms shown for clarity. Legend for both parts: purple, Li; orange, Cu; light blue, Fe; gray, carbon.
quintessential R2CuLi type organocuprate has not occurred, there being two fewer CuI centers in the cluster than requisite. Although organocuprates are often considered to adopt stoichiometric 1:1 Cu:Li aggregates in both their resting and reactive states,20 differing Cu:Li stoichiometry organocuprate complexes have also been documented in the literature and may themselves possess interesting reactivity.12 Of the twelve carbanion centers on the six ferrocenylene units present in 1, eight are coordinated to both CuI and Li centers and can thus be considered lithium cuprate units, whereas the remaining four are solely coordinated to Li centers and are best considered standard organolithium units. The anion in 1 can be regarded as being constructed from two [(1,10 -Fc)3Cu2]4- units assembled perpendicular to one another around a central Li4 core, with two additional Li cations (Li5 and Li6) bridging the Cp rings of the dicuprated ferrocenylene units (see Figure 1b). Within the [(1,10 Fc)3Cu2]4- units the C-Cu-C bond angles approach linearity, in the range 169.26(13)-170.20(13)° (mean 169.56°), and Cu-C distances lie in the range 1.906(3)-1.927(3) A˚ (20) Gschwind, R. M. Chem. Rev. 2008, 108, 3029–3053.
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Figure 2. Molecular structure of 2 viewed (a) perpendicular to and (b) along the Fe 3 3 3 Fe axis. Hydrogen atoms are omitted for clarity. For the color legend, see the caption to Figure 1.
(mean 1.915 A˚). The Cu-C distances are therefore comparable to those observed in other diaryl cuprates such as [CuPh2]- (range 1.900(11)-1.931(11) A˚).14,21 The Cp rings on each ferrocenylene unit are bridged by a lithium cation in a fashion similar to that observed in the crystal structure of (1,10 -Fc)Li2(tmeda)2/33 and deviate from an eclipsed conformation by a maximum of 14.9° (minimum 1.9°). In addition, they are approximately coplanar with tilt angles in the range 1.1-4.6°. The Li4 tetrahedral core is a familiar structural motif in organolithium chemistry, being common to many organolithium aggregates, including (MeLi)422 and (tBuLi)4.23 Each triangular face of the Li4 tetrahedron in 1 is capped by an organolithium carbanion (for example, C6 and C26 in Figure 2b) to give a four-center-two-electron bond with Li-C distances in the range 2.165(6)-2.322(6) A˚ (mean 2.252 A˚). In addition, longer apical C-Li interactions (range 2.466(6)-2.544(6) A˚, mean 2.490 A˚) are present (21) Hope, H.; Olmstead, M. M.; Power, P. P.; Sandell, J.; Xu, X. J. Am. Chem. Soc. 1985, 107, 4337–4338. (22) Weiss, E.; Lucken, E. A. C. J. Organomet. Chem. 1964, 2, 197– 205. (23) Kottke, T.; Stalke, D. Angew. Chem., Int. Ed. Engl. 1993, 32, 580–582.
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connecting each Li center with a further cuprate carbanion center (see for example Li1-C1 and Li2-C21 in Figure 2b). The two [(1,10 -Fc)3Cu2]4- units are also connected through Li5 and Li6 via Li5-C36 (2.266(9) A˚) and Li6-C26 (2.354(11) A˚) interactions (see Figure 1a). Note that both the Li5 and Li6 sites are disordered, with only the major occupancy sites discussed here (see the Supporting Information for more details). It should be noted that no mesityl groups are present in complex 1, and these groups must therefore be incorporated into other reaction products. In order to probe the identity of the mesityl-containing byproduct, freshly prepared (1,10 Fc)Li2(tmeda)2/3 was treated with 1 equiv of CuIMes in C6D6 in an NMR tube and the products characterized with the aid of 7Li NMR spectroscopy. Three main peaks were observed in the 7Li NMR spectrum with chemical shift values of 3.16, -2.42, and -9.42 ppm. The highly shielded upfield peak at -9.42 ppm is indicative of a lithium cation sandwiched directly between two aromatic rings, thus experiencing the magnetic shielding effect of both aryl ring currents. On the basis of our previous 1D and 2D NMR studies of lithium mesityl cuprates,11,12 it is possible to confidently assign this peak to the aryl cuprate species [Cu2Li2Mes4]. Credible assignments for the other two peaks in the 7Li NMR spectrum are the ferrocenylene-bound Li cations in the dianionic cluster in 1 (3.16 ppm) and the tmeda-solvated lithium cations associated with this dianion (-2.42 ppm). The observation of just one apparent 7Li NMR peak for [(1,10 -Fc)6Cu4Li6]2- can be explained either by overlapping peaks for the two similar lithium environments Li1-Li4 and Li5-Li6 or by fast Li interchange as a result of fluxionality within the cluster. The reaction of 1 equiv of (1,10 -Fc)Li2(tmeda)2/3 with 2 equiv of CuIMes was also investigated, as this corresponds to a Cu:Li ratio of 1:1 and is thus congruent with the formation of R2CuLi Gilman-type cuprates. In this case four peaks were observed in the 7Li NMR spectrum with chemical shift values of 3.18, -2.17, -9.42, and -11.03 ppm. The highly shielded upfield peaks at -9.42 and -11.03 ppm can be assigned to the aryl cuprate species [Cu2Li2Mes4] and [Cu3LiMes4], respectively, in accordance with previous studies:12 in [Cu2Li2Mes4] the Li adopts a η1,η6 coordination mode, whereas in [Cu3LiMes4] the Li is η6,η6 coordinated and thus feels the full effect of both ring currents, resulting in a higher upfield shift. The remaining peaks can be assigned as before to the dianionic cluster in 1 (3.18 ppm) and the tmeda-solvated lithium cations associated with this dianion (-2.17 ppm). On the basis of these NMR experiments, a balanced equation for the synthesis of 1 is shown in Scheme 1. In an attempt to obtain the related ferrocenyl monometalated cuprate Fc2CuLi, monolithioferrocene was first prepared by reaction of ferrocene with 2 equiv of tBuLi in thf (24) (a) Malessa, M.; Heck, J.; Kopf, J.; Garcia, M. H. Eur. J. Inorg. Chem. 2006, 857–867. (b) Guillaneux, D.; Kagan, H. B. J. Org. Chem. 1995, 60, 2502–2505. (25) Synthesis of 2: freshly sublimed ferrocene (0.37 g, 2 mmol) in THF (5 mL) was treated with tBuLi (1.6 M in pentane, 2.6 mL, 4 mmol) to give an orange solution, which was stirred for 2 h. A solution of Cu5Mes5 3 C6H5CH3 (0.40 g, 0.4 mmol) in 5 mL of toluene was added, and the mixture was stirred for 1 h. The resultant red solution was reduced to a minimum volume under vacuum and redissolved in 5 mL of hexane and 5 mL of thf. After storage for 2 days at -30 °C red block crystals of 2 were formed and subsequently isolated. Yield: 0.36 g (38%). 1 H NMR (d6-dmso): δ 1.76 (12 H, thf), 3.60 (12 H, thf), 4.15 (2 H, R-CH), 4.19 (2 H, β-CH). 7Li NMR (d6-dmso): δ -1.04.
Bomparola et al. Scheme 1. Synthesis of 1
according to literature procedures (note that even with a large excess of tBuLi the literature yield for this step is often low24) and subsequently reacted directly with CuIMes. The resultant orange solution yielded a batch of red crystals on standing for 2 days at -30 °C.25 X-ray characterization of these crystals show them to be the cuprate complex [(1,10 Fc)2Cu2Li2(thf)6] (2), in which ferrocene has again been dimetalated similar to the case for 1. Despite repeated attempts, we were unable to obtain the monometalated ferrocenyl complex even when employing stoichiometrically equivalent quantities of tBuLi. A possible explanation for this could be that although tBuLi by itself, even when used in large excess, forms predominately the monolithioferrocene with only a minor amount of dimetalation observed,24b when used in combination with CuI its reactivity is enhanced due to cooperative effects between the two metals, analogous to those reported for other mixed-metal organometallic ate complexes.26 The heterotrimetallic complex 2 (Figure 2) conforms to the accustomed R2CuLi formulation for lithium organocuprates, 2R = (C5H4)2Fe, in which all carbanions are cuprated. The tetrahedral twist of the two ferrocenyl units and the CpCu-Cp linkages (vide infra) and the placement of the two Li(thf)3 moieties (Figure 2b) results in the complex having conformational chirality, crystallizing in the chiral space group P41212. The twisting of the two ferrocenyl units would appear to be driven by steric forces within the complex, since no noteworthy intermolecular interactions are observed. The Cu-C bonds in 2 (Cu-C1 = 1.907(3), Cu-C6 = 1.922(3) A˚) are comparable in length to those in 1, and the C-Cu-C angle is almost linear at 176.56(12)°. The Cp rings are close to parallel (tilt angle 2.9°) and, unlike the case for 1, are much nearer to staggered than eclipsed in conformation (deviation from staggered 6.7°). This results in the ferrocenylene units lying almost orthogonal to one another (torsion angle 81.44°) (see Figure 2b). Despite the formation of a (1,10 -Fc)2Cu2 ring, the Cu 3 3 3 Fe distances are fairly long (3.220, 3.522 A˚) and there is no evidence of any Cu 3 3 3 Fe interactions similar to those reported by J€ akle in [(FcCu)2(C6F5Cu)2] and [(1,2-Fc)4(C6F5)8Cu16].18b The Li cations in 2 lie outside the central (1,10 -Fc)2Cu2 dimeric ring and are associated with a ferrocenylene carbanion via a 2.266(6) A˚ Li-C6 bonding interaction. Each Li is additionally coordinated by three thf molecules with Li-O distances in the range 1.943(6)-1.972(6) A˚ (mean 1.957 A˚). Lithium diorganocuprates usually adopt solvent-separated ion pairs in thf in which the Li cations are fully solvated by thf to give [Li(thf)4]þ cations and [R2Cu]- or similar anions.27 The structure of 2 is therefore unusual, being the first example of a thf-solvated diorganocuprate to adopt a contact ion pair (26) (a) Mulvey, R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y. Angew. Chem., Int. Ed. 2007, 46, 3802–3824. (b) Mulvey, R. E. Acc. Chem. Res. 2009, 42, 743–755. (27) John, M.; Auel, C.; Behrens, C.; Marsch, M.; Harms, K.; Bosold, F.; Gschwind, R. M.; Rajamohanan, P. R.; Boche, G. Chem. Eur. J. 2000, 6, 3060–3068.
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Organometallics, Vol. 28, No. 16, 2009 Scheme 2. Synthesis of 2
structure in the solid state. This can most likely be accounted for by the strength of the C6-Li interaction;similar arguments account for the formation of contact ion pairs by amido(organo)cuprates in thf.15 The 1H NMR spectrum of 2 in (CD3)2SO is consistent with that expected for a 1,10 -dimetalated ferrocenylene complex, with pseudo triplets corresponding to the R- and β-protons observable at 4.15 and 4.19 ppm. The low chemical shift difference between the signals for the R- and β-protons indicates that the Cp rings within the ferrocenylene units lie planar or close to planar with minimal ring tilt. One signal is observed in the 7Li NMR spectrum of 2 in (CD3)2SO at -1.04 ppm. In order to investigate the identity of the other reaction products formed concurrently with 2, ferrocene was reacted with 2 equiv of tBuLi in toluene and then treated with 2 equiv of CuIMes as in the original reaction procedure. An aliquot of this reaction mix was then analyzed by 7Li NMR spectroscopy (in toluene doped with C6D6), revealing resonances at 0.93 and -9.38 ppm. Although it is difficult to unequivocally assign the resonance at 0.93 ppm to any particular species (a lithium ferrocenylenecuprate similar to 2 seems likely), the signal at -9.38 ppm can be confidently assigned to [Cu2Li2Mes4].12 A probable balanced reaction scheme for the preparation of 2 is therefore shown in Scheme 2. In order to investigate the reactivity of cuprates 1 and 2 in oxidative coupling reactions, both complexes were prepared in situ according to Schemes 1 and 2, respectively, and treated with nitrobenzene. It was initially envisaged that this would lead to formation of polyferrocenylenes and thereby confirm the key role of ferrocenylenecuprates in the Cu2þpromoted oxidative coupling of 1,10 -dilithioferrocene.16,17 However, in both cases the presence of the mesityl-containing organocuprates in the reaction mixture resulted in the (28) Synthesis of polyferrocenylene: a suspension of (1,10 -FcLi2)(tmeda)2/3 (2.75 g, 10 mmol) in THF (20 mL) was added to a suspension of CuBr 3 SMe2 (4.11 g, 20 mmol) in THF (20 mL) at 0 °C. PhNO2 (2.05 mL, 20 mmol) was subsequently added to the dark red mixture at -78 °C, before warming to room temperature and stirring for 12 h. The solid was filtered off, washed with copious amounts of ammonia (6% solution) and distilled water, and dried under vacuum.
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Scheme 3. Synthesis of Polyferrocenylenes
main products being mesitylferrocene and 1,10 -dimesitylferrocene. Hence, although the low nucleophility of the mesityl group makes it a suitable spectator ligand in organocuprate conjugate addition reactions,12 in oxidative cross-coupling it participates fully in the reaction, leading to a range of mesityl-containing products. The reaction was therefore repeated using CuBr 3 SMe2 instead of CuMes as the CuI source, according to Scheme 3. Using this new procedure, linear polyferrocenylenes up to at least six ferrocene units in length were prepared28 and characterized using mass spectroscopy (m/z 1106). Further analysis of the product using LCMS revealed a mixture of linear polyferrocenes (n = 26), with no evidence of internal cyclization to give the dinuclear ferrocenophane. As was also the case in previous studies,16,17 the very low solubility of the polyferrocenylene products made further purification and characterization difficult; however, the above observations are sufficient to confirm the role CuI species such as 1 and 2 play in the oxidative coupling of ferrocenylenes. In summary, two novel lithium ferrocenylenecuprate complexes, 1 and 2, have been prepared and structurally characterized using X-ray crystallography to reveal unusual trimetallic clusters. In addition, lithium ferrocenylenecuprate is shown to undergo oxidative addition to give poly-1,10 ferrocenylene, although full characterization of the polymer products is inhibited by their extremely low solubility. Further studies on ferrocenylenecuprate derivatives containing additional organo groups on the ferrocene rings is underway for the preparation of more soluble polyferrocenylenes.
Acknowledgment. We gratefully acknowledge the EPSRC (R.B.) and Imperial College London (T.G.) for financial support. Supporting Information Available: Text, figures, tables, and CIF files giving full crystallographic data for 1 and 2. This material is available free of charge via the Internet at http:// pubs.acs.org.
