Published on Web 09/02/2010
Synthesis, Electronic Structure, and Reactivity of Strained Nickel-, Palladium-, and Platinum-Bridged [1]Ferrocenophanes Inmaculada Matas,† George R. Whittell,† Benjamin M. Partridge,† Jason P. Holland,‡ Mairi F. Haddow,† Jennifer C. Green,*,‡ and Ian Manners*,† School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. and Chemistry Research Laboratory, Department of Chemistry, UniVersity of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K. Received April 28, 2010; E-mail:
[email protected];
[email protected] Abstract: The group 10 bis(phosphine)metalla[1]ferrocenophanes, [{Fe(η5-C5H4)2}M(Pn-Bu3)2] [M ) Ni (4a), Pd (4b), and Pt (4c)], have been prepared by the reaction of Li2[Fe(η5-C5H4)2] · tmeda (5, tmeda ) N,N,N′,N′tetramethylethylenediamine) with trans-[MCl2(Pn-Bu3)2] [M ) Ni (trans-6a) and Pd (trans-6b)] and cis-[PtCl2(Pn-Bu3)2] (cis-6c), respectively. Single crystal X-ray diffraction revealed highly tilted, strained structures as characterized by R angles of 28.4° (4a), 24.5° (4b), and 25.2° (4c) and a distorted square planar environment for the group 10 metal center. UV/visible spectroscopy and cyclic voltammetry indicated that all three compounds had smaller HOMO-LUMO gaps and were more electron-rich in nature than ferrocene and other comparable [1]ferrocenophanes. DFT calculations suggested that these differences were principally due to the electron-releasing nature of the M(Pn-Bu3)2 metal-ligand fragments. Attempts to induce thermal or anionic ring-opening polymerization of 4a-c were unsuccessful and were complicated by, for example, competing ligand dissociation processes or unfavorable chain propagation. In contrast, these species all reacted rapidly with acids effecting clean extrusion of the bis(phosphine)metal fragment. Carbon monoxide inserted cleanly into one of the palladium-carbon bonds of 4b to afford the ring-expanded, acylated product [{Fe(η5-C5H4)(η5-C5H4)(CO)}Pd(Pn-Bu3)2] (10). The nickel analogue 4a, however, afforded [Ni(CO)2(Pn-Bu3)2] whereas the platinum-bridged complex 4c was inert. Remarkably, all compounds 4a-c were readily oxidized by elemental sulfur to afford the [5,5′]bicyclopentadienylidene (pentafulvalene) complexes [{η4:η0-C5H4(C5H4)}M(Pn-Bu3)2] [M ) Ni (11a)] and [(η2-C10H8)M(Pn-Bu3)2] [M ) Pd (11b) and Pt (11c)] by a formal 4-electron oxidation of the carbocyclic ligands. Compounds 11b and 11c represent the first examples of [5,5′]bicyclopentadienylidene as a neutral η2-ligand. The relative energies of η2-coordination with respect to that of η4:η0 bonding were investigated for 11a-c by DFT calculations.
1. Introduction
Strained metallocenophanes and related organometallic rings containing π-hydrocarbon ligands have received much attention due to their interesting structures, bonding, and enhanced reactivity, especially with respect to their role as monomers in ring-opening polymerization (ROP) reactions.1-3 ROP of a range of these strained metallorings has now been achieved,
†
University of Bristol. University of Oxford. (1) For a recent review see: Herbert, D. E.; Mayer, U. F. J.; Manners, I. Angew. Chem., Int. Ed. 2007, 46, 5060. (2) For examples of [1]ferrocenophanes, see: (a) Osborne, A. G.; Whiteley, R. H.; Meads, R. E. J. Organomet. Chem. 1980, 193, 345. (b) Foucher, D. A.; Manners, I. Makromol. Chem. Rapid Commun. 1993, 14, 63. (c) Pudelski, J. K.; Gates, D. P.; Rulkens, R.; Lough, A. J.; Manners, I. Angew. Chem., Int. Ed. Engl. 1995, 34, 1506. (d) Rulkens, R.; Lough, A. J.; Manners, I. Angew. Chem., Int. Ed. Engl. 1996, 35, 1805. (e) Sharma, H. K.; Cervantes-Lee, F.; Mahmoud, J. S.; Pannell, K. H. Organometallics 1999, 18, 399. (f) Schachner, J. A.; Lund, C. L.; Quail, J. W.; Mueller, J. Organometallics 2005, 24, 4483. (g) Schachner, J. A.; Lund, C. L.; Quail, J. W.; Mueller, J. Organometallics 2005, 24, 785. (h) Seyferth, D.; Withers, H. P., Jr. Organometallics 1982, 1, 1275. (i) Sa¨nger, I.; Heilmann, J. B.; Bolte, M.; Lerner, H.W.; Wagner, M. Chem. Commun. 2006, 2027. ‡
10.1021/ja103367e 2010 American Chemical Society
and this provides a convenient route to high molecular weight metal-containing polymers with a series of potential applications as functional materials.1-5 To date, the most extensively studied system involves strained sila[1]ferrocenophanes (1, ERx ) (3) For examples of non-iron metallocenophanes and related species, see: (a) Vogel, U.; Lough, A. J.; Manners, I. Angew. Chem., Int. Ed. 2004, 43, 3321. (b) Lund, C. L.; Schachner, J. A.; Quail, J. W.; Mueller, J. Organometallics 2006, 25, 5817. (c) Lund, C. L.; Schachner, J. A.; Quail, J. W.; Mueller, J. J. Am. Chem. Soc. 2007, 129, 9313. (d) Berenbaum, A.; Manners, I. Dalton Trans. 2004, 2057. (e) Braunschweig, H.; Homberger, M.; Hu, C.; Zheng, X.; Gullo, E.; Clentsmith, G.; Lutz, M. Organometallics 2004, 23, 1968. (f) Elschenbroich, C.; Schmidt, E.; Gondrum, R.; Metz, B.; Burghaus, O.; Massa, W.; Wocadlo, S. Organometallics 1997, 16, 4589. (g) Braunschweig, H.; Kupfer, T.; Radacki, K. Angew. Chem., Int. Ed. 2007, 46, 1630. (h) Tamm, M.; Kunst, A.; Bannenberg, T.; Randoll, S.; Jones, P. G. Organometallics 2007, 26, 417. (i) Braunschweig, H.; Lutz, M.; Radacki, K.; Schaumlo¨ffel, A.; Seeler, F.; Unkelbach, C. Organometallics 2006, 25, 4433. (j) Chadha, P.; Dutton, J. L.; Sgro, M. J.; Ragogna, P. J. Organometallics 2007, 26, 6063. (k) Braunschweig, H.; Kupfer, T. Acc. Chem. Res. 2010, 43, 455. (l) Sharma, H. K.; Cervantes-Lee, F.; Pannell, K. H. J. Am. Chem. Soc. 2004, 126, 1326. (m) Tamm, M. Chem. Commun. 2008, 3089. (4) (a) Archer, R. D. Inorganic and Organometallic Polymers; John Wiley & Sons, Inc.: New York, 2001. (b) Whittell, G. R.; Manners, I. AdV. Mater. 2007, 19, 3439. (5) Bellas, V.; Rehahn, M. Angew. Chem., Int. Ed. 2007, 46, 5082. J. AM. CHEM. SOC. 2010, 132, 13279–13289
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SiRR′) which are monomeric precursors to polyferrocenylsilanes (2, ERx ) SiRR′).1,4b,5
The appreciable ring tilting of the two cyclopentadienyl rings in ferrocenophanes, which can be described by the angles depicted in Figure 1, has been shown to constitute the major contribution to the strain present.1,5-7 It is this strain that represents the most important thermodynamic factor in determining the propensity of these species to undergo ring-opening reactions. For example, sila[1]ferrocenophanes react readily with protic reagents, such as HCl or MeOH, leading to the cleavage of the Cp-Si bond to yield ring-opened species such as 3 (X ) Cl or H).1,8 Moreover, the kinetic barrier to the ring-opening process can be overcome by thermal, anionic, or catalytic means, thereby enabling the preparation of high molecular weight polyferrocenylsilanes via versatile routes.1,5,9-11
R ) 14°-15°).1,2 A diverse range of reactivity has been identified for these species, including ring opening via cleavage of either the E-Cp or M-Cp bonds.1,2,13 Although a wide variety of metallocenophanes with main group elements in the bridge has now been studied in detail, analogous species with transition metals in the bridge are rare. Until recently, examples were limited to [1]ferrocenophanes 1 (ERx ) MCp2, Cp ) η5-C5H5) containing the early d-block metals (M ) Ti, Zr, Hf).14 The trinuclear ferrocenophanes prepared by the alkali-metal-mediated manganation of ferrocene, however, also contain components where a single Mn(II) ion bridges a 1,1′-ferrocenediyl fragment.15 In a recent preliminary communication16 we reported the synthesis and characterization of a nickela[1]ferrocenophane (4a) and a platina[1]ferrocenophane (4c), the first examples of strained metallocenophanes with late transition metals in the bridge. In this paper, we describe the full details of our work on the synthesis, structures, and electronic properties of the three group 10 bridged [1]ferrocenophanes (Ni, 4a; Pd, 4b; Pt, 4c). In addition, we describe the unprecedented reactivity of these interesting strained molecules.
2. Results and Discussion Figure 1. Common set of angles used to describe [1]metallocenophanes.
The identity of the bridging atom E in metallocenophanes and related species is one of the most important factors contributing to the degree of ring tilt (R), and consequently, the strain present. Thus, alteration of the covalent radius of the bridging element would be expected to allow the tilt angle and associated strain of [1]ferrocenophanes to be modulated. Indeed, since the report of the first sila[1]ferrocenophane (1, ERx ) SiPh2) in 1975,12 related species containing non-metallic main group elements (e.g., 1, E ) B, Ge, P, As, S, Se) and also main group metals (1, E ) Al, Ga, Sn) in the bridge have been described.1,2 This has allowed studies of highly strained metallorings with tilt angles of over 30° (1, E ) B or S) through to analogs with much lower degrees of strain (e.g., 1, E ) Sn, (6) Green, J. C. Chem. Soc. ReV. 1998, 27, 263. (7) Barlow, S.; Drewitt, M. J.; Dijkstra, T.; Green, J. C.; O’Hare, D.; Whittingham, C.; Wynn, H. H.; Gates, D. P.; Manners, I.; Nelson, J. M.; Pudelski, J. K. Organometallics 1998, 17, 2113. (8) (a) Fischer, A. B.; Kinney, J. B.; Staley, R. H.; Wrighton, M. S. J. Am. Chem. Soc. 1979, 101, 6501. (b) Bourke, S. C.; MacLachlan, M. J.; Lough, A. J.; Manners, I. Chem.sEur. J. 2005, 11, 1989. (9) (a) Foucher, D. A.; Tang, B. Z.; Manners, I. J. Am. Chem. Soc. 1992, 114, 6246. (b) Go´mez-Elipe, P.; Macdonald, P. M.; Manners, I. Angew. Chem., Int. Ed. Engl. 1997, 36, 762. (c) Temple, K.; Jakle, F; Sheridan, J. B.; Manners, I. J. Am. Chem. Soc. 2001, 123, 1355. (d) Reddy, N. P.; Yamashita, H.; Tanaka, M. Chem. Commun. 1995, 22, 2263. (10) (a) Rider, D. A.; Cavicchi, K. A.; Power-Billard, K. N.; Russell, T. P.; Manners, I. Macromolecules 2005, 38, 6931. (b) Tanabe, M.; Vandermeulen, G. W. M.; Chan, W. Y.; Cyr, P. W.; Vanderark, L.; Rider, D. A.; Manners, I. Nat. Mater. 2006, 5, 467. (11) Polymetallocenes and related materials are also available through polycondensation approaches. For some recent examples, see: (a) Miles, D.; Ward, J.; Foucher, D. A. Macromolecules 2009, 42, 9199. (b) Scheibitz, M.; Li, H.; Schnorr, J.; Perucha, A. S.; Bolte, M.; Lerner, H.-W.; Ja¨kle, F.; Wagner, M. J. Am. Chem. Soc. 2009, 131, 16319. (12) Osborne, A. G.; Whiteley, R. H. J. Organomet. Chem. 1975, 101, C27. 13280
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2.1. Synthesis and Characterization of the Group 10 Bridged [1]Ferrocenophanes 4a, 4b, and 4c. The availability of
dihalide complexes containing the elements of group 10 suggested that compounds 4a, 4b, and 4c may be prepared by simple extension of the main synthetic route to mononuclear [1]ferrocenophanes.1 Thus, the reaction of Li2[Fe(η5C5H4)2] · tmeda (5, tmeda ) N,N,N′,N′-tetramethylethylenediamine) with trans-[NiCl2(Pn-Bu3)2] (trans-6a) was attempted and this resulted in a cloudy, dark red mixture in hexanes at -20 °C. Analysis by 31P{1H} NMR spectroscopy immediately after the mixture had warmed to ambient temperature revealed ca. 20% conversion to a single new phosphorus-containing product (δ ) 13.5 ppm). Prolonged stirring at room temperature did not result in further consumption of 6a. Changing the reaction solvent to Et2O, however, had a profound effect with 31 P{1H} NMR spectroscopy indicating complete consumption of 6a after warming to room temperature. In this instance, removal of solvent and recrystallization from hexanes afforded 4a as dark orange crystals and in much improved isolated yield (40% compared to 11% in hexanes). Analysis of the mother liquors by 31P{1H} NMR spectroscopy indicated the presence of a new phosphorus-containing compound, the chemical shift (δ ) -2 ppm) of which suggesting an acyclic formulation (Vide infra). In order to investigate the generality of the chloride metathesis reaction, trans-[PdCl2(Pn-Bu3)2] (trans-6b) and cis-[PtCl2(Pn(13) (a) Mizuta, T.; Imamura, Y.; Miyoshi, K. J. Am. Chem. Soc. 2003, 125, 2068. (b) Herbert, D. H.; Tanabe, M.; Bourke, S. C.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 2008, 130, 4166. (14) Broussier, R.; Da Rold, A.; Gautheron, B.; Dromzee, Y.; Jeannin, Y. Inorg. Chem. 1990, 29, 1817. ´ lvarez, J.; Kennedy, A. R.; Klett, J.; Mulvey, R. E. Angew. (15) (a) Garcı´a-A Chem., Int. Ed. 2007, 46, 1105. (b) Blair, V. L.; Carrella, L. M.; Clegg, W.; Klett, J.; Mulvey, R. E.; Rentschler, E.; Russo, L. Chem.sEur. J. 2009, 15, 856. (16) Whittell, G. R.; Partridge, B. M.; Presly, O. C.; Adams, C. J.; Manners, I. Angew. Chem., Int. Ed. 2008, 47, 4354.
Strained Ni-, Pd-, and Pt-Bridged [1]Ferrocenophanes
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Scheme 1. Preparation of Group 10 Metal-Bridged [1]Ferrocenophanes
Bu3)2] (cis-6c) were treated with compound 5 (Scheme 1). In hexanes at -20 °C, both reactions afforded the respective dark red, crystalline metalla[1]ferrocenophanes, [{Fe(η5-C5H4)2}Pd(PnBu3)2] (4b) and [{Fe(η5-C5H4)2}Pt(Pn-Bu3)2] (4c), albeit in modest to low yields (4b: 34% and 4c: 13%). Unlike the nickel case however, exchanging the solvent for Et2O had a negative effect on the reaction yields and even prevented the isolation of the pallada[1]ferrocenophane, 4b. When 4a, 4b, and 4c were recrystallized from hexanes, analysis of the mother liquors by 31 P{1H} NMR spectroscopy revealed detectable quantities of these species, suggesting that their high solubilities in hexanes at low temperature also contributed to the observed low isolated yields. Interestingly, the treatment of trans-[PtCl2(Pn-Bu3)2] (trans-6c) with 5 afforded no reaction under the same conditions that proved successful for the conversion of cis-6c, as evidenced by 31P{1H} NMR spectroscopy. The trans- to cis-isomerization of 6a and 6b upon formation of the metalla[1]ferrocenophanes presumably reflects the increased stability of the cis-isomer on exchange of the chloride for a stronger trans influence ligand.17 Although the electrostatic interactions between the ligands are calculated to be smaller in 6c, thus reducing the energetic preference for the trans-isomer, it would appear that the activation barrier to isomerization is sufficiently high to prevent formation of 4c from trans-6c. Surprisingly, the relatively bulky Pn-Bu3 ligand proved to be important for the stabilization and isolation of these complexes. Attempts to synthesize nickel-, palladium- and platinum-bridged [1]ferrocenophanes bearing trimethylphosphine or chelating diphosphines as the ligand or ligands failed, and no isolable products were obtained. 2.1.1. Characterization of 4a, 4b, and 4c. 1H, 13C{1H}, and 31 P{1H} NMR spectra recorded for the new [1]ferrocenophanes were in accord with the proposed structures. All spectra displayed resonances at predictable chemical shifts and as such were consistent with the incorporation of a diamagnetic squareplanar group 10 metal center. In the 1H NMR spectra of compounds 4a, 4b, and 4c, the two sets of chemically inequivalent cyclopentadienyl protons give rise to different resonances. It should be noted, however, that the difference in the chemical shifts of the two sets of Cp protons for the three complexes (∆δ ) 0.27,
