Unexpected Synthesis of Oligomeric (Dimethylsilyl)-Bridged

Jan 29, 2010 - Sandwich Complex-Containing Macromolecules: Property Tunability Through Versatile Synthesis. Alaa S. Abd-El-Aziz , Christian Agatemor ...
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Organometallics 2010, 29, 1057–1060 DOI: 10.1021/om100041k

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Unexpected Synthesis of Oligomeric (Dimethylsilyl)-Bridged Ferrocenes from the Desilylative Coupling of Tertiary 1,10 -Bis(dimethylsilyl)ferrocene Damion Miles, Jon Ward, and Daniel A. Foucher* Department of Chemistry and Biology, Ryerson University, 350 Victoria Street, Toronto, Ontario, Canada M5B-2K3 Received January 17, 2010 Summary: The solvent-free, Pt-catalyzed desilylative coupling reaction of 1,10 -bis(dimethylsilyl)ferrocene (12) at elevated temperature leads to the elimination of one dimethylsilane unit and the concomitant oligomerization of ferrocenes with a single dimethylsilyl bridge in low to moderate yields. Airand moisture-stable oils of the dimer (13) and trimer (14) species possessing dimethylsilyl bridges and terminal dimethylsilane end groups have been isolated by preparative TLC and characterized by HR-MS, elemental analysis, and NMR spectroscopy.

Polymetallocenes comprised of a repeat unit of ferrocene and a single heteroatom (e.g., Si,1,2 Ge,3,4 Sn,5 P,6,7 S,8 B,9 Al,10 and Ga10) have been prepared by variety of ring-opening routes including the thermal, anionic, transition metal, or UV light-induced polymerizations of strained[1] metallocenophanes.11 The majority of the resulting polymers possess unusual electronic properties, including semiconducting behavior when oxidatively doped and an extensive degree of metal-metal interaction as demonstrated by cyclic voltammetry.12 Polyferrocenes with a backbone containing

Our interest lies in the development of alternative coupling (non-ROP) routes to polyferrocenes with two (6, 7, 8) or more group 14 bridging elements in the backbone.20 Model

ferrocene and two group 14 or group 15 bridging elements are less common and generally more difficult to prepare than their singly bridged counterparts. The visibly ring-strained ethane- and ethylene-bridged [2]ferrocenophanes undergo thermal ROP and catalyzed ROMP, respectively, leading to broadly dispersed, low to moderate molecular weight polymers 113 and 2.14 The unstrained tetramethyldisilane-bridged [2]ferrocenophane 3 resists both thermal and metal-catalyzed polymerization15 to polymer 6, while the digermane analogue 4 is resistant to thermal ROP, but undergoes metalcatalyzed ROP to 7.16 Ring-opening reactions of the distannyl-bridged ferrocenophane,17 5, leading to the polymer 8 have not yet been reported. Ring-strained [2]ferrocenophanes with either P-C or S-C bridges have also been polymerized to low molecular weight polyferrocenes (i.e., 9, 10) by thermal ROP processes.18 There have been only a few examples of alternative (non-ROP) routes to polyferrocenes containing two bridging elements. An unusual example is the stoichiometric atom abstraction of the central sulfur atom of the trisulfur-bridged [3]ferrocenophane by PPh3, leading to a broad, high molecular weight polymer (11) consisting of a repeating ferrocene unit and disulfide linkage.19

systems for these polymers, such as 3 and 5, readily undergo metal-catalyzed insertion of alkynes in their Si-Si21 or Sn-Sn17 bonds. Employing a similar strategy to the macromolecular intermediates of type 6-8, access to conductive, air- and

*Corresponding author. E-mail: [email protected]. (1) Foucher, D. A.; Tang, B. Z.; Manners, I. J. Am. Chem. Soc. 1992, 114, 6246–6248. (2) (a) Nguyen, M. T.; Diaz, A. F.; Dement’ev, V. V.; Pannell, K. H. Chem. Mater. 1993, 5, 1389–1394. (b) Nguyen, M. T.; Diaz, A. F.; Dement'ev, V. V.; Pannell, K. H. Chem. Mater. 1994, 6, 952–954. (3) Foucher, D. A.; Manners, I. Makromol. Chem. Rapid. Commun. 1993, 14, 63–66. (4) Kapoor, R. N.; Crawford, G. M.; Mahmoud, J.; Dement’ev, V. V.; Nguyen, M. T.; Diaz, A. F.; Pannell, K. H. Organometallics 1995, 14, 4944–4947. (5) J€ akle, F.; Rulkens, R.; Zech, G.; Foucher, D. A.; Lough, A. J.; Manners, I. Chem.;Eur. J. 1998, 4, 2117–2128.

(6) Withers, H. P.; Seyferth, D.; Fellmann, J. D.; Garrou, P. E.; Martin, S. Organometallics 1982, 1, 1283–1288. (7) Honeyman, C. H.; Foucher, D. A.; Dahmen, Y.; Rulkens, R.; Lough, A. J.; Manners, I. Organometallics 1995, 14, 5503–5512. (8) Rulkens, R.; Gates, D. P.; Pudelski, J. K.; Balaishis, D.; McIntosh, D. F.; Lough, A.; Manners, I. J. Am. Chem. Soc. 1997, 119, 10976–10986. (9) Braunschweig, H.; Dirk, R.; M€ uller, M.; Nguyen, P.; Resendes, R.; Gates, D. P.; Manners, I. Angew. Chem., Int. Ed. Engl. 1997, 36, 2338–2340. (10) Lund, C. L.; Schachner, J. A.; Quail, J. W.; M€ uller, J. Organometallics 2006, 25, 5817–5823.

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moisture-stable polymers containing ferrocene, group 14 elements, and an unsaturated hydrocarbon is viable.22 The preparation of such species was inspired by the recent discovery of the dehydrogenative coupling of dithienophospholes with two tertiary dimethylsilane groups.23 In that work, an attempt to prepare olefins via a Pt-mediated alkyne insertion reaction into Si-H bonds leads instead to dehydrogenative coupling and the exclusive formation of Si-Si bonds; these are moderate molecular weight polymers with an alternating backbone of bisthienophospholes and disilane units. To investigate whether this discovery was more general to other disubstituted tertiary silanes, we report herein on the similar reaction protocols of the reactive monomer, 1,10 -bis(dimethylsilyl)ferrocene (12), and on a model species, (dimethylsilyl)ferrocene (16), with a common Pt catalyst. The treatment of 12 with Karstedt’s catalyst under solventfree conditions (Scheme 1) was investigated at both ambient and elevated temperatures (100 °C). No detectable changes were observed under these conditions at ambient temperature. By contrast, the high-temperature reaction mixtures were observed to darken with a concomitant increase in viscosity over a 5-day reaction period. Reaction mixtures were found (NMR, MS) to contain unreacted 12 and oligomeric species with two and three ferrocene units, in addition to traces of higher molecular weight species with a single dimethylsilyl bridge and terminal Si-H groups. The reaction materials were purified initially by column chromatography to remove unreacted monomer (12) and insoluble Pt-containing species (11) Selected references for metal-containing polymers : Frontiers in Transition Metal-Containing Polymers; Abd-El-Aziz, A. S., Manners, I., Eds.; John Wiley and Sons: Hoboken, NJ, 2007. (12) Foucher, D. A.; Nelson, J. M.; Honeyman, C.; Tang, B. Z.; Manners, I. Angew. Chem., Int. Ed. Engl. 1993, 32, 1709–1711. (13) Nelson, J. M.; Rengel, H.; Manners, I. J. Am. Chem. Soc. 1993, 115, 7035–7036. (14) Buretea, M. A.; Tilley, T. D. Organometallics 1997, 15, 1507– 1510. (15) Finckh, W.; Tang, B. Z.; Foucher, D. A.; Zamble, D.; Manners, I. Organometallics 1993, 12, 823–829. (16) Mochida, K.; Shibayama, N.; Goto, M. Chem. Lett. 1998, 339– 340. (17) Heberhold, M.; Steffl, U.; Wrackmeyer, B. J. Organomet. Chem. 1999, 577, 76–81. (18) Rescendes, R.; Nelson, J. M.; Fischer, A.; J€akle, F.; Bartole, A.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 2001, 12, 2116–2126. (19) Brandt, P. F.; Rauchfuss, T. B. J. Am. Chem. Soc. 1992, 114, 1926–1927. (20) A related organometallic polymer [(η5-C5H4)Fe(CO)2SiMe2SiMe2]n was prepared by Pannell and co-workers using a base-catalyzed condensation route, yielding moderate molecular weight polymers containing a backbone of alternating -SiMe2SiMe2- and -Fp(CO)2units. See: Sharma, H. K.; Pannell, K. H. Chem.Commun. 2004, 2556– 2557. (21) Finckh, W.; Tang, B. Z.; Lough, A.; Manners, I. Organometallics 1992, 11, 2904–2911. (22) Polymers containing ferrocene, silicon, and an unsaturated hydrocarbon were prepared by Sheridan and co-workers, who successfully carried out hydrosilylation reactions of 12 with diethynylbenzene spacers to yield poly[ferrocene(phenylene)bis(silylenevinylene)]s. In this work, metal insertion is exclusively through the Si-H bonds. See: Jain, R.; Lalancette, R. A.; Sheridan, J. B. Organometallics 2005, 24, 1458– 1467. (23) Baumgartner, T.; Wilk, W. Org. Lett. 2006, 8, 503–506. (24) A recent report of the permethylated [(HSiMe2C5Me4)2TiCl2] compound describes an intramolecular dehydrogenative coupling of the two SiMe2H groups in the presence of Mg metal to yield a disilane bridge. The favorable ring-tilt of cyclopentadienyl rings toward each other in this structure suggests that formation of an intramolecular Si-Si bond is likely, as the distance between silicon atoms is considerably shorter than in the case of 12. Horacek, M.; Pinkas, J.; Gyepes, R.; Kubista, J.; Mach, K. Organometallics 2008, 27, 2635–2642.

Miles et al. Scheme 1. Desilylative Coupling of 12

and further purified by preparative TLC to isolate the dimer (13) and trimer (14) ferrocenylsilanes. There was no evidence of intramolecular dehydrogenative coupling to give the bridged disilane24 3 nor intermolecular polymerization to a disilane-bridged species such as 15.25-27 Reaction of the neat (dimethylsilyl)ferrocene (16) was undertaken using the same conditions used for 12 (Scheme 2) to study the coupling in more detail and to minimize the number of possible products requiring isolation and analysis. Additionally, a coupling reaction of 16 was carried out in refluxing dichloromethane. After 5 days, reactions carried out using both solventfree and refluxing dichloromethane conditions yielded the monosilyl-bridged biferrocene [(η5-C5H5)Fe(η5-C5H4)]2SiMe2 (17) as the only reaction product. Compound 17 gave a characteristic 29Si NMR resonance signal at δSi = -7.16 ppm (CDCl3) for the bridging silicon group.25,28 The slow rate of conversion to oligomers (13, 14) and dimer (17) in the neat reactions of 12 and 16 appears to show little enhancement by increased catalyst concentration, temperature, or reaction time. The unexpected elimination of one of the dimethylsilane groups from the ferrocene unit may stem from competing insertion reaction mechanisms of Pt into both the Si-ipso-Cp carbon of the ferrocene and the Si-H unit as shown in Scheme 3. Manners et al. have shown that [Pt(cod)2] complexes will readily undergo oxidative insertions into strained Si-ipso-C bonds of silicon-bridged [1]ferrocenophanes.29 In the case of dimethylsilyl mono- (16) and disubstituted ferrocenylsilanes (12), insertion with Pt catalyst is expected to occur at the Si-H bond. The dehydrocoupling reaction that lead to polymers with Si-Si bridges23 in work by Baumgartner et al. was not (25) A series of model biferrocenes were prepared by Pannell et al., including compound 17 and the model compound for polymer 6 with a disilane bridge. See: Sharma, H.; Pannell, K. H. Organometallics 1991, 10, 954–959. (26) Recently, the dehydrogenative coupling of 12 using an iron catalyst [(CpFe(CO2)Me] in DMF yielded disilane-bridged polymers, including 6. See: Itazaki, M.; Ueda, K.; Nakazawa, H. Angew. Chem., Int. Ed. 2009, 48, 3313-3316. The findings of this work were found to be in error (Sharma, H. K.; Pannell, K. H. Angew. Chem., Int. Ed. 2009, 48, 7052-7054), and a correction was issued by the original authors (Itazaki, M.; Ueda, K.; Nakazawa, H. Angew. Chem., Int. Ed. 2009, 48, 6938). (27) Our own work with Pt catalyst shows that dehydrogenative reactions are not occurring. Instead oxygen abstraction from the reduction of DMF leads to oligoferrocenylsiloxanes and polyferrocenylsiloxanes. See: Miles, D.; Ward, J.; Foucher, D. Macromolecules 2009, 42, 9199-9203. (28) Rulkens, R.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 1994, 116, 797–798. (29) Ni, Y.; Rulkens, R.; Pudelski, J. K.; Manners, I. Makromol. Chem. Rapid Commun. 1995, 16, 637–638.

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Scheme 2. Desilylative Coupling of 16

Scheme 3. Proposed Mechanism of Oligomerization of Ferrocenylsilanes

observed for the coupling reactions of 12 and 16.26,27 Instead, we propose that the elimination of the volatile Me2SiH2 species is followed by the oligomerization to ferrocenylsilanes with a single dimethylsilyl bridge. Our attempts to trap the Me2SiH2 gas and other possible volatile materials from the solvent-free reaction at high temperature were unsuccessful; however, after 5 days of reaction, the sample mass decreased by an amount that corresponded to the expected loss of Me2SiH2 for products converted to 13 and 14, respectively. We propose a mechanism that involves two sequential oxidative additions of the Pt catalyst when two ferrocenylsilane units (12) are added to the complex. This is followed by two successive reductive eliminations that result first in the dehydrosilylation of the Me2SiH2 fragment from the Pt center and second in the elimination of the growing oligoferrocene chain with regeneration of the catalyst. The mechanism proceeds by an initial insertion of Pt between the Si-H bond of 12, which occurs easily at lower temperatures. These Pt complexes are essentially inactive toward oligomerization. A second type of Pt

complex, involving Pt insertion between the Si-ipso-C bond of a second molecule of 12, forms at elevated temperatures even in the absence of strain in the monomer. Propagation occurs through a condensation elimination of a molecule of Me2SiH2 from the two Pt complexes, as shown in Scheme 3. More detailed mechanistic studies are now underway that should shed more light on the nature of the oligomerization. Our initial efforts to identify alternative routes to polyferrocenes containing two group 14 bridging elements in the repeat unit have lead instead to the unexpected desilylative coupling of 12 to oligomeric ferrocenylsilanes containing a single silicon bridge (13, 14). Our current efforts are focused now on looking at different reactive ferrocene monomers, alternative catalysts, solvents, and reaction conditions that will lead to the desired polyferrocenes such as 6 or 8.

Experimental Section Equipment and Procedures. 1H NMR (400 MHz), 13C NMR (100.6 MHz), and 29Si NMR (79.5 MHz) spectra were recorded on

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a Bruker Avance 400 MHz NMR spectrometer. 1H and 13C were referenced internally to the deuterated solvent resonances, while 29 Si was referenced to TMS. Mass spectrometry was performed at the University of Toronto AIMS Laboratory with a Waters GC TOF mass spectrometer with an EI/CI source mass spectrometer in electron impact mode. The calculated isotopic distribution for each ion was in agreement with experimental values. Elemental analysis was performed by Analest Laboratories at the University of Toronto. All experiments were performed under inert atmosphere conditions (N2) using Schlenk conditions. Materials. Solvents were dried by standard procedures prior to use. Chlorodimethylsilane and Karstedt’s catalyst (2.0% in xylenes) were purchased from Aldrich and used as received. Dilithioferrocene 3 TMEDA was prepared according to a literature preparation.30 Preparation of 1,10 -Bis(dimethylsilyl)ferrocene (12) and (Dimethylsilyl)ferrocene (16). The disubstituted ferrocenylsilane (12), isolated as red oil, was prepared by a previous literature route31 and purity confirmed in relation to previously published data.31,32 Product 12 was purified by multiple vacuum distillations (150 °C, 5  10-3 mmHg) to recover unreacted ferrocene and the monosubstituted ferrocenylsilane 16. 29Si NMR (C6D6): δSi -18.58 (s, SiMe2H) ppm. Rf: 0.82 (3:1 n-hexane/CH2Cl2). Product 16 was also isolated as red oil, and characterization data for this product were in good agreement with published values.25,33 29Si NMR (C6D6): δSi -18.38 (s, SiMe2H) ppm. Rf: 0.82 (3:1 n-hexane/ CH2Cl2). General Synthesis and Characterization of the Oligomerization Products 13 and 14. In a 10 mL round-bottom flask equipped with condenser and stir bar, compound 12 (5.00 g, 16.55 mmol) and Karstedt’s catalyst (10 drops, 100 mg, 0.01 mmol) were added. The mixture was heated to 100 °C and left for 5 days. Initial purification, using silica column chromatography to remove residual Pt species and unreacted monomer, was achieved by eluting with n-hexane/CH2Cl2 (1:1). Further purification was achieved by preparative TLC methods and development of the preparative TLC plate in neat n-hexane. The isolated yields of all products from the reaction were 0.65 g of starting material 12 (13%), 2.43 g of the dimer compound 13 (54%), and 1.3 g of the trimer compound 14 (30%). Approximately 0.5-0.7 g of material was lost on the separation media. Evidence for higher oligomers (>3) was observed by TLC and MS, but these materials were not isolated. 13: red oil. 1H NMR (C6D6): δH 0.38 (d, J = 3.6 Hz, 12H, SiMe2H), 0.64 (s, 6H, CpSiMe2Cp), 4.17 (t, J = 1.6 Hz, 4H, Cp), 4.19 (t, J = 1.6 Hz, 4H, Cp), 4.33 (t, J = 1.6 Hz, 4H, Cp), 4.34 (t, J = 1.6 Hz, 4H, Cp), 4.79 (septet, J = 3.6 Hz, 2H, SiMe2H) ppm. 13C NMR (CDCl3): δC -3.09 (s, SiMe2H), -0.89 (s, CpSiMe2Cp), 67.90 (s, ipso-C, Cp), 71.47 (s, Cp), 71.68 (s, ipso-C, Cp), 71.74 (s, Cp), 73.49 (s, Cp), 73.66 (s, Cp) ppm. 29 Si NMR (CDCl3): δSi -18.58 (s, SiMe2H), -6.97 (s, CpSiMe2-Cp) ppm. HR-TOF MS EIþ: calcd for C26H36Si3Fe2 544.0824; found 544.0825 g 3 mol-1. Rf: 0.74 (3:1 n-hexane/ CH2Cl2). Anal. Calcd for C24H36Si3Fe2: C 57.35, H 6.66. Found: C 57.95; H 6.73. 14: red oil. 1H NMR (C6D6): δH 0.39 (d, J = 3.6 Hz, 12H, SiMe2H), 0.64 (s, 12H, CpSiMe2Cp), 4.17 (t, 4H, Cp), 4.18 (t, 4H, Cp), 4.19 (t, 4H, Cp), 4.34 (t, 4H, Cp), 4.36 (t, 8H, Cp), 4.81 (septet, J = 3.6 Hz, 2H, SiMe2H) ppm. 1H NMR (CDCl3): δH 0.30 (d, 12H, SiMe2H), 0.49 (s, 12H, CpSiMe2Cp), 4.03 (t, J = 1.6 Hz 4H, Cp), 4.08 (m, 8H, Cp), 4.23 (t, J = 1.6 Hz, 4H, Cp), 4.25 (t, J = 1.6 Hz, 4H, Cp), 4.30 (t, J = 1.6 Hz 4H, Cp), 4.45 (septet, J = 3.6 Hz, 2H, SiMe2H) ppm. 13C NMR (C6D6): δC (30) Rausch, M. D.; Ciappenelli, D. J. J. Organomet. Chem. 1967, 10, 127–136. (31) Jain, R.; Choi, H.; Lalancette, R. A.; Sheridan, J. B. Organometallics 2005, 24, 1468–1476. (32) Kong, Y. K.; Lee, J. J. Korean Chem. Soc. 2002, 46, 139–144. (33) Rausch, M. D.; Schoemer, G. C. Org. Prep. Proc. 1969, 1, 131– 136.

Miles et al. -3.07 (s, SiMe2H), -0.84 (s, CpSiMe2Cp), 68.1 (s, ipso-C, Cp), 71.69 (s, Cp), 71.72 (s, Cp), 71.79 (s, ipso-Cp), 71.94 (s, Cp), 71.97 (s, ipso-C, Cp), 73.50 (s, Cp), 73.72 (s, Cp), 73.88 (s, Cp) ppm. 29Si NMR (C6D6): δSi -18.70 (s, SiMe2H), -7.06 (s, CpSiMe2Cp) ppm. HR-TOF MS EIþ: calcd for C38H50Si4Fe3 786.1038; found 786.1025 g 3 mol-1. Rf: 0.58 (3:1 n-hexane/ CH2Cl2). Anal. Calcd for C38H50Si4Fe3: C 58.02, H 6.41. Found: C 57.63, H 6.43. Thermal Treatment of 12 in the Absence of Catalyst. In a 10 mL round-bottom flask equipped with condenser and stir bar, compound 12 (4.00 g, 13.24 mmol) was added. The mixture was heated to 100 °C and left for 5 days. No color change or decomposition of the compound was observed. Analysis of the heated material by 1 H NMR revealed only unreacted starting material. Preparation of the Model Dimer 17 under Solvent-Free Conditions. In a 50 mL Schlenk flask were added 1.040 g (3.44 mmol) of ferrocene dimethylsilane, 16, and 7 drops (70 mg, 0.007 mmol) of Karstedt’s catalyst (2.0 wt % solution in xylenes). The reaction mixture was heated to 100 °C for 5 days. To the flask was added 5 mL of CH2Cl2, and the products of the reaction were first filtered to remove insoluble materials and then separated by thin-layer chromatography eluting with 3:1 n-hexane/CH2Cl2. Two fractions were isolated. The first fraction, and the bulk of the materials, was identified by 1H NMR as unreacted starting material. The second fraction was characterized through 1H NMR and 29Si NMR as product 17, which agreed with reported literature values for this compound.25,28 It was isolated as a crystalline solid (0.094 g, 9% yield), mp 74 °C. Preparation of the Model Dimer 17 in Refluxing Dichloromethane. In a 50 mL Schlenk flask were added 0.900 g (2.98 mmol) of ferrocene dimethylsilane, 16, and 17 drops (170 mg, 0.017 mmol) of Karstedt’s catalyst (2.0 wt % solution in xylenes) in approximately twice the volume of CH2Cl2. This reaction was allowed to react for 5 days at reflux. Products of the reaction were first filtered to remove insoluble materials and then separated by thin-layer chromatography eluting with 3:1 nhexane/CH2Cl2. Two products were isolated. The first product fraction was identified by 1H NMR as starting material 16. The second fraction was characterized through 1H NMR and 29Si NMR as product 17, in good agreement with reported literature values for this compound.25,28 It was isolated as a crystalline solid (0.73 g, 81% yield). Compound 17: 1H NMR (CDCl3): δH 0.55 (s, 6H, CpSiMe2Cp), 4.15 (s, 10H, Cp), 4.22 (t, J = 1.6 Hz, 4H, Cp), 4.33 (t, J = 1.6 Hz, 4H, Cp) ppm. 29Si NMR (CDCl3): δSi -7.16 ppm. Attempted Dimethylsilane Trapping Experiments. A shortpath coldfinger cooled with liquid N2 was connected to a heated (100 °C) 50 mL Schlenk flask under a slight positive pressure of N2. In the Schlenk flask were added 1.00 g (3.32 mmol) of 12 and 15 drops (150 mg, 0.015 mmol) of Karstedt’s catalyst (2.0 wt % solution in xylenes). This reaction was heated for 5 days, with intermittent refilling of the cold trap. NMR analysis of the trapped materials consisted only of the xylenes solvent from the catalyst and no detectable trace of Me2SiH2. The isolated yields of all products from the reaction products were 0.13 g of starting material 12 (13%), 0.54 g of the dimer compound 13 (54%), and 0.3 g (30%) of the trimer compound 14. Reweighing of sample after reaction indicated that a weight loss corresponding approximately to the expected loss of Me2SiH2 (calcd 44.3 mg, actual 40.1 mg) had occurred.

Acknowledgment. This work was supported by a Ryerson Research Start-up Fund and the NSERC Discovery Program. D.M. was supported by a Ryerson Summer Research Assistant Grant and a graduate student scholarship. Supporting Information Available: A more detailed experimental section; NMR and MS spectral data for compounds 13 and 14. This material is available free of charge via the Internet at http://pubs.acs.org.