Article pubs.acs.org/Organometallics
Rare-Earth Half-Sandwich Dialkyl and Homoleptic Trialkyl Complexes for Rapid and Stereoselective Polymerization of a Conjugated Polar Olefin Yangjian Hu,† Xiufang Wang,‡ Yaofeng Chen,*,‡ Lucia Caporaso,*,§ Luigi Cavallo,*,∥ and Eugene Y.-X. Chen*,† †
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China § Dipartimento di Chimica e Biologia, Università di Salerno, Via Ponte don Melillo, I-84084 Fisciano (SA), Italy ∥ Physical Sciences and Engineering, Kaust Catalysis Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ‡
S Supporting Information *
ABSTRACT: Under ambient conditions, discrete half-sandwich rare-earth (RE) dialkyls, [η5-(1,3-(SiMe3)2C9H5)]]RE(CH2SiMe3)2(THF) (RE = Sc, Y, Dy, Lu), catalyze rapid and stereoselective coordination polymerization of β-methyl-αmethylene-γ-butyrolactone (βMMBL), a conjugated polar olefin and a member of the naturally occurring or biomassderived methylene butyrolactone family. Within the present RE series, the complex of the largest ion (Dy3+) exhibits the highest activity, achieving a high turnover frequency of 390 min−1, and also produces the highly isotactic polymer PβMMBL (mm = 91.0%). This stereoregular polymer is thermally robust, with a high glass-transition temperature of 280 °C, and is resistant to all common organic solvents. Other half-sandwich RE catalysts of the series are also highly active and produce polymers with a similarly high isotacticity. Intriguingly, even simple homoleptic hydrocarbyl RE complexes, RE(CH2SiMe3)3(THF)2 (RE = Sc, Y, Dy, Lu), also afford highly isotactic polymer PβMMBL, despite their much lower polymerization activity, except for the Lu complex, which maintains its high activity for both types of complexes. Computational studies of both half-sandwich and simple hydrocarbyl yttrium complexes have revealed a stereocontrol mechanism that well explains the observed high stereoselectivity of βMMBL polymerization by both types of catalysts. Specifically, the experimental stereoselectivity can be well rationalized with a monometallic propagation mechanism through predominantly chain-end stereocontrol in the coordination−addition polymerization. In this mechanism, formation of an isotactic polymer chiefly originates from interactions between the methyl groups on the chiral β-C atom of the five-membered ring of both the coordinated monomer and the last inserted βMMBL unit of the chain, and the auxiliary ligand on the metal makes a negligible contribution to the stereocontrol of the polymerization.
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classes of bis(η5-cyclopentadienyl) and related sandwich metallocene catalysts, cationic group IV metallocenium catalysts have the advantage of achieving high activity and precise control over stereochemistry of polymerization, often at ambient temperature, while neutral RE metallocene catalysts typically require no cocatalysts and are more effective for acrylate polymerization and are more tolerant toward polar donor media.1,2 Such important features of RE catalysts render them especially effective in polymerizing biomass-derived,
INTRODUCTION
Coordination−addition polymerization of conjugated polar olefins, or acrylics, by discrete, single-site metal catalysts has attracted growing interest due to its precision in the catalystsite-regulated stereochemical and architectural control as well as its ability to produce new classes of polymeric materials unattainable by other means of polymerization.1 In this type of polymerization catalysis, cationic group IV metal complexes and their isoelectronic, neutral rare-earth (RE) complexes are the two best-known classes of highly active, efficient, and controlled catalysts for the coordination−addition polymerization of acrylics such as methyl methacrylate (MMA), acrylates, acrylonitrile, and acrylamides.1,2 On comparison of these two © 2013 American Chemical Society
Special Issue: Recent Advances in Organo-f-Element Chemistry Received: November 22, 2012 Published: January 23, 2013 1459
dx.doi.org/10.1021/om301128g | Organometallics 2013, 32, 1459−1465
Organometallics
Article
oxygenated polar feedstocks,3−5 such as α-methylene-γbutyrolactone (MBL)6 and γ-methyl-α-methylene-γ-butyrolactone (γMMBL),7 because such renewable monomers resemble more acrylates than methacrylates in reactivity and polar donor solvents, such as N,N-dimethylformamide (DMF), are needed for the polymerization due to solubility constraints of the resulting polymers. Such sustainable polymers offer not only an alternative to petroleum-based acrylic polymers such as PMMA8 but also superior materials properties.3−5 Of many types of RE catalysts that have been developed for molecular catalysis9 and for polymerization catalysis of acrylics,10 bis(η5-pentamethylcyclopentadienyl (Cp*) and related sandwich lanthanocenes, such as [Cp*2SmH]2 and Cp*2LnMe(THF), typically exhibit the highest polymerization activity and control.10 In comparison, single-Cp-type, halfsandwich RE catalysts,11 despite their remarkable versatility demonstrated in the polymerization and copolymerization of olefins, especially styrene and isoprene, leading to a series of new polymeric materials,12 have been shown to be much less active and effective than the prototype sandwich lanthanocene catalysts for polymerization of acrylics.1 Likewise, several different types of ansa-RE metallocenes are also considerably less active and effective than the unbridged RE metallocene system.1 However, for the biomass-derived monomers MBL and γMMBL, which can be considered as cyclic analogues of MMA, we found that RE half-metallocenes4 and ansa-halfmetallocenes5 can promote extremely rapid polymerization of such cyclic acrylics without lactone ring-opening side reactions, with a high turnover frequency (TOF) up to 500 min−1. Polymers derived from free-radical polymerization of the βmethyl derivative of methylene butyrolactones, β-methyl-αmethylene-γ-butyrolactone (βMMBL), have been shown to exhibit superior materials properties as plastic optical fibers in comparison to other acrylic polymers.13 Radical polymerization of βMMBL initiated by 2,2-azobis(isobutyronitrile) (AIBN) yields an atactic polymer, which is soluble in common organic solvents such as DMF and DMSO.14 We reported recently that coordination polymerization of βMMBL by chiral C2-ligated zirconocenium ester enolate catalysts afford highly isotactic (95% mm) PβMMBL to stereoperfect (>99% mm) PβMMBL.15 The highly isotactic polymer is thermally robust, with a high glass-transition temperature (Tg) up to 290 °C, and resistant to all common organic solvents at room temperature or refluxing conditions. However, such chiral zirconocenium catalysts exhibit only modest activity, with TOF = 4 min−1.15 On the other hand, we found that ansa-half-sandwich RE dialkyl complexes supported by an ethylene-bridged fluorenyl-Nheterocyclic carbene (NHC) ligand, C2H4(η5-Flu-κ1-NHC)RE(CH2SiMe3)2 (RE = Y, Lu), also promote stereoselective polymerization of βMMBL to produce the isotactic polymer PβMMBL, with an isotacticity of 91% mm.5 Although the isotacticity of the polymer is lower than that by the chiral zirconocenium catalysts, the ansa-half-sandwich RE catalyst exhibits much higher activity, with a high TOF of 75 min−1.5 These earlier interesting observations prompted us to investigate the characteristics of βMMBL polymerization by unbridged half-sandwich RE dialkyl complexes incorporating a disilylated indenyl ligand: [Ind]RE(CH2SiMe3)2(THF), [Ind] = η5-(1,3-(SiMe3)2C9H5), RE = Sc,16 Y,17 Dy,17 Lu17 (Scheme 1). As controls and comparative examples, we also examined simple homoleptic hydrocarbyl RE complexes, RE(CH2SiMe3)3(THF)2 (RE = Sc, Y, Dy, Lu). Most significantly, our combined experimental and theoretical study described in
Scheme 1. Stereoselective Polymerization of βMMBL by Half-Sandwich RE Dialkyls and Simple RE Hydrocarbyl Complexes
the current report has revealed that (a) half-sandwich RE catalysts promote rapid and stereoselective coordination polymerization of βMMBL at ambient temperature, (b) simple hydrocarbyl RE complexes also produce the highly isotactic polymer PβMMBL, and (c) the stereoselectivity of this monometallic coordination polymerization originates chiefly from interactions between the methyl groups on the chiral β-C atom of the five-membered ring of both the coordinated monomer and the last inserted βMMBL unit of the chain.
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EXPERIMENTAL SECTION
Materials and Methods. All syntheses and manipulations of airand moisture-sensitive materials were carried out in flamed Schlenktype glassware on a dual-manifold Schlenk line, on a high-vacuum line, or in an argon or nitrogen-filled glovebox. HPLC-grade organic solvents were sparged extensively with nitrogen during filling of the solvent reservoir and then dried by passage through activated alumina (for Et2O, THF, and CH2Cl2) followed by passage through Q-5supported copper catalyst (for toluene and hexanes) stainless steel columns. HPLC-grade N,N-dimethylformamide (DMF) was degassed and dried over CaH2 overnight, followed by vacuum transfer (no distillation). The NMR solvents CDCl3 and DMSO-d6 were dried over activated Davison 4 Å molecular sieves, and NMR spectra were recorded on a Varian Inova 300 (FT 300 MHz, 1H; 75 MHz, 13C), a Varian Inova 400 MHz, or an Inova 500 MHz spectrometer. Chemical shifts for 1H and 13C spectra were referenced to internal solvent resonances and are reported as parts per million relative to tetramethylsilane. Acetaldehyde (freshly distilled), methyl acrylate, N-bromosuccinimide, dimethyl sulfide, 1,4-diazabicyclo[2.2.2]octane (DABCO), formaldehyde (37 wt % in water), tin powder, and p-toluenesulfonic acid were purchased from Sigma-Aldrich and used as received. AIBN (2,2-azobis(isobutyronitrile)) was purchased from Alfa Aesar. Literature procedures were employed to prepare the following compounds: [η5-(1,3-(SiMe3)2C9H5)]RE(CH2SiMe3)2(THF) (RE = Sc,16 Y,17 Dy,17 Lu17) and RE(CH2SiMe3)3(THF)2 (RE = Sc, Y, Dy, Lu).18 Preparation of βMMBL. Literature procedures5,14 were modified to prepare β-methyl-α-methylene-γ-butyrolactone (βMMBL) as detailed below. The purified monomer was stored in brown bottles inside a glovebox freezer at −30 °C. Step 1: Methyl 2-Methylidene-3-hydroxybutanoate. Freshly distilled acetaldehyde (44.1 g, 1.00 mol), methyl acrylate (95.5 g, 1.11 mol), and DABCO (12.3 g, 0.110 mol) were stirred at room temperature for 10 days. The solution was diluted with diethyl ether (200 mL), washed with water (2 × 100 mL) and 10% aqueous hydrochloric acid (50 mL), and then rinsed with water (2 × 50 mL). 1460
dx.doi.org/10.1021/om301128g | Organometallics 2013, 32, 1459−1465
Organometallics
Article
Table 1. Selected Results of βMMBL Polymerization by RE (Sc, Y, Dy, Lu) Catalystsa run no.
RE cat.
gelation time (min)
reaction time (min)
isolated yield (%)
TOF (min−1)
Tg (°C)
1 2 3 4 5 6 7 8
[Ind]Sc(CH2SiMe3)2(THF) [Ind]Y(CH2SiMe3)2(THF) [Ind]Dy(CH2SiMe3)2(THF) [Ind]Lu(CH2SiMe3)2(THF) Sc(CH2SiMe3)3(THF)2 Y(CH2SiMe3)3(THF)2 Dy(CH2SiMe3)3(THF)2 Lu(CH2SiMe3)3(THF)2
