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Metal and Counteranion Nuclearity Effects in Organoscandium Catalyzed Isoprene Polymerization and Copolymerization Jiazhen Chen, Yanshan Gao, Shuoyan Xiong, Massimiliano Delferro, Tracy L. Lohr, and Tobin J. Marks ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01621 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 7, 2017
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ACS Catalysis
Metal and Counteranion Nuclearity Effects in Organoscandium Catalyzed Isoprene Polymerization and Copolymerization Jiazhen Chen,a Yanshan Gao,a Shuoyan Xiong,a,‡ Massimiliano Delferro,a,† Tracy L. Lohr,a * and Tobin J. Marksa * a
Department of Chemistry, Northwestern University, Evanston, IL 60208-3113, USA
ABSTRACT: The binuclear organoscandium half-sandwich complexes (Me3SiCH2)2(THF)Sc[C5Me4-Si(CH3)2-(CH2)n-Si(CH3)2C5Me4]Sc(CH2SiMe3)2(THF) (n = 0, Sc-C0-Sc; n = 2, Sc-C2-Sc) and monometallic C5Me4SiMe3Sc(CH2SiMe3)2(THF) (Sc1) were prepared and fully characterized by conventional spectroscopic, analytical, and diffraction techniques. These complexes are active catalysts for isoprene polymerization and ethylene/isoprene copolymerization upon activation by the cocatalysts trityl perfluroarylborate (Ph3C+)B(C6F5)4- (B1) and trityl bisperfluoroarylborate (Ph3C+)2[1,4-(C6F5)3BC6F4B(C6F5)3]2- (B2). Marked catalyst and cocatalyst nuclearity effects on product polymer microstructure are achieved in isoprene polymerization. Thus, the % of cis-1,4- units in the polyisoprene products increases from 24% (Sc1) to 32% (Sc-C2-Sc) to 48% (Sc-C0-Sc) as catalyst nuclearity increases and Sc···Sc distance contracts. The binuclear catalysts regulate the isometric unit distributions and favor 3,4~3,4~3,4 blocks. Furthermore, the percentage of polyisoprene trans-1,4- units increases ~5x when binuclear cocatalyst (B2) is used in comparison to B1. In ethylene/isoprene copolymerizations, the binuclear catalysts produce polymers with higher molecular weights (Mn = 3.4 – 6.9 ×104; Ð = 1.4 – 2.0) and with comparable isoprene enchainment selectivity versus Sc1 under identical reaction conditions. However, isoprene incorporation is curiously reduced by ~50% when B2 is used versus B1. These results highlight the importance of both ionpairing and imposed nuclearity in these polymerizations, and indicate that both catalyst and cocatalyst nuclearities can be used to access specific polyisoprene polymer/copolymer microstructures. KEYWORDS: scandium, ion pairing, nuclearity effects, isoprene polymerization, bimetallic catalysis Inspired by multimetallic enzymes, where multiple metal centers are poised in close proximity and proper orientation to achieve exceptional activity and selectivity,1 extensive efforts are underway to develop analogous abiotic multimetallic catalysts that exhibit these beneficial properties.2 In transition metal-catalyzed olefin polymerizations,3 multimetallic complexes are known to significantly enhance polymer molecular weight, chain branching density,4 comonomer incorporation,5 and stereoselectivity,6 as well as to increase tolerance to polar additives7 and functionalized comonomers.8 Despite the many group 4 metal-based binuclear catalysts that have been reported, only a limited number of olefin polymerization studies using binuclear lanthanide complexes have been described.9 In contrast to μ-bridging ligands that can coordinate to two metal centers via amine,9d, 9h phosphine,9d, 9g pyrrolidene,9e, 10 or halide moieties,11 an alternative nucleation strategy would be to implement covalent ligand-linkers to bring two monometallic units into close proximity.9a-c, 9i, 12 This strategy offers control of metal-metal distance by varying the linker dimensions with precise modulation of the metal coordination environment, and opens opportunities for asymmetric catalysis. In addition, it would be desirable to expand the monomer scope beyond simple mono-enes. Currently, approximately 95% of all isoprene output is used for polyisoprenes production, particularly the cis-1,4- isomer which constitutes one of the most widely utilized commercial synthetic rubbers.13 Recently, increasing efforts have been devoted to creating more efficient catalysts for isoprene/diene polymers having diversified microstructures for new applications.9d, 9f, 14 For example, copolymers having polyisoprene
blocks show distinctive physical and mechanical properties, and are useful as adhesives and viscosity modifiers.15 Hou and co-workers reported Sc half-sandwich alkyl complexes that catalyze isoprene homo- and ethylene/isoprene copolymerizations after activation with perfluoroarylborane/borate cocatalysts.9d They also reported a μ-phosphido binuclear cyclopentadienyl Y complex that is promising for isospecific 3,4-polymerization of isoprene (Chart 1).9f Additionally, Anwander et al. described half-sandwich tetramethylaluminate lanthanide complexes (Chart 1) that exhibit excellent trans-1,4-selectivity in isoprene polymerizations after activation with perfluoroarylborate/borane reagents.14h Given the unique polymerization characteristics of group 3 half-sandwich catalysts,16 the intriguing question arises as to whether cooperative effects can operate and be modulated by two metal centers held in variable spatial proximity and by varying both the ion-pairing catalyst and cocatalyst nuclearity. Here we report the synthesis and
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Hou et al Si P Cy Me 3Si
reacts with the binuclear ligand were observed in situ over the course of the reaction by 1H NMR spectroscopy but were not isolated (Figure S7). These intermediates react with the second equiv. of Sc precursor to form the desired bimetallic products after 48 - 72 hrs at room temperature. No metallocene-type sandwich complexes are observed over the course of the reaction.18 Single crystals of Sc-C0-Sc suitable for X-ray diffraction were obtained by cooling n-pentane solutions to -30 °C (Figure 1a). Each Sc center is coordinated to one Cp’ ligand, two CH2SiMe3 groups, and one THF molecule in a distorted tetrahedral geometry, similar to the corresponding monometallic analogue (Sc1).19 Both Cp’Sc units are linked to the disilane bridge in a gauche conformation, with a torsion angle of 53.46o and an intramolecular Sc···Sc distance of 8.57 Å. Single crystals of Sc-C2-Sc suitable for X-ray diffraction were obtained by slow diffusion of n-pentane into a toluene solution at room temperature (Figure 1b and S12). The two conformations observed in the crystal structure have intramolecular Sc···Sc distances of 11.03 Å and 12.12 Å, respectively. The presence of two conformers is in accord with the substantial linker flexibility expected in Sc-C2-Sc. The bond lengths and angles of these bimetallic complexes are similar to those in the monometallic analogue (C5Me4SiMe3)Sc(CH2SiMe3)2(THF) (Sc1) despite slightly longer Sc-O bond average lengths of 2.171(2) Å for Sc-C0-Sc and 2.172(2) Å for Sc-C2-Sc (average of two conformations) vs. 2.158(2) Å for Sc1, which may indicate slightly increased steric repulsion accompanying the THF coordination.
Anwander et al
Y2 Y1
SiMe 3 Cy P Si
Me Me
Al Me
Ln Me Me Me Al Me Me
Ln = Y, La, Nd
This work Si
Si
Si ( )n
Sc Sc Sc Me3 Si Me3 Si SiMe3 THF THF THF Me 3Si Me3 Si SiMe 3 n = 0, Sc-C0-Sc
Sc1
Ph3 C
F
n = 2, Sc-C2-Sc F
F
Ph3 C
(C 6 F5 )3 B
F F
F
CPh3
(C 6F 5) 3B
F
B(C 6F5 )3 F
F
B1 B2 Chart 1. Structures of catalysts and cocatalysts for isoprene polymerization full characterization of two new binuclear scandium complexes (Sc-C0-Sc and Sc-C2-Sc), where two half-sandwich ligands are connected by variable length -Si(CH3)2-(CH2)n-Si(CH3)2(n = 0, 2) linkages (Chart 1). Using these Sc complexes and perfluoroarylborate cocatalysts, mononuclear B1 (Ph3C+)B(C6F5)4and binuclear B2 (Ph3C+)2[1,4(C6F5)3BC6F4B(C6F5)3]2-, we assess nuclearity effects on isoprene polymerization activity, stereoselectivity, isomeric unit distribution, and polymer molecular mass, as well as isoprene incorporation in isoprene + ethylene copolymerizations. It will be seen that these effects can be large and are subject to sequential modulation. The ligands C5Me4H-Si(CH3)2-(CH2)n-Si(CH3)2-C5Me4H (n = 0, 2) were synthesized by the reaction of ClSi(CH3)2-(CH2)nSi(CH3)2Cl (n = 0, 2) with 2 equiv. of tetramethylcyclopentadienyl lithium in 51% (n = 0) and 83% (n = 2) yields, and were fully characterized by 1H NMR, 13C NMR and elemental analysis (See Scheme 1 and SI).17 The molecular structure of C5Me4H-Si(CH3)2-Si(CH3)2-C5Me4H was Cl Si
Si Cl + ( )n
THF Li 48h r.t.
Si
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Si ( )n
n = 0, 2 Si
Sc(CH 2 SiMe3) 3 (THF)2 n-hexane 48-72h r.t. -SiMe4 -THF
Me 3Si Me 3Si
Sc
THF
Si ( )n THF
Sc
SiMe3 SiMe 3
n = 0 Sc-C0-Sc yield 68% n = 2 Sc-C2-Sc yield 65%
Scheme 1. Synthesis of bimetallic Sc half-sandwich complexes Figure 1. ORTEP plots (50% thermal ellipsoids) of the X-ray crystal structures of binuclear catalysts, (a) Sc-C0-Sc and (b) Sc-C2-Sc (H atoms omitted for clarity) Selected bond distances (Å) and angles (deg) for Sc-C0-Sc: Sc1–O1 = 2.166(4) Sc1– C1 = 2.231(6) Sc1–C5 = 2.222(6) Sc1–Cp(centroid) = 2.185 Sc2–O2 = 2.177(4) Sc2–C39 = 2.224(6) Sc2–C35 = 2.227(6)
confirmed by single crystal X-ray diffraction (Figure S10). Bimetallic complexes Sc-C0-Sc and Sc-C2-Sc were obtained in high yield by metallation of corresponding free ligands with Sc(CH2SiMe3)3(THF)2 in n-hexane at room temperature (Scheme 1). Intermediates in which 1 equiv. of Sc precursor
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ACS Catalysis
Sc2–Cp(centroid) = 2.189 O1–Sc1–C1 = 101.2(2) O1–Sc1–C5 = 95.4(2) C1–Sc1–C5 = 105.8(2) O2–Sc2–C35 = 102.6(2) O2–Sc2–C39 = 93.6(2) C35–Sc2–C39 = 104.9(2) Selected bond distances (Å) and angles (deg) for Sc-C2-Sc: Sc1–O1 = 2.1660(16) Sc1–C13 = 2.223(2) Sc1–C17 = 2.244(2) Sc1– Cp(centroid) = 2.177 O1–Sc1–C13 =102.50(8) O1–Sc1–C17 = 95.46(8) C13–Sc1–C17 = 105.63(9) No obvious ligand redistribution is observed by 1H NMR for either Sc-Cn-Sc (n = 0, 2) complex in benzene-d6. The diastereotopic alkyl methylene CH2 signals are found at δ 0.27 and -0.25 ppm for Sc-C0-Sc and δ -0.26 and -0.23 ppm for Sc-C2-Sc, respectively, with geminal coupling constants of 10.5 Hz (Sc-C0-Sc) and 13.3 Hz (Sc-C2-Sc).20 As indicated by in situ 1H NMR spectroscopy, both binuclear Sc catalysts undergo rapid activation with 2 equiv. cocatalyst B1 in toluene-d8 at room temperature. The Sc catalysts and borate react completely within minutes to quantitatively form Ph3CCH2SiMe3 and the corresponding cationic active species (see discussion, peak assignments, and activation procedure on page S24 of the SI). To obtain information on group 3 nuclearity effects on catalytic activity and selectivity, isoprene homopolymerizations by activated Sc-Cn-Sc, n = 0 and 2, as well as by activated monometallic analogue Sc1 were investigated in toluene or toluene + o-difluorobenzene (DFB). Results are summarized in Table 1. When activated with B1, monometallic Sc1 displays moderate selectivity towards 3,4-polyisoprene (59 and 66%, respectively) with lower selectivity towards cis-1,4-polyisoprene (26 and 24%, Table 1, entries 1 and 4). Note that the binuclear catalysts exhibit significantly increased cis-1,4 selectivity versus Sc1, with Sc-C0-Sc producing polyisoprene with the highest cis-1,4- selectivity (48%) (Table 1, entries 2-3 and 5-6). Potential nuclearity effects on the polymerization process induced by cocatalyst B2 versus B1 were also investigated.
After activation with B2, the bisborate dianion may spatially confine the two metal cations via tight ion pairing which, in turn, may enhance selectivity for reactions requiring cooperation between two proximate catalytic centers.21 In the present systems, when binuclear B2 is used as the cocatalyst, trans-1,4 selectivity increases ~5x versus the reactions using B1 (Table 1, entries 7-9 vs. 4-6) for all catalysts under identical reaction conditions. Note that the selectivity for cis-1,4 units mediated by the bimetallic catalysts versus Sc1 is preserved when B2 is used (20% and 19%, respectively, vs. 4%). However the activity of these catalysts declines significantly for B2 versus B1, presumably reflecting the steric constraints of the bulky dianion. Polymerizations in toluene or in a toluene + DFB mixture yield similar results, indicating that solvation effects on the catalyst-cocatalyst ion-pairing are catalytically minor. The distributions of isomeric units in the polyisoprene products at the diad and triad levels were quantified by detailed NMR analysis22 (Table 2). Note that there is a marked preference for forming 3,4- short blocks when the bimetallic catalysts + B1 are used. Thus, the polyisoprene produced by Sc-C2-Sc contains ~20% more 3,4~3,4~3,4 triads versus that predicted for a fully random distribution.22 This preference is even more significant when Sc-C0-Sc and B1 are used, and ~50% more 3,4~3,4~3,4 triads are obtained versus the statistical value. In contrast, Sc1 + B1 yields ~10% more (see SI for more details), and curiously, no significant preference for 3,4blocks is observed using binuclear B2 as the cocatalyst. To address the origin of the nuclearity effects arising from the binuclear catalysts, isoprene homopolymerizations were also carried out at a Sc-Cn-Sc : B1 ratio lower than 1:2. With Sc-Cn-Sc : B1 = 1:1, where presumably only one of the two metal centers is activated, negligible polymer is formed, possibly reflecting THF migration from the unactivated Sc center and
Table 1. Data for isoprene homopolymerization mediated by mononuclear and binuclear organoscandium catalysts a Entry
Catalyst
Cocatalyst
Solvent
d
Polymer microstructureb
Yield (g)
Mnc (×103)
Ð
Trans-1,4-
Cis- 1,4-
3,4-
1.02
15
26
59
80
1.1
1
Sc1
B1
tol
2
Sc-C2-Sc
B1
tol
1.02
4
39
57
95
1.3
3
Sc-C0-Sc
B1
tol
1.02
8
46
45
50
1.8
e
4
Sc1
B1
DFB/tol
1.02
10
24
66
31
1.3
5
Sc-C2-Sc
B1
DFB/tol
1.02
6
32
62
59
1.2
6
Sc-C0-Sc
B1
DFB/tol
0.653
8
48
44
32
2.8
7
Sc1
B2
DFB/tol
0.154
55
4
41
7.4
2.5
8
Sc-C2-Sc
B2
DFB/tol
0.119
21
20
59
8.1
1.7
9
Sc-C0-Sc
B2
DFB/tol
0.070
35
19
46
8.3
1.6
a
Conditions: 21 ߤmol Sc1 (10.5 ߤmol Sc-Cn-Sc), 21 ߤmol B1 or 10.5 ߤmol B2, 15 mmol isoprene, 25 °C, 3 h. b By 13C NMR. cBy GPC versus polystyrene standards. d 20 mL toluene e 2.0 mL DFB (o-difluorobenzene) + 18 mL toluene. DFB was used to dissolve the B2.
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coordination/saturation of the active cationic species.23 Increasing Sc-Cn-Sc : B1 to 1:1.6 yields polyisoprene but with lower activity versus that with 2 equiv. of B1. NMR analysis indicates that the ratio of isomeric units in these polymers is intermediate between Sc1 : B1 =1:1 and Sc-Cn-Sc : B1 =1:2.24 These results argue that the selectivity of the binuclear catalysts must arise from the presence of two proximate cationic centers. Note also that isoprene homopolymerizations at ScCn-Sc : B1 ratios higher than 1:2 yield cross-linked polymers consistent with trityl-induced cationic polymerization.25 Based on previous mechanistic and theoretical findings9d, 9f, , a possible binuclear half-sandwich Sc catalyst-mediated isoprene polymerization mechanism is proposed in Scheme 2. Note that the Sc-Cn-Sc catalysts exhibit significantly enhanced cis-1,4- selectivity, possibly involving a weak agostic interaction3,21g between the second cationic Sc center and the growing polymer chain via an anti-η3-π-allylic intermediate (Scheme 2, e). The preference for 3,4- blocks can be rationalized by the interaction between the second Sc center and the terminal C=C bond on the side chain that forms after a 3,4- isoprene insertion. B2 is sterically more demanding, and isomerization of the σ-allyl intermediate (Scheme 2, c) to the anti-η3-allyl species (i.e., formation of cis-1,4 units) may be kinetically less favored versus formation of the syn-η3-allyl species (Scheme 2, f). This would afford a higher percentage of trans-1,4- over cis-1,4- units with B2 as the cocatalyst, as observed. Binuclear catalysts/cocatalysts may also influence cis/trans selectivity by stabilizing the stereochemistry of a particular isoprene insertion pathway. 26
Table 2. Polyisoprene 3,4~3,4~3,4 triad analysisa
CoEntry
a
catalyst
3,4- Selectivity
Catalyst
3,4~3,4~3,4 Triad percentage Calc.b
Obs.c
1
Sc1
B1
65
27.5
28.6
2
Sc-C2-Sc
B1
61
22.7
27.0
3
Sc-C0-Sc
B1
45
9.1
14.6
4
Sc1
B2
41
6.9
3.6
5
Sc-C2-Sc
B2
59
20.5
22.4
6
Sc-C0-Sc
B2
46
9.7
8.0
Conditions: 21 ߤmol Sc1 (10.5 ߤmol Sc-Cn-Sc), 21 ߤmol B1 or 10.5 ߤmol B2, 15 mmol isoprene, 25 °C, 3 h 2.0 mL DFB + 18 mL toluene. b based on fully random distribution c 3,4~3,4~3,4 triad values observed on the C3 signal at δ = 42.
Scheme 2. Plausible scenarios for isoprene polymerization catalyzed by binuclear half-sandwich organoscandium complexes (anions not shown for visibility). Copolymerizations were carried out by feeding isoprene under an ethylene atmosphere (1 atm) in toluene or DFBtoluene at 25 oC, (Table 3).27 Ethylene/isoprene copolymers were obtained with a random sequence distribution (Figure S17). Consistent with the selectivity observed in the isoprene homopolymerizations, when activated by B1, monometallic Sc1 yields large proportions of 3,4-polyisoprene (>60%, Table 3 entries 1 and 4). In contrast, the binuclear catalysts exhibit decreased catalytic activity and produce higher Mn polymers (Table 3 entries 2, 3, 5 and 6 vs 1 and 4). The lower polymerization activity for the binuclear catalysts attributable to a slower ethylene insertion, reflecting greater steric bulk surrounding the two Sc catalytic centers. Indeed we observe that the activity in ethylene homopolymerizations follows the same trend Sc1 > Sc-C2-Sc > Sc-C0-Sc. The higher product molecular weights produced by the binuclear catalysts may be attributed to slower chain termination processes, possibly influenced by the aforementioned agostic interactions. A similar interplay of these two factors has been observed in other bimetallic polymerization systems reported by our group.3Using B2 also results in lower activity and produces 3,4- and trans-1,4rich copolymers with less than 1% cis-1,4- units (Table 3 entries 7-9). This is consistent with the preference of trans-1,4over cis-1,4- insertion products observed for B2 in isoprene homopolymerization (Table 1). The copolymers produced by the mono- and bimetallic catalysts have similar levels of isoprene incorporation in the presence of the same cocatalyst. However, polymerizations with B2 produce copolymers with far less isoprene incorporation (20%) than the polymerizations with B1 (40%), which may be attributed to the more sterically demanding B2 counteranion that favors the insertion of the smaller ethylene monomer over isoprene. GPC analysis reveals that these copolymers have high molecular weights and unimodal molecular weight distributions (1.4 - 2.9), consistent with a homogenous single-site catalytic process. The Tg values of these polymers are in the range -56 to -30 oC by DSC. An endotherm corresponding to polyethylene melting in the range 110 – 130 oC is observed in samples with high ethylene contents (Table 3 entries 7-9) but is absent in samples with high isoprene content (Table 3 entries 1-6; see Figures S16-S44 for details).
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In conclusion, the effects of catalyst and cocatalyst nuclearity were studied for isoprene polymerization and ethylene/isoprene copolymerization with organoscandium halfsandwich complexes. The binuclear catalysts produce polyisoprenes with higher cis-1,4- content and a preference for 3,4~3,4~3,4 triads, both of which are amplified with decreasing Sc···Sc distance. Using binuclear cocatalyst B2 results in a higher density of trans-1,4- isomeric units with more random
incorporation. Copolymerizations with binuclear cocatalyst B2 yield reduced isoprene incorporation for both mono- and binuclear scandium catalysts.
Table 3. Data for ethylene/isoprene copolymerization mediated by mononuclear and binuclear organoscandium catalystsa Entry
Catalyst
Cocatalyst
Solvent
Yield (g)
Activityb
Isoprene cont. (mol %)
Polymer microstructurec Trans1,4-
Cis1,4-
3,4-
Mnd (×104)
Ð
T ge (oC)
1
Sc1
B1
tolf
0.97
554
37
33
3
63
3.4
2.0
-46
2
Sc-C2-Sc
B1
tol
0.67
383
40
35
9
56
6.9
1.5
-30
3
Sc-C0-Sc
B1
tol
0.62
354
43
35
12
53
5.4
1.6
-46
g
4
Sc1
B1
DFB/tol
1.01
576
38
33
4
62
3.0
2.0
-46
5
Sc-C2-Sc
B1
DFB/tol
0.84
479
40
35
9
56
3.6
2.9
-48
6
Sc-C0-Sc
B1
DFB/tol
0.51
291
40
42
4
54
6.4
1.4
-50
7
Sc1
B2
DFB/tol
0.22
126
21
39
