Organolanthanide Complexes Supported by Thiazole-Containing

Dec 8, 2014 - Organolanthanide Complexes Supported by Thiazole-Containing Amidopyridinate Ligands: Synthesis, Characterization, and Catalytic Activity...
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Organolanthanide Complexes Supported by Thiazole-Containing Amidopyridinate Ligands: Synthesis, Characterization, and Catalytic Activity in Isoprene Polymerization Lapo Luconi,† Dmitrii M. Lyubov,‡ Andrea Rossin,† Tatyana A. Glukhova,‡ Anton V. Cherkasov,‡ Giulia Tuci,† Georgy K. Fukin,‡ Alexander A. Trifonov,*,‡,§ and Giuliano Giambastiani*,†,⊥ †

Institute of Chemistry of OrganoMetallic Compounds, ICCOM-CNR, Via Madonna del Piano, 10, 50019 Sesto Fiorentino (Florence), Italy ‡ G. A. Razuvaev Institute of Organometallic Chemistry of the Russian Academy of Sciences, Tropinina 49, GSP-445, 603950 Nizhny Novgorod, Russian Federation § A. N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences, 28 Vavilova Street, 119991, Moscow, GSP-1, Russian Federation ⊥ Kazan Federal University, 420008 Kazan, Russian Federation S Supporting Information *

ABSTRACT: Neutral bis(alkyl)-organolanthanide complexes supported by tridentate {N−,N,N} monoanionic 5-methylthiazole- or benzothiazoleamidopyridinate ligands have been prepared and completely characterized: (LThiaMe2)Ln(CH2SiMe3)2 [Ln = Lu3+ (3), Er3+ (7), Yb3+ (8)] and (LBnThMe2)Lu(CH2SiMe3)2 (5). Similarly to related Y3+ systems, the nature of the thiazole unit controls the ultimate catalyst stability in solution. In the diamagnetic Lu3+ complex 5, a progressive and complete rearrangement of its metal coordination sphere takes place through a metal-to-ligand alkyl migration with subsequent benzothiazole ring-opening and generation of the Lu3+ mono(alkyl)-arylthiolate species stabilized by a tetradentate {N−,N,N,S−} dianionic ligand. On the other hand, the 5-methylthiazolecontaining complexes 3, 7, and 8 showed no evidence of any ligand rearrangement. Complexes 3−8 have been tested as homogeneous catalysts in isoprene (IP) polymerization, after activation with selected organoborates. Binary systems 3/TB and 7/TB [TB = tritylium tetrakis(pentafluorophenyl)borate] show the highest activity and living character toward IP polymerization, affording polymers with relatively high trans-1,4-selectivity (up to 76.4%), moderate molecular weights (Mn up to 146 000 g/mol), and narrow polydispersities (Mw/Mn). Depending on the rare-earth ion of choice, a prevalent trans-1,4 (Lu3+, Er3+, Yb3+; up to 76.4%) or a dominant 3,4 (Y3+; 92.7%) polymer structure is observed. The influence of the ligand type, metal ion, and activator(s) on the ultimate catalyst activity and selectivity is discussed.

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INTRODUCTION

for the preparation of polyolefin materials with specific physical, thermal, and mechanical properties deriving from a careful control of their microstructure is still a hot topic in homogeneous catalysis. In particular, the stereospecific polymerization of 1,3-conjugated dienes such as isoprene (IP) is one of the most important industrial processes (surely one of the most investigated in the literature)4 for the preparation of polyisoprenes (PIPs) with different microstructures.5−7 On this basis, a relatively high number of cationic mono(alkyl) rare-earth catalysts tested in diene polymerization have appeared in the literature in the past few years.5−8 They are based on tailor-made ligands featuring flexible coordination modes and showing from moderate to high stereospecificity for

Recent years have witnessed an impressive progress toward the design and synthesis of novel organolanthanides, in light of their ability to promote thermodynamically unfavorable reactions such as hydrocarbon activation1 and alkane functionalization.2 A great deal of attention is currently paid to the improvement of the catalytic systems through a finetuning of the stereoelectronic properties of their ancillary ligands, a key tool for controlling and investigating the reactivity, stability, and catalytic performance at the coordinated metal center. The synthesis of tailored ancillary ligands suitable for coordination to rare-earth ions, the isolation of the corresponding alkyl species, and the investigation of their complex structure−reactivity relationship for an optimal catalyst design represent a still challenging area of research in the field of organo-rare-earth chemistry.3 The exploitation of the catalytic properties of selected single-site organolanthanides © XXXX American Chemical Society

Received: September 5, 2014

A

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the reaction of interest; some of them also possess a living polymerization character.8c,d We have recently found that, after a suitable activation by organoborates, selected cationic monoalkyl Y3+ complexes stabilized by tridentate {N−,N,N} thiazole-containing amidopyridinate ligands (Figure 1, compounds 4 and 6) served as

Scheme 1. Synthesis of the Bis(alkyl) Organolanthanide Complexes 3, 5, 7, and 8a

a

Complexes 4 and 6 have already been discussed elsewhere9 and are cited here for completeness.

literature procedure.9 As previously reported for 4 and 6, the reaction between one selected ligand {HL Thia Me 2 or HLBnThMe2} and the trisalkyl precursor Ln(CH2SiMe3)3(thf)2 (Ln = Lu3+, Er3+, Yb3+) in toluene at 0 °C affords the expected bis(alkyl) compounds 3, 5, 7, and 8 as dark red microcrystals, with good isolated yields (from 65% to 78%, Scheme 1). For complexes 3 and 5, the reaction course is conveniently monitored by 1H NMR spectroscopy until complete ligand consumption, which normally occurs within a few minutes (for the NMR spectra of the isolated compounds see Figures S1− S4). Afterward, the concentrated reaction mixtures are cooled to −30 °C overnight, when dark red microcrystals of 3, 5, 7, and 8 separate off. Crystals are separated from the mother liquors by decantation, and they are rapidly washed with cold hexane before being dried under vacuum to constant weight. All bis(alkyl) species are highly air- and moisture-sensitive compounds and almost insoluble in common aliphatic hydrocarbons (n-pentane or n-hexane); they can be stored under an inert atmosphere at room temperature for months without any apparent decomposition. All complexes show fairly good solubility in aromatic hydrocarbons, e.g., toluene and benzene, where their reactivity is commonly exploited (vide inf ra). They are highly soluble also in thf, although this solvent accelerates, in the case of complex 5 only, the occurrence of an (undesired) reorganization of the metal coordination sphere. This rearrangement that proceeds through an alkyl metal-toligand migration followed by benzothiazole ring opening has already been discussed elsewhere for the yttrium species (6), both experimentally and theoretically.9 According to the NMR spectroscopy and microanalysis data, the isolated 5 does not contain any coordinated thf molecule and its 1H NMR spectrum shows large regions that are superimposable to those of 69 (Figure S3 vs S5). Its most relevant 1H NMR spectral features (benzene-d6) consist of a pair of diastereotopic isopropyl methyl resonances [two doublets at δH = 1.30/1.48 ppm] with a single −CHMe2 septet (δH = 3.79 ppm) and two doublets centered at δH = −0.46 (d, 2JHH = 11.5 Hz)/−0.18 ppm (d, 2JHH = 11.5 Hz) assigned to the diastereotopic methylene groups of the alkyl fragments bound to the lutetium center. The six methyl groups of the −SiMe3 moieties appear as a sharp singlet at δH = 0.05 ppm. In the 13C{1H} NMR spectrum, the alkyl −CH2− groups appear as a unique singlet centered at 43.9 ppm, while the −SiMe3 resonances fall at δC = 4.2 ppm. The treatment of a benzene solution of 5 with a few drops of THFleads to a relatively fast reorganization (within a few hours) of the

Figure 1. Bis(alkyl) organolanthanide complexes 3, 5, 7, and 8 stabilized by tridentate {N−,N,N} thiazole-containing amidopyridinate ligands. Complexes 4 and 6 have already been discussed elsewhere9 and are cited here for completeness.

effective single-component initiators for the intramolecular hydroamination of primary and secondary amino alkenes already at moderate catalyst loadings (5 mol %) and under relatively mild (time and temperature) reaction conditions.9 Aimed at widening the catalysts scope, this paper describes the synthesis and characterization of novel bis(alkyl) organolanthanide complexes from the same series (Lu3+, 3 and 5; Er3+, 7; Yb3+, 8; Figure 1) and the ability of their cationic counterparts to perform the catalytic IP polymerization efficiently. Depending on the rare-earth ion of choice, a prevalent trans-1,4 monomer enchainment or a dominant 3,4 motif in the polymer microstructure is observed. Similarly to what was observed in the yttrium benzothiazole complex 69 the Lu3+ analogue 5 undergoes a rapid metal-toligand alkyl migration followed by the heterocycle ring opening, with the generation of a mono(alkyl)-arylthiolate species stabilized by a tetradentate {N−,N,N,S−} dianionic system (vide inf ra). At odds with the benzothiazole complexes 6 and 5, the bis(alkyl) organolanthanide systems bearing the simplest 5methylthiazole fragment (3, 4, 7, 8) generate more stable species, with no evidence for further rearrangements at the metal coordination sphere. Upon activation with an organoborate, some of them serve as valuable cationic catalysts for the production of trans-1,4-enriched (activated complexes 3, 7, and 8) PIPs, showing polymers with moderate molecular weights (Mn) and narrow polydispersities (Mw/Mn). Among this series, the cationic yttrium complex 4 is only moderately active, while it exhibits a distinguished 3,4 regioselectivity (up to 92.7%). The influence of the organoborate used for the generation of the cationic systems, the ability of the latter to initiate a precisely controlled or even living polymerization, and other parameters affecting the catalysts’ activity and selectivity will be discussed in detail.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Bis(alkyl)organolanthanide Complexes 3−8. The tridentate {N,N,N} benzothiazole {HLBnThMe2} (1) and 5-methylthiazole {HLThiaMe2} (2) ligands (Scheme 1) as well as the bis(alkyl) yttrium derivatives 4 and 6 are prepared according to the B

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Scheme 2. Alkyl Metal-to-Ligand Migration and Benzothiazole Ring-Opening to Give the Lutetium Thiolate Complex 9a

a

The analogue yttrium thiolate complex 10 deriving from 6 has already been discussed elsewhere9 and cited here for completeness.

opening generates an alkoxy-imino ligand that finally coordinates to the metal ion with a dianionic tetradentate framework. In this regard it appears that metal-to-ligand alkyl migrations on either oxazolinyl or thiazolyl moieties (having more than one coordinative site on equivalent positions and free to switch their coordination mode) preferentially occur through selective heterocycle ring-openings instead of undergoing the more classical imino-group reductions.11 If alkyl metal-to-ligand migrations on pyridine and imine-based ligands are rather common with a number of main group,12 transition,13 and rare-earth metals,14 processes like those described for the benzothiophene- or the oxazolinyl-containing systems are rather rare instead.8f,9,15 In line with our previous studies on the yttrium complex 4,9 compounds 3, 7, and 8 stabilized by the 5-methylthiazole unit show higher stability, with no evidence of any ligand rearrangement even upon standing in a benzene/thf solution for days. 1H NMR and 13C{1H} NMR spectra of diamagnetic 3 (Figures S1 and S2) show similar patterns to those previously recorded for the yttrium analogue 4, with large superimposable spectral regions.9 The most relevant spectral features are given by a pair of diastereotopic isopropyl methyl resonances [two doublets at δH = 1.28/1.46 ppm] with a single −CHMe2 septet centered at 3.77 ppm. The related 13C{1H} NMR signals appear as three singlets at 24.2, 26.7 and 28.1 ppm, respectively. Methylene protons of the alkyl groups attached to lutetium appear as two doublets at −0.50 and −0.31 ppm (d, 2JHH = 11.4 Hz), while a sharp single resonance for the two −SiMe3 groups is given at 0.14 ppm; the corresponding carbon atoms generate two singlets at 43.3 and 4.2 ppm, respectively. Finally, the methyl groups on the quaternary −CMe2− bridge between the amido and the pyridine moiety appear as a sharp singlet at 1.52 ppm, while their carbon signals give a singlet at 32.9 ppm. Dark red crystals of 3, 7, and 8 suitable for X-ray analyses are also obtained by prolonged cooling of the respective concentrated solutions in toluene at −30 °C. A perspective view of all compounds is given in Figures 2, 3, and 4, while Table S1 lists their main crystal data and structural refinement details. The complete crystallographic data set is reported in Tables S2−S10 in the Supporting Information. Complexes 3, 7, and 8 are isomorphous and isostructural; they crystallize in the monoclinic Pn space group, with two molecules per unit cell. On the basis of the index of trigonality τ16 (3 = 0.24; 7 = 0.26; 8 = 0.25), all metal centers adopt a distorted square-pyramidal coordination geometry (coordination number 5), with the three ligand N-donor atoms and one −CH 2 SiMe 3 group lying equatorially and a residual −CH2SiMe3 fragment occupying the axial position. All chemical structures show a slight deviation of the amido

complex coordination sphere with the quantitative generation of the monoalkyl lutetium thiolate complex 9 (Scheme 2). At odds with the yttrium analogue 10,9 all our attempts to grow crystals of 9 suitable for X-ray diffraction analysis failed. Due to the tricky material handling and its scarce propensity to crystallize in a pure form, its chemical identity has unambiguously been confirmed spectroscopically looking at the many similarities between its 1H and 13C{1H} NMR spectra and those recorded for 10 (Figure S6 vs S8). The most relevant 1 H NMR spectral features of 9 consist of two doublets at δH = −0.93 and −0.87 ppm (2JHH = 11.5 Hz) from the residual methylene group bound to the metal center and two additional doublets at δH = 2.02 and 2.90 ppm (2JHH = 12.5 Hz) for the diastereotopic methylene protons from the migrated −CH2SiMe3 group; the −SiMe3 protons appear as sharp singlets at δH = −0.22 and 0.04 ppm for the migrated one and the metal-bound alkyl residue, respectively. The −SiMe3 carbons in the 13C{1H} NMR spectrum of 9 give rise to two sharp singlets at δC = −0.8 and 4.2 ppm, assigned to the migrated and the residual metal-bound alkyl groups, respectively (to be compared with δC = −0.8 and 4.1 ppm from 10).9 The methylene carbon of the migrated alkyl group in 9 generates a singlet at δC = 20.9 ppm (δC = 20.8 in 10), while the residual −CH2− group attached to the metal center appears at δC = 43.6 ppm. Finally, the singlet at δC = 153.4 ppm (δC = 153.6 ppm in 10) is attributed to the ipso quaternary carbon resulting from the −CH2SiMe3 group metalto-ligand migration on the carbon atom of the benzothiazole fragment in the parent dialkyl complex 5 (δC = 146.6 ppm). It can be inferred that the electronic requirements of the electronpoor Lu3+ ion drive the process toward the generation of a sterically and electronically saturated system, favoring the selective thiazole ring-opening and the generation of a tetradentate dianionic benzothiolate ligand. Although no evidence for a dynamic process associated with Lu3+−NThia bond dissociation and concomitant Lu3+−SThia bond formation can be given on spectroscopic grounds,10 its occurrence can be assumed on the basis of the final nature of the rearranged compound. Indeed, a transient “S” coordination followed by the metal-to-ligand migration of one alkyl group to the ipso carbon of the heterocycle would induce the observed chemoselective C−S bond cleavage while keeping the imino moiety deriving from the thiazolic core untouched (Scheme 2). S. Zhang, X. Li, and co-workers have recently demonstrated how a similar rearrangement occurs on dialkyl organolanthanide systems containing oxazolinyl moieties;8f a metalto-ligand alkyl migration takes place on one of the oxazolinyl groups, leading to the chemoselective N,O-heterocycle ringopening instead of the imino group reduction. The ringC

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Figure 4. Crystal structure of 8. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Yb(1)−N(3) 2.195(2), Yb(1)− C(29) 2.349(2), Yb(1)−C(25) 2.381(2), Yb(1)−N(2) 2.401(2), Yb(1)−N(1) 2.482(2); C(29)−Yb(1)−C(25) 107.71(8), N(3)− Yb(1)−N(2) 68.96(6), N(3)−Yb(1)−N(1) 130.58(6), N(2)− Yb(1)−N(1) 66.45(6).

Figure 2. Crystal structure of 3. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Lu(1)−N(3) 2.195(1), Lu(1)− C(29) 2.344(2), Lu(1)−C(25) 2.376(2), Lu(1)−N(2) 2.386(1), Lu(1)−N(1) 2.461(2); C(29)−Lu(1)−C(25) 107.76(7), N(3)− Lu(1)−N(2) 69.12(5), N(3)−Lu(1)−N(1) 130.98(5), N(2)− Lu(1)−N(1) 66.79(5).

2.217(1) Å for 7; 2.195(2) Å for 8] than the others [Lu(1)− N(2) 2.386(1) Å and Lu(1)−N(1) 2.461(2) Å; Er(1)−N(2) 2.424(1) Å and Er(1)−N(1) 2.497(1) Å; Yb(1)−N(2) 2.401(2) Å and Yb(1)−N(1) 2.482(2) Å]. This structural feature confirms the stronger donating ability of N(3) with respect to N(1) or N(2) (anionic N− amido vs neutral N pyridinic, respectively). Neutral-Dialkyl vs Cationic-Monoalkyl Organolanthanide Complexes 3+−8+. Three different organoborate activators have been used for the generation of the cationic organolanthanides to be scrutinized for the catalytic IP polymerization (vide inf ra): the Lewis acids [Ph3C][B(C6F5)4] [tritylium tetrakis(pentafluorophenyl)borate, TB] and B(C6F5)3 [tris(pentafluorophenyl)borane, BN] and the Brønsted acid [PhNHMe2][B(C6F5)4] [N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate, HNB]. Unlike neutral dialkyl species, the monoalkyl cationic forms are less stable in benzene-d6 solution, where brown semisolid sticky materials separate off already at room temperature. The process is relatively fast, as shown for the diamagnetic complexes 3+ and 5+, where their most diagnostic 1H NMR signals are completely suppressed within 5 and 1 h, respectively, and irrespective of the nature of the activator used. All attempts to characterize the resulting semisolid compounds failed because of their untreatable nature even in the most commonly used aprotic polar solvents (thf-d8, DMF-d7). A 10:90 v/v mixture of bromobenzene-d5 and benzene-d6 is used to improve the solubility of the cationic forms in aromatic hydrocarbons while retarding their decomposition as much as possible. On the other hand, the use of pure bromobenzene-d5 induces a rapid catalyst death. Accordingly, the activation by TB of the diamagnetic compounds 3 and 4 is conveniently followed by in situ 1H NMR spectroscopy using diluted benzene-d6/C6D5Br 90:10 v/v solutions; a quantitative conversion of the neutral forms 4 and 3 to give the respective cationic monoalkyl counterparts {[Ln(κ3-N,Npy,N−)CH2SiMe3][B(C6F5)4]; Ln = Y3+, Lu3+} is observed almost instantaneously upon addition of

Figure 3. Crystal structure of 7. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Er(1)−N(3) 2.217(1), Er(1)− C(29) 2.377(2), Er(1)−C(25) 2.417(2), Er(1)−N(2) 2.424(1), Er(1)−N(1) 2.497(1); C(29)−Er(1)−C(25) 107.86(6), N(3)− Er(1)−N(2) 68.23(5), N(3)−Er(1)−N(1) 129.78(5), N(2)−Er(1)− N(1) 66.07(4).

nitrogen atom N(3) from the plane defined by the pyridine and 5-methylthiazole rings. As a result, a deviation of the lanthanide ion from the {N(1), N(2), N(3)} plane is measured (0.624 Å for 3; 0.623 Å for 7; 0.625 Å for 8). All Ln−C distances in 3, 7, and 8 are close to each other and similar to those measured in related five-coordinated Lu3+,17 Er3+,18 and Yb3+-bis(alkyl) complexes.19 For all complexes, Ln− N distances fall in the typical range observed for related systems,20−22 and they are nonequivalent, with the Ln(1)− N(3) bonds being substantially shorter [2.195(1) Å for 3; D

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Table 1. Isoprene Polymerization Data and Polymer Characterizationa microstructurec (%) i

b

entry

cat.

activator/ Bu3Al (equiv)

t [h]

yield [%]

1 2 3i 4 5 6 7 8 9i 10 11 12 13 14 15f 16g 17i 18 19 20 21 22 23 24 25 26 27

3−8e 4 4 4 4 5, 6 3 3 3 3 3 3 3 7 7 7 7 7 7 7 7 7 7 8 8 8 8

−/− TB/− TB/− −/(10) TB/(10) TB or BN/− TB/− TB/− TB/− TB/− TB/(10) BN/− HNB/− TB/− TB/− TB/− TB/− TB/− TB/− TB/− TB/(10) BN/− HNB/− TB/− TB/(10) BN/− HNB/−

6 12 12 6 6 6 6 4 4 2 6 6 12 1,5 1 1 0.5 0.75 0.5 0.33 6 6 6 6 12 12 12

0 28 31 0 81 0 >99 74 89 36 43 25 12 >99 >99h 89h >99 76 61 25 34 18 0 40 0 0 4

trans-1,4

cis-1,4

3,4-motif

Mn [10−3]d

Mw/Mnd

3.1 3.8

4.2 4.7

92.7 91.5

1937.87

1.82

14.0

24.3

61.7

23.20

1.98

76.4 76.2 74.4 72.9 71.6 76.7 76.2 66.4

79.55 56.82

1.65 1.52

35.28

1.57

8.2

23.6 23.8 25.6 27.1 28.4 23.3 23.8 25.4

65.4 60.5 63.6 71.9 67.4 76.2

6.3 7.7 7.0 1.7 1.4 1.2

28.3 31.8 29.4 26.4 31.2 22.6

24.30 80.72 149.98 80.95 138.24 147.11 111.31 87.65 44.21 8.49 44.10

2.46 1.61 1.39 1.61 1.52 1.41 1.34 1.42 1.40 1.73 1.39

19.7

71.16

1.85

20.6

26.53

3.11

80.3

78.5

0.9

Polymerization conditions: temp = rt (20−21 °C); toluene (3.5 mL); 10 mmol of IP [IP]; 10 μmol of catalyst [cat.] (3−8); [cat.]:[IP] = 1:1000; activators: TB, [Ph3C][B(C6F5)4]; BN, B(C6F5)3; HNB, [PhNHMe2][B(C6F5)4]; [cat.]:[activator] = 1:1.05. bAverage value calculated over three independent runs. cDetermined by 1H NMR and 13C{1H} NMR spectroscopy in CDCl3 at rt. dDetermined by GPC in thf at 40 °C against a polystyrene standard. ePolymerization runs performed at either rt (20−21 °C) or 50 °C. f5 mmol of [IP]. gAdditional 5 mmol of [IP] to run 15. h Estimated by GC analysis of the crude mixture against 1-hexene as internal standard after sampling and quenching 0.2 mL of the reaction mixture. i Polymerization runs performed at 50 °C. a

one equivalent of the Lewis acid (Figures S9 and S10). In all cases, the complex activation is accompanied by the formation of organic side-products. While one species is clearly identified as Ph3CH (Ph3CH: δH = 5.43 ppm), each 1H NMR spectrum features two SiMe3 resonances (−0.05 and −0.50 ppm) and two methylene resonances (1.92 and 2.05 ppm) tentatively ascribed to isomeric “Ph3CCH2SiMe3” forms.23 The generation of similar organic byproducts has already been documented in the literature for the activation of related group 424 and organolanthanide complexes9,28 with TB. In spite of that, the real nature of the two “Ph3CCH2SiMe3” isomers as well as the mechanistic details leading to the formation of the Ph3CH species still remain unknown.24 Isoprene Polymerization Tests. Rare-earth metal complexes have been intensively investigated in the past few years as effective catalysts for the polymerization of either simple olefins or conjugated dienes. To date, a relatively high number of Cp (cyclopentadienyl) and Cp-free rare-earth metal alkyl or halide complexes have been successfully scrutinized under variable reaction conditions for regio- and stereoselective diene polymerization and copolymerization.8 In this regard, catalyst precursors 3−8 have been studied for IP polymerization in toluene under variable reaction conditions (Table 1). The ligand type, the metal ion, and the catalyst activator play an essential role in the control of the ultimate catalyst perform-

ance, as well as in the resulting polymer microstructure. As Table 1 shows, none of the neutral dialkyl species (3−8) (entry 1) induces any appreciable diene polymerization, even at temperatures higher than ambient (50 °C). On the contrary, the addition of an appropriate activator (cocatalyst) and the subsequent complex cationization triggers the polymerization process almost immediately. Benzothiazole-based complexes 5 and 6 and their relative cationic counterparts do not show any appreciable polymerization activity irrespective of the nature of the activator used (Table 1, entry 6). Such behavior is in contrast with that of selected cationic species from this series containing the smaller 5-methylthiazole group, which serve as valuable catalysts for the production of trans-1,4-enriched polyisoprenes with moderate molecular weights (M n). Although any rationale of this reactivity trend can be hardly attributed to simple stereoelectronic reasons, possible explanations are either the accelerated reorganization of the metal coordination sphere of the benzothiazole-containing complexes (likely through an undesired alkyl metal-to-ligand migration) or the fast catalyst decomposition upon monomer treatment. Among the 5-methylthiazole-containing complexes, the erbium derivative 7 activated by TB exhibits the highest catalytic performance, leading to a complete monomer conversion within 1.5 h (entry 14). Under the same conditions, the lutetium analogue 3 provides a quantitative monomer E

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14) but regio- and stereoselectivity similar or even higher than those measured for the parent Lu3+ derivative (Table 1, entry 24 vs 7). Among this organolanthanide series, the Y3+ binary system 4/TB is the least active, with only 28% monomer conversion after 12 h (Table 1, entry 2). On the other hand, it exhibits a distinguished 3,4 selectivity (92.7%), not significantly altered at higher temperatures (entry 2 vs 3) together with an appreciable increase of the final polymer Mn value (up to 1.9 × 105 g/mol) while maintaining a rather narrow molecular weight distribution (Mw/Mn = 1.82). The 13C{1H} NMR spectrum of the PIP produced shows only a moderate tacticity with 21% of the syndiotactic triad rr (in the δC 146.0−146.8 ppm range) and 6% pentad rrrr (at δC 146.67 ppm), 50% of the atactic mr 3,4sequence (in the δC 147.0−148.3 ppm range), and 29% ascribable to the isotactic mm triad (Figure 6).

conversion only upon prolonged reaction times (6 h; entry 7). A moderately improved reactivity is finally observed for the two most active catalysts from this series (3 and 7) upon increasing the temperature from ambient to 50 °C (Table 1, entries 8 vs 9 and 14 vs 17). As Table 1 shows, the PIPs obtained with the TB-activated complexes 3 and 7 in the 21−50 °C temperature range show similar microstructures, predominantly made of 1,4trans units (Figures S13 and S14) and 3,4-units (vide inf ra), as spectroscopically assessed. The absence of detectable signals at δC = 23.6, 26.6, and 32.4 ppm (characteristic of the 1,4-cis enchainment of the IP units) of the 13C{1H} NMR spectra of the PIPs obtained with the binary system 3/TB indicates a high 1,4-trans stereospecificity (1,4-trans microstructure diagnostic signals at δC = 16.2, 26.9, 39.9, 124.4, and 135.1 ppm). PIPs prepared starting from 7/TB display a slightly reduced 1,4-trans vs 1,4-cis selectivity, due to the appearance of a 1,4-cis component (from 6.3% to 10.8%). The rationale for the observed selectivity trend can be explained on the basis of the well-established mechanism for the polymerization of conjugated dienes.4 Indeed, the appearance of the 1,4-cis component is ascribable to the larger size of the Er3+ ion coordination sphere with respect to the smaller Lu3+ ion. This allows for a competitive η4-cis over η2-trans monomer coordination. With both activated systems (3/TB and 7/TB), an increase of the monomer conversion translates into an almost linear increase of the PIP molecular weights (from 35 000 to 80 000 g/mol Mn with 3/TB and from 44 000 to 146 000 g/mol Mn with 7/TB), whereas the molecular weight distribution (PDI) remains rather narrow in both cases (Mw/ Mn = 1.52−1.65 for 3/TB and 1.34−1.42 for 7/TB) [Table 1, entries 7, 8, 10 (3/TB); entries 14, 18−20 (7/TB) and Figure 5]. Moreover, as shown for the most active system 7/TB, after

Figure 6. 13C{1H} NMR spectrum of the PI obtained with the binary system 4/TB, exhibiting a prevalent 3,4-selectivity. Left inset: 13C{1H} NMR expansion of the δC 145.0−150.0 ppm region and 3,4-PI tacticity assignment. Right inset: 1H NMR olefinic region of the relative PI for the calculation of the 1,4- vs 3,4-motif ratio.

As an additional study, the catalyst activation by TB in combination with 10 equiv of iBu3Al was also performed. According to the literature, the addition of the aluminum cocatalyst may have either a positive or a negative effect on the catalyst polymerization performance. When a reversible exchange of the growing PIP chain between the lanthanide ion and aluminum transfer agent is thought to occur, a decrease of the polymerization rate25 together with a broadening of the PIPs’ molecular weight distribution and modification of the ultimate polymer microstructure is observed.26 However, an increase of the catalyst activity upon addition of main-group metal-alkyls is also well documented.8h Indeed, organoaluminum compounds are known to act as impurities scavengers, besides playing a role as alkylating and chain transfer agents. This is likely the case of our ternary system, where the combination of the TB cocatalyst with 10 equiv of i Bu3Al leads to a significant improvement of the catalyst activity (81% monomer conversion after 6 h; see Table 1, entry 5). At odds with several literature results,8k the addition of iBu3Al to 4/TB translates into an appreciable decrease of the Mn and 3,4regioselectivity along with a remarkable activity increase (Table 1, entry 2 vs 5); no polymerization activity is finally observed when the binary 4/iBu3Al system is used instead (entry 4). Unlike 4, ternary systems based on Lu3+ and Er3+ (3/TB/iBu3Al and 7/TB/iBu3Al) show reduced catalyst efficiency and

Figure 5. IP polymerization with 3/TB and 7/TB at variable reaction times [Mn vs IP conversion (%) for 3/TB (◆, green) and 7/TB (■, blue); Mw/Mn of the isolated PIPs for 3/TB (●, gray) and 7/TB (▲, red)]. Black arrows indicate the respective “Y” axis.

consumption of 500 equiv of IP in 1 h, the polymer chain ends are still active and capable of inserting an additional 500 equiv of IP into the final polymer. For this trial the PI molecular weight increases from 81 000 g/mol (Mw/Mn = 1.61) to 138 000 g/mol (Mw/Mn = 1.52), whereas the polydispersity remains narrow. These results, graphically outlined in Figure 5, demonstrate that both complexes behave as living polymerization catalysts. At odds with that of 3 and 7, the Yb3+-based binary system 8/TB shows reduced catalyst efficiency with only 40% monomer conversion after 6 h (Table 1, entries 24 vs 7 and F

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000 g/mol) and relatively high trans-1,4-selectivity (up to 76.4%). With an increase of the metal ion radius (which facilitates an η4-cis IP coordination) the cis-1,4 content increases. Thus, a minor cis-1,4 component appears in the polymer microstructure once moving from the Lu3+ catalyst 3/ TB to the catalyst 7/TB, containing the larger Er3+ ion. Quite unexpectedly, the binary catalyst system 4/TB, based on the larger Y3+ counterpart, gives, under identical reaction conditions, PIPs with an essentially 3,4 atactic structure (3,4 content, 92.7%; rr %, 21; mr %, 50; mm %, 29). Such a trend, slightly altered by further addition of a main-group metal-alkyl activator (ternary system with iBu3Al), can be rationalized on the basis of a prevailing syn−anti isomerization of the Y3+ π-allyl intermediate that produces the 3,4-units.4f,8c

negligible alteration of the PIP microstructures compared to the binary counterparts (Table 1, entries 8 vs 11 and 14 vs 21); the already moderate activity of the Yb3+ complex 8/TB is definitively suppressed upon treatment with the aluminum alkyl (8/TB/iBu3Al; entry 24 vs 25). To evaluate the role of different catalyst activators, the initiating trityl borate was replaced by the Brønsted acid organoborate HNB ([HNMe2Ph][B(C6F5)4]), and the formation of the respective cationic species is established by 1H NMR experiments for the diamagnetic 3/HNB system only (Figure S11). Catalytic trials show that the process selectivity is only slightly affected by the activator used, in line with the identical nature of the weakly coordinating counterion. On the other hand, the catalyst efficiency is remarkably affected by the activator choice (Table 1, entries 7 vs 13, 14 vs 23, and 24 vs 27). In particular, 8/HNB provides only small IP conversions even after prolonged reaction times (entry 24 vs 27), while the 7/HNB is almost inactive (entry 14 vs 23). This behavior can be ascribed to the generation on the N,N-dimethyl aniline sideproduct, whose subsequent coordination to the metal center may hinder the regular monomer uptake.27 Finally, the catalysts activation by the neutral tris(pentafluorophenyl)borane (BN) generates from poorly active to totally inactive species (Table 1, entries 7 vs 12, 14 vs 22, and 24 vs 26). This reactivity trend is typically observed with related systems of the state-of-the-art,27 and it can be rationalized through a number of concomitant side processes whose occurrence is witnessed by the complexity of the 1H NMR spectra of the BN-activated diamagnetic species 3 (Figure S12). According to the literature,17a,28 a concerted silyl-methyl group abstraction by BN and metal-alkyl migration can be invoked for the generation of alternative cationic species of the type {[Ln3+(κ3-N,Npy,N−)CH2SiMe2CH2SiMe3][MeB(C6F5)3]}, in lieu of a simple −CH2SiMe3 group abstraction. Whatever the nature of the cationic form, the less sterically crowded (and consequently “more coordinating”) counterion can be assumed as the key reason for the observed reactivity drop (Table 1, entries 7 vs 12; 14 vs 22; 24 vs 26).

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EXPERIMENTAL SECTION

General Considerations and Materials Characterization. All air- and/or moisture-sensitive reactions were performed under an inert atmosphere in flame-dried flasks using standard Schlenk-type techniques or in a drybox filled with nitrogen. Tetrahydrofuran (thf) was purified by distillation from sodium/benzophenone ketyl, after drying over KOH. Benzene, n-hexane, and toluene were purified by distillation from sodium/triglyme benzophenone ketyl or were obtained by means of an MBraun solvent purification system. Benzene-d6 and toluene-d8 were dried over sodium/benzophenone ketyl, condensed in vacuo over activated 4 Å molecular sieves, and degassed by several freeze−pump−thaw cycles prior to use. CD2Cl2 and C6D5Br were dried over activated 4 Å molecular sieves. IP was purchased from Aldrich, dried over CaH2, distilled under nitrogen at atmospheric pressure, and degassed by two freeze−pump−thaw cycles prior to polymerization experiments. The Lewis acid activator B(C6F5)3 (BN) was purchased from Strem Chemicals Inc., and water contamination has been ruled out through 19F NMR characterization (376.5 MHz, toluene-d8, rt). All the other reagents and solvents were used as purchased from commercial suppliers. 1H and 13C{1H} NMR spectra were obtained on either a Bruker Avance DRX-400 (400.13 and 100.62 MHz, respectively) or a Bruker Avance 300 MHz instrument (300.13 and 75.47 MHz for 1H and 13C, respectively). Chemical shifts are reported in ppm (δ) relative to TMS, referenced to the chemical shifts of residual solvent resonances (1H and 13C), and coupling constants are given in Hz. Lanthanide metal analyses were carried out by complexometric titration. The N, C, H elemental analyses were carried out in the microanalytical laboratory of IOMC or at ICCOM by means of a Carlo Erba model 1106 elemental analyzer with an accepted tolerance of 0.4 unit on carbon (C), hydrogen (H), and nitrogen (N). GC analyses were performed on a Shimadzu GC-17 gas chromatograph equipped with a flame ionization detector and a Supelco SPB-1 fused silica capillary column (30 m length, 0.25 mm i.d., 0.25 μm film thickness) or an HP-PLOT Al2O3 KCl column (50 m length, 0.53 mm i.d., 15 μm film thickness). The GC/MS analyses were performed on a Shimadzu QP2010S apparatus equipped with a column identical with that used for GC analysis. The molecular weights and the molecular weight distributions of the polyisoprene samples were determined at 35 °C by gel permeation chromatography (GPC) on a Waters GPC 2000 system equipped with a set of three columns, Styragel HT6, HT5, and HT3, and refractive index detector. thf was employed as the eluent at a flow rate of 0.35 mL/min. The calibration was made by polystyrene standard EasiCal PS-1 (PL Ltd.). X-ray Diffraction Data. X-ray diffraction intensity data for compounds 3, 7, and 8 were collected on an Agilent Xcalibur E diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å) using ω scans. The structures were solved by direct methods and were refined on F2 using SHELXTL29 and Crysalis Pro30 packages. All non-hydrogen atoms were found from Fourier syntheses of electron density and were refined anisotropically. All hydrogen atoms were placed in calculated positions and were refined in the riding model. SADABS31 and ABSPACK (Crysalis Pro)31 were used to

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CONCLUSIONS In summary, a series of neutral bis(alkyl) rare-earth complexes stabilized by tridentate monoanionic thiazole-amidopyridinate ligands are easily prepared by the reaction of an equimolar amount of the {N,N,NH} ligand with the appropriate tris-alkyl metal precursor [Ln(CH2SiMe3)3(thf)2; Ln = Lu3+ (3), Er3+ (7), Yb3+ (8)] under mild reaction conditions. As already shown for organolanthanides of this series (Y3+),9 the nature of the thiazole unit (benzothiazole vs 5methylthiazole) controls the catalyst stability in solution. At odds with the 5-methylthiazole-based systems, the benzothiazole-containing complexes show a progressive and complete rearrangement of the metal coordination sphere through a 1,3metal-to-ligand alkyl migration. This process results in a chemoselective ring-opening of the heterocycle with the generation of mono(alkyl)-arylthiolate Ln complexes supported by a tetradentate {N−,N,N,S−} dianionic system. As for the stable organolanthanides containing the 5-methylthiazole unit, their activation with a selected organoborate (TB, BN, HNB) generates homogeneous single-site binary systems capable of inducing controllable and living IP polymerization with activity and selectivity depending on the nature of the metal ion and activator(s) used. Binary systems 3/TB and 7/TB show the highest activity and a living character toward IP polymerization, affording PIPs with moderate molecular weights (Mn up to 146 G

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perform area-detector scaling and absorption corrections. The details of crystallographic, collection, and refinement data are shown in Table S1, and corresponding cif files are available as Supporting Information. Molecular plots were produced by the program ORTEP3. CCDC1021045 (3), 1021046 (7), and 1021047 (8) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via ccdc.cam.ac.uk/community/requestastructure. General Procedure for the Synthesis of Organolanthanide Complexes (LThiaMe2)Ln(CH2SiMe3)2 (3, 7, and 8). To a solution of the proper metal precursor [Ln(CH2SiMe3)3(thf)2; Ln = Lu3+ (3), Er3+ (7), Yb3+ (8)] (0.95 mmol) in dry and degassed toluene (7 mL) was added at 0 °C a solution of HLThiaMe2 (2) (0.38 g; 0.95 mmol) in dry and degassed toluene (3 mL). The resulting mixture was stirred at the same temperature for 1 h; then it was concentrated to reduced volume (approximately 1/10 of its initial volume) and stored at −30 °C overnight. Crystals were recovered from the mother liquors, washed with cold hexane, and dried under vacuum to constant weight. Dark brown and analytically pure crystals of 3 [(LThiaMe2)Lu(CH2SiMe3)2], 7 [(LThiaMe2)Er(CH2SiMe3)2], and 8 [(LThiaMe2)Yb(CH2SiMe3)2] were isolated from the mother liquors in 75%, 72%, and 65% yield, respectively. [(LThiaMe2)Lu(CH2SiMe3)2] (3). Anal. Calcd for C32H52LuN3SSi2 (741.98 g·mol−1): C, 51.80; H, 7.06; Lu, 23.58; N, 5.66. Found: C,

Calcd for C35H52LuN3SSi2 (778.01 g·mol−1): C, 54.03; H, 6.74; Lu, 22.49; N, 5.40. Found: C, 54.19; H, 6.98; Lu, 22.34; N, 5.25. 1H NMR (400 MHz, 293 K, C6D6): −0.46 (d, 2JHH = 11.3 Hz, 2H, LuCH2), −0.16 (d, 2JHH = 11.3 Hz, 2H, LuCH2), 0.06 (s, 18H, SiMe3), 1.31 (d, 3 JHH = 6.9 Hz, 6H, CH3iPr, H19,20,21,22), 1.49 (d, 3JHH = 6.9 Hz, 6H, CH3iPr, H19,20,21,22), 1.55 (s, 6H, CMe2 H23,24), 3.79 (sept, 3JHH = 6.9 Hz, 2H, CHiPr, H17,18), 6.74−6.95 (complex m, together 4H, CH, H2,3,4,25,26), 7.12−7.31 (complex m, together 5H, CH, H9,10,11,16,25,26,27), 8.90 (d,3JHH = 8.3 Hz, 1H, CH, H16,27) ppm. 13C{1H} NMR (100 MHz, 293 K, C6D6): 4.2 (s, SiMe3), 23.9 (s, CH3iPr, C19,20,21,22), 26.7 (s, CH3iPr, C19,20,21,22), 28.1 (s, CH iPr, C17,18), 33.2 (s, C(CH3)2, C23,24), 43.4 (s, LuCH2), 66.3 (s, C(CH3)2, C6), 118.9 (s, CH, C2,4), 121.7 (s, C16,27), 122.3 (s, CH, C2,4), 123.6 (s, CH, C9,11), 123.7 (s, CH, C10), 124.7 (s, C16,27), 127.6 (s, C25,26), 128.2 (s, C25,26) 133.2 (s, C15), 139.6 (s, CH, C3), 146.6 (s, C13), 147.1 (s, C7), 148.5 (s, C8,12), 150.6 (s, C14), 170.2 (s, C5), 179.6 (s, C1) ppm. In Situ Synthesis of 9. In a typical procedure, Lu(CH2SiMe3)3(THF)2 (0.027 g, 0.046 mmol) and the ligand

51.71; H, 6.99; Lu, 23.40; N, 5.59. 1H NMR (400 MHz, 293 K, C6D6): −0.50 (d, 2JHH = 11.4 Hz, 2H, LuCH2), −0.31 (d, 2JHH = 11.4 Hz, 2H, LuCH2), 0.14 (s, 18H, SiMe3), 1.28 (d, 3JHH = 6.9 Hz, 6H, CH3iPr, H19,20,21,22), 1.46 (d, 3JHH = 6.9 Hz, 6H, CH3iPr, H19,20,21,22), 1.52 (s, 6H, CMe2 H23,24), 1.61 (d,4JHH = 1.0 Hz, 3H, CH3, H16), 3.77 (sept, 3 JHH = 6.9 Hz, 2H, CHiPr, H17,18), 6.60 (d, 3JHH = 7.4 Hz, 1H, CH, H4), 6.81 (d, 3JHH = 7.8 Hz, 1H, CH, H2), 6.88 (t, 3JHH = 7.8 Hz, 1H, CH, H3), 7.20 (dd, 3JHH = 8.6 Hz, 3JHH = 6.3 Hz, 1H, CH, H10), 7.26 (m, 2H, CH, H9,11), 7.83 (d,4JHH = 1.0 Hz, 1H, CH, H14) ppm. 13 C{1H} NMR (100 MHz, 293 K, C6D6): 4.2 (s, SiMe3), 11.0 (s, CH3, C16), 24.2 (s, CH3iPr, C19,20,21,22), 26.7 (s, CH3iPr, C19,20,21,22), 28.1 (s, CH iPr, C17,18), 32.9 (s, C(CH3)2, C23,24), 43.3 (s, LuCH2), 66.6 (s, C(CH3)2, C6), 117.1 (s, CH, C4), 121.5 (s, CH, C2), 123.4 (s, CH, C9,11), 123.7 (s, CH, C10), 137.7 (s, C15), 139.6 (s, CH, C3), 141.4 (s, CH, C14), 146.3 (s, C7), 146.4 (s, C13), 148.6 (s, C8,12), 168.7 (s, C5), 179.0 (s, C1) ppm. [(LThiaMe2)Er(CH2SiMe3)2] (7). Anal. Calcd for C32H52ErN3SSi2 (734.27 g·mol−1): C, 52.34; H, 7.14; Er, 22.78; N, 5.72. Found: C, 52.48; H, 7.18; Er, 22.50; N, 5.60. [(LThiaMe2)Yb(CH2SiMe3)2] (8). Anal. Calcd for C32H52N3SSi2Yb (740.05 g·mol−1): C, 51.93; H, 7.08; N, 5.68; Yb, 23.38. Found: C, 52.05; H, 7.22; N, 5.73; Yb, 23.15. Synthesis of (LBnThMe2)Lu(CH2SiMe3)2 (5). To a solution of Lu(CH2SiMe3)3(thf)2 (0.439 g, 0.76 mmol) in toluene (7 mL) was added at 0 °C a toluene solution (3 mL) of HLBnThMe2 (1) (0.320 g, 0.76 mmol). The reaction mixture was stirred at the same temperature for 1 h before being concentrated to reduced volume (approximately 1/10 of its initial volume). Afterward, the resulting mixture was maintained at −30 °C overnight to give dark purple crystals of 5. Crystals were recovered from the mother liquors, washed with cold hexane, and dried under vacuum to constant weight. Analytically pure crystals of 5 were isolated from the mother liquor in 78% yield. Anal.

HLBnThMe2 (1) (0.019 g, 0.045 mmol) were dissolved in C6D6 (0.7 mL) in an NMR tube. The process occurs spontaneously upon standing the mixture in the dark at room temperature for few days. The formation of complex 5 and its conversion into 9 occur quantitatively within 2 weeks; the addition of a few drops of thf to the mixture accelerates the conversion of 5 into 9 remarkably, so that a quantitative conversion occurs within a few hours. 9 was completely characterized by in situ 1H and 13C{1H} NMR spectroscopy; the material was neither isolated nor further processed/characterized. 1H NMR (200 MHz, 293 K, C6D6): −0.93 (d, 2JHH = 11.5 Hz, 1H, LuCH2), −0.87 (d, 2JHH = 11.5 Hz, 1H, LuCH2), −0.22 (s, 9H, CCH2SiMe3), 0.04 (s, 9H, LuCH2SiMe3), 1.14 (d, 3JHH = 6.8 Hz, 3H, CH3iPr, H19,20,21,22), 1.25−1.39 (complex m, CMe2, H23,24, CH3iPr, H19,20,21,22 and β-CH2 THF), 1.49 (d, 3JHH = 6.8 Hz, 3H, CH3iPr, H19,20,21,22), 1.53 (d, 3JHH = 6.7 Hz, 3H, CH3iPr, H19,20,21,22), 1.64 (s, 3H, CMe2 H23,24), 2.02 (d, 2JHH = 12.5 Hz, 1H, CCH2SiMe3), 2.90 (d, 2 JHH = 12.5 Hz, 1H, CCH2SiMe3), 3.45 (br s, α-CH2 THF), 3.83 (sept, 3 JHH = 6.8 Hz, 1H, CHiPr, H17,18), 4.27 (sept, 3JHH = 6.8 Hz, 1H, CH i Pr, H 17,18 ), 6.88−7.11 (complex m, together 7H, CH, H2,3,4,16,25,26,27), 7.22−7.20 (complex m, 2H, CH, H9,11), 7.80 (dd, 3 JHH = 7.4 Hz, 4JHH = 1.5 Hz, 1H, CH, H16,27) ppm. 13C{1H} NMR (50 MHz, 293 K, C6D6): −0.8 (s, CCH2SiMe3), 4.2 (s, LuCH2SiMe3), 20.9 (s, CCH2SiMe3), 23.9 (s, CH3iPr, C19,20,21,22), 24.8 (s, CH3iPr, C19,20,21,22), 25.2 (s, β-CH2 THF), 27.2 (s, CH3 iPr, C19,20,21,22), 27.6 (s, CH iPr, C17,18), 27.8 (s, CH iPr, C17,18), 28.4 (s, CH3 iPr, C19,20,21,22), H

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31.9 (s, C(CH3)2, C23,24), 32.3 (s, C(CH3)2, C23,24), 43.6 (s, LuCH2SiMe3), 68.7 (br s, α-CH2 THF), 69.8 (s, CMe2, C6), 120.0 (s, CH, C25,26), 120.4 (s, CH, C2,4), 121.5 (s,CH, C2,4), 123.7 (s, CH, C16,27), 123.8 (s, CH, C9,11), 123.9 (s, CH, C9,11), 124.3 (s, CH, C10), 127.2 (s, C25,26), 135.2 (s, C16,27), 138.5 (s, CH, C3), 144.8 (s, C14,15), 145.4 (s, C7), 146.3 (s, C14,15), 150.1 (s, C8,12), 150.2 (s, C8,12), 153.4 (s, C13), 167.5 (s, C5), 176.9 (s, C1) ppm. IP Polymerization. All polymerization tests were conducted under an inert atmosphere in a nitrogen-filled drybox. In a typical procedure, 10 μmol of the selected catalyst precursor was dissolved in toluene (3.5 mL) and treated with a solution of the proper activator {10.0 μmol; [Ph3C][B(C6F5)4], (TB)/B(C6F5)3, (BN)/[PhNHMe2][B(C6F5)4], (HNB)} in toluene (1.5 mL). The reaction mixture was stirred for 2 min; then 1.0 mL (0.68 g, 10.0 mmol) of IP was added via syringe in one portion. The reaction was maintained under vigorous stirring at room temperature for the desired reaction time. Afterward, polymerization was stopped by quenching the mixture with an excess of methanol [70 mL, containing 0.5% butylhydroxytoluene (BHT) as a stabilizing agent] and maintained under vigorous stirring for 3 h, while PIPs separated off from the reaction mixture. Polymers were isolated by decantation, washed several times with fresh methanol, and dried at 50 °C under vacuum to constant weight. The polymer microstructures were determined by 1H and 13C{1H} NMR spectroscopy in CDCl3 at rt. Catalyst Activation by Treatment of the Dialkyl Precursors with [Ph3C][B(C6F5)4] [TB]. For the diamagnetic species (Y3+, Lu3+), the generation of the cationic monoalkyl catalysts by addition of TB was monitored by 1H NMR spectroscopy using benzene-d6 or a benzene-d6/C6D5Br mixture as solvent. Cationic monoalkyl species are quantitatively formed almost immediately upon addition of TB, while a dark red rubbery material starts to precipitate within a few minutes upon standing at room temperature. Polar aromatics such as mono- or dihalobenzenes (halo = Cl, Br) increase the complexes’ solubility, although they promote their rapid degradation with formation of untreatable dark brown semisolid compounds. 1H NMR spectra of the TB-activated 3 and 4 (see Supporting Information) are recorded at rt from a benzene-d6/C6D5Br 90:10 v/v solution, and the precatalyst conversion into the cationic monoalkyl derivatives takes place immediately. (4+) 1H NMR (300 MHz, C6D6/C6D5Br, 293 K, selected data): −0.40 (br s, 2H, YCH2SiMe3), −0.15 (s, 9H, YCH2SiMe3), −0.12 (s, Me3SiCH2CPh3 of isomer A), 0.10 (s, Me3SiCH2CPh3 of isomer B), 1.10 (m, 12H, CH(CH3)2), 1.38 (s, 6H, CH3), 2.03 (s, Me3SiCH2CPh3 of isomer B), 2.15 (s, 3H, CH3), 2.16 (s, Me3SiCH2CPh3 of isomer A), 3.02 (m, 2H, CH(CH3)2), 5.52 ppm (s, Ph3CH), Me3SiCH2CPh3 (58.7% as two isomers in a ca. 2:1 ratio), and Ph3CH. (41.3%). (3+) 1H NMR (400 MHz, C6D6/C6D5Br, 293 K, selected data): −0.40 (br s, 2H, LuCH2SiMe3), −0.14 (s, 9H, LuCH2SiMe3), −0.12 (s, Me3SiCH2CPh3 of isomer A), 0.10 (s, Me3SiCH2CPh3 of isomer B), 1.23 (m, 12H, CH(CH3)2), 1.35 (s, 6H, CH3), 2.03 (s, Me3SiCH2CPh3 of isomer B), 2.15 (s, 3H, CH3), 2.16 (s, Me3SiCH2CPh3 of isomer A), 3.36 (m, 2H, CH(CH3)2), 5.52 ppm (s, Ph3CH), Me3SiCH2CPh3 (73.3% as two isomers in a ca. 3:1 ratio), and Ph3CH (26.7%). Activation of paramagnetic precatalysts 7 and 8 is similarly performed. Catalyst Activation by Treatment of the Dialkyl Precursor 3 with [PhNHMe2][B(C6F5)4] [HNB]. For the diamagnetic 3 (Lu3+) the generation of the cationic monoalkyl species upon treatment with HNB was monitored by 1H NMR spectroscopy using THF-d8 as solvent. The activation proceeds in a few minutes, during which the most diagnostic 1H NMR signals of 3 disappeared. (3+) 1H NMR (300 MHz, THF-d8, 293 K, selected data): −0.89 (br s, 2H, YCH2SiMe3), −0.49 (s, 9H, YCH2SiMe3), 1.48−1.20 (21H, CH(CH3)2, C(CH3)2, CH3), 2.90 (6H, Me2NPh), 3.73 (m, 2H, CH(CH3)2).

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prepared by 3/TB. Crystal data and structure refinement for complexes 3, 7, and 8. A CIF file of the three crystal structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (A. A. Trifonov): [email protected]. *E-mail: [email protected]. Fax: +39 055 5225203 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Italian authors thank the Fondazione Cariplo (“Crystalline Elastomers” project), the Groupe de Recherche International (GDRI) “Homogeneous Catalysis for Sustainable Development”, and the COST action CM1006: “EUFEN: European FElement Network” for supporting this work. The Russian team acknowledges the Russian Science Foundation (Project 14-1300742) for the financial support of this work. G.G. would like to thank Dr. G. Ricci for fruitful discussions.

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REFERENCES

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S Supporting Information *

1

H NMR and 13C{1H} NMR spectra of complexes 3−6, 9, and 10, those of the activated cationic 4/TB, 3/TB, and 3/HNB species, and 1H NMR and 13C{1H} NMR spectra of the PIP I

dx.doi.org/10.1021/om500918y | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

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dx.doi.org/10.1021/om500918y | Organometallics XXXX, XXX, XXX−XXX