(Arylimido)Vanadium(V)-Alkylidenes Containing Chlorinated Phenoxy

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(Arylimido)Vanadium(V)-Alkylidenes Containing Chlorinated Phenoxy Ligands: Thermally Robust, Highly Active Catalyst in RingOpening Metathesis Polymerization of Cyclic Olefins Sapanna Chaimongkolkunasin and Kotohiro Nomura* Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, 1-1 minami Osawa, Hachioji, Tokyo 192-0397, Japan

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S Supporting Information *

ABSTRACT: A series of (imido)vanadium(V)-alkylidene complexes containing pentachlorophenoxy ligand of type, V(CHSiMe3)(N-2,6-R2C6H3)(OC6Cl5)(PMe3)2 [R = H, Cl, F, CH3], have been prepared, and the structure of V(CHSiMe3)(N-2,6-Me2C6H3)(OC6Cl5)(PMe3)2 was determined by X-ray crystallographic analysis. Ring-opening metathesis polymerization (ROMP) of cyclic olefins such as norbornene (NBE), cyclopentene (CPE), cycloheptene (CHPE), and cis-cyclooctene (COE) using these alkylidene catalysts have been explored, and V(CHSiMe3)(N-2,6Cl2C6H3)(OC6Cl5)(PMe3)2 showed higher activities in the ROMP of CPE, CHPE, and COE than those of the reported V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5)(PMe3)2. The activity in the ROMP of COE increased at high temperature until 120 °C, and the ROMPs of CHPE and COE proceeded without chain-transfer or termination (nor catalyst decomposition); the (quasi) living nature thus maintained even at 80 °C. The activities in the ROMPs of CHPE and COE (at 25 °C) increased upon addition of 1.0 equiv of B(C6F5)3, whereas the activity in the ROMP of NBE became negligible upon the addition. The order in the activity in the ROMP of cyclic olefins displayed as COE ≪ CHPE < CPE ≪ NBE, which is different from not only that in the ring strain energy but also that reported in the ROMPs using ruthenium−carbene catalysts.

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INTRODUCTION Olefin metathesis is the useful method for synthesis of various organic compounds (basic, fine chemicals, pharmaceuticals, etc.) and polymeric, advanced materials,1−5 and the metal− carbene (alkylidene) complexes play a key role in this catalysis. Ring-opening metathesis polymerization (ROMP, addition type) and acyclic diene metathesis (ADMET) polymerization (condensation type) have been employed as the effective methods especially for synthesis of polymeric, advanced materials.5 Both ruthenium−carbene (so-called Grubbs type) and molybdenum-alkylidene (so-called Schrock type) catalysts are the well-known successful examples.1−5 High-oxidation-state early transition metal-alkylidene (called Schrock type carbene) complexes are known to play a key role as catalysts in the olefin metathesis and Wittig type coupling, as exemplified by molybdenum and tungsten,2,3 and some of these alkylidenes (and alkylidyne) are also known as the catalyst for alkane C−H bond activation.3h,6 Synthesis and reaction chemistry of vanadium-alkylidenes are also considered as an attractive subject,3f−h because classical Ziegler type vanadium catalyst systems demonstrated promising characteristics (notable reactivity toward olefins) in olefin polymerization.7 It has been reported that (imido)vanadium(V)alkylidene complexes containing ketimide,8a aryloxo,9 and imidazolidin-2-iminato8b ligands, shown in Chart 1, exhibited from moderate to high catalytic activities for ROMP of norbornene (NBE). In particular, the arylimido complexes containing fluorinated phenoxy ligand, V(CHSiMe3)(N-2,6© XXXX American Chemical Society

R2C6H3)(OC6F5)(PMe3)2 [R = Me, Cl (1)], exhibited the remarkable activities for the living ROMP of NBE affording ultrahigh molecular weight polymers with narrow molecular weight distributions (e.g., Mn = 2.05 × 106, Mw/Mn = 1.12 by 1).9d Complex 1 also polymerized cyclopentene (CPE) and ciscyclooctene (COE),9d,10,11 and the activity in the ROMP of COE increased at high temperature.9d,e Since reported examples in ROMP of COE (and CPE, socalled low strain monomers) using alkylidene complexes with early transition metals still have been limited9d,e,10−14 and since the reason for exhibiting the high activity by 1 would probably due to an electronic effect (formation of more electrondeficient metal-alkylidene),9c,d in this paper, we thus prepared a series of (arylimido)vanadium(V)-alkylidene complexes containing pentachlorophenoxy ligand, V(CHSiMe3)(N-2,6R2C6H3)(OC6Cl5)(PMe3)2 [R = H (2), Cl (3), F (4), CH3 (5), Scheme 1] and explored their catalyst performances in the ROMP of (so-called low strain) cyclic olefins. In particular, we demonstrate that the dichlorophenylimido analogue (3) exhibited the highest activities for ROMP of CPE, cycloheptene (CHPE), and COE, and the ROMPs of CHPE and COE proceeded in a (quasi) living manner with remarkably high activities (even at 80 °C).15 Through this study, we explored effects of arylimido ligand as well as cyclic olefins (ring strain) toward the activity. Received: April 15, 2018

A

DOI: 10.1021/acs.organomet.8b00231 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Chart 1. Reported (Imido)vanadium(V)-alkylidene ROMP Catalysts

Scheme 1. ROMP of Cyclic Olefins by V(CHSiMe3)(N-2,6R2C6H3)(OC6Cl5)(PMe3)3 [R = H (2), Cl (3), F (4), CH3 (5)]

Scheme 2. Synthesis of V(CHSiMe3)(N-2,6R2C6H3)(OC6Cl5)(PMe3)3 [R = H (2), Cl (3), F (4), CH3 (5)]

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RESULTS AND DISCUSSION Synthesis of (Imido)vanadium-Alkylidene Complexes Containing Pentachlorophenoxy Ligand, V(CHSiMe3)(N-2,6-R2C6H3)(OC6Cl5)(PMe3)2 (R = H, Cl, F, CH3). A series of (imido)vanadium(V)-alkylidene complexes containing pentachlorophenoxy ligand of type V(CHSiMe3)(N-2,6R2C6H3)(OC6Cl5)(PMe3)2 [R = H (2), Cl (3), F (4), CH3 (5)] were prepared by one-pot synthesis from the corresponding (imido)vanadium(V) trialkyl complexes, V(N-2,6R2C6H3)(CH2SiMe3)3, upon addition of C6Cl5OH and then PMe3 without isolation/purification of the corresponding dialkyl complexes, V(N-2,6-R2C6H3)(CH2SiMe3)2(OC6Cl5), as shown in Scheme 2.16 This is the analogous method for synthesis of V(CHSiMe3)(N-2,6-Me2C6H3)(O-2,6-iPr2C6H3)(PMe3),9a reported previously due to difficulty for removal of uncertain impurities, and the resultant complexes were isolated as microcrystals upon placing the chilled n-pentane solution at −30 °C.16 The isolated complexes were identified by NMR spectra, elemental analysis, and structure of 5 could be determined by X-ray crystallography (Figure 1).16 The crystallographic result shows that complex 5 folds a distorted trigonal bipyramidal geometry around vanadium consisting of two phosphorus axis [P(1)−V−P(2) 168.66(4)°] and a N−C−O plain [total 359.81°: N(1)−V(1)−C(15)

Figure 1. ORTEP drawing for V(CHSiMe3)(N-2,6-Me2C6H3)(OC6Cl5)(PMe3)2 (5). Thermal ellipsoids are drawn at the 50% probability level, and H atoms are omitted for clarity.16 Selected bond distances (Å): V(1)−C(15) 1.845(3), V(1)−P(1) 2.4702(7), V(1)− P(2) 2.4500(7), V(1)−N(1) 1.687(2), V(1)−O(1) 1.983(2), N(1)− C(1) 1.381(3). Selected bond angles (deg): P(1)−V(1)−O(1) 88.37(5), P(2)−V(1)−O(1) 81.85(5), P(2)−V(1)−C(15) 94.52(7), P(1)−V(1)−C(15) 95.05(7), P(2)−V(1)−N(1) 90.71(6), P(1)−V(1)−N(1) 91.57(6), N(1)−V(1)−C(15) 110.73(12), O(1)−V(1)−C(15) 117.24(10), P(1)−V(1)−P(2) 168.66(4), O(1)−V(1)−N(1) 131.84(10), V(1)−N(1)−C(1) 174.93(17).

110.73(12)°, O(1)−V(1)−C(15) 117.24(10)°, O(1)−V(1)− N(1) 131.84(10)°] from the imido, the alkylidene, and the pentachlorophenoxy ligands.16 The structure indicates that the B

DOI: 10.1021/acs.organomet.8b00231 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. ROMP of Norbornene (NBE) Using V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5)(PMe3)2 (1), V(CHSiMe3)(NR)(OC6Cl5)(PMe3)2 [R = C6H5 (2), 2,6-Cl2C6H3 (3), 2,6-F2C6H3 (4), 2,6-Me2C6H3 (5)]a run

cat.

time (min)

yield (mg)

yield (%)

TONb

TOF (min−1)c

Mnd

Mw/Mnd

cis (%)e

1 2f 3 4 5 6 7 8 9 10

1 1 2 2 3 3 4 4 5 5

2 3 2 3 2 3 2 3 2 3

197 200 38 50 156 184 150 180 23 33

97 >99 19 25 78 92 75 90 11.5 16.5

20600 21200 4000 5300 16600 19500 15900 19100 2400 3500

10300 7070 2000 1770 8300 6500 7950 6370 800 1750

1040000 1100000 1070000 1140000 1280000 1320000 1520000 1580000 467000 625000

1.31 1.27 1.61 1.81 1.57 1.60 1.55 1.66 1.12 1.12

45 45 49 51 40 39 46 46 49 49

Conditions: complex 0.1 μmol, NBE 2.12 mmol, and benzene 4.8 mL (initial NBE 0.44 M), 25 °C. bTON(turnover) = NBE reacted (mmol)/ vanadium (mmol). cTOF = TON/time. dGPC data in THF vs polystyrene standards (g/mol). eCis percentage (%) estimated by 1H NMR spectra. f Cited from ref 9d. a

Table 2. ROMP of Cyclopentene (CPE) Using V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5)(PMe3)2 (1), V(CHSiMe3)(NR)(OC6Cl5)(PMe3)2 [R = 2,6-Cl2C6H3 (3), 2,6-F2C6H3 (4), 2,6-Me2C6H3 (5)], V(CHSiMe3)(NR)[OC(CF3)3](PMe3)2 [R = C6H5 (7), 2,6-Cl2C6H3 (8)]a run

cat.

μmol

time (min)

yield (mg)

yield (%)

TONb

TOF (min−1)c

Mnd

Mw/Mnd

11 12 13 14 15 16 17 18e 19 20e 21 22e 23 24e 25 26 27 28 29 30

1 1 1 1 3 3 3 3 3 3 3 3 3 3 4 4 4 4 7 8

0.1 0.1 0.1 0.1 1.0 1.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 1.0 1.0

1 5 10 15 5 10 1 1 5 5 10 10 15 15 1 5 10 15 12 h 12 h

48 118 145 171 251 270 100 90 180 199 222 230 228 233 32 76 94 95 trace trace

7.0 17.3 21.3 25.1 36.9 39.6 14.7 13.2 26.4 29.2 32.6 33.8 33.5 34.2 4.7 11.2 13.8 14.0

7050 17300 21300 25100 3690 3970 14700 13200 26400 29200 32600 33800 33500 34200 4700 11200 13800 14000

7050 3460 2130 1670 738 397 14700 13200 5280 5840 3260 3380 2230 2280 4700 2240 1380 933

389000 448000 513000 552000 338000 339000 1490000 1400000 1400000 1490000 1290000 1210000 969000 989000 453000 635000 669000 736000

1.51 1.65 1.47 1.52 1.45 1.72 1.70 1.67 1.72 1.69 1.87 1.88 1.88 1.89 1.24 1.23 1.20 1.16

a

Conditions: CPE 10.0 mmol, CPE+benzene total 4.8 mL. bTON(turnover) = CPE reacted (mmol)/vanadium (mmol). cTOF = TON/time. GPC data in THF vs polystyrene standards (g/mol). eIndependent experimental runs to check the reproducibility.

d

alkylidene has a syn isomer which was observed as a major resonance in the 1H NMR spectrum (15.87 ppm).17 The V(1)−C(15) bond distance in 5 [1.845(3) Å] is close to that in V(CHSiMe3)(NAd)(OC6F5)(PMe3)2 [6, 1.845(4) Å], clearly indicating the presence of nucleophilic carbene (metal-alkylidene) species.9b The V(1)−N(1) bond distance in 5 [1.687(2) Å] is slightly longer than that in 6 [1.660(3) Å], whereas the V(1)−N(1)−C(1) angle in 5 [174.93(17)°] is slightly larger than that in 6 [173.3(3)°].9b The V(1)−O(1) bond distance in 5 [1.983(2) Å] is shorter than that in 6 [2.003(3) Å]9b but longer than V(CHSiMe3)(NAd)(O-2,6Me2C6H3)(PMe3)2 [1.937(4) Å].18 These V−O distances are apparently longer than those in V(NAd)Cl2(O-2,6-Me2C6H3) [1.7633(17) Å]19 and V(NAd)(O-2,6-Me2C6H3)3 [1.783(2)1.799(2) Å],18 clearly suggesting that π-donation of the pheoxy ligands in these alkylidenes is very weak (the ligand plays a role as one electron donor). It seems that V−P bond distances in 5

[2.4702(7), 2.4500(7) Å] are rather shorter than those in 6 [2.4869(10), 2.4665(11) Å].9b Complex 5 is thus a 16-electron species containing nucleophilic alkylidene ligand. ROMP of Cyclic Olefins. Effect of Cyclic Olefins and Substituent on the Imido Ligands toward the Activity. As described in the introduction, the arylimido complexes containing pentafluorophenoxy ligand, V(CHSiMe3)(N-2,6R2C6H3)(OC6F5)(PMe3)2 [R = Me, Cl (1)], showed remarkably high catalytic activities for ROMP of norbornene (NBE) affording ultrahigh molecular weight polymers with narrow molecular weight distributions. Complex 1 polymerized cyclopentene (CPE) and cis-cyclooctene (COE),9c,d and the activity in the ROMP of COE increased at high temperature.9e The driving force in the ROMP is known to be release of the ring strain,1,5,20 and several reports concerning effect of ring strain toward the catalytic activity in the ROMP (of so-called low ring strain monomers) using ruthenium−carbene cataC

DOI: 10.1021/acs.organomet.8b00231 Organometallics XXXX, XXX, XXX−XXX

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Organometallics lysts14 and Fischer-type tungsten−carbene catalyst13 were known. Therefore, the effects of ligand substituent (in the imido ligand, OC6F5 vs OC6Cl5) and monomers (cyclic olefins) toward the activity and the polymerization behaviors in the ROMP using a series of V(CHSiMe3)(N-2,6-R2C6H3)(OC6Cl5)(PMe3)3 [R = H (2), Cl (3), F (4), CH3 (5)] have been explored. ROMP of Norbornene, Cyclopentene, Cycloheptene, and cis-Cyclooctene: Effect of Ligand Substituents toward the Activity. Table 1 summarizes selected results in the ROMP of NBE in benzene using a series of (imido)vanadium(V)alkylidene catalysts (1−5) at 25 °C. It turned out that the activity was affected by the imido ligand employed, and the activity (after 2 min) increased in the following order: TOF = 5 (800 min−1) < 2 (2000) ≪ 4 (7950), 3 (8300) < 1 (10 300). Both the 2,6-dichlorophenylimido analogue (3) and the 2,6difluorophenylimido analogue (4) showed higher activities than the phenylimido (2) and 2,6-dimethylphenylimido analogue (5), suggesting that an electronic factor plays a role toward the activity. It seems that the OC6F5 analogue (1) showed higher activity than the OC6Cl5 analogue (3) under these conditions. As reported previously by 1,9c,d the ROMPs afforded ultrahigh molecular weight polymers with relatively narrow molecular weight distributions. The resultant polymers possessed a mixture of cis-/trans-olefinic double bonds, as also reported previously in the ROMP of NBE by 1. Table 2 summarizes ROMP of cyclopentene (CPE) using 1, 3, and 4 which exhibited high catalytic activities for ROMP of NBE (Table 1). ROMP using the fluorinated alkoxo analogues V(CHSiMe3)(NR)[OC(CF3)3](PMe3)2 [R = C6H5 (7), 2,6Cl2C6H3 (8)],9d which afforded highly cis ring-opened polymers in the ROMP of NBE,9d was also conducted for comparison. It turned out that the activity was affected by the ligand substituents employed, and it increased in the order (after 5 min): TOF = 4 (2240 min−1) < 1 (3460) < 3 (5280, 5840). The OC6Cl5 analogue (3) showed higher activity than the OC6F5 analogue (1), and the results by 3 were reproducible (confirmed by conducting independent runs, runs 17−24). It also turned out that the fluorinated alkoxo analogues (7 and 8) showed negligible activities under these conditions (runs 29 and 30). The Mn value in the resultant polymers prepared by 4 seems increasing upon increasing the polymer yields (runs 25− 28) with low PDI values (Mw/Mn = 1.16−1.24). The similar trend was observed in the ROMP by 1, although the molecular weight distributions were rather broad (runs 11−14). The polymer yield (TON, turnover number), however, did not increase after 10 min in the ROMP by 4 (runs 27−28). In contrast, the resultant polymers prepared by 3 after 1 min possessed ultrahigh molecular weights with broad molecular weight distributions probably due to high viscosity in the reaction mixture (runs 17−18), but the Mn values decreased after 5 min with broadening of the molecular weight distributions (runs 19−24). Moreover, the polymer yields in these ROMPs by 3 did not increase after 10 min (runs 21−24) with decreasing in the Mn value. These results thus suggest that certain degree of intramolecular chain transfer or backbiting, in which the active vanadium-alkylidene species containing polymer chain reacted with a double bond in its polymer chain forming cyclic species (Scheme 3),5b,d,k occurred leading decrease of the Mn values; the fact was reproducible (run 21− 24). The reason why only 3 showed this catalyst behavior is not clear at this moment.

Scheme 3. Assumed Reaction Scheme for Depolymerization of Ring-Opened Poly(CPE)

Table 3 summarizes results in ROMP of cycloheptene (CHPE) using 1 and 3−5 in benzene at 25 °C.16 It turned out that the OC6Cl5 analogue (3) showed the higher activity than the OC6F5 analogue (1). As shown in Figure 2a, the Mn value increased upon increasing the polymer yields (TON) with rather large PDI (Mw/Mn) values (Mw/Mn = 1.19−1.77, runs 31−38), probably due to slow initiation or high viscosity in the reaction mixture. Similarly, the linear relationship was also observed when the ROMPs of CHPE by 3 were conducted under rather low concentration conditions (initial CHPE conc. 1.04 M, runs 39−43) and the PDI values became rather low (Mw/Mn = 1.11−1.38, Figure 2b). These results clearly suggest that the ROMP of CHPE by 1 and 3 proceeded without chain transfer or termination (nor catalyst decomposition). The observed facts are unique contrast to those in in the ROMP of CPE by 3, in which certain degree of chain transfer (backbiting) was observed (Table 2). It also turned out that polymer yields in the ROMP of CHPE by the 2,6difluorophenylimido analogue (4) and the 2,6-dimethylphenylimido analogue (5) were negligible after 10 min under the similar conditions probably due to their low activities; polymer could be collected in the ROMP by 4 after 30 min (run 45). Table 4 summarizes the results in ROMP of cis-cyclooctene (COE) using V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6X5)(PMe3)2 [X = F (1), Cl (3)] in benzene under high COE initial concentration conditions (5.59 M, COE 10.0 mmol, and benzene 0.5 mL). As reported previously,9e the polymer yield in the ROMP by 1 increased over time and activity increased at high temperature (25−80 °C, runs 47−53), but the activities at 100 °C were lower than those conducted at 80 °C (runs 52−55). Moreover, the Mn value in the resultant polymer increased over time course even at 50−80 °C (and 100 °C) without significant changes in the PDI (Mw/Mn) values. It turned out that as observed in the ROMP of CPE and CHPE (Tables 2 and 3) the OC6Cl5 analogue (3) showed higher catalytic activity than the OC6F5 analogue (1); the observed activities calculated on the basis of polymer yields became high when these ROMPs by 3 were conducted under low catalyst concentrations [runs 56−61 versus runs 62−69: ex. 169 turnovers (run 59) versus 889 turnovers (run 64) after 5 min at 50 °C, 224 turnovers (run 61) versus 1062 turnovers (run 68) after 2 min at 80 °C]. It should be noted that the activity of 3 increased at high temperature (runs 62−72), especially even at 100 °C but decrease in the activity of 1 was observed. The initial activities at 120 °C were high (runs 73 and 74) but seemed decreasing probably due to catalyst decomposition in certain degree, and the activity after 5 min thus became lower than those conducted at 100 °C (run 71 vs 75). However, as observed in the ROMP of COE by 1 (at 100 °C),9e the activity at 120 °C after 5 min was higher than that conducted at 100 °C upon presence of small amount of PMe3 (run 76); further addition of PMe3 led to rapid decrease in the activity (run 77). D

DOI: 10.1021/acs.organomet.8b00231 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 3. ROMP of Cycloheptene (CHPE) Using V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5)(PMe3)2 (1), V(CHSiMe3)(NR)(OC6Cl5)(PMe3)2 [R = 2,6-Cl2C6H3 (3), 2,6-F2C6H3 (4), 2,6-Me2C6H3 (5)]a run

cat.

CHPE (M)b

time (min)

yield (mg)

TONc

TOF (min−1)d

Mne

Mw/Mne

31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

1 1 1 1 3 3 3 3 3 3 3 3 3 4 4 5

2.08 2.08 2.08 2.08 2.08 2.08 2.08 2.08 1.04 1.04 1.04 1.04 1.04 1.04 1.04 1.04

1 5 10 15 1 5 10 15 1 5 10 15 20 10 30 10

20 73 128 166 29 129 202 280 51 59 115 160 171 trace 49 trace

208 759 1330 1730 300 1340 2100 2910 530 613 1200 1660 1780

208 152 133 115 300 268 210 194 530 123 120 111 89.0

46000 212000 353000 400000 50000 316000 563000 792000 146000 164000 429000 653000 800000

1.24 1.37 1.40 1.59 1.19 1.55 1.54 1.77 1.11 1.12 1.31 1.29 1.38

510

17.0

172000

1.55

a Conditions: cat 1.0 μmol, CHPE 10.0 mmol (initial conc. 2.08 M) or 5.0 mmol (initial conc. 1.04 M), CHPE + benzene total 4.8 mL. bInitial CHPE conc. in mmol/mL (M). cTON (turnover) = CPE reacted (mmol)/vanadium (mmol). dTOF = TON/time. eGPC data in THF vs polystyrene standards (g/mol).

Figure 2. Plots of Mn and Mw/Mn vs turnover numbers (TON, polymer yield on the basis of V) in ROMP of cycloheptene (CHPE) using (a) V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6X5)(PMe3)2 [X = F (1), Cl (3); Mn (plotted as closed blue circles and closed brown circles for 1 and 3, respectively) and Mw/Mn (plotted as open blue circles and open brown circles for 1 and 3, respectively)], (b) using 3 with different initial CHPE concentrations [Marks with blue closed and open circles are Mn and Mw/Mn values conducted at initial concentration 1.04 M; Marks with brown closed and open circles are Mn and Mw/Mn values conducted at initial concentration 2.08 M]. Detailed data are shown in Table 2.

demonstrated until relatively high COE conversion (ca. 50%). The results clearly demonstrate the high temperature ROMP of COE by 3 at 80 °C without catalyst decomposition or chain transfer. Effect of B(C6F5)3 and PMe3 toward the Activity in ROMP of Cyclic Olefins. We previously reported that the activities in the ROMP of COE by the OC6F5 analogue (1) became negligible upon addition of 3.0 equiv of PMe3 at 25 and 50 °C.9e This is a unique contrast to the fact that the activity by 1 in the ROMP of NBE (conducted at 25−80 °C) increased upon addition of PMe3.9d As also shown in Table 4, the activity in the ROMP of COE by the OC6Cl5 analogue (3) at 120 °C increased upon presence of 3.0 equiv of PMe3 (run 76), and the further addition led to rapid decrease in the activity (run 77). Since we previously observed that addition of 1.0 (or 3.0) equiv of B(C6F5)3 (attempted for dissociation of PMe3) rapidly stopped ROMPs of NBE (at 25 °C) by 1 and V(CHSiMe3)(NC6H5)[OC(CF3)3](PMe3)2 (7),21 the effects of PMe3, bis(dimethylphosphino)ethane (dmpe), and B(C6F5)3 were

Interestingly, as also observed in the ROMP of COE by 1, both the polymer yield and Mn value increased over time course (runs 56−61 and 64−72) even at 50−100 °C without significant changes in the PDI values, suggesting that termination/chain transfer during polymerization is negligible and that TONs increased linearly over time course. Since a possibility of (quasi) living polymerization was suggested even at high temperature, ROMP of COE by 3 was conducted at 80 °C under rather COE concentration conditions, and the results are summarized in Table 5. Figure 3 shows plots of Mn values with polymer yields (TON) to monitor the ROMP. In order to check the reproducibility (because the 3 is sensitive toward impurities in solvent and COE), the independent polymerization runs were conducted twice at each polymerization time. It turned out that both the polymer yields (TON) and the Mn values increased over time course without significant changes in the PDI (Mw/Mn) values, a linear relationship between the Mn values and polymer yields was clearly E

DOI: 10.1021/acs.organomet.8b00231 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Table 4. ROMP of Cis-Cyclooctene (COE) Using V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6X5)(PMe3)2 [X = F (1), Cl (3)]a run e

47 48e 49f 50f 51f 52f 53f 54f 55f 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

cat. (μmol) 1 1 1 1 1 1 1 1 1 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3

temp (°C)

time (min)

polymer (mg)

TONb

TOF (min−1) (h−1)c

Mnd

Mw/Mnd

25 25 50 50 50 80 80 100 100 25 25 50 50 80 80 25 25 50 50 50 80 80 80 100 100 100 120 120 120 120 120

30 120 5 10 15 5 10 5 10 120 360 1 5 1 2 10 30 5 10 15 1 2 5 1 5 10 1 2 5 5 10

100 290 52 121 176 181 285 130 154 324 639 183 372 247 494 43 88 98 183 212 40 117 274 89 305 711 109 224 264 320

45 132 47 110 160 164 259 118 140 147 290 83 169 112 224 390 799 889 1660 1920 363 1062 2486 808 2770 6450 989 2030 2400 2900 trace

1.5 (90) 1.1 (66) 9.4 11.0 (660) 10.7 (640) 32.8 25.9 23.6 14.0 1.2 (74) 0.8 (73) 83.0 (4980) 33.8 (2030) 112 112 39.0 (2340) 26.6 (1600) 178 (10700) 166 (9960) 128 (7690) 363 531 497 808 554 645 989 1015 480 580

10500 27000 19300 32000 41900 52000 62100 42200 54600 47800 71200 5800 28200 27700 53700

1.34 1.59 1.51 1.59 1.60 1.46 1.64 1.60 1.36 1.61 1.54 1.45 1.51 1.56 1.68

95000 55400 80300 90100 77900 126000 226000 59000 126000 509000 22100 44400 113000 126000

1.15 1.45 1.47 1.49 1.09 1.18 1.24 1.35 1.59 1.32 1.43 1.55 1.57 1.60

(20) (20) (10) (10) (10) (10) (10) (10) (10) (20) (20) (20) (20) (20) (20) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) + PMe3 (3.0 equiv) (1.0) + PMe3 (10.0 equiv)

a

Conditions: COE 10.0 mmol, benzene 0.5 mL. bTON(turnover) = COE reacted (mmol)/vanadium (mmol). cTOF = TON/time. dGPC data in THF versus polystyrene standards (g/mol). eCited from ref 9d. fCited from ref 9e.

Table 5. ROMP of Cis-Cyclooctene (COE) by V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6Cl5)(PMe3)2 (3) at 80 °Ca yield run

time (min)

(mg)

(%)

TONb

Mnc

Mw/Mnc

78 79 80 81 82 83 84 85 86 87

3 3 5 5 7 7 10 10 15 15

159 156 273 296 350 342 466 434 563 591

14.4 14.2 24.8 26.9 31.8 31.0 42.3 39.4 51.1 53.6

289 283 495 537 635 621 846 788 1020 1070

70800 67200 98900 106000 131000 129000 175000 161000 194000 197000

1.41 1.39 1.54 1.66 1.53 1.55 1.55 1.48 1.64 1.72

Figure 3. Plots of Mn (plotted as ●) and Mw/Mn (plotted as ○) versus turnover numbers (TON, polymer yield on the basis of V) in living ROMP of COE by 3. Detailed data are shown in Table 5.

Reaction conditions: cat. 3 (5.0 μmol), COE 10.0 mmol, COE +benzene total 4.8 mL (initial monomer conc. 2.08 M), 80 °C. bTON (turnovers) = monomer reacted (mmol)/V(mmol). cGPC data in THF versus polystyrene standards (g/mol). a

addition of dmpe diminished the activity (runs 89 and 90). In contrast, it should be noted that the activity increased upon addition of 1.0 equiv of B(C6F5)3 [TOF: 210 min−1 (run 37) → 748 (run 91)] affording ultrahigh molecular weight ringopened polymer (Mn = 2.82 × 106), whereas the activity became negligible upon the further addition (3.0 equiv, run 92). Moreover, it should also be noted that the activity in the ROMP of COE by 3 increased upon addition of 1.0 equiv of B(C6F5)3 and increased upon further addition (3.0 equiv); the

thus explored in the ROMP of CHPE. The results are summarized in Table 6. As observed in the ROMP of COE by 1, the polymers yields were negligible in the ROMPs of CHPE by 3 upon addition of 1.0 equiv of PMe3 (run 88); the further addition (3.0 equiv) or F

DOI: 10.1021/acs.organomet.8b00231 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Table 6. ROMP of Cycloheptene (CHPE) and cis-Cyclooctene (COE) Using V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6Cl5)(PMe3)2 (3): Effect of B(C6F5)3, PMe3, and 1,2-Bis(dimethylphosphino)ethane (dmpe)a run

3 (μmol)a

monomer

37 88 89 90 91 92 93 94 95 96 97 98 99 6f 100f 101f

1.0 1.0 1.0 1.0 1.0 1.0 5.0 5.0 5.0 5.0 5.0 0.0e 0.0e 0.1 0.1 0.1

CHPE CHPE CHPE CHPE CHPE CHPE COE COE COE COE COE COE COE NBE NBE NBE

additive (equiv) PMe3 (1.0) PMe3 (3.0) dmpe (1.0) B(C6F5)3 (1.0) B(C6F5)3 (3.0) B(C6F5)3 B(C6F5)3 B(C6F5)3 B(C6F5)3 B(C6F5)3 B(C6F5)3

(1.0) (3.0) (1.0) (3.0) (1.0) (3.0)

B(C6F5)3 (1.0) B(C6F5)3 (3.0)

time (min)

yield (mg)

TONb

TOF (min)c

Mnd

Mw/Mnd

10 10 10 10 10 10 30 30 30 15 15 30 30 3 3 3

202 trace 0 0 719 0 113 253 364 133 265 0 0 184 0 0

2100

210

563000

1.54

7480

748

2820000

1.71

64000 123000 168000 75000 155000

1.48 1.57 1.61 1.43 1.61

1320000

1.60

205 459 661 241 481

19500

6.8 15.3 22.0 16.1 32.1

6500

a

Conditions: CHPE or COE 10.0 mmol, monomer and benzene total (4.8 mL, initial monomer conc. 2.08 M). bTON (turnover) = monomer reacted (mmol)/vanadium (mmol). cTOF = TON/time. dGPC data in THF versus polystyrene standards (g/mol). eExperiments without 3. fNBE 2.12 mmol in benzene (4.8 mL, initial monomer conc. 0.44 M).

Mn values after 30 min were higher than those after 15 min without decreases in the activity (TOF) and changes in the PDI (Mw/Mn) values (runs 94−97). In contrast, the activity in the ROMP of NBE by 3 became negligible by adding B(C6F5)3 (runs 100 and 101), as observed by 1 and 7.21 Taking into account these results, it is clear that role of PMe3 and B(C6F5)3 should be different. Resonance ascribed to PMe3 in 3 was disappeared in the 31P NMR spectrum upon addition of 1.0 equiv of B(C6F5)3 (in C6D6, 25 °C), whereas the resonance remained when 1.0 equiv of B(C6F5)3 was added into the mixed solution of 3 and COE (10.0 equiv, Figure S20).21 Moreover, a new resonance (in addition to that in 3) was observed in the 51V NMR spectrum (in C6D6, 25 °C) of the mixed solution, whereas the intensity ascribed to 3 seemed decreasing upon addition of B(C6F5)3 without COE (Figure S21).21 Since at least one PMe3 should be dissociated to induce the ROMP of cyclic olefins by these (imido)vanadium-alkylidene catalysts (including 3),3g,h,9c,d these results might provide some useful information concerning the role of PMe3, cyclic olefin, and B(C6F5)3 in the reaction solution. Effect of Cyclic Olefins (Ring Strains) toward the Activity. It has been recognized that release of ring strain is a driving force in the ROMP of cyclic olefins,1,5 and ROMP of norbornene (NBE) generally showed higher catalytic activity than those of cyclopentene (CPE) and cis-cyclooctene (COE). Indeed, ring strain energy of NBE is 27.2 kcal/mol, higher than those of CPE and COE (6.8 and 7.4 kcal/mol, respectively);20 therefore, CPE, cycloheptene (CHPE), and COE are called low-strain monomers in ROMP. As shown in Table 7, which summarizes results in ROMP of NBE, CPE, CHPE, and COE by using V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6X5)(PMe3)2 [X = F (1), Cl (3)], the activities in ROMPs of NBE showed higher than those in the ROMPs of the other cyclic olefins. These results well correspond with above explanation that a release of ring strain plays a role in this catalysis.22 Ring strain energies of CPE, CHPE, and COE are 6.8, 6.7, and 7.4 kcal/mol, respectively,20 suggesting they should behave similarly with the olefin metathesis catalysts. However, as

Table 7. Summary of Results in ROMP of Cyclic Olefins Using V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6X5)(PMe3)2 [X = F (1), Cl (3)] Conducted at 25 °Ca run

cat. (μmol)

1 5 12 19 32 36 48e 56 63

1 3 1 3 1 3 1 3 3

(0.1) (0.1) (0.1) (0.1) (1.0) (1.0) (20) (20) (1.0)

monomer

conc. (M)b

time (min)

TONc

TOF (min−1)d

NBE NBE CPE CPE CHPE CHPE COE COE COE

0.44 0.44 2.08 2.08 2.08 2.08 5.59 5.59 5.59

2 2 5 5 5 5 120 120 30

20600 16600 17300 26400 759 1340 132 147 799

10300 8300 3460 5280 152 268 1.1 1.2 26.6

a

Conditions: shown in Tables 1−4. bInitial monomer concentration, mmol/mL. cTON(turnover) = monomer reacted (mmol)/vanadium (mmol). dTOF = TON/time. eCited from ref 9d.

shown in Table 7, their activities were highly dependent upon the cyclic olefins employed.14 For instance, the catalytic activity in the ROMP of cyclic olefins using 1 and 3 can be displayed in the order: COE ≪ CHPE < CPE < NBE. It should be noted that the activity in the ROMP of CPE showed apparently higher than that in the ROMP of CHPE, although the ring strain energy is close.20 Moreover, the activities in the ROMPs of COE are extremely lower than those in the ROMP of CPE and CHPE although the ring strain energy of COE is larger than those in CPE and CHPE. These results suggest that a release of ring strain is not the crucial factor in this catalysis. Moreover, importantly, the observed trend is different (opposite) from that observed in ROMPs of these cyclic olefins using ruthenium−carbene catalysts, whereas the activity in the ROMP (using so-called Hoveyda−Grubbs secondgeneration catalyst) increased in the order: CPE < CHPE < COE.14b Therefore, these results suggest that factors affecting the activity in the ROMP of cyclic olefins among these (socalled) low strain monomers are different between ruthenium− carbene14 (neutral carbene with late transition metals) and G

DOI: 10.1021/acs.organomet.8b00231 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Cl2C6H3)(CH2SiMe3)3,9c V(N-2,6-F2C6H3)Cl3,23 and V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5)(PMe3)2 (1)9c were prepared according to the reported procedure. All NMR spectra were recorded on a Bruker AV500 spectrometer (500.13 MHz for 1H, 125.77 MHz for 13C, 470.59 MHz for 19F, 202.47 MHz for 31P, 131.55 MHz for 51V). All spectra were obtained in the solvent indicated at 25 °C unless otherwise noted. Chemical shifts are given in ppm and are referenced to SiMe4 (δ 0.00 ppm, 1H, 13 C). Coupling constants are given in Hz. Chemical shift was determined with reference to chloroform (7.26 ppm for 1H, 77.16 ppm for 13C) and benzene-d6 (7.16 ppm for 1H, 128.06 ppm for 13C). Molecular weights and molecular weight distributions of the resultant polymers were measured by gel-permeation chromatography (GPC). HPLC grade THF was used for GPC and was degassed prior to use. GPC was performed at 40 °C on a Shimadzu SCL-10A using a RID-10A detector (Shimadzu Co. Ltd.) in THF (containing 0.03 wt % of 2,6-di-tert-butyl-p-cresol, flow rate 1.0 mL/min). GPC columns (ShimPAC GPC-806, 804 and 802, 30 cm × 8.0 mm diameter, spherical porous gel made of styrene/divinylbenzene copolymer, ranging from