Reaction of [V(X)(OR)(3)] (X = O, Np-tolyl, R = nPr, tBu) with 5,11,17,23-tBu-25,27-dihydroxycalix[4]arene (LH(2)) led to the formation of [V(X)(OR)L](2) X = O, R = nPr (1); X = Np-tolyl, R = nPr (2); X = Np-tolyl, R = tBu (3) as the major product. In the case of X = O, the minor hydrolysis product {[VO(OnPr)](2)(μ-O)L}(2) (4) has also been characterized. Complexes (1)-(4), in the presence of the co-catalyst dimethylaluminum chloride and the reactivator ethyltrichloroacetate, are highly active (≤16,400 g/mmol h bar), thermally stable pro-catalysts for the polymerization of ethylene. The use of silica supports with (1) and (2) under slurry conditions yielded polymer with activities ≤30 g/mmol h. Complexes (1)-(3) have also been screened as pro-catalysts for the ring-opening polymerization of ε-caprolactone; the conversion rate order (1) (94%) > (2) (46%) > (3) (20%) was observed at 80 °C over 72 h.
Calixarenes bridged by thia (−S−), sulfinyl (−SO−), or sulfonyl (−SO 2 −) linkers have been employed as ancillary ligands in vanadium-based ethylene polymerization and in the ring-opening polymerization of ε-caprolactone. For ethylene polymerization, all pro-catalysts [PPh 4 ][VOCl 2 (R-calix[4]areneH 2 )] (R = p-tert-butylthia (−S−) (1), sulfinyl (−SO−) ( 2), sulfonyl (−SO 2 −) (3)) were highly active, with the system bearing the sulfonyl (−SO 2 −)-bridged calixarene displaying enhanced activity. The molecular structures of pro-catalysts 2 and 3 are reported. Pro-catalysts 1−3 were inactive for the ROP of ε-caprolactone.O ver the past decade or so, the search for new ligand sets capable of forming efficient catalytic systems for either α-olefin polymerization or the ring opening of lactones has met with considerable success. 1 Both early-and late-transition-metal complexes have been shown to be capable of impressive catalytic performance. In terms of the ligands employed, the use of the calixarene family has been somewhat limited. 2 However, notable catalytic performance, particularly in vanadium-based systems, has been achieved when the more common methylene-bridged (−CH 2 −) calixarenes are replaced by those bearing dimethyleneoxa-type-bridged (−CH 2 OCH 2 −) calixarenes. 3 The improved performance (near 100-fold increases in observed activity) was thought to arise partly via stabilization of the active species by the additional bridging ether groups of the calixarene linkers. There have also been a number of reports in the literature concerning the beneficial presence of sulfur in ligand backbones, particularly in diphenolate-type ligation. 4 Given this and the availability of a range of other calixarenes which bear functionality at the linker, we have embarked upon a program to screen the catalytic potential of such systems. Herein, we report vanadium-based systems bearing thia (−S−)-, sulfinyl (−SO−)-, or sulfonyl (−SO 2 −)-bridged calixarenes and compare their performance in ethylene polymerization catalysis and in the ring opening of ε-caprolactone. We note that the coordination chemistry of such "sulfur calixarene" ligands is limited, 5 and their use in catalysis is restricted to reports by Limberg et al., in which the thia-bridged ligand set was employed in vanadium-catalyzed alcohol oxidations. 6 Furthermore, the same group has also recently deployed sulfur-bridged (−S−, −SO−, −SO 2 −) diphenolate ligation to mimic those parts of thiacalixarenes relevant to the binding at vanadium. The −SO 2 − bridged system was shown to be an efficient catalyst for the sulfoxidation of thioethers with t-BuOOH (haloperoxidase activity). 7 A dinuclear titanium complex of the −S− bridged ligand set has been employed as a Lewis acid catalyst in the Mukaiyama-aldol reaction of aryl aldehydes with silyl enol ethers, 8 as a catalyst in the cyclotrimerization of alkynes, 9 and more recently in combination with MAO (methylaluminoxane) for ethylene polymerization, albeit with low observed activity (≤25 g/(mmol h bar)). 10 By ex...
The reactions of MCl5 or MOCl3 with imidazole-based pro-ligand L(1)H, 3,5-tBu2-2-OH-C6H2-(4,5-Ph2-1H-)imidazole, or oxazole-based ligand L(2)H, 3,5-tBu2-2-OH-C6H2 (1H-phenanthro[9,10-d])oxazole, following work-up, afforded octahedral complexes [MX(L(1,2))], where MX=NbCl4 (L(1), 1a; L(2), 2a), [NbOCl2(NCMe)] (L(1), 1b; L(2), 2b), TaCl4 (L(1), 1c; L(2), 2c), or [TaOCl2(NCMe)] (L(1), 1d). The treatment of α-diimine ligand L(3), (2,6-iPr2C6H3N=CH)2, with [MCl4(thf)2] (M=Nb, Ta) afforded [MCl4(L(3))] (M=Nb, 3a; Ta, 3b). The reaction of [MCl3(dme)] (dme=1,2-dimethoxyethane; M=Nb, Ta) with bis(imino)pyridine ligand L(4), 2,6-[2,6-iPr2C6H3N=(Me)C]2C5H3N, afforded known complexes of the type [MCl3(L(4))] (M=Nb, 4a; Ta, 4b), whereas the reaction of 2-acetyl-6-iminopyridine ligand L(5), 2-[2,6-iPr2C6H3N=(Me)C]-6-Ac-C5H3N, with the niobium precursor afforded the coupled product [({2-Ac-6-(2,6-iPr2C6H3N=(Me)C)C5H3N}NbOCl2)2] (5). The reaction of MCl5 with Schiff-base pro-ligands L(6)H-L(10)H, 3,5-(R(1))2-2-OH-C6H2CH=N(2-OR(2)-C6H4), (L(6)H: R(1)=tBu, R(2)=Ph; L(7)H: R(1)=tBu, R(2)=Me; L(8)H: R(1)=Cl, R(2)=Ph; L(9)H: R(1)=Cl, R(2)=Me; L(10)H: R(1)=Cl, R(2)=CF3) afforded [MCl4(L(6-10))] complexes (M=Nb, 6a-10a; M=Ta, 6b-9b). In the case of compound 8b, the corresponding zwitterion was also synthesised, namely [Ta(-)Cl5(L(8)H)(+)]·MeCN (8c). Unexpectedly, the reaction of L(7)H with TaCl5 at reflux in toluene led to the removal of the methyl group and the formation of trichloride 7c [TaCl3(L(7-Me))]; conducting the reaction at room temperature led to the formation of the expected methoxy compound (7b). Upon activation with methylaluminoxane (MAO), these complexes displayed poor activities for the homogeneous polymerisation of ethylene. However, the use of chloroalkylaluminium reagents, such as dimethylaluminium chloride (DMAC) and methylaluminium dichloride (MADC), as co-catalysts in the presence of the reactivator ethyl trichloroacetate (ETA) generated thermally stable catalysts with, in the case of niobium, catalytic activities that were two orders of magnitude higher than those previously observed. The effects of steric hindrance and electronic configuration on the polymerisation activity of these tantalum and niobium pre-catalysts were investigated. Spectroscopic studies ((1)H NMR, (13)C NMR and (1)H-(1)H and (1)H-(13)C correlations) on the reactions of compounds 4a/4b with either MAO(50) or AlMe3/[CPh3](+)[B(C6F5)4](-) were consistent with the formation of a diamagnetic cation of the form [L(4)AlMe2](+) (MAO(50) is the product of the vacuum distillation of commercial MAO at +50 °C and contains only 1 mol% of Al in the form of free AlMe3). In the presence of MAO, this cationic aluminium complex was not capable of initiating the ROMP (ring opening metathesis polymerisation) of norbornene, whereas the 4a/4b systems with MAO(50) were active. A parallel pressure reactor (PPR)-based homogeneous polymerisation screening by using pre-catalysts 1b, 1c, 2a, 3a and 6a, in combination with MAO, revealed only moderate-to-good activities...
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