New pre-catalysts derived from calixarenes : synthesis, structural and polymerization studies

Xing, Tian

Chemistry
June 2021

Thesis or dissertation


Rights
© 2021 Tian Xing. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
Abstract

In this thesis, a series of V, Ti, Li and Pb compounds have been synthesized and fully characterized. The catalytic performance of these pre-catalysts towards different polymerization reactions, including ring opening polymerization (ROP) of cyclic esters, ethylene polymerization and co-polymerization of propylene oxide and CO2 are studied.
Chapter 1 presents the history and research achievements of both calixarene and metallocalixarene compounds, with particular reference to polymerization reactions catalyzed by such coordination complexes, including ethylene polymerization, ring opening polymerization of cyclic esters and co-polymerization of propylene oxide and CO2.
In Chapter 2, reactions of larger calix[n]arenes with vanadium precursors are studied. In particular, the reaction of Na[VO(tBuO)4] (generated in-situ from VOCl3 and NaOtBu) with p-tert-butyltetrahomodioxacalix[6]areneH6 (LO6H6) afforded, after work-up (in MeCN), the mixed-metal complex [(VO)2(μ-O)Na2(LO6)(MeCN)4]·5(MeCN) (1·5MeCN), whilst the oxo complex {[VO]4LO6} (2·6MeCN) was isolated via the use of [VO(OnPr)3]. Reaction of LO6H6 with [V(Np-CH3C6H4)(OtBu)3] afforded the complex {[V(Np-CH3C6H4)]2LO6} (3·7MeCN·0.5CH2Cl2). Use of similar methodology afforded the imido complexes {[V(Np-RC6H4)]2LO6} (R = OMe (4); CF3 (5); Cl (6); F (7)); on one occasion, reaction of [V(Np-CH3C6H4)(OEt)3] with LO6H6 afforded the product [VO(L6O’)]2·4MeCN (8·4MeCN) (L6O’ = 2-(p-CH3-C6H4NCH)-4-tBu-C6H2O-6-CH2)-4-tBuC6H2OH) in which LO6 has been cleaved. For comparative catalytic ring opening polymerization (ROP) studies, the known complexes [VOL3] (L3H3 = oxacalix[3]arene) (I), [V(Np-CH3-C6H4)L3]2 (II), [Li(MeCN)4][V2(O)2Li(MeCN)(L6H2)2] (L6H6 = p-tert-butylcalix[6]areneH6) (III) and [(VO)2L8H] (L8H8 = p-tert-butylcalix[8]areneH8) (IV) have also been prepared. ROP studies, with or with or without external alcohol present, indicated that complexes 1 to 8 exhibited moderate to good conversions for ε-Cl, δ-VL and the co-polymerization thereof. Within the imido series, a positive influence was observed when electron withdrawing substituents were present. These systems afforded relatively low molecular weight products and were also inactive toward the ROP of rac-lactide. In the case of ethylene polymerization, complexes 3, 5 and 7 exhibited highest activity when screened in the presence of dimethylaluminium chloride/ethyltrichloroacetate; the activity of 4 was much lower. The products were highly linear polyethylene with Mw in the range 74-120x103 Da.
In Chapter 3, a number of metallocalix[n]arenes, where n = 4, 6, or 8, of titanium and vanadium have been screened for their ability to act as catalysts for the co-polymerization of propylene oxide and CO2 to form cyclic/polycarbonates. The vanadium-containing catalysts, namely [VO(L4Me)] (V), [(VO2)L8H6] (VI), [Na(NCMe)6]2[(Na(VO)4L8)(Na(NCMe))3]2 (VII), [VO(-OH)L4S/H2]2∙6CH2Cl2 (9/), [(VO)2(μ-O)Na2(LO6)(MeCN)4] (1), {[V(Np-CH3C6H4)]2LO6} (3) and [V(Np-RC6H4)Cl3] (R = Cl (VIII), OMe (IX), CF3 (X)), where L4H4 = p-tert-butylcalix[4]areneH4, L8H8 = p-tert-butylcalix[8]areneH8, L4SH4 = p-tert-butylthiacalix[4]areneH4, performed poorly, affording, in the majority of cases, TONs less than 1 at 90 oC over 6 h. In the case of the titanocalix[8]arenes, {(TiX)2[TiX(NCMe)]2(μ3-O)2(L8)} (X = Cl (10), Br (XI), I (XII)), which all adopt a similar ladder-type structure, the activity under the same conditions is somewhat higher (TONs > 6) and follows the trend Cl > Br > I; by comparison the non-calixarene species [TiCl4(THF)2] (XIII) was virtually inactive. The molecular structures of the complexes [HNEt3]2[VO(-O)L4SH2]2∙3CH2Cl2 (9∙3CH2Cl2), [VO(-OH)L4S/H2]2∙6CH2Cl2 (9/) (where L4S/H2 is a partially oxidized form of L4SH4) and {(TiCl)2[TiCl(NCMe)]2(μ3-O)2(L8)}·6.5MeCN (10·6.5MeCN) are reported.
In Chapter 4, the coordination chemistry of azacalix[n]arenes is studied focusing on reactions with titanium precursors. Reaction of excess [Ti(OiPr)4] with L6OH6 afforded, after work-up (MeCN), the complex [Ti2(OiPr)2(MeCN)L6O]∙3.5MeCN (11∙3.5MeCN), whilst the oxo complex [Ti4O4(L6O)2]∙MeCN (12∙MeCN) was isolated via a fortuitous synthesis involving the use of two equivalents of [Ti(OiPr)4]. Reactions of p-methyl-dimethyldiazacalix[6]areneH6 (L6NH6) with [TiF4] (four equivalents), [TiCl4(THF)2] (two equivalents) or [TiBr4] (>four equivalents) resulted in the titanium-based azacalix[n]arene complexes [Ti4F14L6NH2(H)2]∙2.5MeCN (13∙2.5MeCN), [Ti2X4(H2O)2OL6NH2(H)2] (X = Cl (14∙5MeCN), Br (15∙4.5MeCN) and [Ti4Br12L6N(H)2(MeCN)6]∙7MeCN (16∙7MeCN), respectively. Reaction of four equivalents of [TiF4] with L3H4 (L4NH4 = p-methyl-dimethyldiazacalix[4]areneH4) afforded the product [Ti2F2(μ-F)3L4N(H)2(SiF5)]∙2MeCN (17∙2MeCN). These complexes have been screened for their potential to act as pre-catalysts in the ring opening polymerization (ROP) of ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and rac-lactide (r-LA). Generally, the titanium complexes bearing oxacalixarene exhibited better activities than the azacalixarene-based pre-catalysts. For ε-CL, δ-VL and r-LA, moderate activity at 130 °C over 24 h was observed for 11-16. In the case of the co-polymerization of ε-CL with r-LA, 11-16 afforded reasonable conversions and high molecular weight polymers; 17 exhibited lower catalytic performance due to low solubility. None of the complexes proved to be active in the polymerization of ω-pentadecalactone (ω-PDL) under the conditions employed herein.
In Chapter 5, a variety of lithiated calix[n]arenes, where n = 6 or 8, have been isolated, structurally characterized and screened for their ability to act as catalysts for the ring opening polymerization (ROP) of the cyclic esters ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and r-lactide (r-LA). In particular, interaction of L6H6 with LiOtBu in THF afforded [Li14(L6H)2(CO3)2(THF)6(OH2)6]·14THF (18·14THF), whilst L8H8 afforded [Li10(L8)(OH)2(THF)8]·7THF (19·7THF). Similar use of de-butylated calix[8]areneH8 (deBuL8H8) led to an elongated dimer [Li18(deBuL8)2(OtBu)2(THF)14]·4THF (20·4THF). Interaction of L8H8 with LiOH·H2O afforded [Li4(L8H4)(OH2)4(THF)6]·5.5THF (21·5.5THF), whilst addition of Me3Al to the solution generated from L8H8 and LiOtBu led to the isolation of [(AlMe2)2Li20(L8H2)2(OH2)4(O2–)4(OH)2(NCMe)12]·10MeCN (22·10MeCN). These complexes have been screened for their potential to act as pre-catalysts in the ring opening polymerization (ROP) of ε-CL, δ-VL and r-LA. For ε-CL, δ-VL and r-LA, moderate activity for ROP at 130 °C over 8 h was observed for 18-21. In the case of ROP using the mixed-metal (Li/Al) system 22, better conversions and high molecular weight polymers were achieved. None of the complexes proved to be active in the ROP of ω-pentadecalactone (ω-PDL) under the conditions employed herein.
In Chapter 6, the lead coordination chemistry of large calix[n]arenes is studied. Reaction of [LiPb(OiPr)3]2 (generated in-situ from Pb(OiPr)2 and LiOiPr) with either L4H4 or L6H6 resulted in the heterometallic lithium/lead complexes [Pb4Li2(L4)4H6(MeCN)3]∙4.5MeCN (23∙4.5MeCN) and [Pb8Li10Cl2(L6)4(H)8(O)4(H2O)2(MeCN)4]∙14MeCN (24∙14MeCN), respectively. Reaction of five equivalents of [Pb(OiPr)2] with L6OH6 afforded [Pb13(L6O)3O4(iPrOH)]∙11MeCN (25∙11MeCN). Reaction of L8H8 with [Pb(OtBu)2] or {Pb[N(TMS)2]} (TMS = SiMe3) afforded the products [Pb12(L8)2O4]∙8.7C7H8 (26∙8.7C7H8) or [Pb6(SiMe3)2(L8)O2Cl2] (27), respectively. Reaction of {Pb[N(TMS)2]} (generated in-situ from (Me3Si)2NH, nBuLi and PbCl2) with L6H6 afforded, after work-up (MeCN), the mixed-metal complex [Pb5Li(L6)O2.5Cl0.5]∙4.75MeCN (28·4.75MeCN). Complexes 23-27 have been screened for their potential to act as pre-catalysts in the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL) and the copolymerization thereof. Generally, the lithiated complexes 23 and 24 exhibited better activities than the other pre-catalysts screened herein. For ε-CL and δ-VL, moderate activity at 130 °C over 24 h was observed for 23-27. In the case of the co-polymerization of ε-CL with δ-VL, 23-27 afforded reasonable conversions and high molecular weight polymers. The catalysts 23-27 also be proved to be active in the ROP of the rac-lactide (r-LA), the activity trend was found to be 24 >23 >25 >26 ≈ 27.
Chapter 7 presents the experimental section.
Chapter 8 Appendix.

Publisher
Department of Chemistry, The University of Hull
Supervisor
Redshaw, Carl; Prior, Timothy J.
Sponsor (Organisation)
China Scholarship Council
Qualification level
Doctoral
Qualification name
PhD
Language
English
Extent
10 MB
Identifier
hull:18429
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