New pre-catalysts for ring opening of lactides/lactones based on earth abundant metals

Al-Khafaji, Yahya

Abstract

In this study, a number of Nb, Ta, Mo, V, Al, Li and Zn complexes have been synthesised and fully characterised. The catalytic behavior of these pre-catalysts towards the ring opening polymerization (ROP) of the cyclic esters is discussed.

Chapter 1 Presents an introduction to the history of polymers (from polyethylene to biodegradable polymers from cyclic esters), early discovery, mechanism of ring opening polymerisation and the use of vanadium, niobium, tantalum and molybdenum, aluiminium, lithium and zinc complexes as polymerization catalysts.

Chpter 2 This chapter discussed results when the pre-ligands α,α,α’,α’-tetra(3,5-di-tert-butyl-2-hydroxyphenyl-p-)xylene-para-tetraphenol(p-L¹H₄) and α,α,α’,α’tetra (3,5-di-tert-butyl-2-hydroxyphenyl-m-)xylene-meta-tetraphenol (m-L²H₄) are reacted with a number of niobium and tantalum precursors such as [NbCl₅], [TaCl₅] or [Nb(O)Cl₃(NCMe)₂]. The resulting products {[NbCl₃(NCMe)]₂(μ-p-L¹)}·6MeCN (1·6MeCN), {[NbCl₂(OEt)(NCMe)]₂(μ-p-L¹)}·3½MeCN·0.614 toluene (2·3½MeCN·0.614 toluene), {[TaCl₂(OEt)(NCMe)]₂(μ-p-L¹)}.5MeCN (3·5MeCN), {[Nb(NCMe)Cl(m-L²H₂)₂]}·3½MeCN(4·3½MeCN) and {[Nb(NCMe)Cl(m-L²H₂)₂]}·5MeCN (4·5MeCN) were structurally characterized. Complexes 1–4 were screened as pre-catalysts for the ROP of ε-caprolactone, both with and without benzyl alcohol or solvent present, and at various temperatures.

Chapter 3 In this chapter, the reaction of the bulky bi-phenols 2,2′-RCH[4,6-(t-Bu)₂C₆H₂OH]₂ (R = Me L³MeH₂, Ph L⁴PhH₂) with the bis(imido) molybdenum(VI) tert-butoxides [Mo(NR¹)(NR²)(Ot-Bu)₂] (R¹ = R² = 2,6-C₆H₃-i-Pr2; R¹ = t-Bu, R² = C₆F₅) has been studied. The complexes [Mo(NC₆H₃i-Pr₂-2,6)2L3Me] (5), [Mo(NC₆H₃i-Pr₂-2,6)2L⁴Ph] (6) and [Mo(Nt-Bu)(μ-NC₆F₅)(L³Me)]₂ (7) were isolated. Similar use of the tri-phenol 2,6-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-4-methylphenol (L⁵H₃) with [Mo(NC₆H₃i-Pr₂-2,6)₂(Ot-Bu)₂] afforded the oxo-bridged product [Mo(NC6H₃i-Pr₂-2,6)(NCMe)(μ-O)L5H]₂ (8), whilst use of the tetra-phenols L¹pH₄/L²mH₄ led to {[Mo(NC₆H₃i-Pr₂-2,6)₂]₂(μ-L¹p)} (9) or {[Mo(NC₆H₃i-Pr₂-2,6)₂]₂(μ-L²m)}(10), respectively. Similar use of [Mo(NC₆F5)2(Ot-Bu)₂] with L¹pH₄ afforded {[Mo(NC₆F₅)(Ot-Bu)₂]₂(μ-L¹p)}·6MeCN (11·6MeCN). The molecular structures of 5, 6·CH₂Cl₂, 7, 8·6MeCN, 10·2C₆H14, and 11·6MeCN are reported. These complexes have been screened for their ability to act as catalysts for the ROP of ε-caprolactone; for comparative studies the complex [Mo(NC₆H₃i-Pr₂ 2,6)₂Cl₂(DME) (12) has also been screened.

Chpter 4 This chapter focuses on the use of the vanadyl complexes. The new complexes [VO(Ot-Bu)L³] (13), {[VO(Oi-Pr)]₂(µ-p-L¹p)} (14) {[VO(OR)]₂(µ-p-L²m)} (R = i-Pr 15, t-Bu 16 have been prepared from [VO(OR)₃] (R = n-Pr, i-Pr or t-Bu) and the respective phenol, namely 2,2/-ethylidenebis(4,6-di-tert-butylphenol) (L³H₂) or Lp/mH4. For comparative studies, the known complexes [VO(µ-On-Pr)L³]₂ (18), [VOL⁶]₂ (19) (L6H₃  2,6-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-4-tert-butylphenol) were prepared. An imido complex {[VCl(Np-tolyl)(NCMe)]₂(μ-p-L¹p)} (17) has also been prepared. The molecular structures of complexes 13 – 19 are reported, and these complexes 13 – 19 have been screened for their ability to ring open polymerise ε-caprolactone, L-lactide or rac-lactide with and without solvent present. The co-polymerization of ε-caprolactone with L-lactide or rac-lactide was also studied.

Chapter 5 describes the reaction of R¹R²CHN=CH(3,5-t-Bu₂C₆H₂-OH-2) (R¹ = R² = Me L⁷H; R¹ = Me, R² = Ph L⁸H; R¹ = R² = Ph L⁹H) with slightly greater than one equivalent of R³₃Al (R³ = Me, Et), which afforded the complexes [(L⁷⁻⁹)AlR³₂] (L⁷, R³ = Me 20, R³ = Et 21; L⁸, R³ = Me 22, R³ = Et 23; L³, R³ = Me 24, R³ = Et 25); complex 20 has been previously reported. Use of the N,O-ligand derived from 2,2/-diphenylglycine afforded either 24 or an amine by-product [Ph₂NCH₂(3,5-t-Bu₂C₆H₂-O-2)AlMe₂] (26). The known Schiff base complex [2-Ph₂PC₆H4CH₂(3,5-t-Bu₂C₆H₂-O-2)AlMe₂] (27) and the product of the reaction of 2-diphenylphosphinoaniline 1-NH₂,2-PPh₂C₆H₄ with Me₃Al, namely {Ph₂PC₆H₄N[(Me₂Al)₂µ-Me](µ-Me₂Al)} (28) were also isolated. For structural and catalytic comparisons, complexes resulting from interaction of Me₃Al with diphenylamine or benzhydrylamine, namely {Ph₂N[(Me₂Al)₂-Me]} (29) and [Ph₂CHNH(µ-Me₂Al)]₂·MeCN (30), were prepared. The molecular structures of the Schiff pro-ligands derived from Ph₂CHNH₂ and 2,2/-Ph₂C(CO₂H)(NH₂), together with complexes 24, 26 and 28 - 30·MeCN were determined. All complexes were screened for their ability to ROP ε-caprolactone, δ-valerolactone or rac-lactide, in the presence of benzyl alcohol, with or without solvent present.

Chapter 6 describes the reaction of lithium alkoxides LiOR (R = t-Bu, Ph) with the acids 2,2/-Ph₂C(X)(CO₂H), where X = OH, NH₂, i.e. benzilic acid (2,2/-diphenylglycolic acid, benzH) or 2,2/-diphenylglycine (dpgH). In the case of benzH, reaction with one equivalent of LiOt-Bu in THF afforded the complex [Li(benz)(THF)]₂·2THF (31·2THF), which adopts a 1D chain structure. If acetonitrile is employed in the work-up under mild conditions, another solvate of 31 is isolated; use of LiOPh also lead to 31. Use of more robust work-up conditions afforded the complex [Li7(benz)7(MeCN)] (32·2MeCN·THF). Increasing the amount of LiOt-Bu (2 equivalents) led to the isolation of the complex {Li8(Ot-Bu)₂[(benz)](OCPh₂CO₂-CPh₂CO₂t-Bu)₂(THF)₄} (33). In the case of dpgH, use of two equivalents of LiOt-Bu in THF afforded [Li₆(Ot-Bu)₂(dpg)₂(THF)₂] (34), which contains an Li₂O₂ 6-step ladder. Similar reaction of lithium phenoxide with dpg afforded the complex [Li8(PhO)₄(dpg)₄(MeCN)₄] (35). The molecular structures of complexes 31 - 35 are reported; all were screened for their potential to act as pre-catalysts for ROP of ε-caprolactone (ε-CL), rac-lactide (r-LA) and δ-valerolactone (δ-VL).

Chapter 7 describes the reaction of the dialkylzinc reagents R2Zn with the acids 2,2-Ph₂C(X)(CO₂H), where X = NH₂, OH, ie 2,2/-diphenylglycine (dpgH) or benzilic acid (benzH₂). With dpgH, the tetra-nuclear ring complexes [RZn(dpg)]₄, where R = Me (36), Et (37), 2-CF₃C₆H₄ (38), 2,4,6-F₃C₆H₂ (39) were isolated; complex 37 has been previously reported. The crystal structures of 36·2MeCN, 37 and 38·4(C7H8) ·1.59(H₂O) are reported, along with that of the intermediate compound (2-CF₃C₆H₄)₃B·MeCN and the known compound [ZnCl₂(NCMe)₂]. Complexes 36– 39, together with the known [(ZnEt)₃(ZnL)₃(benz)₃] (40; L = MeCN), have been screened, in the presence and absence of benzyl alcohol, for their potential to act as catalysts for the ROP of ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and rac-lactide (rac-LA); the co-polymerization of ε-CL with rac-LA was also studied.

Chapter 8 This chapter presents the experimental section.

Chapter 9 Appendix.

Citation

Al-Khafaji, Y. (2016). New pre-catalysts for ring opening of lactides/lactones based on earth abundant metals. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4219115

• Thesis Type: Thesis
• Deposit Date: Apr 13, 2017
• Publicly Available Date: Feb 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4219115
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Dec 1, 2016