Trinuclear zinc calix[4]arenes: synthesis, structure, and ring opening polymerization studies

The trinuclear zinc calix[4]arene complexes [Zn₃(O₂CCH₃)₂(L(O)₂(OMe)₂)₂·xMeCN (x = 7.5, 1; x = 6, 1'), [Zn₃(O₂CCH₃)₂(L(O)₂(OnPr)₂)₂·5MeCN (2·5MeCN), [Zn₃(OEt)₂(L(O)₂(OMe)₂)₂]·4MeCN (3·4MeCN), [Zn₃(OEt)₂(L(Opentyl)₂)₂]·4.5MeCN (4·4.5MeCN) and [Zn₃(OH)₂(L(O)₂(On-pentyl)₂]·8MeCN (5·8MeCN) have been isolated from reaction of [(ZnEt)₂(L(O)₂(OR)₂)₂] (L(OH)₂(OR)₂ = 1,3-dialkoxy-4-tert-butylcalix[4]arene; R = methyl, n-propyl or pentyl) and the reagents acetic acid, ethanol, and presumed adventitious water, respectively. Attempts to make 5via a controlled hydrolysis led only to the isolation of polymorphs of (L(OH)₂(Opentyl)₂·MeCN. Reaction of [Zn(C₆F₅)₂] with L(OH)₂(Opentyl)₂, in the presence of K₂CO₃, led to the isolation of the complex [Zn₆(L(On-pentyl))₂(OH)₃(C₆F₅)₃(NCMe)₃]·3MeCN (6·3MeCN). The molecular structures of 1-6 reveal they all contain a near linear (163 to 179°) Zn₃ motif. In 1-5, a central tetrahedral Zn centre is flanked by trigonal bipyramidal Zn centres, whilst in 6, for the linear Zn₃ unit, a central distorted octahedral zinc centre is flanked by trigonal planar and a tetrahedral zinc centres. Screening for the ring opening polymerization (ROP) of ε-caprolactone at 90 °C revealed that they are active with moderate to good conversion affording low to medium molecular weight products with at least two series of ions. For comparative studies, the trinuclear aminebis(phenolate) complex [Zn₃(Oi-Pr)₂L^/] (L^/ = n-propylamine-N,N-bis(2-methylene-4,6-di-tert-butylphenolate) I was prepared. Kinetics revealed the rate order I > 4 > 6 ≈ 2 ≈ 1 > 3.

Lead calix[n]arenes (n = 4, 6, 8): structures and ring opening homo-/co-polymerization capability for cyclic esters

Reaction of [LiPb(OiPr)₃]₂ (generated in situ) with either p-tert-butylcalix[4]areneH₄ (L⁴H₄) or p-tert-butylcalix[6]areneH₆ (L⁶H₆) resulted in the heterometallic lithium/lead complexes [Pb₄Li₂(L⁴)₄H₆(MeCN)₃]·4.5MeCN (1·4.5MeCN) and [Pb₈Li₁₀Cl₂(L⁶H₂)₃(L⁶)(OH)₂(O)₂(H₂O)₂(MeCN)₄]·14MeCN (2·14MeCN), respectively. Use of the dimethyleneoxa-bridged p-tert-butyltetrahomodioxacalix[6]areneH₆ (L^6'H₆) with five equivalents of [Pb(OiPr)₂] afforded [Pb₁₃(L^6')₃O₄(iPrOH)]·11MeCN (3·11MeCN). Use of the larger p-tert-butylcalix[8]areneH₈ (L⁸H₈) with [Pb(OtBu)₂] or {Pb[N(TMS)₂]} (TMS = SiMe₃) afforded the products [Pb₁₂(L⁸)₂O₄]·8.7C₇H₈ (4·8.7C₇H₈) or [Pb₆(SiMe₃)₂(L⁸)O₂Cl₂] (5), 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 [Pb₁₀Li₂(L⁶)₂(OH)Cl(O)₄]·9.5MeCN (6·9.5MeCN). Reaction of distilled {Pb[N(TMS)₂]} (six equivalents) with L⁸H₈ resulted in the complex [Pb₁₂(L⁸)₂O₄]·12MeCN (7·12MeCN). Complexes 1-7, Pb(OiPr)₂ and [Pb(N(TMS)₂)₂] 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 1 and 2 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 1-7. In the case of the co-polymerization of ε-CL with δ-VL, 1-7, Pb(OiPr)₂ and [Pb(N(TMS)₂)₂] afforded reasonable conversions and high molecular weight polymers. The systems 1-7, Pb(OiPr)₂ and [Pb(N(TMS)₂)₂] also proved to be active in the ROP of the rac-lactide (r-LA); the activity trend was found to be 1 > 2 ≈ Pb(OiPr)₂ ≈ [Pb(N(TMS)₂)₂] > 4 > 5 ≈ 6 ≈ 7 > 3.

Scandium calix[n]arenes (n = 4, 6, 8): structural, cytotoxicity and ring opening polymerization studies

Interaction of [Sc(OR)3] (R = iPr or triflate) with p-tert-butylcalix[n]arenes, where n = 4, 6, or 8, affords a number of intriguing structural motifs, which are relatively non-toxic (cytotoxicity evaluated against cell lines HCT116 and HT-29) and a number were capable of the ring opening polymerization (ROP) of cyclohexene oxide.

INSIGHTS into the structures adopted by titanocalix[6 and 8]arenes and their use in the ring opening polymerization of cyclic esters

Interaction of p-tert-butylcalix[6]areneH6, L1H6, with [TiCl4] afforded the complex [Ti2Cl3(MeCN)2(OH2)(L1H)][Ti2Cl3(MeCN)3(L1H)]·4.5MeCN (1·4.5MeCN), in which two pseudo-octahedral titanium centres are bound to one calix[6]arene. A similar reaction but employing THF resulted in the THF ring-opened product [Ti4Cl2(μ3-O)2(NCMe)2(L)2(O(CH2)4Cl)2]·4MeCN (2·4MeCN), where LH4 = p-tert-butylcalix[4]areneH4. Interaction of L1H6 with [TiF4] (3 equiv.) led, after work-up, to the complex [(TiF)2(μ-F)L1H]2·6.5MeCN (3·6.5MeCN). Treatment of p-tert-butylcalix[8]areneH8, L2H8, with [TiCl4] led to the isolation of the complex [(TiCl)2(TiClNCMe)2(μ3-O)2(L2)]·1.5MeCN (4·1.5MeCN). From a similar reaction, a co-crystallized complex [Ti4O2Cl4(MeCN)2(L2)][Ti3Cl6(MeCN)5(OH2)(L2H2)]·H2O·11MeCN (5·H2O 11MeCN) was isolated. Extension of the L2H8 chemistry to [TiBr4] afforded, depending on the stoichiometry, the complexes [(TiBr)2(TiBrNCMe)2(μ3-O)2(L2)]·6MeCN (6·6MeCN) or [[Ti(NCMe)2Br]2[Ti(O)Br2(NCMe)](L2)]·7.5MeCN (7·7.5MeCN), whilst use of [TiF4] afforded complexes containing Ca2+ and Na+, thought to originate from drying agents, namely [Ti8CaF20(OH2)Na2(MeCN)4(L2)2]·14MeCN (8·14MeCN), [Na(MeCN)2][Ti8CaF20NaO16(L2)2]·7MeCN (9·7MeCN) or [Na]6[Ti8F20Na(MeCN)2(L2)][Ti8F20Na(MeCN)0.5(L2)]·15.5(C2H3N) (10·15.5MeCN). In the case of [TiI4], the ladder [(TiI)2(TiINCMe)2(μ3-O)2(L2)]·7.25CH2Cl2 (11·7.25CH2Cl2) was isolated. These complexes have been screened for their potential to act as catalysts in the ring opening polymerization (ROP) of ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and rac-lactide (r-LA), both in air and N2. For ε-CL and δ-VL, moderate activity at 130 °C over 24 h was observed for 1, 9 and 11; for r-LA, only 1 exhibited reasonable activity. In the case of the co-polymerization of ε-CL with δ-VL, the complexes 1 and 11 afforded reasonable conversions and low molecular weight polymers, whilst 4, 6, and 9 were less effective. None of the complexes proved to be active in the co-polymerization of ε-CL and r-LA under the conditions employed herein.

Use of titanocalix[4]arenes in the ring opening polymerization of cyclic esters

Titanocalix[4]arenes are shown to be active for the ROP of cyclic esters under N₂ or air with the mono-methoxy complex [Ti(NCMe)Cl(p-tert-butylcalix[4]arene(O)₃(OMe))] performing best.

Mono-oxo molybdenum(vi) and tungsten(vi) complexes bearing chelating aryloxides: synthesis, structure and ring opening polymerization of cyclic esters

Mono-oxo Mo and W complexes bearing di- or tetra-phenolates were active for the ROP of small cyclic esters, albeit with poor control. Related metallocalix[4]arenes were less active.

Metallocalixarene catalysts: α-olefin polymerization and ROP of cyclic esters

This perspective review discusses metallocalix[n]arene complexes that have been employed in either α-olefin polymerization or in the ring opening polymerization (ROP) of cyclic esters over the last 5 years.