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

Abstract

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 (L6′H6) with five equivalents of [Pb(OiPr)2] afforded [Pb13(L6′)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.