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

Reaction of [LiPb(OPr)] (generated ) with either -butylcalix[4]areneH (LH) or -butylcalix[6]areneH (LH) resulted in the heterometallic lithium/lead complexes [PbLi(L)H(MeCN)]·4.5MeCN (1·4.5MeCN) and [PbLiCl(LH)(L)(OH)(O)(HO)(MeCN)]·14MeCN (2·14MeCN), respectively. Use of the dimethyleneoxa-bridged -butyltetrahomodioxacalix[6]areneH (LH) with five equivalents of [Pb(OPr)] afforded [Pb(L)O(PrOH)]·11MeCN (3·11MeCN). Use of the larger -butylcalix[8]areneH (LH) with [Pb(OBu)] or {Pb[N(TMS)]} (TMS = SiMe) afforded the products [Pb(L)O]·8.7CH (4·8.7CH) or [Pb(SiMe)(L)OCl] (5), respectively. Reaction of {Pb[N(TMS)]} (generated from (MeSi)NH, BuLi and PbCl) with LH afforded, after work-up (MeCN), the mixed-metal complex [PbLi(L)(OH)Cl(O)]·9.5MeCN (6·9.5MeCN). Reaction of distilled {Pb[N(TMS)]} (six equivalents) with LH resulted in the complex [Pb(L)O]·12MeCN (7·12MeCN). Complexes 1-7, Pb(OPr) and [Pb(N(TMS))] 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(OPr) and [Pb(N(TMS))] afforded reasonable conversions and high molecular weight polymers. The systems 1-7, Pb(OPr) and [Pb(N(TMS))] also proved to be active in the ROP of the -lactide (-LA); the activity trend was found to be 1 > 2 ≈ Pb(OPr) ≈ [Pb(N(TMS))] > 4 > 5 ≈ 6 ≈ 7 > 3.