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.

Guest-driven unusual conformations in two calix[6]arene solvates and a new calix[8]arene

Unusual conformations have been found in a new calix[8]arene and in new solvates of two known calix[6]arenes. The chair-like conformation with 2/m point group symmetry was found for the first time in the dimethylformamide (DMF) disolvate of the basic calix[6]arene (1) without substituents in the lower and upper rims. Such symmetry is driven by the guest geometry allowing for two equivalent hydrogen bonding patterns in the chair seat. This avoids cone distortion found in the other chair-like conformers, although they have energies lower than that of new symmetrical conformer. The molecular conformation of hexa(carboxymethoxy)calix[6]arene (2) is also described as a dimethylsulfoxide (DMSO) pentasolvate. Its conformation can be described as a 1,3,5-closed cone with three alternate phenyl rings inclined inwards to the cone, thereby closing the cone entrance. Such a conformation also suggests five acid groups are pointed towards the same side of the calyx base and are able to bind metal ions or basic compounds in the lower rim, while inclusion of guests into the cone cavity is hindered. Both inclusion and cooperative acid binding/coordination abilities are still more hindered in the lowest energy 1,2,3-alternate cone conformer of 2. The role of the solvent in avoiding cone distortion was highlighted by inspecting the conformations of 5,11,17,23,29,35,41,47-octanitro-49,50,51,52,53,54,55,56-octa-n-butoxycalix[8]arene (3) and the known nitro analogues having methyl instead of n-butyl groups. Cone distortion is found in the non-solvated crystal form of 3, while non-classical hydrogen bonds with tetrahydrofuran preclude this in the literature analogue.

Molecular tectonics: high dimensional coordination networks based on methylenecarboxylate-appended tetramercaptothiacalix[4]arene in the 1,3-alternate conformation

Tetrasubstituted methylenecarboxylic-appended tetra-mercaptotetrathiacalix[4]arene (TMTCA), combined with transition metals, lead to high-dimensional coordination polymers.

A ladder-type coordination polymer constructed from two macrocyclic units – calix[4]arene tetracarboxylate and azamacrocyclic nickel(ii) complex

Spatially-separated polar channels and non-polar voids were shown to coexist in the crystal of a new type of coordination polymer formed by a hexaazamacrocyclic nickel(ii) cation and a lower rim substituted calix[4]arene tetracarboxylate trianion.

Molecular tectonics: generation of grid and porous diamondoid coordination networks by calixarene based tectons

Combinations of tetrakis meta-pyridyl appended tetrathia- or tetrathiamercapto-calix[4]arene in 1,3-A conformation with MX₂ (M = Cd, Co or Fe, X = Cl or Br), behaving as a neutral metallatecton, lead to 2D grid type and 3D porous diamondoid coordination networks.