p-tert-Butylcalix[8]arene: An Extremely Versatile Platform for Cluster Formation

Construction of zirconium/hafnium-oxo clusters based on a new protection-calix[8]arene

Calix[8]arene has been used as a promising type of macrocyclic ligand for the construction of multinuclear metal-oxo clusters (MOCs), but not for zirconium/hafnium-oxo clusters (Zr/HfOCs). In this paper, we report the first series of ZrOCs (HfOCs) based on calix[8]arene: Zr4, Zr8, Hf4, and Hf8. Zr8/Hf8 has a rhombohedral conformation and can be regarded as a derivative of the octahedral Zr6 cluster. Remarkably, I2 adsorption experiments indicate that Zr4 (Zr8) adsorbs much faster than Hf4 (Hf8). Density functional theory (DFT) calculations show that metallic Zr atoms interact more strongly with I2 than metallic Hf atoms. The successful application of calix[8]arene for the synthesis of well-defined ZrOCs (HfOCs) shows a bright future for MOCs protected by macrocyclic ligands.

Tandem templating strategies facilitate the assembly of calix[8]arene-supported Ln18 clusters

Calix[n]arenes offer ideal chemical functionality through the polyphenolic lower rim to construct nano-sized coordination clusters with lanthanide (Ln) metal ions (e.g., NdIII10, GdIII8). However, the number of metal centers they can accommodate is still limited compared to that achievable with smaller ligands (e.g., GdIII140, GdIII104). Here, we exploit a combination of the "anion template strategy" and "templating ligands" to synthesise three highly symmetric (D3h, trigonal planar) LnIII18 (Ln = La, Nd, and Gd) systems, representing the largest calix[n]arene-based coordination clusters yet. The LnIII18 fragment is templated by a chloride anion located at the center of the cluster, wherefrom twelve μ3-OH- ligands bind 'internally' to the eighteen LnIII ions. 'Externally' the metallic skeleton is connected by p-tert-butylcalix[8]arene, oxo, chloro and carbonate ligands. The crystal packing in the lattice reveals large cylindrical channels of ∼26 Å in diameter, whose pore volume corresponds to ∼50% of the unit cell volume (using a 1.2 Å spherical probe radius). Magnetic measurements reveal the predominance of weak antiferromagnetic exchange in the Gd analog. Heat capacity data of GdIII18 reveal a high magnetic entropy with -ΔSm = 23.7 J K-1 kg-1, indicating potential for engineering magnetic refrigerant materials with calix[8]arenes.

Electron delocalization of robust high-nuclear bismuth-oxo clusters for promoted CO2 electroreduction

The integration of high activity, selectivity and stability in one electrocatalyst is highly desirable for electrochemical CO2 reduction (ECR), yet it is still a knotty issue. The unique electronic properties of high-nuclear clusters may bring about extraordinary catalytic performance; however, construction of a high-nuclear structure for ECR remains a challenging task. In this work, a family of calix[8]arene-protected bismuth-oxo clusters (BiOCs), including Bi4 (BiOC-1/2), Bi8Al (BiOC-3), Bi20 (BiOC-4), Bi24 (BiOC-5) and Bi40Mo2 (BiOC-6), were prepared and used as robust and efficient ECR catalysts. The Bi40Mo2 cluster in BiOC-6 is the largest metal-oxo cluster encapsulated by calix[8]arenes. As an electrocatalyst, BiOC-5 exhibited outstanding electrochemical stability and 97% Faraday efficiency for formate production at a low potential of -0.95 V vs. RHE, together with a high turnover frequency of up to 405.7 h-1. Theoretical calculations reveal that large-scale electron delocalization of BiOCs is achieved, which promotes structural stability and effectively decreases the energy barrier of rate-determining *OCHO generation. This work provides a new perspective for the design of stable high-nuclear clusters for efficient electrocatalytic CO2 conversion.

Oxidation state variation in bis-calix[4]arene supported decametallic Mn clusters

The reaction of MnCl₂·4H₂O, H₈L (2,2'-bis-p-^tBu-calix[4]arene) and NEt₃ in a dmf/MeOH solvent mixture results in the formation of a mixed valent decametallic cluster of formula [MnII6MnIII4(L)₂(μ₃-OH)₄(μ-OH)₄(MeOH)₄(dmf)₄(MeCN)₂]·MeCN (3). Complex 3 crystallises in the monoclinic space group P2₁/n with the asymmetric unit comprising half of the compound. Structure solution reveals that the bis-calix[4]arene ligands are arranged such that one TBC[4] moiety in each has undergone inversion in order to accommodate a [MnIII4MnII6] metallic skeleton that describes three vertex-sharing [MnIII2MnII2] butterflies. The structure is closely related to the species [MnIII6MnII4(L)₂(μ₃-O)₂(μ₃-OH)₂(μ-OMe)₄(H₂O)₄(dmf)₈]·4dmf (4), the major difference being the oxidation level of the Mn ions in the core of the compound. DFT calculations on the full structures reveal that replacing the Mn^III ions in 4 for Mn^II ions in 3 results in a significant decrease in the magnitude of some antiferromagnetic exchange contributions, a switch from ferromagnetic to antiferromagnetic in others, and the loss of significant spin frustration.

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.

4f-Metal Clusters Exhibiting Slow Relaxation of Magnetization: A {Dy7} Complex with An Hourglass-like Metal Topology

The reaction between Dy(NO₃)₃∙6H₂O and the bulky Schiff base ligand, N-naphthalidene-2-amino-5-chlorobenzoic acid (nacbH₂), in the presence of the organic base NEt₃ has led to crystallization and structural, spectroscopic and magnetic characterization of a new heptanuclear [Dy₇(OH)₆(OMe)₂(NO₃)_1.5(nacb)₂(nacbH)₆(MeOH)(H₂O)₂](NO₃)_1.5 (1) compound in ~40% yield. Complex 1 has a unique hourglass-like metal topology, among all previously reported {Dy₇} clusters, comprising two distorted {Dy₄(μ₃-OH)₃(μ₃-OMe)}⁸⁺ cubanes that share a common metal vertex (Dy2). Peripheral ligation about the metal core is provided by the carboxylate groups of four η¹:η¹:η¹:μ single-deprotonated nacbH⁻ and two η¹:η¹:η²:η¹:μ₃ fully-deprotonated nacb²⁻ ligands. Complex 1 is the first structurally characterized 4f-metal complex bearing the chelating/bridging ligand nacbH₂ at any protonation level. Magnetic susceptibility studies revealed that 1 exhibits slow relaxation of magnetization at a zero external dc field, albeit with a small energy barrier of ~5 K for the magnetization reversal, most likely due to the very fast quantum-tunneling process. The combined results are a promising start to further explore the reactivity of nacbH₂ upon all lanthanide ions and the systematic use of this chelate ligand as a route to new 4f-metal cluster compounds with beautiful structures and interesting magnetic dynamics.

Constructing [CoII6] hexagon-centered heterometallic {Ln6Co6} (Ln = Y, Eu and Dy) clusters with a calix[8]arene ligand

Three isostructural 3d–4f polynuclear clusters {(LnIII6CoII6)(C8A)₃} (Ln = Y, Eu, Dy; C8A = p-tert-butylcalix[8]arene) feature some tripod-like entities in which three Ln₂-C8A SBUs bolster a [Co₆] hexagon.

A [Ce21] keplerate

The solvothermal reaction between Ce(NO₃)₃·6H₂O, 2-amino-isobutyric acid, 2-hydroxy-1-naphthaldehyde and 2-amino-2-methyl-1,3-propanediol in MeOH, in the presence of base, leads to the formation of a unique [CeCe ] keplerate cage.

Halogen dependent symmetry change in two series of wheel cluster organic frameworks built from La18 tertiary building units

Two series of wheel cluster organic frameworks (WCOFs) built from La18 tertiary building units are hydrothermally made, which show halogen-dependent structural symmetry, and demonstrate different chiral performances.

Structural characterization of inclusion complexes of para-sulphonato-calix[8]arene with 1,2-bis(4-pyridyl)-ethane and 1,3-bis(4-pyridyl)-propane. New ‘double cone’ and ‘up–flat–down’ conformations of para-sulphonato-calix[8]arene

Two new conformations of para-sulphonato-calix[8]arene have been found in supramolecular complexes with BPE and BPP, the gas take-up has been demonstrated for S8C–BPE complex.

A new family of Ln7 clusters with an ideal D3h metal-centered trigonal prismatic geometry, and SMM and photoluminescence behaviors

A new family of emissive Ln₇ clusters with ideal D_3h metal-centered trigonal prismatic topology and SMM behavior has been obtained.

Heptanuclear lanthanide [Ln7] clusters: from blue-emitting solution-stable complexes to hybrid clusters

A family of heptanuclear lanthanide [Ln₇] complexes was isolated and fully characterized by magnetic measurements, NMR and absorption–emission spectroscopy.