Monocarboxylate-driven structural growth in Calix[n]arene-polyoxotitanate hybrid systems: utility in hydrogen production from water

Sulfur atom-directed metal-ligand synergistic catalysis in zirconium/hafnium-oxo clusters for highly efficient amine oxidation

The performance applications (e.g., photocatalysis) of zirconium (Zr) and hafnium (Hf) based complexes are greatly hindered by the limited development of their structures and the relatively inert metal reactivity. In this work, we constructed two ultrastable Zr/Hf-based clusters (Zr9-TC4A and Hf9-TC4A) using hydrophobic 4-tert-butylthiacalix[4]arene (H4TC4A) ligands, in which unsaturated coordinated sulfur (S) atoms on the TC4A4- ligand can generate strong metal-ligand synergy with nearby active metal Zr/Hf sites. As a result, these two functionalized H4TC4A ligands modified Zr/Hf-oxo clusters, as catalysts for the amine oxidation reaction, exhibited excellent catalytic activity, achieving very high substrate conversion (>99%) and product selectivity (>90%). Combining comparative experiments and theoretical calculations, we found that these Zr/Hf-based cluster catalysts accomplish efficient amine oxidation reactions through synergistic effect between metals and ligands: (i) The photocatalytic benzylamine (BA) oxidation reaction was achieved by the synergistic effect of the dual active sites, in which, the naked S sites on the TC4A4- ligand oxidize the BA by photogenerated hole and oxygen molecules are reduced by photogenerated electrons on the metal active sites; (ii) in the aniline oxidation reaction, aniline was adsorbed by the bare S sites on ligands to be closer to metal active sites and then oxidized by the oxygen-containing radicals activated by the metal sites, thus completing the catalytic reaction under the synergistic catalytic effect of the proximity metal-ligand. In this work, the Zr/Hf-based complexes applied in the oxidation of organic amines have been realized using active S atom-directed metal-ligand synergistic catalysis and have demonstrated very high reactivity.

Encapsulation Enhances the Catalytic Activity of C-N Coupling: Reaction Mechanism of a Cu(I)/Calix[8]arene Supramolecular Catalyst

Development of C-N coupling methodologies based on Earth-abundant metals is a promising strategy in homogeneous catalysis for sustainable processes. However, such systems suffer from deactivation and low catalytic activity. We here report that encapsulation of Cu(I) within the phenanthroyl-containing calix[8]arene derivative 1,5-(2,9-dimethyl-1,10-phenanthroyl)-2,3,4,6,7,8-hexamethyl-p-tert-butylcalix[8]arene (C8PhenMe6 ) significantly enhances C-N coupling activity up to 92 % yield in the reaction of aryl halides and aryl amines, with low catalyst loadings (2.5 % mol). A tailored multiscale computational protocol based on Molecular Dynamics simulations and DFT investigations revealed an oxidative addition/reductive elimination process of the supramolecular catalyst [Cu(C8PhenMe6)I]. The computational investigations uncovered the origins of the enhanced catalytic activity over its molecular analogues: Catalyst deactivation through dimerization is prevented, and product release facilitated. Capturing the dynamic profile of the macrocycle and the impact of non-covalent interactions on reactivity allows for the rationalization of the behavior of the flexible supramolecular catalysts employed.

Ancillary ligand-regulated Ti(IV)-based metallocalixarene coordination cages for photocatalytic H2 evolution

High-valence Ti(IV)-based metallocalixarene coordination cages that are linked by oriented ancillary ligands are unknown so far. Herein, the first family of tunable calixarene-based coordination cages of Ti(IV) with a framework formula [Ti₁₂(OⁱPr)₁₂(TBC[4])₆L₆] have been assembled from six {Ti₂(OⁱPr)₂(TBC[4])}²⁺ nodes and six pyridinedicarboxylic ligands. Furthermore, the {Ti12L6} cage showed strong photocatalytic H₂ evolution activity, and DFT studies were performed to explore its electronic structure.

Heterometallic Polyoxotitanium Clusters as Bifunctional Electrocatalysts for Overall Water Splitting

Incorporating heterometal into titanium-oxygen clusters (TOCs) is an effective way to improve its catalytic activity. Herein, we synthesize three novel heterometallic TOCs with the formula of [Ti₆Cu₂O₇(Dmg)₂(OAc)₄(ⁱPrO)₆][H₂Ti₆Cu₂O₇(Dmg)₂(OAc)₄(ⁱPrO)₈] ({Ti₆Cu₂}), [Ti₈Cu₂O₉(Dmg)₂(OAc)₂(ⁱPrO)₁₂] ({Ti₈Cu₂}), and [Ti₁₀Co₂O₆(Dmg)₂(Pdc)₄(ⁱPrO)₁₈Cl₃] ({Ti₁₀Co₂}, DmgH₂ = dimethylglyoxime; PdcH₂ = pyridine-2,3-dicarboxylic acid) using dimethylglyoxime and different carboxylates as the synergistic ligands. By depositing the clusters {Ti₆Cu₂} and {Ti₁₀Co₂} on carbon cloth as electrodes, we investigated the electrocatalytic performance of TOCs for full water splitting for the first time. To reach a 10 mA cm^-2 current density in an alkaline solution, the {Ti₁₀Co₂}@CC electrode needs an overpotential as low as 120 and 400 mV for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively. In addition, full water-splitting equipment with {Ti₁₀Co₂}@CC as a cathode and an anode need only 1.67 V to deliver a current density of 10 mA cm^-2. Our work confirmed the potential of noble metal-free TOCs as bifunctional cluster-based electrocatalysts for water splitting, and their activities can be tuned by doping with different metal ions.

Heterometallic cobalt(ii) calix[6 and 8]arenes: synthesis, structure and electrochemical activity

Heterometallic cobalt p-tert-butylcalix[6 and 8]arenes have been generated from the in situ reaction of lithium reagents (n-BuLi or t-BuOLi) or NaH with the parent calix[n]arene and subsequent reaction with CoBr₂. The reverse route, involving the addition of in situ generated Li[Co(Ot-Bu)₃] to p-tert-butylcalix[6 and 8]arene, has also been investigated. X-ray crystallography reveals the formation of complicated products incorporating differing numbers of cobalt and lithium or sodium centers, often with positional disorder, as well as, in some cases, the retention of halide. The electrochemical analysis revealed several oxidation events related to the subsequent oxidation of Co(ii) centers and the reduction of the metal cation at negative potentials. Moreover, the electrochemical activity of the phenol moieties of the parent calix[n]arenes resulted in dimerized products or quinone derivatives, leading to insoluble oligomeric products that deposit and passivate the electrode. Preliminary screening for electrochemical proton reduction revealed good activity for a number of these systems. Results suggest that [Co₆Na(NCMe)₆(μ-O)(p-tert-butylcalix[6]areneH)₂Br]·7MeCN (6·7MeCN) is a promising molecular catalyst for electrochemical proton reduction, with a mass transport coefficient, catalytic charge transfer resistance and current magnitude at the catalytic turnover region that are comparable to those of the reference electrocatalyst (Co(ii)Cl₂).

Reactions between p-tert-butylcalix[6 and 8]arenes and lithium or sodium reagents led to complex structures often with positional disorder. Such systems are capable of electrochemical proton reduction.

Self-Assembly of Chiral Ferrocene-Functionalized Polyoxotitanium Clusters for Photocatalytic Selective Sulfide Oxidation

Here, we systematically studied the self-assembly behavior of chiral polyoxytitanium clusters for the first time. Through the cooperative assembly of ferrocenecarboxylic acid and ketoxime ligands, we successfully incorporated the planar chirality of ferrocene (Fc) into the layered {Ti₅} building blocks. The resulting {Ti₅Fc} clusters can be used as structural units to assemble into large ordered structures in various ways; either a pair of {Ti₅Fc} enantiomers are bridged by organic adhesive to form sandwich structures or two homochiral {Ti₅Fc} units participate in the assembly to form the large clusters. Depending on the assembly modes, the chirality of {Ti₅Fc} can be transferred to large nanoclusters or disappear to form mesostructures. The difference of the assembly modes between the {Ti₅Fc} units can also tune the photoelectric activity of the resulting clusters, which has been verified by using {Ti₁₀Fc-6/7} as catalysts for photocatalytic selective sulfide oxidation. This work not only is an important breakthrough in the study of the self-assembly of chiral nanoclusters but also provides an important reference for understanding of chiral transfer on the nanoscale.

Metal-Directed Self-Assembly of {Ti8L2} Cluster-Based Coordination Polymers with Enhanced Photocatalytic Alcohol Oxidation Activity

Cooperative assembly of the neutral cluster {Ti₈O₅(OEt)₁₈L₂} (L = pyrazine-2,3-dicarboxylic acid) with different metal units of Mn(NO₃)₂, CuCl₂, Zn(OEt)₂, Cd(NO₃)₂, Ce(NO₃)₃, Lu(NO₃)₃, and Lu(NO₃)₂(OEt), or the [Cu₂I₂] cluster, generates a family of titanium-oxygen cluster (TOC)-based coordination polymers. These one-dimensional (1D) linear structures contain the same {Ti₈L₂} cluster but with variable bridging metal units. The regulation of the heterometal not only affects the chain geometries of the {MTi₈} but also affects the way the 1D chains are stacked in the crystal lattice. Investigation of the catalytic activities toward alcohol oxidation demonstrated the synergetic effect of combining the metal site and the photosensitive {Ti₈L₂} cluster in the tailored structure. Under light illumination, the {MTi₈} with dual catalytic sites shows greatly enhanced catalytic activity in the selective oxidation of alcohols to aldehydes. Because the compositions and structures of {MTi₈} are highly tunable, this work spotlights the potential of utilizing such metal-bridged multidimensional Ti-oxo materials for cooperative photoredox catalysis for organic transformation.

Cd-Doped Polyoxotitanium Nanoclusters with a Modifiable Organic Shell for Photoelectrochemical Water Splitting

Incorporating heterometal and chromogenic groups into the titanium oxo cluster (TOC) nanomaterials is one of the effective strategies for the development of new high-performance photoelectrically active materials. In this Article, we report the structures and photoelectrochemical (PEC) performances of a family of TOCs, including pure [Ti₁₂O₈(OEt)₁₆L₈] ({Me-Ti₁₂}) and six Cd-doped clusters formulated as [H₄Cd₂Ti₁₀O₈(OEt)₁₆(L)₈(H₂O)₂] ({Cd₂Ti₁₀}; L = salicylic acid and their derivatives). The six Cd-doped clusters are isostructural, containing the same {Cd₂Ti₁₀O₈} core, but are protected by salicylic ligands modified with different functional groups. The compositions, structures, and solution stability of these clusters have been studied in detail by single-crystal X-ray diffraction and electrospray ionization mass spectrometry measurements. The embedding of heterometallic Cd(II) and chemical modification of organic protective shells can effectively regulate the PEC water oxidation activity of those clusters, with {F-Cd₂Ti₁₀} having the highest turnover number of 518.55 and the highest turnover frequency of 172.85 h^-1. Our work highlights the potential of using TOCs that do not contain noble metals as water oxidation catalysts, and their catalytic activity can be regulated by structural modification.

Calixarene-Protected Titanium-Oxo Clusters and Their Photocurrent Responses and Photocatalytic Performances

Three photosensitive tert-butylcalix[n]arene (TBC[n], n = 4, 6, 8)-protected titanium-oxo clusters (TOCs), formulated as [Ti₄(μ₃-O)₂(TBC[4])₂(OⁱPr)₄(DEF)₂]·DEF (1, TBC[4]-Ti₄, DEF = N,N-diethylformamide), [Ti₄(μ₄-O)TBC[6](OCH₃)₉]·H₂O (2, TBC[6]-Ti₄), and [Ti₄(μ₃-O)₂(OⁱPr)₄TBC[8](DEF)₂]·DEF (3, TBC[8]-Ti₄), were successfully synthesized and characterized. Because of the generation of charge transfer from TBC[n] to the TiO core, the three TBC[n]-decorated TOCs show a broadened visible-light absorption and narrowed optical band gap based on the UV-visible spectra and density functional theory calculations. The corresponding photosensitive electrodes prepared using these three TOCs exhibit stable photocurrent responses. Furthermore, their photocatalytic performances for hydrogen evolution and methylene blue degradation were evaluated, and all of the materials display excellent photocatalytic activity and stability. The calixarene-Ti coordination is therefore an effective strategy to enlarge the visible-light absorption band of Ti-O materials and improve their photoelectric/photocatalytic performances.

Titanium complexes bearing oxa- and azacalix[4, 6]arenes: structural studies and use in the ring opening homo-/co-polymerization of cyclic esters

Reaction of excess [Ti(OiPr)₄] with p-tert-butyltetrahomodioxacalix[6]areneH₆ (L¹H₆) afforded, after work-up (MeCN), the complex [Ti₂(OiPr)₂(MeCN)L¹]·3.5MeCN (1·3.5MeCN), whilst the oxo complex [Ti₄(μ₃-O)₂(H₂O)(L¹)₂]·MeCN (2·MeCN) was isolated via a fortuitous synthesis involving the use of two equivalents of [Ti(OiPr)₄]. Reactions of p-methyl-dimethyldiazacalix[6]areneH₆ (L²H₆) with [TiF₄] (four equivalents), [TiCl₄(THF)₂] (two equivalents) or [TiBr₄] (>four equivalents) resulted in the titanium-based azacalix[n]arene complexes [Ti₄F₁₄L²H₂(H)₂]·2.5MeCN (3·2.5MeCN), [Ti₂X₄(H₂O)₂OL²H₂(H)₂] (X = Cl (4·5MeCN), Br (5·4.5MeCN) and [Ti₄Br₁₂L²(H)₂(MeCN)₆]·7MeCN (6·7MeCN), respectively. Reaction of four equivalents of [TiF₄] with L³H₄ (L³H₄ = p-methyl-dimethyldiazacalix[4]areneH₄) afforded the product [Ti₂F₂(μ-F)₃L³(H)₂(SiF₅)]·2MeCN (7·2MeCN). These complexes have been screened for their potential to act as pre-catalysts in the ring opening polymerization (ROP) of ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and rac-lactide (r-LA). Generally, the titanium complexes bearing oxacalixarene exhibited better activities than the azacalixarene-based pre-catalysts. For ε-CL, δ-VL and r-LA, moderate activity at 130 °C over 24 h was observed for 1-6. In the case of the co-polymerization of ε-CL with r-LA, 1-6 afforded reasonable conversions and high molecular weight polymers; 7 exhibited lower catalytic performance due to low solubility. None of the complexes proved to be active in the polymerization of ω-pentadecalactone (ω-PDL) under the conditions employed herein.

Accurate assembly of ferrocene-functionalized {Ti22Fc4} clusters with photocatalytic amine oxidation activity

We report here the synthesis of a ferrocene-functionalized {Ti22Fc4} cluster with a 'dimer-of-clusters' topology, which represents the largest Ti-oxo cluster (TOC) modified with organometallic groups ever reported. The exact assembly path of {Ti22Fc4} can be inferred from its two substructures, {Ti11Fc2} and {Ti5Fc}, which can also be synthesized independently through subtle changes in reaction conditions. Furthermore, we used these clusters as photocatalysts, and have studied, for the first time, the photocatalytic activity of TOCs in the oxidative coupling of amines.

Microwave-Assisted Synthesis of Tris-Anderson Polyoxometalates for Facile CO2 Cycloaddition

Four new tris-Anderson polyoxometalates (POMs), (NH₄)₄[ZnMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (1), (NH₄)₄[CuMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (2), (TBA)₃(NH₄)[ZnMo₆O₁₇(C₅H₉O₃)₂(OH)]·10H₂O (3) (TBA = n-C₁₆H₃₆N), and (NH₄)₄[CuMo₆O₁₈(C₅H₉O₃)₂]·16H₂O (4), were synthesized by a microwave-assisted method. Single-crystal X-ray diffraction revealed that 1 and 2 contained a tris (trihydroxyl organic compounds) ligand grafted on one side, while two tris ligands were grafted on two sides to form χ/δ and δ/δ isomers in 3 and 4, respectively. ¹H and ¹³C NMR spectra of the χ/δ isomer 3 were obtained for the first time, with six methylenes showing six peaks in the ¹H NMR spectrum and only four peaks in the ¹³C NMR spectrum. Mass spectrometry monitoring revealed that during the microwave-assistant process the tris ligand can graft onto POMs to form 1, while tris directly coordinates with metallic heteroatoms to form isopolymolybdates during the conventional reflux synthesis process. In addition, 1-4 can catalyze CO₂ with epoxides into cyclic carbonates with high selectivity and yields at an atmospheric pressure of CO₂, which is lower than the pressure of CO₂ in other catalysis using POMs as catalysts. Furthermore, 1-4 showed good catalytic stability and cycling properties. Mechanism studies substantiated POMs cocatalyzed with Br^- to improve the catalytic yields.