Conversion of Metal Pyrazolate/(Hydr)oxide Clusters into Nanojars: Solution vs Solid-State Structure and Magnetism

Nanojars are a class of anion binding and extraction agents composed of a series of [Cu(μ-OH)(μ-pz)]n (pz = pyrazolate; n = 26-36) supramolecular metal-organic complexes. In contrast to other anion binding agents amenable to liquid-liquid extraction, nanojars only form by self-assembly around the target anion, and guest-free nanojar hosts cannot be isolated. An extraordinary binding strength toward highly hydrophilic anions such as carbonate and sulfate was demonstrated by the inability of Ba2+ ions to precipitate the corresponding insoluble barium salts from nanojars. Herein, we provide an additional proof for the superior robustness of the nanojar framework based on competition experiments with other transition metal pyrazolate/(hydr)oxide complexes. In addition to the mass spectrometric characterization, we present variable-temperature nuclear magnetic resonance studies with an emphasis on the influence of the paramagnetic Cu2+ centers on 1H hyperfine shifts, along with X-ray crystallographic analysis of two polymorphs of (MePh3P)2[CO3⊂{Cu(OH)(pz)}27], including the highest (cubic) symmetry nanojar crystal lattice obtained to date as well as magnetism studies for the first time. Furthermore, we provide evidence for the first molybdate-incarcerating nanojars, [MoO4⊂{Cu(μ-OH)(μ-pz)}n]2- (n = 28, 31-33), formed by rearrangement from [MoVI8O12(μ-O)9(μ-pz)6(pzH)6·3pzH] in the presence of Cu2+ ions.

Structural Chemistry of Titanium (IV) Oxo Clusters, Part 2: Clusters without Carboxylate or Phosphonate Ligands

Homometallic titanium oxo clusters (TOC) are one of the most important groups of metal oxo clusters. In a previous article, TOC structures with carboxylato and phosphonato ligands were reviewed and categorized. This work is now extended to clusters with other ligands. Comparison of the different cluster types shows how the interplay between condensation of the titanium polyhedra by means of bridging oxygen atoms and the coordination characteristics of the ligands influences the cluster structures and allows working out basic construction principles of the cluster core.

Photoactivation of titanium-oxo cluster [Ti6O6(OR)6(O2C t Bu)6]: mechanism, photoactivated structures, and onward reactivity with O2 to a peroxide complex

The molecular titanium-oxo cluster [Ti₆O₆(OⁱPr)₆(O₂C ^t Bu)₆] (1) can be photoactivated by UV light, resulting in a deeply coloured mixed valent (photoreduced) Ti (iii/iv) cluster, alongside alcohol and ketone (photooxidised) organic products. Mechanistic studies indicate that a two-electron (not free-radical) mechanism occurs in this process, which utilises the cluster structure to facilitate multielectron reactions. The photoreduced products [Ti₆O₆(OⁱPr)₄(O₂C ^t Bu)₆(sol)₂], sol = ⁱPrOH (2) or pyridine (3), can be isolated in good yield and are structurally characterized, each with two, uniquely arranged, antiferromagnetically coupled d-electrons. 2 and 3 undergo onward oxidation under air, with 3 cleanly transforming into peroxide complex, [Ti₆O₆(OⁱPr)₄(O₂C ^t Bu)₆(py)(O₂)] (5). 5 reacts with isopropanol to regenerate the initial cluster (1) completing a closed cycle, and suggesting opportunities for the deployment of these easily made and tuneable clusters for sustainable photocatalytic processes using air and light. The redox reactivity described here is only possible in a cluster with multiple Ti sites, which can perform multi-electron processes and can adjust its shape to accommodate changes in electron density.

Coordination-Delayed-Hydrolysis Method for the Synthesis and Structural Modulation of Titanium-Oxo Clusters

ConspectusAtomically precise titanium-oxo clusters (TOCs) are the structure and reactivity model compounds of technically important TiO₂ materials, which could help build structure-property relationships and achieve property modulation at the molecular level. However, the traditional formation of TOCs has relied on the poorly controllable hydrolysis of titanium alkoxide in the solvent for a long time, limiting the development of TOC structural chemistry to a great extent. In addition, easily hydrolyzable alkoxy groups would be still coordinated on the surface of the TOCs generated by this method, making the clusters sensitive and unstable to the moisture. To achieve controllable preparation of TOCs, we believe it is crucial to attenuate the hydrolysis of titanium ions in the formation process of a cluster. To this end, we have recently applied an effective coordination-delayed-hydrolysis (CDH) strategy for TOC synthesis, which provides powerful tools for tuning their structures.In this Account, at the beginning, a brief introduction to the coordination-delayed-hydrolysis strategy is supplied, and its predominant features for constructing novel TOCs are highlighted. In subsequent sections, we discuss how the applied chelating organic/inorganic ligands (named hydrolysis delayed ligands) influence the hydrolysis process of Ti⁴⁺ ions to form a large family of TOCs with various nuclearities and core structures. Various hydrolysis delayed ligands have been explored, ranging from common O-donor ligands (carboxylate, phenol, or sulfate) to rarely used N-donor ligands (pyrazole) or bifunctional O/N-donor ones (quinoline, oxime, or alkanolamine). Breakthroughs in the symmetry, configuration, and cluster nuclei of TOCs have been accordingly achieved. Then, we show that this CDH method can be used to tune the surface structure of TOCs by modifying functional organic ligands. As a result, the physicochemical properties of TOCs, especially optical band gaps, can be optimized, and their stability under ambient conditions is significantly improved. In addition, we illustrate that the reversible bonds between hydrolysis delayed ligands and Ti ions further allows us to introduce active heterometal ions or clusters upon or inside the Ti-O cores to prepare heterometallic TOCs with unprecedented structures and properties. In particular, noble metal (Ag ions or clusters) has been incorporated into Ti-O clusters for the first time. As a summary, the coordination-delayed-hydrolysis strategy has realized the controllable hydrolysis of Ti⁴⁺ ions to some extent, breaking through the limitations of traditional synthesis methods and producing fruitful results in the field of titanium-oxo clusters. It is believed that this CDH method would also be effective for synthesizing oxo clusters of other easily hydrolyzed metal ions (Al³⁺, Sn⁴⁺, In³⁺, etc.) to afford significant contribution for the cluster community.

Organic-Inorganic High-Valence Sn18-oxo Clusters: Direct Utilization of an Inorganic Sn(IV) Source to Improve the Nuclearity and Electrocatalytic CO2 Reduction Properties

The high-valence tin-oxo clusters are of great significance because of their structural diversity and potential applications in many fields, e.g., catalysis, extreme ultraviolet (EUV) lithography, and so on. The synthesis of high-nuclearity tin-oxo clusters remains a great challenge currently, since the key inorganic Sn_xO_y core with Sn⁴⁺ ions could not be obtained only by the in situ Sn-C bond cleavage in organic tin sources. In this context, we synthesize three organic-inorganic hybrid Sn₁₈-oxo clusters, [(BuSn)₁₂Sn₆(μ₃-O)₂₀(ba)₁₂(PhPO₃)₄] (Bu = butyl, Hba = benzoic acid), [(BuSn)₁₂Sn₆(μ₃-O)₂₀(pmba)₁₂(PhPO₃)₄]·2CH₃CN·2H₂O (Hpmba = p-toluic acid), and [(BuSn)₁₂Sn₆(μ₃-O)₂₀(ptba)₁₂(PhPO₃)₄]·2CH₃CN·2iPrOH·2H₂O (Hptba = p-tert-butyl benzoic acid), as well as one Sn₆-oxo cluster [(BuSn)₆(μ₃-O)₂(μ₂-OH)₄(pnba)₆(PhPO₃)₂] (Sn₆) (Hpnba = p-nitrobenzoic acid) by combining an inorganic precursor (SnCl₄) with an organic one (butyltin hydroxide oxide). It is shown that an inorganic dicyclo-chain-like Sn₆O₈ core encapsulated in a U-shaped dodecanuclear butyltin-oxo ring plays an important role in the construction of Sn₁₈-oxo clusters and that the use of a ligand with an electron-withdrawing group reduces the nuclearity of clusters to Sn₆. Moreover, electrocatalytic CO₂ reduction studies confirm that the electrocatalytic activities of the Sn₁₈ clusters are superior to those of the Sn₆ cluster, probably due to the hybrid organotin-inorganotin structures. Our work not only opens a new way for constructing high-nuclearity tin-oxo clusters but also is helpful in deeply revealing the structure-properties relationship of tin-oxo clusters.

Synthesis, Structure, and Light Absorption Behaviors of Prismatic Titanium-Oxo Clusters Containing Lacunary Lindqvist-like Species

Exploring new structural types of polyoxotitanium clusters (PTCs), especially those containing classical polyoxometalates structures, has always been the focus of research in the field of metal-oxo clusters. In this work, we present the synthesis and characterization of three prismatic PTCs: namely, Ti₈(μ₂-O)₃(μ₄-O)₂(OnPr)₆(HOnPr)₂(L1)₈ (PTC-237; H₂L1 = 3,5-di-tert-butylcatechol), Ti₁₂(μ₂-O)₆(μ₃-O)₈(OnPr)₆(L2)₁₂(L3)₂ (PTC-238; HL2 = 1-adamantanecarboxylic acid, HL3 = 2-picolinic acid), and [Ti₁₈(μ₂-O)₄(μ₃-O)₁₆(μ₅-O)₂(OiPr)₁₈(L3)₈](L3)₂ (PTC-239). Single-crystal X-ray diffraction analyses indicate that the construction of these prismatic PTCs is based on a stepwise interlayer assembly of {Ti₃} and {Ti₄} substructures. The diameters of their core skeletons are in the range between 0.9 and 1.3 nm. In particular, lacunary Linqvist-like {Ti₄} and {Ti₅} building units are found to exist in the structures of PTC-237 and PTC-239. According to the solid-state UV-vis diffuse reflectance measurements, the absorption band of 3,5-di-tert-butylcatecholate-functionalized PTC-237 shifts toward the visible-light region, giving a smaller optical band gap of 1.56 eV in comparison to PTC-238 (3.36 eV) and PTC-239 (3.25 eV).

Single-Crystal Syntheses and Properties of Indium-Organic Frameworks Based on 1,1'-Ferrocenedicarboxylic Acid

Presented here are a series of indium-organic frameworks synthesized by the self-assembly of In³⁺ salts and 1,1'-ferrocenedicarboxylic acid (H₂FcDCA). Nitrogen-containing organic additives played various roles in the diversity of the structures. These compounds exhibit diverse frameworks with rich supramolecular interactions, which show good photoelectronic and redox activity together with active FcDCA ligands. Moreover, the indium-based MIL-53 analogue exhibited permanent porosity and gas separation.

Synthesis, Structural, and Physicochemical Characterization of a Ti6 and a Unique Type of Zr6 Oxo Clusters Bearing an Electron-Rich Unsymmetrical {OON} Catecholate/Oxime Ligand and Exhibiting Metalloaromaticity

The chelating catechol/oxime ligand 2,3-dihydroxybenzaldehyde oxime (H₃dihybo) has been used to synthesize one titanium(IV) and two zirconium(IV) compounds that have been characterized by single-crystal X-ray diffraction and ¹H and ¹³C NMR, solid-state UV-vis, and ESI-MS spectroscopy. The reaction of TiCl₄ with H₃dihybo and KOH in methanol, at ambient temperature, yielded the hexanuclear titanium(IV) compound K₂[Ti^IV₆(μ₃-O)₂(μ-O)₃(OCH₃)₄(CH₃OH)₂(μ-Hdihybo)₆]·CH₃OH (1), while the reaction of ZrCl₄ with H₃dihybo and either ⁿBu₄NOH or KOH also gave the hexanuclear zirconium(IV) compounds 2 and 3, respectively. Compounds 1-3 have the same structural motif [M^IV₆(μ₃-Ο)₂(μ-Ο)₃] (M = Ti, Zr), which constitutes a unique example with a trigonal-prismatic arrangement of the six zirconium atoms, in marked contrast to the octahedral arrangement of the six zirconium atoms in all the Zr₆ clusters reported thus far, and a unique Zr₆ core structure. Multinuclear NMR solution measurements in methanol and water proved that the hexanuclear clusters 1 and 3 retain their integrity. The marriage of the catechol moiety with the oxime group in the ligand H₃dihybo proved to be quite efficient in substantially reducing the band gaps of TiO₂ and ZrO₂ to 1.48 and 2.34 eV for the titanium and zirconium compounds 1 and 3, respectively. The application of 1 and 3 in photocurrent responses was investigated. ESI-MS measurements of the clusters 1 and 3 revealed the existence of the hexanuclear metal core and also the initial formation of trinuclear M₃ (M = Ti, Zr) building blocks prior to their self-assembly into the hexanuclear M₆ (M = Ti, Zr) species. Density functional theory (DFT) calculations of the NICS_zz scan curves of these systems revealed that the triangular M₃ (M = Ti, Zr) metallic ring cores exhibit pronounced metalloaromaticity. The latter depends upon the nature of the metallic center with NICS_zz(1) values equal to -30 and -42 ppm for the Ti (compound 1) and Zr (compound 2) systems, respectively, comparable to the NICS_zz(1) value of the benzene ring of -29.7 ppm calculated at the same level of theory.