A Hexanuclear Niobium Cluster Compound Crystallizing as Macroscopic Tubes

Strong Cluster-Supported Brønsted Acids: Hexanuclear Niobium Cluster Compounds with Protonated Crown Ether Cations: (Crown-H)2[Nb6Cl12iX6a] (X = Cl or Br) and the Intermediate [Nb6Cl16(H2O)2]·4 dioxane

Six cluster salts which consist of hexanuclear cluster anions [Nb₆Cl₁₂ⁱX₆^a]^2- (X = Cl or Br) and protonated crown ether molecules (15-crown-5 (15cr5) and 12-crown-4 (12cr4)) or crown ether-stabilized oxonium cations as well as one compound consisting of neutral cluster units, [Nb₆Cl₁₆(H₂O)₂]·4 dioxane, were synthesized in good to high yields. The single-crystal X-ray structures of six of these compounds were determined. The cation/anion ratios and the bond distances confirm in all cases oxidized cluster cores with 14 cluster-based electrons. The cations of the cluster salts are either sandwich-type dimers of the formula [(15cr5)H]₂²⁺ or [(15cr5)(H₃O)]₂²⁺ with the protons or oxonium ions embedded in between the crown ether rings or monomeric units in the case of [(12cr4)H]⁺. ¹H NMR investigations show that the cluster salts are strong Brønsted acids. The fact that the cluster core of [Nb₆Cl₁₆(H₂O)₂]·4 dioxane is oxidized but still carries water ligands indicates that within the multi-step reaction sequence of the formation of the cluster-supported acids, the oxidation step happens much faster than the ligand exchange steps. Temperature-dependent ²H MAS NMR spectra of deuterium-exchanged [(15cr5)H]₂[Nb₆Cl₁₈]·2 CHCl₃ are indicative of dynamic processes of the hydrogen-bonded protons within the crown ether molecule.

Cluster Compounds with Oxidised, Hexanuclear [Nb6 Cli 12 Ia 6 ]n- Anions (n=2 or 3)

Four mixed-halide cluster salts with chloride-iodide-supported octahedral Nb6 metal atoms cores were prepared and investigated. The cluster anions have the formula [Nb6 Cli 12 Ia 6 ]n- with Cl occupying the inner ligand sites and I the outer one. They are one- or two-electron-oxidized (n=2 or 3) with respect to the starting material cluster. (Ph4 P)+ and (PPN)+ function as counter cations. The X-ray structures reveal a mixed occupation of the outer sites for only one compound, (PPN)3 [Nb6 Cli 12 Ia 5.047(9) Cla 0.953 ]. All four compounds are obtained in high yield. If in the chemical reactions a mixture of acetic anhydride, CH2 Cl2 , and trimethylsilyl iodide is used, the resulting acidic conditions lead to form the two-electron-oxidised species (n=2) with 14 cluster-based electrons (CBEs). If only acetic anhydride is used, the 15 CBE species (n=3) is obtained in high yield. Interesting intermolecular bonding is found in (Ph4 P)2 [Nb6 Cli 12 Ia 6 ] ⋅ 4CH2 Cl2 with I⋅⋅⋅I halogen bonding and π-π bonding interactions between the phenyl rings of the cations in (PPN)3 [Nb6 Cli 12 Ia 5.047(9) Cla 0.953 ]. The solubility of (Ph4 P)2 [Nb6 Cli 12 Ia 6 ] ⋅ 4CH2 Cl2 has been determined qualitatively in a variety of solvents, and good solubility in the aprotic solvents CH3 CN, THF and CH2 Cl2 has been found.

Nanoarchitectonics of Glass Coatings for Near-Infrared Shielding: From Solid-State Cluster-Based Niobium Chlorides to the Shaping of Nanocomposite Films

The high potential of [{Nb₆Clⁱ₁₂}L^a₆] cluster-based building blocks as near-infrared radiation blockers for energy saving applications is exposed in the present paper (i = inner edge-bridging ligand, a = apical ligand of the Nb₆; L = H₂O and/or Cl). To do so, a combined experimental and theoretical investigation of edge-bridged [{Nb₆Clⁱ₁₂}Cl^a_6-x(H₂O)_x]^m+/0/n- cluster unit series (x = 0, 4, 6; m = 2, 3, 4; n = 2, 3, 4) has been carried out. By using the K₄[{Nb₆Clⁱ₁₂}Cl^a₆] starting solid-state precursor, we explored the behavior of the [{Nb₆Clⁱ₁₂}Cl^a₆]^4- cluster unit during the different steps of its integration as a building block into a polyvinylpyrrolidone (PVP) matrix to form a glass coating composite denoted {Nb₆Clⁱ₁₂}^m+@PVP (m = 2 or 3). The optical, vibrational and redox properties [{Nb₆Clⁱ₁₂}Cl^a_6-x(H₂O)_x]^m+/0/n- building blocks have been interpreted with the support of electronic structure calculations and simulation of properties. The chemical modifications and oxidation properties have been identified and studied thanks to various techniques in solution. Combining Raman and ultraviolet-visible spectroscopies, electrochemistry, and quantum chemical simulations, we bring new knowledge to the understanding of the evolution of the properties of the [{Nb₆Clⁱ₁₂}Cl^a_6-x(H₂O)_x]^m+/0/n- cluster units as a function of the number of valence electron per cluster (VEC) and the nature of terminal ligands (x = 0, n = 4; x = 4, charge = 0; x = 6, m = 4). The fine understanding of the physical properties and vibrational fingerprints depending on the VEC and chemical modifications in solution are mandatory to master the processing of cluster-based building blocks for the controlled design and shaping of glass coating nanocomposites. On the basis of this acquired knowledge, [{Nb₆Clⁱ₁₂}Cl^a_6-x(H₂O)_x]^m+/0/n- building blocks were embedded in a PVP matrix. The resulting {Nb₆Clⁱ₁₂}²⁺@PVP nanocomposite film shows excellent ultraviolet (UV, 280-380 nm) and near-infrared (NIR, 780-1080 nm) blocking ability (>90%) and a highly visible light transmittance thanks to the controlled integration of the {Nb₆Clⁱ₁₂}²⁺ cluster core. The figures of merit (FOM) value of T_vis/T_sol (T_vis = visible transmittance and T_sol = solar transmittance) as well as the haze, clarity, and the NIR shielding values (S_NIR) were measured. After optimization of the integration process, a {Nb₆Clⁱ₁₂}²⁺@PVP nanocomposite on glass substrate has been obtained with a high FOM equal to 1.29. This high value places the transparent green olive {Nb₆Clⁱ₁₂}²⁺@PVP nanocomposites at the top system in the benchmark in the field of glass coating composites for energy-saving applications.

A Hexanuclear Niobium Cluster Compound Crystallizing as Macroscopic Tubes

Compounds with ordered structures in one or two dimensions are exciting materials and investigated intensively in many different areas of science. Many activities have been put into the preparation of low‐dimensional nano‐scaled structures and many compounds in this size regime are known. Contrary, the number of known compounds that have low‐dimensional macroscopic sized structures that form directly in a chemical reaction is very limited. Here, we present the synthesis of the niobium cluster compound [(Et₂O)₂H]₂[Nb₆Cl₁₈], crystals of which grow in form of large hexagonal empty tubes of several centimeter length and diameters in the range of 2 mm. The single‐crystal X‐ray structure of this compound has been refined. Under warming, the compound readily eliminates diethyl ether molecules and decomposes. From a closer look at the crystallization process a step‐by‐step scheme of the procedure of the tube growth is proposed. The overall conclusion from this proposal is that a crucial balance between the cluster solution concentration, the crystal growth speed and the ether diffusion speed results in the formation of macroscopic crystal tubes.