“Chimie douce”: A land of opportunities for the designed construction of functional inorganic and hybrid organic-inorganic nanomaterials

“Chimie douce” based strategies allow, through the deep knowledge of materials chemistry and processing, the birth of the molecular engineering of nanomaterials. This feature article will highlight some of the main research accomplishments we have performed during the last years. We describe successively the design and properties of: sol–gel derived hybrids, Nano Building Blocks (NBBs) based hybrid materials, nanostructured porous materials proceeds as thin films and ultra-thin films, aerosol processed mesoporous powders and finally hierarchically structured materials. The importance of the control of the hybrid interfaces via the use of modern tools as DOSY NMR, SAXS, WAXS, Ellipsometry that are very useful to evaluate in situ the hybrid interfaces and the self-assembly processes is emphasized. Some examples of the optical, photocatalytic, electrochemical and mechanical properties of the resulting inorganic or hybrid nanomaterials are also presented.     

Chemistry-structure-simulation or chemistry-simulation-structure sequences? The case of MIL-34, a new porous aluminophosphate

A new aluminophosphate, MIL-34, is investigated from its as-synthesized structure to its calcined microporous form. Single-crystal X-ray diffraction measurements on the as-synthesized MIL-34 (Al(4)(PO(4))(4)OH x C(4)H(10)N, space group P-1, a = 8.701(3) A, b = 9.210(3) A, c = 12.385(3) A, alpha = 111.11(2) degrees, beta = 101.42(2) degrees, gamma = 102.08(2) degrees, V = 863.8(4) A(3), Z = 2, R = 3.8%) reveal a 3-D open framework where Al atoms are in both tetrahedral and trigonal bipyramidal coordinations. It contains a 2-D pore system defined by eight rings where channels along [100] cross channels running along [010] and [110]. CBuA molecules are trapped at their intersection. (27)Al, (31)P, and (1)H MAS NMR spectroscopies corroborate these structural features. Calcination treatments of a powder sample of the as-synthesized MIL-34 indicate its transformation into the related template-free structure that is stable up to 1000 degrees C. Lattice energy minimizations are then used in order to anticipate the crystal structure of the calcined MIL-34, starting with the knowledge of the as-synthesized structure exclusively. Energy minimizations predict a new regular zeotype structure (AlPO(4), space group P-1, a = 8.706 A, b = 8.749 A, c = 12.768 A, alpha = 111.17 degrees, beta = 97.70 degrees, gamma = 105.14 degrees, V = 846.75 A(3), Z = 2) together with a thermodynamic stability similar to that of existing zeotype AlPOs. Excellent agreement is observed between the diffraction pattern calculated from the predicted calcined MIL-34 and the experimental X-ray powder diffraction pattern of the calcined sample. Finally, the atomic coordinates and cell parameters of the calcined MIL-34 predicted from the simulations are used to perform the Rietveld refinement of the calcined sample powder pattern, further corroborated by (27)Al and (31)P NMR measurements. This unique combination of experiment and simulation approaches is an interesting and innovative strategy in materials sciences, where simulations articulate the prediction of a possible template-free framework from its as-synthesized templated form. This is especially valuable when straightforward characterizations of the solid of interest with conventional techniques are not easy to carry out.     

Mo5VMo7VIO30(BPO4)2(O3P-Ph)6]5-: a phenyl-substituted molybdenum(V/VI) boro-phosphate polyoxometalate

The title polyanion is the first hybrid borophosphate-phenylphosphonate polyoxometalate. It was structurally characterized as its imidazolium salt, (C(3)N(2)H(5))(5)[Mo(12)O(30)(BPO(4))(2)(O(3)P-Ph)(6)].H(2)O (monoclinic, P2(1)/c, a = 22.120(3) A, b = 13.042(2) A, and c = 32.632(4) A, beta = 101.293(3) degrees ), which was synthesized hydrothermally from imidazole, molybdenum oxide and metal, and boric, phosphoric, and phenylphosphonic acids. The anion is the second example of a new class of polyoxometalates that resemble Dawson anions but where the two pole caps of three edge-sharing MoO(6) octahedra in the latter are replaced by other units, in this case tetrahedral borate sharing corners with three phenylphosphonic groups, [(OB)(O(3)P-Ph)(3)]. The 12 molybdenum atoms forming the two equatorial belts of the cluster are of mixed-valence, five are Mo(V) and seven are Mo(VI), and the resulting five electrons are delocalized. Four of these electrons are paired according to the temperature dependence of the magnetic susceptibility. The new compound is soluble in a mixture of water and pyridine (in equal volumes) as well as in nitromethane, and the anions are intact in these solutions.     

Crystal Structure and Thermal Behaviour of K2[CrF5·H2O]

K₂[CrF₅·H₂O] is monoclinic: a = 9.6835(3) Å, b = 7.7359(2) Å, c = 7.9564(3) Å, β = 95.94(1)°, Z = 4, space group C2/c (n^o 15). Its crystal structure was solved from its X‐ray powder pattern recorded on a powder diffractometer, using for the refinement the Rietveld method. It is built up from isolated octahedral [CrF₅·OH₂]^2? anions separated by potassium cations. The dehydration of K₂[CrF₅·H₂O] leads to anhydrous orthorhombic K₂CrF₅: a = 7.334(2) Å, b = 12.804(4) Å, c = 20.151(5) Å, Z = 16, space group Pbcn (n^o 60), isostructural with K₂FeF₅.     

Synthesis and characterization of [Mo(7)O(16)(O(3)PCH(2)PO(3))(3)]:(8-) a mixed-valent polyoxomolybdenum diphosphonate anion with octahedrally and tetrahedrally coordinated molybdenum

Two new compounds containing the title diphosphono-polyoxometalate anion and diprotonated ethylenediamine (enH(2)) or piperazine (ppzH(2)) countercations have been hydrothermally synthesized and structurally characterized ((enH(2))(4)[Mo(7)O(16)(O(3)PCH(2)PO(3))(3)].7H(2)O, triclinic, P(-)1, Z = 2, a = 10.3455(7) A, b = 13.136(1) A, and c = 20.216(3) A, alpha = 93.247(6) degrees, beta = 96.434(6) degrees, and gamma = 111.900(6) degrees; (ppzH(2))(4)[Mo(7)O(16)(O(3)PCH(2)PO(3))(3)].8H(2)O, triclinic, P(-)1, Z = 2, a = 13.255(2) A, b = 13.638(2) A, and c = 16.874(4) A, alpha = 93.20(2) degrees, beta = 101.27(2) degrees, and gamma = 105.87(1) degrees). The anion is a ring of three pairs of edge-sharing octahedra of Mo(V)O(6) (with Mo(V)-Mo(V) bonds) that share corners with each other. The diphosphonate groups connect the pairs at the periphery. The ring is "capped" by a tetrahedron of Mo(VI)O(4). According to magnetic measurements, the compounds are diamagnetic.     

Molten Salts-Driven Discovery of a Polar Mixed-Anion 3D Framework at the Nanoscale: Zn4Si2O7Cl2, Charge Transport and Photoelectrocatalytic Water Splitting

Mixed-anion compounds widen the chemical space of attainable materials compared to single anionic compounds, but the exploration of their structural diversity is limited by common synthetic paths. Especially, oxychlorides rely mainly on layered structures, which suffer from low stability during photo(electro)catalytic processes. Herein we report a strategy to design a new polar 3D tetrahedral framework with composition Zn₄Si₂O₇Cl₂. We use a molten salt medium to enable low temperature crystallization of nanowires of this new compound, by relying on tetrahedral building units present in the melt to build the connectivity of the oxychloride. These units are combined with silicon-based connectors from a non-oxidic Zintl phase to enable precise tuning of the oxygen content. This structure brings high chemical and thermal stability, as well as strongly anisotropic hole mobility along the polar axis. These features, associated with the ability to adjust the transport properties by doping, enable to tune water splitting properties for photoelectrocatalytic H₂ evolution and water oxidation. This work then paves the way to a new family of mixed-anion solids