Low-Temperature Solvothermal Synthesis, Structure And Properties Of Inorganic-Organic Hybrid Borates

The study of inorganic-organic hybrid materials is a hot research field in recent years. Due to their special structure features, the inorganic-organic hybrid materials act a potential role not only in the molecule engineering, but also in adsorption, separation, catalysis, host-guest chemistry, hydrogen storage, etc. Borate is one of the most important branches in inorganic materials, with various structures, diverse compositions and important applications in industry. After decades of development, borate materials with alkali metal, alkaline earth metal, transition metal, rare earth metal and mixed metal have been widely studied. A large number of borates were used as nonlinear optical materials and luminescence materials. In contrast, less work has been carried out on inorganic-organic hybrid borates, especially the metal-organic templated borates. In this paper, a series of metal-organic templated borates and organically templated borates have been synthesized under low-temperature solvothermal conditions under new solvent systems and new synthetic route by changing the metal ion or organic ligand to modulate the metal-organic template microstructure and the type of organic template. Meanwhile the syntheses of transition metal borate and borophosphate, including the reactions, structures, thermal stabilities and their fluorescence properties were discussed.1. The research of inorganic-organic hybrid borates under low-temperature solvothermal conditions was carried out, and as solvents the 1-methyl-2-pyrrolidinone and N,N’-dimethylacetamide are firstly introduced into the synthesis of borates. The synthesis results indicated that both are available for synthesis of inorganic-organic hybrid borate. In the new solvent system, reaction time and temperature was much less/lower than the reaction conditions in the reported literatures, and high-quality single crystals were obtained under the new conditions. Compounds [Cd(en)3][B5O6(OH)4]2·2H2O (1) and [Cd(DETA)2][B5O6(OH)4]2 (2) are the first reported two examples of metal-organic templated borates contained CdⅡ. Compound 1 and 2 have the same three-dimensional hydrogen-bonded framework with 12-membered boron rings formed by [B5O6(OH)4] uints. Compounds [Cd(TETA)(C2H3O2)][B5O6(OH)4] (3), [Cu(en)2(C5H9NO)][B5O6(OH)4]2·C5H9NO (4) and [Cu(C12N2H8)2(C2H3O2)] [B5O6(OH)4]2·C4H9NO (5) are three new examples of metal-organic templated borates with mixed ligands. Compound 3 and 5 are the second and the third example of borates involving organic acids. In compound 3, [B5O6(OH)4] uints are linked together through O-H…O hydrogen bonds to form a two-dimensional zigzag-like sheet structure with rectangle-like 12-membered boron rings, and the [Cd(TETA)(C2H3O2)]+ connect the adjacent borate layers through N-H…O hydroden-bonds to form a three-dimensional supramolecular framework. In compound 4, the three-dimensional hydrogen-bonded supramolecular framework formed by [B5O6(OH)4] units has alternate square-like channels and rectangle-like channels along the c-axis as well as ellipse-like channels with 16-membered boron rings along the a-axis which are the largest channels in the norganic-organic hybrid pentaborate reported so far. In compound 5, neighbouring zero-dimensional supramolecular aggregates {2[B5O6(OH)4]} connect to two [Cu(C12N2H8)2(C2H3O2)]+ cations through hydrogen-bonds to form a one-dimensional ribbon with 20-membered ring, and the [Cu(C12N2Hg)2(C2H3O2)]+ cations are extended throughπ-πstacking interactions between the phen forming a chain. From the supramolecular frameworks of compound 1-5 and the relationships between the frameworks and fluorescence, it was concluded that the fluorescence are closely related to the structures and dimensions of the supramolecular frameworks of the metal-organic templated borates, and the fluorescence comes from the B-O supramolecular framework. Only when the polyborate anions connect to forming a two-dimensional or three-dimensional supramolecular framework through hydrogen bonding or sharing of oxygen atoms, the fluorescence appears.2. A pyridine borate [C5H6N][B5O6(OH)4]·0.5C5H5N (6) has been synthesized under the mild solvothermal conditions. Isolated [B5O6(OH)4] units connect to three-dimensional supramolecular framework through hydrogen-bonds with large channels along the c and a-axis. Compounds [C6N2H18]0.5[B5O6(OH)4] (7), [C6N4H20]0.5[B5O6(OH)4] (8) and [C10N2H10][B5O10H4]·H3BO3·H2O (9) are three examples of organically templated borates, synthesized by the solvent evaporation method using water as solvent. The three compounds used the chain-like organic amine,1,6-diaminohexane, triethylenetetramine and 4,4’-bipyridine, as templates respectively, and both have the isolated [B5O6(OH)4]- units. In compound 7, the three-dimensional hydrogen-bonded supramolecular framework formed by [B5O6(OH)4] units has ring-like channels along the a-axis, rhombus-like channels along the [101] direction and unidirectional rectangle-like channels along the b-axis. In compound 8, the three-dimensional hydrogen-bonded supramolecular framework formed by [B5O6(OH)4] units has ring-like channels along the a-axis. In compound 9, the three-dimensional hydrogen-bonded supramolecular framework formed by [B5O6(OH)4] units has rectangle-like channels along the a-axis, approximated square-like channels along the c-axis and rectangle-like channels along the b-axis with slightly deviation. Fluorescence analysis revealed that the excitation and emission spectra of compounds 7-9 were observed red-shift by the increasing temperature. The fluorescence properties may com from the borated framework and the self-activating photoluminescence of the samples after heat-treatment probably originates come from the structure defects caused by heat treatment. Compounds 7 and 9 could be used as promising blue component and compound 8 could be promising single-emitting-component phosphors for display and lighting application in the white LED. 3. Compound Zn(H2O)[B3O3(OH)5] (10) were synthesized by the solvent evaporation method using water as solvent at room temperature. Isolated [B5O6(OH)4] units provides two terminal oxygen atoms and one bridge oxygen atom to the coordination of Zn atom to formed a "zigzag" chain. A new protonated borophosphate (H3O)Mg(H2O)2[BP2O8]-H2O (11) contained H3O+ actions were synthesized under mild hydrothermal condition, which has an infinite helical 1∞{[BP2O8]3-} ribbons built up from corner-sharing PO4 and BO4. The fluorescence properties of compound 11 were also studied.4. The types of supramolecular framework formed by [B5O6(OH)4] units through hydrogen-bonds have been summarized for all the compounds in the CSD. The [B5O6(OH)4] units can exist in isolation, and can also form ID chain,2D sheet and 3D supramolecular framework by hydrogen-bonds. The dimension of supramolecular framework has a direct relationship with the size and structure of templates. The supramolecular frameworks of compounds in this paper are also discussed.