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40555-GB10
Synthesis of Organically Templated Molybdenum Sulfates under Hydrothermal Conditions
The initial focus of this proposal was to study several variables in the formation of novel organically templated molybdenum sulfates. These variables include template structure, reaction pH, reactant concentration and temperature. Work in the first year of this grant was focused upon the formation of organically templated molybdenum sulfates, which resulted in the formation of several new compounds. The focus of this work later shifted from the study of sulfated molybdena compounds to templated molybdates. While the chemistry of templated molybdates has been studied extensively, true control over product composition and structure often remains elusive. While a host of reaction variables are known to influence the products, we initially focused our attention on the role of the organic amine.
Two series of amines were used, linear diamines (with the formula H2N(CH2)nNH2 (n = 2 – 7)) and rigid core diamines (4-aminopyridine and 1,4-xylylenediame). Fifteen new organically templated molybdena compounds were synthesized in these systems, eleven of which have been recently reported in two publications. Composition space analysis was used to isolate the role of the amine in these reactions. It was found that the compound whose composition most closely matches that of the reaction gel. The net charge density of the reaction gel dictates the speciation of the inorganic component by shifting the solution equilibria. Specific inorganic frameworks crystallize through the formation of stable hydrogen bonds from template nitrogens to nucelophilic oxide ligands. The charge density of the template dictates the dimensionality and packing of these inorganic species. The inorganic framework groups obtained are a function of the net reaction gel charge density, while the specific identity of the inorganic portion found in the solid state depends more strongly on the structure of the template.
Work in the past year has focused on investigation of another reaction variable, the mineralizer. We observed that b-[Mo8O26]4- molecular anions could be transformed to [Mo16O53F2]12- anions in the presence of very small amounts of fluoride. [Mo16O53F2]12- anions had not been previously reported, and are approximately twice the size of the next largest polyoxofluoromolybdate anion. We recently published this work in Inorganic Chemistry. While the ability of octamolybdates to transform into other polyoxomolybdates species is well known; an important intermediate in many of these transformations is g-[Mo8O26]4-. It has been demonstrated that g-[Mo8O26]4- is not present in appreciable amounts in solution over a wide range of experimental conditions. However, these anions have been observed as secondary building units in the extended structures of several other compounds, most notably those that include [Mo8O27]n6n- chains. In addition, this cluster connectivity has also been observed in the polyoxofluoromolybdate anion, [Mo8O26F2]6-.
Inspection of [Mo8O27]n6n- chains and [Mo8O26F2]6- and [Mo16O53F2]12- molecular anions reveals that modifications to the g-[Mo8O26]4- structure occur at specific sites. The fluorides and bridging oxides ligands in [Mo8O27]n6n-, [Mo8O26F2]6- and [Mo16O53F2]12- are bound to the two five-coordinate molybdenum centers in g-[Mo8O26]4-. The bonding in these MoO5 coordination polyhedra creates a substantial overbonding in selected bridging oxides, and subsequent coordination is directed to relieve these bond stresses. The presence of fluoride in the reaction mixture allowed the isolation of a g-[Mo8O26]4- species through the passivation of the two reactive sites.
Our investigations into the roles of both the amine and mineralizer led us to investigate their interdependence. A series of new compounds were synthesized under identical conditions, save the structure of the amine. All other reaction variables were held constant, including mole fraction ratios of all reagents, solvent level, pH and reaction time and temperature. We used four amines; N,N'-diethylethylenediamine (deed), N,N-dimethyl-N'-ethylethylenediamine (dmeed), N,N,N',N'-tetramethylethylenediamine (tmed) and 1-tris(2-aminethyl)amine (tren). These reactions produced compounds that contain three different inorganic structure types. See Figure 3. Reactions containing deed and dmeed both produced b-[Mo8O26]4- molecular anions, while reactions containing tmed and tren gave [Mo8O26]4- and [Mo8O27]6- chains, respectively. These differences are related to the charges and packing efficiencies of the amines. Tmed can pack more efficiently than deed or dmeed, resulting in a higher charge density structure, while the charge of the [trenH3]3+ cations is reflected in the [Mo8O27]6- chains.
We then re-preformed these same reactions, with one small difference. A small variable amount of NaF was added to each reaction. It was observed that, despite great differences in the inorganic structures without F-, [Mo8O26F2]6- anions were observed from each system. This suggests that in the absence of strong complexing agents (F-), the amine dictates the form of the inorganic component. When a critical concentration of F- is reacted, the trend is reversed. F- now is the dominate factor, leading to a series of identical molecular anions. A manuscript detailing this work is in preparation.
