The first decavanadate-based microporous hybrid, namely, [Cu(cyclam)][{Cu(cyclam)}2(V10O28)]·10H2O(1, cyclam = 1,4,8,11-tetraazacyclotetradecane) has been prepared by reaction of (VO3) -anions and {Cu(cyclam)} 2+ complexes in NaCl (aq) at pH 4.6-4.7, and characterized by elemental analyses, thermogravimetry, and X-ray diffraction techniques (powder, single-crystal). Compound 1 exhibits a POMOF-like supramolecular open-framework built up of covalent decavanadate/metalorganic layers with square-like voids, the stacking of which is aided by interlamellar cementing complexes and generates water-filled channels with approximate cross-sections of. 10.4 × 8.8 Å 2 . The framework is robust enough to remain virtually unaltered upon thermal evacuation of all water molecules of hydration, as demonstrated through single-crystal X-ray diffraction studies on the anhydrous phase 1a. This permanent microporosity renders interesting functionality to 1, such as selective adsorption of CO2 over N2 and remarkable activity as heterogeneous catalyst toward the H2O2-based oxidation of the highly-stable, tricyclic alkane adamantane.Porous crystalline materials such as metal organic frameworks (MOFs) have attracted great attention due to their wide range of relevant applications. 1 These materials are constructed by coordination of metal ions or metalcontaining units (nodes) to organic bridging ligands (linkers) to form open crystalline frameworks with permanent porosity. This feature qualifies them as suitable candidates for gas storage and separation, ion exchange, host-guest chemistry, magnetism, biomedicine and catalysis. 2 However, the synthesis of MOFs usually requires harsh conditions (e.g. high temperature or pressure, prolonged reaction times, harmful solvents, etc.), and removal of guest molecules from their cavities often leads to the collapse of the porous structure when flexible linkers are used. In this context, the incorporation of rigid and voluminous species such as metal clusters could increase the overall robustness
The compound tetrakis(tert-butylammonium)-cyclo-tetrametavanadate(V), the first oxovanadate salt isolated from aqueous solution which contains a discrete unprotonated [V 4 O 12 ] 4anion, undergoes an irreversible, reconstructive, continuous, and heterogeneous phase transition in the solid state by means of a nucleation and growth mechanism. This transformation yields a polymeric metavanadate of tert-butylammonium and it has been followed by X-ray powder diffraction and FTIR techniques. The polymeric compound has also been isolated from aqueous solution and its crystal structure resolved by single-crystal X-ray diffraction methods, this compound crystallizes in the monoclinic space group P2 1 , a ) 10.397(2) Å, b ) 5.873(1) Å, c ) 13.879(4) Å, β ) 111.02(3)°, V ) 791.1(3) Å 3 , Z ) 4, and R ) 0.026 for 1926 observed reflections with I g 3σ(I). Comparison of the polymeric and cyclic metavanadate structures reveals several structural similarities such as V‚‚‚V contacts and V-O-V angles. A possible transformation mechanism has been suggested on the basis of these structural analogies.
TheE porous hybrid metavanadate ij{CuIJcyclam)}IJVO 3 ) 2 ]·5H 2 O (1) (cyclam = 1,4,8,11tetraazacyclotetradecane) undergoes thermally-triggered sequential single-crystal-to-single-crystal (SCSC) transformations upon gradual dehydration to produce three new porous crystalline phases, namely ij{CuIJcyclam)}IJVO 3 ) 2 ]·3H 2 O (2), ij{CuIJcyclam)}IJVO 3 ) 2 ]·1.3H 2 O (3) and ij{CuIJcyclam)}IJVO 3 ) 2 ] (4). Compound 1 has a three-dimensional structure formed by metavanadate chains linked by {CuIJcyclam)} moieties in ahybrid open framework with two different types of hexagonal channels where water molecules of hydration are hosted. The SCSC transformations cause a rearrangement o/f the metavanadate chains in such a way that they contract when going from 1 to 2 and stretch back when 2 transforms into 3. The size of the channels is also modified as the hybrid dehydrates. The transition from 2 to 3 leads to the cleavage of a Cu-O bond and consequent coordination of a {CuIJcyclam)} moiety to a different {VO 4 } unit, which drastically decreases the size of the channel in the process. In contrast, total dehydration of 3 enlarges the channel in the anhydrous phase 4 due to the migration of another Cu atom. The reversibility of such transformations has been monitored by a combination of thermogravimetric and powder X-ray diffraction analyses. While 1 and 3 are stable in open-air conditions, 2 transforms back into 1 upon exposure to air for three weeks. The anhydrous phase 4 rapidly rehydrates into 3 when in contact with ambient moisture. Furthermore, 3 can also be transformed into the parent hybrid 1 when soaked in water for seven days. Therefore, all crystal transitions described herein are fully reversible by applying the appropriate conditions.
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