Structural Diversity of the Oxovanadium Organodiphosphonate System:  A Platform for the Design of Void Channels

The hydrothermal reactions of a vanadium source, an appropriate diphosphonate ligand, and water in the presence of HF provide a series of compounds with neutral V−P−O networks as the recurring structural motif. When the {O<sub>3</sub>P(CH<sub>2</sub>)<i><sub>n</sub></i>PO<sub>3</sub>}<sup>4-</sup> diphosphonate tether length <i>n</i> is 2−5, metal−oxide hybrids of type <b>1</b>, [V<sub>2</sub>O<sub>2</sub>(H<sub>2</sub>O){O<sub>3</sub>P(CH<sub>2</sub>)<i><sub>n</sub></i>PO<sub>3</sub>}]·<i>x</i>H<sub>2</sub>O, are isolated. The type <b>1</b> oxides exhibit the prototypical three-dimensional (3-D) “pillared” layer architecture. When <i>n</i> is increased to 6−8, the two-dimensional (2-D) “pillared” slab structure of the type <b>2</b> oxides [V<sub>2</sub>O<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>{O<sub>3</sub>P(CH<sub>2</sub>)<sub>6</sub>PO<sub>3</sub>}] is encountered. Further lengthening of the spacer to <i>n</i> = 9 provides another 3-D structure, type <b>3</b>, constructed from the condensation of pillared slabs to give V−P−O double layers as the network substructure. When organic cations are introduced to provide charge balance for anionic V−P−O networks, oxides of types <b>4</b>−<b>7</b> are observed. For spacer length <i>n</i> = 3, a range of organodiammonium cations are accommodated by the same 3-D “pillared” layer oxovanadium diphosphonate framework in the type <b>4</b> materials [H<sub>3</sub>N(CH<sub>2</sub>)<i><sub>n</sub></i>NH<sub>3</sub>][V<sub>4</sub>O<sub>4</sub>(OH)<sub>2</sub> {O<sub>3</sub>P(CH)<sub>3</sub>PO<sub>3</sub>}<sub>2</sub>]·<i>x</i>H<sub>2</sub>O [<i>n</i> = 2, <i>x</i> = 6 (<b>4a</b>); <i>n</i> = 3, <i>x</i> = 3 (<b>4b</b>); <i>n</i> = 4, <i>x</i> = 2 (<b>4c</b>); <i>n</i> = 5, <i>x</i> = 1 (<b>4d</b>); <i>n</i> = 6, <i>x</i> = 0.5 (<b>4e</b>); <i>n</i> = 7, <i>x</i> = 0 (<b>4f</b>)] and [H<sub>3</sub>NR]<i><sub>y</sub></i>[V<sub>4</sub>O<sub>4</sub>(OH)<sub>2</sub> {O<sub>3</sub>P(CH)<sub>3</sub>PO<sub>3</sub>}<sub>2</sub>]·<i>x</i>H<sub>2</sub>O [R = −CH<sub>2</sub>(NH<sub>3</sub>)CH<sub>2</sub>CH<sub>3</sub>, <i>y</i> = 1, <i>x</i> = 0 (<b>4g</b>); R = −CH<sub>3</sub>, <i>n</i> = 2, <i>x</i> = 3 (<b>4h</b>); R = −CH<sub>2</sub>CH<sub>3</sub>, <i>y</i> = 2, <i>x</i> = 1 (<b>4i</b>); R = −CH<sub>2</sub>CH<sub>2</sub>CH<sub>3</sub>, <i>y</i> = 2, <i>x</i> = 0 (<b>4j</b>); cation = [H<sub>2</sub>N(CH<sub>2</sub>CH<sub>3</sub>)<sub>2</sub>], <i>y</i> = 2, <i>x</i> = 0 (<b>4k</b>)]. These oxides exhibit two distinct interlamellar domains, one occupied by the cations and the second by water of crystallization. Furthermore, as the length of the cation increases, the organodiammonium component spills over into the hydrophilic domain to displace the water of crystallization. When the diphosphonate tether length is increased to <i>n</i> = 5, structure type <b>5</b>, [H<sub>3</sub>N(CH<sub>2</sub>)<sub>2</sub>NH<sub>3</sub>][V<sub>4</sub>O<sub>4</sub>(OH)<sub>2</sub>(H<sub>2</sub>O){O<sub>3</sub>P(CH<sub>2</sub>)<sub>5</sub>PO<sub>3</sub>}<sub>2</sub>]·H<sub>2</sub>O, is obtained. This oxide possesses a 2-D “pillared” network or slab structure, similar in gross profile to that of type <b>2</b> oxides and with the cations occupying the interlamellar domain. In contrast, shortening the diphosphonate tether length to <i>n</i> = 2 results in the 3-D oxovanadium organophosphonate structure of the type <b>7</b> oxide [H<sub>3</sub>N(CH<sub>2</sub>)<sub>5</sub>NH<sub>3</sub>][V<sub>3</sub>O<sub>3</sub>{O<sub>3</sub>P(CH<sub>2</sub>)<sub>2</sub>PO<sub>3</sub>}<sub>2</sub>]. The ethylenediphosphonate ligand does not pillar V−P−O networks in this instance but rather chelates to a vanadium center in the construction of complex polyhedral connectivity of <b>7</b>. Substitution of piperazinium cations for the simple alkyl chains of types <b>4</b>, <b>5</b>, and <b>7</b> provides the 2-D pillared layer structure of the type <b>6</b> oxides, [H<sub>2</sub>N(CH<sub>2</sub>CH<sub>2</sub>)NH<sub>2</sub>][V<sub>2</sub>O<sub>2</sub>{O<sub>3</sub>P(CH)<i><sub>n</sub></i>PO<sub>3</sub>H}<sub>2</sub>] [<i>n</i> = 2 (<b>6a</b>); <i>n</i> = 4 (<b>6b</b>); <i>n</i> = 6 (<b>6c</b>)]. The structural diversity of the system is reflected in the magnetic properties and thermal behavior of the oxides, which are also discussed.