Alkali
Metal Directed Assembly of Heterometallic V<sup>v</sup>/M (M = Na,
K, Cs) Coordination Polymers: Structures, Topological Analysis, and
Oxidation Catalytic Properties
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Abstract
The
reactions of [VO(acac)<sub>2</sub>] with bis(salicylaldehyde)-oxaloyldihydrazone
(H<sub>4</sub>L) and an alkali metal carbonate M<sub>2</sub>CO<sub>3</sub> (M = K, Na, Cs), in EtOH/H<sub>2</sub>O medium upon reflux,
resulted in the generation of three new heterometallic V<sup>V</sup>/M materials, namely the 1D [(VO<sub>2</sub>)<sub>2</sub>(μ<sub>4</sub>-L){Na<sub>2</sub>(μ-H<sub>2</sub>O)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>}]<sub><i>n</i></sub> (<b>1</b>), 2D [{V(μ-O)<sub>2</sub>}<sub>2</sub>(μ<sub>4</sub>-L){K<sub>2</sub>(μ-H<sub>2</sub>O)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>}]<sub><i>n</i></sub> (<b>2</b>), and
3D [{V(μ-O)(μ<sub>3</sub>-O)}<sub>2</sub>(μ<sub>8</sub>-L){Cs<sub>2</sub>(μ-H<sub>2</sub>O)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>}]<sub><i>n</i></sub> (<b>3</b>) coordination polymers. They were isolated as air-stable solids
and fully characterized by IR, UV–vis, <sup>1</sup>H, and <sup>51</sup>V NMR spectroscopy, ESI–MS(±), elemental, thermal,
and single-crystal X-ray diffraction analyses, the latter showing
that <b>1</b>–<b>3</b> are constructed from the
resembling [(VO<sub>2</sub>)<sub>2</sub>(μ<sub>4/8</sub>-L)]<sup>2–</sup> blocks assembled by the differently bound aqua-metal
[M<sub>2</sub>(μ-H<sub>2</sub>O)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>2+</sup> moieties (M = Na, K, Cs). The main distinctive
features of <b>1</b>–<b>3</b> arise from the different
coordination numbers of Na (5), K (7), and Cs (9) atoms, thus increasing
the complexity of the resulting networks from the ladder-like 1D chains
in <b>1</b> to double 2D layers in <b>2</b>, and layer-pillared
3D framework in <b>3</b>. The topological analysis of <b>2</b> disclosed a uninodal 4-connected underlying net with a rare <b>kgm</b> [Shubnikov plane net (3.6.3.6)/kagome pattern] topology,
while <b>3</b> features a trinodal 4,7,8-connected underlying
net with an unprecedented topology. Compounds <b>1</b>–<b>3</b> also show solubility in water (<i>S</i><sub>25 °C</sub> ≈ 4–7 mg mL<sup>–1</sup>) and were applied
as efficient precatalysts for the homogeneous oxidation of cyclohexane
by aqueous H<sub>2</sub>O<sub>2</sub>, under mild conditions (50 °C)
in MeCN/H<sub>2</sub>O medium and in the presence of an acid promoter.
Total yields (based on substrate) of cyclohexanol and cyclohexanone
up to 36% and turnover numbers (TONs) up to 5700 were achieved
