Alkali Metal Directed Assembly of Heterometallic Vv/M (M = Na, K, Cs) Coordination Polymers: Structures, Topological Analysis, and Oxidation Catalytic Properties

Abstract

The reactions of [VO(acac)2] with bis(salicylaldehyde)-oxaloyldihydrazone (H4L) and an alkali metal carbonate M2CO3 (M = K, Na, Cs), in EtOH/H2O medium upon reflux, resulted in the generation of three new heterometallic V(V)/M materials, namely the 1D [(VO2)2(μ4-L){Na2(μ-H2O)2(H2O)2}]n (1), 2D [{V(μ-O)2}2(μ4-L){K2(μ-H2O)2(H2O)2}]n (2), and 3D [{V(μ-O)(μ3-O)}2(μ8-L){Cs2(μ-H2O)2(H2O)2}]n (3) coordination polymers. They were isolated as air-stable solids and fully characterized by IR, UV-vis, (1)H, and (51)V NMR spectroscopy, ESI-MS(±), elemental, thermal, and single-crystal X-ray diffraction analyses, the latter showing that 1-3 are constructed from the resembling [(VO2)2(μ(4/8)-L)](2-) blocks assembled by the differently bound aqua-metal [M2(μ-H2O)2(H2O)2](2+) moieties (M = Na, K, Cs). The main distinctive features of 1-3 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 1 to double 2D layers in 2, and layer-pillared 3D framework in 3. The topological analysis of 2 disclosed a uninodal 4-connected underlying net with a rare kgm [Shubnikov plane net (3.6.3.6)/kagome pattern] topology, while 3 features a trinodal 4,7,8-connected underlying net with an unprecedented topology. Compounds 1-3 also show solubility in water (S(25 °C) ≈ 4-7 mg mL(-1)) and were applied as efficient precatalysts for the homogeneous oxidation of cyclohexane by aqueous H2O2, under mild conditions (50 °C) in MeCN/H2O 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.