Successive constructions of regular tetra-, hexa- and octanuclear microporous polyoxovanadates(III) for gas adsorption.

Pyrazole-assisted tetranuclear microporous polyoxovanadates(III) (POVs) (NH)K[V(μ-OH)(ox)(pz)]·9HO (1, ox = oxalate and pz = pyrazole) and (NH)Na[V(μ-OH)(ox)(4-mpz)]·7HO (2, 4-mpz = 4-methylpyrazole) have been constructed in reduced media, along with their triazole neutral hexa- and octanuclear products K[V(μ-OH)(ox)(Hdatrz)]Cl·29.5HO (3) and [V(μ-OH)(SO)(Hdatrz)]·38HO (4, Hdatrz = 1-1,2,4-triazole-3,5-diamine) successively. Both polyanionic structures of 1 and 2 share similar inorganic building blocks that consist of regular {V(μ-OH)} skeletons with an inner diameter of 2.8 Å, while a paddle wheel-shaped cluster 3 contains a {V(μ-OH)} skeleton with two chlorides encapsulated around the center of the ring, occupying a hole of 3.7 Å. An interesting isolated intrinsic polyoxometalate-based metal-organic framework (POMOF) 4 exists as an octanuclear petaloid-like skeleton {V(μ-OH)(SO)} with an inner diameter of 5.2 Å. Bond valence sum calculations manifest that all V ions have severely reduced +3 oxidation states in 1-4, which are supported by charge balance, structural and magnetic data. Moreover, gas adsorptions indicate that 1, 2 and 4 can adsorb CO and O more favorably than N, CH and H gases. Compared with 1 and 2, due to the functionalization of microchannels with Lewis base amino and hydroxy groups and uncoordinated azolate N-donors inside POMOF 4, it should have notable affinities toward CO adsorption.