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

Pyrazole-assisted tetranuclear microporous polyoxovanadates(III) (POVs) (NH 4 ) 2 K 2 [V 4 (μ 2 -OH) 4 (ox) 4 (pz) 4 ]·9H 2 O (1, ox = oxalate and pz = pyrazole) and (NH 4 ) 2 Na 2 [V 4 (μ 2 -OH) 4 (ox) 4 (4-mpz) 4 ]·7H 2 O (2, 4-mpz = 4-methylpyrazole) have been constructed in reduced media, along with their triazole neutral hexa- and octanuclear products K 2 [V 6 (μ 2 -OH) 6 (ox) 6 (Hdatrz) 6 ]Cl 2 ·29.5H 2 O (3) and [V 8 (μ 2 -OH) 8 (SO 3 ) 8 (Hdatrz) 8 ]·38H 2 O (4, Hdatrz = 1 H -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 4 (μ 2 -OH) 4 } skeletons with an inner diameter of 2.8 Å, while a paddle wheel-shaped cluster 3 contains a {V 6 (μ 2 -OH) 6 } 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 8 (μ 2 -OH) 8 (SO 3 ) 8 } 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 2 and O 2 more favorably than N 2 , CH 4 and H 2 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 2 adsorption.