Isolated Mixed-Valence Iron Vanadium Malate And Its Metal Hydrates (M = Fe2+, Cu2+, Zn2+) With Reversible And Irreversible Adsorptions For Oxygen

Isolated octanuclear iron-vanadium malate (NH4)(3)(CH3NH3)(3)[(Fe2V2V4O11)-V-III-V-IV-O-V(mal)(6)]center dot 7.5H(2)O (1; H(3)mal = malic acid) and its family of metal hydrates M'(3n)[M-II(H2O)(2)](1.5n)[(Fe2V2V4O11)-V-III-V-IV-O-V(mal)(6)](n)center dot xnH(2)O (2 or 2-Fe, M' = NH4+, M = Fe, x = 7.5; 3 or 3-Cu, M' = K+, M = Cu, x = 10; 4 or 4-Zn, M' = K+, M = Zn, x = 6.5) have been obtained by self-assembly in water. The cluster anion [Fe2V6O11(mal)(6)](6-) (1a) shows an interesting iron bicapped-triangular-prismatic structure, which is bridged by M2+ hydrates (M = Fe, Cu, Zn) to construct isostructural metal organic frameworks (MOFs) 2-4. The mixed-valence vanadium systems in 1-4 were determined by theoretical bond valence calculations (BVS) and charge balance. The magnetic susceptibilities are further elucidated as high spin for Fe3+ in 1a and bridging Fe2+ in 2-Fe, respectively. A strong ferromagnetic interaction was also observed for 2-Fe at 3 K. 2-Fe, 3-Cu, and 4-Zn have similar hydrophilic channels with diameters of 6.8, 6.5, and 6.6 angstrom, respectively, which show obvious affinity for O-2 in comparison with no adsorption of N-2, H-2, CO2, and CH4 at room temperature under different pressures. Moreover, 2-Fe and 4-Zn exhibit irreversible O-2 absorptions, which may be attributed to charge transfer between O-2 and open metal sites (OMSs) formed during vacuum heating pretreatment. UV-vis and EPR spectra show a change in electronic structure of 2-Fe after O-2 adsorption. The reversible adsorption observed in 3-Cu suggests a weak interaction between O-2 and Cu2+ due to the Jahn-Teller effect. The properties of gas adsorption provide an insight into the performances of small molecules in the channels constructed by synthetic octanuclear model compounds, which are related to the interactions between the gas substrate and the heterometal cluster in biology.