Finely Tuning Metal Ion Valences of [FeM(μ-OH)(Carboxyl)(pyridyl)] Cluster-Based -MOFs for Highly Improved CO Capture Performances.

Solvothermal reactions of different trinuclear precursors and 5-(pyridin-4-yl)isophthalic acid (HL) successfully led to four anionic topological MOFs as FeM(μ-OH)(CHCOO)(L)·(DMA)·DMF [M = Mn(II), Fe(II), Co(II), = 0, 1, 2 and 3], namely, NJTU-Bai79 [NJTU-Bai = Nanjing Tech University Bai's group, Mn(μ-OH)], NJTU-Bai80 [FeMn(μ-OH)], NJTU-Bai81 [Fe(μ-OH)], and NJTU-Bai82 [FeCo(μ-OH)], which possess the narrow pores (2.5-6.0 Å). NJTU-Bai80-82 is able to be tuned to the neutral derivatives [NJTU-Bai80-82(-ox), ox = oxidized] with M ions oxidized to M ones in the air and the OH ions coordinated on M ions. Very interestingly, selective CO/N adsorptions of NJTU-Bai80-82(-ox) are significantly enhanced with the CO adsorption uptakes more than about 6 times that of NJTU-Bai79. GCMC simulations further revealed that neutral NJTU-Bai80-82(-ox) supplies more open frameworks around the -CH groups at separate spaces to the CO gas molecules with relatively more pores available to them after the removal of counterions. For the first time, finely tuning metal ion valences of metal clusters of ionic MOFs and making them from electrostatic to neutral were adopted for greatly improving their CO capture properties, and it would provide another promising strategy for the exploration of high-performance CO capture materials.