A Cu-Based Near-IR Active MOF with an Ion-Pair Guest Exhibiting Versatile and Selective Gas-Solid Reactivity

Abstract

Metal-organic frameworks (MOFs) that react spontaneously and selectively with gases or vapors are useful in separation and purification processes, but are very scarce since only a few MOFs are capable of undergoing structural transformations. Simultaneous incorporation of cations and anions in a MOF is, as far as is known, never been observed. Here a new MOF: (Me2NH2)(CuICl2)@[CuII4(INA)4Cl2O]·1.5dmf (3) (INA = isonicotinate, dmf = N,N'-dimethylformamide) is presented that simultaneously incorporates Me2NH2+ cations and CuCl2− anions. Furthermore, to the best of our knowledge, MOF 3 is the first transition metal-based near-IR active MOF. A systematic reactivity study with highly reactive gases and vapors shows that, in the presence of NH3, MOF 3 presents a single-crystal-to-single-crystal transformation into [Cu(INA)(OH)]·H2O (4-NH3), a 3D MOF containing unprecedented [Cu(μ3-OH)]nn- chains. With HCl or HCOOH vapors, MOF 3 shows a dissolution-recrystallization structural transformation into (HINA)2[Cu2Cl6(H2O)2] (5-HCl) or the 3D MOF (Me2NH2)[Cu(HCOO)3] (6-For). In contrast, MOF 3 remains stable under vapors of H2SO4, H3PO4, Et3N, acetone, acetonitrile, methanol, ethanol, and pyridine. Theoretical calculations confirm the spontaneous nature of these transformations. Density functional theory (DFT) calculations show that the reduction of CuII to CuI in the host helps to explain such exceptional optical properties