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)(Cu^ICl₂)@[Cu^II ₄(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)]_n ^n- 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 Cu^II to Cu^I in the host helps to explain such exceptional optical properties.