Solvent-modified dynamic porosity in chiral 3D kagome frameworks

Abstract

Dynamically porous metal-organic frameworks (MOFs) with a chiral quartz-based structure have been synthesized from the multidentate ligand 2,2′-dihydroxybiphenyl-4,4′-dicarboxylate (H₂diol). Compounds [Ni(II)(H₂diol)(S)₂]·xS (where S = DMF or DEF) show marked changes in 77 K N₂ uptake between partially desolvated [Ni(II)(H₂diol)(S)₂] (only the pore solvent is removed) and fully desolvated [Ni(II)(H₂diol)] forms. Furthermore, [Ni(II)(H₂diol)(DMF)₂] displays additional solvent-dependent porosity through the rotation of DMF molecules attached to the axial coordination sites of the Ni(II) centre. A unique feature of the four coordinate Ni(II) centre in [Ni(II)(H₂diol)] is the dynamic response to its chemical environment. Exposure of [Ni(II)(H₂diol)] to H₂O and MeOH vapour leads to coordination of both axial sites of the Ni centres and to the generation of a solvated framework, whereas exposure to EtOH, DMF, acetone, and MeCN does not lead to any change in metal coordination or structure metrics. MeOH vapour adsorption was able to be tracked by time-dependent magnetometry as the solvated and desolvated structures have different magnetic moments. Solvated and desolvated forms of the MOF show remarkable differences in their thermal expansivities; [Ni(II)(H₂diol)(DMF)₂]·DMF displays marked positive thermal expansion (PTE) in the c-axis, yet near to zero thermal expansion, between 90 and 450 K, is observed for [Ni(II)(H₂diol)]. These new MOF architectures demonstrate a dynamic structural and colourimetric response to selected adsorbates via a unique mechanism that involves a reversible change in the coordination environment of the metal centre. These coordination changes are mediated throughout the MOF by rotational mobility about the biaryl bond of the ligand.