Tunable Self-Assembly of Decanuclear Ni(II) Carbonato Clusters with a Hydroxyquinolinato Shell: Robust Porous Networks with Reversible Solvent-/Temperature-Driven Phase Transitions and Selective Gas Separation

The utilization of molecular metal clusters as building units of noncovalent porous materials (NPMs) is a promising strategy, combining the versatile functionality of organic and inorganic subunits with the softness and flexibility of molecular solids controlled by noncovalent interactions. However, the development of robust porous functional frameworks based on self-assembly driven by noncovalent forces is still highly challenging. Herein, we report the synthesis and characterization of a discrete decanuclear Ni(II) hydroxyquinolinato-carbonato cluster, [Ni₁₀(μ₆-CO₃)₄(L)₁₂], which, depending on the crystallization conditions, self-assembles into either of two microporous frameworks: diamondoid WUT-1(Ni) and pyrite WUT-2(Ni). The transitions between both polymorphs can also be selectively triggered by temperature or exposure to vapors of a particular organic solvent, which is accompanied by the easy recovery of crystallinity by the materials from the noncrystalline phase. Moreover, both materials show excellent robustness toward various chemical environments, including air/moisture and water stability, and demonstrate interesting gas adsorption properties. Remarkably, WUT-1(Ni) exhibits significant enhancement in gas uptake compared to the previously reported isostructural Zn(II) analogue, WUT-1(Zn), representing one of the highest H₂ uptakes among NPMs. In turn, tighter voids of the ultramicroporous WUT-2(Ni) framework facilitate selective interactions with gas molecules, resulting in outstanding selectivity in the adsorption of CO₂ over CH₄ and N₂. The presented studies demonstrate the profound role of the character of metal centers on the self-assembly of isostructural nanoclusters as well as properties of the resulting microporous frameworks.