Metal-organic frameworks displaying single crystal-to-single crystal transformation through postsynthetic uptake of metal clusters

Abstract

Three metal-organic frameworks (MOFs) formulated as [Co₃O(BTB)₂(H₂O)_x(DMF)_y]·zDMF·nH₂O (BTB = 1,3,5-benzenetribenzoate; 1a, x = y = 1, z = 7.5, n = 12; 1b, x = 2, y = 0, z = 8.5, n = 8; 1c, x = 2, y = 1, z = 7, n = 8) have been synthesized under different temperatures; they crystallize as two-fold interpenetrated analogous structures with the same structural trinuclear Co₃O(CO₂)₆ secondary building units (SBUs), and their stabilities depend on the temperature under which they formed. Upon immersion in the filtrate of 1a (namely 1a-s, freshly filtered) or 1b (namely 1b-s, filtered and then activated in air for three days), single crystal-to-single crystal (SCSC) transformations of 1a–1c to a new MOF, formulated as [Co₆O₂(OH)₄(BTB)_8/3(H₂O)₄]·14DMF·4EtOH·2H₂O (2), occur along with dramatic color change from blue purple to red. Crystallographic studies reveal that the new MOF is also two-fold interpenetrated but is constructed with hexanuclear Co₆O₂(CO₂)₈ SBUs. This SCSC transformation from 1a–1c to 2 can take place only in 1a-s or 1b-s but not in 1c-s and is accompanied with an increase of metal-to-ligand ratio. ESI-MS studies unveil the formation of free {Co₃O} units in situ formed only in 1a-s and 1b-s, and evidence that the SCSC transformation involves a postsynthetic uptake of free metal clusters, {Co₃O}. On the other hand, the SCSC transformation speed decreases from 1a to 1c, which is determined by the chelate carboxylate groups undergoing cleavage of metal-carboxylate bonds during the SCSC transformation and the shape of the 1D channels affecting the diffusion of free metal clusters. Furthermore, the resulting MOF (2) can maintain its crystallinity upon activation and adsorbs iodine up to 38 wt%. Combined with the significant SCSC transformation through postsynthetic uptake of free metal clusters and the resulting MOF capable of adsorbing iodine, the present approach not only provides a true route to construct SBUs-based MOF materials with pre-existing “real” SBUs but also represent a new type of SCSC transformation in the field of MOFs that is able to efficiently tune structures and physicochemical properties of MOFs in the solid states.