Post-synthetic modifications of framework metal ions in isostructural metal-organic frameworks: Core-shell heterostructures via selective transmetalations

The transmetalation (the replacement of metal ions) of a family of highly porous isostructural metal-organic frameworks (MOFs), M ₆(BTB) ₄(BP) ₃ (where M = Zn(II) (1), Co(II) (2), Cu(II) (3), and Ni(II) (4), BTB = 1,3,5-benzenetribenzoate, and BP = 4,4′-dipyridyl) with an ith-d net topology has been investigated. These compounds have different framework stabilities depending on the framework metal ions. The transmetalation and the reverse transmetalation reactions of the framework metal ions were observed between the MOFs, 1 and 2, having a similar thermodynamic stability. While the transmetalation from thermodynamically less stable 1 and 2 to more stable 3 and 4 were achieved by soaking single crystals of 1 and 2 in a solution of N,N′-dimethylformamide (DMF) containing Cu(II) and Ni(II) ions, respectively, no reverse transmetalation was observed. By simply controlling the soaking time, not only could homogeneously transmetalated crystalline framework structures be prepared via the thermodynamically controlled complete replacement of the framework metal ions but also selectively transmetalated core-shell heterostructures were formed via kinetically controlled replacement that was mainly restricted to the external shell region of the crystal. The fully transmetalated MOFs showed significantly improved framework stabilities compared with the parent MOFs. A marked improvement in the framework stability was observed, even in the selectively transmetalated Co(II)/Cu(II)- and Co(II)/Ni(II)-core-shell heterostructures. Although the frameworks are partially transmetalated, the framework stability of not only the external shell region but also of the internal core region was significantly affected.