Post-synthetic shaping of porosity and crystal structure of Ln-Bipy-MOFs by thermal treatment

Abstract

The reaction of anhydrous lanthanide chlorides together with 4,4′-bipyridine yields the MOFs ∞ 2 [Ln 2 Cl 6 (bipy) 3 ]·2bipy, with Ln = Pr - Yb, bipy = 4,4′-bipyridine, and ∞ 3 [La 2 Cl 6 (bipy) 5 ]·4bipy. Post-synthetic thermal treatment in combination with different vacuum conditions was successfully used to shape the porosity of the MOFs. In addition to the MOFs microporosity, a tuneable mesoporosity can be implemented depending on the treatment conditions as a surface morphological modification. Furthermore, thermal treatment without vacuum results in several identifiable crystalline high-temperature phases. Instead of collapse of the frameworks upon heating, further aggregation under release of bipy is observed. ∞ 3 [LaCl 3 (bipy)] and ∞ 2 [Ln 3 Cl 9 (bipy) 3 ], with Ln = La, Pr, Sm, and ∞ 1 [Ho 2 Cl 6 (bipy) 2 ] were identified and characterized, which can also exhibit luminescence. Besides being released upon heating, the linker 4,4′-bipyridine can undergo activation of C-C bonding in ortho-position leading to the in-situ formation of 4,4′:2′,2″:4″,4⌄-quaterpyridine (qtpy). qtpy can thereby function as linker itself, as shown for the formation of the network ∞ 2 [Gd 2 Cl 6 (qtpy) 2 (bipy) 2 ]·bipy. Altogether, the manuscript elaborates the influence of thermal treatment beyond the usual activation procedures reported for MOFs.