Homochiral Metal-Organic Frameworks Embedding Helicity Based on a Semirigid Alanine Derivative

Five novel homochiral metal-organic frameworks (HMOFs) have been synthesized using a semirigid alanine-derived ligand and varied auxiliary N-donor ligands, namely, [Cu₂(TMAla)₂(bipy)₂]·10H₂O (1) and [Cu(TMAla)(bpea)_0.5(H₂O)]·H₂O (2), Ln(TMAla)(HTMAla)(phen) (Ln = Eu, 3; Tb, 4; Gd, 5), where H₂TMAla = terephthaloyl-mono(l-alanine), bipy = 4,4′-bipyridine, bpea = 1,2-bis(4-pyridyl)ethane, phen = 1,10-phenanthroline. Crystallographic analysis indicates that all the complexes contain homochiral left- and/or right-handed helical chains, which are constructed by TMAla fragments and metal ions. Meanwhile, the introduction of auxiliary N-donor ligands gives rise to the formation of HMOFs with different structural topologies. Complex 1 shows a three-dimensional (3D) quartz-dual net with a 4-connected (7⁵·9) topology in the presence of bipy. Complex 2 possesses a novel 3D pillared-layer framework with a (3,4,4)-connected (6·10⁴·12)(6³)₂(6⁴·10²) topology by using bpea instead of bipy. Complexes 3-5 are isomorphous and feature a 2-fold interpenetrating 3D quartz net with a 4-connected (6⁴·8²) topology due to the participation of phen. Complexes 3 and 4 exhibit characteristic Eu(III) and Tb(III) emissions in the red and green light regions, respectively. The solid-state circular dichroism (CD) spectra of all complexes exhibit strong CD signals. Our results highlight that the semirigid chiral linker is successful for the construction of interesting HMOFs with unique helical chains.