Helix-mediated over 1 nm-range chirality recognition by ligand-to-ligand interactions of dinuclear helicates

Abstract

Long-range chirality recognition between the two chiral guest ligands can be tuned based on the helix distances (dLn-Ln= 11.5 and 14.0 Å) of bis-diketonate bridged dinuclear lanthanide complexes (2Thand3Th, respectively) used as mediators. Both2Thand3Thform one-dimensional (1D) helical structures upon terminal binding of two chiral guest co-ligands (LRor LS). Long-range chiral self-recognition is achieved in self-assembly of2Thwith LRand LSto preferentially form homochiral assemblies,2Th-LR·LRand2Th-LS·LS, whereas there is no direct molecular interaction between the two guest ligands at the terminal edges. X-ray crystal structure analysis and density functional theory studies reveal that long-range chiral recognition is achieved by terminal ligand-to-ligand interactions between the bis-diketonate ligands and chiral guest co-ligands. Conversely, in self-assembly of3Thwith a longer helix length, statistical binding of LRand LSoccurs, forming heterochiral (3Th-LR·LS) and homochiral (3Th-LR·LRand3Th-LS·LS) assemblies in an almost 1 : 1 ratio. When phenyl side arms of the chiral guest co-ligands are replaced by isopropyl groups (L′Rand L′S), chiral self-recognition is also achieved in the self-assembly process of3Thwith the longer helix length to generate homochiral (3Th-L′R·L′Rand3Th-L′S·L′S) assemblies as the favored products. Thus, subtle modification of the chiral guests is capable of achieving over 1.4 nm-range chirality recognition.