Ya-Jun Hou

Homochiral D4-symmetric metal-organic cages from stereogenic Ru(II) metalloligands for effective enantioseparation of atropisomeric molecules.

Absolute chiral environments are rare in regular polyhedral and prismatic architectures, but are achievable from self-assembly of metal–organic cages/containers (MOCs), which endow us with a promising ability to imitate natural organization systems to accomplish stereochemical recognition, catalysis and separation. Here we report a general assembly approach to homochiral MOCs with robust chemical viability suitable for various practical applications. A stepwise process for assembly of enantiopure ΔΔΔΔΔΔΔΔ- and ΛΛΛΛΛΛΛΛ-Pd6(RuL3)8 MOCs is accomplished by pre-resolution of the Δ/Λ-Ru-metalloligand precursors. The obtained Pd–Ru bimetallic MOCs feature in large D4-symmetric chiral space imposed by the predetermined Ru(II)-octahedral stereoconfigurations, which are substitutionally inert, stable, water-soluble and are capable of encapsulating a dozen guests per cage. Chiral resolution tests reveal diverse host–guest stereoselectivity towards different chiral molecules, which demonstrate enantioseparation ability for atropisomeric compounds with C2 symmetry. NMR studies indicate a distinctive resolution process depending on guest exchange dynamics, which is differentiable between host–guest diastereomers.

Photoluminescence and white-light emission in two series of heteronuclear Pb(II)–Ln(III) complexes

Two series of Pb(II)–Ln(III) heteronuclear coordination complexes are assembled from a tripodal ligand triCB-NTB ((4,4′,4′′-(2,2′,2′′-nitrilotris(methylene)tris(1H-benzo[d]imidazole-2,1-diyl)tris(methylene))tribenzoic acid)). In the Pb2LnL2 series, the Ln3+ ion is encapsulated in a highly symmetrical {LnO6} octahedron with an inversion center, and Pb–Ln–Pb clusters are formed by the linkage of carboxyl groups on triCB-NTB ligands to Pb2+ and Ln3+ simultaneously. However, in the PbLn2L2 series, the Ln3+ ion is encapsulated in a distorted {LnO9} polyhedron without an inversion center. Pb2+ ions are coordinated with benzimidazole and apical N atoms on the ligand in isolation, and the carboxyl groups link two Ln3+ ions into a Ln–Ln cluster. This structural variation leads to different photoluminescence behaviour in these two series of complexes. Most importantly, the linkage of the Pb–Ln–Pb clusters causes more perturbation to the excited states of the ligand. Therefore, a more obvious ligand-to-metal charge transfer (LMCT) process is observed in the Pb2LnL2 series, and the energy transfer to the accepting levels of Ln3+ ions becomes more efficient. Furthermore, the combination of LC (ligand-centered) + LMCT + MC (metal-centered) emissions in the Pb2EuL2 complex results in single component white light emission.

Semidirected versus holodirected coordination and single-component white light luminescence in Pb(II) complexes

2-Methyl-8-hydroxyquinoline (HMq) and the tripodal ligands 4,4′,4′′-(2,2′,2′′-nitrilotris (methylene)tris(1H-benzo[d]imidazole-2,1-diyl)tris(methylene))tribenzonitrile (triBZ-NTB) and 4,4′,4′′-(2,2′,2′′-nitrilotris(methylene)tris(1H-benzo[d]imidazole-2,1-diyl)tris(methylene))tribenzoic acid (H3triCB-NTB) were used individually to assemble a hetero-nuclear, a tetra-nuclear and two mono-nuclear Pb(II) complexes. The Pb(II) coordination centers in two of these complexes were observed to display semidirected coordination with the ligands and counter anions (small solvent molecules), whereas the other two complexes showed holodirected coordination, together leading to varied coordination geometries. The combination of ligand-to-metal charge-transfer (LMCT) and metal-centered (MC) emissions in the semidirected Pb(II) complexes resulted in single-component white light luminescence.