Li-Peng Zhou

Stereocontrolled Self-Assembly and Self-Sorting of Luminescent Europium Tetrahedral Cages

Coordination-directed self-assembly has become a well-established technique for the construction of functional supramolecular structures. In contrast to the most often exploited transition metals, trivalent lanthanides Ln(III) have been less utilized in the design of polynuclear self-assembled structures despite the wealth of stimulating applications of these elements. In particular, stereochemical control in the assembly of lanthanide chiral cage compounds is not easy to achieve in view of the usually large lability of the Ln(III) ions. We report here the first examples of stereoselective self-assembly of chiral luminescent europium coordination tetrahedral cages and their intriguing self-sorting behavior. Two pairs of R and S ligands are designed on the basis of the pyridine-2,6-dicarboxamide coordination unit, bis(tridentate) L1 and tris(tridentate) L2. Corresponding chiral Eu4(L1)6 and Eu4(L2)4 topological tetrahedral cages are independently assembled via edge- and face-capping design strategies, respectively. The chirality of the ligand is transferred during the self-assembly process to give either Δ or Λ metal stereochemistry. The self-assembled cages are characterized by NMR, high-resolution ESI-TOF-MS, and in one case by X-ray crystallography. Strict control of stereoselectivity is confirmed by CD spectroscopy and NMR enantiomeric differentiation experiments. Narcissistic self-sorting is observed in the self-assembly process when two differently shaped ligands L1 and L2 are mixed. More impressively, distinct self-sorting behavior between Eu4(L1)6 and Eu4(L2)4 coordination cages is observed for the first time when racemic mixtures of ligands are used. We envisage that chiral luminescent lanthanide tetrahedral cages could be used in chiroptical probes\sensors and enantioselective catalysis.

Evolution of Luminescent Supramolecular Lanthanide M2nL3n Complexes from Helicates and Tetrahedra to Cubes

Lanthanide-containing molecules have many potential applications in material science and biology, that is, luminescent sensing/labling, MRI, magnetic refrigeration, and catalysis among others. Coordination-directed self-assembly has shown great power in the designed construction of well-defined supramolecular systems. However, application of this strategy to the lanthanide edifices is challenging due to the complicated and greatly labile coordination numbers and geometries for lanthanides. Here we demonstrate a sensitive structural switching phenomenon during the stereocontrolled self-assembly of a group of Ln2nL3n (Ln for lanthanides, L for organic ligands, and n = 1, 2, 4) compounds. Systematic variation of the offset distances between the two chelating arms on the bis(tridentate) ligands dictated the final outcomes of the lanthanide assembly, ranging from Ln2L3 helicates and Ln4L6 tetrahedra to Ln8L12 cubes. Remarkably, the borderline case leading to the formation of a mixture of the helicate and the tetrahedron was clearly revealed. Moreover, the concentration-dependent self-assembly of an unprecedented cubic Ln8L12 complex was also confirmed. The luminescent lanthanide cubes can serve as excellent turn-off sensors in explosives detection, featuring high selectivity and sensitivity toward picric acid. All complexes were confirmed by NMR, ESI-TOF-MS, and single crystal X-ray diffraction studies. Our results provide valuable design principles for the coordination self-assembly of multinuclear functional lanthanide architectures.

Adaptive self-assembly and induced-fit transformations of anion-binding metal-organic macrocycles

Container-molecules are attractive to chemists due to their unique structural characteristics comparable to enzymes and receptors in nature. We report here a family of artificial self-assembled macrocyclic containers that feature induced-fit transformations in response to different anionic guests. Five metal-organic macrocycles with empirical formula of MnL2n (M=Metal; L=Ligand; n=3, 4, 5, 6, 7) are selectively obtained starting from one simple benzimidazole-based ligand and square-planar palladium(II) ions, either by direct anion-adaptive self-assembly or induced-fit transformations. Hydrogen-bonding interactions between the inner surface of the macrocycles and the anionic guests dictate the shape and size of the product. A comprehensive induced-fit transformation map across all the MnL2n species is drawn, with a representative reconstitution process from Pd7L14 to Pd3L6 traced in detail, revealing a gradual ring-shrinking mechanism. We envisage that these macrocyclic molecules with adjustable well-defined hydrogen-bonding pockets will find wide applications in molecular sensing or catalysis.

Water-Soluble Redox-Active Cage Hosting Polyoxometalates for Selective Desulfurization Catalysis

Transformations within container-molecules provide a good alternative between traditional homogeneous and heterogeneous catalysis, as the containers themselves can be regarded as single molecular nanomicelles. We report here the designed-synthesis of a water-soluble redox-active supramolecular Pd4L2 cage and its application in the encapsulation of aromatic molecules and polyoxometalates (POMs) catalysts. Compared to the previous known Pd6L4 cage, our results show that replacement of two cis-blocked palladium corners with p-xylene bridges through pyridinium bonds formation between the 2,4,6-tri-4-pyridyl-1,3,5-triazine (TPT) ligands not only provides reversible redox-activities for the new Pd4L2 cage, but also realizes the expansion and subdivision of its internal cavity. An increased number of guests, including polyaromatics and POMs, can be accommodated inside the Pd4L2 cage. Moreover, both conversion and product selectivity (sulfoxide over sulfone) have also been much enhanced in the desulfurization reactions catalyzed by the POMs@Pd4L2 host-guest complexes. We expect that further photochromic or photoredox functions are possible taking advantage of this new generation of organo-palladium cage.

Intraligand Charge Transfer Sensitization on Self-Assembled Europium Tetrahedral Cage Leads to Dual-Selective Luminescent Sensing toward Anion and Cation

Luminescent supramolecular lanthanide edifices have many potential applications in biology, environments, and materials science. However, it is still a big challenge to improve the luminescent performance of multinuclear lanthanide assemblies in contrast to their mononuclear counterparts. Herein, we demonstrate that combination of intraligand charge transfer (ILCT) sensitization and coordination-driven self-assembly gives birth to bright EuIII tetrahedral cages with a record emission quantum yield of 23.1%. The ILCT sensitization mechanism has been unambiguously confirmed by both time-dependent density functional theory calculation and femtosecond transient absorption studies. Meanwhile, dual-responsive sensing toward both anions and cations has been demonstrated making use of the ILCT transition on the ligand. Without introduction of additional recognition units, high sensitivity and selectivity are revealed for the cage in both turn-off luminescent sensing toward I- and turn-on sensing toward Cu2+. This study offers important design principles for the future development of luminescent lanthanide molecular materials.

A supramolecular lanthanide separation approach based on multivalent cooperative enhancement of metal ion selectivity

Multivalent cooperativity plays an important role in the supramolecular self-assembly process. Herein, we report a remarkable cooperative enhancement of both structural integrity and metal ion selectivity on metal-organic M4L4 tetrahedral cages self-assembled from a tris-tridentate ligand (L1) with a variety of metal ions spanning across the periodic table, including alkaline earth (CaII), transition (CdII), and all the lanthanide (LnIII) metal ions. All these M4L14 cages are stable to excess metal ions and ligands, which is in sharp contrast with the tridentate (L2) ligand and bis-tridentate (L3) ligand bearing the same coordination motif as L1. Moreover, high-precision metal ion self-sorting is observed during the mixed-metal self-assembly of tetrahedral M4L4 cages, but not on the M2L3 counterparts. Based on the strong cooperative metal ion self-recognition behavior of M4L4 cages, a supramolecular approach to lanthanide separation is demonstrated, offering a new design principle of next-generation extractants for highly efficient lanthanide separation.Lanthanide ions possess similar chemical properties, making their separation from one another challenging. Here the authors show that a tris-tridentate ligand causes high-precision metal ion self-sorting, leading to the selective assembly of tetrahedral M4L4 cages across the lanthanide series.

Ligand-enabled site-selectivity in a versatile rhodium( ii )-catalysed aryl C–H carboxylation with CO 2

Although carbon dioxide (CO2) is an attractive renewable carbon source, its utilization to produce fine chemicals through the catalytic carboxylation of unactivated carbon–hydrogen (C–H) bonds is still very limited and remains a challenge, largely because CO2 is thermodynamically and kinetically stable. In particular, the generation of (hetero)aromatic carboxylic acids via a transition-metal-catalysed C–H carboxylation of arenes with CO2 is extremely rare. Here we report a ligand-enabled site-selective carboxylation of 2-arylphenols under atmospheric pressure of CO2 through a Rh2(OAc)4-catalysed and chelation-assisted C–H bond activation. Remarkably, the reaction occurs selectively on the less nucleophilic phenyl group with the promotion of a phosphine ligand, which overrides the site selectivity dictated by the well-known Kolbe–Schmitt type reaction. The non-acidic C–H bonds within several important classes of heterocycles were also efficiently carboxylated with this method. A mechanistic investigation revealed complexes of active catalysts and that this reaction proceeds under redox-neutral reaction conditions.Catalytic arene carboxylation with CO2 is a challenging reaction, with limited substrate scope and selectivity. Now, a redox neutral method based on a ligand-assisted Rh-catalysed C–H activation is shown, which features broad scope and provides access to important and biologically relevant structural motifs

Self-Assembly of a Tetraphenylethylene-Based Capsule Showing Both Aggregation- and Encapsulation-Induced Emission Properties

Functional molecular capsules have attracted a lot of attention in recent years because of their potential applications as chemosensors, catalysis, drug carriers, and so on. We report here the coordination-directed self-assembly of a fluorescent-lantern-type molecular capsule from four tetraphenylethylene-based ditopic ligands and two square-planar palladium(II) ions. The capsule has been thoroughly characterized by UV-vis, 1D/2D NMR, electrospray ionization time-of-flight mass spectrometry, and single-crystal X-ray diffraction studies. The aggregation-induced emission performance of the capsule has been studied by tuning the ratio of mixed solvents. Moreover, with an open cavity, the fluorescence of the capsule also displays anion sensitivity, with the best turn-on responsiveness observed for HCO3-, demonstrating for the first time an encapsulation-induced emission property.

Coordination-Enhanced Luminescence on Tetra-Phenylethylene-Based Supramolecular Assemblies

Materials with aggregation-induced emission (AIE) properties have received increased attention recently due to their potential applications in light-emitting devices, chemo/biosensors and biomedical diagnostics. In general, AIE requires the forced aggregation of the AIEgens induced by the poor solvent or close arrangement of AIEgens covalently attached to polymer chains. Here, we report two coordination-enhanced fluorescent supramolecular complexes featuring hierarchically restricted intramolecular motions via the self-assembly of tetraphenylethylene (TPE)-based tetra-dentate (La) and bidentate (Lb) ligands and the cis-Pd(en)(NO3)2 (en = ethylenediamine) unit. While the free ligands are non-emissive in dilute solution and show typical AIE properties in both mixed solvent system and the solid state, the self-assembled complexes maintain their fluorescent nature in the solution state. In particular, the Pd4(La)2 complex shows remarkable 6-fold fluorescent enhancement over La in dilute solution. We anticipate that these kinds of coordination-enhanced emissive supramolecules will find applications in biomedical sensing or labeling.

Hierarchical Self-Assembly and Chiroptical Studies of Luminescent 4d–4f Cages

Multinuclear lanthanide-containing supramolecular cages have received increasing attention recently because of their unique electroptical and magnetic properties. Here we report the hierarchical self-assembly and chiroptical studies of a group of 4d-4f heterometallic cages synthesized from a preformed dimetalloligand [(bpy)2Pd212]2+ (2) (bpy = 2,2-bipyridine) and a variety of trivalent lanthanide ions (Ln = NdIII, EuIII, YbIII). The programmable self-assembly process leading to the trigonal bipyramidal cages formulated as {Ln2[(bpy)2Pd212]3}12+ (3) has been confirmed by one- and two-dimensional NMR, electro-spray-ionization time-of-flight mass-spectroscopy, and in one typical case by single-crystal X-ray diffraction studies. Circular dichroism and circular polarized luminescence spectra confirmed the strict control of stereoselectivity on the heterometallic cages, dictated by the chiral amide groups on the ligands. Excitation (up to 420 nm) on the dipalladium chromophores on these cages leads to the characteristic lanthanide luminescence at both the visible and the near-infrared regions, depending on the lanthanide ions used. Through the assembly-disassembly process, luminescent turn-off sensing toward penicillin among several widely used antibiotics has also been demonstrated with the Europium cage, featuring a limit of detection as low as 0.88 ppb (S/N = 3). Our results pave the way for the construction of chiral 4d-4f supramolecular cages which may find potential applications in luminescent sensing and/or labeling reagents.