Cheng He

Homochiral metal-organic frameworks for heterogeneous asymmetric catalysis.

Homochiral crystallizations of two enantiomeric metal-organic frameworks (MOFs) Ce-MDIP1 and Ce-MDIP2 were achieved by using L- or D-BCIP as chiral inductions, respectively, where the chiralities were characterized by solid state CD spectra. Ce-MDIPs exhibit excellent catalytic activity and high enantioselectivity for the asymmetric cyanosilylation of aromatic aldehydes; the homochiral Cd-TBT MOF having L-PYI as a chiral adduct exhibits stereochemical catalysis toward the Aldol reactions.

Photoactive Chiral Metal–Organic Frameworks for Light-Driven Asymmetric α-Alkylation of Aldehydes

Chiral metal-organic frameworks (MOFs) with porous and tunable nature show promise as heterogeneous asymmetric catalysts. Through incorporating the stereoselective organocatalyst L- or D-pyrrolidin-2-ylimidazole (PYI) and a triphenylamine photoredox group into a single framework, we have developed two enantiomeric MOFs, Zn-PYI1 and Zn-PYI2, to prompt the asymmetric α-alkylation of aliphatic aldehydes in a heterogeneous manner. The strong reductive excited state of the triphenylamaine moiety within these MOFs initiated a photoinduced electron transfer, rendering an active intermediate for the α-alkylation. The chiral PYI moieties acted as cooperative organocatalytic active sites to drive the asymmetric catalysis with significant stereoselectivity. Control experiments using the lanthanide-based metal-organic frameworks Ho-TCA and MOF-150, assembled from 4,4,4-nitrilotribenzoic acid, as catalysts suggested that both the photosensitizer triphenylamine moiety and the chiral organocatalyst D-/L-PYI moiety were necessary for the light-driven α-alkylation reactions. Further investigations demonstrated that the integration of both photocatalyst and asymmetric organocatalyst into a single MOF makes the enantioselection superior to that of simply mixing the corresponding MOFs with the chiral adduct. The easy availability, excellent stereoselectivity, great separability, and individual components fixed with their well-defined porous and repeating structures make the MOF a versatile platform for a new type of tandem catalyst and cooperative catalyst.

Highly Sensitive Multiresponsive Chemosensor for Selective Detection of Hg2+ in Natural Water and Different Monitoring Environments

A new chemosensor RF1 that combines a ferrocene unit and a rhodamine block via the linkage of a carbohydrazone binding unit was designed and prepared for the highly selective detection of Hg (2+) in natural water. This chemosensor displays great brightness and fluorescence enhancement following Hg (2+) coordination within the limit of detection for Hg (2+) at 1 parts per billion (ppb). The fluorescence intensities are nearly proportional to the amount of Hg (2+) at the ppb level. It is capable of distinguishing between the safe and the toxic levels of inorganic mercury in drinking water. Hg (2+)-binding also arouses the absorption of the rhodamine moiety in RF1 significantly with the chromogenic detection limit for Hg (2+) at 50 ppb. The conventional UV-vis spectroscopic method thus has the potential to provide the critical information about the mercury hazard assessment for industrial wastewater discharging. The obvious and characteristic color change of the titration solution from colorless to pink upon the addition of Hg (2+) demonstrates that RF1 can be used for naked-eye detection of Hg (2+) in water. The Hg (2+) complexation also causes a significant shift of the redox potential about the ferrocene/ferrocenium couple. The electrochemical responses provide the possibility to quantitative analysis of Hg (2+) at the parts per million (ppm) level. Preliminary investigations in natural water samples including seawater and freshwater indicate that RF1 offers a direct and immediate Hg (2+) detection in complex media, pointing out its potential utility in environment monitoring and assessment. The responses of RF1 are Hg (2+) specific, and the chemosensor exhibits high selectivity toward Hg (2+) over other Group 12 metals, alkali, alkaline earth metals, and most of the divalent first-row transition metals. The RF1-Hg (2+) complex is successfully isolated and the Hg (2+)-binding is reversible. The crystal structure and spectral properties of its congener RF2 that contains one ferrocene group and two rhodamine 6G moieties were also investigated for a comparison.

Recognition preference of rhodamine-thiospirolactams for mercury(II) in aqueous solution.

This work presents the design, syntheses, photophysical properties and Hg(2+)-binding of the red-emitting rhodamine derivatives RS1, RS2, and RS3 with different coordination ability and different spatial effects that derived from rhodamine thiohydrazone chromophores and respective carboxaldehydes (benzaldehyde, pyridine-2-carboxaldehyde, ferrocenecarboxaldehyde). Chemosensors RS2 and RS3 afford turn-on fluorescence enhancement and display high brightness in water with the EC(50) for Hg(2+) of 0.5 ppb. The fluorescence intensities are nearly proportional to the amount of Hg(2+) at ppb level, when employing 100 nM probes in water. The fluorescence responses of these two chemosensors are Hg(II) specific, and the probes are selective for Hg(II) over alkali, alkaline earth metals, divalent first-row transition metal ions, and Group 12 congeners Zn(II) and Cd(II), as well as heavy metals Pb(II) and Ag(I). X-ray crystal structure analyses exhibit the thioether derivative of the spirolactone in these compounds. Hg(II)-specific binding in water would make the opening of the spirolactam ring and consequently causes the appearance of strong absorption at visible range, and the obvious and characteristic color change from colorless to pink. Compared to the thioamides, the improved selectivity for Hg(2+) is attributed to the poorer coordination affinity of the thioether over other interference metal ions.

Crystal Structures and Properties of Large Protonated Water Clusters Encapsulated by Metal−Organic Frameworks

A large ionic water cluster H(H(2)O)(28)(+), consisting of a water shell (H(2)O)(26) and an encaged species H(H(2)O)(2)(+) as a center core, was trapped in the well-modulated cavity of a porous metal-organic framework (MOF) {[Co(4)(dpdo)(12)(PMo(12)O(40))(3)](-)}(infinity) and structurally characterized. Degeneration of the protonated water cluster H(H(2)O)(28)(+) into a smaller cluster H(H(2)O)(21)(+) and recovery of H(H(2)O)(28)(+) from the resulting H(H(2)O)(21)(+) cluster in a reversible way demonstrated the unusual stability of the protonated water clusters H(H(2)O)(28)(+) and H(H(2)O)(21)(+) in the robust crystal host. Proton transport and proton/potassium ion exchange through the channels of the crystal host have been investigated by a well-established fluorometry method. X-ray fluorescence experiments and X-ray structural analyses of the exchanged crystals confirmed the occurrence of the proton/potassium ion-exchange reaction and the transformation of the protonated water cluster H(H(2)O)(28)(+) to an ionic cluster K(H(2)O)(27)(+). Comparison of the H(+)/K(+) exchange of H(H(2)O)(28)(+) with that of its neighboring protonated water cluster H(H(2)O)(27)(+) suggested that the abundance of hydrogen bonds associated with the hydronium/water cluster in the H(H(2)O)(28)(+) cluster was essential for proton transport through the Grotthuss mechanism. On the basis of the results, our porous network could be described as a synthetic non-peptide ion channel, in terms of not only structural features but also the functions addressed. Direct observation of the structures of various large ionic water clusters trapped by porous MOFs, coupled with the proton/ion-exchange processes and the reversible dehydration/rehydration, provided valuable insights into the aqueous proton transfer and its mobility pertaining to the large protonated water clusters in the condensed phase.

An Amide-Containing Metal–Organic Tetrahedron Responding to a Spin-Trapping Reaction in a Fluorescent Enhancement Manner for Biological Imaging of NO in Living Cells

Metal-organic polyhedra represent a unique class of functional molecular containers that display interesting molecular recognition properties and fascinating reactivity reminiscent of the natural enzymes. By incorporating a triphenylamine moiety as a bright blue emitter, a robust cerium-based tetrahedron was developed as a luminescent detector of nitronyl nitroxide (PTIO), a specific spin-labeling nitric oxide (NO) trapper. The tetrahedron encapsulates molecules of NO and PTIO within the cavity to prompt the spin-trapping reaction and transforms the normal EPR responses into a more sensitively luminescent signaling system with the limit of detection improved to 5 nM. Twelve-fold amide groups are also functionalized within the tetrahedron to modify the hydrophilic/lipophilic environment, ensuring the successful application of biological imaging in living cells.

“Turn-On” Fluorescent Sensor for Hg2+ via Displacement Approach

A new Cu2+ compound Cu- NB, (where H2 NB is bis(2-hydroxyl-naphthalene-carboxaldehyde) benzil dihydrazone) was synthesized as a highly selective fluorescence chemosensor for the detection of Hg2+ in aqueous media through a displacement turn-on signaling strategy. Whereas the coordination of Cu2+ resulted in a considerable quenching of the typical luminescence of the naphthol rings in Cu-NB, the addition of Hg2+ ion led to a dramatic increase in the emission intensity of Cu-NB at about 530 nm (excitation at 430 nm). The competitive fluorescent experiments showed that alkali, alkaline earth metal ions, the group 12 metals Zn2+, Cd2+, the first-row transition-metal ions such as Mn2+, Fe2+, Co2+, and Ni2+, as well as Pb2+ could not inhibit the Hg2+-binding fluorescent enhancement. It is postulated that the existence of Cu2+ in the luminescent probe Cu-NB could turn away the interferences of other metal cations from Hg2+ detection. The optical responses of the free ligand upon addition of Cu2+ ion, and of the Hg-H2NB compound upon the addition of Cu2+ were also investigated for comparisons.

Structural and Catalytic Performance of a Polyoxometalate-Based Metal−Organic Framework Having a Lanthanide Nanocage as a Secondary Building Block

A polyoxometalate-based lanthanide-organic framework was achieved using the {[Ho(4)(dpdo)(8)(H(2)O)(16)BW(12)O(40)] (H(2)O)(2)}(7+) nanocage as a secondary building block for the heterogeneous catalysis of phosphodiester cleavage in an aqueous solution.

Metallohelical Triangles for Selective Detection of Adenosine Triphosphate in Aqueous Media

Metallohelical triangles consisting of chromophore units and hydrogen-bonding trigger sites were prepared by modulating two tridentate N(2)O units containing amide groups within a central benzene ring at the meta sites, for the selective detection of adenosine trisphosphate in aqueous media over other ribonucleotides.

A mixed-valence (FeII)2(FeIII)2 square for molecular expression of quantum cellular automata

A di-mixed-valence molecular square (Fe(II))(2)(Fe(III))(2) with two extra mobile electrons (or holes) occupying the opposite corners is achieved via self-assembly as a pure phase with remarkable stability for molecular expression of quantum cellular automata (QCA).