Shijun Li

Supramolecular polymers with tunable topologies via hierarchical coordination-driven self-assembly and hydrogen bonding interfaces

A powerful strategy to obtain complex supramolecular materials is the bottom-up construction of noncovalently bound materials by hierarchical self-assembly. This assembly process involves stepwise, uniform increases to the architectural complexity of a substrate, starting from discrete precursors and growing in dimensionality through controlled reactivity to a final product. Herein, two orthogonal processes are exploited: coordination-driven self-assembly and hydrogen bonding. The former relies on the predictable formation of metal–ligand bonds wherein the directionalities of the rigid precursors used determines the structural outcome. The latter uses 2-ureido-4-pyrimidinone interfaces that are structurally robust by virtue of the quadruple hydrogen bonding that can occur between subunits. By combining these two processes into a single system, it is possible to generate hierarchical materials that preserve the attractive tunability associated with discrete supramolecular coordination complexes. For instance, the synthesis of a one-dimensional chain comprising linked metalla-rhomboids is readily adapted to a 2D cross-linked hexagonal network by simply selecting a different metal acceptor precursor as an assembly component. The specific interactions between subunits, in this case platinum(II)-pyridyl bonds and the quadruple H-bonding of ureidopyrimidinone, are unchanged, establishing a unique strategy to obtain supramolecular polymers with marked topological differences with minimal synthetic redesign. In addition, the structural rigidity imposed by the inclusion of the platinum metallacycles serves to minimize the formation of cyclic oligomers, increasing the efficacy of formation and improving the properties of the resultant materials. Furthermore, this study taps the potential of organoplatinum(II) metallacycles in materials science.

Hierarchical self-assembly: well-defined supramolecular nanostructures and metallohydrogels via amphiphilic discrete organoplatinum(II) metallacycles.

Metallacyclic cores provide a scaffold upon which pendant functionalities can be organized to direct the formation of dimensionally controllable nanostructures. Because of the modularity of coordination-driven self-assembly, the properties of a given supramolecular core can be readily tuned, which has a significant effect on the resulting nanostructured material. Herein we report the efficient preparation of two amphiphilic rhomboids that can subsequently order into 0D micelles, 1D nanofibers, or 2D nanoribbons. This structural diversity is enforced by three parameters: the nature of the hydrophilic moieties decorating the parent rhomboids, the concentration of precursors during self-assembly, and the reaction duration. These nanoscopic constructs further interact to generate metallohydrogels at high concentrations, driven by intermolecular hydrophobic and π-π interactions, demonstrating the utility of coordination-driven self-assembly as a first-order structural element for the hierarchical design of functional soft materials.

Deletion of PrBP/δ impedes transport of GRK1 and PDE6 catalytic subunits to photoreceptor outer segments

The mouse Pde6d gene encodes a ubiquitous prenyl binding protein, termed PrBP/δ, of largely unknown physiological function. PrBP/δ was originally identified as a putative rod cGMP phosphodiesterase (PDE6) subunit in the retina, where it is relatively abundant. To investigate the consequences of Pde6d deletion in retina, we generated a Pde6d−/− mouse by targeted recombination. Although manifesting reduced body weight, the Pde6d−/− mouse was viable and fertile and its retina developed normally. Immunocytochemistry showed that farnesylated rhodopsin kinase (GRK1) and prenylated rod PDE6 catalytic subunits partially mislocalized in Pde6d−/− rods, whereas rhodopsin was unaffected. In Pde6d−/− rod single-cell recordings, sensitivity to single photons was increased and saturating flash responses were prolonged. Pde6d−/− scotopic paired-flash electroretinograms indicated a delay in recovery of the dark state, likely due to reduced levels of GRK1 in rod outer segments. In Pde6d−/− cone outer segments, GRK1 and cone PDE6α′ were present at very low levels and the photopic b-wave amplitudes were reduced by 70%. Thus the absence of PrBP/δ in retina impairs transport of prenylated proteins, particularly GRK1 and cone PDE, to rod and cone outer segments, resulting in altered photoreceptor physiology and a phenotype of a slowly progressing rod/cone dystrophy.

Formation of [3]catenanes from 10 precursors via multicomponent coordination-driven self-assembly of metallarectangles.

We describe the formation of a suite of [3]catenanes via multicomponent coordination-driven self-assembly and host-guest complexation of a rectangular scaffold comprising a 90° Pt-based acceptor building block with a pseudorotaxane bis(pyridinium)ethane/dibenzo-24-crown-8 linear dipyridyl ligand and three dicarboxylate donors. The doubly threaded [3]catenanes are formed from a total of 10 molecular components from four unique species. Furthermore, the dynamic catenation process is reversible and can be switched off and on in a controllable manner by successive addition of KPF(6) and 18-crown-6, as monitored by (1)H and (31)P NMR spectroscopy.

Self-Assembly of Triangular and Hexagonal Molecular Necklaces

The formation of catenated systems can be simplified greatly if one or more rings are generated via self-assembly. Herein we exploit the orthogonality of coordination-driven self-assembly and crown-ether host-guest complexation to obtain a [4]molecular necklace and a [7]molecular necklace based on a well-developed recognition motif of 1,2-bis(pyridinium)ethane/dibenzo[24]crown-8. By adapting the bis(pyridinium) motif into the backbone of a donor building block, the resulting semirigid dipyridyl species can serve both as a structural element in the formation of metallacycles and as a site for subsequent host-guest chemistry. The pseudolinear nature of the donor precursor lends itself to the formation of triangular and hexagonal central metallacycles based on the complementary acceptor unit used. This exemplary system organizes up to 18 molecules from three unique species in solution to afford a single supramolecular ensemble.

Template-directed synthesis of flexible porphyrin nanocage and nanorings via one-step olefin metathesis

We describe the fabrication of a suite of flexible porphyrin cages and nanorings from a simple tetraalkene-derived zinc porphyrin monomer via a highly efficient template-directed strategy. The zinc porphyrin monomers were preorganized together by coordination with N atoms of multidentate ligands. Subsequent one-step olefin metathesis furnished a bisporphyrin cage, a triporphyrin nanoring, and a hexaporphyrin nanoring. In the case of the hexaporphyrin nanoring, 24 terminal olefins from six porphyrin monomers reacted with each other to form 12 new double bonds, delivering the final product. The triporphyrin and hexaporphyrin nanorings were further employed as hosts to encapsulate C60 and C70.

Self-Assembly of Chiral Metallacycles and Metallacages from a Directionally Adaptable BINOL-Derived Donor.

We present the formation of a series of chiral metallacycles and metallacages by the use of a BINOL-derived dicarboxylate as a donor that is capable of affording a variety of coordination angles between its two Lewis basic sites. Two squares, two rhomboids, two tetragonal prisms, and one hexagonal prism were successfully formed when the chiral dicarboxylate donor self-assembled with one of four ditopic Pt(II) complexes, including two bimetallic 180° Pt-based acceptors, a 120° bimetallic Pt-based acceptor, and a 90° mononuclear Pt-based acceptor. Their structures were well characterized by (31)P{(1)H} NMR, ESI-MS, CD, and optical rotation analyses.

CuII‐Catalyzed Asymmetric Hydrosilylation of Diaryl‐ and Aryl Heteroaryl Ketones: Application in the Enantioselective Synthesis of Orphenadrine and Neobenodine

With certain amounts of sodium tert-butoxide and tert-butanol as additives, catalytic amounts of an inexpensive and easy-to-handle copper source Cu(OAc)(2)⋅H(2)O, a commercially available and air-stable non-racemic dipyridylphosphine ligand, as well as the stoichiometric desirable hydride donor polymethylhydrosiloxane (PMHS), formed a versatile in situ catalyst system for the enantioselective reduction of a broad spectrum of prochiral diaryl and aryl heteroarylketones in air, in high yields and with good to excellent enantioselectivities (up to 96 %). In particular, the practical viability of this process was evinced by its successful applications in the asymmetric synthesis of optically enriched potent antihistaminic drugs orphenadrine and neobenodine.

Self-assembly of [3]catenanes and a [4]molecular necklace based on a cryptand/paraquat recognition motif

Hierarchical self-assembly centered on metallacyclic scaffolds greatly facilitates the construction of mechanically interlocked structures. The formation of two [3]catenanes and one [4]molecular necklace is presented by utilizing the orthogonality of coordination-driven self-assembly and crown ether-based cryptand/paraquat derivative complexation. The threaded [3]catenanes and [4]molecular necklace were fabricated by using ten and nine total molecular components, respectively, from four and three unique species in solution, respectively. In all cases single supramolecular ensembles were obtained, attesting to the high degree of structural complexity made possible via self-assembly approaches.

Nickel(II)-Dipyridylphosphine-Catalyzed Enantioselective Hydrosilylation of Ketones in Air

Out of thin air: Catalytic amounts of nickel(II) salt and non-racemic dipyridylphosphine ligand, as well as the stoichiometric hydride source PhSiH(3), formed an effective catalyst system for the Ni(II)-catalyzed asymmetric hydrosilylation of a diverse range of electron-deficient aryl alkyl ketones with enantioselectivities up to 90% ee. The practical potential of the protocol was evinced by its good air-stability.