Chemical Recycling of Polyethylene by Tandem Catalytic Conversion to Propylene.

Although polyethylene (PE) and polypropylene (PP) are by far the world's largest volume plastics, only a tiny fraction of these energy-rich polyolefins are currently recycled. Depolymerization of PE to its constituent monomer, ethylene, is highly endothermic and conventionally accessible only through unselective, high-temperature pyrolysis. Here, we provide experimental demonstrations of our recently proposed tandem catalysis strategy, which uses ethylene to convert PE to propylene, the commodity monomer used to make PP. The approach combines rapid olefin metathesis with rate-limiting isomerization. Monounsaturated PE is progressively disassembled at modest temperatures via many consecutive ethenolysis events, resulting selectively in propylene. Fully saturated PE can be converted to unsaturated PE starting with a single transfer dehydrogenation to ethylene, which produces a small amount of ethane (1 equiv per dehydrogenation event). These principles are demonstrated using both homogeneous and heterogeneous catalysts. While selectivity under batch conditions is limited at high conversion by the formation of an equilibrium mixture of olefins, high selectivity to propylene (≥94%) is achieved in a semicontinuous process due to the continuous removal of propylene from the reaction mixture.

Catalyst coaxed to make branched α-olefins.

Process conditions allow a titanium catalyst to add just two ethylene molecules to an α-olefin.

Duplex-selective ruthenium-based DNA intercalators.

We report the design and synthesis of small molecules that exhibit enhanced luminescence in the presence of duplex rather than single-stranded DNA. The local environment presented by a well-known [Ru(dipyrido[3,2-a:2',3'-c]phenazine)L2 ](2+) -based DNA intercalator was modified by functionalizing the bipyridine ligands with esters and carboxylic acids. By systematically varying the number and charge of the pendant groups, it was determined that decreasing the electrostatic interaction between the intercalator and the anionic DNA backbone reduced single-strand interactions and translated to better duplex specificity. In studying this class of complexes, a single Ru(II) complex emerged that selectively luminesces in the presence of duplex DNA with little to no background from interacting with single-stranded DNA. This complex shows promise as a new dye capable of selectively staining double- versus single-stranded DNA in gel electrophoresis, which cannot be done with conventional SYBR dyes.

Allosteric supramolecular coordination constructs.

Coordination chemistry is regularly used to generate supramolecular constructs with unique environments around embedded components to affect their intrinsic properties. In certain cases, it can also be used to effect changes in supramolecular structure reminiscent of those that occur within stimuli-responsive biological structures, such as allosteric enzymes. Indeed, among a handful of general strategies for synthesizing such supramolecular systems, the weak-link approach (WLA) uniquely allows one to toggle the frameworks' structural state post-assembly via simple reactions involving hemilabile ligands and transition metal centers. This synthetic strategy, when combined with dynamic ligand sorting processes, represents one of the few sets of general reactions in inorganic chemistry that allow one to synthesize spatially defined, stimuli-responsive, and multi-component frameworks in high to quantitative yields and with remarkable functional group tolerance. The WLA has thus yielded a variety of functional systems that operate similarly to allosteric enzymes, toggling activity via changes in the frameworks' steric confinement or electronic state upon the recognition of small molecule inputs. In this Perspective we present the first full description of the fundamental inorganic reactions that provide the foundation for synthesizing WLA complexes. In addition, we discuss the application of regulatory strategies in biology to the design of allosteric supramolecular constructs for the regulation of various catalytic properties, electron-transfer processes, and molecular receptors, as well as for the development of sensing and signal amplification systems.

General strategy for the synthesis of rigid weak-link approach platinum(II) complexes: tweezers, triple-layer complexes, and macrocycles.

Air-stable, heteroligated platinum(II) weak-link approach (WLA) tweezer and triple-layer complexes that possess P,X-Aryl hemilabile ligands (P^ = Ph2PCH2CH2-, X = chalcoethers or amines) have been synthesized via the halide-induced ligand rearrangement (HILR) reaction, using a one-pot, partial chloride-abstraction method. The approach is general and works with a variety of phosphine-based hemilabile ligands; when a P,S-Ph ligand is used as the relatively strongly chelating ligand, heteroligated complexes are formed cleanly when an ether- (P,O-Ph), amine- (P,N-Ph2), or fluorinated thioether-based (P,S-C6F4H) hemilabile ligand is used as the weakly chelating counterpart. The HILR reaction has also been used to synthesize bisplatinum(II) macrocycles free of oligomeric material without having to resort to the high-dilution conditions typical for macrocycle synthesis. This approach is complementary to the traditional WLA to the synthesis of macrocyclic complexes which typically proceeds via fully closed, chloride-free intermediates. The structures of the complexes may be toggled between semiopen (with only one chelating ligand) and fully closed (with both ligands chelating) via the abstraction and addition of chloride.

Elucidating the mechanism of the halide-induced ligand rearrangement reaction.

The formation of heteroligated Rh(I) complexes containing two different hemilabile phosphinoalkyl ligands, (κ(2)-Ph(2)PCH(2)CH(2)S-Aryl)(κ(1)-Ph(2)PCH(2)CH(2)O-C(6)H(5))RhCl, through a halide-induced ligand rearrangement (HILR) reaction has been studied mechanistically. The half-life of this rearrangement reaction depends heavily on the Rh(I) precursor used and the chelating ability of the phosphinoalkyl thioether (PS) ligand, while the chelating ability of the phosphinoalkyl ether (PO) ligand has less of an effect. An intermediate complex which contains two PO ligands, (nbd)(κ(1)-Ph(2)PCH(2)CH(2)O-C(6)H(5))(2)RhCl (nbd = norbornadiene), converts to (nbd)(κ(1)-Ph(2)PCH(2)CH(2)O-C(6)H(5))RhCl resulting in a free PO ligand. The free PO ligand can then react with a homoligated PS complex [(κ(2)-Ph(2)PCH(2)CH(2)S-Aryl)(2)Rh](+)Cl(-) producing the heteroligated product. The PS ligand generated during the reaction pathway can be trapped by the monoligated PO complex (nbd)(κ(1)-Ph(2)PCH(2)CH(2)O-C(6)H(5))RhCl, leading to the formation of the same heteroligated product. In this study, some of the key intermediates and reaction steps underlying the HILR reaction have been identified by variable temperature (31)P{(1)H} NMR spectroscopy and in two cases by single-crystal X-ray diffraction studies. Significantly, this work provides mechanistic insight into the HILR process, which is a key reaction used to prepare a large class of highly sophisticated three-dimensional metallosupramolecular architectures and allosteric catalysts.

A coordination chemistry dichotomy for icosahedral carborane-based ligands.

Although the majority of ligands in modern chemistry take advantage of carbon-based substituent effects to tune the sterics and electronics of coordinating moieties, we describe here how icosahedral carboranes-boron-rich clusters-can influence metal-ligand interactions. Using a series of phosphine-thioether chelating ligands featuring meta- or ortho-carboranes grafted on the sulfur atom, we were able to tune the lability of the platinum-sulfur interaction of platinum(II)-thioether complexes. Experimental observations, supported by computational work, show that icosahedral carboranes can act either as strong electron-withdrawing ligands or electron-donating moieties (similar to aryl- or alkyl-based groups, respectively), depending on which atom of the carborane cage is attached to the thioether moiety. These and similar results with carborane-selenol derivatives suggest that, in contrast to carbon-based ligands, icosahedral carboranes exhibit a significant dichotomy in their coordination chemistry, and can be used as a versatile class of electronically tunable building blocks for various ligand platforms.

Chelating effect as a driving force for the selective formation of heteroligated Pt(II) complexes with bidentate phosphino-chalcoether ligands.

The halide-induced ligand rearrangement reaction (HILR) has been employed to provide selective and exclusive in situ formation of heteroligated Rh(I), Pd(II), and Pt(II) complexes with bidentate phosphino-chalcoether ligands. To gain insights on the nature of this unique reaction, we explored this process via the stepwise addition of bidentate phosphino-chalcoether (P, X; X = S or Se) and relevant monodentate phosphine ligands with a Pt(II) metal precursor. The corresponding monoligated complexes were obtained in quantitative yields by reacting 1 equiv of a P, X bidentate ligand with Pt(II) and were fully characterized via single crystal X-ray diffraction studies and heteronuclear ((31)P, (77)Se, and (195)Pt) NMR spectroscopy in solution. These species were further reacted with a second equivalent of either a bidentate ligand or the monodentate ethyl diphenylphosphine ligand, resulting in the clean formation of the heteroligated species or, in the case of the monodentate ligand with an electron-withdrawing bidentate ligand, a mixture of products. On the basis of competitive exchange reactions between these heteroligated, homoligated, and monoligated complexes, we conclude that ligand chelation plays a crucial role in the Pt(II) HILR. The in situ preferable formation of the stable monoligated complex allows for ligand sorting to occur in these systems. In all cases where the heteroligated product results, the driving force to these species is ligand chelation.

Selective formation of heteroligated Pt(II) complexes with bidentate phosphine-thioether (P,S) and phosphine-Selenoether (P,Se) ligands via the halide-induced ligand rearrangement reaction.

Bidentate phosphine-selenoether (P,Se) ligands were synthesized, and their heteroligated Pt(II) complexes were made and studied. The unique "P,S/P,Se" ligand coordination to Pt(II) can be realized via the halide-induced ligand rearrangement reaction. In all cases, the exclusive formation of semi-open heteroligated complexes was achieved as shown by (31)P and (77)Se NMR spectroscopy and from single crystal X-ray diffraction studies. This is the first example of the use of (77)Se NMR spectroscopy to characterize these types of structures through direct observation of the weak-link interaction with the metal center. Heteroligated structure formation is believed to be driven by the relative electron-donating ability of the substituent groups on the seleno or thioether moieties. This effect is studied by comparing the structures of corresponding "P,SMe" and "P,SeMe" complexes bearing a hemilabile "P,SCH(2)CF(3)" group, which is less sterically demanding than "P,SPh" but is similar in terms of electron withdrawing ability.

Allele-specific inhibition of divergent protein tyrosine phosphatases with a single small molecule.

A central challenge of chemical biology is the development of small-molecule tools for controlling protein activity in a target-specific manner. Such tools are particularly useful if they can be systematically applied to the members of large protein families. Here we report that protein tyrosine phosphatases can be systematically 'sensitized' to target-specific inhibition by a cell-permeable small molecule, Fluorescein Arsenical Hairpin Binder (FlAsH), which does not inhibit any wild-type PTP investigated to date. We show that insertion of a FlAsH-binding peptide at a conserved position in the PTP catalytic-domain's WPD loop confers novel FlAsH sensitivity upon divergent PTPs. The position of the sensitizing insertion is readily identifiable from primary-sequence alignments, and we have generated FlAsH-sensitive mutants for seven different classical PTPs from six distinct subfamilies of receptor and non-receptor PTPs, including one phosphatase (PTP-PEST) whose three-dimensional catalytic-domain structure is not known. In all cases, FlAsH-mediated PTP inhibition was target specific and potent, with inhibition constants for the seven sensitized PTPs ranging from 17 to 370 nM. Our results suggest that a substantial fraction of the PTP superfamily will be likewise sensitizable to allele-specific inhibition; FlAsH-based PTP targeting thus potentially provides a rapid, general means for selectively targeting PTP activity in cell-culture- or model-organism-based signaling studies.