Outfitting an aza-crown ether with an organotransition-metal pendant provides a mechanism for tuning its supramolecular properties. The binding affinity can be tuned by more than 2 orders of magnitude by changing the identity of the transition metal-center, altering the overall charge of the complex, or engaging in organometallic ligand substitution reactions. High Li + selectivity (up to 29-fold higher affinity in comparison to Na + ), proton-responsive behavior, and ion pair (ditopic) binding capabilities are observed in the metallacrown ethers.

The catalytic transposition of double bonds holds promise as an ideal route to alkenes with value as fragrances, commodity chemicals, and pharmaceuticals; yet, selective access to specific isomers is a challenge, requiring independent development of different catalysts for different products. In this work, a single cation-responsive iridium catalyst is developed for the selective production of either of two different internal alkene isomers. In the absence of salts, a single positional isomerization of 1-butene derivatives furnishes 2-alkenes with exceptional regioselectivity and stereoselectivity. The same catalyst, in the presence of Na+, mediates two positional isomerizations to produce 3-alkenes. The synthesis of new iridium pincer-crown ether catalysts based on an aza-18-crown-6 ether proved instrumental in achieving cation-controlled selectivity. Experimental and computational studies guided the development of a mechanistic model that explains the observed selectivity for various functionalized 1-butenes, providing insight into strategies for catalyst development based on non-covalent modifications. File list (9) download file view on ChemRxiv Camp_SwitchableRegioselectivity_Manuscript_ChemRxiv... (2.31 MiB) download file view on ChemRxiv Camp_SwitchableRegioselectivity_SI_ChemRxiv_Submis... (3.22 MiB) download file view on ChemRxiv 215c5b.cif (1.97 MiB) download file view on ChemRxiv 218c6a.cif (1.67 MiB) download file view on ChemRxiv 315c5b.cif (1.89 MiB) download file view on ChemRxiv 318c6a.cif (3.47 MiB) download file view on ChemRxiv 318c6b.cif (1.97 MiB) download file view on ChemRxiv Na118c6a.cif (1.31 MiB) download file view on ChemRxiv 218c6b.cif (3.23 MiB)

Cationic gold(I) enamine complexes with the (t-Bu)2(o-biphenyl)phosphine ligand were isolated and characterized by NMR spectroscopy and X-ray crystallography. The complexes display highly distorted coordination modes that are consistent with characterization as α-metalated iminium ions. The barrier to rotation around the formal enamine C–C double bond has been measured in a geminally disubstituted enamine complex, and it is comparable to the barrier to C–C single-bond rotation in electronically restricted alkanes. With additional substitution on the enamine double bond, the complexes remain highly distorted, and the reaction of a mixture of E and Z enamines results in formation of a stereochemically pure gold complex. A survey of binding constants reveals enamines to be significantly stronger donors than any alkenes examined to date, and in the case of a geminally disubstituted enamine, the coordination is stronger even than that of triethylamine. The high stability drives the isomerization of an internal enamine complex generated from an intramolecular hydroamination reaction, to the exocyclic double-bond isomer.

A protocol for identifying ligand binding modes in a series of iridium pincer complexes bearing hemilabile aza-crown ether ligands has been developed using readily accessible NMR methods. The approach was tested on a collection of 13 structurally diverse pincer-crown ether complexes that include several newly characterized species. New synthetic routes enable facile interconversion of coordination modes and supporting ligands. Detailed structural assignments of five complexes reveal that the difference in chemical shift (Δδ) between geminal protons in the crown ether is influenced by diamagnetic anisotropy arising from halides and other ligands in the primary coordination sphere. The average difference in chemical shift between diastereotopic geminal protons in the crown ether macrocycle (Δδ), as determined through a single H-C HSQC experiment, provides information on the pincer ligand binding mode by establishing whether the macrocycle is in close proximity to the metal center. The Δδ values for binding modes that involve chelating ether(s) bound to iridium are roughly 2-fold larger than those for tridentate complexes with no Ir-O bonds.

Quantum dots (QDs) are semiconductor nanocrystals with optical properties that can be tuned through postsynthetic ligand exchanges. Importantly, the stability of QD surfaces in optoelectronic devices is influenced by the ligand shell composition and the structure of the exchange ligand. QDs incorporated into such devices are frequently exposed to excess electronic charges that can localize at the surface via doping, charge hopping, etc. However, changes in the reactivity and stability of QDs upon surface reduction as a function of ligand shell composition are not well understood. In this work, we evaluated the impacts of both surface-binding head group and ligand backbone on the properties and reactivity of PbS QDs through a partial exchange of native oleate ligands with undec-10-enoic acid, p-toluate, and undec-10-ene-1-thiol to access mixed-shell QDs. We compared the reactivity and stability of these mixed-shell QDs in response to surface reduction via the addition of a molecular reductant, cobaltocene (CoCp2). Upon reaction with CoCp2, X-type ligand displacement from the QD surface was observed in each of the mixed-shell systems and monitored via 1H NMR spectroscopy. Comparative studies reveal that indiscriminate and moderate ligand displacement occurs from QDs capped with long-chain carboxylate ligands (ca. 10% ligands displaced), while more dramatic (ca. 20–30%) and preferential displacement of aryl ligands occurs with a mixed shell of alkyl and aryl carboxylates. In contrast, QDs capped with a mix of thiolate, thiol, and carboxylate ligands only exhibit displacement of carboxylate ligands. Overall, this work demonstrates that the extent of surface reduction induced by the addition of a molecular reductant is highly sensitive to the composition of the QD ligand shell.