We have investigated the influence of electron density on the three-center [N–I–N]⁺ halogen bond. A series of [bis(pyridine)iodine]⁺ and [1,2-bis((pyridine-2-ylethynyl)benzene)iodine]⁺ BF₄^- complexes substituted with electron withdrawing and donating functionalities in the para-position of their pyridine nitrogen were synthesized and studied by spectroscopic and computational methods. The systematic change of electron density of the pyridine nitrogens upon alteration of the para-substituent (NO₂, CF₃, H, F, Me, OMe, NMe₂) was confirmed by ¹⁵N NMR and by computation of the natural atomic population and the π electron population of the nitrogen atoms. Formation of the [N–I–N]⁺ halogen bond resulted in >100 ppm ¹⁵N NMR coordination shifts. Substituent effects on the ¹⁵N NMR chemical shift are governed by the π population rather than the total electron population at the nitrogens. Isotopic perturbation of equilibrium NMR studies along with computation on the DFT level indicate that all studied systems possess static, symmetric [N–I–N]⁺ halogen bonds, independent of their electron density. This was further confirmed by single crystal X-ray diffraction data of 4-substituted [bis(pyridine)iodine]⁺ complexes. An increased electron density of the halogen bond acceptor stabilizes the [N–I–N]⁺ bond, whereas electron deficiency reduces the stability of the complexes, as demonstrated by UV-kinetics and computation. In contrast, the N–I bond length is virtually unaffected by changes of the electron density. The understanding of electronic effects on the [N–X–N]⁺ halogen bond is expected to provide a useful handle for the modulation of the reactivity of [bis(pyridine)halogen]⁺-type synthetic reagents.

A novel NHC‐catalyzed annulation protocol for the asymmetric synthesis of biologically important β‐lactam fused spirocyclopentane oxindoles with four contiguous stereocenters, including two quaternary carbon centers, was developed. Alternatively, spirocyclopentane oxindoles containing an enaminone moiety can be achieved using the same starting materials, isatin‐derived enals, and N‐sulfonyl ketimines, in the presence of a slightly different NHC catalytic system. This switchable annulation strategy enables the selective assembly of both heterocyclic scaffolds with good yields and excellent enantioselectivities for a broad range of substrates.

Halogen bonding (XB), a non-covalent interaction between an electron-deficient halogen atom and a Lewis base, is widely adopted in organic synthesis and supramolecular crystal engineering. However, the roadmap towards materials applications is hindered by the challenges in harnessing this relatively weak intermolecular interaction to devise human-commanded stimuli-responsive soft materials. Here, we report a liquid crystalline network comprising permanent covalent crosslinks and dynamic halogen bond crosslinks, which possess reversible thermo-responsive shape memory behaviour. Our findings suggest that I···N halogen bond, a paradigmatic motif in crystal engineering studies, enables temporary shape fixation at room temperature and subsequent shape recovery in response to human body temperature. We demonstrate versatile shape programming of the halogen-bonded polymer networks through human-hand operation and propose a micro-robotic injection model for complex 1D to 3D shape morphing in aqueous media at 37 °C. Through systematic structure-property-performance studies, we show the necessity of the I···N crosslinks in driving the shape memory effect. The halogen-bonded shape memory polymers expand the toolbox for the preparation of smart supramolecular constructs with tailored mechanical properties and thermoresponsive behaviour, for the needs of, e.g., future medical devices.