Preferential Formation of Three-layered Host-Guest Complexes of a Planar Dinuclear Metallohost with Alkali Metal Ions

We synthesized a planar macrocyclic dinuclear nickel(II) metallohost from the corresponding macrocyclic imine ligand containing two N2O2 chelate coordination sites and an O6 cation binding site like 18-crown-6 as well as peripheral hexyl groups. Due to the lipophilic nature of the hexyl groups, the metallohost was soluble in less polar media where its interaction with alkali metal ions was enhanced. The binding studies by NMR spectroscopy clearly showed its strong tendency to form multi-layered structures. The metallohost formed 2 : 1 and 1 : 1 (host/guest) complexes with Na+ with the two-step binding constants of logK1=6.6 and logK2=3.0. In contrast, its complexation with larger alkali metal ions (K+, Rb+, Cs+) preferentially gave 3 : 2 (host/guest) complexes when 2/3 equiv. of the guest was present. The three-layered structures of these 3 : 2 complexes were well characterized by mass spectrometry and 2D COSY/ROESY experiments as well as DFT calculations, elucidating their unique structural feature with three chemically different environments due to the oppositely curved two [Ni(saloph)] moieties of the metallohost. Therefore, the three-layered structures were preferentially formed when larger alkali metal ions (K+, Rb+, Cs+) were complexed with the metallohost.

Stack bonding in polyaromatic hydrocarbons

Parallel displacement of π-stacked component molecules enhances the efficiency of organic semiconductors by maximizing interpenetration of the π-densities. Dimers of symmetric polyaromatic hydrocarbons coronene, hexabenzo[bc,de,gh,kl,no,qr]coronene, circumcoronene, kekulene, and circumcircumcoronene are examined using density functional theory from the stack bonding perspective which considers π-stacking interactions in terms of contributions of monomer π-orbital overlap to the character of dimer orbitals. Energetically favored parallel displaced and/or twisted dimer conformations are consistent with patterns of mixing of the monomer molecular orbitals (MOs) that maximize interpenetration of the π densities. The multiple minima found along parallel displacement (PD) coordinates coincide with the formation of dimer MOs formally antibonding between the monomers at the sandwich conformation to bonding at the PD minima. Minima identified with favorable stack bonding are consistent with polymorphs found in large polyaromatic hydrocarbons.

Cyclophane with eclipsed pyrene units enables construction of spin interfaces with chemical accuracy

Advanced functionality in molecular electronics and spintronics is orchestrated by exact molecular arrangements at metal surfaces, but the strategies for constructing such arrangements remain limited. Here, we report the synthesis and surface hybridization of a cyclophane that comprises two pyrene groups fastened together by two ferrocene pillars. Crystallographic structure analysis revealed pyrene planes separated by ∼352 pm and stacked in an eclipsed geometry that approximates the rare configuration of AA-stacked bilayer graphene. We deposited this cyclophane onto surfaces of Cu(111) and Co(111) at submonolayer coverage and studied the resulting hybrid entities with scanning tunnelling microscopy (STM). We found distinct characteristics of this cyclophane on each metal surface: on non-magnetic Cu(111), physisorption occurred and the two pyrene groups remained electronically coupled to each other; on ferromagnetic Co(111) nanoislands, chemisorption occurred and the two pyrene groups became electronically decoupled. Spin-polarized STM measurements revealed that the ferrocene groups had spin polarization opposite to that of the surrounding Co metal, while the pyrene stack had no spin polarization. Comparisons to the non-stacked analogue comprising only one pyrene group bolster our interpretation of the cyclophane's STM features. The design strategy presented herein can be extended to realize versatile, three-dimensional platforms in single-molecule electronics and spintronics.

A chemical strategy for the bottom-up construction of 3D spin interfaces is presented. Scanning tunnelling microscopy reveals distinct electronic features of a cyclophane with precisely designed pi-stacking on ferromagnetic Co(111) nanoislands.

Mild deprotection of the N-tert-butyloxycarbonyl (N-Boc) group using oxalyl chloride

We report a mild method for the selective deprotection of the N-Boc group from a structurally diverse set of compounds, encompassing aliphatic, aromatic, and heterocyclic substrates by using oxalyl chloride in methanol. The reactions take place under room temperature conditions for 1–4 h with yields up to 90%. This mild procedure was applied to a hybrid, medicinally active compound FC1, which is a novel dual inhibitor of IDO1 and DNA Pol gamma. A broader mechanism involving the electrophilic character of oxalyl chloride is postulated for this deprotection strategy.

We report a mild method for the selective deprotection of the N-Boc group from a structurally diverse set of compounds, encompassing aliphatic, aromatic, and heterocyclic substrates by using oxalyl chloride in methanol.

Controlling the structure and photophysics of fluorophore dimers using multiple cucurbit[8]uril clampings

A modular strategy has been employed to develop a new class of fluorescent molecules, which generates discrete, dimeric stacked fluorophores upon complexation with multiple cucurbit[8]uril macrocycles. The multiple constraints result in a “static” complex (remaining as a single entity for more than 30 ms) and facilitate fluorophore coupling in the ground state, showing a significant bathochromic shift in absorption and emission. This modular design is surprisingly applicable and flexible and has been validated through an investigation of nine different fluorophore cores ranging in size, shape, and geometric variation of their clamping modules. All fluorescent dimers evaluated can be photo-excited to atypical excimer-like states with elongated excited lifetimes (up to 37 ns) and substantially high quantum yields (up to 1). This strategy offers a straightforward preparation of discrete fluorophore dimers, providing promising model systems with explicitly stable dimeric structures and tunable photophysical features, which can be utilized to study various intermolecular processes.

Dimerisation of a wide range of fluorophores through multiple CB[8] clampings leads to constrained intracomplex motion and distinct photophysical properties.