Zhongrui Chen

N‐Substituted Azacalixphyrins: Synthesis, Properties, and Self‐Assembly

Pre- and postintroduction of substituents with respect to the macrocyclization step leads to previously unknown N-substituted azacalixphyrins. The stepwise synthetic approach has been studied in detail to highlight the key role of the N-substituents of the precursors and/or intermediates in terms of reactivity. Based on a combined experimental and theoretical investigation, the relationship between the properties of the macrocycles and their degree of substitution is rationalized. Depending on the nature of the N-substituents, the formation of supramolecular ribbon-like structures could also be observed, as demonstrated by combined TEM, SEM, AFM, and FTIR experiments.

Exceptional Stability of Azacalixphyrin and Its Dianion

Azacalixphyrin is a recently synthesized precursor of potentially highly versatile analogues of porphyrins. Surprisingly, this macrocyclic compound is stable to such an extent that it could be exposed to air for months as a solid or for days in solution without detectable changes. However, no rationalization of this specific property has been established yet as the structure/electronic features of this novel compound are still very much unknown. Here, we present an extensive theoretical study on the stability of azacalixphyrin. We use density functional theory to quantify both its aromaticity and its reactivity, or more precisely its lack thereof, with water. In addition, since we find that neutral azacalixphyrin has the ability to capture easily two electrons, we also discuss the results obtained for its dianion. We show that the azacalixphyrin core is strongly aromatic despite the antiaromatic peripheric rings, and we find that all reactions with water are extremely endergonic, hence explaining the stability of azacalixphyrin. Upon reduction, the aromatic signature is reversed as the center becomes antiaromatic while the antiaromatic character of the peripheric rings is either remarkably reduced or completely annihilated. Accordingly, we find that the reactivity of the dianion with water is considerably inhibited.

Transamination at the Crossroad of the One-Pot Synthesis of N-Substituted Quinonediimines and C-Substituted Benzobisimidazoles

A green and very efficient synthesis of N-substituted benzoquinonediimines or C-substituted benzo-bis(imidazole) derivatives is described under similar conditions. The different reaction pathway is only controlled by the nature of the primary amines, which tunes the reactivity of the intermediates.

1,3-Alternate Tetraamido-Azacalix[4]arenes as Selective Anion Receptors.

Six tetraaza[1.1.1.1]cyclophane derivatives bearing peripheral amide groups were prepared according to two distinct synthetic strategies that depend on the connection pattern between the aryl units. NMR experiments combined with the X-ray structures of two tetraamide derivatives 4 b and 10 show that these cavitands adopt a 1,3-alternate conformation both in solution and in the solid state. Consequently, the four amide groups of the aza[1.1.1.1]-m,m,m,m-cyclophane isomer 10 can contribute to the same recognition process towards neutral water molecules or anion guests. NMR experiments, mass spectrometry analyses and single-crystal X-ray structures confirm the anion-binding ability of this receptor. Absorption spectrophotometric titrations in nonpolar solvents provided evidence for the selectivity of 10 to chloride anions in the halide series, with a corresponding association constant Ka reaching 2.5 × 10(6) m(-1).

Synthesis and Combined Experimental and Theoretical Characterization of Dihydro-tetraaza-acenes

We present a combined experimental and theoretical study of electronic and optical properties of dihydro-tetraaza-acenes (DHTAn). Using solvent-free condensation, we are able to synthesize not only DHTA5 but also the longer DHTA6 and DHTA7 molecules. We then investigate their gas-phase electronic structures by means of ab initio density functional calculations employing an optimally tuned range-separated hybrid functional. By comparing with the parent linear oligoacenes (nA) and based on computed ionization potentials and electron affinities, we predict DHTAn molecules to be more stable than acenes of the same length, where we expect DHTAn molecules to be persistent at least up to n = 7 rings. We further exploit the analogy with nA by analyzing the entire intramolecular π-band structure of the DHTAn molecules. This clearly reveals that the additional two electrons donated by the dihydropyrazine group are delocalized over the entire molecule and contribute to its π-electron system. As a consequence, the symmetry of the frontier orbitals of DHTAn differs from that of the parent nA molecule. This also affects the UV–vis absorption spectra which have been measured for DHTA5, 6, and 7 dissolved in dimethyl sulfoxide and analyzed by means of excited state calculations within a time-dependent density functional theory framework.

Regioselective addition of DDQ on a quinoid ring: an entry into chiral zwitterionic bridging ligands

The regioselective insertion of DDQ into a C–H bond of the 6π + 6π electron zwitterionic benzoquinonemonoimines 4a–c results in the formation of novel chiral C-substituted quinoid ligands 11a–c. These Michael adducts feature a preserved zwitterionic form and a quaternary stereogenic carbon center as evidenced by the single crystal X-ray structure of the derivative 11a. The ECD spectra, optical rotations and racemization barriers of the two enantiomers of 11a were measured subsequently to their separation using preparative chiral HPLC. Because the racemization of 11a is stopped in basic media, compounds 11a–c give a new entry in chiral zwitterionic bridging ligands.

Hetero-Bimetallic Effect as a Route to Access Multinuclear Complexes

We report the synthesis of a key mononuclear intermediate complex based on a quinoid ligand and its further metalation to afford the corresponding hetero-bimetallic compound that revealed unique properties. An unprecedented hetero-bimetallic effect in coordination chemistry could be indeed observed and exploited to prepare, through selective ligand exchange, a tetranuclear complex (Pd-Ni-Ni-Pd) absorbing light up to the far-red region. Most importantly, we describe here to the best of our knowledge the first use of bischelating ligand for ligand exchange, and this approach can be considered as a new route for incorporating planar units to access multi-heteronuclear complexes. The origin of this specific ligand exchange as well as of the nature of the electronic excited states of the relevant structures were investigated by first-principle calculations.

Straightforward metal-free synthesis of an azacalix[6]arene forming a host–guest complex with fullerene C60

Electronic Supplementary Information (ESI) available: Details of the TD-DFT calculations, NMR spectra of 3b and 4b, preliminary spectrophotometric titration experiments. See a Contrasting with the well mastered azacalix[4]arene, the synthesis and characterization of azacalix[6]arene derivatives remains a challenge for chemists. In this framework, bulky n-octylamino groups appended on the 1,3-meta-diaminobenzene 2b can lead to the concomittant and straightforward one-pot formation of the azacalix[6]arene 4b and of the thermodynamically favoured azacalix[4]arene 3b through simple consecutive nucleophlic aromatic substitutions. The conformations of 3b and 4b were studied in solution and in the solid state using their molecular structures, their NMR and UV-visible spectra combined with first principle TD-DFT calculations. Preliminay solution experiments show that 4b can form a 1:1 host-guest complex with C60.