Synthesis, Structure, Thermal Behavior and cis/trans Isomerization of 2,2'-(EMe₃)₂ (E = C, Si, Ge, Sn) Substituted Azobenzenes.

Jonas Hoffmann,Anne Staubitz,Thomas Kuczmera,Enno Lork

doi:10.3390/molecules24020303

Abstract

The synthesis of a series of 2,2′-bis(trimethyltetrel) azobenzenes is reported, evaluating the different synthetic approaches that different group 14 element substituents individually require. The synthetic access to the carbon substituted congener is very different from the heavier tetrels, in that the key step is the formation of the N=N bond in azobenzene, rather than the azobenzene-C bond. Sn could be introduced with a cross-coupling route, whereas the Si and Ge congeners were prepared by a stannylation-lithiation-electrophilic quenching sequence. Iodo-lithium exchange was also a possible route to obtain the dilithiated species, which can be attributed to the chelating effect of the nitrogen atoms. However, the organo-lead species could not be obtained via these routes. The resulting structures were fully characterized (NMR, FTIR, HRMS and XRD). Furthermore, their thermal properties (TGA and DSC) and their photoswitching behavior in solution (UV-VIS & NMR experiments) were investigated and compared for the different tetrels (C, Si, Ge, Sn).

Highlights

In main group chemistry, trends within a group play an important role for the understanding of how particular properties of a molecule can be tuned
Due to our interest in both main group chemistry and photoswitchable molecules, we systematically investigated the influence that could be exerted by tetrels in the ortho-position of azobenzenes, as well as the structural influences this substitution might trigger: Azobenzenes are thermally and photochemically stable molecular switches that can undergo a reversible trans–cis isomerization under illumination
Since we developed a method for the lithiation of azobenzenes by halogen-lithium exchange reactions for para- and meta-substituted azobenzenes [23,24], we were interested to transfer this methodology towards disubstituted ortho-azobenzenes

Summary

Introduction

Trends within a group play an important role for the understanding of how particular properties of a molecule can be tuned. In which an atom is replaced by another element from the same group, the geometric shape of the molecules can be changed. It has been shown that for cyclopentadiene analogs, in which the sp substituted C atom is replaced by a heavier group 14 element, the HOMO-LUMO gap (HOMO = highest occupied molecular orbital; LUMO = lowest unoccupied molecular orbital) is reduced due to σ*-π* conjugation [2,3,4,5,6]. Due to our interest in both main group chemistry and photoswitchable molecules, we systematically investigated the influence that could be exerted by tetrels in the ortho-position of azobenzenes, as well as the structural influences this substitution might trigger: Azobenzenes are thermally and photochemically stable molecular switches that can undergo a reversible trans–cis isomerization under illumination. The properties of Molecules 2019, 24, 303; doi:10.3390/molecules24020303 www.mdpi.com/journal/molecules

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