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

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).


Reagents and Solvents
All reagents were used without purification unless stated otherwise.
All solvents for purification and extraction were used as received. In the case of n-pentane, the solvent was distilled prior use.
All solvents which were used for synthesis under inert conditions were dried by a solvent purification system from Inert Technologies. Scheme SI1. Direct lithiation of the iodo-azobenzene 2 with n-butyllithium and further reaction with trimethylsilyl chloride (9).

Via tin-lithium-exchange
In a J. Young's tube, 8 (100 mg, 200 µmol) was dissolved in THF (10 mL) and cooled to -78 °C. Then MeLi (0.36 mL, 0.60 mmol, 1.66 M in Et 2 O) was added over the course of 5 min. The reaction mixture turned black. After 60 min at this temperature, Me 3 PbBr (200 mg, 1.26 mmol, dissolved in 2 mL of THF) was added in one portion. After 10 min at this temperature, the color changed to brown. The reaction mixture was warmed to 23 °C for 14 h, stirred for 2 d, to result in an orange color with a dark precipitate in the flask. Afterwards the solvent was removed; 1 H NMR analysis revealed an undefined mixture of products. The crude product was subjected to column chromatography (silica, pentane) but this purification attempt did not give any product.
Scheme SI4. Attempt to synthesize plumbinated azobenzene 11 via a tin-lithium exchange reaction followed by transmetallation to copper, followed by quenching with a lead electrophile.
In a J. Young's tube, 8 (406 mg, 800 µmol) was dissolved in THF (20 mL) and cooled to -78 °C.  Figure SI1: 1 H NMR spectrum of compound 2. The less intense signals in the aromatic region can be assigned to the cis-isomer.

UV/Vis Spectra and 1 H NMR Spectra of the Switching Experiments
The solutions for the UV-vis spectra were prepared using a stock solution with the appropriate amount of compound in cyclohexane and diluting it to obtain different concentrations.
For the switching experiments we used the maximum of the ππ*-band and not a absorption value at a fixed wavelength, since the position of the absorption maximum shifted slightly during the switching process.
2,2'-Di(tert-butyl)azobenzene (10) Figure SI15: Absorption spectra of compound 10 and linear fitting to according to Lambert-Beer's-law.                    Due to the fact that the TG analyses showed a constant loss of mass at higher temperature we were interested in the nature of this process. Therefore we interrupted the measurement at 220 °C at measured 1 H and 13 C NMR spectra.

S31
The obtained spectra are in agreement with the spectra of the purified materials indicating no decomposition but evaporation of the molten azobenzene.