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Conjugated Hybrid Polymers from Unsaturated Heavier Group 14 Building Blocks

Periodic Reporting for period 1 - POLY-SAAR (Conjugated Hybrid Polymers from Unsaturated Heavier Group 14 Building Blocks)

Reporting period: 2019-07-01 to 2021-06-30

The introduction of main-group elements into the main chain of polymers generally leads to the emergence of hitherto unknown, and often useful physical and chemical properties that are not attainable with purely organic systems. The incorporation of heavier group 14 double bonds (E=E) (E = Si, Ge) into the main chain of polymers however poses considerable synthetic challenges. Our work builds on the emerging catalyst- and byproduct-free protocol for the generation of E=E double bonds from the organo silane/germane precursors.
Our investigations started with the use of 2,4,6-trimethoxyphenyl (TMOP) substituent as a protecting group for compounds with Si=Si double bonds (disilenes), which was met with limited success. By using 2,6-dimethoxyphenyl (DMOP) substituent, however, we were able to synthesize both cis- and trans-disilenes with the DMOP groups in 1,2-position. The reduction of these disilenes with alkali metals unfortunately led to the isolation of disilane.

In order to synthesize Janus head-type bis(germylenes) (and eventually apply them in polymer synthesis), a reported NHC-coordinated four-membered germylene with an exocyclic imino functionality and a chloro substituent was subjected to 2e– reduction by potassium graphite (KC8), which afforded potassium silagermenide instead of the targeted sp3 silyl anion. X-ray diffraction analysis on single crystals revealed that potassium silagermenide forms a 1D coordination polymer in the solid state. The potassium silagermenide is also used as valuable synthon for the synthesis of functional silagermenes.

With a view to hitherto unknown organometallic polymers with silagermenylidene repeat units, the reactivity of a literature-known silagermenylidene towards a variety of transition metal precursors was investigated. Specifically, the treatment of silagermenylidene [R2(Cl)Si−(R)Si(NHC)Ge:] with [Ni(COD)2] (R = 2,4,6-triisopropylphenyl; NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; COD= 1,5-cyclooctadiene) afforded the first transition metal complex of a heavier allylic pi-system. The reaction of the latter with one equivalent of NHC produced the first heavier cyclopropene-transition metal complexes. Notably, the coordination mode in these complexes is different from the metallacyclopropane and pi-complex extremes of the Dewar-Chatt-Duncanson model: the sigma-component of the Ge=Si double bond acts as donor and acceptor leaving behind a nearly unsupported Si–Ge pi-bond.
The project was highly successful yielding exciting results and resulted in three high-profile publications (see below). Two further publications are currently being finalized. The fellow (PKM) has received advanced training and acquired significant hands-on experience in the field of low-valent silicon and germanium chemistry. PKM has also gained new expertise in the application of advanced computational calculations, which complements his previous expertise. The results obtained during the lifetime of this project was presented by PKM in three workshops (one in Switzerland and other two in Germany). In total, the experience of Saarbrücken as MSC fellow and the high-profile publications resulted from this MSC project have equipped the fellow well to obtain an independent faculty position which he is currently looking for.

List of publications:

1. P. K. Majhi, M. Zimmer, B. Morgenstern, V. Huch, D. Scheschkewitz, “Transition Metal Complexes of Heavier Vinylidenes: Allylic Coordination vs Vinylidene-Alkyne Rearrangement at Nickel” J. Am. Chem. Soc. 2021, DOI:10.1021/jacs.1c06453.

2. P. K. Majhi, V. Huch, D. Scheschkewitz, “A Mixed Heavier Si=Ge Analogue of a Vinyl Anion” Angew. Chem. 2021, 133, 246-250; Angew. Chem. Int. Ed. 2021, 60, 242-246.

3. P. K. Majhi, M. Zimmer, B. Morgenstern, D. Scheschkewitz, “Transition Metal Complexes of Heavier Cyclopropenes: non-Dewar-Chatt-Duncanson Coordination and Facile Si=Ge Functionalization” J. Am. Chem. Soc. 2021, 143, 8981-8986. (This work was also highlighted by David Schilter in Nature Reviews Chemistry, 2021, 5, 446).