Coordination sphere stabilization by macrocyclization of polypyridyl ligands

This thesis is a continuing work on a project, which started already in 2010 within our group. The syntheses of the terpyridine metal complexes was mainly based on the direct interconnection of different commercially available boronic acids with the terpyridine ligand via Suzuki cross-coupling reaction to obtain metal coordination complexes for their fundamental investigation in a mechanically controlled break junction (MCBJ). However, the remaining challenge was the instability of the Fe(II) terpyridine complexes, leading to an exciting question:
Is there a chemical design that stabilizes the geometry around the metal ion and prevents the decomposition of the complexes?
This thesis approaches this challenge with different molecular designs whose success has been undoubtedly proven through various publications in international scientific journals. The publications are ordered with increasing complexity of the molecular structures, starting with shielding of the active center to stabilize the complex and interlinking approaches to stabilize the geometry with their physical measurements. Four interconnections around the metal center lead to the final target structure.
The remarkable improvement of the previously synthesized complexes with shielding groups was published in the European Journal of Organic Chemistry (2019) where the MCBJ measurements are included. Another strategy that improves the previous strategy immensely is published in Chemical Science (2018) where the concept of interlinking the ligands with its detailed synthesis, the chiroptical studies and the dynamic behavior of the molecule is presented. The geometrical stability of this system was tested by immobilization on a gold surface and the results were published in The Journal of Physical Chemistry C (2019). The interlinking of the ligands was further elucidated on Cu(I) phenanthroline systems and was published in Chemistry – A European Journal (2020) with photophysical measurements included. Finally, the interlinking of an Fe(II) complex was increased to four bridging units, which was accepted for publication in Angewandte Chemie International Edition (2020) where the detailed synthesis is described, the increased stiffness of the ligand sphere is shown by NMR experiments and first evidence of switching between two conductance states has been shown by scanning tunneling microscopy. The supplementary information of the manuscripts is given in the same order at the very end of this thesis for the reader’s convenience.
Preceding these five manuscripts, the reader will find a review entitled “Emerging Functionality from Interlinking Polypyridyl Ligands in Mononuclear Complexes”, which has been prepared for publication in ChemPlusChem. It gives an overview of the published concepts to interlink bipyridine, phenanthroline and terpyridine ligands – including the ones applied in this thesis. It links the topic of this thesis to the wider world of interlinked metal complexes that are based on hexadentate pyridine ligands.