Spin-crossover complexes are attractive for their spinswitching functionality. However, only few compounds have been found to remain intact in direct contact to metal surfaces. For the design of new spin-crossover complexes, it is important to understand the mechanisms leading to fragmentation. Here, we investigate, using low-temperature scanning tunneling microscopy along with density functional theory calculations, two Fe(terpyridine) 2 complexes deposited on Au(111) by electrospray ionization with in-line mass selection. Only fragments of the first compound are observed on the surface, while the second compound is strongly flattened. Based on a detailed analysis of the adsorbates on the surface, possible mechanisms for the fragmentation and molecular distortion are proposed.
We introduce a design principle to stabilize helically chiral structures from an achiral tetrasubstituted [2.2]paracyclophane by integrating it into a macrocycle. The [2.2]paracyclophane introduces a three-dimensional perturbation into a nearly planar macrocyclic oligothiophene. The resulting helical structure is stabilized by two bulky substituents installed on the [2.2]paracyclophane unit. The increased enantiomerization barrier enabled the separation of both enantiomers. The synthesis of the target helical macrocycle 1 involves a sequence of halogenation and cross-coupling steps and a high-dilution strategy to close the macrocycle. Substituents tuning the energy of the enantiomerization process can be introduced in the last steps of the synthesis. The chiral target compound 1 was fully characterized by NMR spectroscopy and mass spectrometry. The absolute configurations of the isolated enantiomers were assigned by comparing the data of circular dichroism spectroscopy with TD-DFT calculations. The enantiomerization dynamics was studied by dynamic HPLC and variable-temperature 2D exchange spectroscopy and supported by quantum-chemical calculations.
Interlinking of two terpyridine ligands results in mononuclear chiral metal complexes (Fe and Ru) which were separated into their enantiomers.
A new concept to improve the reliability of functional single molecule junctions is presented using the E‐field triggered switching of FeIIbis‐terpyridine complexes in a mechanically controlled break junction experiment as model system. The complexes comprise a push‐pull ligand sensing the applied E‐field and the resulting distortion of the FeII ligand field is expected to trigger a spin‐crossover event reflected in a sudden jump of the transport current. By molecular engineering, the active centre of the complex is separated from the gold electrodes in order to eliminate undesired side‐effects. Two aspects are considered to isolate the central metal ion, namely the spacing by introducing additional alkynes, and the steric shielding achieved by bulky isopropyl groups. With this small series of model complexes, a pronounced correlation is observed between the occurrence of bistable junctions and the extent of separation of the central metal ion, affirming the hypothesized Enhanced Separation Concept (ESC).
The coordination sphere of the Fe(II) terpyridine complex 1 is rigidified by fourfold interlinking of both terpyridine ligands. Profiting from an octa‐aldehyde precursor complex, the ideal dimensions of the interlinking structures are determined by reversible Schiff‐base formation, before irreversible Wittig olefination provided the rigidified complex. Reversed‐phase HPLC enables the isolation of the all‐trans isomer of the Fe(II) terpyridine complex 1, which is fully characterized. While temperature independent low‐spin states were recorded with superconducting quantum interference device (SQUID) measurements for both, the open precursor 8 and the interlinked complex 1, evidence of the increased rigidity of the ligand sphere in 1 was provided by proton T2 relaxation NMR experiments. The ligand sphere fixation in the macrocyclized complex 1 even reaches a level resisting substantial deformation upon deposition on an Au(111) surface, as demonstrated by its pristine form in a low temperature ultra‐high vacuum scanning tunneling microscope experiment.
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