Spin Crossover and Fluorine-Specific Interactions in Metal Complexes of Terpyridines with Polyfluorocarbon Tails

In coordination chemistry and materials science, terpyridine ligands are of great interest, due to their ability to form stable complexes with a broad range of transition metal ions. We report three terpyridine ligands containing different perfluorocarbon (PFC) tails on the backbone and the corresponding Fe-II and Co-II complexes. The Co-II complexes display spin crossover close to ambient temperature, and the nature of this spin transition is influenced by the length of the PFC tail on the ligand backbone. The electrochemical properties of the metal complexes were investigated with cyclic voltammetry revealing one oxidation and several reduction processes. The fluorine-specific interactions were investigated by EPR measurements. Analysis of the EPR spectra of the complexes as microcrystalline powders and in solution reveals exchange-narrowed spectra without resolved hyperfine splittings arising from the Co-59 nucleus; this suggests complex aggregation in solution mediated by interactions of the PFC tails. Interestingly, addition of perfluoro-octanol in different ratios to the acetonitrile solution of the sample resulted in the disruption of the F MIDLINE HORIZONTAL ELLIPSIS ${\cdots }$ F interactions of the tails. To the best of our knowledge, this is the first investigation of fluorine-specific interactions in metal complexes through EPR spectroscopy, as exemplified by exchange narrowing.

Automated summary

Terpyridine ligands are widely studied in coordination chemistry and materials science due to their stable complex formation ability with various transition metal ions. In this study, three terpyridine ligands with different perfluorocarbon (PFC) tails on the backbone, along with their Fe-II and Co-II complexes, were reported. The Co-II complexes exhibited spin crossover near room temperature, which was influenced by the length of the PFC tail. Electrochemical properties of the metal complexes were investigated using cyclic voltammetry, revealing oxidation and reduction processes. EPR measurements were used to study fluorine-specific interactions, showing exchange-narrowed spectra without hyperfine splittings from the Co-59 nucleus, indicating complex aggregation mediated by PFC tail interactions in solution. Interestingly, the addition of perfluoro-octanol disrupted the F...F interactions of the tails. To our knowledge, this is the first EPR spectroscopy study on fluorine-specific interactions in metal complexes, demonstrated by exchange narrowing.