Vibrational properties of the trinuclear spin crossover complex [Fe3(4-(2′-hydroxy-ethyl)-1,2,4-triazole)6(H2O)6](CF3SO3)6: a nuclear inelastic scattering, IR, Raman and DFT study

The vibrational properties of the trimeric iron complex [Fe3(4-(2′-hydroxy-ethyl)-1,2,4-triazole)6(H2O)6](CF3SO3)6 which serves as a model of the 1D iron coordination polymers based on 1,2,4-triazoles have been investigated by nuclear inelastic scattering of synchrotron radiation (NIS), as well as by Raman and infrared (IR) spectroscopy. The system reveals a soft spin crossover involving only the central iron atom with its FeN6 core, while the terminal FeN3O3 units show no spin transition. The NIS spectra of the central low-spin isomer exhibit a number of marker bands in the 350–450 cm−1 region which have not been detected in the Raman spectra. The density functional theory (DFT) calculations allowed the assignment of these bands to Fe–N bending and stretching modes. A characteristic high-spin marker mode has been identified and discriminated from the iron-ligand modes of the terminal iron atoms. This characteristic central Fe–N mode has been observed experimentally at 245 cm−1 and theoretically at 255 cm−1. Contrary to mononuclear centrosymmetric Fe complexes, some of the symmetric vibrations of the trimeric complex involving iron movements are observed by NIS. Furthermore the DFT calculations displayed the importance of the coulombic repulsion between metal ions for the geometry and stability of a given spin isomer.