Vibrational properties of the mononuclear Fe[HBpz3]2 spin crossover complex

Within this work, we report the results of nuclear inelastic scattering experiments of the low-spin phase of the iron(II) mononuclear SCO complex Fe[HBpz3]2 and density functional theory based calculations performed on a model molecule of the complex. We show that the calculated partial density of vibrational states based on the structure of a single iron(II) center which is linked by three pyrazole rings to borat is in good accordance with the experimentally obtained 57Fe-pDOS and assign the molecular vibrations to the prominent optical phonons.


Introduction
The spin crossover (SCO) phenomenon refers to a reversible spin crossover (SCO) transition between the colored low-spin state (LS) and the typically colorless high-spin state (HS) in some materials based on 3d 4 -3d 7 transition metal complexes. The spin transition can be triggered upon external stimuli like temperature, pressure or illumination with light. SCO materials display the properties for novel potential applications such as contrast agents [1], temperature/pressure threshold indicators [2,3] and memory devices [4][5][6]. Iron(II) complexes with tris(pyrazolyl)methane or tris(pyrazole)borate ligands are promising classes of SCO compounds due to their relatively simple synthesis and almost endless possibility for modifications to tune the SCO properties [7,8] This work focuses on the SCO compound Fe[HBpz 3 ] 2 which was studied at low temperature by nuclear inelastic scattering (NIS) experiments. In addition, simulations based on density functional theory (DFT) calculations on a model molecule are presented which permits the assignment of molecule vibrations to the vibrational modes observed by nuclear inelastic scattering.
The 57 Fe NIS experiments were conducted in 40 bunch mode with a bunch separation of 192 ns at the Dynamics Beamline P01, DESY in Hamburg. The synchrotron beam was monochromatized to the 14.4125 keV nuclear resonance transition energy with a bandwidth of ca. 1.5 meV using a two-step monochromatization setup. Cooling of the powder samples to T = 4.2 K was performed by a dedicated cryostat from Janis Research. The NIS data were collected during several scans within the energy range of −20 to 80 meV with a 0.25 meV step size with an avalanche photodiode. The experimentally determined partial density of vibrational states (pDOS) was calculated with a binning of 0.5 meV. The evaluation of the NIS data was performed using the software isdos2019. The temperature of the sample was regulated with a dedicated cryostat (Janis Research).
The DFT calculations were conducted using the B3LYP [10] density functional and the cep-31G [11] basis set.  440 cm −1 . The multiband at 407 cm −1 has the highest intensity in the pDOS. Moreover, a broad vibrational multiband with its maximum located at 230 cm −1 in the pDOS is identified between 200 and 240 cm −1 . Typically, vibrations above 300 cm −1 are reported to be characteristic of the LS configuration of a spin crossover complex [12]. Thus, the experimental pDOS suggests that the spin crossover is in the LS state due to the very intense vibrational multibands in this energy region. Furthermore, Table 1 displays the thermodynamic parameters extracted from the NIS data of the bulk powder of 1 recorded at 4.2 K. According to the analysis, the Lamb-Mößbauer f LM is 0.89, the mean force constant D is 331 Nm −1 and the Debye temperature Θ D is 63.46 K. Additionally, the specific heat c v is calculated to be 0.01 k B , the entropy S is 0.004 k B and the sound velocity v m is 3.3 kms −1 .

Results and discussion
In addition to the experimental 57 Fe-pDOS, Fig. 1b contains the simulated Fe-pDOS obtained by performing normal mode analysis on the depicted model molecule in the LS state. The model molecule consists of a single iron(II) center which is linked by three pyrazole rings to borat on two sides. This iron(II) center in the LS state was used for the calculation of the theoretical pDOS. The simulated pDOS yields multibands at 220, 327, 355, 409 and 435 cm −1 . Generally, the simulation reproduced the experimental 57 Fe-pDOS very well with minor deviations of up to 10 cm −1 of the position of the multibands between Table 1 Lamb-Mößbauer factor f LM , thermodynamic parameters and mean force constant D extracted from the experimental 57 Fe-pDOS shown in Fig. 2

Conclusion
In conclusion, we report the 57 Fe-pDOS of this SCO complex at 4.2 K as well as the thermodynamical parameters which were extracted from the experimental NIS data. Furthermore, we show that theoretical DFT simulations based on one molecule of this SCO complex reproduce the experimental data well and illustrate the vibrations assigned to the experimental vibrational modes.