New tool for extraction of ¹⁸⁷Os M\"ossbauer parameters with biologically relevant detection sensitivity

A large number of osmium complexes with osmium in different oxidation states (II, III, IV, VI) have been reported recently to exhibit good antiproliferative activity in cancer cell lines. Herein, we demonstrate new opportunities offered by $^{187}$Os nuclear forward scattering (NFS) and nuclear inelastic scattering (NIS) of synchrotron radiation for characterization of hyperfine interactions and lattice dynamics in a benchmark Os(VI) complex K$_2$[OsO$_2$(OH)$_4$], by accurate extraction of M\"ossbauer parameters and the determination of Os-projected density of phonon states confirmed by first-principles phonon calculations. The values of isomer shift ($\delta$ = 3.3(1) mm/s) relative to [Os$^{IV}$ Cl$_{6}$]$^{2-}$ and quadrupole splitting ($\Delta E_Q$ = 12.0(2) mm/s) were determined with NFS, while the Lamb-M\"ossbauer factor (0.55(1)), the density of phonon states (DOS), and a full thermodynamics characterization was carried out using the NIS data combined with first principle theoretical calculations. In more general terms, this study provides strong evidence that $^{187}$Os nuclear resonance scattering is a reliable technique for the investigation of hyperfine interactions and Os specific vibrations in osmium(VI) species, which might be potentially applicable for measuring such interactions in osmium complexes of other oxidation states, including those with anticancer activity such as Os(III) and Os(IV).