Energy transport via coordination bonds

Vibrational energy transport in transition metal complexes involves stages where energy crosses relatively weak coordination bonds between a coordinated metal atom and the ligands. Understanding the energy transport rules on a molecular level is fundamentally important; it is also essential in relation to a recently proposed structural method, the relaxation-assisted two-dimensional infrared (RA 2DIR) technique, where the vibrational population transport time across the molecule of interest is linked to the transport distance. In this study we report on the energy transport across coordination bonds in tetraethylammonium bis(maleonitriledithiolate)iron(III)nitrosyl complex, studied using dual-frequency RA 2DIR spectroscopy. Three mode pairs, C ≡ N and N = O , N = O and C ≡ N , and N = O and C-C, were interrogated. All three cross-peaks show substantial amplification due to vibrational energy transport from the initially excited mode toward the "probed" mode, including a record amplification of 27-fold observed for the C ≡ N / N = O cross-peak. A ninefold amplification measured for the N = O / C ≡ N cross-peak, where the "probed" CN mode has higher frequency than the initially excited NO, proves unequivocally that the excitation of the "probed" mode via energy transport is not essential for observing stronger cross-peaks and that lower frequency modes serve as the energy accepting modes. A simple modeling of the energy transport is presented highlighting the role of a spatial overlap of the interacting modes. The observed strong cross-peak amplifications and a correlation between the energy transport time and the intermode distance, the distance between atom pairs on which vibrational excitations predominantly reside, demonstrate an applicability of the RA 2DIR method for structural interrogation of transition metal complexes.