Characterization of a Genuine Iron(V)−Nitrido Species by Nuclear Resonant Vibrational Spectroscopy Coupled to Density Functional Calculations

The characterization of high-valent iron species is of interest due to their relevance to biological reaction mechanisms. Recently, we have synthesized and characterized an [Fe(V)-nitrido-cyclam-acetato]+ complex, which has been characterized by Mössbauer, magnetic susceptibility data, and XAS spectroscopies combined with DFT calculations (Aliaga-Alcade, N.; DeBeer George, S.; Bill, E.; Wieghardt, K.; Neese, F. Angew. Chem., Int. Ed. 2005, 44, 2908−2912). The results of this study indicated that the [Fe(V)-nitrido-cyclam-acetato]+ complex is an unusual d3 system with a nearly orbitally degenerate S = 1/2 ground state. Although the calculations predicted fairly different Fe−N stretching frequencies for the S = 1/2 and the competing S = 3/2 ground states, a direct experimental determination of this important fingerprint quantity was missing. Here we apply synchrotron-based nuclear resonance vibrational scattering (NRVS) to characterize the Fe−N stretching frequency of an Fe(V)−nitrido complex and its Fe(III)−azide precursor. The NRVS data show a new isolated band at 864 cm-1 in the Fe(V)−nitrido complex that is absent in the precursor. The NRVS spectra are fit and simulated using a DFT approach, and the new feature is unambiguously assigned to a Fe(V)−N stretch. The calculated Fe−N stretching frequency is too high by ∼75 cm-1. Anharmonic contributions to the Fe−N stretching frequency have been evaluated and have been found to be small (−5.5 cm-1). The NRVS data provided a unique opportunity to obtain this vibrational information, which had eluded characterization by more traditional vibrational spectroscopies.