125Te and 57Fe nuclear resonance vibrational spectroscopic characterization of intermediate spin state mixed-valent dimers

Abstract

Iron-sulfur clusters fulfill numerous roles throughout biology. The reduced [2Fe-2S]⁺ cluster offers unique electronic and magnetic properties due to its mixed-valent nature and can serve as an essential model for understanding electron transfer, electron delocalization, and accessible spin states not only in mixed-valent dimers, but potentially larger iron sulfur clusters. Recently a series of mixed-valent diiron dichalcogenide complexes [L₂Fe₂Q₂]^- (Q = S (1), Se (2), Te (3), L = 2,6-diisopropylphenyl β-diketiminate ligand) were synthesized and characterized, where complex 1 showed a typical S = 1/2 spin state, while complexes 2 and 3 exhibited intermediate S = 3/2 spin states, potentially enabled by the minimization of vibronic coupling. Here we studied the vibrational dynamics of the Fe and Te centers in these complexes using ⁵⁷Fe and ¹²⁵Te nuclear resonance vibrational spectroscopy (NRVS), coupled with DFT calculations. The findings suggest that heavy character of larger chalcogen atoms results in decreased vibronic coupling. The observation of an intermediate spin state is shown to be unattainable for lighter Fe₂Q₂ cores. This highlights the crucial role of vibronic coupling in modulating the electronic structure of mixed-valence systems and should enhance understanding of the electronic structure in more complex biological Fe-S clusters.

Fig. 1. Structures of studied complexes and example of potential energy surfaces.

a Structures of previously synthesized mixed-valent diiron dichalcogenide clusters. b Energy levels determined by Eq. 1, as a function of B/|J| ratio, for antiferromagnetically coupled systems (J < 0). c Representation of PKS vibration for studied complexes. d, e Energy levels determined by Eq. 2 in the PKS coordinate for d S = 1/2 localized [Fe₂S₂]⁺ complex and e S = 9/2 delocalized [Fe₂(OH)₃(tmtacn)₂]^2+,.

Fig. 2. Experimental ⁵⁷Fe and ¹²⁵Te nuclear resonance vibrational spectroscopy (NRVS) partial vibrational density of states (PVDOS).

⁵⁷Fe (black line) and ¹²⁵Te (blue line) NRVS PVDOS spectra of complexes 1–3. Gray dashed lines depict the shifting of the Fe-Q stretching (A) and lower energy core (B) vibrations.

Fig. 3. Calculated ⁵⁷Fe NRVS and PKS vibrational modes.

a Comparison of experimental (black line) and calculated (red line) ⁵⁷Fe NRVS spectra of complexes 1–3. Individual transitions are depicted by red vertical lines. b ⁵⁷Fe PVDOS along x (red line), y (blue line), and z (brown line) axes, for complexes 1–3. The x axis is defined along Fe-Fe bond, and z axis is perpendicular to the [Fe₂Q₂]⁺ core. c Representation of calculated PKS normal modes for complexes 1–3. For clarity, only the [Fe₂Q₂]⁺ cores and coordinating N atoms are shown.

Fig. 4. Comparison of ⁵⁷Fe and ¹²⁵Te NRVS and selected vibrational modes.

a Comparison of ¹²⁵Te (top) and ⁵⁷Fe (bottom) NRVS spectra of complex 3. Experimental spectra are depicted by black lines, calculated spectra by blue (¹²⁵Te) and red (⁵⁷Fe) lines, with selected individual transitions depicted by blue and red vertical lines. b Representations of selected calculated normal modes of complex 3. For clarity, only the [Fe₂Te₂]⁺ core and coordinating N atoms are shown. Magenta colored rectangles represent regions containing normal modes with significant Te displacements (a) and corresponding normal modes (b). Orange colored rectangles represent regions containing normal modes with significant Fe displacements (a) and corresponding normal modes (b).

Fig. 5. Potential energy surfaces and influence of vibronic coupling.

a Ground and excited spin states in the PKS coordinate for the complexes 1–3. b Spin states in the PKS coordinate for the complex 1 with a decreased amount of vibronic coupling. c Spin states in the PKS coordinate for the complex 3 with an increased amount of vibronic coupling.