Vibrational characterization of a diiron bridging hydride complex - a model for hydrogen catalysis

Abstract

A diiron complex containing a bridging hydride and a protonated terminal thiolate of the form [(μ,κ₂-bdtH)(μ-PPh₂)(μ-H)Fe₂(CO)₅]⁺ has been investigated through ⁵⁷Fe nuclear resonance vibrational spectroscopy (NRVS) and interpreted using density functional theory (DFT) calculations. We report the Fe-μH-Fe wagging mode, and indications for Fe-μD stretching vibrations in the D-isotopologue, observed by ⁵⁷Fe-NRVS. Our combined approach demonstrates an asymmetric sharing of the hydride between the two iron sites that yields two nondegenerate Fe-μH/D stretching vibrations. The studied complex provides an important model relevant to biological hydrogen catalysis intermediates. The complex mimics proposals for the binuclear metal sites in [FeFe] and [NiFe] hydrogenases. It is also an appealing prototype for the 'Janus intermediate' of nitrogenase, which has been proposed to contain two bridging Fe-H-Fe hydrides and two protonated sulfurs at the FeMo-cofactor. The significance of observing indirect effects of the bridging hydride, as well as obstacles in its direct observation, is discussed in the context of biological hydrogen intermediates.

Fig. 1. Schematic structures of (a) the μHS(Me/H) synthetic complexes studied in this work; (b) the [NiFe] hydrogenase Ni–R intermediate; (c) the [FeFe] hydrogenase H_hyd intermediate; (d) the nitrogenase Janus intermediate. The iron hydrides and sulfur protonations are labelled in red.

Fig. 2. Top: the NRVS-derived ⁵⁷Fe-PVDOS spectra for μHSH () and its deuterium isotopologue μDSD (), vertically offset by 1 × 10⁻³ cm from each other in the high-energy region. Bottom: the ⁵⁷Fe-PVDOS spectra for μHSMe (). In all cases the high-energy region intensity above 650 cm⁻¹ is multiplied by 5 for visibility. The bands are labelled with their top positions, with those above 800 cm⁻¹ assigned tentatively.

Fig. 3. DFT-based ⁵⁷Fe-PVDOS for different chemical models and their isomers. Left (a): μHSH or μDSD. Right, top to bottom: (b) μHS− or μDS−, (c) μHSMein or μDSMein, (d) [μHSH]2 or [μDSD]2, (e) μHSHinvs.μHSH (= μHSHout) or μDSDinvs.μDSD (= μDSDout). The high energy region >650 cm⁻¹ (b–e) is multiplied by 2 for visibility. In (a), the intensities <210 cm⁻¹ are based on the [μHSH]2 or [μDSD]2 dimer calculations, as explained in the main text and shown in Fig. 5.

Fig. 4. Top: comparison of DFT-predicted ⁵⁷Fe_d and ⁵⁷Fe_p PVDOS for models μHSH and μDSD. Bottom: comparison of DFT-predicted μH/D PVDOS for the same models. The relative intensity multiplication factors (×) are applied for visibility.

Fig. 5. (A) The normal mode calculated at 38 cm⁻¹ showing relative displacements of the two enantiomers comprising the [μHSH]2 dimer. Actual amplitude of this [μHSH]–[μHSH]m vibration is ∼0.05 Å. (B) Low-frequency (<350 cm⁻¹) ⁵⁷Fe-PVDOS spectra for the μHSH compound from NRVS experiment (blue) and DFT calculations using [μHSH]2 dimer (black) and μHSH monomer (orange) models; for the full-range spectra, see Fig. S10.

Fig. 6. DFT ⁵⁷Fe-PVDOS spectra for the (a) H- (top) and (b) D- (bottom) isotopologues in the iron-hydride bands region >650 cm⁻¹: μHSH monomer (blue), averaged between the three μHSH/μHSHin/μHS− monomers (green), and the [μHSH]2 dimer (red) models. The relative intensity multiplication factors (×) are applied for visibility.