Vibrational analysis of the model complex (μ-edt)[Fe(CO) 3]2 and comparison to Iron-only hydrogenase: The activation scale of hydrogenase model systems

Mary Grace I. Galinato, C. Matthew Whaley, Nicolai Lehnert

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Abstract

Research on simple [FeFe] hydrogenase model systems of type (μ-S 2R)[Fe(CO)3]2 (R = C2H4 (edt), C3H6 (pdt)) which have been shown to function as robust electrocatalysts for proton reduction, provides a reference to understand the electronic and vibrational properties of the active site of [FeFe] hydrogenases and of more sophisticated model systems. In this study, the solution and solid state Raman spectra of (μ-edt)[Fe(CO)3] 2 and of the corresponding 13CO-labeled complex are presented and analyzed in detail, with focus on the v(C=O) and v(Fe-CO)/δ(Fe-C=O) vibrational regions. These regions are specifically Important as vibrations Involving CO ligands serve as probes for the "electron richness" of low-valent transition metal centers and the geometric structures of the complexes. The obtained vibrational spectra have been completely assigned In terms of the v(C=O), v(Fe-CO), and δ(Fe-C=O) modes, and the force constants of the important C=O and Fe-CO bonds have been determined using our Quantum Chemistry Centered Normal Coordinate Analysis (QCC-NCA). In the 400-650 cm-1 region, fifteen mixed v(Fe-CO)/δ;(Fe-C=O) modes have been Identified. The most prominent Raman peaks at 454, 456, and 483 cm-1 correspond to a combination of v(Fe-CO) stretching and O(Fe-C=O) linear bending modes. The less intense peaks at 416 cm-1 and 419 cm-1 correspond to pure δ(Fe-C=O) linear bends. In the v(C=O) region, the v(C=O) normal modes at lower energy (1968 and 1964 cm-1) are almost pure equatorial (eq) v(C=O)8q stretching vibrations, whereas the remaining four v(C=O) normal modes show dominant (C=O)eq (2070 and 1961 cm-1) and (C=O) ax (2005 and 1979 cm , ax = axial) contributions. Importantly, an Inverse correlation between the f(C=O)ax/eq and f(Fe-CO) ax/eqforce constants Is obtained, in agreement with the idea that the Fe(I)-CO bond In these types of complexes is dominated by π-backdonation. Compared to the reduced form of [FeFe] hydrogenase (Hred ), the v(C=O) vibrational frequencies of (μ-edt)[Fe(CO)3]2 are higher In energy, indicating that the dinuclear Iron core In (μ-edt)[Fe(CO)3]2 is less electron rich compared to Hred in the actual enzyme. Finally, quantum yields for the photodecomposltion of (μ-edt)[Fe(CO)3]2 have been determined.

Original languageEnglish (US)
Pages (from-to)3201-3215
Number of pages15
JournalInorganic chemistry
Volume49
Issue number7
DOIs
StatePublished - Apr 5 2010

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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