Understanding the [NiFe] Hydrogenase Active Site Environment through Ultrafast Infrared and 2D-IR Spectroscopy of the Subsite Analogue K[CpFe(CO)(CN)₂] in Polar and Protic Solvents

The [CpFe(CO)(CN)₂]⁻ unit is an excellent structural model for the Fe(CO)(CN)₂ moiety of the active site found in [NiFe] hydrogenases. Ultrafast infrared (IR) pump–probe and 2D-IR spectroscopy have been used to study K[CpFe(CO)(CN)₂] (M1) in a range of protic and polar solvents and as a dry film. Measurements of anharmonicity, intermode vibrational coupling strength, vibrational relaxation time, and solvation dynamics of the CO and CN stretching modes of M1 in H₂O, D₂O, methanol, dimethyl sulfoxide, and acetonitrile reveal that H-bonding to the CN ligands plays an important role in defining the spectroscopic characteristics and relaxation dynamics of the Fe(CO)(CN)₂ unit. Comparisons of the spectroscopic and dynamic data obtained for M1 in solution and in a dry film with those obtained for the enzyme led to the conclusion that the protein backbone forms an important part of the bimetallic active site environment via secondary coordination sphere interactions.