Probing the electronic structure of the Di-iron subsite of [Fe]-hydrogenase: A photoelectron spectroscopic study of Fe(I)-Fe(I) model complexes

The electronic structures of a series of Fe(I)-Fe(I) model complexes of the di-iron subsite of [Fe]-hydrogenase, [(mu-PDT)Fe-2(CO)(4)(CN)(2)](2-) (1), [Fe-2(CO)(4){MeSCH2C(Me)(CH2S)(2)}(CN)(2)](2-) (II), [Fe-2(CO)(4){PhCH2SCH2C(Me)-(CH2S)(2)}(CN)(2)](2-) (III), [Fe-2(CO)(4){PhCH2SCH2C(Me) (CH2S)(2)}(CN)](-) (IV), and [Fe-2(CO)(4){MeSCH2C(Me)(CH2S)(2)}(CN)](-) (V), were investigated in the gas phase using photodetachment photoelectron spectroscopy. The adiabatic electron detachment energy (ADE) of each species and the intramolecular Coulomb repulsion for the doubly charged species were obtained. The ADEs correspond to the intrinsic redox potentials (in vacuo) of reactions involving the Fe(I)-Fe(I)/Fe(I)-Fe(II) couples in these compounds. The photoelectron spectra were understood and qualitatively assigned by comparing with that of Fe-2(CO)(6)S-2, which has been well studied previously and exhibits similar valence spectral features as I-V. A "normal level scheme" is suggested for the electronic structure of these low spin di-iron compounds, in which all occupied 3d levels lie above all occupied ligand levels. We also observed subtle differences in the electronic structures of the five di-iron complexes due to the slightly different ligand environments.