Structural and Spectroscopic Features of Mixed Valent FeIIFeI Complexes and Factors Related to the Rotated Configuration of Diiron Hydrogenase

The compounds of this study have yielded to complementary structural, spectroscopic (Mössbauer, EPR/ENDOR, IR), and computational probes that illustrate the fine control of electronic and steric features that are involved in the two structural forms of (μ-SRS)­[Fe­(CO)2PMe3]20,+ complexes. The installation of bridgehead bulk in the −SCH2CR2CH2S– dithiolate (R = Me, Et) model complexes produces 6-membered FeS2C3 cyclohexane-type rings that produce substantial distortions in FeIFeI precursors. Both the innocent (Fc+) and the noninnocent or incipient (NO+/CO exchange) oxidations result in complexes with inequivalent iron centers in contrast to the FeIFeI derivatives. In the FeIIFeI complexes of S = 1/2, there is complete inversion of one square pyramid relative to the other with strong super hyperfine coupling to one PMe3 and weak SHFC to the other. Remarkably, diamagnetic complexes deriving from isoelectronic replacement of CO by NO+, {(μ-SRS)­[Fe­(CO)2PMe3] [Fe­(CO)­(NO)­PMe3]+}, are also rotated and exist in only one isomeric form with the −SCH2CR2CH2S– dithiolates, in contrast to R = H (Olsen, M. T.; Bruschi, M.; De Gioia, L.; Rauchfuss, T. B.; Wilson, S. R. J. Am. Chem. Soc. 2008, 130, 12021−12030). The results and redox levels determined from the extensive spectroscopic analyses have been corroborated by gas-phase DFT calculations, with the primary spin density either localized on the rotated iron in the case of the S = 1/2 compound, or delocalized over the {Fe­(NO)} unit in the S = 0 complex. In the latter case, the nitrosyl has effectively shifted electron density from the FeIFeI bond, repositioning it onto the spin coupled Fe–N–O unit such that steric repulsion is sufficient to induce the rotated structure in the FeII-{FeI(•NO)}8 derivatives.