Hyperfine interactions and electron distribution in FeIIFeI and FeIFeI models for the active site of the [FeFe] hydrogenases: Mössbauer spectroscopy studies of low-spin FeI

Mössbauer studies of [{μ-S(CH 2 C(CH 3 ) 2 CH 2 S}(μ-CO)Fe II Fe I (PMe 3 ) 2 (CO) 3 ]PF 6 ( 1 OX ), a model complex for the oxidized state of the [FeFe] hydrogenases, and the parent Fe I Fe I derivative are reported. The paramagnetic 1 OX is part of a series featuring a dimethylpropanedithiolate bridge, introducing steric hindrance with profound impact on the electronic structure of the diiron complex. Well-resolved spectra of 1 OX allow determination of the magnetic hyperfine couplings for the low-spin distal Fe I ( Fe D I ) site, A x , y , z = [−24 (6), −12 (2), 20 (2)] MHz, and the detection of significant internal fields (approximately 2.3 T) at the low-spin ferrous site, confirmed by density functional theory (DFT) calculations. Mössbauer spectra of 1 OX show nonequivalent sites and no evidence of delocalization up to 200 K. Insight from the experimental hyperfine tensors of the Fe I site is used in correlation with DFT to reveal the spatial distribution of metal orbitals. The Fe–Fe bond in [Fe 2 {μ-S(CH 2 C(CH 3 ) 2 CH 2 S}(PMe 3 ) 2 (CO) 4 ] ( 1 ) involving two d z 2 -type orbitals is crucial in keeping the structure intact in the presence of strain. On oxidation, the distal iron site is not restricted by the Fe–Fe bond, and thus the more stable isomer results from inversion of the square pyramid, rotating the d z 2 orbital of Fe D I . DFT calculations imply that the Mössbauer properties can be traced to this d z 2 orbital. The structure of the magnetic hyperfine coupling tensor, A , of the low-spin Fe I in 1 OX is discussed in the context of the known A tensors for the oxidized states of the [FeFe] hydrogenases.