Sterically Stabilized Terminal Hydride of a Diiron Dithiolate

The kinetically robust
hydride [t-HFe2(Me2pdt)­(CO)2(dppv)2]+ ([t-H1]+) (Me2pdt2– = Me2C­(CH2S–)2; dppv = cis-1,2-C2H2(PPh2)2) and related derivatives were prepared with 57Fe enrichment for characterization by NMR, FT-IR, and NRVS. The experimental
results were rationalized using DFT molecular modeling and spectral
simulations. The spectroscopic analysis was aimed at supporting assignments
of Fe–H vibrational spectra as they relate to recent measurements
on [FeFe]-hydrogenase enzymes. The combination of bulky Me2pdt2– and dppv ligands stabilizes the terminal
hydride with respect to its isomerization to the 5–16 kcal/mol
more stable bridging hydride ([μ-H1]+) with t1/2(313.3 K) = 19.3 min. In agreement
with the nOe experiments, the calculations predict that one methyl
group in [t-H1]+ interacts
with the hydride with a computed CH···HFe distance
of 1.7 Å. Although [t-H571]+ exhibits multiple NRVS features in the 720–800
cm–1 region containing the bending Fe–H modes,
the deuterated [t-D571]+ sample exhibits a unique Fe-D/CO band at ∼600 cm–1. In contrast, the NRVS spectra for [μ-H571]+ exhibit weaker bands near 670–700
cm–1 produced by the Fe–H–Fe wagging
modes coupled to Me2pdt2– and dppv motions.