Sterically Stabilized Terminal Hydride of a Diiron Dithiolate

The kinetically robust hydride [ t -HFe 2 (Me 2 pdt)(CO) 2 (dppv) 2 ] + ([ t -H 1 ] + ) (Me 2 pdt 2– = Me 2 C(CH 2 S – ) 2; dppv = cis -1,2-C 2 H 2 (PPh 2 ) 2 ) and related derivatives were prepared with 57 Fe 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 Me 2 pdt 2– and dppv ligands stabilizes the terminal hydride with respect to its isomerization to the 5–16 kcal/mol more stable bridging hydride ([μ-H 1 ] + ) with t 1/2 (313.3 K) = 19.3 min. In agreement with the nOe experiments, the calculations predict that one methyl group in [ t -H 1 ] + interacts with the hydride with a computed CH···HFe distance of 1.7 Å. Although [ t -H 57 1 ] + exhibits multiple NRVS features in the 720–800 cm –1 region containing the bending Fe–H modes, the deuterated [ t -D 57 1 ] + sample exhibits a unique Fe-D/CO band at ∼600 cm –1 . In contrast, the NRVS spectra for [μ-H 57 1 ] + exhibit weaker bands near 670–700 cm –1 produced by the Fe–H–Fe wagging modes coupled to Me 2 pdt 2– and dppv motions.