Sterically Stabilized Terminal Hydride of a Diiron Dithiolate

The kinetically robust hydride [t-HFe2(Me(2)pdt)(CO)(2)(dppv)(2)](+) ([t-H1](+)) (Me(2)pdt(2-) = Me2C-(CH2S-)(2); dppv = cis-1,2-C2H2(PPh2)(2)) and related derivatives were prepared with Fe-57 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 ([mu-H1](+)) 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-H1](+) interacts with the hydride with a computed CH center dot center dot center dot HFe distance of 1.7 angstrom. 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 similar to 600 cm(-1). In contrast, the NRVS spectra for [mu-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.