High-Frequency Fe–H and Fe–H₂ Modes in a trans-Fe(η²‑H₂)(H) Complex: A Speed Record for Nuclear Resonance Vibrational Spectroscopy

Nuclear resonance vibrational spectroscopy (NRVS) and density functional theory (DFT) are complementary tools for studying the vibrational and geometric structures of specific isotopically labeled molecular systems. Here we apply NRVS and DFT to characterize the trans-[⁵⁷Fe­(η²-H₂)­(H)­(dppe)₂]­[BPh₄] [dppe = 1,2-bis­(diphenylphosphino)­ethane] complex. Heretofore, most NRVS observations have centered on the spectral region below 1000 cm^–1, where the ⁵⁷Fe signal is strongest. In this work, we show that state-of-the-art synchrotron facilities can extend the observable region to 2000 cm^–1 and likely beyond, in measurements that require less than 1 day. The ⁵⁷Fe–H stretch was revealed at 1915 cm^–1, along with the asymmetric ⁵⁷Fe–H₂ stretch at 1774 cm^–1. For a small fraction of the H₂-dissociated product, the ⁵⁷Fe–H stretch was detected at 1956 cm^–1. The unique sensitivity to ⁵⁷Fe motion and the isolated nature of the Fe–H/H₂ stretching modes enabled NRVS to quantitatively analyze the sample composition.