Synthesis, Spectroscopy, and Hydrogen/Deuterium Exchange in High-Spin Iron(II) Hydride Complexes

Abstract

Very few hydride complexes are known in which the metals have a high-spin electronic configuration. We describe the characterization of several high-spin iron(II) hydride/deuteride isotopologues and their exchange reactions with one another and with H2/D2. Though the hydride/deuteride signal is not observable in NMR spectra, the choice of isotope has an influence on the chemical shifts of distant protons in the dimers through the paramagnetic isotope effect on chemical shift. This provides the first way to monitor the exchange of H and D in the bridging positions of these hydride complexes. The rate of exchange depends on the size of the supporting ligand, and this is consistent with the idea that H2/D2 exchange into the hydrides occurs through the dimeric complexes rather than through a transient monomer. The understanding of H/D exchange mechanisms in these high-spin iron hydride complexes may be relevant to postulated nitrogenase mechanisms.

High‐Frequency Fe–H Vibrations in a Bridging Hydride Complex Characterized by NRVS and DFT

“…The Mössbauer spectrum at 170 K (Figure S3) showed nearly identical doublets with isomer shifts of 0.47 ± 0.02 mm s −1 and quadrupole splittings of 1.25 ± 0.02 mm s −1 for both isotopologues, suggesting a high-spin electronic configuration at the iron sites in 1 and 1-D 2 . These values are very similar to related high-spin iron(II) hydride and alkyl complexes, [13, 24] and the isomer shift is higher than in known intermediate-spin or low-spin iron(II) compounds. [25] The solid-state magnetization of 1 is consistent with high-spin iron sites (Figure S5).…”

“…[12] In our extensive experience with diketiminate-supported high-spin iron hydride complexes like the iron(II) species 1 (Figure 1) we have never been able to detect an isotope-sensitive vibration attributable to a bridging hydride using IR or RR. [13] Murray has prepared Fe 3 H 3 species with a related ligand environment and high-spin iron(II) ions, but no direct spectroscopic detection of hydrides was reported. [14] Limberg and Yang have studied related nickel hydride complexes, [15],[16] but neither report assigned hydride bands in the IR spectra.…”

“…[23] The isotopologue of 1 with deuterium at the bridging positions ( 1-D 2 ) was prepared by exchange of the hydrides with D 2 . [13] The relative intensities of the 1 H NMR resonances for the ligands were unchanged after deuteration (Figure S1), indicating that D does not exchange into the supporting ligands. The Mössbauer spectrum at 170 K (Figure S3) showed nearly identical doublets with isomer shifts of 0.47 ± 0.02 mm s −1 and quadrupole splittings of 1.25 ± 0.02 mm s −1 for both isotopologues, suggesting a high-spin electronic configuration at the iron sites in 1 and 1-D 2 .…”

Molecular Calcium Hydride: Dicalcium Trihydride Cation Stabilized by a Neutral NNNN‐Type Macrocyclic Ligand

“…AK IE of k H /k D = 1.40 was found for a b-phase palladium hydride [16] for the H/D exchange reaction and similar reactivities were observed for iron hydride complexes, [17] amagnesium, [18] and ac alcium borohydride. [19] When the cationic calcium hydride 3 was deuterated in [D 8 ]THF at 25 8 8Cinthe presence of one equivalent of HSiPh 3 , the Si À Hr esonance had disappeared after 12 ha nd DSiPh 3 was formed.…”

Synthesis and Mechanism of Formation of Hydride–Sulfide Complexes of Iron

“…This paramagnetic isotope effect on chemical shift (PIECS) has been observed for the deuterium isotopologues of other bridging hydride complexes. 36, 37 Compound 3 also displayed PIECS, but it could only be observed when the sample was cooled to −80 °C. 31 …”