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. 2019 May 7;10(21):5539-5545.
doi: 10.1039/c9sc00561g. eCollection 2019 Jun 7.

Tuning ligand field strength with pendent Lewis acids: access to high spin iron hydrides

John J Kiernicki  1 , James P Shanahan  1 , Matthias Zeller  2 , Nathaniel K Szymczak  1
Affiliations

Tuning ligand field strength with pendent Lewis acids: access to high spin iron hydrides

John J Kiernicki et al. Chem Sci. .

Abstract

Geometrically flexible 9-borabicyclo[3.3.1]nonyl units within the secondary coordination sphere enable isolation of high-spin Fe(ii)-dihydrides stabilized by boron-hydride interactions and a rare example of an isolable S = 3/2 reduction product. The borane-capped Fe(ii)-dihydride: (1) rapidly deprotonates E-H (E = N, O, P, S) bonds to afford borane-stabilized Fe adducts and (2) releases H2 upon exposure to π-acids. The Lewis acids provide an avenue for redox-leveling in analogy to the near constant operating potential for N2 reduction in nitrogenase.

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Figures

Fig. 1
Fig. 1. FeMo-cofactor and our approach to hydride accumulation with boron Lewis acids.
Fig. 2
Fig. 2. Synthesis of borane-capped metal–dihydrides.
Fig. 3
Fig. 3. (A) Reduction of 1. (B) Molecular structures of 1 and 1-K(crypt) (50% probability ellipsoids). For clarity, the 9-BBN substituents are displayed in wireframe. (C) Electrochemical comparison between 1 and (BBNPDPtBu)FeCl2. (D) X-band EPR of 1-K(crypt) at 10 K in 1 : 1 toluene/THF. (E and F) Spin-density isosurface plots (0.002 a.u.) of 1 and 1-K(crypt), respectively.
Fig. 4
Fig. 4. Formation of 2-E and molecular structures (50% probability ellipsoids) and averaged bond distances. For clarity, the 9-BBN substituents are displayed in wireframe.
Fig. 5
Fig. 5. Reversible acid/base interaction in 2-SPh and molecular structure of 3 (50% probability ellipsoids). For clarity, the 9-BBN and phenyl substituents are displayed in wireframe.
Fig. 6
Fig. 6. π-acid induced H2 extrusion from 1.
See this image and copyright information in PMC

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