Oral Presentation 21st International Conference on Biological Inorganic Chemistry 2025

High Spin Iron(I) Dinitrogen Complexes in Three-Fold Symmetry (116609)

Joshua Telser 1 , Hao Yang 2 , Juan A. Valdez-Moreira 3 , Yafei Gao 3 , Austin D. Chivington 3 , Veronica Carta 3 4 , W. Hill Harman 4 , Brian M. Hoffman 2 , Jeremy M. Smith 3
  1. Roosevelt University, Chicago, IL, US
  2. Northwestern University, Evanston, IL, US
  3. Indiana University, Bloomington, IN, US
  4. University of California-Riverside, Riverside, CA, US

The ability in Nature of the nitrogenase enzymes to reduce atmospheric dinitrogen to ammonia remains an evergreen topic of interest.1 In parallel, the analogous human-made Haber-Bosch process is also a major area of research in heterogeneous catalysis.2 In both processes, N2 coordinates to Fe, although the nature of the Fe-N2 species in both cases is not fully established. A variety of dinitrogen complexes of iron has been reported,3 but one that is of particular interest is that with the widely used “scorpionate” (hydridotrispyrazol-1-ylborate, Tp) ligand: (TpAd,Me)FeI(N2), 1, where the superscripts indicate the substituents on the 3- and 5-positions of the pyrazole rings (Ad = 1-adamantyl; Me = methyl).4 Another ligand platform that has been widely used is the N-heterocyclic carbene (NHC) analog of Tp, namely the trisimidazol-2-ylborate ligand.5 The complex PhB(ImAd)3FeI(N2), 2, has been reported by Deng and co-workers.6,7 We describe here a detailed spectroscopic and computational study of these two dinitrogen complexes to compare and contrast their binding and potential activation of N2. These complexes contain FeI (3d7) in a high-spin (S = 3/2) ground state, as found for more routine TpR,R′CoIIX (X = halide, pseudohalide) complexes.8 As such, 1 and 2 are EPR active and X- and Q-band (35 GHz) EPR studies are described. More important is the use of Q-band electron-nuclear double resonance (ENDOR) spectroscopy which provides information on magnetic nuclei that are hyperfine coupled to the paramagnetic center.9 Besides the ubiquitous 1H, ENDOR from the coordinated nitrogen atoms, both 14N and 15N introduced via the use of 15N2(g), provide information on the nature of N2 bonding. Additionally, we describe studies on PhB(ImAd)3FeI(CO), making the traditional contrast between these two neutral backbonding ligands. In parallel with these experimental studies, we use both classical ligand-field theory (LFT, locally written and Ligfield software10) and quantum chemical theory (ORCA software, version 5.011) to understand the electronic absorption (UV-Vis-NIR), vibrational (IR), and EPR/ENDOR spectra of the FeI complexes of interest. These results show that dinitrogen is significantly activated (i.e., the N≡N bond weakened) in both, but more so in 2.

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