Poster Presentation 21st International Conference on Biological Inorganic Chemistry 2025

Over 20000 Fold activation of a S = 1 Nonheme Iron(IV)Oxo Complex for C-H bond cleavage Reaction in a Tris imidazole ligand backbone (#543)

Nabhendu Pal 1 , Faiza Ahsan 2 , Jin Xiong 3 , Victor G. Young, Jr 4 , Yisong Guo 3 , Marcel Swart 2 , Prof. Larry Que 4
  1. University of Michigan, Ypsilanti, MICHIGAN, United States
  2. IQCC and Department of Chemistry, University of Girona, 17003 Girona, Spain, PhD, Girona, Spain
  3. Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, United States, Postdoc, Pittsburgh, PA, United State
  4. Chemistry, University of Minnesota, Minneapolis, Minnesota, United States

However, a paradox has arisen in the literature that requires further scrutiny, namely the existence of an S = 1 FeIV=O complex that is capable of oxidizing hydrocarbon substrates at rates just slightly below those found for the TQA complex. This S = 1 complex  [FeIV(O)(Me3NTB)(MeCN)]2+ first reported by Nam would appear to challenge the tacit notion that the S = 2 FeIV=O spin state is the only factor that determines the observed high oxidative reactivity. The existence of this pair of comparably reactive complexes but with different spin states has led us to look for other complexes with unusually high HAT reactivity. The new FeIV=O complex is reported herein that exhibit C–H bond cleavage rates comparable to those for the S = 2 [FeIV(O)(TQA)(MeCN)]2+, the most reactive nonheme complex found to date, despite the fact that this oxidant has S = 1 FeIV(O) units. This new oxidant [FeIV(O)TMIMA)(MeCN)]2+ (2) is synthesized using standard procedures  by reaction with an equimolar amount of [FeII(OTf)2(CH3CN)2] in MeCN under inert atmosphere with oxidant. Corresponding Fe(II) complexes  [FeII(TMIMA)(CH3CN)2](OTf)2 (1) in ~80% yield in MeCN at RT. Their crystal structures display 5-coordinate high-spin iron(II) centers (Figure 1). Corresponding FeIV(O) complexes are generated from the ferrous precursors at −40 °C in MeCN upon the addition of 2-(tBuSO2)C6H4IO (s-ArIO) in  2,2,2-trifluoroethanol. The intermediate has only a few seconds lifetime at 233 K. Using Acetone/MeCN (8:2) at 193K enhanced the intermediate's lifetime (t1/2 ~ 4 min). Treatment of 1 with 2-(tBuSO2)C6H4IO (s-ArIO) at low temperature leads to the formation of highly reactive intermediates 2 that exhibit weak d-d bands at 750 nm (εM = 250 M−1cm−1) as observed for other FeIV=O complexes. At 4 K, 2 exhibit Mossbauer doublets with δ = 0.07 mm/s,  ΔEQ = 1.77 mm/s (30 % of Fe), similar to values observed for other S=1 oxoiron complexes. We introduced a biomimetic imidazole-based ligand, inspired by the crucial role imidazole plays in biological systems. This modification led to the synthesis of complex 2, which exhibited exceptional reactivity. Notably, despite possessing an S = 1 iron–oxo core, complex 2 displayed remarkably high reactivity. 

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