2024/12/19 by Abhishek Das, Nabhendu Pal, Jin Xiong +4 · 1 voice · 2 citations
Chemistry · Medicine · Biochemistry, Genetics and Molecular Biology · #Metal-Catalyzed Oxygenation Mechanisms #Metal complexes synthesis and properties #Photosynthetic Processes and Mechanisms
paper · doi:10.1021/jacs.4c10120
Nonheme iron enzymes utilize S = 2 iron(IV)-oxo intermediates as oxidants in biological oxygenations. In contrast, corresponding synthetic nonheme Fe IV ═O complexes characterized to date favor the S = 1 ground state that generally shows much poorer oxidative reactivity than their S = 2 counterparts. However, one intriguing exception found by Nam a decade ago is the S = 1 [Fe IV (O)(Me 3 NTB)] 2+ complex (Me 3 NTB = [tris(( N -methyl-benzimidazol-2-yl)methyl)amine], 1O ) with a hydrogen atom transfer (HAT) reactivity that is 70% that of the S = 2 [Fe IV (O)(TQA)] 2+ complex (TQA = tris(2-quinolylmethyl)amine, 3O ). In our efforts to further explore this direction, we have unexpectedly uncovered a family of new S = 1 complexes with HAT reaction rates beyond the currently reported limits in the tripodal ligand family, surpassing oxidation rates found for the S = 2 [Fe IV (O)(TQA)] 2+ complex by as much as an order of magnitude. This is achieved simply by replacing the secondary sphere methyl groups of the Me 3 NTB ligand with larger cycloalkyl-CH 2 (R groups in 2O R ) moieties ranging from c -propylmethyl to c -hexylmethyl. These 2O R complexes show Mössbauer data at 4 K and 1 H NMR spectra at 193 and 233 K that reveal S = 1 ground states, in line with DFT calculations. Nevertheless, they give rise to the most reactive synthetic nonheme oxoiron(IV) complexes found to date within the tripodal ligand family. Our DFT study indicates transition state stabilization through entropy effects, similar to enzymatic catalysis.