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Effect of Ligand Fields on the Reactivity of O 2 ‐Activating Iron(II)‐Benzilate Complexes of Neutral N5 Donor Ligands
Author(s) -
Bhattacharya Shrabanti,
Singh Reena,
Paine Tapan Kanti
Publication year - 2020
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.202000142
Subject(s) - chemistry , ligand (biochemistry) , reactivity (psychology) , stereochemistry , denticity , diphenylamine , ligand field theory , decarboxylation , medicinal chemistry , photochemistry , crystallography , crystal structure , catalysis , organic chemistry , medicine , ion , biochemistry , alternative medicine , receptor , pathology
Three new iron(II)‐benzilate complexes [(N4Py)Fe II (benzilate)]ClO 4 ( 1 ), [(N4Py Me2 )Fe II (benzilate)]ClO 4 ( 2 ) and [(N4Py Me4 )Fe II (benzilate)]ClO 4 ( 3 ) of neutral pentadentate nitrogen donor ligands have been isolated and characterized to study their dioxygen reactivity. Single‐crystal X‐ray structures reveal a mononuclear six‐coordinate iron(II) center in each case, where benzilate binds to the iron center in monodentate mode via one carboxylate oxygen. Introduction of methyl groups in the 6‐positions of the pyridine rings makes the N4Py Me2 and N4Py Me4 ligand fields weaker compared to that of the parent N4Py ligand. All the complexes ( 1 – 3 ) react with dioxygen to decarboxylate the coordinated benzilate to benzophenone quantitatively. The decarboxylation is faster for the complex of the more sterically hindered ligand and follows the order 3 > 2 > 1 . The complexes display oxygen atom transfer reactivity to thioanisole and also exhibit hydrogen atom transfer reactions with substrates containing weak C−H bonds. Based on interception studies with external substrates, labelling experiments and Hammett analysis, a nucleophilic iron(II)‐hydroperoxo species is proposed to form upon two‐electron reductive activation of dioxygen by each iron(II)‐benzilate complex. The nucleophilic oxidants are converted to the corresponding electrophilic iron(IV)‐oxo oxidant upon treatment with a protic acid. The high‐spin iron(II)‐benzilate complex with the weakest ligand field results in the formation of a more reactive iron‐oxygen oxidant.