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On the Lewis Acidity of the Oxoiron(IV) Unit in a Tetramethylcyclam Complex
Author(s) -
Klein Johannes E. M. N.,
Draksharapu Apparao,
Shokri Alireza,
Cramer Christopher J.,
Que Lawrence
Publication year - 2018
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201704977
Subject(s) - unit (ring theory) , psychology , mathematics education
The correlation between oxidation state and Lewis acidity is well established for hexaquairon complexes in the +II and +III oxidation state, in which the higher oxidation state leads to a lower p K a for the bound H 2 O ligand. This article addresses the Lewis acidity of the oxoiron(IV) complex [Fe IV (O)(TMC)(OH 2 )] 2+ ( 1‐OH 2 ; TMC=1,4,8,11‐tetramethylcyclam) by determining the p K a of the H 2 O ligand. We establish that 1‐OH 2 has a p K a of 6.9±0.5, a value that falls in between those found for [Fe III (OH 2 ) 6 ] 3+ and [Fe II (OH 2 ) 6 ] 2+ . This intermediate value can be readily rationalized by the presence of the highly basic oxide ligand that mitigates the Lewis acidity of the iron(IV) center. Although the oxo ligand occupies only one position in 1‐OH 2 , anti to all four methyl groups that protrude from the same face of the nonplanar TMC ligand, its conjugate base 1‐OH exists as a mixture of syn and anti tautomers, which are related by proton transfer between the oxo and the hydroxo ligands.