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Is the μ‐Oxo‐μ‐Peroxodiiron Intermediate of a Ribonucleotide Reductase Biomimetic a Possible Oxidant of Epoxidation Reactions?
Author(s) -
de Visser Sam P.
Publication year - 2008
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.200701802
Subject(s) - ribonucleotide reductase , chemistry , singlet state , reactivity (psychology) , ligand (biochemistry) , stereochemistry , ground state , crystallography , spin states , density functional theory , photochemistry , excited state , computational chemistry , inorganic chemistry , atomic physics , protein subunit , medicine , biochemistry , physics , alternative medicine , receptor , pathology , gene
Abstract Density functional calculations on a μ‐oxo‐μ‐peroxodiiron complex ( 1 ) with a tetrapodal ligand BPP (BPP= N , N ‐bis(2‐pyridylmethyl)‐3‐aminopropionate) are presented that is a biomimetic of the active site region of ribonucleotide reductase (RNR). We have studied all low‐lying electronic states and show that it has close‐lying broken‐shell singlet and undecaplet ( S =0, 5) ground states with essentially two sextet spin iron atoms. In strongly distorted electronic systems in which the two iron atoms have different spin states, the peroxo group moves considerably out of the plane of the μ‐oxodiiron group due to orbital rearrangements. The calculated absorption spectra of 1,11 1 are in good agreement with experimental studies on biomimetics and RNR enzyme systems. Moreover, vibrational shifts in the spectrum due to 18 O 2 substitution of the oxygen atoms in the peroxo group follow similar trends as experimental observations. To identify whether the μ‐oxo‐μ‐1,2‐peroxodiiron or the μ‐oxo‐μ‐1,1‐peroxodiiron complexes are able to epoxidize substrates, we studied the reactivity patterns versus propene. Generally, the reactions are stepwise via radical intermediates and proceed by two‐state reactivity patterns on competing singlet and undecaplet spin state surfaces. However, both the μ‐oxo‐μ‐1,2‐peroxodiiron and μ‐oxo‐μ‐1,1‐peroxodiiron complex are sluggish oxidants with high epoxidation barriers. The epoxidation barriers for the μ‐oxo‐μ‐1,1‐peroxodiiron complex are significantly lower than the ones for the μ‐oxo‐μ‐1,2‐peroxodiiron complex but still are too high to be considered for catalytic properties. Thus, theory has ruled out two possible peroxodiiron catalysts as oxidants in RNR enzymes and biomimetics and the quest to find the actual oxidant in the enzyme mechanism continues.