Properties and reactivity of μ-nitrido-bridged dimetal porphyrinoid complexes: how does ruthenium compare to iron?

Abstract

Methane hydroxylation by metal-oxo oxidants is one of the Holy Grails in biomimetic and biotechnological chemistry. The only enzymes known to perform this reaction in Nature are ironcontaining soluble methane monooxygenase and copper-containing particulate methane monooxygenase. Furthermore, few biomimetic iron-containing oxidants have been designed that can hydroxylate methane efficiently. Recent studies reported that -nitrido bridged diiron(IV)-oxo porphyrin and phthalocyanine complexes hydroxylate methane to methanol efficiently. To find out whether the reaction rates are enhanced by replacing iron by ruthenium we performed a detailed computational study. Our work shows that the -nitrido bridged diruthenium(IV)-oxo reacts with methane via hydrogen atom abstraction barriers that are considerably lower in energy (by about 5 kcal mol -1 ) as compared to the analogous diiron(IV)-oxo complex. An analysis of the electronic structure implicates similar spin and charge distributions for the diiron(IV)-oxo and diruthenium(IV)-oxo complexes, but the strength of the O-H bond formed during the reaction is much stronger for the latter.As such a larger hydrogen atom abstraction driving force for the Ru complex than for the Fe complex is found, which should result in higher reactivity in the oxidation of methane.

From mononuclear iron phthalocyanines in catalysis to μ-nitrido diiron complexes and beyond

“…The particular electronic structures of these µ-nitrido diiron based species confer them unusual reactivity in challenging reactions such as the oxidation of methane and oxidative defluorination of heavily fluorinated aromatic compounds. Not surprisingly that the electronic structures of these µ-nitrido bridged diiron species have been a subject of extensive computational studies [64][65][66][67]82,[129][130][131].…”

“…It is of great interest to determine how the nature of metal sites in µ-nitrido binuclear fragment might influence on the catalytic properties. De Visser and co-workers have performed computational studies on a µ-nitrido bridged diruthenium (IV) porphyrazine oxo species as well as its reaction with methane in comparison with corresponding diiron complex [67]. Both complexes are in a doublet spin ground state which is well-separated from other spin states and oxo group has significant radical character.…”

“…Although, the electronic configurations of oxo species of µ-nitrido diruthenium(IV) and diiron(IV) complexes are similar, the former reacts with methane with H-atom abstraction barrier that is considerably lower in energy (by ~ 5 kcal mol -1 ). The diruthenium(IV)-oxo and diiron(IV)-oxo species exhibit similar spin and charge distributions, but the strength of the O-H bond formed is much stronger for Ru complex that should result in high reactivity of diruthenium species compared with its diiron counterpart [67].…”