A diiron(iv) complex that cleaves strong C–H and O–H bonds

Abstract

The controlled cleavage of strong C-H bonds like those of methane poses a significant challenge for chemists. In nature methane is oxidized to methanol by soluble methane monooxygenase via a diiron (IV) intermediate called Q. To model the chemistry of MMO-Q, an oxo-bridged diiron(IV) complex has been generated by electrochemical oxidation and characterized by several spectroscopic methods. This novel species has an Fe IV/III redox potential of +1.50 V vs. ferrocene (>2 V vs. NHE), the highest value thus far determined electrochemically for an iron complex. This species is quite an effective oxidant. It can attack C-H bonds as strong as 100 kcal mol −1 and reacts with cyclohexane a hundredto a thousand-fold faster than mononuclear Fe IV =O complexes of closely related ligands. Strikingly, this species can also cleave the strong O-H bonds of methanol and tert-butanol instead of their weaker C-H bonds, representing the first example of O-H bond activation for iron complexes.The controlled cleavage of strong C-H bonds like those of methane remains an elusive goal and one of great industrial and practical importance. In nature, methane hydroxylation occurs by oxygen activation through the action of soluble and particulate methane monooxygenases (MMOs) found in methane-metabolizing bacteria [1][2][3][4] . The much better characterized soluble MMOs have a nonheme diiron active site that reacts with O 2 to produce an oxo-bridged diiron (IV) oxidant called Q that effects the transformation [5][6][7][8] . The generation of a diiron(IV) complex that can oxidize strong C-H bonds thus represents an appealing challenge for synthetic chemists.To date, only two types of (μ-oxo)diiron(IV) complexes have been characterized. The first example, represented by two complexes of tetraamido macrocyclic ligands (TAML) characterized by Ghosh et al., is stable at room temperature, but no oxidation chemistry involving C-H bond cleavage has been reported 9 . The second example, recently reported by Xue et al., utilizes the neutral tetradentate N4 ligand tris(3,5-dimethyl-4-methoxypyridylmethyl)amine to form [Fe IV 2 (μ-O) 2 (N4) 2 ] 4+ and serves as a synthetic precedent for the Fe IV 2 (μ-O) 2 core proposed for MMO-Q 10 . This complex is stable for only 1 hour at −40 °C and can attack the weak C-H bonds of dihydroanthracene. These (μ-oxo)diiron(IV) complexes thus exhibit reactivities that fall far short of that associated with the diiron active site of soluble MMO. In this study, we report a much more strongly oxidizing (μ-oxo)diiron(IV) complex, obtained by electrochemical oxidation of its (μ-oxo)diiron(III) precursor, that is capable of cleaving not only the strong C-H bonds of cyclohexane but also the even stronger O-H bonds of aliphatic alcohols. The starting point for this study isIts structure (Fig. 1a) shows that L acts as a pentadentate ligand with an N 4 O donor set where the N 4 subset binds to one iron center while the oxygen atom coordinates to the other iron site. This structure resembles that of a recently described c...

Nuclear resonance vibrational spectroscopic and computational study of high-valent diiron complexes relevant to enzyme intermediates

“…Thus, the bonds are shorter in the latter and their vibrational frequencies are higher. This is consistent with previous EXAFS and Mössbauer studies of 1 (30).…”

“…30 and obtained with >95% and 85% yield, respectively. The 5-mM Fe(III)Fe(IV) and 3-mM Fe(IV) 2 complexes in CH 3 CN solutions were frozen for NRVS samples, and the Fe(III) 2 precursor sample contained pure solid.…”

“…The energy-minimized Fe(III) 2 structure of 1 was then used to obtain the optimized structures of its Fe(III)Fe (IV) and Fe(IV) 2 complexes. Although the Mössbauer data on the Fe(III)Fe(IV) complex of 1 suggested a electron delocalized Fe 3.5+ Fe 3.5+ core (30), DFT calculations produced either a localized Fe(III)Fe(IV) or Fe(IV)Fe(III) core at essentially equivalent energies. Using their average geometry raised the free energy by only 2.6 kcal/mol and did not create imaginary frequencies; thus this average geometry structure was used for the NRVS simulation of the Fe(III)Fe(IV) complex of 1.…”

“…The similarity between the NRVS spectra of the Fe(IV) 2 and Fe(III)Fe(IV) species demonstrates that the NRVS spectral difference between the Fe(III) 2 precursor and the high-valent species is not due to the change in oxidation state of the Fe centers but to their change in spin state. Although the 2Fe(III)s of the precursor are in the S = 5/2 high-spin states, the 2 Fes of the high-valent complexes are low spin (30). Thus, the bonds are shorter in the latter and their vibrational frequencies are higher.…”

“…1). (30), contains a monoμ-oxo bridged diiron center that has two additional amide bridges that model the carboxylate bridges of the biFe enzymes. Its Fe(III)Fe(III) form has been crystallized, and its conversions to the Fe(III)Fe(IV) and Fe(IV) 2 2 ] can be converted to the Fe(IV) 2 form by either electrolysis or chemical oxidation using WCl 6 , without undergoing any structural disruption according to their EXAFS spectra (31).…”