A bis-Phenolate Carbene-Supported\nbis-μ-Oxo\nIron(IV/IV) Complex with a [Fe<sup>IV</sup>(μ-O)<sub>2</sub>Fe<sup>IV</sup>] Diamond Core Derived from Dioxygen Activation

The diiron(II) complex, [(OCO)Fe(MeCN)]<sub>2</sub> (<b>1</b>, MeCN = acetonitrile), supported by the bis-phenolate carbene\npincer\nligand, 1,3-bis(3,5-di-<i>tert</i>-butyl-2-hydroxyphenyl)benzimidazolin-2-ylidene\n(OCO), was synthesized and characterized by single-crystal X-ray diffraction, <sup>1</sup>H nuclear magnetic resonance, infrared (IR) vibrational, ultraviolet/visible/near-infrared\n(UV/vis/NIR) electronic absorption, <sup>57</sup>Fe Mössbauer,\nX-band electron paramagnetic resonance (EPR) and SQUID magnetization\nmeasurements. Complex <b>1</b> activates dioxygen to yield the\ndiferric, μ-oxo-bridged complex [(OCO)Fe(py)(μ-O)Fe(O(CO)O)(py)]\n(<b>2</b>) that was isolated and fully characterized. In <b>2</b>, one of the iron–carbene bonds was oxidized to give\na urea motif, resulting in an O(C<sub>NHC</sub>O)O binding\nsite, while the other Fe(OCO) unit remained unchanged. When the reaction\nis performed at −80 °C, an intensively colored, purple\nintermediate is observed (<b>INT</b>, λ<sub>max</sub> =\n570 nm; ε = 5600 mol L<sup>–1</sup> cm<sup>–1</sup>). <b>INT</b> acts as a sluggish oxidant, reacting only with\neasily oxidizable substrates, such as PPh<sub>3</sub> or 2-phenylpropionic\naldehyde (2-PPA). The identity of <b>INT</b> can be best described\nas a dinuclear complex containing a closed diamond core motif [(OCO)Fe<sup>IV</sup>(μ-O)<sub>2</sub>Fe<sup>IV</sup>(OCO)]. This proposal\nis based on extensive spectroscopic [UV/vis/NIR electronic absorption, <sup>57</sup>Fe Mössbauer, X-band EPR, resonance Raman (rRaman),\nX-ray absorption, and nuclear resonance vibrational (NRVS)] and computational\nstudies. The conversion of the diiron(II) complex <b>1</b> to\nthe oxo diiron(IV) intermediate <b>INT</b> is reminiscent of\nthe O<sub>2</sub> activation process in soluble methane monooxygenases\n(sMMO). Most importantly, the low reactivity of <b>INT</b> supports\nthe consensus that the [Fe<sup>IV</sup>(μ-O)<sub>2</sub>Fe<sup>IV</sup>] diamond core in sMMO is kinetically inert and needs to\nopen up to terminal Fe<sup>IV</sup>O cores to react with the\nstrong C–H bonds of methane.