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The diiron(II) complex, [(OCO)Fe(MeCN)]2(1, MeCN = acetonitrile), supported by the bis-phenolate carbene pincer ligand, 1,3-bis(3,5-di-tert-butyl-2-hydroxyphenyl)benzimidazolin-2-ylidene (OCO), was synthesized and characterized by single-crystal X-ray diffraction, 1H nuclear magnetic resonance, infrared (IR) vibrational, ultraviolet/visible/near-infrared (UV/vis/NIR) electronic absorption, 57Fe Mössbauer, X-band electron paramagnetic resonance (EPR) and SQUID magnetization measurements. Complex 1activates dioxygen to yield the diferric, μ-oxo-bridged complex [(OCO)Fe(py)(μ-O)Fe(O(C═O)O)(py)] (2) that was isolated and fully characterized. In 2, one of the iron–carbene bonds was oxidized to give a urea motif, resulting in an O(CNHC═O)O binding site, while the other Fe(OCO) unit remained unchanged. When the reaction is performed at −80 °C, an intensively colored, purple intermediate is observed (INT, λmax= 570 nm; ε = 5600 mol L–1cm–1). INTacts as a sluggish oxidant, reacting only with easily oxidizable substrates, such as PPh3or 2-phenylpropionic aldehyde (2-PPA). The identity of INTcan be best described as a dinuclear complex containing a closed diamond core motif [(OCO)FeIV(μ-O)2FeIV(OCO)]. This proposal is based on extensive spectroscopic [UV/vis/NIR electronic absorption, 57Fe Mössbauer, X-band EPR, resonance Raman (rRaman), X-ray absorption, and nuclear resonance vibrational (NRVS)] and computational studies. The conversion of the diiron(II) complex 1to the oxo diiron(IV) intermediate INTis reminiscent of the O2activation process in soluble methane monooxygenases (sMMO). Most importantly, the low reactivity of INTsupports the consensus that the [FeIV(μ-O)2FeIV] diamond core in sMMO is kinetically inert and needs to open up to terminal FeIV═O cores to react with the strong C–H bonds of methane.

Rational innovation of electrocatalysts requires detailed knowledge of spatial property variations across the solid–electrolyte interface. We introduce correlative atomic force microscopy (AFM) to simultaneously probe, in situand at the nanoscale, electrical conductivity, chemical-frictional, and morphological properties of a bimetallic copper–gold system for CO2electroreduction. In air, water, and bicarbonate electrolyte, current–voltage curves reveal resistive CuOxislands in line with local current contrasts, while frictional imaging indicates qualitative variations in the hydration layer molecular ordering upon change from water to electrolyte. Nanoscale current contrast on polycrystalline Au shows resistive grain boundaries and electrocatalytically passive adlayer regions. In situconductive AFM imaging in water shows mesoscale regions of low current and reveals that reduced interfacial electric currents are accompanied by increased friction forces, thus indicating variations in the interfacial molecular ordering affected by the electrolyte composition and ionic species. These findings provide insights into how local electrochemical environments and adsorbed species affect interfacial charge transfer processes and support building in situstructure–property relationships in catalysis and energy conversion research.

A modular approach for the synthesis of Kekulé diradicaloids is reported. The key step is the insertion of a carbene, namely, a cyclic (alkyl)(amino)carbene (CAAC), into the C–H bonds of two terminal alkynes linked by a spacer. Subsequent hydride abstraction, followed by two-electron reduction of the corresponding bis(iminium) salts, affords the desired diradicaloids. This synthetic route readily allows for the installation of communicating spacers, featuring different degrees of aromaticity and lengths, and gives the possibility of generating unsymmetrical compounds with two different CAACs. Electron paramagnetic resonance (EPR), NMR, UV–vis, and X-ray studies in combination with quantum-chemical calculations give insight into the electronic nature of the deeply colored Kekulé diradicaloids. They feature a singlet ground state with varying degrees of diradical character in combination with small singlet/triplet gaps. Upon lengthening of the spacer, the properties of the compounds approach those of monoradicals in which steric protection of the propargyl radical moiety is necessary to inhibit decomposition pathways. Most of these diradicaloids are stable at room temperature, both in solution and in the solid state, but are highly oxygen-sensitive. They represent the first diradicaloids derived from iminium salts.