Cytochrome P450s and Galactose Oxidases exploit redox active ligands to form reactive high valent intermediates for oxidation reactions. This strategy works well for the late 3d metals where accessing high valent states is rather challenging. Herein, we report the oxidation of Ni II (salenwith mCPBA (meta-chloroperoxybenzoic acid) to form a fleeting Ni III bisphenoxyl diradical species, in CH 3 CN and CH 2 Cl 2 at À 40 °C. Electrochemical and spectroscopic analyses using UV/Vis, EPR, and resonance Raman spectroscopies revealed oxidation events both on the ligand and the metal centre to yield a Ni III bisphenoxyl diradical species. DFT calculations found the electronic structure of the ligand and the dconfiguration of the metal center to be consistent with a Ni III bisphenoxyl diradical species. This three electron oxidized species can perform hydrogen atom abstraction and oxygen atom transfer reactions.

We report the synthesis and crystallographic characterization of a novel Au(III)À OH complex featuring a N^N^N-pincer ligand. Reactivity studies towards oxygen atom transfer (OAT), a type of reactivity previously reported for a Au(III)À OH complex, indicate that this complex provides both a sterically encumbered Au atom and a sterically poorly accessible OH group leading to no reactivity with a series of phosphines. The steric encumbrance sets this example apart from the known examples of Au(III)À OH (pincer) complexes, which commonly feature planar ligands that provide little control over steric accessibility of the Au and O atoms in these complexes. Implications for the mechanism of OAT from AuÀ OH complexes are briefly discussed.Only few examples of structurally defined Au(III) hydroxide complexes have been reported featuring pincer-type ligands, [1] and for most, little is known regarding their reactivity (Figure 1). [2] A notable feature of all reported complexes thus far is that the ligands are planar and therefore provide comparable steric environments. AuÀ OH complexes in general, both Au(I) and Au(III), can be used as synthons to introduce for example anionic ligands, such as a hydride, aryls, and N-heterocycles by ligand exchange. [2c,3] Among the Au(III)À OH complexes an exceptionally wellstudied example is the (C^N^C)AuÀ OH complex from the Bochmann group, which not only serves as a synthon, but also undergoes oxygen atom transfer (OAT) with various phosphines to form the corresponding phosphine oxides and Au(III)À H (Figure 2). [4] Based on a series of experiments, the Bochmann group proposed a concerted mechanism for OAT in which there is planar attack of the phosphine directly onto the oxygen leading to both P=O bond formation and proton reduction giving a AuÀ H (Figure 2). The possibility of an initial AuÀ P interaction was judged unlikely based on DFT calculations [4] and further corroborated by a computational study on the corre-[a

Bis(formazanate)iron(II) complexes undergo a thermally induced S = 0 to S = 2 spin transition in solution. Here we present a study of how steric effects and π-stacking interactions between the triarylformazanate ligands affect the spin-crossover behavior, in addition to electronic substituent effects. Moreover, the effect of increasing the denticity of the formazanate ligands is explored by including additional OMe donors in the ligand ( 7 ). In total, six new compounds ( 2 – 7 ) have been synthesized and characterized, both in solution and in the solid state, via spectroscopic, magnetic, and structural analyses. The series spans a broad range of spin-crossover temperatures ( T 1/2 ) for the LS ⇌ HS equilibrium in solution, with the exception of compound 6 which remains high-spin ( S = 2) down to 210 K. In the solid state, 6 was shown to exist in two distinct forms: a tetrahedral high-spin complex ( 6a , S = 2) and a rare square-planar structure with an intermediate-spin state ( 6b , S = 1). SQUID measurements, 57 Fe Mössbauer spectroscopy, and differential scanning calorimetry indicate that in the solid state the square-planar form 6b undergoes an incomplete spin-change-coupled isomerization to tetrahedral 6a . The complex that contains additional OMe donors ( 7 ) results in a six-coordinate (NNO) 2 Fe coordination geometry, which shifts the spin-crossover to significantly higher temperatures ( T 1/2 = 444 K). The available experimental and computational data for 7 suggest that the Fe···OMe interaction is retained upon spin-crossover. Despite the difference in coordination environment, the weak OMe donors do not significantly alter the electronic structure or ligand-field splitting, and the occurrence of spin-crossover (similar to the compounds lacking the OMe groups) originates from a large degree of metal–ligand π-covalency.

Recent desires to develop environmentally benign procedures for electrophilic chlorinations have encouraged researchers to take inspiration from nature. In particular, the enzyme chloroperoxidase (CPO), which is capable of electrophilic chlorinations through the umpolung of chloride by oxidation with hydrogen peroxide (H 2 O 2 ), has received lots of attention. CPO itself is unsuitable for industrial use because of its tendency to decompose in the presence of excess H 2 O 2 . Biomimetic complexes (CPO active-site mimics) were then developed and have been shown to successfully catalyze electrophilic chlorinations but are too synthetically demanding to be economically viable. Reported efforts at generating the putative active chlorinating agent of CPO (an iron hypochlorite species) via the umpolung of chloride and using simple meso-substituted iron porphyrins were unsuccessful. Instead, a meso -chloroisoporphyrin intermediate was formed, which was shown to be equally capable of performing electrophilic chlorinations. The current developments toward a potential method involving this novel intermediate for environmentally benign electrophilic chlorinations are discussed. Although this novel pathway no longer follows the mechanism of CPO, it was developed from efforts to replicate its function, showing the power that drawing inspiration from nature can have.

C(sp 3 )-H and OÀ H bond breaking steps in the oxidation of 1,4-cyclohexadiene and phenol by a Au(III)-OH complex were studied computationally. The analysis reveals that for both types of bonds the initial XÀ H cleavage step proceeds via concerted proton coupled electron transfer (cPCET), reflecting electron transfer from the substrate directly to the Au(III) centre and proton transfer to the Au-bound oxygen. This mechanistic picture is distinct from the analo-gous formal Cu(III)-OH complexes studied by the Tolman group (J. Am. Chem. Soc. 2019, 141, 17236-17244), which proceed via hydrogen atom transfer (HAT) for CÀ H bonds and cPCET for OÀ H bonds. Hence, care should be taken when transferring concepts between CuÀ OH and AuÀ OH species. Furthermore, the ability of AuÀ OH complexes to perform cPCET suggests further possibilities for one-electron chemistry at the Au centre, for which only limited examples exist.[a] S. Engbers, I.