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Abstract Manganese catalysts that activate hydrogen peroxide have seen increased use in organic transformations, such as olefin epoxidation and alkane C−H bond oxidation. Proposed mechanisms for these catalysts involve the formation and activation of Mn^III‐hydroperoxo intermediates. Examples of well‐defined Mn^III‐hydroperoxo complexes are rare, and the properties of these species are often inferred from Mn^III‐alkylperoxo analogues. In this study, we show that the reaction of the Mn^III‐hydroxo complex [Mn^III(OH)(^6Medpaq)]⁺(1) with hydrogen peroxide and acid results in the formation of a dark‐green Mn^III‐hydroperoxo species [Mn^III(OOH)(^6Medpaq)]⁺(2). The formulation of2is based on electronic absorption,¹H NMR, IR, and ESI‐MS data. The thermal decay of2follows a first order process, and variable‐temperature kinetic studies of the decay of2yielded activation parameters that could be compared with those of a Mn^III‐alkylperoxo analogue. Complex2reacts with the hydrogen‐atom donor TEMPOH two‐fold faster than the Mn^III‐hydroxo complex1. Complex2also oxidizes PPh₃, and this Mn^III‐hydroperoxo species is 600‐fold more reactive with this substrate than its Mn^III‐alkylperoxo analogue [Mn^III(OO^tBu)(^6Medpaq)]⁺. DFT and time‐dependent (TD) DFT computations are used to compare the electronic structure of2with similar Mn^III‐hydroperoxo and Mn^III‐alkylperoxo complexes. 

Abstract Coordination complexes of general formulatrans‐[MX₂(R₂ECH₂CH₂ER₂)₂] (M^II=Ti, V, Cr, Mn; E=N or P; R=alkyl or aryl) are a cornerstone of coordination and organometallic chemistry. We investigate the electronic properties of two such complexes,trans‐[VCl₂(tmeda)₂] andtrans‐[VCl₂(dmpe)₂], which thus representtrans‐[MX₂(R₂ECH₂CH₂ER₂)₂] where M=V, X=Cl, R=Me and E=N (tmeda) and P (dmpe). These V^IIcomplexes haveS=3/2 ground states, as expected for octahedral d³. Their tetragonal distortion leads to zero‐field splitting (zfs) that is modest in magnitude (D≈0.3 cm⁻¹) relative to analogousS=1 Ti^IIand Cr^IIcomplexes. This parameter was determined from conventional EPR spectroscopy, but more effectively from high‐frequency and ‐field EPR (HFEPR) that determined the sign ofDas negative for the diamine complex, but positive for the diphosphine, which information had not been known for anytrans‐[VX₂(R₂ECH₂CH₂ER₂)₂] systems. The ligand‐field parameters oftrans‐[VCl₂(tmeda)₂] andtrans‐[VCl₂(dmpe)₂] are obtained using both classical theory andab initioquantum chemical theory. The results shed light not only on the electronic structure of V^IIin this environment, but also on differences between N and P donor ligands, a key comparison in coordination chemistry. 

Abstract The reduction of dioxygen to produce selectively H₂O₂or H₂O is crucial in various fields. While platinum‐based materials excel in 4H⁺/4e⁻oxygen reduction reaction (ORR) catalysis, cost and resource limitations drive the search for cost‐effective and abundant transition metal catalysts. It is thus of great importance to understand how the selectivity and efficiency of 3d‐metal ORR catalysts can be tuned. In this context, we report on a Co complex supported by a bisthiolate N2S2‐donor ligand acting as a homogeneous ORR catalyst in acetonitrile solutions both in the presence of a one‐electron reducing agent (selectivity for H₂O of 93 % and TOFi=3 000 h⁻¹) and under electrochemically‐assisted conditions (0.81 V <η<1.10 V, selectivity for H₂O between 85 % and 95 %). Interestingly, such a predominant 4H⁺/4e⁻pathway for Co‐based ORR catalysts is rare, highlighting the key role of the thiolate donor ligand. Besides, the selectivity of this Co catalyst under chemical ORR conditions is inverse with respect to the Mn and Fe catalysts supported by the same ligand, which evidences the impact of the nature of the metal ion on the ORR selectivity. 

Abstract High‐valent Fe(IV)=O intermediates of metalloenzymes have inspired numerous efforts to generate synthetic analogs to mimic and understand their substrate oxidation reactivities. However, high‐valent M(IV) complexes of late transition metals are rare. We have recently reported a novel Co(IV)−dinitrate complex (1‐NO₃) that activates sp³C−H bonds up to 87 kcal/mol. In this work, we have shown that the nitrate ligands in1‐NO₃can be replaced by azide, a more basic coordinating base, resulting in the formation of a more potent Co(IV)−diazide species (1‐N₃) that reacts with substrates (hydrocarbons and phenols) at faster rate constants and activates stronger C−H bonds than the parent complex1‐NO₃. We have characterized1‐N₃employing a combination of spectroscopic and computational approaches. Our results clearly show that the coordination of azide leads to the modulation of the Co(IV) electronic structure and the Co(IV/III) redox potential. Together with the higher basicity of azide, these thermodynamic parameters contribute to the higher driving forces of1‐N₃than1‐NO₃for C−H bond activation. Our discoveries are thus insightful for designing more reactive bio‐inspired high‐valent late transition metal complexes for activating inert aliphatic hydrocarbons. 

The reaction of a Mn^III-hydroxo complex with ceric ammonium nitrate generates a product with a Mn^III–O–Ce^IVcore that has enhanced reactivity in one-electron oxidation reactions and new reactivity in two-electron oxidation reactions. 

The ability of earth-abundant metals to serve as catalysts for the oxygen reduction reaction is of increasing importance given the prominence of this reaction in several emerging technologies. It is now recognized that both the primary and secondary coordination environments of these catalysts can be modulated to optimize their performance. In this present work, we describe two CoII complexes [CoII(PaPy2Q)](OTf) (1) and [CoII(PaPy2N)](OTf) (2) that catalyze chemical and electrochemical dioxygen reduction. Both 1 and 2 contain CoII centers in a N5− coordination environment, but 2 has a naphthyridine group that places a nitrogen atom in the secondary coordination sphere. Solid-state X-ray crystallography and solution-state spectroscopic measurements reveal that, apart from this second-sphere nitrogen in 2, complexes 1 and 2 have essentially identical properties. Despite these similarities, 2 performs the chemical reduction of dioxygen ~10-fold more rapidly than 1. In addition, 2 has an enhanced performance in the electrochemical reduction of dioxygen compared to 1. Both complexes yield a significant amount of H2O2 in the chemical reduction of dioxygen (>25%). The enhanced catalytic performance of 2 is attributed to the presence of the second-sphere nitrogen atom, which might enable the efficient protonation of cobalt–oxygen intermediates formed during turnover.