Manganese(IV)-oxo complexes are often invoked as intermediates in Mn-catalyzed C-H bond activation reactions. While many synthetic MnIV-oxo species are mild oxidants, other members of this class can attack strong C-H bonds. The basis for these reactivity differences is not well understood. Here we describe a series of MnIV-oxo complexes with N5 pentadentate ligands that modulate the equatorial ligand field of the MnIV center, as assessed by electronic absorption, electron paramagnetic resonance, and Mn K-edge X-ray absorption methods. Kinetic experiments show dramatic rate variations in hydrogen-atom and oxygen-atom transfer reactions, with faster rates corresponding to weaker equatorial ligand fields. For these MnIV-oxo complexes, the rate enhancements are correlated with both i) the energy of a low-lying 4E excited state, which has been postulated to be involved in a two-state reactivity model, and ii) the MnIII/IV reduction potentials.

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The synthesis and a detailed reactivity study of a binuclear zinc(II) bis(benzenethiolate) complex, [Zn 2 (BPMP)(SPh) 2 ] + (4), and an unprecedented binuclear zinc(II) pentasulfido complex, [Zn 2 (BPMP)(μ 2 -S 5 )] + (6), are presented. While one-electron oxidation of the coordinated benzenethiolate ligands in 4 by Cp 2 Fe + produces diphenyl disulfide and [Zn 2 (BPMP)(μ 2 -OH)] 2+ (5), a two-electron redox reaction between coordinated benzenethiolate ligands in 4 and elemental S (S 8 ) generated diphenyl disulfide and the binuclear zinc(II) pentasulfido complex 6. Complex 6 features a chelating, dianionic, pentasulfido (S 5 2− ) chain and can consume up to a maximum of 3 equiv of PPh 3 to generate Ph 3 PS and 5, while the reaction of 6 with 1 equiv of diphenylphosphinoethane allowed the isolation of [Zn 2 (BPMP)(μ 2 -S 4 )] + (7). A proteolysis reaction of the coordinated S 5 2− chain in 6 with fluoroboric acid (HBF 4 ), benzoic acid (PhCOOH), and thioacetic acid (MeCOSH) generates the complexes [Zn 2 (BPMP)(MeCN) 2 ] 3+ (1), [Zn 2 (BPMP)(μ 2 -PhCOO) 2 ] + (8), and [Zn 2 (BPMP)(μ 2 -SCOMe) 2 ] + (9), respectively, while the protonated S 5 2− chain liberates S 8 and hydrogen sulfide (H 2 S). Finally, the transfer of the coordinated benzenethiolate ligands in 4 and the S 5 2− chain in 6 to selected organic compounds, namely, PhCH 2 Br and PhC(O)Cl, for the generation of various organosulfur compounds is demonstrated.

The synthesis of four molybdenum and tungsten complexes bearing tetradentate tripodal amino bisphenolate ligands with either hydroxyethylene (1a) or hydroxyglycolene (1b) substituents is reported. The molybdenum dioxo complexes [MoO2L] (L = 2a, 2b) and tungsten complexes [WO2L] (3a, 3b) were synthesized using [MoO2(acac)2] and [W(eg)3] (eg = 1,2‐ethanediolato, ethylene glycolate), respectively, as precursors. All complexes were characterized by spectroscopic means as well as by single‐crystal X‐ray diffraction analyses. The latter reveal, in all cases, hexacoordinate complexes in which the hydrogen atom of the hydroxy group is involved in hydrogen bonding with one of the metal oxo groups. In the case of the glycol substituent, the ether oxygen atom is coordinated to the metal whereas the hydroxy group remains uncoordinated. The complexes were tested as catalysts in the epoxidation of cyclooctene under eco‐friendly conditions, using an aqueous solution of H2O2 as the oxidant and dimethyl carbonate (DMC) as solvent or neat conditions, as substitutes for chlorinated solvents. Molybdenum complexes 2a and 2b showed good catalytic activity using H2O2 without added solvent, and tungsten complexes 3a and 3b showed very high activity in the epoxidation of cyclooctene using H2O2 and DMC as solvents.

Apparent high-spin complex with FeN5Cl coordination sphere exhibits a complete transition to the low-spin state on cooling.