We present a highly efficient cobalt-mediated hydroboration reaction of alkynes and alkenes enabled by a πacidic and redox-active pyrimidinediimine (P Pym DI) core. The entry point in the catalytic cycle is on a cobalt dinitrogen complex rather than cobalt hydride species, which are commonly postulated in hydroelementation reactions. Stoichiometric studies have demonstrated that both cobalt hydrides and dinitrogen complexes can be generated from the same precursors and under the same reaction conditions, with the sole difference being the reaction atmosphere (argon vs N 2 ). Nevertheless, while the P Pym DI-based cobalt dinitrogen complex is highly active (TOF = 1100 h −1 at t 1/2 , RT), the hydride analogue displays only modest conversions at slow reaction rates. Under the optimized conditions, a wide range of vinyl and alkyl organoboron derivatives can be obtained with high catalytic efficiency. Mechanistic studies suggest that, due to the increased π-acidity of the P Pym DI core, initial formation of Co-π-complexes is preferred, followed by the oxidative addition of borane, which is the turnover-limiting step. This sequence of catalytic steps is supported by a Hammett analysis, which shows that the hydroboration reaction proceeds more rapidly for electron-rich substrates. The electronic structure of all relevant species was investigated in detail by computational, crystallographic, and spectroscopic means, revealing ligand involvement in the redox processes.

A small macrocycle comprising ether‐bridged naphthyl units was prepared in a two‐step synthesis. Single‐crystal X‐ray diffraction of two polymorphs are reported, one of which showed multiple C−H⋅⋅⋅πnaphthyl interactions of a solvent molecule in the cavity of the macrocycle. Chemical reduction led to C−O bond cleavages accompanied by a Z/E isomerization. The resulting twofold negatively charged (E)‐1,2‐bis(2‐naphthyl)ethylene fragment was isolated as its potassium salts. Electronic characterization revealed a singlet ground state, and a marked distortion of the central ethylene unit was observed upon electron uptake.