Closed Synthetic Cycle for Nickel-Based Dihydrogen Formation.

Dihydrogen evolution was observed in a two-step protonation reaction starting from a Ni0 precursor with a tripodal N-heterocyclic carbene (NHC) ligand. Upon the first protonation, a NiII monohydride complex was formed, which was isolated and fully characterized. Subsequent protonation yields H2 via a transient intermediate (INT) and an isolable NiII acetonitrile complex. The latter can be reduced to regenerate its Ni0 precursor. The mechanism of H2 formation was investigated by using a deuterated acid and scrutinized by 1 H NMR spectroscopy and gas chromatography. Remarkably, the second protonation forms a rare nickel dihydrogen complex, which was detected and identified in solution and characterized by 1 H NMR spectroscopy. DFT-based computational analyses were employed to propose a reaction profile and a molecular structure of the Ni-H2 complex. Thus, a dihydrogen-evolving, closed-synthetic cycle is reported with a rare Ni-H2 species as a key intermediate.

Evaluation of Manganese Cubanoid Clusters for Water Oxidation Catalysis: From Well-Defined Molecular Coordination Complexes to Catalytically Active Amorphous Films.

With a view to developing multimetallic molecular catalysts that mimic the oxygen-evolving catalyst (OEC) in Nature's photosystem II, the synthesis of various dicubanoid manganese clusters is described and their catalytic activity investigated for water oxidation in basic, aqueous solution. Pyridinemethanol-based ligands are known to support polynuclear and cubanoid structures in manganese coordination chemistry. The chelators 2,6-pyridinedimethanol (H2 L1 ) and 6-methyl-2-pyridinemethanol (HL2 ) were chosen to yield polynuclear manganese complexes; namely, the tetranuclear defective dicubanes [MnII 2 MnIII 2 (HL1 )4 (OAc)4 (OMe)2 ] and [MnII 2 MnIII 2 (HL1 )6 (OAc)2 ] (OAc)2 ⋅2 H2 O, as well as the octanuclear-dicubanoid [MnII 6 MnIII 2 (L2 )4 (O)2 (OAc)10 (HOMe/OH2 )2 ]⋅3MeOH⋅MeCN. In freshly prepared solutions, polynuclear species were detected by electrospray ionization mass spectrometry, whereas X-band electron paramagnetic resonance studies in dilute, liquid solution suggested the presence of divalent mononuclear Mn species with g values of 2. However, the magnetochemical investigation of the complexes' solutions by the Evans technique confirmed a haphazard combination of manganese coordination complexes, from mononuclear to polynuclear species. Subsequently, the newly synthesized and characterized manganese molecular complexes were employed as precursors to prepare electrode-deposited films in a buffer-free solution to evaluate and compare their stability and catalytic activity for water oxidation electrocatalysis.