Synthesis, oligomerization and catalytic studies of a redox-active Ni<inf>4</inf>-cubane: A detailed mechanistic investigation

A robust tetrameric nickel complex [Ni4((Oal-)2L-Me)4(s)4] (3) (s = solvent) with cubane-like Ni4O4 core topology was isolated as a light greenish-orange crystalline solid in excellent yield. The mechanism of formation of 3 involving the two chloride-containing precursors [Ni4((Oal-)2L-Me)4(s)4]·2MeOH (1) and [Ni4((O-)2L-Me)3((Oal-)(OH)L-Me)Cl] (2) was studied by ESI mass spectrometry and confirmed by the solid state isolation and single-crystal X-ray diffraction. The challenging ligand fields containing mono/di-anionic O2N donating atoms and/or chloride ions stabilized the pentacoordinate Ni(ii) ions in 1-2 upon controlling the experimental conditions. Complexes 1-3 have been characterized by NMR, UV-Vis and mass spectrometric analysis. Complex 3 was found to be redox active by cyclic voltammetry (CV) studies. Theoretical calculations were carried out to shed light on the effects of ligand fields on the stability of complexes 1-3. Complex 3 was found to be a potential catalyst for the diastereoselective cyclopropanation of heteroarenes with good to excellent yields. The ESI mass spectrometric analysis revealed the existence of solution dynamics and oligomerization of 3 in solution. Mechanistic investigation of the catalytic cycle revealed that complex 3 and its various oligomers bind to the diazoester employed, followed by dissociative insertion of the respective carbene moieties to the C2-C3 double bond of the involved aromatic heterocycle, leading to the diastereoselective cyclopropanation. This journal is