Unlocking Coordination Chemistry and Photocatalytic Activity of Mn(IV) Complexes with 1,4,6,10-Tetraazaadamantane-Based Tris-Hydroxylamine Ligand TAAD

Manganese(IV) species play an essential role in natural photoredox processes, making the development of artificial biomimetic catalysts a promising goal, which is, however, complicated by the lack of stable and soluble Mn(IV) complexes. Cyclic tris(hydroxylamine) ligands were recently shown to stabilize atypical +4 oxidation states for a variety of 3d metals due to a strong σ-donating ability of three deprotonated N-OH moieties. Here, we report a comprehensive study of manganese coordination chemistry with the 1,4,6,10-tetraazaadamantane-based tris(hydroxylamine) ligand (TAAD). A series of mono- and dinuclear Mn(IV)-TAAD complexes were obtained and studied by SC-XRD analysis, HRMS, NMR (Evans method), UV-Vis spectroscopy, cyclic voltammetry, and DFT calculations. The TAAD ligand was found to stabilize [Mn2(μ-OR)3]-dimeric motifs uncommon in Mn(IV) chemistry. These dinuclear motifs were shown to disassemble into mononuclear species upon treatment with chelating N-ligands. Mononuclear Mn(IV)-TAAD complexes exhibited photocatalytic activity in the oxidative azidation/ring-opening of 1-arylcyclohexenes with TMSN3 and molecular oxygen.