Synthesis, characterization, and reactivity of high-valent manganese complexes of macrocyclic tetraamido-N ligands

A strategy will be described in which the reactivity of a manganese-oxo is significantly altered by a ligand design process. The approach arises from the desire to understand how one can instigate the reactivity of a manganese-oxo species supported in a robust tetraamido macrocyclic ligand. This tetraamido macrocyclic ligand was designed to have in close proximity to the manganese-oxo unit a secondary site that could reversibly or irreversibly bind a secondary ion. The secondary site of tetraamido macrocyclic ligand is defined by the lone pairs of the pyridyl nitrogen and the closest amide oxygen. The donor atoms of this bidentate site are capable of binding Lewis acids. The compound $MnvOSwitch-$ was synthesized by oxidation of its Mn(III) precursor with t-butyl-hydroperoxide in $H2O;$; it was isolated as the tetraphenylphosphonium salt. $MnvOSwitch-$ was characterized by X-ray crystallography and by spectroscopic means (UV/visible, IR, $H1$ and $C13$ NMR, and mass spectrometry).; The binding of simple inorganic mono-, di-, and trivalent cations by the secondary binding site was studied by UV/visible, IR, $H1$ and $C13$ NMR spectroscopy and mass spectrometry. It was found that secondary ion binding depends on both ionic size and charge. For comparably sized mono- and divalent cations, the divalent ones bind more strongly. The susceptibility of the manganyl moiety of the $MnvOSwitch-$ system to secondary ion perturbation is illustrated by noting that the $Mn\equiv O18$ stretch in the IR spectrum shifts from 939 $cm-1$ when no cation is present in the secondary site to 954 $cm-1$ when Li+ occupies the secondary site.; As hoped, insertion of a cation into the secondary site had the desired dramatic effect on the reactivity of $MnvOSwitch-$ as an O-atom transfer agent. The reactivity was investigated by examining the oxidation of $Ph3P$ to $Ph3PO$ by $MnvOSwitch-$. It was found that the rate of $Ph3P$ oxidation could be increased by >1500 fold compared to the parent complex when a cation was included in the secondary site.; The O-atom transfer activity towards tetramethylethylene was studied by 13C-NMR spectroscopy. When the $MnvOSwitch-$ is employed as the catalyst a mixture of oxidation products is obtained, tetramethylethylene oxide and 2,3-dimethylbut-3-en-2-ol. Interestingly, when the zinc bound form of the $MnvOSwitch-$ catalyst is used only the 2,3-dimethylbut-3-en-2-ol...