Multinuclear Clusters of Manganese and Lithium with Silsesquioxane-Derived Ligands: Synthesis and Ligand Rearrangement by Dioxygen- and Base-Mediated Si–O Bond Cleavage

The synthesis of manganese cluster complexes templated by polyhedral oligomeric silsesquioxane-derived ligands is described. MnII3(Ph7Si7O12)2Pyr4 (1) and MnII4(Ph4Si4O8)2(Bpy)2(Py)2 (3) are prepared by replacement of the amide ligands of Mn(NR2)2 (R = SiMe3) via ligand protolysis by the acidic proton of the respective silsesquioxane-derived silanols. Complex 1 is shown to undergo ligand rearrangement by reaction with O2, which results in oxidation of the cluster to a mixed MnII/III cluster, concomitant with cleavage of the Si-O bonds of the ligand, releasing a [Ph2Si-O]+ unit, opening a new ligating siloxide group, and resulting in the formation of Mn3(Ph6Si6O11)2Pyr4 (2). The ligand framework of 1 can also be perturbed by a base. The addition of LiOH/BuLi delivers a soluble equivalent of Li2O to 1, resulting in cleavage of the Si-O bonds and linkage of the resulting exposed silicon atoms by the new oxide, giving a linked ligand variant that templates a Li2Mn3 cluster, Mn3Li2(Ph7Si7O12OPh7Si7O12)DMF5Pyr (4). These systems are characterized by single-crystal X-ray diffraction, absorption spectroscopy, Fourier transform infrared, cyclic voltammetry, and CHN combustion analysis. Mechanistic implications for the Si-O bond cleavage events are discussed.