Exploring Cagelike Silsesquioxane Building Blocks for the Design of Heterometallic Cu₄/M₄ Architectures

Within the sweeping research on the design of new coordination polymers and related metal–organic architectures, the use of silsesquioxane derivatives as important organosilicon building blocks has been poorly explored, despite a number of unique structural and functional characteristics of the resulting products. The present study thus describes an extended series (eight examples) of heterometallic Cu₄Cs₄ and Cu₄Rb₄ coordination polymers with the common formula [(PhSiO_1.5)₁₂­(CuO)₄­(AO_0.5)₄(Solv)_x]_n·nSolv (A is Cs or Rb; Solv refers to ligands and/or solvate molecules including H₂O, EtOH, BuOH, DMF, and DMSO in various combinations), which are based on cagelike coppersilsesquioxanes as nontrivial secondary building units. The concept of supramolecular design was implemented in a straightforward way by the assembly of coppersilsesquioxane cages using large and coordination-versatile cesium or rubidium cations. The structures of all products were established by single-crystal X-ray diffraction studies mainly using synchrotron radiation. The resulting Cu₄Cs₄- and Cu₄Rb₄-silsesquioxanes exhibit an extracage location of the alkali-metal cations, which enables the interconnectivity of neighboring cages into 1D, 2D, or 3D coordination polymer architectures. The unique feature of such architectures is a realization of metallocene Cs···π (Rb···π) joints, providing tightly connected nonporous coordination polymers. A topological classification of cages and coordination polymer networks was performed. Some of the obtained compounds also represent the first examples of Rb-containing silsesquioxanes. The selected products were also tested as homogeneous catalysts in the oxidation and hydrocarboxylation of C₅–C₈ cycloalkanes. This study extends the structural types of heterometallic silsesquioxane cages that can be efficiently applied to the design of functional coordination polymers.