Stabilizing Gamma-Alkyltin-Oxo Keggin Ions By Borate Functionalization

We report a hierarchical self-assembly engineering of tin-oxo clusters from nanosized hydrophobic clusters to a single-layer film of assembled clusters. These clusters are derivatives of the previously reported Na-centered butyltin Keggin ions, but they are bicapped with butyltin and with borate ligands. The formulas gamma-[(n-BuSn)(14)(OCH3)(10)(OH)(3)O-9(NaO4)(HBO3)(2)] and gamma-[(n-BuSn)(14)(OCH3)(10)(OH)(3)O-9(NaO4)(PhBO2)(2)] were determined from single crystal X-ray diffraction and bulk solution characterization including small-angle X-ray scattering, electrospray ionization mass spectrometry, and multinuclear and multidimensional NMR (Sn-119, C-13, and H-1). Solution characterization confirms that borate functionalization inhibits the solution-phase beta-gamma Keggin isomer interconversion that was recognized prior for uncapped butyltin clusters, and in this case, the gamma isomer is favored. The assembly of the gamma-NaSn14BO3 clusters into a homogeneous Langmuir-Blodgett monolayer is the first step toward creating nanopatterned films for microelectronic devices.