Synthesis and Properties of Hypercoordinate Stannanes and Polystannanes

Asymmetric tetraorganotin (IV) compounds containing either a flexible propyl alcohol ligand (10) or a semi-flexible ethyl 2-pyridyl ligand (11) were synthesized in good yields via select hydrostannylation reactions. The triphenyl derivatives were converted in good yields to their dihalide stannane intermediates (6, 7), respectively, by mild sequential chlorination with HCl or by reaction with Br₂. The dihalides were converted to reactive dihydride monomers (8, 9) by a reaction with LiAlH₄ or NaBH₄ in good yields. All stannane intermediates and monomers were fully characterized by NMR (¹H, ¹³C, ¹¹⁹Sn) spectroscopy and high-resolution mass spectrometry. DFT calculations were also performed on the ethyl 2-pyridyl stannane intermediates 2 and 7, which revealed optimized structure geometry and electronic energies. From the three methods that were used, M05-2X-GD3 showed the lowest mean sum of squared distances (MSSD) value and the lowest electronic energies of the hypercoordinate structures. Polymerization of the corresponding dihydrides in the presence of both early and late transition metal catalysts produced moderate molecular weight polymers 10a and 11a. Compound 10a was cast onto a transparent film for the first time and was additionally coated on a PET film. While the cast film on PET showed good flexibility, an initial evaluation of its electronic properties using a 4-probe conductivity device revealed no intrinsic semi-conductivity. Optimization of the polymerization of 10a involved a catalyst screening, using alternative dehydrocoupling catalysts. A new ethyl 2- pyridyl containing polystannane 11a was synthesized with a Mw of 11,200 Da using RhCl(PPh₃)₃, and ¹¹⁹Sn NMR revealed the presence of both 4- and 5-coordinate Sn centers randomly distributed along the backbone.