Linear Oligostannanes: A Synthetic and TD-DFT Study

A series of model linear tristannanes [((R-1)(3)Sn)(2)(R-2)(2)Sn (1: R-1 = R-2 = n-Bu, 2: R-1 = Ph, R-2 = n-Bu, 3: R-1 = Me, R-2 = n-Bu, 4: R-1 = n-Bu, R-2 = Ph, 5: R-1 = Me, R-2 = Ph, 6: R-1 = n-Bu, R-2 = Me, 7: R-1 = Ph, R-2 = Me)] were prepared by condensation reactions of one equivalent of a dialkyl or diaryl tin diamide or dihydride and two equivalents of a trialkyl or triaryl tin hydride or amide respectively, and characterized by NMR (H-1, C-13, Sn-119) and UV-Vis spectroscopy, and elemental analysis. An LSDA/SDD level of DFT and TD-DFT for trimers 1-7 predicts significant structural differences as a function and nature of the substituents bound to tin and calculated electronic transitions for these compounds in fair relative agreement with experimental results. Calculated electronic spectra obtained from equivalent DFT studies of the sequentially lengthened oligomers Ph3Sn[t-Bu2Sn](n)SnPh3 (8-11 where n = 1-4) previously prepared by Drager are in good agreement with the reported experimental UV-Visible spectroscopic and X-ray diffraction data which show a red shift of the sigma-sigma* absorption transition (lambda(max)) and bond lengthening and bond angle widening of the central tin atoms as the oligostannane is extended. TD-DFT studies of the alkoxy terminated di(n-butyl) oligostannanes 12a-m (CH3CH2OCH2CH2[Sn(n-Bu)(2)](n)CH2CH2OCH2CH3); n = 3-15) indicate lower limiting energy gaps as the polymer chain size is increased. The calculated data deviates significantly from results found experimentally by Sita and suggests that even smaller energy gaps are possible if the polymer (13) resides in an all trans configuration. TD-DFT studies at the LSDA/SDD level were also carried out on a model polystannane 16 (H[SnH2](n)H, n = 1-40) to investigate the impact of Sn catenation on the intrinsic band gap of the material and compared to both the Si and Ge analogues 14 and 15 (H[MH2](n)H, M = Si, Ge: n = 1-40). The modelling of the polymetallanes reveals the expected limiting energy gap of the sigma-sigma* transition in a trans-planar configuration with Sn (2.681 eV) < Ge (3.096 eV) < Si (3.719 eV) in good agreement with previous studies by Takeda. Finally, comparison of the TD-DFT calculated data for model alternating oligostannanes (17-19) with the UV-Vis spectroscopic data for alternating polystannanes (20-22) recently prepared by polycondensation is made.