Preparation and DFT Studies of κ2C,N-Hypercoordinated Oxazoline Organotins: Monomer Constructs for Stable Polystannanes

Abstract

Tetraorganotin tin(IV) compounds containing a flexible or rigid (4: Ph3Sn-CH2-C6H4-R; 7: Ph3SnC6H4-R, R = 2-oxazolinyl) chelating oxazoline functionality were prepared in good yields by the reaction of lithiated oxazolines and Ph3SnCl. Reaction of 7 with excess HCl resulted in the isolation of the tin monochlorido compound, 9 (ClSn[Ph2]C6H4-R). Conversion of the triphenylstannanes 7 and 4 into their corresponding dibromido species was successfully achieved from the reaction with Br2 to yield 10 (Br2Sn[Ph]C6H4-R) and 11 (Br2Sn[Ph]-CH2-C6H4-R), respectively. X-ray crystallography of 4, 7, 9, 10, and 11 reveal that all structures adopt a distorted trigonal bipyramidal geometry around Sn in the solid state. Compound 4, with an additional methylene spacer group, displays a comparatively long Sn–N bond distance compared to the dibromido tin species, 11. Several DFT methods were compared for accuracy in predicting the solid-state geometries of compounds 4, 7, 9–11. Compounds 10 and 11 were further converted into the corresponding dihydrides (12: H2Sn[Ph]C6H4-R, 13: H2Sn[Ph]-CH2-C6H4-R), via Br–H exchange, in high yield by reaction with NaBH4. Polymerization of 12 or 13 with a late transition metal catalyst produced a low molecular weight polystannane (14: –[Sn[Ph]C6H4-R]n–, Mw = 10,100 Da) and oligostannane (15: –[Sn[Ph]-CH2-C6H4-R]n–, Mw = 3200 Da), respectively.

New Hypercoordinating Organostannanes for the Modular Functionalization of Mono‐ and Polystannanes: Synthetic and Computational Studies**

“…distorted tetrahedral geometry around the Sn centre (τ 4 = 0.52, where the τ 4 measure ranges from 1 for tetrahedral to 0 for square planar geometries). [24] The Sn(1)À O(1) distance of 12 was 3.030(1) Å, falling within the van der Waals radii for Sn and O (3.69 Å) but exceeding typical SnÀ O covalent bond lengths (2.05 Å). [18] The new model stannanes 22 and 23 were prepared using established protocols.…”

“…The four DFT methods used in geometry optimizations gave similar results, with the PBE0-GD3BJ method generally providing the best prediction of solid-state molecular geometries (Table S1), as previously observed. [23,24] The M05-2X method also performs well.…”

“…In two previous studies, [23,24] we compared a total of 19 X-ray crystallographic structures of Sn hypercoordinate compounds with those calculated by a selection of computational DFT methods to assess which level of theory provided the best structural predictions. Although comparisons between experimental solid state and calculated gas phase structures are problematic for a variety of reasons, [23,25,26] there are few other practical options.…”

“…The B3PW91 [28] and PBE0 ("PBE1PBE") [29] functionals supplemented with Grimme's D3 empirical dispersion function and Becke-Johnson damping (GD3BJ) [30] were the best of ten methods studied; [23] the M05-2X [31] method with Grimme's D3 empirical dispersion (GD3) performed comparably. [24] In this study, nine crystallographic structures were compared using the three best DFT methods from the previous work as well as the MN15 [32,33] functional with GD3BJ empirical dispersion (see SI, Table S1 for details). The four DFT methods used in geometry optimizations gave similar results, with the PBE0-GD3BJ method generally providing the best prediction of solid-state molecular geometries (Table S1), as previously observed.…”

“…Compound 12 was recrystallized from DCM/hexanes and its solid-state structure established by X-ray crystallography (Figure 4: top left). This model triphenylstannane assumes a distorted tetrahedral geometry (τ4 = 0.52) 24 around the Sn centre. The new model stannanes 22 and 23 were prepared using established protocols.…”