Structural and Spectroscopic Characterization of Tin–Tin Double Bonds in Cyclic Distannenes

Three cyclic distannenes, 1, 3, and 4, and one spacer-bridged bis(stannylene), 2, were prepared and thoroughly investigated by single-crystal X-ray diffraction in the solid state, by variable-temperature (VT) 119Sn NMR, VT 1H NMR, 13C NMR, and UV–vis spectroscopy in solution, and by quantum chemical calculations. The tin(II) compounds feature rigid 9,9-dimethylxanthene or naphthalene backbones and very bulky m-terphenyl substituents ArR [=C6H3-2,6-{C6H2-2,4,6-R3}2; R = Me (1, 3), i-Pr (2, 4)]. For distannenes 3 and 4, the strain of the naphthalene backbone results in rather short tin–tin distances of 2.7299(3) and 2.7688(2) Å, respectively, whereas the xanthene backbone produces long tin–tin distances of 3.0009(7) Å for distannene 1 and 4.2779(7) Å for the spacer-bridged bis(stannylene) 2. In comparison to the AriPr substituents, the less bulky ArMe substituents give rise to stronger trans-bending of the distannenes; moreover, DFT calculations indicate that, in contrast to AriPr, the ArMe substituents allow for asymmetric distortion of the trans-bending in dynamic processes. The oxidation products of distannene 1 and bis(stannylene) 2 reveal rare structural motifs: dihydroxydistannoxane 5 and bis(dihydroxystannane) 6, respectively, which feature terminal Sn–OH functionalities. The reaction of distannene 1 with 1 equiv of potassium chloride in the presence of the cryptating agent 222 results in the formation of the unusual stannyl stannide 7. A modified synthesis protocol for the preparation of distannene 1 yields in one step the stannyl stannylene 8 with a center of chirality at the stannyl tin atom. The series 1, 7, and 8 represents a variation of electronic tin–tin interactions.