Molecular structure of 1,8-bis(trimethylelement)naphthalenes

The X-ray structures of 1,8-bis(trimethylgermyl)naphthalene (4) and 1,8-bis(trimethylstannyl)naphthalene (5) have been determined. The structures of 4 and 5 are critically compared with those of 1,3,6,8-tetra-tert-butylnaphthalene (1) and 1,8-di-tert-butylnaphthalene (2). It is found that familial traits are shared by all the 1,8-bis(trimethylelement)naphthalenes: in each case a twist of the C1-C9-C8 and C4-C10-C5 planes about the C9-C10 axis imparts near C 2 symmetry to the molecule, and the trimethylelement groups in the peri positions are further deflected away from each other and out of the average molecular plane. In each case, the effects of internal strain, brought about by repulsion between the (CH 3) 3Z groups, are felt most strongly near the peri positions, and it is there that the greatest distortions are localized. In this family of compounds, the skeletal distortion decreases in the order 1 (2) ≫ 4 > 5, i.e., in the inverse order of the covalent radius of Z. The conformations adopted by the (CH 3) 3Z groups in 4 and 5 are almost the same, but distinctively different from those in 1 (2). Empirical force field calculations yield structures in good agreement with those obtained by X-ray diffraction; these calculations also predict that the conformation of the as yet unknown 1,8-bis(trimethylsilyl)naphthalene (3) resembles 4 and 5 more closely than 1 (2).