Successive Change in Conformation Caused by p-Y Groups in 1-(MeSe)-8-(p-YC6H4Se)C10H6: Role of Linear Se···Se−C Three-Center−Four-Electron versus n(Se)···n(Se) Two-Center−Four-Electron Nonbonded Interactions

Abstract

The structure of 1-(methylselanyl)-8-(p-anisylselanyl)naphthalene (2) and 1-(methylselanyl)-8-(p-chlorophenylselanyl)naphthalene (3) was studied by X-ray crystallographic analysis. The structures around the Se−Me and Se−C6H4OMe-p (Se−CAn) groups in 2 are close to type A and type B, respectively: type A if the Se−C bond is almost perpendicular to the naphthyl plane and type B when the Se−C bond is placed on the plane. Those around the Se−Me and Se−C6H4Cl-p (Se−CAr) groups in 3 are type B and type A, respectively. The nonbonded Se···Se distances of 2 and 3 are 3.0951(8) and 3.1239(7) Å, respectively. The structure of 3 is very different from that observed in 1-(methylselanyl)-8-(phenylselanyl)naphthalene (1), the structure of which is type C, where the two Se−C bonds decline by about 45° from the naphthyl plane. The structure of 3 strongly suggests the contribution of the nonbonded n(SeAr)···σ*(Se−CMe) three-center−four-electron (3c−4e) type interaction. The nonbonded n(SeMe)···σ*(Se−CAn) 3c−4e type interaction must partly contribute to the structure of 2. To clarify the nature of the n(Se)···σ*(Se−C) 3c−4e type interaction observed in 2 and 3, together with the π type two-center−four-electron interaction in 1, ab initio MO calculations with the B3LYP/6-311++G(3df,2pd) method were performed on model a, HaHbSe···SeHa‘Hb‘, where the naphthylidene group was replaced by Ha and Ha‘ and the Me and Ar groups by Hb and Hb‘. Two structures are optimized to be energy minima with ∠SeSeHb = ∠SeSeHb‘ = ca. 74° and 155°. The latter corresponds to the conformation observed in 1, which is controlled by HOMO (π*(Se···Se)); the former is HOMO-1 (σ(Se···Se))-controlled. On the contrary, similar calculations with the B3LYP/6-311+G(2d,p) method on naphthalene-l,8-diselenol show that the type A−type B pairing is evaluated to be most stable, which explains the conformations observed in 2 and 3: the n(Se)···σ*(Se−C) 3c−4e interaction, as well as π-orbitals of the naphthyl group, play an important role in the pairing. The resonance effect of OMe and the inductive effect of Cl must also be important to determine the structures of 2 and 3, respectively. The character of CT calculated by the natural population analysis for the diselenol supports further the n(Se)···σ*(Se−C) 3c−4e interaction.