Structural studies on diaryl selenide dihalides in solution: molecular complex formation of substituted diphenyl selenides with bromine

Abstract

Diaryl selenides that yield molecular complexes (MC) with bromine are prepared by modulating the effective electronegativity of the selenium atom and the steric environment around the atom in diphenyl selenide (1) with various substituents at the 2-, 3- and/or 4-positions. Halogen induced ¹H and ¹³C NMR chemical shifts of the diaryl selenides are examined. The chlorine and iodine adducts of the selenides are shown to be trigonal bipyramidal adducts (TB) and MC, respectively. In the case of bromine adducts, the structures of (3- and 4-YC₆H₄)₂SeBr₂ are demonstrated to be MC if Y is CN and NO₂, contrary to the general rule: they are TB if Y is less electron-withdrawing than the ethoxycarbonyl group. The CN and NO₂ groups increase the effective electronegativity of the Se atom in (3- and 4-YC₆H₄)₂Se and do not give TB with bromine. The four chloro groups at all meta-positions in 1 are also effective for MC formation. However, 3,5-(O₂N)₂C₆H₃SeBr₂Ph is TB. Ab initio MO calculations show that the structures of (3-O₂NC₆H₄)₂Se and 3,5-(O₂N)₂C₆H₃SePh are close to the C₂ and C_s symmetries, respectively, which reveals that the conformational change is also important when the structures of the bromine adducts are determined. The steric congestion must be more severe for TB formation than for MC formation. The structures of 2,6-Cl₂C₆H₃SeBr₂C₆H₄Y-p (Y = H and Br) are MC, which shows that the steric effect of 2,6-Cl₂C₆H₃ group is effective for MC formation. (2-MeC₆H₄)₂SeBr₂ and (2-ClC₆H₄)₂SeBr₂ are TB and MC, respectively: the electronic effect of the Cl group must play an additional role in the MC formation since the bulkiness of the Me and Cl groups are expected to be similar. The bromine adduct of (2,4,6-Me₃C₆H₂)₂Se is also concluded to be MC: the steric effect of the four Me groups at the ortho-positions is large enough to give MC with bromine. The oxidation potentials (E_ox) of the diaryl selenides explain well the structures of the bromine adducts: the electronic effect is directly correlated with E_ox and the importance of the steric effect has been brought into sharp relief by E_ox. Results of MO calculations support the outline of the observations. The structural behavior of some halogen adducts such as 2,6-Cl₂C₆H₃SeCl₂C₆H₄Y-p (TB: Y = H and Br) is also discussed in some detail.