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

摘要

Diaryl selenides that yield molecular complexes (MC) with bromine are prepared by modulating the effectivenelectronegativity of the selenium atom and the steric environment around the atom in diphenyl selenide (1) withnvarious substituents at the 2-, 3- and/or 4-positions. Halogen induced n1nH and n13nC NMR chemical shifts of the diarylnselenides are examined. The chlorine and iodine adducts of the selenides are shown to be trigonal bipyramidalnadducts (TB) and MC, respectively. In the case of bromine adducts, the structures of (3- and 4-YC6H4)2SeBr2nare demonstrated to be MC if Y is CN and NO2, contrary to the general rule: they are TB if Y is less electron-nwithdrawing than the ethoxycarbonyl group. The CN and NO2 groups increase the effective electronegativity of thenSe atom in (3- and 4-YC6H4)2Se and do not give TB with bromine. The four chloro groups at all meta-positions in 1nare also effective for MC formation. However, 3,5-(O2N)2C6H3SeBr2Ph is TB. Ab initio MO calculations show thatnthe structures of (3-O2NC6H4)2Se and 3,5-(O2N)2C6H3SePh are close to the C2 and Cs symmetries, respectively, whichnreveals that the conformational change is also important when the structures of the bromine adducts are determined.nThe steric congestion must be more severe for TB formation than for MC formation. The structures of 2,6-Cl2C6H3-nSeBr2C6H4Y-p (Y = H and Br) are MC, which shows that the steric effect of 2,6-Cl2C6H3 group is effective for MCnformation. (2-MeC6H4)2SeBr2 and (2-ClC6H4)2SeBr2 are TB and MC, respectively: the electronic effect of the Clngroup must play an additional role in the MC formation since the bulkiness of the Me and Cl groups are expected tonbe similar. The bromine adduct of (2,4,6-Me3C6H2)2Se is also concluded to be MC: the steric effect of the four Mengroups at the ortho-positions is large enough to give MC with bromine. The oxidation potentials (Eox) of the diarylnselenides explain well the structures of the bromine adducts: the electronic effect is directly correlated with Eox andnthe importance of the steric effect has been brought into sharp relief by Eox. Results of MO calculations supportnthe outline of the observations. The structural behavior of some halogen adducts such as 2,6-Cl2C6H3SeCl2C6H4Y-pn(TB: Y = H and Br) is also discussed in some detail.