On the equilibrium between molecular complexes and trigonal bipyramidal adducts of diaryl sesenide dibromides in solution

A method to visualize the equilibrium between molecular complexes (MCs) and trigonal bipyramidal (TB) adducts for ArSeBr2Ar1 has been devised. The TB formation causes large downfield shifts for the ipso carbons delta (C-i), w hereas the upfield shifts are characteristic for delta (C-i) in the MC form ation. Therefore, the equilibrium is clearly proved if such an Ar-SeAr' is prepared that shows an upfield shift at one of ipso carbons, whereas the ot her goes downfield when bromine is added to the ArSeAr' solution. The halog en induced chemical shifts (Delta delta) are measured for 4-YC6H4SePh (1a-1 g), 2-YC6H4SePh (2d, 2e, 2h), 2-YC6H4SeC6H4Br-p (3d, 3e, 3h), 2-O2NC6H4SeC6 H4Y-p (4a, 4e, 4i), and 2, 4- (O2N)(2)C6H3SeC6H4Y-p (5a, 5e, 5i), where Y = H (a), OMe rb), Me (c), Cl (d), Br (e), COOEt (f), NO2 (g), I (h), and t-B u (i). Positive values (downfield shifts) ave observed for both delta (C-i) in 2 and 3, positive and negative values are recorded for each delta (C-i) in 4; and negative values ave detected for both delta (C-i) in 5. The equi librium between MC's and TB adducts is clearly shown in 4. The equilibrium constants K (K = [MC]/[TB] ave also calculated: they are ca. 0.20 for 2 and 3 and 0.16, 0.33, and 0.73 for 4i, 4a, and 4 e, respectively. The contribu tion of MC is negligible in la, and the contribution of the TB adduct is ne gligible in 5. The results show that TB adducts become unstable relative to MCs, when the effective electronegativity of the selenium atom and/or the bulkiness around the atom in a selenide are increased. (C) 2001 John Wiley & Sons, Inc.