Reversible oxidative addition of a diaryl diselenide to a diorganopalladium(II) complex, carbon–selenium bond formation at palladium(IV), and structural studies of palladium(II) and platinum(IV) selenolates

Methyl(4-methoxyphenyl)(2,2'-bipyridine)palladium(II) (1) reacts with bis(4-chlorophenyl) diselenide in dichloromethane to form an equilibrium with the Pd(IV) complex Pd(SeC6H4Cl)2Me(C6H4OMe)(bpy) (2) for which the forward reaction exhibits DH = -130 +/- 12 kJ mol-1 and DS = -472 +/- 49 JK-1 mol-1, and with K = 754 +/- 145 at -25 C. The Pd(IV) complex is isolable at -40 C, and when the equilibrium mixture is kept at -25 C, a temperature at which the Pd(II) complex is stable, selective reductive elimination of Me–SeC6H4Cl occurs very slowly from the Pd(IV) complex to form Pd(SeC6H4Cl)(C6H4OMe)(bpy) (3). In contrast, (ClC6H4Se)2 reacts with PdMe2(dmpe) (4) [dmpe = 1,2-bis(dimethylphosphino)ethane] to form Pd(SeC6H4Cl)Me(dmpe) (5) and Me–SeC6H4Cl. A second equivalent of (ClC6H4Se)2 reacts with 5 to cleave the second Pd–Me bond to give Pd(SeC6H4Cl)2(dmpe) (6) and Me–SeC6H4Cl. Similarly, PdMeTol(dmpe) (7) (Tol = 4-tolyl) forms predominantly Pd(SeC6H4Cl)Tol(dmpe) (8) together with some Pd(SeC6H4Cl)Me(dmpe) (5), and 8 reacts with (ClC6H4Se)2 to form Pd(SeC6H4Cl)2(dmpe) (6) and Tol-SeC6H4Cl. Bis(4-chlorophenyl) diselenide reacts with PtTol2(bpy) (9) (Tol = 4-tolyl) to form Pt(SeC6H4Cl)2Tol2(bpy) (10) which, together with 2, has a trans-configuration for the selenolate ligands. X-ray structural studies of octahedral 10 as the solvate 10.3CHCl3 and square planar 5 are reported