Synthesis of Mono-, Di-, and Tripalladated 1,3,5-Benzenetricarboxaldehyde Complexes

The oxidative addition of 2,4,6-tribromo-1,3,5-benzenetricarboxaldehyde (C6(CHO)3Br3) to 1, 2, or 3 equiv of [Pd(dba)2] ([Pd2(dba)3]·dba) and N∧N affords, respectively, the mono-, di-, or trinuclear complexes [Pd{C6(CHO)3Br2}Br(N∧N)] (N∧N = tmeda = N,N,N′,N′-tetramethylethylenediamine (1a), tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine (1b), bpy = 2,2′-bipyridine, (1c)); [{PdBr(N∧N)}2{μ2-C6(CHO)3Br}] (N∧N = tmeda (2a), tbbpy (2b)); or [{PdBr(N∧N)}3{μ3-C6(CHO)3}] (N∧N = tmeda (3a), tbbpy (3b)). The reactions of C6(CHO)3Br3 with a mixture of [Pd(dba)2] and PR3 lead, depending on the nature of the phosphine and the molar ratio of the reagents, to the mononuclear trans-[Pd{C6(CHO)3Br2}Br(PR3)2] (R = Ph (1d), R3 = Me2Ph (1e)) or the dinuclear [{trans-PdBr(PMe2Ph)2}2{μ2-C6(CHO)3Br}] (2e) complex. All attempts to prepare trinuclear phosphine complexes using this method of synthesis have been unsuccessful. However, displacement of the tbbpy ligand in 3b by an excess of PMe3 affords the trinuclear complex [{trans-PdBr(PMe3)2}3{μ3-C6(CHO)3}] (3f). The crystal structures of 1b, 1d, 2a·CDCl3, and 2e have been determined by X-ray diffraction studies.