Synthesis, characterization and reactivity of ionic palladium(II) complexes containing bidentate nitrogen ligands in a unidentate coordination mode

The novel ionic complexes [Pd(Me)(p-An-BIAN)(L-L)]SO3CF3 (L-L = p-An-BIAN (bis(anisylimino)acenaphthene) (1a), phen (2a), dmphen (3a), dppe (4a), dppp (5a)) have been synthesized via the reaction of [Pd(Me)(NCMe)(p-An-BIAN)]SO3CF3 with L-L. The X-ray crystal structure of complex 1a has been determined and shows a distorted square planar geometry in which one BIAN ligand is coordinated in a bidentate fashion (Pd-N(1) = 2.037(4) Angstrom; Pd-N(2) = 2.189(4) Angstrom) and, interestingly, the other BIAN ligand in a unidentate fashion (Pd-N(3) = 2.066(4) Angstrom; Pd-N(4) = 2.714(6) Angstrom). Spectroscopic data of the mixed ligand complexes [Pd(Me)(p-An-BIAN)(L-L)]SO3CF3 (L-L = phen (2a), dppe (4a), dppp (5a)) indicate that the L-L ligand is coordinated in a bidentate fashion and the p-An-BIAN ligand in a unidentate fashion, which is in agreement with the larger complexation strength of the phen, dppe and dppp ligands, as compared with that of the p-An-BIAN ligand. In contrast, complex 3a (L-L = dmphen) contains a bidentate p-An-BIAN ligand and a unidentate dmphen ligand, which can be explained by the sterically demanding methyl groups of the dmphen ligand. Complexes 1a-4a underwent insertion of carbon monoxide, resulting in the formation of acetylpalladium complexes [Pd(C(O)Me)(p-An-BIAN)(L-L)]SO3CF3 (1b-4b). Since mass-law retardation by excess p-An-BIAN has been observed for CO insertion for complexes 1a and 4a, it is proposed that the mechanism involves dissociation of the unidentate nitrogen ligand. Complexes 1a-5a and 1b-4b show fluxional behavior due to flipping of the unidentate nitrogen ligand. Complexes 1a-3a and 1b-3b also show fluxional behavior due to site exchange of the nitrogen atoms of the bidentate nitrogen ligand. A mechanism for this exchange process has been proposed. This mechanism involves (a) substitution of a nitrogen atom of the bidentate nitrogen ligand by the uncoordinated nitrogen atom of the unidentate nitrogen ligand, (b) flipping of the unidentate nitrogen ligand, and (c) a second nitrogen substitution reaction. Reaction of the acetylpalladium complexes 1b-4b with norbornadiene led to dissociation of the unidentate nitrogen ligand and formation of the known alkylpalladium complexes [Pd(C7H8C(O)Me)(p-An-BIAN)]SO3CF3 (1c), [Pd(C7H8C(O)Me)(phen)]SO3CF3 (2c), 1c, and [Pd(C7H8C(O)Me)(dppe)]SO3CF3 (4c), respectively. (C) 1999 Elsevier Science S.A. All rights reserved.