Synthesis and Fluxional Behavior, Including a Comparative Analysis of the Pd−N Bond Rupture, of New Chiral Complexes of Palladium(0) and -(II) with a Rigid (Aminoferrocenyl)phosphine Ligand. Crystal Structure of the Two Rotamers of a Palladium(0) Maleic Anhydride Complex

Abstract

The (aminoferrocenyl)phosphine ligand 1-diphenylphosphino-2,1‘-(1-dimethylaminopropanediyl)ferrocene, 1, was used to synthesize new palladium(0) and -(II) complexes. The reaction of Pd2(dba)3·CHCl3 with 1 in the presence of the electron-withdrawing olefins dimethylfumarate (DMFU) and maleic anhydride (MA) gave the new complexes Pd(1)(DMFU) (2) and Pd(1)(MA) (3). The allyl complex [Pd(η3-2-Me-C3H4)(1)]Tf (4) was obtained from the reaction of 1 with [Pd(η3-2-Me-C3H4)(Cl)]2 in the presence of AgTf. In solution all these compounds exist as mixtures of two diastereomers, with either the alkene or the allyl group differently oriented with respect to the aminophosphine ligand. The orientation of these ligands in the major isomers has been determined by means of NOEs. Alkene rotation takes place in complexes 2 and 3 with free energies of activation ΔG340⧧ = 73.1 kJ mol-1 (2) and ΔG368⧧ = 79.9 kJ mol-1 (3), respectively. These barriers are compared with those of some analogous ferrocenyl aminophosphine ligands, PPFA (2-(1-dimethylaminoethyl)-1-diphenylphosphinoferrocene) and PTFA (1-diphenylphosphino-2,3-endo-(α-dimethylamino)tetramethyleneferrocene) complexes. For 3, the alkene rotation leads to isomer interconversion, while the observed isomerization of 2 must proceed via an olefin face exchange. Some experiments in relation to the nature of this process are discussed. Starting from PdRR‘L‘ precursors and 1 or PPFA, other Pd(II) derivatives of formulas PdRR‘(1), R = Cl, R‘ = Me, L‘ = cod, 5; R = R‘ = Me, L‘ = tmeda, 6; R = R‘ = C6F5, L‘ = cod, 7, or PdClMe(PPFA), 8, were prepared. For 5 and 8, only the isomers with the methyl group trans to nitrogen were obtained. The Pd−N bond rupture in the new complexes of 1 and in similar derivatives of PPFA and PTFA has been analyzed by variable-temperature 1H NMR studies, and in some cases a line shape analysis has been carried out. The influence of the ferrocenyl aminophosphine and ancillary ligands as well as of the oxidation state of the palladium center on this process is discussed. The molecular structures of both rotamers of 3, present in the same crystal, were determined by X-ray structure analysis.