New chiral palladium(0) and -(II) complexes of (aminoferrocenyl)phosphine ligands PPFA and PTFA. X-ray crystal structure analysis and fluxional behavior involving alkene rotation, Pd-N bond rupture, and selective η3-η1-η3 allyl isomerization

Abstract

(Aminoferrocenyl)phosphine ligands 2-(1-(dimethylamino)ethyl)-1-(diphenylphosphino)ferrocene (PPFA) and [η5-cyclopentadienyl][η5-4-(endo-dimethylamino)-3-(diphenylphosphino)-4,5,6,7-tetrahydro-1H-indenyl]iron(II) (PTFA), were used as ligands in palladium(0) and -(II) complexes. The reaction of Pd2(dba)3·CHCl3 with PPFA or PTFA in the presence of the electron-withdrawing olefins maleic anhydride (MA) and dimethyl fumarate (DMFU) gave the complexes Pd(PTFA)(DMFU) (2), Pd(PPFA)(MA) (3), and Pd(PPFA)(DMFU) (4). Allylic complexes [Pd(η3-2-Me-C3H4)(PTFA)]Tf (5) and [Pd(η3-2-Me-C3H4)(PPFA)]Tf (6) (Tf = triflate) were obtained by reaction of PTFA or PPFA 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. 1H NMR variable-temperature studies have been carried out for 2−6 and for Pd(PTFA)(MA) (1). Rotation of the alkene was observed for complexes 1−4 on the NMR time scale. ΔG⧧c has been calculated and values between 57.6 kJ mol -1 (298 K) and 76.6 kJ mol -1 (373 K) have been obtained. A Pd−N bond rupture which interchanges the two amino methyl groups is observed (ΔG⧧328 = 63.9 kJ mol-1 to ΔG⧧368 = 74.9 kJ mol-1) for derivatives of PPFA, but not for complexes containing PTFA. An EXSY experiment carried out on complex 5 has evidenced a selective η3−η1−η3 (carbon cis to phosphorus) allyl isomerization. Molecular structures of 4 and 6 were determined by X-ray structure analysis.