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 ΔG 340 ⧧ = 73.1 kJ mol-1 (2) and ΔG 368 ⧧ = 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.

Synthesis, stability and reactivity of palladium(0) olefin complexes bearing labile or hemi-labile ancillary ligands and electron-poor olefins

“…In contrast with the compounds just described, hemi-labile auxiliary ligands containing phosphorus and nitrogen were more extensively studied [26,42,43,[90][91][92][93][94]. Again, irrespectively of the stabilizing olefin the bond lengths are very similar among the fourteen complexes examined with the Pd-N bond length usually spanning within 2.11 and 2.25 Å.…”

“…On the contrary, when N-N, N-S and N-P molecules are employed as ancillary ligands their 1 H NMR signals are all shifted downfield, indicating the predominant r-nature of the ensuing bonds. The formation of different isomers as a function of the olefin structure and of the symmetry of the ancillary ligand can be observed (Scheme 3) [4,30,32,34,[40][41][42][43]. Usually these species undergo extensive fluxionality and the RT 1 H NMR spectra of the complexes are characterized by rapid interconversion among isomers and only averaged spectra are detectable.…”

“…This is the earliest and most frequently proposed mechanism [30,42,43,[46][47][48]; (2) olefin-metal dissociation followed by recombination [30]; (3) intermolecular associative process with free alkene, via (4) metal-L or metal-L 0 bond dissociation followed by rotation of the ancillary ligand and recombination (olefin pseudorotation) [42,43]; (5) intermolecular exchange with the free ancillary ligand L-L 0 . In this case, a mechanism has been proposed which involves an olefin-metal dissociation step [30].…”

“…Taking advantage from the peculiar steric characteristics of the tetramethyl-ethene-tetracarboxylate (tmetc) the olefin exchange equilibria were also studied from the kinetic point of view [26,31,32,34,39,43,[63][64][65]. It has been shown that the rates of exchange are strongly dependent on the steric requirement of the ancillary ligand and the mechanism is associative in nature.…”

(Perfluoro)alkylsilyl-Substituted 2-[Bis(4-aryl)phosphino]pyridines: Synthesis and Comparison of Their Palladium Complexes in Methoxycarbonylation of Phenylacetylene in Regular Solvents and Supercritical CO2

“…Apparently the interchange process between two different isomers of the complex, possibly caused by alkene rotation around the palladium(CdC) bond, dissociation/reassocation reaction of the alkene, or Pd-P rotation, becomes fast at room temperature. 40 Catalytic methoxycarbonylation of phenylacetylene using the in situ prepared catalysts has demonstrated that the influence of funtionalization of the P,N-ligand with a fluoroalkyl group is especially evident when catalysis was performed in a 1:1 (v/v) mixture of methanol and R,R′,R′′-trifluorotoluene, where the use of 10 led to a higher conversion compared to 2. A possible explanation might be that the actual catalyst is formed with different rates and efficiency.…”