Synthesis, Characterization and Dynamic Behavior of (π‐Allyl)palladium Complexes with Polydentate Nitrogen Ligands, Evidence of a Dissociative Mechanism

Abstract

New examples of the fluxional behavior of (q3-ally1)palladium complexes are described. Variable-temperature experiments and line-shape analysis of two (q3-allyl)palladium complexes containing 2-( 1H-pyrazol-1-y1)azine ligands were performed in order to determine the activation free energies, enthalpies and entropies of the process. A dissociative mechanism pointing to a selective Pd-N cleavage is proposed on the basis of these results and NOE experiments above and below the coalescence temperature.The synthesis and catalytic reactivity of (q3-ally1)palladium complexes have been widely studied [']. For instance, the catalytic allylation is a process of synthetic interest[le-"I. When a chiral group, for example a chelating nitrogen or a phosphorus-nitrogen ligand, is involved, relatively high enantiomeric excesses are obtainedr21. There is a growing interest in understanding the solution structures ['] and the fluxional behaviorL41 of allylpalladium compounds as they may be related with the catalytic results. Some of us reported recently[5a] on the stabilization of PF; hydrolysis intermediates by the [2-methylallylpalladium]+ fragment. This unsaturated species may be stabilized in solutions of coordinating solvents such as acetone and reacts with different polypyrazolylmethane and borate ligands to give cationic complexes with interesting fluxional The fluxionality phenomena are very common in palladium(I1) chemistry and, in many cases, the dynamic behavior is explained as a result of preliminary ligand dissociations enabling different processes to occud6I. For instance, in complexes with N-donor chelating ligands, the Pd-N bond breaking is at this moment an accepted path to explain different interconversion processes. In this paper we study the fluxional behavior of new ally1 complexes containing asymmetric N-donor ligands. In this case, the equivalence of the two halves of the allylic group would be an indication of a fluxional process and, if a dissociative mechanism was found, the selectivity towards the Pd-N bond cleavage could be explored. Another aim in our work was to investigate if the study of NOE effects between the allylic group and the ligand could be a way to test the existence of a Pd-N bond cleavage. Results and DiscussionIn this paper we report on the preparation and dynamic behavior of (2-methyla1lyl)palladium complexes with asymmetric ligands such as 2-( lH-pyrazol-l-y1)pyridine (PzPy) and 2-( 1H-pyrazol-1 -yl)pyrimidine (PzPm). The new complexes [Pd(q3-2-Me-C3H4)PzPy]Tf (1) and [Pd(q3-2-Me-C3H4)PzPm]Tf (2) (Tf = CF3S0,, Scheme 1) were prepared by treatment of the dimer complex [Pd(q3-C4H7)C1I2 with AgCF3S03. The addition of the ligands to the filtered solutions allowed the isolation of complexes 1 and 2.Complexes 1 and 2 were characterized by their NMR spectra, elemental analyses and MS FAB. 'H-NMR and 13C-NMR spectra of 1 and 2 (Tables 1-2) at 293 K were assigned by comparison with the spectra of the ligdnd and with several related c0mplexes[~1. Coordination with the (q3-allyl)palladium fragme...

Apparent Allyl Rotation and Pd−N Bond Rupture in Allylpalladium Complexes with N‐Donor Ligands − Evidence of an Associative Mechanism

“…In the other cases these protons give rise to a single broad signal at this temperature. This phenomenon clearly indicates that the H 4 ϪH 6 interconversion process previously detected in [Pd(η 3 -2Me-C 3 H 4 )-(pzpm)]CF 3 SO 3 (I) [23] is also operating (see section on fluxional behaviour). In general, a shift towards higher frequency is observed for the corresponding resonances of the complexes with respect to the free ligands due to the ligandto-metal σ-donation.…”

“…This information and the study of the influence of different factors could also shed light on the controversy surrounding mechanisms (associative or dissociative). In the case of derivative I, [23] the corresponding representation of free energy of activation versus coalescence temperature did not allow a conclusion to be drawn as to whether the two points for the apparent allyl rotation and the one for the H 4 ϪH 6 interconversion were in a straight line -the infor- Table 1). An increase in the temperature allowed the corresponding coalescence temperatures to be determined.…”

“…The assignment for the nitrogen-ligand resonances was made on the basis of previous studies with similar ligands. [23,37] The H 3Ј and H 5Ј protons of the pyrazolyl rings appear as singlets both in the ligands and complexes as a result of the presence of the substituent in the 4-position. In the free ligands the H 5 proton of the pyrimidine ring appears as a triplet coupled to H 4 and H 6 , which are equivalent.…”

“…[31a,35] In recent years we have been interested in the synthesis, structural characterisation and dynamic behaviour of allylpal-ladium(ii) complexes with N-donor ligands containing pyrazolyl groups. [23Ϫ28,35] We have described the synthesis and fluxional behaviour of the complexes [Pd(η 3 -2Me-C 3 H 4 )(NNЈ)]CF 3 SO 3 [NNЈ ϭ 2-(1H-pyrazol-1-yl)pyrimidine ϭ pzpm (I); NNЈ ϭ 2-(1H-pyrazol-1-yl)pyridine ϭ pzpy (II)] [23] (see Scheme 1) where the syn-syn, anti-anti interconversion was detected along with an interchange process between the H 4 and H 6 protons of the pyrimidine ring in complex I. This last interchange is only possible through PdϪN(pm) bond rupture.…”

(Polypyridyl)ruthenium(II) Complexes Based on a Back‐to‐Back Bis(pyrazolylpyridine) Bridging Ligand

“…The presence of s-trans,trans conformations of the pyrazolyl groups with respect to the central pyridine groups can be concluded from the observed chemical shifts for L in the uncoordinated form (Figure 1a). [38,39] Due to the substitution at the N 1 atom of the pyrazole, the chemical shift of H pz5 is largely downfieldshifted. This shift is attributed to the interaction of the H pz5 proton with the electron density of the pyridine lone pair, causing local deshielding of the magnetic field.…”