Influence of the Phosphine Ligand on the Stability of [L 3 PdBr] –

Knowing that the strongly electron-withdrawing ligand L allowed the formation of stable pal-ladate complexes, the generation of analogous complexes from [Pd(PPh3)4] as a precursor was attempted. It could be seen in the³¹P-NMR spectra of [Pd(PPh3)4] in THF-D8that the signal representing the neutral palladium complex was shifted upon the addition of one equivalent of LiBr, suggesting that some kind of interaction takes place in this case as well, which is in line with previous findings by Negishi and coworkers.^[13]Unfortunately, the resulting NMR spectra had a very low overall signal intensity and the observed signal was broadened in such a way as to make a quantitative analysis impossible. ESI-mass spectrometric analysis of a mixture of [Pd(PPh₃)₄] and five equivalents of LiBr showed no palladium-containing complexes. This indicates that the triphenylphosphine ligand does not withdraw enough electron density from the palladium center to allow the formation of stable, free [(PPh3)3PdBr]^– ions in the gas phase.

Any palladate complexes that are potentially formed are likely to form contact-ion pairs with Li⁺cations and therefore cannot be probed by ESI mass spectrometry.

To determine the minimum electron-withdrawing capacity of an applied phosphine ligand which is necessary to form stable, detectable palladate complexes [(PR3)3PdBr]^– (R = aryl sub-stituent), ligands bearing phenyl groups as well as electron-withdrawing Ar^Fgroups, L^Ph1and L^Ph2, were synthesized (chart 3, section 6.3).

Inorganic Palladates [L_nPdX]^– and Their Reactivities Towards Electrophiles

Chart 3:Ligands used for the determination of the influence of the electronic properties of the used phosphine onto the formation and stability of palladate complexes.

First, L^Ph1 and L^Ph2 were combined with Pd(OAc)₂ and lithium bromide to yield the corre-sponding palladate complexes, yet the ESI mass spectra of the mixtures only showed palladium(II)-containing species. The addition of hydrazine as reductant led to the formation of [L^Ph1₃ PdBr]^– and, in small amounts, also [L^Ph2₃ PdBr]^–, although the signal intensities were quite low and the mass spectra still displayed a lot of palladium(II)-based anions. Collision-induced dissociation experiments of [L^Ph1₃ PdBr]^– and [L^Ph2₃ PdBr]^– showed that the loss of the less electron-withdrawing phosphine ligands occured at significantly lower acceleration ener-giesE_LAB, indicating a weaker palladium-ligand bond in these cases (figures 4.19 and 4.20, see also figure 4.11 for [L3PdBr]^–).

Figure 4.19: Mass spectrum of mass-selected [L^Ph1₃ PdBr]^– and its fragment ions pro-duced upon collision-induced dissociation (E_LAB= 10.0 eV).

Figure 4.20: Mass spectrum of mass-selected [L^Ph2₃ PdBr]^– and its fragment ions pro-duced upon collision-induced dissociation (E_LAB= 5.0 eV).

To compare the tendencies of the different phosphine ligands to be incorporated in palladate complexes directly, competition experiments with L and L^Ph1 or L^Ph2, respectively, were con-ducted. To this end, solutions of [L3Pd] in THF were treated with three equivalents of L^Ph1 or L^Ph2 and LiBr and stirred for 10 min. As could be expected due to the electronic properties of the applied ligands, the mass spectra recorded from these solutions demonstrated a strong

42

Results and Discussion preference for the coordination of the more electron-withdrawing phosphine ligand L to the palladium center of the anionic complexes, compared to the less electron-withdrawing L^Ph1. The above-shown [L₃PdBr]^– dominated the spectrum, while mixed complexes [L^Ph1L₂PdBr]^– and [L^Ph1₂ LPdBr]^– were present at lower intensities. [L^Ph1₃ PdBr]^– could only be detected with a very low signal intensity. The relative signal intensities of this complex and of the other L^Ph1-containing species increased, as one would expect, when larger amounts of L^Ph1 were added. The favored incorporation of L into the considered palladate species was even more pronounced when L^Ph2 was added as second ligand. In this case, the only detectable L^Ph2 -containing palladate complex was [L^Ph2L2PdBr]^–, even when the amount of added L^Ph2 was increased to 20 equivalents relative to [L3Pd].

Gas-phase fragmentation experiments with the mixed complexes [L^Ph2L2PdBr]^–, [L^Ph1L₂PdBr]^–, and [L^Ph1₂ LPdBr]^–confirmed the stronger bond between the electron-poor phos-phine L and the palladium center: The phenyl-substituted ligands L^Ph1 and, especially, L^Ph2 were dissociated preferably upon collision-induced dissociation, as energy-dependent CID ex-periments showed (equations (4.35) to (4.37), figures 4.21 and 4.22).

[L^Ph2L2PdBr]⁻−−→ [L2PdBr]⁻+L^Ph2 (4.35) [L^Ph1L2PdBr]⁻−−→ [L2PdBr]⁻+L^Ph1 (4.36) [L^Ph1₂ LPdBr]⁻−−→ [L^Ph1LPdBr]⁻+L^Ph1 (4.37)

Figure 4.21:Normalized signal intensities of mass-selected [L^Ph1L2PdBr]^– (black) and its frag-ment ions [L2PdBr]^– (red), [LPdBr]^– (blue), [L^Ph1LPdBr]^– (green), and [L^Ph1PdBr]^– (ma-genta) produced upon collision-induced disso-ciation at varying acceleration energiesE_LAB.

Figure 4.22:Normalized signal intensities of mass-selected [L^Ph1L2PdBr]^– and its fragment ions [L2PdBr]^– (red) and [LPdBr]^– (blue) produced upon collision-induced dissociation at varying acceleration energiesE_LAB.

Inorganic Palladates [L_nPdX]^– and Their Reactivities Towards Electrophiles

The alternative dissociation of L was only observed in the case of [L^Ph1L2PdBr]^–, where [L^Ph1LPdBr]^– was formed in very small amounts (equation (4.38)). When higher acceleration energies were applied, the dissociation of a second phosphine ligand could be induced in all cases (equations (4.39) and (4.40)).

[L^Ph1L₂PdBr]⁻−−→ [L^Ph1LPdBr]⁻+L (4.38) [L^Ph1LPdBr]⁻−−→ [LPdBr]⁻+L^Ph1 (4.39) [L₂PdBr]⁻−−→ [LPdBr]⁻+L (4.40) In the competition experiments, the acceleration energies needed to bring about the disso-ciation of L were generally higher than those needed for the dissodisso-ciation of L^Ph1 and L^Ph2, as was already seen for [L^Ph1₃ PdBr]^– and [L^Ph2₃ PdBr]^–. This finding, together with the pre-ferred incorporation of the more electron-withdrawing complexes into the regarded palla-date complexes, confirms the assumption that the bond between the palladium center and the phosphine ligand is stronger when the electron density at the aryl substituents of the phosphine ligand is reduced. This effect can be attributed to the π-backbonding interaction of the palladium center with the ligand, which can be expected to account for a substan-tial proportion of the bonding interaction between a very electron-rich metal center, such as a low-valent palladium center in an anionic complex, and a relatively electron-poor phos-phine ligand. The more electron-withdrawing substituents are present in the phosphos-phine lig-ands, the stronger the π-back-bonding becomes, resulting in a more stable palladium-ligand bond.