Computational studies (performed by Dr. Lucia Caporaso)

DFT calculations were performed with Gaussian09¹³⁰ using the BP86131,132,133

functional and the LANL2DZ ECP C5¹³⁴ with the associated valence basis set on the Pd atom and the 6-31G (d) basis set on all the other atoms. All geometries were localized in the gas phase at the BP86 level. Minima were localized by full optimization of the starting structures, while transition states were approached through a linear transit procedure and then located by a full transition state search. All structures were confirmed as minimum or transition state through frequency calculations. The natural bond orbital analyses were performed by using NBO version 3.135,136,137,138,139,140,141,142

The reported energies were obtained through single point

Deactivation of Neutral Palladium(II) Phosphinesulfonato Polymerization Catalysts

energy calculations on the BP86 geometries using the M06 functional, the SDD ECP with the associated valence basis set on the Pd atom, and the TZVP basis set on all other atoms.

dichloroethane solvent effects were included with the continuum solvation model PCM.¹⁴³

Scheme 5.10 Energetics (kJ mol^-1) of the reductive elimination of 1-H-P^tBu₃, 1-CH₃-P^tBu₃ and 1-Et-P^tBu₃ to the corresponding phosphonium salts and Pd(0)

Since experimental studies suggested that reductive elimination is the key reaction as it forms protonated ligand H[P,O] which is responsible for deactivating catalytically active species, the mechanistic pathway for reductive elimination of the hydride complex 1-H-P^tBu₃ in presence of one equivalent of P^tBu3 was investigated. Calculations exhibit a transition state (1-H-TS1) lying 124 kJ mol^-1 higher than 1-H-P^tBu3 + P^tBu3 (Scheme 5.10). The transition state shows a slightly elongated Pd-H distance of 1.58 Å in comparison to the initial distance of 1.48 Å in 1-H-P^tBu3 as well as an elongated Pd-P distance of 2.96 Å as compared to 2.31 Å in 1-H-P^tBu₃ suggesting a subsequent cleavage of these bonds. In addition, an interaction between the phosphorus and the hydride is formed with a distance of 1.86 Å which is however still long in comparison to literature-known phosphonium salts (typical distances 1.33–1.40 Å) (Scheme 5.11).¹⁴⁴ The departing Pd-P^tBu₃ fragment is stabilized by an incoming P^tBu₃. This reaction is all in all nearly thermoneutral with 5 kJ mol^-1 what seems plausible since the back reaction, namely oxidative addition was used to synthesize the model compound 1-H-P^tBu₃. The similar reaction pathways employed for CH₃[P,O] and Et[P,O] from 1-CH₃-P^tBu₃ and 1-Et-P^tBu3, respectively, were also found to be accessible with 164 kJ mol^-1 and 171 kJ mol^-1, thus require higher energies than the corresponding reductive elimination from 1-H-P^tBu3. The products formed are much more stable than the educts by 65 and 66 kJ mol^-1; respectively (Scheme 5.10).

Deactivation of Neutral Palladium(II) Phosphinesulfonato Polymerization Catalysts

Scheme 5.11 Transition state geometries of 1-H-TS1, 1-CH₃-TS1 and 1-Et-TS1

The thermodynamic difference in reductive elimination of 1-H-P^tBu3 in comparison to 1-CH₃/Et-P^tBu₃ may be explained by considering homolytic bond dissociation energies (BDE).

Pd-H bonds were calculated to be approximately 100 kJ mol^-1 stronger than Pd-C bonds, while in case of the resulting phosphonium salts, P-H is merely ca 30 kJ mol^-1 stronger than P-C bonds, thus the difference in thermodynamics can be explained by the difference in Pd-H and Pd-C bond energies and can be reasonably approximated by the calculated differences of the BDE of the Pd−R and P−R bonds. In addition, a steric factor for the reductive elimination of 1-CH3-P^tBu3 and 1-Et-P^tBu3 may be present as close distances between the ipso-carbon of the anisyl and the migrating alkyl group of 2.94 Å and 2.80 Å were detected.

The thermodynamic preference for reductive elimination of 1-CH₃-P^tBu₃ and 1-Et-P^tBu₃ makes the oxidative addition of Me[P,O] and Et[P,O] to [Pd⁰(P^tBu3)2] less likely as it would cost 229 and 237 kJ mol^-1, respectively, to reach the corresponding transition state. A lower transition state was found to be the oxidative addition of P-anisyl of the ligand Me[P,O] with the transition state 1-anisyl-TS1 lying only 157 kJ mol^-1 higher than the educts Me[P,O] and [Pd⁰(P^tBu₃)₂]. In the absence of the additional P^tBu₃ which would stabilize the Pd-P^tBu₃ fragment, the oxidative addition of P-anisyl turns energetically barrierless considering the complete reaction pathway starting from 1-CH3-P^tBu3 (Scheme 5.12). This supports the observation of phosphorus scrambling which was described earlier in this chapter.

Deactivation of Neutral Palladium(II) Phosphinesulfonato Polymerization Catalysts

Scheme 5.12 Energetics (kJ mol^-1) of the reductive elimination of 1-CH3-P^tBu3 to Me[P,O] and subsequent oxidative addition of P-anisyl to Pd(0) in presence and absence of one additional P^tBu₃

Since decomposition was experimentally identified to occur via protonation of the catalytically active species by the P-protonated ligand H[P,O] which is formed by reductive elimination of the hydride species [1-H], the reaction pathways 1-H-P^tBu₃, 1-CH₃-P^tBu₃ and 1-Et-P^tBu3 with H[P,O] were theoretically investigated. With respect to the P-protonated ligand H[P,O], it has to be noted that the sulfonate protonated tautomer [P,O]H lies only 12 kJ mol^-1 higher. The protonation reaction can now proceed by exchange of the coordinated P^tBu3

by the O-protonated ligand [P,O]H and forms a transition state (1-H-TS2) in which a hydrogen-hydrogen bond is formed (Scheme 5.13 and Scheme 5.14). The distance of 0.80 Å between the two hydrogens in 1-H-TS2 is very close to the distance in dihydrogen (0.74 Å).

Subsequent formation of dihydrogen and [κκκκ²-(2-anisyl)2P,O)]2Pd lies 43 kJ mol^-1 higher than the educts. A rather similar energetic profile was calculated for the reaction of O-protonated ligand [P,O]H with 1-CH3-P^tBu3 and 1-Et-P^tBu3. Displacement of the initially coordinated P^tBu3 was found to only lie 2 kJ mol^-1 higher than the educts 1-CH3-P^tBu3 and [P,O]H. In case of 1-Et-P^tBu₃, formation of 1-Et-(P,O)H is even slightly preferred by 3 kJ mol^-1. However, displacement of PPh3 in 1-Et-PPh3 by [P,O]H to form 1-Et-(P,O)H requires 18 kJ mol^-1. This may be considered as the reason that the ratios of decomposition products from thermolysis of 1-Et-PPh3 is less affected by the presence of H[P,O] in comparison to the alkyl complexes 1-alkyl-dmso since replacement of the ligand at the fourth coordination site (dmso or PPh₃) becomes a crucial issue.

Deactivation of Neutral Palladium(II) Phosphinesulfonato Polymerization Catalysts

In contrast to the protonation reaction of fragment [1-H], the corresponding transition states for the protonation of [1-CH3] and [1-Et] lie significantly lower with 84 and 104 kJ mol^-1, respectively (Scheme 5.13). Moreover, the products, methane or ethane and [κκκκ² -(2-anisyl)2P,O)]2Pd, are thermodynamically strongly favored while the reaction to hydrogen is disfavored by 43 kJ mol^-1. Again, this can be explained by considering the calculated homolytic bond dissociation energies in 1-H-P^tBu₃ and 1-CH₃-P^tBu₃, which indicate that Pd-CH3 bond is ca 100 kJ mol^-1 weaker than the Pd-H, while the respective CH3-H and H-H bonds are reported to be 440 and 436 kJ mol^-1.¹⁴⁵ Thus the energetic difference in formation of hydrogen and methane originates mainly in the difference of homolytic bond dissociation energies of Pd-H and Pd-CH3.

Scheme 5.13 Energetics (kJ mol^-1) of the tautomerization of H[P,O] to [P,O]H and reaction with 1-H-P^tBu₃, 1-CH₃-P^tBu₃ and 1-Et-P^tBu₃.

Scheme 5.14 Transition state geometry of 1-H-TS2

According to these calculations, the rate-limiting step for the deactivation of [1-alkyl] is the reductive elimination of 1-H-P^tBu₃ to H[P,O] with 124 kJ mol^-1 as compared to the barriers calculated for the protonation of 1-CH -P^tBu (84 kJ mol^-1) and 1-Et-P^tBu (104 kJ mol^-1)