Mechanistic Studies of the Insertion of CO₂into Palladium(I) Bridging Allyl Dimers

Abstract

In contrast to the chemistry of momomeric η1-Pd allyls, which act as nucleophiles, and monomeric η3-Pd allyls, which act as electrophiles, relatively little is known about the reactivity of Pd complexes with bridging allyl ligands. Recently we demonstrated that PdI dimers containing two bridging allyl ligands react with one equivalent of CO2 to form species with one bridging allyl and one bridging carboxylate ligand. In this work we have prepared complexes from three different classes of PdI bridging allyl dimers: (i) dimers containing two bridging allyl ligands, (ii) dimers with one bridging allyl and one bridging chloride ligand, and (iii) dimers with one bridging allyl and one bridging carboxylate ligand. Complexes from all three groups have been characterized by X-ray crystallography, and their structures compared. Complexes with two bridging allyl ligands have the longest Pd bridging allyl bond lengths due to the high trans influence of the opposing bridging allyl ligand. For these species the HOMO is located almost entirely on the bridging allyl ligands, whereas for chloride- and carboxylate-bridged species the HOMO is primarily Pd based. A combined experimental and theoretical study has been performed to investigate the reactivity of the three different types of bridging allyl dimers with CO2. Complexes with one bridging allyl and one bridging chloride ligand and complexes with one bridging allyl and one bridging carboxylate ligand do not insert CO2 because the reaction is thermodynamically unfavorable. In contrast, in most cases the reaction of CO2 with species containing two bridging allyl ligands is facile and involves nucleophilic attack of the bridging allyl ligand on electrophilic CO2. An alternative pathway for CO2 insertion, which involves a monomer/dimer equilibrium, can occur in the presence of a weakly coordinating ligand. Overall, our results suggest that although the bridging allyl ligand is likely to be unreactive in carboxylate- and chloride-bridged species, complexes with two bridging allyl ligands can act as nucleophiles like monomeric η1-Pd allyls.

Nickel(I) Monomers and Dimers with Cyclopentadienyl and Indenyl Ligands

“…(1) and (2)] 7b, 9 Furthermore, in some systems which use Pd II precatalysts, the formation of Pd I dimers is detrimental to catalysis as it sequesters the active Pd 0 species into a less reactive form 8f. 9a, 10…”

“…However, these complexes were not structurally characterized and the synthetic routes involved highly thermally unstable Ni 0 precursors. Furthermore, although we previously serendipitously crystallized a Ni complex with two bridging allyl ligands,7b there are no reproducible methods to synthesize complexes of this type. In pioneering work, Johnson and co‐workers12 very recently described the first examples of Ni I dimers with bridging Cp and indenyl ligands, which were supported by P i Pr 3 ligands, but did not explore the reactivity or electronic structures of these complexes.…”

Trinuclear Nickel Complexes with Metal–Arene Interactions Supported by Tris- and Bis(phosphinoaryl)benzene Frameworks

“…The Ni I 2 motif with bridging allyl ligands has been structurally characterized. 12 The strong interactions between the metal centers and the arene are not only supported by the short distances in the solid-state, but also by NMR spectroscopy. 31 P- 31 P coupling through five bonds is observed in both 2 and 3 , indicating electronic communication across metals coordinated to opposite faces of the arene.…”

An Unusual Example of Hypervalent Silicon: A Five‐Coordinate Silyl Group Bridging Two Palladium or Nickel Centers through a Nonsymmetrical Four‐Center Two‐Electron Bond

“…Square pyramidal is a rare geometry for hypervalent fivecoordinate Si, which in general has a marked preference for trigonal-bipyramidal geometry. [2] The structure contains a puckered bicyclic core, Pd 2 (5) is consistent with a Pd À Pd bond, [11] and all Si À C bond distances are similar (ca. 1.9 ).…”