Palladium-based complexes comprise some of the most important catalysts in synthetic chemistry. Its ability to reliably and cyclically effect two-electron bond-forming and bond-breaking processes has made palladium the go-to metal for cross coupling reactions (Kumada, Suzuki, Negishi, Sonogashira, Heck, Stille, Hiyama). One of the most widely used catalyst precursors is tris(dibenzylideneacetone)dipalladium(0).[1] It is a commercially available Pd(0) source with labile ligands, easily replaced with others such as phosphines or carbenes. Furthermore, it can be easily synthesized from Pd(II) sources and sodium acetate in the presence of dibenzylideneacetone (DBA). A 2012 study actually found that store-bought Pd2(dba)3 samples were often contaminated with palladium nanoparticles, which will form upon gradual decomposition of the reagent at room temperature.[2] This observation implies that it is unwise to rely on store-bought Pd2(dba)3 and it is instead more advisable to synthesize it in-house to ensure maximum purity. The procedure works equally well if starting from Pd(OAc)2 or PdCl2.
Procedure:[2] A 50mL round bottom flask with sir bar was charged with Pd(OAc)2 (0.100g, 0.445mmol), sodium acetate (0.365g, 4.45mmol), DBA (0.208g, 0.890mmol), and 10mL of anhydrous methanol. The flask was attached to the Schlenk line under argon, and heated to 40degC for 3h. The precipitate was subsequently filtered off in air, and washed with methanol (2 x 3mL), followed by water (3 x 3mL). The filter flask was swapped for a clean one, and the solids on the frit were extracted with 25mL of anhydrous, non-acidic chloroform. This filtrate was concentrated to dryness in vacuo (at room temperature), and the residue dissolved in a minimum amount of chloroform (ca. 5mL). After adding an additional 20mL of acetone, the mixture was capped and placed in the freezer overnight. The crystallized product was subsequently isolated by vacuum filtration, washed with cold acetone (2 x 5mL), and dried in vacuo. Pd2(dba)3-CHCl3 solvate was isolated as a dark purple solid (reported yield: 0.219g, 95%) and was transferred into the glovebox freezer for storage.
Notes:
- Reference 2 describes a method to determine the purity of Pd2(dba)3 samples by 1H NMR. The measured samples from three commercial sources were found to be 77, 92, and 64% pure.[2]
- The Ananikov procedure (described here) affords Pd2(dba)3 in 99% purity by the method of reference 2. A detailed pictorial description of the procedure can be found in its Supporting Information.
- Solid samples of Pd2(dba)3 solvates are moderately stable in air and can thus be handled briefly. In solution, the complex decomposes slowly to DBA and metallic palladium. The procedure described above affords the chloroform solvate. It is the one that crystallizes in the highest yield.
- 1H NMR of Pd2(dba)3 is quite complex, displaying an absurd number of signals (>50) in the 4-8ppm range. This is because the fluxional DBA is bound to palladium in three different conformations (s-cis, s-cis; s-trans, s-trans; and s-cis, s-trans). The solution spectrum in CDCl3 shows a major isomer in which the three DBA ligands adopt the s-cis, s-cis thermodynamically preferred conformation, and a minor isomer. However, conformation of the DBA ligands in the solid-state varies depending on crystallization conditions: All DBAs in s-cis, s-trans for Pd2(dba)3-CHCl3;[3] and two s-cis, s-trans with one s-cis, s-cis for Pd2(dba)3.[4]
- Disregarding solvates, there are two forms of the Pd-DBA motif: Pd2(dba)3 and Pd(dba)2. The difference is that the latter is actually Pd2(dba)3-dba, with one cocrystallized DBA molecule that does not bind palladium but affect the stoichiometry.[4] Both species seemed to offer similar catalytic performance in several cross-coupling reactions.[5]
[1] Tsuji, J.; Fairlamb, I. J. S. Tris(dibenzylideneacetone)dipalladium-Chloroform. EROS, 2008. DOI: 10.1002/047084289X.rt400.pub2.
[2] Zalesskiy, S. S.; Ananikov, V. P. Organometallics 2012, 31, 2302-2309.
[3] Ukai, T.; Kawazura, H.; Ishii, Y.; Bonnet, J. J.; Ibers, J. A. J. Organomet. Chem. 1974, 65, 253-266.
[4] Pierpont, C. G.; Mazza, M. C. Inorg. Chem. 1974, 13, 1891-1895.
[5] Cong, M.; Fan, Y.; Raimundo, J.-M.; Tang, J.; Peng, L. Org. Lett. 2014, 16, 16, 4074-4077.