NMR Studies of the Species Present in Cross-Coupling Catalysis Systems Involving Pd(η3-1-Ph-C3H4)(η5-C5H5) and [Pd(η3-1-Ph-C3H4)Cl]2 Activated by PBu3t, XPhos, and Mor-Dalphos: Nonexistence of Pd(XPhos)n and Pd(Mor-Dalphos)n (n=1, 2) at Moderate Temperatures

The compounds Pd(eta(3)-1-Ph-C3H4)(eta(5)-C5H3) (I), Pd-2(dba)(3) (II), Pd(OAc)(2) (III), and [Pd(eta(3)-1-Ph-C3H4)Cl](2) (IV) are frequently utilized as catalyst precursors for a variety of cross-coupling processes, including Suzuki-Miyaura, Heck-Mizoroki, Sonogashira, and Buchwald-Hartwig reactions. In the preceding paper in this issue, we assess and compare catalyst systems based on I-IV activated with PBu3t, XPhos, and/or Mor-Dalphos for the prototypical Buchwald-Hartwig amination reactions of 4-bromo- and 4-chloroanisole with morpholine, noting several apparent incongruities which seem to indicate mechanistic dissimilarities for various reactant/precatalyst combinations. In this paper we investigate by NMR spectroscopy the solution chemistry of I and IV with PBu3t, XPhos, and Mor-Dalphos, noting similarities and differences in the respective abilities of these precursor-ligand combinations to generate palladium(0) catalyst systems. We find inter alia that steric requirements prevent Xphos and Mor-Dalphos from forming 2:1 palladium(0) complexes and, surprisingly, that 1:1 palladium(0) complexes of Xphos and Mor-Dalphos are unstable with respect to dissociation to free ligand and palladium metal. In other words, these two ligands and, by implication, other sterically demanding phosphine ligands do not form palladium(0) compounds.