Pd-mediated Negishi cross-coupling reactions were studied by a combination of kinetic measurements, electrospray-ionization (ESI) mass spectrometry, (31)P NMR and UV/Vis spectroscopy. The kinetic measurements point to a rate-determining oxidative addition. Surprisingly, this step seems to involve not only the Pd catalyst and the aryl halide substrate, but also the organozinc reagent. In this context, the ESI-mass spectrometric observation of heterobimetallic Pd-Zn complexes [L2 PdZnR](+) (L=S-PHOS, R=Bu, Ph, Bn) is particularly revealing. The inferred presence of these and related neutral complexes with a direct Pd-Zn interaction in solution explains how the organozinc reagent can modulate the reactivity of the Pd catalyst. Previous theoretical calculations by González-Pérez et al. (Organometallics- 2012, 31, 2053) suggest that the complexation by the organozinc reagent lowers the activity of the Pd catalyst. Presumably, a similar effect also causes the rate decrease observed upon addition of ZnBr2 . In contrast, added LiBr apparently counteracts the formation of Pd-Zn complexes and restores the high activity of the Pd catalyst. At longer reaction times, deactivation processes due to degradation of the S-PHOS ligand and aggregation of the Pd catalyst come into play, thus further contributing to the appreciable complexity of the title reaction.

Palladium ate complexes are frequently invoked as important intermediates in Heck and cross-coupling reactions, but so far have largely eluded characterization at the molecular level. Here, we use electrospray-ionization mass spectrometry, electrical conductivity measurements, and NMR spectroscopy to show that the electron-poor catalyst [L Pd] (L=tris[3,5-bis(trifluoromethyl)phenyl]phosphine) readily reacts with Br ions to afford the anionic, zero-valent ate complex [L PdBr] . In contrast, more-electron-rich Pd catalysts display lower tendencies toward the formation of ate complexes. Combining [L Pd] with LiI and an aryl iodide substrate (ArI) results in the observation of the Pd ate complex [L Pd(Ar)I ] .

Palladium-At-Komplexe gelten als wichtige Zwischenstufen in Heck-und Kreuzkupplungsreaktionen, haben sich aber bisher einer Charakterisierung auf molekularer Ebene weitgehend entzogen. Mithilfe der Elektrosprayionisierungs-Massenspektrometrie, Messungen der elektrischen Leitfähigkeit und NMR-Spektroskopie wird hier gezeigt, dass der elektronenarme Katalysator [L 3 Pd] (L = Tris[3,5-bis(trifluormethyl)phenyl]phosphan) bereitwillig mit Br À zum anionischen, nullwertigen At-Komplex [L 3 PdBr] À reagiert. Elektronenreichere Pd-Katalysatoren weisen dagegen eine geringere Tendenz zur Bildung von At-Komplexen auf.D ie Kombination von [L 3 Pd] mit LiI und einem Aryliodid ArI führt zur Entstehung des Pd II -At-Komplexes [L 2 Pd(Ar)I 2 ] À .