Pd-catalyzed cross-coupling reactions have become one of the key processes for carbon-carbon bond formations. Even though their organometallic coupling partners vary, it is understood that they all follow a common general catalytic cycle in which a transmetalation reaction is one of the key steps. The mechanism of transmetalation reactions has been most thoroughly studied for the Stille reaction and for the Suzuki-Miyaura coupling, and it has been proposed that the reaction proceeds via either an open (S_N2 like) or closed transition state.  Gas phase and computational studies on the mechanisms of the Sonogashira (Pd/Cu) and Negishi (Pd/Zn) transmetalations suggest that these reactions proceed somewhat different. [1] The main difference is the existence of direct metal-metal (d⁸-d¹⁰) interactions in calculated structures of transition states and intermediates.

To study the crucial Pd(II)-Cu(I) [2] and Pd(II)-Zn(II) [3] interactions experimentally we have prepared isolable heterobimetallic d⁸-d¹⁰ complexes.  The crystal structures resemble computed transition states for the Sonogashira and Negishi transmetalation steps and we have investigated the bonding qualitatively and quantitatively by X-ray, NMR, mass spectrometry (ESI-MS/MS) and calculations (DFT). 

Structural and thermochemical work on the isolable model complexes will be used to predict and optimize substrates, ligands, and reaction conditions for cross-coupling reactions, especially the Negishi coupling, so as to suppress undesired side reactions, e.g. homocoupling, by rational design.

[1] Raphael J. Oeschger, David H. Ringger, Peter Chen,  Organometallics, 2015, 34, 3888-3892
[2] Raphael J. Oeschger, Peter Chen, J. Am. Chem. Soc. 2017, 139, 1069-1072.
[3] Raphael J. Oeschger, Peter Chen,  Organometallics, 2017, 36, 1465–1468