The Elusive Structure of Pd2(dba)3. Examination by Isotopic Labeling, NMR Spectroscopy, and X‑ray Diffraction Analysis: Synthesis and Characterization of Pd2(dba-Z)3 Complexes

Pd0 2(dba)3 (dba = E,E-dibenzylidene acetone) is the most widely used Pd0 source in Pd-mediated transformations. Pd0 2(dba-Z)3 (Z = dba aryl substituents) complexes exhibit remarkable and differential catalytic performance in an eclectic array of cross-coupling reactions. The precise structure of these types of complexes has been confounding, since early studies in 1970s to the present day. In this study the solution and solid-state structures of Pd0 2(dba)3 and Pd0 2(dba-Z)3 have been determined. Isotopic labeling (2H and 13C) has allowed the solution structures of the freely exchanging major and minor isomers of Pd0 2(dba)3 to be determined at high field (700 MHz). DFT calculations support the experimentally determined major and minor isomeric structures, which show that the major isomer of Pd0 2(dba)3 possesses bridging dba ligands found exclusively in a s-cis,s-trans conformation. For the minor isomer one of the dba ligands is found exclusively in a s-trans,s-trans conformation. Single crystal X-ray diffraction analysis of Pd0 2(dba)3·CHCl3 (high-quality data) shows that all three dba ligands are found over two positions. NMR spectroscopic analysis of Pd0 2(dba-Z)3 reveals that the aryl substituent has a profound effect on the rate of Pd-olefin exchange and the global stability of the complexes in solution. Complexes containing the aryl substituents, 4-CF3, 4-F, 4-t-Bu, 4-hexoxy, 4-OMe, exhibit well-resolved 1H NMR spectra at 298 K, whereas those containing 3,5-OMe and 3,4,5-OMe exhibit broad spectra. The solid-state structures of three Pd0 2(dba-Z)3 complexes (4-F, 4-OMe, 3,5-OMe) have been determined by single crystal X-ray diffraction methods, which have been compared with Goodson’s X-ray structure of Pd0 2(dba-4-OH)3.