Synthesis, Structures, and Solution Behavior of Bis(sulfoxide)-Pincer Complexes of Palladium(II)

Cyclometalation of bis(sulfoxide)-substituted m-xylene derivatives, rac/meso-1,3-(iPrS(O)CH2)2C6H4, occurred readily with Pd(II) using two synthetic routes. The first route utilized
[Pd(NCMe)4][(BF4)2] as starting material and led to the exclusive isolation of the C2-symmetric
diastereomer [rac-2,6-(iPrS(O)CH2)2C6H3Pd(NCMe)][BF4], rac-3, even though spectroscopic
measurements confirmed the formation of meso-3. The second palladation pathway used
Pd(BF4)Cl(NCPh)2 and both diastereomers rac- and meso-2,6-(iPrS(O)CH2)2C6H3PdCl, rac-
and meso-4, were separable by fractional crystallization. Characterization of the isolated
complexes using 1H and 13C NMR, FT-IR, and X-ray crystallography illuminated subtle
differences between the two diastereomers of 4 and provided a rationale for the greater
solution stability of rac-4 relative to meso-4. The source of the instability is due to the
stereochemical configuration at sulfur. Additional solution studies, examined by variable-temperature 1H NMR (25 to −130 °C), probed the dynamic exchange behavior of the three
complexes. In addition, electrospray mass spectrometry experiments of rac-4 in methanol
solutions detected the presence of the unsupported μ-chloro-bridged dimer, [2,6-(iPrS(O)CH2))2C6H3Pd]2-μ2-Cl.