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Cyclopalladated complexes or palladacycles are successfully applied in many areas of material science and biological chemistry. Their preparation is usually performed in solution but number of reports on the solid-state synthesis of these organometallic complexes is steadily growing. Recently, our group reported a successful solid-state synthesis of azobenzene palladacycles in a ball mill that was followed by a detailed mechanistic study of C–H bond activation in azobenzene by various palladium (Pd) precursors, i.e. polymeric PdCl2, monomeric [PdCl2(MeCN)2], trimeric [Pd(OAc)2]3 and ionic [Pd(MeCN)4][BF4]2. Herein we report on X-ray diffraction identification and characterization of palladacycles obtained by C–H bond activation in 4-chloro-4’-(N, N- dimethylamino)azobenzene by various Pd(II) precursors in a ball mill (Figure 1). Each type of the employed Pd(II) precursor afforded a structurally different mono- and/or dipalladated complex. Structural analysis shows that all monopalladated products have a palladated p-(N, N-dimethylamino)phenyl ring, which agrees with a regioselective solid- state reaction. Reactions with acetate and chloride Pd(II) precursors afford dimeric and monomeric dicyclopalladated complexes, respectively. Obtained results show a strong influence of the ancillary ligands on the molecular structure of the analyzed palladacycles and support the proposed mechanistic scheme for double palladation of azobenzenes in a ball mill by various Pd(II) precursors.

C–H bond activation is often the first step in metal-catalyzed functionalization of the organic compounds usually performed using palladium (Pd) catalysts. Number of reports on successful solid state C–H bond functionalization by ball milling is growing, but no deeper insight into their mechanism has been obtained. Recently, a detailed mechanistic study of the solid-state activation of two C–H bonds in an asymmetric azobenzene substrate by Pd(II) chloride precursors, e.g. PdCl2 and PdCl2(MeCN)2, has been elucidated using in situ collected Raman spectra and ex situ spectroscopic and synthetic experiments. Results support a regioselective two-step reaction in which each step involves a coordination of the substrate to the Pd(II) precursor forming an adduct that then undergoes the C–H bond cleavage. Herein we report single- crystal X-ray diffraction experiments that allowed for structural identification of the azobenzene reactant as well as reaction intermediates and products of the double C–H bond activation in the solid state (Figure 1). Structural analysis proved as a key step toward determination of the reaction course and supported the proposed mechanism of the C–H bond activation in the azobenzene substrate, which was found analogous to the proposed mechanism for these reactions in solution.

The direct solid-state mechanochemical (Juribašić et al., Chem. Commun. 2014) or accelerated aging (Monas et al., Chem. Commun. 2016) palladium-mediated activation of strong phenyl C–H bonds by palladium(II) acetate results in mono- or dicyclopalladated complexes. Reactions are quantitative and significantly faster than solution synthesis and allow highly regioselective activation of one or two C–H bonds in asymmetrically substituted azobenzene. Organopalladium products were fully spectroscopically characterized and their structures have been confirmed by X-ray crystallographic structure determination. Mono- and dicyclopalladated complexes are anti dimers with transoid (1A and 2A) or cisoid (2B) geometry in which each palladium atom is bonded in a slightly distorted square-planar geometry to one carbon atom of the phenyl ring, one azo-nitrogen, and two oxygen atoms of the bridging acetate ligands mutually in cis position. The molecular structure of complexes 1A and 2A is dimeric with two monocyclopalladated azobenzene units bonded by two bridging acetate ligands in an anti open-book arrangement. The molecular structure of 2B is a centrosymmetric tetranuclear acetato-bridged complex. It is a unique dimer with two dicyclopalladated azobenzene units forced to lie exactly above one another by four cis-bonded acetate ligands.