Tuning of the catalytic properties of PdCl2(XnPy)2 complexes by variation of the basicity of aromatic ligands

• Substituent effect in XnPy on physico-chemical properties of PdCl2(XnPy)2 is studied.
• Thermal and structural properties of Pd(II) catalyst are correlated with its activity.
• Results allow us to get insights into the mechanism of nitrobenzene carbonylation.
• A detailed mechanism of catalytic carbonylation of nitrobenzene is elucidated.

The position and number of substituents in pyridine ligands (XnPy) were correlated with structural, physical, and chemical properties of PdCl2(XnPy)2 complexes applied as catalysts for the carbonylation of aromatic nitrocompounds (phosgene-free method of carbamates production). Thermal stability and catalytic activity of PdCl2(XnPy)2 complexes without steric hindrance increases with increasing XnPy's basicity whereas a decrease of thermal stability and catalytic activity of the complexes is observed for sterically crowded complexes (with the ortho-substituted XnPy). The complexes with X = Cl in meta- position of XnPy decompose to a mixture of PdCl2 and metallic Pd (similarly to complexes with MenPy) whereas complexes with ortho-chlorine (in XnPy) decompose to the organopalladium products. Therefore, two different mechanisms of thermal decomposition are proposed for PdCl2(ClnPy)2 and PdCl2(MenPy)2. The results of complex thermal and structural analysis of a series of PdCl2(XnPy)2 complexes allow us to get insight into the mechanism of nitrobenzene (NB) carbonylation catalyzed by PdCl2(XnPy)2 at 150–180 °C. We conclude that the electron transfer from Pd(0) to nitrobenzene is the rate determining step of catalytic cycle of NB carbonylation.

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