Synthesis and structural characterizations of Pd(II) complexes bearing the new hexahydropyrimidine and tetrahydropyrimidinium based bis(pyrazole) ligands with DFT studies

Abstract

The one pot reaction between 1,3­-diaminopropane, 3,5-dimethylpyrazole and paraformaldehyde afforded 1,3­-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)hexahydropyrimidine 1 in an excellent yield. The reaction of 1 with one equivalent of N-bromosuccinimide in DME gave the oxidized product 2; the same reaction in the presence of 4 equivalents of NBS yielded the pyrazole ring brominated product. Treatment of 2 with NH4PF6 gave its PF6− salt 3. Conversely, its BF4− salt was obtained directly from the reaction of AgBF4 with 1. Treatment of 3 with palladium(II) precursors gave two monomeric trans chelated complexes containing acetate and chloride ions as their PF6− salts. Conversely, 1 reacts with palladium(II) source to give the dimeric 20-membered palladacycle. The monomeric complexes are rigid molecules, whereas the dimer complex is fluxional as shown by their 1H NMR spectra. From the low temperature NMR study of the dimer complex the broad signals in the room temperature NMR spectrum are attributed to the fast interconversion between its conformers. All compounds were characterized by both spectroscopic and X-ray methods, and in some cases by CHN analysis. DFT calculations were performed to understand the formation of the chelate complex against its anticipated NHC complex containing the 6-membered ring and the binuclear complex against a monomeric complex in terms of 5- and 6-membered ring strain energies.