Synthesis, structure and catalytic properties of CNN pincer palladium(ii) and ruthenium(ii) complexes with N-substituted-2-aminomethyl-6-phenylpyridines

Abstract

N-substituted-2-aminomethyl-6-phenylpyridines 2a–c have been easily prepared from commercially available 6-bromo-2-picolinaldehyde in two steps. Reaction of 2a–c with PdCl₂ in toluene in the presence of triethylamine gave the CNN pincer Pd(II) complexes 3a–c in 18-28% yields. The CNN pincer Ru(II) complex 5 containing a Ru-NHR functionality could be obtained in a 71% yield by treatment of 2c with a Ru(II) precursor instead of PdCl₂. Additionally, the related CNN pincer Ru(II) complex 7 containing a Ru–NH₂ functionality has been synthesized by the reaction of 2-aminomethyl-6-phenylpyridine with the same Ru(II) precursor in a 68% yield. All the new compounds were characterized by elemental analysis (MS for ligands), ¹H, ¹³C NMR, ³¹P{¹H} NMR (for Ru complexes) and IR spectra. Molecular structures of Pd complex 3c as well as Ru complexes 5 and 7 have been determined by X-ray single-crystal diffraction. The obtained Pd complexes 3a–c were effective catalysts for the allylation of aldehydes as well as for three-component allylation of aldehydes, arylamines and allyltributyltin and their activity was found to be much higher than a related NCN Pd(II) pincer in the allylation of aldehyde. On the other hand, the two new CNN pincer Ru(II) complexes 5 and 7 displayed excellent catalytic activity in the transfer hydrogenation of ketones in refluxing 2-propanol with the latter being much more active. The final TOF values were up to 4510 h⁻¹ with 0.01 mol% of 5 and 220 800 h⁻¹ with 0.005 mol% of 7, respectively.