Synthesis and characterization of metallomacrocyclic palladium(II) complexes with new hybrid pyrazole ligands. Diffusion nmr studies and theoretical calculations

Three new 3,5-dimethylpyrazolic hybrid ligands N1-substituted by polyether chains and phenyl groups have been synthesized: 1,2-bis[4-(3,5-dimethyl-1H- pyrazol-1-yl)-2-oxabutyl]benzene(L1), 1,3-bis[4-(3,5-dimethyl-1H-pyrazol-1-yl)- 2-oxabutyl]benzene (L2), and 1,4-bis[4-(3,5-dimethyl-1H-pyrazol-1-yl)-2- oxabutyl]benzene (L3). The reaction of these ligands with [PdCl 2(CH3CN)2] gives two kinds of complexes, monomer or dimer, depending on the solvent. Monomeric chelated complexes [PdCl22(L)] [L = L1 (1), L2 (2), L3 (3)] are obtained when the solvent of the reaction is acetonitrile, whereas when the reaction takes place in tetrahydrofuran binuclear compounds [PdCl2(L)]2 [L = L1 (4), L2 (5), L3 (6)] are formed. The solid-state structures for 1 and 4 were determined by single-crystal X-ray diffraction methods. All of the palladium(II) complexes 1-6 were isolated and fully characterized. Diffusion NMR studies have been performed to characterize monomeric and dimeric species in solution. Dimeric compounds present complex 1H NMR spectra, especially 4. Theoretical calculations on this molecule suggest that it is due to the coexistence of different conformers that do not interconvert to each other at room temperature. Finally, it has been observed that dimers are converted into the corresponding monomers in an acetonitrile reflux, thus indicating that the latter are thermodynamically more stable than dimers. © 2008 American Chemical Society.