Palladium(ii) complexes with highly basic imidazolin-2-imines and their reactivity toward small bio-molecules. Issue 39 (21st September 2015) - Bogojeski, Jovana
Abstract : Pd(ii ) complexes with chelating imidazolin-2-imine ligands were synthesized, and the p K a values and reactivity of these complexes were investigated. Abstract : A series of novel Pd(ii ) complexes with chelating mono(imidazolin-2-imine) and bis(imidazolin-2-imine) ligands were synthesized. The crystal structures of [Pd(DMEAIm iPr )Cl2 ] and [Pd(DPENIm iPr )Cl2 ] were determined by X-ray diffraction analysis. The reactivity of the six Pd(ii ) complexes, namely, [Pd(en)Cl2 ], [Pd(EAIm iPr )Cl2 ], [Pd(DMEAIm iPr )Cl2 ], [Pd(DPENIm iPr )Cl2 ], [Pd(BL iPr )Cl2 ] and [Pd(DACH(Im iPr )2 )Cl2 ], were investigated. Spectrophotometric acid–base titrations were performed to determine the p K a values of the coordinated water molecules in [Pd(en)(H2 O)2 ] 2+, [Pd(EAIm iPr )(H2 O)2 ] 2+, [Pd(DMEAIm iPr )(H2 O)2 ] 2+, [Pd(DPENIm iPr )(H2 O)2 ] 2+, [Pd(BL iPr )(H2 O)2 ] 2+ and [Pd(DACH(Im iPr )2 )(H2 O)2 ] 2+ . The substitution of the chloride ligands in these complexes by TU, l -Met, l -His and Gly was studied under pseudo-first-order conditions as a function of the nucleophile concentration and temperature using stopped-flow techniques; the sulfur-donor nucleophiles have shown better reactivity than nitrogen-donor nucleophiles. The obtained results indicate that there is a clear correlation between the nature of the imidazolin-2-imine ligands and the acid–base characteristics and reactivity of the resulting Pd(ii ) complexes; the order of reactivity of the investigated Pd(ii ) complexes is: [Pd(en)Cl2 ] > [Pd(EAIm iPr )Cl2 ] > [Pd(DMEAIm iPr )Cl2 ] > [Pd(DPENIm iPr )Cl2 ] > [Pd(BL iPr )Cl2 ] > [Pd(DACH(Im iPr )2 )Cl2 ]. The solubility measurements revealed good solubility of the studied imidazolin-2-imine complexes in water, despite the fact that these Pd(ii ) complexes are neutral complexes. Based on the performed studies, three unusual features of the novel imidazolin-2-imine Pd(ii ) complexes are observed, that is, good solubility in water, very low reactivity and high p K a values. The coordination geometries around the palladium atoms are distorted square-planar; the [Pd(DMEAIm iPr )Cl2 ] complex displays Pd–N distances of 2.013(2) and 2.076(2) Å, while the [Pd(DPENIm iPr )Cl2 ] complex displays similar Pd–N distances of 2.034(4) and 2.038(3) Å. The studied systems are of interest because little is known about the substitution behavior of imidazolin-2-imine Pd(ii ) complexes with bio-molecules under physiological conditions.