Palladium(ii) complexes with highly basic imidazolin-2-imines and their reactivity toward small bio-molecules

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 pK_a values of the coordinated water molecules in [Pd(en)(H₂O)₂]²⁺, [Pd(EAIm^iPr)(H₂O)₂]²⁺, [Pd(DMEAIm^iPr)(H₂O)₂]²⁺, [Pd(DPENIm^iPr)(H₂O)₂]²⁺, [Pd(BL^iPr)(H₂O)₂]²⁺ and [Pd(DACH(Im^iPr)₂)(H₂O)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 pK_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.