Platinum(ii) complexes with hybrid amine-imidazolin-2-imine ligands and their reactivity toward bio-molecules

Abstract

Two new Pt(II) complexes with imidazolin-2-imines as ancillary ligands, [Pt(DMEAIm^iPr)Cl₂] and [Pt(DPENIm^iPr)Cl₂], were synthesized and characterized. Substitution reactions of these complexes with nucleophiles – thiourea (TU), L-methionine (L-Met), L-histidine (L-His) and guanosine-5′-monophosphate (5′-GMP) – were carried out in 25 mM Hepes buffer in the presence of 30 mM NaCl. The reactions were monitored using variable-temperature UV-Vis spectrophotometry and were followed under pseudo-first-order conditions with a large excess of nucleophiles. A slightly higher reactivity was found for [Pt(DMEAIm^iPr)Cl₂], while the reactivity of the nucleophiles decreased in the order TU > L-Met > L-His > 5′-GMP. The negative values reported for the entropy of activation confirmed an associative substitution mode. Spectrophotometric acid–base titrations were performed to determine the pK_a values of the coordinated water molecules in the diaqua complexes [Pt(DMEAIm^iPr)(H₂O)₂]²⁺ and [Pt(DPENIm^iPr)(H₂O)₂]²⁺. Solubility measurements revealed good solubility of the studied imidazolin-2-imine complexes in water. The crystal structure of [Pt(DMEAIm^iPr)Cl₂] was determined by X-ray diffraction analysis. The coordination geometries around the platinum atoms are distorted square-planar; the [Pt(DMEAIm^iPr)Cl₂] complex displays Pt–N distances of 2.0162(19) and 2.0663(19) Å. Attempts to coordinate Au(III) ions to different imidazolin-2-imine ligands did not result in the formation of coordination complexes, but rather in the reduction of the Au(III) precursor. This was evidenced by the X-ray crystal structure of [(DACH(Im^iPrH)₂)(AuCl₂)₂], which formed during the reaction of KAuCl₄ with the ligand DACH(Im^iPr)₂.