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(DMEAImiPr)Cl2] and [Pt(DPENImiPr)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(DMEAImiPr)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 pKa values of the coordinated water molecules in the diaqua complexes [Pt(DMEAImiPr)(H2O)2]2+ and [Pt(DPENImiPr)(H2O)2]2+. Solubility measurements revealed good solubility of the studied imidazolin-2-imine complexes in water. The crystal structure of [Pt(DMEAImiPr)Cl2] was determined by X-ray diffraction analysis. The coordination geometries around the platinum atoms are distorted square-planar; the [Pt(DMEAImiPr)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(ImiPrH)2)(AuCl2)2], which formed during the reaction of KAuCl4 with the ligand DACH(ImiPr)2.