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

Two new Pt(II) complexes with imidazolin-2-imines as ancillary ligands, [Pt(DMEAIm(iPr))Cl-2] and [Pt(DPENIm(iPr))Cl-2], 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))Cl-2], 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))Cl-2] was determined by X-ray diffraction analysis. The coordination geometries around the platinum atoms are distorted square-planar; the [Pt(DMEAIm(iPr))Cl-2] complex displays Pt-N distances of 2.0162(19) and 2.0663(19) angstrom. 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(iPr)H)(2))(AuCl2)(2)], which formed during the reaction of KAuCl4 with the ligand DACH(Im(iPr))(2).