Aqua substitution by biologically relevant nucleophiles in dinuclear platinum(II) complexes linked by diamino linkers with cyclohexyl groups

Substitution of the aqua ligand from three platinum(II) complexes, namely [Pt(H2O)(N,N-bis(2-pyridylmethyl)cyclohexylamine](ClO4)(2) (Pt1); [{Pt(H2O)}(2)(N,N,N',N'-tetrakis(2-pyridylmethyl)-trans-1,4-cyclohexyldiamine)](ClO4)(4) (Pt2); and [{Pt(H2O)}(2)(N,N,N',N'-tetrakis(2-pyridylmethyl)-4,4'-ethylenedicyclohexyldiamine)](ClO4)(4) (Pt3), by three biologically relevant nucleophiles [viz., glutathione (Glu); DL-penicillamine (Pen); and l-histidine (His)] was studied in aqueous 0.1 M perchloric acid medium. The substitutions were investigated under pseudo-first-order conditions as a function of nucleophile concentration and temperature using UV-visible spectrophotometry. The reactions of these complexes with the nucleophiles proceeded via a single step whose reactivity decreased in the order: Pt1 > Pt3 > Pt2 and was controlled by steric and electronic influences of the complexes. Sulfur donor nucleophiles (Glu and Pen) reacted much faster than the nitrogen bearing His, with Glu appearing to be more nucleophilic than Pen. The large and negative activation entropies and low but positive enthalpies of activation affirm an associative mode of activation.