Understanding the role of flexible alkyl-α,ω-diamine linkers on the substitution behaviour of dinuclear trans-platinum(II) complexes: A kinetic and mechanistic study

Abstract

An investigation of the role of flexible alkyl-α,ω-diamine linkers on the substitution behaviour of dinuclear trans-platinum(II) complexes. The substitution reactions of four dinuclear trans-platinum(II) complexes viz. [PtNH3Cl2]2-µ-NH2(CH2)2NH2] (P12), [PtNH3Cl2]2-µ-NH2(CH2)3NH2] (P13), [PtNH3Cl2]2-µ-NH2(CH2)4NH2] (P14) and [PtNH3Cl2]2-µ-NH2(CH2)5NH2] (P15) with three neutral thiourea-based nucleophiles specifically: thiourea (TU), N-methyl-2-thiourea (MTU) and N,N-dimethyl-2-thiourea (DMTU) were studied quantitatively under pseudo-first-order condition as function of concentration and temperature by conventional UV–Visible and stopped-flow spectrophotometers. The ligand substitution reaction of the complexes proceeds in veritably three consecutive steps. Each step follows first-order kinetics with the respective complex and nucleophile. The pseudo first order rate constants, kobs(1/2/3), for sequential substitution of the chlorido ligands, the ammine, and subsequent displacement of the linker obeyed the rate law: kobs(1/2/3) = k(1/2/3)[NU]. The ligand substitution reactions were driven by both electronic and steric factors. However, our findings revealed that upon the substitution of the chlorido ligands by the nucleophiles at the platinum centres, the σ-donor capacity via inductive effect of these electron-rich nucleophiles over compensate the steric strain imposed by the nucleophiles and by the alkanediamine linker at the substitution sites. Consequently, electronic factors governed the overall reaction pattern of these complexes. 195Pt NMR results confirmed the simultaneous substitution of all the chlorido ligands by thiourea-based nucleophiles, followed by the subsequent but successive displacement of the ammine groups and the flexible alkanediamine linker from the metal centres. The order of reactivity of the nucleophiles with the complexes decreases with the increase in steric bulk in the nucleophiles: TU > MTU > DMTU. The small positive enthalpy and the large but negative entropy confirm the associative mode of activation for all the studied complexes. Computational modelling using density functional theory (DFT) calculations was employed to rationalise the kinetic trends.