A detailed quantum chemical study of the interactions of [Pt(dien)Cl]+ with a series of S-donor ligands: A computational approach

The mechanism of substitution reaction of [Pt(dien)Cl]+ (dien = diethylenetriamine) with a series of endogenous and exogenous S-containing ligands such as H2S, sodium methanethiolate (NaSMe), dimethylsulfane (Me2S), diethylsulfane (Et2S), thiourea (TU), 1,3-dimethylthiourea (DMTU), 1,3-diethylthiourea (DMTU), sodium-2-mercaptoethanesulfonate (mesna), sodium diethyldithiocarbamate (Naddtc), l-methionine (Met) and l-cysteine (Cys) were studied by computational methods. Within the framework of transition state theory, DFT-based calculations were performed in gas phase using the B3LYP exchange–correlation functionals as implemented in the Gaussian 09 program. Pentacoordinated square pyramidal (SP) and trigonal–bipyramidal (TBP)-like structure transition states (TS) along with other stationary points on potential energy surface were optimized and characterized. Results led us to conclude that for platination of S-ligands as protecting agents Naddtc, mesna and TU are more suitable than thiol and thioether ligands. Natural bond orbital (NBO) analysis method was performed for the investigation of major stabilizing orbital interactions. To obtain accurate energies for the reaction surfaces, single-point energies were calculated by conductor-like polarisable calculation model (CPCM) using larger basis sets at MP2 and B3LYP levels of theory. The sequence of rate constants found in our study in aqueous phase is: Naddtc > mesna > TU > Met > Cys > Me2S > Et2S ≈ MeSH > H2S ≈ DETU > NaSMe > DMTU.