Kinetics and mechanism of the substitution reactions of some monofunctional Pt(II) complexes with heterocyclic nitrogen donor molecules. Crystal structure of [Pt(bpma)(pzBr)]Cl2·2H2O
Substitution reactions of [Pt(terpy)Cl]+ (terpy = 2,2′;6′,2′′-terpyridine), [Pt(bpma)Cl]+ (bpma = bis(2-pyridylmethyl)amine), [Pt(dien)Cl]+ (dien = diethylenetriamine or 1,5-diamino-3-azapentane) and [Pt(tpdm)Cl]+ (tpdm = tripyridinedimethane) with nitrogen donor heterocyclic molecules, such as 3-amino-4-iodo-pyrazole (pzI), 5-amino-4-bromo-3-methyl-pyrazole (pzBr) and imidazole (Im), were studied in aqueous 0.10 M NaClO4 in the presence of 10 mM NaCl using variable-temperature UV–vis spectrophotometry. The second-order rate constants k2 indicate decrease in reactivity in the order [Pt(terpy)Cl]+ > [Pt(bpma)Cl]+ > [Pt(tpdm)Cl]+ > [Pt(dien)Cl]+. The most reactive nucleophile among the heterocyclic compounds is imidazole, while pzI shows slightly higher reactivity than pzBr. Activation parameters were also determined and the negative values for entropies of activation, ΔS≠, support an associative mode of substitution for all substitution processes. Crystal structure of [Pt(bpma)(pzBr)]Cl2·2H2O was determined by single-crystal X-ray analysis. The coordination geometry of the complex is distorted square-planar while the bond distance Pt–N2(pzBr) is longer than the other three Pt–N distances.
