The role of annular nitrogen in tuning the reactivity of bifunctional platinum(II) complexes appended to pyridyl spacers; A kinetic and mechanistic investigation

A kinetic and mechanistic study on ligand substitution reactions of pyridine functionalized dinuclear Pt(II) complexes visa-viz., N,N,N’,N’-tetrakis(2-pyridyl)-2,3 pyridinediaminetetraquaplatinum(II) (PtL1); N,N,N’,N’-tetrakis(2-pyridyl)-2,4-pyridinediaminetetraquaplatinum(II) (PtL2); N,N,N’,N’-tetrakis(2-pyridyl)-2,6-pyridinediaminetetraquaplatinum(II) (PtL3); N,N,N’,N’-tetrakis(2-pyridyl)-2,5-pyridinediaminetetraquaplatinum(II) (PtL4) and N,N,N’,N’-tetrakis(2-pyridylaminomethyl)-2,6-pyridinediaminetetraquaplatinum(II) (PtL5) complexes with thiourea (TU), N,N’-dimethylthiourea (DMTU) and N,N,N’N’-tetramethylthiourea (TMTU) as the entering nucleophiles was carried out under pseudo-first-order conditions by stopped-flow and UV–Vis spectrophotometric techniques. Substitution reactions proceeds via a two-step pathway with the rate law k_obs(1,2) = k_2(1st/2nd)[Nu]. The first substitution step is attributed to simultaneous displacement of one water molecule at each platinum(II) center. The second step is attributed to the slow phase substitution of the second water molecule occurring concurrently with the slow displacement of the bridging linker. The steps were confirmed by ¹H and ¹⁹⁵Pt NMR spectroscopy. The reactivity of the complexes is governed by the position of the annular nitrogen in the pyridine spacer. The Low positive values of enthalpy of activation and significantly large negative values of entropy of activation parameters for both steps indicate an associative mechanism of substitution.