A kinetics and mechanistic study on the role of the structural rigidity of the linker on the substitution reactions of chelated dinuclear Pt(ii) complexes

Abstract

Substitution reactions of platinum complexes bearing cyclohexylamine/diamine moieties viz., [Pt(H₂O)(N,N-bis(2-pyridylmethyl)cyclohexylamine)](CF₃SO₃)₂, bpcHna; [{Pt(H₂O)}₂(N,N,N′,N′-tetrakis(2-pyridylmethyl)-trans-1,4-cyclohexyldiamine)](CF₃SO₃)₄, cHn and [{Pt(H₂O)}₂(N,N,N′,N′-tetrakis(2-pyridylmethyl)-4,4′-dicyclohexylmethanediamine)](CF₃SO₃)₄, dcHnm and phenylamine/diamine moieties viz., ([Pt(H₂O)N,N-bis(2-pyridylmethyl)phenylamine)](CF₃SO₃)₂, bpPha; [{Pt(H₂O)}₂(N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,3-phenyldiamine)](CF₃SO₃)₄, mPh; [{Pt(H₂O)}₂(N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,4-phenyldiamine)](CF₃SO₃)₄, pPh and [{Pt(H₂O)}₂(N,N,N′,N′-tetrakis(2-pyridylmethyl)-4,4′-diphenylmethanediamine)](CF₃SO₃)₄), dPhm with thiourea nucleophiles were studied in acidified 0.01 M LiCF₃SO₃ aqueous medium under pseudo-first-order conditions using stopped-flow and UV-visible spectrophotometric techniques. The rate of substitution follows a similar trend in the two sets of complexes and decreases in the order: bpcHna > dcHnm > cHn and bpPha > dPhm ≈ pPh ≈ mPh), respectively. The result of this study has shown that the rigidity and/or the planarity of a diamine bridge linking the two (2-pyridylmethyl)amine-chelated Pt(II) centres, influences the reactivity of the metal centres by protracting similar symmetry elements within the complexes, which determines the amount of steric influences felt on the coordination square-plane. Hence, the order of reactivity is controlled by both the steric hindrance and the magnitude of the trans σ-inductive effect originating from the linker towards the metal centre. These two factors also impact on the acidity of the complexes. The high negative entropies and low positive enthalpies support an associative mode of activation.