Tuning the reactivity of chelated dinuclear Pt(ii) complexes through a flexible diamine linker. A detailed kinetic and mechanistic study

Abstract

The rate of displacement of the aqua ligands by three neutral nucleophiles (Nu) of different steric demands, namely thiourea (tu), N,N′-dimethylthiourea (dmtu) and N,N,N′,N′-tetramethylthiourea (tmtu) and an anionic nucleophile (I⁻) in complexes of the form [{Pt(H₂O)}₂(N,N,N′,N′-tetrakis(2-pyridylmethyl)-N(CH₂)_nN](CF₃SO₃)₄, n = 2 (En); 3 (Prop); 4 (But); 6 (Hex); 8 (Oct) and 10 (Dec), was studied under pseudo first-order conditions as a function of concentration, temperature and pressure using stopped-flow techniques and UV-visible spectrophotometry. The pseudo first-order rate constants, k_obs(1^st_/2^nd₎, for the simultaneous substitution of the aqua ligands and the proposed subsequent dechelation of the pyridyl units, respectively, agreed well to the rate law: k_obs(1^st_/2^nd₎ = k₂₍₁^st_/2^nd₎[Nu]. High negative activation entropies, negative volumes of activation and second-order kinetics for the displacement reactions all support an associative mode of activation. Except for Prop, the rate of the simultaneous substitution of the aqua ligands in the complexes was found to increase as the chain length of the linker increases from En to Hex, beyond which any further increase in chain length is not accompanied by a further increase in reactivity. The reactivity trend of the even-bridged complexes with C_2h symmetry is ascribed to a concomitant decrease in axial steric influences imposed on one side of the square-planar picolyl chelates by the other as the chain length increases. Based on the model structures of the complexes, this kind of steric imposition occurs only in complexes with an even number of CH₂ groups within the linker. The Prop complex, having a C_2v symmetry showed exceptional high reactivity towards the nucleophiles. A cage effect, evolving from its bowl-shaped molecular structure, is proposed to explain this high reactivity. The order of reactivity of the nucleophiles increased in the order I⁻≫ tu ≈ dmtu > tmtu, in line with the strong electrostatic interactions between the highly polarizable iodide nucleophile and the Pt centers, steric retardation effects in the case of tmtu and dominating positive inductive effects for the dmtu nucleophile.