Understanding the role of the flexible bridging linker through kinetics and mechanistic study of Pt(ii) amphiphiles derived from a bis(2-pyridylmethyl)amine chelate head group

Abstract

The substitution of aqua ligands of mononuclear Pt(II) complexes of the general form [Pt(H₂O)(N,N-bis(2-pyridylmethyl)-N(CH₂)_n-CH₃; -NC(CH₃)₃; -NH](CF₃SO₃)₂, n = 1 (bpea); 2 (bppa); 3 (bpba); 5 (bpha), 9 (bpda) -NC(CH₃)₃ (bpbta) and -NH (bpma) by thiourea nucleophiles was investigated under pseudo first-order conditions as a function of concentration and temperature using the stopped-flow technique and UV-vis spectroscopy. The substitution reactions occur via two separate reaction steps, each fitting to a single exponential curve. In the two reaction steps, the thiourea nucleophiles first substitute the coordinated aqua ligand followed by ring opening via dechelation of one of the pyridyl units. The mode of activation for both steps remains associative in nature and the observed rate constants can be fitted to the equation k_obs(1^st_/2^nd₎ = k₂₍₁^st_/2^nd₎[Nu]. Appending a primary alkyl hydrocarbon group on the trans-N donor atom of the chelate head group marginally increases the rate of substitution of the aqua leaving group due to the weaker trans-influence of its alkyl amine donor group. However, when a tert-butyl group is the pendant group, reactivity increases by a factor of about two, reiterating the inductive nature of the flow of electron density from the tailing groups towards the Pt(II) metal centres. A comparison of the reactivities of the studied complexes with their dinuclear analogues bridged by alkyl diamines has demonstrated that the electronic effect of the alkyl diamine bridge on the overall reactivity of the multinuclear Pt(II) complexes is weak and insignificant when compared to steric effects due to the constraining bridge.