A 195Pt Nuclear magnetic resonance and molecular dynamics computer simulation study of the solvation of simple platinum (IV) chlorido complex anions in water and water-miscible solvent mixtures

Engelbrecht, Leon de Villiers (2017-03)

Thesis (PhD)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: A combined 195Pt NMR spectroscopy and Molecular Dynamics (MD) computer simulation study of the solvation of the octahedral Pt(IV) complex [PtCl6]2‒ in binary mixtures of water and the fully water-miscible organic solvents methanol, 2-methoxyethanol and 1,2-dimethoxyethane has been carried out. A recent 195Pt NMR chemical shift-trends study indicated a preferential solvation of the aforementioned platinum complex by the organic solvent component in such solvent mixtures. The solvent dependence of the intrinsic 1Δ195Pt(37/35Cl) NMR isotope shifts of [PtCl6]2‒ in pure solvents indicate a slight increase in magnitude ~7 ppb in the order water < methanol < 2-methoxyethanol < 1,2-dimethoxyethane. In selected equimolar binary mixtures of water and organic solvents, 1Δ195Pt(37/35Cl) is found to be similar in magnitude to that in the pure organic solvents, supporting the proposed preferential solvation of the platinum complex. 195Pt NMR T1 spin relaxation times and pulsed gradient spin echo (PGSE) translational diffusion measurements were performed for [PtCl6]2‒ in selected solvents and binary mixtures. The results were interpreted in the context of hydrodynamic continuum models of molecular diffusion; while these were found to be not strictly appropriate, the rotational and translational dynamics results appear to be notionally consistent with the preferential solvation phenomenon as indicated. A series of classical MD computer simulations were performed for [PtCl6]2‒ in these equimolar binary solvent mixtures, using a recently revised force field developed by Naidoo et al. The results using the standard force field indicate a strong preference for water in the primary solvation shell region of the complex in all solvent mixtures studied. A similar result is obtained for [PtCl4]2‒ in an equimolar water‒methanol mixture. Simulations were repeated with ionic charges scaled according to the recently developed Molecular Dynamics in Electronic Continuum (MDEC) theory, which is intended to account for the dielectric screening of charges in condensed phases. In these MDEC simulations, a significant reduction in the contribution of water to the primary solvation shells of both complexes is observed; this is particularly evident in the solvation shell of [PtCl6]2‒ in mixtures of water with 2-methoxyethanol and 1,2-dimethoxyethane, for which a strong preferential solvation by the organic components has been indicated. Dynamic properties were also computed from MD trajectories, and are qualitatively consistent with experimental trends, but deviate due to the solvent model combination. Finally, an interpretation of the fascinating 35/37Cl and 16/18O isotope-induced fine structure in the 195Pt NMR spectra of complexes of the type [PtCln(OH)6‒n]2‒, n = 0‒5, is presented, based on the expected trans-influence series of ligands for Pt(IV) OH‒ > Cl‒ > H2O in aqueous solution.

AFRIKAANSE OPSOMMING: ʼn Gekombineerde 195Pt KMR spektroskopie en Molekulêre Dinamika (MD) rekenaar-simulasie studie van die solvasie van die oktahedriese Pt(IV) kompleks [PtCl6]2‒ in binêre mengsels van water en volledig water mengbare organiese oplosmiddels metanol, 2-metoksiëtanol en 1,2-dimetoksiëtaan is uitgevoer. ʼn Onlangse studie van 195Pt KMR chemiese verskuiwings (frekwensies) het ʼn voorkeur solvasie vir solvasie van die platinum kompleks deur die organiese komponent in sulke mengsels aangedui. Die 1Δ195Pt(37/35Cl) KMR isotoopverskuiwing van [PtCl6]2‒ in suiwer oplosmiddels toon ʼn relatief klein toename van ~7 ppb in die orde van oplosmiddels, water < metanol < 2-metoksiëtanol < 1,2-dimetoksiëtaan. Die 1Δ195Pt(37/35Cl) verskuiwing gemeet in sekere ekwimolare binêre mengsels van water en organiese oplosmiddels is soortgelyk in grootte aan die verskuiwing in die suiwer organiese oplosmiddel, in ooreenkoms met die voorgestelde voorkeur vir die organiese komponent. 195Pt KMR T1 kenmerkende tye en translasionele diffusiekoëffisiente m.b.v. die gepulsde gradient spin-echo (PGSE) tegniek, is ook gemeet vir [PtCl6]2‒ in geselekteerde oplosmiddels en binêre mengsels. Die resultate word geïnterpreteer in die konteks van hidrodinamiese modelle van molekulêre diffusie, en alhowel daar gevind word dat sulke modelle nie geskik is vir hierdie doel, word getoon dat die resultate nietemin algemeen konsistent is met die voorgestelde oplosmiddelvoorkeur, soos aangedui. ʼn Reeks klassieke MD rekenaarsimulasies van [PtCl6]2‒ in binêre oplosmiddelmengsels is uitgevoer deur gebruik te maak van ʼn onlangs-hersiende model wat deur Naidoo et al. ontwikkel is. Die resultate van simulasies, wat met die standaard model vir die platinum kompleks uitgevoer is, toon ʼn sterk voorkeur vir water in die onmiddelike omgewing van die kompleks in alle gebestudeerde mengsels. ʼn Soortgelyke resultaat is verkry vir die vierkantig-planêre [PtCl4]2‒ in ʼn ekwimolare water‒metanol mengsel. Die simulasies is herhaal deur ioniese ladings aan te pas volgens die onlangs ontwikkelde Molekulêre Dinamika in Elektroniese Kontinuum (MDEK, oorspronklik MDEC) teorie, wat deels voosiening maak vir die diëlektriese afskerming van gelaaide partikels in gekondenseerde fases. In hiedie MDEK simulasies word ʼn merkbare afname in die bydrae deur water tot die primêre oplosmiddel omgewing van beide komplekse waargeneem: die effek is veral ooglopend in die geval van [PtCl6]2‒ in mengels van water met 2- metoksiëtanol en 1,2-dimetoksiëtaan, waarin ʼn sterk voorkeur vir die organiese komponent eksperimenteel waargeneem is. Dinamiese einskappe is ook van MD trajekte bereken, en toon tendense soortgelyk aan eksperimentele waardes, maar wyk af weens die oplosmiddelmodel. Verder is ʼn interpretasie van die fassinerende 35/37Cl en 16/18O isotoopgeïndusseerde fynstruktuur in die 195Pt KMR spektra van komplekse met vorm [PtCln(OH)6‒n]2‒, n = 0‒5, voorgestel, gebasseer op die verwagte trans-invloedreeks van ligande vir Pt(IV) H2O < Cl‒ < OH‒ in waterige oplossing.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101362
This item appears in the following collections: