Guest-induced flexibility in crystals

Heyns, Anneli (2012-03)

Thesis (PhD)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: The primary goal of the work presented here was to prepare both organic and metal-organic porous crystals (using crystal engineering strategies) in order to study the guest-induced flexibility of such seemingly rigid materials. The first section describes the structural modification of a known oxacalix[4]arene compound and the ability of the novel derivatives to encapsulate guest molecules in the solid state. Although it was not possible to obtain porous guest-free forms of any of the hosts, an apohost phase of a 2,3-naphthalene-derivative was obtained from dimethyl sulfoxide. This host uses the same principle as a molecular tweezer by capturing the guests between its offset eclipsing naphthalene moieties; a series of solvate structures were obtained. The flexible nature of the host molecule, when enclosing guests of different shapes and sizes, has been illustrated by a systematic conformational study. The work dealing with metal-organic compounds was shown to be the more successful of the two parts and forms the largest portion of the work. A series of discrete metallocycles was synthesised and their ability to function as porous materials was investigated. In all instances the metallocyclic hosts included the solvent of crystallisation within the ring. Desolvation of the included solvent did not occur as a single-crystal to single-crystal process and in some instances desolvation coincided with decomposition of the metallocyclic host even though the guests are situated within continuous channels. Although it was not possible to examine the permeability of the empty host, single-crystal to single-crystal guest exchange occurs rapidly when exposing the acetonitrile solvated metallocycle to several solvents. Significant adjustment of the host conformation, as well as the guest-accessible volume, accompanies the uptake of the different guests. Remarkably, this exchange process can also occur upon exposure to small gaseous guests such as I2, CO2, C2H2 and C2H3Cl. The physico-chemical properties of a known seemingly nonporous metallocycle were investigated in order to formulate a mechanism of transport from one discrete cavity to the next. Crystals of the apohost were shown to be permeable to a series of solvents despite the lack of conventional channels in the host structure. Accurate sorption isotherms measured at four different temperatures revealed host:guest ratios that are comparable to the host:guest ratio inferred from the single-crystal structures. The thermodynamic parameters of sorption ΔHad and ΔSad could be derived from the isotherms and revealed essential information on the affinity of the hosts for particular guests. Significant deviation in the centroid-to-centroid distance between the imidazole rings suggest that the transport of molecules is facilitated by “flapping” of the imidazole rings. The extent of the host flexibility was explored by studying the permeation of relatively large volatile solids such as naphthalene and p-dichlorobenzene. Kinetic sorption isotherms in conjunction with precise single-crystal data revealed a possible mechanism of transport, which was confirmed by molecular mechanics calculations.

AFRIKAANSE OPSOMMING: Die primêre doel van die werk wat hier aangebied word, is om die soepelheid van skynbaar rigiede gasheerkonformasies, wanneer die gasheer verskillende gasmolekules omsluit, te ondersoek. Gedurende die studie is kristalmanipulasie strategieë gebruik om beide organiese- en organometaliese poreuse gasheermateriale voor te berei. In die eerste afdeling ondersoek ons die strukturele modifisering van 'n bekende oxacalix[4]arene verbinding asook die vermoë van die nuwe gasheermateriale om gasmolekules in die vastetoestand te omsluit. Hoewel ons nie in staat was om poreuse gasvrye vorms van enige van die gasheermateriale voort te bring nie, is 'n gasvrye fase van 'n 2,3-naftaleenafgeleide verbinding vanuit 'n dimetielsulfoksiedoplossing verkry. Hierdie gasheer maak gebruik van dieselfde beginsel as 'n molekulêre “knyper” om die gasmolekules tussen sy naftaleen arms vas te vang en 'n reeks gas-gasheerstrukture is gevolglik verkry. Die dinamiese aard van die gasheermolekuul, wanneer dit gasmolekules met verskillende vorms en groottes omsluit, is deur 'n sistematiese konformasiestudie geïllustreer. Die organometalliese afdeling was meer suksesvol, siende dat poreuse materiale wel geproduseer is, en vorm dus die grootste deel van die studie. Gedurende hierdie deel is 'n reeks metallosiklieseverbindings gesintiseer en hulle potensiaal om as poreuse spesies te funksioneer is ondersoek. Die oplosmiddel waarvan die metallosikliese kristalle gegroei is, is in alle gevalle binne die kompleksringholte vasgevang as 'n gasspesie. Desorpsiestudies het bewys dat die gasspesies eers by hoë temperature vrygelaat word en in sekere gevalle vind desorpsie van die oplosmiddel en ontbinding van die metallosikliese verbinding gelyktydig plaas, alhoewel die gasspesies in oop kanale vasgehou word. Die deurdringbaarheid van die gasvrye metallosikliesekomplekse kon dus nie met behulp van enkelkristaldiffraksie bestudeer word nie, maar die tegniek het bewys dat enkelkristal-tot-enkelkristal gasuitruiling spoedig plaasvind wanneer 'n asetonitrielgevulde metallosikliese kristal blootgestel word aan 'n reeks ander oplosmiddels. Uitruiling van die gas molekules vind plaas deur middel van noemenswaardige modifikasie van die gasheerkonformasie en die gastoeganklike spasie. Die buitengewone verwisseling van klein gasagtige molekules soos CO2, I2, C2H2 en C2H3Cl is ook as 'n enkelkristal-tot-enkelkristal proses, met behulp van 'n hoë druk gassel, bestudeer. Gedurende die finale gedeelte van die studie is die fisiese-chemiese eienskappe van 'n skynbaar-nieporeuse metallosikliese verbinding bestudeer sodat 'n meganisme van die beweging van gas molekules van een holte na die volgende voorgestel kon word. Aangesien dit reeds voorheen bevind is dat dié verbinding poreus is vir klein gasagtige molekules, is die deurdringbaarheid van 'n reeks groter oplosmiddels as gaste ondersoek deur middel van enkelkristaldiffraksie, desorpsiestudies en sorpsiestudies. Akkurate sorpsie isoterme is by vier verskillende temperature gemeet en toon dat die gas:gasheer verhoudings vergelykbaar is met die verhoudings afgelei van die enkelkristalstrukture. Verder is termodinamiese parameters soos ΔHad en ΔSad ook bereken en waardevolle inligting in verband met die affiniteit van die metallosikliese gasheer kon oorgedra word. Merkwaardige veranderinge in die posisies van die imidasoolringe het voorgestel dat die meganisme van gasoordrag moontlik 'n proses behels waarin die gasheer die imidasool ringe “flapper”. Die aard van die gasheer dinamika is uitgebeeld deur die deurlaatbaarheid van groot vlugtige vaste stowwe soos naftaleen en p-dichlorobenseen te bestudeer. Kinetiese sorpsie isoterme en akkurate enkelkristal data in samewerking met molekulêre meganika berekeninge het dié voorgestelde meganisme van vervoer bevestig.

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