Synthesis and Characterisation of Fluorous Compounds and Ligands for Catalysis and Surface Modification


Student thesis: Doctoral Thesis

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Award date10 Oct 2018


Perfluoroalkanes, perfluorodialkylethers, perfluorotrialkylamines and similar perfluoroalkyl derivatives have limited miscibility with aqueous and organic media. Compounds with high fluorine content (>60 w%) were defined as “fluorous”, having one or more linear or branched perfluoroalkyl chains attached to their hydrocarbon domains. A fluorous reagent and/or catalyst can be designed to dissolve preferentially in fluorous phase or the fluorous phase of the biphasic system, depending on the attachment of perfluoro-alkyl group(s) containing substituents to the hydrocarbon domains of fluorous reagents and catalysts in appropriate size and number. However, the use of perfluorohexyl or longer perfluoroalkyl ponytails are no longer recommended due to the persistency and adverse effects of their degradation products (for example, PFOA). Therefore, the aim of the project was to develop fluorous reagents and ligands containing fluorous ponytail(s) with short perfluoroalkyl chain.
A series of novel fluorous reagents and ligands with linear short-chain C3F7- and C4F9-, and nonafluoro-t-butoxy groups are synthesised and characterised. Nonafluoro-tert-butoxy-1-butene (53), nonafluoro-tert-butoxy-1-pentene (54), and nonafluoro-tert-butoxy-1-hexene (55), were successfully prepared. A series of fluorous triflates 1,1,1,2,2,3,3-heptafluoro-4-butyl triflate (56), 1,1,1,2,2,3,3-heptafluoro-6-hexyl triflate (57) and 1,1,1,2,2,3,3,4,4-nonafluoro-5-pentyl triflate (58); linear perfluoroalkyl mono-substituted imidazole, 1-(1,1,1,2,2,3,3-heptafluoro-4-butyl) imidazole (59), 1-(1,1,1,2,2,3,3-heptafluoro-5-pentyl) imidazole (60), 1-(1,1,1,2,2,3,3-heptafluoro-6-hexyl) imidazole (61), 1-(1,1,1,2,2,3,3,4,4-nonafluoro-5-pentyl) imidazole (62), 1-(1,1,1,2,2,3,3,4,4-nonafluoro-6-hexyl) imidazole (63), symmetric and asymmetric imidazolium salts 1,3-bis(1,1,1,2,2,3,3-heptafluoro-4-butyl) imidazolium triflate (64), 1,3-bis(1,1,1,2,2,3,3-heptafluoro-5-pentyl) imidazolium iodide (65), 1,3-bis-(1,1,1,2,2,3,3-heptafluoro-6-hexyl) imidazolium triflate (66), 1,3-bis(1,1,1,2,2,3,3,4,4-nonafluoro-5-pentyl) imidazolium triflate (67), 1,3-bis(1,1,1,2,2,3,3,4,4-nonafluoro-6-hexyl) imidazolium iodide (68), 1-(1,1,1,2,2,3,3-heptafluoro-4-butyl)-3-(1,1,1,2,2,3,3,4,4-nonafluoro-6-hexyl) imidazolium triflate (69) and 1-(1,1,1,2,2,3,3-heptafluoro-5-pentyl)-3-(1,1,1,2,2,3,3,4,4-nonafluoro-6-hexyl) imidazolium iodide (70) were prepared via two-step alkylation reactions. The radical addition reaction of perfluorobutyl iodide to 53 – 55 in the presence of AIBN resulted in the corresponding secondary fluorous iodides, 1-(nonafluoro-tert-butoxy)-3-iodo-4-perfluorobutyl-butane (71), 1-(nonafluoro-tert-butoxy)-4-iodo-5-perfluorobutyl-pentane (72) and 1-(nonafluoro-tert-butoxy)-5-iodo-6-perfluorobutyl-hexane (73).
Palladium nitrogen-heterocyclic carbene (NHC) complexes bearing linear perfluoroalkyl short-chain bis-[1,3-bis(1,1,1,2,2,3,3-heptafluoro-5-pentyl)imidazol-2-ylidene]diiodo-palladium(II) (74) and bis-[1,3-bis(1,1,1,2,2,3,3,4,4-nonafluoro-6-hexyl)imidazol-2-ylidene]diiodo-palladium(II) (75) were obtained and characterised. Their preliminary catalytic activity was explored in the Pd-catalysed Suzuki-Miyaura cross-coupling reaction of 4-bromoanisoles or 4-iodotoluene and phenyl boronic acid was catalysed by complexes 74 and 75 in low yields, these complexes have limited stability under catalytic reaction.
Fluorous partition coefficients of olefins 53 – 55, triflates 56 – 58, and iodides 71 – 73 were measured in biphasic solvent system perfluoro(methylcyclohexane) /toluene (50 w%:50 w%).
The cytotoxicity of olefins 53 – 55, together with nonafluoro-tert-butyl propyl ether (49), allyl ether (76), propargyl ether (77), fluorous diethers 78 - 80, have been tested via MTT assays in three mammalian carcinoma cell lines (A549, HepG2 and MDA-kb2 cells). Overall, fluorous diether 78 was the most consistently cytotoxic of all the newly-synthesised fluorous compounds. The significant increases in cell viability observed in response to some of the branched short-chain fluorous compounds were not dose-dependent and were generally small compared to those reported for perfluorocarboxylic acids.
Further application of the use of short-chain perfluoroalkyl group was extended to the synthesis of fluorous silanes as the precursors of hydrophobic materials. 1-(Nonafluoro-tert-butoxy)-4-butyl triethoxysilane (82), 1-(nonafluoro-tert-butoxy)-5-pentyl triethoxysilane (83), and 1-(nonafluoro-tert-butoxy)-6-hexyl triethoxysilane (84), were prepared by hydrosilylation reaction of fluorous olefins 53 – 55 and triethoxysilane in the presence of a platinum complex. By attaching fluorous silanes 82, 83 and 84 to silica aerogel via sol-gel method, three fluorous silica nanoparticles (FSiNPs) 85, 86 and 87 were synthesised and characterised by FT-IR spectroscopy, SEM-EDX spectroscopy. 
Different weight% of fluorous silica nanoparticles (FSiNPs) of 85 – 87 were coated on to the surface of commercially available PVDF membrane via electrospraying method to produce hydrophobic membranes [85]25%/PVDF, [85]50%/PVDF, [85]100%/PVDF, [86]25%/PVDF, [86]50%/PVDF, [87]25%/PVDF and [87]50%/PVDF, where % represents the weight% of FSiNPs added to the membrane. The measured contact angles of the membranes indicated that the hydrophobicity was significantly improved and even superhydrophobicity (>150o) can be achieved. A series of branched fluorous modified membranes were characterized FT-IR-ATR, SEM, EDX spectroscopy and optical profilometry. Preliminary test on three 25w% FSiNPs-modified membrane in direct contact membrane distillation showed stable and durable performances in terms of flux and thermal conductivity in high salinity feed solution condition.