Edinburgh Research Explorer A Comparison of the Selectivity of Extraction of [PtCl6](2-) by Mono-, Bi-, and Tripodal Receptors That Address Its Outer Coordination Sphere

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INTRODUCTION
Solvent extraction of metals from chloride solutions underpins many processes which recover metals from the acidic chloride leaching of ores and metal wastes. 1 In order to gain an understanding of the design features which favour binding in the outer coordination spheres of chloridometalates rather than "ion pairing" to chloride, it is easier to study the extraction of kinetically inert chloridometalates such as [PtCl6] 2because they 3 will not exchange coordinated chloride for water or for the basic, protonatable, atom in the receptor on the time scale involved in the phase transfer reaction shown in equation 1.
We have reported the use of tripodal ionophores incorporating multiple hydrogen-bond donors linked to a protonatable bridgehead nitrogen centre (L 1 -L 5 ; Figure 1) to extract [PtCl6] 2into water-immiscible solvents. [2][3][4] Efficient extraction (>85%) from acidic chloride solutions was achieved with these tripodal reagents, and the quantitative stripping and release of the metalate by base (equation 3), provides the basis for a process in which the separation and concentration of platinum with recycling of the extractant and minimal reagent consumption (2 equivalents of HCl and of NaOH) and the generation of 2 mol equiv. of NaCl as a by-product. 2,3 Structural studies suggest that although each extractant contained three arms functionalised with potential hydrogen bond donors, only one or two of these arms participated in direct hydrogen-bond donation to the chloridometalate anion. In the majority of cases, the redundant arms in the solid state structures formed intra-and/or intermolecular hydrogen-bonds to neighbouring amide groups. 2 Tripodal aminoamide reagents with a different sequence of atoms linking the amide groups to the bridgehead nitrogen atom, RnN(CH2CONR'2)3-n, have been shown recently to act as efficient extractants for [RhCl5(H2O)] 2from acidic chloride solutions. 5,6 [(LH)2PtCl6](org) + 2NaOH ⇌ Na2[PtCl6] + 2L + 2H2O We report herein the synthesis and characterisation of the novel mono-and bipodal ionophores L 6 -L 18 (Schemes 2, 3 and 5). These retain the same hydrogen bond donor groups (urea, amide and sulfonamide), and the same solubilising alkyl and methoxy groups and a protonatable nitrogen centre that are present in the tripodal extractants L 1 -L 5 ( Figure 1) but introduce mono-, bi-as well as tri-functional pendant 4 arms to facilitate the study of the effect of different numbers of hydrogen bond donors.
The compounds L 19 -L 24 (Scheme 6) have a tertiary amine nitrogen atom carrying one, two or three 3-N-hexylpropanamide groups, and whilst similar to L 1 -L 18 , the sequence of the CO/NH components in the pendant amide group is reversed. These two series of extractants, L 6 -L 18 and L 19 -L 22  The amido analogue L 13 was also prepared from N,N-bis(3-aminopropyl)-methylamine by reaction with two equivalents of the 3,4,5-trimethoxybenzoyl chloride in the presence of a base (Scheme 3). All extractants were purified by column chromatography.

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Amine 5 was prepared, in high yield, by adaptation of a literature procedure (Scheme 4), 9 and was converted to L 14 -L 18 (Scheme 5) using similar procedures to those described above for L 6 -L 13 . Extractants L 14 and L 15 are white solids, whereas L 16 -L 18 were obtained as yellow oils after purification by column chromatography.   Table 5

Synthesis and characterisation of extractants
Apart from L 11 which was too insoluble in chloroform and L 23 Table 2 and Supporting Information) and have thus been largely excluded from the discussion below.
The general trend in the strength as an extractant for a particular type of amide in TREN-based systems, L 1 -L 18 , varies in the order tripodal > bipodal > monopodal extractant. The efficiency of loading falls off particularly sharply for the "simple" amides, e.g. when a 3-fold excess of the reagent is present the loadings are for L 3 (tripodal) 87%, L 8 (bipodal) 30% and for L 16 (monopodal) 13% [for their 3,5-methoxy isomers L 4 , L 9 and L 17 the values vary similarly: 86, 25 and 9%].

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In terms of the type of hydrogen bonding substituent present in the arms of the receptor, the incorporation of urea units leads to the strongest extractants. For both the tripodal and monopodal systems the distribution coefficients for [PtCl6] 2loading ( Table   2) follow the order ureas > amides > sulfonamides. Thus for the tripodal urea, amido and sulfonamido extractants L 1 , L 3 and L 5 the recovery of Pt by chloroform solutions containing a 50% excess of extractant is 98, 87 and 77% ( Figure 7a and Table 2), and for the monopodal analogues, L 14 , L 16 and L 18 , under similar conditions (Table 2) recoveries of 50, 13 and 10% were recorded. For the bipodal series L 6 (65%), L 8 (30%) and L 10 (53%) the order ( Table 2) is different with the amide being the weakest extractant.
The extractant series containing the reversed CO/NH amido functionality (L 19 - Results from the titrations are presented in Figure 9. In all cases the ammonium proton, shown in black, experiences a significant upfield shift but there is no clear pattern for the direction of the shifts for the other protons in L 4 H + , L 9 H + and L 17 H + . Whilst the analysis of results is complicated by the number of species present (see below and SI, Fig   SI7), the values for formation of the 2 : l assemblies (Table 3), evaluated using a purposewritten computer program, 22  that inter-and intra-molecular hydrogen bonding between amido groups occurs, particularly at high concentration of receptor relative to metalate. 7,19 These may need to be broken to adopt the optimum conformation to bind to the PtCl6 2guest and the energies required to do this could vary considerably between the mono-, di-and tri-amido extractants. Evidence for changes to inter-or intramolecular hydrogen bonding in L 4 H + is provided by monitoring the chemical shifts of the amido NH and the adjacent aromatic and methoxy hydrogen atoms when it is titrated with [(Oct4NH)2PtCl6] (see Figure SI7).