Marco
Wenzel
,
Quintin W.
Knapp
and
Paul G.
Plieger
*
Institute of Fundamental Sciences, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand. E-mail: p.g.plieger@massey.ac.nz; Fax: +64 6 350 5682; Tel: +64 6 356 9099 Ext 7825
First published on 22nd October 2010
The preferential uptake of sulfate over the industrially important anions, chloride and nitrate and structurally similar dihydrogen phosphate has been achieved in aqueous media with a dicopper salicylaldoxime complex.
The ligand L, N,N′-dimethyl-N,N′-hexamethylenedi-(3-hydroxyiminomethyl-2-hydroxy-5-tert-butylbenzylamine), consisting of two 5-tert-butylsalicylaldoxime molecules, forms a helical twisted 2∶2 metallo-macrocycle upon Cu(II)-complexation, which can encapsulate anions (Fig. 1).8 In this work, we show that the extraction of the hydrophilic sulfate anion from aqueous solution into a water-immiscible organic phase is possible using this receptor; even in the presence of high concentrations of industrially relevant anions such as chloride and nitrate, which, based on their solvation energies should be preferentially extracted. In addition, we show unequivocally that anion shape is not a factor in this anti-Hofmeister behaviour as the structurally similar yet more hydrophobic dihydrogenphosphate anion is not captured in preference to sulfate within this system.
Fig. 1 Anion encapsulated Cu2L2 metallo-macrocycle. |
In order to probe sulfate uptake over chloride, nitrate or phosphate, two sets of liquid–liquid extraction experiments were undertaken. The first was chosen to match the industrially relevant, Bulong circuit,9e.g. an aqueous solution containing sodium sulfate and sodium chloride was vigorously stirred for 24 h with an equal volume of chloroform containing the “copper only” complex [Cu2(L-2H)2].8 The intramolecular hydrogen bonding present around the copper centre within these pseudo-macrocyclic systems imparts improved stability to these complexes to the degree that ESMS analysis in this experiment gave a representative indication of the organic solution composition. Thus, anion uptake within the core could be conveniently analysed by ESMS with a strong isotopic peak pattern at 660 (m/z) indicative of the presence of the dicationic complex species [SO4⊂Cu2L2]2+. In a second set of experiments a large 100-fold excess of chloride, nitrate or phosphate was used with parallel results, i.e. the presence of the dicationic complex species was again observed in the ESMS in all cases with no observation of the alternative anion within the core.
The similarity in the molecular mass and the isotopic distribution of sulfate and dihydrogen phosphate prevents their discrimination via low resolution ESI-MS. However, support for the preferred uptake of sulfate over dihydrogen phosphate is given by a carefully calibrated HR-MS study. The observed signal of the +2 fragment at 665.2882 m/z (lowest-mass) and the isotopic pattern is in agreement to the theoretical model of [SO4⊂Cu2L2]2+ (C64H100N8O8Cu2SO4)2+m/z = 665.2886 (Fig. 2), with no indication of a peak associated with encapsulated dihydrogen phosphate species at ∼+0.01.
Fig. 2 Experimental high resolution ESI-MS isotope pattern for the dicationic fragment [SO4Cu2L2]2+ (top) and the expected theoretical isotope pattern (bottom). |
UV-vis titration experiments of the “copper only” complex [Cu2(L–2H)2]‡ in a 1∶1 mixture of isopropanol/1,2-dichloroethane were undertaken with nitrate and the halide anions to underline the observed stronger affinity towards sulfate. The calculated stability constants§ show the expected trend with smaller log K values for both nitrate and the halides as compared with the recently reported values for the tetrahedral anions sulfate and dihydrogen phosphate.8 It is worth noting that based on the complex stability data for sulfate and phosphate, a preference in binding these anions is not expected (Table 1).
To emphasise the preference of the receptor to bind sulfate over either chloride, nitrate or phosphate, L in methanol was mixed in a 1∶1∶4 ratio of CuSO4 and either NaCl, NaNO3 in water or in a 1∶1∶2 ratio with CuSO4 and K2HPO4 in water. Crystallisation of the resulting bulk materials resulted in the isolation of the mixed anion complexes [SO4⊂Cu2L2]Cl2, [SO4⊂Cu2L2](NO3)2 and [SO4⊂Cu2L2](H2PO4)2.¶ An addition experiment utilising a mixture of three potential anions SO42−, NO3− and HPO42− also resulted in the formation of [SO4⊂Cu2L2](NO3)2 as evidenced by a space group check which matched exactly the data obtained for the above sulfate/nitrate complex. Each of the structures exhibit the 2∶2 metal to ligand helical assembly common to these lengthy bis-salicylaldoxime ligands (Fig. 3). In all cases the four tertiary amines on the complex are protonated as evidenced from identification of the accompanying anions. With the exception of the phosphate containing complex, X-ray structures of the complexes confirmed the encapsulation of SO42− at the complex core in preference to the other anions (Fig. 3). There are subtle differences between the basic [SO4⊂Cu2L2]2+ dicationic core based on the Cu–Cu distances and helical twist angles of the individual complexes (Table 2). Interestingly, direct reaction of L with Cu(NO3)2·3H2O results in a NO3− encapsulated complex [NO3⊂Cu2L2](NO3)3 (Fig. S5, ESI‡) with similar Cu–Cu distances (Table 2), indicating that the Cu2L2 core with its additional intra-hydrogen bonding has limited flexibility and probably plays a part in the overall stability of the bound sulfate complex. For the phosphate complex, both data quality and the similarity in size between the dihydrogen phosphate and sulfate anions preclude direct identification of sulfate at the core of this helicate, however, additional evidence for encapsulation of SO42− over H2PO4− is given by HR-MS and elemental analysis on the bulk crystalline sample as used in the X-ray analysis. A detailed description of the structures is given in the ESI.‡
Fig. 3 Comparison of the [SO4⊂Cu2L2]2+ dication for (a) the chloride, (b) nitrate and (c) dihydrogen phosphate mixed anion complexes emphasising the strong similarities between each. |
The stability of the sulfate core is due to a number of factors including, copper sulfate coordination and an electropositive cavity of protonated ammonium groups. However, it is the enhanced stability of the basic helicate structure brought about by intramolecular hydrogen bonding around the copper centre, coupled with an appropriate span between the copper metal centres that allows this complex to bind sulfate strongly.
Investigations of any sulfate/phosphate discrimination point to a preferred uptake of sulfate over phosphate which is noteworthy given the minor difference between the association constants (Δlog = 0.05).
Work continues in the development of systems related to [Cu2(L–2H)2] which have both a reduced strap size and more rigidity. Such minor variations appear to drastically change the anion preference.
This work was supported by grants from Massey University. For the high resolution mass spectroscopy data we thank Mrs Pat Gread and Prof. Bill Henderson from the University of Waikato. The MacDiarmid Institute for Advanced Materials and Nanotechnology and the Tertiary Education Commission of New Zealand provided funding for the Rigaku Spider optics and detector.
Footnotes |
† This article is part of the ‘Emerging Investigators’ themed issue for ChemComm. |
‡ Electronic supplementary information (ESI) available: Ligand synthesis, crystal packing, table of weak interactions of the crystal structures, crystal structure of [NO3⊂Cu2L2](NO3)3·2H2O, Job plots of the UV-Vis experiments and HR-MS spectra of [SO4⊂Cu2L2](H2PO4)2. CCDC 783210–783213. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0cc02230f |
§ Multi-varied analysis of the spectra (SPECFIT/32™),10 assuming a 1∶1 anion to [Cu2L2]4+ binding model as supported by Job plots (see ESI‡). |
¶ Characterisation of complexes: [SO4⊂Cu2L2]Cl2·2.5H2O: green solid, yield 41%, MS(ESI) m/z 666 [Cu2L2SO4]2+; anal. calc. for C64H100Cu2N8O12Cl2S·2.5H2O: C 53.06, H 7.31, N 7.74%; found: C 53.02, H 7.18, N 7.65%. Green plate crystals suitable for X-ray diffraction analysis were isolated by slow diffusion of diethyl ether into an acetonitrile/ethanol solution (1∶1) containing the complex over one week. Crystal data for C66H108Cl2Cu2N8O14S, M = 1467.64, monoclinic, a = 19.805(4) Å, b = 20.320(4) Å, c = 21.840(4) Å, β = 109.52(3)°, V = 8283(3) Å3, T = 100 K, P21/c (no. 14), Z = 4, 72361 reflections measured, 14014 unique (Rint = 0.0821) of which 14014 were used in the calculations, R1 = 0.0650 (9908 with F > 2σ(F)). Disordered solvent regions in this complex were treated in the manner described by van der Sluis and Spek,11 as 234 e− per cell, approx. to 3.25(CH3OH) (=72 e−) per formula unit. [SO4⊂Cu2L2](NO3)2·H2O·3EtOH·MeCN: green solid, yield 88%, MS(ESI) m/z 666 [Cu2L2SO4]2+; anal. calc. for C64H100Cu2N10O18S·H2O·3EtOH·MeCN: C 52.29, H 7.50, N 9.32%, found: C 52.43, H 7.21, N 9.11%. Green block crystals suitable for X-ray diffraction analysis were grown by slow diffusion of diethyl ether into an acetonitrile/ethanol mixture (1∶1) over 3 days. Crystal data for C72H121Cu2N11O22S, M = 1651.94, triclinic, a = 10.464(2) Å, b = 18.668(4) Å, c = 21.678(4) Å, α = 78.77(3)°, β = 85.67(3)°, γ = 85.96(3)°, V = 4135.0(14) Å3, T = 123 K, P (no. 2), Z = 2, 71567 reflections measured, 12207 unique (Rint = 0.0921) of which 12207 were used in the calculations, R1 = 0.0667 (8871 with F > 2σ(F)). [SO4⊂Cu2L2](H2PO4)2: yield 55%, MS(ESI) m/z 666 [Cu2L2SO4]2+, anal. calc. for C64H104Cu2N8O20P2S: C 50.35, H 6.87, N 7.34%, found: C 50.20, H 7.10, N 7.20%. Green chip crystals suitable for X-ray diffraction analysis were grown by slow diffusion of diethyl ether into an acetonitrile/methanol/ethanol mixture (1∶1∶1) over 2 days. Crystal data C64H100Cu2N8O20P2S, M = 1522.60, orthorhombic, a = 20.2187(4) Å, b = 21.9011(4) Å, c = 38.781(3) Å, V = 17172.6(13) Å3, T = 123 K, Pbca (no. 61), Z = 8, 126497 reflections measured, 13160 unique (Rint = 0.1223) of which 13160 were used in the calculations, R1 = 0.0758 (7141 with F > 2σ(F)). Disordered solvent regions were treated in the manner described by van der Sluis and Spek,11 as 590 e− per cell, approximating to 1(CH3CH2OH), 2(CH3OH) and 1(H2O) (=72 e−) per formula unit. [NO3⊂Cu2L2](NO3)3·2H2O: yield 92%, MS(ESI) m/z 649 [Cu2L2NO3–H]2+; anal. calc. for C64H100Cu2N12O20·2H2O: C 50.55, H 6.89, N 11.05%, found: C 50.59, H 6.79, N 10.89%. Green chip crystals suitable for X-ray diffraction analysis were grown by slow diffusion of diethyl ether into an acetone/ethanol mixture (1∶1) over 1 week. Crystal data for C64H100Cu2N12O20, M = 1484.64, monoclinic, a = 11.847(2) Å, b = 19.317(4) Å, c = 37.831(8) Å, β = 93.95(3)°, V = 8637(3) Å3, T = 123 K, P21/c (no. 14), Z = 4, 84887 reflections measured, 13237 unique (Rint = 0.1092) of which 13237 were used in the calculations, R1 = 0.0860 (9141 with F > 2σ(F)). Disordered solvent regions were treated in the manner described by van der Sluis and Spek,11 as 250 e− per cell, approximating to 2(CH3CH2OH) and 0.75(H2O) (=61.5 e−) per formula unit. |
This journal is © The Royal Society of Chemistry 2011 |