Tetraphenylborate salts of alkali and alkaline earth metal complex cations

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TETRAPHENYLBORATE SALTS OF ALKALI AND ALKALINE EARTH METAL COMPLEX CATIONS P (;. DEI,DUCA. A. M. Y. JABER. G. J. MOODY and J. D. R. THOMAS* Chemistry Department, UWIST, Cardiff CFI "~NU.Wales (Receil:ed 4 May 19771

Abstract--An introductory review summarises complex formation between poly(alkyleneoxy)adducts and inorganic ~alts. This is followed by preparative and IR and NMR spectroscopicfeatures of the tetraphenylboratesof complexesof polyethyleneglycols,nonylphenoxy(polyethyleneoxylethanolsand polypropyleneglycolswith sodium, magnesium,calcium,strontium and barium ions. Generally.an alkyleneoxide:cation ratio of 8,5 : I is indicatedfor the complexes with sodium, and 12:1 i ~ 10.5:1 for the polyethyleneglycols)for the complexeswith the alkalineearth metals INTRODUCTION Poty(alkyleneoxy) systems may be regarded as acyclic polyethers with the -(C HCH20)- repeating unit. They

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include polyethylene glycols (PEGs) [HO(CH2 (TH20),,H], polyglycol dimethyl ethers (glymes) [CH30. (CH2CH:()),,CH~], polypropylene glycols (PPGs) [HO. (CHCHeO),,HI. and alkylphenoxypoly(ethyleneoxy)-

ethanols--otherwise known as polyoxyethylene alkylphenyl ethers and of which the nonylphenoxy poly (ethyleneox y)ethanols (N Ps) C,~H,9C~H40(CH2CH20),H] are typical. The materials have various applications. For example, the low molecular mass PEGs are solvents and being less hygroscopic can be used as blenders to alter the humecrant properties of simple glycols and glycerine[l]. The NPs are important non-ionic surfactants and variation in the number of ethyleneoxy units can produce useful changes in wetting, detergency, emulsification, solubility and foam propertiesl2]. Complexes with metal ions facilitate their analysis. Thus, gravimetric methods for determining PEGs are based on precipitating their cationic complexes with reagents comprising large anions like those in potassium hexacyanoferrate(ll)[3], potassium bismuth iodide[4], heteropolyacids[5-9], and sodium tetraphenylborate (TPB)110-141, the latter also being adapted to titrimetric procedures[15-17]. Spectrophotometric methods developed for poly(ethyleneoxy) non-ionic surfactants[1825] depend on salts of the cationic complexes with large polarisable anions like hexathiocyanatocobaltate(II) [18, 19] and picrate125]. The viscosity [26], cloud-point curves [271, crystallinity[26, 28,291 and electrical conductivity [30, 31] of poly(alkyleneoxyl materials are disrupted when contaminated or reacted with inorganic salts, such as sodium iodide, potassium halides, rubidium iodide, mercury(ll) halides, and lhiocvanates of sodium, potassium and ammonium. Their polyelecm)lyte properties suggest an association between polymer and cations, a feature reported in NMR studies of ion-dipole interactions between potassium iodide and poly(ethylene oxide)f32]. Such interactions are also indicated by many of the above studies as well as by the reaction between potassium alcoholate and ~\lllh~r ['~r ~ o r r c s p o n d e n c e

polyalkylene glycols[33], by spectrophotometric data on polyalkylene glycols in the presence of sodium, potassium and ammonium thiocyanates[34], and by X-ray diffraction and other studies of complexes of mercury(II) halides with poly(ethylene oxide)f35,36] and related ethylene oxide oligomers[37-40]. Temperature-dependent optical and NMR spectroscopic data have shown the effectiveness of glymes as coordinating agents for the alkali metal ions and for the ion pairs of flurenyl carbanion alkali metal ion salts[41,42]. The tetraphenylborate salts of complexes between the NPs and alkaline earth metal cations, especially barium, have been exploited for their response in ion-selective electrode membranes [43-49]. This has led to rather more direct interest in synthesising salts of the complexes than in many of the studies cited above where the formation of the complex has been incidental--whether it be in an analytical procedure or in obtaining salt-contaminated polymer ribbons for assessing variations in selected properties of the polymer. Nevertheless, more convenient preparations have been sought. Thus, the one year (45 days for the type I 4:1 (CH=CH2()h. HgCi2 complex[28]) that it takes to obtain the type II 1:1 CH=CH~O:HgCI2 complex by exposing poly(ethylene oxide) (Polyox WSR 301 of the Carbide and Carbon Chemicals Company product range) to a saturated dry ether solution of mercury(ll) chloride at room temperature[35] may be reduced to about 2 days by soaking the type I complex in a saturated boiling ether solution of mercury(II) chloride[35]. The type I complex of 4:1 ethylene oxide : mercury(ll) halide has been prepared in aqueous solution by titrating poly(ethylene oxide) (Carbowax 6000 of the Carbide and Carbon Chemicals Company product range) with mercury(II) chloride and bromide, but mercury(II) iodide gave a 5 : 1 ethylene oxide :mercury(I1) iodide ratio[29]. The 4:1 ethylene oxide:salt ratio has also been obtained for preparations in methanolic solution of complexes of alkali metal ions with poly(ethylene oxide) in the cases of sodium iodide, sodium thiocyanate and potassium thiocyanate [50]. Other ethylene oxide:cation ratios are known and 9:1 ratios have been reported for materials precipitated with potassium and rubidium halides[26], 10.4117] or ~12[15, 16] for barium tetraphenylborate, in each case with polyethylene glycols. For the NPs an ethylene oxide:cation ratio of 12:1 appears to hold for calcium[49], strontium[49], and barium[16,49] tetraphenylborates respectively. Of the several methods metnioned for precipitating

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poly(alkyleneoxy) adducts, the conversion with tetraphenylborate ions is mostly convenient for the materials used in ion-selective electorde membranes [14, 4349]. In view of their importance in the ion-selective electrode field and in connection with the general properties of the parent polymers, an extended range of the tetraphenylborate salts of alkali and alkaline earth cation complexes with PEGs, PPGs and NPs has been prepared.

chloromethane and deuterated dimethylsulphoxide. Silicon tetramethyl could not be used as internal standard because of the formation of a multiphase system, and other reference standards interfered with the proton resonance positions of the complexes. Hence, the solvent spectra were scanned with silicon tetramethyl as reference and the known positions of the main proton resonances used for locating the principal NMR features of the complexes in the solvents. These are shown in Table 3.

EXPERIMENTAL

Chemicals. Chemicals were of reagent grade. The Antarox CO series of NPs and the PEGs were gifts from GAF (Great Britain) Ltd., Manchester and Union Carbide (UK) Ltd., London respectively. Preparation of TPB salts of the complexes. The method previously described[48] for preparing the TPB salts of NP complexes of alkaline earth cations was employed. Thus, an aqueous metal chloride solution (10cm3 of 0.1 moldm 3 or l moldm 3 solution) was added to an aqueous solution (0.154 g per 100 cm3) of the appropriate poly(alkyeneoxy) adduct (NP, PEG or PPG). The resulting oxonium ion was precipitated with excess sodium TPB (10 3tool din-3), filtered, washed well with water and vacuum dried at 35-50°C. The waxy, thermally unstable magnesium complexes need to be dried over phosphorus pentoxide. Although the same general method was used for TPB salts of sodium complexes, concentrated sodium chloride solutions (1 tool dm 3) were needed to coagulate the precipitates which were finally dried over phosphorus pentoxide. The white products were assessed by elemental analyses, titration and by spectroscopic measurements. For titration purposes 5 cm3 barium acetate (120g dm ~) buffered to pH 4.6 with acetic acid (63 g dm 3) are added to an aqueous solution (20cm3) containing 0.2 g of the appropriate poly(alkyleneoxy) adduct and titrated with 0.1 moldm -3 sodium tetraphenylborate. The NPs were titrated to a visual end-point with Congo red indication[15,16] and the polyethylene and polypropylene glycols to a potentiometric endpoint[17] with a silver wire indicator electrode, used in conjunction with an Orion 90-02 double junction saturated calomel reference electrode and where the potassium nitrate junction was replaced by 0.1 tool dm 3 sodium nitrate. IR spectra were obtained on Nujol mulls with a PerkinElmer t57G instrument and NMR spectra were run on a PerkinElmer R32 (90 MHz) spectrometer in dichloromethane or deulerated dimethylsulphoxide with silicon tetramethyl standard. RESULTS

Table 1 summarises the principal characteristics of the TPB salts of stoichiometries deduced by titration and considerably extends the range previously reported[48]. Attempts to prepare TPB salts of complexes with beryllium and the higher alkali metal ions gave just metalTPB precipitates. For example, a product obtained with caesium chloride and Antarox CO-880 treated by the general preparative procedure matched CsTPB. Furthermore, it is stressed that the preparative procedure is the most straightforward for the barium complexes, followed by strontium and calcium. The TPB salts are stable for several months without apparent decomposition. Spectroscopic features are summarised in Tables 2 and 3, and deductions concerning the number of - ( C H C H 2 0 ) - u n i t s associated with each metal cation

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obtained by chemical analysis and appropriate peak integration of the NMR spectra in Table 4. The NMR studies were bedevilled by insolubility problems although the complexes were soluble in di-

DISCUSSION OF IR AND NMR SPECTRA

X-ray diffraction studies[35-49] of poly(ethylene oxide)mercury(II) chloride complexes and similar extensive studies[51-53] of alkali metal ion-cyclic polyether complexes, indicate direct interaction of ether oxygen atoms with the cations and not anions as reported in a solution viscosity study[26]. Such interactions can be expected to influence the helical structures of the poly(alkyleneoxy) chains and so affect the IR spectra. Indeed, the most significant spectral differences between the reference poly(alkyleneoxy) adducts and the tetraphenylborate salts of the complexes concern the carbon ether oxygen stretching frequencies. Thus, for the NP-barium, -strontium and -calcium complexes (represented in Table 2 by a barium- CO-880 complex) the ethyleneoxy bands at 1149 and 1060cm ~ have disappeared and the main band at l l l 5 c m ~ has shifted to 1085 cm 1. The IR spectra of the NP-sodium complexes are similar to those of the alkaline earth metal ion complexes, but with the main l l l 5 c m -~ C-O-C stretching frequency appearing at 1095 and the 1060cm -1 band being again evident as in the uncomplexed polymer. Similar IR spectroscopic changes were observed for PEGs and PPGs on complexation. For example, with barium and calcium complexes of PPG (1025) the very strong C-O-C stretching frequency of the uncomplexed polymer at 1105 cm ~ is represented by a very strong band at 1065 cm -~ and a strong broad band at 1130 cm 1. These IR studies strongly support previous IR spectra recorded for poly(ethylene oxide) complexes with mercury(II) chloride[28, 29, 35]. However, IR studies alone give no clear indication about conformation without supporting X-ray diffraction studies. This has only been done for mercury(II) chloride complexes with poly(ethylene oxide) and ethylene oxide oligomers[3539]. It was not possible to determine the precise structure of the PEO complexes[35, 36] but those concerned with the ethylene oxide oligomer complexes point to oxygen atoms being on the inner side of circular (but not closed) molecules and enclosing the HgCI2 with close interatomic distances (2.66-2.96 A) between the Hg and O atoms [37, 39]. The TPB frequencies observed at 741 and 711 cm ~for sodium TPB which shifts to -731 and -703 cm t respectively in the TPB salts of the complexes indicate at least some anionic interaction. The main NMR features are accountable as shown in Table 3. They and the assignments made from NMR tables[54] are consistent with published data for polyalkylene glycols[32, 55-57]. However, the differences in shifts between poly(ethylene oxide) and the polymer plus potassium iodide observed by Lui[32] for NMR spectra in deuterated methanol were not obtained here. Confirmation of the number of alkyleneoxy units associated with each metal ion is of interest. Towards this end, the total number of protons of an alkylene oxide

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Table 4. Comparison of alkylene oxide units (AOU) cation ratios obtained by chemical analysis on NMR studies of TPB salts of cation adducts with poly(alkyleneoxy) material AOU/cation ratios by chemical analysis

Adduct

NMRt

NPs Na Mg Ca ~Antarox CO-880.TPB Sr Ba Na Mg Antarox CO-890.TPB Ba Na , Antarox CO-850.TPB Ba

8.4 12 12 12 12 8.4 12 12 8.4 12

8.67 8.22 8.24 12.1 13.7 12.4 12.2 12.0 11.9 11.8 11.9 8.68 8.3 8.7 13.3 12.4 7.07.1 12.3

PEGs Na | Mg / PEG(1540).TPB Ba

8.5 10.5 10.5

8.42 13.7 10.4 10.6

PPGs] Na | Mg j PPG(1025).TPB Ca Ba

8.5 12 12 12

8.77 13.6 13.55 13.6 12.1 12.0

C : } PPG(2025)'TPB

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chain of each complex molecule has been related to the height of their integrated area in the NMR spectrum and used to compare with integrations of the TPB protons which are shifted far downfield from those of the alkyleneoxy protons. Due allowance has been made for the phenyl protons associated with the NPs in deducing the number of TPBs associated with each polymer molecule of predetermined molecular mass. From this information the number of alkyleneoxy units associated with each metal ion follows (Table 4). In general, the number of alkylene oxy units associated with each metal ion deduced from NMR data and chemical analysis is similar. For the barium complexes, the agreement is very consistent and bearing in mind that the barium complexes are by far the easiest to prepare, the NMR data fully support the earlier assessments of 12 alkyleneoxy units associated each barium atom in the NP complexes[43,48] and now with PPGs, and the other alkaline earth cation complexes with these materials. For PEGs the previously reported[17] ratio of -10.5 appears to hold. Although the sodium complexes are also difficult to prepare the alkyleneoxy:sodium ion ratio of -8.5 indicated by elemental analysis is with one exception supported by the NMR data. Acknowledgement--The authors are grateful to the University of Wales for a Research Studentship to A.M.Y.J. REFERENCES

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