New bis(15-Crown-5) derivatives and their physicochemical properties

Two novel bis-crown ethers 1 and 2 were obtained. Both compounds form complexes with alkali metal ions. Such formation process was investigated using mass spectrometry (ESI MS). Compounds 1 and 2 can form both mono-and bimetallic complexes. Detected also are ions that reflect formation of complexes between two molecules of bis-crown ethers and one metal ion. Cesium and rubidium ions form with compound 2 only complexes of 1:1 type.


Introduction
It is somewhat difficult to enumerate all practical applications of crown-ethers, not only chemical ones but also in various areas of material sciences.The diversity of crown ethers and their sheer number reflect their popularity in contemporary chemistry.Bis(crown-ethers) and their applications have been widely described in the literature [1][2][3] .The first compounds of this type appeared three decades ago 4 .Since then, numerous bis(crown-ethers) were obtained, in which they are linked to each other in various manners.Bis(crown-ethers) found numerous applications, e.g. in ion-selective electrodes 5 .In compounds of this type a great role is played the effect of cooperative behavior of both ether rings upon complexed cations.Different types of complexes formed between bis(crown-ethers) and cations are described in 3 .
Almost all crown ethers form complexes with alkali metal cations.These complexes often are formed between two molecules of crown-ether and one metal ion found sandwiched between the two rings of crown-ethers.Formation of such complexes and their potential applications led to the synthesis of many different bis(crown-ethers).Compounds of this kind often were more selective in complexing a given kind of ions compared to their mono analogues.There are two types of bis(crown-ethers) described in the literature: those based on benzo-crown-ethers and crown-ethers.
Among many bis(ether) compounds there are bis(crown-ethers) based on benzophenone 6 .They belong to the first kind of compounds mentioned above.
The present study describes synthesis of two bis(crown-ethers) 1 and 2 (Fig. 1), which belong to the second group of compounds.The synthetic procedure used two starting compounds which, presumably, would facilitate formation of sandwich-type complexes with alkali metals: 4,4'dihydroxybenzophenone and α,α'-bis(para-hydroxyphenyl)-1,4-diisopropylbenzene.These compounds were used to obtain crown-ether derivatives.Structural formulas of compounds obtained are shown on Fig. 1.
Benzophenone is a good substrate for synthesis of sandwich-forming compounds.Aromatic rings rigidly bound via carbonyl bridge do not allow crown ethers to be separated significantly.This eases formation of intramolecular sandwich complexes with metal ions.In the case of the second starting compound two conformations of molecules are possible: syn and anti 7 (Scheme 1).
Compounds 1 and 2 can form complexes of CE × Me type (CE denotes bis(crown-ether) and Me is alkali metal), as well as CE × 2 Me.Owing to the lability of both ether rings also 2 CE × Metype complexes are possible.All of the possible alkali metal ion complexes with the investigated crown ethers were confirmed using mass spectrometry (ESI MS).
Compound 1, in which both crown ether rings are positioned closely to each other, forms with ions of sodium, rubidium and cesium strong complexes with apparent ESI MS molecular ion peaks of 100% intensity (see Table 1).The most intensive ion peak in the potassium complex 1 spectrum is the ion formed by two molecules of crown-ether and one potassium ion (m/z = 1395).Lithium ions, owing to their small ionic radius form most frequently bimetallic complexes ([1+2×Li] +2 and [1 + Li + LiI] + , the intensity of which is 100 %).Bimetallic complexes are also formed by sodium and potassium ions.Peak intensity in ESI MS spectra for sodium ions depends on the accompanying anion.Particularly intensive ions are recorded for acetate anions.Ions with a larger ionic radius , such as rubidium and cesium tend to form monometallic complexes, albeit 2 CE × Me type ions were also observed.Bimetallic complexes for these ions are not observed.A much simpler situation occurs in the spectra of compound 2 complexes.From Table 2 it can be seen that peaks for [CE + 2 × Me] +2 and [2 CE + Me] + -type ions are observed only for lithium and sodium ions, i.e. those with smallest ionic radii.For these ions formation of bimetallic complexes is observed also but one metal ion is coupled to anion (Li + with I -and Na + with HCOO -).Potassium forms complexes of all types considered here although the contribution of ions with [2 + 2 × K] +2 and [2 × 2 + K] + structure in the spectrum is minor.In the spectra of rubidium and cesium complexes only one type of ions is observed, that of [2 + Me] + (see scheme 1).This allows to assume that the formation of these complexes involves a cis conformer of compound 2. The trans conformer for rubidium and cesium ions should primarily give ion with [2 × 2 + Me] + or polymerlike structure.
UV spectrum of compound 1 has two almost identical bands: one at λ = 222 nm (log ε = 4.17), and the other, long -wave, at λ = 293 nm (log ε = 4.31).The spectrum is almost identical with that of the model compound 4,4'-diethoxybenzophenone. The spectrum of this compound has also bands at 225 and 291 nm (cyclohexane solution), whereas in chloroform there is only one band at λ = 295.5 (log ε = 4.37).Both spectra lack bands from n → πtype transitions, which in benzophenone occur at λ = 340 nm 11 .
UV spectra of compound 2 are characterized by the occurrence of three distinct bands at λ = 240, 278 and 284 nm and the shoulder at λ = 270 nm.These bands result from π → π* -type transitions.There is no band above 300 nm, i.e. a band resulting from n → πtype transition.In the IR spectrum of compound 1 there is a band at ca. 1645 cm -1 , which may be ascribed to the C=O fragment vibration.This band is redshifted by 7 cm -1 compared to identical vibration in the model 4,4'-diethoxybenzophenone, and blueshifted by 5 cm -1 compared to benzophenone 7 .Vibrations characteristic for ether fragments (νC-O-C) are very strong in the 1249 -1251 cm -1 and 1119 -1123 cm -1 ranges and strong in the 1032 -1035 cm -1 range for both compounds (1 and 2).The remaining bands for both compounds 1 and 2 are given in the Experimental section.

Conclusions
Two biscrown ethers were obtained that can form complexes with alkali metal ions, at the ionization source of a mass spectrometer.For metal ions with lesser mass one can observe formation of various complexes with the investigated bisethers, whereas for rubidium and cesium ions preferential formation of biscrownether / metal ion complexes is observed.In case of lithium, sodium and potassium ions formation of bimetallic complexes is observed.The obtained compounds belong to the group of ligand compounds in which particular fragments can cooperate with each other when forming complexes with metal ions.

Experimental Section
General. 1 H NMR spectra were recorded in CDCl3 or DMSO -d6 using a Bruker spectrometer (500 MHz), with TMS as reference.UV spectra were recorded in methanol or chloroform solutions using Genesys 6 (ThermoSpectronic) spectrophotometer.IR spectra were recorded with the Nicolet FT -IR Magna 560 spectrometer.ESI MS experiments were performed on a LCQ DUO FINNINGAN THERMOQUEST instrument.The mixtures of the crowns and tenfold more concentrated alkali metal ions in chloroformmethanol solutions were directly injected into the ion sources using a syringe pump.Complex solutions were prepared directly before the analysis.Stock solutions of 1 and 2 were prepared by dissolving ca.0.1 mmol of each into 5 mL of chloroformmethanol (3:1, vol/vol) mixture. 1 Mmol alkali-metal salts (iodides or acetate) in methanol were prepared in each.100 μL aliquots of 1 or 2 were mixed with 100 μL of cation solution and the obtained mixture was ten-fold diluted.4,4'-Dihydroxybenzophenone was purchased from Aldrich.α,α'-Bis(para-hydroxyphenyl)-1,4-diisopropylbenzene was synthesized according to 8 .Alkali metal salts were purchased from POCh (Poland) or Aldrich, and they were used as received.

1 H
NMR spectra shown in the experimental section have at the expected positions peaks due to aromatic protons.The signals of protons linking aromatic fragments with crown ethers occur at δ = 4.00 -4.20 ppm for 1 and at δ = 3.90 -4.05 ppm for 2 as multiplets.Among the remaining signals one might distinguish multiplets due to single protons attached to tertiary carbon atoms in ether fragments.The remaining peaks occur for both compounds as multiplets, at δ = 3.40 -3.80 ppm.

Table 1 .
ESI mass spectra for mixture of 1 and alkali metals (Me).Diagnostic ions (m/z) and their abundance (%) in positiveion experiment

Table 2 .
ESI mass spectra for mixture of 2 and alkali metals (Me).Diagnostic ions (m/z) and their abundance (%) in positiveion experiment O Scheme 1. Synform of 2 can form dimerlike complexes.