Ionic Liquid Analogues Formed from Hydrated Metal Salts

Andrew P. Abbott,* Glen Capper, David L. Davies, and Raymond K. Rasheed[a]

Introduction

Ambient temperature ionic liquids have received extensiveinterest for both electrochemical and synthetic applica-tions.[111] The studies to date can broadly be divided intotwo types of ionic liquids; those based on chlorometallateanions, such as Al2Cl7

, and those based on non-metal-con-taining anions, such as BF4

and (CF3SO2)2N . Most of the

initial work focused on imidazolium or pyridinium salts withaluminium chloride. Osteryoung[1] showed that n-butylpyri-dinium salts with AlCl3 gave liquids over a wide range ofcomposition at ambient temperatures. Similar results werealso found using dialkylimidazolium chloroaluminatemelts.[2] The properties of these ionic liquids are controlledby various chloroaluminate anions, whose relative concen-trations change considerably with the melt composition. Thedensity, viscosity and electrical conductivity of these systemshave been characterised by Hussey et al.[3] The electrochem-istry of aluminium and a range of transition-metal salts havebeen studied in a variety of chloroaluminate melts and thishas found application for batteries and the electrodepositionof aluminium and its alloys.[47] The chemical and electro-chemical properties of AlCl3-containing melts have recentlybeen reviewed in a number of articles.[811]

It has recently been shown that ionic liquids can beformed by using zinc chloride with pyridinium,[12] dimethyl-

ethylphenylammonium[13] and imidazolium salts.[1416] Thesehave higher melting points than the corresponding chloro-aluminate melts, but are still fluid at ambient temperatures.In recent work[17] we have also shown that choline (2-hy-droxyethyltrimethylammonium) chloride also forms ionicliquids with metal halides, for example, with ZnCl2 andSnCl2. Not only are these ionic liquids easy to prepare, butin contrast to the AlCl3-containing liquids, they are waterand air insensitive; in addition their low cost enables theiruse in large-scale applications such as metal finishing.[18]

Analogous to AlCl3, ZnCl2 forms complex anions with Cl

and the change in properties of the ionic liquids with com-position and temperature can be predominantly ascribed tothe equilibria given in Equations (1) and (2).[19]

ZnCl3 ZnCl2 Zn2Cl5 1

Zn2Cl5 ZnCl2 Zn3Cl7 2

Other metal salts, such as SnCl2 and FeCl3, could also beused to form room-temperature ionic liquids.[17,19] However,the limited range of anhydrous metal salts that form ionicliquids with choline chloride, restricts the use of such mediafor a variety of important electrochemical processes. Thecurrent work reports the inclusion of other metals into ionicliquids as hydrate salts and focuses on the system formedbetween choline chloride and hydrated chromium chloridewith the specific application of electrodepositing chromium.

[a] Dr. A. P. Abbott, Dr. G. Capper, Dr. D. L. Davies, Dr. R. K. RasheedChemistry Department, University of LeicesterLeicester, LE1 7RH (UK)Fax: (+44)116-252-3789E-mail : Andrew.abbott@le.ac.uk

Abstract: A dark green, viscous liquidcan be formed by mixing choline chlo-ride with chromium(iii) chloride hexa-hydrate and the physical properties arecharacteristic of an ionic liquid. Theeutectic composition is found to be 1:2choline chloride/chromium chloride.The viscosity and conductivity aremeasured as a function of temperatureand composition and explained in

terms of the ion size and liquid voidvolume. The electrochemical responseof the ionic liquid is also characterisedand it is shown that chromium can beelectrodeposited efficiently to yield a

crack-free deposit. This approach couldcircumvent the use of chromic acid forchromium electroplating, which wouldbe a major environmental benefit. Thismethod of using hydrated metal saltsto form ionic liquids is shown to bevalid for a variety of other salt mix-tures with choline chloride.

Keywords: chromium electrochemistry green chemistry ionic liquids

Chem. Eur. J. 2004, 10, 3769 3774 DOI: 10.1002/chem.200400127 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 3769

FULL PAPER

Results and Discussion

Table 1 shows the freezing point of choline chloride mix-tures with CrCl36H2O. As with the anhydrous ZnCl2 and

SnCl2 cases reported previously,[17] the eutectic composition

occurs at a composition of 1:2 choline chloride/CrCl36H2O.At this composition it is a dark green viscous liquid at ambi-ent temperature. The liquid cannot be formed by adding an-hydrous CrCl3 to choline chloride with six equivalents ofwater, and the anhydrous salt is also insoluble in the ionicliquid formed from the hydrated salt. This shows that thewaters of hydration are vital in controlling the co-ordinationchemistry around the chromium centre and the liquid is notsimply a concentrated aqueous solution. It could be arguedthat a system containing water molecules is not strictly anionic liquid, but since all of the physical properties are com-parable to the anhydrous analogues (see below) and thewater molecules behave as ligands rather than solvent mole-cules, it seems appropriate to classify them as fluids analo-gous to ionic liquids.

The physical properties change markedly with tempera-ture and composition, and above 6570 8C the colour of theliquid changes to purple showing that the coordinationsphere around the chromium centre is changing. Thermog-ravimetry shows that the waters of hydration are released intwo steps; the first starts at about 85 8C, which equates toapproximately three water molecules, and the second atabout 180 8C, corresponding to the other three water mole-cules. A gradual weight loss is observed above 270 8C, prob-ably due to the decomposition of the choline cation and nofurther mass loss is observed between 500 and 1000 8C. Ther-mogravimetry of CrCl36H2O shows that three waters of hy-dration are released at 71 8C, that is, slightly lower than inthe ionic liquid, suggesting that the quaternary ammoniumsalt affects the coordination sphere around the chromiumcentre.

The chemistry of CrCl36H2O is extremely complex and avariety of species could be present in the ionic liquid. Inaqueous solutions the main equilibria include those in Equa-tion (3), and the dominant species are dependent upon thesolution environment.[21]

CrH2O6Cl3 0 CrH2O5ClCl2 H2O I CrH2O4Cl2Cl 2H2O II CrH2O3Cl3 3H2O III

3

Elving and Zemel showed that the UV-visible spectrumof the chromium species is affected by the number of chlo-

ride and water ligands bound to the metal. For each com-plex two bands are observed; the first in the range 407 to475 nm and the second in the range 575 to 665 nm. In eachrange the lower value corresponds to the hexaaqua complex[0] and the upper value to the trichloro species [III]. The ad-dition of each chloride ligand shifts the lower wavelengthband by about 20 to 25 nm and the upper band by 30 nm.[21]

In a neutral aqueous solution species [I] and [II] dominateand species [III] is only observed when the solution contains12m HCl. In the 2:1 CrCl36H2O/choline chloride mixture,two bands are also seen, one at 470 nm and the other at669 nm. This suggests that in the ionic liquid the trichlorospecies is present. A pronounced shoulder is observed at700 nm suggesting a species with four chlorides is formed inthe high chloride environment [Eq. (4)]:

CrCl3 3H2O Cl CrCl4 2H2O H2O 4

Electrospray MS of the eutectic mixture shows some evi-dence for [CrCl4]

(m/z : 192/194*/196) in the ionic liquid(the waters of hydration are bound too weakly to be ob-served and [Cr(H2O)3Cl3] is neutral and therefore not de-tected) although no evidence is observed in FAB MS. In themixtures of choline chloride with ZnCl2 and SnCl2, di- andtrimetal-containing anions with bridging chloride ligandswere observed.[19] No evidence was obtained for dichromiumcontaining complexes; however, these may be very weakcomplexes and hence not detectable by using FAB-MS. Noevidence is observed for the dianion [CrCl5(H2O)]

2. In ad-dition, no major chromium-containing ions were seen in thepositive ion spectra. Hence we conclude that the main chro-mium complex in the ionic liquid is [Cr(H2O)3Cl3].

Figure 1 shows the viscosity of ChCl/CrCl36H2O mixturesas a function of temperature and composition. It is notice-able that the viscosity of these ionic liquids is considerablylower than the ChCl/ZnCl2 ionic liquids reported previous-ly,[19] suggesting that the waters of hydration play a signifi-cant role in solvation. The viscosity of the medium is re-duced as the mole fraction of CrCl36H2O is increased, andthe minimum viscosity is not observed at the eutectic com-position.

Table 1. Freezing temperatures of choline chloride/CrCl36H2O mixturesas a function of composition.

ChCl/ Freezing ChCl/ FreezingCrCl36H2O point [K] CrCl36H2O point [K]

1:1 300.2 1:1.5 290.31:2 286.9 1:2.5 292.61:3 321.5

Figure 1. Viscosity of ChCl/CrCl36H2O mixtures as a function of temper-ature and composition

2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.chemeurj.org Chem. Eur. J. 2004, 10, 3769 37743770

FULL PAPER

www.chemeurj.org

The change in viscosity (h) with temperature is given byEquation (5):

ln h ln h0 EhRT

5

in which h0 is a constant and Eh is the energy for activationof viscous flow.[22] Table 2 shows that all the data in Figure 1fit well to Equation (5); the values obtained for Eh are alsogiven in Table 2.

It has been shown empirically that Eh values are relatedto the melting point of liquids (Tm) by Equation (6).

[22]

Eh3:74RTm 6

This has been found to be valid for a range of molecularliquids and high-temperature molten salts.[22] This wouldsuggest an Eh value of approximately 9 kJmol

1 for theChCl/CrCl36H2O mixtures, which is significantly smallerthan that shown in Table 2. The values for the chromium-containing liquid are, however, similar to other room-tem-perature ionic liquids. Branco et al.[23] reported the viscosityof a range of imidazolium salts and using these data theenergy for activation of viscous flow was found to rangefrom 37.2 (C4mimBF4) to 59.7 kJmol

1 ((C5O2mimCl).Figure 2 shows the plot of Eh verus RT for the data inFigure 1 and that extracted from the Branco study.[23] Aroughly linear trend is observed with a constant of propor-

tionality of 20.7. This suggests that the empirical relationshipobserved in Equation (6) for molecular liquids and high-temperature molten salts, is not valid for ambient-tempera-ture ionic liquids and may result from the large size of thesolvent ions found in the latter.

The eutectic mixture has a very high refractive index(1.5470), which is larger than that of most molecular sol-vents and comparable to many of the imidazolium-basedionic liquids and high-temperature molten salts.[2426] Thedensity is also extremely large (1.54 gcm3 at 60 8C) and isgreater than other imidazolium-based ionic liquids and sug-gests a small void volume in the liquid. The surface tension(77.680.05 mNm1 at 40 8C) is higher than water and iscomparable to high temperature molten salts, for example,KBr g=77.3 mNm1 at 900 8C.[26]

In high-temperature molten salts hole theory has beenused to explain the mobility of ions. It can be shown that ina molten salt the radius of the average sized void (r) is relat-ed to the surface tension of the liquid (g) by the expressiongiven in Equation (7):[22]

4phr2i 3:5 kTg

7

in which k is the Boltzmann constant and T the absolutetemperature. Hence, in the ionic liquid containing chromiumchloride the average void radius is 1.24 , while the radiusof the choline ion is 3.29 . By contrast, for KBr at 900 8Cthe average void radius is 2.1 , whereas the cation radius is1.3 .[22] This explains the significant disparity between theviscosity of molten salts and ionic liquids, namely, the ratioof ionic radius to void radius is considerably larger for ionicliquids than molten salts making ionic motion considerablyeasier in the latter case. We therefore suggest that the largespread of Eh values could be due to the inordinately largesolvent/hole ratio, which appears to be characteristic ofroom-temperature ionic liquids. This is, however, difficult toprove at present, because of the lack of consistent viscosityand Eh data available.

Figure 3 shows the conductivity of choline chloride/chro-mium chloride mixtures as a function of temperature andcomposition. The data show liquids that have conductivitiesthat are an order of magnitude larger than the ZnCl2/ChClsystem and comparable to imidazolium salts. This is proba-bly related to the viscosity of the ionic liquids, since theZnCl2/ChCl system is an order of magnitude more viscousand the imidazolium salts have similar viscosities to thoseshown in Figure 1. Analogous to the viscosity data, conduc-tivity (s) of ionic liquids have been fitted to Equation (8).[22]

ln s ln s0ELRT

8

The activation energy for conduction (EL) is listed inTable 2 as a function of composition. As with the Eh values,they are significantly larger than high-temperature moltensalts (typically group I halides have values of 5 to20 kJmol1).[26]

Table 2. Activation energies for viscous flow (Eh) and conductivity (EL)as a function of liquid composition. Coefficients of correlation (r) for thefits to Equations (5)[a] and (8)[b] are also given.

ChCl/ Eh r[a] EL r[b]

CrCl36H2O [kJmol1] [kJmol1]

1:1 58.5 0.997 43.7 0.9991:1.5 55.5 0.994 50.5 0.9951:2 54.2 0.998 41.2 0.9961:2.5 52.4 0.996 40.6 0.9921:3 37.8 0.999

Figure 2. Energy for activation of viscous flow as a function of ionicliquid freezing point.

Chem. Eur. J. 2004, 10, 3769 3774 www.chemeurj.org 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 3771

Ionic Liquid Analogues 3769 3774

www.chemeurj.org

For ionic fluids, the conductivity is generally governed bythe mobility of the charge carriers. Hence a plot of s versush1 should be linear for a given charge carrier. The case ofchromium chloride is somewhat complex because of the var-ious ligand permutations that are possible [Eq. (9)].

ChCl CrH2O4Cl2Cl 2H2O CrH2O3Cl3 3H2OChCl ChCrH2O2Cl4 4H2O

9

Figure 4 shows a strong linear correlation between con-ductivity and the reciprocal of viscosity, which shows thatthe ionic mobility is controlling the conductivity of the

liquid. It also shows that since the slope is approximatelyconstant at all compositions, the charge-carrying species isdominated by one species. The UV-visible data suggests that[Cr(H2O)3Cl3]3H2O is the major chromium containing spe-cies and hence the charge carriers are presumably Ch+ andCl .

The clear application of an ionic liquid containing chromi-um is for the study of chromium electrodeposition. Figure 5shows the cyclic voltammogram for the 1:2 ChCl/CrCl36H2O system at 60 8C on a Pt microelectrode at a

sweep rate of 20 mVs1. The reduction process that is initi-ated at approximately 0 V is most probably due to the re-duction of Cr3+ to Cr2+ . At fast sweep rates (>100 mVs1)the process appears to be quasi-reversible, but at 20 mVs1

the reduction current decreases markedly when E

allic form. The deposition process is apparently irreversible;this is surprising since the deposition of aluminum in anionic liquid is reversible.[28]

Electrodeposition of chromium is usually carried out fromaqueous solutions based either on CrO3/H2SO4 or trivalentchromium salts/complexing agents.[29] Both of these solutionshave their limitations primarily due to the competing hydro-gen evolution process, which limits the current efficiencyand leads to hydrogen embrittlement of the substrate, butalso because of the highly cracked deposit that is pro-duced.[29] The high toxicity of CrVI is also an issue in its use.

Bulk deposition from the 1:2 ChCl/CrCl36H2O system at60 8C for 2 h onto a nickel electrode at a current density of0.345 mAcm2 resulted in an amorphous pale blue/grey de-posit of 27 mm thickness. Negligible gas evolution was ob-served confirming that the waters of hydration are less elec-trochemically active than bulk water. The current efficiencyfor the process was found to be in excess of 90%. Exactquantification was not possible due to the incorporation of asmall, but undetermined, amount of chloride in the film.Hard chrome plating based on aqueous chromic acid resultsin an inherently inefficient process; only 1020% of theenergy supplied is utilised for electrodeposition.[29] Also, thedeposition of the metal from CrVI naturally necessitatestwice as much current as the corresponding CrIII complex.Power consumption is an important factor in the practicaloperation and the very high efficiency of the ionic-liquid-based processes could have a greatly beneficial impact onthe industrial sustainability of chrome plating. Also, the ab-sence of hydrogen production during plating will eliminatethe potential for hydrogen embrittlement, a significantfactor for component failure.

Figure 6 shows the scanning electron micrograph of thedeposit formed from the bulk electrolysis of the chromium-containing ionic liquid, and it can clearly be seen that it isnot micro-cracked as is typical of the deposits formed fromaqueous chromic acid solutions. A defect-free coating is im-portant, as it is these micro-cracks that can lead to pittingcorrosion in the substrate material. EDAX analysis showedthat the deposit was predominantly chromium with a small

amount of residual chloride. The hardness of the film was242 Vickers, which is less than the chrome hydride formedby the chromic acid process (typically 800 to 900 Vickers)but greater than pure chromium (typically 220 Vickers).This shows that chromium(iii)-containing ionic liquids couldbe a suitable alternative to hexavalent chrome baths for thedeposition of crack-free chromium coatings.

The mixture of hydrated metal salts with substituted qua-ternary ammonium salts is by no means limited toCrCl36H2O. A wide variety of hydrated salt mixtures withcholine chloride have been found to form these ionic liquids,including CaCl26H2O, LaCl36H2O, CoCl26H2O,LiNO34H2O and Zn(NO3)24H2O.

[30] This represents anovel method of incorporating other metals into ionic-liquid-like fluids.

Conclusion

It has been shown that ionic liquids can be formed betweencholine chloride and a variety of hydrated metal salts. Awide variety of metals can be included into ionic liquids bythis method and they are shown to have improved transportproperties over anhydrous metal salt systems. Specificallythe choline chloride/CrCl36H2O system has been character-ised as a function of composition and temperature and isfound to have properties similar to imidazolium-based ionicliquids. The temperature effects on the viscosity and conduc-tivity of these liquids are explained in terms of the ion/holeratio. The electrodeposition of thick, adherent, crack-freechromium films is demonstrated from this ionic liquid, andit is shown that the waters of hydration are highly coordinat-ed in the liquid and are not significantly electroactive withinthe potential window of the liquid. This could offer an effi-cient and environmentally more acceptable process bywhich to electrodeposit chromium than the current chromicacid based solutions.

Experimental Section

Choline chloride [ChCl] (Aldrich, 99%) was dried under vacuum priorto use and chromium(iii) chloride hexahydrate (Fluka, 98+%) was usedas received to prepare the ionic liquid. Choline chloride and chromi-um(iii) chloride hexahydrate in a molar ratio of 1:2 were combined andheated at approximately 70 8C until a green liquid was obtained, (approx-imately 30 minutes for 0.03 moles of 1:2 mixture). With prolonged heat-ing the liquid turned purple due to the loss of water molecules from thecoordination sphere around the chromium ion. This process was, howev-er, reversible on cooling the mixture.

The viscosity was determined using a Brookfield DV-E viscometer fittedwith a thermostated jacket. The conductivity and its temperature depen-dence were determined by using a Jenway 4071 conductivity meter withtemperature and conductivity probes (probe cell constant=1.01 cm1).Refractive index measurements were made with a Bellingham and Stan-ley refractometer, which was calibrated using dichloromethane and tolu-ene as reference standards. A Kruss K11 tensiometer equipped with athermostatted jacket was used for the surface tension experiments.

The equilibrium geometry and volumes of the ions were calculated usinga HartreeFock method utilizing an STO-3G model provided by com-mercially available software.[20] Thermogravimetric analysis was carriedout using PerkinElmer Pyris 6 TGA at a heating rate of 5 8Cmin1.

Figure 6. SEM image of the deposit formed following the electrolysis ofthe 1:2 ChCl/CrCl36H2O system at 60 8C for 2 hours onto a nickel elec-trode at a current density of 0.345 mAcm2 (bar=10 mm).

Chem. Eur. J. 2004, 10, 3769 3774 www.chemeurj.org 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 3773

Ionic Liquid Analogues 3769 3774

www.chemeurj.org

Voltammetry and chronoamperometry were carried out using an AutolabPGSTAT12 potentiostat controlled with GPES software. A three-elec-trode system consisting of a platinum microelectrode (5x104 cm radius),a platinum counter electrode and a chromium wire reference electrodewere used. The working electrode was polished with 0.3 mm aluminapaste, rinsed and dried prior to all measurements. Electrochemical meas-urements were performed at 60 8C and a scan rate of 20 mVs1 was usedin voltammetric experiments.

The effect of current density on chromium deposit morphology was de-termined by using a Hull Cell. Two nickel plates were abraded with glasspaper (P600), degreased with hexane and dried. Bulk electrolysis experi-ments were performed by using a Thurlby Thander power supply. Follow-ing electrolysis the cathode was washed with deionised water and dried.Surface analysis was carried out by means of scanning electron microsco-py (SEM) and energy dispersive analysis by X-rays (EDAX).

Acknowledgement

The authors would like to acknowledge the EPSRC (GR/R59199) andScionix for funding this work, and Dr. Gregor Rozenberg and Mr. ColinJohn for helpful discussions.

[1] R. A. Osteryoung, Molten Salt Chemistry, NATO ASI Series C, Vol.202, 1987, pp. 329364.

[2] J. Robinson, R. A. Osteryoung, J. Am. Chem. Soc. 1979, 101, 323327.

[3] R. A. Carpio, L. A. King, R. E. Lindstrom, J. C. Nardi, C. L. Hussey,J. Electrochem. Soc. 1979, 126, 16441650.

[4] T. P. Wier, US Patent 2446350, 1948.[5] E. Peled, E. Gileadi, J. Electrochem. Soc. 1976, 123, 1519.[6] J. Robinson, R. A. Osteryoung, J. Electrochem. Soc. , 1980, 127, 122

128.[7] C. J. Dymek, Jr., J. L. Williams, D. J. Groeger, J. J. Auborn, J. Elec-

trochem. Soc. 1984, 131, 28872892.[8] Y. Zhao, T. J. Vandernoot, Electrochim. Acta 1997, 42, 313.[9] T. Welton, Chem. Rev. 1999, 99, 20712083.

[10] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH,Weinheim (Germany), 2003.

[11] P. Wasserscheid, W. Keim Angew. Chem. 2000, 112, 39263945;Angew. Chem. Int. Ed. 2000, 39, 37723789.

[12] N. Koura, T. Endo, Y. Idemoto, J. Non-Cryst. Solids 1996, 205, 650655.

[13] L. Simanavicius, A. Stakenas, A. Starkis, Electrochim. Acta 1997, 42,15811586.

[14] Y. F. Lin, I. W. Sun, Electrochim. Acta 1999, 44, 27712777.[15] P. Chen, M. Lin, I. Sun, J. Electrochem. Soc. 2000, 147, 33503355.[16] P. Y. Chen, I. W. Sun, Electrochim. Acta 2001, 46, 11691177.[17] A. P. Abbott, G. C. Capper, D. L. Davies, H. Munro, R. Rasheed V.

Tambyrajah, Chem. Commun. 2001, 20102011.[18] A. P. Abbott, G. C. Capper, D. L. Davies, R. Rasheed V. Tambyrajah

Trans. Inst. Met. Finish. 2001, 79, 204207.[19] A. P. Abbott, G. C. Capper, D. L. Davies, R. Rasheed, Inorg. Chem.

2004, in press.[20] Spartan Pro, Wavefunction, Irvine, CA, (USA).[21] P. J. Elving, B. Zemel, J. Am. Chem. Soc. 1957, 79, 12811285.[22] J. OM. Bockris, A. K. N. Reddy, Modern Electrochemistry Vol. 1,

Plenum, New York, 1970, Chapter 6.[23] L. C. Branco, J. N. Rosa, J. J. M. Ramos, C. A. M. Afonso, Chem.

Eur. J. 2002, 8, 36713677.[24] J. A. Riddick, W. B. Bunger, T. K. Sakano, Organic Solvents: Solvent

Properties and Methods of Purification, 4th ed., Wiley, New York,1986.

[25] J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. Willauer,G. A. Brooker, R. D. Rogers, Green Chem. 2001, 3, 156164.

[26] G. J. Janz, Molten Salt Handbook, Academic Press, New York, 1967.[27] A. J. Bard, L. R. Faulkener, Electrochemical Methods, Wiley, New

York, 1980.[28] Y. Zhao, T. J. Vandernoot, Electrochim. Acta 1997, 42, 16391643.[29] The Canning Handbook on Electroplating, Canning, Birmingham,

NewYork, Spon, 1982.[30] A. P. Abbott, G. C. Capper, D. L. Davies, R. Rasheed V. Tambyrajah

Int. Pat. WO 0226701, 2001.

Received: February 9, 2004Published online: June 15, 2004

2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.chemeurj.org Chem. Eur. J. 2004, 10, 3769 37743774

FULL PAPER A. P. Abbott et al.

www.chemeurj.org