PAPER www.rsc/dalton |Dalton Transactions
Solution and solid state studies on the binding of isomeric carboranes C 2B 10H 12by p -Bu t -calix[5]arene†‡
Thomas E.Clark,Mohamed Makha,Colin L.Raston*and Alexandre N.Sobolev
Received 7th August 2006,Accepted 25th September 2006
First published as an Advance Article on the web 9th October 2006DOI:10.1039/b611399k
p -Bu t -calix[5]arene forms crystalline inclusion complexes with o -and m -carboranes in toluene or dichloromethane–hexane,but not with the p -isomer,the extended structures being bad on 1:1host–guest supermolecules,with the p -Bu t -substituents creating a snug fit for o -and m -carborane;p -carborane forms a highly hexane soluble complex,induced by grinding,which crystallizes as fibres.Solution pha studies showed the prence of a 1:1host–guest stoichiometry with all three isomeric carboranes as determined from Job plots.The association constants for the o -and m -carborane complexes are 6.4±0.3M −1and 3.8±0.1M −1respectively,whereas the p -isomer is only weakly associated.Competition experiments involving all three isomers show rapid exchange on the N
MR time scale,and no lectivity in solution is evident.Selective association involving the o -and m -isomers in the solid state is therefore remarkable,and it is a manifestation of crystal packing forces which embodies the differences in dipole moments of the carboranes.
Introduction
Calixarenes and related molecules form inclusion complexes with icosahedral carboranes,for example o -carborane (1,2-C 2B 10H 12).The complexes show a remarkable structural diversity which depends on the size of the cavity,the substituents on the calixarene and the electronic complementarity along with the choice of solvent or the prence of other solutes which are important in the crystallisation process.1In general carboranes and other globular molecules are attractive building blocks in supramolec-ular chemistry particularly in binding with receptor molecules posssing symmetrical cavities such as calix[4and 5]arenes in their preferred cone conformations.The larger calix[6and 8]arenes are more conformationally flexible and their host–guest chemistry with globular molecules is more problematic.2Calix[4]arenes have relatively small cavities and binding of globular molecules results in a perched structure with the phenyl rings splayed apart in a flattened cone conformation,for example in the binding of [2.2.2]cryptand with p -sulfonatocalix[4]arene,3and o -carborane with p -benzylcalix[4]arene.4In the ca of calix[5]arenes t
he cavity is larger,with an ability to bind o -carborane,5and larger globular fullerenes including C 606,7and C 70.8
排山倒海英文版The parent calix[5]arene,1,R =H,forms a variety of complexes with o -carborane,2(a ),all bad on the supermolecule [(o -carborane)∩(calix[5]arene)],and contain solvent molecules,other carborane molecules,or indeed another calix[5]arene in the extended structures.5The first reported inclusion complexes of carboranes were the 1:1complexes of a -,b -and c -cyclodextrin with o -carborane,and the 2:1complex of a -School of Biomedical,Biomolecular and Chemical Sciences,University of Western Australia,Crawley,W.A.,6009,Australia.E-mail:clraston@chem.uwa.edu.au;Fax:+61864881005;Tel:+61864883045
摩洛幼儿英语†The HTML version of this article has been enhanced with additional colour images.
‡Electronic supplementary information (ESI)available:Experimental details for the Job plots.See DOI:10.1039/b611399k
cyclodextrin with the same carborane.9Complexation with cy-clodextrins renders the carborane water soluble,which has also been achieved by Kusukawa and Fujita in the binding of four o -carboranes within a spectacular lf-asmbled ionic capsule,which has a large internal volume crea
ted by the interplay of (ethylenediamine)palladium(II )2+ions and four triply bridging pyridyl ligands.10Other relevant studies include the binding of o -carborane to aza-crown ethers,11and to cyclotriveratrylene,12,13which has a symmetrical hydrophobic cavity like calix[5]arene,albeit with three fold rather than five fold symmetry.Interestingly hexamethylphosphoramide (HMPA)forms 1:1complexes with o -carborane,m -carborane,2(b ),and p -carboranes,2(c ),all involving C–H ···O interactions 14which reprents the only systematic study on the supramolecular chemistry for the m -and p -isomeric C 2B 10H 12compounds.
Results and discussion
Herein we explore the host–guest chemistry of all the isomeric carborane compounds,2(a–c ),with p -Bu t -calix[5]arene,3,R =Bu t ,and find remarkable lectivity towards the icosahedral molecules,2(b )over 2(a )for the isolated complexes.The choice of calixarene,beyond its availability,relates to the size and shape of the cavity,Fig.1.Complexes of 1,with o -carborane have the guest molecule offt from the centre of the cavity,the interplay between the two components being associated with two C–H ···p interactions between the C–H groups of the carborane and two adjacent aromatic rings of the calixarene 5as non-classical hydrogen bonding involving a Coulombic interaction between a polarized C–H bond and the basic p -electrons of an aromatic ring.12Incorporating the bulky Bu t su
bstituents in the para -positions of the calixarene should circumvent the offt of the carborane relative to the centre of the calixarene cavity and bind the carborane deeper in the cavity.
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Fig.1General synthetic procedure for calix[5]arene/carborane com-plexes bad on [2(a )∩3](4and 5from toluene and CH 2Cl 2–hexane respec-tively),[2(b )∩3](6and 7from toluene and CH 2Cl 2–hexane respectively),and [2(b )∩1](8from CH 2Cl 2–hexane).
For comparison we also report (i)the formation of the m -carborane complex of 1,Fig.1,and (ii)the formation of a highly hexane soluble complex of the p -isomer with 3which crystallis as fibres.This complex forms when the components are subjected to solventless grinding conditions or by crystallisation from hexane.The lective binding of the o -and m -isomers for calixarenes,1and 3is noteworthy,and has implications in the paration of the isomers which is traditionally achieved using chromatography.15The results reprent the first comprehensive study on the binding of the three carboranes in a host–guest system.
Crystal structures
Slow evaporation of a toluene or dichloromethane (DCM)–
hexane solution of each carborane in various mole ratios afforded crystalline complexes only for the o -and m -isomers,as complexes 4–8,which are bad on the supermolecules,[2∩3]and [2∩1],Fig.1.In the ca of the p -isomer no complexation is evident with the isolation of a toluene or d
ichloromethane solvate.16The same carborane complex is obtained from toluene for o -and m -carborane using 1,2,3or additional excess equivalents of the carborane relative to the calixarene.Complexes 4–8were authenticated using single crystal X-ray diffraction data with the uniformity of samples checked by determining cell dimensions on crystals from the same preparation,and from different preparations.Structures 4–7have a condary carborane inclusion component,which interestingly has previously been noted for calix[5]arene/o -carborane inclusion complexes.5
Complexes 4–7crystallize in the hexagonal space group P 6/m ,Z =12,where one host–guest supermolecule is distributed between two different mirror planes in the asymmetric unit with some disorder of the molecular fragments and solvents,whereas complex 8crystallis in space group P 21/n ,Z =4.The toluene containing complexes 4and 6were modeled as a system of [(carborane)∩(p -Bu t -calix[5]arene)]with 1/4of water,1/12of carborane and 1/4of toluene as solvent in the asymmetric unit.Complexes 5and 7containing DCM were modeled as a system of [(carborane)∩(p -Bu t -calix[5]arene)]with one water,1/12of carborane and 1/12of DCM as solvent in the asymmetric unit.Complex 8is [(m -carborane)∩(p -H-calix[5]arene)]with a slight disorder of included DCM solvent molecule.
The key feature of all four structures is the host–guest complex with a globular molecule residing over the centre of the calixarene,Fig.2.This is in contrast to the offt arrangement in complexes of o -carborane with p -H-calix[5]arene.5In the two o -carborane structures,4and 5,the carborane is drawn significantly deeper
into the cavity by 0.2A
˚,compared to the o -carborane in the calix[5]arene structure,Fig.2,ignoring the incread depth of the cavity associated with the Bu t -substituents.This presumably aris from the snug fit of the carborane in the confines of the Bu t -substituents in attempting to gain two C–H ···p interactions.The supermolecules lie on mirror planes and for each aromatic ring there is either a C–H ···p or a B–H ···p contact.However,there is no expected difference associated with attractive C–H ···p interactions compared with B–H ···p interactions,the C–H ···p
distances (2.55(4),2.55A
˚(5)),and B–H ···p distances (2.55and 2.74A
˚(4),2.15and 2.52A ˚(5),Fig.3.In contrast there are significant differences in such distances in the o -carborane
complex of 1,having C–H ···p distances at 2.64A ˚and 2.71A ˚,and B–H ···p clo contacts at 2.79,3.08and 3.19A
˚.
5Fig.2Space filling for the host–guest complexes of p -H-calix[5]arene 5(a)and p -Bu t -calix[5]arene with o -carborane (b),as found in complex 4,showing the carborane embedded deeper into the calixarene
asia hot
cavity.
Fig.3Top and side view projections for the host–guest complexes of p -Bu t -calix[5]arene and p -H-calix[5]arene structures with o -carborane [(i),4(similarly for 5),and (ii)5]and m -carborane [(iii),6(similarly for 7),(iv),8,(v),6](dotted lines highlight the C–H ···p interactions).
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The m -carborane complexes 6–7have similar geometry of docking with the substituted calix[5]arene.The supermolecules are also on mirror planes but the carborane is disordered along the principle axis of the supermolecules such that there is no distinction between C–H ···p and B–H ···p which vary from 2.56
to 2.58A
˚,6,and 2.57,2.60A ˚,7.For the complex involving p -H-calix[5]arene with m -carborane,complex 8,the carborane resides symmetrically and deeper in the cavity relative to that of the o -carborane complex.This aris from two C–H ···p interactions arranged 1,3-relative to the aromatic rings of the calixarene in the abnce of steric effects associated
with the Bu t -substituents.The distances are 2.36and 2.63A
˚,with the B–H ···p contacts at 2.74,2.85and 3.03A
˚.In the ca of the p -carborane with 1,the formation of only one C–H ···p interaction is possible (unlike the dual effect for the other isomers with 1,and 3),and this would be associated with four less favourable B–H ···p interactions.Moreover,the fact that the carborane can form a complex with the substituted calixarene under solventless conditions is consistent with the expected more favorable energetics of interaction of C–H ···p for associated aromatic molecules,as would be expected in the ca of the toluene inclusion complex of the calixarene.Similarly the C–H ···p interaction for DCM binding with the calixarene is more energetically favoured than binding of p -carborane with its four less favourable B–H contacts.Rather the carborane forms a 1:1complex bad on the host–guest supermolecule under solventless conditions involving grinding the two components in a 1:1ratio.It is noteworthy that a single C–H ···p interaction between benzene and o -carborane lowers the total energy by 2.72kcal mol −1with
a C–H ···benzene ring centroid distance of 2.69A
˚.12In addition,a similar interaction between chloroform and the benzene ring is energetically favoure
d by 3.94kcal mol −1with a chloroform C–H ···benzene ring centroid distance of 2.46.17
Competition experiments involving mixtures of o -/m -,o -/p -,and m -/p -carborane with either 1(DCM–hexane)or 3(toluene)were investigated using IR on the isolated crystalline material;o -and m -carboranes are incorporated over p -carborane,and m -carborane is incorporated over o -carborane for both systems.This can be rationalized on the basis of the C–H ···p interactions,or lack thereof,between the carborane in the cavity of the calixarene.In the ca of p -carborane the acidic C–H protons cannot simultaneously form two C–H ···p interactions,unlike for the o -and m -isomers which can form two such interactions,either with the aromatic rings 1,2or 1,3-relative to each other (e above).In all structures 4–8the cavity bound carborane molecule esntially has a local five fold symmetry (ignoring the position of the carbon atoms)aligned with the principle axis of the calixarene.Presumably the lectivity for the m -isomer relates to a more rigid grip for the 1,3-C–H ···p interactions or could only be related to the relative solubilities of the complexes.Moreover,the m -isomer has the dipole moment more aligned with the axis of the supermolecules;dipole moments for o -and m -respectively 4.53(5)D,2.85(5)D.18
The IR spectra show a clear shift in m C–H and m B–H of the carboranes in the complexes 4–8,which is indicative of the involvement of the carborane in non-classical hydrogen bonding.Shifts for
m C–H and m B–H in the complexes relative to uncomplexed carborane are −15.3to −5.9cm −1,and −6.7to −6.0cm −1respectively.That is,there is evidence for B–H ···p and C–H ···p interactions.In contrast for the o -carborane complex of 1,where the B–H groups are pushed slightly out of the cavity (repulsion),the shift in m B–H is +17.5cm −1,whereas the m C–H is indicative of a pronounced C–H ···p interaction,−3.5cm −1.In the ca of the p -carborane complex with 3,the shifts in m C–H and m B–H are −8.9and −3.9cm −1respectively,which are consistent with complex formation.
customersSolution studies
Solution pha studies on p -Bu t -calix[5]arene with all three isomeric carboranes involved the u of Job plots associated with NMR titrations.Stock solutions (10mM)of the p -Bu t -calix[5]arene,and each carborane 2(a–c )were made in CDCl 3.Using conditions of Job’s method of continuous variation,19a ries of samples were made which contained both components,host 3and guest 2in varying amounts,but such that the volume,and concentration of the total species in solution (10mM)remained constant.1H-{11B }NMR spectra were recorded for each sample in the ries,monitoring the level of chemical shift change in the guest as the host–guest ratios were varied.The monitored chemical shifts ari from the C–H (signal 1)and B–H environment (signal 2)of each carborane (e ESI‡).
NMR titrations ud solutions prepared by mixing 84mM stock solutions of host and 4mM stock solutions of the guest to ensure constant guest concentration.To a solution of guest 2(800l L)in an NMR tube fitted with a rubber ptum,a solution of the above mixture was added via a syringe in the state quantities for all the experiments.The titration solution was added ensuring constant guest concentration throughout the experiment and 1H-{11B }MMR spectra were recorded.The chemical shift changes for the C–H (signal 1)and one B–H environment (signal 2)of the carborane were recorded and analyzed by non-linear regression methods to determine the association constants.
Job plot and NMR tiration for p -Bu t -calix[5]arene and o -carborane.Results for studies on p -Bu t -calix[5]arene and o -carborane are depicted in the Job plot and titration graphs,Fig.4.The Job plot showed a 1:1host–guest stoichiometry and titration experiments gave the association constants of 6.4±0.3M −1.Job plot and NMR tiration for p -Bu t -calix[5]arene and m -carborane.The results for p -Bu t -calix[5]arene and m -carborane are depicted in the Job plot and titration graphs,Fig.5.The Job plot showed a 1:1host–guest stoichiometry and titration experiments gave the association constants of 3.8±0.1M −1.Job plot and NMR tiration for p -Bu t -calix[5]arene and p -carborane.Fig.6shows the Job plot for the p -carborane which is consistent with the 1:1host–guest stoichiometry,but the binding is relatively weak.
Preferential binding of the carborane isomers was also studied and competition experiments involving all three isomers show rapid exchange on the NMR time scale,and thus no lectivity in solution is evident.The C–H of o -carborane,m -carborane and p -carborane showed chemical shifts of 0.01to 0.2ppm indicating that all three species are bound at the same time in solution.
D o w n l o a d e d o n 09 S e p t e m b e r 2011P u b l i s h e d o n 09 O c t o b e r 2006 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 611399K
Fig.4Job plot showing a 1:1host–guest stoichiometry of p -Bu t -calix[5]arene and o -carborane in CDCl 3and titration experiments afford-ing an association constant of 6.4±0.3M −1.
Conclusions
We have established lectivity at various levels for the three isomeric carboranes,using either p -H-calix[5]arene or its p -Bu t -substituted analogue,and also the ability to form a complex under solventless conditions,for which conventional solution chemistry fails.Solution studies are consistent with the solid state results showing the preferential binding of the isomeric carboranes at least for p -Bu t -calix[5]arene.The results have implications in fine tuning the molecular recognition of other globular molecules capable of forming p -interactions with the walls of the calixarene.
Experimental
Crystallography
Crystals were grown from a solution of equimolar quantities of the two components in a minimum of toluene,or a mixture of 1:1hexane–dichloromethane.All structures were measured at 153K on a Bruker ASX CCD diffractometer using monochromatized国庆节英语手抄报图片
babel
温州少儿英语提高班Mo-K a radiaiton(k =0.71073A
˚.)Data were corrected for Lorentz and polarization effects and absorption correction applied using multiple symmetry equivalent reflections.The structures were solved by direct methods and refined on F 2using the SHELX97crystallographic package.
right here waiting for you
20,21
Fig.5Job plot for the 1:1host–guest stoichiometry of p -Bu t -calix[5]arene and m -carborane and titration experiment affording the association constant of 3.8±0.1M −1
.
Fig.6Job plot showing a 1:1host–guest stoichiometry of p -Bu t -calix[5]arene and p -carborane in CDCl 3.
Complex 4.[2(a )∩(p -Bu t -calix[5]arene)]·[2(a )]0.0833·(H 2O)0.25·(toluene)0.25,C 58.92H 85.5B 10.83O 5.25,M =994.88,F (000)=6422e ,
hexagonal,P 6/m ,Z =12,a =35.416(4),c =18.021(2)A
˚,V =19575(3)A
˚3;D c =1.013Mg m −3;l =0.060mm −1;sin h /k max =0.5969;N (unique)=11975(merged from 143969,R int =0.095,R sig =0.048),N o (I >2r (I ))=6417;R =0.1171,wR 2=0.2734
(A ,B =0.08,110.0),GOF =1.036;|D q max |=1.2(1)e A
˚−3.D o w n l o a d e d o n 09 S e p t e m b e r 2011P u b l i s h e d o n 09 O c t o b e r 2006 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 611399K
effection
Complex 5.[2(a )∩(p -Bu t -calix[5]arene)]·[2(a )]0.0833·(H 2O)·(CH 2Cl 2)0.0833,C 57.25H 85.17B 10.83Cl 10.17O 6,M =992.44,F (000)=
6404e ,hexagonal,P 6/m ,Z =12,a =35.477(3),c =17.920(2)A
˚,V =19533(3)A
˚3;D c =1.012Mg m −3;l =0.067mm −1;sin h /k max =0.5946;N (unique)=11666(merged from 125693,R int =0.068,R sig =0.041),N o (I >2r (I ))=6485;R =0.1089,wR 2=0.2820
(A ,B =0.19,47.5),GOF =1.024;|D q max |=1.2(1)e A
˚−3.Complex 6.[2(b )∩(p -Bu t -calix[5]arene)]·[2(b )]0.0833·(H 2O)0.25·
(toluene)0.25,C 58.92H 85.5B 10.83O 5.25,M =994.88,F (000)=6422e ,
hexagonal,P 6/m ,Z =12,a =35.444(9),c =17.990(5)A
˚,V =19573(7)A
˚3;D c =1.013Mg m −3;l =0.060mm −1;sin h /k max =0.5955;N (unique)=11728(merged from 122623,
R int =0.246,R sig =0.161),N o (I >2r (I ))=4487;R =0.1341,wR 2=0.3179
(A ,B =0.20,91.9),GOF =1.033;|D q max |=0.9(1)e A
˚−3.Complex 7.[2(b )∩(p -Bu t -calix[5]arene)]·[2(b )]0.0833·(H 2O)·
(CH 2Cl 2)0.0833,C 57.25H 85.17B 10.83Cl 10.17O 6,M =992.44,F (000)=
6404e ,hexagonal,P 6/m ,Z =12,a =35.564(3),c =17.908(2)A
˚,V =19615(3)A
˚3;D c =1.008Mg m −3;l =0.067mm −1;sin h /k max =0.5946;N (unique)=11821(merged from 80946,R int =0.222,R sig =0.182),N o (I >2r (I ))=4088;R =0.1080,wR 2=0.2564
(A ,B =0.15,55.0),GOF =1.002;|D q max |=0.61(8)e A
˚−3.Complex 8.[2(b )∩(p -H-calix[5]arene)]·(CH 2Cl 2),C 38H 44B 10-Cl 2O 5,M =759.73,F (000)=1584e ,monoclinic,P 21/n ,Z =4,a =12.425(1),b =12.008(1),c =26.694(2)A
˚,b =98.814(1)◦,V =3935.7(5)A
˚3;D c =1.282Mg m −3;l =0.208mm −1;sin h /k max =0.5955;N (unique)=6763(merged from 24364,R int =0.025,R sig =0.026),N o (I >2r (I ))=5275;R =0.0450,wR 2=0.1157(A ,B =
0.06,3.46),GOF =1.021;|D q max |=0.76(5)e A
˚−3.All structures were solved and refined using the SHELX-97suite of programs 20and the X-Seed 21interface.CCDC reference numbers 606110–606114.
For crystallographic data in CIF or other electronic format e DOI:10.1039/b611399k NMR studies
All spectra were run on a DRX 300MHz spectrometer using 99.8%chloroform-d 1and the general procedure for Job plots 19was bad on 10mM solutions of host and guest prepared in CDCl 3and mixed in the stated ratios.1H-{11B }NMR spectra of the mixtures were recorded and the chemical shift changes for the C–H (signal 1)and one B–H environment (signal 2)of the carborane were analyzed by Job’s method to determine the
host–guest stoichiometry.NMR titrations for determining binding constants ud 800l L of a 4mM solution of the carborane in CDCl 3which was added to an NMR tube fitted with a rubber ptum.The titration solution (84mM)was added via syringe in the stated quantities and ensuring con
stant guest concentration throughout the experiment.The total volume was kept constant for each experiment and 1H-{11B }MMR spectra were recorded.The chemical shift changes for the C–H (signal 1)and one B–H environment (signal 2)of the carborane were recorded and analyzed by non-linear regression methods to determine the association constants (e ESI‡).
References
1J.L.Atwood,L.J.Barbour and C.L.Raston,Cryst.Growth Des.,2002,2,3–6;C.L.Raston and G.W .V .Cave,Chem.–Eur.J.,2004,10,279–282;M.J.Hardie,C.L.Raston and B.Wells,Chem.–Eur.J.,2000,6,3293–3298.
2C.D.Gutsche,in Calixarenes ,Royal Society of Chemistry,Cambridge,1989;S.J.Dalgarno,M.J.Hardie,M.Makha and C.L.Raston,Chem.–Eur.J.,2003,9,2834–2839;M.Makha,A.N.Sobolev and C.L.Raston,Chem.Commun.,2006,511–513.
3S.J.Dalgarno and C.L.Raston,Chem.Commun.,2002,2216–2217.4M.Makha,C.L.Raston,A.N.Sobolev,L.J.Barbour and P .Turner,CrystEngComm ,2006,8,306–308.
5M.J.Hardie and C.L.Raston,Eur.J.Inorg.Chem.,1999,195–200.6J.L.Atwood,L.J.Barbour,M.W .Heaven and C.L.Raston,Angew.Chem.,Int.Ed.,2003,42,3254–3257.
7R.Haino,M.Yana and Y .Fukazawa,Angew.Chem.,Int.Ed.,1998,37,997–998.
8J.L.Atwood,L.J.Barbour,M.W .Heaven and C.L.Raston,Chem.Commun.,2003,2270–2271.
9A.Harada and S.Takahashi,J.Chem.Soc.,Chem.Commun.,1988,1352–1353.
10T.Kusukawa and M.Fujita,Angew.Chem.,Int.Ed.,1998,37,3142–3144.
11P .D.Godfrey,W .J.Grigsby,P .J.Nichols and C.L.Raston,J.Am.Chem.Soc.,1997,119,9283–9284.
12R.J.Blanch,M.Williams,G.D.Fallon,M.G.Gardiner,R.Kaddour and C.L.Raston,Angew.Chem.,Int.Ed.Engl.,1997,36,504–506.13M.J.Hardie,P .D.Godfrey and C.L.Raston,Chem.–Eur.J.,1999,5,1828–1833.
14M.J.Davidson,T.G.Hibbert,A.K.Howard,A.Mackinnon and K.Wade,Chem.Commun.,1996,2285–2286.
15R.N.Grimes,in Carboranes ,Academic Press,New Y ork,1970.
16T.E.Clark,M.Makha,C.L.Raston and A.N.Sobolev,unpublished results.
17W .L.Jorgenn and D.L.Severance,J.Am.Chem.Soc.,1990,112,4768–4774.
18A.W .Laubengayer and W .R.Rysz,Inorg.Chem.,1965,4,1513–1514.19P .Job,Ann.Chim.Appl.,1928,9,113–203.
20M.Sheldrick,SHELX-97:Structure solution and refinement programs ,University of G¨ottingen,Germany,1997.leoni
21L.J.Barbour,J.Supramol.Chem.,2001,1,189.
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