stimulated by the promise of mechanically interlocked molecular architectures with potential employment in future nanotechnological applications such as in the development of molecular machines and switches , the interest being shown in their construction is ever increasing .
rotaxane and catenane species can also , however , be designed to function as selective host systems whereby the topologically unique interlocked three - dimensional cavities are exploited to selectively recognise specific guest species . in previous work , we have utilised anion templation to construct a range of rotaxanes and catenanes , which , upon removal of the template , bind anions strongly and selectively in competitive solvent mixtures .
furthermore , selective anion binding can be exploited to bring about triggered motion within the interlocked supramolecular architectures , underlining the possible application of these systems in the preparation of molecular switches . to sense and monitor anion binding ( and/or binding - triggered motion ) , it is desirable to integrate a redox- or photoactive reporter group in proximity to the interlocked anion - binding site .
examples of interlocked hosts with the capability of sensing anions by electrochemical or optical[3b , 4 , 6 ] methods are rare .
herein , we report the preparation of the first anion - templated rotaxane incorporating an optically- and electro - active osmium(ii ) tris(bipyridine ) reporter group . after removal of the anion template
, selective anion binding is investigated by monitoring the optical and electronic output from the osmium(ii ) tris(bipyridine ) reporter moiety . to develop and fabricate a prototype molecular sensory system ,
surface association removes complications associated with brownian motion , typically increases conformational rigidity , and will underpin potential applications of such molecular architectures and , ultimately , device integration.[1b ] to date , only a limited number of surface - bound interlocked structures have been reported.[5b , c , 7 ] hence , we additionally report the anion - templated assembly of these electrochemically active osmium(ii ) bipyridyl rotaxane structures on gold substrates and , after template removal , the specific reporting of anion recruitment from solution ( figure 1 ) .
schematic representation of the recognition and sensing of anions for a surface - bound rotaxane .
axle in blue , macrocycle in green , with anion binding functionalities shown in red .
design of the system : the target design for the mechanically bonded host features the incorporation of the osmium(ii ) tris(bipyridine ) reporter group into the macrocyclic component of a rotaxane molecular framework containing a convergent hydrogen - bond - donor anion - binding interlocked cavity .
the macrocycle contains the 4,4-bis(amide)-2,2-bipyridyl motif for coordination to the osmium(ii ) metal centre and electron - rich hydroquinone units to facilitate supplementary secondary aromatic donor  acceptor interactions with the electron - deficient positively charged pyridinium axle.[3a ] the choice of the osmium(ii ) bipyridyl reporter group comes from its established electro- and photochemical properties , making the system an attractive probe to sense the anion - binding event .
synthesis of the rotaxanes : three distinct osmium(ii ) bipyridyl ( bipy ) rotaxanes , 13 , were prepared by clipping and stoppering anion - templated synthetic methodologies , comprising the same macrocycle component and different axle lengths ( scheme 1 ) .
rotaxane 1 was obtained in two steps by a chloride - anion - templated  clipping  reaction between axle[3a ] components bis(amine ) 8 and 4,4-bis(chlorocarbonyl)-2,2-bipyridine 9 in the presence of et3n in dry dichloromethane ( scheme 2 and scheme s1 in the supporting information ) .
the crude metal - free rotaxane intermediate was then treated with [ os(bipy)2cl2 ] , 16 , in an etoh / h2o mixture at reflux to give , after anion exchange using 0.1 m aqueous nh4pf6 , rotaxane 1 in 6 % overall yield .
macrocycle 19 was synthesised in 37 % yield by condensation of the bis(amine ) 8 with bis(acid chloride ) 9 in the presence of et3n and template 17 in dry dichloromethane .
reaction of macrocycle 19 with 16 in an etoh / h2o mixture at reflux afforded , after anion exchange , macrocycle 4 in 74 % yield .
the reaction of carboxy  terphenyl amide pyridine derivative 10[3f ] with oxalyl chloride produced the corresponding acid chloride , which upon condensation with 3-bromopropylamine hydrobromide in the presence of et3n in dry dichloromethane gave bis(amide ) 11 .
this was converted to its azide derivative 12 by reaction with sodium azide in dimethylformamide .
reaction of compound 12 with methyl iodide at reflux , followed by anion exchange using 0.1 m aqueous nh4pf6 , gave the desired axle precursor 13 .
an analogous condensation reaction between the acyl chloride derivative of 10 and 4-(azidomethyl)biphenyl-4-yl - methanamine , followed by methylation and anion exchange , produced axle precursor 14 ( scheme 2 ) .
rotaxanes 2 and 3 were obtained in 16 and 72 % yield , respectively , by  mono stoppering 
reactions followed by washing with nh4pf6/h2o to remove the chloride template ( see the supporting information , scheme s1 ) .
axle precursors 13 and 14 were added to macrocycle 4 leading to chloride - anion - templated pseudo - rotaxane assembly .
alkyne cycloaddition ( cuaac ) reactions in dry dichloromethane using alkyne stopper 15 , copper(i ) tetrakis(acetonitrile ) hexafluorophosphate , tris(benzyltriazolylmethyl)amine ( tbta ) and diisopropylethylamine afforded the desired rotaxanes 2 and 3 .
rotaxanes 13 were characterised by nmr spectroscopy ( h , c , f and p ) and by mass spectrometry ( maldi - tof ) .
h nmr spectra of rotaxanes 13 ( figure 2 ) reveal splitting and an upfield shift of hydroquinone protons of the macrocycle component , owing to aromatic donor  acceptor interactions between the electron - rich hydroquinone groups and the electron - deficient pyridinium moiety of the axle , characteristic of an interlocked structure .
h nmr spectra ( 500 mhz , [ d6]acetone / d2o ( 7:3 ) , 293 k ) of rotaxanes 1 ( a ) , 2 ( c ) , 3 ( e ) and their chloride complexes 1 cl ( b ) , 2 cl ( d ) , and 3 cl ( f ) after addition of one equivalent of chloride .
anion - binding studies in solution : anion binding was probed by using nmr , luminescence and electrochemical methods in assorted solvents as dictated by various factors .
these techniques all have dramatically different limits of detection and different requirements from the system . in the case of nmr titration ,
initial assessments in acetonitrile revealed strong association with anions , compounded by the low solubility of the generated complexes , a fact that necessitated the use of a more competitive aqueous solvent mixture , [ d6]acetone / d2o ( 7:3 ) .
luminescence spectroscopy , offering much lower detection limits , presents a quantum yield weighted average ; as a consequence , the response of the observed luminescence to anion concentrations varies between solvents .
this is particularly true in the case of osmium tris(bipy ) complexes , in which solvation of the excited state plays an important role in determining the form of the spectrum . small quantities of water were added to the acetonitrile mixtures ; it was found that the addition of 3 % water produced optimal and reproducible changes to the osmium emission spectrum . in the case of electrochemistry , the need for a large potential window to resolve bipy - ligand - based voltammetry and the use of relatively concentrated electrolytes dictated the use of acetonitrile as a solvent system .
h nmr investigations : preliminary h nmr anion titration experiments between rotaxanes 1 , 2 and 3 and various anions ( cl , aco and h2po4 ) were performed in competitive [
the addition of one equivalent of tetrabutylammonium ( tba ) chloride resulted in a downfield shift of inner protons  ( 0.13 , 0.05 and 0.29 ppm ) and c ( 0.10 , 0.05 and 0.34 ppm ) for rotaxanes 1 , 2 and 3 , respectively , indicative of halide binding within the rotaxane s interlocked binding cavity ( figure 2 ) .
in addition , modest upfield perturbations of the macrocyclic hydroquinone protons of the respective rotaxane were observed , g ( 0.02 , 0.02 and 0.06 ppm ) and h ( 0.03 , 0.02 and 0.03 ppm ) for 1 , 2 and 3 , respectively . when oxoanions such as acetate or dihydrogen phosphate where added , only a small perturbation was observed for inner protons . in the case of rotaxane 3 , the  protons move upfield , an indication of the oxoanion binding on the periphery of the rotaxane .
wineqnmr2 analysis of the binding isotherms with chloride and acetate anions , obtained by monitoring the chemical - shift perturbation of proton  of the axle component of the respective rotaxane versus equivalent of anion ( figure 3 ) , determined 1:1 stoichiometric association constants ( table 1 ) .
it proved impossible to obtain quantitative binding data from any of the dihydrogen phosphate titration experiments , indicative of a complex equilibrium binding process ( chemical shifts observed were too small , association constants < 100 m , to be sensibly fitted within error ) .
a ) chemical - shift perturbation of proton  upon addition of cl (  ) , aco (  ) and h2po4 (  ) ( as the tba salt ) to a solution of   rotaxane 1 ( unfilled ) , 2 ( half - filled ) and 3 ( filled ) in 7:3 [ d6]acetone / d2o at 293 k. b ) chemical - shift perturbation of proton c upon addition of cl ( as the tba salt ) to a solution of macrocycle 4 in 7:3 [ d6]acetone / d2o (  ) and in 9:1 [ d6]acetone / d2o (  ) at 293 k. symbols represent experimental data ; continuous lines represent calculated curves .
association constants [ m ] of rotaxanes 1 , 2 and 3 in 7:3 acetone / d2o and macrocycle 4 in 9:1 acetone / d2o with cl and aco ( as tba salts ) at 293 k [ a ] obtained by monitoring proton . error < 15 % ( for all values ) .
[ b ] determined by monitoring proton c. error < 10 % . [ c ] in 9:1 acetone / d2o .
the results indicate selectivity towards cl , with only weak and peripheral association with aco and h2po4 .
the oxoanions are too large to penetrate the interlocked binding domain , whereas chloride anions are of complementary size and shape leading to strong binding even in this competitive 30 % aqueous solvent mixture . comparing the strength of chloride anion binding for the three rotaxanes , the interlocked host 3 , containing a rigid biphenyl axle linkage , exhibits significantly stronger binding .
this may reflect the optimal degree of preorganisation of the interlocked binding domain of the respective rotaxane as determined by the nature of the axle component .
the relatively more preorganised binding sites of rotaxanes 1 and 3 , as compared with the flexible propyl - linked axle component present in 2 , potentially enhances chloride binding .
steric congestion could be responsible for 1 being a relatively inferior halide - complexing reagent in comparison with 3 .
the binding of chloride anions by macrocycle 4 was also investigated ( table 1 and figure 3 b ) .
as expected , the binding is very weak in 7:3 acetone / water , as shown by the small downfield shift of protons c , 0.03 ppm after addition of one equivalent of chloride .
the h nmr titration experiment was repeated in 9:1 acetone / water , a less competitive solvent mixture , which enabled a 1:1 stoichiometric association constant of 210 m to be determined .
analogous titrations with acetate anions revealed only small perturbations of the macrocycle proton , indicative of weak binding .
these observations serve to highlight the rotaxanes potency for chloride anion recognition as a consequence of their three - dimensional binding cavities being capable of encapsulation of this guest .
macrocycle 4 and the rotaxanes ( 13 ) all display two emission bands in the near - ir region centred at approximately 800 and 925 nm ( figure 4 ) following excitation into metal - to - ligand charge transfer ( mlct ) absorption bands at 430 nm , with the lifetimes of the two bands determined to be 44 and 35 ns at 800 and 950 nm , respectively .
a ) titration of rotaxane 1 with tbacl in 97:3 acetonitrile / water , showing emission spectra uncorrected for detector sensitivity ; (  ) increasing chloride anion concentration .
b ) binding isotherms and fits from the same titration following the two emission bands .
these observations are consistent with those of nozaki and co - workers , who assigned the presence of two bands in the emission spectra of osmium bipyridyl complexes to subtle variations in the osmium environment arising from solvation - induced polarisation of the triplet state . upon titration with tbacl in 97:3 acetonitrile / water ,
an increase in emission intensity was observed for all four species , with the increases being more pronounced for the higher - energy bands .
given the conclusions of nozaki regarding solvent - induced distortions to the triplet - state structure , it is plausible that anion binding will also influence local structure to differing degrees .
representative spectra are shown in figure 4 along with the corresponding binding isotherms and fits .
association constants were obtained with the dynafit software using a 1:1 binding model ( table 2 ) .
association constants [ m ] determined by luminescence titration of macrocycle 4 and rotaxanes 1 , 2 and 3 in 97:3 acetonitrile / water , with chloride , dihydrogen phosphate and acetate anions , as tba salts [ a ] 99 % confidence intervals given in square brackets . a 1:1 binding model
acetate and dihydrogen phosphate ( as their tba salts ) were also titrated with rotaxane 1 and the macrocycle 4 for comparison , although with the latter , phosphate anion binding could not be determined owing to precipitation .
the data indicate that the binding of chloride anions with the rotaxanes is two orders of magnitude greater than that with the macrocycle .
the chloride anion association constants obtained from luminescence measurements for the three rotaxanes confirm the observations from nmr spectroscopy of strong binding of chloride ions by rotaxane 3 .
it should be noted that the change in solvent system will also be reflected in differences in binding owing to different solvation of the host . in this relatively uncompetitive solvent system , it is likely that many anions will associate with the cationic receptors ; indeed , studies on dihydrogen phosphate and acetate bear out this hypothesis .
it is important to point out that it is likely such optical titrations are notably less reflective of binding selectivity in that they report anion association , be it cavity confined or peripheral .
cyclic voltammetric scans ( figure 5 ) of an acetonitrile solution containing macrocycle 4 revealed an electrochemical redox system at 0.605 v vs. ag / ag , assigned to the reversible os(+2/+3 ) redox couple .
in addition , the three bipyridyl - ligand - centred redox systems were observed at 1.375 , 1.715 and 1.975 v vs. ag / ag ( labelled x , y , z , respectively ) consistent with previous reports of a tris(bipy ) ruthenium moiety .
these assignments were confirmed by equivalent integration of the charges associated with the os(+2/+3 ) redox system and the bipyridyl - ligand - centred redox events , as expected as all are one - electron redox systems on the same species . in accordance with our previous report , the least - cathodic bipy redox couple ( x , figure 5 )
is assigned to the amide - substituted bipyridyl , located next to the anion - binding site ( in light of the electron - withdrawing nature of the carbonyl amide moieties ) .
cyclic voltammetric scans of 0.2 mm of macrocycle 4 in 0.15 m tbapf6/ch3cn ; scan rate=0.1 vs. insert : electrochemical titrations of 4 with tbacl monitoring bipy couples x , y and z. titrations of macrocycle 4 with tbacl or tbaoac were carried out to determine the ability of ligand- and metal - centre - based redox potentials to report on anion association . as shown in figure 6 a , cathodic perturbations of approximately 20 mv and 40 mv of the metal - centred osmium - based ( + 2/+3 ) couple are observed upon the addition of 5 molar equivalents of tbacl and tbaoac , respectively .
the bipy - ligand - centred responses scale in accordance with their vicinity to the anion - association site , with x showing the strongest perturbations and z the weakest , in the presence of 5-fold excess of cl and aco ( see insert figure 5 , schemes s2 and s3 in the supporting information ) .
these observations are fully consistent with both anionic species being bound in the vicinity of the bis(amide ) bipy cavity .
dynafit modelling analysis of a simultaneous fit of both the metal - centred os(+2/+3 ) couple and the least - cathodic ligand - centred ( x ) potential perturbations as a function of anion concentration gave 1:1 stoichiometric association constants of 2.510 [ 2.22.710 ] and 1.510 [ 1.12.110 ] m , for chloride and acetate anions , respectively .
the larger association constant ( and potential shifts ) observed after addition of tbaoac , compared with tbacl , can be attributed to the stronger association of aco to the amide - based binding site than cl , owing to the relative basicities of the anions .
e1/2 of a ) os(+2/+3 ) and b ) bipy couple versus equivalents of tbacl (  ) and tbaaco (  ) added .
we assign the higher luminescence and electrochemically defined association constants , in comparison to those obtained by nmr spectroscopy , to the more competitive aqueous solvent system used for the latter , but we acknowledge also that optical and redox transitions may not be exactly equivalent in their response to specific anion association . indeed , optical transition energies are likely to be sensitive to broad electrostatic / dielectric change in the vicinity of relevant chromophore orbitals ; redox signatures are more specifically sensitive to electron density at the bipyridyl and osmium centres and , hence , may be more reflective of specific anion association .
resolved fundamental electrochemical characteristics of the rotaxane hosts are similar to those of the macrocycle and are reported in the supporting information ( page 4 ) .
unfortunately , a combination of sluggish diffusion and strong physical - adsorption - based electrode fouling precluded the attainment of reliable electrochemical anion - binding data with the rotaxanes .
surface immobilisation of the rotaxane : the cui - catalysed huisgen cycloaddition of axles 13 and 14 to prepared alkyne - terminated molecular films on gold ( see the experimental section ) in the presence of templating chloride anions and a 5-fold excess of macrocycle 4 , generates surface - confined rotaxane assemblies ( scheme 3 ) .
accompanying ellipsometry - defined increases in film thickness ( ( 1.30.1 ) nm ) are consistent with axle - to - surface orientations being normal or close to normal .
control surface analyses , in the absence of cui catalyst , confirm this surface assembly to be specifically click - chemistry driven .
the associated template locking of the osmium bipy macrocycle at the surface by this process is confirmed by resolved osmium - based ( + 2/+3 ) electrochemical signatures ( figure 7 ) .
an integration of these signals resolves surface macrocycle concentrations of 1.05 and 2.510 mol cm for axles 13 and 14 , respectively , corresponding to 20 and 45 % of a densely packed monolayer , based on a macrocycle footprint of 3.0 nm .
surface association is both stable to ultrasonic washing and only observed with the chloride anion template specifically involved ( importantly , no macrocycle - osmium - based ( + 2/+3 ) electrochemical response is observed if the pf6 salts of 13 or 14 are used ) .
the linear scaling of os(+2/+3 ) redox peak current with voltage scan rate is further confirmatory of surface entrapment .
the surface - density differences between axle - only and rotaxane films are , additionally , resolvable through both electrode access of a solution - phase faradaic redox probe and surface - capacitance analysis ; an axle - only film being significantly more densely packed ( 10 f cm ) than a rotaxane film with a  mushroom - shaped  component ( 25 and 15 f cm for surfaces prepared with 13 and 14 , respectively ; scheme s4 in the supporting information ) .
estimated surface concentration as 50 % of theoretical maximum ( calculated from a molecular footprint of the macrocycle , 3.0 nm ) .
distance of faradaic exchange estimated at approximately 2 nm based on localisation of the macrocycle at the pyridinium station and a perpendicular orientation of the axle to the underlying gold surface .
insert : potential shifts of surface - bound os(+2/+3 ) couple , after immersion of rotaxane interface in tbaaco / ch3cn and tbacl / ch3cn solutions following washing with nh4pf6/h2o to remove cl template .
it is notable that axle 14 generates a significantly higher ( > 200 % ) macrocycle capture on the surface than that observed with axle 13 , consistent with both increased axle rigidity and a higher associated chloride anion template association constant ( table 1 ) .
these films constitute not only an addition to the few surface - confined tethered interlocked systems but , to the best of our knowledge , are also the first incorporating a redox - active osmium(ii ) bipy reporter motif .
we have also investigated the possibility that surface - presented vacant interlocked cavities ( generated by using axle 14 ) can provide a means of selectively recruiting and electrochemically sensing chloride anions .
the chloride anion template was initially removed by washing with copious amounts of 0.1 m nh4pf6/h2o .
pleasingly , subsequent halide anion recruitment is both detectable and selective , exhibiting no cathodic perturbation upon immersion in a 50 m solution of acetate anions ( the observed 2 mv shift is less than the 3 mv error of the reference electrode ) , but a 14 mv cathodic shift upon immersion in a chloride - anion - containing solution of the same concentration(with both solutions being formed from the tba salt in anhydrous ch3cn , figure 7 ) .
no potential shift was observed when the cavity was not depopulated prior to immersion in the chloride solution , as expected for a cavity exhibiting 1:1 binding stoichiometry .
these results are consistent with those presented in table 1 and confirm the selectivity of the three - dimensional cavity towards chloride anion binding .
furthermore , the smaller potential shift observed here upon binding of chloride anions , in comparison with that observed of the macrocycle , is attributed to the electron - withdrawing nature of the positively charged pyridinium motif of the axle component , mitigating donation of electron density to the osmium centre , and is indicative of the interlocked surface - bound structure .
interlocked structures can be engineered to bind specific guests within the topologically constrained three - dimensional cavities created during their template - driven syntheses .
this binding ability , when coupled to the signal - transduction capabilities associated with appended reporter groups , make catenanes and rotaxanes highly promising candidates for the development of molecular sensors .
we have reported herein the solution and surface synthesis of a number of anion - templated rotaxanes incorporating the photo- and electroactive tris(bipy ) osmium moiety . upon removal of the chloride anion template ,
h nmr titration of the solution rotaxanes with a number of anions ( chosen on the basis of their contrasting size , shape and basicity ) showed selectivity for chloride anions over acetate anions and dihydrogen phosphate oxoanions , observations broadly supported by associated perturbations in osmium luminescence or redox character .
rotaxane immobilisation onto an alkyne - modified gold electrode substrate by copper(i)-catalysed huisgen cycloaddition produced molecular films capable of responding electrochemically and selectively to chloride anions .
this work clearly demonstrates the successful application of self - assembled monolayers to the electrochemical sensing of halide ions .
general procedure : commercially available solvents and chemicals were used without further purification unless otherwise stated .
where dry solvents were used , they were degassed with nitrogen , dried by passing through an mbraun mpsp-800 column and then used immediately .
h , c , f and p nmr spectra were recorded on a varian mercury - vx 300 , and a bruker avii500 spectrometer .
synthesis : macrocycle 19,[6b ] axle 5,[3a ] bis(amine ) 8 , thread 17,[3a ] and axle 14 have been synthesized according to reported procedures .
the syntheses of 11 , 12 and 13 along with electrochemical , surface analysis and luminescence anion titration details are given in the supporting information .
rotaxane 1 : in a 50 ml round - bottom flask , 2,2-bipyridine-4,4-dicarboxylic acid ( 60 mg , 245 mol , 1.2 equiv ) was suspended in 10 ml of thionyl chloride , a drop of dmf was added and the solution was heated at reflux under n2 for 16 h. after removal of the solvent , the residue was dissolved in 10 ml of dry dichloromethane and added dropwise to a 50 ml dry dichloromethane solution containing bis(amine ) 8 ( 86 mg , 204 mol , 1 equiv ) , axle 5 ( 220 mg , 204 mol , 1 equiv ) and triethylamine ( 71 l , 510 mol , 2.5 equiv ) .
the reaction mixture was stirred at room temperature ( 213 c ) for 2 h , washed with 10 % hcl(aq . )
after removal of the solvent , the residue was dissolved in 10 ml of acetonitrile , filtered and solvent removed in vacuo to give 121 mg of a crude mixture containing rotaxane and macrocycle 19 . this crude material ( 121 mg , 71 mol , 1 equiv ) was suspended in 20 ml of a 4:1 etoh / h2o mixture and [ os(bipy)2cl2 ] 16 ( 82 mg , 142 mol 2 equiv ) was added .
after removal of the solvent , the crude product was purified by preparative tlc ( sio2 , ch3cn / h2o / kno3(sat .
rotaxane 1 ( 30 mg , 6 % ) was obtained as a brown solid after anion exchange to the hexafluorophosphate salt by washing a dichloromethane solution of the rotaxane with 0.1 m nh4pf6(aq . )
h nmr ( 500 mhz , [ d6]acetone / d2o ( 7:3 ) ) : =9.40 ( 2 h , s ; py ) , 9.33 ( 1 h , s ; py ) , 9.29 ( 2 h , s ; arhc ) , 8.75 ( 2 h , d , j=8.4 hz ; bipy ) , 8.71 ( 2 h , d , j=8.4 hz ; bipy ) , 8.05 ( 2 h , d , j=6.2 hz ; arha ) , 8.02 ( 2 h , t , j=8.1 hz ; bipy ) , 7.90 ( 2 h , t , j=8.1 hz ; bipy ) , 7.78 ( 2 h , d , j=5.6 hz ; bipy ) , 7.71 ( 2 h , d , j=5.6 hz ; bipy ) , 7.66 ( 2 h , dd , j=6.1.4 hz , j=1.7 hz ; arhb ) , 7.59 ( 4 h , s , j=8.9 hz ; arh ) , 7.46 ( 2 h , t , j=6.7 hz ; bipy ) , 7.067.27 ( 28 h , m ; arhstopper and bipy ) , 6.50 ( 4 h , m , j=8.9 hz ; arhg ) , 6.39 ( 4 h , m , j=9.1 hz ; arhh ) , 4.61 ( 3 h , s ; h ) , 3.92 ( 4 h , m ; ch2 ) , 3.86 ( 4 h , m ; ch2 ) , 3.79 ( 4 h , m ; ch2 ) , 3.77 ( 4 h , s ; ch2 ) , 3.54 ( 4 h , m ; ch2 ) , 1.18 ppm ( 36 h , s ; tbuh ) ; c nmr ( 125 mhz , [ d6]acetone ) : =164.0 , 160.7 , 159.7 , 159.5 , 152.4 , 151.7 , 149.5 , 147.9 , 144.7 , 138.9 , 138.8 , 132.3 , 131.7 , 131.4 , 129.4 , 129.3 , 128.4 , 127.4 , 126.9 , 125.7 , 125.7 , 125.3 , 123.2 , 121.1 , 116.5 , 115.4 , 71.4 , 70.9 , 67.5 , 64.7 , 40.4 , 34.9 , 31.7 ppm ; f nmr ( 282.5 mhz , [ d6]acetone ) : =72.4 ppm ( d , j=707 hz ; pf6 ) ; p nmr ( 121.6 mhz , [ d6]acetone ) : =144.3 ppm ( sept , j=707 hz ; pf6 ) ; ms ( maldi - tof ) : m / z calcd for [ c128h130f18n11o10osp3 ] [ mh3(pf6 ) ] : 1085.98 ; found : 1085.98 .
rotaxane 2 : in a 50 ml round - bottom flask , osmium macrocycle 4 ( 100 mg , 70 mol , 2 equiv ) and thread 13 ( 26 mg , 35 mol , 1 equiv ) were slowly stirred for 1 h in 25 ml of dry dichloromethane .
after removal of the solvent , the residue was dissolved in 25 ml of dry and degassed dichloromethane .
stopper 15 ( 19 mg , 35 mol , 1 equiv ) , cu(ch3cn)4pf6 ( 2.6 mg , 7 mol , 0.2 equiv ) , tbta ( 3.7 mg , 7 mol , 0.2 equiv ) and diisopropylethylamine ( dipea ) ( 12 l , 70 mol , 2 equiv ) were successively added and the reaction mixture stirred at room temperature ( 213 c ) for 3 d. then , 10 ml of kno3(sat .
after removal of the solvent , the crude product was purified by preparative tlc ( sio2 , ch3cn / h2o / kno3(sat .
rotaxane 2 ( 16 mg , 16 % ) was obtained as a brown solid after anion exchange to the hexafluorophosphate salt by washing a chloroform solution of the rotaxane with 0.1 m nh4pf6(aq . )
h nmr ( 500 mhz , [ d6]acetone ) : =10.06 ( 1 h , s ; nhd ) , 9.42 ( 1 h , s ; pyg ) , 9.36 ( 3 h , m ; pyb / b , arhc and arhc ) , 9.20 ( 1 h , s ; pyb / b ) , 8.82 ( 2 h , d , j=7.5 hz ; bipy ) , 8.77 ( 2 h , d , j=8.2 hz ; bipy ) , 8.74 ( 2 h , d , j=8.2 hz ; bipy ) , 8.36 ( 1 h , s ; nhd ) , 8.20 ( 2 h , m ; arha ) , 8.04 ( 1 h , s ; hc ) , 7.968.03 ( 8 h , m ; bipy ) , 7.78 , ( 2 h , d , j=6.1 hz ; arhbj ) , 7.74 ( 2 h , d , j=5.8 hz ; arhe ) , 7.51 ( 4 h , m ; bipy ) , 7.147.39 ( 29 h , m ; arhstopper ) , 6.90 ( 2 h , d , j=8.8 hz ; arhe ) , 6.63 ( 4 h , s ; arhg ) , 6.47 ( 4 h , s ; arhh ) , 5.03 ( 2 h , s ; hl ) , 4.65 ( 3 h , s ; ha ) , 4.44 ( 2 h , t , j=6.8 hz ; hnj ) , 4.04 ( 4 h , m ; och2 ) , 3.99 ( 4 h , m ; och2 ) , 3.91 ( 8 h , s ; nch2 and och2 ) , 3.63 ( 4 h , m ; och2 ) , 3.48 ( 2 h , m ; hr ) , 2.19 ( 2 h , m ; hk ) , 1.29 ( 18 h , s ; tbuh ) , 1.27 ppm ( 27 h , s ; tbuh ) ; c nmr ( 125 mhz , [ d6]acetone ) : =164.4 , 160.8 , 159.8 , 159.7 , 159.6 , 157.4 , 153.3 , 152.5 , 152.4 , 152.0 , 151.7 , 149.6 , 149.3 , 148.0 , 145.3 , 144.8 , 144.2 , 142.3 , 140.8 , 138.9 , 132.9 , 132.4 , 131.8 , 131.5 , 131.4 , 129.6 , 129.4 , 128.5 , 127.2 , 126.9 , 125.7 , 125.4 , 125.2 , 125.2 sic , 125.1 , 123.4 , 121.0 , 120.9 , 116.6 , 115.5 , 115.5 sic , 114.3 , 71.5 , 70.9 , 68.5 , 67.5 , 67.4 , 64.8 , 64.0 , 62.2 , 55.0 , 50.2 , 48.5 , 40.3 , 36.2 , 34.9 , 31.7 ppm ; f nmr ( 282.5 mhz , [ d6]acetone ) : =72.5 ppm ( d , j=708 hz ; pf6 ) ; p nmr ( 121.6 mhz , [ d6]acetone ) : =144.3 ppm ( sept , j=709 hz ; pf6 ) ; ms ( maldi - tof ) : m / z calcd for [ c138h147f18n14o11osp3 ] [ m(pf6 ) ] : 2658.03 ; found : 2658.26 .
rotaxane 3 : in a 50 ml round - bottom flask , osmium macrocycle 4 ( 100 mg , 70 mol , 2 equiv ) and thread 14 ( 31 mg , 35 mol , 1 equiv ) were slowly stirred for 1 h in 25 ml of dry dichloromethane .
after removal of the solvent , the residue was dissolved in 25 ml of dry and degassed dichloromethane .
stopper 15 ( 19 mg , 35 mol , 1 equiv ) , cu(ch3cn)4pf6 ( 2.6 mg , 7 mol , 0.2 equiv ) , tbta ( 3.7 mg , 7 mol , 0.2 equiv ) and dipea ( 12 l , 70 mol , 2 equiv ) were successively added and the reaction mixture was stirred at room temperature ( 213 c ) for 3 d. then , 10 ml of kno3(sat .
after removal of the solvent , the crude product was purified by preparative tlc ( sio2 , ch3cn / h2o / kno3(sat .
rotaxane 3 ( 74 mg , 72 % ) was obtained as a brown solid after anion exchange to the hexafluorophosphate salt by washing a chloroform solution of the rotaxane with 0.1 m nh4pf6(aq . ) ( 815 ml ) and h2o ( 215 ml ) .
h nmr ( 500 mhz , [ d6]acetone ) : =9.92 ( 1 h , s ; nh ) , 9.44 ( 1 h , s ; py ) , 9.34 ( 1 h , s ; py/ ) , 9.27 ( 1 h , s ; arhc / c ) , 9.23 ( 1 h , s ; py/ ) , 9.12 ( 1 h , s ; arhc / c ) , 8.78 ( 4 h , d , j=8.5 hz ; bipy ) , 8.36 ( 1 h , s ; nh ) , 8.18 ( 2 h , d , j=6.1 hz ; arha ) , 8.17 ( 1 h , s ; h ) , 8.03 ( 4 h , m ; bipy ) , 7.87 ( 4 h , m ; bipy ) , 7.80 ( 2 h , m ; arhb ) , 7.70 ( 2 h , s ; arh ) , 7.48 ( 4 h , m ; bipy ) , 7.097.42 ( 37 h , m ; arhstopper and arhbiphenyl ) , 6.92 ( 2 h , d , j=9.0 hz ; arh ) , 6.61 ( 4 h , m ; arhg ) , 6.41 ( 4 h , m ; arhh ) , 5.67 ( 2 h , s ; h ) , 5.16 ( 2 h , s ; h ) , 4.67 ( 3 h , s ; h ) , 4.08 ( 4 h , m ; och2 ) , 4.04 ( 4 h , m ; och2 ) , 3.94 ( 4 h , m ; och2 ) , 3.87 ( 8 h , m ; nch2 and och2 ) , 3.71 ( 2 h , s ; h ) , 1.30 ( 18 h , s ; tbuh ) , 1.28 ppm ( 27 h , s ; tbuh ) ; c nmr ( 125 mhz , [ d6]acetone ) : =163.8 , 162.4 , 160.3 , 159.4 , 159.1 , 157.1 , 152.8 , 152.2 , 151.9 , 151.4 , 151.3 , 151.2 , 149.1 , 148.8 , 147.6 , 146.4 , 144.9 , 144.3 , 141.8 , 140.3 , 138.5 , 137.6 , 135.8 , 132.4 , 132.0 , 131.3 , 131.1 , 131.0 , 129.2 , 129.1 , 129.0 , 128.7 , 128.1 , 127.6 , 127.5 , 127.0 , 126.5 , 125.3 , 125.0 , 124.7 , 123.2 , 122.7 , 120.5 , 116.3 , 116.2 , 115.0 , 113.9 , 71.0 , 70.4 , 68.0 , 67.2 , 67.0 , 64.4 , 63.4 , 61.9 , 54.6 , 53.6 , 49.7 , 39.8 , 35.8 , 34.6 , 34.5 , 31.3 ppm;f nmr ( 282.5 mhz , [ d6]acetone ) : =72.5 ppm ( d , j=708 hz ; pf6 ) ; p nmr ( 121.6 mhz , [ d6]acetone ) : =144.3 ppm ( sept , j=709 hz ; pf6 ) ; ms ( maldi - tof ) : m / z calcd for [ c149h153f18n14o11osp3 ] [ m(pf6 ) ] : 2796.07 ; found : 2796.10 .
macrocycle 4 : in a 250 ml round - bottom flask , macrocycle 19 ( 168 mg , 270 mol , 1 equiv ) was suspended in 100 ml of a 4:1 etoh / h2o mixture and [ os(bipy)2cl2 ] 16 ( 153 mg , 142 mg , 2 equiv ) was added .
the brown solution was left to cool to room temperature ( 213 c ) and then filtered through a celite bed before the solvent was removed in vacuo .
the brown residue was then redissolved in 20 ml of h2o and nh4pf6(s ) was added to the solution until precipitation ceased .
the precipitate was collected by vacuum filtration and dried under vacuum to give macrocycle 4 as a black solid ( 282 mg , 198 mol , 74 % ) .
h nmr ( 300 mhz , [ d6]acetone ) : =9.14 ( 2 h , s ; arhc ) , 8.82 ( 4 h , m ; bipy ) , 8.51 ( 2 h , t , j=5.3 hz ; nhd ) , 8.19 ( 2 h , d , j=5.6 hz ; arha ) , 8.05 ( 4 h , t , j=7.9 hz ; bipy ) , 7.97 ( 4 h , t , j=6.0 hz ; bipy ) , 7.80 ( 2 h , dd , j=6.1 hz , j=1.7 hz ; arhb ) , 7.53 ( 2 h , t , j=6.8 hz ; bipy ) , 7.47 ( 2 h , t , j=6.8 hz ; bipy ) , 6.88 ( 8 h , m ; arhg , h ) , 4.14 ( 4 h , t , j=5.3 hz ; ch2 ) , 4.06 ( 4 h , t , j=4.5 hz ; ch2 ) , 3.733.85 ( 8 h , m ; ch2 ) , 3.67 ppm ( 4 h , s ; ch2 ) ; c nmr ( 75.5 mhz , [ d6]acetone ) : =163.6 , 160.8 , 159.7 , 153.9 , 152.4 , 152.2 , 151.7 , 142.4 , 138.8 , 129.4 , 127.5 , 125.6 , 122.7 , 116.5 , 116.3 , 71.4 , 70.4 , 68.9 , 67.4 , 40.5 ppm ; f nmr ( 282.5 mhz , [ d6]acetone ) : =72.5 ppm ( d , j=708 hz ; pf6 ) ; p nmr ( 121.6 mhz , [ d6]acetone ) : =144.3 ( sept , j=709 hz ; pf6 ) ; ms ( maldi ) : m / z calcd for [ c54h52f12n8o8osp2 ] [ m(pf6)2 ] : 1132.35 ; found : 1132.38 .
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 we report the preparation of [ 2]rotaxanes containing an electrochemically and optically active osmium(ii ) bipyridyl macrocyclic component mechanically bonded with cationic pyridinium axles . 
 such interlocked host systems are demonstrated to recognise and sense anionic guest species as shown by 1h nmr , luminescence and electrochemical studies . 
 the rotaxanes can be surface assembled on to gold electrodes through anion templation under click copper(i)-catalysed huisgen cycloaddition conditions to form rotaxane molecular films , which , after template removal , respond electrochemically and selectively to chloride . 
Introduction
Results and Discussion
Conclusion
Experimental Section
Supporting Information