Article pubs.acs.org/Organometallics
Branched Redox-Active Complexes for the Study of Novel Charge Transport Processes Michael S. Inkpen, Tim Albrecht,* and Nicholas J. Long* Department of Chemistry, Imperial College London, London SW7 2AZ, U.K. S Supporting Information *
ABSTRACT: The syntheses and electrochemical/optical properties of some branched and linear 1,1′-substituted ferrocene complexes for molecular electronics are described. Metal centers were extended (and where relevant, connected) by arylethynyl spacers functionalized with m-pyridyl, tert-butylthiol (StBu), and trimethylsilyl (TMS) moieties. Such systems provide two welldefined molecular pathways for electron transfer and hold interesting prospects for the study of new charge transport processes, such as quantum interference, local gating, and correlated hopping events.
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INTRODUCTION Significant interest in the potential for single or small groups of molecules to replace existing semiconductor technologies may be attributed to the myriad of distinctive charge transfer processes they can exhibit. While three primary mechanisms of through-molecule electron transport are typically discussed (tunneling, hopping, and Coulomb blockade),1 conductance may be affected by a wide range of factors, including temperature,2 local molecular environment,3 redox state,4 and sample irradiation5 (these may also determine the relative contributions of each transport mechanism to the whole). In addition to preparing molecular versions of existing wires and devices, by improving our understanding of such processes it may ultimately be possible to access molecular-based components that do not yet have solid-state counterparts. Toward this end, it is worth noting that despite extensive investigations of linear molecules (including those with single or multiple redox centers “wired” in series), the properties of branched and cyclic structures remain largely unexplored. This disparity is even more remarkable given the variety of unusual and interesting transport phenomena that this class of materials could be expected to exhibit. Should coherent tunneling (transmission of electron waves through the molecule without dephasing or scattering) dominate transport, then quantum interference effects may prove significant. Magoga and Joachim have suggested that the total conductance, Gt , for a branched species (that is, a compound containing two well-defined molecular pathways) can be greater than the summed conductance of its individual branches.6 They accordingly included a constructivelike interference term (2√G1G2) in their conductance expression, where Gt = G1 + G2 + 2√G1G2 (Gn = conductance of a single branch). A related theoretical study by Baer and Neuhauser explored destructive patterns in a series of alkene wires linked at two points by a shorter wire.7 Conductance was found to be © 2013 American Chemical Society
dependent on the length of the linking wire, as this had an impact upon the phase difference between propagating electron waves at recombination. Very recently,8 constructivelike interference effects were observed in a first-of-its-kind experimental investigation by Vazquez et al., using a set of all-organic structures. Only a handful of related studies have been conducted to date.9 If, in contrast, hopping were to dominate transport, then local gating (where the redox state of one branch has an impact upon transport through the other, via electrostatics or modulation of the electronic structure) or electron correlation effects might come into play. The latter have been discussed for single-center redox systems by Kuznetsov.10 Correlation in the present context implies that electrons transferred via the redox center(s) may be coupled to some degree. If the correlation is strong, electrons may be transferred in pairs; if there is no correlation, they would be transferred independently. To investigate these and related processes experimentally, we considered the linear and branched synthetic targets shown in Figure 1 and herein describe their preparation and properties. Critical features include (i) ferrocene-1,1′-diyl moieties as stable redox-active centers, (ii) conjugated “conducting” backbones, and (iii) m-pyridyl,49 tert-butylthiol (StBu) (toward SAc), and trimethylsilyl (TMS) substituents for surface binding. (Though TMS groups are more normally utilized en route to more advanced materialsfor example via tin-mediated syntheses11their application in forming molecule−gold contacts has very recently been presented.12) In general, we wondered if the inclusion of redox centers might also provide a mechanism by which electron transport through individual Special Issue: Ferrocene - Beauty and Function Received: June 21, 2013 Published: September 5, 2013 6053
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components. In a seminal paper, Getty et al. found that a ferrocene-containing oligo(phenylethynyl)dithiolate compound demonstrated a higher (indeed, “near-perfect”) conductance than an all-organic analogue in gold nanogap junctions.13 Xiao and co-workers later investigated the properties of cysteamineterminated ferrocene complexes using electrochemical STM, where changes in current were attributed to switching between low-conductance reduced and high-conductance oxidized states.14 Further examples of relevant materials utilize thioacetyl,11b,15 tert-butylthiol,16 m-pyridyl,17 p-pyridyl,17 or a m i no 1 8 en d g r ou ps , t y pi cal l y b as ed o n oligo(phenyleneethynylene) frameworks.
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Figure 1. Our proposed ferrocene-containing “redox-active” (a) linear and (b) branched analogues of the purely “all-organic” systems studied theoretically by Magoga and Joachim6 and experimentally by Vazquez et al. (R = SiMe3, p-C6H4-StBu, m-C5H4N).8 It is considered that redox events at the ferrocene-1,1′-diyl motif will facilitate the study of novel hopping transport mechanisms or quantum interference effects by providing a mechanism by which the conductance of individual branches may be affected in situ within a macroscopic experimental setup.
RESULTS AND DISCUSSION (a). Synthesis. Retrosynthetic Considerations. Many of the idiosyncrasies associated with preparations of ferrocenyl− alkynes have been described elsewhere.15,16,19 However, in light of recent findings and the context of this work it seems appropriate to briefly recap and extend these discussions. In doing so, we also hope to rationalize our approach. 1,1′-Diiodoferrocene (fcI2, a typical starting material) and its cross-coupled alkynyl−ferrocene intermediates do not always behave as might be expected, given their apparent similarities to dihalo- or bis(arylethynyl)benzenes. It is worth noting, for example, that synthetic routes via 1,1′-diethynylferrocene are not readily accessible. Desilylation of 1,1′-bis(trimethylsilyl)ethynylferrocene typically results only in ferrocenophanes via base-mediated intramolecular cyclization.20 To date, only a bulky, tBu-substituted analogue has been isolated21though some research groups have been successful in utilizing in situ deprotection/onward reaction strategies.22 While 1-ethynyl-1′-
molecular branches of a double-branched or cyclic compound could be affected in situ within a macroscopic experimental setup (redox switching). In theoretical studies this approach has proved useful for comparisons of Gt and Gn and achieved artificially by rotating a phenyl ring out of planarity,6 raising the potential energy of a specific site, or changing the coupling strength of one bond.7 Our designs were substantiated by previous reports regarding linear ferrocene-1,1′-diyl complexes as molecular electronic
Scheme 1. Synthesis of Linear and Branched Ferrocene−Alkyne Compoundsa
a
3-EP = 3-ethynylpyridine; 3,5-DEP = 3,5-diethynylpyridine. 6054
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reasonably nonpolar CH2Cl2/n-hexane eluent combinations, affording superior resolution between components. Cp-H regions of 1H NMR spectra provided particular insight into the identity of complexes, being highly diagnostic of the number and substitution of ferrocene centers (Figure S-1, Supporting Information). In mass spectra [M + H]+ was always observed, and IR studies showed characteristic bands at 2207− 2219 cm−1 for ν(CC)aryl and ∼2150 cm−1 for ν(CC)TMS. Complexes containing Cp-I frequently appeared to be contaminated with small quantities of their hydrodehalogenated analogues (Cp-I → Cp-H), as indicated by accurate mass analyses and singlet 1H NMR resonances in the Cp-H region. This side reaction was also noticed in cross-couplings of fcI2 and phenylacetylene.24 In our hands, attempts to convert the tert-butylthiol functionalities of 2b and 5b to thioacetyl using the BBr3 method29 proved unsuccessful. While the dark red solid formed from 2b showed an initially promising 1H NMR spectrum (resonances attributable to tBu now absent, with a new signal at ∼2.5 ppm arguably due to Ac), mass spectrometry showed no clear molecular ion for the desired product (or 2b). Even considering the possibility of additional borane adduct formation (via the pyridyl nitrogen) did not aid peak assignments (though the presence of boron was confirmed by 11 B NMR). Similar experiments using 5b proved even more problematic. We ascribe these difficulties to complications relating to the ferrocene moiety or ferrocene−alkyne linkage but note two relevant reports of successful procedures elsewhere.30 (b). Electrochemistry. The redox properties of selected materials were investigated via cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in CH2Cl2, using 0.1 M [(nBu)4N]PF6 or ∼0.02 M Na[B(C6H3(CF3)2)4] (a saturated solution) as supporting electrolyte. Relevant data are summarized in Tables 1 and 2.
(trimethylsilyl)ethynylferrocene was very recently prepared by reaction of a dibromoalkene precursor with LDA,23 it has been suggested that 1-ethynyl-1′-alkynylferrocenes also exhibit instabilities similar to those of 1,1′-diethynylferrocenes upon desilylation.15 It is also not yet possible to cross-couple iodoferrocenes with terminal alkynes in the presence of thioacetate moieties (a popular terminal group for surface binding). We have very recently shown that, under Sonogashira conditions, syn addition of S−Ac across CC occurs prior to any crosscoupling reaction between CCH and Fc-I.24 Given that aryl bromides/iodides are readily cross-coupled in the presence of thioacetate moieties,25 this implies that rates of oxidative addition to Pd(0) follow the series kAr−I > kAr−Br > kS−Ac > kFc−I (assuming oxidative addition to Pd(0) is always the ratelimiting step). Despite relatively slow reactivity, we found that yields for Sonogashira cross-couplings with iodoferrocenes can be maximized using high reaction concentrations/temperatures with Pd0−(PPh3)n, or alternatively using Pd0−(PtBu3)n at room temperature.24 Elsewhere, microwave conditions have also proved advantageous over normal heating procedures,16a and an increased reactivity between iodoferrocenes and silylacetylenes or 2,5-dimethoxybenzyl-derived alkynes (versus benzylalkynes in general) has been observed.15 Of additional interest, in cross-couplings from fcI2 the second iodo functionality is sometimes found to be less accessible than the first.15,19,26 When controlled preparations of asymmetrically substituted 1,1′-ferrocenes are in fact desired, a reliable approach appears to be the utilization of 1-ethynyl-1′iodoferrocene as an asymmetrical precursor.15,16,27 It is then possible to capitalize on the difference in reactivity between FcI and Ar-I in subsequent cross-couplings. In this work, we were inspired to explore an alternative strategy, given the new oxidative purification procedures for halogenated ferrocenes28 and the large quantities of pure fcI2 at our disposal. We find that reaction of a large excess of fcI2 with either mono- or dialkynes is effective in favoring monosubstitution at the ferrocene center, facilitating the formation of advanced asymmetric intermediates in one step from fcI2, rather than three (via 1-ethynyl-1′-iodoferrocene). This direct method provides yields typically around 60−70% per bond formed, with unreacted fcI2 easily recovered by chromatography. Synthesis of Linear and Branched Compounds. In accordance with the previous discussion, linear and branched complexes were prepared via successive Sonogashira crosscouplings starting from fcI2 (Scheme 1).50 Compounds 115 and 3 were obtained by reaction of 3-ethynylpyridine and 3,5diethynylpyridine, respectively, with a 5-fold excess of fcI2 (in attempts with lower fcI2-to-alkyne ratios, yields were significantly reduced). The desired 1,1′-bis(arylethynyl)ferrocenes (linear, 2b,c; branched, 5a−c) were subsequently synthesized from 1 and 3, respectively, via reaction with the appropriate terminal alkynes. The symmetrical linear complex 2a17 was conveniently prepared directly from fcI2 using an excess of 3ethynylpyridine. Yields from cross-couplings of the second iodo functionality were typically lower (15−37%), particularly in the branched series, as monosubstituted products were also formed (4a,b). These two-terminal complexes contain a free Cp-I moiety and could readily react to produce materials containing three different terminal groups. Key to the isolation of most of these pyridyl-containing materials via column chromatography was the use of neutral grade V alumina as a stationary phase. This permitted the use of
Table 1. Electrochemical Data for Cyclic Voltammetry Experiments with 0.1 M [(nBu)4N]PF6/CH2Cl2a compd
Epa (V)
Epc (V)
ΔE (mV)
ipa/ipc
E1/2b (V)
fcI2 1 2a 2b 2c 3 5ac 5b 5c
0.37 0.32 0.31 0.29 0.28 0.33 0.36 0.29 0.31
0.29 0.25 0.25 0.23 0.23 0.27 0.28 0.23 0.25
80 73 68 60 48 51 81 63 65
0.98 0.94 1.05 1.05 1.06 0.92 1.05 1.05 1.00
0.33 0.28 0.28 0.26 0.26 0.30 0.32 0.26 0.28
a Conditions: scan rate 0.1 V s−1; working electrode, glassy carbon; reference and counter electrodes, Pt. All potentials are reported relative to an internal [Cp2Fe]+/[Cp2Fe] reference and corrected for iRs unless otherwise stated. bE1/2 = 1/2(Epa + Epc). cMeasured against an internal [Cp*2Fe]+/[Cp*2Fe] reference (−0.495 V vs [Cp2Fe]+/ [Cp2Fe] in our system).
CV experiments in [(nBu)4N]PF6/CH2Cl2 showed close to reversible behavior (ipa/ipc ≈ 1, ip ∝ Vs1/2) for all complexes. Even after potentials were corrected for solution resistance (Rs) effects (using values of Rs estimated from ac impedance spectroscopy),31 ΔE was typically found to vary with scan rate (Vs) (Figure S-22, Supporting Information). In the reversible case, this relationship should be independent with ΔE ≈ 59 6055
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“statistical factor” should also be accounted for in quantitative work.33b,38 Our observations are consistent with voltammetric studies of related complexes elsewhere. For example, with 3,5-diethynylpyridine-bridged Fe(dppe)2Cp* complexes ΔE1/2 = 0.11 V (compared with 0.72 V with butadiynyl bridges),39 or with ferrocene centers linked by 3,5-benzene40 or 2,5-diethynylpyridine ΔE1/2 ≅ 0.15 meta-Substituted bridges are known to be poor mediators of electron transfer,9a and in general the C5H5− Fe−C5H4− moiety exhibits a lower ΔE1/2 value (through the same bridge) in comparison to other redox centers.41 In an attempt to realize the singly oxidized species depicted in Figure 1, we conducted CV and DPV experiments for the double-branched series in Na[B(C6H3(CF3)2)4]/CH2Cl2. The small cation and large anion of this electrolyte has been shown by Barrière and Geiger to minimize ion-pairing effects, maximizing ΔE1/2 for sequential oxidation processes based on electrostatic arguments (ion pairing is considered to confer a greater stability to the fully oxidized state than to the mixedvalence state; therefore, by reducing such interactions the comproportionation equilibrium is shifted in favor of the latter).35a,c Indeed, in this medium a small splitting of the original 2e Fe2+/Fe3+ redox feature was now observed for 5b (ΔE1/2 ≠ 0; Figure 2b), and to a lesser extent 5c (perhaps best indicated by the asymmetry of the DPV wave; Figure 2c). The two overlapping waves are attributed to the [Fe12+Fe22+]0/ [Fe13+Fe22+]+ (E1) and [Fe13+Fe22+]+/[Fe13+Fe23+]2+ (E2) redox processes. Though only a single wave was seen for compounds 3 (Figure S-24, Supporting Information) and 5a (Figure 2a), the peak−peak separation was noticeably larger than the expected theoretical value. This may be attributed to an unresolved separation of the two 1e redox waves, broadening the features to provide an observed ΔE ≫ 59 mV. Further evidence of reduced ion pairing in this reaction medium is provided by the increased equilibrium potentials of all species in comparison to their values in [(nBu)4N]PF6/CH2Cl2. Though notoriously difficult to interpret,33a,34b,35b,36 the observed changes in ΔE1/2 (ΔΔE1/2) are particularly intriguing, given that all complexes share the same 3,5-diethynylpyridyl bridge motif. Evidently, the terminal groups play a critical role, with −CCC6H4StBu providing a more stable mixed-valence state than −CCSiMe3 and the −I/−CCC5H4N functionalized mixed-valence species proving the least stable of all. Notably, this trend appears to follow the electron-donating ability of these groups, as indicated by the order of equilibrium potentials observed in cyclic voltammetry studies using [(nBu)4N]PF6 (Table 1). As the oxidation of these complexes results in the removal of an electron from a HOMO primarily metallic in character42 assuming qualitative similarity to the ferrocene parentit is not straightforward to attribute ΔΔE1/2 to changes in electrostatic repulsion within the different fully oxidized species (the positive charges being essentially localized on the Fe center in all instances). In any case, electron-donating groups should serve to attenuate the effect (reducing ΔΔE1/2), yet this runs contrary to the observed trend. As is discussed in the next section, only small energetic changes in MLCT/LMCT and d− d transitions are observed across the series (≤7 nm), suggesting that inductive contributions to ΔE1/2 are negligible. Returning to the impact of ion pairing on ΔE1/2, we considered the role that steric effects might play in affecting analyte cation−electrolyte anion interactions (and so modulating ΔE1/2). As discussed by Barrière and Geiger,35c decreased
Table 2. Electrochemical Data for Cyclic Voltammetry Experiments with ∼0.02 M Na[B(C6H3(CF3)2)4]/CH2Cl2a compd
Epa (V)
Epc (V)
ΔE (mV)
ipa/ipc
E1/2b (V)
3 5a 5bc
0.58 0.57 0.51 (sh) 0.57 (pk) 0.56 (br)
0.42 0.43 0.39 (br)
158 144
0.98 0.99
0.50 0.50 0.48
5cc
0.35 (br)
0.46
Conditions: scan rate 0.1 V s−1; working electrode, glassy carbon; reference and counter electrodes, Pt. All potentials are measured against an internal [Cp*2Fe]+/[Cp*2Fe] reference, corrected for iRs, and reported relative to the [Cp2Fe]+/[Cp2Fe] redox couple (0.495 V vs [Cp*2Fe]+/[Cp*2Fe] in our system). bE1/2 = 1/2(Epa + Epc). c Estimated values due to poorly resolved overlapping redox waves. a
mVdeviations from this ideal value may be related to ion pairing and other effects. Redox features were assigned to the Fe2+/Fe3+ couple, with E1/2 values comparable to those observed for (arylethynyl)ferrocenes reported elsewhere.15,16,32 All complexes proved more difficult to oxidize than ferrocene, indicating that the iodo/alkynyl substituents are to some extent electron withdrawing. The relative trend −I ≥ −CCC5H4N > −CCSiMe3 ≥ −CCC6H4StBu was observed in both linear and branched series. Redox waves for 2a and 5a rapidly became irreversible if too strong an oxidizing voltage was applied (plausibly due to further reaction of the oxidized species in solution or modification of the electrode surface). Other complexes were generally more tolerant of the applied voltage range. Use of ferrocene as an internal reference for 5a was not possible due to an apparent reaction between the oxidized species in [(nBu)4N]PF6/CH2Cl2 (Figure S-23, Supporting Information). Replacing ferrocene with decamethylferrocene (−0.495 V vs [Cp2Fe]+/[Cp2Fe]) in this experiment provided a convenient solution to the problem. Interestingly, no splitting of the Fe2+/Fe3+ redox feature was observed for the bimetallic complexes (3 and 5a−c) in this system (ΔE1/2 ≅ 0). Such species, containing two or more chemically identical redox centers, can exhibit separate 1e redox processes at different potentials, rather than a single feature (comprising superimposed 1e processes) at the same potential. Though this is often attributed to an “interaction” or “communication” between the sites, it is more accurately discussed in terms of the stability of the mixed-valence state(s) vs the isovalent states. For two redox sites the position of the comproportionation equilibrium (eq 1) is defined by the Kc
[Fe12+Fe2 2+]0 + [Fe13+Fe2 3+]2 + ⇌ 2[Fe13+Fe2 2+]+
(1)
comproportionation constant (Kc), which in turn can be related to the Gibbs free energy of comproportionation (ΔcG) and to the difference between the first and second equilibrium potentials, ΔE1/2 (=E2 − E1), via eq 2. The magnitude of Kc ΔcG = −RT ln Kc = −F ΔE1/2
(2)
is determined by multiple (often intimately related) factors, including electrostatic, inductive/synergistic, and resonance contributions,33 antiferromagnetic superexchange interactions,34 ion-pairing/medium effects,35 solvation energetics,36 and enthalpic/structural changes.37 Though invariable, the 6056
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Figure 2. Cyclic (middle, corrected for iRs) and differential pulse (bottom) voltammograms recorded in Na[B(C6H3(CF3)2)4]/CH2Cl2 for (a) 5a, (b) 5b, and (c) 5c. Potentials are reported vs [Cp2Fe]+/[Cp2Fe].
(c). UV−Vis Spectroscopy. Intrigued by the trends and questions raised by electrochemical experiments, the electronic structures of selected materials were probed spectroscopically. To assist assignments, we first noted key spectral features for the parent complex.44 With ferrocene, three spin-allowed d−d transitions are expected (formally Laporte forbidden and weak). 1A1g → a1E1g and 1A1g → 1E2g are unresolved and assigned to a band at 442 nm (measured in THF), with 1A1g → b1E1g responsible for a band at 325 nm. A more intense, higher energy band is also observed at 200 nm (measured in isopentane), provided by a ligand-to-metal charge transfer (LMCT) transition, with shoulders at 240 and 265 nm attributed to metal-to-ligand charge transfer (MLCT) and LMCT transitions, respectively. Additional features have been observed in electron-accepting solvents (such as halogenated hydrocarbons and ethyl 2-cyanoacrylate), provided by intermolecular charge-transfer-to-solvent (CTTS) transitions (forming ferrocenium).44c Following substitution of the Cp ring, some notable spectral differences may be anticipated. For some 1-ferrocenes, Zhang et al. found that staggered conformations (D5d symmetry) were preferred over the eclipsed conformation favored by ferrocene (D5h symmetry), changing the symmetry of interactions between Fe and the Cp ligand(s).45 They also observed that substitution of the Cp ligand results in significant changes to the energies of its frontier orbitals, ultimately reducing 1ferrocene HOMO (a1g)−LUMO (e1g) gaps relative to the parent compound. This is in agreement with work by Dowben and co-workers concerning 1,1′-dichloro- and 1,1′-dibromoferrocenes (symmetrical systems with electron-withdrawing substituents), which indicated that the greater the electronwithdrawing power of the Cp ring substituent, the greater the
ion pairing between the analyte cation and electrolyte anion (in addition to increased competitive ion pairing between the electrolyte cation and electrolyte anion) should result in a more “naked” positively charged analyte. This will (i) make the analyte monocation harder to oxidize, being essentially more positively charged than in the strongly ion-paired case (where electron-donating anions reduce the effective positive charge) and (ii) increase the effective electrostatic repulsion between positive charges of the analyte dication (from reduced screening). Consequentially, the second electron becomes harder to remove than the first, increasing ΔE1/2. Extending these concepts, for a given series of complexes we suggest that sterically hindered redox centers may, on time average, demonstrate further reduced ion-pairing interactions (the electrolyte anion cannot easily get close to the positive charge) in comparison to those which are less hindered. Such analytespecific changes in ion pairing would accordingly also regulate ΔE1/2. In the absence of other obvious contributing factorsand the clear relationship between ion pairing and ΔE1/2 observed here and elsewhere35this explanation fits well with our observations, where 5b (comprising the bulky −C CC6H4StBu moiety) demonstrates the largest redox splitting. In this context, it is worthy of note that all complexes containing the −CCC6H4StBu group (2b, 4b and 5b) and 5c (containing two −CCSiMe3 substituents) exhibit broadened NMR resonances (see the Supporting Information). This is a well-known characteristic of bulky metallocenes, where hindered rotation around the Cp−Fe−Cp axis renders otherwise equivalent nuclei nonequivalent on the NMR time scale.43 6057
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Table 3. Electronic Spectral Data of Selected Compoundsa
mixing of Fe dx2−y2,xy and Cp (e2g)π orbitals, reducing the a1g− e1g energy gap and increasing the e2g−a1g and e1g(a)−e1g(b) spacings.46 Spectra of the asymmetrical, 1,1′-substituted linear complexes 1 and 2a−c are shown in Figure 3, with those of the
λmax/nm (ε/M−1 cm−1)b
compd fcI2 3-ethynylpyridine fcI(CC-m-Py) (1) fc(CC-m-Py)2 (2a) fc(CC-m-Py)(CC-pC6H4-StBu) (2b) fc(CC-m-Py)(C CSiMe3) (2c) 3,5-diethynylpyridine (μ-3,5-Py)(CC-[fc]-I)2 (3) (μ-3,5-Py)(CC-[fc]-C C-m-Py)2 (5a) (μ-3,5-Py)(CC-[fc]-C C-p-C6H4-StBu)2 (5b) (μ-3,5-Py)(CC-[fc]-C C-SiMe3)2 (5c)
290 (3782), 319 sh (1169), 444 (246) 233 (13221), 244 (10844), 260 (2459), 269 (3015), 275 (3305), 282 (2454) 253 (16171), 307 (14000), 353 sh (1768), 451 (624) 259 (24248), 287 sh (20447), 312 (20226), 348 sh (3995), 455 (1076) 262 (28484), 292 (23123), 317 (25158), 356 (4040), 456 (1085) 254 (20076), 281 sh (17308), 313 (14878), 350 sh (2248), 454 (839) 248 infl (15443), 276 (2186), 284 (2839), 292 (3331), 301 (3112) 251 (23918), 310 (21687), 352 sh (4232), 449 (1211) 263 (56422), 289 sh (45177), 313 (47038), 354 sh (10212), 456 (2486) 263 (66375), 293 (52553), 317 (60307), 356 sh (12128), 456 (2693) 259 (45793), 295 infl (34770), 316 (35807), 344 (9090), 456 (2554)
a
Figure 3. UV−vis spectra (in CH2Cl2) for linear complexes 1 and 2a− c, with data for 3-ethynylpyridine and fcI2 included for comparison. Inset: absorptions in the visible region magnified.
Recorded at room temperature in CH2Cl2, using quartz cells with a path length of 10 mm. bWhere possible, spectra were deconvoluted into composite Gaussian bands to obtain λmax values. All extinction coefficients were taken from the experimental data at these wavelengths.
benzoyl-substituted derivatives. Due to their intensity (ε ≈ 12500−67500 M−1 cm−1), and the significantly weaker bands for fcI2 and the free ethynylpyridine ligands, these are attributed to LMCT/MLCT transitions facilitated by alkyne modification of Cp. Interestingly, ε follows the trend −CCC6H4StBu > −CCC5H4N > −CCSiMe3 > −I in both the linear and branched series. For bands around 257 and 312 nm, λmax follows a small similar trend, except −CCSiMe3 > −C CC5H4N for the latter. Intensities for bands in the branched series are approximately double those for the linear complexes.
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CONCLUSION We have demonstrated a rapid and general synthetic approach to unsymmetrically substituted 1,1′-bis(arylethynyl)ferrocene complexes through the use of fcI2 in large stoichiometric excess. These materials exhibit essentially reversible redox behavior and also display an intriguing medium- and substituent-dependent redox splitting in the bimetallic series (the latter plausibly due to steric effects relating to ion pairing). This work marks a starting point for the experimental study of electron transport processes in branched redox-active compoundsan area which has received remarkably little attention to date. Efforts toward cyclic analogues of these materials are underway, and some promising preliminary data have been obtained. In addition to exploring the utility of different bridging motifs, redox centers, and surface-binding groups in this context, subsequent investigations will consider molecular junction measurements of selected compounds, toward the exploitation of molecular electronic components with novel functionality.
Figure 4. UV−vis spectra (in CH2Cl2) for branched complexes 3 and 5a−c, with data for 3,5-diethynylpyridine and fcI2 included for comparison. Inset: absorptions in the visible region magnified.
branched compounds 3 and 5a−c given in Figure 4 (both figures include spectra of the relevant ethynylpyridine ligand and fcI2 for comparison). The obtained data are summarized in Table 3. With reference to features observed for ferrocene, and assuming an analogous electronic structure, bands at 290 and 444 nm in the spectrum of fcI2 may be assigned to LMCT and d−d transitions, respectively. Interestingly, the latter band undergoes a subtle bathochromatic shift upon substitution of iodide, such that in mixed alkynyl/iodo complexes (1 and 3) it is observed at 449−451 nm and in 1,1′-bis(arylethynyl)ferrocenes at 454−456 nm. In all alkyne-functionalized complexes, intense bands are apparent between 250 and 350 nm, overshadowing the (much weaker) d−d transitions expected in that region. Similar features have been observed elsewhere in 1-substituted FcC CR complexes (R = H, Ph, naphthyl, anthryl, pyrenyl, perylenyl),47 1,1-ferrocenedicarboxylate M(II) salts,48 and
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ASSOCIATED CONTENT
S Supporting Information *
Text and figures giving full experimental procedures/analytical characterization, 1H/13C{1H} NMR spectra of all new compounds, and additional solution voltammograms. This 6058
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Organometallics
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material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for T.A.:
[email protected]. *E-mail for N.J.L.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are most grateful to the EPSRC and the Leverhulme Trust for funding, and would like to thank Prof P. J. Low for useful discussions.
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REFERENCES
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Information were missing or illegible. In the version of this paper that appears as of October 2, 2013, the Supporting Information contains the entire spectra.
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NOTE ADDED AFTER ASAP PUBLICATION In the version of this paper that appeared on the web on September 5, 2013, parts of the spectra in the Supporting 6060
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