Divinylphenylene- and Ethynylvinylphenylene-Bridged Mono-, Di

Article pubs.acs.org/Organometallics

Divinylphenylene- and Ethynylvinylphenylene-Bridged Mono‑, Di‑, and Triruthenium Complexes for Covalent Binding to Gold Electrodes Evelyn Wuttke,† Yves-Marie Hervault,‡ Walther Polit,† Michael Linseis,† Philipp Erler,§ Stéphane Rigaut,*,‡ and Rainer F. Winter*,† †

Fachbereich Chemie, Universität Konstanz, Universitätsstraße 10, D-78453 Konstanz, Germany Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Campus de Beaulieu, F-35042 Rennes Cedex, France § Fachbereich Physik, Universität Konstanz, Universitätsstraße 10, D-78453 Konstanz, Germany ‡

S Supporting Information *

ABSTRACT: In this work, we describe the preparation and the properties of the novel bis(vinylphenylene)-bridged diruthenium complexes {Ru(CO)(η2-O2C-p-C6H4SAc)(PiPr3)2}2(μ-CHCHC 6 H 4 -CHCH-1,3 and -1,4) (6 and 7), the bis(ethynylphenylene)-bridged complex trans-[AcS-p-C6H4-CCRu(dppe)2-CC-p-C6H4-CC-Ru(dppe)2-CC-p-C6H4-SAc] (11), the bis(1-ethynyl-4-vinylphenylene)-bridged triruthenium complex trans-[{Ru(dppe)2}{−CC-p-C6H4-CHCH-Ru(CO)(η2-O2C-p-C6H4SAc)(PiPr3)2}2] (8), and the monometallic congeners Ru(CHCH-p-C6H4SAc)(CO)(η2-O2C-pC6H4SAc)(PiPr3)2 (4) and trans-[Ru(dppe)2(−CC-p-C6H4-SAc)2] (10). These mono-, bi-, and trimetallic complexes feature terminal acetyl-protected thiol functions for covalent binding to gold surfaces or for bridging the gaps of gold nanoelectrodes. All complexes display low oxidation potentials, and IR studies of the neutral complex 8 and of its various oxidized forms 8n+ indicate the high vinyl/ethynyl bridging ligand contribution to the oxidation processes and complete charge delocalization in all available oxidation states (n = 1−3). Strong delocalization of the relevant occupied frontier MOs over the entire π-conjugated {Ru}− bridge−{Ru′}−bridge−{Ru} backbone is also supported by DFT calculations on the parent complexes V8 and V8OMe. The benzoate ligand bearing the functional group for gold binding is outside the conjugation path and insulates the wirelike central portion of these molecules from their periphery. Upon insertion into molecular junctions, these molecules are expected to enhance sequential tunneling and to facilitate Coulomb blockade behavior. They will thus contribute to our understanding of structure−property relationships for metal-containing molecular wires.

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INTRODUCTION

Particularly interesting in this respect are electron-rich metal−organic complexes with extended π-conjugated backbones that provide a strong electronic interaction between the remote redox-active metal centers and allow for easy charge injection and high charge carrier (hole) mobilities. Such compounds are exemplified by 1,4-diethynylphenylene-29,30,38−59 and 1,4-divinylphenylene-bridged59−70 dimetal complexes. With a proper choice of the respective transitionmetal end groups, such complexes are commonly stable in several adjacent oxidation states, including mixed-valent ones, each with their own specific (charge carrier transport) properties. This is particularly the case for ruthenium, where the σ-bonded bis(carbyl) bridge actively participates in the redox processes and hence acts as a noninnocent ligand.35,71−76 Among few studies on such complexes acting as molecular wires in an electrode/molecule/electrode junction,29−32,77,78

1

Feynman’s now historic talk has seeded the idea of individual molecules performing the tasks of electronic devices or their basic constituents.2−12 Today molecular electronics is also viewed as one means to overcome the physical limitations inherent to the top-down approach of further decreasing the size of macroscopic devices. A conducting wire is the most basic constituent of any electronic device. Hence, much research has been directed to fabricating and testing molecule-based analogues of macroscopic wires. The key challenge of connecting individual molecules to the macroscopic world has been successfully addressed with different strategies,11,13−19 including mechanically controlled break junctions (MCBJs),20−24 nanogaps,25,26 and the utilization of transmission electron microscopy (TEM),27 conducting probe atomic force microscopy (CP-AFM),3,28−31 or scanning tunneling microscopy (STM).31−34 Studies of metal electrode/molecule/metal electrode junctions have meanwhile unearthed new features such as Coulomb blockade and Kondo resonances.35−37 © XXXX American Chemical Society

Special Issue: Organometallic Electrochemistry Received: July 1, 2013

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dx.doi.org/10.1021/om400642j | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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Scheme 1

functional group. Experimental and computational studies of six-coordinated vinyl complexes Ru(CHCHR1)(CO)(Cl)(PiPr3)2(pyR) (pyR = substituted pyridine) and Ru(CH CHR)(CO)(OOCR2)(PiPr3)2 indicate that the HOMO, while fully delocalized over the entire conjugated metal−organic π system, is virtually decoupled from the ancillary ligands delivering the point of attachment to the electrodes.62,80,81 This particular feature can be expected to enhance the localization of charges on the central π-conjugated metal− organic pathway and to favor a hopping mechanism with the possible observation of a higher temperature Coulomb blockade over a metal/molecule/metal junction. Here we report the synthesis and our findings on the new bis(vinylphenylene) complexes {Ru(CO)(η2-O2C-p-C6H4SAc)(PiPr3)2}2(μ-CHCH-C6H4-CHCH-1,4 and -1,3) (6 and 7), the bis(ethynylphenylene)-bridged complex trans-[AcS-pC6H4-CC-Ru(dppe)2-CC-p-C6H4-CC-Ru(dppe)2-C C-p-C6H4-SAc] (11), the bis(1-ethynyl-4-vinyl-phenylene)bridged triruthenium complex trans-[{Ru(dppe)2}{−CC-pC6H4-CHCH-Ru(CO)(η2-O2C-p-C6H4SAc)(PiPr3)2}2] (8), and the monometallic congeners Ru(CHCH-p-C6H4SAc)(CO)(η2-O2C-p-C6H4SAc)(PiPr3)2 (4) and trans-AcS-p-C6H4CC-Ru(dppe)2-CC-p-C6H4-SAc (10) as well as on the corresponding derivatives Ru(CHCHPh)(CO)(η2-O2C-pC6H4SAc)(PiPr3)2 (2), and Ru(CHCH-p-C6H4SAc)(CO)Cl(PiPr3)2 (3), bearing acetyl-protected forms of 4-thiobenzoate and/or 4-thiostyryl ligands. We compare them to their chloro complex precursors and to similar complexes lacking the thioacetyl functionality on the benzoate ligands, some of which are also new, in order to probe how each unit affects the electrochemical and electronic properties of these complexes. Given the easy deprotection of the thioacetyl functionality and the long success story of thiolate anchors,8,9,28,78,82,83 these derivatives should be well suited for binding to gold surfaces or bridging the gaps of nanoelectrodes using MCBJs. In addition, we provide IR studies of the different redox states of complexes 1 and 8 to gain insight into the oxidation processes and the charge and spin delocalization upon charge injection and to probe how the thioacetyl functionalities affect the electron density distributions and their changes upon oxidation. Our experimental results are augmented by quantum chemical calculations on 4 and the previously reported V871 to study the effects of thioacetyl functionalization and to further support our notions as to the wirelike behavior of this class of extended trinuclear complexes. Therefore, this work addresses the influence of the linkers, including the modification of the coordination sphere and the valence electron count of the vinyl ruthenium centers, on the electronic properties of the

some of us recently reported on the transport behavior of a series of redox-active conjugated molecular Ru(II) bis(σarylacetylide) wires as a function of molecular length ranging from 26 to 60 Å (Scheme 1).29,30 It is well understood that, with increased molecular length, there is a transition of the conduction mechanism from direct tunneling to hopping. In the latter mechanism, a charge is injected into the molecular wire.28 Inserting electron-rich metal centers into the wire renders this process particularly facile, due to the ready accessibility of its oxidized state(s). As a result, studies on these compounds have unearthed a small attenuation factor for the decrease of molecular conductance with increasing molecule length and a gradual change of the principal conduction mechanism from direct to sequential tunneling as a function of length. Coulomb blockade behavior is also observed when the charging energy overcomes thermal energy. Such behavior was observable at 5 K for trimetallic complexes with isocyanide head groups.30 Other candidates of great potential interest for such measurements can be identified among the divinylphenyleneand 1-ethynyl-4-vinylphenylene-bridged di- and triruthenium complexes of Scheme 2 and Chart 1.63,70,79 These were found Scheme 2. Synthesis of the Trinuclear Ruthenium Complex 8

to exhibit partial (complex V7) or even full charge delocalization (complexes V6, V8, and V11) in up to five different oxidation states over distances of up to 26 Å.70 The complexes we synthesized so far, however, lack suitable functionalities that would allow for their covalent attachment to electrodes. The 5-coordinated 16-valence-electron ruthenium end groups offer a vacant coordination site that allows for the introduction of an additional ligand bearing an appropriate B


Organometallics

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Chart 1

Scheme 3. Synthetic Pathway for the Adduct 4

complexes toward acids,84 in situ deprotonation of the carboxylic acid is required before the respective chloro complex is added. Model compounds 1 and 5 could be prepared from V4 and V6 in this manner in yields of 88 and 83%. In THF as the solvent and with K2CO3 as the base, this procedure is also compatible with the acetyl-protected thiol function, as shown by the successful synthesis of complex 2 from V4 and of diruthenium complexes 6 and 7 from precursors V6 and V7 (Chart 1).63,85 Recently we reported the synthesis of the trinuclear ruthenium complexes trans-[{Ru(dppe)2}{−CC1,4-C6H2R2-CHCH-Ru(CO)Cl(PiPr3)2}2]n+ (R = H, V8; R = OMe, V8OMe), which exhibit complete electron delocalization

complexes and provides a series of complexes containing each type of ruthenium unit for future rationalization of their electrical properties.

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RESULTS AND DISCUSSION Synthesis and Characterization. Treatment of 5coordinated alkenyl complexes (RCHCH)Ru(CO)Cl(PiPr3)2 with a carboxylate results in the displacement of the chloro ligand and the formation of 18-valence-electron carboxylato complexes (RCHCH)Ru(η2-O2CR2)(CO)(P i Pr 3 ) 2 , where R 2 COO − acts as a bidentate chelate ligand.84,89,90 In light of the sensitivity of these alkenyl C


Organometallics

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Scheme 4. Synthesis of the Alkynyl Complexes 10 and 11

over a length of about 26 Å. 7 0 The target bis(ethynylvinylphenylene)-bridged triruthenium complex 8 with thioacetyl-decorated benzoate coligands was likewise prepared by reacting the precursor V8 with a suspension of 4(thioacetyl)benzoic acid 86 and K 2 CO 3 in THF in a stoichiometric ratio slightly larger than 1:2 over 14 h (Scheme 2). The unsymmetrical mononuclear complex 4 with two different terminal thioacetyl-functionalized ligands was prepared following the two-step reaction of Scheme 3. By reaction of the hydride ruthenium complex Ru(CO)ClH(PiPr3)287 with 1 equiv of 4-ethynyl-1-thioacetylbenzene88 in dichloromethane, the square-pyramidal complex 3 was formed within 30 min. This reaction involves the classical regio- and stereospecific insertion of the terminal alkyne into the Ru−H bond of the hydride complex and provides a substituted vinyl ligand with a trans disposition of the metal atom and the aryl substituent. The introduction of the 4-thioacetyl -functionalized benzoate ligand according to the aforementioned method finally afforded complex 4 in a yield of 77%. In order to obtain a complete set of complexes for investigating the electronic properties of each units, we also synthesized the acetylide complexes 10 and 11 by reacting trans-[(dppe)2ClRuCCH-p-C6H4-SAc)](OTf) (9)26 with the appropriate alkyne in the presence of NaPF6 as a mild halide-abstracting agent with a weakly coordinating anion and a base (Et3N), according to the general procedure for obtaining bis(σ-arylacetylide)ruthenium complexes (Scheme 4).89 All of these compounds were fully characterized by multinuclear NMR and IR spectroscopy and by combustion analysis (see the Supporting Information). NMR resonances of the new complexes are similar to those of other known ruthenium vinyl and acetylide complexes, including the immediate precursors given in Chart 1.40,63,70,79,80,85,90 Particularly informative for the identification of 8 are the two sharp singlets at δ 53.9 (dppe) and 37.4 (PiPr3) ppm in 31P NMR spectroscopy, whose shifts compare well with those of its immediate precursor V8. 1H NMR spectroscopy shows the presence of trans-disubstituted vinyl groups with signals at δ 9.01 (RuCHCH) and 6.78 (RuCHCH) ppm, along with the presence of the phenylene, dppe, PiPr3, and SAc protons in the correct integral ratios. In the 13C NMR spectrum, the typical resonance signals of Ru(CO) (209.1 ppm), SC(O)CH3 (190.7 ppm), RuCHCH (156.2 ppm), RuCHCH (134.8 ppm), and RuCC (117.1 ppm) are observed. Note that all complexes with an octahedral coordination geometry (1, 2, 4−8) show the expected downfield shift of the Ru(CO) resonance by 5−7 ppm relative

to the resonances for the corresponding square-pyramidal complexes 3 and V4−V8. The IR spectra of all vinyl complexes in CH2Cl2 are quite similar (see the Supporting Information). The strong band of the CO stretching vibration in the octahedral complexes 1, 2, and 4−8 is found between 1901 and 1906 cm−1 and thus is at slightly lower energy than those of the corresponding fivecoordinated, square-pyramidal complexes 3 and V4−V8 (Table 1). The red shift by ∼8 cm−1 indicates a moderate increase of Table 1. Characteristic FT-IR Data (cm−1) of the Vinyl Complexes in CH2Cl2 ν̃(C C)

ν̃(C O)

3 V485 V663

1913 1911 1910

V763 V870

2058

1910 1910

V1145

2071

ν̃(C O)

ν̃(C C)

ν̃(C O)

ν̃(C O)

1 2

1902 1903

1707

4 5 6 7 8 10 11

1906 1901 1902 1902 1902

1700

2058 2056 2059

1704 1707 1707 1710 1701 1701

electron density at the metal atom upon substitution of the monodentate chloro by a bidentate benzoate donor and the concomitant increase of the valence electron (VE) count from 16 to 18. In addition, we can clearly identify the ν(CO) band of the newly introduced thioacetyl functions at energies between 1701 and 1710 cm−1 (1689 and 1713 cm−1 for solid samples), the ν(CC) band of the acetylide units between 2056 and 2071 cm−1 (2045 and 2052 cm−1 for solid samples), and the CH stretches of the phosphine ligands between 2800 and 3000 cm−1 (methyl and methine groups). Crystallographic Studies. The solid-state structures of the six-coordinated vinyl complexes 2, 5, and 6 and of the fivecoordinated precursors V4 and V6 (Figure 1, Table 2, and the Supporting Information) were determined by X-ray diffraction. The quality of the crystals of 6 was somewhat poor, which resulted in an incomplete data set. Particular difficulties were encountered in the refinement of the toluene cosolvent molecule. The adverse effects on the structure parameters of the complex itself are, however, minor and only result in somewhat larger standard deviations in comparison to those for the other structures of this study. In all six-coordinated complexes the ruthenium atom is octahedrally coordinated with the two phosphine ligands in mutually trans positions, as already deduced from the appearance of only one singlet D


Organometallics

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Figure 1. Drawings of molecules of complexes 2, 5, 6, and V4. Hydrogen atoms and cosolvent molecules have been omitted for clarity; ellipsoids are shown at the 50% probability level.

Table 2. Selected Bond Lengths (Å) of Complexes 2, 5, 6, V4, and V6 Ru−CO CO Ru−C1 C1−C2 C2−C3 a

2

5

6

V4

V670

1.793(3) 1.175(4) 2.007(3) 1.338(4) 1.472(4)

1.801(5) 1.162(6) 2.015(4) 1.350(6) 1.467(6)

1.794(8) 1.172(8) 2.021(6) 1.329(10) 1.484(8)

1.878(5)a 0.978(5)a 1.986(3) 1.294(5) 1.501(5)

1.791(3) 1.148(3) 1.973(2) 1.319(4) 1.464(3)

These parameters suffer from disorder between the CO and Cl ligands.

resonance in the 31P NMR spectra. The structures closely resemble those of the corresponding five-coordinated complexes in many respects, such as the cisoid arrangement of the vinyl and the carbonyl ligands owing to secondary stabilizing interactions between the filled vinyl π and CO π* orbitals.91 As in other divinylphenylene-bridged diruthenium complexes, the entire {Ru}-CHCH-C6H4-CHCH-{Ru} π-conjugated path of complexes 5 and 6 is planar.61,66−68,70 Specifically, for centrosymmetric complexes 5/6/V6, the Ru−C(1)−C(2)− C(3) torsional angles are 179.8/175.7/173.4°, while the interplanar angle between the Ru-CHCH moieties and the phenylene ring amounts to 17.9/15.4/17.2°. For complexes V4 and 2 the Ru−C(1)−C(2)−C(3) torsional angle is 171.1 and 172.9° and the interplanar angle between the Ru-CHCH moiety and the phenylene ring amounts to 7.8 and 9.1°, respectively. The interplanar angles between the phenyl ring of the benzoate ligands and the Ru,O(2),O(3) plane are 6.7° (2), 5.7° (5), and 8.3° (6). The Ru−C(1), C(1)C(2), and C(2)− C(3) bond lengths of complexes V4/2/5/6/V6 of 1.986(3)/ 2.007(3)/2.015(4)/2.021(6)/1.973(2) Å, 1.294(5)/1.338(4)/ 1.350(6)/1.329(10)/1.319(4) Å, and 1.501(5)/1.472(4)/ 1.467(6)/1.484(8)/1. 464(3) Å are in th e usual range.70,81,85,92−97 UV/vis Spectroscopy and (TD-)DFT Calculations. UV/ vis data for the new complexes are compiled in Table 3, and the spectra obtained from CH2Cl2 solutions are displayed in Figures 43−47 of the Supporting Information. The bisacetylide complexes 10 and 11 show an intense broad absorption band at 364 or 380 nm with a shoulder at higher energy. With reference to other studies on similar systems,

Table 3. UV/Vis Data for Complexes 1−8 and V4−V8 in CH2Cl2 Solution λmax, nm (ε, M−1 cm−1)

complex 1 2 3 4 V481 5 6 V6a 63 7 V7a 8 V870 V8OMe71 10 11

316 310 342 346 308 362 360 358 312 312 381 383 308 364 380

(19100) (16500), (26300), (28100) (23000), (33800) (37500) (35700), (30800), (38300), (81000) (69500), (30000), (65300) (82000)

380 (sh, 1740) 400 (sh, 2660), 510 (sh, 1000) 406 (3700), 510 (290)

388 (18500), 542 (880) 400 (sh, 2500) 389 (6550), 518 (1340) 517 (2050) 400 (70000), 549 (2400)

a

The extinction coefficients reported in ref 63 have since been found to be erroneous and are corrected here.

these transitions are assigned as multiconfigurational MLCT excitations involving a considerable mixing of Ru dπ orbitals with alkynyl π orbitals, and the low-energy transitions should assume a RuII(dπ)−L(π*) (MLCT) character admixed with a moderate to strong π−π* intraligand (IL) contribution.98,99 The electronic spectra of the 5- and 6-coordinated alkenyl complexes of Chart 1 resemble those of other complexes of this type. They are dominated by a strong band peaking at 310−381 E


Organometallics

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Table 4. Calculated One-Electron Energies of Selected Frontier MOs of Complexes V4Me, 1Me, and 4Me and Percent Contributions of Individual Fragmentsa V4Me LUMO+4 LUMO+3 LUMO+2 LUMO+1 LUMO HOMO HOMO-1 HOMO-2 HOMO-3 HOMO-4

1Me

4Me

energy, eV

[Ru]

styryl

L

energy, eV

[Ru]

styryl

L

energy, eV

[Ru]

styryl

L

0.28 0.24 0.07 −0.44 −1.48 −5.43 −6.54 −6.57 −7.04 −7.14

98 1 97 8 83 32 79 64 1 78

1 99 0 92 14 67 5 1 99 20

1 0 3 0 3 0 16 34 0 1

0.24 −0.05 −0.32 −0.39 −1.02 −5.25 −6.48 −6.62 −6.89 −6.98

96 1 8 88 2 34 90 73 70 1

1 0 90 7 1 65 5 11 16 99

3 99 1 5 97 1 5 16 14 0

−0.28 −0.44 −0.48 −0.88 −1.42 −5.42 −6.59 −6.69 −6.96 −7.18

0 10 78 6 2 35 90 63 55 28

100c 1 7 92 1 64 5 10 29 0

0 89b 16 1 97 1 5 27 16 72

a

[Ru] denotes the Ru(CO)(PR3)2 fragment, and L denotes the chloride or benzoate coligand. bLocal MO of the SAcetyl function at the styryl ligand. cLocal MO of the SAcetyl function at the benzoate ligand.

accord with the value of the Hammett parameter of the thioacetyl functionality of +0.44, which indicates that it behaves as a veritable electron acceptor.101 The thioacetyl substituents have the further effect of stabilizing an unoccupied styryl-based MO which constitutes the LUMO+1 in 4Me but the LUMO+2 in 1Me. Irrespective of the coordination number and the substituent on the styryl or the benzoate ligand, the main band is largely due to a π → π* excitation within the ruthenium/styryl entity along with some more or less pronounced MLCT admixture (see Table 6) and an additional d−d component in 1Me. Introduction of the thioacetyl functionality to the styryl ligand causes a distinct red shift of the main absorption band, while substitution at the benzoate ligand has no such effect (cf. complexes 1−4). This experimental observation is adequately reproduced by our calculations, as is seen on comparison of the experimental with the calculated spectra (see Figures 43 and 47 of the Supporting Information), and can be traced to the lowering of the energy gap between the mixed metal/styryl HOMO and the entirely styryl based acceptor orbital. The spectrum of the five-coordinated complex 3 also features two weak bands at 510 and 400 nm, respectively, which have analogously been observed in the closely related complex V4. In that latter complex, these bands are due to transitions between the metal/styryl-based HOMO and the metal-based LUMO, which essentially constitutes the dσ orbital pointing to the vacant coordination site, and to a d−d type transition from the metal/phosphine donor orbital HOMO-3 to the same acceptor orbital.81 Similar low-energy absorptions are seen as a weak band or a shoulder at the low-energy side of the main band in the benzoate complexes 1, 2, 4, and 7. They here assume the character of styryl → benzoate ligand to ligand charge transfer (LLCT) and styryl to metal charge transfer (LMCT) transitions (see Tables 3 and 6). Charge density differences concomitant with the low-energy transitions in model complexes 1Me and 4Me are visualized in Figures 49 and 50 of the Supporting Information. The substantial red shift from ca. 315 nm in the monoruthenium complexes 1 and 2 to ca. 360 nm in the 1,4divinylphenylene-bridged complexes 5 and 6 and to 380−400 nm in the triruthenium complexes 8, V8, and V8OMe is obviously a consequence of the increased conjugation length of the π chromophore and the concomitant lowering of the energy gap between the relevant orbitals. In an attempt to substantiate that hypothesis, calculations were performed on the slightly

nm, the extinction coefficient of which increases with the extension of the metal−organic π system. For complexes of the same nuclearity and architecture, the position of this prominent absorption band is almost invariant with respect to coordination number and valence electron count, as is exemplarily seen for complexes 1 (λ 316 nm) and 2 (λ 310 nm), 3 (λ 342 nm) and 4 (λ 346 nm), or V6, 5, and 6 (λ 358, 362, and 360 nm, respectively). This already signals that the ancillary donor ligand lies outside the main π-conjugated path of these molecules (vide infra). Revealingly, the position of the band in complex 7 with its less conjugated 1,3-divinylphenylene linkage strongly resembles that in complexes 1 and 2. The effective chromophore in 7 does obviously not extend beyond a single ruthenium styryl unit, as was also seen for the corresponding five-coordinated precursors V6 and V7.63 These experimental findings are readily explained by the results of previous TD-DFT calculations on complexes V4Me,81 complex (PhCHCH)Ru(CO)(κO,O-OOCC6H3CCH-4)(PiPr3)2, a close analogue to complex 1,80 and the present DFT and TD-DFT (Gaussian 09;100 for details see Supporting Information) calculations on model complexes 1Me and 4Me with PMe3 instead of PiPr3 ligands. Those calculations also probe for the effects of introducing the acetyl-protected thiol functionalities to both types of attachment sites: i.e., the styryl and benzoate ligands. Graphical representations of the orbitals HOMO-4 to LUMO+4 of complex 4Me are shown in Figure 42 of the Supporting Information, while Table 4 summarizes the energies and compositions of these frontier molecular orbitals (FMOs). Apart from a slight enhancement of the metal contributions, chloride substitution by benzoate has not much effect on the composition of the HOMO and the HOMO-1 orbitals, which retain their highly mixed πstyryl or their dominant dRu character. The lower-lying orbitals assume mixed metal/ ligand character with larger contributions of either the benzoate or the styryl ligand or are confined to the styryl ligand. The main difference between the five- and six-coordinated styryl complexes is the character of the LUMO, which changes from a d-orbital pointing toward the vacant coordination site to a benzoate-based MO, and the availability of occupied, benzoatebased MOs below HOMO-4, which lead to additional electronic transitions. Increasing the VE count from 16 in V4Me to 18 in 1Me has the expected effect of raising the HOMO energy, as is evident from Table 4. On introduction of the thioacetyl functionalities, the energies of all benzoate and styryl based MOs decrease (cf. complexes 4Me and 1Me). This is in F


Organometallics

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Figure 2. Graphical representations of relevant MOs of complex V8.

simplified PMe3-substituted model complexes V8Me and V8OMeMe while keeping the complete dppe ligands at the central ruthenium atom. Unfortunately, similar calculations on model complex 8Me failed to converge. One remarkable property of these ethynylvinylphenylene-bridged triruthenium complexes is complete charge delocalization over the entire system, including the (alkenyl)Ru(CO)Cl(PiPr3)2 end groups, in all available oxidation states up to the di- or even trication in the case of V8OMe. Figure 2 shows graphical representations of

some relevant frontier MOs that form the basis of the wirelike behavior and of the optical properties of complex V8, while the contributions of individual fragments to the overall electron density of the respective MOs for these three complexes are compared in Table 5. The occupied FMOs of the triruthenium complexes V8 and V8Me are indeed delocalized over the entire {Ru}−bridge−{Ru′}−bridge−{Ru} array. In complete agreement with our experimental results, all high-lying occupied FMOs receive strong (HOMO-2, HOMO-3) or even dominant G


Organometallics

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Table 5. Calculated One-Electron Energies of Selected Frontier MOs of Complexes V8Me and V8OMeMe with Percent Contributions of Individual Fragmentsa V8Me LUMO+4 LUMO+3 LUMO+2 LUMO+1 LUMO HOMO HOMO-1 HOMO-2 HOMO-3 HOMO-4

V8OMeMe

energy, eV

[Ru]

B

[Ru]B

B′

[Ru′]

energy, eV

[Ru]

B

[Ru]B

B′

[Ru′]

−0.57 −0.69 −0.76 −1.37 −1.43 −4.40 −4.84 −5.05 −5.60 −5.90

0 5 0 0 87 2 7 0 14 0

0 62 6 0 13 28 44 26 26 1

88 21 93 0 0 24 7 47 23 1

11 12 1 14 0 42 36 26 28 80

1 1 0 86 0 4 6 0 9 18

−0.59 −0.65 −0.79 −1.44 −1.46 −4.55 −4.94 −5.18 −5.64 −6.11

0 2 0 0 86 3 5 3 12 8

3 30 0 0 14 32 35 30 25 22

95 23 99 0 0 27 20 35 22 19

2 42 0 13 0 34 34 29 27 34

0 3 0 87 0 3 5 4 14 17

a

[Ru] and [Ru′] denote the left and right {Ru(CO)Cl(PMe3)2} moieties, [Ru]B denotes the central {Ru(dppe)2} moiety, and B and B′ denote the 4-vinyl-1-ethynylphenylene bridges connecting [Ru] and [Ru′] to [Ru]B.

Table 6. TD-DFT Calculated Transitions for Complexes 1Me, 4Me, and 8Me in CH2Cl2 (CP-PCM) and Comparison to Experimental Data λmax, nm (osc strength) 1

Me

358 (0.007) 348 (0.062) 297 (0.272) 292 (0.434)

4Me

V8Me

367 (0.010) 351 (0.007) 321 (1.000) 548 (0.003) 541 (0.004) 385 (0.2955)

V8OMeMe

558 (0.005) 556 (0.003) 400 (1.240) 395 (0.696)

λmax, nm (ε, M−1 cm−1)

main character HOMO → LUMO+1 (85%) HOMO → LUMO (98%) HOMO-1 → LUMO+1 (64%), HOMO → LUMO+2 (27%) HOMO → LUMO+2 (68%), HOMO-1 → LUMO+1 (23%) HOMO → LUMO (97%) HOMO → LUMO+2 (74%), HOMO → LUMO+3 (10%) HOMO → LUMO+1 (96%) HOMO-1 → LUMO+1 (22%), HOMO → LUMO+1 (44%) HOMO-1 → LUMO (24%), HOMO→LUMO (46%) HOMO → LUMO+3 (30%), HOMO → LUMO+4 (52%) HOMO-1 → LUMO (32%), HOMO → LUMO (44%) HOMO-1 → LUMO+1 (22%), HOMO → LUMO+1 (58%) HOMO → LUMO+3 (70%) HOMO → LUMO+4 (74%)

316 (19100)

not obsd not obsd 346 (28100) 517 (2050)

383 (69500) 549 (2400)

400 (70000) 308 (30000)

Electrochemical Studies. Cyclic voltammetry (CV) was used to study the electrochemical behavior of the new 6coordinated vinyl complexes 1, 2 and 4−8 (CH2Cl2, 0.1 M Bu4NPF6) and of the bis(σ-arylacetylide) complexes 10 and 11. Redox potentials are reported in Table 7 along with those of their corresponding precursors (3, V4−V11). Representative voltammograms of complexes 4−6 and 8 are shown in Figure 3. A more complete set of voltammograms recorded for the studied complexes are provided as Figures 13−33 in the Supporting Information. In agreement with previous findings on this family of complexes55,63,70 the number of electrochemical processes observed at low potentials is equivalent to the number of ruthenium units present in the molecules, with the exception of 8, for which four events are observed, as in V8. For our present purpose, we particularly focused our attention on the fully reversible processes at lower potentials. The data in Table 7 reveal the following features. (i) The mononuclear ruthenium complexes 1−4, 10, and V4 undergo a single reversible one-electron oxidation at moderately positive

(HOMO, HOMO-1) contributions from the ethynylvinylphenylene bridges. Bridge contributions and overall delocalization even increase somewhat on introduction of the electronreleasing methoxy substituents in V8OMe. In further agreement with our experiments, the HOMO is biased toward the central bis(alkynyl)ruthenium moiety with only small contributions from the terminal sites. Slightly larger contributions of the alkenyl ruthenium moieties (including the CHCH “linker”) are found for HOMO-1 and HOMO-3. The results of TD-DFT calculations on complexes V8Me and V8OMeMe in Table 6 indicate that the intense band at ∼380 nm originates from the HOMO → LUMO+3 and HOMO → LUMO+4 excitations. With reference to Figure 2, these bands are of π → π* parentage along with a considerable bridge → metal charge transfer contribution to the central Ru(dppe)2 fragment for the HOMO → LUMO+4 transition. Much weaker bands at lower energies are again seen for the prototypical LMCT and LLCT transitions, as discussed before. H


Organometallics

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Table 7. Electrochemical Data of All Complexes at v = 0.1 V/sa E1/20/+, V

E1/2+/2+, V

E1/22+/3+, V

E1/23+/4+, V

ΔE1/2,b V

Kc or Kc

d

1 2 3 4 5 6 7 8

0.11 0.13 0.33 0.19 −0.25 −0.24 0.04 −0.31

0.98 1.08d 1.02d 0.98d 0.02 0.04 0.32 −0.04

10 11 V481 V663 V763 V870

0.04 −0.24 0.28 −0.09 0.18 −0.26

0.95d 0.05 0.85d 0.17 0.45 0.08

V1145

−0.33

0.01

0.15

0.56

0.84d

0.26

0.63

0.96d

× × × × × ×

104 104 104 104 103 106

0.27 0.28 0.28 0.27 0.20 0.40

3.7 5.4 5.4 3.7 2.4 5.8

0.29

8.0 × 104

0.26 0.27 0.34 0.18 0.37 0.34

2.5 3.7 5.6 1.1 1.8 5.6

× × × × × ×

104 104 105 103 106 105

a

Data in CH2Cl2/0.1 M NBu4PF6 at a Pt electrode and at room temperature unless stated otherwise. Potentials are calibrated against the internal Cp2Fe0/+ couple, which is set as E = 0.00 V. bDifference between half-wave potentials. cEquilibrium constant for the reaction A(n+1)+ + A(n−1)+ ⇆ 2An+, as calculated by the expression K = exp{FΔE1/2/(RT)}. dPeak potential (Epa) of an irreversible process.

Figure 3. Cyclic voltammograms of complexes 4 (top left), 5 (top right), 6 (bottom left), and 8 (bottom right) at room temperature in 0.1 M NBu4PF6/CH2Cl2 and at v = 100 mV/s.

potential (E1/20/+ = 0.04−0.33 V vs Cp2Fe0/+), while a second, completely irreversible one-electron oxidation81 occurs at substantially higher potential (Epa+/2+ = 0.98−1.08 V vs Cp2Fe0/+). (ii) Similar to their corresponding precursor complexes V6−V8 and V11, the di- and triruthenium complexes 5−8 and 11 are oxidized in two or four sequential one-electron steps with potential separations of 180−380 mV for each step. The corresponding values of the individual

comproportionation constants (or, in the case of 8, equilibrium constants for the formation of species 8n+ from 8(n−1)+ and 8(n+1)+ with n = 1−3) of 1.1 × 103 to 1.8 × 106 indicate that the monooxidized forms of the dinuclear complexes and the monoto trioxidized forms of complex 8 are thermodynamically stable and hence can be generated and characterized. (iii) The replacement of the chloro ligands by bidentate carboxylate donors and the accompanying increase of the VE count from I


Organometallics

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Table 8. Characteristic IR Data (cm−1) of Complexes 1, 8, and V8 in Their Various Oxidation States in 0.2 M NBu4PF6/1,2-C2H4Cl2

16 to 18 shift the oxidation potentials of all redox processes to more cathodic values. This matches the trends in the CO band shifts. However, considering that the oxidation processes mainly involve the bridging ligand (vide infra), the redox potentials are only an indirect probe of the electron densities at the metal atoms; rather, they track the electron-releasing properties of the {Ru(CO)(L)(PiPr)2} entities (L = Cl−, ArCOO−) toward the vinyl ligands. (iv) Introduction of the electron-withdrawing thioacetyl functionality at the styryl ligand shifts the oxidation potential anodically by 50−60 mV (cf. complexes 2 and 4 or complexes 3 and V4). A similar effect is seen for the acetylide complexes (cf. the oxidation potential of −0.05 V for Ru(dppe)2(−CCPh)255 compared to 0.04 V for 10). Attaching that substituent to the benzoate ligand has a much smaller effect, if any (cf. complexes 1 and 2 or complexes 5 and 6). This general behavior is readily explained by the strong contribution of the styryl or the phenylethynyl ligands to the relevant redox orbital, as is indicated by our quantum chemical calculations. Due to the negligible contribution of the benzoate ligand to the immediate occupied FMOs, 4substitution on the benzoate has only a minor inductive effect. (v) We also note that the first and the second oxidation potentials of the di- and triruthenium complexes 5, 6, and 8 are lower than the first oxidation potentials of the monometallic congeners 1 and 2, whereas the effect on complex 7 with its less conjugated 1,3-divinylphenylene linkage is much smaller. This fact supports the idea that the oxidized species gain considerable stabilization when the two vinyl ruthenium donors are in direct conjugation. For several of the thioacetyl-functionalized complexes, complications arising from electrode poisoning were observed during repetitive scans on platinum electrodes which led to a broadening of the redox waves. Wiping of the working electrodes after each scan was therefore necessary in order to obtain reproducible results. These difficulties were largely circumvented by using a glassy-carbon working electrode as it is exemplarily shown for complex 6 (see Figure 25 of the Supporting Information). IR Spectroelectrochemical Experiments. Owing to the prototypical π-acid properties of the carbonyl ligand, the Ru(CO) stretching band provides a valuable tag to monitor the electron density changes at the metal upon ligand substitution or a redox process. Here, the oxidation-induced CO band shifts allow us to elucidate to what extent oxidation affects those metal centers. Here it was of interest to first probe experimentally how the benzoate ligand and the acetylthiolate functionalities affect the metal contribution to the HOMO and hence the Ru(CO) band shift upon oxidation. This was done by means of IR spectroelectrochemistry in a transparent thinlayer electrochemical cell.102 The results of these studies are collected in Table 8. The Ru(CO) band shift of 73 cm−1 for the first oxidation of complex 1 (see Figure 34 of the Supporting Information) is basically identical with that of 72 cm−1 for the analogous 4-ethynylbenzoate complex (1904 → 1976 cm−1)84 and clearly surpasses that of 65 cm−1 for the five-coordinated styryl complex V4.85 In complexes with more than one {Ru(CO)(L)(PiPr3)2} site, the charge-sensitive ν̃(CO) IR label at the vinyl ruthenium subunit plays an important role in the interpretation of the IR spectroelectrochemistry (SEC) patterns and even allowed us to quantify the degree of charge delocalization between them. Thus, the appearance of just one Ru(CO) band in complexes V6n+ (n = 0−2),63 V8n+ (n = 0−3), and V8OMen+ (n = 0−4)70

ν̃(CC) 1 1•+ 8 8•+ 82+ 8•3+ V870 V8•+ V82+ V8•3+

n.a. n.a. 2058 2032 2012 1734 2058 2023 2012 1775

(m) (w), 1978 (w), 1890 (vs) (w), 1970 (m), 1866 (w) (w) (m) (w), 1888 (vs) (m), 1960 (sh), 1860 (m) (w)

ν̃(CO)

ν̃(CO)

1904 1976 1902 (s) 1921 (s) 1925 (s) 1967 (s) 1910 (s) ∼1916 (s) 1929 (vs) 1967 (s)

n.a. n.a. 1710 1710 1710 1710

(w) (w) (w) (w)

evidenced that the charge(s) are completely delocalized over the entire wirelike metal−bridge−metal array, even when the oxidation states of the individual alkenyl ruthenium moieties are formally different. Changing the topology of the central bridge from 1,4 in V6 to 1,3 in V7 was found to induce partial charge localization on one of these sites.63 The question of main interest in the present context is to evaluate the impact of the introduction of the thioacetyl-functionalized benzoate electrode linking ligands on the electronic delocalization in complex 8 in its various states. The results of our IR spectroelectrochemical studies on 8 are displayed in Figure 4 and Figures 35−37 of the Supporting Information and are gathered in Table 8 along with those of V8.70 As for this latter analogue, the new trimetallic complex 8 offers the chargesensitive ν(CO) IR label at the vinyl ruthenium subunit as well as the ν(CC) label of the bridging ligands. Overall, the behavior of 8 varies only slightly from that of V8. The difference lies in the larger blue shift of the Ru(CO) band of 19 cm−1 for 8 in comparison to that of 6 cm−1 for V8 on the first oxidation, indicating a larger contribution of the vinyl ruthenium termini to the first oxidation process. This is again in line with the fact that the more electron-rich {Ru(CO)(η2OOCR)(PiPr3)2} moieties enhance delocalization along the πconjugated {Ru}−bridge−{Ru′}−bridge−{Ru} backbone by distributing the HOMO more evenly over the three metal sites. As for V8, the ν(CC) band strongly gains in intensity upon oxidation and shifts to 1890 cm−1, thus overlapping with the Ru(CO) band (see Figure 37 of the Supporting Information for a deconvoluted spectrum). We note that redox-induced intensity changes of ν(CC) are rather common for alkynediyl-bridged diruthenium complexes.59,103−106 This confirms the still heavy involvement of the central bis(alkynyl) ruthenium site in the first oxidation process. In contrast, the shift of the Ru(CO) bands of just 4 cm−1 during the second oxidation of 8 is smaller than that for V8 and reveals a weaker contribution of the ruthenium termini to that process. The total shift of the ν(CO) band upon oxidation to the dication is, however, similar for both complexes and only the shift sequence inverts. The Ru(CO) band shift of 42 cm−1 upon the third oxidation signals a larger contribution of the vinyl ruthenium sites to that process and agrees well with the shift of 38 cm−1 observed for V8. Concomitantly, the shift of the ν(CC) bands upon the second oxidation are more modest than those observed during the first oxidation, showing that the redox sites that are involved in that process receive lower contributions from the central bis(alkynyl) ruthenium entity. Importantly, our results indicate that all species have just one J


Organometallics

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Figure 4. IR spectroscopic changes of 8 during the first (top left), second (top right), and third (bottom) oxidations in 0.2 M NBu4PF6/1,2-C2H4Cl2.

Table 9. EPR Data at Room Temperature and at 103 K in Solution (CH2Cl2) and in the Solid State T = room temp •+

1

T = 103 K

31

99/101

solid

giso = 2.042 (A( H) = 8.9 G (1 H), 5.4 G (1H), A( P) = 8.6 G (2P); A( (1Ru))c g⊥ = 2.033, g|| = 2.017, Δg = 0.016, ⟨gav⟩ = 2.022c,d

solution

giso = 2.042

solid

g⊥ = 2.035, g|| = 2.018, Δg = 0.017, ⟨gav⟩ = 2.024c,d

3•+

solution

4•+

solution

5•+

solution solid solution solid solution solid solution solid solution

giso = 2.038 (A(1H) = 8.4 G (1 H), 2.9 G (1H), A(31P) = 18.4 G (2P); A(99/101Ru) = 9.6 G (1Ru))c giso = 2.042 (A(1H) = 9.3 G (1 H), 5.0 G (1H), A(31P) = 7.9 G (2P); A(99/101Ru) = 10.7 G (1Ru))c giso = 2.023 giso = 2.024 giso = 2.025 giso = 2.023 giso = 2.035 giso = 2.023 no signal ⟨gav⟩ = 2.034 giso = 2.045 (A(31P) = 21.5 G)c

solution solution solution solid

giso = 2.028 (A(31P) = 9.5 G; A(99/101Ru) = 4.5 G)c giso = 2.027 (A(31P) = 10 G; A(99/101Ru) = 6 G)b,c no signal giso = 2.030

1

2•+

6•+ 7•+ 8•+ 4•+ 81 V6•+ 63 V7•+ 63 V8•+ 70

solution

Ru) = 9.6 G

g⊥ = 2.054, g|| 2.036c,d g⊥ = 2.033, g|| 2.022c,d g⊥ = 2.054, g|| 2.018c,d g⊥ = 2.035, g|| 2.024c,d g⊥ = 2.079, g|| 2.066c,d g⊥ = 2.059, g|| 2.036c,d giso = 2.023 giso = 2.024 giso = 2.023 giso = 2.023 giso = 2.021 giso = 2.023 ⟨gav⟩ = 2.023 ⟨gav⟩ = 2.032 g⊥ = 2.072, g|| 2.046c,d giso = 2.022a giso = 2.011a giso = 2.018 giso = 2.032

= 2.027, Δg = 0.027, ⟨gav⟩ = = 2.017, Δg = 0.016, ⟨gav⟩ = = 2.029, Δg = 0.025, ⟨gav⟩ = = 2.018, Δg = 0.017, ⟨gav⟩ = = 2.041, Δg = 0.038, ⟨gav⟩ = = 2.025, Δg = 0.034, ⟨gav⟩ =

= 2.033, Δg = 0.039, ⟨gav⟩ =

a T = 110 K. bT = 230 K. cData based on the spectrum simulation (MATLAB/EasySpin). d⟨gav⟩ = [(1/3)(2g∥2 + g⊥2)]1/2 for spectra of the axial type or (gx2 + gy2 + gz2)1/3 for those of the rhombic type.

K


Organometallics

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show such features. All of these conclusions are fully supported by previous quantum chemical calculations on such complexes59,61−63,75,80,81,109 and the present calculations on complexes 4Me, V8Me, and V8OMeMe (for a spin density plot of 4Me•+ see Figure 59 of the Supporting Information). Interestingly the six-coordinated vinyl complexes 4•+−7•+ exhibit no well-resolved hyperfine splitting to 31P nuclei, in contrast to their chloro-substituted 16-VE precursors. The experimental EPR spectrum of radical cation 4•+ is reproduced by invoking hyperfine splittings to the 31P nuclei of the phosphine coligands of 7.9 G, to the vinylic protons of 5.0 and 9.3 G, and to the 99/101Ru nuclei of 9.6 G (Figure 5). It thus

single Ru(CO) band in their IR spectra such that both termini remain electronically equivalent. Therefore, we are still dealing with highly bridge-centered oxidized forms with the positive charge(s) delocalized over the entire π-conjugated path, though involving the central and peripheral parts of the structure to somewhat different degrees. These experimental results are again in good agreement with our quantum chemical studies and show that all relevant frontier molecular orbitals are delocalized over the entire conjugated path, a point of importance for charge transport over such molecular architectures. We also note that the ν(CO) band of the acetyl protected thiol groups remains unaffected during the different redox events. This confirms that the benzoate ligand is effectively decoupled from the delocalized π-conjugated path and hence blocks electronic communication between the electrode attachment sites and the π-conjugated wirelike core. EPR Spectroscopy. EPR spectroscopy is a very powerful method in order to clarify the metal versus bridge contributions to the SOMO of paramagnetic species. Such information can be obtained from the g value and from the hyperfine coupling constants, either to the metal atoms or to other EPR-active nuclei of other coligands. Organic ligand-centered radicals typically give isotropic signals at g values close to that of the free electron ge at 2.0023. Genuine Ru(III) species are usually EPR silent in fluid solution and exhibit axially or rhombically split g tensors at low temperatures as solids or frozen solutions with average g values that differ strongly from ge. Thus, intense isotropic EPR signals at room temperature make a strong case for a ligand-dominated oxidation, whereas strong deviations of the g value from ge and large g anisotropies suggest a large metal contribution.107,108 This fact was recently underpinned by theoretical calculations on corresponding acetylide and vinyl systems.75,98 Samples of the monooxidized paramagnetic species 1•+−8•+ were obtained via chemical oxidation of the neutral precursors with ferrocenium or acetylferrocenium hexafluorophosphate in CH2Cl2, and their EPR spectra were recorded at room temperature and at 103 K (Table 9 and Figures 51−58 of the Supporting Information). The resonance lines of the radical cations 1•+−8•+ and V4•+−V8•+ are broad but retain their isotropy or low degree of g anisotropy as frozen solutions or as solid samples. From the data in Table 8, we do clearly see that the SOMO of all paramagnetic species has dominant ligand character. The EPR signals of the mononuclear complexes (giso ≈ 2.040) show styryl ligand parentages of the SOMO slightly smaller than those of the diruthenium complexes (giso ≈ 2.025). We note that g values and g anisotropies Δg of the 6coordinated benzoate complexes are very similar to those of comparable 5-coordinated ones. This indicates that chloride substitution by a benzoate ligand and the concomitant increase of the VE count from 16 to 18 has no measurable influence on the spin density distributions in their associated radical cations. The notion of a ligand-dominated oxidation also pertains to radical cations of the triruthenium complexes 8 and V8, which are EPR silent in fluid solution at room temperature but exhibit isotropic signals at low T and even at room temperature as solids. Associated g values are only slightly larger than those of the dinuclear divinylphenylene-bridged complexes 6•+ and V6•+ owing to a somewhat increased metal character through insertion of the central ruthenium bis(ethynyl) moiety. The heavy involvement of the central ruthenium bis(ethynyl) part of this structure is also indicated by the loss of the A(31P) hyperfine couplings in V8OMe•+, while V6•+ and V7•+ clearly

Figure 5. Simulated (top, blue) and experimental (bottom, black) EPR spectra of chemically oxidized complex 4•+ in CH2Cl2 at T = 293 K. Hyperfine coupling constants are A(31P) = 7.9 G (2 P), A(1H) = 5.0 G (1 H), A(1H) = 9.3 G (1 H), and A(99/101Ru) = 10.6 G (1 Ru).

seems that, on introduction of the benzoate ligand, hyperfine splittings to the phosphorus donor ligands decrease while those to the hydrogen atoms at the vinyl ligand increase with only minor effects on those to the metal atom itself. This results in broadened individual lines such that no hyperfine splittings are immediately apparent. Concluding Remarks. In this work, we have synthesized novel carbon-rich ruthenium complexes for molecular electronics. These ruthenium(II) complexes bearing bridging ethynylphenylene and/or vinylphenylene as well as benzoate ligands with terminal acetyl-protected thiol functionalities for electrode attachment will allow for the construction of new molecular junctions with extended π-conjugated paths including one, two, or three metal centers. As for their parent compounds without these linking functions, cyclic voltammetry reveals the presence of one to four consecutive oxidations at conveniently low potentials. Significantly, our electrochemical and spectroscopic studies show that the electrode-binding functionalities do not interfere with the wirelike properties of the inner, π-conjugated part of these molecules and alter their intrinsic redox potentials only slightly. The relevant oxidation processes are still dominated by the vinyl/ethynyl bridging ligand, and the relevant frontier molecular orbitals are strongly delocalized over the entire conjugated path. In contrast to previously reported oligometal complexes where the electrodebinding functionalities are attached to terminal arylethynyl ligands,29,52,110 the benzoate ligands bearing the functionalities for electrode attachment are outside the conjugation paths and insulate the wirelike central portions of these molecules from their peripheries, in very much the same way as would be the case for an alkyl connector, but with the rigidity of a conjugated L


Organometallics

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(2) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277. (3) Choi, S. H.; Kim, B.; Frisbie, C. D. Science 2008, 320, 1482. (4) Low, P. J. Dalton Trans. 2005, 2821. (5) Metzger, R. M. J. Mater. Chem. 2008, 18, 4364. (6) Joachim, C.; Ratner, M. A. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8801. (7) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541. (8) James, D. K.; Tour, J. M. Chem. Mater. 2004, 16, 4423. (9) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384. (10) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378. (11) McCreery, R. L.; Bergren, A. J. Adv. Mater. 2009, 21, 4303. (12) Coskun, A.; Spruell, J. M.; Barin, G.; Dichtel, W. R.; Flood, A. H.; Botrosghi, Y. Y.; Stoddart, J. F. Chem. Soc. Rev. 2012, 41, 4827. (13) Ulgut, B.; Abruna, H. D. Chem. Rev. 2008, 108, 2721. (14) Chen, F.; Tao, N. J. Acc. Chem. Res. 2009, 42, 429. (15) Song, H.; Reed, M. A.; Lee, T. Adv. Mater. 2011, 23, 1583. (16) Shen, Q.; Guo, X.; Steigerwald, M. L.; Nuckolls, C. Chem. Asian J. 2010, 5, 1040. (17) Kiguchi, M.; Kaneko, S. Phys. Chem. Chem. Phys. 2013, 15, 2253. (18) Wang, G.; Kim, T.-W.; Lee, T. J. Mater. Chem. 2011, 21, 18117. (19) Chen, F.; Hihath, J.; Huang, Z.; Li, X.; Tao, N. J. Annu. Rev. Phys. Chem. 2007, 58, 535. (20) Natelson, D. ACS Nano 2012, 6, 2871. (21) Hong, W.; Li, H.; Liu, S.-X.; Fu, Y.; Li, J.; Kaliginedi, V.; Decurtins, S.; Wandlowski, T. J. Am. Chem. Soc. 2012, 134, 19425. (22) Kim, Y.; Pietsch, T.; Erbe, A.; Belzig, W.; Scheer, E. Nano Lett. 2011, 11, 3734. (23) Zotti, L. A.; Kirchner, T.; Cuevas, J.-C.; Pauly, F.; Huhn, T.; Scheer, E.; Erbe, A. Small 2010, 6, 1529. (24) Egle, S.; Bacca, C.; Pernau, H.-F.; Huefner, M.; Hinzke, D.; Nowak, U.; Scheer, E. Phys. Rev. B 2010, 81, 134402. (25) Qin, L.; Park, S.; Huang, L.; Mirkin, C. A. Science 2005, 309, 113. (26) (a) Meng, F.; Hervault, Y.-M.; Norel, L.; Costuas, K.; Van Dyck, C.; Geskin, V.; Cornil, J.; Hng, H. H.; Rigaut, S.; Chen, F. Chem. Sci. 2012, 3, 3113. (b) Meng, F.; Hervault, Y.-M.; Shao, Q.; Hu, B.; Norel, L.; Rigaut, S.; Chen, X. Nat. Commun. 2014, 5:3023 DOI: 10.1038/ ncomms4023. (27) Dadosh, T.; Gordin, Y.; Krahne, R.; Khivrich, I.; Mahalu, D.; Frydman, V.; Sperling, J.; Yacoby, A.; Bar-Joseph, I. Nature 2005, 436, 677. (28) Luo, L.; Choi, S. H.; Frisbie, C. D. Chem. Mater. 2011, 23, 631. (29) Luo, L.; Benameur, A.; Brignou, P.; Choi, S. H.; Rigaut, S.; Frisbie, C. D. J. Phys. Chem. C 2011, 115, 19955. (30) Kim, B.; Beebe, J. M.; Olivier, C.; Rigaut, S.; Touchard, D.; Kushmerick, J. G.; Zhu, X.-Y.; Frisbie, C. D. J. Phys. Chem. C 2007, 111, 7521. (31) Liu, K.; Wang, X.; Wang, F. ACS Nano 2008, 2, 2315. (32) Marqués-González, S.; Yufit, D. S.; Howard, J. A. K.; Martín, S.; Osorio, H. M.; Garcı ́a-Suárez, V. M.; Nichols, R. J.; Higgins, S. J.; Ceac, P.; Low, P. J. Dalton Trans. 2013, 42, 338. (33) Szuchmacher Blum, A.; Ren, T.; Parish, D. A.; Trammell, S. A.; Moore, M. H.; Kushmerick, J. G.; Xu, G.-L.; Deschamps, J. R.; Pollack, S. K.; Shashidhar, R. J. Am. Chem. Soc. 2005, 127, 10010. (34) Inkpen, M. S.; Albrecht, T. ACS Nano 2012, 6, 13. (35) Danilov, A.; Kubatkin, S.; Kafanov, S.; Hedegard, P.; StuhrHansen, N.; Moth-Poulsen, K.; Bjornholm, T. Nano Lett. 2007, 8, 1. (36) Kubatkin, S.; Danilov, A.; Hjort, M.; Cornil, J.; Brédas, J.-L.; Stuhr-Hansen, N.; Hedegård, P.; Bjørnholm, T. Nature 2003, 425, 698. (37) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoski, M.; Sethna, J. P.; Abruña, H. D.; McEuen, P. L.; Ralph, D. C. Nature 2002, 417, 722. (38) Le Narvor, N.; Lapinte, C. Organometallics 1995, 14, 634. (39) Lavastre, O.; Even, M.; Dixneuf, P. H.; Pacreau, A.; Vairon, J.-P. Organometallics 1996, 15, 1530. (40) Lavastre, O.; Plass, J.; Bachmann, P.; Guesmi, S.; Moinet, C.; Dixneuf, P. H. Organometallics 1997, 16, 184.

ligand. All of these factors are expected to enhance sequential tunneling within such molecular junctions (e.g., of direct charge injection into the molecular wires). In addition, compounds such as 4, 10, and 11 will allow for comparison and rationalization of the performance of each type of building blocks in future conductivity measurements and give valuable insights into the structure−property relationships of electrode/ molecule/electrode junctions with metal-containing molecular wires. This area is likely to rapidly develop in the future, as original properties and high levels of functionality may be integrated into such a molecular system through molecular engineering. In this respect, it is encouraging to see that such molecules can be deposited onto gold surfaces by means of electrospray ion beam deposition, as shown by the results of preliminary experiments (see Figures 60−62 of the Supporting Information), and their electronic properties are the subject of current investigations.

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AUTHOR INFORMATION

S Supporting Information *

Text, figures, tables, and CIF files giving details of the synthesis and characterization data for all complexes, X-ray crystallographic data for complexes 2, 5, 6, and V4, representative voltammetric scans and IR, UV/vis/near-IR and EPR spectra, IR spectroelectrochemical studies on complexes 1, 3, and 4, plots of the crucial frontier molecular orbitals of complexes 4 and V8 as well as calculated spin densities of radical cation 4•+, and charge density difference plots for the low-energy transitions in complexes 1Me and 4Me. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data have been deposited with the Cambridge Structure Data Base as CCDC 948194 (complex 5), CCDC 948195 (complex 6), CCDC 948193 (complex V4), and CCDC 948192 (complex 2). They can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ; fax (+44) 1223-336-033, e-mail [email protected]. Corresponding Author

*E-mail: [email protected] (R.F.W.); stephane. [email protected] (S.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support of this work by the CNRS, the Université de Rennes 1, the ANR (No. ANR-09-JCJC0025), and the German French intergovernmental S&T cooperation program PROCOPE (DAAD/Egide) and by the state of Baden-Württemberg for making the bwGrid computational facilitites available to us. We thank Bernhard Weibert for the X-ray data collection and refinement, Gernot Haug for the preparation of some starting materials, and Priv. Doz. Dr. Mikhail Fonin for helpful discussions and his help with the ion beam deposition.

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REFERENCES

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