Article pubs.acs.org/JPCC
Redox-Active Molecular Wires Derived from Dinuclear Ferrocenyl/ Ruthenium(II) Alkynyl Complexes: Covalent Attachment to Hydrogen-Terminated Silicon Surfaces Guillaume Grelaud,†,§ Nicolas Gauthier,† Yun Luo,† Frédéric Paul,*,† Bruno Fabre,*,† Frédéric Barrière,† Soraya Ababou-Girard,‡ Thierry Roisnel,† and Mark G. Humphrey*,§ †
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS/Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France ‡ Institut de Physique de Rennes, UMR 6251 CNRS/Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France § Research School of Chemistry, Australian National University, Canberra, ACT 0200 Australia S Supporting Information *
ABSTRACT: After their brief characterization, a voltammetric study of the Fe(II)/ Ru(II) heterobinuclear organometallic complexes [Fe]CC-1,4-(C6H4)CC[Ru]C C-1,4-(C6H4)CCH (2), FcCC[Ru]CC-1,4-(C6H4)CCH (3), and PhC C[Ru]CC-1,1′-Fc(CCH) (4) ([Fe] = Fe(κ2-dppe)(η5-C5Me5), [Ru] = transRu(κ2-dppe)2; Fc = ferrocenyl; dppe =1,2-bis(diphenylphosphino)ethane) is reported in solution. These complexes which possess pendant ethynyl groups have then been grafted onto hydrogenated silicon surfaces using a mild photochemical protocol, to yield redoxactive functional interfaces (Si-2/Si-3/Si-4) that were characterized by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. The resulting interfaces Si-2, Si-3, and Si-4 possess surface coverages and electrochemical responses that differ significantly and that depend on the nature of the grafted molecule. While the coverages calculated for the more rigid of these systems (Si-2 and Si-3) are similar (around 10−10 mol cm−2), the significantly lower coverage achieved for Si-4 (4.3 × 10−11 mol cm−2) can be ascribed to the presence of a flexible linker between the redox sites in 4. The two grafted redox centers can be electrochemically addressed in a stepwise fashion, the species exhibiting apparent charge transfer rate constants with the Si surface that are considerably higher than those reported for many related systems (>200 s−1). Comparison between Si-2 and Si-3 reveals that the use of the ferrocenyl group instead of a “Fe(κ2-dppe)(η5-C5Me5)” moiety improves the kinetic stability of the first oxidized state and its apparent charge transfer rate constant, while also increasing the first oxidation potential. However, an enhanced fragility is observed for Si-3 and even more so for Si-4 at higher oxidation potentials; this may be related to the larger spin density present on the ferrocenyl terminus of the grafted dications 32+ and 42+, as evidenced by molecular modeling calculations. Several ways to solve this important issue are discussed in the conclusion.
1. INTRODUCTION The modification of conducting surfaces at the molecular level with redox-active “building blocks” constitutes a powerful approach to the fabrication of smart materials, particularly when the goal is to obtain integrated systems devoted to information storage or transfer.1−4 For such applications, technologically relevant semiconducting surfaces, such as doped monocrystalline silicon, constitute particularly attractive substrates. With this in mind, oxide-free, hydrogen-terminated silicon (Si−H) surfaces covalently derivatized with redox-active molecules such as for instance ferrocene,5−12 or metal-complexed porphyrins,1,13−16 have been prepared and examined, in order to determine if molecular memories could result from such electrically addressable hybrid junctions. Moreover, when such surfaces are functionalized by monolayers incorporating distinct reversible redox-active systems, multibit information storage media are obtained.17 In this area, the integration of metalcomplexed porphyrins with silicon afforded a breakthrough in © 2014 American Chemical Society
the drive toward high charge density molecular-based information storage devices.1,13,15 Indeed, metalloporphyrins display very exciting electrochemical characteristics, such as multiple electron transfer steps at relatively low potentials, chemical stability of the different redox forms under ambient conditions, and a versatility of their redox properties that depends on the nature of the complexed metal. More recently, redox-active group 8 metal-alkynyl complexes on various surfaces were also shown to be attractive candidates for reversible charge storage at the molecular level,6,18−24 particularly when homo- and heteropolynuclear representatives of this class of molecules were anchored on conducting silicon3,4,25,26 or on gold.27−33 Moreover, far superior conduction through these organometallic substrates than Received: December 20, 2013 Revised: January 20, 2014 Published: January 27, 2014 3680
dx.doi.org/10.1021/jp412498t | J. Phys. Chem. C 2014, 118, 3680−3695
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Scheme 1. Redox-Active Dinuclear Organometallic Complexesa
a
The terminal unit binding to the Si(111)-H surface is highlighted in red.
sites in the heterobinuclear precursor complexes (2, 3) and also on the position of the linkage to the surface.
through purely organic analogues of similar length has been evidenced on gold,27−30 while the studies on silicon surfaces have shown that a given redox state of the immobilized molecules can be generated by application of a suitably chosen electrical potential to this hybrid junction.3,4 In this connection, we have recently shown that complexes 1n and 2 (Scheme 1) can be readily grafted onto Si−H surfaces through SiCC bonds.25 Furthermore, the alkynederived monolayers obtained from 1n34 possess remarkably fast charge-transfer kinetics with the underlying silicon chip.26 Due to the presence of interfacial vinyl bonds, the resulting monolayers are densely packed and are more ordered, and exhibit a higher surface coverage than alkene-derived monolayers, as previously demonstrated for purely organic substrates.35−38 Based on these observations, interfaces incorporating 2 or related heterobinuclear complexes could possess attractive features for multibit information storage. In the pursuit of these investigations, we report herein the successful covalent assembly on Si(111)-H surfaces of novel redox-active dinuclear ferrocenyl/Ru(II) alkynyl complexes (3 and 4, Scheme 1) that are related to 2, and address the question of the electrochemical stability of these hybrid junctions. The use of the ferrocenyl center in 3 and 4, instead of the organoiron(II) center in 1n and 2, has been motivated by the possibility of increasing the kinetic stability of the oxidized states in these electroactive assemblies. Furthermore, the ethynylferrocenyl center in 4 does not possess the rigidity of the phenylethynyl group in 2 or 3 because the cyclopentadienyl (Cp) ring can freely rotate around the Fe-Cp axis. We were therefore also interested in ascertaining if this structural difference would impact the efficiency of the grafting on Si(111)-H or the electrochemical properties of the resulting modified surfaces. In the following, we disclose the synthesis of the complex 2, along with those of the new compounds 3 and 4, and present our results concerning the grafting of these heterobinuclear species on p-doped silicon hydride, before comparing the properties of the resulting new interfaces with that previously obtained from 2. Finally, with the help of density functional theory (DFT) calculations, we rationalize our experimental findings pertaining to these modified Fe(II)/Ru(II) surfaces. As we shall see, their electron-transfer properties appear to be highly dependent on the actual nature of the organometallic
2. EXPERIMENTAL SECTION 2.1. General. Acetone (MOS electronic grade, Erbatron from Carlo Erba), anhydrous ethanol (RSE electronic grade, Erbatron from Carlo Erba), and acetonitrile (>99.5%, puriss, over molecular sieves, Sigma-Aldrich) were used without further purification. All other solvents were freshly distilled and purged with argon before use. All reactions and workup procedures were carried out under dry, high purity argon using standard Schlenk techniques.39 Infrared spectra were obtained on a Bruker IFS28 FT-IR spectrometer (400−4000 cm−1). Raman spectra of the solid samples were obtained by diffuse scattering on the same apparatus and recorded in the 100− 3300 cm−1 range (Stokes emission) with a laser excitation source at 1064 nm (25 mW) and a quartz separator with a FRA 106 detector. NMR spectra were acquired at 298 K on a Bruker DPX200, a Bruker Ascend 400 MHz NMR, or a Bruker AVANCE 500, with a 5 mm broadband observe probe equipped with a z-gradient coil. Chemical shifts are given in ppm and referenced to the residual nondeuterated solvent signal for 1H and 13C and external H3PO4 (0.0 ppm) for 31P NMR spectra. Cyclic voltammograms were recorded in dry CH2Cl2 solutions (containing 0.1 M [n-Bu4N][PF6], purged with argon and kept under an argon atmosphere) using a EG&G-PAR model 263 potentiostat/galvanostat. The working electrode was a 1 mm diameter Pt disk, the counter electrode a Pt wire, and the reference electrode a saturated calomel electrode. The ferrocene/ferrocenium (FcH/FcH+) couple (E1/2 = 0.46 V, ΔEp = 0.09 V; Iap/Icp = 1) was used as an internal reference for the electrochemical measurements.40 Elemental analyses and high-resolution mass spectra were performed at the “Centre Regional de Mesures Physiques de l′Ouest” (CRMPO), University of Rennes 1. ((4-Ethynylphenyl)ethynyl)trimethylsilane (6)41 and 1-trimethylsilylethynyl-1′-ethynylferrocene (10)42 were obtained following published procedures. Other chemicals were purchased from commercial suppliers and used as received. [Fe(κ 2 -dppe)(η 5 -C 5 Me 5 ){CC-1,4-(C 6 H 4 )CC}-transRuCl(κ2-dppe)2] (5)43 (dppe =1,2-bis(diphenyl-phosphino)ethane) and the ruthenium vinylidene trans-[Ru{CCH(C6H5)}Cl(κ2-dppe)2][PF6] complex (8)44 were obtained 3681
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C6H4), 4.00 and 3.99 (2 × s, 4H, C5H4), 3.98 (s, 5H, C5H5), 2.64 (m, 8H, CH2/dppe), 0.26 (s, 9H, SiMe3). 31P{1H} NMR (121 MHz, CDCl3): δ 54.0 (s, PPh2). 13C{1H} NMR (75 MHz, CDCl3): 139.5 (quint, 2JP,C = 15 Hz, RuCC), 137.4 and 136.7 (2 × m, Cipso/dppe), 134.6 − 126.9 (m, Caromatics), 121.8 (quint, 2JP,C = 15 Hz, RuCC), 116.4 (s, RuCC), 111.7 (s, RuCC), 106.5 (s, CCSiMe3), 93.4 (s, CCSiMe3), 77.2, 68.6, 68.4, and 65.9 (4 × s, Fc), 31.5 (m, CH2/dppe), 0.1 (s, SiMe3). UV−vis [CH2Cl2; λmax, nm (ε, 103 M−1 cm−1)]: 376 (52.7), 272 (46.7). [(Me 3 SiCC-η 5 -C 5 H 4 )Fe(η 5 -C 5 H 4 )CC-trans-Ru{CC(C6H5)}(κ2-dppe)2] (11). The reaction, workup and purification were performed as described above for 9, but employing trans[Ru{CCH(C6H5)}Cl(κ2-dppe)2][PF6] (8; 0.531 g, 0.45 mmol), NaPF6 (0.302 g, 1.80 mmol), 1-trimethylsilylethynyl-1-̀ ethynylferrocene (10; 0.134 g, 0.675 mmol), and Et3N (0.50 mL, 3.60 mmol), affording the title complex as an air-stable orange solid (0.54 g; 92%). Calcd. for C77H70FeP4RuSi·1/2C4H8O·1/2CH3OH: C: 70.40%, H: 5.65%; found: C: 70.22%, H: 5.39%. HRMS (ESI): calcd.: 1304.2585 [M]+, found: 1304.2597. IR (KBr, cm−1): 2142 (m, CC), 2060 (vs, RuCC), 1246 (m, SiCH3). 1H NMR (300 MHz, CDCl3): δ 7.62 − 6.71 (m, 43H, Haromatics), 6.72 (d, 2H, 3JH,H = 6 Hz, Ph), 4.28, 4.06, 4.00, and 3.95 (4 × s, 4 × 2H, C5H4), 2.67 (m, 8H, CH2/dppe), 0.23 (s, 9H, SiMe3). 31P{1H} NMR (121 MHz, CDCl3): δ 53.8 (s, PPh2). 13C{1H} NMR (75 MHz, CDCl): δ 137.1 (m, Cipso/dppe), 134.5 − 122.7 (m, Caromatics), 131.9 (quint, 2JP,C = 15 Hz, RuCC), 124.9 (quint, 2JPC = 15 Hz, RuCC), 116.1 (s, Ru−CC), 110.3 (s, Ru−CC), 105.1 (s, CCSiMe3), 90.1 (s, CCSiMe3), 78.0, 71.9, 70.1, 70.0, 69.3, 64.0 (6 × s, Fc), 31.7 (m, CH2/dppe), 0.4 (s, SiMe3). Desilylation of 9 and 11. The reaction, workup, and purification were performed as described in the Supporting Information section. [(η 5 -C 5H 5)Fe(η 5 -C 5 H 4)CC-trans-Ru{CC(1,4-C 6 H 4C CH)}(κ2-dppe)2] (3). Yield 97%. Calcd. for C74H62FeP4Ru: C: 72.14%, H: 5.07%; found: C: 72.05%, H: 5.28%. HRMS (ESI): calcd.: 1232.2186 [M]+, found: 1232.2195. IR (KBr, cm−1): 3290 (m, C−H), 2101 (w, CCH), 2044 (s, RuCC), 693 (s, C−H). 1H NMR (300 MHz, CDCl3): δ 7.76 − 6.93 (m, 44H, Haromatics), 6.59 (d, 2H, 3JH,H = 8 Hz, C6H4), 4.00 (s, 7H, C5H5 + C5H4), 3.99 (s, 2H, C5H4), 3.15 (s, 1H, CH), 2.69 (m, 8H, CH2/dppe). 31P{1H} NMR (121 MHz, CDCl3): δ 54.1 (s, PPh2). 13C{1H} NMR (75 MHz, CDCl3): δ 139.7 (quint, 2 JP,C = 15 Hz, RuCC), 137.4 and 136.7 (2 × m, Cipso/dppe), 134.5 − 126.9 (m, Caromatics), 121.8 (quint, 2JP,C = 15 Hz, RuCC), 116.3 (s, RuCC), 115.3 (s, CCH), 111.6 (s, RuCC), 84.9 (s, CCH), 76.7, 68.6, 68.4, and 65.9 (4 × s, Fc), 31.5 (m, CH2/dppe). [(HCC-η5-C5H4)Fe(η5-C5H4)CC-trans-Ru{CC(C6H5)}(κ2-dppe)2] (4). Yield 95%. Calcd. for C74H62FeP4Ru: C: 72.14%, H: 5.07%; found: C: 71.75%, H: 4.83%. HRMS (ESI): calcd.: 1232.2189 [M]+, found: 1232.2186. IR (KBr, cm−1): 3296 (m, C−H), 2103 (w, CCH), 2060 (vs, RuCC), 667 (s, C−H). 1H NMR (300 MHz, CDCl3): δ 7.82 − 6.85 (m, 43H, Haromatics), 6.72 (d, 2H, 3JH,H = 9 Hz, Ph), 4.42, 4.10, 4.02, and 3.96 (4 × s, 8H, C5H5), 2.78 (m, 8H, CH2/dppe), 2.67 (s, 1H, CH). 31P{1H} NMR (121 MHz, CDCl3): δ 53.8 (s, PPh2). 13C{1H} NMR (75 MHz, CD2Cl2): δ 137.3 (m, Cipso/dppe), 134.7 − 122.8 (m, Caromatics), 132.0 (quint, 3JP,C = 15 Hz, RuCC), 125.0 (quint, 3JP,C = 15 Hz, RuCC), 116.0 (s, RuCC), 110.0 (s, RuCC), 83.1 (s, CCH), 73.3 (s, C
following published procedures. The synthesis of 7 is given in the Supporting Information. The chemicals used for cleaning and etching of silicon wafer pieces (30% H2O2, 96−97% H2SO4, and 40% NH4F aq. solutions) were of VLSI semiconductor grade (Riedel-deHaën). 2.2. Synthesis of the Heterobimetallic Complexes. [Fe(κ2-dppe)(η5-C5Me5){CC-1,4-(C6H4)CC}-trans-Ru(κ2dppe)2{CC-1,4-(C6H4)CCH}] (2). In a Schlenk tube, [Fe(κ2dppe)(η 5 -C 5 Me 5){CC-1,4-(C 6 H 4 )CC}-trans-RuCl(κ 2 dppe)2] (5; 0.220 g, 0.134 mmol) and KPF6 (0.098 g, 0.534 mmol) were dissolved in 20 mL of dichloromethane and stirred for 2 h at room temperature. ((4-Ethynylphenyl)ethynyl)trimethylsilane (6; 0.079 g, 0.401 mmol) dissolved in 10 mL of dichloromethane was subsequently added, and the mixture was stirred for 2 days at room temperature. After removal of the solvent, the red residue was dissolved in THF in the presence of an excess of KOtBu (0.030 g, 0.267 mmol) and stirred for 2 h. After removal of the solvent, the product was then extracted with toluene (3 × 10 mL) and filtered, and the filtrate was concentrated to dryness. The residual solid was washed with npentane (2 × 5 mL) to afford the title complex as a red solid (0.190 g, 0.109 mmol, 81%). Calcd for C108H96FeP6Ru: C, 74.69%; H, 5.57%. Found C, 74.28%; H, 5.71%. HRMS (ESI) m/z 1736.4377 (M+•) calc. for C108H96FeP6Ru: 1736.4330 (M+•). IR (KBr, cm−1): 3314 (sh) and 3286 (m, H−C), 2052 (vs, FeCC, RuCC and C CH). 1H NMR (200 MHz, C6D6): δ 8.10 (m, 4H, HAr/dppe), 7.83 (m, 8H, HAr), 7.53 (m, 10H, HAr), 7.35−6.70 (m, 46H, HAr/dppe), 2.91 (s, 1H, CCH), 2.88−2.50 (m, 10H, CH2/dppe), 1.88 (m, 2H, CH2/(dppe)Fe), 1.59 (s, 15 H, C5(CH3)5). 31P{1H} NMR (81 MHz, C6D6): δ 101.7 (s, 2P, (dppe)Fe), 54.8 (s, 4P, (dppe)2Ru). 13C{1H} (125 MHz, CD2Cl2): δ 140.1 (q, 1C, 2 JC,P = 15 Hz, RuCC), 139.8−126.6 (CAr + CArH + Fe-CC + Ru-CC), 125.6 (CArH), 125.2, 120.2, 118.2, 117.0, 115.8 (s, CAr and MCC), 87.8 (s, C5(CH3)5), 85.0 (s, CCH), 77.1 (s, CCH), 31.8 (m, CH2/(dppe)2Ru), 31.0 (m, CH2/(dppe)Fe), 10.3 (s, C5(CH3)5). UV−vis [CH2Cl2; λmax, nm (ε, 103 M−1 cm−1)]: 380 (53.9). [(η5-C5H5)Fe(η5-C5H4)CC-trans-(κ2-dppe)2Ru{CC(1,4C 6 H 4 CCSiMe 3 )}] (9). In a Schlenk flask, trans-[(κ 2 dppe) 2ClRu{CCH(η 5 -C 5 H 4 )Fe(η 5 -C 5 H 5 )}][PF6 ] (7; 0.58 g, 0.45 mmol), NaPF6 (0.302 g, 1.80 mmol), and ((4ethynylphenyl)ethynyl)trimethylsilane (6; 0.134 g, 0.675 mmol) were dissolved in CH2Cl2 (25 mL) under nitrogen. Et3N (0.50 mL, 3.60 mmol) was added and the brown-orange solution was stirred for 24 h at room temperature, giving an orange solution with a white precipitate. The suspension was removed by filtration, the inorganic salts were washed with CH2Cl2 (3 × 10 mL), and the combined filtrates were concentrated under reduced pressure to 5 mL and adsorbed on top of a chromatographic column (basic alumina, 3 × 10 cm). An orange band was collected after eluting with CH2Cl2 (containing 1% Et3N) and concentrated by rotary evaporation to ∼5 mL. The title complex was precipitated by addition of methanol, collected by filtration, washed with methanol (3 × 10 mL) and hexanes (3 × 10 mL), and finally dried in vacuo to give the product as an air-stable orange solid (0.52 g; 89%). Calcd. for C77H70FeP4RuSi: C: 70.91%, H: 5.41%; found: C: 70.72%, H: 5.41%. HRMS (ESI): calcd.: 1327.2488 [M+Na]+, found: 1327.2485. IR (KBr, cm−1): 2150 (m, CC), 2058 (vs, RuCC), 1249 (w, νSi‑CH3). 1H NMR (300 MHz, CDCl3): δ 7.73 − 6.87 (m, 42H, Haromatics), 6.50 (d, 2H, 3JH,H = 8 Hz, 3682
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Scheme 2. Syntheses of 2, 3, and 4
CH), 78.1, 71.9, 70.2, 69.9, 68.9, and 63.1 (6 × s, Fc), 31.6 (m, CH2/dppe). 2.3. Preparation of the 2-, 3-, and 4-Modified Si(111) Surfaces. A single-side-polished silicon(111) shard (1−5 Ω cm, p-type, boron doped, thickness = 525 ± 25 μm, Siltronix) was cut to afford a 1.5 × 1.5 cm2 piece, which was then sonicated for 10 min successively in acetone, ethanol, and ultrapure 18.2 MΩ cm water (Veolia Water STI). It was then cleaned in a 3:1 v/v concentrated H2SO4/30% H2O2 mixture at 100 °C for 30 min, followed by copious rinsing with ultrapure water. Caution: The concentrated H2SO4/H2O2(aq) piranha solution is very dangerous, particularly in contact with organic materials, and should be handled extremely carefully. The surface was etched with ppb grade argon-deaerated 40% aqueous NH4F solution for 20 min to obtain atomically flat Si(111)-H. It was then dipped in argon-deaerated ultrapure water for several seconds, dried under an argon stream, and transferred immediately into a quartz Schlenk tube containing ca. 8−10 mM of 2, 3, or 4 in ca. 10 mL of deoxygenated, anhydrous toluene. The solution was thoroughly purged with argon for 30 min, the Schlenk tube was sealed with paraffin film (Parafilm M), and then irradiated for 4 h in a Rayonet photochemical reactor (254 nm). The modified surface was then rinsed copiously with anhydrous toluene and dichloromethane and dried under an argon stream. 2.4. Characterization Techniques. Electrochemical Characterization. Cyclic voltammetry (CV) measurements were performed with an Autolab electrochemical analyzer (PGSTAT 30 potentiostat/galvanostat from Eco Chemie B.V.) equipped with GPES software and a homemade three-electrode Teflon cell. To avoid photoinduced charge-transfer processes on the silicon surfaces, all of the electrochemical measurements were performed in the dark. The working electrode, modified Si(111), was pressed against an opening in the cell bottom using a FETFE (Aldrich) O-ring seal. An ohmic contact was made on the previously polished rear side of the sample by applying a drop of an In−Ga eutectic (Alfa-Aesar, 99.99%). The electrochemically active area of the Si(111) surface was 0.1 cm2 and was estimated by measuring the charge under the voltammetric peak corresponding to the ferrocene oxidation
on Si(111)-H and compared to that obtained with a 1 cm2 Pt electrode under the same conditions. The counter electrode was a platinum grid and 10−2 M Ag+ | Ag in acetonitrile was used as the reference electrode (+0.29 V vs aqueous SCE). All reported potentials are referred to SCE (uncertainty ±5 mV). Tetra-n-butylammonium perchlorate [n-Bu4N][ClO4] was purchased from Fluka (puriss, electrochemical grade) and was used, as received, at 0.1 mol L−1 as supporting electrolyte in acetonitrile. The electrolytic medium was dried over activated, neutral alumina (Merck) for 30 min with stirring under an argon atmosphere. All electrochemical measurements were carried out inside a homemade Faraday cage, at room temperature (20 ± 2 °C) and under a constant argon flow. The resistance of the electrolytic cell was compensated by positive feedback. X-ray Photoelectron Spectroscopy (XPS) Analysis. After a few minutes exposure to an ambient atmosphere, the modified surfaces were introduced to the UHV chamber and kept at 1 × 10−9 mbar for several hours before XPS analysis. XPS measurements were performed with a Mg Kα (hν = 1254 eV) X-ray source, using a VSW HA100 photoelectron spectrometer with a hemispherical photoelectron analyzer, working at an energy pass of 22 eV. The experimental resolution was 1.0 eV. Spectral analysis included a Shirley background subtraction and peak separation using mixed Gaussian−Lorentzian functions. The Si 2p signal measured before and after grafting was averaged over several emission angles around normal emission, in order to eliminate photoelectron diffraction effects. Binding energies are referenced to the main C 1s peak which is set at 285.0 eV. 2.5. Crystallography. A single orange needle of 11 was grown by slow diffusion of methanol into a THF solution of this complex at room temperature. Data collection of crystals of 11 was performed on a APEX diffractometer, at 120(2) K, with graphite monochromatized Mo Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods using the SIR97 program45 and then refined by full-matrix least-squares methods based on F2 (SHELX-97)46 with the aid of the WINGX program.47 The complex 11 (C77H70Fe1P4Ru1Si1; M = 1304.22 g) crystallizes in a triclinic space group with a = 9.5320(10) Å, b = 13.7554(15) Å, c = 27.635(3) Å and α = 3683
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81.394(5); β =83.466(4); γ =71.709(4) (V = 3393.0(6) Å3); space group P-1; Z = 2, 41503 reflexions measured, 15295 unique (Rint = 0.0384) which were used in calculations (Supporting Information). The contribution of the disordered solvent molecules (THF and presumably MeOH) to the calculated structure factors was estimated using the BYPASS algorithm, implemented as the SQUEEZE option in PLATON.48 A new data set, free of solvent contribution, was then used in the final refinement. The complete structures were refined with SHELXL9748 by the full-matrix least-squares technique. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were included in the final cycle of refinement in their calculated positions. The final refinement on F2 with 15295 unique reflections and 757 parameters converged at wR(F2) = 0.1605 (R(F) = 0.0680) for 11850 observed reflections with I > 2σ(I). Atomic scattering factors were taken from the literature.49 CCDC deposition no.: CCDC 981254. 2.6. Computational Details. Gas phase geometry optimizations of 3, 4, and models of Si-3 and Si-4 (using a Si10H15 cluster for the silicon surface) were performed using DFT (spin unrestricted for all open shell systems) using the Gaussian 03 (revision D.02) program package and the default convergence criteria implemented in the program, the B3LYP functional, and the LanL2DZ basis set. The latter employs the Dunning/Huzinaga valence double-ζ D95 V basis set for first row atoms and the Los Alamos Effective Core Potential plus DZ on atoms from Na−Bi. Full parameters for common basis sets, including the LanL2DZ basis set, are available at https:// bse.pnl.gov/bse/portal. The figures were drawn with Molekel 4.3. Full details and references for the theoretical modeling are provided as Supporting Information.
of this compound was definitively established by single-crystal X-ray diffraction (Figure 1).
Figure 1. ORTEP representation of 11 at the 50% probability level (hydrogen atoms have been removed for clarity). Selected bond distances (Å) and angles (°): Ru1−P1, 2.3536(11); Ru1−P2, 2.3565(11); Ru1−P11, 2.3531(11); Ru1−P12, 2.3574(11); Ru1− C91, 2.070(4); C91−C92, 1.211(6); C92−C93, 1.439(6); C105− C106, 1.345(11); C106−C107, 1.272(12); C107−Si1, 1.878(10); Ru1−C81, 2.066(4); C81−C82, 1.216(6); C82−C83, 1.435(6); Fe1Cp1, 1.661; Fe1-Cp2, 1.620; P1−Ru1−P2, 82.97(4); P2−Ru1−P12, 96.91(4); Ru1−C81−C82, 173.6(4); C81−C82−C83, 173.2(4); Ru1−C91−C92, 173.2(4); C91−C92−C93, 170.0(4); C105− C106−C107, 176.4(11); C106−C107−Si1, 171.2(11); Ru1-Cp1/ Cp2-Si1, 80.8.
3. RESULTS 3.1. Syntheses of 2, 3, and 4. The synthesis of complex 2 was achieved from the known precursor compound 5 by reacting this compound with excess ((4-ethynylphenyl)ethynyl)trimethylsilane 6,25 following a methodology also successfully used for preparing other heterobinuclear compounds.50 In this procedure, the desilylation step takes place concomitantly with the deprotonation of the vinylidene complex intermediate,51,52 which was not isolated or characterized in the present studies (Scheme 2).34 The synthesis of the new dissymmetrical ruthenium bis-alkynyl complexes 3 and 4 was also achieved in several steps but using a different approach. In this case, the trimethylsilyl-protected precursors 9 and 11 were first isolated from the vinylidene precursors 7 and 844 and the dissymmetrical bis-alkynes 641,53 and 1042 following a well-established methodology (Scheme 2).44 These alkyne complexes were then desilylated using TBAF in THF to give the desired complexes 3 and 4 in excellent yields. In contrast to 2, all these organometallic species are air stable. Particularly noteworthy is the fact that 3 and 4 are stable in solution over extended periods of time (>15 h), in spite of the high reactivity often reported for 1,1′-ferrocene derivatives featuring a pendant ethynyl bond,54−56 and this behavior possibly stems from the steric hindrance created by the “Ru(κ2-dppe)2” fragment around the alkynyl ligand, which thereby prevents any unwanted polymerization or cyclization.57−61 As a result, complexes 2−4, 9, and 11 could also be fully characterized by the usual spectroscopic methods such as NMR and IR. In addition, crystals of 11 were grown, and the solid-state structure
3.2. Molecular Structure of 11. Complex 11 crystallizes in the centrosymmetric triclinic space group P-1 with disordered molecules of solvent, which were removed during the data treatment (Figure 1). In the solid state, the protected triple bond is nearly orthogonal to the metal-acetylide axis, forming an angle of 82.7°, a feature which most likely originates from the packing of the complexes (Figure S1a in Supporting Information). As far as the complex itself is concerned, the octahedral coordination sphere around the ruthenium is slightly deformed. The bonding within the complex is unexceptional compared to available data in the literature62 with, however, a C106−C107 bond length (1.271 Å) that is longer than those in propionylferrocene63 and trimethysilylethynylferrocene.64 As previously observed in other bis-alkynyl complexes, the P−Ru− P angles between the two phosphorus atoms of the same dppe ligand are narrower (83.3° and 83.0°) than those between the two cis phosphorus atoms of two distinct dppe ligands (96.9° and 96.9°), due to the bite angle of the diphosphine ligands. The Fc-CC−Ru−CC−Ph backbone adopts a global Sshaped conformation (Figure S1b), which also possibly results from packing interactions.65,66 3.3. Cyclic Voltammetry Studies of 2−4, 9, and 11. Cyclic voltammetry (CV) studies of these complexes in dichloromethane medium revealed two reversible one-electron waves along with a third, nearly irreversible, one close to the solvent discharge (Supporting Information; Figures S2 and S3). Partial loss of the reversibility of the first two processes was observed when the potential was scanned to the third process, 3684
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but reversibility was restored when a narrower potential range was then used (Figure S3). The observed redox processes can be tentatively assigned based on ongoing work from our group67,68 and related studies available in the literature for this type of complex.69 Thus, a reversible ferrocene-based oxidation apparently takes place at lowest potentials, followed by a reversible mono-oxidation localized at the Ru(II) center. Finally, the irreversible process at highest potentials can be attributed to a subsequent ruthenium-centered (RuIII/IV) oxidation.70,71 Interestingly, changing the location of the terminal trimethylsilyl or ethynyl substituents does not significantly influence the reversibility of these electrochemical processes but does slightly affect the value of the ferrocenecentered oxidation potential (Table 1). When appended to the
Scheme 3. Grafting of the Heterobinuclear Complexes 2, 3 and 4 onto Si−H Surfaces
revealing characteristic peaks from the silicon substrate itself and from the C 1s, Fe 2p, Ru 3d, and P 2p core levels of the attached molecule (Figure S4). Similar observations were made for Si-3 and Si-4 (Figure 2). For both surfaces, the C 1s spectra display a principal component at 285.0 ± 0.1 eV, attributed to different CC, CC, and CC bonds of the grafted molecules. Ru 3d, which is close to C 1s, has a double peak structure due to spin−orbit splitting (3d5/2 and 3d3/2) with the peaks separated by 4.2 eV (see below and Figure 2c,d). For the C 1s region, lower and higher binding energy contributions are also visible at 282.9 ± 0.1 eV and 286.2 ± 0.1 eV, which can be assigned to the C−Si interfacial bond73−76 and phosphorusbonded carbons (C−P) of the diphenylphosphine units surrounding the Ru(II) atoms,77,78 respectively. Major and well-resolved components of the Fe 2p3/2 level are located at 708.5 and 707.7 eV for Si-3 and Si-4, respectively, these values being consistent with those previously reported for some ferrocene derivatives.79,80 We also note that the change from mono- to disubstitution of the ferrocene group in the grafted chain results in a 0.8 eV shift toward lower binding energies for the Fe 2p3/2 electrons; such a trend is in line with earlier XPS studies of ferrocene derivatives, which demonstrated that methyl substitution of the cyclopentadienyl ring had a major influence on the electronic characteristics of the Fe atom.79 Two other unresolved components, located at 710.3 and 712.7 eV for Si-3 and 711.1 and 712.7 eV for Si-4, have been used to fit the Fe 2p3/2 spectra. These are believed to correspond to differently coordinated ferrocene and ferrocenium Fe(III) species, respectively.3,10,12 From the area of the Fe(III) peak, we estimate its contribution to be ca. 10% of total Fe, irrespective of the nature of the grafted molecule. The presence of ferrocenium is thought to result from the oxidation of ferrocenyl groups by traces of residual oxygen in the reaction vessel, as previously reported for ferrocene-terminated monolayers bound to Si−H surfaces.10−12,81 Consistent with this suggestion, the Si 2p spectra show a very weak peak attributable to silicon oxides at a binding energy of 103−104 eV, indicating that Si-3 and Si-4 were very weakly oxidized during the grafting process (Figure 2e,f). The presence of the second Ru metallic center is evidenced by the XPS signal of the Ru 3d5/2 level at 280.7 ± 0.1 eV for both Si-3 and Si-4, which is characteristic of a Ru(II) species.25,82−86 It should be noted that the Ru 3d3/2 component at ca. 285 eV overlaps the intense C 1s peak and consequently cannot be clearly resolved. The phosphorus peak P 2p is very close to the broad Si 2p plasmon peak at 134 eV (Figure 2g,h). To deconvolute the contribution of P 2p, the data have been fitted by comparison to a reference signal on clean Si(111) surface. The Rutotal/P atomic ratio (determined from the areas under the Ru 3d and P 2p peaks) was estimated at 0.20 for both surfaces, which is close to the theoretical ratio of 1:4. Moreover, the Fetotal/Rutotal atomic ratio was found to be 1.3 and 1.0 for Si-3 and Si-4, in excellent agreement with the theoretical value of 1. The surface coverage of the molecular chains has been estimated comparing the Rutotal signal to that of a clean Si(111)
Table 1. Cyclic Voltammetry Data for 2−4, 9, and 11a compound 2 3 4 9 11
E°′b/V vs SCE
ΔEpc/V
Ipa/Ipcd
ΔE°′e/V
−0.24 0.45 0.17 0.75 0.27 0.79 0.17 0.76 0.26 0.79
0.07 0.07 0.12 0.12 0.07 0.08 0.09 0.09 0.08 0.08
1.0 1.0 1.0 0.9 1.0 0.9 0.9 0.9 1.0 0.9
0.69
this work and ref 25
0.58
this work
0.52
this work
0.59
this work
0.53
this work
refs
a
Conditions: 1 mM in CH2Cl2 + 0.1 M [n-Bu4N][PF6]; platinum (1 mm diameter) disk electrode. bAverage of anodic and cathodic peak potentials. cPeak-to-peak separation. dRatio between the anodic and cathodic peak current intensities. eDifference in formal potentials for the two reversible redox systems.
ferrocene, the alkyne groups render the first oxidation of 4/11 slightly more difficult than for 3/9, in line with their weak electron-withdrawing character (σp = 0.23 for CCH and 0.03 for CCMe).72 In contrast, the substituent effect remains relatively moderate for the Ru(II)-centered oxidation owing to the similar chemical environment around the Ru(κ2-dppe)2 group in 4/11 and 3/9. Finally, comparison of the CV data obtained for 3, 4, 9, and 11 with that for 2 evidence that replacement of the “Fe(κ2dppe)(η5-C5Me5)” group by a ferrocenyl unit results in a ≥ 400 mV increase in the first oxidation potential of these heterobinuclear complexes. This is presumably the main reason for the increased air-stability of 3, 4, 9, and 11 over 2. Furthermore, the electrochemical response of all molecules studied was not significantly changed upon cycling over the first two ferrocene- and Ru(II)-based oxidation processes. In contrast, degradation of the complexes in their dicationic state was observed when attempts were made to isolate them by undertaking electrolysis experiments at constant potential. 3.4. Preparation and Characterization of Dinuclear Assembly-Modified Silicon Surfaces. The deprotected alkyne-terminated complexes 3 and 4 were photochemically reacted at 254 nm for 4 h in the presence of a p-type Si(111)-H surface (Scheme 3). This hydrosilylation reaction, previously used for 2, 25 yielded the redox-active Si-3 and Si-4 submonolayers featuring the corresponding complexes covalently bound on Si−H. XPS Characterization. The XPS characterization of Si-2 has been previously briefly described in a communication,25 3685
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Figure 2. High-resolution XP Fe 2p3/2 (a and b), C 1s and Ru 3d (c and d), Si 2p (e and f), P 2p, and Si plasmons (g and h) spectra of Si-3 (left column) and Si-4 (right column) surfaces. Black circles are experimental data and black lines are the fitted curves that were generated using combinations of Gaussian−Lorentzian functions, each corresponding to a different contribution. Spectra have been recorded at normal incidence. The inset in panel e highlights the silicon oxides region.
centers in Si-2 are relatively close to those found for 2 in dichloromethane (Table 1). Apparent rate constants for electron transfer at the two bound metallic centers, kapp, were then calculated using Laviron’s formalism (eq 1)87 based on classical Butler−Volmer theory. Upon increasing the potential scan rate v, the anodic and cathodic peak potentials (Epa and Epc) corresponding to the two redox processes shift in positive and negative directions relative to E°′, respectively, which reflects control of the voltammetry by the rate of electron transfer at the metallic centers (Figure S5).
surface as reference, and taking into account the attenuation of the Ru signal due to its location in the molecular layer. Values of (8.3 ± 0.5) × 1013 and (6.9 ± 0.5) × 1013 molecular chains per cm2 were calculated for Si-3 and Si-4, respectively, i.e., (1.3 ± 0.1) × 10−10 and (1.1 ± 0.1) × 10−10 mol cm−2, respectively. Globally, the XPS results obtained for Si-3 and Si-4 are consistent with those reported for Si-2.25 For the latter system, the XPS signals of Fe 2p3/2 and Ru 3d5/2 were located at ca. 708 and 280−281 eV, respectively, together with ca. 10% of the oxidized form Fe(III) (Table S2 in the Supporting Information). However, unlike Si-3 and Si-4, the grafting of 2 onto Si−H induced more surface oxidation, confirmed by the presence of a more intense peak arising from silicon oxides at 103−104 eV in the Si 2p spectrum. Cyclic Voltammetry Studies. The CV characterization of Si2, which has been briefly communicated,25 is now revisited in order to extract accurate kinetic data (Figure S5). The formal potentials E1°′ and E2°′ determined for the bound metallic
kapp = (1 − α)nFva /RT
(1)
where α is the charge transfer coefficient and va is the intersect of the linear regions in the graph Epa vs log v that are obtained at low and high scan rates.87 Assuming α = 0.5 for both redox processes, the slope of the linear Epa − log v region at high v is therefore equal to 2.3RT/{(1 − α)nF}, i.e., 116 mV, per decade of v at 293 K. Values of 200 ± 50 and 290 ± 80 s−1 are thus 3686
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Table 2. Cyclic Voltammetry Data for Si-2, Si-3, and Si-4 Grafted onto Si(111)-Ha organometallic interface Si-2 Si-3 Si-4
Ebound°′b (V vs SCE) −0.16 0.48 0.23 0.56 0.27 f
surf. coveragec (mol cm−2)
ΔEbound°′ (V)
−11
atom/surf.d (Si atom)
0.64
(9 ± 2) × 10
0.33
(10 ± 2) × 10−11
0.08 ± 0.01
(4.3 ± 0.2) × 10−11
0.035 ± 0.005
0.07 ± 0.01
kappe (s−1) 200 290 560 320 640 f
± ± ± ± ±
50 80 60 80 80
a
Conditions: CH3CN + 0.1 M [n-Bu4N][ClO4]. bAll E°′ values are taken as the average of the anodic and cathodic peak potentials (±5 mV). Determined electrochemically (see text). dThe atomic density of Si(111) is 7.8 × 1014 atoms cm−2. eApparent charge transfer rate constants for the bound Fe(II)/Fe(III) (Si-2) or Fc/Fc+ (Si-3, Si-4) and Ru(II)/Ru(III) couples, respectively. fIrreversible system.
c
Figure 3. Cyclic voltammograms in CH3CN + 0.1 M [n-Bu4N][ClO4] of Si-3 at scan rates of 0.1, 0.2, 0.4, 0.6, and 1 V s−1 with a switching upper potential of 0.6 (a) and 0.8 V vs SCE (b). Corresponding Ipa, Ipc − v (c) and Epa, Epc − log v plots (d) for the two attached metallic centers.
determined for the Fe2+/Fe3+ and Ru2+/Ru3+ couples, respectively (Table 2). Typical cyclic voltammograms of the Si-3 surface are shown in Figure 3a as a function of v. Again, two reversible and stable processes of the same magnitude, attributed to the one-electron oxidation of bound ferrocene and Ru(II) centers, were observed at E1°′ = 0.23 V and E2°′ = 0.56 V vs SCE, respectively, when the potential was scanned to the anodic peak potential corresponding to the oxidation of Ru2+, i.e., 0.6 V vs SCE. However, excursion to 0.8 V led to a strong decrease in intensity of the Fc/Fc+ system while the Ru2+/Ru3+ system was found to be electrochemically stable (Figure 3b). A possible reason for this electroactivity decrease in the first redox process will be discussed in the next section. As was observed with Si-2, the redox potential determined for the bound Fc/Fc+ system was relatively close to that for 3 in dichloromethane solution (Figure S2). In contrast, the Ru2+/Ru3+ couple was observed at a ca. 0.2 V less positive potential. The easier oxidation of the bound Ru2+ can be tentatively ascribed to the stabilization of Ru3+ due to the formation of relatively strong ion pairs between the dicationic state of the immobilized molecule and the counteranions (i.e., ClO4−) from the electrolyte. As expected for surface-confined redox species,88 both anodic and cathodic peak current intensities (Ipa and Ipc) corresponding to the two redox systems were found to be directly proportional to v (Figure 3c). From the area under the voltammetric peak
corresponding to the oxidation of the Fc units, the surface coverage of the molecular chains was estimated at (1.0 ± 0.2) × 10−10 mol cm−2, in excellent agreement with that calculated from XPS data (vide supra). This value is smaller than those obtained for Si-1n (1.5−2.2 × 10−10 mol cm−2)26 and purely ferrocene-terminated monolayer-modified Si surfaces (3.0−5.0 × 10−10 mol cm−2)5 but compares well with that determined for Si-2. Such a result can be at least partially explained by the larger steric hindrance of the coordination sphere around the Ru(II) center, if one compares the specific area occupied by one Ru(κ2-dppe)2 synthon (≥85 Å2)25 with that occupied by one ferrocene (35 Å2).89 Note that the value for the Ru(II) complex was determined by considering the area delimited by a cylinder englobing all the hydrogens of CH2 groups of dppe, but not the phenyl groups. Thus, this constitutes a lower bound value that implies some interpenetration of these phenyl groups when such a superficial coverage is attained. From the variation of Epa with log v, and assuming α = 0.5 for both processes, the kapp values for the Fc/Fc+ and Ru2+/Ru3+ redox couples of Si-3 were estimated to be 560 ± 60 and 320 ± 80 s−1, respectively. These high values, which can be ascribed to the presence of the conjugated bridge between the attached metallic centers and the underlying silicon surface, are considerably higher than those previously reported for ferrocene monolayers5 and for dinuclear systems bound to Si surfaces through a saturated spacer.3,26 Comparison with the 3687
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Figure 4. Cyclic voltammograms in CH3CN + 0.1 M [n-Bu4N][ClO4] of Si-4 (a) at scan rates of 0.1, 0.2, 0.4, 0.6, and 1 V s−1 with a switching upper potential of 0.4 V vs SCE (solid lines), and at a scan rate of 0.1 V s−1 with a switching upper potential of 0.8 V (first scan: dashed line; second scan: dotted line). Epa, Epc − log v plots corresponding to the Fc/Fc+ system (b).
Scheme 4. Rotational Flexibility of 4 Grafted on Si(111)-H
because it considers the molecules of 4 oriented perpendicular to the surface. A specific area of 385 ± 20 Å2 per bound 4 was extracted from the electrochemical data; the area occupied by grafted 4 is therefore much greater than that occupied by grafted 3, namely 170 ± 30 Å2, a result that can be ascribed to the increased flexibility of 4 on the surface. As a matter of fact, the rigid-rod nature of 3 bound to the surface through a phenyleneethynylene group restricts its lateral motions on the surface. In contrast, substituting the phenyl ring by one ferrocenyl unit introduces some flexibility in the complex. As observed for the trimethylsilyl-protected derivative 11, the trimethylsilylethynyl-substituted Cp ring is free to rotate with respect to the Fc-Ru vector. It is likely that such motion, in combination with rotation of the complex around the Si−C axis, also occurs for the complex 4 grafted onto the silicon surface, thereby increasing its apparent specific area (Scheme 4). In the crystal structure of 11, the ruthenium alkynyl axis and the Fe−Si axis are almost orthogonal and the distance between the iron atom and the proton on the para position of the phenylalkynyl ligand is 14.7 Å. Considering that this situation represents the extreme case in terms of rotation of the ruthenium alkynyl axis, the higher value calculated for the specific area of bound 4 compared to bound 3 is therefore not surprising. At more oxidizing potentials, the cyclic voltammogram of Ru2+ in Si-4 reveals an irreversible anodic peak at 0.68 V, which disappears on the second scan (Figure 4a). Instead, only the reversible Fc/Fc+ process is observed, but it is shifted 180 mV to more positive potentials when compared to the first scan. As discussed in the next section, this electrochemical behavior is consistent with either (i) the cleavage of the ligand bridging the two metallic centers in the dicationic state due to the presence
values determined for Si-2 (Table 2) reveals that replacement of the remote electron-rich “Fe(κ2-dppe)(η5-C5Me5)” end group by a much less electron-rich ferrocenyl unit located spatially closer to the Ru(II) center increases the rate of electron-exchange with the surface while keeping approximately constant the kapp value ascribed to the second Ru2+/Ru3+ redox couple. Finally, Si-4 showed a significantly different electrochemical stability from Si-3. When the potential was scanned over the first redox process, corresponding to the Fc/Fc+ couple, Si-4 exhibited a stable electrochemical response, showing characteristics of a surface-confined redox species (Figure 4). This process was observed at 0.27 V, which is identical to the value determined for 4 in dichloromethane solution (Table 1 and Figure S2). From the variation of Epa with log v, the kapp value for the Fc/Fc+ couple was estimated to be 640 ± 80 s−1 (Figure 4b), which is within the error margins of the value calculated for bound 3. In comparison to Si-3, the signal due to Fc in Si-4 was broader and less intense, indicating a lower surface coverage and the presence of strong repulsive electrostatic interactions between the redox centers87 and the inhomogeneity of those centers.89 Such electrochemical features have also been reported for ferrocene monolayers from ethynylferrocene covalently attached on Si−H;90 however, these were related to significant amounts of silicon oxides produced during the grafting step, which was not the case in our study, as confirmed by the absence of a peak corresponding to silicon oxides in the Si 2p spectrum of Si-4 (Figure 2f). The surface coverage of the molecular chains was electrochemically estimated at (4.3 ± 0.2) × 10−11 mol cm−2, which is smaller than that determined by XPS (ca. 1.0 × 10−10 mol cm−2). In the latter case, XPS overestimates the coverage 3688
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Chart 1. Atom Numbering Scheme Used in Tables 3 and 4
Table 3. Selected Bond Distances Found in the Optimized Structures of 3n+, 4n+, Si-3n+, and Si-4n+a
a
compound
C1−C2
C2−C3
C3−Ru
Ru−C4
C4−C5
C5−C6
3 3+ 32+ (T) 32+ (S) Si-3 Si-3+ Si-32+ (T) Si-32+ (S) 4 4+ 42+ (T) 42+ (S) Si-4 Si-4+ Si-42+ (T) Si-42+ (S)
1.4325 1.4335 1.4145 1.413 1.4325 1.4315 1.4085 1.406 1.4365 1.4345 1.4195 1.393 1.4375 1.4365 1.4215 1.420
1.2495 1.2485 1.2625 1.261 1.2495 1.2505 1.2635 1.264 1.2485 1.2495 1.2615 1.271 1.2485 1.2475 1.2605 1.258
2.0685 2.0535 1.9905 1.999 2.0695 2.0485 1.9965 1.991 2.0725 2.0465 1.9905 1.976 2.0735 2.0585 1.9915 2.009
2.0825 2.0095 2.0565 1.979 2.0845 2.0135 2.0495 1.994 2.0825 2.0165 2.0595 1.999 2.0825 2.0075 2.0625 1.972
1.2475 1.2615 1.2545 1.276 1.2475 1.2595 1.2555 1.265 1.2485 1.2595 1.2545 1.296 1.2485 1.2615 1.2535 1.272
1.4285 1.4055 1.4055 1.394 1.4285 1.4075 1.4025 1.399 1.4275 1.4105 1.4065 1.417 1.4275 1.4035 1.4105 1.390
n = 1, 2 [triplet (T) and singlet (S)]. For the atom numbering, see Chart1.
and second redox processes is based on experimental evidence obtained on closely related systems,68,69 the theoretical data from the present study, calculated in the absence of counterion, in the absence of a solvent model and on a single conformation, should be considered with caution, especially when considering the highly polarizable nature of the FMOs,50,91 along with the well-established bridge conformational-dependence of the spin distribution in such species.92−96 We have indeed shown that, for such kind of organometallic heterodinuclear Fe/Ru compounds featuring polarizable atoms, the Ru character was often overemphazised relative to Fe in the frontier MOs when computations were performed in absence of solvent and counterion.50 These computations nevertheless confirm the largely delocalized nature of the FMOs over the Fc−CC− Ru−CC backbone in the first redox states and highlight the fact that a description of the oxidation as essentially localized on one metallic atom is a simplistic formalism with these compounds.18 Finally, the two SOMOs of the dications (in the triplet state) are quasi-degenerate in energy, both exhibiting a strong ruthenium character consistent with the assignment of the third (and irreversible) redox process as Ru(III/IV) in nature. Regardless of the spin state, these computations are consistent with the iron and the ruthenium sites formally being oxidized by one electron each at the dicationic stage. The calculations provide a qualitative comparison of the relative electronic changes taking place between 22+, 32+, and 42+ (Table 3), and thereby afford insight into the surface reactivity of these species.
of the nearby positively charged surface, resulting in a loss of the Ru-based moiety or (ii) the transformation of the Ru-based electrophore into an electrochemically silent system within the potential range investigated. 3.5. DFT Computations. DFT calculations (Gaussian 03, B3LYP/LanL2DZ: see details in the SI) were carried out to afford insight into the redox properties and reactivity of the bound species 3 and 4, in comparison to those of bound 2. Thus, structures of 3 and 4 and models of Si-3 and Si-4 (using a Si10H15 cluster for the silicon surface) were optimized in the gas phase. The bond lengths of the optimized structure corresponding to 4 are consistent with those determined in the single-crystal X-ray study of the parent compound 11 (Figure 1 and Supporting Information). The spin distribution on these four compounds was then computed in their mono- and dioxidized (triplet) states (Table 2 and Chart 1). In all cases, the triplet states of 32+ and 42+ were found to be more stable than the closed-shell singlet states by ca. 0.50−0.75 eV (48−72 kJ mol−1), suggesting an open-shell ground state (GS) for these dications. This contrasts with a previous study on related compounds for which these two states were much closer in energy (0.03 eV) and for which the ground state was computed to be the singlet state.67 The frontier molecular orbitals (FMOs) computed for 3 and 4 in their neutral and cationic states reveal a slightly dominant contribution of ruthenium to the HOMO of the neutral state and of iron to the SOMO of the cationic state (Supporting Information). These features appear at variance with the localized assignment proposed in the text for the successive oxidation sites. However, given that our assignment of the first 3689
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Table 4. Computed Atomic Spin Density (Electrons) for Compounds 3n+, 4n+, Si-3n+ and Si-4n+ Compared to Those Previously Found for 2n+a
a
cpnd
2+
3+
Si-3+
4+
Si-4+
22+
32+
Si-32+
42+
Si-42+
C0 C1 C2 C3 Ru C4 C5 C6 C7 C8 C9 Fe total
0.02 −0.01 0.04 −0.02 0.13 0.07 0.05 0.04 0.01 0.13 0.02 0.52 1.00
0.04 −0.03 0.15 −0.03 0.49 −0.02 0.22 −0.04
0.05 −0.03 0.15 −0.04 0.50 −0.02 0.22 −0.04
0.05 −0.03 0.17 −0.04 0.54 −0.03 0.23 −0.04
0.03 −0.03 0.15 −0.05 0.50 −0.03 0.22 −0.05
0.14 −0.03 0.25 0.04 0.45 −0.06 0.11 −0.06
0.15 0.01 0.18 0.10 0.32 −0.03 0.08 −0.03
0.14 −0.04 0.28 0.03 0.50 −0.03 0.11 −0.03
0.13 −0.03 0.17 0.02 0.51 −0.04 0.12 −0.04
0.19 0.97
0.19 0.98
0.15 1.00
0.26 1.00
0.08 −0.03 0.25 −0.01 0.54 −0.03 0.16 108 cycles) reported for the ferrocene-based system Si-14.13 The surface coverage obtained with the more rigid systems 2 and 3 is higher than that reported by Fehlner and co-workers 3690
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Scheme 6. Reactions Undergone by Ru(III) Alkynyl Complexes
Scheme 7. Tentative Proposal to Explain the Side Reaction Undergone by the Dications 32+ (R = H) and 42+ (R = CHCHSin) Tethered to the Si Surface
for analogous dinuclear complexes such as 15+ (Scheme 5).3 In the latter example, the Fc-Ru(κ2-dppm)2-containing complex cation (dppm =1,1′-bis(diphenylphosphino)methane) was covalently attached to a chlorinated Si(111) surface through Si−N interfacial bonds. Because of the lack of robustness of the Si−N bond and possible hydrolysis of the starting Si−Cl bond, a significant amount of silicon oxides was introduced during the preparation of the molecular monolayer. Furthermore, the rate constants of electron transfer between Si and 3 or 4 were considerably higher than those reported for Fehlner’s system, demonstrating the potential of group 8 metal alkynyl complexfunctionalized interfaces for the development of fast chargestorage devices. However, Si-3 exhibits two closely spaced redox events limiting the potential window available to sequentially address both processes, whereas Si-4 only allows observation of a single redox process upon redox cycling on the surface, as a result of the loss of the Ru(II)/Ru(III) electrochemical signal while the Fc/Fc+ system remains intact. This is believed to originate from an enhanced chemical reactivity for the dications 32+ and 42+ when linked to the surface, compared to Si-22+. DFT calculations performed on 3 or 4 and on models of the immobilized molecules Si-3 and Si-4 (using a Si10H15 cluster as a model for the silicon surface) suggest that the observed reactivity of the bound species is not due to a change in reactivity induced by grafting. Rather, it reflects the intrinsic reactivity of the grafted molecules and is not attributable to an electronic change induced by the nearby surface nor a consequence of the Si-CHCH linkage. Accordingly, chemical isolation of 3[PF6]2 and 4[PF6]2 or analogues67 turned out to
be extremely difficult, in contrast to dications related to 2[PF6]2.50 Several explanations that are founded on literature data may permit rationalization of our experimental observations. First, the observed instability might derive from the Ru(III)-alkynyl character of the grafted dications 32+ and 42+, which are present in their triplet states, as evidenced by DFT. The reactions classically undergone by trans-[Ru(κ2-dppe)2CC-]+• fragments are usually of three types (Scheme 6):98,99 (i) oxidation with molecular oxygen, leading to a cationic ruthenium carbonyl complex (A),62,100,101 (ii) hydrogen abstraction at the alkynyl Cβ position to give a vinylidene complex such as (B),102 and (iii) radical−radical coupling at the para-phenyl and β-alkynyl positions to give heterodimers such as (C).98 However, none of these reactions convincingly explain the electrochemical behavior of Si-32+ and Si-42+. Thus, reaction (i), occurring with traces of exogenous oxygen, would possibly sever the linker connecting the dication to the surface, and would in any event lead to an electrochemically silent ruthenium complex, since the ruthenium−carbonyl complex (A) has no electroactivity within the potential range of a ruthenium alkynyl complex. Likewise, reaction (ii) with solvent or unreacted Si−H bonds should lead to an electrochemically silent ruthenium vinylidene complex. Thus, both side-reactions (i) and (ii) should result in a partial loss of the Ru-based process upon dioxidation of Si-3, in contrast to experimental observation. Reaction (iii) can also be disregarded, since the complex is tethered to the surface and the packing at the silicon interface should prevent any intermolecular reaction between grafted dications, especially in the presence of bulky counter3691
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anions (moreover its para-phenylene position is occupied by the ethene linker to the surface). Alternatively, this instability might be attributed to the enhanced spin density present on the ferrocenium side of 32+ and 42+ in the triplet state, which is also associated with a larger cumulenic character of the carbon-rich spacer and an electrophilic Cγ atom (compared to 22+). The high spin density and the positive partial charge evidenced by DFT can be explained by including a VB mesomer such as B′ in the VB description of the triplet dications (Scheme 7). Nucleophilic species such as water or surface hydrides on silicon might therefore attack at the exposed Cγ atom, similar to reactions reported for related electron-rich Ru(II) allenylidenes, and thereby lead to the corresponding Ru(III) alkynyl complex (Scheme 7);51,103,104 related reductions of cationic Ru(II) allenylidene complexes by hydrides have also been reported.70 Interestingly, such a process with 32+ and 42+ would preserve the apparent electroactivity of the ruthenium electrophore, since a new Ru(III) alkynyl complex is formed, while the electroactivity of the now 17-electron Fe(III) center is likely to be significantly modified, as observed for Si-3. Depending on the evolution of the surface bound species, the process might even lead to cleavage of the dinuclear complex 4, resulting in a loss of the electroactivity associated with the ruthenium center, as experimentally observed for Si-4. Under such an assumption, the overall fragility of these interfaces will therefore also be linked to the number of Si−H sites remaining after the grafting step, given that a limiting coverage is always attained due to the fairly large volume occupied by the organometallic fragments bearing the terminal alkyne functionality. Thus, the reason for the lower kinetic stability of Si-32+ and Si-42+ can be traced back to the enhanced ″radical″ character gained by the Fc end upon oxidation (B) and to the lower steric protection of the Fe(III) end group compared to that of Si-22+. As has already been demonstrated experimentally (by IR) in previous work on related mono- and dications, the oxidation of these species is also accompanied by a concomitant enhancement in the cumulenic character of the carbon-rich bridge, a feature more pronounced in their singlet state.67,96,105,106 These features are confirmed by the present DFT calculations. Based on this interpretation, one strategy to increase the stability of such interfaces would be to sterically shield the γ-carbon atom and the ferrocenyl end of the bridge. Another strategy would be to limit the availability and proximity of surface-bound nucleophilic species such as hydrides. This work also reveals that flexible ferrocenyl-linked species such as 4 should not be pursued in this regard in the future. To access more fatigueresistant redox-active interfaces, both the quality and the packing density of the Fc/Ru organometallic monolayers could be improved by diluting the redox-active chains with electrochemically inert organic chains (e.g., derived from phenylacetylene). This would enable control of the surface coverage of the redox centers while maintaining the molecular orientation of the monolayer and limiting the monolayer’s interaction with the unreacted Si−H sites.
interfaces. The modified surfaces Si-2, Si-3, and Si-4 show coverages and electrochemical responses that differ significantly and that depend on the nature of the grafted molecule (Table 2). Compared to data gathered for Si-1n and Si-2, all electrochemical data obtained for Si-3 and Si-4 demonstrate that the use of the ferrocenyl group instead of the “Fe(κ2dppe)(η5-C5Me5)” moiety positively increases the kinetic stability of the first oxidized state in these electroactive assemblies while lowering significantly the oxidation of the underlying silicon surface. Furthermore, we have shown that the various redox systems in 2 and 3 can be electrochemically addressed in a stepwise manner, with high charge-transfer rates. Indeed, values of the apparent charge-transfer rate constants with the Si surface found for Si-2, Si-3, and Si-4 in the present studies are considerably higher than those reported for many oxide-free silicon-bound related systems. This demonstrates the potential of such complexes for the development of fast chargestorage devices. The surface coverage calculated for the more rigid of these systems, i.e., 3, is similar to that obtained for the dinuclear complex 2. In contrast, the significantly lower coverage achieved for Si-4 can be rationalized by the presence of a flexible linker between the redox sites in 4. The two redox processes for Si-3 and Si-4 were found to be sensitive to the potential window scanned. Thus, significant fragility was found for Si-3 and even more so for Si-4 at high potentials. As evidenced by DFT calculations, this likely originates from the large spin density residing on the Fe(III) end in the grafted dications 32+ and 42+. To address the sensitivity of these modified surfaces to very oxidizing potentials, several possibilities can now be envisaged. These are (i) to increase the steric bulk around the alkynyl-ferrocenyl terminus in the heterobimetallic precursors, in order to sterically limit any side-reaction in the (di)oxidized state(s), (ii) to ensure that the ethenyl linker to the surface tethers the heterobimetallic unit in a rigid and perpendicular fashion, as revealed by the dramatic difference in the electrochemical behavior of Si-3 and Si-4, and (iii) to limit the number of Si−H sites remaining following the grafting step. Work directed toward these stimulating prospects is currently in progress.
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ASSOCIATED CONTENT
S Supporting Information *
Additional structural data for complex 11, electrochemical data of 3, 4, 11, and Si-2, XPS data for Si-2, and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Tel. (+33) 02 23 23 59 62. E-mail:
[email protected]. *Tel. (+33) 02 23 23 65 50. E-mail:
[email protected]. *Tel. (+61) 2 61 25 29 27. E-mail:
[email protected]. au. Notes
The authors declare no competing financial interest.
5. CONCLUSIONS We have reported herein the synthesis and characterization of three new heterobinuclear complexes 2−4, each possessing a pendant ethynyl group. As previously demonstrated for 2,25 these complexes can be readily grafted onto hydrogenated silicon surfaces under mild photochemical conditions, thereby opening facile access to multistate redox-active functional
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ACKNOWLEDGMENTS G.G. thanks Region Bretagne for financial support. The ANR (09-BLAN-0109), C.N.R.S. (PAI No. 19817 and PICS program No. 5676), and the Australian Research Council (ARC) are acknowledged for financial support, and the CINES (Mont3692
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dx.doi.org/10.1021/jp412498t | J. Phys. Chem. C 2014, 118, 3680−3695
