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Assembly of Platinum Diimine Dithiolate Complexes onto HydrogenTerminated Silicon Surfaces Gilles Yzambart,† Bruno Fabre,*,† Thierry Roisnel,† Vincent Dorcet,† Soraya Ababou-Girard,‡ Cristelle Meriadec,‡ and Dominique Lorcy*,† †

Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, 35042 Rennes Cedex, France Institut de Physique de Rennes, Département Matériaux et Nanosciences, Campus de Beaulieu, UMR 6251 CNRS-Université de Rennes 1, 35042 Rennes Cedex, France

‡

S Supporting Information *

ABSTRACT: The synthesis and structural characterization of the platinum diimine dithiolate complexes [Pt(R2bipy)(dmipi)] (dmipi = 4,5-dimercapto-l,3-dithiol-2-propargylimino and R = H, tBu) are described together with the X-ray crystal structure of the dithiolate proligand. These heteroleptic Pt complexes have been covalently bound to hydrogen-terminated silicon (100) surfaces using either a one-step or two-step procedure. The redox-active organometallic film modified surfaces were prepared from a hydrosilylation reaction at 90 °C of either the Pt complex bearing an ethyne terminal group or an ethyne-terminated dithiolate precursor followed by the subsequent anchoring of the Pt complex. Cyclic voltammetry measurements showed the presence of a single reversible one-electronoxidation process corresponding to the oxidation of the complex into its radical cation species at 0.42 and 0.46 V vs SCE for the unsubstituted and tBu-substituted bipyridine dithiolate Pt complex-modified Si(100) surfaces, respectively. Such values compare well with those determined for the electroactive molecules in solution. Moreover, FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements were consistent with the expected structure of grafted molecular chains and revealed a significant oxidation of the underlying silicon surface. Nevertheless, the one-step procedure was found to lead to redox-active films of density higher than those produced from the two-step procedure. From XPS data, the surface coverage was estimated at 0.10 and in the range 0.03−0.06 Pt complex per surface silicon atom for the one-step and two-step procedures, respectively.

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Pt complexes of the type [Pt(NN)(S∧S)] with a diimine ligand (NN) such as bipyridine (bipy) or 4,4′-di-tert-butyl-2,2′bipyridine (tBu2bipy) have received a great deal of attention as solar cell sensitizers25 and second-order nonlinear optical (SONLO) materials.26 These Pt complexes exhibit multistage electrochemical processes; for instance, [Pt(tBu2bipy)(dmid)] (dmid = 4,5-dimercapto-l,3-dithiol-2-one)27 (Chart 1) undergoes two sequential reversible monoelectronic oxidation processes and one reversible monoelectronic reduction step

INTRODUCTION The integration of redox-active molecules into highly densely packed monolayers covalently bound to hydrogen-terminated silicon (Si−H) surfaces has received intense attention due to the large extent of potential applications of controlled and robust organic/Si interfaces.1−5 Various electrochemically oxidizable electrophores, such as ferrocene,6−13 metal-complexed porphyrins,14−18 and tetrathiafulvalenes (TTF),19−21 have been immobilized onto Si−H using different grafting procedures (hydrosilylation, aryldiazonium, “click”, and carbodiimide chemistry). The two latest electrophores exhibit multiple electron transfer steps at relatively low potentials and chemical stability of the different redox states. Such attractive characteristics are appealing for the fabrication of electrically addressable devices, particularly when the goal is integrated systems devoted to information storage or transfer. In addition to metalloporphyrins, which exhibit a versatility of their redox properties depending on the nature of the complexed metal, heteroleptic metal dithiolate complexes of the type [M(S∧S)(L)n] in this aspect open wide possibilities as multiredox systems. Indeed, their electrochemical properties can be finely tuned depending on both the nature of the complexed metal and the second type of coordinated ligand (L).22−24 Among the various heteroleptic dithiolate complexes, © XXXX American Chemical Society

Chart 1

Special Issue: Organometallic Electrochemistry Received: January 14, 2014

A

dx.doi.org/10.1021/om5000369 | Organometallics XXXX, XXX, XXX−XXX

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at easily accessible potentials (Ered = −1.32 V; Eox1 = 0.55 V, and Eox2 = 1.44 V vs Ag/AgCl in CH2Cl2/Bu4NPF6 medium). Therefore, we sought to synthesize an analogous Pt complex for further grafting to hydrogen-terminated silicon (Si−H) surfaces. Among different anchoring groups studied for the derivatization of Si−H surfaces, it has been demonstrated that the molecules functionalized with terminal alkynes led to densely packed and ordered monolayers covalently bound to Si−H with a high surface coverage.28−31 Therefore, we investigated the synthesis of a dithiolate Pt complex bearing a propargyl imino group on the 2-position of the dithiole ring, [Pt(R2bipy)(dmipi)] (dmipi = 4,5-dimercapto-l,3-dithiol-2propargylimino and R = H, tBu) (Chart 1). We select a propargyl group in order to introduce a nonconjugated alkyne and also to allow a certain degree of flexibility between the bound complex and the Si surface. Our ongoing interest in the design of well-defined redoxactive interfaces prompted us to investigate the covalent derivatization of Si−H surfaces by these dithiolate complexes using two approaches. The first is based on the direct grafting of [Pt(R2bipy)(dmipi)] complexes onto Si−H through a hydrosilylation route (route a, Scheme 1). The second is a

Scheme 2. Synthetic Route for the Synthesis of the Target Molecules

oborate formed in situ from triethyl orthoformate and BF3· Et2O.33 As already reported, 1,3-dithiolium cations substituted by a leaving group can react with primary amine to give 2imino-1,3-dithiole derivatives.34 Reaction of propargylamine with the dithiolium salt 2 in the presence of ammonium acetate led to the formation of dithiole 3 in moderate yield. Dithiole 3 is the organic dithiolate hidden form of the ligand where the two thiolate functions are protected by cyanoethyl groups. Deprotection of the dithiolate ligand was performed in basic medium using Cs2CO3,32 and subsequent addition of the Pt(bipy)Cl2 complex in the medium afforded the [Pt(bipy)(dmipi)] complex 4, which precipitated in the medium in excellent yield (90%). Complex 4 was found to be poorly soluble in halogenated solvents such as CH2Cl2 and CHCl3 but soluble in DMSO. In order to increase the solubility of the target Pt complex, we also prepared the analogous complex substituted by two tBu groups using Pt(tBu2bipy)Cl2 as the starting material. [Pt(tBu2bipy)(dmipi)] (5) was obtained in 80% yield. Single crystals of dithiole 3 and complexes 4 and 5 were obtained by slow evaporation of mesitylene, DMSO, and dichloromethane solutions of the compounds, respectively. The molecular structures are given in Figure 1, and selected bond lengths and angles are summarized in Table 1. Both complexes present similar trends such as a square-planar geometry around the platinum center, a planar skeleton, and the alkyne anchoring group pointing out of the plane. The observed Pt−N and Pt−S bond lengths as well as the ligand bite angles, N−Pt−N and S−Pt−S, are in the same range as that found for the Pt(tBu2bipy)(dmid) analogue.27 This indicates that there is no influence of the nature of the bound group on the 2-position of the dithiole ring. The alkyne bond lengths and the C25− C26C27 angles have comparable values for the three derivatives 3−5 (Table 1). Complexes 4 and 5 are air stable in their solid form, as evidenced by an 1H NMR study performed on the same sample 6 months later (NMR in the Supporting Information), and are stable in solution with particular precautions (in the dark and under argon). Indeed, in solution, we noticed upon light and air exposure a slow discoloration of the deep purple solution into a colorless solution for 4 and light yellow solution for 5. In order to get insights into what we assume to be oxidation,25a a

Scheme 1. Chemical Pathways for the Preparation of Hydrogen-Terminated Si(100) Surfaces Covalently Bound to Pt Complexesa

a

The one-step procedure is given at the top (route a) and the two-step procedure is given at the bottom (route b). R = H, tBu.

multistep method and consists first of a hydrosilylation reaction of the alkyne-substituted precursor of the dithiolate ligand with Si−H followed by the formation of the Pt complex after deprotection of the dithiolate ligand (route b, Scheme 1). In this work, we describe the synthesis and the characterization of the [Pt(R2bipy)(dmipi)] complexes (R = H, tBu) as well as the preparation of Si−H surfaces covalently bound to the redoxactive Pt complexes using the direct and multistep routes. The formed monolayers have been characterized by various experimental techniques including cyclic voltammetry, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS).

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RESULTS AND DISCUSSION Synthesis of the Pt Complexes. The Pt diimine dithiolate complexes were prepared starting from 4,5-bis(cyanoethylthio)1,3-dithiole-2-thione (1)32 as outlined in Scheme 2. In our synthetic approach, we converted the dithiole-2-thione 1 into a tetrafluoroborate dithiolium salt 2 thanks to the alkylation of the exocyclic sulfur atom with diethoxycarbonium tetrafluorB


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Figure 1. ORTEP drawings showing the atom labeling scheme for compound 3 (left) and Pt complexes 4 (middle) and 5 (right). Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms are omitted for clarity.

Table 1. Selected Bond Distances (Å) and Angles (deg) for 3−6 3 Pt−N1 Pt−N11 Pt−S1 Pt−S2 S1−C21 S2−C22 C21−C22 C23−N24 N24−C25 C25−C26 C26−C27 N1−Pt−N11 S1−Pt−S2 C23−N24−C25 N24−C25−C26 C25−C26−C27

4

5

6

1.761(7) 1.749(6) 1.340(8) 1.285(8) 1.467(7) 1.475(10) 1.188(11)

2.057(3) 2.048(3) 2.270(1) 2.271(2) 1.745(4) 1.743(4) 1.202(2) 1.261(5) 1.468(5) 1.459(6) 1.190(7)

2.052(13) 2.046(15) 2.273(8) 2.263(7) 1.749(19) 1.744(16) 1.342(25) 1.280(24) 1.451(23) 1.426(34) 1.196(39)

2.043(2) 2.046(2) 2.273(1) 2.272(1) 1.735(4) 1.721(3) 1.515(5)

78.79(48) 90.54(17) 121.14(160) 108.88(189) 175.38(289)

79.10(9) 88.64(3)

114.76(46) 111.31(46) 177.69(69)

79.35(12) 90.32(3) 117.02(33) 111.55(31) 177.27(47)

(Scheme 3). Selected bond lengths and angles of 6 are also given in Table 1. This result is reminiscent of the photo-

solution of 5 in CHCl3 was kept for 1 week with exposure to light and air. After slow evaporation of the solvent, yellow crystals suitable for an X-ray diffraction study were isolated. The resulting X-ray crystal structure is depicted in Figure 2. Actually, upon air and light exposure, the complex 5 was converted into Pt(tBu2bipy)(dto) (6;35 dto = dithiooxalate)

Scheme 3. Oxidative Degradation of the Pt Complex 5 upon Light Exposure

chemical oxidation of Pt(bipy)(1,2-benzenedithiolate) in the presence of oxygen, but in this case sulfinate and disulfinate products were obtained.36 It is worth mentioning that the presence of both oxygen and light is necessary for this transformation.36 Therefore, by working under an inert atmosphere upon light exposure or by avoiding light exposure in an air atmosphere these complexes do not suffer this photoreaction. Electrochemical Properties of the Pt Complexes. The redox properties of these complexes have been studied by cyclic voltammetry (CV) in DMSO for 3−5 and in CH3CN for 3 and

Figure 2. Molecular structure of Pt(tBu2bipy)(dto) (6; dto = dithiooxalate). Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms are omitted for clarity. C


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(e.g., p-xylene and toluene) which have been demonstrated to be efficient solvents for producing high-quality and densely packed organic monolayers on Si−H.38,39 As an alternative method, we have tested DMSO as the grafting solvent, in which 4 is highly soluble, and mesitylene as the solvent for complex 5 (Scheme 4). Moreover, as both 4 and 5 are light sensitive, the grafting reaction was performed at 90 °C in the dark to avoid any decomposition of the starting complexes (vide supra).

5 using Bu4NPF6 as supporting electrolyte (Figure 3). The redox behavior of 3 in CH3CN and DMSO has also been

Scheme 4. One-Step Covalent Grafting of Pt Complexes onto Hydrogen-Terminated Si(100) Surfaces Figure 3. Cyclic voltammograms in DMSO (a) or CH3CN (b) + 0.1 M Bu4NPF6 at 0.1 V s−1 of proligand 3 (green) and complexes 4 (blue) and 5 (red).

investigated for comparison. The electrochemical data are reported in Table 2. The CVs of 4 and 5 in DMSO are characterized by two reversible waves at E°′red = −1.18 V and E°′ox = 0.52 V vs SCE for 4 and at E°′red = −1.27 V and E°′ox = 0.49 V for 5 (average of anodic and cathodic peak potentials) and irreversible reduction processes at Epc = −1.86 V and Epc = −1.95 V (cathodic peak potential) for 4 and 5, respectively, which is superimposed on the onset of the solvent discharge. The reversible and monoelectronic character of the first two systems is supported by a peak-to-peak separation of close to 60 mV and a ratio between the anodic and cathodic peak current intensities close to 1 within the investigated potential scan range (i.e., 0.1−1.0 V s−1). They correspond to the reduction of the neutral species into the monoanionic species and to the oxidation of the neutral species into its cation radical. The reduction of 5 is rendered more difficult by the introduction of two tBu groups on the bipyridine ligand in comparison to 4, while the oxidation of 5 is easier compared to that of 4. The CV of 5 in CH3CN allows the observation of two reversible one-electron waves and an ill-defined quasi-reversible oxidation system characterized by a larger peak-to-peak separation (namely 254 mV). This difference of the CV for 5 in DMSO is due to the shift of the second oxidation process outside the accessible potential window in DMSO. The two oxidation processes which are ascribed to the formation of the radical cation and dication forms are observed at E°′ox1 = 0.43 V and E°′ox2 = 1.22 V, respectively, and the reduction into the monoanionic species occurs at E°′red = −1.31 V. Globally, complexes 4 and 5 exhibit an electrochemical behavior similar to that of Pt(tBu2bipy)(dmid)27 (Table 2). The oxidation processes are assigned to the Pt dithiolate moiety, while the reduction processes are attributed to the bipy or tBu2bipy ligand.37 One-Step Preparation of Pt Dithiolate Complex Modified Si(100) Surfaces. Unlike 5, 4 is not soluble in mesitylene or other aromatic solvents with high boiling points

FTIR Spectroscopy Characterization. The derivatization of oxide-free, hydrogen-terminated Si(100) surfaces with the two redox-active complex-based films was monitored by FTIR spectroscopy using porous silicon and monocrystalline Si(111) substrates in the transmission and attenuated total reflectance (ATR) modes, respectively (see the Experimental Section for details). We are aware that the efficiencies of the hydrosilylation reaction can be different for porous silicon substrates. Indeed, the reagents need to penetrate within the pores to react with the hydrogenated sites. Due to these diffusional constraints, the grafting efficiency is usually lower with porous silicon. Moreover, because of its high surface area and its complex morphology, hydrogenated porous silicon is often more sensitive to oxidation than flat silicon. In the present work, transmission FTIR spectroscopy has been used to extract qualitative information on the chemical composition of the grafted film produced after the hydrosilylation reaction using porous silicon and these data have been compared with those obtained for modified ATR Si crystals. It was not our purpose to determine quantitatively by IR the surface concentrations of complexes. FTIR spectra of 4 and 5 modified porous Si(100) and 5 modified ATR Si(111) crystals are shown in Figure 4. The two surfaces show clear differences, particularly in the 2000−3100 cm−1 region. The presence of one intense band at

Table 2. Cyclic Voltammetry Data of Compounds 3−5a compound

E°′red2/V (ΔE/mV)

E°′red1/V (ΔE/mV)

c

proligand 3 proligand 3d [Pt(bipy)(dt)] (4)c [Pt(tBu2bipy)(dt)] (5)c [Pt(tBu2bipy)(dt)] (5)d [Pt(tBu2bipy)(dmid)]e a

−1.86b −1.95b

−1.18 (70) −1.27 (67) −1.31 (96) −1.32

E°′ox1/V (ΔE/mV) 1.34 1.28 0.52 0.49 0.43 0.55

E°′ox2/V (ΔE/mV)

b

(80) (50) (59) (59)

1.65 (110)

1.22 (254) 1.44

E in V vs SCE. bIrreversible process; peak potential. cIn DMSO. dIn CH3CN. ein CH2Cl2/Bu4NPF6; E in V vs Ag/AgCl (from ref 27). D


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The FTIR spectrum of the 5 modified ATR Si(111) crystal shown in Figure 4C was subtracted from that of the Si−H surface taken as the reference spectrum. Under these conditions, the negative peaks at ca. 2142−2087 cm−1 are consistent with the consumption of Si−H units upon the hydrosilylation reaction. In addition, the bands observed in the 3100−2850 cm−1 region confirm the presence of the bound Pt complex, as these are characteristic of the Car−H stretching modes of the bipy moiety in the 3020−3100 cm−1 range as well as those of the CH2 and CH3 groups at lower energy. The band at 1618 cm−1 is assigned to the imine bond, CN, while the large band at 1442 cm−1 corresponds to the contribution of the νCC of the pyridine and the dithiole rings. Moreover, no band characteristic of the interfacial vinyl bond is observed around 1580−1600 cm−1, which is unsurprising because the spectral signature for such a bond is usually quite weak. Globally, all IR spectroscopic data are consistent with the complex 5 bound to the Si surface via the hydrosilylation of the terminal alkyne. Electrochemical Characterization of the Pt Complex 5 Modified Si(100) Surface. The CV characterization of the 5 modified flat Si(100) surface (Si-5) was performed in the dark, in order to avoid the decomposition of the bound complex 5. In thoroughly dried acetonitrile medium, Si-5 undergoes a reversible one-electron oxidation process at E°′ = 0.46 V vs SCE corresponding to the oxidation of the complex into its radical cation species (Figure 5). This value is relatively close to

Figure 4. (a, b) Transmission FTIR spectra of hydrogenated porous Si(100) before (red line) and after reaction with 4 (a) or 5 (b) (blue line) at 90 °C overnight. (c) ATR FTIR spectrum of 5 modified Si(111) crystal. The negative peaks at ca. 2100 cm−1 in (c) correspond to νSi−Hx stretching modes of the Si(111)−H surface that was used as the reference for background subtraction.

2250 cm−1 together with the disappearance of the stretching modes of νSi−Hx with x = 1−3 at 2050−2150 cm−1 in the spectrum of the surface produced from 4 in DMSO reveals that most of the Si−H sites have been oxidized into OSi−H (Figure 4a). Additionally, the presence of one broad band centered at 1108 cm−1, characteristic of the νSi−O stretching vibration, confirms the oxidation of porous silicon. Furthermore, no bands characteristic of the complex such as the Car−H stretching modes of the bipy moiety are detected in the 3020−3100 cm−1 range. The unsuccessful grafting of 4 in DMSO and the resulting strong silicon oxidation observed in the presence of this solvent can be explained by the oxidizing character of DMSO, which acts as a mildly self-limiting oxidizing agent for porous silicon (Scheme 4).40 In contrast, the IR spectrum obtained after the reaction of 5 in mesitylene shows several vibration bands characteristic of the complex together with a weakly intense band centered at 2241 cm−1 due to the presence of OSi−H resulting from the oxidation of some Si−H sites. The presence of this band is due to an incomplete grafting reaction with porous silicon, as evidenced by the persistence of the band centered at ca. 2105 cm −1 corresponding to the νSi−Hx stretching modes (Scheme 4). Moreover, the bands observed in the 2800−3000 cm−1 region are attributed to the CH3 groups bound to the bipy moiety: the symmetric CH3 mode at 2874 cm−1 and the two nondegenerate asymmetric CH3 stretching modes at ca. 2968 and 2915 cm−1 (Figure 4b). The absence of the band corresponding to the terminal alkyne νCH, observed at 3286 cm−1 in the powder spectrum of 5, is worth noting; this is consistent with the binding of complex 5 to the surface via the hydrosilylation of the terminal alkyne (Scheme 1). Other bands are also observed in the 1650−1350 cm−1 region. The band at 1616 cm−1 can be unambiguously attributed to the CN bond within the complex moieties, while the weaker band at 1584 cm−1 can be ascribed to the C C units bound to the silicon surface. However, as already noted by others,20,41−43 the spectral signature for the interfacial vinyl bond is usually quite weak.

Figure 5. (a) Cyclic voltammograms in the dark of Si-5 in CH3CN + 0.1 M Bu4NClO4, as a function of the potential scan rate v within the range 0.1−1 V s−1 and (b) corresponding Ipa and Ipc vs v plots for the redox process.

that determined for 5 in solution, namely 0.43 V, indicating that its redox properties are not significantly altered after the immobilization step. Another weaker oxidation wave can be observed at −0.2 V. Such a redox process is associated with the presence of SiOH units coming from some surface oxidation. As expected for surface-confined redox species,44 anodic and cathodic peak current intensities Ipa and Ipc are both found to be directly proportional to the potential scan rate v within the range 0.0−1.0 V s−1 (Figure 5b). Due to the poorly resolved redox system at 0.46 V, it was not possible to estimate electrochemically the surface coverage of attached 5 from the area under either the background-corrected anodic or cathodic wave. Two-Step Preparation of Pt Diimine Dithiolate Complex 4 and 5 Modified Si(100) Surfaces. The second approach used for the elaboration of Pt diimine dithiolate complex monolayers grafted to silicon is a two-step route which consists first of derivatization of the Si−H surface with the precursor of the alkyne-substituted dithiolate 3 and the subsequent formation of the Pt dithiolate complex onto the surface (Scheme 5). E


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Scheme 5. Two-Step Procedure (Route b) for the Preparation of Hydrogen-Terminated Si(100) Surfaces Covalently Bound to Pt Complexes, Si-4 and Si-5

grafting of dithiole 3 (Si-3) and the subsequent formation of the bound Pt complex 4 (Si-4) or 5 (Si-5) was investigated by cyclic voltammetry in CH3CN medium. The CVs shown in Figure 7 provide clear evidence of the presence of the different

Thanks to the presence of the alkyne anchoring group in 3, the hydrosilylation reaction with Si−H was achieved in mesitylene and was monitored by FTIR spectroscopy in order to characterize the surface before the conversion of the dithiole unit into the Pt complex (vide infra). In the first step, the Si-3 monolayer was reacted with a solution of DMF and MeOH containing Cs2CO3 for the deprotection of the dithiolate, and then the addition of Pt(R2bipy)(Cl)2 enabled the formation of the Pt complex onto the surface. The ATR FTIR spectrum of the Si(111) crystal after reaction with 3 shows the presence of bands in the range 2960−2720 cm−1 which are characteristic of the CH2 stretching modes of the derivative 3 (Figure 6). Moreover, we note the absence of the

Figure 6. ATR FTIR spectra of Si(111) (a) after reaction with 3 at 90 °C overnight in mesitylene and (b) after deprotection of the bound dithiolate ligand and further reaction with Pt(tBu2bipy)(Cl)2 (Si-5).

band corresponding to the terminal alkyne in the 3200−3300 cm−1 region observed at 3247 cm−1 in the spectrum of 3. We also noticed the presence of a weak band centered at 2250 cm−1 which can be ascribed to the signature of the nitrile stretching vibration band of 3 and/or to the presence of oxidized silicon sites OSi−H. The band at 1587 cm−1 can be ascribed to the stretching vibration of the imine bond (CN) of dithiole 3 bound to the surface, as this band is observed at 1583 cm−1 in the spectrum of 3. After deprotection and reaction of the bound dithiolate ligand with Pt(R2bipy)(Cl)2, the FTIR spectrum shows some significant changes (Figure 6). First, we observe the disappearance of the band at 2250 cm−1, which is consistent with the loss of the cyanoethyl group during the deprotection of the ligand. Second, the stretching vibration band of the imine bond (CN) is now shifted to 1612 cm−1: i.e., at a energy relatively close to that corresponding to the same band observed in the spectrum of Si-5 prepared in one step (1618 cm−1) or the starting complex 5 (1616 cm−1). Electrochemical Characterization of the Modified Surfaces. The redox behavior of the modified surfaces after the

Figure 7. Cyclic voltammograms in the dark of (a) Si-3 at 0.1 V s−1 and of (b) Si-4 and (d) Si-5 as a function of the potential scan rate and the corresponding Ipa and Ipc vs v plots for the oxidation processes of (c) Si-4 and (e) Si-5. Conditions: CH3CN + 0.1 M Bu4NClO4.

electroactive species on the surface. Indeed, for Si-3, a single irreversible oxidation peak at 1.18 V is observed, whereas Si-4 and Si-5 show a reversible oxidation process at 0.42 and 0.46 V, respectively. These oxidation processes can be undoubtedly ascribed to the bound dithiole 3 and Pt complexes 4 and 5, respectively, as the measured oxidation potentials are approximately the same as those observed for these derivatives in solution (Table 2). Furthermore, it must be noted that the complex 5 grafted either in one step or two steps exhibits the same redox system at 0.46 V. For Si-4 and Si-5, Ipa and Ipc are found to be directly proportional to v within the range 0.05− 1.5 V s−1, as expected for surface-confined redox species.44 F


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In the dark, the underlying p-doped silicon electrode behaves as an insulator for reduction processes.45 Consequently, it is not possible to observe the reduction processes assigned to the bipy ligand under such conditions. Light irradiation is required to photogenerate electron−hole pairs and thus promote the reduction processes. It must be recalled that both 4 and 5 are not stable in solution upon light exposure, and it is expected that the light sensitivities of the immobilized complexes will be rather similar. Nevertheless, we investigated the redox signature of the tBu2bipy ligand of Si-5 in CH3CN upon light irradiation, and the CVs are reported in Figure 8. Under these conditions,

supra). Concerning the two-step procedure, in addition to the Si substrate and O 1s signals, only C 1s, S 2s, and N 1s components appear after the grafting step of dithiole 3. The presence of these components is consistent with the monolayer composition of Si-3. After the formation of the Pt complex onto the surface, additional signals appear such as Pt 4f (Figure 9). From the high-resolution spectra of the N 1s signal, it is possible to determine the changes in the surface chemical composition. For instance for Si-5 generated in one step, the high-resolution XP spectrum of the N 1s shows a broad signal which was fitted and deconvoluted into two peaks: the most intense component at 401.1 ± 0.1 eV and the smallest one at 399.2 ± 0.1 eV can be reasonably assigned to the CN of the bipy ligand and CN connected to the dithiole ring, respectively, with a ratio 2:1 as expected for the composition of the monolayer (Figure 10a). With regard to Si-3, the high-

Figure 8. Cyclic voltammograms in CH3CN + 0.1 M Bu4NClO4 upon light exposure of Si-5 as a function of v.

the reduction of the tBu2bipy ligand occurs irreversibly at −1.18 V vs SCE. Analysis of the CVs as a function of v indicates that this system is shifted toward more cathodic potentials upon increasing v. In comparison with the electrochemical response of 5 in solution, the irreversible nature of the reduction of the immobilized ligand is somewhat surprising. It can be explained either by a light-induced decomposition of the grafted reduced ligand impeding the related oxidation step in the reverse scan to take place or by a reduced charge transfer kinetics by photogenerated electrons coming from the underlying silicon surface. XPS Characterization of the Modified Si Surfaces. Further information on the composition of the organometallic films deposited in either one step or two steps can be provided by X-ray photoelectron spectroscopy (XPS) analysis. In the latter case this analysis is a valuable tool in order to evaluate the changes in the surface chemical composition during the surface derivatization. Typical survey scans for the formation of Si-5 in one step or in two steps, via Si-3, as well as the formation of Si4 after the reaction of the bound dithiolate ligand with Pt(bipy)Cl2 are shown in Figure 9. The XPS analysis of Si-5, prepared in one step, reveals characteristic peaks of the Si substrate itself (Si 2s and Si 2p), as well as characteristic signals of C 1s, N 1s, S 2s, and Pt 4f, confirming the successful grafting of complex 5. The presence of oxygen is due to the partially oxidized surface, as demonstrated by the FTIR analysis (vide

Figure 10. High-resolution XPS spectra of N 1s for (a) Si-5 prepared in one-step and (b) Si-3 and (c) Si-4.

resolution N1 s signal was deconvoluted into three peaks with the two main peaks at 399.0 eV (CN of the dithiole ligand) and 400.2 eV (C ≡ N of the protecting group) in a ratio of 1:2 as expected. It is worth mentioning that the third weak signal at 402.1 eV could be attributed to some CN bonds formed from the reaction of the nitrile units with Si−H (Figure 10b). After the conversion of Si-3 into Si-4 or Si-5, the observed spectral changes are consistent with the expected structure of the novel monolayer. The peak corresponding to the nitrile groups disappears, and the two peaks located at 401.1 and 399.2 eV indicate the presence of the CN bonds of the bipyridine and the dithiole ligands, respectively, as previously observed for Si-5 prepared in one step (Figure 10a). Using the total area under the N 1s signal and comparing this value to a reference signal obtained on a clean hydrogenated silicon surface, the surface molecular coverage, which is the ratio of the number of grafted molecules to the Si(100) surface atom density (6.78 × 1014 atoms cm−2 46), can be estimated as 0.10 for Pt complex 5 (grafted in one step), 0.13 for ligand precursor 3, 0.06 for Pt complex 4 (grafted in two steps), and 0.03 for Pt complex 5 (grafted in two steps). Such values give a specific area of approximately 145 ± 10 Å2 per 5 bound in one step, 110 ± 10 Å2 per bound 3, and 230 ± 20 and 490 ± 40 Å2 per 4 and 5, respectively, both bound in two steps. Accordingly, as demonstrated for Si-5, it is obvious that the one-step attachment of the Pt complex is more efficient than the twostep procedure. Nevertheless, as highlighted by Si-4, the twostep approach allows the production of a functionalized monolayer which could not be obtained in one step, owing to the insolubility of the metallic salt in most organic solvents commonly used for the grafting on silicon surfaces.

Figure 9. Survey XP spectra (a) of Si-5 prepared in one step and (b) of Si-3 (blue trace) and after deprotection followed by the attachment of the Pt complex Si-4 (red trace) or Si-5 (green trace). G


Organometallics

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Table 3. Crystallographic Data for Compound 3 and Complexes 4−6 formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) T (K) Z Dcalcd (g cm−3) μ (mm−1) total no. of rflns abs cor no. of unique rflns (Rint) no. of unique rflns (I > 2σ(I)) R1, wR2a R1, wR2 (all data)a GOF a

3

[Pt(bipy)(dmipi)] (4)

[Pt(tBu2bipy)(dmipi)] (5)

[Pt(tBu2bipy)(dto)] (6)

C12H11N3S4 325.48 monoclinic P21/n 5.2048(8) 11.4426(16) 24.329(4) 90 91.116(7) 90 1448.7(4) 150(2) 4 1.492 0.644 12446 multiscan 3307 (0.0927) 2061 0.0782, 0.2352 0.1233, 0.2611 1.11

C16H11N3PtS4 568.61 triclinic P1̅ 8.2410(2) 8.8161(2) 11.7669(3) 97.166(1) 102.267(1) 90.086(1) 828.52(3) 150(2) 2 2.279 8.975 13 313 multiscan 3678 (0.0305) 3470 0.0207, 0.0465 0.0228, 0.0473 1.067

C49H56Cl2N6Pt2S8 1446.56 triclinic P1̅ 12.4205(18) 15.252(2) 15.807(2) 73.738(4) 70.722(5) 71.165(4) 2624.3(6) 150(2) 2 1.831 5.787 25240 multiscan 11643 (0.0974) 5726 0.0747, 0.1591 0.1758, 0.2067 0.95

C20H24N2O2PtS2, CHCl3 702.99 monoclinic P21/a 12.8524(3) 11.9482(2) 16.5506(3) 90 94.615(1) 90 2533.32(9) 150(2) 4 1.843 6.041 21746 multiscan 5799 (0.0289) 5163 0.0212, 0.0467 0.0259, 0.0487 1.043

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(Fo2 − Fc2)2/∑wFo4]1/2.

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All of the syntheses were performed under an argon atmosphere. The solvents were purified and dried by standard methods. 4,5Bis(cyanoethylthio)-1,3-dithiole-2-thione (1)32 and the Pt complexes Pt(bipy)Cl2 and Pt(tBu2bipy)Cl247 were prepared according to literature procedures. NMR spectra were recorded on a Bruker AV300III spectrometer. Chemical shifts are reported in ppm referenced to TMS for 1H NMR and 13C NMR. Melting points were measured on a Kofler hot-stage apparatus and are uncorrected. Mass spectra were recorded with a Bruker MicrOTOF-Q II instrument by the Centre Régional de Mesures Physiques de l’Ouest, Rennes, France. Elemental analyses were performed at the Centre Régional de Mesures Physiques de l’Ouest, Rennes, France. Column chromatography was performed using silica gel Merck 60 (70−260 mesh). Powder FT-IR spectra were recorded using a Varian-640 FT-IR spectrometer equipped with a diffuse reflectance accessory. Synthesis of Tetrafluoroborate Dithiolium Salt 2. To a solution of 4,5-bis(cyanoethylthio)-1,3-dithiole-2-thione (1; 500 mg, 1.64 mmol) in CHCl3 (30 mL) were added HC(OEt)3 (1.09 mL, 6.48 mmol) and Et2O·BF4 (1.04 mL, 8.23 mmol). The reaction mixture was stirred at room temperature under an inert atmosphere for 14 h. The CHCl3 was removed in vacuo, and the crude product was washed several times with Et2O. Dithiolium salt 2 was obtained as an oily product in 94% yield and used in the next step without further purification. 1H NMR (CD3CN, 300 MHz): δ 1.58 (t, 3H, CH3, 3J = 7.4 Hz), 2.86 (t, 4H, 2CH2, 3J = 6.8 Hz), 3.37 (t, 4H, 2CH2, 3J = 6.8 Hz), 3.62 (q, 2H, CH2, 3J = 7.4 Hz). 13C NMR (CD3CN, 75 MHz): δ 12.4 (CH2-CN), 19.2 (CH3), 33.8 (S-CH2), 37.1 (CH3), 118.9 (CN), 148.2 (C), 206.5 (C+). HRMS: calcd for C11H13N2S5 332.9682, found 332.9678. Synthesis of Dithiole 3. To a solution of 2 (800 mg, 1.9 mmol) in CH3CN (8 mL) under an inert atmosphere was added propargylamine (0.15 mL, 1.14 mmol). The reaction mixture was stirred at room temperature for 24 h, and then ammonium acetate (146 mg, 1.9 mmol) was added. The reaction mixture was stirred at reflux for an additional 24 h. The solvent was removed in vacuo, and the residue was purified by column chromatography on a silica gel using CH2Cl2/ Et2O (8/1) as eluent (Rf = 0.31). 3 was isolated as white crystals in 30% yield after slow evaporation of a mesitylene solution. Mp: 79 °C. 1 H NMR (CDCl3, 300 MHz): δ 2.34 (t, 1H, CH, 4J = 2.6 Hz), 2.77 (m, 4H, 2CH2), 3.11 (m, 4H, 2CH2), 3.98 (d, 2H, CH2, 4J = 2.6 Hz).

CONCLUSIONS In this study, we have synthesized and characterized two novel redox-active platinum diimine dithiolate complexes possessing an alkyne terminal unit for further grafting on hydrogenterminated silicon surfaces. The hydrosilylation reaction of the alkyne allowed us to prepare monolayers of these platinum complexes covalently bound to silicon, using either a one-step or two-step procedure. The latter method consisted of the covalent grafting of the dithiolate precursor followed by the formation of the platinum complex. Cyclic voltammetry measurements showed that the platinum complexes retain their redox activity after anchoring on the semiconducting surface. The one-step procedure led to redox-active films of density higher than those produced from the two-step procedure. However, the two-step approach allowed the immobilization of metallic films which could not be grafted in one step. This postattachment functionalization of Si−H with metallic complexes opens wide perspectives, as it could be extended to other metal dithiolate complexes, the electrochemical characteristics of which could be finely tuned depending on the nature of the ligand, the complexed metal, and the substituent(s) present on the ligand.

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EXPERIMENTAL SECTION

Reagents. Acetone (MOS electronic grade, Erbatron from Carlo Erba), anhydrous ethanol (RSE electronic grade, Erbatron from Carlo Erba), and trichloroethylene (VLSI electronic grade from Carlo-Erba) were used without further purification. The chemicals used for cleaning and etching of silicon wafer pieces (30% H2O2, 96−97% H2SO4, and 50% HF aqueous solutions) were of VLSI semiconductor grade (Riedel-de-Haën). Mesitylene (>99%, Sigma-Aldrich) was passed through a neutral, activated alumina column and distilled under vacuum over sodium. Acetonitrile (>99.5%, puriss, over molecular sieves, Sigma-Aldrich) was used without further purification. DMSO (>99.99%, puriss, Fisher Chemicals) was distilled under vacuum and stored over molecular sieves under an inert atmosphere. H


Organometallics

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C NMR (CDCl3, 75 MHz): δ 18.8 (CH2-CN), 31.2 (S-CH2), 45.2 (N-CH2), 72.4 (CH), 78.2 (−C), 117.3 (CN), 125.2 (C), 128.2 (C), 162.5 (CN). IR: νCC 2125 (s), νCN 2250 (s), νC−H 3247 cm−1 (s). HRMS: calcd for C12H11N3S4 325.99141, found 325.9914. Anal. Calcd for C12H11N3S4: C, 44.28; H, 3.41; N, 12.91. Found: C, 44.33; H, 3.40; N, 12.75. Synthesis of Pt Complexes 4 and 5. To a solution of dithiole 3 (100 mg, 0.31 mmol) in DMF (6 mL) under an inert atmosphere was added a solution of Cs2CO3 (300 mg, 0.92 mmol) in MeOH (4 mL). The reaction mixture was stirred at room temperature for 30 min, and the platinum complex was added (0.31 mmol; 130 mg for Pt(bipy)Cl2 and 166 mg for Pt(tBu2bipy)Cl2). The reaction mixture was stirred at room temperature for 2 h. The resulting purple complex was collected by filtration and washed with water and MeOH. 4 was isolated as a purple powder in 90% yield; after slow evaporation of DMSO solution crystals were obtained. 1H NMR (DMSO, 300 MHz): δ 3.23 (t, 1H, CH, 4J = 2.5 Hz), 3.96 (d, 2H, CH, 4 J = 2.5 Hz), 7.77 (t, 2H, CH, 3J = 6.5 Hz), 8.42 (t, 2H, CH, 3J = 7.7 Hz), 8.68 (t, 2H, CH, 3J = 7.7 Hz), 8.92 (d, 2H, CH, 3J = 6.3 Hz). IR: νCC 2111 (s), νN−C 1577 (s), νC−H 3246 cm−1 (s). UV−vis (DMSO; λ (nm) (ε (L mol−1 cm−1)): 260 (10240), 290 (12820), 575 (2050). HRMS: calcd for C16H11N3PtS4 567.95409, found 567.9539. Anal. Calcd for C16H11N3PtS4: C, 33.80; H, 1.93; N, 7.39. Found: C, 33.31; H, 2.16; N, 7.30. 5 was isolated as a purple powder in 80% yield; after slow evaporation of CH2Cl2 solution crystals were obtained. 1H NMR (CDCl3): δ 1.45 (s,18H, 6CH3), 2.29 (t, 1H, CH, 4J = 2.6 Hz), 3.94 (d, 2H, CH2, 4J = 2.6 Hz), 7.44 (d, 1H, CH, 3J = 1.5 Hz), 7.46 (d, 1H, CH, 3J = 1.5 Hz), 7.91 (s, 1H, CH), 7.92 (s, 1H, CH), 8.89 (d, 1H, CH, 3J = 1.5 Hz), 8.90 (d, 1H, CH, 3J = 1.5 Hz). IR: νCC 2116 (s), νN−C 1576 (s), νC−H 3286 cm−1 (s). UV−vis (CH2Cl; λ (nm) (ε (L mol−1 cm−1)): 236 (30460), 295 (28610), 587 (4630). HRMS: calcd for C24H27N3195PtS4 680.07302, found 680.0725. Anal. Calcd for C24H27N3PtS4: C, 42.35; H, 3.97. Found: C, 41.84; H, 4.16. Crystallography. Single-crystal X-ray diffraction data were collected on an APEX II Bruker AXS diffractometer, with Mo Kα radiation (λ = 0.71073 Å) (Centre de diffractométrie X, Université de Rennes 1, Rennes, France). Structures were solved by direct methods using the SIR97 program48 and then refined with full-matrix leastsquares methods based on F2 (SHELX-97)49 with the aid of the WINGX50 program. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were finally included in their calculated positions. Details of the final refinements are given in Table 3. Preparation of Pt Complex Modified p-Type Si(100) Surfaces. Pt Complex Modified Flat Si(100) Surfaces. Double side polished silicon Si(100) samples (p-type, boron doped, 1−5 Ω cm, Siltronix) were cut into 1.5 × 1.5 cm2 pieces and sonicated for 10 min in acetone, ethanol, and 18.2 MΩ cm ultrapure water. They were then cleaned in 3/1 v/v concentrated H2SO4/30% H2O2 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 dipped in ca. 5% HF for 2 min and dried under an argon stream without rinsing. The Si−H surface was immediately transferred into a Pyrex Schlenk tube containing 5 (20 mM) in 5 mL of deoxygenated mesitylene. The solution was thoroughly purged with argon for 30 min at room temperature in the dark (aluminum foil around the glassware) and then kept at 90 °C under argon in the dark for 20 h. The complex-modified surface (Si-5) was rinsed with acetone and trichloroethylene and then dried under argon. According to the similar procedure described above, a solution of 3 at 20 mM in 5 mL of mesitylene was used to prepare Si-3. Then, the Si-3 surface was dipped into a deoxygenated solution of Cs2CO3 (15 mg, 4.6 × 10−2 mmol) in a mixture of DMF (6 mL) and MeOH (4 mL). After 30 min of stirring at room temperature in the dark, Pt(bipy)Cl2 (6.54 mg) or Pt(tBu2bipy)Cl2 (8 mg) was introduced to the medium. The medium was again stirred for 14 h, and the resulting 13

complex-modified surface (Si-4 or Si-5) was successively rinsed with DMF, MeOH, and acetone and then dried under argon. Pt Complex Modified Porous Si(100) for FTIR Characterization. A flat Si−H surface was prepared as described above. It was pressed against an opening in the bottom of a Teflon electrochemical cell using a FETFE (fluoroelastomer with special tetrafluoroethylene additives, Aldrich) O-ring seal, and an ohmic contact was made on the polished rear side of the sample with the steel bottom cap (care was taken to avoid surface contamination for subsequent FTIR investigation). A platinum counter electrode was used. The hydrogen-terminated porous Si surface was produced by applying a current density of 20 mA cm−2 for 5 min in 50% HF/ethanol/ultrapure 18.2 MΩ cm water (2/2/1 v/v/v). The surface was then rinsed with ethanol and dried under an argon stream. After monitoring of the FTIR spectrum of hydrogen-terminated porous Si, the sample was again dipped in ca. 5% HF for 2 min and dried under an argon stream without rinsing. The reaction conditions of porous Si−H with 5 were identical with those described for the flat Si(100) surface. Characterization Techniques. Electrochemical Characterization. Cyclic voltammetry measurements were performed with an Autolab electrochemical analyzer (PGSTAT 30 potentiostat/galvanostat from Eco Chemie BV) equipped with the GPES software in a homemade three-electrode Teflon cell. To avoid photoinduced electron transfer processes on silicon surfaces, most electrochemical measurements have been performed in the dark. The working electrode, modified Si(100), 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(100) surface was 0.3 cm2. 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 ±0.005 V). Tetra-n-butylammonium perchlorate (Bu4NClO4) and tetra-nbutylammonium hexafluorophosphate (Bu4NPF6) were purchased from Fluka (puriss, electrochemical grade) and were 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, under stirring and an argon atmosphere. All electrochemical measurements were carried out inside a homemade Faraday cage, at room temperature (20 ± 2 °C) and under constant argon flow. The resistance of the electrolytic cell was compensated by positive feedback. FTIR Spectroscopy Characterization. Modified Porous Si(100). FTIR spectra were acquired using a Brüker Optics Vertex 70 FT-IR spectrometer in the transmission mode (100 scans, 2 cm−1 resolution, and automatic gain) using a DTGS detector. The porous silicon was mounted on a homemade Teflon sample mount. Modified 45° Beveled Trapezoidal Si(111). FTIR spectra of modified Si(111) crystals (50 × 5 × 2 mm3, Synchrotronix) were acquired using a Brüker Optics Vertex 70 FT-IR spectrometer in the attenuated total reflectance (ATR) mode (25 reflections, 1000 scans, 4 cm−1 resolution, and automatic gain; accessory from Specac) equipped with a sensitive liquid-nitrogen-cooled MCT photovoltaic detector. The different grafting reactions of these substrates were similar to those described above. XPS Analysis. The grafted surfaces (approximative size 1.0−1.3 cm2) were introduced in the UHV chamber and kept under high vacuum (2 × 10−10 mbar) before and during the XPS analysis. XPS spectra were measured using the Mg Kα (1253.6 eV) anode source operating at 120 W and electron pass energy set either at 40 eV for survey spectra or at 22 eV for resolved spectra. In this latter case, the overall resolution is 1.0 eV. The binding energy scale of the XPS spectra is such that the Au 4f7/2 peak is set at 84.0 ± 0.1 eV, and all the given binding energies are then referenced to the Fermi level at the surface of conductive samples. Spectral analysis included a Shirley background subtraction and peak separation using mixed Gaussian− Lorentzian functions. I


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ASSOCIATED CONTENT

S Supporting Information *

CIF files giving X-ray crystallographic data for 3−6 and figures giving NMR spectra of complex 5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*B.F.: tel, 33 2 2323 6550; fax, 33 2 2323 6732; e-mail, fabre@ univ-rennes1.fr. *D.L.: tel, 33 2 2323 6273; fax, 33 2 2323 6738; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.Y. thanks the Ministère de la Recherche for his Ph.D grant. REFERENCES

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