1150
J. Phys. Chem. C 2011, 115, 1150–1155
Access to Well-Defined Ruthenium Mononuclear Species Grafted via a Si-Ru Bond on Silane Functionalized Silica† Fernando Rasco´n,‡ Romain Berthoud,‡ Raphae¨l Wischert,‡ Wayne Lukens,§ and Christophe Cope´ret*,‡ UniVersite´ de Lyon, Institute de Chimie de Lyon, C2P2, ESCPE Lyon, 43, Bd. du 11 NoVembre 69616, Villeurbanne, France, and Chemical Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed: July 13, 2010; ReVised Manuscript ReceiVed: August 28, 2010
A functionalized silica with T3 silane surface groups (i.e., (tSiO)3Si-H) was prepared and interacted with Ru(cod)(cot), resulting in the formation of monometallic surface species attached to the surface via a Si-Ru bond, according to EXAFS spectroscopy, infrared spectroscopy, and solid-state NMR. Introduction The controlled immobilization of organometallic precursors on metal oxide surfaces (silica, alumina...) has given access to a deeper understanding of several surface phenomena and catalytic processes,1 where reaction intermediates,2 enhanced activity,3 selectivity,4 or even novel catalytic processes have been discovered.5 It has even been exploited to generate highly reactive isolated metal hydride single-sites; by treatment under H2, such species display unprecedented activities toward alkanes.6 Although this chemistry is generally effective for earlytransition metals because of the formation of strong SiO-M bonds via grafting on OH functionalities, it is, in contrast, not the case for late-transition complexes because of either unfavorable thermodynamics of grafting or the formation of unstable surface species, which are susceptible to metal leaching or selfaggregation into particles in the presence of dihydrogen.7 Avoiding or controlling this aggregation process for latetransition metals on supports is a current challenge and could lead to entities with unprecedented reactivity. We have recently shown that it is possible to graft and stabilize Ru complexes on silica surfaces by implementing silane functionalities, namely, {(tSiO)Si(Me)2(H)}.8 Here, we want to investigate further the formation and the stability of species anchored via a Ru-Si bond, through a grafting directly at a silicon atom from the support, here, a silane having three oxygen atoms, {(tSiO)3SiH}, so-called T3 sites. Such surface functionalities are accessible from silane-modified silicas9 and have been exploited to prepare functionalized stationary phases for chromatography10,11 or to anchor organic ligands for supported catalysts.12 Herein, we investigate the structure and the stability of Ru species obtained via grafting of a Ru organometallic complex, Ru(cod)(cot), on these T3 sites and compare them in terms of structure and stability with those obtained on {(tSiO)Si(Me)2(H)}.8 Experimental Procedure General. All experiments were carried out under dry and oxygen-free argon, using either standard Schlenk or glovebox techniques for organometallic synthesis. For the syntheses and †
Part of the “Alfons Baiker Festschrift”. * To whom correspondence should be addressed. Fax: (+33)472431795. E-mail:
[email protected]. ‡ Universite´ de Lyon. § Lawrence Berkeley National Laboratory.
the treatments of the surface species, reactions were carried out using high-vacuum lines (10-5 mbar) and glovebox techniques. Ru(cod)(cot) was prepared according to literature procedures.20 (EtO)3SiH was purchased from ABC-R and used as received. Silica (Aerosil Degussa, 200 m2 g-1) was used as received. SBA15 was prepared from Si(OEt)4 according to literature conditions,11 calcined at 500 °C under air for 5 h and then treated under vacuum at 500 °C for 15 h (support referred to as SBA15500). Pentane and dioxane were distilled from NaK under N2. Elemental analyses were performed at the CNRS Central Analysis Department of Solaize (Ru) or at the University of Bourgogne (C, H, N). Gas-phase analyses were performed on a Hewlett-Packard 5890 series II gas chromatography (GC) apparatus equipped with (i) a KCl/Al2O3 column (50 m × 0.32 mm) and a flame ionization detector (FID) for hydrocarbons or (ii) a molecular sieve column and a thermal conductivity detector (TCD) for H2. Liquid-phase analyses (cyclooctane/cyclooctene) were performed on a Hewlett-Packard 5890 series II gas chromatography (GC) apparatus equipped with a flame ionization detector (FID) and an HP-5 column (30 m × 0.32 mm × 0.25 µm). Infrared spectra were recorded on a Nicolet 5700 FT-IR spectrometer by using a custom infrared cell equipped with CaF2 windows, allowing in situ studies. Typically, 16 scans were accumulated for each spectrum (resolution ) 4 cm-1). EXAFS Spectroscopy. Extended X-ray fine structure spectroscopy data were acquired in transmission at Stanford Synchrotron Radiation Lightsource on beamline 4-2. Data analysis was performed using Athena, Artemis, ifeffit, and FEFF7, as previously reported (see the Supporting Information for full details). Solid-State NMR Spectroscopy. Spectra were recorded on a Bruker Avance or Avance II 500 wide-bore or DSX 300 spectrometer with conventional double resonance 4 and 3.2 mm CP-MAS probes. The MAS frequency was set to 10 and/or 5 kHz. The samples were introduced in a zirconia rotor in the glovebox and tightly closed. Chemical shifts are reported in parts per million downfield from liquid SiMe4 (0 ppm) for 1H, 13C, and 29Si NMR. For the cross-polarization (CP) experiments, a ramp radio frequency (rf) field centered at 60 kHz was applied on protons, while the carbon rf field was matched to obtain an optimal signal. The contact time for CP was set to 1 ms, and the proton decoupling field strength was set to 83 kHz (TPPM decoupling) during acquisition. A total of 250 t1 increments with 1024 scans each were collected. The spinning frequency was
10.1021/jp1064962 2011 American Chemical Society Published on Web 09/20/2010
Well-Defined Ruthenium Mononuclear Species
J. Phys. Chem. C, Vol. 115, No. 4, 2011 1151
TABLE 1: Silane-Modified Silica T3SiHA-500 and T3SiHSBA-500 Materials surface pore silane content/ area/m2 g-1 diameter/nm mmol g-1 SiH/nm-2 T3SiHA-500 T3SiHSBA-500
154.1 619.1
5.4
0.9a 3.4a
0.8b 3.3b
3.7c 3.3c
a Quantified by gas chromatography. b Quantified by solid-state MAS 1H NMR spectroscopy. c Calcd from results in footnote a.
10 kHz, and the recycle delay was 2 s. Quadrature detection in ω1 was achieved using the TPPI method. More experimental details concerning NMR data acquisition can be found in the Supporting Information. Preparation of T3SiHsupp-500 (supp ) Aerosil (A) and SBA15 (SBA)). Silanization of the silica support was performed following the procedure described by Chu, et al.10 Afterward, the silica support T3SiHsupp was dried at room temperature under vacuum (10-5 mbar) for 1 h, then at 140 °C for 12 h, and finally at 500 °C for 12 h to yield T3SiHsupp-500. IR (disk, cm-1): 3747 ν(residual SiO-H), 3674 ν(bridging OH), 2261 ν(Si-H), 2210 ν(Si-H) on (tSiO)2SiH(OH). NMR MAS recorded at 10 kHz, 1 H (δ, ppm): 1.6 (residual Si-OH), 4.0 (Si-H). 29Si (δ, ppm): -110 (Si-O-Si), -86 ((tSiO)3SiH), -75 ((dSiO)2SiH(OH)). Grafting of Ru(cod)(cot) on T3SiHA-500, Representative Procedure. A mixture of Ru(cod)(cot) (117 mg, 0.37 mmol,) and T3SiHA-500 (1.12 g) in pentane (15 mL) was stirred at 25 °C for 15 h. The solid initially turned orange, then orangebrown, while the yellow color of the solution did not totally vanish. The reaction mixture was filtered off. The solid was washed with pentane (3 × 15 mL) and finally dried under vacuum (10-5 mbar) at 25 °C for 1 h to yield a pale orange solid, [T3SiRuL]A-500. All the solutions were gathered, and the cyclooctane/cyclooctene released was quantified by GC using an external standard. IR (disk, cm-1): 3030-2840 ν(Csp2-H/ Csp3-H), 1460-1450 δ(Csp2-H/Csp3-H), 2235 ν(Si-H), 2210 ν(Si-H) on (tSiO)2SiH(OH). NMR MAS recorded at 10 kHz, 1H (δ, ppm): 0.5, 1.2 (Csp3-H of (tSiO)3Si(C8Hx), x ) 13, 15), 2.6 (Csp2-H (tSiO)3SiRu(C8Hy), y ) 9, 11), 4.0 (SiH). 13C CP (δ, ppm): 26 (Csp3-H from Si(C8Hx) and SiRu(C8Hy), 81 (Csp2-H from (tSiO)3SiRuC8Hy). 29Si at 5 kHz (δ, ppm): -110 (Si-O-Si), -86 ((tSiO)3SiH), -65 ((tSiO)3Si(C8Hx)). Elemental Anal. (%): Ru, 2.31; C, 5.11; H, 0.75. Grafting of Ru(cod)(cot) on T3SiHSBA-500. The same procedure as described above was used, replacing T3SiHA-500 by T3SiHSBA-500. Results and Discussion Characterization of the T3SiH Silica Supports. The preparation of silica materials containing T3 surface silane functionalities, that is, (tSiO)3SiH, was performed by a sol-gel technique using hydrolysis/condensation of (EtO)3SiH on silica using acidic conditions11 to yield the material T3SiH, which, upon treatment under high vacuum at 500 °C, gave T3SiHsupp-500. Two different silica materials were studied: Aerosil (ca. 200 m2 g-1) and SBA-15 (ca. 800 m2 g-1), leading to T3SiHA-500 and T3SiHSBA-500, respectively. Both materials were fully characterized prior to grafting Ru(cod)(cot), and the main characteristics for both support materials are summarized in Table 1. First, in terms of texture, nitrogen adsorption experiments were performed (Table 1 and Figures S1 and S2, Supporting Information) and showed a decrease of the Brunauer-Emmet-Teller (BET) surface area to 154 and 619 m2 g-1, for T3SiHA-500 and T3SiHSBA-500, respectively, compared with the original silica material. In the case of the mesoporous material,
Figure 1. (a) Isolated material. (b) T3SiHA-500, obtained after dehydroxylation at 500 °C.
this process is accompanied by a decrease of the mean pore diameter (Barrett-Joyner-Halenda (BJH) model at desorption/ adsorption) from 6.5 nm in the SBA-15 material to 5.4 nm in T3SiHSBA, which is consistent with the formation of an extra layer(s) of tSi-H at the walls of the pores (cf. with SBA-15 pore mean diameter, 6.5 nm). Both supports display similar spectroscopic features, the only difference being in surface area and porosity; the concentrations of silane sites per square nanometer are comparable (see the Supporting Information for a full characterization of the supports). Transmission IR spectra of the thus-prepared material prior to thermal treatment, T3SiHA (Figure 1, spectrum a) and T3SiHSBA, show a very intense band at 2261 cm-1 associated with νSi-H, as well as νO-H bands at 3747 and 3500 cm-1, the former belonging to isolated silanols and the latter, a broad signal, associated with -OH groups in interaction with adjacent functionalities (OH and/or SiH). Further treatment at 500 °C under high vacuum yielded T3SiHsupp-500 and resulted in the almost complete disappearance of the broad band associated with -OH interactions (Figure 1, spectrum b, and Figure S3 (Supporting Information) for the SBA material). Note also the appearance of an additional weak band at 2210 cm-1, which is tentatively attributed to a minor amount of (tSiO)2SiH(OH) surface groups. Finally, νC-H bandssobserved at ca. 3000-2800 cm-1sare present in a minute amount, suggesting the almost complete hydrolysis of the ethoxy groups from the starting alkoxysilane reagent. The 1H MAS NMR spectrum of T3SiHA-500 (Figure S4, Supporting Information) shows a sharp resonance at 4.0 ppm, assigned to T3 surface silane hydrides, along with a weak signal at 1.6 ppm attributed to surface silanol groups, (tSiO)3SiOH as well as to (tSiO)2SiH(OH), previously observed by FT-IR. Note that this latter peak is slightly more intense in T3SiHSBA-500 (Figure S5, Supporting Information). All other NMR spectra are essentially identical between Aerosil and SBA materials. The 29Si cross-polarization (CP) MAS NMR spectrum of T3SiHA-500 (Figure S6, Supporting Information) displays resonances at -86 and -75 ppm, attributed to (tSiO)3SiH (T3) and (tSiO)2SiH(OH) (T2) surface groups, respectively; those at -100 and -110 ppm can be readily attributed to the silica network: Q3, comprising the isolated (SiO)3SiOH silanols, and Q4, for the (SiO)4Si groups. The total concentration of Si-H sites was evaluated by a quantitative NMR experiment (see the Supporting Information), indicating a concentration of 0.8 and 3.3 mmol g-1 for T3SiHA-500 and T3SiHSBA-500, respectively. These results
1152
J. Phys. Chem. C, Vol. 115, No. 4, 2011
Figure 2. NMR MAS double-quantum 1H at 10 kHz spectrum of T3SiHA-500 showing (a) silane-silane and (b) silane-silanol correlations.
Figure 3. Depiction of the functional group distribution at the T3SiHsupp-500 surface.
were corroborated by titration of these Si-H functionalities by treatment of the support with a standard solution of KOH in ethanol, followed by a quantification of the dihydrogen evolved (0.9 and 3.4 ( 0.2 mmol g-1 for T3SiHA-500 and T3SiHSBA-500, respectively; see Table 1).13 This shows that the concentration of Si-H sites per gram is roughly proportional to the BET specific surface area and that the density of sites is similar for both supports, ca. 3.5 Si-H sites/nm2. To further understand the local environment of the silane fuctionalities, double-quantum (DQ) solid-state NMR experiments were performed (Figure 2).14 The DQ spectrum of T3SiHsupp-500 solids shows a very strong self-correlation peak (signals lying on the diagonal, read in the double-quantum dimension F1; see Figure 2) at 8.0 ppm (a), showing that Si-H groups must be in close proximity to each other. An autocorrelation peak associated with SiOH groups is not observed (at 3.2 ppm in F1), which confirms the spacing
Rasco´n et al. among residual silanol groups after surface modification. Finally, a correlation between silane groups and silanols is also present (b ) 5.6 ppm in F1, i.e., 4.0 + 1.6 ppm), which is consistent with both, the presence of T2 sites and neighboring (Si-H · · · H-OSi) interactions. Therefore, a schematical description of the surface consistent with all experimental data can be sketched, as in Figure 3. Reactivity of T3SiHsupp-500 toward a Ru0 Organometallic Complex. Both T3SiHsupp-500 surfaces (Aerosil(200) and SBA15) were reacted with Ru(cod)(cot), a Ru0 organometallic complex that does not react with the OH groups of silica.8b When a pellet of T3SiHA-500 was contacted with a solution of Ru(cod)(cot) (3 mmol of Ru/g of T3SiHA-500) in pentane, the solid turned slowly yellow, then orange, in agreement with a chemical reaction between the surface and the Ru0 complex. After 12 h of reaction at room temperature, the IR spectrum of the solid (Figure 4) showed new bands in the 3030-2840 and 1460-1450 cm-1 regions, typically associated with ν(Csp2H/Csp3-H) and δ(Csp2-H/Csp3-H), whereas the ν(Si-H) band at 2260 cm-1 had been partially consumed (29% of the initial SiH content according to the transmission IR spectra integrals). Concomitantly, the ν(O-H) band of free silanols disappeared, whereas the ν(O-H) band attributed to OH in interaction at 3705 cm-1 increased. Moreover, a new ν(Si-H) band at 2235 cm-1 appeared. The red shifts observed for the ν(O-H) and ν(Si-H) bands suggest the existence of an interaction of these groups with the ligand of the grafted complex. The orange-brown solid obtained is stable under an inert atmosphere at ambient temperature and does not show any significative changes when it is heated to 100 °C in vacuum or an argon atmosphere. Chemical grafting of the Ru complex on the T3SiHA-500 surface and the absence of aggregation of the metal is supported by Ru K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy.15 The EXAFS spectrum of [T3SiRuL]A-500 (Figure 5) is thus consistent with a Ru directly attached to a Si at 2.41 Å and to ca. six C at 2.19 Å (see Table 2). On the basis of the reactivity of Ru(cod)(cot) toward silanes and the various spectroscopic data,16 we propose the formation of a surface complex of RuII directly attached to the support by one covalent Si-Ru bond, with its coordination sphere further completed by a 1-5-η-C8H11 (I) or a 1-3,5-6-η-C8H11 (III)
Figure 4. IR tracking of the grafting reaction. (a) T3SiHA-500. (b) After addition of Ru(cod)(cot). (c) Subtraction of (b) - (c).
Well-Defined Ruthenium Mononuclear Species
J. Phys. Chem. C, Vol. 115, No. 4, 2011 1153 SCHEME 1: Grafting of Ru(cod)(cot) on the T3SiHsupp-500 Surface
Figure 5. EXAFS spectrum (at the Ru K-edge) and Fourier transform of [T3SiRuL]A-500.
TABLE 2: Fitting Parameters for [T3SiRuL]A-500 at the Ru K Edgea neighbor
no. of neighbors
distance (Å)
σ2 (Å2)
p(F)b
C Si
6 1 (fixed)
2.19 (1) 2.41 (2)
0.005 (1) 0.008 (3)
