Local Structures and Heterogeneity of Silica-Supported M(III) Sites

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Local Structures and Heterogeneity of Silica-Supported M(III) Sites Evidenced by EPR, IR, NMR, and Luminescence Spectroscopies Murielle F. Delley,† Giuseppe Lapadula,† Francisco Núñez-Zarur,†,‡ Aleix Comas-Vives,† Vidmantas Kalendra,†,§ Gunnar Jeschke,† Dirk Baabe,∥ Marc D. Walter,∥ Aaron J. Rossini,⊥ Anne Lesage,# Lyndon Emsley,⊥ Olivier Maury,◊ and Christophe Copéret*,† †

Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5, CH-8093 Zürich, Switzerland Facultad de Ciencias Básicas, Universidad de Medellín, Carrera 87 N 30-65, 050026 Medellín, Colombia § Faculty of Physics, Vilnius University, Sauletekio 9, LT-10222 Vilnius, Lithuania ∥ Institut für Anorganische und Analytische Chemie, TU Braunschweig, Hagenring 30, 38106 Braunschweig, Germany ⊥ Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland # Centre de RMN à Tres Hauts Champs, Institut de Sciences Analytiques, Université de Lyon (CNRS/ENS Lyon/UCB Lyon 1), 69100 Villeurbanne, France ◊ Laboratoire de Chimie de l‘ENS Lyon, Université de Lyon (CNRS/ENS Lyon/UCB LyonUMR 5182), 46 alleé d’Italie, 69007 Lyon, France ‡

S Supporting Information *

ABSTRACT: Grafting molecular precursors on partially dehydroxylated silica followed by a thermal treatment yields silica-supported M(III) sites for a broad range of metals. They display unique properties such as high activity in olefin polymerization and alkane dehydrogenation (M = Cr) or efficient luminescence properties (M = Yb and Eu) essential for bioimaging. Here, we interrogate the local structure of the M(III) surface sites obtained from two molecular precursors, amides M(N(SiMe3)2)3 vs siloxides (M(OSi(OtBu)3)3·L with L = (THF)2 or HOSi(OtBu)3 for M = Cr, Yb, Eu, and Y, by a combination of advanced spectroscopic techniques (EPR, IR, XAS, UV−vis, NMR, luminescence spectroscopies). For paramagnetic Cr(III), EPR (HYSCORE) spectroscopy shows hyperfine coupling to nitrogen only when the amide precursor is used, consistent with the presence of nitrogen neighbors. This changes their specific reactivity compared to Cr(III) sites in oxygen environments obtained from siloxide precursors: no coordination of CO and oligomer formation during the polymerization of ethylene due to the presence of a Ndonor ligand. The presence of the N-ligand also affects the photophysical properties of Yb and Eu by decreasing their lifetime, probably due to nonradiative deactivation of excited states by N−H bonds. Both types of precursors lead to a distribution of surface sites according to reactivity for Cr, luminescence spectroscopy for Yb and Eu, and dynamic nuclear polarization surfaceenhanced 89Y NMR spectroscopy (DNP SENS). In particular, DNP SENS provides molecular-level information about the structure of surface sites by evidencing the presence of tri-, tetra-, and pentacoordinated Y-surface sites. This study provides unprecedented evidence and tools to assess the local structure of metal surface sites in relation to their chemical and physical properties.

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formation of a large amount of active sites.8 When coordinated with an appropriate ligand, supported metal cations such as Zn9 or Yb10 are efficient luminescence emitters. These properties have been used to generate very bright nano-objects emitting in the near-IR for Yb, making imaging at a single-particle level possible.10 More recently, Dy(III) sites have been shown to display unique magnetic memory at low temperatures.11 These relatively well-defined and isolated M(III) sites are best generated via a two-step process involving grafting of a

INTRODUCTION Site isolation of metal cations at the surface of oxide materials has recently emerged as a route to obtain species that are highly reactive toward hydrocarbons.1 For instance, well-defined silicasupported Cr(III) species polymerize ethylene in the absence of coactivator.2 The dehydrogenation of alkanes or the hydrogenation of alkenes is also catalyzed by a broad range of supported metal cations: M = Co(II),3 Fe(II),4 Ga(III),5 and Cr(III).6 Such sites also catalyze H/D exchange reactions.7 Furthermore, W(VI) oxo species, generated by this approach, are highly active in olefin metathesis at low temperature (70 °C) upon activation with an organoreductant because of the © 2017 American Chemical Society

Received: March 3, 2017 Published: June 5, 2017 8855

DOI: 10.1021/jacs.7b02179 J. Am. Chem. Soc. 2017, 139, 8855−8867

Article

Journal of the American Chemical Society

Scheme 1. Preparation of M(III) on Silica via Use of Amide Molecular Precursors (Top) or Siloxide Molecular Precursors (Bottom)a

a

X and X′ = O or NR.

Scheme 2. Reaction of Cr(N(SiMe3)2)3 (1-Cr) with SiO2‑700 Gives 1-Cr/SiO2 Followed by a Thermal Treatment To Yield 1-Cr@ SiO2

lifetimes on the order of milliseconds.16 This study provides important information about the structure of the supported metal cations using the metal site itself as a probe and establish a method to evaluate metal site heterogeneity. In particular, we show that the chemical environment is borrowed in part from the amorphous character from the silica surface but also depends on the molecular precursor employed.

metal siloxide or a metal amide precursor followed by a thermolysis at high temperatures (Scheme 1). This approach builds on the combination of surface organometallic chemistry (SOMC),1e,12 which provides access to site isolation of metal sites, and the thermolytic molecular precursor (TMP)13 approach, which helps in generating bare metal ions at the surface of a material. In the case of Cr(III), the use of Cr(OSi(OtBu)3)3(THF)2 generates Cr(III) sites on silica free of any organic ligand after thermolysis.2b,6a,14 CO adsorption studies show the presence of two main Cr(III) sites: ca. 60% tricoordinated Cr(III) sites, which are responsible for the ethylene polymerization activity, and ca. 40% Cr(III) sites coordinated by additional siloxide bridges, which are probably inactive in polymerization. In addition, the large polymer weight distribution indicates the presence of a distribution of sites, which has been explained by the amorphous nature of the silica support.15 In the case of Yb (and Y), the surface metal ions prepared from the corresponding amides (Scheme 1) also show a distribution of surface sites (heterogeneity) evidenced by the partial binding of ancillary ligands and a distribution of luminescence decay rates (vide infra). However, in the case of the amide route, it was not possible to fully evaluate the first coordination sphere because of the similar EXAFS signatures for O and N.10a Similar heterogeneity has recently been evidenced for the magnetic property of the corresponding Dy material.11 Here, we interrogate the local structure of supported metal(III) cations and the differences associated with the use of siloxide vs amide molecular precursors. We compare supported Cr(III), Yb(III) and Y(III) sites in terms of reactivity (Cr) and their spectroscopic response: EPR for Cr, luminescence properties for Yb, and NMR for 89Y. We also investigate supported Eu(III), because it can be directly probed without additional antenna ligand by luminescence measurements via Eu f−f transitions due to its long luminescence

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RESULTS Cr(III)−Amide vs Siloxide Routes. We previously reported the preparation of the silica-supported Yb(III) and Y(III) surface species from the corresponding lanthanide amides, Ln(N(SiMe3)2)3 (1-Ln).10 We therefore prepared the corresponding Cr(III) sites by the same approach using Cr(N(SiMe3)2)3 (1-Cr). The complex 1-Cr17 was first reacted with partially dehydroxylated silica, SiO2−700, in pentane at RT for 3 h to yield a bright green material, 1-Cr/SiO2 (Scheme 2).18 Elemental analysis of 1-Cr/SiO2 gives 0.12 mmol of Cr/g and 3.2 N/Cr, 16.3 C/Cr, and 47.3 H/Cr, corresponding to an average Cr site density of ca. 0.4 Cr·nm−2. The infrared (IR) spectrum contains a broad band between 3750 and 3650 cm−1 associated with SiOH groups interacting with organic moieties and bands at 2959, 2904, 1444, and 1405 cm−1 associated with C−H stretching and deformation vibrations of the −N(SiMe3)2 groups (ESI Figure S1). The IR spectrum also contains weak bands between 3450 and 3250 cm−1 which are associated with N−H stretching vibrations of adsorbed H− N(SiMe3)2 species. 1-Cr/SiO2 was then treated at 400 °C under high vacuum (10−5 mbar) to yield light blue 1-Cr@SiO2 (Scheme 2) with 0.12 mmol of Cr/g and 2.1 N/Cr according to elemental analysis. The infrared spectrum of 1-Cr@SiO2 shows bands at 3450 and 3412 cm−1 consistent with N−H stretching vibrations and bands at 2965 and 2906 cm−1 which are blue shifted by 6 and 2 cm−1 compared to the corresponding 8856


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Figure 1. (a) X-ray near-edge absorption spectroscopy (XANES) at the Cr K-edge of (A) 1-Cr, (B) 1-Cr/SiO2 and (C) 1-Cr@SiO2. (b) X-band CW-EPR (9.5 GHz) spectra taken at 110 K of (A) 1-Cr in methylcyclohexane (2.5 mM), (B) 1-Cr/SiO2, and (C) 1-Cr@SiO2. The blue arrow denotes a weak feature at g = 3.87. Simulated spectra can be found in the ESI Figure S5. (c) HYSCORE spectrum of 1-Cr@SiO2 displaying signals at 14.90 MHz from hyperfine coupling with 1H and at 1.05 MHz from a coupling of 4 MHz with 14N.

infrared bands of −SiMe3 in 1-Cr/SiO2 (ESI Figure S1). This is consistent with −SiMe3 groups bound to oxygen instead of nitrogen in 1-Cr@SiO2 as found for the grafting of other silyl amide metal complexes.19 The −OSiMe3 and Cr−N(H)− are formed from −N(SiMe3)2 via reaction of nearby silanol groups or the opening of siloxane bridges. In particular, the presence of N−H groups suggests the formation of ammonia, a product of protonolysis of −N(SiMe3)2, and its subsequent reaction with the surface siloxane bridges.10a,20 X-ray near-edge absorption spectroscopy (XANES) at the Cr K-edge of 1-Cr, 1-Cr/SiO2, and 1-Cr@SiO2 reveals spectra with almost identical edge energies which indicate the conservation of the oxidation state +3 in all three species (Figure 1a).2b,21 The spectrum of 1-Cr displays pre-edge features at 5989.1 and 5991.2 eV (1s to 3d transitions) and a

near-edge feature at 5995.0 eV (1s to 4p transition). The preedge feature at 5989.1 eV can be assigned to transitions to the molecular orbitals with mainly metal d character of e″ and a′1 symmetry of D3h symmetry, while the pre-edge feature at 5991.2 eV probably corresponds to transitions to the molecular orbitals of e′ symmetry, consistent with UV−vis absorption spectrum of 1-Cr,22 which shows an absorption maximum at 685 nm (ESI Figure S2). Similar pre-edge and near-edge features at 5989.5, 5991.3, and 5995.0 eV are present in the spectrum of 1-Cr/SiO2, though weaker in intensity. The difference in energy between the two pre-edge features is slightly lower than in the case of 1-Cr, consistent with the UV− vis spectrum of 1-Cr/SiO2 showing an absorption maximum at 720 nm and also consistent with the smaller ligand field strength due to the presence of a siloxy ligand instead of a third 8857


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Table 1. Ethylene Polymerization Activity, Polymer Properties, and Formation of Oligomeric Species Using 1-Cr@SiO2 or 2Cr@SiO2 at 325 mbar Ethylene Pressure and at 70 °C polymer properties

a

material

activitya

Mnb

Mwb

Đ

branches

1-Cr@SiO2 2-Cr@SiO22b

15.7 15.2

23 600 33 600

139 600 415 100

5.9 12.3

3/100 C 1/2. However, it is also possible that some Cr sites are not observable by EPR but contribute to the magnetic properties of the sample. Thus, it cannot be excluded that alternative Cr species, for instance, dinuclear Cr species, may be present. The hyperfine sublevel correlation (HYSCORE) spectrum of 1-Cr@SiO2 displays peaks at 14.90 MHz in the weak coupling region associated with 1H coupling and at 1.05 MHz in the strong coupling quadrant from a hyperfine coupling with 14N of ca. 4 MHz (Figure 1c). 14N hyperfine couplings of similar magnitude have been observed for nitrogen atoms directly coordinated to Cr(III).28 The HYSCORE spectrum thus strongly suggests that chromium is directly bonded to at least one nitrogen atom, which together with the elemental analysis result of 2.1 N/Cr further suggests that a Cr(III) with two nitrogen next neighbors and one oxygen neighbor is formed via

amide ligand as in 1-Cr (ESI Figure S2). In the spectrum of 1Cr@SiO 2 the pre-edge and near-edge features almost disappear. The decreased intensity of the pre-edge and nearedge features in 1-Cr/SiO2 and 1-Cr@SiO2 in comparison to 1-Cr may be due to the heterogeneity of the support or due to decreased 3d−p mixing from less covalent metal−ligand bonds, which could potentially result from a geometry enforced by the surface leading to less efficient overlap of metal and ligand orbitals.23 Probing Cr(III) Surface Sites with CO. When 1-Cr@SiO2 is treated with CO, no CO bands can be observed by IR spectroscopy. This is in contrast to what is observed for the related Cr(III)/silica material obtained via the siloxide route (2Cr@SiO2) for which two CO IR bands at 2202 and 2188 cm−1 appear.2b The difference in CO adsorption ability could be a surface polarity effect (−OSiMe3 groups on the surface of 1Cr@SiO2 vs −OH groups for siloxide route on 2-Cr@SiO2) or could indicate different structures of the Cr sites (N in 1-Cr@ SiO2 vs O in 2-Cr@SiO2, vide infra). To evaluate the influence of the polarity of the support on the CO coordination ability of Cr(III), we reacted 2-Cr@SiO2 with HN(SiMe3)2 to replace silanols by −OSiMe3 groups while retaining the local environment of Cr (material 2-Cr@SiO2-TMS, ESI Figure S3). Exposure of 2-Cr@SiO2-TMS to CO leads to two CO bands in the IR spectrum at 2197 and 2182 cm−1 (ESI Figure S4). This result shows that the −OSiMe3 groups in 2-Cr@ SiO2-TMS do not prevent coordination of CO to Cr(III), implying different structures for the Cr sites in 1-Cr@SiO2 and 2-Cr@SiO2. EPR Signatures. The Cr materials were also characterized by EPR spectroscopy. The X-band (9.5 GHz) CW EPR spectrum taken at 110 K of 1-Cr in methylcyclohexane (2.5 mM) shows a hyperfine triplet due to coupling to 14N (A⊥ = 145 MHz) at an effective g value g⊥ ≈ 4 and a negative signal at an effective g value g∥ = 1.995 (Figure 1b).24 No hyperfine coupling is resolved at the g∥ feature, and from the width of this feature it can be concluded that the 14N hyperfine coupling in this direction must be smaller than 25 MHz. This type of spectrum is expected for a zero-field splitting (ZFS) with axial symmetry that is significantly larger than the microwave frequency.25 The spectrum can be simulated with ZFS parameters D = 23 GHz and E = 0 as well as with an isotropic g value of 1.999 (ESI Figure S5). Note, however, that such a large ZFS can be determined only approximately at a much smaller microwave frequency. The axial symmetry of the ZFS tensor is consistent with the expected existence of a C3 symmetry axis in the molecule. In the X-band (9.5 GHz) CW EPR powder spectrum of 1-Cr/SiO2 also taken at 110 K, the 14N hyperfine splitting is lost due to line broadening that arises from a distribution of ZFS parameters (Figure 1b). Furthermore, the spectrum no longer corresponds to axial symmetry. A fairly good fit can be obtained with mean values D = 27.5 GHz, E = 2.75 GHz as well as D and E strain of 4 and 1.3 GHz, respectively, again assuming an isotropic g value g = 1.999 (ESI Figure S5). The 8858


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Scheme 3. (a) Preparation of 2-Ln@SiO2 Materials (Ln = Yb, Eu, Y) via Grafting of Siloxide Molecular Precursors 2-Ln on Partially Dehydroxylated Silica and Thermal Treatment; (b) Preparation of 1-Ln@SiO2 Materials (Ln = Yb, Eu, Y) via Grafting of Amide Molecular Precursors 1-Ln on Partially Dehydroxylated Silica and Thermal Treatment

the amide synthetic route. Such coordination would also be consistent with the observed lack of axial symmetry in the ZFS. In contrast, no coupling with nitrogen can be observed in the HYSCORE spectrum for Cr(III) obtained by the siloxide route, 2-Cr@SiO2, as expected (ESI Figure S9). Reactivity toward Ethylene. In view of the high activity of 2Cr@SiO2 toward ethylene polymerization,2,6a 1-Cr@SiO2 was also exposed to ethylene (325 mbar, 390 equiv per Cr) at 70 °C. 1-Cr@SiO2 polymerizes ethylene with an initial rate of 15.7 kg of PE (mol Cr h)−1, and the polyethylene formed has a number-average molecular weight of Mn = 23 600 g/mol, a weight-average molecular weight of Mw = 139 600 g/mol, and a dispersity of Đ = Mw/Mn = 5.9, according to HT-SEC (Table 1). Differential scanning calorimetry on the polyethylene gives a broad melting temperature range between 110 and 135 °C, and the 1H NMR and 13C NMR spectra suggest a branched polymer with 3 branches per 100 backbone carbon atoms (ESI Figure S10). In addition, a significant amount of oligomers (0.1−2.0 equiv of isomeres of butene and hexene per Cr) can be observed in the gas phase via in situ GC-MS, in contrast to what is observed for 2-Cr@SiO2, for which, despite similar activities in ethylene polymerization, no oligomer is formed and the associated polyethylene is highly linear with

Local Structures and Heterogeneity of Silica-Supported M(III) Sites

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