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Revealing Structure Reactivity Relationships in Heterogenized Dirhodium Catalysts by Solid State NMR Techniques Jiquan Liu, Pedro B. Groszewicz, QingBo Wen, Aany Sofia Lilly Thankamony, Bin Zhang, Ulrike Kunz, Grit Sauer, Yeping Xu, Torsten Gutmann, and Gerd Buntkowsky J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06807 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

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The Journal of Physical Chemistry

Revealing Structure Reactivity Relationships in Heterogenized Dirhodium Catalysts by Solid State NMR Techniques Jiquan Liua, Pedro B. Groszewicza, Qingbo Wenb, Aany Sofia Lilly Thankamonya,c, Bin Zhangd, Ulrike Kunze, a Grit Sauer , Yeping Xua, Torsten Gutmanna*, Gerd Buntkowskya* a

Eduard-Zintl-Institute for Inorganic Chemistry and Physical Chemistry, Technical University Darmstadt, Alarich-Weiss-Straße 8, D-64287 Darmstadt, Germany b

c

d

e

Institut für Materialwissenschaft, Technische Universität Darmstadt, Jovanka-Bontschits-Straße 2, D-64287, Darmstadt, Germany

Institute for Physical and Theoretical Chemistry, University Frankfurt, Max-von-Laue-Straße7, 60438, Frankfurt, Germany

Institute of Coal Chemistry, Chinese Academy of Sciences, South Taoyuan Road 27, 030001, Taiyuan, P.R.China Department of Materials and Earth Sciences, Technical University Darmstadt, Alarich-Weiss-Straße 2, D-64287, Darmstadt, Germany

Abstract Heterogenized dirhodium catalysts were prepared by anchoring the rhodium (II) trifluoroacetate dimer (Rh2(TFA)4) (TFA=trifluoroacetate) on the surface of SBA-15 material bearing either amine, or amine and carboxyl groups. After basic characterizations, the binding geometries of the catalysts were analyzed by 13C CP MAS and 15N CP MAS DNP NMR. 19F MAS NMR measurements were employed to quantify active catalyst sites in the heterogenized catalysts, which enable one to understand their catalytic performance in the model cyclopropanation between styrene and ethyl diazoacetate (EDA). The heterogeneous nature of the catalysts was confirmed by leaching and recyclability tests.

1. Introduction Homogeneous transition metal catalysts possess high performance in both activity and selectivity, but have the disadvantages of difficult recovery and recycling, especially when it comes to the tight legislation on metal contamination of pharmaceuticals.1-4 Dirhodium (II) complexes, containing a Rh-Rh bond and four carboxyl ligands at equatorial positions (ESI Scheme S1), have been employed as efficient catalysts in organic synthesis to produce carbon-carbon (or carbon-heteroatom) bonds via the formation of intermediary rhodium carbenoids. These complexes can be utilized for example as catalysts in cyclopropanation or C-H insertion reactions,4-6 which have a high application potential in the synthesis of pharmaceutical products such as the antidepressant milnacipran.7 Owing to this high application potential, there were recently major attempts to 1

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heterogenize dirhodium complexes and thus overcome the disadvantages of the homogeneous catalysts. For example, dirhodium complexes were anchored on mesoporous silica or polymer supports via ligand exchange 8-15 or axial coordination. 16-20 Recently, some of us reported the immobilization of dirhodium catalysts on cellulose nanocrystals (CNCs) via carboxyl binding,21 and the self-supported heterogenization of dirhodium complexes in a coordination polymer.22 Bifunctional mesoporous silica materials bearing carboxyl and amine groups were firstly obtained by Che and co-workers.23 In parallel, Davis et al. introduced various acid-base pairs into SBA-15 type materials, which showed enhanced activity in the aldol condensation between 4-nitrobenzaldehyde and acetone due to the cooperation of acidic (eg. -COOH) and basic (-NH2) functional groups.24 Furthermore, Vallet-Regí and co-workers reported that the zwitterionic SBA-15 material exhibited high resistance for protein adsorption owing to the coexistence of carboxyl and amine groups.25-26 Very recently, some of us presented a tentative study on the immobilization and characterization of a dirhodium complex on bifunctionalized material. 27 The structure elucidation of such heterogenized catalysts is an essential step to understand their catalytic performance. While for homogeneous dirhodium catalysts containing amine or pyridine linkers 15N HMBC were employed to determine their structure, 28 for heterogenized catalysts this has practically not done up to now. 16-20 Thus, the clear identification of such sites for heterogenized dirhodium catalysts is of great interest, although it is more challenging due to low grafting densities of the catalysts on the surface. In the past few years, solid-state NMR combined with dynamic nuclear polarization has been employed to overcome this obstacle. With this technique, it was feasible to detect even nuclei in low natural abundance such as 15N or 13C on the surface of silica materials.29-38 Employing DNP NMR, two types of binding sites were identified for dirhodium complexes grafted on bifunctional material, namely equatorial bound carboxyl groups and axial coordinated amine groups. 27 However, with the employed model system based on Rh2(OAc)4 it was not feasible to answer the following questions: (i) How to distinguish various combinations of possible amine binding sites? (ii) Can solid-state NMR experiments help to explain the catalytic performance of these heterogenized dirhodium catalysts? To answer these questions, in the present work a novel immobilized catalyst model system is developed, employing bifunctional SBA-15 as support material and Rh2(TFA)4 (TFA=trifluoroacetate) as homogeneous catalyst precursor. This system has several advantages: On the one hand, the carboxyl chemical shift of parent Rh2(TFA)4 (ca. 173 ppm) is lower than that of Rh2(OAc)4 (192 ppm) and the carboxyl groups in SBA-15~COOH~NH2 (ca. 183 ppm).27 Thus, the carboxyl species on SBA-15~COOH~NH2 may be easier distinguishable from the Rh2(TFA)4 precursor. On the other hand, Rh2(TFA)4 contains trifluoroacetate ligands which strongly influence the electron density of nuclei involved in the heterogenization process. Changes in the chemical shifts of 13C or 15N nuclei during this process are thus expected to be more distinctive. Furthermore, the 19F nucleus is an excellent probe for quantification of different chemical environments as shown in previous works.21-22 Hence, this model system is expected to be suitable to clarify the binding situation in complex heterogenized dirhodium catalysts and to explain the differences in their catalytic performance. The rest of the paper is organized as follows. After this brief introduction, the synthesis of the carrier materials and heterogenized catalysts is discussed and their structural properties are investigated by TEM, small-angle XRD, 2


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N2-adsorption-desorption and SEM measurements. The nature of the dirhodium unit in the obtained catalysts is studied by UV-vis diffusion reflectance spectroscopy (DR-UV-vis), XPS and quantified by ICP-OES. Thereafter, structural changes referring to the immobilization of the catalyst are identified by 13C CP MAS NMR and 15N CP MAS DNP NMR. By combination of 15N CP MAS DNP NMR and quantitative 19F MAS NMR, the binding sites of the catalysts are analyzed in detail. Finally, the cyclopropanation reaction between styrene and ethyl diazoacetate (EDA) is carried out and the relationship between catalyst structure and its catalytic efficiency in cyclopropanation is discussed.

2. Results and Discussion 2.1 Synthesis and basic characterization In the first step, SBA-15~COOH~NH2 (see structure (0), Scheme 1a) was prepared via co-condensation of TEOS, APTES and CES (molar ratio n(TEOS):n(APTES):n(CES) 8:1:1) by modifying the previously presented procedures,27, 39 to improve the structural order of the mesopores. In this modified route, pre-hydrolysis of TEOS was carried out before adding APTES and CES. Similarly, SBA-15~NH2 was obtained by employing TEOS and APTES (molar ratio n(TEOS):n(APTES) 9:1). In the next step, the dirhodium precursor Rh2(TFA)4 was introduced into the pore system of the SBA-15 materials via wet impregnation according to ref. 27 Rh2(TFA)4 is assumed to be captured by the APTES linker at axial position (i.e. [Rh2(TFA)4·2APTES] and [Rh2(TFA)4·APTES] (see structures (1) and (2) in Scheme 1b). Additionally, in the case of the SBA-15~COOH~NH2 supporting material, ligand exchange between the carboxyl groups of the CES linker and [Rh2(TFA)4·APTES] is expected to generate a third structure namely [Rh2(TFA)3(CES)·APTES] (see structure (3) in Scheme 1b) when the byproduct trifluoroacetic acid is removed from the system.27 Since the Rh-N bond seems to be stable even at 80°C according to Dikarev et. al. 20 it is assumed that a structure containing coordination of only CES is not formed during the synthesis. All of these three proposed structures are assumed to co-exist in the bifunctional system, and have to be clearly identified and distinguished.

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Scheme 1. Schematic representation of the synthesis of SBA-15~COOH~NH2 (0) (a), and immobilization of Rh2(TFA)4 (b) including three reasonable combinations of binding sites: (1) [Rh2(TFA)4·2APTES], (2) [Rh2(TFA)4·APTES] and (3) [Rh2(TFA)3(CES)1·APTES] The obtained SBA-15 type functional materials were structurally characterized by small-angle XRD, TEM, SEM and N2-adsorption-desorption measurements (for details see ESI Figures S1-S3). The ordered organization of the mesopores is reflected in the small-angle XRD spectra (ESI Figures S1I a,b) and TEM images (ESI Figures S1II a,b). The worm-rod like morphology is visible in the SEM images (ESI Figures S1III a,b). The diameter of pores is in the mesoporous region as confirmed by analysis of the N2-adsorption-desorption isotherms (ESI Figures S2 a,b) and related BJH curves (ESI Figures S3 a,b). After introducing the dirhodium precursor Rh2(TFA)4, the texture feature and morphology of the porous supporting materials were preserved as reflected by small-angle XRD, TEM and SEM (ESI Figures S1 c,d), respectively. In addition, a reduction of the BET surface area was obtained (ESI Table S1) after the immobilization of the dirhodium catalyst. This observation is not very surprising since the introduction of Rh2(TFA)4 into mesopores of functional SBA-15 fills the pores and thus reduces the specific surface area. The loading of amine groups (i.e. APTES linker) was investigated via elemental analysis. For the supporting materials, the amounts of APTES linker in SBA-15~COOH~NH2 and SBA-15~NH2 were calculated to be 0.915 mmol·g-1 and 1.075 mmol·g-1, respectively, according to the N-fraction obtained from elemental analysis (see ESI Quantitative Analysis). Thereafter, Rh2(TFA)4 was immobilized on SBA-15~COOH~NH2 and SBA-15~NH2, respectively. The success of this process 4


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was confirmed by DR-UV-vis measurements (ESI Figure S4) and XPS (ESI Figure S9). In DR-UV-vis, the adsorption bands at ca. 609 nm (transition of Rh-Rh π*→ Rh-Rh σ*) and 425 nm (transition of Rh-Rh π*→Rh-O σ*) clearly show the intactness of the dirhodium unit after the immobilization process. The oxidation state of the Rh of +2 is revealed by XPS, which underlines the presence of the dirhodium unit. The loading of dirhodium units on the supporting materials was determined by ICP-OES. The experimental Rh2 fractions for SBA-15~COOH~NH2+Rh2 and SBA-15~NH2+Rh2 were both found to be approximately 0.17 mmol·g-1 (see ESI Quantitative Analysis).

2.2 Characterization of the functionalized carrier materials via solid-state NMR 13

C and 29Si CP MAS NMR To get deeper insights into the functionalization and the interactions of the linkers with the supporting materials, 13C and 29Si solid-state NMR experiments were performed (Figures 1 a,b and ESI Figure S12 a,b). The 13C CP MAS NMR spectrum of SBA-15~COOH~NH2 is displayed in Fig. 1a. In the aliphatic region, the signals at 42, 21 and 8 ppm assign the α, β, γ methylene carbons of the APTES linker (i.e., -CH2(γ)CH2(β)CH2(α)NH2). These signals are also observed in the spectrum of SBA-15~NH2 (Figure 1b). Furthermore, signals at 28 and 15 ppm (Figure 1a) are obtained referring to α and β methylene carbons in the CES linker (i.e., -CH2(β)CH2(α)COOH).27, 40 In the carbonyl region, two signals at 178 ppm and 180 ppm are observed, indicating different carbonyl-containing functional groups on the bi-functionalized system. The peak at 178 ppm most probably refers to free carboxyl groups, while the other peak at 182 ppm is assigned to carboxylate groups. This assignment is in agreement with our previous results on CNC materials, where the deprotonation of carboxyl groups caused a slight deshielding of 13C nuclei.21, 41 Most probably, APTES and CES linkers form carboxyl-amine (COO-NH3+) ion pairs,25 which explains the signal at 182 ppm. Signals centered at 70 ppm refer to template molecules (P123) that were not fully removed during the synthesis. Additionally, the resonances at 59 ppm and 14 ppm are attributed to remaining ethoxy carbons of -SiOCH2CH3 of the linker groups. Finally, the analysis of the 29Si CP MAS NMR spectra (ESI Figures S12a,b) shows the appearance of T-groups next to Q-groups. This clearly underlines that APTES and/or CES linker molecules are grafted on the surface of the SBA-15 material. 15

N CP MAS DNP NMR For SBA-15~COOH~NH2 (Figure 1e), a signal appeared at ca. -347 ppm together with a shoulder at -334.5 ppm, which represents the presence of free amine and protonated amine, respectively.42-44 Due to the co-existence of carboxyl and amine groups in SBA-15~COOH~NH2, the peak at -334.5 ppm is attributed to protonated amine of carboxyl-amine (COO-NH3+) ion pairs. In the case of SBA-15~NH2 (Figure 1f), only one signal at ca. -347 ppm is obtained that refers to free amine.27

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Fig. 1 13C CP MAS NMR and 15N CP MAS DNP NMR spectra of SBA-15~COOH~NH2 (a,e), SBA-15~NH2 (b,f), SBA-15~COOH~NH2+Rh2 (c,g) and SBA-15~NH2+Rh2 (d,h)

2.3 Identification of binding sites of immobilized catalysts 13

C CP MAS NMR The 13C CP MAS NMR spectra of the immobilized catalysts are displayed in Figures 1c,d. For SBA-15~COOH~NH2+Rh2 (Figure 1c), the α-methylene signal of the APTES linker (-CH2(γ)CH2(β)CH2(α)NH2) changed its line shape compared to the bi-functionalized supporting material (Figure 1a). Next to the signal at 42 ppm, a shoulder peak appeared at 47 ppm. Furthermore, signals at 163 ppm and 114 ppm with low intensity become visible. Such line shape changes were also observed in the spectrum of SBA-15~NH2+Rh2 (Figure 1d). Finally, a novel peak emerged at 195 ppm in the carbonyl region of SBA-15~COOH~NH2+Rh2 (Figure 1c). Initial evidence for the binding of the dirhodium unit via the amine group of the APTES linker for SBA-15~COOH~NH2+Rh2 and SBA-15~NH2+Rh2 are obtained from the methylene signal at 42 ppm for which a shoulder peak at 47 ppm in the 13C spectra appeared (Figures 1 c,d). This 5 ppm higher chemical shift most probably refers to an inductive effect which is induced by the axial coordination of the amine group. This is in line with the 13C liquid NMR data of a series of adducts between Rh2(TFA)4 and different N-species.28 For the SBA-15~COOH~NH2+Rh2 sample, the carboxyl binding site is identified by the peak at 195 ppm in the 13C spectrum (Figure 1c), which has a 17 ppm higher chemical shift than that of the free carboxyl groups, and a 13 ppm higher chemical shift than that of carboxylate anions in the supporting material (Figure 1a). This observation is in agreement with our previous report. When anchoring the Rh2(TFA)4 on CNC materials via carboxyl groups the C=O signal for CNC~COORh2 was shifted by 17 ppm and 14 ppm to higher chemical shift than those of CNC~COOH and 6


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CNC~COO- groups, respectively.21 Finally, the 13C signals with low intensity at ca. 163 and 114 ppm obtained for SBA-15~COOH~NH2+Rh2 and SBA-15~NH2+Rh2 (Figures 1c,d) are assigned to trifluoroacetate moieties coordinated on the dirhodium unit. Their low intensity and resolution most probably refers to the (i) limited efficiency of 1H→13C polarization transfer (since no protons are in close distance to the CF3 groups) and (ii) non-decoupled C-F scalar interactions. 15

N CP MAS DNP NMR Direct information on nitrogen containing species were obtained from 15N CP MAS DNP spectra (Figures 1g,h). For both heterogenized catalysts a broad signal containing several sub-signals appeared centered at a 15N chemical shift of ca. -404 ppm. This difference of 57 ppm compared to the amine chemical shift (-347 ppm, Fig. 1g) confirms the axial coordination. This is in line with our previous results obtained for the immobilized Rh2(OAc)4 on SBA-15~COOH~NH2, where a chemical shift difference of 54 ppm was observed.27 The detailed analysis of the 15N CP MAS DNP spectrum of SBA-15~COOH~NH2+Rh2 showed that the broad signal of the amine-dirhodium adducts centered at ca. -404 ppm, (Figure 1g) contains sub-signals centered at -389, -398 and -410 ppm. For the amine-dirhodium adduct in the 15N CP MAS DNP spectrum of SBA-15~NH2+Rh2 one signal centered at ca. -406 ppm (Figure 1h) is clearly identified, while at -389 ppm no signal is visible at the obtained noise level. The signals at -398 and -410 ppm most probably correspond to [Rh2(TFA)4·2APTES] and [Rh2(TFA)4·APTES] binding sites, respectively. This assignment is based on previous results obtained for amine-Rh2(TFA)4 2:1 and 1:1 adducts, where the 15N nuclei in the 2:1 adduct are more deshielded than that in the 1:1 adduct.28 In the case of SBA-15~NH2+Rh2 the signal at ca. -406 ppm seems to be related to both [Rh2(TFA)4·2APTES] and [Rh2(TFA)4·APTES] binding sites, which are not clearly resolved at the noise level of the spectrum. Finally, the signal at -389 ppm (Figure 1g) is attributed to dirhodium complexes where one carboxyl group is replaced by an CES linker and in addition the amine linker is ligating to the rhodium, forming an [Rh2(TFA)3(CES)·APTES] binding geometry. Obviously, this binding situation is only feasible in the bifunctional system. In this binding geometry (ESI Scheme S2), two trifluoroacetate groups are coordinated trans- to each other. Thus, the electron withdrawing effect of trifluoroacetate groups is counteracted. After replacing one trifluoroacate group with a CES linker, the electron density on its corresponding trans-trifluoroaceate group will increase at the cost of a reduced electron density on the amine-dirhodium unit [-NH2-Rh2]. Thus, the 15N nuclei in the [Rh2(TFA)3(CES)·APTES] are expected to be deshielded. Finally, for SBA-15~COOH~NH2+Rh2 (Figure 1g), an additional signal at -252 ppm became visible assigning to amide nitrogen.23, 27, 40 This amide may be formed by reaction of an amine group with either a surface carboxyl group of the CES linker or residues of trifluoroacetic acid stemming from ligand replacement. Detailed investigation of binding sites by 19F MAS NMR To get deeper insights into the various binding sites, 19F solid-state NMR measurements were performed. On the one hand, 19F is a very sensitive nucleus due to its 100% natural abundance and its high gyromagnetic ratio (25.181×107 rad·T-1·s-1).45 On the other hand, quantitative analysis via 19F MAS NMR is a powerful tool to obtain the fraction of different binding sites, as shown in a recent paper.22 7



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Fig. 2 19F MAS NMR SBA-15~NH2+Rh2 (b)

spectra

of

SBA-15~COOH~NH2+Rh2

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(a)a

and

The 19F MAS NMR spectrum of SBA-15~COOH~NH2+Rh2 is depicted in Figure 2a. It contains a broad asymmetric signal which can be deconvoluted into three sub-signals centered at -77, -80 and -82 ppm. In the 19F MAS NMR spectrum of SBA-15~NH2+Rh2 (Figure 2b) only two signals (-77 and -80 ppm) are present. Both 19 F spectra differ in their line shapes from the 19F spectrum of neat Rh2(TFA)4 which shows a broad signal containing several sub-signals probably due to polymorphism of the solid complex (see ESI Figure S13).21 This observation excludes the presence of significant amounts of neat Rh2(TFA)4 on the surface functional SBA-15 carriers. The 19F chemical shifts at -77 and -80 ppm are assigned to [Rh2(TFA)4·APTES] and [Rh2(TFA)4·2APTES], respectively, referring to the work of Jaźwiński.28 When an amine group coordinates the dirhodium unit, the electron density will increase in the amine-dirhodium unit as well as at the equatorial coordinating trifluoroacetate groups. Thus, shielding of 19F nuclei is expected in [Rh2(TFA)4·2APTES] compared with that in [Rh2(TFA)4·APTES]. Finally, the signal at -82 ppm (Figure 2a) is attributed to the trifluoroacetate group in trans- position to the CES linker in [Rh2(TFA)3(CES)·APTES], where the dirhodium unit is bound to both carboxyl and amine moieties. As illustrated in ESI Scheme S2, the electron density at the trifluoroacetate group trans- to the CES linker increases at the cost of reduced electron density at the amine-dirhodium unit. Hence, the 19F nuclei in the trifluoroacetate group trans- to the CES linker are expected to be shielded. Such shielding effect of 19F nuclei in TFA (trans to carboxyl group) was observed in both solid state and solution state NMR (see ESI Figure S14). Finally, the 19F signal for the two remaining trifluoroacetate groups in [Rh2(TFA)3(CES)·APTES] appears at -77 ppm, since the electron withdrawing effect of the trifluoroacetate groups is counteracted.22 To perform a quantitative analysis of the structures described above, the relative area of each 19F signal was determined. The [Rh2(TFA)4·2APTES], [Rh2(TFA)4·APTES] and [Rh2(TFA)3(CES)·APTES] fractions in the sample SBA-15~COOH~NH2+Rh2 8


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were calculated as 15%, 29% and 56% (see ESI Quantitative Analysis and Table S3), respectively. In the same way for the SBA-15~NH2+Rh2 catalyst, the fractions of the binding geometries [Rh2(TFA)4·2APTES] and [Rh2(TFA)4·APTES] were calculated as 70% and 30%. These fractions are essential for understanding the catalytic performance of the catalysts, and will be discussed in detail in the catalytic part below.

2.4 Catalytic evaluation The catalytic performance of the heterogenized dirhodium catalysts was investigated by employing the cyclopropanation between styrene and EDA (Figure 3I). Cis- and trans-1-ethoxycarbonyl-2-phenylcyclopropane are formed via inserting the rhodium carbenoid (intermediate of EDA and dirhodium unit) into the C=C double bond of styrene.46-47 After a reaction time of 300 min, the yields of products were determined as 91% for SBA-15~COOH~NH2+Rh2 and 35% for SBA-15~NH2+Rh2, respectively (Figure 3II a,b). The diastereoselectivity of parent Rh2(TFA)4 is preserved after anchoring on the functionalized SBA-15 materials as demonstrated by the cis-isomer fraction (cis/(cis+trans)) that stayed slightly below 50% for both catalysts (Figure 3III a,b). This observation is in agreement with our previous work on CNC based dirhodium catalysts.21 The turnover frequencies (TOF) and related TOF ratios for the two catalysts were calculated for each reaction time (see ESI Quantitative Analysis and Table S4). The initial TOF values are 209 and 81 h-1 for SBA-15~COOH~NH2+Rh2 and SBA-15~NH2+Rh2, respectively. Averagely, SBA-15~COOH~NH2+Rh2 exhibited 2.6 times higher catalytic efficiency than that of SBA-15~NH2+Rh2 according to the TOF ratios. (ESI Table S4) The most probable explanation for this difference is based on the binding geometries present in these catalysts. According to the 15N and 19F NMR data discussed above, three types of binding geometries occur in SBA-15~COOH~NH2+Rh2, namely [Rh2(TFA)4·2APTES], [Rh2(TFA)4·APTES] and [Rh2(TFA)3(CES)·APTES] (Figure 3 IV a). For [Rh2(TFA)4·2APTES], the dirhodium unit is axially coordinated via two amine groups. In this case, the substrate cannot reach the axial sites that are blocked by two APTES linkers. On the contrary, for [Rh2(TFA)4·APTES] and [Rh2(TFA)3(CES)·APTES] there is one axial site available for the substrate to coordinate and convert into the reactive intermediate carbene. Based on the quantitative analysis of 19F MAS NMR, the overall fraction of these net catalytic sites (Rh2(TFA)4·APTES] and [Rh2(TFA)3(CES)·APTES]) covers 85% of the dirhodium units in SBA-15~COOH~NH2+Rh2. In the case of SBA-15~NH2+Rh2 the fraction of the net catalytic sites ([Rh2(TFA)4·APTES]) was estimated to be 30% (Figure 3IV b). These results show that the fraction of net catalytic sites in SBA-15~COOH~NH2+Rh2 is approximately 2.8 times higher than that in SBA-15~NH2+Rh2, which coincides well with the difference of the TOFs for both catalysts. This result clearly demonstrates that the relationship between the catalytic efficiency and the dirhodium binding sites on functional SBA-15 can be explained by quantitative analysis of 19F MAS NMR.

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Fig. 3 (I) Schematic representation of the model cyclopropanation reaction between styrene and EDA, (II) total yield of cis- and trans-isomers as well as (III) diastereoselectivity (represented as cis-isomer fraction cis/(cis+trans)) of the reaction employing SBA-15~COOH~NH2+Rh2 (a) and SBA-15~NH2+Rh2 (b) as catalysts, respectively. (IV) Schematic drawing of the open catalytic sites (structures 2 and 3) in SBA-15~COOH~NH2+Rh2 (a) and (structure 2) in SBA-15~NH2+Rh2 (b), as well as the fractions of these sites (in parentheses) calculated from quantitative 19F MAS NMR measurements. Leaching and recyclability properties To verify the heterogeneous nature of the obtained catalysts, both leaching and recyclability tests were carried out. For the leaching test, the catalysts were removed from the reaction mixture after 15 min of reaction time. Within the experimental error the yield of product (Figure 4I a’ and b’) in the filtered solutions stayed constant for SBA-15~COOH~NH2+Rh2 and SBA-15~NH2+Rh2. Furthermore, the ICP-OES showed that rhodium contents of 1.17 ppm and 0.45 ppm (see ESI) were left in the reaction medium of SBA-15~COOH~NH2+Rh2 and SBA-15~NH2+Rh2, respectively. These tiny amounts of rhodium in the filtered solutions mainly refer to incomplete separation of the catalysts from the reaction medium, since certain precipitation was observed in the filtered solutions. Finally, an exemplary recyclability test was carried out for both catalysts (Figure 4 IIa,b). Only a slight decrease of yield (ca. 2.5%) was observed for both catalysts after three reaction cycles. All these results clearly indicate that the reaction was indeed catalyzed by the 10


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heterogeneous catalysts. The robustness of these heterogeneous catalysts is attributed to the fact that the dirhodium moieties are chemically bound via carboxyl and/or amine groups on functional SBA-15 as proven by 13C CP MAS and 15N CP MAS DNP spectra.

Fig. 4 (I) Leaching test: reaction in the presence of SBA-15~COOH~NH2+Rh2 (a) and SBA-15~NH2+Rh2 (b), and after removal of SBA-15~COOH~NH2++Rh2 (a’) and SBA-15~NH2+Rh2 (b’) at 15 min. (II) Recyclability test of SBA-15~COOH~NH2+Rh2 (a) and SBA-15~NH2+Rh2 (b).

3. Conclusion Heterogenized Rh2(TFA)4 catalysts based on SBA-15~COOH~NH2 and SBA-15~NH2 materials were synthesized, and employed as suitable model systems to characterize the structure-reactivity relationship. Basic characterization by small-angle XRD, TEM and N2-adsorption-desorption techniques revealed ordered structures of the carrier materials and heterogenized catalysts. By means of multinuclear solid-state NMR techniques (13C, 15N and 19F), a clear picture of the structure and binding sites of the heterogenized catalysts was derived. Three different binding geometries were identified, namely [Rh2(TFA)4·2APTES] (1), [Rh2(TFA)4·APTES] (2) and [Rh2(TFA)3(CES)·APTES] (3). Apart from understanding of binding geometries, the establishment of the structure-reactivity relationship was achieved by quantitative analysis of 19F NMR spectra, that allowed a quantification of the active binding sites [Rh2(TFA)4·APTES] (2) and [Rh2(TFA)3(CES)·APTES] (3). The catalytic efficiencies of the catalysts given as TOF in model cyclopropanation agree perfectly with the ratio of accessible catalytic sites. This clearly demonstrates the power of solid-state NMR to investigate structure-reactivity relationship of immobilized catalysts. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acs.jpcb.xxxxxxx Structures of dirhodium complexes, experimental details on synthesis, characterization (elemental analysis, XRD, TEM, SEM, N2 adsorption, UV-vis, XPS, 13 C, 19F, 29Si solid-state NMR, 15N solid-state DNP NMR) and catalytic activity of immobilized dirhodium catalysts, small-angle XRD spectra, TEM and SEM images, BET and BJH analyses, DR-UV-vis and solution UV-vis spectra, XPS and ATR-IR spectra, 29Si CP MAS NMR spectra, and their analyses, quantitative analysis of 19F MAS data, 19F NMR spectra of Rh2(TFA)4 and derived catalysts, ICP-OES analysis of rhodium content 11



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Author Information Corresponding Authors * E-mail: [email protected] (T.G.) * E-mail: [email protected] (G.B.) ORCID Torsten Gutmann: 0000-0001-6214-2272 Gerd Buntkowsky: 0000-0003-1304-9762 Notes The authors declare no competing financial interest. Acknowledgments Financial support by the Deutsche Forschungsgemeinschaft (DFG) under contracts Bu-911-20-1 (DNP spectrometer) and Bu-911-26-1 is gratefully acknowledged. The authors thank Niels Rothermel, Mayke Werner and Sara Hadjiali for their technical support.

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