Solid-state NMR Studies of Supported Transition Metal Catalysts and Nanoparticles
The following book chapter reviews recent advances in solid-state NMR spectroscopy of heterogenized transition metal catalysts that have widespread application potential for technical reactions, i.e., to produce structural building blocks for pharmaceuticals. Catalysts based on mesoporous solid support materials such as silica or crystalline nanocellulose (CNC) as well as catalysts based on inorganic organic hybrid nanoparticles are discussed in terms of their synthesis, application, and physicochemical characterization. The power of 1D and 2D multinuclear solid-state NMR techniques of sensitive nuclei such as 31P as well as of quantitative 19F solid-state NMR is demonstrated at selected examples of heterogeneous rhodium and iridium catalysts. For less sensitive nuclei especially of 15N, the combination of high-field solid-state NMR with dynamic nuclear polarization (DNP) is presented as an effective method to dramatically boost the sensitivity of NMR and allow measurements of samples with natural abundance of 15N.
KeywordsSolid-state NMR Dynamic nuclear polarization Iridium Dirhodium Mesoporous silica Polymer hybrid Heterogeneous catalyst Nanoparticle HETCOR Surface functionalization
In everyday life, the production of consumables as well as of basic commodities requires highly efficient manufacturing processes which are scalable. Many of them include catalytic reaction steps. For example, the 1925 developed Fischer-Tropsch synthesis to produce hydrocarbons includes the catalytic conversion of H2 and CO. This reaction has an enormous technical and economic application potential in the efficient management of energy resources [1, 2]. Another example is the Haber and Bosch process which was developed at the beginning of the twentieth century to produce ammonia. In this process, atmospheric nitrogen is catalytically converted into biologically accessible nitrogen, which is the basis of modern food production [3, 4, 5]. Finally, in the synthesis of pharmaceuticals, catalytic conversion steps are often essential. The Monsanto process is a prominent example for manufacture of L-DOPA, a therapeutic agent for Parkinson’s disease, which includes a catalytic hydrogenation step employing a chiral rhodium catalyst .
Due to this high technical importance of transition metal catalysts, strong efforts have been made to heterogenize them (see [7, 8, 9, 10] and references therein), mainly to save resources and avoid environmental pollution but also to minimize the risk of poisoning of fine chemicals or pharmaceuticals. The strategies to heterogenize catalysts range from immobilization on solid supports such as silica or resins [11, 12, 13, 14, 15, 16], over binding via ionic interactions [17, 18, 19, 20, 21, 22] to the development of highly reactive transition metal nanoparticles (see [23, 24, 25, 26, 27] and references therein).
However, the design and optimization of tailor-made heterogeneous catalysts is still strongly based on chemical intuition and empirical methods. A directed design of these systems requires the detailed understanding of the surface structures at the molecular level and chemical processes at the surfaces of heterogenized catalysts. While surface chemistry techniques deliver this knowledge for well-defined and clean model systems, real-world systems containing a number of defect sites require techniques which analyze local structures and surface species.
In recent years, the combination of solid-state NMR techniques and high-field dynamic nuclear polarization (DNP ) has paved new pathways for the structure determination of heterogenized catalysts. While solid-state NMR spectroscopy provides the spectral resolution to identify chemical species, DNP delivers the sensitivity to detect them even in low concentration. In DNP, the strong polarization of unpaired electron spins is transferred to NMR nuclei [28, 29]. While for a long time DNP was limited to low magnetic fields, the situation changed with the development of commercial high-power microwave sources . Starting with the pioneering works of Griffin and co-workers [31, 32], the number of DNP applications increased exponentially within the past few years as shown in recent reviews [33, 34, 35, 36, 37, 38, 39].
In the following, a short overview on investigations of the structure of heterogenized catalysts on solid supports employing conventional solid-state NMR and DNP is given. This book chapter focusses recent studies performed in our lab in Darmstadt. In the first section, mononuclear rhodium and iridium catalysts are investigated, which were immobilized on the surface of porous silica materials or polymer/silica hybrid nanoparticles , and their characterization employing conventional solid-state NMR techniques is described. In the second section, dirhodium catalysts immobilized on porous silica materials or crystalline nanocellulose and their characterization by conventional multinuclear solid-state NMR methods are reviewed. Finally, it is demonstrated how the tremendous increase in sensitivity of solid-state NMR by DNP can help to study these systems without expensive or unfeasible isotope labeling. As a real-world example, the immobilization of a dirhodium catalyst system, grafted via a linker on mesoporous silica , is proven by DNP natural abundance 15N solid-state NMR. It is shown that with DNP enhancement, it is possible to obtain a spectrum within half a day with very good S/N ratio, which otherwise would have taken more than a year to acquire.
Solid-state NMR Investigations of Mononuclear Rhodium and Iridium catalysts on Solid Supports
With the introduction of porous silica materials such as Mobil Crystalline Material (MCM)  by Mobil Corporation laboratories and few years later of Santa Barbara Amorphous type material (SBA) , the interest came up to employ these materials as solid supports for transition metal complexes. Various works dealt with the functionalization of these type of materials and finally with the binding of platinum group metal catalysts (see [42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52] and references therein). While several of the early works mainly focused on the synthesis and catalytic characterization of the heterogenized catalyst, studies which provided a deeper look on the catalyst binding at a molecular level were limited. Since these materials mostly contained amorphous structures, characterization techniques were required which did not depend on long range order. With the establishment of multinuclear 1D and 2D solid-state NMR techniques, structural details at the molecular level became accessible. As an important technique, 29Si solid-state NMR enabled the clear identification of silica containing surface species on surfaces via the appearance of M, D, or T groups in the spectra that can be distinguished from bulk silica material commonly called Q groups . Looking on the transition metal catalyst binding by solid-state NMR is a more demanding task, since many functional groups coordinate the metal via nitrogen, oxygen, or sulfur sites, which have low NMR sensitivity and are poorly amenable for conventional solid-state NMR investigations. For this reason, initially the main emphasis was set on the solid-state NMR investigation of phosphorous-containing linker systems via the 31P nucleus, which is a highly sensitive NMR probe. As important examples, the synthesis and characterization of a variety of phosphine-functionalized silica materials and heterogenized rhodium catalysts was demonstrated [54, 55, 56, 57, 58, 59]. As prominent examples, phosphine-immobilized Wilkinson’s catalysts showing significant catalytic activity in hydrogenation reactions and good separability were investigated [60, 61]. For characterization of the materials, 1D and 2D 31P solid-state NMR techniques were employed (an overview is given in ) which were previously developed on homogeneous rhodium complexes by Wasylishen and co-workers [63, 64].
Based on these works, a variety of heterogenized Wilkinson’s type catalysts were synthesized and characterized by 31P solid-state NMR techniques with the aim to understand and to optimize the heterogenization for applications in catalysis such as the hydrogenation of olefins [65, 66]. As example, the binding of Wilkinson’s catalyst RhCl(PPh3)3 on the surface of amine-functionalized SBA-3 was investigated . The combination of 29Si and 31P solid-state NMR revealed the functionalization of the silica surface with amine linkers and indirectly confirmed the binding of the Wilkinson’s catalyst on the surface of the material. Extended, J-resolved 31P NMR allowed the determination of the binding mode of the catalyst and the number of PPh3 ligands that are replaced by bonds to the amine groups. The experiments strongly suggested that two PPh3 groups were replaced by two amine linker groups, binding the catalyst to the silica surface. This interpretation was supported by the absence of the strong (ca. 400 Hz) 31P-31P trans-J-coupling present in the neat catalyst, which was monitored by off-magic-angle-spinning and slow-spinning MAS experiments.
However, there are two main issues which came up with the previous immobilization strategies. Depending on the linker, the stability of the resulting catalyst was reduced, and leaching occurred as shown in [68, 69]. Furthermore, relevant ligands of the catalyst were replaced by linker molecules. This changed the local coordination of the catalytic center and might influence the catalytic efficacy.
This observation indirectly confirmed that the pyridyl group has indeed coordinated the rhodium (Fig. 4b). Next to these signals, a broad peak is observed at 28.7 ppm in f2 correlating with Si-OH proton signals at ca. 1–2 ppm, with aliphatic signals at ca. 3–4 ppm and with aromatic signals at 7–8 ppm in the f1 dimension. This signal clearly assigns the phenyl groups of OPPh3, adsorbed on the silica surface. Another surprising observation for this catalyst system was the increasing catalytic activity, when several reaction cycles were performed (Fig. 4c). This indicated that the catalytic activity of the catalyst requires an activation step. From the 31P CP MAS spectrum and the 31P liquid NMR spectra recorded after each reaction cycle, it was concluded that this activation refers to a release of PPh3 groups from the catalyst which were oxidized to OPPh3 in solution.
Solid-State NMR Investigations of Heterogenized Binuclear Rhodium Complexes
Binuclear rhodium (II) catalysts, containing paddle-wheel-like structures with catalytic sites at axial position, have been utilized in a variety of organic transformations such as in synthesis of fine chemicals and drugs employing the intermediate carbenoid formation by such catalysts as key step [76, 77, 78, 79]. For example, the dirhodium (II)-catalyzed cyclopropanation is highly efficient to form cyclopropyl moieties present in a number of pharmaceuticals such as the antidepressant milnacipran . However, uncomplete recovery and recycling of homogeneous dirhodium (II) complexes limit their applications  for synthesis of pharmaceuticals which requires routes that overcome heavy metal contamination. Thus, many efforts toward the heterogenization of dirhodium (II) catalysts have been proposed ranging from anchoring of the complexes on polymer chains [82, 83, 84, 85, 86, 87, 88, 89], over immobilization on porous silica supports [90, 91, 92] to possibilities of self-supporting in form of a coordination polymer [93, 94, 95]. While the reactivity of these heterogenized dirhodium (II) catalyst systems has been extensively studied, there was a lack of structural characterization of these materials on a molecular level, due to the limited number of characterization methods that allow deeper insights into their complex disordered structures. To overcome this issue, different types of novel heterogenized dirhodium catalysts were developed and structurally characterized employing solid-state NMR techniques.
In the first step, the linking process of the bdc to the dirhodium units was studied employing 13C CP MAS NMR. Comparing the spectrum of the employed ligand with the spectrum after ligand exchange (see  Fig. 3), it was evident that the binding of the ligand system was successful. Nevertheless, since the ligand exchange was not complete as suggested by preliminary results from elemental analysis, the question arose at this point which kind of defect sites are present in the structure. To solve this puzzle, a quantitative 19F MAS NMR spectrum of the sample was recorded (Fig. 6b). A broad signal containing two sub-signals with chemical shifts of −76.2 and −78.2 ppm was obtained. These signals indicate the presence of coordinating trifluoroacetate groups which contain another bdc (or an acetate) ligand (−78.2 ppm) or a trifluoroacetate ligand (−76.2 ppm) in trans-position, respectively. To confirm this interpretation, DFT calculations were performed. From the experimental and theoretical results, a clear picture of the defect sites was derived as shown by the two scenarios Tf defect 1 and Tf defect 2 in (Fig. 6c). These observations from solid-state NMR revealed that the structure of the coordination polymer, which originally was expected to be relatively well ordered, is strongly disordered, which may explain the high reactivity of this heterogenized catalyst.
Signal Enhanced DNP for Characterization of Immobilized Catalysts on Solid Support Materials
For the previously described examples, conventional solid-state NMR techniques were employed to characterize the structures of the materials. These experiments were feasible since sensitive nuclei such as 31P, 19F, and 29Si were employed for the characterization. However, the signals in the carbonyl region of the CNC-Rh2 sample (Fig. 5b) already showed that for less sensitive nuclei such as 13C, almost the limit is reached to record spectra at an economic time scale with a S/N ratio that allows a clear analysis of the spectrum. To overcome such obstacles, in recent years, high-field dynamic nuclear polarization (DNP) has been developed. DNP techniques open up completely new application fields for surface NMR as was demonstrated in the pioneering MAS NMR experiments of the Emsley group [98, 99]. In their experiments, they found an enhancement of more than 56 for 13C. Following these seminal examples, an increasing number of works are found in the field of material science that use DNP to study the structure and surface functionalization of mesoporous materials and catalysts [100, 101, 102, 103, 104, 105], nanoparticles [106, 107], oxides [108, 109], polymers [110, 111, 112], cellulose systems [113, 114, 115, 116, 117], and MOFs [118, 119, 120]. Many of these systems contained spins even less sensitive than 13C as, for example, 15N but also 17O, which suddenly became accessible even in natural abundance by DNP-enhanced solid-state NMR. This makes DNP the tool of choice to investigate low amounts of functional groups on surfaces at an economic time scale and gave rise also to investigate catalytic surface species in heterogenized catalyst systems. First examples are found in the works by Coperet and Emsley who studied heterogenized ruthenium and palladium catalysts on silica support materials [121, 122]. Employing a combination of 31P NMR and DNP enhanced 1H-29Si HETCOR techniques , they were able to explain the stabilization of Ru−NHC-based catalysts by metal surface interactions which has a strong influence on the catalytic activity in alkene metathesis reactions . In the case of the palladium-containing catalyst, the molecular environment of the immobilized catalyst was monitored by surface-enhanced DNP (SENS DNP) techniques. 1D 13C and 29Si CP MAS and 2D HETCOR experiments gave detailed insights into surface interactions of the catalyst that influence the reactivity and selectivity of the catalyst . Next to these examples, also catalyst systems containing no platinum group metals as catalytic centers were investigated by surface-enhanced DNP. Conley et. al demonstrated the use of 13C and 119Sn solid-state NMR to monitor the reaction of allyltributylstannane with silica material dehydroxylated at 700 °C . In this study, they identified an unexpected reaction behavior of the SiO2 material by identifying surface sites which they underlined by DFT calculations. Next to this study, Pruski and co-workers analyzed an amidozirconium catalyst supported on silica material for carbonyl hydroboration . Measuring 1H-15N HETCOR experiments at natural abundance for different contact times, they were able to identify different nitrogen-containing species via the 1H-15N distance which they assigned to dimethylamide and dimethylamine coordinated to surface-bound zirconium.
Furthermore, it was shown by the groups of Emsley and Pruski that DNP is a powerful tool to detect reactive intermediates on heterogenized catalysts surfaces [107, 125]. The work by Ong et al.  impressively demonstrated the use of 2D 13C correlation spectroscopy in combination with isotope labeling and DNP to monitor reaction intermediates for a supported tungsten metathesis catalyst which shed more light on the reaction mechanism of this important class of catalysts. In a very recent example by Johnson et al. , DNP-NMR was applied to detect organic molecules adsorbed on Pd nanoparticle supported on γ-Al2O3. Employing the >2500-fold time savings, they were able to obtain low coverage species. 13C–13C correlation spectra obtained with DNP made it feasible to identify products from degradation of methionine.
The detailed analysis of the 13C CP MAS spectrum of the immobilized catalyst (Fig. 7b), the bifunctionalized carrier material (Fig. 7a), and comparison with the 13C CP MAS spectrum of the neat Rh2(ac)4 catalyst (not shown) confirmed that a part of the catalyst molecules were bound via a carboxylic group on the surface. Furthermore, the splitting of the signal in the carbonyl region in two sub-signals at 183.3 ppm and 178.0 ppm suggested the existence of carbonyl functions in different chemical environments such as free carboxyl groups, carboxylate in an ion pair with NH3 +, or in an amide bond. An analysis of the recorded 15N CP MAS DNP spectra of the functionalized material and the immobilized catalyst (Fig. 7d, e) showed the appearance of an additional high-field signal, which indicated the successful binding of the amine function to the dirhodium unit. This result was corroborated by quantum chemical DFT calculations on model systems of the Rh2(ac)4 complex containing propylamine ligands that showed similar chemical shifts. In summary, the 13C and 15N CP MAS DNP investigations of the catalyst system demonstrated the feasibility of both carboxyl coordination (Fig. 7f) and amine coordination (Fig. 7e) of the Rh2(ac)4 complex.
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