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Structural and Dynamics Characterization of a New Generation of Single Site Heterogeneous Metathesis Catalysts by Solid-State NMR Spectroscopy

Final Report Summary - DNP SSNMR STUDIES (Structural and Dynamics Characterization of a New Generation of Single Site Heterogeneous Metathesis Catalysts by Solid-State NMR Spectroscopy)

Solid-state NMR is one of the most powerful atomic level spectroscopic probes of structure and dynamics for solid materials. However, like all magnetic resonance methods (MRI, solution NMR, etc.) it is hampered by intrinsically low sensitivity. It is often impossible to apply solid-state NMR to many interesting materials because the concentration of the active sites/analytes are below the detection limit. Therefore, methods for enhancing the sensitivity of solid-state NMR are required. Dynamic nuclear polarization (DNP) potentially represents a powerful approach for enhancing the sensitivity solid-state NMR experiments by several orders of magnitude. In a DNP experiment the large polarization of unpaired electrons (usually stable free radicals) are transferred to the magnetically active nuclei in a sample providing maximum theoretical sensitivity enhancements of a factor 658 for protons.1 Although DNP was first invented in the 1950’s, high field DNP compatible with modern magnetic field strengths become a possibility with the introduction of high power gyrotron THz frequency sources in the mid 1990’s.1 The initial development and commercialization of DNP technology primarily targeted characterization of biological systems. However, as demonstrated by the Emsley research group, the technique is also well suited for characterizing surfaces of materials.2,3 In the DNP surface enhanced NMR spectroscopy (DNP SENS) approach materials are impregnated with radical solutions to bring the radical proximate to the surface and facilitate DNP enhancement of the surface NMR signals.2,3 While the technique had been demonstrated for model silica materials, the objective of the “DNP SSNMR Studies” project was to further develop DNP SENS as a spectroscopic technique. This will enable the comprehensive characterization of a variety of advanced functional materials with applications in catalysis, separation and energy conversion.
To expand the scope of DNP SENS several technical obstacles first had to be addressed. In particular, DNP was initially focused on biological systems so experiments were designed for aqueous media and stable radicals were water soluble. However, many materials are incompatible with water either because they are hydrophobic or highly reactive. We demonstrated that there are many possible organic solvent-radical combinations which yield very high DNP enhancements.4 In a separate study we thoroughly investigated the optimal sample preparation conditions for DNP SENS experiments.5 Notably, we observed that a large fraction of the DNP signal enhancements were negated by a reduction of the NMR signal by the presence of radicals in the sample (“quenching” or “bleaching” of the NMR signal). Our observations prompted the development of further polarizing agents and sample preparations which reduce or eliminate quenching of the NMR signal. For example by passivating the silica surface with deuterated methyl groups it is possible to maintain high DNP enhancements and reduce paramagnetic quenching effects.6
We have also detailed the application and design of a new biradical polarizing agent for DNP (bCTbK).7 When combined with the previously investigated organic solvents bCTbK provides DNP enhancements (ε) up to 100. The high enhancements available from bCTbK enable the rapid acquisition of natural abundance 13C, 15N and 29Si solid-state NMR spectra of the immobilized molecular species and the silica support material, respectively.7 We also demonstrated that is was possible to rapidly acquire DNP enhanced 2D hetero-nuclear 1H-13C/15N/29Si correlation (1H-X HETCOR) solid-state NMR spectra at natural isotopic abundances.7 Such spectra enable the comprehensive characterization of molecular surface species (Figure 1) demonstrating the power of DNP SENS. In a forthcoming study we will detail the design and synthesis of radicals providing ε up to 200.
We also demonstrated that DNP could be applied to other classes of materials. It is straightforward to apply DNP to metal-organic framework (MOF) materials, an important class of materials employed in catalysis and gas separation and purification.8 In this case the organic linkers of the MOF materials were functionalized with amine or proline groups and with DNP SENS it was possible to confirm incorporation of the proline into the MOFs.8 DNP SENS was also applied to other porous materials such as periodic mesoporous (Inagaki-type) organo-silica hybrid materials that serve as supports for catalysts.9 DNP SENS 15N CPMAS spectra allowed us to distinguish surface and core phenylpyridine functionalities and monitor the fraction of the surface pheynlpyridine groups functionalized with an iridium organometallic complex.9 We also demonstrated how DNP SENS could be applied to nano-particulate alumina.10 This enabled the acquisition of 27Al CP-MQMAS solid-state NMR spectra, something which would likely be impossible without the sensitivity enhancement provided by DNP.10 The applicability of DNP SENS for the characterization of the alumina surface is important since this is one of the most commonly employed support materials for heterogeneous catalysts. In a forthcoming publication we will demonstrate the comprehensive structural characterization of a novel palladium based alkyne hydrogenation catalyst in collaboration with the Coperét group (ETH Zurich).
We have also undertaken several other projects which were un-anticipated in the original proposal. In a major breakthrough we have recently demonstrated how DNP enhanced solid-state NMR spectra of ordinary organic solids can be acquired.11 Large sensitivity enhancements on the order of 150 can be obtained for solids which are characterized by long proton spin-lattice relaxation times, such as pure pharmaceuticals. A manuscript describing the application of DNP for characterization of formulated pharmaceuticals is currently under preparation.
In summary, this project has been very successful and all of the originally proposed research goals have been achieved. Many of the findings have been documented in peer-reviewed scientific publications in high-impact international chemistry journals, clearly demonstrating the success of the project to date. Future work will focus on developing inert atmosphere techniques to use DNP SENS to characterize reactive catalysts and materials. By the end of 2013, there should be ~12 commercial high field DNP instruments installed within the EU, several of which will be aimed at the characterization of advanced materials using many of the methods and techniques developed here. This should aid and accelerate in the rational development of many advanced functional materials.

(1) Maly, T.; Debelouchina, G. T.; Bajaj, V. S.; Hu, K. N.; Joo, C. G.; Mak-Jurkauskas, M. L.; Sirigiri, J. R.; van der Wel, P. C. A.; Herzfeld, J.; Temkin, R. J.; Griffin, R. G. J. Chem. Phys., 2008, 128, 052211-052211-052219.
(2) Lelli, M.; Gajan, D.; Lesage, A.; Caporini, M. A.; Vitzthum, V.; Mieville, P.; Heroguel, F.; Rascon, F.; Roussey, A.; Thieuleux, C.; Boualleg, M.; Veyre, L.; Bodenhausen, G.; Copéret, C.; Emsley, L. J. Am. Chem. Soc., 2011, 133, 2104-2107.
(3) Lesage, A.; Lelli, M.; Gajan, D.; Caporini, M. A.; Vitzthum, V.; Mieville, P.; Alauzun, J.; Roussey, A.; Thieuleux, C.; Mehdi, A.; Bodenhausen, G.; Copéret, C.; Emsley, L. J. Am. Chem. Soc., 2010, 132, 15459-15461.
(4) Zagdoun, A.; Rossini, A. J.; Gajan, D.; Bourdolle, A.; Ouari, O.; Rosay, M.; Maas, W. E.; Tordo, P.; Lelli, M.; Emsley, L.; Lesage, A.; Copéret, C. Chem. Commun., 2011, 48, 654-656.
(5) Rossini, A. J.; Zagdoun, A.; Lelli, M.; Gajan, D.; Rascon, F.; Rosay, M.; Maas, W. E.; Coperet, C.; Lesage, A.; Emsley, L. Chem. Sci., 2012, 3, 108-115.
(6) Zagdoun, A.; Rossini, A. J.; Conley, M. P.; Gruning, W. R.; Schwarzwalder, M.; Lelli, M.; Franks, W. T.; Oschkinat, H.; Coperet, C.; Emsley, L.; Lesage, A. Angew. Chem.-Int. Edit., 2013, 52, 1222-1225.
(7) Zagdoun, A.; Casano, G.; Ouari, O.; Lapadula, G.; Rossini, A. J.; Lelli, M.; Baffert, M.; Gajan, D.; Veyre, L.; Maas, W. E.; Rosay, M.; Weber, R. T.; Thieuleux, C.; Coperet, C.; Lesage, A.; Tordo, P.; Emsley, L. J. Am. Chem. Soc., 2012, 134, 2284-2291.
(8) Rossini, A. J.; Zagdoun, A.; Lelli, M.; Canivet, J.; Aguado, S.; Ouari, O.; Tordo, P.; Rosay, M.; Maas, W. E.; Coperet, C.; Farrusseng, D.; Emsley, L.; Lesage, A. Angew. Chem. Int. Ed., 2012, 51, 123-127.
(9) Grüning, W. R.; Rossini, A. J.; Zagdoun, A.; Gajan, D.; Lesage, A.; Emsley, L.; Copéret, C. 2013, 15, DOI:10.1039/C1033CP00026E.
(10) Vitzthum, V.; Mieville, P.; Carnevale, D.; Caporini, M. A.; Gajan, D.; Coperet, C.; Lelli, M.; Zagdoun, A.; Rossini, A. J.; Lesage, A.; Emsley, L.; Bodenhausen, G. Chem. Commun., 2012, 48, 1988-1990.
(11) Rossini, A. J.; Zagdoun, A.; Hegner, F. S.; Schwarzwälder, M.; Gajan, D.; Copéret, C.; Lesage, A.; Emsley, L. J. Am. Chem. Soc., 2012, 134, 16899−16908.