We are involved in the design, development and implementation of nuclear magnetic resonance methods and the application of them to the study of chemical processes. This work is supported by the synthesis of inorganic complexes, ligands and substrates which may be enriched in NMR active nuclei such as 13C and 15N. Our methods feature the use of hyperpolarised molecules which sensitise the NMR technique sufficiently to allow the detection of species that are normally invisible. Examples of such species can be found among true reaction intermediates that play a role in transition metal catalysis and metabolites that are involved in enzyme based transformations. In order to carry out these studies it is often necessary to develop new experimental methodologies. Our most recent success in this regard involved the design of a new NMR probe that allows samples to be irradiated with UV light whilst data are recorded. We therefore have interests that span inorganic chemistry, catalysis, organic synthesis and biochemistry. For a detailed description of the group’s activities please visit the Duckett Group website. The research group has relocated to the Centre for Hyperpolarisation in Magnetic Resonance.
In terms of methodology the following areas are of particular interest:
This work has involved the examination of the reaction chemistry of metal dihydrides and clusters1-7 that catalyse the hydrogenation1-3, 5, 6, 8-11 and hydroformylation12-14 reactions. Our most recent successes have involved the detection of palladium alkyl, hydride and vinyl complexes that have been shown to play a direct role in the semihydrogenation of alkynes.8, 15 Other work with HCo(CO)4 has demonstrated that key linear and branched acyl containing intermediates can be seen during hydroformylation.12-14
Parahydrogen is itself NMR silent as a consequence of the fact that the two hydrogen atoms exist in the same chemical and magnetic environments. When the symmetry of the two dihydrogen nuclei is broken in a chemical reaction, for example by forming CHBr2CHI2, the NMR signals for these two protons can be 31,000 times larger than normal.16-20 The symmetry of the two dihydrogen nuclei is also broken when they enter a magnetically inequivalent environment such as that found in all cis-Ru(H)2(CO)2(PMe3)2 and cis-Pt(H)2(PEt3)2.21 These products therefore also show substantial signal enhancements in the associated hydride resonances. We have probed fluxional processes in a range of such systems and through appropriate collaborations rationalised the behaviour through density functional studies. We have extended this approach to looking at normally invisible complexes and dynamic processes by preparing specific ligand environments that break the molecular symmetry element which prevents their observation. For example, in cis-Pt(H)2(PPh3)(PCy3) we can see that hydride site interchange competes with reductive H2 elimination. In the case of CpRh(C2H4)(H)2 the two hydride ligands are equivalent but the corresponding product CpRh(CH2=CHSiMe3)(H)2 shows substantial parahydrogen enhancement as a consequence of the change in molecular symmetry.22 Alternatively, when the chiral complex CpRh(C2H4)(H)(SiEt3)23 is prepared the two enantiomers are indistinguishable but when CpRh(C2H3R)(H)(SiEt3) is formed the alkene substituent breaks the molecular symmetry and the resultant isomers can be shown to interconvert via an η2-silane based intermediate.24
One key use of NMR spectroscopy in our group corresponds to the probing of fluxional processes and chemical reactions that occur on the NMR timescale.25 NMR spectroscopy can achieve this because it is possible to label the magnetisation present at specific molecular sites and then read out the information after a specific time interval. The data then collected identifies not only the new molecular site but also provides precise rate information for the associated transformation. We have used this method to accurately quantify the role of key reaction intermediates in both hydrogenation and hydroformylation catalysis by intermolecular magnetisation transfer. We have also studied a number of intramolecular rearrangements and thereby demonstrated roles for η2-H2 and η2-SiH ligands.26, 27, 10, 28, 29
Work on a novel photochemical approach to study reactions started in 1997 with the development of an NMR probe for use on an MSL 300 that allowed in-situ sample photolysis while recording 1H NMR spectra. In 1999 we moved to a wide-bore 400 MHz spectrometer where a second NMR probe allowed high resolution multinuclear NMR spectra to be recorded on unstable materials that were prepared at low temperatures.29 When long irradiation times were required the transmission properties of the liquid light guide degraded.30 In collaboration with Perutz we have replaced the liquid light guide with a laser source. This equipment now forms a cornerstone of the groups research efforts and has been used to detect solvent complexes using both conventional and parahydrogen based approaches, to probe the electronic spin states of metal complexes, and to produce pure spin states suitable for quantum information based applications.10, 16-20, 24, 28, 30-32
The detection of biological substrates such as amino acids, at picomolar levels, has been demonstrated via the detection of a receptor complex that simultaneously coordinates both the amino acid and parahydrogen. In addition, we have produced an NMR experiment that removes the signals that should be present for species where the magnetisation is at thermal equilibrium but leaves intact signals from nuclei that originate in parahydrogen. We are in the process of applying the OPSY filter to imaging experiments using the imaging probe on the university’s newly acquired 600 MHz wide bore NMR spectrometer.33
The University of York has recently purchased a Hypersense dynamic nuclear polariser from Oxford Instruments. We have shown that we are able to successfully polarise samples of tryptophan, diphenylacetylene and iodobenzene attaining enhancement factors between 103-104. We have also demonstrated that the DNP enhancement of a secondary reaction product can be achieved by detection of a palladium vinyl complex.
We have a strong interest in exploring how the catalytic activity of metal complexes are changed when they are brought into the close proximity of a second metal centre. For example when CpRh(C2H4)2 is irradiated in the presence of Et3SiH CpRh(C2H4)(SiEt3)H and CpRh(SiEt3)2(H)2 are formed. However, when [CH2(η5-C5H4)2][Rh(C2H4)2]2 is employed the expected products [CH2(η5-C5H4)2][Rh(C2H4)2][Rh(C2H4)(SiEt3)H], [CH2(η5-C5H4)2][Rh(C2H4)(SiEt3)H]2, [CH2(η5-C5H4)2][Rh(C2H4)(SiEt3)H][Rh(SiEt3)2(H)2] and [CH2(η5-C5H4)2][Rh(SiEt3)2(H)2]2 are formed. But when this reaction is completed in d6-benzene, deuteration of the α-ring proton sites and all the silyl protons occurs. In addition, the formation of the two Si-C bond activation products, [CH2(η5-C5H4)2][RhH(µ-SiEt2)]2 and [CH2(η5-C5H4)2][(RhEt)(RhH)(µ-SiEt2)2] occurs.30 We have also found evidence for a dinuclear based hydrogenation pathway during catalysis by Wilkinson’s complex.11, 34-36
I am very grateful to the research support that the following companies have provided.
Astra-Zeneca, Bruker UK Limited, BP Chemicals, CSL, Dow Corning, Oxford Instruments, SASOL, Shell and YNI Limited.
I am very grateful to the research support that the following collaborators have provided.
Doctor Karina Q. Almeida Leñero, Doctor Hillary Carteret, Doctor Eite Drent, Professor Gary Green, Professor Richard Eisenberg, Doctor Jonathon Jones, Professor Augsti Ledos, Doctor Joost Lohman, Doctor Roger Mawby, Professor John McGrady, Professor Robin Perutz, Professor Rinaldo Poli, Dr Robert Tooze and Doctor Ian Wilson.
I am very grateful to the research efforts of the following scientists who completed the work described.
Mr R Adams, Dr J Aguila, Dr. S. Aiken, Dr K Ampt, Dr D Blazina, Mr P Caldwell, Dr. P. Callaghan, Dr S Colebrooke, Dr J Cunningham, Dr D Chan, Dr J Dunne, Dr P Elliott, Dr C. Elkington, Dr C Godard, Ms A Gomes, Mr J Grace, Dr S K Hasnip, Mr J Holmes, Ms R Kandiha, Dr I Khazal, Dr P. Kaye, Dr. C. Kozak, Dr J. Lowe, Dr S Matthews, Dr. P. Morran, Dr. M. Partridge, Dr D Schott, Dr J Serrano, Ms C Sexton, Ms N Smith, Mr D Taylor, Ms J Welch, Dr N Wood and Dr R Zhou.