Many reactions in organometallic chemistry and homogeneous catalysis take place in the presence of a gas under pressure of a few to several hundred bar. In situ monitoring of such reactions by means of NMR is highly desirable for obtaining details concerning the structure of intermediate compounds, the mechanism of such reactions. Also, study of equilibria involving gaseous reactants by NMR is relevant. Some general aspects and subjects relating to the research in the group will be highlighted.

After initial experiments involving a suitable titanium-sapphire cell,1,2 which has recently been improved by incorporating a pressure sensor,3 we have been able to follow reactions under pressure of e.g. dihydrogen2,3,4 and carbon monoxide.5 These excercises have been very helpful indeed to obtain detailed information about exchange of small molecules in the coordination sphere of metals, the structure of intermediates (e.g. 1H, 31P and 103Rh NMR of rhodium species which are present under syngas pressure), the resting state in carbonylation and copolymerization reactions as well as the kinetics of hydrogenation reactions. A number of such applications will be presented.

We have a particular interest in chemistry and catalysis involving organometallic (notably palladium) compounds containing nitrogen donor ligands as the stabilizing and activating entity. 4,6 For such compounds, 15N and 14N NMR are important. 15N NMR is accessible by gradient enhanced HSQC or HMQC. Access to 14N (abundance 99.7%) and other quadrupolar nuclei is highly desirable, but is often hampered by the broadness of their resonances.
We have explored the line-narrowing of 14N resonances and other nuclei, employing supercritical media as a low-viscosity solvent in titanium-sapphire tubes. The low viscosity of the medium results in longer longitudinal relaxation times and hence narrowing of the resonance signals by a factor of 2 to 8 is obtained.7 In principle, a straightforward, semi-routine method for 14N NMR has been developed. Unfortunately, the solubility of many coordination compounds in supercritical CO2 is limited. NMR of other quadrupolar nuclei such as 99Ru and 53Cr has also been obtained.

1. D.C. Roe, Magn. Reson. Chem., 1985, 63, 388.
2. C.J. Elsevier, J. Mol. Cat., 1994, 92, 285 and references therein.
3. S. Gaemers, H. Luyten, J.M. Ernsting, C.J. Elsevier, Magn. Reson. Chem., 1999, 37, 25.
4. M.W. van Laren, C.J. Elsevier, Angew. Chemie Int. Ed., 1999, 38, 3715-3717.
5. I Tóth, C.J. Elsevier, J. Am. Chem. Soc., 1993, 115, 10388.
6. R. van Belzen, H. Hoffmann, C.J. Elsevier, Angew. Chemie, Int. Ed., 1997, 36, 1743.
7. S. Gaemers, C.J. Elsevier, Chem. Soc. Rev., 1999, 135.
8. S. Gaemers, C.J. Elsevier, Magn. Reson. Chem., 2000, 38, 650-654.

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