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CSIRO PUBLISHING
Aust. J. Chem. 2006, 59, 78
www.publish.csiro.au/journals/ajc
PGSE Diffusion NMR—An Emerging Technique for Inorganic/Organometallic Chemists P. G. Anil KumarA A
Nanoscale Organization and Dynamics Group, College of Science, Technology and Environment, University of Western Sydney, Penrith South DC NSW 1797, Australia. Email:
[email protected] Manuscript received: 14 September 2005. Final version: 16 December 2005. P. G. Anil Kumar is a post-doctoral fellow with William S. Price at the University of Western Sydney. He received his Ph.D. from the ETH, Switzerland (April 2005) under the supervision of Paul S. Pregosin. His current research interests are focussed on the development and applications of diffusion NMR in nanobiotechnology.
Background For a long time, X-ray and mass spectroscopic methods have been the techniques of choice for determining the size of organometallic/coordination complexes. However, the chemistry community has awoken to the recent advances in NMR, among which pulsed field gradient spin-echo (PGSE) diffusion NMR is attracting considerable interest.[1–3] PGSE is a powerful technique for probing intermolecular interactions in solution. Importantly, NMR is a non-invasive technique that does not perturb the thermodynamics of the system.
PGSE diffusion NMR measurements were introduced about 40 years ago and the technique is a promising addition to the field of inorganic chemistry. This has been possible because of the use of multinuclear PGSE studies for obtaining the translational diffusion coefficients (D) of various nuclei. In addition, the range of accessibility of diffusion coefficients using current commercial equipment is from 10−9 down to ∼10−11 m2 s−1 with the magnetic gradient strengths typically found on high resolution probes. With specialized probes, it is possible to extend this to 10−15 m2 s−1 under favourable conditions. The theory and procedures for performing the PGSE diffusion NMR experiment has been explained in detail elsewhere.[2–4] The present article highlights some of the latest applications of the PGSE diffusion NMR technique to organometallic chemistry.
Applications
Y⫺
Ion Pairing The interactions between anions and transition metal-based cations can play an important role in the chemistry of the resulting salts. Using gas chromatography techniques, Kundig et al. recently showed that the Ru-catalyzed (1a in Fig. 1) enantioselective Diels–Alder reaction was anion dependent.[5] Using PGSE diffusion NMR (i.e., to compare Dcation and Danion ) on suitable model complexes (1b),[6] it was shown that the strongly ion-paired anion, BF− 4 , inhibits the important product/adduct exchange step in the catalysis. In contrast, the bigger anion BArF− is not so strongly ion paired, thus it facilitates faster reaction kinetics. Furthermore, the diffusion studies also showed the importance of solvent effects and their impact on the extent of ion pairing.
⫹ Ru
(C6F5)2P O
L
P(C6F5)2 O
C6H5 C6H5
L ⫽ acetone (1a), acrylonitrile (1b), water (1c) Y ⫽ BF4, BArF
Structures of the Ru complexes.
Fig. 1.
Hydrogen Bonding
Cl
P Ru P
⫹
P
CO
vac, 120⬚C
Cl CO
CO (atm), 120⬚C
Ru P
2
Acknowledgments The author thanks Professors W. S. Price and P. S. Pregosin for their useful comments and supervision.
References [1] P. S. Pregosin, P. G. A. Kumar, I. Fernandez, Chem. Rev. 2005, 105, 2977. doi:10.1021/CR0406716
Ru(dtbpe)(CO)2(Cl)2
⫺1.0
Cl
P
Ru P
Cl CO
Molecular Volumes/Aggregation/Encapsulation The most obvious application of PGSE diffusion NMR involves detecting unexpected molecular volumes/aggregation, which is reflected by smaller diffusion coefficients. X-Ray measurements showed the Ru complex 3 to be a monomeric coordinatively unsaturated species (Fig. 2a); however, PGSE NMR measurements revealed that it existed as a dimer in solution (see Fig. 2b).[8] The D values (at 223 K) were found to be 2.84 × 10−10 and 2.23 × 10−10 m2 s−1 for complexes 2 and 3, respectively, which indicated the slower translational diffusion of the monocarbonyl Ru complex. The applications of diffusion NMR are rapidly expanding, especially to biological studies. For example, it is known that certain platinum complexes are useful in the treatment of cancer. PGSE diffusion NMR will serve as a versatile tool for detecting the interaction of these drugs with a protein, and our future research will focus on such phenomena.
Ru(dtbpe)(CO)(Cl)2
0.0
Cl
CO
(a)
Cl
ln(E)
The diffusion of small molecules (or small anions) can easily be influenced by the presence of a hydroxy group, either from a complexed alcohol or bound water. When the anion is strongly held by hydrogen bonds, the translational diffusion as measured by PGSE diffusion NMR decreases markedly and approaches that of the cation.[6,7] As an example, hydrogen bonding results in nearly equal diffusion coefficients for both the anion and cation in Ruii –aquo (1c) complexes.[6]
⫺2.0
CO
⫺3.0
3
⫺4.0 0.0
0.1
0.2
0.3 0.4 0.5 G2 (T2 m⫺2)
0.6
0.7
(b)
Fig. 2. (a) Formation of the coordinatively unsaturated species 3. (b) A plot of the echo attenuation E versus square of the gradient amplitude, which shows complex 3 diffusing slower than the neutral Ru-complex 2 (note that the slope is proportional to D).
[2] W. S. Price, Aust. J. Chem. 2003, 56, 855. doi:10.1071/CH03128 [3] W. S. Price, Y. Aihara, K. Hayamizu, Aust. J. Chem. 2004, 57, 1185. doi:10.1071/CH04155 [4] W. S. Price, Concepts Magn. Reson. 1997, 9, 299. doi:10.1002/(SICI)10990534(1997)9:53.0.CO;2-U [5] E. P. Kündig, C. M. Saudan, G. H. Bernardinelli, Angew. Chem. Int. Ed. 1999, 38, 1220. [6] P. G. A. Kumar, P. S. Pregosin, M. Vallet, G. Bernardinelli, R. F. Jazzar, F. Viton, E. P. Kundig, Organometallics 2004, 23, 5410. doi:10.1021/ OM049559O [7] P. G. A. Kumar, P. S. Pregosin, J. M. Goicoechea, M. K. Whittlesey, Organometallics 2003, 22, 2956. doi:10.1021/OM030145P [8] J. M. Goicoechea, M. F. Mahon, M. K. Whittlesey, P. G. A. Kumar, P. S. Pregosin, Dalton Trans. 2005, 588. doi:10.1039/B415812A
© CSIRO 2006
10.1071/CH05251
0004-9425/06/010078
