Research Interests
 

Organomentallic Application of Phosphorous K-edge X-ray Absorption Spectroscopy

        The role of organic phosphines in coordination chemistry is indisputable1, in particular in organometallic chemistry2. It is well known that they can stabilize transition metals in a low oxidation state and control the ligand coordination sterically and stereochemically3, which are of great importance in asymmetric synthesis4 and catalysis5. The majority of the phosphine ligands PR3 contain alkyl or aryl substituents. Unique phosphine ligands optimized to certain chemical function are also known with other substituents such as alkoxy OR groups (phosphites) or ylidene type groups R2C=PR3 (phosphoranes)1.

          The phosphine ligands are generally considered to be s-donor ligands due to their lone-pair to metal donor capabilities6-8. In complexes with electron rich metals (Ir, Os, Pt), they can act as p acceptors taking electron density out of a metal d orbital into P-C s* or P 3d orbitals (Figure 1).
        Spectroscopic techniques such as electronic absorption9 and nuclear magnetic resonanceare extensively applied to characterize organophosphorous ligands, which help to gain insight into their role in catalysis. Due to the nuclear spin 1/2 and the rich natural abundance of the 31P isotope, the NMR technique10 is routinely applied and of great interest beyond organometallic chemistry11. It is promising to extend the use of spectroscopic techniques with the X-ray absorption spectroscopy12,13 in the range of 2000-2300eV that can directly probe individual, phosphorous-based orbitals, e.g. the metal-phosphorous bond. In addition, structural information can be obtained by analyzing the post-edge EXAFS region14 of their XAS spectra. The introduction of the XAS technique intends to provide complimentary ground state information to 31P NMR experiments. NMR will remain as a bulk and rapid spectroscopy, while XAS can give insight into chemical problems directly at the level of molecular orbitals.
        Earlier attempts of employing XAS to describe near-edge structure (XANES) in organophosphorous compounds are known in the literature15; however, those studies have been restricted to limited organophosphorous compounds and their derivatives with chalcogenes. It is also important to emphasize that the correlation of computational studies is much more difficult with experimental 31P NMR chemical shifts and coupling constants than with XAS. In order to do this, a sophisticated and elaborate computational algorithm is required, although notable progress has been made in the field of computational magnetic spectroscopy16. Quantitation of ligand K- and metal L-edge XAS spectra directly give ground state orbital coefficients and, therefore, the complete wave function. The comparison of computational results and experimental data is crucial in understanding the geometric and electronic structures of short-lived, catalytically active species, which can be described properly by spectroscopically calibrated computational methods.
Figure 1
          Proposed research: Due to the lack of a reference set to describe and quantitate P K-edge XAS spectra, as a first step, a well defined spectroscopic series will be established for the various oxidation states and types of bonding in phosphorous compounds, such as elemental phosphorous, aliphatic and aromatic phosphines and phosphites, oxophosphines, phosphonium ylides, phosphate and [PF6]- anions. These simple inorganic and organic compounds are commercially available. The calibration and data workup procedures will be established and computer programming will be carried out to obtain data at the beamlines instantaneously after data collection. The quantitation of the P K-edge spectral features will be based on other experimental techniques such as EPR and PES to establish P 1s® 3p transition dipole integrals, which will provide the orbital coefficients. Once the theory is established, P K-edge XAS will be applied for catalytically important systems such as asymmetric synthesis, hydroformylation, olefin metathesis, and ring-opening polymerization. As a case study, the roles of phosphonium ylides in Mo/W-carbonyl complexes are open research questions, which could be addressed by combined XAS and computational studies. The formation and decay of catalytically active species W(CO)4(py)2 will be monitored and the changes quantitated. The results of XAS studies will be correlated with NMR-based observations as well as with computations. The catalysts, intermediates and products will be obtained from collaborators. The combined methods of XAS, NMR, and spectroscopically calibrated DFT will be applied to design novel catalysts with modified phosphine ligands.
        In addition to applications in material science, the P K-edge spectroscopy can be beneficial in biological sciences, for example, by measuring phosphate concentration or speciation or even pH, in vivo, on the basis of different spectral features of phosphate anions in various protonated forms.
 
 

Research questions:

  1. How do the phosphorous near-edge X-ray absorption spectral features vary upon changing the oxidation-, protonation-state and atomic environment of the absorber in solid and solution phases?
  2. What are the key quantitation factors (fitting parameters, dipole integrals) for phosphorous pre-edge features in transition metal phosphine/phosphite/phosphorane complexes?
  3. What is the role of phosphorous containing ligands and initiators in Mo-, Ru-, and W-based olefin metathesis reactions and ring-opening polymerizations?
 
 

REFERENCES

 (1)The Chemistry of Organophosphorus Compounds; Hartley, F. R., Ed.; Wiley: New York, 1990; Vol. 1-4.
 (2)McAuliffe, C. A.; Levanson, W. Phosphine. Arsine and Stibine Complexes of the Transition Elements; Elsevier: Amsterdam, 1979.
 (3)Montilla, F.; Monger, A.; Gutierrez-Puebla, E.; Pastor, A.; del Rio, D.; Hernandez, N. C.; Sanz, J. F.; Galindo, A. Inorg. Chem. 1999, 38, 4462.
 (4)Comprehensive Asymmetric Catalysis; Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H., Eds.; Springer: New York, 1999.
 (5)Homogeneous Catalysis with Metal Phosphine Complexes; Pignolet, L. H., Ed.; Plenum Press: New York, 1983.
 (6)Morris, R. J.; Girolami, G. S. Inorg. Chem. 1990, 29, 4167.
 (7)Pacchioni, G.; Bagus, P. S. Inorg. Chem. 1992, 31, 4391.
 (8)Dias, P. B.; Depiedade, M. E. M.; Simoes, J. A. M. Coord. Chem. Rev. 1994, 135, 737-807.
 (9)Vogler, A.; Kunkely, H. Coord. Chem. Rev. 2002, 230, 243-251.
 (10)Multinuclear NMR; Mason, J., Ed.; Plenum: New York, 1987.
 (11)Picard, F.; Paquet, M. J.; Levesque, J.; Belanger, A.; Auger, M. Biophys. J. 2000, 77, 888.
 (12)Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 1990, 112, 1643-1645.
 (13)Glaser, T.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. Acc. Chem. Res. 2000, 33, 859-868.
 (14)Zhang, H. H.; Hedman, B.; Hodgson, K. E. X-ray absorption spectrosocpy and EXAFS analysis. In Inorganic Electronic Structure and Spectroscopy; E.I. Solomon, A. B. P. L., Ed.; Wiley: New York, 1999; Vol. 1: Methodology; pp 513-554.
 (15)Engmann, C.; Franke, R.; Hormes, J.; Lauterbach, C.; Hartman, E.; Clade, J.; Jansen, M. Chem. Phys. 1999, 243, 61-75.
 (16)Patchkovskii, S.; Ziegler, T. J. Chem. Phys., A. 2002, 106, 1088-1099.