Structural Chemistry from the Edge(s): An Introduction

1. Structural Chemistry from the Edge(s): An Introduction Georgina Rosair Department of Chemistry, Heriot-Watt University .

2. Overview • Why use absorption edges? • X-ray absorption: XANES, EXAFS • Anomalous scattering : Diffraction at different wavelengths • Case Histories: • Molecular magnets • Excited states • Metalloproteins • Catalysis • Conclusions

3. Why use absorption edges? • Study of local environment in liquids and amorphous solids, including surfaces • Electronic and magnetic structure • Element specific: Can use elements above Ca in atomic weight • Below Ca: vacuum environmental cell needed for P and S K edges • Changes in anomalous components of the Scattering factor

4. Edge origins • Edge: Ionisation of a core electron • K edge: electron originates from 1s orbital • L edge electron from 2s (LI) and 2p (LII and LIII) • LII: state 2P1/2 • LIII: state 2P3/2

5. Fe K-edge X-ray Absorption Spectrum of Trevorite, Fe2NiO4 Pre-edge: core to valence level Edge: Ionisation of a core electron XANES and EXAFS: scattering of ejected photoelectron XANES Absorbance EXAFS oscillations Pre-edge Eo Energy / eV

6. Features of the Absorption Edge • The higher the frequency of the oscillations the lower the distance between absorber and scatterer • Phase of the EXAFS and shape of the amplitude are dependent on the identity of the scatterer, but weakly so - O and S can be distinguished but not O and N • Intensity of oscillations proportional to the number of neighbours i.e. coordination no. • The EXAFS function is dampened by thermal motion. Debye Waller factor (similar to Ueq) Structural disorder also influences this parameter. • The pre-edge height is proportional to the number of vacancies in the valence levels

7. Fourier Transform • The FT of the EXAFS spectrum : approximate radial distribution of scatterers around the absorbing atom, after correction for phase and amplitude • The theoretical fit is generated by adding shells of scatterers and refining the model to get the best fit Fe…Fe, Fe..Ni Fe..O Transform Amplitude R / Å

8. Some limitations • Reference compounds needed • If there's a high uncertainty in a distance then the peak may not be visible in the EXAFS • Low data:parameter ratio, therefore accurate models are required to act as constraints in refinement • J.E. Penner-Hahn, Coord. Chem. Revs., 1999, 1101

9. Anomalous scattering • Collect diffraction data at two or more wavelengths near the absorption edge • Chosen wavelengths e.g.maximise the change in the real part (f') of the anomalous scattering and minimise the change in the imaginary (f") part • Position of anomalous scatterer found by f’ difference Patterson or Fourier maps http://www.bmsc.washington.edu/scatter/AS_index.html

10. Some Applications of Anomalous Scattering • Distinguish between neighbouring elements in the periodic table: particularly when a site is disordered and occupied by two different elements • A change in valence states shift the position of the absorption edge • Many macromolecular crystal structures are solved by using MAD (Multiwavelength Anomalous Dispersion) or SAD if they contain an anomalous scatterer

11. Diffraction Anomalous Fine Structure • The detector is set at the right scattering angle 2θ for a particular hkl value and a DAFS spectrum is measured. • The contribution of each component to the total absorption spectrum can be separated • Example: Co3O4 Tetrahedral Co sites are high spin Co(II) Octahedral Co sites are low spin Co(III) • Because the Co atoms are on special positions, the hkl reflections 2 2 2 and 4 2 2 were used for the octahedral site and tetrahedral sites respectively. • I.J. Pickering, M. Sansome, J. Marsch, G. N. George, J. Am Chem. Soc. 1993, 115, 6302

12. Light-induced low spin to high spin transition in [Fe(NCS)2(phen)2] • XAS of the Fe K, LII and LIII edges are measured after the sample is irradiated with He/Ne laser • Fe-N distances from the K edge • Metal spin state - ratio between the intensities of the LII and LIII edges • J-J Lee, H-S. Sheu, C-R Lee, J-M Chen, J-F Lee, C-C. Wang, C-H Huang and Y.Wang, J. Am. Chem. Soc, 2000, 122, 5742 and refs therein

13. Study of the excited state • The compound [Fe(NCS)2(phen)2] has two spin states; low spin, S=0; high spin S = 2 • Two high spin states, thermal and light-induced • Light-induced HS state trapped at 17K • K edge: • Fe-N(Phen): 1.985(5) at 17K to: 2.12(1) Å on light excitation at 17 K 2.190(5) Å at 300K. • L edge: relaxation of high spin to low spin • Crystal field multiplet calculations : theoretical fit

14. Metal cyanide complexes as molecular magnets • X-ray Magnetic Circular Dichroism • Direction and magnitude of the local magnetic moment • Collect data with magnetic field • Need circularly polarised X-rays - synchrotron radiation

15. XMCD at the V and Cr K edges for Cs(I)V(II) V(III)1½[Cr(III)(CN)6 ] ·nH2 O • Antiferromagnetic coupling between V and Cr ions is shown by the inversion of the dichroic signal at the V and Cr K edges • M. Verdaguer et al. Coord. Chem. Rev., 1999, 190–192, 1023–1047 Vanadium K edge Chromium K edge

16. XANES • XANES region: distance travelled by photoelectron longer than in EXAFS region • Multiple scattering provides angular as well as radial information - 3D structure around a photoabsorber, even determine chirality • Multiple scattering analysis to simulate the spectrum. e.g. FEFF, ab initio multiple scattering calculations of EXAFS and XANES spectra • Accurate models needed to provide a constraint in refinement. • FEFF: http://leonardo.phys.washington.edu/feff/

17. XANES and EXAFS: Metal environment in metalloproteins • Cytochrome-c on oxidation: ΔFe-N negligible ΔFe-S 2.29 to 2.33(2) Å • Greater precision than previous single crystal structure determination • Sulfur K pre-edge:Degree of covalency in M-L bonds • E.I. Solomon et al. Acc. Chem. Res., 2000, 33,959 • M-C Cheng, A. M. Rich, R. S. Armstrong, P.J. Ellis and P. A. Lay, Inorg. Chem., 1999, 38, 5703

18. Reduction by H2 of Pt(acac)2 and Ge(Bu)4 to form Pt particles on a silica support • The catalytic activity of Pt is enhanced by the presence of Ge • Multi edge energy dispersive EXAFS (EDE) follows the changes in the Pt LIII edge and Ge K edge simultaneously as the temperature is increased from 300 to 630 K • Ideally, an elliptically bent monochromator delivers a focused X-ray beam containing a range of X-ray energies • The detector is a photodiode array • S. G. Fiddy,M. A. Newton,A. J. Dent,I. Harvey, G. Salvini,J. M. Corker,S. Turin,T. Campbell and J. Evans, Chem. Commun.,2001, 445.

19. EDE spectra for the Pt LIII and Ge K edges; 298–670 K. • Above 460 K Pt..Pt coordination declines • Above 540 K: Evidence of Pt-Ge interactions and alloy formation • C/O coordination to Ge retained up to 650 K

20. Conclusions • Absorption edges can be used for: • Determining the spin state of metals • Resolution of disorder • Local structure around the metal in metalloproteins • Follow the change in local environment around a metal during a chemical reaction in the bulk and/or on a surface • Thanks to: Dr Andrew Dent at Daresbury and research groups who carried out the work