Determination of the electronic and structural configuration of coordination compounds by synchrotron-radiation techniques

TLDR: In this article, the potential of synchrotron radiation techniques to understand the structural and electronic properties of coordination compounds is discussed, including the contribution arising from more specialized techniques that have become more widely used in the last years, such as the total scattering approach in the XRPD data analysis and X-ray emission spectroscopy.

About: This article is published in Coordination Chemistry Reviews.The article was published on 2014-10-01 and is currently open access. It has received 89 citations till now. The article focuses on the topics: X-ray scattering techniques & X-ray absorption spectroscopy.

Figures

Figure 25. (a) N3 molecule adsorbed on TiO2, calculated at the DFT-GGA level (see ref. [966] for details), and (b) chemical structure of the N3 molecule. Reproduced with permission from ref. [966]. Copyright (2008), AIP Publishing LLC.

Figure 8. Example of X-ray anomalous scattering in contrast mode application to mixed-valent metal complexes. (a) Solid-state structure of the Fe0.99Co1.01(LPh)4 complex investigated by Zall et al. [570], at 50% probability (colour code: Co: cyan; Fe, yellow, N, blue, C, light grey; H atoms are omitted for clarity). The pie charts depict the percentages of Fe and Co present at each metal binding site, as quantified by the analysis of X-ray anomalous scattering data. (b) Real Δf’ (λ) and imaginary Δf ” (λ) parts of the anomalous contribution Δf (λ) to the X-ray scattering factor for Fe (red lines) and Co (blue lines) as a function of the incident Xray wavelength. The four dotted vertical lines indicate the experimental wavelengths selected by Zall et al. for the anomalous experiments, spanning the Fe and Co absorption edges. Selected wavelengths are those that maximize the difference between the Δf’value of the two elements. The cross marks () indicate the Δf’ (λ) and Δf ” (λ) values for Fe and Co employed by the authors in the least-squares refinement to obtain the metal occupancies shown in the pie charts reported in part (a). Reproduced with permission from ref. [570]. Copyright (2013) American Chemical Society.

Figure 3. (a) X-ray absorption coefficient μ(E)/ (where is the sample density) as a function of the incident X-ray energy E, in the 1–30 keV range, for a selection of elements commonly found in coordination compounds (μ(E)/ can be approximately expressed as: μ(E)/ ~ Z4/AE3, where Z the atomic number and A the atomic mass of the element of interest). In particular, μ(E)/ is reported for three “light” atoms, i.e. H (Z = 1), O (Z = 8), and S (Z = 16) and three metals belonging to the 1 st , 2 nd and 3 rd row of the transition metal series, i.e. Fe (Z = 26), Ru (Z = 44), and Os (Z = 76). The X-ray absorption edges encountered in the reported range are indicated for each element. Data obtained from NIST: http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html.

Figure 9. Effect of the local symmetry of Ti(IV) sites hosted in microporous titanosilicates on the intensity of the pre-edge peak of the Ti K-edge XANES spectra. Part (a): experimental Ti K-edge XANES spectra of thermally activated TS-1 (black line) and ETS-10 (grey line) titanosilicate molecular sieves. The red and the violet curves report respectively the theoretical spectra of pentacoorinated (C4v-like) and octahedral (Oh-like) Ti sites of ETS-4 molecular sieve, computed with the FEFF8.2 code [664]. Part (b): 3D representation of TS-1 molecular sieve (left); zoom on the local environment of Ti(IV) sites exhibiting Td-like symmetry. Part (c): 3D representation of ETS-10 molecular sieve (top); zoom on the local environment of Ti(IV) in the monoatomic –Ti–O–Ti–O–Ti– quantum wire showing the more abundant regular Oh-like sites and the defective C4V-like sites, representing the terminal Ti atoms of the chain. Unpublished figure reporting spectra published by Prestipino et al. [691].

Figure 15. Part (a): N 1s synchrotron XPS for Ta(NEtMe)5 (1), t-BuN=Ta(CH2Bu-t)3 (2), t-BuN=Ta(NEtMe)3 (3), tBuN=Ta(NEt2)3 (4), (t-BuN=)2W(NHBu-t)2 (5), and NCr(OBu-t)3 (6), adsorbed on Cu(111) surface at 300 K. Part (b): correlation of N 1s binding energies to parameter for Ta(NEtMe)5 (1), t-BuN=Ta(CH2Bu-t)3 (2), t-BuN=Ta(NEtMe)3 (3), tBuN=Ta(NEt2)3 (4), (t-BuN=)2W(NHBu-t)2 (5), and NCr(OBu-t)3 (6). Adapted with permission from ref. [795]. Copyright (2003) American Chemical Society.

Figure 12. Parts (a-d): simplified representation, using a one electron picture, of: (a) the ground state |g> and the (b) vtc-XES; (c) rvtc-XES; and (d) rctc-XES processes. V n indicates the number (n) of electrons in the valence molecular orbitals (V) just below the EF. U indicates the unoccupied molecular orbitals just above the Fermi energy (EF) and C the continuum excitations. The blue arrows represent the transition of the 1s electron due to the adsorption of an incoming photon of energy ħ to reach the intermediate state |i> (XAS or XANES spectra in part (e). The red arrow indicates the decay of an electron from a higher level to the core hole with final state |f>, accompanied by the emission of a photon of energy ħ (XES spectra in part e). The energy transfer ħ() is indicated with a dashed black arrow. Part (e): the combination of TFY-XAS (blue spectra) with vtc-XES (red

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Determination of the electronic and structural configuration of coordination compounds by synchrotron-radiation techniques" ?

In this paper, the basic concepts of synchrotron radiation techniques and their potentialities in understanding the structural and electronic properties of coordination compounds are discussed. 

Q2. What future works have the authors mentioned in the paper "Determination of the electronic and structural configuration of coordination compounds by synchrotron-radiation techniques" ?

( ii ) Time resolved techniques, such as laser pump-X-ray probe and fast data recording methods ( quick XAS and energy dispersive XAS ) have shown a great development in the last decade and will further develop in the future, conjugating a faster response with a number of independent characterization techniques available on line and allowing e. g. parallel IR, UV-Vis, and Raman investigations. Hopefully, improved and readily available dataanalysis programs will be developed to take full advantage of the rapid data-taking. These methods will allow to access XPS-like information on coordination complexes in interaction with gases and liquids. The authors foresee that the simulation of the XANES spectra will be used more and more frequently to confirm or discard local structures hypothesized from the refinement of EXAFS or diffraction data [ 717 ]. 

Q3. Why is silica used as a support for many heterogeneous catalysts?

The large use of amorphous silica as support for many heterogeneous catalysts is due to its high surface area, thermal and mechanical stability. 

Q4. Why is XES spectroscopy used only at high brilliance beamlines?

Due to the low efficiency of the process (particularly for valence to core transitions) and because of the high E/E requested (<10 –4 ) the potentialities of XES spectroscopy can be fully exploited only at high brilliance beamlines hosted in III-generation synchrotrons. 

Q5. What were the complementary methods needed to yield the structure of a rhodium complex?

Besides X-ray absorption, complementary methods (notably infrared spectroscopy and detailed kinetic analysis) were necessary to yield the catalyst structures and the structure – performance relations. 

Q6. What is the role of the dye in the efficiency of DSSCs?

Interactions between the dye and the semiconductor material play a key role in the efficiency of DSSCs because they influence the electron transfer process. 

Q7. What is the molecular orbital bonding picture?

The molecular orbital bonding picture indicates that the N atom of the ligands contributes more to the M–Nbonding orbitals than the metal atom. 

Q8. What is the importance of using complementary methods in the analysis of synchorotron data?

the use of complementary methods, such as laboratory techniques (XRD, SAXS, luminescence and UV-Vis) and DFT calculations, is greatly helpful in assisting the analysis of synchorotron data, reinforcing the robustness of the derived results. 

Q9. What is the significance of the enhancement in the intensity of the incoming X-ray beam?

In conjunction with the boom in computational power and the widespread introduction of area detectors and efficient systems for low-temperature collection, the enhancement in the intensity of the incoming X-ray beam allowed a remarkable extension of the applicability of single-crystal XRD and XRPD analysis. 

Q10. How can the authors extend the acquisition over a wide range of momentum transfer?

to achieve an adequate r-space resolution of the peaks in the G(r), it is crucial to extend the acquisition over a wide range of momentum transfer in q-space. 

Q11. What is the longstanding project to mimic plants and other photosynthetic organisms to make?

The longstanding project is to mimic plants and other photosynthetic organisms to make high-energy chemicals, such as H2 or reduced forms of carbon. 

Q12. Why was the anomalous scattering technique unexploited for more than three decades?

due to the availability of more practical approaches allowing successful phasing in small molecules (heavy atom methods, direct methods, Patterson methods), this possibility remained unexploited for more than three decades. 

Q13. How was the structure of the complexes determined?

The three complexes were characterized in solution by spectroscopic methods, while their solid state structures were determined by single crystal XRD. 

Q14. What was the first idea of using anomalous scattering in crystallography?

The idea of exploiting anomalous scattering (in particular the phase-shift term Δf ”) to elegantly solve the phase problem in crystallography, was firstly proposed by Bijvoet, in an article dating back to 1949 [551]. 

Q15. What is the application of HXPES characterization on metal complexes?

The application on the study of metal complexes is much more limited, whereas HXPES characterization is mainly performed on thin films or heterogeneous catalysts.