Structural and Electronic Effects of X-Ray Radiation on Prototypical Catalysts

X-rays are essential to a wide variety of advanced characterisation techniques for probing properties of matter. However, inelastic scattering interactions of X-rays with crystalline matter are known to induce sample change, both global (overall degradation of crystal structure) and specific chemical change (e.g. photoreduction, bond cleavage). The undesirable consequences of radiation during characterisation, has been well documented in biological macromolecular crystallography since it was first studied in the 1960s^1,2. These early studies inspired many advancements in the field over the following decades, with the categorisation of damage into primary, secondary, and tertiary phases, the application of X-ray dose calculations as a damage ″predictive″ tool for experiment planning and notably, the development of sample damage mitigation strategies. Despite the enormous progress made in protein crystallography, the effect of this ionising radiation on small molecular crystals, which are integral to countless modern applications for instance, catalysis, is still poorly understood.<br/><br/>In recent years, there has been a rapid expansion of X-ray characterisation capabilities. The shift towards modern micro- and nano-focused laboratory sources, extremely high brilliance synchrotron sources, as well as the move to more complex, longer <i>in-situ</i> or <i>in-operando</i> studies, has exacerbated the problem of unwanted radiation effects. Therefore, there is great urgency for a better understanding of its influence on matter and characterisation results. Observing changes caused by irradiation allows us to not only design and implement preventative measures, but also to fully understand the radiation damage process and how it occurs in the materials being studied.<br/><br/>Here we present a combined experimental and computational approach of high-resolution synchrotron powder X-ray Diffraction (XRD) at beamline I11 at Diamond Light Source, UK, with laboratory-based X-ray Photoelectron Spectroscopy (XPS) to study the effects of X-ray radiation on model small molecular crystals. Applying these X-ray methods, in addition to Density Functional Theory (DFT) calculations allows us to understand changes to the global structure, the local chemical environments, as well as the electronic structure of a family of industrially important prototypical catalysts.³These have the general formula [M(COD)Cl]₂ where M= group 8-11 transition metals and COD= cyclooctadiene. The progression of radiation-induced changes to these small molecular crystals is monitored over considerable timescales and is discussed as a function of radiation dose. Values of X-ray dose are obtained using a recent development of the RADDOSE-3D utility⁴ for the estimation of radiation dose in small molecular systems.⁵ In addition, the significant contribution of various experimental conditions to radiation induced sample change, such as X-ray energy, temperature, and measurement environment, is investigated.<br/><br/>Approaching this topic with the combination of XRD, XPS and DFT allows for a compelling, novel, multi-modal way to probe effects of X-ray irradiation, by way of a direct correlation of structural changes with changes of the electronic structure and chemical state of the metal. There is enormous potential to extend the application of these three advanced techniques beyond this family of organometallic catalysts, to follow X-ray induced effects and apply conclusions drawn, to other small molecular systems of scientific and industrial relevance.<br/><br/><b>References</b><br/>¹ C. Blake and D. Philips, <i>Symposium of the International Atomic Energy Agency</i>, 1962, 183.<br/>² E. F. Garman, <i>Acta Crystallogr., Sect. </i>D, 2010, <b>66</b>, 339–351.<br/>³N. K. Fernando et al., <i>J. Phys. Chem. A, </i>2021<i>, </i><b>125</b>, 7473–7488<br/>⁴ C.S. Bury et al., <i>Protein Sci.,</i> 2018, <b>27</b>, 217-228.<br/>⁵J. Christensen et al.,<i> IUCrJ</i>, 2019, <b>6</b>, 703-713.