Hard X-ray spectroscopy: an exhaustive toolbox for mechanistic studies (?)

Established and recent hard X-ray spectroscopic methods in the form of conventional X-ray absorption near edge structure spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS), and the photon-in/photon-out techniques high energy resolution fluorescence detection XANES and valence-to-core X-ray emission spectroscopy (VtC-XES) provide unique opportunities to study mechanisms in metal-organic reactions. The combination of these techniques allows the determination of the local geometric and electronic structures in the form of the numbers of nearest neighbours, their types and distances around an X-ray absorbing atom and the highest occupied and lowest unoccupied molecular levels. Different sample cells for this purpose, which allow high pressure, electrochemical or multi-spectroscopic measurements under inert conditions, are presented and discussed. The potential of HERFD-XANES and VtC-XES to eliminate limitations of conventional EXAFS spectroscopy is established with case studies on the Hieber anion [Fe(CO)3(NO)]- and the iron hydride complex [Fe(CO)H(NO)(PPh3)2]. With VtC-XES the formation of an allyl complex by reaction of [Fe(CO)3(NO)]- in a catalytic nucleophilic substitution reaction can be followed. Combination of HERFD-XANES and VtC-XES allows the identification and investigation of hydride species, as well as their fate in chemical reactions. On the other hand, in order to investigate the active species formation in iron-catalysed cross coupling reactions, conventional XANES and EXAFS are the method of choice for the moment. For all examples, the advantages and limitations of the hard X-ray toolbox are commented on and the value of the individual methods are compared.