Developments of time-resolved XAFS spectroscopy techniques – applications in homogeneous catalysis

Abstract

Catalysis is one of the most important methods to obtain products in a selective and sustainable manner, i.e. in an environmental responsible manner. To be able to modify and optimize these catalytic production pathways, it is important to obtain knowledge on the reaction mechanisms occurring. X-ray Absorption Fine Structure spectroscopy is a very powerful technique to obtain detailed electronic and structural information about the homogeneous catalysts in their chemical active environment, in situ and time-resolved, and derive structure – performance relationships and reaction mechanisms. In this thesis the application of XAFS spectroscopy in homogeneous catalytic systems is explored. Theory and data-analysis procedures to reliable analyze and interpret the obtained XAFS data, and instrumentation to monitor homogeneous catalytic reactions in situ and in a time-resolved mode were developed. A number of important homogeneous reactions are studied in detail. In the standard scanning data acquisition mode, EXAFS spectra are obtained in timescales that vary from minutes to hours. An alternative data acquisition method, i.e. the energy dispersive (ED) mode, has been developed which allows short collection times and thus enables structural information on dynamic systems. The ED-XAFS technique is further developed to apply to homogeneous catalytic systems. Moreover, a new set-up was developed to allow simultaneous time-resolved UV-Vis and ED-XAFS measurements (Chapter 2). The limited amount of applications of XAFS spectroscopy in homogeneous catalysis so far is most likely due to the complicated data-analysis of EXAFS data of organometallic compounds. In Chapter 3 a new refined data-analysis procedure is developed which accurately analyzes these EXAFS data. Application of the difference file technique while fitting in R-space, allows one to examine the different individual contributions to the total EXAFS spectrum in detail. In combination with the use of different k-weightings, antiphase behavior of different contributions can be detected and accounted for. The procedure is solely based on EXAFS parameters making it applicable to every EXAFS spectrum. Although crystal structures of many homogeneous catalysts are known, the structures in reaction medium are often not clear. Characterizing the organometallic compounds in reaction medium (solution) reveals essential structural information, which directly establishes structure-selectivity relationships in important Cu and Pd catalyzed reactions as shown in Chapters 3 and 6a. In Chapter 4 we demonstrate that X-ray absorption near edge spectroscopy (XANES) directly probes molecular orbitals. The negative second derivative of these XANES data provides direct information on the energy and charge distribution within the different molecular orbitals. The corresponding orbital interaction diagrams are determined. Theoretical density functional and FEFF8 calculations validate the results obtained. The XANES spectroscopy technique described can be applied to in principle every kind of sample. The introduction of substituents at the para-position of the NCN-pincer ligand benzene ring allows one to tune the metal center of the complex electronically and consequently the reactivity of the catalytically active pincer. In Chapter 5 we have used the Atomic XAFS contributions in the Pt L2,3 XAFS spectra of [PtCl(NCN)-Z] pincer complexes to probe the electron density changes on the Pt atom. The results validate the AXAFS technique and the AXAFS theoretical interpretation in particular. Theoretical FEFF8 calculations predict the proper effects and trends. AXAFS is thus a powerful technique to probe the electronic properties of Pt and principally, of every atom, similar yet complementary to NMR. Moreover, XAFS measurements can be performed in situ and time-resolved so the changes in electronic properties can be monitored during reaction. The novel time-resolved ED-XAFS/UV-Vis set-up was used to elucidate the deactivation pathway of palladium catalysts and to gain insights into the catalytic cycle of a copper catalyzed amination reaction. In Chapter 6b, the size and nature of the different inactive Pd-clusters formed during the allylic substitution reaction as a function of ligand and solvent are studied in detail. In Chapter 7 it is shown that the application of a wide range of spectroscopic techniques gives detailed structural and electronic information on the reaction intermediates involved in the Cu(II)-catalyzed arylation reaction. In combination with catalytic results, a novel, unexpected reaction mechanism for this important Cu(II) catalyzed arylation of aza-compounds has been proposed.

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