The palladium (Pd) and ruthenium (Ru) species in several attractive catalysts have been probed using X-ray absorption spectroscopy (XAS). The study of catalyst evolution in suspension- and solution-based reactions was the primary aim. It was achieved by performing in situ XAS experiments on Pd and Ru over the course of the reactions. A custom-made reactor was employed which allowed the catalysts to be mixed with other reaction components under desired conditions.

The first system investigated was the Heck coupling reaction catalyzed by Pd(II) complexes embedded on metal-organic frameworks. It was realized that the as-synthesized catalysts go through an instant ligand substitution process when added to the reaction mixture. Mononuclear Pd complexes are the active species at the first stage of the measurement which then gradually transform into Pd nanoclusters. At a later stage of the measurement, chloride ligands start to bind to surface atoms of the Pd nanoclusters, leading to a deactivation of the catalyst. Following the first successful in situ XAS experiment, Pd(II) carbene complexes catalyzing undirected C–H acetoxylation of benzene in the presence of an oxidant were explored. A gradual ligand substitution occurs, and the mean oxidation state of Pd increases at the same time. At a later stage, Pd nanoclusters form, while the mean oxidation state of Pd returns to the start value. Deactivation of a heterogeneous Pd(II) catalyst during cycloisomerization of acetylenic acids was then investigated using in situ XAS. The choice of substrates showed to significantly influence the nature of Pd species, and the reduction of Pd(II) forming Pd(0) aggregates causes the deactivation. Moreover, strategies of reactivating the catalyst and prevention of the deactivation were developed and examined. In the end, the activation process of a Ru catalyst was studied and the structure of the intermediate was determined by in situ XAS. It was demonstrated that an electron-donating substituent on the cyclopentadiene ligand exhibits a promoting effect on the activation, while an electron-withdrawing substituent inhibits the activation.

The work presented in this thesis is based on methodology development and mechanistic investigations using heterogeneous palladium and nickel catalysts. Following the introduction (Chapter 1), Chapter 2 presents a summary of the synthesis and characterization of the MOF-supported Pd catalysts (Pd @ MOF) that are used in this thesis. Chapters 3, 4 and 5 are based on the use of heterogeneous Pd @ MOF catalysts for CC bond forming reactions, whilst Chapter 6 deals with the use of nickel foam for hydrogenation reactions. 

In Chapter 3, the speciation of the ligandless Suzuki-Miyaura reaction catalyzed by Pd @ MOF is investigated. Here, questions regarding the composition, structure and reactivity of leached palladium are studied by means of electrospray ionization mass spectrometry (ESI-MS), density-functional theory (DFT) calculations, nuclear magnetic resonance (NMR) spectroscopy and scanning-transmission electron microscopy (STEM). (Paper I)

The next chapter (Chapter 4) deals with the study of the Mizoroki-Heck reaction catalyzed by Pd @ MOF under working conditions. In this study, catalyst activation, catalyst deactivation and the role of the MOF support are studied. (Paper II) 

Chapter 5 concerns the use of MOF-supported Pd (II) complexes for the aerobic homocoupling of boronic acids. A mild oxidation method for regenerating active catalytic palladium species, which enables its recyclability, is described. (Paper III)

The last chapter (Chapter 6) describes the use of a commercially available nickel foam for the stereoselective semireduction of alkynes using electrochemically generated hydrides from acidic water. As the method tolerates numerous functional groups, it could be applied to a large variety of alkynes. The use of deuterated solvents provided easy access to a library of deuterated Z -olefins. (Paper IV)