Structure and Activity Investigation of New Immobilized Wilkinson's Type Catalysts

In this thesis, the development of three new immobilized catalysts is presented employing novel strategies of Wilkinson's catalyst immobilization on silica surface. These catalysts are [SiO2~DA-Wilk], [SiO2~Py-Wilk], and [SiO2~PvPy-Wilk] which are immobilized via diamine silane linker, pyridyl silane linker, and silica supported poly4-vinylpyridine, respectively. The hydrogenation catalytic activities of the obtained catalysts range from a moderate catalytic activity in the case of [SiO2~DA-Wilk] catalyst up to an excellent catalytic activity comparable to the homogeneous Wilkinson's catalyst [RhCl(PPh3)3], in the case of both [SiO2~Py-Wilk] and [SiO2~PvPy-Wilk] catalysts. All these developed catalysts do not feature any considerable leaching. Furthermore, their reusability is demonstrated, even with a higher activity in the following catalytic run. Explanations are proposed for this effect, based on the structural changes of the catalysts prior to and after their use. These findings highlight the advantages of pyridyl linkers over amine linkers in the immobilization of rhodium-based catalysts. The thesis is introduced by highlighting the importance of Wilkinson's catalyst in different reactions and applications particularly in parahydrogen induced polarization (PHIP)), which is an NMR signal enhancement technique. The major goal in this study is to improve solutions for the long-standing issue of separation and recovery of this homogeneous catalyst. Previously suggested solutions included the immobilization of Wilkinson's catalyst on silica and/or polymer supports via Rh-P or Rh-N bonds. However, these attempts have encountered challenges, such as the leaching of the catalytic species and/or the deactivation of the immobilized catalyst. Therefore, novel immobilization strategies are utilized here to obtain the immobilized Wilkinson's catalysts with considerable catalytic performance, and high resistance against catalyst leaching. These strategies are based on functionalizing silica nanoparticles with different linking groups. In a first approach, the functionalization of silica nanoparticles with diamine silane linkers is described. A following ligand exchange reaction is performed to immobilize [RhCl(PPh3)3] on the diamine functionalized silica [SiO2~DA]. The success of functionalization and immobilization steps to obtain [SiO2~DA-Wilk] catalyst is demonstrated via multi-nuclear (29Si, 13C and 31P) solid state NMR, ICP-OES, and elemental analysis. The disappearance of two peaks of PPh3 groups of Wilkinson's catalyst from 31P NMR spectrum of [SiO2~DA-Wilk] confirms their replacement with two immobilizing groups within [SiO2~DA-Wilk] catalyst. In a second approach the [SiO2~Py-Wilk] catalyst is developed. For this, silica nanoparticles are functionalized with pyridyl silane linker. The pyridyl groups of the functionalized silica [SiO2~Py] immobilize [RhCl(PPh3)3] by replacing one of its phosphine groups. The success of theses processes and the structure of [SiO2~Py-Wilk] are followed by analyzing the solid state NMR spectra of [SiO2~Py] and [SiO2~Py-Wilk]. The splitting patterns of phosphine groups signals in 1D 31P NMR spectrum, and their correlation peaks in 2D [1H-31P] HETCOR spectrum demonstrate unambiguously that [SiO2~Py-Wilk] consists of the rhodium center coordinating to one chloride, two phosphine groups in cis position to each other, and to the immobilizing pyridyl group. Finally in a third approach, Wilkinson's catalyst is immobilized on silica microparticles via poly4-vinylpyridine. For that, aminopropyl functionalized silica is modified with acryloyl chloride yielding acrylamidopropylsilica [SiO2~AcAP], which ends with a polymerizable functional groups. Subsequent grafting polymerization of 4-vinylpyridine monomers is performed later in the presence of the radical initiator benzoyl peroxide to obtain the silica supported poly4-vinylpyridine [SiO2~PvPy]. The catalyst is then obtained by binding Wilkinson's complex to the pyridyl groups present on the polymer brushes of poly4-vinylpyridine [SiO2~PvPy]. It is confirmed via 1D 31P NMR and 2D [1H-31P] HETCOR solid state NMR that one pyridyl group displaces one of the trans phosphine ligands of [RhCl(PPh3)3] yielding the immobilized catalyst [SiO2~PvPy-Wilk]. The catalytic activities of all prepared catalysts are monitored by gas chromatography (GC) employing the hydrogenation of styrene as a model reaction. In addition to that, the occurrence of catalyst leaching, as well as the reusability of the catalysts are investigated. While both catalysts [SiO2~Py-Wilk] and [SiO2~PvPy-Wilk] show significant catalytic activities up to a factor of 1.3 of the catalytic activity of [RhCl(PPh3)3], [SiO2~DA-Wilk] catalyst has exhibited very low catalytic activity (2% of Wilkinson's catalyst activity). Moreover, all the obtained catalysts have shown high resistance against catalytic moieties leaching with less than 1 ppm rhodium leaching (the lower detection limit of ICP-OES). Furthermore, the reusability tests confirm that both [SiO2~Py-Wilk] and [SiO2~PvPy-Wilk] catalysts can be reutilized for at least three cycles. The PHIP catalytic activities of all prepared immobilized catalysts are investigated employing the model hydrogenation reaction of styrene by means of para-enriched hydrogen. While both [SiO2~Py-Wilk] and [SiO2~PvPy-Wilk] catalysts are efficient to produce hyperpolarization in solution, [SiO2~DA-Wilk] catalyst is found to be unable to produce such effect.
The overall outcomes show that the goal of preparing three new immobilized Wilkinson's catalysts with high resistance against catalyst leaching was achieved. Two of them, i.e. [SiO2~Py-Wilk] and [SiO2~PvPy-Wilk], have exhibited significant catalytic performance compared to Wilkinson's catalyst. Therefore, both of them are qualified to be further developed for sensitivity enhancement applications in NMR and MRI techniques due to their parahydrogenation catalytic activity.