A new POCOP pincer-type ligand was synthesized and characterized. This new ligand gave rise to the first POCOP pyridylidene pincer-type ruthenium and iridium complexes. Investigations into structural and electronic parameters of these newly generated compounds were conducted. Furthermore, we prepared ruthenium dihydrogen and bis-dihydrogen complexes and studied them via deuterium labelling and low temperature NMR spectroscopy. These complexes were screened for catalysis in C-C coupling, C-H bond activation and ketone hydrogenation reactions.

A new POCOP pincer-type ligand was synthesized and characterized. This new ligand gave rise to the first POCOP pyridylidene pincer-type ruthenium and iridium complexes. Investigations into structural and electronic parameters of these newly generated compounds were conducted. Furthermore, we prepared ruthenium dihydrogen and bis-dihydrogen complexes and studied them via deuterium labelling and low temperature NMR spectroscopy. These complexes were screened for catalysis in C-C coupling, C-H bond activation and ketone hydrogenation reactions.

Cyclometalated phosphine-based PNP and PCP ‘pincer’ complexes of the platinum group metals have been the subject of intense research in recent years, owing to the remarkable stoichiometric and catalytic reactivity exhibited by such complexes. With the goal of discovering new metal-mediated reactivity patterns and extending the versatility of metal pincer chemistry, significant effort has been devoted to the synthesis of structurally and/or electronically related systems where strategic alterations have been introduced to the pincer ligand architecture, including variation of the central and peripheral donor fragments, as well as the ancillary ligand backbone. In this context, the synthesis and study of Ru and Pd complexes supported by pincer-like tridentate ancillary ligands that feature a central anionic phosphorus ([NPN]) or silicon ([PSiP]) donor in the pincer ligand backbone are described herein. The decreased propensity for forming ?-bonds to P was anticipated to lead to a higher degree of electronic unsaturation in complexes supported by tridentate phosphido ligation relative to structurally related metal amido (M-NR2) species. In the case of [PSiP] ligation, the reduced electronegativity of Si relative to C should promote the formation of electron-rich late metal species that can readily undergo oxidative addition reactions. The trans-labilizing silyl donor was also expected to stabilize coordinatively and electronically unsaturated late metal complexes. The synthesis and reactivity of Ru complexes featuring bis(phosphino)silyl ligation of the type [?3-(2-R2PC6H4)2SiMe] ([R-PSiP]; R = Ph, Cy) are described. The 5-coordinate complex [Ph-PSiP]RuCl(PPh3) was shown to be catalytically active for the transfer hydrogenation of ketones in basic isopropanol. These transfer hydrogenation studies are among the first catalytic studies of silyl-pincer complexes and establish [R-PSiP]M species as viable candidates for catalysis. The synthesis and reactivity of 4- and 5-coordinate RuII complexes featuring the [Cy-PSiP] ligand were explored. Reaction of [Cy-PSiP]H with [(p-cymene)RuCl2]2 in the presence of NEt3 and PCy3 resulted in the formation of ([Cy-PSiP]RuCl)2, which serves as a precursor to a series of unprecedented 4-coordinate, formally 14-electron [Cy-PSiP]RuX (X = NHAr, N(SiMe3)2, OtBu) complexes that feature an unusual trigonal pyramidal geometry at Ru. The reactivity of these novel diamagnetic complexes is described, including the reaction of [Cy-PSiP]RuOtBu with amine-boranes resulting in the formation of rare bis(?-BH) complexes. Computational studies confirmed the key role of the strongly ?-donating silyl group of the Cy-PSiP ligand in facilitating the synthesis of such low-coordinate Ru species and enforcing the unusual trigonal pyramidal geometry. The mechanism of ammonia-borane activation was also examined computationally. Lastly, the synthesis and structural characterization of PdII complexes supported by the pincer-like bis(amino)phosphido ligand [?3-(2-Me2NC6H4)2P]- ([NPN]) is described. Examples of ?1-, ?2-, and ?3-NPN coordination to Pd are described, as is the catalytic activity of ([NPN]PdX)2 (X = Cl, OAc, OTf) complexes in the Heck olefin arylation reaction. In an effort to discourage the formation of phosphido-bridged dinuclear complexes, pre-coordination of the Lewis acid BPh3 to [NPN] was pursued. Upon reaction of [N(P?BPh3)N]K with [PdCl(C3H5)]2, the ?1-allyl complex [?3-N(P?BPh3)N]Pd(?1-C3H5) was isolated, which establishes the coordination of a Lewis acid to the phosphido donor of the [NPN] ligand as a viable strategy for encouraging the formation of mononuclear ?3-NPN complexes.

Abstract: The choice ofsuitable ligand platforms is crucial to organometallic coordination chemistry and homogeneous catalysis. Among the various ligand platforms available, pincer ligands offer a convenient route to manipulate the properties of the resulting complexes. The pincer chemistry of rhodium has attracted attention for over 40 years, and Rh complexes are dominated by Rh(I) and Rh(III) low-spin states, thus they are more predictable than other paramagnetic species. Compared to other pincer ligand platforms, pyridine-based pincer complexes are particularly attractive as they exhibit diverse reactivities. Our group realized a new class of the PN3 (P) pincer system, with altered the unique catalytic performances, thermodynamic and kinetic properties due to their pseudo-dearomatized nature. In Chapter 2, selective carbonylation of benzene to benzaldehyde using a phosphorus nitrogen PN3P Rh(I) complex was realized. The PN3P Rh pincer chloride complex cPePN3PRhCl was capable of activating C−H bond of benzene to give the phenyl complex cPePN3PRh(C6H5) using KN(SiMe3)2 as a base. Furthermore, the benzoyl complex cPePN3PRh(CO)(C6H5) was obtained by treating a benzene solution of cPePN3PRh(C6H5) with CO gas. In dilute HCl, a high yield of 90% benzaldehyde was formed with regeneration of the cPePN3PRhCl. This is the first example of selective carbonylation of benzene into benzaldehyde accomplished by directly inserting CO without irradiation. In Chapter 3, the ligand-centered reactivity of a pseudo-dearomatized PN3P *rhodium complex towards molecular oxygen wasrealized. For the dearomatized rhodium carbonyl complex (tBuPN3P*RhCO), one of the C−H bonds of the pseudo-dearomatized pyridine ring was oxidized by O2 to create an α, β-unsaturated carbonyl functionality. Moreover, the resulting metal complex with the post-modified PN3P ligand could react with thiophenol and 4-methylaniline to afford the corresponding oxidative Michael addition products. In Chapter 4, to further explore the ligand-centered reactivity of tBuPN3P *RhCO, a series of second-generation diimine-amido PN3P-pincer carbonyl complexes were synthesized by reaction of tBuPN3P*RhCO and various alkyl/benzyl halides via a post-modification strategy, and these complexes were well characterized by NMR, HRMS, FT-IR, and single crystal diffraction. Moreover, a plausible mechanism for the formation of 2nd -generation PN3P complexes was proposed

Die vorliegende Arbeit beschäftigt sich mit der Synthese von Pincer-Ligandenvor-läufern mit fluorierten Alkylketten –(CH2)2Rf6 (Rf6 = C6F13) an Sauerstoff- bzw. Phosphorhaftatomen. Darüber hinaus stehen die Bildung hochfluorierter Lithium-, Palladium-, Ruthenium- sowie Aluminium-Pincer-Komplexe und die Reaktivitäts-studien für diese neuartigen Komplexe im Fokus. Für vergleichende Untersuchungen war ebenso die Synthese der analogen, nicht fluorierten Verbindungen von Interesse. Eine Mischung aus in situ hergestelltem (NC5H3)-1,3-(CH2P((CH2)2(CF2)5CF3)2)2 (13) und [Ru(2Me-C3H4)2(cod)] kann die Dehydrogenierung von Cyclooctan bei vergleichsweise niedrigen Temperaturen von 80 °C katalysieren. Interessant ist die je nach Lösungsmittel unterschiedliche Produktbildung. Die Lithium- und Aluminiumkomplexe [Li(C6H3-2,6-(CH2O(CH2)2(CF2)5CF3)2)] (21), [Li(C6H3 2,6 (CH2OCH3)2)] (22), [Al((C6H3)-2,6-(CH2O(CH2)2(CF2)5CF3)2)(CH3)2] (28), [Al((C6H3)-2,6-(CH2OCH3)2)I2] (29), [Al((C6H3)-2,6-(CH2O(CH2)2(CF2)5CF3)2)I2] (31) wurden erfolgreich synthetisiert und charakterisiert. Mittels [Al((C6H3)-2,6-(CH2O(CH2)2(CF2)5CF3)2)I2] (31) konnten diverse aromatische Verbindungen wie Benzol, Toluol oder Pentafluorbenzol dehydrogenierend gekuppelt werden. Weiterhin wurden die Palladiumkomplexe [Pd(Cl)((C6H3)-2,6-(CH2O(CH2)2(CF2)5CF3)2)] (34) und [Pd(NCCH3)((C6H3)-2,6-(CH2O(CH2)2(CF2)5CF3)2)][PF6] (36), [Pd(Cl)((C6H3)-2,6-(CH2OCH3)2] (37) und [Pd(NCCH3)((C6H3)-2,6-(CH2OCH3)2][PF6] (38) hergestellt und charakterisiert. / The current thesis is concerned with the syntheses of pincer ligand precursors with fluorinated alkyl chains –(CH2)2Rf6 (Rf6 = C6F13), the so called „ponytails“, at oxygen or phosphorous donor atoms. Furthermore, this work focuses on the formation of highly fluorinated lithium, palladium, ruthenium or alumina pincer complexes and considering reactivity studies of these novel compounds. For comparative investigations the syntheses of the analog non-fluorinated compounds was of great interest. A mixture of in situ synthesized (NC5H3)-1,3-(CH2P((CH2)2(CF2)5CF3)2)2 (13) and [Ru(2Me-C3H4)2(cod)] catalyses the dehydrogenation of cyclooctane at relatively low temperatures of 80 °C. Depending on the used solvent cyclooctene or cyclooctatriene can be received as the single product, respectively. The lithium and alumina complexes [Li(C6H3-2,6-(CH2O(CH2)2(CF2)5CF3)2)] (21), [Li(C6H3 2,6 (CH2OCH3)2)] (22), [Al((C6H3)-2,6-(CH2O(CH2)2(CF2)5CF3)2)(CH3)2] (28), [Al((C6H3)-2,6-(CH2OCH3)2)I2] (29), and [Al((C6H3)-2,6-(CH2O(CH2)2(CF2)5CF3)2)I2] (31) were synthesized and characterized succesfully. With the complex [Al((C6H3)-2,6-(CH2O(CH2)2(CF2)5CF3)2)I2] (31) a diversity of aromatic compounds like benzene, toluene or pentafluorobenzene can be coupled after dehydrogenation. Moreover, the palladium complexes [Pd(Cl)((C6H3)-2,6-(CH2O(CH2)2(CF2)5CF3)2)] (34), [Pd(NCCH3)((C6H3)-2,6-(CH2O(CH2)2(CF2)5CF3)2)][PF6] (36), [Pd(Cl)((C6H3)-2,6-(CH2OCH3)2] (37) and [Pd(NCCH3)((C6H3)-2,6-(CH2OCH3)2][PF6] (38) were synthesized and characterized.

The development of a palladium-catalyzed intramolecular carboesterification of unactivated olefins is described. Olefin difunctionalization is a powerful tool for adding complexity to a molecule, and this formal [3+2] cycloaddition generates highly functionalized fused ring systems. Initially discovered by Dr. Yang Li in our group, it was found that when propiolic acids with a pendant terminal olefin were treated with 1 mol % Pd(MeCN)2Cl2, 3 equivalents of copper (II) chloride, and 3 equivalents of lithium chloride in acetonitrile at 50 °C, cyclization occurred in up to 90% yield. The optimization of this reaction and the extension to propiolamides and propargyl alcohols is described in this thesis. A mechanism involving a novel palladium-carboxylate species is proposed. Preliminary investigations into the synthesis of chiral sulfoxide pincer ligands are also described. The nucleophilic aromatic substitution of 1,3-dibromobenzene and 2,6-dichloropyridine with various thiols, followed by oxidation of the sulfides to sulfoxides is investigated as a route to the desired proligands.

The development of a palladium-catalyzed intramolecular carboesterification of unactivated olefins is described. Olefin difunctionalization is a powerful tool for adding complexity to a molecule, and this formal [3+2] cycloaddition generates highly functionalized fused ring systems. Initially discovered by Dr. Yang Li in our group, it was found that when propiolic acids with a pendant terminal olefin were treated with 1 mol % Pd(MeCN)2Cl2, 3 equivalents of copper (II) chloride, and 3 equivalents of lithium chloride in acetonitrile at 50 °C, cyclization occurred in up to 90% yield. The optimization of this reaction and the extension to propiolamides and propargyl alcohols is described in this thesis. A mechanism involving a novel palladium-carboxylate species is proposed. Preliminary investigations into the synthesis of chiral sulfoxide pincer ligands are also described. The nucleophilic aromatic substitution of 1,3-dibromobenzene and 2,6-dichloropyridine with various thiols, followed by oxidation of the sulfides to sulfoxides is investigated as a route to the desired proligands.

Transition metals are known to be the essential part in most of the catalysts, the heterogeneous and the homogenous catalysts; however, the ligands that attached to the metal centers can also alter the reactivity of the catalyst, and that is widely observed in nature. In our project, we are interested in the metal-ligand cooperation of a special type of ligand called the pincer ligand. Our focus is mainly on the tridentate Pincer Ligands with a pyridine backbone. Also, it contains a spacer that could be deprotonated and protonated during the aromatization and dearomatization process. Aromatization and dearomatization of the pincer ligand are responsible for the unique reactivity of the pincer complexes, especially in the hydrogenation and dehydrogenation reactions. Recently, huge developments have been made in the dehydrogenative coupling of aniline and benzyl alcohol via manganese pincer complexes. The most recent papers on that subject have been done by Beller in 2016[1], Kempe 2018 [2], and Hultzsch 2019 [3]. However, rhenium complexes have not been studied enough even though it is in the same seventh row of the transition metal. Therefore, the rhenium was studied as a possible alternative. Then, the synthesis of a well-defined PN3 rhenium complex was performed from the bipy-tBu ligand and the metal precursor Re(CO)5Cl. The ligand has a unique deformity as the phosphine sidearm is not attached to the metal center. Further investigation of the aniline and benzyl alcohol dehydrogenative coupling via PN3 rhenium pincer complex has been done. An optimal reaction condition was achieved, and the substrate scope was further examined with various alcohols and amines, and the result shows good to moderate conversion with decent selectivity towards the imine. Except for the secondary alcohols.

N-Heterocyclic carbenes (NHCs) are one of the few ligand systems that can finely tune transition metal catalysts via sterics and electronics. The strong sigma-donating properties of these ancillary ligands allow the development of robust tridentate NHC pincer framework, which has emerged as an alternative to the phosphine pincer ligands. The combination of NHC and pincer systems has resulted in a new generation of catalytically active organometallic complexes reported throughout the literature. -NHC Rh pincer complexes were found to be catalytically active in C-C and C-B bond formation via 1,4dition reactions. In addition, the in-situ generated -NHC Ir(H) pincer complex demonstrated catalytic activity in borylation of arene C-H bonds. Preliminary results are comparable to the C-H borylation results published by Hartwig and co-workers. The -NHC Ir(H) pincer complex may also prove to be a suitable catalyst for alkane dehydrogenation, due to framework similarities of the highly active and durable PCP and POCOP pincer hydride systems. Expansion of group 9 metal sources for transmetalation of the -NHC Zr pincer complex afforded the development of -NHC Rh(CO) and -NHC Co complexes. Group 9 metal carbonyl complexes have been reported as active catalysts in photocatalytic C-H activation of small molecules. Testing of Co sources for transmetalation afforded three rare Co pincer complexes, and the first examples of -NHC Co pincer complexes to date. Development of -NHC pincer complexes with base metals provide cost-effect alternatives to pincer systems with precious metal centers, and is reported herein.

The first stable “free” N-heterocyclic carbene (NHC) was isolated in 1991, since then many classes of NHCs were synthesized and isolated. N-heterocyclic carbene ligands have become a common ligand in the synthesis of metal complexes, due to their strong σ-donation with poor πeptor ability. Among all the NHC metal complexes, Hollis group was interested in exploring -NHC pincer complexes. In 2005, Hollis and co-workers synthesized and isolated first achiral -NHC zirconium pincer amido complex and studied its catalytic ability to perform intramolecular hydroamination of aminoalkenes. Later, Hollis group designed and developed new NHCs with a saturated chiral backbone, and used these new ligands to generate a new series of metal complexes. This dissertation is focused on the design, synthesis and characterization of several chiral bi-dentate NHC ligands with varying substituents on the backbone. These NHC ligands were used to synthesize several chiral mono, bis and tris amido -NHC zirconium pincer complexes. Further, transmetallation to cobalt complexes were achieved. Intramolecular hydroamination was measured for several chiral bis and tris amido -NHC pincer zirconium catalysts. Variants of these catalytic systems were generated with differing steric hinderance and electronic structure. The enantioselectivity of these novel new molecules were studied.