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

As an extension of the previous work on the post-modification strategy of pyridine-based PN3P group 10 metal complexes, the triazine-based PN3P pincer ligands incorporating nickel complexes, Me-Et-PN3PNiCl, and Me-Et-PN3PNiI, were synthesized and characterized. The solid state structures suggest that the N-donor atom of triazine-based PN3P ligands have more electron donating than the pyridine-based PNP pincer ligands. A new non-symmetric PONNP pincer ligand system was developed to: (1) give more parameters for electronic and steric properties (2) to block the influence of acidic proton. However, the unstable metal-hydride complexes indicate the degradable O–P bond or N–P bond of the PONNP ligand. Treating the three ligands, including tert-butyl, phenyl, and cyclopentyl substituents with NiCl2(DME) individually all resulted in the production of [(PtBuONNPtBu)NiCl]+Cl-. It suggests that both O‒P and N‒P can cleave and rearrange during the complexation. The solvent effect and time tracing experiments demonstrated that the O‒P and N‒P bond rearrangement occurs after forming the nickel complex. The finding of [(ONNPtBu)NiCl]22+(Cl-)2 indicates the weakness of the O‒P bond, suggesting the degradation of an oxygen-phosphorus bond may be the initial step of substituents rearrangement. To ensure the ligand-centered reactivity of (PtBuONNPtBu)*NiCl, a nickel-silver bimetallic complex, [(PtBuONNPtBu)*NiCl]2[AgOTf]2, was produced. In contrast to pyridine-based PN3P*NiH, there are two active sites of [(PtBuONNPtBu)*NiCl]2[AgOTf]2, the N atoms on the imine arm and pyrimidine ring. The solid-structure of the acid-base adduct compound, {[(PtBuONNPtBu)*NiCl][B(C6F5)3], demonstrates that the nitrogen atom on the pyrimidine ring is a better active site than the imine arm because of the steric effect. The molecular structures of [(PtBuONNPtBu)*NiCl]2[AgOTf]2 and {[(PtBuONNPtBu)*NiCl][B(C6F5)3] exhibit that the pyrimidine ring of [(PtBuONNPtBu)*NiCl]2[AgOTf]2 tends to be an aromatic zwitterionic form. The ligand backbone of {[(PtBuONNPtBu)*NiCl][B(C6F5)3] favors the dearomatized form.