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Abstract:  The choice of ligand platforms is crucial to the organometallic coordination chemistry and homogeneous catalysis. Among various ligand platforms, the pincer ligands offer a convenient route to manipulate the steric and electronic properties of resulting  complexes. The pincer chemistry of rhodium has attracted attention over 40 years, and these complexes are dominated by Rh(I) and Rh(III) low-spin states, thus they are more predictable than other paramagnetic species. Compared to the other pincer ligand platforms, the pyridine-based pincer complexes are particularly attractive, exhibiting the diversified reactivities. Our group realized the new class of PN3(P) pincer system, altering the unique catalytic performances, thermodynamic and kinetic properties due to the pseudo-dearomatized nature, showing huge potential for different reactivities in bond activation and catalysis.

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) with KN(SiMe3)2 as the base. Furthermore, a benzoyl complex cPePN3PRh(CO)(C6H5) was furnished by treating a benzene solution of cPePN3PRh(C6H5) with CO gas. In a diluted-HCl condition, a high yield of 90% benzaldehyde was formed with the regeneration of cPePN3PRhCl. This is the first example of selective carbonylation of benzene into benzaldehyde accomplished by inserting CO gas directly without irradiation.
In Chapter 3, the ligand-centered reactivity of a pseudo-dearomatized  PN3P* rhodium complex towards molecular oxygen was realized. For the dearomatized rhodium carbonyl complex (tBuPN3P*RhCO), one of the C−H bonds of pseudo-dearomatized pyridine ring was oxidized by O2 to create an α, β-unsaturated carbonyl functionality, showcasing an unprecedented ligand-centered reactivity with the central metal Rh(I) remaining untouched. 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 the tBuPN3P*RhCO, a series of second-generation diimine-amido PN3P-pincer carbonyl complexes were synthesized by the reaction of tBuPN3P*RhCO and various alkyl/benzyl halides via the post-modification strategy, and these complexes were characterized well by NMR, HRMS, FTIR, and single crystal diffraction.

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• CONTACTLinda J. Sapolu
• EMAILLinda.Sapolu@kaust.edu.sa