Structure, Electronics and Reactivity of Ce(PNP) Complexes

Abstract

Synthetic methods for the coordination of the monoanionic bis[2-(diisopropylphosphino)-4-methylphenyl]amido (PNP) ligand framework to the cerium(III) cation have been developed and applied for the isolation of a series of {(PNP)Ce} and {(PNP)₂ Ce} type complexes. The structures of the complexes were studied by X-ray diffraction and multinuclear NMR spectroscopy. We found that the cerium(III) ion can induce the elimination of one of the iPr groups at phosphorus to yield a new dianionic PNP tridentate framework (PNP^-iPr ) featuring a phosphido-donor functionality, which is bound to the cerium ion with the shortest known Ce-P bond of 2.7884(14) Å for molecular compounds. The reaction of the complex [(PNP)Ce{N(SiMe₃ )₂ }₂ ] (1) with Ph₂ CO gave the Ce-bound product of C-C coupling, ^- N(SiMe₃ )SiMe₂ CH₂ -CPh₂ O^- , through the C-H bond activation of a SiMe₃ group. ³¹ P NMR spectroscopy was used to estimate the presence of a vacant coordination position at the cerium ion in the Ce^III -PNP complexes by the examination of the δ(³¹ P) shift recorded both in non-polar (C₆ D₆ ) and polar ([D₈ ]THF) solvents. Moreover, ³¹ P NMR spectroscopy was also found to be a useful tool for the estimation of the Ce-P bond distances in {(PNP)Ce^III } and {(PNP)₂ Ce^III } systems. Electrochemical and computational studies for 1 and its lanthanum analogue containing a redox-innocent metal center revealed the stabilization of the Ce^III oxidation state by the PNP ligand.

Keywords: 31P NMR; PNP ligand; X-ray diffraction; cerium; lanthanide.