Diverse C-Coordination Modes of NHC-Tricyclohexylphosphonium Ylide Ligands in Palladium(II) Complexes

Thanks to the +PCy3 substituents which are unlikely to undergo intramolecular C-H activation, the Pd(II) complex [(C,C,C)PdCI] (OTf) having a neutral LX2-type NHC, diphosphonium bis(ylide) pincer ligand was selectively prepared by a double deprotonation of the [(NHC)PdCl2(Py)] precursor with tBuOK. The influence of the (+)PCy(3)substituents on the overall electronic properties of this pincer scaffold was evaluated by IR spectroscopy data of the corresponding Pd-CO adduct, revealing its stronger electron-donating character in comparison to the structurally related NHC core pincer bearing +PPh3 extremities. Treatment of the electron-rich pincer complex [(C,C,C)PdCI](OTf) with AgOTf led to the C,C-chelating NHC-phosphonium ylide Pd(II) complex [(C,C)Pd(OTOCIYOTO via the oxidatively induced homolytic cleavage of a Pd-ylide bond. In the presence of a stronger base (KHMDS or LDA) cyclometalation of a P+-Cy substituent was observed, affording a constrained Pd(11) complex featuring the unique anionic LX3-type C,C,C,C-NHC, diphosphonium tris(ylide) ligand.

Automated summary

In this study, different Pd(II) complexes were synthesized and their electronic properties were evaluated by IR spectroscopy data. The impact of different substituents on the overall electronic properties of the complexes was investigated. Furthermore, the effect of oxidatively induced reactions on the structure of the complexes was studied.