Phosphine-NHC-Phosphonium Ylide Pincer Ligand: Complexation with Pd(II) and Unconventional -Coordination of the Ylide Moiety.

An efficient synthesis of two pincer preligands [PhPCH(R)ImCHCHCHPPh]X (R = H, X = OTf; R = Ph, X = BF) was developed. Subsequent reactions with PdCl and an excess of CsCO led to the formation of highly stable cationic metalated Pd(II) complexes [(,,,)Pd]X exhibiting phosphine, NHC, phosphonium ylide, and σ-aryl donor extremities. The protonation of one of the latter complexes with R = H affords the Pd(II) complex (,,)Pd(MeCN) bearing an unprecedented nonsymmetrical NHC core pincer scaffold with a 5,6-chelating framework. The overall donor properties of this phosphine-NHC-phosphonium ylide ligand were estimated using the experimental ν stretching frequency in the corresponding (,,)Pd(CNBu derivative and were shown to be competitive with the related bis(NHC)-phosphonium ylide and phenoxy-NHC-phosphonium ylide pincers. The presence of a phenyl substituent in the bridge between phosphine and NHC moieties in the metalated complex (,,,)Pd makes possible the deprotonation of this position using LDA to provide a persistent zwitterionic complex [(,,,)Pd] featuring a rare -coordinated phosphonium ylide moiety in addition to a conventional -coordinated one. The comparison of the P and C NMR data for these - and -bound phosphonium ylide fragments within the same molecule was performed for the first time, and the bonding situation in both cases was studied in detail by QTAIM and ELF topological analyses.