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)](OTf) bearing an unprecedented nonsymmetrical NHC core pincer scaffold with a 5,6-chelating framework.

Direct Access to Palladium(II) Complexes Based on Anionic ,,-Phosphonium Ylide Core Pincer Ligand.

The reaction of readily available imidazolium-phosphonium salt [MesIm(CH)PPh](OTf) with PdCl in the presence of an excess of CsCO afforded selectively in one step the cationic Pd(II) complex [(,,)Pd(NCMe)](OTf) exhibiting an LX-type NHC-ylide-aryl ,,-pincer ligand via formal triple C-H bond activation. The replacement of labile MeCN in the latter by CNBu and CO fragments allowed to estimate the overall electronic properties of this phosphonium ylide core pincer scaffold incorporating three different carbon-based donor ends by IR spectroscopy, cyclic voltammetry, and molecular orbital analysis, revealing its significantly higher electron-rich character compared to the structurally close NHC core pincer system with two phosphonium ylide extremities. The pincer complex [(,,)Pd(CO)](OTf) represents a rare example of Pd(II) carbonyl species stable at room temperature and characterized by X-ray diffraction analysis.

NHC Core Pincer Ligands Exhibiting Two Anionic Coordinating Extremities.

The chemistry of NHCcore pincer ligands of LX type bearing two pending arms, identical or not, whose coordinating center is anionic in nature, is here reviewed. In this family, the negative charge of the coordinating atoms can be brought either by a carbon atom via a phosphonium ylide (RP-CR) or by a heteroatom through amide (RN), oxide (RO), or thio(seleno)oxide (RS, RSe) donor functionalities. Through selected examples, the synthetic methods, coordination properties, and applications of such tridentate systems are described.

NHC Core Phosphonium Ylide-based Palladium(II) Pincer Complexes: The Second Ylide Extremity Makes the Difference.

The coordinating properties of N-heterocyclic carbene (NHC) (A), phenolate (B), and phosphonium ylide (C) moieties were investigated systematically through the preparation of a family of NHC, phosphonium ylide-based pincer ligands, where the third donor extremity can be either an NHC, a phenolate, or a phosphonium ylide. The overall donor character of such ligands [NHC(ABC)] ( + + = 2) was analyzed by comparison of the molecular orbitals (energy and shape), oxidation potentials (), and IR ν and ν stretching frequencies of their isostructural pincer Pd(II) complexes [NHC(ABC)PdL][OTf] (L = NCCH, CO, or CNBu). The three categories of pincer complexes based on phosphonium ylides were easily obtained by acidic treatment of their highly stable -metalated Pd(II) precursors prepared in a single step from readily available N-phosphonio-substituted imidazolium salts.

Palladium(ii) pincer complexes of a C,C,C-NHC, diphosphonium bis(ylide) ligand.

A new family of pincer palladium(ii) complexes bearing an electron-rich C,C,C-NHC, diphosphonium bis(ylide) ligand of LX-type was prepared through the dual N-functionalization of 1H-imidazole by (3-bromopropyl)triphenylphosphonium bromide. Selected basic conditions allowed the sequential coordination of the NHC and phosphonium ylide moieties to Pd(ii). This strategy led to an original ortho-metallated complex where the Pd center is bonded to four carbon atoms of three different natures: carbenic (sp), arylic (sp), and chiral ylidic (sp).

A convenient access to N-phosphonio-substituted NHC metal complexes [M = Ag(i), Rh(i), Pd(ii)].

A series of NHC pre-ligands featuring a pendant phosphonium moiety attached to N' position through a propyl linker was readily obtained upon quaternization of N-mesityl- and N-methyl-imidazole, or N-mesityl-imidazoline using (3-bromo-propyl)-triphenylphosphonium bromide. Reactions of the resulting dicationic salts with [PdCl(allyl)] in the presence of KCO afforded the anticipated NHC-ligated Pd(ii) metal complexes in high yields. Further treatment of the latter complexes with tBuOK induced a deprotonation of the carbon atom adjacent to the phosphonium moiety and the formation of the corresponding C,C-chelating NHC-phosphonium ylide Pd(ii) complexes.