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

Abstract

The reaction of readily available imidazolium−phosphonium salt [MesIm(CH 2 ) 3 PPh 3 ](OTf) 2 with PdCl 2 in the presence of an excess of Cs 2 CO 3 afforded selectively in one step the cationic Pd(II) complex [(C,C,C)Pd(NCMe)]-(OTf) exhibiting an LX 2 -type NHC-ylide-aryl C,C,C-pincer ligand via formal triple C−H bond activation. The replacement of labile MeCN in the latter by CNtBu 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 [(C,C,C)Pd(CO)](OTf) represents a rare example of Pd(II) carbonyl species stable at room temperature and characterized by X-ray diffraction analysis. The treatment of isostructural cationic complexes [(C,C,C)Pd(NCMe)](OTf) and [(C,C,C)Pd-(CO)](OTf) with (allyl)MgBr and nBuLi led to the formation of zwitterionic phosphonium organopalladates [

Carbenes and phosphonium ylides: a fruitful association in coordination chemistry

“…[60][61][62] imidazolium salt 25 bearing a mesityl group under orthometallation conditions, namely the addition of PdCl2 in the presence of Cs2CO3 led selectively in one step to the cationic Pd(II) complex 26 featuring the C,C,C-NHC-ylide-aryl pincer ligand (Scheme 11). 63 The formation of this complex results from a formal triple C-H bond activation of the cationic precursor. The overall donor character of this new phosphonium ylide core pincer ligand evaluated on the basis of nCO and nCN stretching frequencies of complexes 27 was found to be even higher than that of the C,C,C-NHC core, bis(phosphonium ylide) ligand of complex 23.…”

“…65 The unique electronic properties of the ylide core pincer backbone were also at the origin of the preparation of rare examples of zwitterionic phosphonium organopalladates 28 featuring bromide and acyl co-ligands obtained by the treatment of cationic pincer complexes 26 and 27b with (allyl)MgBr and nBuLi, respectively. 63 The X nature of the trans-ylide coordinated ligand accounts for the transfer of the negative charge initially located at the C-ylidic extremity to the Pd atom with partial delocalization over the bromide and acyl co-ligands.…”

“…Preparation of phosphonium ylide core NHC, aryl pincer Pd(II) complexes 26-28 from the N-phosphonio substituted NHC precursor 25. 63 The generalization of this straightforward approach to the preparation of other transition metal pincer complexes should be explored as well as their reactivity towards small molecules and catalytic activity in transformations requiring strongly donating complexes.…”

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

“…In accordance with the second scenario, chelating NHC-triphenylphosphonium ylide ligands featuring an insulating C 3 propyl spacer were recently devised in the bi-, tri-, and tetradentate series and coordinated to various Pd­(II) centers. In the pincer series, the electron-rich character of the ligand was found to be beneficial for the stabilization of Pd–CO adducts , and the Pd-catalyzed allylation of aldehydes .…”

Experimental and theoretical insights into thetransinfluence of organo-sulfur and -selenium ligands in 5,6-membered palladium(ii) cationic pincer complexes based on iminophosphoranes

“…Analyzing the ratio |V(r c )|/G(r c ) for the four palladium-ligand bonds on the four complexes refer to the intermediate bond regime (1 o |V(r c )|/G(r c ) o 2) located between ionic and covalent bonding. 52,53 In addition, the negative energy density of these bonds [V(r c ) o 0 and H(r c ) o 0] further defines the Pd-L bonds as dative bonds in 4a-d according to Bianchi's classification. 51 The values of electron density r(r c ) at the BCP of the four Pd-L bonds in complexes 4a-d are reported in Table 4.…”