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

Abstract

The coordinating properties of NHC (A), phenolate (B) and phosphonium ylide (C) moieties have been investigated systematically through the preparation of a family of NHC, phosphonium ylide-based pincer ligands where the third donor extremity can be either a NHC, a phenolate or a phosphonium ylide. The overall donor character of such ligands [NHC(AaBbCc)] (a + b + c = 2) has been analyzed by comparison of the MOs (energy and shape), oxidation potentials (Ep ox), and IR nCO and nCN stretching frequencies of their isostructural pincer Pd(II) complexes [NHC(AaBbCc)PdL][OTf] (L = NCCH3, CO or CNtBu). The three categories of pincer complexes based on phosphonium ylides were easily obtained by acidic treatment of their highly stable ortho-metallated Pd(II) precursors prepared in a single step from readily available N-phosphonio-substituted imidazolium salts. Analysis of IR data indicated that NHC and phenolate ligands have a similar donor character but which remains lower than that of the phosphonium ylide. The impact on catalytic performance of the incorporation of a second strongly donating phosphonium ylide into the ligand architecture has been illustrated in the Pd-catalyzed allylation of aldehydes.

NHC Core Pincer Ligands Exhibiting Two Anionic Coordinating Extremities

“…With the previous cases 47-48, the Pd(II) complex 52 constitutes one of the very rare examples of NHC core pincer complex of LX2-type exhibiting two different peripheral coordinating extremities reported to date [93]. The latter, featuring pending phenolate and phosphonium ylide moieties, was readily obtained from the ortho-metallated Pd complex 51 in 94% yield, thanks to the selective acid cleavage of a Car-Pd bond using HOTf in MeCN (Scheme 18).…”

“…The electronic properties of these phosphonium ylide-based pincer Pd(II) complexes were exploited in homogeneous catalysis for the Pd-catalyzed allylation of aldehydes. It was in particular observed that the Pd complex bearing the most donor pincer ligand, namely the NHC, bis(ylide), wasthe most active in this catalytic process [93].…”

“…Notably, the IR ν CN frequency values of Pd complexes 53 (2207 cm −1 ) and 55 (2206 cm −1 ) indicate that the C,C,O-NHC, phenolate, phosphonium ylide, and the C,C,Cbis(NHC) phosphonium ylide have similar electronic properties. These IR values, which appear at higher frequency than that of the Pd-CNtBu complex 41 (2194 cm −1 ), bearing the C,C,Cbis(ylide) NHC ligand lead to the conclusion that the substitution of a NHC or a phenolate for a phosphonium ylide increases significantly the donor character of corresponding pincer ligands (Scheme 19) [93]. The Pd−CO complex 42 only experimentally observed in the case of the bis(ylide) ligand is in perfect agreement with these findings.…”

“…[94,95] Scheme 19. Experimental IR νCO and νCN stretching frequencies (cm -1 ) for NHC core phosphonium ylide-based pincer Pd(II) complexes (L = CNtBu or CO) [93].…”

“…Scheme 18. Preparation of the C,C,O-phenolate, NHC, phosphonium ylide pincer Pd(II) complex 52 from the imidazolium salt 49 following two different routes[93].…”