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

Abstract

Thanks to the + PCy3 substituents unlikely to undergo intramolecular C−H activation, a Pd(II) complex [(C,C,C)PdCl](OTf) exhibiting the 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 on the + PCy3 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 compared to the structurally related NHC core pincer bearing + PPh3 extremities. Treatment of the electron-rich pincer complex [(C,C,C)PdCl](OTf) with AgOTf led to the C,C-chelating NHC-phosphonium ylide Pd(II) complex [(C,C)Pd(OTf)Cl](OTf) via the oxidatively induced homolytic cleavage of a Pd−ylide bond. In the presence of stronger base (KHMDS or LDA) cyclometalation of a + P−Cy substituent was observed affording a constraint Pd(II) complex featuring the unique anionic LX3-type C,C,C,C-NHC, diphosphonium tris(ylide) ligand.

Exploring the Coordination Properties of Phosphonium‐Functionalized N‐Heterocyclic Carbenes Towards Gold

“…The deprotonation of the methylene group in alpha position to the phosphonium group has been already reported in the literature for palladium(II) complexes with similar ligands. [33][34][35]39] In our case, the formation of an ylide-metal bond was indeed suggested on the basis of the 31 P NMR spectrum, which exhibited two resonances around δ 29 ppm (29.6 (minor) and 29.5 (major) ppm for complex 5; 29.6 (major) and 28.8 (minor) ppm for complex 6) shifted of ca. 5 ppm to lower fields relative to phosphonium precursors or the gold(I) complexes 3 and 4.…”

“…The presence of two signals in the 31 P NMR spectrum can be justified by the occurrence of two conformational isomers in solution, [54] characterized by different orientation of the bridge between the two donors or of two diasteroisomers due to the presence of the stereogenic ylidic carbon atoms. [35,36,[38][39][40] Finally, the molecular structure of dinuclear complex 6 was definitively established by X-ray diffraction analysis on colourless crystals isolated by slow diffusion of diethyl ether into an acetonitrile solution of 6 (Figure 3).…”

“…[32] In the series of functionalized NHCs, those bearing a phosphonium group have recently been extensively investigated by Canac and co-workers with particular interest towards palladium(II) and rhodium(I) centres. [33][34][35][36][37][38][39][40] Importantly, this type of compounds can act as bidentate chelating ligands through the formation of a phosphonium ylide in addition to the NHC donor. Although phosphonium ylides are known since the early 20 th century, there is now a renaissance in the study of their coordination properties.…”

“…In the series of functionalized NHCs, those bearing a phosphonium group have recently been extensively investigated by Canac and co‐workers with particular interest towards palladium(II) and rhodium(I) centres [33–40] . Importantly, this type of compounds can act as bidentate chelating ligands through the formation of a phosphonium ylide in addition to the NHC donor.…”

The first synthesis of an isocyanide-functionalized imidazolium salt and transition metal complexes thereof

“…8 Cyclopentadienyl-functionalized NHCs and their related indenyl- and fluorenyl-NHCs have been used as ligands for early and late transition metal complexes, 9 and remarkable Pd( ii ) complexes containing phosphonium ylide-functionalized NHCs have been described. 10 With these precedents in mind and considering the ubiquity of isocyanide and NHC ligands in organometallic chemistry, we aimed to prepare an isocyanide-functionalized N-heterocyclic carbene, in order to have both functionalities fused in the same molecule. We have successfully found an approach for the synthesis of an isocyanide functionalized imidazolium salt and proved its suitability for the modular formation of mixed ligand isocyanide–NHC heterometallic complexes.…”