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

Abstract

A new family of pincer palladium(II) complexes bearing an electron-rich C,C,C-NHC, diphosphonium bis(ylide) ligand of LX₂-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 (sp²), arylic (sp²), and chiral ylidic (sp³). Protonation of the latter afforded NHC, diphosphonium bis(ylide) pincer Pd(II) complexes as a mixture of meso- and dl-diastereomers (de = 50%). The selectivity of C-coordination was rationalized on the basis of DFT calculations, evidencing the quasi-degeneracy of the two diastereomeric forms.