Synthesis and Ligand Properties of Chelating Bis(N-heterocyclic carbene)-Stabilized Bis(phosphinidenes)

NHC-phosphinidene (NHCP) adducts are an emerging class of ligands with proven binding ability for main group and transition metal elements. They possess electron-rich P atoms with two lone pairs (LPs) of electrons, making them interesting platforms for the formation of multimetallic complexes. We describe herein a modular, high-yielding synthesis of bis(NHCP)s, starting from alkylidene-bridged bis(NHC)s ((IMe)₂C_nH_2n; n = 1,3) and triphosphirane (PDip)₃ (Dip = 2,6-iPr₂C₆H₃) as phosphinidene transfer reagent. The coordination chemistry of [{DipP(IMe)}₂CH₂], 1, was studied in detail, and complexes [1·FeBr₂] and [1·Rh(cod)]Cl were prepared, showing that the ligand has a flexible bite angle. The dicarbonyl complex [1·Rh(CO)₂]Cl, with an average value for the CO stretching frequency of 2029 cm^-1, indicates a strongly donating ligand when compared to related complexes. The binding ability of the remaining two phosphorus LPs was demonstrated with AuCl(SMe₂), giving the heterotrimetallic complex [1·(AuCl)₂·Rh(cod)]Cl. Moreover, [1·Rh(cod)]X (X^- = Cl, B(3,5-(CF₃)₂-C₆H₃)₄) was tested in the catalytic hydrogenation of methyl-Z-α-acetamidocinnamate (MAC) and dimethyl itaconate (ItMe₂), revealing that the chloride complex was inactive, while the BAr^F complex demonstrated moderate activity. Additionally, [1·Rh(cod)]Cl was shown to be moderately air- and moisture-stable, slowly decomposing to the corresponding NHC-stabilized bis-dioxophosphorane, which was independently synthesized by treating the free ligand with dry O₂.