Synthesis and Ligand Properties of Chelating Bis( <i>N</i> -heterocyclic carbene)-Stabilized Bis(phosphinidenes)

Abstract

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) 2 C n H 2n; n = 1,3) and triphosphirane (PDip) 3 (Dip = 2,6- i Pr 2 C 6 H 3 ) as phosphinidene transfer reagent. The coordination chemistry of [{DipP(IMe)} 2 CH 2 ], 1, was studied in detail, and complexes [ 1 ·FeBr 2 ] 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(SMe 2 ), giving the heterotrimetallic complex [ 1 ·(AuCl) 2 ·Rh(cod)]Cl. Moreover, [ 1 ·Rh(cod)]X (X – = Cl, B(3,5-(CF 3 ) 2 -C 6 H 3 ) 4 ) was tested in the catalytic hydrogenation of methyl- Z -α-acetamidocinnamate (MAC) and dimethyl itaconate (ItMe 2 ), 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 O 2 .

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