Synthesis and ligand properties of chelating bis(N-heterocyclic carbene)-stabilized bis(phosphinidenes)

Abstract

N-heterocyclic carbene-phosphinidene adducts (NHPs) are an emerging class of ligands with proven binding ability for main group and transition elements. They can be considered as inversely polarized phosphaalkenes with electron-rich P atoms possessing two lone pairs of electrons, which render them interesting platforms for the formation of multi-metallic complexes. We describe herein a modular, high-yielding synthesis of bis(NHP)s, starting from alkylidene-bridged bis(NHC)s ((IMe)}2CnH2n; n = 1,3) and a triphosphirane (cyclo-P3Dip3) (Dip = 2,6-iPr2C6H3) as phosphinidene transfer reagents. The coordination chemistry of [{DipP(IMe)}2CH2], 1, was studied in detail, and the complexes [1∙FeBr2] and [1∙Rh(cod)]Cl were prepared, showing that the ligand has a flexible bite-angle ranging between 86-106°. [1∙Rh(cod)]Cl was converted into the corresponding dicarbonyl complex [1∙Rh(CO)2]Cl, with an average value for the CO stretching frequency of 2029 cm−1, which indicates a strongly donating ligand, when compared to related bis(NHC) rhodium complexes. The binding ability of the remaining two phosphorus lone pairs was demonstrated via addition of AuCl(SMe2), which generated the hetero-trimetallic complex [1∙(AuCl)2∙Rh(cod)]Cl. Moreover, [1∙Rh(cod)]X (X− = Cl, B(3,5-CF3-C6H3)4) were tested in the catalytic hydrogenation of the α,β-unsaturated esters methyl-Z-α-acetamidocinnamate (MAC) and dimethyl itaconate (ItMe2), revealing that the chloride complex was inactive, while the BArF-complex demonstrated moderate activity, lower than that of analogous complexes incorporating classical diphosphines such as DiPAMP. In addition, [1∙Rh(cod)]Cl was shown to be moderately air- and moisture stable, slowly decomposing to the corresponding NHC-stabilized bisdioxophosphorane, which was independently synthesized by treating the free ligand with dry O2.