Donor-Free Phosphenium–Metal(0)–Halides with Unsymmetrically Bridging Phosphenium Ligands

Reactions of (cod)­MCl₂ (cod = 1,5 cyclooctadiene, M = Pd, Pt) with <i>N</i>-heterocyclic secondary phosphines or diphosphines produced complexes [(NHP)­MCl]₂ (NHP = <i>N</i>-heterocyclic phosphenium). The Pd complex was also accessible from a chlorophosphine precursor and Pd₂(dba)₃. Single-crystal X-ray diffraction studies established the presence of dinuclear complexes that contain μ-bridging NHP ligands in an unsymmetrical binding mode and display a surprising change in metal coordination geometry from distorted trigonal (M = Pd) to T-shaped (M = Pt). DFT calculations on model compounds reproduced these structural features for the Pt complex but predicted an unusual <i>C</i>_2<i>v</i>-symmetric molecular structure with two different metal coordination environments for the Pd species. The deviation between this structure and the actual centrosymmetric geometry is accounted for by the prediction of a flat energy hypersurface, which permits large distortions in the orientation of the NHP ligands at very low energetic cost. The DFT results and spectroscopic studies suggest that the title compounds should be described as phosphenium–metal(0)–halides rather than conventional phosphido complexes of divalent metal cations and indicate that the NHP ligands receive net charge donation from the metals but retain a distinct cationic character. The unsymmetric NHP binding mode is associated with an unequal distribution of σ-donor/π-acceptor contributions in the two M–P bonds. Preliminary studies indicate that reactions of the Pd complex with phosphine donors provide a viable source of ligand-stabilized, zerovalent metal atoms and metal(0)–halide fragments

Phosphenium Hydride Reduction of [(cod)MX₂] (M = Pd, Pt; X = Cl, Br): Snapshots on the Way to Phosphenium Metal(0) Halides and Synthesis of Metal Nanoparticles

The outcome of the reduction of [(cod)­PtX₂] (X = Cl, Br; cod = 1,5-cyclooctadiene) with N-heterocyclic phosphenium hydrides ^RNHP–H depends strongly on the steric demand of the <i>N</i>-aryl group R and the nature of X. Reaction of [(cod)­PtCl₂] with ^DippNHP–H featuring bulky N-Dipp groups produced an unprecedented monomeric phosphenium metal(0) halide [(^DippNHP)­(^DippNHP–H)­PtCl] stabilized by a single phosphine ligand. The phosphenium unit exhibits a pyramidal coordination geometry at the phosphorus atom and may according to DFT calculations be classified as a Z-type ligand. In contrast, reaction of [(cod)­PtBr₂] with the sterically less protected ^MesNHP–H afforded a mixture of donor-ligand free oligonuclear complexes [{(^MesNHP)­PtBr}_<i>n</i>] (<i>n</i> = 2, 3), which are structural analogues of known palladium complexes with μ₂-bridging phosphenium units. All reductions studied proceed via spectroscopically detectable intermediates, several of which could be unambiguously identified by means of multinuclear (¹H, ³¹P, ¹⁹⁵Pt) NMR spectroscopy and computational studies. The experimental findings reveal that the phosphenium hydrides in these multistep processes adopt a dual function as ligands and hydride transfer reagents. The preference for the observed intricate pathways over seemingly simpler ligand exchange processes is presumably due to kinetic reasons. The attempt to exchange the bulky phosphine ligand in [(^DippNHP)­(^DippNHP–H)­PtCl] by Me₃P resulted in an unexpected isomerization to a platinum(0) chlorophosphine complex via a formal chloride migration from platinum to phosphorus, which accentuates the electrophilic nature of the phosphenium ligand. Phosphenium metal(0) halides of platinum further show a surprising thermal stability, whereas the palladium complexes easily disintegrate upon gentle heating in dimethyl sulfoxide to yield metal nanoparticles, which were characterized by TEM and XRD studies