Conformationally Constrained N‑Heterocyclic Phosphine–Diimine with Dual Functionality

Condensation of octahydro-2,2′-bipyrimidine with P­(NMe₂)₃ gave a 1,3,2-diazaphospholidine–4,5-diimine <b>4a</b> in which the “open” (exo/exo) conformation of the diimine unit was enforced by incorporation into a tricyclic molecular backbone. The coordination behavior of this potentially ambident ligand was sampled in reactions with ([(nbd)­W­(CO)₄] and [CpCo­(CO)₂]) and pnictogen halides ECl₃ (E = P, As, Sb). While PCl₃ reacted under ring metathesis, all other reactions gave isolable complexes of composition (<b>4a</b>)­ML_<i>n</i> (ML_<i>n</i> = W­(CO)₅, CpCo­(CO), AsCl₃, SbCl₃); attempted recrystallization of the As-adduct yielded a complex (<b>4a</b>)­(AsCl₃)₂ which was also accessible from reaction of <b>4a</b> with 2 equiv of AsCl₃. Single-crystal X-ray diffraction studies revealed that the ligand in [(<b>4a</b>)­W­(CO)₅] and [(<b>4a</b>)­CpCo­(CO)] binds through its phosphorus lone-pair; [(<b>4a</b>)­SbCl₃] and [(<b>4a</b>)­(AsCl₃)₂] contain a T-shaped ECl₃ unit which binds to the chelating diimine moiety, and associate further via chloride bridges to give centrosymmetric dimers. Reactions of <b>4a</b> with excess metal substrates gave no evidence that formation of bimetallic complexes with μ-bridging 1κ²(N,N′)-2κP-coordination is feasible; the extra AsCl₃ moiety in [(<b>4a</b>)­(AsCl₃)₂] avoids this coordination mode by interacting with the peripheral chlorides of the central core. The observed selectivity suggests that ligand <b>4a</b> specifically addresses transition metal centers with low positive charge and some back-bonding capacity through the phosphorus lone-pair, and electrophiles that behave essentially as “pure” Lewis acids through the diimine unit. This assumption was confirmed by DFT studies which disclosed further that binding of the first metal center deactivates the opposite binding site and thus strongly inhibits the formation of dinuclear complexes

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