Sterically Controlled Synthesis and Nucleophilic Substitution Reactions of Di- and Trimeric N-Heterocyclic Phosphenium Metal(0) Halides

The reaction of symmetrical bis(diazaphospholenyl) compounds with [MCl2(cod)] (M = Pd, Pt; cod = 1,5‐cyclooctadiene) has been used to prepare N‐heterocyclic phosphenium (NHP) metal(0) halides [M(NHP)Cl]n with diverse N substituents. Characterisation by ESI‐MS and NMR spectroscopy revealed that N‐tBu‐ and N‐aryl‐substituted NHP precursors yield trimeric (n = 3) and dimeric (n = 2) products, respectively; the degree of aggregation is presumably controlled by the different steric requirements of the NHP moiety. A single‐crystal XRD study of a trimeric Pd complex confirmed the proposed constitution of the complexes with μ2‐bridging NHP and terminal chlorido ligands and allowed their metrical parameters to be analysed for the first time undisturbed by crystallographic disorder. Studies of ligand substitution and redistribution processes revealed that the complexes withstand exchange of the μ‐bridging NHP units but tolerate substitution of the terminal chlorido ligands by other strong nucleophiles (SCN–, I–) with conservation of the oligomeric framework. Attempts to replace chlorido by thiolato ligands led to the discovery of a reaction between a phosphenium metal thiolate [Pd(NHP)(SR)]3 and CH2Cl2 to give a formaldehyde dithioacetal H2C(SR)2 (R = benzyl). This reaction could be developed into a protocol for a C–S cross‐coupling reaction in the presence of a catalytic amount of the phosphenium complex. The size of the bridging ligand controls whether N‐heterocyclic phosphenium metal(0) chlorides [MCl(NHP)]n (M = Pd, Pt) form dimers or trimers. The oligomers resist attempts to exchange bridging NHP units, but easily undergo displacement of the chlorido terminal ligands. This reaction can be employed in a catalytic synthesis of formaldehyde dibenzylthioacetal.