Novel access to carbodiphosphoranes in the coordination sphere of group 10 metals: template synthesis and protonation of PCP pincer carbodiphosphorane complexes of C(dppm)2

Abstract

In a novel template synthesis of carbodiphosphoranes (CDPs), the phosphine functionalized CDP ligand C(dppm)₂ (dppm = Ph₂PCH₂PPh₂) is formed in the coordination sphere of group 10 metals from CS₂ and 4 equivalents of dppm. The products are the PCP pincer complexes [M(Cl)(C(dppm)₂-κ3P,C,P)]Cl (M = Ni, Pd, Pt) and 2 equivalents of dppmS. The compound C(dppm)₂, which is composed of a divalent carbon atom and two dppm subunits, represents a new PCP-type pincer ligand with the formally neutral carbon Lewis base of the CDP functionality as the central carbon. Treatment of [M(Cl)(C(dppm)₂-κ3P,C,P)]Cl (M = Pd, Pt) with hydrochloric acid results in protonation at the CDP carbon atom and the formation of the PCP pincer complexes [M(Cl)(CH(dppm)₂-κ3P,C,P)]Cl₂ (M = Pd, Pt). The PCP pincer ligand [CH(dppm)₂]⁺ involves a formally cationic central carbon donor. The reaction of [Ni(Cl)(C(dppm)₂-κ3P,C,P)]Cl with HCl leads to the extrusion of NiCl₂ and formation of the diprotonated CDP compound [CH₂(dppm)₂]Cl₂, from which the monoprotonated conjugate base [CH(dppm)₂]Cl is obtained upon addition of bases, such as NH₃. The crystal structures of [M(Cl)(C(dppm)₂-κ3P,C,P)]Cl (M = Ni, Pd, Pt), [Ni(Cl)(C(dppm)₂-κ3P,C,P)]₂[NiCl₄], [M(Cl)(CH(dppm)₂-κ3P,C,P)]Cl₂ (M = Pd, Pt) as well as [CH₂(dppm)₂]Cl₂ and [CH(dppm)₂]Cl are presented. A comparison of the solid state structures reveals interesting features, e.g. infinite supramolecular networks mediated by C–H⋯Cl hydrogen bond interactions and an unexpected loss of molecular symmetry upon protonation in the complexes [M(CH(dppm)₂-κ3P,C,P)(Cl)]Cl₂ (M = Pd, Pt) as a result of the flexible ligand backbone. Additionally the new compounds were characterized comprehensively in solution by multinuclear ³¹P, ¹³C and ¹H NMR spectroscopy: Several spectroscopic parameters show a striking variability in particular regarding the carbodiphosphorane functionality. Furthermore the compound [Ni(Cl)(C(dppm)₂-κ3P,C,P)]Cl was examined by cyclic voltammetry (CV) and could be shown to display quasi-reversible oxidative as well as reductive behaviour.