Multi-site reduction of hexachlorophosphazene to low-valent PN heterocycles and extension to the reduction of poly-chlorophosphazene

Abstract

Facile one and two site reduction of hexachlorophosphazene using cyclic (alkyl)(amino)carbene substituents are shown to yield P-CAAC^Me-cyclo-(PNP(Cl)₂NP(Cl)₂N) 1 and P,P′-bis-CAAC^Me-cyclo-(PNPNP(Cl)₂N) 2 (CAAC^Me = 1-[2,6-bis(isopropyl)phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene), respectively. Compound 1 is characterized by its predominantly phosphorus-centered HOMO, which results in typical phosphine-type nucleophilic and reductive reactivity; however, the resultant compounds of such reactions feature properties distinct from their classical phosphine analogues due to the CAAC-centered LUMO, which acts as an acceptor for both intramolecular interactions and photophysical excitations. In contrast, compound 2 exhibits π-conjugation spanning the endocyclic PNP moiety and the two CAAC^Me substituents, despite its non-planar structure. Treatment of 2 with [Cp*RuCl]₄ results in the electrophilic displacement of one of the CAAC^Me moieties by two Cp*RuCl fragments to yield the spirocyclic compound 3. Preliminary results show that the methodology used to reduce hexachlorophosphazene to 1 can be directly transposed to the regiospecific reduction of poly-chlorophosphazene, to yield poly-1, a fundamentally new class of inorganic polymer that possesses a phosphorus center with chemically active lone pairs in the main chain.