The quantitative displacement of triflate groups in 1,3-ditriflato-2,4-bis(2,6-dimethylphenyl)cyclodiphospha-2,4-diazane by DMAP (4-dimethylaminopyridine) or Me3P gives dicationic complexes containing bifunctional diphosphorus Lewis acceptors.

Graphical abstract: Bifunctional diphosphorus Lewis acids from cyclodiphosphadiazanes
Cationic phosphine centers are readily prepared from phosphines1 bearing good anionic leaving groups that are displaced by neutral ligands. Chlorophosphines (in the presence of Me3SiOTf) or Mes*NPOTf (Mes* = 2,4,6-tri-tert-butylphenyl; OTf = trifluoromethanesulfonate)2 react with a wide variety of Lewis bases to give phosphenium cationic complexes of type 13–8 and phosphadiazonium complexes of type 2,6,9–12 respectively.

Analogous complexes with two charges have been realized using diphosphine and diamineligands that tether two phosphorus cation acceptors, generically represented by 3.4,10 Moreover, reductive coupling of chloro- derivatives of 1 provide dications that behave as complexes involving a bifunctional P–P diphosphenium acceptor, generically represented by 4,3 demonstrating the ability of such compounds to accommodate more than one charge.

We have now exploited the well-established series of cyclodiphospha-2,4-diazanes 5 as synthetic origins for complexes of cyclophosphenium 6 and cyclodiphosphenium 7 frameworks that represent dimers of phosphadiazonium complexes of type 2. Isolation of derivatives of 7 introduces the potential for development of oligomeric and polymeric polycationic systems.

Solution 31P NMR spectra of reaction mixtures containing DmpNH2 (Dmp = 2,6-dimethylphenyl) and PCl3 in the presence of Et3N13 show two signals (intensity ratio ∼95 : 5) assigned to 1,3-dichlorocyclodiphospha-2,4-diazane,14–165b, with the chlorine substituents in a cis [δ(31P) = 210.1 ppm] and trans [δ(31P) = 295.4 ppm] configuration with respect to the approximate plane of the P2N2 ring. Reaction of 5b with AgOTf in hexane gives an almost quantitative yield of 1,3-ditriflato-2,4-bis(2,6-dimethylphenyl)cyclodiphospha-2,4-diazane 5c as a cis (98%) [δ(31P) = 182.8 ppm] and trans (2%) [δ(31P) = 272.4 ppm] mixture. A crystalline sample of the cis isomer 5c (Fig. 1a, Table 2) exhibits two 31P CP/MAS NMR signals (Table 1), consistent with the observation of two crystallographically non-equivalent phosphorus centers.