1,2-diphosphaacenaphthene 1,2-dications: synthetic, stereochemical and computational study of the stabilising role of naphthalene-1,8-diyl backbone.

Syntheses and full characterisation data (including single crystal diffraction) of three 1,2-diphosphonium dicationic species with the naphthalene-1,8-diyl (Nap) backbone are reported. The oxidation of Nap[P(NMe(2))(2)](2) with P(2)I(4) to its 1,2-dication was achieved. meso- and rac-forms of "all carbon" 1,2-diphosphonium dications were obtained in good yields and purity by double alkylation of the parent diphosphine (1,2-diphenyl-1,2-diphosphaacenaphthene) with methyl triflate or trimethyloxonium tetrafluoroborate. Each methylating reagent produces one of the rac- or meso-forms of the dication diastereospecifically. Structural parameters of the new dications are discussed with respect to other phosphorus 1,2-dications. DFT (B3LYP) computations revealed the significant role of the naphthalene backbone in stabilisation of the dicationic motif and helped to assess the energy cost of the steric clash of a variety of groups attached to the peri-positions of naphthalene. The synthesis and single crystal X-ray data of the extremely crowded Nap[P(Se)(OiPr)(2)](2) are discussed, and are contrasted with the unsuccessful synthesis of Nap(PtBu(2))(2) from NapLi(2) and ClPtBu(2).