Insignificance of P-H⋯P hydrogen bonding: Structural chemistry of neutral and protonated 1,8-di(phosphinyl)naphthalene

While there is extensive information on 1,8-di(amino)naphthalene (i.e., the parent compound of the "proton sponge" series), the corresponding phosphorus compound has not been described. A high-yield synthesis of 1,8-di(phosphinyl)naphthalene (9) and the 1-naphthylphosphine reference compound (4) is now reported. Thermal decomposition of 9 leads to intramolecular dehydrogenative P-P coupling to afford 1,2-dihydro-1,2-diphosphaacenaphthene (10). Protonation of 9 and 4 with CF₃SO₃H gives quantitative yields of the monophosphonium salts 11 and 5, respectively. With excess acid and traces of moisture, the hydronium salt [C₁₀H ₆(PH₂)(PH₃)]⁺[H₃O] ⁺2[CF₃SO₃]^- (13) is obtained. The structures of 9, 11, and 13 have been determined. Molecules of 9 have a planar naphthalene skeleton, C₁₀H₆P₂, with the two -PH₂ groups in a transoid conformation. The molecules form loose dimers in the crystal, the individual chiral enantiomers of which are related by a center of inversion. In contrast to the situation for the amino analogue, and despite the proximity of the two -PH₂ functions, there is no intra- or intermolecular hydrogen bonding. Solutions of 9 (in CD₂Cl ₂) show equivalent P-bound hydrogen atoms due to conformational fluctionality. By analysis of the ABCD₂XX′D′ ₂C′B′A′ spin system, it was shown that, in 9, there are strong through-space pericouplings [ⁿJ(P_XP _X′) = 221.6 Hz, ⁿJ(P_XH _D′) = 31.7 Hz, ⁿJ(H_DH_D′) = 3.9 Hz]. In the cations of 11, the C₁₀H₆P₂ skeleton is also planar (by C_s symmetry), with the -PH₂ and -PH₃⁺ groups in a conformation which rules out any P-H⋯P hydrogen bonding. The hydronium cation and the two triflate anions in 13 are associated into an anionic network through extensive hydrogen bonding surrounding stacks of the phosphonium cations. In solution, the cations of 11 and 13 show separate ³¹P resonances for the two phosphorus atoms with fully resolved ¹J(PH) couplings, which indicate that there is no intra- or intercationic proton exchange. By contrast, the NMR spectra of solutions of [C₁₀H₆(NH₂)(NH₃)] ⁺X^- salts show proton scrambling equilibrating all five N-bound hydrogen atoms, and in the crystal, the conformations of the cations feature intramolecular N-H⋯N hydrogen bonding.