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 [C10H6(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 CD2Cl2) 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(PXPX‘) = 221.6 Hz, nJ(PXHD‘) = 31.7 Hz, nJ(HDHD‘) = 3.9 Hz]. In the cations of 11, the C10H6P2 skeleton
is also planar (by Cs 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.