Reaction of the 1,8-Bis(diphenylmethylium)naphthalenediyl Dication with Fluoride: Formation of a Cation Containing a C−F→C Bridge

Treatment of 1,8-bis(diphenylhydroxymethyl)naphthalene with a mixture of [HBF4]aq and
(CF3CO)2O affords the corresponding dication, 1,8-bis(diphenylmethylium)naphthalenediyl (12+), which was
isolated as the [BF4]- salt. This dication has been fully characterized, and its structure has been studied
computationally. The 13C NMR resonance of the methylium centers appears at 207.7 ppm. As indicated by
an X-ray single-crystal analysis, the vicinal methylium centers are separated by 3.112(4) Å. Dication (12+)
reacts with fluoride to afford [1−F]+ which has been isolated as the [BF4]- salt. The fluorine atom of [1−F]+
is connected to one of the former methylium centers through a typical C−F bond of 1.424(2) Å and forms
a long interaction of 2.444(2) Å with the other methylium center. While the structure of [1−F]+ can be
largely accounted for by considering a simple methylium formulation, density functional calculations followed
by an Atom In Molecules analysis as well as a calculation of the Boys localized orbitals indicate that the
long C−F interaction of 2.444(2) Å corresponds to a dative bond. Hence, formulation of [1−F]+ as an
unsymmetrical fluoronium must also be considered. As indicated by 1H NMR spectroscopy, the structure
of this ion is fluxional; the fluorine atom oscillates between the former methylium centers with apparent
activation parameters of ΔH⧧ = 52(±3) kJ mol-1 and ΔS⧧ = −18(±9) J K-1 mol-1 as derived from line
shape analysis. This dynamic process, which has also been studied theoretically by B3LYP density functional
theory and Møller−Plesset second-order perturbation theory methods, involves symmetrical fluoronium
ions as low-energy transition states.