Hydrogen-Bonded Homoleptic Fluoride–Diarylurea Complexes: Structure, Reactivity, and Coordinating Power

Hydrogen bonding with fluoride is
a key interaction encountered
when analyzing the mode of action of 5′-fluoro-5′-deoxyadenosine
synthase, the only known enzyme capable of catalyzing the formation
of a C–F bond from F–. Further understanding
of the effect of hydrogen bonding on the structure and reactivity
of complexed fluoride is therefore important for catalysis and numerous
other applications, such as anion supramolecular chemistry. Herein
we disclose a detailed study examining the structure of 18 novel urea–fluoride
complexes in the solid state, by X-ray and neutron diffraction, and
in solution phase and explore the reactivity of these complexes as
a fluoride source in SN2 chemistry. Experimental data show
that the structure, coordination strength, and reactivity of the urea–fluoride
complexes are tunable by modifying substituents on the urea receptor.
Hammett analysis of aryl groups on the urea indicates that fluoride
binding is dependent on σp and σm parameters with stronger binding being observed for electron-deficient
urea ligands. For the first time, defined urea–fluoride complexes
are used as fluoride-binding reagents for the nucleophilic substitution
of a model alkyl bromide. The reaction is slower in comparison with
known alcohol–fluoride complexes, but SN2 is largely
favored over E2, at a ratio surpassing all hydrogen-bonded complexes
documented in the literature for the model alkyl bromide employed.
Increased second-order rate constants at higher dilution support the
hypothesis that the reactive species is a 1:1 urea–fluoride
complex of type [UF]− (U = urea) resulting from
partial dissociation of the parent compound [U2F]−. The dissociation processes can be quantified through a combination
of UV and NMR assays, including DOSY and HOESY analyses that illuminate
the complexation state and H-bonding in solution.