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

Area of Science:

• Supramolecular Chemistry
• Organometallic Chemistry
• Chemical Biology

Background:

• Hydrogen bonding with fluoride is crucial for understanding enzyme catalysis, such as 5'-fluoro-5'-deoxyadenosine synthase, and has applications in supramolecular chemistry.
• Investigating fluoride complexation by hydrogen bonding is vital for advancing catalysis and anion recognition.

Purpose of the Study:

• To synthesize and characterize novel urea-fluoride complexes.
• To explore the tunability of fluoride binding and reactivity by modifying urea substituents.
• To evaluate these complexes as fluoride sources in nucleophilic substitution (S_N2) reactions.

Main Methods:

• Synthesis of 18 novel urea-fluoride complexes.
• Solid-state structural analysis using X-ray and neutron diffraction.
• Solution-phase characterization using UV-Vis spectroscopy, NMR (including DOSY and HOESY), and kinetic studies.
• Hammett analysis to correlate electronic properties of urea substituents with fluoride binding affinity.

Main Results:

• Structural and coordination properties of urea-fluoride complexes are tunable via urea substituents.
• Electron-deficient urea ligands exhibit stronger fluoride binding.
• Defined urea-fluoride complexes act as effective fluoride sources in S_N2 reactions, largely favoring S_N2 over E2.
• Kinetic data suggest the reactive species is a 1:1 urea-fluoride complex, supported by dissociation studies.

Conclusions:

• Urea-fluoride complexes offer a tunable platform for fluoride binding and delivery.
• These complexes demonstrate significant potential as reagents in nucleophilic fluorination reactions.
• The study elucidates the role of hydrogen bonding in modulating fluoride reactivity and provides insights into dissociation mechanisms.