Site-Specific Description of the Enhanced Recognition Between Electrogenerated Nitrobenzene Anions and Dihomooxacalix[4]arene Bidentate Ureas

Abstract

Electron transfer controlled hydrogen bonding was studied for a series of nitrobenzene derivative radical anions, working as large guest anions, and substituted ureas, including dihomooxacalix[4]arene bidentate urea derivatives, in order to estimate binding constants (Kb) for the hydrogen-bonding process. Results showed enhanced Kb values for the interaction with phenyl-substituted bidentate urea, which is significantly larger than for the remaining compounds, e.g., in the case of 4-methoxynitrobenzene a 28-fold larger Kb value was obtained for the urea bearing a phenyl (Kb ∼ 6888) vs tert-butyl (Kb ∼ 247) moieties. The respective nucleophilic and electrophilic characters of the participant anion radical and urea hosts were parametrized with global and local electrodonating (ω(-)) and electroaccepting (ω(+)) powers, derived from DFT calculations. ω(-) data were useful for describing trends in structure–activity relationships when comparing nitrobenzene radical anions. However, ω(+) for the host urea structures lead to unreliable explanations of the experimental data. For the latter case, local descriptors ωk(+)(r) were estimated for the atoms within the urea region in the hosts [∑kωk(+)(r)]. By compiling all the theoretical and experimental data, a Kb-predictive contour plot was built considering ω(-) for the studied anion radicals and ∑kωk(+)(r) which affords good estimations.

Competition between Hydrogen Bonding and Proton Transfer during Specific Anion Recognition by Dihomooxacalix[4]arene Bidentate Ureas

“…These results suggest that there is an enhanced interaction ( K b in eq ) of the host–guest system with a mutually increased rate of proton transfer ( k f2 in eq ) in systems involving the urea residue bearing phenyl substituents. These effects are in accordance with the changes in electrophilicity of the reactive species, specifically with the local electroaccepting power (∑ k ω k + ( r )) in the vicinity of the urea region for compounds 4 and 5 . For these systems, electronic structure calculations showed ∑ k ω k + ( r ) values for compound 5 (−3.298 × 10 –1 ) higher than those for compound 4 (−3.95 × 10 –2 ) .…”

“…The binding reaction should occur as a fast and reversible process for which the relative rates of both formation and dissociation reactions ( k f1 and k b1 ) of the adduct (eq ) should be very large, as the system is in equilibrium during the binding process. ,, For this purpose, both rate constants were fixed at values close to the diffusion limit rate constant for a bimolecular reaction (10 8 s –1 ) . Classically, the presence of the binding reaction leads to a shift in the corresponding reduction potentials toward less negative values, exhibiting diffusion-controlled voltammetric waves. ,,, This case can be identified as a DE process …”

“…From the latter, K b values can be obtained (e.g., by nonlinear regressions between Epc vs log [(DH) 2 – R 2 ] Figure ). ,, In the case of the systems studied, interactions between urea 4 and dinitrobenzene isomers 2 and 3 would be classified in this zone in addition to the experimental results for the interaction between urea 5 and 3 (Figure C).…”

“…However, in this case, proton transfer occurred at the methylene group linking the pyridinium fragment to the receptor skeleton, while the urea moieties of the receptor formed intermolecular hydrogen bonds with the corresponding anions (fluoride and acetate), leading to competition between H-bonding and proton transfer pathways. The anion affinity in these processes can be increased using more preorganized structures of the receptor, for example, calix­[ n ]­arene skeletons, ,− which allows incorporation of more than one urea binding site in its structure and favors its selectivity. This was observed by Jin and collaborators by comparing the binding constants of different anions with calix[4]­arene ureas and simple urea and thiourea derivatives.…”

“…From the above discussion, more information is required about the conditions driving H-bonded systems into proton transfer reactions to fully understand anion recognition processes by these urea derivatives. Also, such a study would complement the growing interest in elucidating H-bonding effects on PCET reactions. − ,,− Dihomooxacalix[4]­arene bidentate ureas are interesting H-bond donor species ((HD) 2 -R 2 ) for studying anion recognition, as previous results have shown that they noticeably increase their binding capacity to anions when the nature of the substituent groups in their structures is changed (from t -Bu to Ph, K b ranges from ∼250 to ∼7000 M –1 , respectively). , Also, by electrogenerating dianions ([NO 2 – ϕ – NO 2 ] 2– ) from dinitrobenzene compounds (NO 2 – ϕ – NO 2 ), electron transfer-controlled hydrogen bonding (ETCHB) can be studied, , leading to increases in the binding affinity when the charge state of the receptor is changed. ,,, ETCHB processes between these latter species can be generally described by the next set of chemical equations: , …”