Self-Assemblies of Positional Isomeric Linear Bis-Urea Ligands with Oxyanions/Hydrated Oxyanions: Evidence of F^– and OH^– Induced Atmospheric CO₂ Fixation

Abstract

A set of three positional isomeric nitro-phenyl functionalized linear bis-urea scaffolds has been purposefully synthesized from p-phenylenediamine, basically for investigating the extensive anion-coordination behavior in their neutral form. The para-isomer (L 1 ) and meta-isomer (L 2 ) can readily form bicarbonate dimer (HCO3)2 entrapped neutral self-assemblies in the solid state by either fluoride or hydroxide induced atmospheric CO2 fixation. In the bulk scale and based on the isolated yield of (HCO3)2 entrapped complexes of L 1 and L 2 , it has been found that 6.7, 7.3, and 6.2 mmol of CO2 can be fixed respectively by 1 mmol of linear bis-urea derivatives. The other planar acetate anions in their bare and hydrated form [(OCOCH3)2­(H2O)] are coordinated by hydrogen-bonding interactions in noncooperative fashion with L 1 and L 2 respectively. Interestingly, the para-isomer L 1 entraps bisulfate dimer (HSO4)2, while the meta-isomer L 2 has been found to self-assemble with divalent sulfate (SO4 2–) and polymeric dihydrogen-phosphate (H2PO4) n by noncooperative hydrogen-bonding interactions also. However, successful crystallization of any anion complexes with ortho-isomer (L 3 ) was not fruitful; instead the free L 3 ligand structure has been attained in most of the cases. This phenomena occurs owing to the steric hindrance provided by the nitro group at the ortho-position, which probably hampers the facile inclusion via coordination of an anion due to electrostatic factors, as observed and confirmed by the presence of intramolecular hydrogen-bonding interactions in the crystal structures of free ligand L 3 .

Templating Bicarbonate in the Second Coordination Sphere Enhances Electrochemical CO2 Reduction Catalyzed by Iron Porphyrins

“…At the higher concentrations used for cyclic voltammetric and preparative scale electrolysis experiments (i.e., 1 mM), k obs is inversely correlated with the concentration of Fe- para -urea , presumably due to a dimerization process occurring in solution (Figure S2). Similar dimers generated through H-bond templating with exogenous anions have been extensively observed for linear bis-urea ligands, which may explain the lower-than-expected observed activity of Fe- para -urea .…”

“…To illustrate the general concept for this current study, we chose iron tetraphenylporphyrin ( Fe-TPP ) as a representative member of a prototypical class of molecular CO 2 RR catalysts. − Inspired by the field of anion receptor chemistry, , we sought to enhance the interaction of the Fe-TPP catalyst with bicarbonate by incorporating a bis-aryl urea moiety into the second coordination sphere (Figure ). (Thio)­ureas are known to be highly effective motifs for selective binding of bicarbonate and other oxyanions and thus have been incorporated into diverse frameworks, such as amide-linked macrocycles, cyclopeptide cages, and tripodal and linear − ligand scaffolds for recognition and sensing applications. Indeed, previous reports from our laboratory ,, and others − have established the ability of second coordination sphere pendants to enhance CO 2 RR catalysis.…”

Templating Bicarbonate in the Second Coordination Sphere Enhances Electrochemical CO2 Reduction Catalyzed by Iron Porphyrins

“…At the higher concentrations used for cyclic voltammetric and preparative scale electrolysis experiments (i.e., 1 mM), kobs is inversely correlated with the concentration of Fe-para-urea, presumably due to a dimerization process occurring in solution (Figures S2). Similar dimers generated through H-bond templating with exogenous anions have been extensively observed for linear bisurea ligands, 27 which may explain the lower-than-expected observed activity of Fe-para-urea.…”

“…[17][18][19][20][21] Inspired by the field of anion receptor chemistry, 22,23 we sought to enhance the interaction of the Fe-TPP catalyst with bicarbonate by incorporating a bis-aryl urea moiety into the second coordination sphere (Figure 1). (Thio)ureas are known to be highly effective motifs for selective binding of bicarbonate and other oxyanions, and thus have been incorporated into diverse frameworks such as amide-linked macrocycles, 24 cyclopeptide cages, 25 and tripodal 26 and linear [27][28][29][30][31] ligand scaffolds for recognition and sensing applications. Indeed, previous reports from our laboratory 6,32,33 and others [34][35][36][37][38][39][40][41][42][43][44][45][46][47] have established the ability of second coordination sphere pendants to enhance CO2RR catalysis.…”

Investigations of Thermally Controlled Mechanochemical Milling Reactions

“…Compounds PNBA (p-nitrobenzoyl azide) and PMBA (p-methoxybenzoyl azide) were prepared using previously published procedures. 1 Compound DU (1,1'-(1,4-phenylene)bis(3-(4-nitrophenyl)urea)) 2 and MA (N-(4-aminophenyl)-4-nitrobenzamide) 3 were previously prepared and characterized, and analyses of herein prepared MA and DU products were compared to published results.…”