Influences of hydrogen-bond promoting hydroxyl group on photophysical and catalytic properties of lanthanide pyridine dicarboxylate frameworks

To study the influences of hydrogen bonding interactions on photophysical properties and catalytic activities with insignificant interference from structural disparity, two new isostructural [Ln^III₂(OH-pda)(ox)₂(H₂O)₄]∙4H₂O coordination polymers (Ln^III = Eu^III (I) and Nd^III (IIa), OH–H₂pda = 4-hydroxypyridine-2,6-dicarboxylic acid, H₂ox = oxalic acid) were synthesized and characterized. They are also isostructural to [Nd^III₂(pda)(ox)₂(H₂O)₄]∙4H₂O (IIb), H₂pda = pyridine-2,6-dicarboxylic acid) containing pda²⁻ instead of OH-pda^2-. The identical framework structure of IIa and IIb with additional hydrogen bonding interactions in IIa due to –OH group of OH-pda^2- has been presented. Based on IIa and IIb, the effects of the –OH group on photophysical properties and catalytic activities were then investigated without structural bias. With reference to the UV–vis absorption and phosphorescent emission spectroscopy, the singlet (S₁) and triplet (T₁) state energies of OH–H₂pda (41,600 and 19,200 cm⁻¹), H₂pda (48,200 and 23,300 cm⁻¹), OH-pda^2- (IIa, 41,400 and 22,500 cm⁻¹) and pda²⁻ (IIb, 47,100 and 23,300 cm⁻¹) were determined from which the influences of the –OH group have been revealed. Important roles of the –OH group as electron-donating group as well as hydrogen bond promoter have been proposed. Catalytic activities of IIa and IIb were comparatively examined using the solvent-free CO₂ cycloaddition reaction at atmospheric pressure with epichlorohydrin in the presence of tetrabutylammonium bromide. The inferior performance of IIa (maximum yield of 62(±2)%) to IIb (maximum yield of 68(±2)%) was unexpectedly disclosed and the drawback of the substantial involvement of the catalytic sites in hydrogen bonding interactions has been demonstrated. Their reusability was additionally explored.