Acidity Scale for Metal Oxides and Sanderson's Electronegativities of Lanthanide Elements

Abstract

Metal oxides are widely used in industry and academia. As their electron-acceptor or acidic strengths play vital roles in their applications, there needs to be a general scale that can quantitatively compare their relative acidic strengths. Conventionally, calorimetric heat measurements during adsorption of probe molecules, infrared spectroscopic analyses of adsorbed bases or acids, application of indicator dyes, and temperature-programmed desorption of the pre-adsorbed bases are standard methods for the analyses of their acidic strengths. However, these methods are not suitable for a quantitative comparison. Thus, unlike metal ions in solution, no such scales have been available for metal oxides. One of the important types of interaction between adsorbates and metal oxides is the formation of coordinate covalent bonding between adsorbates and the surface metal ions. For instance, in the case of TiO2, those compounds that have enediol, carboxylate, and nitrile groups have been shown to form coordinate covalent bonding with the surface Ti ions. In this type of interaction, the adsorbateto-metal charge-transfer interaction is often the lowestenergy electronic transition. However, in the case of alizarin (Figure 1a, inset) on TiO2, a theoretical study has suggested that the intramolecular charge-transfer (IMCT) band from the catechol moiety to the entire ring system is the lowestenergy transition. Electronegativity (EN) is one of the most important fundamental properties of an atom, which represents “the power of an atom in a compound to attract electrons to itself”. Among various EN scales that have been developed, Sanderson s scale and the associated EN equalization principle are successful in calculating the bond energies of various compounds , elucidating the acidic and basic properties of zeolites, and establishing the relationship between the reactivity and the composition of the zeolite that served as the guideline for the preparation of optimum zeolite catalysts. These methods have also been used for various other purposes. However, owing to a lack of experimental data, Sanderson s EN scale has not been extended to lanthanides (Ln) during the last five decades, despite the fact that lanthanide-containing compounds are widely used. Herein, we report that the IMCT transition of alizarin is still the lowest-energy transition when it is adsorbed on various metal oxides and sulfides, regardless of the nature of the metal ion. The charge-transfer transition serves as a highly sensitive and accurate probe for the quantitative comparison of the acidic strengths of the metal oxides and sulfides. We also report the factors that govern the surface acidity, which allows us to assign for the first time the important Sanderson s EN values of Ln ions (SLn3+) and Ce . To experimentally verify the IMCT nature of the lowestenergy electronic transition from the catechol moiety to the entire ring, we also synthesized 4-methoxyalizarin (Figure 1a, inset; Supporting Information, SI-1). The UV/Vis spectra of the two compounds (Figure 1a) show that the lowest-energy transition (2.856 eV) shifts to the lower energy region (2.617 eV) upon introducing a methoxy group at the 4 position; that is, upon increasing the donor strength of the catechol moiety, which verifies the IMCT nature of the transition. The IMCT bands of alizarin and 4-methoxyalizarin adsorbed on various metal oxides and sulfides are shown in Figure 1b, with the order of energy increasing from bottom to top. The IMCT bands appear at 2.322–2.713 eV for alizarin and 2.288–2.536 eV for 4-methoxyalizarin (Supporting Information, Table SI-2). The red-shift from alizarin to 4-methoxyalizarin also suggests that the lowest-energy transition of the adsorbed alizarin is IMCT. Furthermore, the IMCT bands of alizarin and 4-methoxyalizarin are red-shifted when they are adsorbed onto oxides and sulfides. Such coordination-induced redshifts were also observed in solution. The IMCT band of alizarin (2.398 eV) red-shifts upon coordinating to Mg in ethanol relative to its uncoordinated state (2.856 eV; Supporting Information, Figure SI-3). We attribute the red shift to a decrease in the degree of electron withdrawal of the two phenoxide groups as a result of the replacement of the two strongly electron-withdrawing protons by a less strongly electron-withdrawing metal cation (Figure 1d, inset). In this context, the gradual blue shift of the IMCT band of the adsorbed alizarin and 4-methoxyalizarin on going from MgO to Ta2O5 is attributed to the increase in the degree of electron withdrawal from the two phenoxide ligands to a surface metal ion in the following order: MgO<PbO<Y2O3<ZnO< ZnS<HfO2<Ga2O3<ZrO2<TiO2< SnO2<Ta2O5. The IMCT energies do not correlate with Sanderson s partial charges of the metal ions (dM) in metal oxides and sulfides (Figure 1c), which are expressed by Equation (1) for metal chalcogenides MxChy (Ch= chalcogen): [32] [*] N. C. Jeong, Dr. J. S. Lee, Dr. E. L. Tae, Y. J. Lee, Prof. Dr. K. B. Yoon Center for Microcrystal Assembly, Center for Nanoporous Materials, Department of Chemistry, and Program of Integrated Biotechnology, Sogang University, Seoul 121-742 (Korea) Fax: (+82)2-706-4269 E-mail: yoonkb@sogang.ac.kr