Communications
DOI: 10.1002/anie.200803837
Analytical Methods
Acidity Scale for Metal Oxides and Sandersonꢀs Electronegativities of
Lanthanide Elements**
Nak Cheon Jeong, Ji Sun Lee, Eunju Lee Tae, Young Ju Lee, and Kyung Byung Yoon*
Metal oxides are widely used in industry and academia.[1,2] 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. Con-
ventionally, calorimetric heat measurements during adsorp-
tion of probe molecules,[3] infrared spectroscopic analyses of
adsorbed bases or acids,[5,6] application of indicator dyes,[4]
and temperature-programmed desorption of the pre-adsor-
bed bases are standard methods for the analyses of their
acidic strengths.[6–8] However, these methods are not suitable
for a quantitative comparison. Thus, unlike metal ions in
solution,[9] 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,[10–14] carboxylate,[15–18] and nitrile[19,20] groups
have been shown to form coordinate covalent bonding with
the surface Ti4+ ions. In this type of interaction, the adsorbate-
to-metal charge-transfer interaction is often the lowest-
energy 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 lowest-
energy transition.[11]
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”.[21,22] Among various EN scales that have been
developed,[21–35] Sandersonꢀs scale and the associated EN
equalization principle[31–35] are successful in calculating the
bond energies of various compounds[32–36], elucidating the
acidic and basic properties of zeolites,[37,38] 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.[39] These methods have also been
used for various other purposes.[40–44] However, owing to a
lack of experimental data, Sandersonꢀs 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 Sandersonꢀs
EN values of Ln3+ ions (SLn3+) and Ce4+.
To experimentally verify the IMCT nature of the lowest-
energy electronic transition from the catechol moiety to the
entire ring,[11] we also synthesized 4-methoxyalizarin (Fig-
ure 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 Infor-
mation, Table SI-2). The red-shift from alizarin to 4-methox-
yalizarin 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 Mg2+ in
ethanol relative to its uncoordinated state (2.856 eV; Sup-
porting 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 Sandersonꢀs
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 Biotech-
nology, Sogang University, Seoul 121-742 (Korea)
Fax: (+82)2-706-4269
E-mail: yoonkb@sogang.ac.kr
[**] We thank the Ministry of Education, Science, and Technology
(MEST) of Korea and Sogang University for supporting this work.
Supporting information for this article is available on the WWW
10128
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 10128 –10132
Jeong, Nak Cheon
