that once the HCHO molecules were adsorbed into a supercage
of NaY, they remained monomeric without oligomerization
even at room temperature.7 Our quantum chemical calculations
revealed that the HCHO was thermodynamically stabilized by
steady coordination with Na+ on the pore surface.14 If the
number of added HCHO molecules per supercage exceeded
three, their trimerization and further oligomerization started.15
In these papers, the carbonyl carbon of HCHO adsorbed on
NaY shows a lower magnetic field shift by 6 ppm in the
13C DD/MAS NMR spectra than that of the unconfined HCHO
in a CDCl3 solution.7 Such a lower magnetic field shift of a
carbonyl carbon is ascribed to the coordination of a carbonyl
oxygen to Na+ on the surface of a supercage, and is similarly
observed for acrolein,8 propynal9 and ethyl diazoacetate.10
Cyclopentadiene also shows lower chemical shifts of the sp2
carbons due to specific π-interactions with Na+.11 In addition,
such low magnetic field shifts were similarly observed when
HCHO and acrolein were adsorbed on Ag+-exchanged Y-type
zeolite (AgY), where the carbonyl oxygens coordinated to the
silver ions.16 Therefore, we can claim that the coordination
modes of an organic molecule having multicoordination sites
to metal ions can be estimated based on the chemical shift
changes in the 13C MAS NMR spectra when the molecule is
adsorbed in the zeolite.
Ketenes are composed of a carbonyl group and an accu-
mulated C-C double bond, and highly reactive toward various
nucleophiles such as water, alcohols, carboxylic acids, amines,
etc. Since the beginning of the 20th century, their synthesis and
chemical behaviors have drawn many chemists’ attention and
been extensively studied in light of organic chemistry, coordi-
nation chemistry, catalysis chemistry and physical chemistry.17
Especially, mononuclear, early to late transition metal com-
plexes18 with ketenes, such as Ti,19 Zr,20 V,21 Nb,22 Cr,23 Mo,24
W,25 Mn,26 Re,27 Fe,28 Os,29 Co,30 Rh,31 Ir,32 Ni33 and Pt,34
have been intensively developed. These ketene complexes
adopt intrinsic coordination modes of η2-C=O or η2-C=C,
depending on the types of metals with different oxophilicities,
ketenes and accompanying ligands. It was also reported that
ketene intermediates were involved in chemical processes
catalyzed by acidic zeolite.35 Ketenes are, therefore, intriguing
adsorbate molecules for zeolite chemistry in light of their com-
plex formations with metals located in the zeolite’s confined
cages: Different coordination modes, such as η2-C=O, η1-C=O
and η2-C=C, can be expected, depending on the types of
metals (Figure 1).
and Bases” theory,36 and investigated how ketene molecules
behaved in the supercages of the zeolite by comparing the
spectra of the 13C CP/MAS and DD/MAS NMR with rela-
tively simple quantum chemical calculations in order to exam-
ine which coordination modes are predominant and what trend
of chemical shift changes in the NMR spectra of the ketene
complexes are recognizable.
2. Experimental
2.1 Preparation of Adsorbed Samples and Analysis with
13C CP/MAS NMR, DD/MAS NMR and FTIR. Powder
NaY (Si/Al = 2.75, HSZ-320NAA) was obtained from the
Tosoh Corporation (Japan). AgY was prepared from NaY by
ion-exchange with aq. AgNO3, and its elemental analysis
shows a >99% ion-exchange with Ag+.
MY (M = Na or Ag) was activated in a vacuum at 400 °C,
<0.2 torr for 4 h. In a nitrogen atmosphere, 1,2-13C-labeled
diphenylketene (1), 1-13C-labeled benzophenone (2) or 1,2-13C-
labeled 1,1-diphenylethene (3) was added to MY at room tem-
perature, respectively. Each sample was determined to contain
one molecule of 1, 2 or 3 per supercage of MY. The resulting
adsorbed samples are abbreviated 1@MY, 2@MY, or 3@MY,
respectively. The 13C CP (cross polarization)/MAS (magic
angle spinning) and DD (dipole decoupling)/MAS NMR of 1
to 3@MY were measured. For the IR measurements, 13C-
unlabeled diphenylketene (1¤) was included in each activated
carrier, and applied to ATR (Attenuated Total Reflection)-FTIR
analysis.
2.2 Quantum Chemical Calculation Using MY/5T Clus-
ters. Based on the framework data of NaY from its X-ray
crystal structure analysis,37 the structure around site II of NaY
was cut out, and the terminal silicon atoms of the resulting NaY
cluster model were capped with hydrogens (Figure 2). The
NaY clusters were then optimized at the B3LYP/6-31G(d,p)
level by fixing the positions of silicon, aluminum and oxygen
using Gaussian 16.38 It is also known that AgY has the same
structure as NaY,39 so AgY clusters were created by replacing
Na with Ag with the atoms other than Ag fixed, and optimiza-
tion. For the Ag atom, SDD was used as the basis function of
the effective core potential (ECP) type.40 The clusters are repre-
sented as “MY/5T clusters,” which stand for M = Na or Ag,
and 5T = the total number (5) of silicon and aluminum atoms.
In this study, we selected zeolite Y containing not only
sodium ions as a “hard acid” for NaY, but also silver ions as a
“soft acid” for AgY that are based on the “Hard and Soft Acids
Figure 1. Possible coordination modes of a ketene mole-
cule in a supercage of zeolite Y.
Figure 2. NaY/5T cluster is cut out of the supercage
framework around site II of NaY zeolite.
© 2020 The Chemical Society of Japan