α-Substituted Bis(octabutoxyphthalocyaninato)Terbium(III) Double-Decker Complexes: Preparation and Study of Protonation by NMR and DFT

Article abstract of DOI:10.1021/acs.inorgchem.5b02391

Synthesis of the anionic, α-substituted, bis(phthalocyaninato)Tb^III complex [Tb(α-obPc)₂]^- ([1]^-) (obPc = α-octabutoxyphthalocyaninato) leads to the isolation of its protonated form [1H]⁰. This complex was characterized by X-ray diffraction (XRD), mass spectroscopy (MS), infrared (IR) and ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy. Crystal structure analysis did not allow localization of the additional proton, which is probably attached to the meso-N atom or isoindole-N atom of the phthalocyaninato ligand. [1H]⁰ can easily be deprotonated or protonated, giving the corresponding anionic and cationic complexes. The three compounds [1H]⁰, [1]^-, and [1HH]⁺ were studied by a combination of paramagnetic NMR experiments (¹H, ¹³C, variable-temperature measurements, two-dimensional nuclear magnetic resonance and DFT calculations (done on Y^III analogues with octamethoxyphthalocyaninato ligands), for the purpose of elucidating the positions of the acidic protons and for understanding the structural changes of the coordination environment of the Tb ion induced by protonation.

Full text of DOI:10.1021/acs.inorgchem.5b02391

Article
pubs.acs.org/IC
α‑Substituted Bis(octabutoxyphthalocyaninato)Terbium(III) Double-
Decker Complexes: Preparation and Study of Protonation by NMR
and DFT
Marko Damjanovic,† Yusuke Horie,‡ Takaumi Morita,‡ Yoji Horii,‡ Keiichi Katoh,*,‡,§
́
Masahiro Yamashita,*,‡,§ and Markus Enders*,†
†Institute of Inorganic Chemistry, University of Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany
‡Department of Chemistry, Graduate School of Science, Tohoku University, 6-3, Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi
980-8578, Japan
§CREST, JST, 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan
S
* Supporting Information
ABSTRACT: Synthesis of the anionic, α-substituted, bis-
(phthalocyaninato)TbIII complex [Tb(α-obPc)2] ([1]) (obPc
= α-octabutoxyphthalocyaninato) leads to the isolation of its
protonated form [1H]0. This complex was characterized by X-ray
diffraction (XRD), mass spectroscopy (MS), infrared (IR) and
ultraviolet−visible−near-infrared (UV−vis-NIR) spectroscopy.
Crystal structure analysis did not allow localization of the
additional proton, which is probably attached to the meso-N
atom or isoindole-N atom of the phthalocyaninato ligand. [1H]0
can easily be deprotonated or protonated, giving the corresponding
anionic and cationic complexes. The three compounds [1H]0,
[1], and [1HH]+ were studied by a combination of paramagnetic
NMR experiments (1H, 13C, variable-temperature measurements, two-dimensional nuclear magnetic resonance and DFT
calculations (done on YIII analogues with octamethoxyphthalocyaninato ligands), for the purpose of elucidating the positions of
the acidic protons and for understanding the structural changes of the coordination environment of the Tb ion induced by
protonation.
around TbIII in TbH(TPP)2 was shown as well. Because of
1. INTRODUCTION
The discovery of single-molecule magnets (SMMs) in 19931
the distortion and low symmetry in TbH(TPP)2, this complex
did not show SMM properties. In contrast, the deprotonated
made a significant impact on the course of materials science.
SMMs possess a magnetic bistability on the molecular level and
complex showed SMM properties, because of the highly
symmetric SAP coordination. Thus, these complexes achieve a
proton-induced switching of SMM properties by a reversible
protonation−deprotonation. In the case of the protonated
TbIII−phthalocyaninato−porphyrinato heteroleptic double-
decker complex Tb(TPP)(HPc), an acidic proton is located
on the meso-N atom of the phthalocyanine ligand.6 Since the
acidic proton did not disturb the SAP coordination geometry
around TbIII ions, Tb(TPP)(HPc) keeps its SMM properties.
Although the positions of the protons of TbH(TPP)2 and
Tb(TPP)(HPc) were determined by single-crystal X-ray
structural analysis, it is still difficult to determine the position
of the acidic proton directly, because of its low electron density.
On the other hand, 1H NMR measurements are highly sensitive
and can directly reveal the position of the proton via analysis of
its paramagnetic NMR shift, as will be shown in this work.
quantum effects that cannot be seen in classical bulk magnets.
Today, great efforts are made toward pushing the frontiers of
SMMs, through the design of complexes with increased
blocking temperatures and suppressed magnetic relaxation
processes.2 SMMs attract great attention, because of their
potential application for ultrahigh density molecular memories
and for quantum bits (qubits). In particular, the unsubstituted
TbIII−phthalocyaninato double-decker complex TbPc2 shows
unparalleled SMM properties.3 Because of its large activation
energy for spin reversal (ca. 400 cm−1) and high chemical
stability, TbPc2 has been used for research in spintronic
devices.4 It is well-known that TbPc2 analogues also show good
SMM properties. Ogawa and co-workers reported the SMM
properties of TbIII−tetraphenylporphyrinato (TPP) double-
decker complexes Tb(TPP)2 and TbH(TPP)2, which are the
analogues of TbPc2.5 With the help of single-crystal X-ray
analysis of TbH(TPP)2, the position of protons connected to
the pyrrole ring was determined for the first time, and the
distorted square antiprismatic (SAP) coordination mode
Received: October 16, 2015
© XXXX American Chemical Society
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Recently, our research group has reported the nuclear
magnetic resonance (NMR) analysis of lanthanide SMMs.7
Aided with various NMR experiments full assignments of the
1H and 13C resonances in TbIII−β-butoxy substituted
phthalocyaninato triple-decker complex Tb2(β-obPc)3 and
solution structure analysis has been achieved. In addition,
redox series of double-decker complexes [Tb(β-obPc)2]−/0/+
were investigated by 1H and 13C NMR measurements. A
combination of NMR measurements and density functional
theory (DFT) calculations enabled us not only to separate
hyperfine shift contributions but also to determine the
interligand distances in the [Tb(β-obPc)2]+ complex, for
which no crystal structure is available. In addition, the analysis
showed that the π-radical and the unpaired f-electrons at TbIII
in the neutral [Tb(β-obPc)2]0 complex are ferromagnetically
coupled at room temperature.
report the synthesis and NMR analysis of [Tb(α-obPcH)2]+
([1HH]+), which is the first example of a doubly protonated
double-decker structure.
2. EXPERIMENTAL DETAILS
2.1. General Information. The required reagents and solvents
were used as provided by the supplier, without further purification. 1,8-
Diazabicyclo[5.4.0]-undec-7-ene (DBU), 1-butanol, and 1-octanol
were purchased from Wako Chemicals. Metal lithium wire (3 mm in
diameter) and dibenzo-18-crown-6 were purchased from Sigma−
Aldrich. TbIII acetylacetonate trihydrate Tb(acac)3·3H2O was
purchased from Strem Chemicals. The solvents used for column
chromatography were special grade from Wako Chemicals. 3,6-
Dibutoxyphthalonitrile was synthesized according to a reported
method.9
2.2. Synthesis of [Tb(α-obPc)2H]0 ([1H]0). The complex was
prepared by modifying a reported procedure for isostructural EuIII and
YIII complexes.8 H2(α-obPc) was prepared according to a previously
reported procedure (see the Supporting Information).9 Figure 1 shows
the synthesis of the H2(α-obPc) ligand and, subsequently, the
synthesis of [1H]0. H2(α-obPc) (171 mg, 0.156 mmol), Tb(acac)3·
3H2O (36 mg, 0.0727 mmol), dibenzo-18-crown-6 (29 mg, 0.0804
mmol) and DBU (3 mL) were refluxed for 10 h in n-octanol (16 mL)
under N2 atmosphere. The mixture was evaporated under reduced
pressure. The residue was chromatographed on a neutral alumina
column (Al2O3, Merck) with CHCl3 as an eluent. A deep green
fraction was collected. After the solvent was evaporated, column
chromatography was repeated over BioBeads S-X1 with tetrahydrofur-
an (THF) for further purification. Recrystallizing from toluene/MeOH
gave a dark green powder (49 mg, 28.8%). ESI-MS: m/z 2338.15
(100%) [M]+ (≡ [1H]+) (see Figure S1 in the Supporting
Information). Elemental analysis for C128H161N16O16Tb (%): Calcd.:
C 65.73, H 6.93, N 9.58. Found: C 65.85, H 6.95, N 9.54.
On the basis of our previous research, here we report the
synthesis, characterization, and a combined NMR- and DFT-
based analysis of the new double-decker complex [1H]0 (Figure
1) and its deprotonated and protonated forms, respectively.
2.3. Characterization of [1H]0. Before NMR measurements were
conducted, [1H]0 was characterized by elemental analysis, electron
spin ionization-mass spectroscopy (ESI-MS), ultraviolet−visible−near-
infrared (UV-vis-NIR) spectroscopy, infrared (IR) analysis, and X-ray
structural analysis. The molecular structure and the recorded spectra
are shown in the Supporting Information (Figures S1−S4, and Table
S1). Elemental analysis and MS measurements were performed at the
Research and Analytical Centre for Giant Molecules, Tohoku
University. UV-vis-NIR spectra were corrected on a Shimadzu
Model UV-3100PC system, by using CH2Cl2 as a solvent. IR
spectroscopy was performed on attenuated total reflection (ATR)
method on Jasco FT/IR-4200. Single crystals suitable for SXRD
measurements were made by slow diffusion of a CHCl3 solution of the
complex into toluene. The XRD data for [1H]0 were collected at 90 K
on a Bruker APEX-II CCD diffractometer with graphite monochro-
mated Mo Kα radiation (λ = 0.7107 Å). The crystal was mounted on
the Micromount with Paratone-N. The initial structure of [1H]0 was
solved by Intrinsic Phasing (Bruker APEX-II). Refinement by a full-
matrix least-squares method on F2 by using SHELX-9710 was
performed. All non-hydrogen atoms were refined anisotropically
using a least-squares method, and hydrogen atoms were fixed at
calculated position.
Figure 1. Schematic representation of the synthesis of H2(α-obPc)
and of [1H]0 ([Tb(α-obPc)2H]0, where α-obPc is the dianion of
1,4,8,11,15,18,22,25-octa(n-butoxy)phthalocyaninato). The position of
the acidic proton could not be determined from crystal structure data.
In 2010, Jiang and co-workers reported the synthesis and
structural determination of [M(α-obPcH)(α-obPc)]0 (M =
EuIII or YIII) and deprotonated [M(α-obPc)2]− complexes.8
Single-crystal XRD measurements of [Eu(α-obPcH)(α-obPc)]0
could not elucidate the position of the acidic proton.
2.4. NMR Measurements. NMR spectra were recorded at a field
strengths of 7.9 T (400 MHz) with a Bruker Avance II instrument
equipped with a BBFO probe and at a field strength of 14.09 T (600
MHz) with a Bruker Avance III instrument equipped with a QNP
Cryoprobe (inner coil tuned to 13C, cold preamplifier). Prior to
variable-temperature measurements, temperature calibration was done
by using the temperature dependence of the residual nondeuterated
methanol signal in deuterated methanol.11 Solvent resonances were
In this article, we chose TbIII ions as a metal center of the
double-decker assembly to determine the position of acidic
proton for the following reasons: (1) toward the improved
understanding of the factors that lead to emergence of SMM
properties, (2) TbIII−phthalocyaninato complexes have strong
uniaxial magnetic anisotropies, which makes NMR analysis
12
1
taken as references for all H and 13C NMR spectra. Deuterated
solvents (CD2Cl2 and toluene-d8, Sigma−Aldrich) were dried with
conventional methods (over CaH2) and degassed before use with the
freeze−pump−thaw technique. DBU and glacial acetic acid were used
as provided by the supplier (Sigma−Aldrich). The sample with the
anionic complex [1]− (D4d point group) was prepared by adding
1
easier. Paramagnetic shifts in H NMR of lanthanoid systems
are dominated by the pseudo-contact shift (PCS), which is
quite sensitive to the atomic coordinates. Therefore, the
position of acidic protons can be determined by analysis of
paramagnetic NMR shifts. In addition to these experiments, we
B
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excess DBU to a toluene-d8 solution of [1H]0. The sample with the
doubly protonated complex, [1HH]+, was prepared by adding an
excess of glacial acetic acid to a toluene-d8 solution of [1H]0. All
samples for NMR measurements were prepared and stored in an inert
gas atmosphere, in NMR tubes with Teflon plugs (J. Young valves).
2.5. DFT Calculations. For the purpose of using computational
resources reasonably, the density functional theory (DFT) calculations
were performed with YIII complexes, rather than with TbIII complexes.
This was done since it is known that the ionic radii of YIII and TbIII are
almost equivalent (115.9 pm for YIII vs 116.7 pm for TbIII, for 8-
coordinated complexes).13 Furthermore, in the DFT calculations,
butoxy substituents on the α-positions of the phthalocyaninato ligands
were replaced with methoxy substituents (abbreviated as omPc, thus
reducing the total number of atoms in each structure by a factor of
∼1.9). DFT calculations were performed with the program Gaussian
09 (revision D.01).14 Geometry optimizations were done with the
restricted B3LYP functional.15 The def2svp16 basis set was used for all
light atoms and Stuttgart ECP was used for YIII.17 In all geometry
calculations, the dispersion interaction was included (Grimme DFT-
D3 with Becke-Johnson damping).18 The “superfine” integration grid
was used throughout, and the quadratically convergent self-consistent
field (SCF) procedure was utilized when convergence could not be
achieved by the first-order SCF methodology.19 Tight criteria were
used for optimization. No symmetry restrictions were imposed during
the optimization procedure. For all structures, stationary points on the
potential energy surface were characterized as minima by the absence
of imaginary frequencies. The validity of this geometry optimization
strategy for phthalocyaninato double-decker complexes was previously
shown by Kitagawa et al.20 and in our recent NMR study of β-
substituted octabutoxyphthalocyaninato TbIII and DyIII double-decker
complexes.7b Single-point calculations of the optimized geometries
were done using the restricted B3LYP functional and using the
def2tzvp16 basis set on all atoms. The “superfine” integration grid was
used, along with the “scf = xqc” procedure. The xyz coordinates of the
optimized structures, the energies from single-point calculations and
the interligand distances of the various optimized structures (defined
as the separation of the centroids of the two N4 basal planes of the
square antiprism) are provided in the Supporting Information.
al.21 and La Mar et al.,22 and elsewhere.23 Recently, a complete
ab initio theory for paramagnetic NMR shifts that goes far
beyond the simple eqs 1−4 has been developed.23d−f The
dominant contribution to the hyperfine shift of 1H nuclei in the
complexes studied in this work is the pseudo-contact term
(δPC), which can be described by a simple point-dipole based
eq 1 for general cases and eq 2 for axially symmetric complexes:
1
12π
δPC
=
[χaG + χrhH]
(1)
χaG
12π
δPC
=
(2)
where χa and χrh are the axial and rhombic components of the
magnetic susceptibility anisotropy tensor, respectively, and G
and H are geometric factors expressed by the following
relationships (eqs 3 and 4):
3 cos2 θ − 1
G =
r3
(3)
2
⎛
⎞
3 sin θ cos 2Ω
⎝
H =
⎜
⎟
⎠
r3
2
(4)
where r is the length of the vector connecting the TbIII ion and
the NMR nucleus, θ the angle between the corresponding r
vector and the magnetic susceptibility main axis, and Ω the
angle that the r vector makes in the plane perpendicular to the
magnetic susceptibility axis. For complexes studied in this work,
the rhombic component of the χ tensor is neglected because of
the axial symmetry of the first coordination sphere of the metal,
which leads to a simplified analysis.
The 1H NMR spectrum of [1H]0 (Figure 2) is quite
complicated, since protonation alleviates the D4d symmetry of
the anionic double-decker complex. The intense eight signals
due to chemically inequivalent methyl groups can clearly be
identified in the region from −5 ppm to −17 ppm. The
diastereotopic CH2α, CH2β, and CH2γ groups, along with eight
CHar groups, give a total of 56 signals, some of which overlap
3. RESULTS AND DISCUSSION
3.1. Characterization of [1H]0. In the IR spectrum (Figure
S2 in the Supporting Information), a weak peak at 3246 cm−1 is
observed for [1H]0, which originates from the acidic proton,
which is connected to a nitrogen atom. The lack of a broad
peak at ∼1500 nm in the UV-vis-NIR spectrum of [1H]0 (see
Figure S3 in the Supporting Information) proves that no π-
radical is present. The crystal structure of [1H]0 is given in
Figure S4 in the Supporting Information (Cambridge Crystallo-
graphic Data Centre (CCDC) No. 1438573). This complex
crystallizes in the monoclinic P21/c group, with no solvent
molecules incorporated in the structure. Despite considerable
effort, the position of the acidic proton in the crystal structure
could not be found. Skeletal π-structures of the α-obPc ligands
are strongly distorted due to the steric hindrance induced via
the n-butoxy chains in α-positions. The stacking angle between
the phthalocyaninato ligands within a single molecule is 43.9°,
corresponding to a staggered conformation. The TbIII ion is
located almost in the middle between the two ligands. As will
be shown in the following sections, the nonprotonated α-obPc
ligand is closer to the metal center than the α-obPcH one. The
intermolecular π−π interactions lead to a slipped column
structure along the c-axis.
1
Figure 2. H NMR spectrum of [1H]0 (295.0 K, CD2Cl2, 14.09 T).
Upper inset: expanded region with signals of 8 methyl groups. Upper
middle inset: expanded region with signals of diastereotopic CH2β and
CH2γ groups. Lower middle inset: expanded region with signals of
diastereotopic CH2α groups. Lower inset: expanded region with the
proton on the meso-N atom. Features marked by an asterisk (*)
represent impurities.
3.2. Analysis of NMR Data. A detailed introduction about
the NMR analysis of TbIII−phthalocyaninato complexes, and
generally about NMR of paramagnetic complexes, has been
given in our previous work,7 in well-known works of Bertini et
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Figure 3. 1H NMR spectrum of [1HH]+[CH3COO] by protonation of [1H]0 (295.0 K, toluene-d8, 14.09 T). Upper inset: expanded region from
−2 ppm to −100 ppm. Middle inset: expanded region with eight methyl group signals. Lower inset: expanded region from −160 ppm to −175 ppm.
but can be distinguished at elevated temperatures (see the
Supporting Information). The most shifted signal in Figure 2,
at −152.50 ppm, belongs to the acidic proton on the
phthalocyaninato ligand. The sign and the magnitude of the
hyperfine shift of the signal confirm that this proton is located
on the meso-N. This is verified by DFT calculations, as will be
shown in the next section. If the proton were located on an
isoindole-N, due to the nature of eq 2, it would experience a
very large and positive δPC. Namely, with the use of eq 2, it can
easily be shown that a protonation on the meso-N atom would
result in a δPC value of approximately −161.5 ppm, whereas an
acidic proton on the isoindole-N atom would experience a δPC
value of +2073.3 ppm (see the Supporting Information). No
1H signal was observed in the chemical shift region at
approximately +2000 ppm. The χa value of the studied [1H]0
complex was estimated to be 9.50 × 10−30 m3 (see the
Supporting Information), which is close to the value
Supporting Information). The deprotonation is shown to be
reversible via the addition of acetic acid. A second protonation
with glacial acetic acid is possible, thereby inducing a color
change of the complex from green to red. The H NMR
spectrum of the doubly protonated, now cationic complex, is
1
shown in Figure 3.
The second protonation could also be observed in CD2Cl2,
but it was not complete, even with a considerably excessive
amount of acid. However, a complete double protonation is
possible in toluene. As can be seen in Figure 3, a second
protonation does not lead to an increase in the symmetry of the
complex. The most negatively shifted signal corresponds to two
1
acidic protons on meso-N atoms, and these H resonances
experience a larger δPC than the proton on the meso-N atom in
[1H]0 (Figure 2). Unlike for [1]−[H-DBU]+, the doubly
protonated complex undergoes a dynamic process at higher
temperatures (see the Supporting Information). This can be
due to either a fast proton exchange process, or it could be due
to a decreased ligand rotation barrier caused by the structural
change induced by protonations. The distinction between these
two dynamic processes cannot be made based solely on NMR
data. However, a more-detailed understanding of the structure
and molecular dynamics of the doubly protonated species is
achieved with the help of DFT calculations, as will be shown in
the next section.
3.3. Analysis of DFT Calculations. 3.3.1. Proton Position.
As the single-point calculations of the optimized geometries
indicate, it is more favorable for the proton in [Y(α-omPcH)(α-
omPc)] to be located on the meso-N, rather than on the
isoindole-N, by ca. 57 kJ/mol. This theoretical outcome is in
agreement with the NMR data, which show that the meso-N
atom is protonated in [1H]0. Our findings are in agreement
with the recent study of a mixed TbIII−(phthalocyaninato)
(porphyrinato) double-decker complex, conducted by Tanaka
et al.6 The DFT structures were also used to determine
structural parameters that are used in pseudo-contact shift (eq
2). The magnetic properties relevant for NMR spectroscopy,
such as electronic g-factors, can be obtained from relativistic
−
determined for the previously studied [Tb(β-obPc)2]
complex.7b An EXSY measurement of the less-crowded region
of the spectrum, containing signals of the CH2α groups, shows
that at 380.0 K (in toluene-d8), the CH2α protons within the α-
obPc ligand are exchanging with each other, which indicates
ligand rotation. Studies of the dynamics in lanthanide
complexes bearing porphyrinato ligands revealed that the
ligand rotation barriers can be large (e.g., Ce(MOTP)2, where
MOTP is a porphyrinato-based ligand, ΔG⧧ = 101
0.4 kJ
mol−1 at 50 °C)24 and inversely proportional to the interplanar
spacing between porphyrinato ligands.25 Hence, we expect that
the rotation barrier in TbIII−phthalocyaninato complexes is also
high. Because of the low symmetry of [1H]0 and to the small
differences in δPC of the resonances of the α-obPcH and α-obPc
ligands, it is not possible to quantify the changes in ligand−
metal distances that are induced by a protonation. However, as
will be shown, DFT provides valuable insights into the
structure of the title complex. Upon deprotonation with an
excess of DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene), [1] [H-
DBU]+ is obtained, which has simple 1H and 13C NMR spectra,
because of the D4d point group (see Figure S15 in the
D
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DFT calculations.26 However, the existing methods are unlikely
to be suitable for open-shell f-elements. An alternative to the
experimental determination of the susceptibility anisotropy are
the newly developed ab initio multireference wavefunction
methods.23e,f The DFT calculations of the four possible
permutations of proton positions on the meso-N atoms in the
doubly protonated complex show that it is unlikely that the
same α-obPc ligand gets protonated twice, and that the two
protons are likely to be on meso-N atoms on different α-obPc
ligands. This is shown in Figure 4, which depicts schematic
representations of the four permutations.
single α-obPcH ligand. In permutations 1 and 2 of [1HH]+, the
two α-obPcH ligands are chemically equivalent due to the
presence of a C2 symmetry axis, which goes through the TbIII
ion and is perpendicular to the vector that connects the two
protons on the meso-N atoms.
3.3.2. Structural Changes Induced by Protonation. All the
optimized structures showed that the α-omPc, α-omPcH, or α-
omPcH2 ligands are not in a completely staggered con-
formation, but rather slightly skewed away from the staggered
conformation. These structures have, as the lowest vibrational
mode, a libration toward the staggered conformation. This
outcome of DFT geometry optimizations and frequency
calculations was observed in our previous studies of β-obPc
double-decker complexes.7b As described previously, the
solution structure of these complexes, as observed on the
NMR time scale, corresponds to a fully staggered conformation.
Generally, the DFT optimized geometries show that a
protonation of only one phthalocyaninato ligand leaves the
interligand distance almost unchanged. However, the metal-
atom-to-ligand distance becomes inequivalent with a shorter
distance to the doubly charged Pc2− and longer to the PcH−
ligand (see the Supporting Information (Figure S24)). The
[Y(α-omPcH)2]+ structures have a d(YIII−(α-omPcH)) of 1.39
Å, which is only slightly smaller than in the [Y(α-omPc)2]− case
(1.40 Å). In structures where only a single α-omPc ligand is
protonated, the planarity of the nonprotonated α-omPc ligand
is distorted to a concave shape, as a consequence of the reduced
distance between this ligand and the metal center.
Previously, studies about the phthalocyaninato ligand
rotation barriers in LaIII−bis(phthalocyaninato) and in
heteroleptic CeIV−(porphyrinato) (phthalocyaninato) double-
decker complexes were conducted.27 Studies of dynamics in
lanthanoid complexes with porphyrinato ligands showed that
the ligand rotation barriers can be large24 and inversely
proportional to the interplanar spacing between porphyrinato
ligands.25 We provide theoretical evidence that double
protonation of the complex [Tb(α-obPc)2]− ([1]) results in
a small contraction of the interligand distance (by ca. 0.02 Å).
This contraction of the interligand distance is substantiated by
an increase of the δPC of the protons on meso-N atoms in
[1HH]+[H-DBU], when compared to [1H]0. The DFT
calculations also show that the two [Y(α-omPcH)2]+ structures
have very similar energies, and a distinction between
permutations 1 and 2 cannot be made based on the NMR
data. With these results in mind, we propose that the dynamic
process of [1HH]+[H-DBU] which is observed at elevated
temperatures, is due to a fast “hopping” of protons from one
meso-N atom to another one, rather than being due to a
lowered rotation barrier of the α-obPcH ligands induced by a
change in the interligand distance. The mechanism of proton
hopping could involve the energetically less favorable [Y(α-
omPcH2)(α-omPc)]+ structures (the structure described by
permutation 3 contributes with 0.05% to the total population at
380.0 K, according to the Boltzmann distribution).
Figure 4. Schematic representation of four permutations that describe
the position of the second proton in the cationic complex. For clarity
reasons, the ortho-butoxy chains are represented by black dots. The
point groups and number of expected signals for the methyl groups of
the butoxy chains are given in the figure, as well as the relative
energies, as obtained from single-point DFT calculations.
The [Y(α-omPcH)2]+ structure with the two protons being
as far apart as possible (permutation 1) has the lowest energy.
The [Y(α-omPcH)2]+ structure in which the two protons are
located on nearby meso-N atoms is ∼6 kJ/mol higher in energy
(permutation 2). The [Y(α-omPcH2)(α-omPc)]+ structures, in
which the protons are located on opposite (permutation 3) or
neighboring (permutation 4) meso-N atoms within the same
ligand are ca. 24 kJ/mol and ca. 35 kJ/mol higher in energy
than the structure with permutation 1, respectively. This
theoretical outcome is in agreement with the experimental
results obtained by NMR. Namely, a second protonation of the
opposite meso-N position in the same α-obPc ligand
(permutation 3, C2v point group) would lead to an increased
symmetry, which would result in a decrease in the total number
of observed signals by NMR. The existence of the permutation
4 structure is unlikely, because of its very high energy cost. In
[1H]0, the eight methyl group signals are due to four
chemically inequivalent groups from each of the two ligands
(Cs point group). On the other hand, in the C2 symmetric
permutations of [1HH]+, the eight methyl group signals stem
from the eight chemically inequivalent methyl groups within a
4. CONCLUSIONS
We report the preparation and characterization of the new
complex [Tb(α-obPc)2H)]0 ([1H]0). The anionic ([Tb(α-
obPc)2]−) and doubly protonated ([Tb(α-obPcH)2]+) com-
plexes are easily obtainable by deprotonation with DBU or by a
second protonation with acetic acid, respectively. Paramagnetic
NMR, with the help of DFT calculations, has shown that the
protonation of the anionic [Tb(α-obPc)2]− double-decker
E
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complex happens on the meso-N atom, which is in agreement
with other studies.6 DFT calculations indicate that this
protonation leads to an increased distance between the
protonated ligand and the metal center, and to a decreased
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ASSOCIATED CONTENT
* Supporting Information
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chem.5b02391.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: kkatoh@m.tohoku.ac.jp (K. Katoh).
*E-mail: yamasita@agnus.chem.tohoku.ac.jp (M. Yamshita).
*E-mail: markus.enders@uni-heidelberg.de (M. Enders).
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We express our gratitude for the following research funds: a
Ph.D. scholarship for M.D. from the Beilstein-Institut zur
■
Forderung der Chemischen Wissenschaften, Grant-in-Aid for
̈
JSPS Fellows from Japan Society for the Promotion of Science
(JSPS) (Y.H., T.M.), Tohoku University Division for Interna-
tional Advanced Research and Education (DIARE) (T.M.), a
Grant-in-Aid for Scientific Research (S) (Grant No. 20225003,
M.Y.), Grant-in-Aid for Young Scientists (B) (Grant No.
24750119, K.K.) and Scientific Research (C) (Grant No.
15K05467, K.K.) from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan (MEXT), and the
German−Japanese University Consortium (HeKKSaGOn) for
travel supports (all authors). We are grateful for the
computational resources provided by the bwForCluster
JUSTUS, funded by the Ministry of Science, Research and
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D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
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