Synthesis, Structural, and Physicochemical Characterization of a Ti6and a Unique Type of Zr6Oxo Clusters Bearing an Electron-Rich Unsymmetrical {OON} Catecholate/Oxime Ligand and Exhibiting Metalloaromaticity

Article abstract of DOI:10.1021/acs.inorgchem.0c02959

The chelating catechol/oxime ligand 2,3-dihydroxybenzaldehyde oxime (H3dihybo) has been used to synthesize one titanium(IV) and two zirconium(IV) compounds that have been characterized by single-crystal X-ray diffraction and 1H and 13C NMR, solid-state UV-vis, and ESI-MS spectroscopy. The reaction of TiCl4 with H3dihybo and KOH in methanol, at ambient temperature, yielded the hexanuclear titanium(IV) compound K2[TiIV6(μ3-O)2(μ-O)3(OCH3)4(CH3OH)2(μ-Hdihybo)6]CH3OH (1), while the reaction of ZrCl4 with H3dihybo and either nBu4NOH or KOH also gave the hexanuclear zirconium(IV) compounds 2 and 3, respectively. Compounds 1-3 have the same structural motif [MIV6(μ3-?)2(μ-?)3] (M = Ti, Zr), which constitutes a unique example with a trigonal-prismatic arrangement of the six zirconium atoms, in marked contrast to the octahedral arrangement of the six zirconium atoms in all the Zr6 clusters reported thus far, and a unique Zr6 core structure. Multinuclear NMR solution measurements in methanol and water proved that the hexanuclear clusters 1 and 3 retain their integrity. The marriage of the catechol moiety with the oxime group in the ligand H3dihybo proved to be quite efficient in substantially reducing the band gaps of TiO2 and ZrO2 to 1.48 and 2.34 eV for the titanium and zirconium compounds 1 and 3, respectively. The application of 1 and 3 in photocurrent responses was investigated. ESI-MS measurements of the clusters 1 and 3 revealed the existence of the hexanuclear metal core and also the initial formation of trinuclear M3 (M = Ti, Zr) building blocks prior to their self-assembly into the hexanuclear M6 (M = Ti, Zr) species. Density functional theory (DFT) calculations of the NICSzz scan curves of these systems revealed that the triangular M3 (M = Ti, Zr) metallic ring cores exhibit pronounced metalloaromaticity. The latter depends upon the nature of the metallic center with NICSzz(1) values equal to-30 and-42 ppm for the Ti (compound 1) and Zr (compound 2) systems, respectively, comparable to the NICSzz(1) value of the benzene ring of-29.7 ppm calculated at the same level of theory.

Full text of DOI:10.1021/acs.inorgchem.0c02959

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Article
Synthesis, Structural, and Physicochemical Characterization of a Ti6
and a Unique Type of Zr6 Oxo Clusters Bearing an Electron-Rich
Unsymmetrical {OON} Catecholate/Oxime Ligand and Exhibiting
Metalloaromaticity
Stamatis S. Passadis, Michael G. Papanikolaou, Alexander Elliott, Constantinos G. Tsiafoulis,
Athanassios C. Tsipis,* Anastasios D. Keramidas,* Haralampos N. Miras,*
and Themistoklis A. Kabanos*
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ABSTRACT: The chelating catechol/oxime ligand 2,3-dihydroxy-
benzaldehyde oxime (H3dihybo) has been used to synthesize one
titanium(IV) and two zirconium(IV) compounds that have been
characterized by single-crystal X-ray diffraction and 1H and 13C NMR,
solid-state UV−vis, and ESI-MS spectroscopy. The reaction of TiCl4
with H3dihybo and KOH in methanol, at ambient temperature,
yielded the hexanuclear titanium(IV) compound K2[TiIV6(μ3-O)2(μ-
O)3(OCH3)4(CH3OH)2(μ-Hdihybo)6]·CH3nOH (1), while the re-
action of ZrCl4 with H3dihybo and either Bu4NOH or KOH also
gave the hexanuclear zirconium(IV) compounds 2 and 3, respectively.
Compounds 1−3 have the same structural motif [MIV6(μ3-Ο)2(μ-
Ο)3] (M = Ti, Zr), which constitutes a unique example with a trigonal-prismatic arrangement of the six zirconium atoms, in marked
contrast to the octahedral arrangement of the six zirconium atoms in all the Zr6 clusters reported thus far, and a unique Zr6 core
structure. Multinuclear NMR solution measurements in methanol and water proved that the hexanuclear clusters 1 and 3 retain their
integrity. The marriage of the catechol moiety with the oxime group in the ligand H3dihybo proved to be quite efficient in
substantially reducing the band gaps of TiO2 and ZrO2 to 1.48 and 2.34 eV for the titanium and zirconium compounds 1 and 3,
respectively. The application of 1 and 3 in photocurrent responses was investigated. ESI-MS measurements of the clusters 1 and 3
revealed the existence of the hexanuclear metal core and also the initial formation of trinuclear M3 (M = Ti, Zr) building blocks prior
to their self-assembly into the hexanuclear M6 (M = Ti, Zr) species. Density functional theory (DFT) calculations of the NICSzz scan
curves of these systems revealed that the triangular M3 (M = Ti, Zr) metallic ring cores exhibit pronounced metalloaromaticity. The
latter depends upon the nature of the metallic center with NICSzz(1) values equal to −30 and −42 ppm for the Ti (compound 1)
and Zr (compound 2) systems, respectively, comparable to the NICSzz(1) value of the benzene ring of −29.7 ppm calculated at the
same level of theory.
In a similar manner, zirconium dioxide has been used in a wide
range of applications such as the degradation of organic
pollutants,9 as well as in photochemical water splitting,10 fuel
cells,11 and photocatalysis.12 The band gap of ZrO2 ranges from
3.25 to 5.1 eV depending on its phase and the method of its
preparation.13 Moreover, the field of zirconium-based MOFs
has exploded in the last few years.14
INTRODUCTION
■
Polyoxo-titanium clusters (PTCs) have drawn unprecedented
attention1 over the last few years due to their potential
applications in photocatalytic hydrogen production,2 in solar
energy conversion,3 in the degradation of environmental
pollutants,4 and in the reduction of carbon dioxide.5
The precise knowledge of the structures of PTCs, since these
oxide clusters are soluble analogues of discrete fragments of solid
TiO2, originates from the necessity to understand the binding
modes of sensitizers to Ti−O surfaces.6 The large band gap
(3.20 eV) of anatase (TiO2) limits its applications in
photocatalysis.7 The employment of organic chelators could
allow the manipulation of the band gap and presumably the
subsequent visible light absorption by PTCs.8
Received: October 7, 2020
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Previous studies demonstrated the strong chelating affinity of
catechol toward high-oxidation-state metals.15 Additionally,
oximes16 are also strong binders to the group IV transition
metals in their highest oxidation state. Thus, it was envisaged
that the incorporation of both functional groups within the same
chelating system as in the case of 2,3-dihydroxybenzaldehyde
oxime (H3dihybo, Scheme 1)17 could potentially allow us to
exploit the structural and electronic effects of both groups and
modulate the material’s band gap.
revealed quite a few examples of compounds bearing all-metal-
based aromatic rings and that were characterized as being
metalloaromatic. The most popular aromaticity/antiaromaticity
criteria are based on structural (ring planarity), energetic
(aromatic stabilization energy, ASE), magnetic (paratropicity/
diatropicity), and electronic (electron delocalization descrip-
tors) properties. As far as the electronic delocalization
descriptors are concerned, the aromaticity of organic com-
pounds arises from electron delocalization supported by the
formation of π ΜΟs (π-type aromaticity), while in all-metal
aromatic rings electron delocalization could take place through
the formation of σ, π, δ, and φ MOs. The aromaticities of three-,
four-, five-, and six-membered metal rings have been extensively
studied.18d For example, it has been reported18c that the [c-X3]−
(X = B, Al, Ga) “bare” metallic rings exhibit double (σ + π)
aromaticity. Other examples of three-membered aromatic
metallic rings are the “ligand stabilized” cyclo-M3(μ-H)3 (M =
Cu, Ag, Au) systems.18d Similar studies have also reported
aromaticity for the “bare” four-membered ring in [c-Al4]− as well
for the “ligand stabilized” [M4(CO)12] (M = Fe, Ru, Os)
systems.18f Similarly, many studies of five- and six-membered
metallic rings have also been reported.18d
Scheme 1. Ligands H3dihybo and H3pidiox
We employed herein almost the full range of aromaticity
descriptors, including structural and magnetic descriptors
(particularly the most powerful descriptor, the nucleus-
independent chemical shift (NICS)) in an effort to investigate
the system’s behavior.20 The NICS has been widely applied as a
magnetic criterion of the aromaticity of organic rings (e.g., the
NICS of the archetypica; benzene molecule at the ring center,
NICS(0), is −9.7 ppm).21 Note that comparable or even higher
NICS(0) values have been theoretically estimated for all-metal
rings.18d
In this contribution, we report the successful synthesis of
three hexanuclear polyoxometalates of the general formula
[MIV6(μ3-O)2(μ-O)3(μ-η1,η2,η1-Hdihybo-O,O′,N)6] (M = Ti,
Zr) by reacting MCl4 (M = Ti, Zr) with H3dihybo in the
presence of either KOH or nBu4NOH as a base in methanol at
room temperature. The three compounds were characterized by
single-crystal X-ray structure analysis and various physicochem-
ical techniques. Not only the core structure of the zirconium
compounds [Zr6(μ3-O)2(μ-O)3] but also the trigonal-prismatic
arrangement of the six zirconium atoms are unique. The cyclo-
M3 (M = Ti, Zr) metallic cores exhibit metalloaromaticity. The
band gap investigation of the compounds showed a very low
values of 1.48 and 2.34 eV for the titanium (1) and zirconium
(3) compounds, respectively.
Research on the band gap tuning of TOCs, via a dye
sensitization method, is an important topic and the marriage of
the catechol moiety with the oxime group in neighboring
positions in the H3dihybo (Scheme 1) ligand would lead to a
substantial reduction of the TiO2 and ZrO2 band gaps.
Moreover, the presence of the electron-withdrawing −C
N−OH group on the aromatic ring is predicted to significantly
lower the catechol protonation constant, making it a much more
effective sequestering ligand at neutral or even acidic pH values.
Here we demonstrate for the first time the effective use of the
H3dihybo ligand for the construction of multinuclear clusters,
allowing the modulation of the cluster’s band gap and the
induction of metalloaromaticity effects. The ligand’s chelating
ability has been reported only once in the case of a palladium(0)
monomeric complex,17c [Pd0(H3dihybo)], where its structure
(Scheme 2) has been proposed only on the basis of NMR and IR
data.
Scheme 2. Proposed Structure of the Palladium(0) Complex
[Pd0(H3dihybo)] on the Basis of IR and NMR Data
EXPERIMENTAL SECTION
■
Materials, Syntheses, and Physical Measurements. All
chemicals and solvents were purchased from Sigma-Aldrich and
Merck, were of reagent grade, and were used without further
purification, except TiCl4, which was distilled under high vacuum just
prior to use. Merck silica gel 60 F254 TLC plates were used for thin-
layer chromatography. C, H, and N analyses were conducted by the
microanalytical service of the School of Chemistry, the University of
Glasgow. FT-IR transmission spectra of the compounds, in KBr pellets,
were acquired using a JASCO Model 460 spectrophotometer in the
4000−400 cm−1 range. The UV−vis diffuse reflectance spectra were
recorded at room temperature on an Agilent Cary 60 UV−vis
spectrophotometer.
Furthermore, the ligand H3dihybo, which also exhibits donor
atoms in three neighboring positions of the ring, in a fashion
similar to that for the ligand H3pidiox16a (Scheme 1) gave rise to
the formation of a PTC containing two cyclo-Ti3 metallic cores
exhibiting metalloaromaticity. Metalloaromaticity is a relatively
new concept used to describe the aromaticity of all-metal rings18
with potential applications in molecular electronics.19a Addi-
tionally, the emergence of metalloaromaticity in metals, metal
oxides, and mixed-metal clusters is of vital importance in
explaining the chemical bonding as well as understanding the
electronic and surface properties of the metals, metal oxides, and
metal alloys.19b The mixed metal Al−Li forms a strong and
lightweight material which has been used extensively in
aerospace engineering.19c The ongoing research in this field
Synthesis of the Organic Molecule 2,3-Dihydroxybenzalde-
hyde Oxime (H3dihybo). To a stirred anhydrous ethanol solution (30
mL) containing NaOH (0.694 g, 17.37 mmol) and NH2OH·HCl
(1.207 g, 17.37 mmol) was added in one portion solid 2,3-
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dihydroxybenzaldehyde (1.500 g, 10.86 mmol). The reaction mixture
was refluxed for 3 h under argon. Then, the solvent was evaporated to
dryness under high vacuum to give a crude light brown mixture.
Dichloromethane (50 mL) was added to the solid, and the solution was
refluxed for 10 min under argon. The boiling solution was filtered and
cooled first to room temperature (∼20 °C) and then to 4 °C for 1 h,
during which period white crystals of the desired product were formed.
The crystals were filtered and dried under vacuum to yield 1.33 g of
H3dihybo. Yield: 80% (based on 2,3-dihydroxybenzaldehyde). Mp: 113
°C. High-resolution electrospray ionization mass spectrometry (+)
[HR-ESI(+)]-MS: calcd for C7H8NO3 ([M + H]+) 154.0504, found
154.0503. Anal. Calcd for C7H7NO3 (Mr = 153.137): C, 54.88; H, 4.61;
N, 9.15. Found: C, 54.90; H, 4.59; N, 9.13. Rf = 0.06 (CHCl3/C6H14 4/
1).
factor. Final unit cell data and refinement statistics for compounds 1−3
are collated in Table S1.
NMR Analysis. A sample solution was transferred into a 5 mm NMR
tube. NMR experiments were performed on a Bruker AV500
spectrometer (Bruker Biospin, Rheinstetten, Germany) at 298 K
using the Topsin 2.1 suite. All 1D 1H NMR spectra were collected using
a 30° flip angle, a spectral width of 14 ppm, a relaxation delay of 5 s, and
an acquisition time of 4.3 s. A total of 64 K data points were collected,
and the FIDs were treated using a line-broadening exponential function
of 0.3 Hz. Phase adjustment and baseline correction were carried out
using the Topspin 2.1 suite. Signal integration was performed manually.
1
1
1
2D H−1H TOCSY, H−13C HSQC, and H−13C HMBC NMR
experiments were recorded using standard Bruker software. T1
measurements were obtained by using the inversion recovery method.
The 2D 1H−1H TOCSY experiments were measured using 256
increments of 2 K size (each consisting of 33 scans) covering the full
spectrum (10 ppm in both dimensions). The standard grNOESY pulse
Synthesis of K2[TiIV6(μ3-O)2(μ-O)3(OCH3)4(HOCH3)2(μ-η1,η2,η1-
Hdihybo-O,O′,N)6]·CH3OH (1). To a stirred methyl alcohol solution
(4 mL) were successively added H3dihybo (69.8 mg, 0.456 mmol) and
TiCl4 (0.05 mL, 86.5 mg, 0.456 mmol). The colorless solution of the
ligand turned dark red upon addition of TiCl4. Then, addition of solid
ΚΟΗ (51.1 mg, 0.912 mmol) in one portion resulted in the formation
of a small quantity of precipitate. The solution was kept overnight at ∼4
°C (to remove most of the formed KCl); the mixture was filtered off,
and the dark red filtrate was kept at room temperature (∼20 °C) for 3−
4 days, during which period of time 77.0 mg of dark red crystals of
compound 1 were formed. The crystals were filtered off and dried under
an ambient atmosphere. Yield: 65% (based on H3dihybo). Anal. Calcd
for (C48H50N6O29K2Ti6·CH3OH, Mr = 1572.381 g mol−1): C, 37.41; H,
3.46; N, 5.35. Found: C, 37.25; H, 3.57; N, 5.25.
sequence was used in the 2D H−1H grEXSY-grNOESY measure-
1
ments. These spectra were acquired using 512 increments of 2 K size
(with 60 scans each) covering the full spectrum (10 ppm in both
dimensions) or partial (3−4 ppm) regions of the spectrum. The delay
time used in the 2D spectra was 3.0 s, on the basis of the measured T1
values. Variable mixing times ranging from 0 to 0.1 s were used. 1H−13C
1
grHSQC and H−13C grHMBC NMR experiments were measured
using 512 increments of 2 K size (each consisting of 40 scans) covering
the full spectrum (10 ppm at f2 and 160 ppm (HSQC) and 220 ppm
(HMBC) at f1 dimensions). All NMR samples were prepared by
dissolving the crystalline compounds in CD3OD at room temperature.
Photocurrent Measurements. The photocurrent measurements
of compounds 1 and 3 were caried out on FTO electrodes by drop-
casting solutions of the clusters of appropriate concentrations according
to the following procedure. Crystals (5 mg) of compounds 1 and 3 were
dissolved in 2 mL of methanol. A 0.2 μL portion of the prepared
solutions was drop-casted onto a fluorine-doped indium−tin oxide
(FTO) glass (2.5 × 5 cm). After evaporation under an ambient
atmosphere, the coated film was used as the working electrode.
Photocurrent measurements were conducted using a CHI 760E
electrochemical workstation in a three-electrode system, with an Ag/
AgCl electrode as the reference electrode and a Pt wire as the auxiliary
electrode. All of the tests were performed at the same bias potential of
+0.4 V, and an aqueous solution of Na2SO4 (0.1 mol L−1) was used as
the electrolyte. A 250 W high-pressure xenon lamp was used as a full-
wavelength light source, located 10 cm away from the FTO electrode.
The on−off irradiation intervals were 10 s.
Synthesis of [ZrIV6(μ3-O)2(μ-O)3(μ-η1,η2,η1-Hdihybo-
O,O′,N)6(OH2)6]Cl2·2Bu4NCl·2CH3OH (2). To a stirred methyl
alcohol solution (2 mL) were successively added 2,3-dihydroxybenzal-
dehyde oxime (65.7 mg, 0.429 mmol), and ZrCl4 (100.0 mg, 0.429
mmol). The colorless solution of the ligand turned orange upon
addition of ZrCl4. Then, the addition of 2.20 mL of tetrabutylammo-
nium hydroxide, 0.39 M in methyl alcohol (222.0 mg, 0.858 mmol) in
one portion, caused the formation of small amount of a white
precipitate, which was filtered off. The light orange filtrate was kept at
room temperature (∼20 °C) for 3−4 days, during which period yellow
crystals of compound 2 were formed. We were unable to prepare an
analytically pure sample on a preparative scale because compound 2 is
very hygroscopic.
Synthesis of [ZrIV6(μ3-O)2(μ-O)3(μ-η1,η2,η1-Hdihybo-
O,O′,N)6(OCH3)2(OH2)4]·2CH3OH (3). To a stirred methyl alcohol
solution (4.0 mL) were successively added 2,3-dihydroxybenzaldehyde
oxime (65.7 mg, 0.429 mmol), ZrCl4 (100.0 mg, 0.429 mmol), and
KOH (48.1 mg, 0.858 mmol). Upon addition of KOH a small amount
of a white precipitate was formed. The solution was kept overnight at
∼4 °C to remove most of the formed KCl, which was filtered off. The
light orange filtrate was kept at room temperature (∼20 °C) for 3−4
days, during which period 45 mg of yellow crystals of compound 3 were
formed. The crystals were filtered off and dried under an ambient
atmosphere. Yield: 37% (based on H3dihybo). Anal. Calcd for
(C44H44N6O29Zr6·2CH3OH, Mr = 1732.28 g mol−1): C, 31.89; H,
2.96; N, 4.85. Found: C, 31.93; H, 3.15; N, 4.80.
X-ray Crystallography. A suitable single crystal was selected and
mounted onto a rubber loop using Fomblin oil. Single-crystal X-ray
diffraction data of 1−3 were recorded on a Bruker Apex CCD
diffractometer (λ(Mo Kα) = 0.71073 Å) at 150 K equipped with a
graphite monochromator. Structure solution and refinement were
carried out with SHELXS-9722 and SHELXL-9723 using the WinGX
software package.24 Data collection and reduction were performed
using the Apex2 software package. Corrections for incident and
diffracted beam absorption effects were applied using empirical
absorption corrections.25 All of the other atoms and most of the
carbon atoms were refined anisotropically. Solvent molecule sites were
found and included in the refinement of the structures. Multiple
crystallization efforts allowed us to obtain single crystals and
subsequent data collection of compound 3. However, the poor quality
of the single crystal led to a structure solution with a slightly elevated R
ESI Mass Spectrometry. All MS data were collected using a Q-trap,
time-of-flight MS (Maxis Impact MS) instrument supplied by Bruker
Daltonics Ltd. The detector was a time-of-flight, microchannel plate
detector, and all data were processed using the Bruker Daltonics Data
Analysis 4.1 software, while simulated isotope patterns were
investigated using Bruker Isotope Pattern software and Molecular
Weight Calculator 6.45. The calibration solution used was an Agilent
ES tuning mix solution, Recorder No. G2421A, enabling calibration
between approximately m/z 100 and 3000. This solution was diluted
60/1 with MeCN. Samples were dissolved in MeOH and introduced
into the MS via direct injection at 180 μL h−1. The ion polarity for all
MS scans recorded was negative, at 180 °C, with the voltage of the
capillary tip set at 4000 V, the end plate offset at −500 V, the funnel 1
RF at 300 Vpp, and the funnel 2 RF at 400 Vpp.
Computational Details. The NICSzz scan curve was calculated
using Cartesian coordinates obtained from the X-ray structure of the
compounds as well from the model compounds employing the GIAO
(gauge-including atomic orbitals) DFT method26,27 as implemented in
the Gaussian09 series of programs28 using the PBE0 functional in
combination with the 6-31G(d,p) and the Def2-TZVP basis sets for the
nonmetal and the Ti and Zr metal atoms, respectively. The
computational protocol will henceforth be denoted as GIAO/PBE0/
Def2-TZVP(M)U6-31G(d,p)(E).
C
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RESULTS AND DISCUSSION
■
Synthesis of the Ligand H3dihybo and Its Ti and Zr
Hexanuclear Compounds. The ligand H3dihybo was
prepared via oximation of 2,3-dihydroxybenzaldehyde. The
synthesis was carried out under argon to avoid the oxidation of
the free hydroxylamine and the ligand. The organic molecule
H3dihybo was synthesized following a modified version of the
reported procedure17 to avoid some unnecessary steps. The
completion of the reaction was determined by TLC. The
extraction of the crude H3dihybo with dichloromethane proved
to be an effective purification method.
The hexanuclear Ti6 cluster 1 was prepared by reacting the
ligand H3dihybo (1 equiv) with TiCl4 (1 equiv, distilled just
prior to its use) and KOH (2 equiv) in methanol. Dark red
crystals of 1 were formed after slow evaporation of the methanol
(3 days) at ambient temperature (∼20 °C). The hexanuclear Zr6
clusters 2 and 3 were prepared according to the procedure for 1
using ZrCl4 instead of TiCl4 and Bu4NOH and KOH as bases for
the synthesis of 2 and 3, respectively.
Figure 3. Ball-and-stick plot of compound 3. Solvent molecules have
been omitted for clarity.
Scheme 3. Coordination Mode of the Ligand Hdihybo2− in
Compounds 1−3
Crystal Structures. Single-crystal X-ray diffraction analysis
revealed that compounds 1−3 have the same structural motif
(Figures 1, 2A, and 3, respectively, for 1−3), and thus, the
defined by the Zr3 structural unit. The Zr−(μ3-O) bond lengths
(2.061(4)−2.072(4) Å, average 2.067 Å) and the Zr−(μ-O)
bond lengths (2.204(4)−2.231(4) Å, average 2.217 Å) are
similar to those reported in the literature.29,30
The Zr···Zr distances in the symmetry-related structural units
Zr3(μ3-O) (3.471(1)−3.481(1) Å, average 3.474 Å) are longer
than those observed in [{(EtMe4C5)ZrIV}6(μ6-O)(μ3-O)8]·
31
C7H8 (3.1542(9)−3.1709(11) Å, average 3.1645 Å) but
shorter than those in [{(Cp*ZrIV)6(μ4-O)(μ-O)4(μ-OH)8]·
32
2C7H8 (3.564(1)−3.608(1) Å, average 3.586 Å). The three
Figure 1. Ball-and-stick plot of the anion of 1.
angles in the symmetry-related Zr3 triangles are almost 60°, and
thus, it is obvious that the Zr3 triangles are equilateral in a
manner similar to the Ti3-based triangles observed in 1. The
d(ZrIV−Ocat)av and d[ZrIV−(μ-Ocat)]av values are 2.120(4) and
2.217(4) Å, respectively, and are very close to those reported in
the literature.33 The deprotonated catecholate moiety adopts a
singly bridging chelate μ-(Ο,Ο′,Ο′) mode.33 The d(ZrIV−Nox)av
value of 2.384(6) Å is much longer in comparison to the
reported value of ∼2.20 Å,34 but in the latter case the oxime is
deprotonated. Compound 2 is the first example of a polyoxo-
catecholate zirconium compound to be reported.
Figure 2. Ball-and-stick plot of the cation of 2 (A). Ball-and-stick plot of
the structural unit [Zr3(μ3-O)(μ-η1:η2:η1-Hdihybo-O,O′,N)3(OH2)3]
(B). Polyhedral/ball-and-stick representation of the cation of 2 (C).
All of the ZrIV discrete polyoxo clusters in the only three
6
different Zr6 core structures ([ZrIV6(μ6-Ο)(μ3-Ο)8],24
[ZrIV6(μ4-Ο)(μ-Ο)4(μ-ΟH)8],32 and [ZrIV6(μ3-Ο)4(μ3-
ΟH)4]35) reported thus far have the six zirconium atoms in an
octahedral arrangement. An octahedral Zr6 arrangement has also
been observed in the ZrIV6-MOFs.14 Thus, the Zr6 discrete
polyoxo cluster [ZrIV6(μ3-Ο)2(μ-Ο)3] of this study is a unique
example with a trigonal-prismatic arrangement of the six
zirconium atoms and a unique Zr6 core structure.
In marked contrast to the missing polyoxo-catecholate
zirconium clusters, there are a few examples of polyoxo-
catecholate titanium clusters.1h,3b,36 In these polyoxo-titanium
catecholate clusters the main binding modes of the deproto-
nated catecholate moiety are the singly bridging chelate μ-
(O,O′,O′) and the singly bridging μ-(O,O′).
structure of compound 2 will be reported in detail. The
centrosymmetric structure of compound 2 consists of six
zirconium atoms, three μ-O, two μ3-O, and six (μ,η1:η2:η1-
Hdihybo2−) ligands (Figure 2B and Scheme 3), and six terminal
water molecules. The six zirconium atoms are seven-coordinate,
with an NO6 coordination environment in a pentagonal-
bipyramidal arrangement. The three edge-sharing zirconium
atoms, via μ3-O and a μ-Ophenoxy atoms (Figure 2B), are linked by
one μ3-O atom to generate a Zr3(μ3-O) subunit. The two
Zr3(μ3-O) subunits, featuring a pyramidal mode, are connected
by three O2− atoms via a vertex-sharing mode to form a Zr6O5
core structure (Figure 2C). The symmetry-related μ3-O atoms
in the two Zr3(μ3-O) subunits are 0.501(3) Å above the plane
D
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Figure 4. Solid-state UV−vis−NIR spectra of compounds 1, 3, and 4 (A). Tauc plots of compounds 1, 3 and 4 (B).
Table 1. 1H NMR Chemical Shifts (ppm) for H3dihybo, 1,
and 3 and the Shielding/Deshielding Effect (Δδ, ppm) upon
Complexation
A comparison of the metrical parameters of the titanium
cluster 1 with those of the zirconium cluster 2 reveals the
following. In the case of the titanium isostructural cluster 1 the
Ti6 metallic core appears slightly contracted with the relevant
Ti−(μ3-O) distances found to be 1.955(3)−1.958(3) Å)
(average 1.957 Å) and the Ti−(μ-O) bond lengths found to
be 2.102(3)−2.072(3) Å (average 2.089 Å). Additionally, the
T···Ti distances in the symmetry-related structural units Ti3(μ3-
O) (3.300(1)−3.292(1) Å, average 3.296 Å) are shorter than
those observed in the Zr analogue (3.471(1)−3.481(1) Å,
average 3.474 Å). The three angles in the symmetry-related Ti3
triangles are almost 60°, forming Ti3 equilateral triangles in a
manner similar to that for the Zr3-based triangles observed in 2.
The metrical parameters of the structure of compound 3 (Figure
3) are very similar to those observed in 2.
At this point, it is worth noting that the ligands H3dihybo and
H3pidiox16a (Scheme 1), which have donor atoms in three
neighboring positions of the rings, gave the hexanuclear titanium
compounds 1 and K6[TiIV6(μ3-O)2(μ2-O)3(OCH3)6(μ2-Hpi-
diox)4(μ2-pidiox)2]·7.5CH3OH (4),16a respectively, with the
same core structure [Ti6(μ3-Ο)2(μ-Ο)3]. This observation
might demonstrate a new strategy to control the formation of
hexanuclear titanium clusters.
H3dihybo
1/Δδ*
3/Δδ*
H(7)
H(4)
H(5)
H(6)
8.19
6.80
6.72
6.76
7.85/−0.34
6.78/−0.02
6.86/+0.16
6.43/−0.33
7.86/−0.33
6.83/+0.03
6.74/+0.02
6.32/−0.41
Table 2. 13C NMR Chemical Shifts for H3dihybo, 1, and 3 and
the Shielding/Deshielding Effect (Δδ) upon Complexation
a
a
H3dihybo
1/Δδ
3/Δδ
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
117.6
146.5
146.4
120.4
118.8
121.7
152.3
114.8/−2.8
152.8/+6.3
153.6/+8.2
115.5/−4.9
121.6/+2.8
123.4/+1.7
148.1/−4.2
115.8/−1.8
154.2/+8.7
153.9/+7.5
118.6/−1.8
121.0/+2.2
123.8/+2.1
151.5/−0.8
a
Δδ = δM‑L − δL.
((1H,13C) HSQC/HMBC spectra, see Figures S1 and S2). The
numbering of the carbon atoms of the ligand H3dihybo is
depicted in Scheme 4.
Solid State UV−Vis−NIR Spectroscopy. Figure 4 shows
the solid-state UV−vis−NIR spectra of compounds 1, 3, and
4.16a A comparison of the spectra of the titanium compounds
416a and 1 (this work) reveals a 200 nm expansion of the
absorbance band of 1 related to 4 toward the red region of the
spectra (Figure 4). The band gaps for the three compounds were
found to be 1.48, 2.34, and 2.13 eV, respectively, and were
calculated from the solid state spectra by the Kubelka−Munk
method.37 These low band gap values for compounds 1 and 3
reveal their potential use as semiconducting photocatalysts, and
thus, it is crystal clear that the marriage of the catechol moiety
with the oxime group was quite successful in the modulation of
the band gaps of TiO2 and ZrO2. At this point, it is worth noting
that the 1.48 eV band gap in 1 constitutes the lowest value
reported thus far for any titanium oxide cluster. The synergistic
effect of the oxime group presumably plays a vital role in
achieving such a low value for the band gap in 1.
1
The H and 13C NMR spectra of compounds 1 and 3 in
solution gave peaks shifted in comparison to the peaks of the free
ligand due to the ligation of Hdihybo2− to the Ti and Zr metal
ions. In particular, C(2) and C(3) show a strong deshielding for
both 1 (+6.3 and +8.2 ppm; see Table 2) and 3 (+8.7 and +7.5
Scheme 4. Numbering of the Carbon Atoms of the Ligand
H3dihybo
NMR Spectroscopy. Table 1 displays the 1H NMR chemical
shifts of the ligand H3dihybo and of compounds 1 and 3 in
solution (CD3OD), and Table 2 displays the 13C NMR chemical
shifts of H3dihybo, 1, and 3 in solution as assigned by 2D spectra
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Figure 5. 2D {1H} NOESY spectrum of compound 1 and the assignments of the NOESY interactions.
Figure 6. Negative mode electrospray ionization mass spectrum (ESI-MS) of the {Zr6} (3) cluster in CH3OH.
ppm; see Table 2) due to the ligation of the deprotonated
catecholate oxygen atoms with the metal atoms. The
coordination of the metal ions to the oxime nitrogen atom
causes shielding of C(7) for both 1 (−4.2 ppm; see Table 2) and
3 (−0.8 ppm; see Table 2). The proton attached to the C(7)
carbon atom is also highly deshielded for both complexes (Δδ ≈
+4.2 ppm) presumably due to the complexation of the metal
atoms with the oxime nitrogen atom and/or the formation of
intramolecular hydrogen bond between the oxime proton of one
ligand with the catecholate oxygen atom (C(3)−O−) of the
neighboring Hdihybo2−.
The differences in the C and H shielding/deshielding values
between 1 and 3 might be due to the different through-space
neighboring anisotropy interactions induced by the applied
magnetic field on either the Ti−O and Zr−O bonds or the Ti3O
and Zr3O metalloaromatic cores, respectively (see Tables 1 and
2).
properties of molecules around the solute of the proton, minor
variance in temperature, etc., alter the “expected trend”.
The 2D {1H} NOESY-EXSY spectra of compounds 1 and 3 in
solution gave strong NOESY cross peaks between the
neighboring protons of the ligand (H(7)−H(6), H(6)−H(5),
H(5)−H(4)) as shown in Figure 5. In addition to the intraligand
proton interactions, the 2D {1H} NOESY-EXSY spectra gave
weaker NOESY peaks between the protons H(7)−H(4),
H(7)−H(5), and H(4)−H(6) (Figure 5). These NOESY
peaks were assigned to the interactions between the protons of
the ligands belonging to the two M3(Hdihybo2−)3 planes in the
complexes, as shown for complex 1 in Figure 5. The intensity of
the NOESY cross peaks suggests that the distances between the
protons 4−7 are H(7)−H(4) < H(4)−H(6) < H(7)−H(5),
which are in agreement with the distances in the crystal
structures of 1 and 3. Thus, compounds 1 and 3, in solution,
retain their structural integrity. The NMR spectra of 1 and 3 in
D2O gave features similar to those observed in deuterated
methanol, and this fact supports that 1 and 3 are hydrolytically
stable.
The Δδ values of H(4) protons in 1 and 3 do not follow the
expected trend: that is, both are either negative or positive. A
reasonable explanation for this is that these values are so small,
close to zero, in marked contrast to those of all hydrogen and
carbon atoms and even slight differences, such as solvent/bulk
ESI-MS Spectrometry. In an effort to characterize further
the hexanuclear zirconium(IV) and titanium(IV) clusters in
solution, we employed high-resolution ESI-MS to determine
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Table 3. Representation of the Experimentally Identified and Simulated m/z Values of Distribution Envelopes of {Zr6} Cluster
a
exptl
theor
charge
formula
1716.36
1512.38
1494.37
1476.36
1458.36
1377.49
1357.47
1341.46
773.75
1716.49
1512.48
1494.47
1476.46
1458.45
1377.59
1357.56
1341.57
773.85
−1
−1
−1
−1
−1
−1
−1
−1
−1
−1
−1
{ZrIV2ZrIII4O5(C7NO3H4)6(OCH3)(OH2)2K3H5}−
{ZrIV2ZrIII4O5(C7NO3H4)5(OH2)3K2H3}−
{ZrIV2ZrIII4O5(C7NO3H4)5(OH2)2K2H3}−
{ZrIV2ZrIII4O5(C7NO3H4)5(OH2)K2H3}−
{ZrIV2ZrIII4O5(C7NO3H4)5K2H3}−
{ZrIV3ZrIII3O5(C7NO3H4)4(OCH3)3(OH2)3H3}−
{ZrIV5ZrIIIO5(C7NO3H4)4(OCH3)3(OH2)2H}−
{ZrIV3ZrIII3O5(C7NO3H4)4(OCH3)3(OH2)H3}−
{Zr3ΙΙΙO(C7NO3H4)2(OCH3)3(OH2)5H}−
{Zr3ΙΙΙO(C7NO3H4)2(OCH3)3(OH2)4H}−
{Zr3ΙΙΙO(C7NO3H4)2(OCH3)3(OH2)3H}−
755.75
737.74
755.85
737.84
a
Contamination of compound 3 with small amounts of KCl during the synthesis can be easily detected by the high-resolution electrospray
ionization Q-TOF instrument with a detection limit much lower than that of the elemental analysis.
Figure 7. Negative mode electrospray ionization mass spectrum (ESI-MS) of {Ti6} (1) cluster in CH3OH.
unambiguously the structural integrity and composition of the
zirconium- and titanium-based species in solution.38 The ESI-
MS studies were performed in methanol. In both cases, the
observation of a higher intensity set of distribution envelopes is
due to the existence of the hexanuclear moiety, resulting from
the combination of protons, counterions, and solvent molecules.
In the case of the zirconium species 3, the region of higher m/z
values is populated by a series of distribution envelopes assigned
to −1 charged species and correspond to the intact {Zr6} cluster
(Figure 6). In this case three of the coordinating axial groups
have been removed during the ionization process (e.g.,
{ZrIV2ZrIII4O5(C7NO3H4)6(OH2)2(OCH3)K3H5}−; Table 3).
In both cases, the observation of different oxidation states for the
metal centers and the presence of other fragments during the
course of the studies is due to the ionization and transfer
processes and has been observed in numerous occasions.39−41
Interestingly, the lower m/z region (ca. 700−800) revealed
the presence of fundamental trimeric building blocks generated
during the ionization process. The distribution envelopes
centered at m/z 773.75, 755.75, and 737.74 have been identified
as the intact Zr−oxo-centered triangles associated with a
different number of counterions and solvent molecules. The
identification of the building blocks that appear in the mass
spectrometry studies suggests the existence of an underlying
sequential mechanism of assembly, where the initial formation
of trimeric clusters takes place prior to their subsequent self-
assembly into the hexanuclear species that finally crystallize. A
similar behavior has been observed in the solution studies
conducted for {Ti6} species reported recently.16a More
specifically, we observed the fully reduced {Zr3ΙΙΙO-
(C7NO3H4)2(OCH3)3(OH2)5H}− (773.75), {Zr3ΙΙΙO-
(C7 NO3 H4 )2 (OCH3 )3 (OH2 )4 H}− (755.75), and
{Zr3ΙΙΙOC7NO3H4)2(OCH3)3 (OH2)3H}− (737.74) Zr-based
oxo-centered molecular triangles, respectively.
In a similar manner, investigation of the titanium adduct in
solution under the same experimental conditions revealed a
series of distribution envelopes with different amounts of solvent
molecules associated with the intact hexanuclear cluster of the
general formula {TiIV3TiIII3O5(C7NO3H4)5(OH2)x}−, where x =
0−8 (Figure 7 and Table 4). Smaller fragments of the cluster
Table 4. Representation of the Experimentally Identified and
Simulated m/z Values of Distribution Envelopes of {Ti6}
Cluster
exptl
theor.
charge
formula
1234.55
1252.56
1270.57
1288.58
1306.59
1342.61
1360.61
1388.61
921.75
1234.66
1252.66
1270.67
1288.57
1306.69
1342.71
1360.71
1388.72
921.81
−1
−1
−1
−1
−1
−1
−1
−1
−1
−1
{TiIV3TiIII3O5(C7NO3H4)5}−
{TiIV3TiIII3O5(C7NO3H4)5(OH2)} −
{TiIV3TiIII3O5(C7NO3H4)5(OH2)2}−
{TiIV3TiIII3O5(C7NO3H4)5(OH2)3}−
{TiIV3TiIII3O5(C7NO3H4)5(OH2)4}−
{TiIV3TiIII3O5(C7NO3H4)5(OH2)6}−
{TiIV3TiIII3O5(C7NO3H4)5(OH2)7}−
{TiIV3TiIII3O5(C7NO3H4)5(OH2)8}−
{TiIV4TiIIIO5(C7NO3H4)4H2}−
939.75
939.82
{TiIV4TiIIIO5(C7NO3H4)4H2(OH2)} −
occurring during the studies were observed at lower m/z values,
such as {TiIV4 TiIII O5(C7 NO3H4)4 H2 }− (921.75),
{TiIV4TiIIIO5(C7NO3H4)4H2(OH2)}− (939.75), and
{TiIV4TiIIIO5(C7NO3H4)4H2(OH2)}− (939.75).
Photocurrent Response. The charge-separation efficiency
of a semiconductor material is one of the important parameters
which define its potential for practical applications. The
photocurrent responses of two photoelectrodes based on
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compounds 1 and 3 have been determined using a three-
electrode setup. As shown in Figure 8, clear photocurrent
ring center (Figure 9). The molecular structures of the model
compounds used in this study along with the positions of the Bq
ghost atoms are depicted in Figure 9.
Figure 8. Photocurrent densities of compounds 1 and 3 (0.1 M Na2SO4
solution).
responses were observed for both compounds. Upon illumina-
tion, the photocurrents were quickly generated without an
obvious intensity decrease and remained constant during the
measurement. When the light source was switched off, the
photocurrents rapidly decayed, indicative of their good
photoelectric response and photophysical stability during the
experiment. The two compounds exhibited comparable photo-
current densities per gram of deposited material and were found
to be approximately 40 mA cm−2 g−1.
Theoretical Study. Having in mind the previously
reported16a observation of the metalloaromaticity phenomenon
exhibited by the hexanuclear titanium compound 4 with the
ligand (2Z,6Z)-piperidine-2,6-dione dioxime (H3pidiox,16a
Scheme 1), we investigated whether the cyclic trinuclear
metallic ring cores MIV (M = Ti, Zr) of 1 and 2 exhibit
3
metalloaromaticity18g as well, employing DFT electronic
structure calculation methods.16a The diatropic/paratropic
ring current signals are associated with the aromaticity/
antiaromaticity of the metallic ring cores. In this context, the
nucleus independent chemical shift (NICS), by computation of
the respective NICSzz scan curves, could be used very effectively
as good indicators of aromaticity/antiaromaticity of cyclic
metallic ring cores.20b,21,42
Accordingly, we calculated the NICSzz scan curve of the cyclo-
M3 trinuclear metallic cores (M = Ti or Zr) with the GIAO
(gauge-including atomic orbitals) DFT method26,27 as imple-
mented in the Gaussian09 series of programs28 employing the
PBE0 functional in combination with the 6-31G(d,p) and the
Def2-TZVP basis sets for the nonmetal and the Ti and Zr metal
atoms, respectively. The computational protocol will henceforth
be denoted as GIAO/PBE0/Def2-TZVP(M)U6-31G(d,p)(E).
The NICSzz curves were calculated for the model systems
[MIV3(μ3-O)(μ-Hdihybo)3)(OCH3)3(OH)3] (M = Ti (1M),
Zr (2M)), where the coordination environments of 1 and 2 were
truncated by the use of methoxy and hydroxyl groups in order to
reduce the computational cost. In addition, we have calculated
the NICSzz curves for the simpler model [MIV3(μ-Hdihybo)3)]
(M = Ti (3M), Zr (4M)) complexes in order to study the effect
of the μ3-O ligand on the aromaticity/antiaromaticity of the
trimetallic ring cores. The NICSzz scan curves were obtained by
placing Bq ghost atoms on a line, coinciding with the z axis,
being vertical to the trimetallic ring core and passing through the
Figure 9. NICSzz scan profiles of 1M ([TiIV3(μ3-O)(μ-Hdihy-
bo)3(OCH3)3(OH)3]) and 2M ([ZrIV3(μ3-O)(μ-Hdihy-
bo)3(OCH3)3(OH)3]) (a) and 3M ([TiIV3(μ-Hdihybo)3]) and 4M
([ZrIV3(μ-Hdihybo)3]) (b) calculated at the PBE0/6-31G(d,p) level
(red lines for 1M/3M, green lines for 2M/4M).
Figure 9a shows the calculated NICSzz scan curves for the
model compounds bearing the μ3-O ligand, 1M and 2M (solid
green and red lines, respectively). The NICSzz scan curves for
the model compounds without the μ3-O ligand, 3M and 4M
(dashed green and red lines, respectively) are depicted in Figure
9b.
Let us first examine the model systems bearing the μ3-O ligand
which is capping the M3 ring. Inspection of Figure 9a (solid
green line) reveals that the NICSzz curve of 1M exhibits a
NICSzz(0) value equal to −14.5 ppm while the NICSzz(1) value
amounts to −18.4 ppm. Both values indicate that the cyclo-Ti3
metallic ring core is aromatic. The NICSzz scan curves exhibiting
two maxima at a certain distance above and below the molecular
plane defined by the three metal atoms and the three Hdihybo2−
ligands are typical for σ + π aromatics where the π-diatropic ring
current overwhelms the σ-type current.43
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Figure 10. 3D plots of the molecular orbitals and significant Tx,y-allowed transitions leading to the diamagnetic ring currents in (a) 1M, (b) 2M, (c)
3M, and (d) 4M.
The NICSzz curve of 2M, depicted in Figure 9a (solid red
line), follows the pattern of the respective NICSzz curves
calculated for the Ti system (Figure 9a, solid green line). Along
these lines the NICSzz(0) value is equal to −40.1 ppm and the
NICSzz(1) value amounts to −37.3 ppm, while the NICSzz(−1)
value is only −5.2 ppm. For the Zr model system we again
observe three aromaticity zones ranging from strongly aromatic
to slightly aromatic in a manner similar to that of its Ti
counterpart model system. It is worth noting that the
aromaticities of these model systems are comparable to that
reported for a similar system16a bearing a cyclo-Ti3 trinuclear ring
core with an NICSzz(1) value of −23.2 ppm. Obviously, the
magnitude of aromaticity of the cyclo-M3 trinuclear ring cores
depends on the nature of the metal centers as well as of the
surrounding ligands.
In order to further delineate the obscured aromaticity of the
metallic ring cores of these systems, we have also calculated the
NICSzz curves for the even simpler model systems, namely
[MIV3(μ-Hdihybo)3)] (M = Ti (3M), Zr (4M)). In these
systems we have removed the μ3-O capping ligands as well as the
methoxy and hydroxy ligands, protruding out of the metallic ring
core. Figure 9b reveals that indeed the trimetallic ring cores in
both 3M and 4M are expected to be aromatic, with the
phenomenon being more pronounced for M = Zr. The
NICSzz(0), NICSzz(1), and NICSzz(−1) values are −23.5,
−13.0, and −18.6 ppm, respectively, for 3M and −87.2, −36.4,
and −45.7 ppm, respectively, for 4M. The much higher aromatic
character of the Zr metallic ring can be attributed probably to the
stronger overlap of the more diffuse 4d orbitals of Zr in
comparison to the 3d orbitals of Ti forming the delocalized d
orbital aromatic system on the respective metallic ring cores.
Upon removal of the μ3-O ligand and the methoxy and
hydroxy ligands a dramatic changes in the NICSzz curves (Figure
9b) of the trimetallic cores of 3M and 4M are observed. More
specifically, for 3M and 4M the NICSzz curves exhibit only one
broad maximum with their shape being indicative of σ
aromaticity.43
cores is further corroborated by examining the shapes of the
MOs involved in the Tx,y-allowed transitions that contribute to
the diamagnetic ring current (Figure 10). For 3M the Tx,y-
allowed transition arises from a HOMO-2 → LUMO transition
where the HOMO-2 is a delocalized σ bonding MO, constructed
mainly from Ti 3d AOs (Figure 10c). For 4M there are two Tx,y-
allowed transitions i.e., HOMO-2 → LUMO and HOMO →
LUMO, where both HOMO-2 and HOMO are σ delocalized
bonding MOs, constructed mainly from Zr 4d AOs (Figure
10d).
For 1M, there is only one Tx,y-allowed transition inducing a
diamagnetic ring current involving HOMO-1 and LUMO
(Figure 10a). The former is a π-type bonding MO, while the
latter is an antibonding MO, both constructed mainly from the
2
in-phase linear combination of dz AOs of the three Ti metal
centers. The HOMO-1 → LUMO transition corresponds tos
relatively small excitation energy.
The shape of the NICSzz curve of 2M (Figure 9a, solid red
line) is only qualitatively like that found for 1M.
In contrast to 1M, the NICSzz(1) and NICSzz(−1) values of
2M differ significantly, being −3 and −42 ppm, respectively.
Obviously, the μ3-O ligand capping the cyclo-Zr3 ring
significantly suppresses the NICS(1) value in comparison to
1M. Note, however, that the NICS(0) valued calculated for 1M
and 2M amount to −38 and −32 ppm, respectively, both
signifying aromaticity. Both, the NICSzz scan curve shapes and
the Tx,y-allowed transitiond inducing s diamagnetic ring current
involving the HOMO-1 and LUMO (Figure 10b) point to a σ +
π aromatic system, where the π-diatropic ring current over-
whelms the σ-type current. Note that the Zr system exhibits
enhanced aromaticity in comparison to its Ti homologue
system. From the results discussed so far, it can be concluded
that the aromaticity of a metallic ring core depends upon the
nature of its metallic center. Moreover, we wish to point out that
this is the very first example of a Zr3 triangle which exhibits
metalloaromaticity.
CONCLUSIONS
Figure 10 shows 3D plots of the highest occupied molecular
orbitals (HOMOs) and the lowest unoccupied molecular
orbitals (LUMOs) along with the lowest Tx,y-allowed transitions
calculated for 3M and 4M. The σ aromaticity of the metallic ring
■
In conclusion, a series of hexanuclear titanium(IV) and
zirconium(IV) compounds were synthesized by a simple
three-component reaction, at room temperature. The three
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compounds have the same core structure [MIV6(μ3-Ο)2(μ-Ο)3]
(M = Ti, Zr), which constitutes a unique example with a
trigonal-prismatic arrangement of the six zirconium(IV) atoms
and a unique ZrIV6 structural motif.
Authors
Stamatis S. Passadis − Section of Inorganic and Analytical
Chemistry and, University of Ioannina, Ioannina 45110,
Greece
The trend observed in the formation of [MIV6(μ3-Ο)2(μ-Ο)3]
(M = Ti, Zr) structural motifs with organic molecules having
donor atoms in three neighboring positions of the ligand’s ring
(ref 16a and this work) might reveal a new design principle to
control the formation of titanium and zirconium hexanuclear
clusters.
Michael G. Papanikolaou − Section of Inorganic and Analytical
Chemistry and, University of Ioannina, Ioannina 45110,
Greece
Alexander Elliott − West CHEM, School of Chemistry,
University of Glasgow, Glasgow G12 8QQ, U.K.
Constantinos G. Tsiafoulis − NMR Center, University of
Ioannina, Ioannina 45110, Greece; orcid.org/0000-0002-
8254-0187
Multinuclear solution NMR showed that these hexanuclear
compounds retain their integrity in both methanol and water
solvents, and thus, their hydrolytic stability makes them
potential building blocks for the preparation of titanium(IV)
and zirconium(IV) metal organic frameworks (MOFs).
The nucleus-independent chemical shift (NICS) magnetic
criterion was used in this study and revealed the aromaticity of
the MIV3 (M = Ti, Zr) rings. ESI-MS measurements also support
the initial formation of the M3 structural units prior to their self-
assembly into the hexanuclear M6 species. The metalloaroma-
ticity of the M3 rings and the ESI-MS measurements suggest that
the M3 rings’ formation play an important role in the synthesis of
these hexanuclear clusters.
The successful marriage of the catechol moiety with the oxime
group in the ligand H3dihybo led to a substantial band gap
reduction of the hexanuclear clusters in comparison to TiO2 and
ZrO2 and induced photocurrent responses in 1 and 3. The
present study opens up new avenues for the rational design of
titanium- and zirconium-based clusters absorbing in the NIR
region of the spectrum for numerous applications.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02959
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research work was supported by the Hellenic Foundation
for Research and Innovation (HFRI) under an HFRI Ph.D.
Fellowship grant (Fellowship Number 1553) and funded by the
European Regional Development Fund and the Republic of
Cyprus through the Research and Innovation Foundation
(Project: EXCELLENCE/1216/0515). H.N.M. thanks the
University of Glasgow for supporting this work and Prof. L.
Cronin for providing access to the X-ray and ESI-MS facilities.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02959.
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sı
UV−vis-NIR, FT-IR, and NMR spectra and final unit cell
data (PDF)
Accession Codes
CCDC 2013476−2013478 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
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AUTHOR INFORMATION
Corresponding Authors
■
Athanassios C. Tsipis − Section of Inorganic and Analytical
Chemistry and, University of Ioannina, Ioannina 45110,
Greece; orcid.org/0000-0002-0425-2235;
Email: attsipis@uoi.gr
Anastasios D. Keramidas − Department of Chemistry,
University of Cyprus, Nicosia 1678, Cyprus; orcid.org/
0000-0002-0446-8220; Email: akeramid@ucy.ac.cy
Haralampos N. Miras − West CHEM, School of Chemistry,
University of Glasgow, Glasgow G12 8QQ, U.K.;
orcid.org/0000-0002-0086-5173;
Email: charalampos.moiras@glasgow.ac.uk
Themistoklis A. Kabanos − Section of Inorganic and Analytical
Chemistry and, University of Ioannina, Ioannina 45110,
Greece; orcid.org/0000-0001-6944-5153;
Email: tkampano@uoi.gr
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