Showcasing research from Professor Awaga’s laboratory,
Department of Chemistry, Nagoya University, Nagoya,Japan.

As featured in:Volume 59
Number 67
28 August 2023
Pages 10035–10178

Ideal trigonal prismatic coordination geometry of Co(II)in a honeycomb MOF with a triptycene-based ligand

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Chemical Communications
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Ideal trigonal prismatic coordination geometry of the
Co ion was achieved in a two-dimensional honeycomb
MOF structure that consists of two simple components,
namely a triptycene-based non-planar 3-fold symmetricbridging ligand and a Co ion.
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Joshua Telser, Daniel J. Mindiola et al.
A mononuclear, terminal titanium(III) imido

See Masahisa Tsuchiizu,
Kunio Awaga et al.,Chem. Commun., 2023, 59, 10105.

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Cite this: Chem. Commun., 2023,
59, 10105

Ideal trigonal prismatic coordination geometryof Co(II) in a honeycomb MOF with atriptycene-based ligand†Received 22nd June 2023,
Accepted 20th July 2023

Yoshiaki Shuku,a

Rie Suizu,abMasahisa Tsuchiizu*c and Kunio Awaga*a

DOI: 10.1039/d3cc02986g
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A metal–organic framework (MOF) comprised of cobalt ions and triptycene-based 3-fold symmetric bridging ligands 9,10-[1,2]benzenoanthracene-2,3,6,7,14,15(9H,10H)-hexaone (o-TT) was prepared.
Single-crystal structure analysis revealed a 2D honeycomb network
structure and the ideal trigonal prismatic geometry of the Co(II) ion.
The magnetic anisotropy of the Co(II) ion in the trigonal prism
coordination geometry was analyzed via magnetic measurements
and model calculations.

Six-coordinate complexes with an octahedral coordination geometry are the most frequently observed for the first row transition metal ions. Another representative six-coordinate geometryis the trigonal prism, although far fewer examples of thisgeometry have been reported.1,2 Ligand field theory predicts
that the ligand field stabilization energy (LFSE) of trigonal
prismatic geometry is equal to that of octahedral geometry only
for the d0, d1, d2 (LS), d5 (HS), d6 (HS) and d10 electronic
configurations, and is less stable for the rest.3 The reported
structural analyses of 6-coordinate complexes indicate thatsteric or electronic eﬀects other than the LFSE are importantto achieve the trigonal prismatic geometry.1,2 One of theeﬀective strategies to form the trigonal prismatic geometry is
to use a multi-dentate (tetra- or hexa-dentate) ligand that cansterically restrain the coordination geometry. In recent years,
the syntheses of distorted trigonal prismatic cobalt complexes
with hexadentate ligands have been reported, and the resulting

complexes show single molecule magnet (SMM) behaviourin the trigonal prismatic geometry.4,5 This unique magnetic
behaviour due to the strong anisotropic nature of the cobalt ion
motivated us to design a trigonal prismatic coordination geometry for the development of magnetic materials.
In our previous studies, we demonstrated that triptycene
derivatives with p–p interactions can be building blocks for thehoneycomb structure in their crystals.6,7 Recently, a triptycene
derivative with three o-quinone moieties was reported to form a
2D honeycomb metal–organic framework (MOF) by forming asquare planar coordination (Scheme 1d),8 with a packing motif
similar to that of the MOFs of planar tris-bidentate bridging
ligands such as HHTP (Scheme 1a and c).9,10 Here we would likea

Department of Chemistry & Integrated Research Consortium on Chemical Sciences(IRCCS), Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.
E-mail: awaga.kunio.h8@f.mail.nagoya-u.ac.jpbJapan Science and Technology Agency (JST), PRESTO, 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japanc
Department of Physics, Nara Women’s University, Kitauoyanishi-machi, Nara 6308506, Japan† Electronic supplementary information (ESI) available: Details of the sample
preparations, crystal structure analysis, model calculation of magnetic anisotropy, magnetic data and PXRD data. CCDC 2255851 and 2281350. For ESI andcrystallographic data in CIF or other electronic format see DOI: https://doi.org/10.
1039/d3cc02986g

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Scheme 1 Molecular structures (a and b) and schematic honeycomb
packing motifs (c–e) of 3-fold symmetric bridging ligands HHTP and oTT. Gray and purple building blocks represent the bridging ligands and thecoordination geometry, respectively.

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to emphasize that the trigonal prismatic coordination geometry is
also allowed as the building block for the 2D honeycomb network
structure when the triptycene-based bridging ligand is used,because of the unique trigonal prismatic arrangement of bidentate coordination sites in the triptycene-based bridging ligand(Scheme 1e). From this point of view, the formation of a honeycomb structure can provide steric effects to stabilize the local
trigonal prismatic coordination structure.
In the present work, we prepared a cobalt complex of the
9,10-[1,2]benzenoanthracene-2,3,6,7,14,15(9H,10H)-hexaone
ligand (o-TT; Scheme 1b), namely [Co(o-TT)], and showed that it
contains a honeycomb network that consists of the o-TT ligand
and the trigonal prismatic coordination geometry of the Co ion.
Since the anisotropic axes of the trigonal prismatic ions werealigned in the same direction in the crystal structure, magnetic
measurements on [Co(o-TT)] clearly revealed the uniaxial magnetic anisotropy of the Co ion, which was consistent with thetheoretical model calculations.
o-TT was prepared using the method described in theliterature.11 The synthesis and crystal growth of [Co(o-TT)]
3CH3CN is described in the ESI.† Details of the X-ray crystal
analysis are also shown in the ESI.† Fig. 1 shows the crystal
structure of [Co(o-TT)]3CH3CN, which belongs to the orthorhombic Pbca space group (#61). The crystal structure consists ofFig. 1 Projections of the crystal structure of [Co(o-TT)] along the b axis (a)and along the a axis (b). Gray, light blue, red and purple spheres represent
carbon, hydrogen, oxygen and cobalt ions, respectively. Acetonitrile
molecules were omitted for clarity.

10106 | Chem. Commun., 2023, 59, 10105–10108

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Fig. 2 (a) Trigonal prismatic coordination geometry of the Co ions in[Co(o-TT)] and (b) definition of the structural parameters of D3h polyhedrawith six vertices.

a slightly distorted honeycomb lattice, which is compressedalong the a axis, with three acetonitrile molecules in the voids.
The honeycomb layers are stacked along the b axis by shifting in
the a-axis direction relative to their neighbours with an interlayer
distance of 5.72 Å. As shown in Fig. 2, the coordination geometryaround the Co ion is a trigonal prism. The reported trigonal
prismatic geometries around cobalt ions with three-claw ligands
are usually distorted with a Bailar twist angle a (Fig. 2b) of
1–301.4,5 However, the very small values of a (0.33, 0.35 and 0.631)
for [Co(o-TT)]3CH3CN indicate an ideal trigonal prismaticgeometry with a D3h local symmetry; this feature is supportedby the finding of almost no variation in the Co–O distance l, thetrigonal side distance s, or the trigonal bite distance 2h (Table S2,
ESI†). Since there are no specific interactions between bidentate
oxygen atoms such as three-claw ligands, the formation of a 2D
honeycomb structure is presumably the driving force underlying
the formation of an ideal trigonal prismatic coordinationgeometry. The Co–O distances are in the range from 2.047 to
2.132 Å, which are comparable to the range of Co–O distances forthe octahedral Co(II) ion in the high-spin state (S = 3/2).12 Fig. S2a(see ESI†) shows the molecular structure of o-TT in [Co(o-TT)]
3CH3CN, which consists of three o-quinone moieties, A, B
and C. Fig. S3 (see ESI†) shows a comparison of the C–O bonddistances in o-TT for [Co(o-TT)] and its neutral species, where thethree C–O bond distance ranges that are typical of benzoquinones (BQ), semiquinones (SQ) and hydroquinones (HQ) areindicated by pale red, green and blue, respectively. The structureof plane A is close to that of benzoquinone, and the structures of
B and C are close to that of semiquinone. We concluded thato-TT in [Co(o-TT)]3CH3CN is in a dianion state, namely (o-TT)2,in which each negative charge is localized on planes B and C.
The g-value anisotropy caused by the crystal electric field(CEF) was calculated for the Co(II) (d7) ion in the trigonal

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prismatic geometry (see Fig. S4, ESI†). The CEF Hamiltonian of
the trigonal prism with D3h symmetry is eﬀectively described byOpen Access Article. Published on 27 July 2023. Downloaded on 9/23/2024 10:34:46 PM.
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HDeff3h ¼ D cos 2y  D0 cos 4y(1)where y is the polar angle expressed in the spherical coordinate
Fig. S4, ESI.† The coeﬃcients D and D 0 depend on the structureof the trigonal prism and can change their signs:
ð1
ð1
D ¼ A drR3;2 ðrÞr4 R3;2 ðrÞ  A0 drR3;2 ðrÞr6 R3;2 ðrÞ
(2)
0

0

D0 ¼ CÐ1
6
0 drR3;2 ðrÞr R3;2 ðrÞ

where A, A 0 and C are defined as
9 2h2  a2 Ze2
;
A
4l 5

(3)

(4)



Ze2 15 24a2 h2  3a4  8h4;
A0  
4pe0
64l 9

(5)



Ze2 105 24a2 h2  3a4  8h4:
4pe0
256l 9

(6)

and
C

Here Z is the atomic number, e is the elementary charge, and a,h, and l are the structural parameters defined in the inset of
Fig. 2b. Under the D3h CEF, the 3d orbitals are classified into
the a01 (dz2), e 0 (dxy, dx2y2), and e00 (dyz, dzx) irreducible representations. For D 4 0, the e 0 orbitals are stabilized while the a01
orbital is destabilized, and vice versa for D o 0. For D 0 4 0, boththe a1 0 and e 0 orbitals are stabilized, while the e00 orbitals aredestabilized, and vice versa for D 0 o 0.
The electronic states were examined exactly for the standard
multi-orbital Hubbard model of the d7 configurations under
the CEF with D3h symmetry (see ESI† for details). The phase
diagram of the high-spin (S = 3/2) and low-spin (S = 1/2) states isshown on the plane of D and D 0 in Fig. S5 (see ESI†). The highspin states are obtained in the regions where the energy levelsof unoccupied and singly occupied d orbitals become closer, as
expected naturally. The g-values were also evaluated for thismodel, and the ratio of gz/gx is also shown in Fig. 3. The
magnetic anisotropy depends significantly on the values of D
and D 0 . There are three parameter regions of high-spin states:(i) the Heisenberg spin with weak anisotropy gz 4 gx, gy when
D 4 0 and D 0 4 0; (ii) a strong uniaxial magnetic anisotropy,
namely gz c gx, gy when D o 0 and D 0 4 0; and (iii) the XYmodel type anisotropy gz E 0 when D E 0 and D 0 o 0.
The temperature dependence of the magnetic susceptibility
for a polycrystalline sample of [Co(o-TT)]3CH3CN was examinedusing a Quantum Design MPMS XL SQUID magnetometer in thetemperature range of 2–300 K under a magnetic field of 0.1 T.
The powder X-ray diffraction pattern of the polycrystalline
sample is consistent with the single-crystal structure (Fig. S12,
ESI†). The temperature-independent contribution of the sample
and cell was estimated by assuming Curie–Weiss behaviour inthe high-temperature region, and the estimated contribution
was subtracted from the molar susceptibility. The obtainedThis journal is © The Royal Society of Chemistry 2023

Fig. 3 Phase diagram and the contour plot of gz/gx for the model of theCo(II) (d7) ion under the CEF with D3h symmetry. The model parameters arechosen as l = 0.01 eV and U = 5 eV, U 0 /U = 0.8, J/U = 0.2 where l is thespin–orbit coupling, U(U 0 ) is the intra-orbital (inter-orbital) Coulombic
repulsion and J is the Hund coupling (see ESI† for details).

paramagnetic susceptibility wp is shown in Fig. S7 (see ESI†),
where the values of wpT are plotted as a function of thetemperature T. The values of wpT monotonically decrease witha decrease in T from 300 K. The temperature dependence of wp1
over the whole temperature range is shown in Fig. S8 (see ESI†).
This temperature dependence above 50 K can be fitted to theCurie–Weiss law with the Curie constant of 3.78 cm3 mol1 K
and the Weiss constant of 14.5 K. It is notable that there is no
ferrimagnetic behaviour in the low-temperature region, such as aturnover of the Weiss constant from negative to positive with adecrease in temperature. This suggests an S = 0 ground state for(o-TT)2. The obtained Curie constant corresponds to g = 2.84 for
the Co(II) ion, which is a reasonable value for the S = 3/2 HS Co(II)ion with a largely unquenched orbital contribution. Fig. S9(see ESI†) shows the temperature dependence of wp below
50 K. The value wp shows a monotonic increase with a decreasein temperature, suggesting that there is no magnetic transition
down to 2 K.
In order to measure the magnetic anisotropy of Co(II) in thetrigonal prismatic coordination, we prepared oriented samples
by aligning the thick platelet crystals of [Co(o-TT)]3CH3CN on aplastic (PVC) plate by hand (see Fig. S10, ESI†) and fixing themwith Apiezon N grease. The z axis in this figure is perpendicular to
the ac plane of the crystal structure, namely the honeycomb
lattice, and the x and y axes are parallel to it. Fig. 4 shows theangular dependence of the magnetization M in the zy plane forthe oriented sample at 2, 5, 10, 15, 20 and 25 K. These measurements were obtained under a field of 2 T, using the sample rotator
option of Quantum Design. The contribution of the empty rotator
was measured and subtracted. The angle j is the dihedral angle
between the magnetic field and the ac plane, so that the magnetic
field is parallel to the b axis at j = 901. Magnetic anisotropy with aperiodicity of 1801 clearly appears, and the magnetization exhibits
maxima and minima at j = 90 and 01, respectively. This behaviour
indicates uniaxial magnetic anisotropy with the easy axis parallel

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Fig. 4 Angular j dependence of the magnetization M in the zy plane
for the oriented sample of [Co(o-TT)] (Fig. S10, ESI†) at 2, 5, 10, 15, 20
and 25 K. The angle j is the dihedral angle between the magnetic field andthe ac plane.

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prismatic coordination geometry by forming a 2D honeycomb
MOF structure. This strategy will be broadly applicable totriptycene-based ligands and has the potential to achieve strong
magnetic anisotropy that aligns in the same direction in the
crystal structure.
Y. S. performed the synthesis and magnetic measurements.
R. S. carried out single crystal structural analysis. M. T. carriedout theoretical model calculations. All authors contributed towriting and reviewing the manuscript. K. A. directed the project.
The authors acknowledge Japan Society for the Promotion of
Science (JSPS) KAKENHI Grants 19K15520 (to Y. S.), 20H02707(to R. S.) and 20H05621 (to M. T. and K. A.), and Japan Scienceand Technology Agency (JST) PRESTO Grant JPMJPR21A9(to R. S.) for funding this work.

Conflicts of interest
to the b axis (j = 901). To confirm the uniaxial magnetic
anisotropy, the temperature dependence of Mx, My and Mz was
examined. The results are shown in Fig. S11 (see ESI†). It is clearthat Mz is always larger than Mx and My, and the diﬀerence
between Mx and My is negligibly small. In addition, the ratio
Mz/Mx (or My) is constant 1.38 (or 1.35) and is without any
dependence on temperature. This gives gz/gx (or gy) = 1.17
(or 1.16), which corresponds to the light-blue region in Fig. 3.
The observed weak uniaxial magnetic anisotropy is quite consistent with theoretical simulations for the Co(II) ion in the present
trigonal prismatic coordination geometry.
In summary, we prepared a MOF with a honeycomb network
structure that consists of a 3-fold symmetric triptycene-based
bridging ligand (o-TT) and Co(II) ion in the trigonal prismaticcoordination geometry. Interestingly, the trigonal prismaticcoordination geometry and its orientation were fixed by a 2D
honeycomb network structure. CEF calculations on a trigonal
prismatic coordination geometry model illustrated the potentialof various spin configurations and magnetic anisotropy, including Ising, XY and gz \ gx. The angular dependent magnetic
measurements of aligned single crystals revealed a uniaxial
magnetic anisotropy of gz/gx = 1.17. Although SMM behaviour
was not observed on [Co(o-TT)] down to 2 K, presumably due to
its weak magnetic anisotropy or intermolecular magnetic interactions, we proposed a novel strategy to prepare an ideal trigonal

10108 | Chem. Commun., 2023, 59, 10105–10108

There are no conflicts to declare.

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