1,2,4-triazolyl-carboxylate-based MOFs incorporating triangular Cu(II)-hydroxo clusters: Topological metamorphosis and magnetism

Article abstract of DOI:10.1021/ic403148f

Bifunctional 1,2,4-triazole-carboxylate ligands, an achiral 1,2,4-triazol-4-yl-acetic acid (trgly-H) and a chiral (d)-2-(1,2,4-triazol-4-yl) -propionic acid (d-trala-H), derived from the corresponding α-amino acid precursors revealed unique binding abilities in the construction of Cu(II)-coordination polymers composing discrete triangular [Cu ₃(μ₃-OH)] clusters. A related series of MOFs, [Cu ₃(μ₃-OH)(trgly)₃(SO₄)] ·2H₂O (1a), [Cu₃(μ₃-OH)(trgly) ₃(H₂O)₃]SO₄·16H₂O (1b), Cu₃(μ₃-OH)(d-trala)₃(ClO ₄)_0.5](ClO₄)_1.5·1.5H ₂O (2), was prepared, and their crystal structures were determined by means of X-ray diffraction. Being singly deprotonated, the organic ligands act as multidentate μ₃- or μ₄-donors using tr and -COO^- moieties. The generated [Cu₃(μ₃-OH) (tr)₃] cluster core is primarily supported by three [-N-N-] triazole heterocycles in a basal plane and tripodal-assisted μ₃-anions (SO₄^2-: 1a; ClO₄^-: 2) capping the axial faces. The carboxylate groups join the units into either two-dimensional (2D) layer (1a, 2) or 3D zeolite-like networks (1b). Compound 1b represents the topology of α-Po (pcu: 4¹².6³) and crystallizes in the noncentrosymmetric space group I43m, in which the six-connected [Cu₃(μ₃-OH)] clusters and trgly self-assemble in an open-channel cubic array possessing ～56% solvent-accessible volume. Upon slight thermal treatment (～60 °C), the structure irreversibly shrinks to the nonporous 2D motif 1a that belongs to a uninodal (3,6) network type. In structure 2 (space group R3₂), due to the [-N-N-] triazole and 1,3-bidentate carboxylate binding mode, each organic ligand bridges three metal clusters affording cross-linking of two adjacent layers with the same (3,6) topology. The resultant 3,9-c net is novel and can be categorized as two-nodal with point symbol {4¹⁸.6¹⁸}{4².6}₃. Spin frustration and antisymmetric exchange effects, resulting in abnormally low g values in the S = 1/2 states, were observed in the magnetic properties and the EPR spectra.

Full text of DOI:10.1021/ic403148f

Article
pubs.acs.org/IC
1,2,4-Triazolyl-Carboxylate-Based MOFs Incorporating Triangular
Cu(II)-Hydroxo Clusters: Topological Metamorphosis and Magnetism
Sergiy I. Vasylevs’kyy,† Ganna A. Senchyk,† Andrey B. Lysenko,*,† Eduard B. Rusanov,‡
Alexander N. Chernega,‡ Julia Jezierska,§ Harald Krautscheid,∥ Konstantin V. Domasevitch,†
and Andrew Ozarowski*,⊥
†Inorganic Chemistry Department, Taras Shevchenko National University of Kyiv, Volodimirska Street 64, Kyiv 01033, Ukraine
‡Institute of Organic Chemistry, 02660, Murmanska Street 5, Kyiv, Ukraine
§Faculty of Chemistry, University of Wroclaw, 14 Joliot-Curie Str., 50-383 Wroclaw, Poland
∥Institut fur Anorganische Chemie, Universitat Leipzig, Johannisallee 29, D-04103 Leipzig, Germany
̈
̈
⊥National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United
States
S
* Supporting Information
ABSTRACT: Bifunctional 1,2,4-triazole-carboxylate ligands, an achiral
1,2,4-triazol-4-yl-acetic acid (trgly-H) and a chiral (d)-2-(1,2,4-triazol-4-
yl)-propionic acid (d-trala-H), derived from the corresponding α-amino
acid precursors revealed unique binding abilities in the construction of
Cu(II)-coordination polymers composing discrete triangular [Cu3(μ3-
OH)] clusters. A related series of MOFs, [Cu3(μ3-OH)(trgly)3(SO4)]·
2H2O (1a), [Cu3(μ3-OH)(trgly)3(H2O)3]SO4·16H2O (1b), Cu3(μ3-
OH)(d-trala)3(ClO4)0.5](ClO4)1.5·1.5H2O (2), was prepared, and their
crystal structures were determined by means of X-ray diffraction. Being
singly deprotonated, the organic ligands act as multidentate μ3- or μ4-
donors using tr and −COO− moieties. The generated [Cu3(μ3-
OH)(tr)3] cluster core is primarily supported by three [-N-N-] triazole
heterocycles in a basal plane and tripodal-assisted μ3-anions (SO42−: 1a;
−
ClO4 : 2) capping the axial faces. The carboxylate groups join the units
into either two-dimensional (2D) layer (1a, 2) or 3D zeolite-like networks (1b). Compound 1b represents the topology of α-Po
(pcu: 412.63) and crystallizes in the noncentrosymmetric space group I43m, in which the six-connected [Cu (μ -OH)] clusters
̅
3
3
and trgly self-assemble in an open-channel cubic array possessing ∼56% solvent-accessible volume. Upon slight thermal
treatment (∼60 °C), the structure irreversibly shrinks to the nonporous 2D motif 1a that belongs to a uninodal (3,6) network
type. In structure 2 (space group R32), due to the [-N-N-] triazole and 1,3-bidentate carboxylate binding mode, each organic
ligand bridges three metal clusters affording cross-linking of two adjacent layers with the same (3,6) topology. The resultant 3,9-c
net is novel and can be categorized as two-nodal with point symbol {418.618}{42.6}3. Spin frustration and antisymmetric exchange
effects, resulting in abnormally low g values in the S = 1/2 states, were observed in the magnetic properties and the EPR spectra.
connecting the triangular motifs into three-dimensional (3D)
coordination arrays through three angularly arranged basal
binding sites (Scheme 1: a, b).8 For these purposes, employing
bi- or multitopic triazole ligands which bear various additional
donor moieties (e.g., py, carboxylate) seems to be a promising
approach for designing solids. In this regard, the mixed-donor
triazolyl-carboxylate ligands as well as their complexes are
subjects of extensive interest.9 Both functions belonging to a
particular class of cluster-supporting and self-complementary
bridges, show high binding efficiency in fabrication of
heteroleptic triazole(ate) and carboxylate complexes.8c,10 The
major advantage of 1,2,4-triazolyl-carboxylates lies in the
INTRODUCTION
■
Metal organic frameworks (MOFs) incorporating trinuclear
hydroxoclusters [Cu3(μ3-OH)] have received intensive research
interest for multiple applications in gas storage and sorption,1
catalysis (like peroxidative oxidation of alkanes,2 CO oxidation,3
etc.) and magnetism (e.g., to test magnetic exchange models in
spin-frustrated systems,4 or for spin-canted ferro- and
antiferromagnetism5). Their structure and redox properties
make them attractive models for the active sites of
biomolecules, like the multicopper oxidases.6 In this sense,
1,2,4-triazole (tr) derivatives are a unique family of N-donor
building blocks which provide a short [-N-N-] linkage between
two metal centers to achieve the molecular triangular topology.7
Deprotonation of a N−H triazole function induces an
important structural evolution in the molecular assemblies
Received: December 26, 2013
Published: March 18, 2014
© 2014 American Chemical Society
3642
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
Scheme 1. (a) Trinuclear Hydrocluster [Cu3(μ3-OH)(tr)3]; (b) a Self-Assembly of [Cu3(μ3-OH)(tr)3] Clusters in a 3D Cubic
Array; (c) 1,2,4-Triazolylcarboxylic Acids Explored in This Paper: the Presence of CH3-Group at the α-Position of trala-H Was
Aimed to Elucidate Its Influence on the Coordination Network Topology; (d) the Binding Mode Was Observed for [Cu3(μ3-
O)(trgly)3(H2O)3]BF4·H2O11
possibility of diverse ligand design reflected in a practical and
reliable synthesis through commercial or readily accessible
precursors. Among them, natural amino acids reveal an almost
unexplored library of molecular building blocks with desirable
side-chain functionality. The aspect of native chirality at tertiary
carbon centers in such molecules opens a straightforward
pathway to homochiral coordination polymers. In a pioneering
work, Garcia et al. successfully applied a simple 1,2,4-triazole
derivative of glycine (1,2,4-triazol-4-yl-acetic acid) to engineer a
microporous zeolite-like framework based on [Cu3(μ3-OH)]
scaffolds for gas sorption application (Scheme 1: c, d).11
EXPERIMENTAL SECTION
■
Caution! Since Cu(ClO4)2·6H2O and compound 2 are
potentially explosive, they must be handled with extreme
care.
Synthesis of the Organic Ligands. All chemicals were of reagent
grade and used as received without further purification. The organic
ligands were prepared in moderate yields up to 30% using the general
method normally applied for aromatic and heterocyclic amines:
refluxing of the corresponding amino acid (gly or (d)-ala) with N,N-
dimethylformamide azine in toluene in the presence of TsOH·H2O.14
The workup procedure with isolation of the potassium intermediates
(trgly-K and trala-K) was found to be convenient and general because
of the highly soluble nature of 1,2,4-triazolyl-carboxylates.
Recently Lassig, Lincke, and co-workers demonstrated
̈
1,2,4-Triazol-4-yl-acetic Acid, trgly-H. In a typical procedure,14
a
another attractive feature of 1,2,4-triazolyl-carboxylates con-
cerning the topological transformation of the molecular
complexes and low dimensional polymeric precursors into the
interpenetrated 3D frameworks through simple thermal
conversion.12 Even more recently,13 reversible crystal-to-
amorphous-like phase transformation of the porous 3D
framework consisting of triangular [Cu3(μ3-OH)(μ2-O)(tr)2]
clusters was reported by Xia et al. Exploiting the solid-state
approach can lead to coordination solids with a sensory
magnetic response, especially interesting in context of novel
magnetic materials.
mixture of gly (4.88 g, 0.0650 mmol), N,N-dimethylformamide azine
(13.85 g, 0.0974 mmol) and TsOH·H2O (0.520 g, 2.73 μmol) in
toluene (15 mL) was stirred under reflux for 36 h (the reaction was
monitored by evolution of dimethylamine). After being cooled to rt,
the solvent was decanted, and the dark viscous residue was dissolved in
hot ethanol (30 mL). To the dark solution was added an ethanolic (20
mL) solution of KOH (7.28 g, 0.129 mmol) that afforded a pale-
yellow precipitate of potassium 1,2,4-triazol-4-yl-acetate, trgly-K. The
solid was filtered off, washed with small portions of ethanol (3 × 5
mL). The white precipitate was redissolved in ethanol, and then 9.1 M
hydrogen chloride in ethanol was added, adjusting the pH to 4. The
resulting white precipitate of KCl was filtered off. The filtrate was
concentrated in vacuo to a volume of 4 mL that led to the desired
white product (2.56 g, 31%). A second crop of the product was not
isolated from the remaining syrup. Anal. for trgly-H·0·5H2O,
C4H6N3O2.5. Calc (%): C, 35.30; H, 4.44; N, 30.87. Found (%): C,
35.10; H, 4.54; N, 30.65. 1H NMR (400 MHz, DMSO-d6: CCl4 = 1: 3,
In this report, we describe the coordination abilities of
bifunctional 1,2,4-trizolyl-carboxylate ligands (trgly-H, d-trala-
H) derived from glycine and (d)-alanine toward rational
construction of [Cu3(μ3-OH)]-based MOFs exhibiting diverse
topologies and tending to form solvatomorphs.
3643
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
TMS): δ = 8.32 (2 H, s, tr), 4.79 (2 H, s, -CH2-). IR (KBr pellet,
cm−1): 422s, 630m, 666s, 786s, 882m, 894m, 958m, 976m, 1012s,
1080m, 1198s, 1240s, 1348s, 1384m, 1428w, 1456m, 1524s, 1544s,
1652w, 1732s, 1942m, 2730w, 2816w, 2858w, 2942w, 2994m, 3062w,
3124s, 3148m, 3420w.
Our attempts aimed at optimizing postsynthetic activation
procedures of compound 1b for further gas sorption measurements,
similar to those recently described by one of us,9b were unsuccessful
due to easy structural conversion processes (Figure S24 in SI).
Cu3(μ3-OH)(d-trala)3(ClO4)0.5](ClO4)1.5·1.5H2O (2). A mixture of
Cu(ClO4)2·6H2O (36.6 mg, 0.0988 mmol), (d)-trala-H (14.1 mg,
0.100 mmol), and 2 mL of water was placed in a Teflon-lined steel
autoclave, stirred for 10 min at rt and then heated at 120 °C for 36 h.
Cooling to rt during the next 60 h led to green-blue plates of the
product that were collected, washed with water, and dried in air. Yield
9.6 mg (34%). Anal. Calc for C15H22Cl2Cu3N9O16.5 (%): C, 21.10; H,
2.60; N, 14.76. Found: C, 21.01; H, 2.50; N, 14.56. IR (KBr pellet,
cm−1): 625s, 665m, 725m, 845m, 880m, 990, 1090ws, 1200s, 1325m,
1525s, 1735w, 2000w, 2860m, 2915m, 2955m, 3130s, 3420m.
Under similar hydrothermal conditions, the interaction of the trgly-
H and (d)-trala-H ligands with some other Cu(II) salts containing
(d)-2-(1,2,4-Triazol-4-yl)propionic Acid, (d)-trala-H. In a typical
procedure,14 a mixture of (d)-ala (4.45 g, 0.0499 mmol), N,N-
dimethylformamide azine (10.65 g, 0.0748 mmol) and TsOH·H2O
(0.400 g, 2.73 μmol) in toluene (15 mL) was stirred under reflux for
40 h. After being cooled to rt, the solvent was decanted, and the dark
viscous residue was dissolved in hot ethanol (20 mL). To the dark
solution was added an ethanolic (20 mL) solution of KOH (5.60 g,
0.100 mmol) that gave a pale-yellow precipitate of potassium 1,2,4-
triazol-4-yl-propionate, trala-K. The solid was filtered off, washed with
small portions of ethanol (3 × 5 mL), and then redissolved with
stirring in 30 mL of hot ethanol (60 °C). To the solution was added
9.1 M hydrogen chloride in ethanol, adjusting the pH to 4. The
resulting white precipitate of KCl was then removed by filtration.
Partial evaporation of the solvent in vacuo to a volume of 3 mL left a
white solid, which was isolated and dried at 70 °C giving the desired
product (1.90 g, 27%). Anal. for C5H7N3O2. Calc (%):C, 42.55; H,
5.00; N, 29.77. Found (%): C, 42.47; H, 5.11; N, 29.60. 1H NMR (400
MHz, DMSO-d6: CCl4 = 1: 3, TMS): 8.57 (2 H, tr), 5.21 (1H, q, J=7.1
Hz, CH), 1.83 (3H, d, J=7.1 Hz, CH3). IR (KBr pellet, cm−1): 448w,
544w, 578w, 628s, 678s, 730m, 796m, 848s, 884s, 948m, 984m, 1034s,
1066s, 1096s, 1192s, 1220s, 1256s, 1286s, 1314s, 1380m, 1404m,
1462m, 1544s, 1674m, 1732s, 2802w, 2826w, 2884w, 2912w, 2948w,
2974m, 2998w, 3110w, 3136s.
Synthesis of the Coordination Compounds. [Cu3(μ3-OH)-
(trgly)3(SO4)]·2H2O (1a). A mixture of CuSO4·5H2O (50.0 mg, 0.200
mmol), trgly-H·0.5H2O (27.2 mg, 0.200 mmol), and 5 mL of water
were placed in a 20 mL Teflon-lined steel autoclave, stirred for 10 min
at rt, and then heated at 120 °C for 36 h. Cooling to rt during the next
60 h led to green-blue prisms of the product that were collected,
washed with water, and dried in air. Yield 28.7 mg (60%). Anal. Calc
for C12H17Cu3N9O13S (%): C, 20.07; H, 2.39; N, 17.56. Found: C,
20.00; H, 2.33; N, 17.47. IR (KBr pellet, cm−1): 424w, 510 m, 612m,
632m, 698m, 786m, 890m, 978s, 1054s, 1100s, 1158s, 1192m, 1218s,
1274s, 1300s, 1328s, 1362s, 1386s, 1496w, 1564s, 1642s, 2964m,
3004m, 3044m, 3136s, 3306m, 3460m, 3524m. At 130−140 °C, the
reaction proceeded with the formation of two products: green-blue
prisms 1a and an unidentified deep-blue powder. The latter is even
more stable under severe conditions (>165 °C).
[Cu3(μ3-OH)(trgly)3(H2O)3]SO4·16H2O (1b). 1b was prepared by a
slow interdiffusion of CH3CN/H2O (1:1) solutions (or ∼0.5%
aqueous solution of benzonitrile) of CuSO4·5H2O and trgly-H. A
CH3CN/H2O (1:1, 0.5 mL) solution was layered onto solid CuSO4·
5H2O (12.5 mg, 0.050 mmol) which was placed in a narrow-diameter
test tube (length/diameter size = 30 mm × 5 mm, ∼0.5 mL). The tube
then was put into a larger size vial (50 mm × 20 mm, ∼12 mL)
containing trgly-H (13.6 mg, 0.100 mmol) in CH3CN/H2O (1:1, ∼8
mL). For interdiffusion of the components, the small test tube was
immersed into the vial followed by slow addition of CH3CN/H2O
(1:1, ∼2 mL) using a Pasteur pipet, and then the vial was capped. The
slowly dissolving reaction components were left undisturbed at rt for
two weeks. The resulting blue octahedral crystals were collected and
dried at rt. (Yield 7.0 mg, 41%, please see Figure S14 in SI). Anal. Calc
for C12H51Cu3N9O30S (%): C, 14.07; H, 5.02; N, 12.31. Found: C,
14.00; H, 4.99; N, 12.00. IR (KBr pellet, cm−1): 604m, 632m, 700m,
794w, 974m, 1052s, 1092s, 1218m, 1304m, 1396s, 1564s, 1630s,
3032s, 3124s, 3400s.
−
−
tetrahedral anions (SO42−, SeO42−, ClO4 , BF4 ) afforded either
−
−
polycrystalline solids (trgly-H: SeO42−, ClO4 , BF4− or trala-H: BF4 )
or clear-blue solutions without any precipitated product (trala-H:
SO42−, SeO42−).
Measurements. IR spectra (400−4000 cm−1) were collected using
a Perkin-Elmer FTIR spectrometer with samples prepared as KBr
pellets. 1H NMR spectra were recorded on a Bruker 400 MHz
spectrometer. Elemental analysis (C, H, and N) was carried out with a
Vario EL-Heraeus microanalyzer. PXRD was carried out on a Stoe
STADIP (Cu Kα1) using a linear PSD detector. The temperature-
dependent X-ray measurements were recorded on a Stoe STADIP
with a high-temperature attachment and image-plate detector system.
Thermogravimetric/differential thermal analysis (TG/DTA) was
carried out on a Netzsch F1 Jupiter device (the sample was heated
at a rate of 10 K min−1).
X-ray Crystallography. Diffraction data were collected at 296 K on
a Bruker APEXII CCD area-detector diffractometer (φ scans)
(graphite-monochromated Mo Kα radiation, λ = 0.71073 Å). The
data were corrected for Lorentz-polarization effects and for the effects
of absorption (multiscans method). The structures were solved by
direct methods and refined by full-matrix least-squares on F2 using the
SHELX-97 package.15 For complex 1b, contribution of all disordered
water molecules was modeled using the Squeeze routine as
implemented in Platon.16
Crystallographic data and experimental details for structural
analyses are summarized in Table S1 in SI. The crystallographic
material can also be obtained from the CCDC, the deposition
numbers being CCDC 949597, 949599 (for 1a and 1b, respectively)
and 949598 (for 2).
Magnetic Properties. Magnetic susceptibility data of powdered
samples were measured on a Quantum Design SQUID magnetometer
MPMSXL-5 (Faculty of Chemistry, Wroclaw University), over the
temperature range 1.8−300 K at the magnetic induction of 0.5 T.
Corrections for the sample holders were applied. Diamagnetic
corrections were determined from Pascal’s constants.17
EPR Spectra. High-field, high-frequency EPR spectra at temper-
atures ranging from ∼3 to 300 K were recorded on a home-built
transmission spectrometer at the EMR facility of NHMFL.18
The instrument is a transmission-type device in which microwaves
are generated by a phase-locked Virginia Diodes source producing
harmonics of its 13 1 GHz base frequency, of which the second,
fourth, sixth, eighth, 16th, 24th, and 32nd were available. A
superconducting magnet (Oxford Instruments) capable of reaching a
field of 17 T was employed. The powder samples were not constrained
and showed no magnetic torquing at high magnetic fields.
Analogously, compound 1b was isolated in 49% yield from a
saturated benzonitrile aqueous solution (∼0.5%) starting from trgly-H
(81.6 mg, 0.600 mmol), CuSO4·5H2O (153.3 mg, 0.614 mmol) and
using 50 and 20 mL test tubes, respectively. The phase purity of the
sample was established by comparison of the observed and simulated
X-ray powder diffraction patterns (see SI). Anal. Calc for
C12H51Cu3N9O30S (%):C, 14.07; H, 5.02; N, 12.31. Found: C,
14.19; H, 5.40; N, 12.28. This method can be applied for the bulk
synthesis of complex 1b.
RESULTS AND DISCUSSION
■
1,2,4-Triazol-4-yl-acetic acid is a known compound and was
synthesized by Garcia in two similar ways starting either from
glycine (one-pot reaction, 50% yield)19 or glycine ethyl ester
hydrochloride (two-step reaction, ∼58% total yield),11 whereas
(d)-2-(1,2,4-triazol-4-yl)-propionic acid had not been described
yet. In the present study we tried to follow the traditional
3644
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
synthetic method using the corresponding amino acid (gly or
(d)-ala; amine source) and N,N-dimethylformamide azine in
boiling toluene. These reaction conditions are frequently
applied for aromatic and heterocyclic amines.14 The trans-
amination reaction afforded a viscous dark oil and thus required
extensive purification. However, a simple workup procedure,
including the preparation of potassium intermediates (trgly-K
and trala-K) and their further conversion to trgly-H and trala-
H by treatment of trgly-K (or trala-K) with HCl in ethanolic
solution, was employed. Despite the low total yield (27−30%)
of the final products, this method allows preparation of
triazolyl-carboxylic acids and their simple salts from suitable
precursors avoiding chromatography. Moreover, the potassium
salts are potentially interesting building blocks for construction
of MOFs.
understanding the resultant structural topologies in compounds
1a, 1b and 2. First, in the [Cu3(μ3-OH)] core each pair of Cu2+
cations is primarily supported by a [−N-N-] triazole fragment
which can be regarded as a complementary unit to the short
inorganic bridge (e.g., μ3-OH). The carboxylate moieties
connect the clusters into extended layer or zeolite-like
structures.
The asymmetric unit in complex [Cu3(μ3-OH)-
(trgly)3(SO4)]·2H2O (1a, space group P21/c) contains one
cluster molecule with three independent Cu atoms, each of
them adopting a square-pyramidal N2O2 + O environment. In
the basal plane the coordination arrangement is achieved by
two trans-coordinated N(tr) atoms and two oxygen atoms from
μ3-OH and singly bound −O−CO groups. The axial sites
above the cluster plane are perfectly suited for cooperative
2−
The reaction of 1,2,4-triazole carboxylic acids (trgly-H, (d)-
trala-H) with Cu2+ salts in water at nearly neutral pH,
accompanied by deprotonation of the organic ligand and
simultaneous hydrolysis of the Cu2+ cations, proceeds efficiently
under hydrothermal conditions (Scheme 2) affording the basic
interactions with tripodal μ3-SO4 anions (Figure 1, Table 1).
Scheme 2. 1,2,4-Triazole Carboxylate Building Blocks,
Derived from α-Amino Acids, Represent a Straightforward
Pathway toward the Incorporation of Triangular [Cu3(μ3-
OH)] Clusters into MOFs
Figure 1. In the structure of 1a, the triangular hydroxo-cluster of
[Cu3(μ3-OH)(trgly)3] is stabilized by a facially coordinating tripodal
2−
SO4 anion, symmetry codes: (i) x, −y+3/2, z−1/2; (ii) x, −y+1/2,
z−1/2; (iii) x, y, z+1.
Each triangular subunit is connected to six surrounding
triangles through O atoms of carboxylate groups. Considering
the [Cu3(μ3-OH)] clusters as 6-fold-connected nodes, the
resultant topologic motif can be classified as a 2D uninodal
uniform (3,6) coordination network. A similar subtopological
type was observed earlier in the related [{Cu3(μ3-OH)}(μ3-
btr)3]n+ cationic framework.20 The small trigonal cavities of the
[Cu3(μ3-OH)(trgly)3(SO4)] network are occupied partially by
uncoordinated water molecules and the apical O-atom of the
sulfate ligand (Figure 2).
A semi-rigid methylene linkage between triazole and
carboxylate groups in the trgly ligand dictates the angular
orientation of Cu−N and Cu−O bonding vectors in the
coordination network. Such ligand−metal interaction leads to
wave-like layers (Figure 2) in which the cluster centers are
positioned at distances of 10.542, 10.642, and 12.096 Å,
whereas the closest interlayer separations between triangular
centroids are significantly shorter (5.294 Å). Being stabilized by
numerous hydrogen bonds (μ3-OH···O−CO 2.806 Å, HO−
μ3-hydroxo species (pure crystalline phases of complexes 1a
and 2). Similar reactions carried out at room temperature led to
immediate formation of blue precipitates (for trgly-H/
H
(aqua)...OSO3 2.777 Å), the two closest neighboring sheets are
tightly stacked in a ABAB manner that unexpectedly results in
the disposition of sulfate anions on only one side of the layers.
The self-assembly of the enantiopure ligand, (d)-trala, and
Cu(ClO4)2 under hydrothermal conditions affords the
coordination network of [Cu3(μ3-OH)(d-trala)3(ClO4)0.5]-
(ClO4)1.5·1.5H2O (2) crystallizing in the non-centrosymmetric
space group R32. At first glance, the local shape and geometry
2‑
−
−
Cu2+:SO4 , ClO4 , BF4 ), which contained a mixture of
products (including compound 1b; please see observed and
calculated XRPD patterns shown in Figure S25 in SI), or deep-
blue solutions (for (d)-trala-H/Cu2+:SO4 , ClO4 , BF4 ).
These triangular [Cu3(μ3-OH)] hydroxoclusters can serve as
self-associating building units; that is one of the key factors in
2‑
−
−
3645
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
Table 1. Selected Bond Lengths (Å) in Compounds 1a, 1b and 2
[Cu3(μ3-OH)(trgly)3 (SO4)]·
2H2O, (1a)
[Cu3(μ3-OH)(trgly)3 (H2O)3]SO4·
[Cu3(μ3-OH)(d-trala)3(ClO4)0.5] (ClO4)1.5
·
bond type
16H2O, (1b)
1.5H2O, (2)
Cu−N(tr)
Cu−O(H)
1.976(3)−1.987(3)
2.027(2)−2.030(2)
1.926(3)−2.004(3)
−
1.967(4)
2.009(3)
1.967(4)
−
1.992(3), 2.001(3)
2.0337(13)
1.954(3)
(eq. plane)
(eq. plane)
Cu−O(CO)
(eq. plane)
Cu−O(C)−
2.359(4)
(axial)
Cu−O(anion) (axial)
2.174(3)−2.190(3)
−
−
2.519(15)
−
Cu−Oaqua
2.241(7)
(axial)
Cu···Cu distance within [Cu3(μ3-
OH)]
3.3562(6)−3.3804(6)
3.3950(15)
3.4447(9)
closest intercluster Cu···Cu
distance
3.499
8.512
4.433
Figure 3. Perchlorate-capped equilateral triangular hydroxo-cluster
[Cu3(μ3-OH)((d)-trala)3] in complex 2 has a cone-shaped geometry.
Symmetry codes: (i) −y+1, x−y, z; (ii) −x+y,−x, z; (iii) y−1/3, x−2/
3, −z+4/3; (iv) −y, x−y, z.
mode that increases the structural integrity of clusters in the
polymer network. In 2, the 2D topological motif reminds us of
a combination of two equivalent uninodal (3,6) nets shifted in
the ab plane (Figure 4). μ2-Carboxylate groups efficiently
interlink two [Cu3(μ3-OH)] units from the upper and lower
sheet, keeping them at a Cu···Cu distance of 4.433 Å. In the
Figure 2. (a) Two-dimensional wave-like layer in 1a having uninodal
uniform (3,6) topology; (b) layered packing showing the ABAB
stacking mode.
of the building block [Cu3(μ3-OH)(μ2-tr)3] remains unchanged
in comparison to those of complex 1a. However, in contrast to
those of 1a, the Cu ions are in a pseudo-octahedral
coordination environment [N2O2 + 2O], in equatorial plane
defined by two triazole N atoms (N1, N2), an oxygen atom of a
carboxylate moiety (O3), and the inorganic bridge μ3-OH
(O1). The axial positions on one face of the C3v symmetric
−
copper cluster are occupied by O-atoms of μ3-ClO4 , whereas
the opposite side is completed by oxygen atom (O2) of another
carboxylate function (Figure 3, Table 1). Such discrimination
toward axially bound ligands results in the specific location of
the methyl groups sitting above the [Cu3(μ3-OH)] plane.
Presumably because of its more basic character, the
deprotonated trala ligand exhibits a tetradentate coordination
Figure 4. In the structure of [Cu3(μ3-OH)((d)-trala)3(ClO4)0.5]-
(ClO4)1.5·1.5H2O (2), the formation of a double-layer motif through
μ2-carboxylate interlinks between two identical (3,6) coordination
layers led to a novel topological type (two-nodal 3,9-c net).
3646
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
Figure 5. (a) The equilateral triangular [Cu3(μ3-OH)(trgly)3(H2O)3] cluster in complex 1b, (b) the construction of an Archimedean polyhedron, a
truncated cube, via self-assembly of eight Cu3(μ3-OH) triangles using a “symmetry-interaction approach”. Symmetry codes: (i) −z+1/2, x+1/2, −y
+1/2; (ii) z, y, x; (iii) x, z, y; (iv) y, z, x; (v) z, x, y.
Figure 6. Fragments of the crystal structure of 1b showing the formation of cages and open channels.
Figure 7. (a) Crystal packing of the pseudo-truncated cubes which are connected via common triangular faces. (b) Two kinds of channels (2 × 2 and
2−
10 × 10 Å) run along the crystallographic axes. Uncoordinated SO4 anions (shown as tetrahedra) and water molecules are lodged within the
channels.
resulting double layer, each equilateral triangular cluster serves
as 9-fold-connected node, whereas the organic ligand acts as 3-
fold-connected one. Thus, the net belongs to a novel two-nodal
In 2, the cationic [Cu3(μ3-OH)((d)-trala)3(ClO4)0.5]n layers
require additional counteranions to maintain the overall charge-
−
balance. The required uncoordinated ClO4 anions are readily
3,9-c topology with Schlafli point symbol {418.618}{42.6} .21
accommodated between the double-layer motifs.
̈
3
3647
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
Figure 8. Thermo-PXRD pattern (2θ = 5−60°) for complex 1b shows a double-step thermal conversion (at 60−70° and at 120−130 °C, red dashed
lines) which is accompanied by visible color changes from blue to green and then deep blue.
Figure 9. The Guinier−Simon diagram (2θ = 5−60°) of complex 1a shows a single-step transformation at ∼120−130 °C (red dash line) that leads
to an unknown crystalline phase. Similar transformations are observed for compound 2 (Figure S23 in SI).
The interaction of CuSO4 with trgly-H proceeding at room
temperature demonstrates another challenging alternative of
the mixed donor triazole-carboxylate systems in the design of
microporous zeolite-like frameworks consisting of Cu3(μ3-OH)
SBUs (secondary building blocks). In the 3D network of
[Cu3(μ3-OH)(trgly)3(H2O)3]SO4·16H2O (1b) with cubic
are further assembled in a cubic array possessing α-Po topology
(pcu: 412.63). The structural motif is identical to [Cu3(μ3-
O)(trgly)3(H2O)3]BF4·H2O described in ref 11. As a matter of
fact, compound 1b should be classified as an Archimedean
solid.23
The unit cell of 1b contains eight trinuclear clusters that form
a convex polyhedron, a giant truncated cube, constructed from
two kinds of faces: eight triangles of two types and six irregular
octagons. The diagonal distance between opposite triangle
centroids is 18.305 Å (Figure 5). The neighboring polyhedra
are condensed through the common triangular faces yielding
two sets of parallel channels, with van der Waals cross sections
of 2 × 2 and 10 × 10 Å, where guest water molecules and
symmetry (space group I43m), the organic ligand, similarly as
̅
in complex 1a, acts as a tridentate module with well-defined
coordination functions of tr and −COO−. The coordination
environment around each copper ion is square-pyramidal
[N2O2+O], but unlike the tripodal binding mode of sulfate in
1a; here the one-side axial positions are occupied only by aqua
ligands with a distant contact Cu−O of 2.241(7) Å (Figure 5a).
Three carboxylate fragments around the C3v symmetric
[Cu3(μ3-OH)(tr)3] core are arranged above or below the
cluster plane. This donor direction of the −COO− groups is
perfectly suited to form spherical polyhedra or cage-like
assemblies according to the so-called ‘directional-bonding’
concept.22 All Cu3(μ3-OH) clusters as 6-fold-connected nodes
2−
−
SO4 anions reside (Figures 6, 7). Similarly to BF4 anions in
[Cu3(μ3-O)(trgly)3(H2O)3]BF4·H2O,11 either SO4-tetrahedra
becomes encapsulated in the small cavity that supported by two
kinds of H-bonding contacts. One belongs to a bifurcated type
between tr-fragments (Ctr−H) and O-(SO3)2−, while the other
3648
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
̂
̂
̂
̂
13
̂
̂
̂
arises from uncoordinated water molecules ( in SI see Table S6
and Figures S7 and S8).
HHDVV = J S1S2 + J S1S3 + J23S2S3
(1)
12
In this notation, the J values are positive for antiferromag-
netic interactions. Three states of the total spin operator of the
system, ST = S1 + S2 + S3, are possible: one quartet state with
the quantum number ST = 3/2 and two doublet states with ST =
1/2. If J12 = J13 = J23 = J, then the quartet state lies 3/2 J above
the degenerate pair of the doublet states. In a situation where
The solvent-accessible volume, after removing the bound and
unbound water molecules, is calculated (PLATON program) to
be 5313.5 Å3 (or 56% of the unit cell), slightly more than for
the [Cu3(μ3-O)] tetrafluoroborate analogue.11 Interestingly, the
similar microporous framework crystallizing in the same space
group was achieved for [Cu3(μ3-O)] clusters with angular 3-
(1,2,4-triazol-4-yl)-pyridine ligands and tetrahedral counter-
̂
̂
̂
̂
J
12 = J23 = J1 ≠ J13 = J2, the doublet states split, and the energies
24
−
−
of the quartet state (Q) and of the two doublet states (D1 and
anions (BF4 and ClO4 ). This may indicate that the
construction of the cage-like motifs is accomplished by
directional bonding being preprogrammed in the ligand
functionality and angular geometry.
D2) are:7,25
EQ = J /2 + J2/4
1
ED1 = −J + J2/4
Thermal Transformations. The thermal behaviors of
complexes 1a and 1b were studied using temperature-
dependent powder X-ray diffractometry (TD PXRD). The
results clearly demonstrate the occurrence of interesting
structural phase transitions. As shown in Figure 8, in the
temperature interval from 20 to 230 °C the microporous
framework 1b undergoes a two-step structural transformation
before the sample loses its crystallinity. The first conversion in
the range of 60−70 °C afforded exclusive formation of the two-
dimensional network 1a, as evidenced by comparison of the
experimental and simulated powder patterns. This phase is
sufficiently stable up to ∼130 °C, while a further gradual
heating leads to sharp changes in the PXRD pattern yielding a
new but, unfortunately, unidentified crystalline phase, which
remains intact up to 230 °C. The observations are correlated
with the Guinier−Simon diagram of complex 1a (Figure 9)
which shows just one thermal phase transformation at 120−130
°C. As indicated in the TGA experiments (see SI), the
structural changes are accompanied by the gradual release of
water molecules in the temperature ranges of 60−200 °C (for
complex 1a) and 30−240 °C (for 1b). However, these DTA
data are not very informative because the dehydration processes
do not proceed in a stepwise manner, synchronously with the
structural transformations.
1
ED2 = −3J2/4
(2)
as shown in Figure 10.
Interestingly, when the synthesis of compound 1a (see
Experimental Section) was carried out at 130−140 °C, a
mixture of two products (green prisms 1a and deep-blue
powder of the unknown phase) were formed in close
proportions. The deep-blue phase is only a product when the
hydrothermal process temperature was set at 165 °C.
Obviously, the thermal behavior suggests that the microporous
[Cu3(μ3-OH)(trgly)3(H2O)3]SO4·16H2O framework with the
3D zeolite-like topology can be regarded as a “kinetic” product
or a metastable phase. For instance, upon standing in water for
over a few months the compound irreversibly transform into
the denser and more stable 2D crystal structure of 1a, i.g.,
thermodynamic product.
Magnetic Properties and EPR Spectra. In the trimeric
units of both compounds 1a and 1b the three copper ions
arranged in a triangle are bridged by triazole molecules and by
the O atom of OH−. The groups bridging different trinuclear
units to each other contain a tetrahedral carbon atom and thus
are unlikely to transmit significant exchange interactions. From
the viewpoint of magnetism, the trinuclear units can thus be
treated separately. The magnetic moment of 1a (per three
copper(II) ions) is indicative of strong antiferromagnetic
interactions. The isotropic exchange interactions in a trinuclear
system can be analyzed by using the Heisenberg−Dirac−Van
Vleck Hamiltonian:7
Figure 10. Energy levels of a trinuclear Cu(II) system according to
Hamiltonian (eq 3), calculated with gx = gy = 2.07, gz = 2.33, J1 = 138
cm−1, J2 = 163 cm−1. The Gz components of all antisymmetric
interactions 1−2, 1−3, and 2−3 are equal to 35 cm−1. Labels Q, D1,
and D2 indicate the quartet and two doublet states (see Formula
2).The red and blue lines were calculated for the magnetic field
perpendicular (orientation Z) and parallel to the Cu3 plane (X,Y),
respectively. It is seen that the Zeeman splitting in the doublet states
D1 and D2 is reduced at the X,Y orientation as a result of the
antisymmetric exchange interactions. When J1 = J2 and Gz ≠ 0, that
splitting vanishes.
At sufficiently low temperatures, only the D2 doublet is
populated, and the magnitude of the magnetic moment referred
to the entire trinuclear entity at low temperatures should be
μ = g 3/4 where g is the average g value, typically about 2.1
for Cu2+. This results in μeff of some 1.8 μB, or χT = 0.4 cm3
mol−1 K, corresponding to one unpaired electron per three
copper ions. The experimental χT magnitudes in Figure 11 are
much smaller than this (∼0.2 cm3 mol−1K) which cannot be
explained on the basis of Hamiltonian (eq 1) only. Two
interesting phenomena must be taken into account when
dealing with the antiferromagnetic triangular systems. One is
the ‘spin frustration’ which is the impossibility of orienting each
pair of spins in the antiparallel fashion at the same time. The
3649
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
exchange interaction in a pair “i,j” of copper atoms of which
atom “j” is in its ground state (g) and the other one is in an
excited state (e).7 These g deviations are large in Cu(II): a
value of gz of 2.30 was found in a linear trinuclear complex with
1,3-bis(1,2,4-triazol-4-yl)adamantane,25 and accordingly, the
antisymmetric exchange may contribute significantly to the
properties of a triangular system. The most interesting
consequence is quenching of the effective g values in the
doublet states of a triangular system. This quenching, occurring
when the magnetic field lies in the triangle plane affects both
the EPR spectra and the magnetic susceptibility.
A program was written by one of us to fit both the magnetic
susceptibility and EPR data on the basis of the Hamiltonian eq
3. The program performed numerical diagonalization of the 8 ×
8 spin Hamiltonian matrix to evaluate the energy levels. The
magnetic susceptibility at the magnetic field B and an
orientation given by the polar angles Θ, Φ was calculated from
Figure 11. The temperature dependence of the product χT for 1a.
Circles: experimental; red solid line: calculated using spin Hamiltonian
3 with gx = gy = 2.07, gz = 2.33, J1 = 138 cm−1, J2 = 163 cm−1. The z
components of the antisymmetric interactions 1−2 and 2−3 (Gz1) as
well as 1−3 (Gz2) are equal to 34 cm−1. These data can be fitted
substantially better when the g parameters are allowed to vary, but
then gz is obtained unacceptably low (gx,y = 2.05, gz = 2.07, J1 = 119
cm−1, J2 = 143 cm−1, Gz1 = 25 cm−1, Gz2 = 25 cm−1), green line. The
green and red lines overlap below ∼80 K.
∂Ei(Θ, Φ)
8
i=1
∑
exp(−Ei(Θ, Φ)/kT)
N
B
∂B
χ(Θ, Φ) = −
8
∑
exp(−Ei(Θ, Φ)/kT)
(4)
i=1
where the derivatives δEi/δB were calculated numerically by
evaluating the energies Ei 5 G below and 5 G above the working
magnetic field of the SQUID magnetometer (5000 G). The
powder susceptibility was then calculated in a manner similar to
the way by which the powder EPR spectra are simulated, that is,
it was numerically integrated with sinΘdΘdΦ. The fit results
for 1a are presented in Figure 11. The spin Hamiltonian
parameters gx = gy = 2.07, gz = 2.33, J1 = 138 cm−1, J2 = 163
cm−1, Gz1 = Gz2 = 34 cm−1 were found. Since the magnetic data
for out three compounds are difficult to present in one figure,
data for 2 and 1b are given in the SI, Figure S26. The spin
Hamiltonian parameters for 2 are gx = gy = 2.07, gz = 2.33, J1 =
154 cm−1, J2 = 193 cm−1, Gz1 = 38 cm−1, Gz2 = 39 cm−1.
Problems were encountered in the case of 1b, whose porosity
and high water contents cause uncertainty in the molar mass at
the time of the magnetic measurement. Chemical analysis of a
sample performed just after the magnetic susceptibility
measurement indicated a formula [Cu3(μ3-OH)-
(trgly)3(H2O)3]SO4·22H2O instead of [Cu3(μ3-OH)-
(trgly)3(H2O)3]SO4·16H2O from X-ray and earlier chemical
analyses. The magnetic fitting was still less successful than for
1a or 2, resulting in parameters which are not reasonable (see
Figure S26 in SI).
second is the antisymmetric exchange interaction, also called
the Dzialoshinskii−Moriya term. The DM interaction between
atoms “i” and “j” is represented by a dot-product of a vector
̂
quantity Gij with the cross-product of the spin operators Si and
̂
̂
̂
Sj: Gij·Si × Sj. There are three such DM terms in the full spin
Hamiltonian for a triangular trimer:
̂
̂
̂
̂
̂
̂
H = μ B
{g}S +
S{J} S+
G ·S × S
∑
∑ ∑
∑ ∑
i<j j
i
i
i
ij
j
ij
i
j
B
i
i<j
j
(3)
The indices i, j number the atoms in the trimer. The
symmetric {J}ij tensors describing interactions in a pair i,j may
be anisotropic and thus may contain the zero-field splitting
effects. The scalar parameters J12 etc. in eq 1 equal one-third of
the trace of a respective tensor, like J12 = ({Jxx}12 + {Jyy}12
+
{Jzz}12)/3. Each of the Gij vectors (for a pair i−j) in our system
is expected to have their largest component Gzij perpendicular
to the plane of the three copper ions.7 This direction is also
very close to the expected direction of the gz component for
each copper. The second largest components, if present, will lie
in the Cu1Cu2Cu3 plane and be perpendicular to the Cu−Cu
vector of a pair. The components along the Cu−Cu direction
are expected to be close to zero and are assumed to be zero.
Further in this paper we will attempt to interpret the
experimental data, assuming that all three copper ions have
the same values of gz, gx and gy. Both the isotropic and
antisymmetric parts of the interactions 1−2 and 2−3 are
assumed to be equal, while the interaction 1−3 may be different
from them. We will thus use J1 = J12 = J23 and J2 = J13, as in eq 2
and G1 = G12 = G23, G2 = G13. The components of the G1 and
G2 vectors will be named G1x etc.
EPR Simulations. Upon finding a resonance field by an
iterative procedure, the eigenvectors ψi and ψj of the levels
involved in a transition were evaluated, and relative transition
probability was calculated. The intensity of a transition was
taken as a product of the probability and the population
difference between the levels ψi and ψj obtained from the
Boltzmann distribution. That procedure was repeated thou-
sands of times for various orientations of the steady magnetic
field B to generate a powder EPR spectrum.
In 1a, the normal to the least-squares planes N2O2 of the
equatorial ligands of each copper ion form angles 7.1°, 7.4°, and
3.0° with the respective Cu−Osulfate vectors. This is a good case
of an elongated square-pyramid and there is no doubt that the
2
2
The theory of the antisymmetric exchange can be found in
many sources.7a,26,27 Here we would like to emphasize that this
interaction disappears when the interacting ions are related by
an inversion center. The components of the Gij vectors are of
ground state of the copper ions is dx −y . The direction of the
largest g component, gz, of each ion is expected to be
perpendicular to its equatorial ligands plane, and it was so
assumed in the following calculations. However, the smaller gx
and gy components are likely not to be equal to each other
because of the mixed N, O coordination in the equatorial plane.
eigj
the order of (Δg/g)·Jgigj where Δg is the deviation of the g
value in a complex from 2.0023 and the symbol Jgigjeigj refers to
3650
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
The distances in two copper pairs in the triangle are 3.380,
3.374, and 3.356 Å, respectively, and the Cu1Cu2Cu3 triangle is
approximately isosceles. The vector perpendicular to the
Cu1Cu2Cu3 plane was taken as the Z axis of the trinuclear
entity. The gz directions of the three copper ions deviate from Z
by 13.3°, 6.8°, and 14.4°, respectively. The symmetry of 1b is
higher yet, as the molecule has C3v symmetry with the hydroxyl
oxygen atom lying on the three-fold axis. The deviation
between the normal to the least-squares plane N2O2 and the
Cu−Owater vector is 1.7° for each metal ion. The Cu1Cu2Cu3
triangle is equilateral, and the Z axis of the trinuclear entity,
defined as above, deviates by just 4.0° from each of the Cu−
Owater vectors.
The high-field EPR spectra of both species 1a and 1b were
very weak because of enormous width of the resonance lines (1
T for resonance D in Figure 12) and noisy, yet they were well
Figure 13. Frequency dependencies of the prominent resonances for
1a shown in Figure 12. The red circles represent resonances assigned
to the trinuclear species, and the red solid lines E and H were
calculated by using spin Hamiltonian 3 with gx,y = 2.07, gz = 2.33, J1 =
138, J2 = 163 cm−1, Gz1 = Gz2 = 34 cm−1. Line D was calculated with
gx,y = 2.07, gz = 2.33, J1 = 135, J2 = 171 cm−1, Gz1 = Gz2 = 34 cm−1.
Blue circles and solid lines refer to the hypothetical dinuclear species.
Its parameters, pertinent to the standard triplet-state spin Hamiltonian
are gx,y = 2.05, gz = 2.25, D = 1.6 cm−1, E = 0.19 cm−1. The green line
C appears to be due to a mononuclear impurity with gx,y = 2.07. The Z
component of that spectrum may overlap with line H (g = 2.33). The
slope of these lines allows estimation of the effective g values
associated with respective transitions. The geff values of 2.22, 2.13, 2.06,
1.00, and 0.91 were obtained for the branches A, B, C, D, and E,
respectively.
seen at any conditions. When analyzing the HF EPR spectra of
1a at 3 K (Figure 12), one has to bear in mind that only the
ground state doublet is populated, which should produce at
most three features in its powder spectrum, corresponding to
the effective gx, gy and gz values of the coupled system. The high
number of resonances present must thus be explained first.
Resonance “A” in Figures 12 and 13 could be followed down to
the microwave frequency of ∼50 GHz, at which point its
position has approached zero magnetic field. The resonance
therefore occurs between two levels which are split in zero
magnetic field by ∼1.6 cm−1, and it cannot be due to a
transition within a doublet spin state. The quartet state is
depopulated at 3 K, and one could be tempted to assume that
this branch represents a transition between the doublets D1 and
D2 (see Figure 10). Such a small difference between the
doublets cannot be reconciled with the magnetic data, however.
A possible explanation could be that at some locations in the
extended layers copper ions may be missing, leading to local
dinuclear systems. Problems of this kind appear to be present
also in ref 7a where too many resonances were observed,
including transitions at zero magnetic field (in X-Band EPR) in
similar triangular systems. Some of the branches in Figure 13
(blue lines and circles), including “A” can be simulated using
the standard spin Hamiltonian for S = 1
Figure 12. (Top) EPR spectrum of 1a recorded at 3 K with ν = 156.0
GHz (upper trace) also showing the integrated spectrum (lower
trace). (Bottom) Spectra recorded at 3 K with microwave frequencies
as indicated. Note that the resonances D and E travel about a double
distance compared to resonances A, B, and C when the frequency is
changed from 84 to 156 GHz. Resonance labeled O2 is due to oxygen,
and it is often observed in the high-frequency EPR at or below 30 K.
reproducible. Figure 12 shows high-frequency EPR spectra of
1a recorded at 3 K with various microwave frequencies. The
prominent resonance lines which could be followed over a wide
microwave frequency range are marked with letters A−H.
Figure 13 shows the frequency dependence of those lines. The
X-Band EPR spectra (∼9.5 GHz) of 1a were very poorly
resolved showing a narrower transition at the effective g value
of 2.08 (corresponding to C) and very broad transitions at geff
of 1.38 and 0.75. No features similar to those reported in ref 7a
for closely related complexes could be observed in X-Band. No
transitions at low geff were seen in the EPR spectra of 2 and 1b.
The HF EPR spectra of 1a, like those in Figure 12 could be
only observed at the temperatures lower than 30 K. At higher
temperatures the resolution decreased, and only a transition at
effective g of 2.29 was observed. No excited quartet state was
2
2
2
̂
̂
̂
̂
̂
H = μBB{g}S + D{Sz − S(S + 1)/3} + E(Sx − Sy)
(5)
with gx,y = 2.07, gz = 2.25, D = 1.6 cm−1, E = 0.19 cm−1. Branch
C in Figure 13 passes through the origin, and its slope indicates
a geff value of 2.07. It must be assigned as the gx branch of a
mononuclear species whose gz feature may overlap with the
branch H which also passes through the origin and its geff
3651
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
calculated from the slope is 2.33. Branch H is assigned to gz of
the trinuclear (and possibly also a mononuclear) species. When
the microwave frequency is changed, two branches, D and E,
shift much faster in magnetic field than all other resonances do.
They represent the ‘perpendicular’ transitions in a spin-
frustrated system, characterized by very low geffective values.
The question now arises, whether they can be due to one
trinuclear systemthat is whether one could be an X and
another a Y transition. Calculations indicate that it is not
possible to find parameters which would result in the
anisotropy in the CuCuCu plane sufficient for the observed
behavior of the D and E features, even if nonzero x and y
components of the G1 and G2 vectors are allowed. The gx, gy
anisotropy of the three copper ions can only contribute to the
width of resonances D and E, but cannot explain the large
splitting between them. We are thus forced to conclude that the
compound is not uniform, as similarly observed in ref 7a, and
resonances D and E belong to different trinuclear entities. The
parameters required to reproduce the E branch differ only
slightly from those reproducing the D branchthe difference
between J1 and J2 needs to be slightly increased, from 25 to 36
cm−1 while keeping 2J1+J2 constant (Figure 13 caption). This
sensitivity of the effective g to parameters may also explain the
enormous line width of the D and E resonances.
In 2, there appear to be signals at very low geff but they are
yet much broader than those in 1a and are barely distinguish-
able from the background (Figure S27 in SI). This may be
caused by wider distribution of J1 and J2 in complex 2 than in
1a, resulting in wider distribution of the geff in the former. In 1b
no such signals could be detected, and its EPR spectrum must
be attributed to defects only.
Surprisingly, the more symmetric compound 1b did not
show EPR spectra similar to those of 1a. The only resonances
observed were of low intensity at g = 2.07 and 2.33 (Figures
S28 in SI) plus a signal of “forbidden” character appearing at
frequencies 200 GHz and above.
systems. A trinuclear case may be handled in two ways by
ORCA. One possibility is to replace one of the copper atoms in
the structure by Zn and calculate J in the system of two copper
ions. Another way is to use the “spin flip” operation of ORCA
on one of the copper ions. The software first performs a SCF
calculation for the high-spin S = 3/2 state, then ‘flips’ the spin
on one copper atom to generate the ‘broken-symmetry’ state
with MS = 1/2, performs another SCF calculation for that state,
and finally uses the two energies found above to calculate the
exchange integral. The calculation utilized Ahlrichs-type basis
set TZVPP30a for Cu, S and all coordinated atoms, while SVP30
functions were used for the remaining atoms. B3LYP31
functional was employed. Ahlrichs polarization functions from
basis H - Kr R and auxiliary bases from the TurboMole library
were also used.30b The exchange integral value reported by
ORCA must be multiplied by −2 because of the form of
Hamiltonian (eq 1) used in this paper. The two methods gave
comparable results. For 1a, J = 88 and 100 cm−1 were obtained
from the “zinc substitution” and spin flip method, respectively.
The analogous numbers for 1b were 106 and 136 cm−1.
CONCLUSION
■
To summarize, the mixed donor triazolyl-carboxylates (trgly-H,
trala-H) derived from natural α-amino acids were explored in
the design of MOFs incorporating [Cu3(μ3-OH)] clusters. The
generation of well-defined triangular motifs exhibiting three-
fold symmetry can be considered as a consequence of a
synergistic ligand combination (short inorganic μ3-OH−, μ3-
−
SO42−/ClO4 as well as three [−N−N−] organic bridges) and
Cu2+ cations undergoing partial hydrolysis in aqueous media.
The trimers self-associate in 2D layered (1a, 2) and 3D zeolite-
like (1b) structures. The porous zeolite-like framework (1b)
easily transforms to 2D motif 1a under simple heating. Further
exploring of the structural metamorphosis can lead to a more
comprehensive understanding of such reaction systems and
reaction mechanisms, e.g., to the development of novel
materials employing solid-state reactions.
DFT Calculations. Broken symmetry calculations have been
performed for both triangular systems in order to estimate the
exchange integral values. The X-ray coordinates were used, but
the trinuclear entity of each complex was cut out of the
extended lattice by breaking the C−C bonds between the acetic
acid fragments and triazoles and putting hydrogen atoms at
appropriate distances from a C atom along the broken bonds.
This resulted for 1a in a neutral molecule shown in Figure 14.
The same procedure produced a dipositive cation for 1b.
The free software package ORCA was employed.28,29 The
“broken symmetry” method is designed to deal with dinuclear
The magnetic properties and EPR spectra are determined by
the antisymmetric (Dzialoshinskii−Moriya) exchange interac-
tions resulting in abnormally low g values (gx,y < 1) of the
trinuclear species. This may be the first case of observing these
low-g resonances in the high-frequency EPR. Resonances due
to dinuclear and mononuclear entities appear also in the EPR
spectra, indicating imperfection of the lattice.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data, experimental details for X-ray structural
analyses and spectral characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: ab_lysenko@univ.kiev.ua (A.B.L.)
*E-mail: ozarowsk@magnet.fsu.edu (A.O.)
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Figure 14. The symmetric (left, lower energy) and antisymmetric
(right, higher energy) single-occupied molecular orbitals (SOMO) of
the two-copper system in the simplified molecule of 1a (the third
copper atom was replaced by Zn).
The high-field EPR spectra were recorded at the NHMFL,
which is funded by the National Science Foundation (NSF)
through the Cooperative Agreement DMR-1157490, the State
3652
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
(10) (a) Senchyk, G. A.; Lysenko, A. B.; Krautscheid, H.; Rusanov, E.
of Florida and the Deparetment of Energy (DOE). We are also
grateful to the Ministry of Science and Higher Education of
Poland for financial support in the purchase of the Bruker
ELEXSYS E 500 EPR spectrometer. Financial support from
Deutsche Forschungsgemeinschaft and the Ukrainian education
program, for the education, training of students, Ph.D. students,
scientific and pedagogical staff abroad is gratefully acknowl-
edged.
B.; Chernega, A. N.; Kramer, K. W.; Liu, S.-X.; Decurtins, S.;
̈
Domasevitch, K. V. Inorg. Chem. 2013, 52, 863−872. (b) Ling, Y.;
Yang, F.; Deng, M.; Chen, Z.; Liu, X.; Weng, L.; Zhou, Y. Dalton
Trans. 2012, 41, 4007−4011. (c) Sarma, D.; Srivastava, V.; Natarajan,
S. Dalton Trans. 2012, 41, 4135−4145. (d) Ren, H.; Song, T.-Y.; Xu,
J.-N.; Jing, S.-B.; Yu, Y.; Zhang, P.; Zhang, L.-R. Cryst. Growth Des.
2009, 9, 105−112. (e) Wang, X.-L.; Qin, C.; Lan, Y.-Q.; Shao, K.-Z.;
Su, Z.-M.; Wang, E.-B. Chem. Commun. 2009, 4, 410−412. (f) Park,
H.; Moureau, D. M.; Parise, J. B. Chem. Mater. 2006, 18, 525−531.
(g) Hulvey, Z.; Falcao, E. H. L.; Eckert, J.; Cheetham, A. K. J. Mater.
Chem. 2009, 19, 4307−4309. (h) Yang, E.-C.; Liang, Q.-Q.; Wang, P.;
Zhao, X.-J. Inorg. Chem. Commun. 2009, 12, 211−213. (i) Mahata, P.;
Prabu, M.; Natarajan, S. Cryst. Growth Des. 2009, 9, 3683−3691.
(j) Zhai, Q.-G.; Lu, C.-Z.; Wu, X.-Y.; Batten, S. R. Cryst. Growth Des.
2007, 7, 2332−2342. (k) Luo, F.; Che, Y.-X.; Zheng, J.-M.
CrystEngComm 2009, 11, 1097−1102. (l) Chowdhuri, D. S.; Rana,
A.; Bera, M.; Zangrando, E.; Dalai, S. Polyhedron 2009, 28, 2131−
2136. (m) Habib, H. A.; Sanchiz, J.; Janiak, C. Dalton Trans. 2008, 13,
1734−1744. (n) Senchyk, G. A.; Lysenko, A. B.; Krautscheid, H.;
Sieler, J.; Domasevitch, K. V. Acta Crystallogr. 2008, C64, m246−
m249.
REFERENCES
■
(1) Aromí, G.; Barrios, L. A.; Roubeau, O.; Gamez, P. Coord. Chem.
Rev. 2011, 255, 485−546.
(2) (a) Di Nicola, C.; Karabach, Y. Y.; Kirillov, A. M.; Monari, M.;
Pandolfo, L.; Pettinari, C.; Pombeiro, A. J. L. Inorg. Chem. 2007, 46,
́
221−230. (b) Di Nicola, C.; Garau, F.; Gazzano, M.; Aatima, C.;
Guedes da Silva, M.; Lanza, A.; Monari, M.; Nestola, F.; Pandolfo, L.;
Pettinari, C.; Pombeiro, A. J. L. Cryst. Growth Des. 2012, 12, 2890−
2901.
(3) Su, S.; Zhang, Y.; Zhu, M.; Song, X.; Wang, S.; Zhao, S.; Song, S.;
Yang, X.; Zhang, H. Chem. Commun. 2012, 48, 11118−11120.
(4) (a) Magnetic Molecular Materials; Gatteschi, D., Kahn, O., Miller,
J. S., Palacio, F., Eds.; Kluwer Academic: Dordrecht, The Netherlands,
1991. (b) Kahn, O.; Pei. Y.; Yournaux, Y. In Inorganic Material; Bruce,
D. W., O’Hare, O., Eds.; John Wiley & Sons: Chichester, U.K., 1992.
(c) Kahn, O. Molecular Magnetism; VCH Publishers: New York, 1993.
(d) Molecule-Based Magnetic Materials: Theory, Techniques, and
Applications; Turnbull, M. M., Sugimoto, T., Thompson, L. K., Eds.;
ACS Symposium Series 644; American Chemical Society: Washington,
DC, 1996.
́
(11) Naik, A. D.; Dîrtu, M. M.; Leonard, A.; Tinant, B.; Marchand-
Brynaert, J.; Su, B.-L.; Garcia, Y. Cryst. Growth Des. 2010, 10, 1798−
1807.
(12) Lassig, D.; Lincke, J.; Gerhardt, R.; Krautscheid, H. Inorg. Chem.
̈
2012, 51, 6180−6189.
(13) Xia, B.; Chen, Z.; Zheng, Q.; Zheng, H.; Deng, M.; Ling, Y.;
Weng, L.; Zhou, Y. CrystEngComm 2013, 15, 3484−3489.
(14) (a) Bartlett, R. K.; Humphrey, I. R. J. Chem. Soc. 1967, C17,
1664−1666. (b) Bondar, O. A.; Lukashuk, L. V.; Lysenko, A. B.;
Krautscheid, H.; Rusanov, E. B.; Chernega, A. N.; Domasevitch, K. V.
CrystEngComm 2008, 10, 1216−1226.
(5) Yang, E.-C.; Liu, Z.-Y.; Shi, X.-J.; Liang, Q.-Q.; Zhao, X.-J. Inorg.
Chem. 2010, 49, 7969−7975.
(6) (a) Solomon, E. I.; Sundaram, U. M.; Machonkin, T. E. Chem.
Rev. 1996, 96, 2563−2606. (b) Messerschmidt, A. In Structure and
Bonding; Hill, H. A. O.; Sadler, P. J., Thomson, A. J., Eds.; Springer:
Berlin/Heidelberg, 1998; Vol. 90, pp 37−68.
(15) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122.
(16) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7−13.
(17) (a) O’Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203−283.
(b) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532−536.
(18) Hassan, A. K.; Pardi, L. A.; Krzystek, J.; Sienkiewicz, A.; Goy, P.;
Rohrer, M.; Brunel, L.-C. J. Magn. Reson. 2000, 142, 300−312.
(19) Naik, A. D.; Marchand-Brynaert, J.; Garcia, Y. Synthesis 2008, 1,
149−154.
(20) Lysenko, A. B.; Govor, E. V.; Krautscheid, H.; Domasevitch, K.
V. Dalton Trans. 2006, 31, 3772−3776.
(21) (a) Blatov, V. A. IUCr CompComm Newsletter 2006, 7, 4−38.
(b) Blatov, V. A.; Shevchenko, A. P.; Serezhkin, V. N. J. Appl.
Crystallogr. 2000, 33, 1193−1193.
(7) (a) Ferrer, S.; Lloret, F.; Pardo, E.; Clemente-Juan, J. M.; Liu-
Gonzal
(b) Ferrer, S.; Lloret, F.; Bertomeu, I.; Alzuet, G.; Borras
Granda, S.; Liu-Gonzalez, M.; Haasnoot, J. G. Inorg. Chem. 2002, 41,
́
ez, M.; García-Granda, S. Inorg. Chem. 2012, 51, 985−1001.
́
, J.; García-
́
5821−5830. (c) Ferrer, S.; Haasnoot, J. G.; Reedijk, J.; Muller, E.;
̈
Biagini Cingi, M.; Lanfranchi, M.; Manotti Lanfredi, A. M.; Ribas, J.
Inorg. Chem. 2000, 39, 1859−1867. (d) Lider, E. V.; Peresypkina, E.
V.; Smolentsev, A. I.; Elokhina, V. N.; Yaroshenko, T. I.; Virovets, A.
V.; Ikorskii, V. N.; Lavrenova, L. G. Polyhedron 2007, 26, 1612−1618.
(e) Zhou, J.-H.; Cheng, R.-M.; Song, Y.; Li, Y.-Z.; Chen, X.-T.; Xue,
Z.-L.; You, X.-Z. Inorg.Chem. 2005, 44, 8011−8022. (f) Wu, X.-Y.;
Kuang, X.-F.; Zhao, Z.-G.; Chen, S.-C.; Xie, Y.-M.; Yu, R.-M.; Lu, C.-Z.
Inorg. Chim. Acta 2010, 363, 1236−1242. (g) Liu, J.-C.; Guo, G.-C.;
Huang, J.-S.; You, X.-Z. Inorg. Chem. 2003, 42, 235−243. (h) Ferrer,
S.; Aznar, E.; Lloret, F.; Castineiras, A.; Liu-Gonzalez, M.; Borras, J.
Inorg. Chem. 2007, 46, 372−374.
(22) (a) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40,
2022−2043. (b) Seidel, S. R.; Stang, P. J. Acc. Chem. Res. 2002, 35,
972−983. (c) Swiegers, G. F.; Malefetse, T. J. Coord. Chem. Rev. 2002,
225, 91−102. (d) Cotton, F. A.; Lin, C.; Murillo, C. A. Acc. Chem. Res.
2001, 34, 759−771.
(23) (a) MacGillivray, L. R.; Atwood, J. L. Angew. Chem., Int. Ed.
1999, 38, 1018−1033. (b) Moulton, B.; Zaworotko, M. J. Chem. Rev.
2001, 101, 1629−1658. (c) Perry, J. J., IV; Perman, J. A.; Zaworotko,
M. J. Chem. Soc. Rev. 2009, 38, 1400−1417.
(8) (a) Ouellette, W.; Yu, M.-H.; O’Connor, C. J.; Hagrman, D.;
Zubieta, J. Angew. Chem., Int. Ed. 2006, 45, 3497−3500. (b) Yamada,
T.; Maruta, G.; Takeda, S. Chem. Commun. 2011, 2, 653−655.
(c) Yang, E.-C.; Liu, Z.-Y.; Wang, X.-G.; Batten, S. R.; Zhao, X.-J.
CrystEngComm 2008, 10, 1140−1143. (d) Huang, X.-C.; Luo, W.;
Shen, Y.-F.; Lin, X.-J.; Li, D. Chem. Commun. 2008, 34, 3995−3997.
(e) Ouellette, W.; Prosvirin, A. V.; Chieffo, V.; Dunbar, K. R.; Hudson,
B.; Zubieta, J. Inorg.Chem. 2006, 45, 9346−9366. (f) Ding, B.; Yi, L.;
Cheng, P.; Liao, D.-Z.; Yan, S.-P. Inorg. Chem. 2006, 45, 5799−5803.
(24) Wang, Y.; Cheng, P.; Song, Y.; Liao, D.-Z.; Yan, S.-P. Chem.
Eur. J. 2007, 13, 8131−8138.
(25) Senchyk, G. A.; Lysenko, A. B.; Rusanov, E. B.; Chernega, A. N.;
Jezierska, J.; Domasevitch, K. V.; Ozarowski, A. Eur. J. Inorg. Chem.
2012, 5802−5813.
(26) Moriya, T. Phys. Rev. 1960, 120, 91−98.
(27) Yoon, J.; Solomon, E. I. Coord. Chem. Rev. 2007, 251, 379−400.
(28) Neese, F. ORCA: An ab Initio, Density Functional and
Semiempirical Program Package, version 2.9.1; 2012, free download
from http://www.cec.mpg.de/forum/downloads.php.
(29) Neese, F. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2, 73−
78.
(9) (a) Lincke, J.; Lassig, D.; Stein, K.; Moellmer, J.; Kuttatheyil, A.
̈
V.; Reichenbach, C.; Moeller, A.; Staudt, R.; Kalies, G.; Bertmer, M.;
Krautscheid, H. Dalton Trans. 2012, 41, 817−824. (b) Lassig, D.;
̈
Lincke, J.; Moellmer, J.; Reichenbach, C.; Moeller, A.; Glaser, R.;
̈
Kalies, G.; Cychosz, K. A.; Thommes, M.; Staudt, R.; Krautscheid, H.
Angew. Chem., Int. Ed. 2011, 50, 10344−10348. (c) Chen, M.; Chen,
M.-S.; Okamura, T.; Lv, M.-F.; Sun, W.-Y.; Ueyama, N. CrystEngComm
2011, 13, 3801−3810.
(30) (a) Schaefer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97,
2571−2577. (b) The Ahlrichs auxiliary basis sets were obtained from
3653
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654

Inorganic Chemistry
Article
the TurboMole basis set library under ftp.chemie.uni-karlsruhe.de/pub/
jbasen. (c) Eichkorn, K.; Treutler, O.; Ohm, H.; Haser, M.; Ahlrichs,
R. Chem. Phys. Lett. 1995, 240, 283−290. (d) Eichkorn, K.; Weigend,
F.; Treutler, O.; Ahlrichs, R. Theor. Chem. Acc. 1997, 97, 119−124.
(31) (a) Becke, D. A. Phys. Rev. A 1988, 38, 3098−3100. (b) Perdew,
J. P. Phys. Rev. B 1986, 33, 8822−8824. (c) Perdew, J. P. Phys. Rev. B.
1986, 34, 7406−7406. (c) Kendall, R. A.; Fruchtl, H. A. Theor. Chem.
̈
Acc. 1997, 97, 158−163.
3654
dx.doi.org/10.1021/ic403148f | Inorg. Chem. 2014, 53, 3642−3654