The synthesis and characterisation of coordination and hydrogen-bonded networks based on 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid

Article abstract of DOI:10.1039/c5dt00011d

The synthesis, structural and thermal characterisation of a number of coordination complexes featuring the N,O-heteroditopic ligand 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoate, HL are reported. The reaction of H₂L with cobalt(II) and nickel(II) ni

Full text of DOI:10.1039/c5dt00011d

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The synthesis and characterisation of coordination
and hydrogen-bonded networks based on 4-(3,5-
dimethyl-1H-pyrazol-4-yl)benzoic acid†
Cite this: Dalton Trans., 2015, 44,
9269
a
b
c
Macguire R. Bryant, Andrew D. Burrows,* Christopher M. Fitchett,
c
a
b
b
c,d
Chris S. Hawes, Sally O. Hunter, Luke L. Keenan, David J. Kelly, Paul E. Kruger,*
Mary F. Mahon and Christopher Richardson*
b
a
The synthesis, structural and thermal characterisation of a number of coordination complexes featuring
the N,O-heteroditopic ligand 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoate, HL are reported. The reaction
of H
mononuclear coordination complexes with the general formula {[M(HL)
2)), whereas the reaction with zinc(II) nitrate gave [Zn(HL) , 3, a coordination polymer with distorted di-
amondoid topology and fourfold interpenetration. Coordination about the tetrahedral Zn(II) nodes in 3 are
furnished by two pyrazolyl nitrogen atoms and two carboxylate oxygen atoms to give a mixed N
donor set. Isotopological coordination polymers of zinc(II), {[Zn(HL) ]·2CH OH·H O} , 4, and cobalt(II),
, 5, are formed when the reactions are carried out under solvothermal conditions in methanol
80 °C) and water (180 °C), respectively. The reaction of H L with cadmium(II) nitrate at room temperature
(MeOH) ]·1.8MeOH} 6, a 2-D (4,4)-connected coordination polymer,
whereas with copper(II) the formation of green crystals that transform into purple crystals is observed. The
metastable green phase [Cu (HL) (μ -SO )(H O) , 7, crystallises with conserved binding domains of the
2 2
L with cobalt(II) and nickel(II) nitrates at room temperature in basic DMF/H O solution gave discrete
2
(H O)
2
4
]·2DMF} (M = Co (1), Ni
(
2 ∞
]
2
O
2
2
3
2
∞
[
(
2 ∞
Co(HL) ]
2
in methanol gives {[Cd(HL)
2
2
∞
3
4
2
4
2
3 ∞
]
heteroditopic ligand and contains two different metal nodes: a dicopper carboxylate paddle wheel motif,
and, a dicopper unit bridged by sulfate ions and coordinated by ligand pyrazolyl nitrogen atoms. The
2 3 2 ∞
resultant purple phase {[Cu(HL) ]·4CH OH·H O} , 8, however, has single copper ion nodes coordinated
by mixed N donor sets with trans-square planar geometry and is threefold interpenetrated. The de-
2 2
O
solvation of 8 was followed by powder X-ray diffraction and single crystal X-ray diffraction which show
desolvation induces the transition to a more closely packed structure while the coordination geometry
about the copper ions and the network topology is retained. Powder X-ray diffraction and microanalysis
were used to characterise the bulk purity of the coordination materials 1–6 and 8. The thermal character-
istics of 1–2, 4–6 and 8 were studied by TG-DTA. This led to the curious observation of small exothermic
events in networks 4, 6, and 8 that appear to be linked to their decomposition. In addition, the solid
Received 3rd January 2015,
Accepted 17th April 2015
2 2 3
state structures of H L and that of its protonated salt, H L·HNO , were also determined and revealed that
DOI: 10.1039/c5dt00011d
H
2
L forms a 2-D hydrogen bonded polymer incorporating helical chains formed through N–H⋯O and
www.rsc.org/dalton
3 3
O–H⋯N interactions, and that [H L]NO forms a 1-D hydrogen-bonded polymer.
a
d
School of Chemistry, Faculty of Science, Medicine and Health, University of
Wollongong, Wollongong, NSW 2522, Australia.
MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of
Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140,
New Zealand. E-mail: paul.kruger@canterbury.ac.nz
E-mail: chris_richardson@uow.edu.au; Fax: +61 2 4221 4287; Tel: +61 2 4221
3
254
†Electronic supplementary information (ESI) available: Synthetic details for the
b
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK.
E-mail: a.d.burrows@bath.ac.uk
2 2
preparation of H L. PXRD patterns of 1–6, 8. Further TG-DTA traces for H L, 1–6,
8. Infrared spectra of 1, 1-desolv, 2, 4, 5, 6-dry, and air- and TGA-dried 8. CCDC
1040598–1040606. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/C5DT00011D
c
Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch
8
140, New Zealand
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Despite these attractive features, H
a few reports. Janiak and co-workers formed a structure isoreti-
2
L has only been used in
Introduction
2
−
The construction of coordination networks by design is a topic cular to MOF-5 where dianionic L
4
units bridge Zn O
1
16
of continuing high interest. This interest stems from the nodes. Zhang et al. have reported different forms of zinc
−
potential of these materials in applications ranging from opto- coordination networks where the anionic HL bridges zinc
2
17
electronics to catalysis. A challenge in the field is to create ions, and studied solid phase micro extraction and the topo-
1
8
coordination networks with more than one functional unit, logical subtlety of the materials. The porous zinc coordi-
which increases the complexity of the framework. One way of nation network MAF-X8 has been further examined for
1
9
achieving such complexity is through the structure of the brid- p-xylene separation capability in silico. A beautiful example
ging ligand component. In this regard, heteroditopic ligands of hard–hard soft–soft interactions dictating structure for-
3
2
0
offer the prospect of constructing complex coordination net- mation has been reported by Zhou and co-workers. The reac-
works, with each end of the linker coordinating to a different tion of H L with copper(II) led to a structure containing
2
type of metal node (also known as Secondary Building Units or conserved binding domains featuring well-known pyrazolate-
SBUs). There is also the potential for the metal nodes in these bridged trigonal copper(I) units [Cu (pyrazolate) ] from in situ
3
3
structures to contain both types of donor atom or to have reduction of the copper(II) ions, and square carboxylate-
nodes with conserved binding domains where all the coordi- bridged copper(II) paddlewheels in a coordination network of
2
0
nating atoms are of one type. By designing ligands with Pt O topology. The versatility and potential of the pyrazole-
3
4
binding domains that are different in nature, it is possible that benzoate synthon is also seen by the incorporation into
coordination networks can be made that exploit “soft–soft” methylene ‘hinged’ bispyrazole-benzoate ligands to success-
2
1,22
and “hard–hard” coordination interactions.
fully target flexible MOFs.
2
In this paper we report the structures of H L and its nitrate
We are interested in ditopic and heteroditopic molecules
featuring pyrazole units as ligands for constructing coordi- salt and describe the syntheses, structures and thermal studies
4
,5
nation networks, and have worked with pyrazole-pyridines,
indazole-carboxylates
of coordination networks and hydrogen-bonded networks of
6,7
8
−
and pyrazole-carboxylates. Pyrazole metal complexes of HL .
ligands are an attractive option for a number of reasons. As
9
,10
commented on by Long and collaborators
the deprotona-
tion of pyrazoles and the binding of the resulting pyrazolates
confers good thermal and chemical stability in coordination
networks. Indeed, Navarro and co-workers have explored stable
Experimental
General
1
1–13
14,15
Unless otherwise specified, all reagents and starting materials
were of reagent grade or better and purchased from standard
suppliers and used as received. Water was purified by reverse
osmosis. Where anhydrous solvents were required, HPLC-
grade solvent was either distilled from standard drying agents
or dried by passing over a sealed column of activated alumina.
Powder X-ray diffraction patterns were obtained using a
GBC-MMA diffractometer operating at 1.0 kW with samples
mounted on 1″ quartz substrates. Simultaneous thermo-gravi-
metric-differential thermal analysis (TG-DTA) traces were
obtained using a Shimadzu DTG-60 instrument fitted with a
FC-60A flow rate controller and TA-60WS thermal analyser
bis-pyrazolate
and pyrazolate-carboxylate
networks for
sequestration-type applications. The pyrazole ring can also
coordinate in a neutral form. This mode of bonding offers
additional hydrogen bonding capacity and we reasoned that
this offers a mechanism for secondary organisation of the
supramolecular structure of coordination materials.
4
2
-(3,5-Dimethyl-1H-pyrazol-4-yl)benzoic acid (H L) is hetero-
ditopic with the potential to bridge metal centres in a number
of ways through pyrazolyl nitrogen and carboxylate oxygen
−
2−
atoms and also in anionic (HL ) or dianionic forms (L
Fig. 1).
)
(
−
1
using measuring parameters of 10 °C min under nitrogen
3
−1
flow of 20 cm min . Infrared spectra were obtained using a
Shimadzu IR Affinity-1 FTIR, fitted with a MIRacle 10 single
reflection ATR accessory. Microanalyses were recorded by Mr
Alan Carver (University of Bath Microanalysis Service) or
Gillian Maxwell (University College London Microanalysis
Service) or the Microanalytical Unit, Australian National Uni-
versity, Australia or the Campbell Microanalytical Laboratory,
University of Otago, New Zealand.
X-ray crystallography
X-ray crystallographic data collection at the University of Can-
terbury was carried out with an Oxford-Agilent SuperNova
instrument with focused microsource Cu-Kα radiation (λ =
1.5418 Å) radiation and ATLAS CCD area detector. Single crys-
L and of its anionic counterparts, HL−
2
Fig. 1 The structure of H
and L2−.
9270 | Dalton Trans., 2015, 44, 9269–9280
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tals were analysed at the University of Bath using a Nonius microanalysis. Yield 9 mg (38%). Found C, 57.14; H, 4.71; N,
Kappa CCD diffractometer and Mo-Kα radiation (λ = 0.71073 Å) 11.14; Zn(HL) ·0.5H O (C H N O4.5Zn) requires C, 57.10; H,
2
2
24 24 4
and CCD area detector. The diffraction data for compounds 7 4.59; N, 11.10.
and 8-dry were collected at the MX1 beamline on the Austra-
lian Synchrotron, Victoria, Australia, operating at 17.4 keV (λ =
Synthesis of [Co(HL) ], 5
2
0
.7108) with data collections conducted using BluIce control H L (50 mg, 0.23 mmol) and CoSO ·7H O (100 mg, 36 mmol)
2
4
2
2
3
software. These datasets were processed using the XDS soft- were added to water (5 mL) in a Teflon lined Parr vessel and
ware suite, with anomalous dispersion corrections for the heated to 180 °C for 54 h and then cooled slowly to room
non-standard wavelength applied using Brennan and Cowan temperature. The contents were a mixture of purple and col-
data. All structures were solved using direct methods with ourless crystals. The majority of the colourless crystals of H2L
SHELXS and refined on F using all data by full matrix least- could be removed by swirling and decanting with portions of
2
4
25
2
6
2
2
7
28
squares procedures with SHELXL-97 within OLEX-2 or water. The remaining purple crystals were cleaned by short
2
9
through the X-Seed interface. Unless otherwise noted, non- bursts of sonication with fresh portions of water. The crystals
hydrogen atoms were refined with anisotropic displacement could also be cleaned by filtration and washing with DMF.
parameters. Unless otherwise noted, hydrogen atoms were Yield 44 mg (78%). Found C, 58.5; H, 4.55; N, 11.3; Co(HL)2
included in calculated positions with isotropic displacement (C H N O Co) requires C, 58.9; H, 4.53; N, 11.5.
2
4
22 4 4
parameters either 1.2 times or 1.5 times the isotropic equi-
valent of their carrier atoms, where appropriate, with the
exception of selected hydrogen bonding sites, in which the
hydrogen atoms were manually located from residual
Fourier peaks and modelled with appropriate bond length
restraints and Uiso dependences. The functions minimized
Synthesis of {[Cd(HL)
A dilute solution of 2,6-dimethylpyridine in MeOH was
diffused into a solution of H L (21.6 mg, 0.10 mmol) and Cd-
NO ) ·4H O (31 mg, 0.1 mmol) in MeOH (2 mL). This solution
was filtered after 2 weeks and again after 4 weeks to remove
small amounts of precipitated solid. After this time large poly-
hedral crystals grew over the course of the next 8 weeks. Yield
2 2
(MeOH) ]·1.8MeOH}, 6
2
(
3
2
2
2
2
2
2
2
−1
were ∑w(Fo − F ), with w = [σ (F ) + aP + bP] , where P =
c
o
2
2
[
max(F
o
) + 2F
c
]/3.
1
3 mg (47%) (after drying under dynamic vacuum at 100 °C).
Found: C, 47.57; H, 5.00; N, 9.13. [Cd(HL) (H O) ]·1.5H
C H CdN O7.5) requires C, 47.58; H, 4.82; N, 9.24.
Synthesis of H
The synthesis of H
pared H L using routes that enabled a final step of ester hydro-
2
L
2
2
2
2
O
3
0
2
L has been reported previously. We pre-
(
2
4
29
4
2
lysis and the synthetic details are provided in the ESI.†
Synthesis of [Cu
3
(HL)
4
(μ
2
-SO
4
)(H
2
3 ∞
O) ] , 7; and
{
[Cu(HL) ]·4CH OH·H O} , 8
2
3
2
∞
General method for the preparation of {[Co
CuSO
4
·5H
2
O (31.7 mg, 0.127 mmol) was dissolved in methanol
(HL)
2 2 4 2 2 4
(H O) ]·2DMF}, 1; {[Ni(HL) (H O) ]·2DMF}, 2; and
(
(
3 mL) and added to a methanolic solution (13 mL) of H2L
109.8 mg, 0.508 mmol). Under these conditions, crystals of
[
Zn(HL) ] , 3
2
∞
A solution of H
2
L (22.7 mg, 0.10 mmol) in DMF (1.25 mL) and
coordination network 7 could be isolated after a few hours. If
the solution was left to stand for a longer period, crystals of 7
transformed into 8 as small flower-like structures. The sample
for elemental analysis was air dried over 13 days. Yield of 8:
45 mg (69%). Found: C, 55.49; H, 4.65; N, 10.77. [Cu-
1
.0 M NaOH (100 μL, 0.10 mmol) was carefully layered on top
of an aqueous solution (1 mL) of metal nitrate hexahydrate
salt (0.05 mmol) separated by a layer of DMF (0.5 mL).
Data for 1: Co(NO ) ·6H O (15.5 mg, 0.05 mmol). Pink
3
2
2
crystals were harvested after a week. Yield 28 mg (74%).
Found: C, 51.01; H, 6.57; N, 12.00. [Co(HL) (H O) ]·2DMF
C H CoN O ) requires C, 50.92; H, 6.26; N, 11.87.
(
1
HL)
0.75.
2
]·1.5H
2
O (C24
H
25CuN
4
O
5.5) requires C, 55.33; H, 4.84; N,
2
2
4
(
3
0
44
6 10
3 2 2
Data for 2: Ni(NO ) ·6H O (15.1 mg, 0.05 mmol).
Small, pale blue crystals were harvested after a week. Yield
Results and discussion
Synthesis and structural characterisation of H L and [H L]NO
2
9 mg (87%). Found: C, 50.76; H, 6.50; N, 11.94. [Ni-
HL) (H O) ]·2DMF (C30 44NiN 10) requires C, 50.92; H, 6.26;
N, 11.87.
Data for 3: Zn(NO ) ·6H O (16 mg, 0.05 mmol). Large, dull of 5 (vide infra) and were analysed by single crystal X-ray diffr-
2
3
3
(
2
2
4
H
6
O
2
Colourless crystals of H L were obtained during the synthesis
3
2
2
crystals were harvested after 1.5 weeks. Yield 17 mg.
action. The structure was solved and refined in the triclinic
space group P ˉ1 with four unique molecules of the ligand in
Synthesis of {[Zn(HL)
L (40.7 mg, 0.19 mmol) and ZnSO
.05 mmol) were heated in methanol (4 mL) in an ACE glass torsion angles, which fall in the range 28.86(9)–38.29(9)°.
pressure tube at 80 °C for 48 h and then cooled slowly to room H L is replete with hydrogen bond donor and acceptor sites
2 3 2
]·2CH OH·H O}, 4
the asymmetric unit (Fig. 2a). The essential differences
H
0
2
4 2 2
·7H O (13.4 mg, between each molecule of H L resides in the pyrazole-phenyl
2
temperature. The reaction solution was replaced with fresh and O–H⋯N and N–H⋯O hydrogen bonds form between pyra-
MeOH and the crystals were kept under solvent until analysis. zolyl and carboxylic acid functionalities in the crystal lattice.
A sample was dried under dynamic vacuum at 100 °C before This takes the form of helical chains of hydrogen bonds
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Fig. 3 (a) The asymmetric unit of [H
3
L]NO
3
with selected hydrogen
3 3
atoms shown; (b) part of the 1D polymeric structure of [H L]NO
showing the two hydrogen-bonding ring systems in the structure, C–H
hydrogen atoms omitted for clarity.
4
R
4
(14) (Fig. 3b). Carboxylic acid groups associate via hydrogen
2
bonding to the equivalent site on another molecule in an R (8)
2
fashion typical of carboxylic acid dimers. The net result of
these interactions is the formation of a 1-dimensional hydro-
gen-bonded tape, which propagates parallel to the [2,1,1]
vector within the crystal lattice (Fig. 3b). The alignment of
dimeric units in [H L]NO can be described as being head-to-
2
Fig. 2 (a) Contents of the asymmetric unit of H L with heteroatoms
labelled and selected hydrogen atoms shown; (b) perspective view of
the sheet structure formed in the ac plane by N–H⋯O and O–H⋯N
hydrogen bonds, C–H hydrogen atoms omitted for clarity.
3
3
head and tail-to-tail and is in contrast to the analogous struc-
ture of the trifluoroacetate salt which aligns in a 1D polymeric
3
0
trifluoroacetate-bridged head-to-tail manner. The polymer
strands in [H L]NO align in a staggered formation with equi-
3
3
valent units above and below each other and face-to-face π–π
interactions between aligned pyrazole rings (average inter-
planar separation 3.302(2) Å) result in the formation of a
sheet-like structure.
running along the crystallographic a-axis which alternate
handedness along the c-direction as ligands link chains into a
2
D sheet in the crystallographic ac plane (Fig. 2b). The X-ray
structure determination confirms no prototropic tautomerism
exists in neutral H L, which is in line with previous solid state
H NMR measurements. Within each sheet, the phenyl rings
of adjacent ligands which propagate along a-axis are offset,
2
1
30
Synthesis and structural characterisation of 1–8
Crystal structures of 1 and 2. Pink coloured crystals suitable
and this permits edge-to-face C–H⋯π interactions between for analysis by single crystal X-ray diffraction formed when the
adjacent phenyl groups to occur. No significant intermolecular sodium salt Na[HL], prepared by the addition of one equi-
interactions were observed between the 2D sheets, most likely valent of 1 M aqueous sodium hydroxide to H L dissolved in
2
due to the alternating twists of the phenyl rings preventing DMF, was layered upon an aqueous solution of cobalt(II)
inter-sheet π–π stacking interactions.
The crystal structure of [H L]NO
nitrate. The structure of 1 crystallises in the space group P2 /c
) was also with the asymmetric unit comprising of a cobalt atom located
1
3
3
(H
2
L·HNO
3
determined. This salt formed in several attempted synthesis of on a crystallographic inversion centre coordinated to one
coordination materials with metal nitrate salts; however, ligand via the N2 atom of the pyrazolyl ring and to two water
single crystals were prepared in a dedicated synthesis by molecules. Also in the asymmetric unit is a solvate DMF mole-
heating a sample of H L in dilute nitric acid under hydro- cule. The pyrazolyl ring is neutral, and the H atom of the pyra-
2
thermal conditions, followed by slow cooling. The asymmetric zolyl ring could be located from Fourier residuals in the
+
unit contains a nitrate anion and one cationic molecule (H
3
L ) crystallographic refinement. To achieve charge balance, the
featuring a pyrazolium ring, in the triclinic space group P ˉ1 ligand exists in a deprotonated carboxylate form resulting in a
−
(
Fig. 3a). The pyrazolium moieties and nitrate anions associate HL binding mode. Thus, the structure exists as discrete mole-
via N–H⋯O hydrogen bonding about inversion centres such cular complexes (Fig. 4a) with DMF solvate molecules in the
that nitrate anions bridge pyrazolium units in a ring structure lattice, with the formula {[Co(HL) (OH ) ]·2DMF}, 1.
2
2 4
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bonded rectangular 2D sheet. The result of the hydrogen-
bonding pattern is to position carboxylate groups closely
aligned to metal centres of other complexes in the sheets.
A 3D hydrogen bonded superstructure is assembled when
H3A interacts with the next sheet (O2″⋯H3A, 1.933 Å). The
supramolecular structure is organised along the crystallo-
graphic a-axis into rectangular channels, in which the DMF
guests are positioned (Fig. 4c). The bulk purity of 1 was
established through powder X-ray diffraction (PXRD) (Fig. S2,
ESI†) and elemental analysis. Pale blue crystals of
{
[Ni(HL) (OH ) ]·2DMF} 2 formed under the same reaction
2 2 4
conditions using nickel(II) nitrate and were shown to be iso-
structural with 1 by PXRD (Fig. S2, ESI†).
Crystal structures of 3 and 4. The reaction of H L with
2
zinc(II) nitrate at room temperature under the same layering
conditions in DMF, aqueous sodium hydroxide and water,
followed a different course. Initially a powder formed and de-
posited and then large dull colourless crystals grew. These
crystals proved suitable for structural analysis by X-ray diffrac-
tion. The structure crystallises in the space group Fdd2 with
two unique zinc atoms lying on rotation axes that each coordi-
−
nate HL ligands, a zinc atom lying on a 2-fold screw axis that
coordinates two HL− ligands via pyrazolyl N2 atoms, and a
non-coordinating water molecule in the asymmetric unit. Each
of the zinc atoms coordinates two pyrazolyl N2 atoms and two
carboxyl O atoms, resulting in mixed N O donor sets in dis-
2
2
torted tetrahedral geometries (Fig. 5a). Hydrogen bonding
interactions between pyrazolyl N1H and the uncoordinated
carboxyl oxygen atoms (N–H⋯O) exist around the coordination
spheres of all zinc centres. The topology of the networks
can be described as being distorted diamondoid (dia) with
each zinc centre being a tetrahedral node and the bridging
ligand acting as a linear link. Each network features large
cavities with windows of ca. 20 × 30 Å and the material is
quadruply interpenetrated (Fig. 5b). Despite the high level of
interpenetration,
a
PLATON analysis suggests there
are potential solvent accessible voids in the structure (25.1%
Fig. 4 (a) The structure of the complexes in 1 with only selected hydro-
void volume). The electron density within these voids is diffuse
gen atoms shown; (b) a perspective view of the hydrogen bonding
around the metal centres in 1; (c) a view down the a-axis of the rect- and no ordered solvent molecules could be modelled
angular channels containing DMF guests in 1 with hydrogen atoms
omitted for clarity. Symmetry codes used to generate equivalent atoms:
crystallographically.
The PXRD patterns of crystals isolated and cleaned by soni-
(
i) −x, −y, 2 − z.
cation contain additional peaks when compared to the pattern
simulated from the crystal structure analysis (Fig. S4, ESI†).
The PXRD pattern for the powder that is formed initially could
The discrete complexes arrange into a 3D hydrogen bonded be recorded and matched well to the additional peaks seen in
polymeric structure through hydrogen bonds from coordinated the pattern obtained from the crystals. Given the cleaning
pyrazolyl and water molecules to carboxylate groups of other process and the relative intensities of the peaks due to each
complexes. The DMF guest interacts with an aqua ligand phase in the PXRD patterns of the crystals, the large crystals
(CvO⋯H4A, 1.726 Å) and this ligand also acts with an adja- contain high proportions of both phases. Interestingly, the
cent pyrazolyl ring of the coordination sphere to cooperatively PXRD pattern of the powder was found to not match any struc-
bind a carboxylate oxygen (O1′) from another complex (O1′ tures previously reported for zinc coordination networks made
⋯
H4B, 1.83 Å, NH1⋯O1′ 1.92 Å, NH1⋯O1′⋯H4B, 72°). The from H L.
2
other unique aqua ligand makes a hydrogen bond to the other
carboxylate oxygen atom (O2′) of the nearby complex (O2′ sulfate in methanol at 80 °C yielded small well-formed colour-
H3B, 1.80 Å) fixing this into position in the network, as less crystals suitable for structural analysis. The structure crys-
shown in Fig. 4b. Extension of this motif reveals a hydrogen tallises in the Fdd2 space group and is isostructural to the
Changing the synthetic procedure to react H L with zinc
2
⋯
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Fig. 6 (a) The structure of one of the metal nodes in the structure of 5
with selected hydrogen atoms shown to highlight the hydrogen
bonding around the coordination sphere; (b) perspective view of the
quadruply interpenetrating nets in 5. Symmetry related nets are red-blue
and green-yellow.
Fig. 5 (a) Structure of a single network of 3/4 within the unit cell
boundaries. Hydrogen atoms, interpenetrating networks and solvent
molecules omitted for clarity. (b) Topological representation of the
extended structure of 3/4, independent networks coloured differently.
study, and the data was solved and refined in the tetragonal
space group I ˉ4 2d. The model determined for the asymmetric
unit is comprised of two crystallographically unique Co(II) ions
laying on rotoinversion axes that are each coordinated to
−
network described above. In the structural analysis of these unique HL ions. As such, each Co(II) coordinates to four crys-
crystals, however, the guest solvate molecules could be located tallographically equivalent ligand molecules in a tetrahedral
and the full formula for this coordination network is {[Zn- fashion with a mixed N O donor set (Fig. 6a). These structural
2
2
(
HL) ]·2CH OH·H O} 4. This synthetic method gives a phase- units are unique due to differences in angles about the coordi-
2 3 2 ∞
pure zinc coordination polymer as judged by the excellent nation spheres of the cobalt atoms, and the torsion angles
match between simulated and experimental PXRD patterns between pyrazole and phenyl rings of the ligands.
(Fig. S5, ESI†). The framework backbones of 3 and 4 are identi-
In the same manner as 3 and 4, N–H⋯O hydrogen bonding
cal to that reported by Zhang who used different synthetic con- between pyrazolyl N1H and carboxyl groups is present around
ditions of zinc nitrate in ethanol/water/DMA. The essential the cobalt coordination spheres in 5, as shown in Fig. 6a, and
difference between the structures lies in the well-ordered when each crystallographically unique unit is extended into
methanol and water guests in coordination network 4 reported three dimensions, distorted dia networks are observed. The
here.
material is also quadruply interpenetrated (Fig. 6b) and 5 can
Crystal structure of 5. To generate coordination networks be viewed as consisting of two sets of two crystallographically
with cobalt we explored reaction conditions at higher tempera- equivalent networks. In contrast to 3 and 4, the networks pack
tures. The coordination polymer [Co(HL) ] 5 formed when closely and 5 contains no void space. This coordination
2 ∞
H L and cobalt sulfate were reacted under hydrothermal con- network is similar to that described by Zhang et al. for a non-
2
−
ditions at 180 °C, with slow cooling to room temperature. A porous zinc network of HL synthesised under hydrothermal
good yield of purple crystals was obtained (78%) after separ- conditions.18
ation from crystallised H L by repeated decanting or by dissol-
Crystal structure of 6. Large polyhedral crystals of {[Cd-
2
ution of unreacted H
crystals were subjected to a single crystal X-ray diffraction weeks when 2,6-dimethylpyridine was diffused at room temp-
2
L in DMF and filtration. The purple (HL) (MeOH) ]·1.8MeOH} 6 formed over a period of several
2
2
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Paper
erature into a methanol solution of H
2
L and cadmium(II) to uncoordinated carboxyl oxygen atoms of HL− (O1) in the
nitrate. The coordination network crystallises in the space next layer (O–H⋯OvC, 1.74 Å) and so the rhomboidal pore
group C2/c with a Cd centre, located on a crystallographic shape is extended into 1D channels in the structure, within
−
2
-fold rotation axis, coordinating a ligand of HL and a metha- which the methanol guests are located.
nol ligand in the asymmetric unit. The hydrogen atom of the
Crystal structures of 7 and 8. The reaction of H L with
2
hydroxyl group in the ligated methanol was readily located and copper(II) sulfate was first carried out in a 2 : 1 ligand to metal
was refined at a distance of 0.95 Å from O3. The asymmetric ratio in methanol solution at room temperature. Pale green
unit is completed with disordered and partially occupied guest plate crystals formed after several hours and this green phase
molecules that were modelled with restraints and without transformed to purple crystals over time. Modifying the prepa-
hydrogen atoms as methanol.
ration conditions to a 4 : 1 ligand to metal ratio sometimes saw
The cadmium(II) centre is 6-coordinate with a highly dis- the formation of the green crystals but more often resulted
torted octahedral geometry. The methanol ligands coordinate only in the purple phase appearing. The green crystals were
in a trans orientation (166.9°) and the equatorial plane consists subjected to single crystal X-ray diffraction using synchrotron
−
of a N O set of donors from HL ligands configured in a cis radiation. The small size and poor quality of individual crystal-
2
2
geometry around the equatorial plane (N2⋯Cd⋯N2′, 97.7°; lites and possibly the metastable nature of the crystals contrib-
N2⋯Cd⋯O2′, 90.2°; O2⋯Cd⋯O2′, 86.7°). Hydrogen bonding uted to poor diffraction and a low resolution structure solution
between pyrazolyl N1H and carboxyl oxygen atom O1 was obtained in the monoclinic space group C2/m. The asym-
(N1H⋯O1, 2.05 Å) is present around the cadmium coordi- metric unit contains three crystallographically unique copper
nation spheres in 6, as shown in Fig. 7a. The network assem- sites, each with coordinated water molecules; Cu1 also coordi-
bles into a (4,4)-sheet in the crystallographic ab-plane. The nates a sulfate anion and the asymmetric unit is completed by
−
−
coordination angles between HL ligands subtended at the two HL ligands that bridge the three copper(II) ions, resulting
metal centres create rhomboidal solvent channels in the in a formula for the network of [Cu (HL) (SO )(OH 7.
3
4
4
2 3 ∞
) ]
sheets (Fig. 7b). Sheets stack in the c-direction through hydro- Intriguingly, there are two types of metal node in 7 that display
gen bonds from the hydroxyl groups of axial methanol ligands conserved binding domains of the ligand. Cu1 exhibits five
coordinate geometry that is distorted from square pyramidal,
if one considers Cu1 to coordinate to two pyrazole N2 atoms,
an aqua ligand, and an oxygen atom of the sulfate anion, in
the basal plane. The apical site is occupied by another sulfate
oxygen atom related by crystallographic symmetry. The sulfate
2
anions engage in a μ -O mode bridging crystallographically
equivalent copper ions about a centre of inversion into a
dicopper unit. This unit is supported by four hydrogen-
bonding interactions between pyrazolyl donors and non-coor-
dinating sulfate oxygen atom O4 and O6 acceptors (N1H⋯O–
S), two of which are shown in Fig. 8a. Copper ions Cu2 and
Cu3 each adopt square pyramidal coordination geometry
within different dicopper paddlewheel clusters, through
−
coordination to carboxylate oxygen atoms of HL and axial
aqua ligands. Each paddlewheel unit is comprised of only one
crystallographically unique copper atom and carboxylate
group. This conserved coordination arrangement is an
example of the potential afforded by heteroditopic ligands to
assemble in this way and create complex structures. The two
−
molecules of HL within the asymmetric unit are geometrically
related by non-crystallographic mirror symmetry through the
Cu1–O3–Cu1(i) plane, however, the lattice symmetry prevents
crystallographic equivalence.
Extension of 7 through the two types of dicopper clusters
results in the formation of a (4,4) two-dimensional network, in
which the Cu paddlewheel and Cu (OSO ) clusters both act
2
2
3 2
−
Fig. 7 (a) A view of the a metal node in the structure of 6 with selected as four-connected nodes, bridged through linear HL links
hydrogen atoms shown to highlight the hydrogen bonding around the
(
1
Fig. 8b). The stacking of the sheets gives rise to two types of
D channels running parallel to the crystallographic c-axis, as
shown in Fig. 8c. The channels make up 42% of the unit cell
volume but the low quality of the diffraction data, and the
diffuse nature of the residual Fourier difference peaks within
coordination sphere; (b) a view down the 1D channels along the c-axis
in the structure of 6 with hydrogen atoms and guest solvent molecules
removed for clarity. Symmetry codes used to generate equivalent
atoms: (i) 1 − x, +y, 1/2 − z; (ii) 1/2 + x, 1/2 + y, +z; (iii) 3/2 − x, 1/2 + y, 1/
2
− z.
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angles are 88.07(9)° and 91.93(9)° in the square-planar geome-
try (Fig. 9a). The hydrogen-bonding interactions that are
present around the metal coordination spheres in other net-
works consisting of a single metal node are not present here.
Instead, there are hydrogen bonds made from pyrazolyl NH1
to the disordered site of the methanol guests (NH⋯O), and the
well-ordered methanol guest makes a hydrogen bond to the
uncoordinated oxygen of the ligand carboxyl group (O–
H⋯OvC, 2.086 Å).
The structure expands through the 4-connected square
planar nodes and linear linking ligands into the lvt network.
The network has large windows when viewed along the a or
b-axis of 25 × 35 Å (Fig. 9b). This network is interpenetrated by
two other networks and therefore 8 is a triply interpenetrated
coordination material (Fig. 9c). Small pores exist in the
material and contain the water and methanol guests.
Crystals of 8 visually appeared to retain single crystallinity
after removal from methanol solution and drying and this
encouraged us to collect single crystal diffraction data on dried
crystals (8-dry). The diffraction experiments revealed high
mosaicity perhaps caused by cracking of the crystals during
desolvation and a low resolution structure model could be
obtained in the monoclinic space group I2/a with the help of
synchrotron radiation. The structure of the asymmetric unit of
8
-dry retains the same connectivity to that seen in 8; that is,
Cu(II) ions coordinating to two pyrazole and two carboxylate
groups from anionic HL− ligands around a square planar
arrangement. This arrangement means the topology of the lvt
network observed in the fully solvated network of 8 is retained.
The structure was modelled with two Cu(II) ions disordered
−
over two positions each, and two unique molecules of HL in
the asymmetric unit, each of which have extensive disorder
around both coordinating sites and torsional disorder between
Fig. 8 (a) A view showing the two types of node in the structure of 7.
Selected hydrogen atoms are shown to highlight the hydrogen bonding
around the dicopper-pyrazolyl node; (b) the extension of the structure the pyrazole and phenyl rings. Connelly surface plots (Fig. 10)
into a 2D sheet with the hydrogen atoms removed for clarity; (c) a view highlight the impact of losing solvent. In 8, channels run
of the solvent channels that exist along the c-axis in the structure of 7,
hydrogen atoms are removed for clarity. Symmetry codes used to gene-
a-direction whereas in 8-dry the connected channel system is
rate equivalent atoms: (i) 1/2 − x, 1/2 − y, −z.
along the b-axis and are connected through portals in the
lost and only small pocket-like voids remain. We sought to
obtain supporting information regarding the process of
desolvation through a range of other techniques.
these channels, meant the contents of the channels were not
Powder X-ray diffraction (PXRD) was used to follow the
able to be modelled and the SQUEEZE routine within PLATON changes when 8 was removed from solvent and allowed to air
was applied to the data in order to provide more meaningful dry (Fig. 11 and S10 ESI†). The PXRD traces show changes to
refinement statistics.
the peak positions indicating a structural transformation
The purple crystals 8 were of good quality and were ana- begins when the crystals are removed from methanol solution.
lysed by single crystal X-ray diffraction. The material crystal- Over the first 9 hours the low angle reflections at 7.5° and
lises in the tetragonal space group I4 /a with the asymmetric 10.5° move to slightly higher 2θ angles and then lose intensity.
1
unit containing a half-occupancy copper atom residing on an A new peak at 8.0° and peaks around 13.8° grow into the pat-
−
inversion centre that is coordinated to a HL ligand. The asym- terns but fall off in intensity around the 24 h mark. A phase
metric unit is completed by a well-ordered methanol guest with reasonable crystallinity is seen between 15–21 h and
molecule, and a second methanol guest molecule disordered these peaks die away by the 24 h mark. The sample continues
in a 57 : 43 ratio, as well as a half occupancy guest water mole- to change after 24 h with new broad peaks growing in to the
cule. This gives the coordination network 8 a formula of {[Cu- pattern around 7°, between 12–13°, 15° and 20° in 2θ. These
(
HL) ]·4CH OH·H O} . By virtue of the inversion centre, the peaks are dominant in the patterns from 39 h onward. From
2 3 2 ∞
angles between equivalent coordinating atoms about the these observations, it is clear that the desolvation process is
copper coordination sphere are 180° and the unique O–Cu1–N complicated and we suspect 8 moves through at least one and
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Fig. 10 Perspective views of Connelly surfaces of 2 × 2 × 2 unit cells
with the b-axis orientated vertically of (a) 8 and (b) 8-dry. Solvate mole-
cules are removed to highlight the channels that exist in 8 and small
void areas in 8-dry.
Fig. 9 (a) A view of the metal node in the structure of 8; (b) a view of
the large channels that form in a single network of 8 with hydrogen
atoms and guest solvent molecules removed for clarity; (c) a perspective
of the network catenation present in 8. Symmetry codes used to gene-
rate equivalent atoms: 1 − x, 1 − y, 1 − z.
perhaps more intermediate structures before the structural
transformation is complete and this process takes around
three days at room temperature.
Fig. 11 PXRD patterns taken at time points over 72 h following the
removal of 8 from solvent.
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IR spectroscopy (Fig. S22, ESI†) indicates the presence of
Having identified in 1 that aqua ligands position carboxy-
water in air-dried samples and microanalysis confirms that late groups proximal to the metal centres by hydrogen bonds,
.5 molecules of water per formula unit are present in the crys- we wondered if dehydration of the metal centres would induce
1
tals after air drying. The impact of losing solvent from the the close-by carboxylates to coordinate and thereby form a
pores on the crystallinity was also studied by heating samples coordination network. Such a conversion has been achieved by
of 8 in a thermogravimetric analyser (a description of the full the thermal treatment of a cadmium complex with a structu-
3
5
thermogravimetrogram is discussed later) with analysis by rally related 1,2,4-triazolyl benzoate ligand. Accordingly, 1
PXRD afterward. We observed that completely removing the was heated until the DMF guests and aqua ligands were
methanol and water by heating gave PXRD traces that were removed and a purple powder was recovered (1-desolv) and was
reasonable matches compared to the pattern simulated from analysed by PXRD. Although the PXRD pattern (Fig. S3, ESI†)
the single crystal analysis of 8-dry (Fig. S9, ESI†). We also showed very little crystallinity in the material, it is notable that
found that the structure did not take up any water once desol- when 1-desolv was exposed to a solution of DMF and water that
vated, indicating this materials response to complete guest 1 was not regenerated and it seems likely that 1-desolv is an
loss is a considerable loss of crystallinity and closure of its amorphous coordination polymer with the formula [Co-
pores.
2 ∞
(HL) ] .
TG-DTA study on 4. Upon isolation, the crystals of 4 quickly
lose solvent and turn opaque. This thwarted our attempts to
record data on fully solvated samples. The TG-DTA recorded
Thermal studies of 1–2, 4–6 and 8
Compounds 1–2, 4–6 and 8 were studied by simultaneous
thermogravimetric-differential thermal analysis (TG-DTA). The
power of TG-DTA lies in its ability to identify energy changes
that occur in the sample with no loss of mass, which TG analy-
(
Fig. S13, ESI†) shows a 5% mass loss of remaining solvent
coming from the structure under 50 °C, which is likely to be
some remaining methanol (calc. loss for one methanol 5.5%)
and this is accompanied by a small endothermic signal in the
DTA. The material continues to lose a further 5% mass out to
nearly 400 °C whereupon it undergoes rapid mass loss in
exothermic decomposition steps. The decomposition is com-
plete just after 500 °C with 11.5% mass remaining, presumably
as ZnO.
3
1,32
sis alone cannot identify.
For example, we have used
TG-DTA to identify molecular rearrangements in porous MOFs
that are not accompanied by mass loss or a change of
3
3,34
phase.
TG-DTA study on 1 and 2. The TG-DT analysis of {[Co-
HL) (OH ]·2DMF} 1 is shown in Fig. 12 (Fig. S11, ESI†). The
(
2
2 4
)
TG-DTA study on 5. Consistent with the dense quadruply
interpenetrated structure of [Co(HL)2] 5, the TG curve
mass loss between 100 and 200 °C is 30.8% and is consistent
with the loss from the crystals of the DMF guests and all water
molecules bound to the cobalt centres (calc. 30.8%). The loss
is endothermic in nature with the peak maximum for this
process occurring at 133 °C. The desolvated structure is stable
until 250 °C whereupon the sample begins to lose mass gradu-
ally and at 375 °C major breakdown of the material occurs in
an exothermic process. The breakdown is complete at 520 °C
with only 10.0% of the mass remaining, most likely as CoO
(
Fig. S14, ESI†) shows that only 1.8% of mass is lost from the
sample up to 400 °C and there are no thermal events detected
by DT analysis until the onset of decomposition. The
decomposition is strongly exothermic and complete by 525 °C
and the mass remaining above this temperature is likely to be
CoO (obs. 14.9%; calc. 15.3%).
TG-DTA study on 6. The extremely rapid loss of methanol
from the pores of [Cd(HL)
capture of this information in our TG-DTA studies. The de-
solvated form, [Cd(HL) (MeOH) ] (6-dry), was studied and
2 2
(MeOH) ]·1.8MeOH 6 prevented the
(Calc. 10.5%). The TG analysis of {[Ni(HL) (OH ) ]·2DMF} 2 is
2 2 4
very similar with a 29.9% mass loss between 100 and 200 °C
2
2
(
(
calc. 30.8%) and major breakdown starting at 370 °C
Fig. S12, ESI†).
TG-DT analysis for 6-dry is shown in Fig. 13 (Fig. S15, ESI†).
Fig. 12 Thermogravimetric (black) and differential response (blue)
curves for 1.
Fig. 13 Thermogravimetric (black) and differential response (blue)
curves for 6-dry.
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The bound methanol ligands are liberated from the metal
centres in the network by 120 °C (obs. mass loss 10.4%; calc.
Conclusions
1
0.5%) and the DT curve shows this to be an endothermic A variety of structure types were observed in this work, includ-
process centred at 112 °C. There is little further loss in mass ing discrete coordination complexes arranged by multiple
until 272 °C at which point the DT curve shows a clear hydrogen bonds into 3D superstructures, 2D sheet structures
exotherm. This seems to trigger the decomposition of 6-dry assembled in the third dimension by inter-sheet hydrogen
with a further 7.5% mass loss observed up to 400 °C and then bonds and a number of highly interpenetrated 3D coordi-
large losses totalling 53% mass up to 550 °C in two exothermic nation networks. The interpenetrated structures 3–5 and 8
decomposition steps.
contain single metal ion nodes and it is likely that these small
TG-DTA study on 8. The TG-DT analysis for 8 supports the nodes contribute to the highly interpenetrated structures
formulation of {[Cu(HL) ]·4MeOH·H O} derived from the observed. The zinc coordination networks 3 and 4 are iso-
2
2
single crystal X-ray structure. The TG curve shows a mass loss structural and form the dia structure type under very
of 19.2% up to 100 °C and a further 2.8% up to 150 °C, fully different synthetic conditions, indicating the strong preference
consistent with the loss of four methanol molecules (calc. for this structure over others. This result adds to the work
2
0.0%) and a water molecule (calc. 2.8%) (Fig. 14 and S16, reported by Zhang on different inclusion isomers of this
1
8
ESI†). Matching peaks are found in the DT curve with peak network.
maxima centred at 77 °C and 135 °C, respectively. The temp-
A significant feature of the metal ion nodes in all the
erature of 135 °C for the removal of water from the pore struc- structures is the interplay of hydrogen and coordinate bond-
ture indicates the strong affinity that water has for the ing around the coordination spheres, specifically NH⋯O
framework and this is a reflection of the position that water hydrogen bonding between coordinated pyrazole and carboxy-
occupies in the structure. The sample mass plateaus until an late. The ligand bridges metal centres in the coordination
exotherm is observed in the DT curve at 222 °C. The sample networks in the anionic HL− form, wherein the carboxylate
begins to lose mass after this point with a further 5% lost up carries the negative charge and the pyrazole ring retains its
to 320 °C and above this temperature there is rapid onset of hydrogen atom and is neutral. The use of the weakly basic
exothermic decomposition until only CuO remains by 445 °C nitrate and sulfate counter ions in the syntheses and, in
(
obs. 10.4%; calc. 10.3%).
Upon standing 8 in air for a day the methanol from the of bonding. The anionic nature of the ligand gives rise to
pore structure is nearly all lost. The TG analysis curve shows a neutral coordination networks and coordination complexes 1
.7% mass loss up to 100 °C, corresponding to half a remain- and 2.
2 2
ing methanol per Cu(HL) formula unit (calc. 2.8%), and a The most prevalent donor set observed is N O on
general, mild reaction conditions seem to favour this mode
2
2
further 4.3% mass loss to 150 °C that accounts for 1.5 water single metal ion nodes in tetrahedral or square planar con-
molecules (calc. 4.3%). The change in the populations of figurations. This node type is often seen in coordination net-
framework guests is reflected in the corresponding DT peaks. works derived from co-ligand systems such as bridging
3
6,37
The peak centred at 75 °C is very broad and low in intensity dicarboxylates and diazoles.
The exception to this is seen
while the DT peak at 135 °C has become more intense. Micro- in coordination network 7 where conserved binding domains
analysis on air dried material confirms that there remains of the ligand are observed in different dicopper aggregates.
1
.5 guest water molecules per formula unit, [Cu(HL)
2 2
]·1.5H O, This structure is meta-stable and intercepted en route to a
consistent with the electron density observed in the crystal thermodynamically stable phase containing single metal ion
structure of the desolvated material.
2 2
nodes with N O donor sets. This highlights, perhaps, an
intrinsic difficulty in using heteroditopic ligands to engineer
coordination networks featuring nodes with conserved
binding domains.
Acknowledgements
CR gratefully acknowledges support from the University of
Wollongong for this research. CSH and PEK gratefully acknowl-
edge the University of Canterbury (College of Science scholar-
ship to CSH) and the Royal Society of New Zealand Marsden
Fund for financial support. Parts of this work were carried out
on the MX1 Macromolecular Crystallography beam line at the
Australian Synchrotron, Victoria, Australia. ADB gratefully
acknowledges the EPSRC and the University of Bath for finan-
cial support.
Fig. 14 Thermogravimetric (black) and differential response (blue)
curves for as synthesised 8; Thermogravimetric (orange) and differential
response (red) curves for air-dried 8.
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