An indirect generation of 1D MII-2,5-dihydroxybenzoquinone coordination polymers, their structural rearrangements and generation of materials with a high affinity for H2, CO2 and CH4

Article abstract of DOI:10.1039/c5dt04095g

A series of solid-state structural transformations are found to accompany desolvation of relatively simple coordination polymers to yield materials that exhibit unexpected gas sorbing properties. Reaction of 1,2,4,5-tetrahydroxybenzene with M^II salts (M = Mg, or Zn) in an alcohol/water solution in the presence of air affords cis-M^II(C₆H₂O₄^-II)(H₂O)₂·2H₂O·xROH, (M = Mg, or Zn), crankshaft-like chains in which the absolute configurations of the chiral metal centres follow the pattern ?Δ Δ Λ Λ Δ Δ Λ Λ?, and are hydrogen bonded together to generate spacious channels. When crystals of the crankshaft chain are air dried the crystals undergo a single crystal-to-powder rearrangement to form linear trans-M^II(C₆H₂O₄^-II)(H₂O)₂ chains. Further dehydration yields microporous solids that reversibly sorb H₂, CH₄ and CO₂ with high sorption enthalpies.

Full text of DOI:10.1039/c5dt04095g

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An indirect generation of 1D MII-2,5-
dihydroxybenzoquinone coordination polymers,
their structural rearrangements and generation of
materials with a high affinity for H2, CO2 and CH4†
Brendan F. Abrahams,*a A. David Dharma,a Brendan Dyett,a Timothy A. Hudson,a
Helen Maynard-Casely,b Christopher J. Kingsbury,a Laura J. McCormick,a
Richard Robson,*a Ashley L. Suttona and Keith F. Whitea
Cite this: DOI: 10.1039/c5dt04095g
Received 19th October 2015,
Accepted 22nd December 2015
DOI: 10.1039/c5dt04095g
www.rsc.org/dalton
A series of solid-state structural transformations are found to capture of environmentally harmful gases.11–14 Research in
accompany desolvation of relatively simple coordination polymers this area has demonstrated that coordination polymers posses-
to yield materials that exhibit unexpected gas sorbing properties. sing nano-scale cavities are capable of sorbing considerable
Reaction of 1,2,4,5-tetrahydroxybenzene with MII salts (M = Mg, or quantities of gas, especially under cryogenic conditions and
Zn) in an alcohol/water solution in the presence of air affords cis- high pressures.15–17 These systems commonly rely upon van
MII(C6H2O4−II)(H2O)2·2H2O·xROH, (M = Mg, or Zn), crankshaft-like der Waals-type interactions between the gas molecules (H2,
chains in which the absolute configurations of the chiral metal CO2, CH4 etc.) and the internal pore surfaces of the framework
centres follow the pattern ⋯Δ Δ Λ Λ Δ Δ Λ Λ⋯, and are hydrogen materials. As a result of the relatively weak interactions
bonded together to generate spacious channels. When crystals of between host and guest, the gas uptake of these materials
the crankshaft chain are air dried the crystals undergo a single under conditions involving more moderate pressures and
crystal-to-powder rearrangement to form linear trans- ambient temperatures can be significantly lower. Nevertheless,
MII(C6H2O4−II)(H2O)2 chains. Further dehydration yields micro- work in this area is progressing towards the development of
porous solids that reversibly sorb H2, CH4 and CO2 with high materials with pore shapes18–23 and surfaces24–28 that enhance
sorption enthalpies.
framework – gas molecule interactions allowing for optimal
uptake, storage and release of gas under practical conditions.
Upon deprotonation, dihydroxybenzoquinone, H2dhbq, is
able to act as a bridge between a variety of metal centres
within discrete and polymeric systems. In regards to the for-
mation of coordination polymers our interest in this readily
available ligand stems from the fact that as a charged ligand
capable of chelation, it is able to bind strongly to metal
centres and thus has the potential to provide robust coordi-
nation polymers. Furthermore, the redox activity of the
dianion, (dhbq2− is capable of undergoing reduction to both a
radical trianion and a tetranion) offers the prospect of forming
redox-active porous coordination polymers that may possess
interesting and perhaps useful magnetic and electronic pro-
perties.29,30 Intermittently since the early 1990s we have been
exploring procedures for obtaining transition metal derivatives
of H2dhbq in the form of single crystals suitable for X-ray dif-
fraction study, but until recently our results had been uniformly
disappointing. Given the great potential interest of dhbq-
based coordination polymers, we consider it very likely that
other groups have had similar discouraging experiences that
have gone unreported. We suspect this is the reason why struc-
tural data on very simple dhbq-based derivatives such as trans-
MII(dhbq)(H2O)2·yH2O (M = Mg, Mn, Fe, Co, Ni and Zn) (Fig. 1)
The field of coordination polymers has undergone immense
growth in the last 25 years and is currently an area of intense
worldwide interest.1–3 Early work provided an indication of the
scope of the type materials that may be produced and focussed
on development of synthetic approaches, the characterization
of the materials and the use of topological representations to
describe the connectivity of networks.4,5 Building on these
foundations, later work was directed towards applications
spanning diverse areas such as electronics, catalysis, lumine-
scence, drug delivery and separation.6–10 One of the most
intensely investigated areas includes the generation of porous
coordination polymers for storage of gaseous fuels or the
aSchool of Chemistry, University of Melbourne, Parkville, Victoria 3010, Australia.
E-mail: bfa@unimelb.edu.au, r.robson@unimelb.edu.au; Fax: +61 3 9347 5180;
Tel: +61 3 8344 0341
bBragg Institute, Australian Nuclear Science and Technology Organisation, Locked
Bag 2001, Kirrawee DC, New South Wales 2234, Australia
†Electronic supplementary information (ESI) available: Synthetic procedures,
general crystallographic details, powder XRD patterns and data, TGA traces, gas
sorption isotherms, sorption enthalpy data, pore size distribution data and
infra-red spectra. CCDC 1431943–1431946. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c5dt04095g
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Fig. 1 The planar strip-like trans-M(dhbq)(H2O)2 chain reported by
H. Kitagawa et al.3
were first reported as recently as 200931,32 and even then,
mainly on the basis of powder data. In our recent work with
H2dhbq and its derivatives however, we have discovered that
the indirect generation of the H2dhbq through the in situ
aerial oxidation of the starting material 1,2,4,5-tetrahydroxy-
benzene (H4thb) has been a productive avenue for the for-
mation of nicely crystalline metal-dhbq2− polymers, which
could not be obtained when employing H2dhbq as the starting
material.33,34
In this work we present the synthesis, structures‡ and gas
sorption properties of two new polymers of dhbq2−, in which
crystals suitable for X-ray analysis were not obtained directly
from H2dhbq itself but by in situ oxidation of the H4thb pre-
cursor when combined with M2+ cations in an alcoholic reac-
tion mixture. The products, of composition cis-MII(dhbq)
(H2O)2·2H2O·xROH (M = Mg, or Zn) (R = Me for the Mg deriva-
tive or Et for Zn), have closely related compositions to the com-
pounds, trans-MII(dhbq)(H2O)2·yH2O, (M = Mg, Mn, Co, Ni,
Zn), described by H. Kitagawa et al.,31,32 but have a structure
significantly different to the strip-like trans-MII(dhbq)(H2O)2
polymer which is depicted in Fig. 1. The structures of cis-M
(dhbq)(H2O)2·2H2O·xROH (M = Mg, or Zn) contain almost
identical crankshaft-like MII(dhbq)(H2O)2 1D polymeric
chains, an example of which (the Mg derivative) is shown in
Fig. 2a. Two types of dhbq2− ligand are present. One type, type
A in Fig. 2a, bridges two metal centres, the line between which
is oriented roughly parallel to the general direction in which
the infinite chain extends, whilst the type B ligands are very
Fig. 2 (a) One cis-Mg(dhbq)(H2O)2 crankshaft-like coordination
polymer. Ligands of types A and B are indicated. (b) The hydrogen
bonding pattern linking cis-Mg(dhbq)(H2O)2 crankshaft-like chains
together. O–H⋯O bonds are represented by black and white banded
connections. Four type A ligands are shown face-on. For clarity only a
fraction of each of the type B ligands (the O–C–C–O system) is shown.
(c) The extended 3D hydrogen bonded network containing spacious
channels.
significantly inclined to that direction. The metal centres are channels (see Fig. 2c) occupied by non-coordinated water and
essentially octahedral, being attached to one chelated ligand methanol molecules (in the case of the magnesium derivative)
A, one chelated ligand B and two cis-aqua ligands. The metal or water/ethanol (present in the zinc derivative) is formed.
centres are thus individually chiral, and their absolute con- Two well-defined, non-coordinated water molecules form
figurations alternate in the fashion ⋯Λ Λ Δ Δ Λ Λ Δ Δ⋯ hydrogen bonds to dhbq2− oxygen centres, whereas the
along the length of the chain. Crankshaft chains are linked alcohol molecules are disordered over a number of sites.
together by hydrogen bonds between the coordinated aqua
Elemental analysis of air-dried crystals of the cis-Mg(dhbq)
ligands and dhbq2− oxygen centres, as shown in Fig. 2b. Every (H2O)2·2H2O·2.5MeOH crankshaft chains (ESI, S.1†) indicates
chain is linked in this way along its entire length to four that upon removal of the sample from its mother liquor,
others, whereby a 3D hydrogen bonded network that contains practically all the channel solvent molecules are lost. Powder
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X-ray diffraction measurements performed on a sample of air-
dried cis-Mg(dhbq)(H2O)2·2H2O·2.5MeOH reveals the powder
pattern no longer resembles the pattern expected for the
crankshaft chains, but instead matches the diffraction pattern
of the trans-Mg(dhbq)(H2O)2 strip-like chains observed by
H. Kitagawa et al. and shown above in Fig. 1. Following this
insight, an experiment was conducted in which powder X-ray
data were initially collected on a bulk slurry of cis-Mg(dhbq)
(H2O)2·2H2O·2.5MeOH, the solid was then isolated from its
mother liquor and air dried for further powder X-ray data
measurements. The resulting X-ray powder patterns (shown in
ESI S.2†) provide resounding evidence that, quite remarkably,
by simply exposing the cis-Mg(dhbq)(H2O)2·2H2O·2.5MeOH
crankshaft-like chains to air, the non-coordinated solvent
molecules are lost and the chains undergo a single crystal-to-
powder structural rearrangement to the planar strip-like
chains (ESI Fig. S4†). Similar experiments conducted on cis-Zn
(dhbq)(H2O)2·2H2O·1.5EtOH indicate the same structural
rearrangement occurs for the Zn derivative. A further experi-
ment in which a sample of the trans-Mg(dhbq)(H2O)2 strips
were immersed in a methanolic solution and subsequently
analysed with powder X-ray diffraction indicated that the trans-
formation from the cis-Mg(dhbq)(H2O)2 crankshaft chains to
trans-Mg(dhbq)(H2O)2 strips is not reversible (ESI Fig. S5†).
Given the propensity of the cis-MII(dhbq)(H2O)2·2H2O·xROH
crankshaft chains to undergo a structural rearrangement to
the strip-like trans-M(dhbq)(H2O)2 chains upon the loss of
alcohol, we also performed synthetic investigations in which
we set out to deliberately prepare the trans-M(dhbq)(H2O)2
strips (M = Mg2+ or Zn2+), in a nicely crystalline form, using
H4thb as a starting material in combination with the appropri-
ate MII salt in an aqueous reaction mixture that is free of
methanol or ethanol. The motivation behind such investi-
gations were to (a) determine if the crankshaft chain formation
was dependent on the presence of alcohol and (b) investigate
whether following the synthetic approach of the in situ oxi-
dation of H4thb would permit the formation of trans-MII(dhbq)
(H2O)2 as high quality single crystals; H. Kitagawa et al.
reported the single crystal structure of only the manganese
derivative, the structures of the remaining compounds were
determined using powder X-ray diffraction. The combination
of H4thb with Mg2+ and Zn2+ in water, did indeed lead to
nicely crystalline samples of trans-M(dhbq)(H2O)2.‡ These
results suggest that the short-chain alcohols (MeOH and
EtOH) play a templating role in the synthesis of cis-MII(dhbq)
(H2O)2·2H2O·xROH crankshaft-type chains. A summary of the
synthetic routes to the formation of both cis-, and trans-
MII(dhbq)(H2O)2 and the structural rearrangements they
undergo upon various levels of desolvation is provided in
Scheme 1.
Scheme 1 A summary of the synthetic routes for both cis-, and trans-
MII(dhbq)(H2O)2 and the structural rearrangements they undergo upon
various levels of desolvation.
powder diffraction indicates that the solvent-free Mg(dhbq)
compound is microcrystalline (ESI, Fig. S6†) although the dif-
fraction peaks are rather broad making unambiguous determi-
nation of cell dimensions difficult. Nevertheless it is possible
to make a tentative unit cell assignment as primitive tetragonal
with cell dimensions a = 9.284, c = 12.680 Å.
The completely desolvated material shows reversible sorp-
tion of H2, CH4 and CO2. The gas uptake results pertaining to
the Mg(dhbq) sample are presented here whilst the sorption
data for Zn(dhbq), which is similar to that found for
Mg(dhbq), is provided in the ESI.† The hydrogen uptake on a
sample of Mg(dhbq) was measured at 77 K. The isotherm, pre-
sented in Fig. 3a, shows the sorption path rises steeply with an
uptake of 80 cm3(STP) g−1 at ∼1 kPa. The uptake of the gas
continues to increase to approximately 100 kPa, when the
uptake is of the order of one H2 molecule per metal centre.
Whilst the total H2 uptake reported here falls below that of a
number of coordination polymers that possess very large pore
volumes,15–17 the high H2 uptake at very low pressures is sug-
gestive of the gas binding favourably to the internal surfaces of
the desolvated compound. A second H2 isotherm collected at
87 K allowed a calculation of the isosteric enthalpies of sorp-
tion (see ESI S.5†). At low H2 loading, the isosteric enthalpy of
sorption is 9.5 kJ mol−1. As further H2 is introduced to the
solid, the value declines to 7 kJ mol−1 and beyond this point
the sorbent material is saturated. A review of the literature
reveals that porous metal–ligand systems typically have hydro-
gen isosteric enthalpy of sorption values in the range 4 to
7 kJ mol−1 11,35–37
In cases where metal–ligand systems have
.
Thermogravimetric analysis (ESI, S.3†) indicates the two
molecules of water bound to the metal centres of trans-
Mg(dhbq)(H2O)2 are lost in the range 125–180 °C. Perhaps not
surprisingly, the removal of the bound water molecules results
in a loss of single crystal character thus preventing structural
characterization of the completely desolvated material. X-ray
sorption enthalpies comparable to those reported here, the
favourable interactions have been attributed to either the pres-
ence of unsaturated metal centres throughout the sorbent
material,38–41 or materials with pores of optimal shape and
size that allow for the maximum interaction of a gas molecule
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to 2D or even 3D. In the compound Mg-MOF74 (also known as
CPO-27-Mg)45,46 Mg2+ centres are linked by the ligand, 2,5-
dioxido-1,4-benzene dicarboxylate (dobdc4−
) which shares
some structural similarities with dhbq2−. Single O donors,
bound directly to the aromatic ring of dobdc4−, provide links
between pairs of Mg2+ centres, which leads to the generation
of a porous 3D network that exhibits excellent gas sorption
properties. If single oxygen atoms in Mg(dhbq) or Zn(dhbq)
are able to bind pairs of metal centres in a similar manner
then it is conceivable that a porous network would be
generated.
In the case of Mg-MOF74 the Mg2+ centres are coordinately
unsaturated, a feature that is responsible for the reported
relatively high magnitude for the CO2 isosteric enthalpy of
sorption (39 to 47 kJ mol−1),13,47–49 values close to that
obtained for Mg(dhbq). Similarly, in the case of H2, our value
of 9.5 kJ mol−1 for Mg(dhbq) is comparable to the value of
10.3 kJ mol−1 found for Mg-MOF74 which was based upon
isotherm data (a value of 12 kJ mol−1 was found for Mg-
MOF74, determined from an IR spectroscopic analysis).41 The
similarity of these results raises the prospect of the Mg2+
centres in Mg(dhbq) being coordinately unsaturated and that
the high binding enthalpies reflects a direct interaction
between the metal centres and the guest molecules. On the
other hand, as indicated above, small pore size can lead to
elevated sorption enthalpies because the guest molecules can
make multiple contacts with the internal pore surface. As a
consequence it is possible that the relatively high sorption
enthalpies reflect the presence of small pores within the host
network.
Fig. 3 (a) A H2 sorption isotherm measured on a sample of Mg(dhbq) at
77 K. Inset: H2 uptake at pressures up to 1 kPa. (b) CO2 (green diamonds)
and CH4 (red squares) sorption isotherms measured on Mg(dhbq) at
258 K.
In summary, this work provides further examples of an
indirect approach to the generation of M2+–dhbq2− coordi-
nation polymers, based on the in situ aerial oxidation of
1,2,4,5-tetrahydroxybenzene, as a means to obtain crystalline
products suitable for X-ray diffraction studies. We recently
reported a family of compounds, in a form suitable for
single crystal structure determination, of composition
(NBu4)2[MI2I(dhbq−II)3] (M = Mn, Fe, Co, Ni, Zn and Cd) in
with the internal surfaces of the framework.20,42,43 Carbon
dioxide and methane uptake measurements also indicate a
high affinity for carbon dioxide and methane at low pressures
(Fig. 3b). The isosteric enthalpy of sorption values for CO2 and
CH4, of 37 kJ mol−1 and 26 kJ mol−1 respectively, are higher
than those commonly encountered with porous coordination
networks; the methane isosteric sorption enthalpy at low
loading in particular is amongst the highest recorded.13,14,18,44
The interesting sorption properties of the desolvated
Mg(dhbq) and Zn(dhbq) compounds provides an opportunity
to speculate on the structural nature of the material that is
responsible for the gas sorption. In the case of the 1D poly-
mers, Mg(dhbq)(H2O)2 and Zn(dhbq)(H2O)2 (either as the cis-
crankshaft or trans strip structures) the octahedral metal
centres are bound by a pair of dhbq2− dianions that each
adopt the commonly observed bridging-chelating coordination
mode. Upon loss of the coordinated water molecules the
coordination number of the metal centre would drop from six
to four unless each dhbq2− has a coordination mode in which
it links to more than two metal centres. Such a change may
lead to an increase in dimensionality of the structure from 1D
which the [MI2I(dhbq−II 2−] coordination framework possessed
)
3
the (10,3)-a topology;33 the dhbq2− component in these cases
was generated, again not directly from H2dhbq itself, but by
in situ hydrolysis of 2,5-diaminobenzoquinone. These results,
together with those reported above, afford encouragement that
indirect routes involving the in situ generation of dhbq2− may
provide access to new classes of potentially interesting dhbq2−
-
based coordination polymers in a form suitable for single
crystal X-ray structure determination, thereby surmounting a
major and long-standing impediment to progress in this area.
Moreover, the products above afford, upon dehydration, micro-
porous materials with surfaces that are suited for the uptake of
significant quantities of H2, CO2 and CH4 at low pressures.
The sorption enthalpies reported here are consistent with
materials that possess either surfaces lined with coordinately
unsaturated metal sites or have pore structures that are of a
shape and size that allow for gas molecules to be in simul-
taneous contact with multiple surfaces.
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The authors gratefully acknowledge the support of the 16 H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi,
Australian Research Council. This research is supported by the
Science and Industry Endowment Fund. Part of this research
was undertaken on the Powder Diffraction beamline at the
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Notes and references
‡Crystal data for cis-Mg(dhbq)(H2O)2·2H2O·2.5MeOH: [C8.5H20MgO10.5] mono-
clinic, C2/c, a = 21.0769(13) Å, b = 15.6680(8) Å, c = 9.2192(4) Å, β = 103.261(12)°,
V = 2963.3(3) Å3, Z = 8, λ = 1.54184 Å, T = 130 K, reflections collected 4769, 2818
unique (Rint = 0.0428). R1 0.0571 (I > 2σ(I)), wR2 0.1562 (all data), GOF = 0.996.
Crystal data for cis-Zn(dhbq)(H2O)2·2H2O·1.5EtOH: [C9H19O9.5Zn], monoclinic,
C2/c, a = 21.4589(10) Å, b = 15.5785(6) Å, c = 9.2643(3) Å, β = 102.941(4)°, V =
3018.4(2) Å3, Z = 8, λ = 1.54184 Å, T = 130 K, reflections collected 4348, 2303
unique (Rint = 0.0329). R1 0.0427 (I > 2σ(I)), wR2 0.1180 (all data), GOF = 0.922.
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Crystal data for trans-Mg(dhbq)(H2O)2: [C6H6MgO6], monoclinic, C2/m,
6.8276(7) Å, b = 7.6461(7) Å, c = 7.8635(7) Å, β = 112.777(12)°, V = 378.50(7) Å3,
Z = 2, λ = 1.54184 Å, T = 130 K, reflections collected 745, 379 unique (Rint
a =
=
0.0303). R1 0.0424 (I > 2σ(I)), wR2 0.1169 (all data), GOF = 1.090. Crystal data for
trans-Zn(dhbq)(H2O)2: [C6H6O6Zn], monoclinic, C2/m, a = 6.715(2) Å, b = 7.7544
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