A heterofunctional ligand approach for the preparation of high connectivity coordination polymers: Combining a "bridge" and "pillar" in one ligand

Article abstract of DOI:10.1039/d0ce01020k

Two of the most successful strategies for the preparation of three-dimensional coordination polymers and MOFs are reticular synthesis and pillaring. Here we present a new approach which combines aspects of both of these by employing a heterofunctional dicarboxylic and dipyridyl ligand, 2,5-di(pyridin-4-yl)terephthalic acid (H2L). The reaction of H2L with zinc(ii) produces a non-interpenetrated 3D coordination polymer [ZnL(H2O)]n. This journal is

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A heterofunctional ligand approach for the preparation of high
connectivity coordination polymers: combining a “bridge” and
“pillar” in one ligand†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Hydar A. AL-Fayaad,a Kasun S. Athukorala Arachchigea and Jack K. Clegg*a
Two of the most successful strategies to the preparation of three-
dimensional coordination polymers and MOFs are reticular
synthesis and pillaring. Here we present a new approach which
combines aspects of both of these by employing a heterofunctional
dicarboyxlic and dipyridyl ligand, 2,5-di(pyridin-4-yl)terephthalic
acid (H2L). The reaction of H2L with zinc(II) produces a non-
interpenetrated 3D coordination polymer [ZnL(H2O)]n.
Experimental
General
Solvents and reagents were purchased from commercial
sources and used without further purifications unless otherwise
stated. The used dry solvents were dried by passing the solvents
through an Innovative Technology PureSolv solvent purification
system then stored under nitrogen. H and 13C NMR spectra
1
Since Robson invented coordination polymers (CPs) in the
1980s,1 their useful properties such as porosity, robustness,
high surface-areas, as well as complexity have found application
were measured on Bruker Avance 500, 400 and 300 MHz high
resolution spectrometers. The NMR spectra were analysed by
using MestReNova software package where the chemical shifts
are reported in ppm relative to the residual solvent peak. ESI-
MS spectra were collected on three machines, Bruker
MicroTOF-Q, Bruker HCT and Thermo LCQ Fleet mass
spectrometers. Samples were prepared in methanol or
acetonitrile unless otherwise specified. Elemental microanalysis
were measured by the SCMB Microanalytical Service, University
of Queensland. FTIR spectra were measured on PerkinElmer
Spectrum II. UV-Visible spectra were collected on Agilent
Technologies Cary 60 UV-Vis spectrophotometer using the
baselines at both 0% and 100% T.
in industrially relevant areas including catalysis, gas separation
2-8
and drug delivery.
Despite myriad reports into the
preparation of CPs, the successful design of these materials still
remains a significant challenge, particularly if 3D connectivity
and a lack of interpenetration are desired.9 Two of the most
successful strategies for the assembly of such materials are i)
reticular synthesis,10 in which bridging organic components
(often carboxylic acids) with well-defined coordination vectors
are combined with similarly well-defined metal clusters, and ii)
pillaring,11-13 in which 2D layered structures (often composed of
carboxylate ligands and metal “paddle-wheels”) are combined
with multi-functional (often dipyridyl) “pillars”, which produces
3D CPs which are reminiscent of Hofmann clathrates.14 Here we
report a hybrid heterofunctional ligand approach that combines
design features common in both bridging and pillaring
components and demonstrate its use to produce a 3D
coordination polymer. We envisaged that 2,5-di(pyridin-4-
yl)terephthalic acid13 (H2L) would be a suitable symmetric
hetero-functional ligand for this purpose as it has the
directional properties of both the commonly used bdc2- and
4,4’-bpy ligands.
Single Crystal X-Ray Diffraction
Data were collected using either an Oxford Diffraction Gemini II
ULTRA diffractometer operating at graphite-monochromated Mo
Kα radiation (0.71073 Å ) generated from a sealed tube at 190(2)
K., or at the MX1 beamline of the Australian synchrotron with
silicon double crystal monochromated radiation (0.7108 Å) at 100
K.15 Data were processed with CrysAlisPro16 or XDS17 respectively.
Processed data was solved using SHELXS-201418 or SHELXT19 within
the Olex220 graphical interfaces. Solutions were refined via a full-
matrix least-squares refinement against F2 using SHELXL-201418.
The hydrogen atoms were placed on appropriate atoms and refined
using a riding model. Crystallographic data for each structure is
summarised below. CIFs have been deposited with the CCDC under
deposition numbers CCDC: 2008988-2008991
a. School of Chemistry and Molecular Biosciences, The University of Queensland, St
Lucia, QLD, 4072, Australia. Email: j.clegg@uq.edu.au
† Electronic Supplementary Information (ESI) available: Crystal structures in CIF
format. CCDC 2008988-2008991. See DOI: 10.1039/x0xx00000x
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Synthesis of 2,5-dibromoterephthalic acid
which the solution was degassed once more. TheVmiewixAtrtuicrleeOwnlianes
DOI: 10.1039/D0CE01020K
2,5-dibromoterephthalic acid was prepared according to a stirred at 90 °C under nitrogen for 24 hours. Solvent was
reported procedure21. 1,4-Dibromo-2,5-dimethylbenzene (1.32 removed under reduced pressure, and the residue was poured
g, 5 mmol) was dissolved in 10 mL pyridine then potassium into water and extracted with chloroform three times. The
permanganate (4.1 g, 26 mmol) in 10 mL water was added and combined organic layers were washed with brine and water
stirred at reflux for 24 hours. After cooling to room before being dried over MgSO4, which was followed by the
temperature, the reaction mixture was filtered and the filtrate removal of the solvent under reduced pressure. The crude
was washed with 1 M sodium hydroxide. Pyridine was removed product was dissolved in dichloromethane and passed through
from the combined aqueous layers under reduced pressure a silica gel plug after which the combined layers were washed
then the aqueous solution was washed with chloroform for with 1M aqueous hydrochloric acid solution (3 X 50 mL). The
three times. The aqueous layer was acidified to pH = 2 with aqueous layer was washed with chloroform (3 X 50 mL)
concentrated hydrochloric acid, the precipitate solid was followed by neutralisation with 2M aqueous sodium hydroxide
extracted with ethyl acetate 3 × 20 mL then dried over MgSO4, solution. The precipitate was collected by mean of extraction
filtered and followed by solvent removal under reduced from dichloromethane. The combined organic layers were dried
pressure to afford 2,5-dibromoterephthalic acid (1.47 g, yield over MgSO4, which was followed by the removal of the solvent
1
91%) white solid. H NMR spectra and EI-MS matched those under reduced pressure to give 4,4'-(2,5-dimethyl-1,4-
previously reported.
phenylene) dipyridine which was purified by column
chromatography (silica gel, dichloromethane 95 %: methanol 5
%) to give as a white microcrystalline powder (247 mg, yield
1
Synthesis of 4-iodopyridine
95%). H NMR (300 MHz, CDCl3, 300 K.) δ (ppm) = 8.67 (dd, J=
To a cold (-10° C) slurry of 4-aminopyridine (6 g, 63.8 mmol) in 4.4 Hz, 1.8 Hz, 4H), 7.3 (dd, J= 4.4 Hz, 1.5 Hz, 4H), 7.15, s, 2H,
50 mL of 48% aqueous tetrafluoroboric acid, sodium nitrate (4.8 2.29, s 6H. 13C NMR (75 MHz, CDCl3, 300 K.) δ (ppm) = 149.73,
g, 69.5 mmol) was added portion-wise and stirred for 30 149.16, 139.15, 132.78, 131.58, 124.14, 19.70. EI-MS; (L+H)+
minutes. The resulting diazonium salt was filtered and added m/z = 261.0. Elemental analysis: calculated for C18H18N2: C=
without delay in portions to a solution of potassium iodide (17 83.04, H= 6.19, N= 10.79; found C= 82.26, H= 6.50, N= 10.42.
g, 102.4 mmol) in 40 mL acetone and 60 mL water. The dark Selected FTIR (ATR, cm-1); 3025, 2900, 1539, 1414, 1385, 824,
brown solution was decolourised by aqueous solution of 743. UV-vis (200-800nm): λmax = 261nm.
sodium thiosulfate 0.1 M followed by neutralisation with
aqueous solution of sodium carbonate. The product was Synthesis of diethyl 2,5-dibromoterephthalate (4)
extracted with diethyl ether three times and the combined Concentrated H2SO4 (1 mL) was added to a mixture of 2,5-
organic layers were dried over magnesium sulfate, filtered then dibromoterephthalic acid (1.07 g, 3.3 mmol) and ethanol 25 mL
the solvent was evaporated under reduced pressure to afford and stirred at reflux and the reaction was monitored by TLC.
1
4-iodopyridine as a pale yellow solid (9.2 g, yield 70%). The H After 48 hours the reaction mixture was cold to room
NMR and EI-MS matched the previous reported procedure22.
temperature and the solvent was removed under reduce
pressure. The residue was purified by column chromatography
Synthesis of 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2- on silica gel eluting with dichloromethane to produce colourless
1
yl)pyridine (2)
solid (1.2 g, yield 96%). H NMR spectra and EI-MS matched
4-Iodopyridine (8 g, 39 mmol) was dissolved in 500 mL dry those previously reported24. Crystals for X-ray diffraction were
diethyl ether followed by cooling the solution to (-78°C) in dry grown from chloroform.
ice – acetone path. A solution of nBuLi (17 mL, 48.9 mmol) of
2.5 M in hexane was then added dropwise, after 30 minutes Formula C12H12Br2O4, Mr = 380.04, monoclinic, space group
tributyl borate (11.2 g, 48.9) was added. The mixture was P21/c, a = 10.0468(3) Å, b = 7.6582(2) Å, c = 9.6509(2) Å, b =
heated to room temperature slowly over two hours followed by 117.188(3)°, a = g = 90°, V = 660.50(3) Å3, T = 190(2) K, Z = 2, , Dc
the addition of pinacol (6.2 g, 52.5 mmol), after 30 minutes the = 1.911 g cm-3 crystal size 0.55×0.43×0.14 mm3, colour
reaction quenched with acetic acid (2.5 mL, 40 mmol). The colourless, habit plate, temperature = 190(2) K, λMoKα =
solution was filtered through celite and washed with diethyl 0.71073 Å, μ(MoKα)= 6.139, 2max = 64.05, hkl range -14 ≤ h ≤
ether, the solvent was removed under reduced pressure. The 14, -11 ≤ k ≤ 11, -14 ≤ l ≤ 14, N = 48241, Nind = 2280 (Rmerge
crude product was purified by hot filtration from cyclohexane 0.0150), R1(all) 0.0396, wR2(F2 , all) 0.0794, GoF(all) 1.117.
1
to afford white solid (6 g, 75%). H NMR spectra and EI-MS
matched those reported22, 23
.
Synthesis of diethyl 2,5-di(pyridin-4-yl)terephthalate (Et2L)
4 (380 mg, 1 mmol), 2 (615.2 mg, 3 mmol) and potassium
Synthesis of 4,4'-(2,5-dimethyl-1,4-phenylene) dipyridine (3)
1,4-Dibromo-2,5-dimethylbenzene (263.9 mg, 1 mmol), 2 carbonate (1.38 g, 10 mmol) were mixed in dry DMF (30 mL).
(615.18 mg, 3 mmol) and potassium carbonate (1.38 g, 10 This solution was degassed three times before
mmol) were mixed in dry DMF (30 mL). This solution was tetrakis(triphenylphosphine)palladium(0) (231.2 mg, 0.2 mmol)
degassed three times before (231.0 mg, 0.2 mmol) was added, after which the solution was degassed once more.
tetrakis(triphenylphosphine)palladium(0) was added, after The mixture was stirred at 90°C under nitrogen for 24 hours.
2 | J. Name., 2012, 00, 1-3
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Solvent was removed under reduced pressure, and the residue 4205 (Rmerge 0.0478), R1(all) 0.1085, wR2(F2 , all) 0.2013, GoF(all)
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DOI: 10.1039/D0CE01020K
was poured into water and extracted with chloroform three 1.268.
times. The combined organic layers were washed with brine and
water before being dried over MgSO4, which was followed by Method B:
the removal of the solvent under reduced pressure. The crude To a solution of Et2L (97.9 mg, 0.26 mmol) in 10 mL ethanol was
product was purified by column chromatography (silica gel, add 5 mL of 2 M aqueous solution of potassium hydroxide and
ethyl acetate 95 %: petroleum spirit 5 %), followed by stirred at reflux and monitored by TLC. After 24 hours the
recrystallisation from cyclohexane to afford the product as a solution was cooled to room temperature and neutralised by
1
white powder (160 g, yield 42.5%). H NMR (300 MHz, CDCl3, concentrated hydrochloric acid then ethanol was removed by
300 K.) δ (ppm) = 8.68 (dd, J= 4.4 Hz, 1.8 Hz, 4H), 7.87 (s, 2H), rotary evaporator followed by acidification of the solution to pH
7.29 (dd, J= 4.4 Hz, 1.8 Hz, 4H), 4.15 (q, J= 7.2 Hz, 4H), 1.05 (t, J= = 5. The solution was left in a fridge at 5° C overnight,
7.2 Hz). 13C NMR (75 MHz, CDCl3, 300 K.) δ (ppm)= 166.51, microcrystalline solid was collected by filtration and washed
149.62, 147.90, 139.79, 133.29, 132.16, 128.58, 123.28, 61.83. with cold water to give compound H2L (40 mg, yield 48%).
EI-MS; (L+H)+ m/z = 377. Elemental analysis: calculated for
(C22H20N2O4): C=70.20, H=5.36, N=7.44; found C=70.48, H=5.35, Synthesis and crystallisation of [ZnL(H2O)]n
N=7.91. Selected FTIR (ATR, cm-1) = 2900, 2812.83, 2637, 2531, A solution of triethylamine (100 µL) in toluene (10 mL) was
1674.14, 4586.43, 1531. UV-vis (200-800nm): λmax = 245 nm.
diffused into a solution of H2L (77.2 mg, 0.24 mmol) and zinc
acetate hexahydrate (73 mg, 0.246 mmol) in DMF (3 mL). The
Formula C22H20N2O4, Mr = 376.40, orthorhombic, space group reaction was carried out in a sealed container and left to stand
P212121, a = 9.4240(19) Å, b = 9.5020(19) Å, c = 20.940(4) Å, a = for crystallisation at room temperature and after one month
b = g = 90°, V = 1875.1(7) Å3, T = 100(2) K, Z = 4, Dc = 1.795 g cm-3 colourless crystals suitable for X-ray diffraction were formed.
crystal size 0.04×0.03×0.03 mm3, colour colourless, habit The product was collected by mean of centrifuging for 4
needle, temperature = 100(2) K, synchrotron = 0.7108 Å , μ = minutes at 4000 rpm, then the solid product was washed for
0.093, 2max = 50.206, hkl range -12 ≤ h ≤ 12, -12 ≤ k ≤ 12, -27 ≤ several times with methanol then with acetone before it was
l ≤ 27, N = 31317, Nind = 4443 (Rmerge 0.0218), R1(all) 0.0439, dried under high vacuum to give (50 mg, 52.1 % yield).
wR2(F2 , all) 0.1128, GoF(all) 1.117.
Elemental analysis calculated for (C18H12N2O5Zn) C = 54.00, H =
3.02, N = 7.00%; found C = 54.17, H = 2.58, N = 6.60 %.
Formula C18H10N2O5Zn, Mr = 399.65, monoclinic, space group P21/n,
a = 10.620(2) Å, b = 8.4200(17) Å, c = 17.350(4) Å, b = 107.64(3)°, a =
g = 90°, V = 1478.5(6) Å3, Z = 4, Dc = 1.795 g cm-3 crystal size 0.12 ×
0.09 × 0.05 mm3, colour colourless, habit plate, temperature = 100(2)
K, synchrotron = 0.7108 Å, μ = 1.698 mm-1, 2max = 50.206, hkl range
-13 ≤ h ≤ 13, -10 ≤ k ≤ 10, -22 ≤ l ≤ 22, N = 33771, Nind = 3265 (Rmerge
0.0545), R1(all) 0.0897, wR2(F2 , all) 0.1394, GoF(all) 1.013.
Synthesis of 2,5-di(pyridin-4-yl)terephthalic acid (H2L)
Method A:
A
solution of 3 (260.34 mg, 1 mmol) and potassium
permanganate (1.58 g, 10 mmol) in 30 mL water was refluxed
for 24 hours. After filtration and washing with aqueous NaOH
1M the filtrate was extracted with chloroform. The aqueous
solution was concentrated under reduced pressure and
acidified with aqueous 1 M HCl to pH = 5 and kept under 5°C
overnight a white microcrystalline powder precipitated which
was collected and washed with cold water (286 mg, yield
Results and Discussion
1
89.3%). H NMR (300 MHz, methanol-d4, 300 K) δ (ppm) =8.56
H2L was produced in 85 % combined yield over two steps, via the
palladium catalysed cross-coupling of 1,4-dibromo-2,5-
dimethylbenzene and 4-pyridylboronic acid pinacol ester followed
by oxidation with potassium permanganate. The same material was
also prepared less efficiently by a similar cross-coupling reaction
using diethyl-2,5-dibromoterephthalate and 4-pyridylboronic acid
pinacol ester and subsequent deprotection. Two synthetic routes (A
& B, Figure 1) were explored for the synthesis of H2L. Although both
were successful, the former produced significantly higher overall
yields.
(dd, J=4.8 Hz, 1.5 Hz, 4H), 7.72, s, 2H, 7.59 (dd, J=4.8 Hz, 1.5 Hz,
4H). 13C NMR (75 MHz, D2O, 300 K.) δ (ppm)= 169.17, 158.45,
140.76, 138.39, 133.98, 132.35, 127.04. EI-MS; (L+Na+4H2O)+
m/z = 413, (L-H)- m/z = 319. UV-vis (200-800nm): λmax = 293 nm.
X-ray quality crystals of [Na2(OH2)10][(H2L)L]·8H2O) were grown
from an aqueous solution adjusted to pH 7 with 1M NaOH.
These crystals were collected by filtration and dried under
vacuum subsequent to X-ray data collection. Elemental analysis:
calculated for NaC18H13N2O4·7.5H2O: C= 45.08, H= 5.89, N= 5.85;
found C= 45.15, H= 3.17, N= 5.56.
Formula C36H58N4Na2O26, Mr = 1008.84, triclinic, space group P-
, a = 10.4109(6) Å, b = 11.1279(7) Å, c = 12.1125(10) Å, a =
101.580(7)°, b = 98.222(6)°, g = 117.146(6)°, V = 1178.13(15) Å3,
T = 190(2) K, Z = 1, Dc = 1.422 g cm-3 crystal size 0.20×0.10×0.06
mm3, colour colourless, habit plate, temperature = 190(2) K,
λMoKα = 0.71073 Å, μ(MoKα) = 0.136 mm-1 , 2max = 50.246,
hkl range -12 ≤ h ≤ 12, -13 ≤ k ≤ 13, -7 ≤ l ≤ 14, N = 4205, Nind
=
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Figure 3: Representation of the crystal structure of Et2L illustrating the hydrogen bonds
in blue line
Figure 1: The two synthetic routes employed for the preparation of H2L. The former
was more successful.
Slow evaporation of a basic aqueous solution of H2L yielded
crystals of a sodium salt of formula [Na2(OH2)10][(H2L)L]·8H2O.
The crystal structure (Figure 4) is composed of two molecules of
H2L in different protonation states. In the first, it is fully
deprotonated, and in the second the ligand is in a neutral, but
zwitterionic form with the carboxylic acid groups deprotonated
and the pyridyl groups protonated. The zwitterionic pyridinium
protons act as H-bond donors which are accepted by the
pyridine nitrogens from L forming one-dimensional chains. The
sodium ions, which balance the charge, form a bridged
[Na2(OH2)10] dimer consisting of two octahedral sodium ions
and ten water ligands with two of them linking the metal
centres in a µ2 fashion.
Crystal structures of the intermediate products 4 and Et2L were
obtained. The slow evaporation of chloroform from a solution
of 4 produced colourless crystals suitable for X-ray diffraction.
The solid state crystal structure of 4 shows it crystallises in the
monoclinic P21/c space group and the asymmetric unit contains
half molecule. The molecules connect together through weak
type I halogen bonding25, 26 between adjacent bromine atoms
distance 3.6 Å (Figure 2).
Figure 2: Representation of the crystal structure of 4 showing the halogen bonds.
Figure 4: ORTEP representation of the crystal structure of [Na2(OH2)10][(H2L)L]·4H2O
drawn with 50 % probability ellipsoids. Solvents and cation not shown.
Evaporation of chloroform slowly from a solution of Et2L
produced crystals suitable for X-ray diffraction. The molecules
interact with each other through hydrogen bonds that form
between the pyridyl nitrogen and hydrogen from the middle
phenyl ring (2.6 Å, 160 °) . In addition to face to edge π−π
interaction between the pyridyl ring and the central phenyl ring
(Figure 3).
The reaction of H2L with zinc(II) acetate tetrahydrate in the
presence of triethylamine produced colourless crystals suitable
for diffraction. The crystal structure was solved in monoclinic
P21/n and shows that the product is three-dimensional
coordination polymer of formula [ZnL(H2O)]n. The Zn(II) centres
are octahedral with a N2O4 coordination sphere consisting of
two axial pyridyl groups, three monodentate carboxylate
groups and an aqua ligand. Two of the carboxylates “bridge” to
adjacent metals to form two dimensional sheets (Figure 5).
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References
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DOI: 10.1039/D0CE01020K
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19.
In summary, we have demonstrated the use of a hetero-
functional ligand approach which combines aspects of both the
well-established pillaring and reticular strategies for the
synthesis of 3D coordination polymers. Further studies will
investigate the use of this approach with different transition
metals.
The authors thank The University of Queensland (UQ) and
The Australian Research Council (DP190102036 and
FT140100273) for support. H. A. AL-Fayaad thanks the Higher
Committee for Education Development in Iraq and Basrah Oil
Company PhD scholarship. Part of this research was undertaken
on the MX1 beamline at the Australian Synchrotron, part of
ANSTO.
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Conflicts of interest
There are no conflicts to declare.
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