Carving Out Pores in Redox-Active One-Dimensional Coordination Polymers

Article abstract of DOI:10.1002/anie.201807506

Reduction of the insulating one-dimensional coordination polymer [Cu(abpy)PF₆]_n, 1 a(PF₆), (abpy=2,2′-azobispyridine) yields the conductive, porous polymer [Cu(abpy)]_n, 2 a. Pressed pellets of neutral 2 a exhibi

Full text of DOI:10.1002/anie.201807506

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International Edition: DOI: 10.1002/anie.201807506
German Edition: DOI: 10.1002/ange.201807506
Microporous Materials Very Important Paper
Carving Out Pores in Redox-Active One-Dimensional Coordination
Polymers
Naomi E. Clayman+, Mary Anne Manumpil+, Daiki Umeyama, Andrey E. Rudenko,
Hemamala I. Karunadasa,* and Robert M. Waymouth*
Abstract: Reduction of the insulating one-dimensional coor-
dination polymer [Cu(abpy)PF6]n, 1a(PF6), (abpy = 2,2’-azo-
bispyridine) yields the conductive, porous polymer [Cu-
(abpy)]n, 2a. Pressed pellets of neutral 2a exhibit a conductiv-
ity of 0.093 ScmÀ1 at room temperature and a Brunauer–
Emmett–Teller (BET) surface area of 56 m2 gÀ1. Fine powders
of 2a have a BET surface area of 90 m2 gÀ1. Cyclic voltam-
metry shows that the reduction of 1a(PF6) to 2a is quasi-
reversible, indicative of facile charge transfer through the bulk
material. The BET surface area of the reduced polymer 2 can
be controlled by changing the size of the counteranion X in the
cationic [Cu(abpy)X]n. Reduction of [Cu(abpy)X]n with X =
Br (2b) or BArF (2c; BArF = tetrakis(3,5-bis(trifluoromethyl)-
phenyl)), affords [Cu(abpy)]n polymers with surface areas of
60 and 200 m2 gÀ1, respectively.
1a(PF6),[19] results in the precipitation of a black insoluble
material (Figure 1). Inductively coupled plasma atomic
emission spectroscopy (ICP-AES) and combustion analysis
of the polymer support assignment of the approximate
formula [Cu(abpy)]n for 2a, with trace contamination from
residual Co and PF6 (Supporting Information). Analysis of 2a
by matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) mass spectrometry reveals ions correspond-
ing to short chains of the copper–azobispyridine repeat unit
Cun(abpy)x (Supporting Information, Figure S1).
C
oordination polymers[1] and other classes of metal-con-
taining macromolecules combine the electronic properties of
inorganic complexes with macromolecular behavior. Owing
to their unique combination of properties, metallopoly-
mers[2–9] have elicited significant interest as materials for
sensors,[3–5] microelectronics,[6,10,11] energy storage,[7,8] gas
separation,[9] and catalysis.[9,12] Although there are many
examples of redox-active and electrically conductive metal-
lopolymers,[6,13–17] few are porous.[12] Whereas porosity in
network solids, such as three-dimensional metal–organic
frameworks, arises from voids in the framework,[1,9,18] porosity
in 1D coordination polymers depends on how the polymer
chains pack in the solid state. Herein, we report a method to
generate porous conductive coordination polymers by reduc-
tion of cationic polymers; extrusion of counterions provides
a means of generating and tuning the porosity.
Figure 1. Non-porous and insulating [Cu(abpy)PF6]n, 1a(PF6), reversibly
transforms to porous and conductive [Cu(abpy)]n, 2a, upon reduction.
Films of 1a(PF6) and 2a are blue (left) and black (right), respectively.
The reduction of 1a(PF6) to 2a results in both an increase
in porosity and a significant increase in conductivity. The
cationic coordination polymer 1a(PF6) exhibits negligible
porosity, whereas powders of the reduced neutral polymer 2a
exhibit a Brunauer–Emmett–Teller (BET) surface area of
90 m2 gÀ1. Four-point-probe conductivity measurements on
pressed pellets of 2a yielded conductivities approaching
0.11 ScmÀ1 at room temperature (Table 1). In contrast,
pressed pellets of the cationic metallopolymer 1a(PF6)
exhibits an upper limit of conductivity of about 10À10 ScmÀ1
(corresponding to instrumental detection limits; Supporting
Information, Figure S2).
Electrochemical or chemical reduction with cobaltocene
(Cp2Co) of an acetonitrile (MeCN) solution of the blue,
insulating, cationic coordination polymer [Cu(abpy)PF6]n,
[*] N. E. Clayman,[+] M. A. Manumpil,[+] D. Umeyama, A. E. Rudenko,
H. I. Karunadasa, R. M. Waymouth
Addition of an acetonitrile solution of ferrocenium
hexafluorophosphate (Cp2FePF6) to the insoluble reduced
Department of Chemistry, Stanford University
Stanford, CA 94305 (USA)
E-mail: hemamala@stanford.edu
Table 1: Surface area and conductivity values for 2.[a]
waymouth@stanford.edu
[Cu(abpy)]n Departed anion BET surface area
[m2 gÀ1
Conductivity (ꢀ
10À2 ScmÀ1) pressed
pellet, 258C
D. Umeyama
]
Present Address: International Center for Materials Nanoarchitec-
tonics (MANA), National Institute for Materials Science (NIMS)
1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan)
powder pellet[a]
a
b
À
[+] These authors contributed equally to this work.
2a
2b
2c
PF6
BrÀ
90
60
200
56
52
110
9.3
1.7
3.1
11
3.5
3.5
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201807506.
BArFÀ
[a] Pellets pressed at a) 14.5 kN for 1.5 h and b) 22 kN, 12 h.
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polymer 2a results in the re-dissolution of all material and re-
speciation of all copper species as 1a(PF6). The reversible
deposition and dissolution can also be performed electro-
chemically: electrochemical reduction of 1a(PF6) by con-
trolled-potential electrolysis at À0.7 V vs. Ag/AgNO3 results
in the deposition of a black film of 2a on the working
electrode (Supporting Information, Figure S4). When the
polarity of the electrochemical cell is reversed, all material
deposited on the electrode is dissolved to regenerate the
cationic polymer 1(PF6).
molecules in the unit cell in a distorted tetrahedral geometry
À
À
with C N bond distances from 1.35–1.41 ꢀ and Cu N
distances of 1.9–2.0 ꢀ (Supporting Information, Table S2).
The PDF of Cu(azpy)2, simulated from the crystal structure, is
similar to that of 2a but lacks the intense peak at about 4.7 ꢀ,
À
which supports the assignment of this peak as the Cu Cu
interatomic distance in 2a (Supporting Information, Fig-
ure S7).
The infrared (IR) spectrum of 1a(PF6) exhibits peaks at
818 cmÀ1 and 554 cmÀ1, corresponding to PF6 stretches
À
Owing to the insolubility of the reduced polymer 2a,
attempts to grow crystals suitable for single-crystal X-ray
diffraction were unsuccessful. However, microcrystalline
sheets of 2a could be deposited directly on indium tin oxide
(ITO) electrodes in an electrochemical cell containing 1(PF6)
by oscillating the potential around the E1/2 of the 1a(PF6)/2a
redox couple. The synchrotron powder X-ray diffraction
pattern of 2a displays some long-range order (Supporting
Information, Figure S5). Scanning electron microscopy
(SEM) reveals that 2a forms as slip-stacked sheets on ITO
(Supporting Information, Figure S6). To provide further
information on the structure of 2a, we performed X-ray
scattering experiments (beamline 11-ID-B, Advanced Photon
Source, Argonne National Laboratory) and analyzed the pair
distribution function (PDF) of the total X-ray scattering data.
The PDF of the [Cu(abpy)BF4]n coordination polymer
(Supporting Information, Figure S8). These peaks are
absent in the IR spectrum of 2a, indicating the departure of
À
PF6 anions to maintain charge balance upon chemical
reduction. Additionally, a broad absorbance centered at
about 2500 cmÀ1 appears in the IR spectrum of 2a; this
feature is absent in the IR spectrum of 1a(PF6) (Figure 2C).
Analysis of the absorbance of 2a in the UV/Vis-NIR region
by diffuse-reflectance spectroscopy (Figure 2D) also reveals
a low lying electronic excitation in the region of about
5000 cmÀ1 (limited by the instrumental detection range;
Figure 2D) that is absent in the spectrum of 1a(PF6). Based
on the observation of a similar band in the mixed-valence
molecule Cu(abpy)2 (which was transiently generated in an
electrochemical cell),[20] this dominant low-energy band in 2a
is assigned as an intervalence charge transfer (IVCT) band.
Maity et al. describe the mixed-valency in the molecular
Cu(abpy)2 as a Robin–Day[21] Class III system with CuI-
(abpy0.5À)2, where one electron is delocalized over the two
abpy ligands.[20] An IVCT band is a signature of mixed-
valence,[22] and we therefore expect substantial electronic
communication between the redox-active abpy ligands and
the Cu centers in 2a. The UV/Vis-NIR spectrum of 1a(PF6)
has an intense band centered around 9100 cmÀ1 and a band
around 26000 cmÀ1 (Figure 2D). Analogous bands had been
observed for the cationic [Cu(abpy)2][PF6] molecule and were
assigned to a metal-to-ligand charge transfer (MLCT) tran-
sition and a p–p* transition, respectively.[20] These bands
(1BF4)[19] with the BF4 anions removed was simulated from
À
the reported crystal structure. This simulated PDF matched
well with the experimental PDF of 2a (Figure 2A), including
À
À
the covalent bond lengths in abpy (C N and C C ca. 1.4 ꢀ),
À
À
coordination bond length (N Cu ca. 2.0 ꢀ), and Cu Cu
interatomic distance (ca. 4.7 ꢀ). The correspondence
between the simulated and experimental PDF data indicates
that the structural parameters of 2a are similar to those of
1(BF4),[19] even though the oxidation state is different. The
peak assigned to the distance between nearest-neighbor
copper centers (ca. 4.7 ꢀ) suggests that the Cu atoms are
bridged by the abpy ligands, consistent with the polymeric
nature of 2a. We also synthesized and crystallographically
characterized Cu(azpy)2 (azpy = phenylazo-2-pyridine; Fig-
ure 2B; Supporting Information, Table S1), a molecular ana-
logue of [Cu(abpy)]n. Molecular Cu(azpy)2 crystallizes in the
space group Pna21 with two crystallographically independent
diminish upon reduction, consistent with electrons being
[20]
=
added to the azo N N p* orbital.
The solid-state electron paramagnetic resonance (EPR)
spectrum of 2a is also indicative of considerable ligand radical
character. The spectrum features a broad peak with a g value
of 2.08 (Figure 3A; Supporting Information, Figure S9),
À
Figure 2. A) Simulated X-ray pair distribution function (PDF) of the previously reported 1(BF4)[10] without BF4 using Qmax =23.7 ꢁÀ1 (red) and the
experimental PDF of 2a (blue) with Qmax =23.7 ꢁÀ1. B) Single-crystal X-ray structure of Cu(azpy)2 (azpy=phenylazo-2-pyridine), a molecular
analogue of 2a.[29] Cu orange, N blue, C gray. H atoms omitted for clarity. C) Mid-IR spectra of 1a(PF6) (blue) and 2a (green). D) Normalized
diffuse reflectance UV/Vis-NIR spectra of 1(PF6) (blue) and 2a (green).
2
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This hypothesis raised the intriguing possibility that the
porosity of these reduced [Cu(abpy)]n coordination polymers
might be tuned simply by choice of the appropriate counter-
ion in the cationic [Cu(abpy)X]n precursor. To test that the
porosity of the [Cu(abpy)]n polymers was associated with the
loss of the counterion upon reduction of 1(X), and to
modulate the porosity of the [Cu(abpy)]n polymers, we
chemically reduced two other compounds of the form [Cu-
(abpy)X]n, XÀ = BrÀ(1b) and X = BArFÀ (1c; BArFÀ
=
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), that contain
counterions of different size. Analysis of the surface areas of
the resulting powders (Table 1) reveal that the surface area of
the [Cu(abpy)]n coordination polymers can be tuned by
changing the identity and size of the anion X. Reduction of
[Cu(abpy)PF6]n 1a(PF6), [Cu(abpy)Br]n 1b(Br), and [Cu-
(abpy)BArF]n 1c(BArF) results in the [Cu(abpy)]n polymers
2a, 2b, and 2c with BET surface areas of 90 m2 gÀ1, 60 m2 gÀ1,
and 200 m2 gÀ1, respectively, with porosity increasing with the
size of the counterion (Table 1, Figure 4A).
Polymers 2a and 2b are microporous and show a combi-
nation of Type I and II isotherms, according to the IUPAC
classification.[26] We observe a filling of micropores in the low-
pressure region, followed by continuous uptake at higher-
pressures, corresponding to the filling of larger pores and
possibly the voids between particles (Figures 4A).[27] In
contrast, 2c possesses a Type IV isotherm shape and hyste-
resis loop, indicating that the bulky BArFÀ anions induce
mesoporosity in the material (Supporting Information, Fig-
ure S11).[26]
To probe the transport properties of these materials, we
performed four-point-probe conductivity measurements on
pellets pressed at two different pressures (Table 1) with
electrodes placed in the van der Pauw configuration.[28] Upon
reduction of 1a(PF6) to 2a, the room-temperature conduc-
tivity increased by nine orders of magnitude (Supporting
Information, Table S2). The conductivity of pellets of 2
(pressed at 22 kN) is 0.11 ScmÀ1 at room temperature and
increases to 0.45 ScmÀ1 at 1188C (Figure 4B). The temper-
ature dependence of the conductivity indicates a thermally
activated conduction process with an Arrhenius energy (Ea)
of 150 meV (Supporting Information, Figure S18). Based on
reports of other conductive coordination polymers and trans-
port in class II mixed-valence systems, the mechanism of
conduction is likely nearest-neighbor hopping.[13] Pellets of
2a, 2b, and 2c pressed under two different conditions are all
electrically conductive (Table 1) with conductivities in the
range of 1.7 ꢁ 10À2 to 0.11 ScmÀ1 (Table 1; Supporting Infor-
mation, Figures S12–S22, Table S5). While the surface areas
of the pressed pellets of 2 are lower than those of the powders
isolated upon reduction of 1(X) (Table 1), the trends in
porosity (2b < 2a < 2c) are retained. Moreover, the magni-
tude of the conductivities of the pressed pellets do not show
a strong correlation with the porosities of the pressed pellets
(Table 1), implicating that the factors which contribute to the
conductivity are decoupled from those which contribute to
the porosity.
Figure 3. A) Solid-state EPR spectrum of 2a measured at 77 K.
B) Solid-state cyclic voltammograms of 2a obtained at a scan rate of
2 mVsÀ1. Scans started in the reductive direction.
consistent with a ligand-based radical with some delocaliza-
tion over the metal. A ligand-based reduction would formally
correspond to a polymer comprised of copper(I) centers, with
azobispyridine radical mono-anions providing charge bal-
ance. Ligand-centered reduction of azopyridine ligands has
precedent.[20,23,24] However, the g value of 2a is slightly shifted
from the free-electron g value (g = 2.002) expected for
organic radicals, indicating that the unpaired electron resides
in a molecular orbital with some Cu character.[20,25]
Electrochemical analysis of 1a(PF6)/2a was accomplished
by electrodeposition of the polymer as a thin film on the
surface of a glassy carbon (GC) working electrode from an
electrolyte solution of 1 mm 1a(PF6) in 0.1m tetrabutylam-
monium hexafluorophosphate (TBAPF6) in MeCN. Cyclic
voltammograms (CVs) of the film reveal a broad redox event
assigned to the oxidation and reduction of each repeat unit,
with an E1/2 of À0.82 V vs. Ag/AgNO3 (Figure 3B). The broad
peak width is most likely due to overlapping redox processes
as each monomer unit of the chain is reduced.
The reported crystal structure of [Cu(abpy)BF4]n 1(BF4)
À
reveals that the BF4 counteranions occupy channels in
between polymer chains.[19] The increase in porosity upon
reduction of 1a(PF6) to 2a (Figure 4A) suggests that the
À
porosity arises from the loss of PF6 anions upon chemical
reduction and that the resulting metallopolymer chains
undergo limited structural relaxation. Voids created by
inefficient packing of polymer chains in the solid state may
also contribute to the observed porosity. Indeed, a pore-size-
distribution analysis shows a variation ranging from micro- to
mesopores (Supporting Information, Figure S10).
In conclusion, we show a new method for generating
conductive porous coordination polymers by sculpting pores
in polymers through templating effects by anions that leave
Figure 4. A) Nitrogen isotherms measured at 77 K of fine powders of
2c (purple), 2a (green), 2b (orange), and 1a(PF6) (blue). B) Current–
voltage curves of 2a.
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We thank the Office of Naval Research (ONR
N000141410551, R.M.W) and the Center for Molecular
Analysis and Design (CMAD, N.E.C, M.A.M.) for graduate
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Conflict of interest
The authors declare no conflict of interest.
Keywords: microporous materials · N-ligands · polymers ·
redox chemistry · semiconductors
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Manuscript received: June 29, 2018
Version of record online: && &&, &&&&
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Communications
Microporous Materials
N. E. Clayman, M. A. Manumpil,
D. Umeyama, A. E. Rudenko,
H. I. Karunadasa,*
R. M. Waymouth*
&&&& — &&&&
Carving Out Pores in Redox-Active One-
Dimensional Coordination Polymers
Templating pores in polymers: Reduction
of non-porous cationic insulating coordi-
nation polymers (see picture, left) yields
porous neutral conductive polymers
(right). The porosity varies with the size
of the anion that departs the parent
polymer.
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