Mixed Valency in a 3D Semiconducting Iron-Fluoranilate Coordination Polymer

Article abstract of DOI:10.1021/acs.inorgchem.7b01038

A pair of coordination polymers of composition (NBu₄)₂[M₂(fan)₃] (fan = fluoranilate; M = Fe and Zn) were synthesized and structurally characterized. In each case the compound consists of a pair of interpenetrat

Full text of DOI:10.1021/acs.inorgchem.7b01038

Article
pubs.acs.org/IC
Mixed Valency in a 3D Semiconducting Iron−Fluoranilate
Coordination Polymer
Ryuichi Murase,†,‡ Brendan F. Abrahams,*,† Deanna M. D’Alessandro,*,‡ Casey G. Davies,§
Timothy A. Hudson,† Guy N. L. Jameson,§,△ Boujemaa Moubaraki,⊥ Keith S. Murray,⊥
Richard Robson,*,† and Ashley L. Sutton†
†School of Chemistry, The University of Melbourne, Parkville, Victoria 3010, Australia
‡School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia
§Department of Chemistry & MacDiarmid Institute for Advanced Materials & Nanotechnology, University of Otago, Dunedin, PO
Box 56, New Zealand 9054
⊥School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
S
* Supporting Information
ABSTRACT: A pair of coordination polymers of composition
(NBu4)2[M2(fan)3] (fan = fluoranilate; M = Fe and Zn) were synthesized
and structurally characterized. In each case the compound consists of a pair
of interpenetrating three-dimensional, (10,3)-a networks in which metal
centers are linked by chelating/bridging fluoranilate ligands. Tetrabuty-
lammonium cations are located in the spaces between the two networks.
Despite the structural similarity, significant differences exist between
(NBu4)2[Fe2(fan)3] and (NBu4)2[Zn2(fan)3] with respect to the oxidation
states of the metal centers and ligands. For (NBu4)2[Fe2(fan)3] the
structure determination as well as Mossbauer spectroscopy indicate the
̈
oxidation state for the Fe is close to +3, which contrasts with the +2 state
for the Zn analogue. The differences between the two compounds extends
to the ligands, with the Zn network involving only fluoranilate dianions,
whereas the average oxidation state for the fluoranilate in the Fe network lies somewhere between −2 and −3. Magnetic studies
on the Fe compound indicate short-range ordering. Electrochemical and spectro-electrochemical investigations indicate that the
fluoranilate ligand is redox-active in both complexes; a reduced form of (NBu4)2[Fe2(fan)3] was generated by chemical reduction.
Conductivity measurements indicate that (NBu4)2[Fe2(fan)3] is a semiconductor, which is attributed to the mixed valency of the
fluoranilate ligands.
of metal centers can serve as nodes. In addition to their
structural role, a particularly attractive feature of these ligands is
their ability to exist in different redox states. The dianion is able
to undergo a one-electron reduction to form a radical trianion
(II) and a second one-electron reduction to form the aromatic
tetraanion (III). In 2011 a synthetic and structural investigation
of 3D networks of composition (NBu4)2[M2(dhbq)3] (M =
Mn, Fe, Co, Ni, Zn, and Cd) and (NBu4)2[Mn2(can)3] was
reported (can2− = the dianion of 3,6-dichloro-2,5-dihydroxy-
1,4-benzoquinone; I, X = Cl).9 In this work the prospect of
creating networks that employ bridging ligands and metal
centers that are both amenable to facile electron transfer was
discussed. This work was followed in 2012 by a report on a
crystalline material of composition (PMePh3)2[Cd2(dhbq)3]
that was able to undergo repeated cycles of electrochemical
reduction and oxidation.10
INTRODUCTION
■
Coordination polymers, otherwise known as metal−organic
frameworks (MOFs), have been recognized as versatile
materials for a broad range of potential applications.1 The
tunability and rational design of the components allow for the
construction of tailor-made porous materials that have found
uses in gas sorption and separation, heterogeneous catalysis,
and molecular sensing, among others. While there has been a
strong emphasis on exploiting the host properties of porous
coordination polymers there has also been interest in exploring
the electrical and magnetic properties of coordination networks.
For example, in recent years enormous strides have been made
toward the realization of electrically conductive framework
materials.2−7
The dianion of 2,5-dihydroxy-1,4-benzoquinone (dhbq2− X =
H; I) and related species have been used extensively, in their
dianionic form, to link metal centers within coordination
polymers.8 The ability of these anionic, chelating ligands to
bridge metal centers has led to robust two-dimensional (2D)
and three-dimensional (3D) networks in which a wide variety
Received: April 24, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01038
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synthesis of two coordination polymers of composition
(NBu4)2[M2(fan)3] (M = Fe (1), Zn (2)). The characterization
of 1 and 2 is supported by investigations of the electronic,
magnetic, spectro-electrochemical (SEC) and optical proper-
ties.
EXPERIMENTAL SECTION
■
General Synthetic Details. The compound 2,3,5,6-tetrafluoro-
1,4-benzoquinone was purchased from Sigma-Aldrich and used
without further purification. FeCl2·4H2O (FSE), Zn(NO3)2·6H2O
(Mallinckrodt), and tetrabutylammonium bromide (Aldrich) were
used without further purification. 2,5-Difluoro-3,6-dihydroxy-1,4-
benzoquinone·dihydrate was synthesized according to a modified
literature procedure.16 Lithium naphthalenide (0.1 M) in tetrahy-
drofuran (THF) was prepared using a modified reported procedure.17
The preparation of Li0.27{(NBu4)2[Fe2(fan)3]} was performed in an
argon-filled glovebox. Characterization of compounds was performed
under ambient conditions in air unless otherwise stated. Tetrahy-
drofuran was obtained from a PuraSolv solvent purification system and
kept under an argon atmosphere. Solution-state 13C{1H} NMR
spectra were recorded on a Bruker AVANCE 300 spectrometer
operating at 75 MHz for 13C. 13C NMR chemical shifts were
referenced internally to residual solvent resonances. Deuterated
solvents were obtained from Cambridge Stable Isotopes and used as
received. Microanalyses were performed at the Chemical Analysis
Facility−Elemental Analysis Service in the Department of Chemistry
and Biomolecular Science at Macquarie University, Australia.
Inductively coupled plasma-optical emission spectrometer (ICP-
OES) analysis was performed at the Mark Wainwright Analytical
Center at the University of New South Wales, Australia.
Synthesis of 2,5-Difluoro-3,6-dihydroxy-1,4-benzoquinone·
Dihydrate. 2,3,5,6-Tetrafluoro-1,4-benzoquinone (2.0 g, 0.11 mol)
was dissolved in 25 mL of 1,4-dioxane. NaOH (8 M, 30 mL) was
added slowly using a pipet while stirring. The dark brown solution was
allowed to stir for 3 h at room temperature. A brown precipitate of
Na2fan was isolated by filtration and air-dried. 2,5-Difluoro-3,6-
dihydroxy-1,4-benzoquinone was synthesized by the acidification of
Na2fan. Na2fan (2.00 g, 9.01 mmol) was stirred in HCl (37%, 5 mL),
whereupon precipitates of H2fan, NaF, and NaCl were collected
subsequently. H2fan·2H2O was recrystallized from acetone as red
blocklike crystals (Yield: 0.71 g, 30.1%), IR: 3452, 2363, 2340, 1640,
1553, 737 cm−1. 13C NMR (CD3CN, 300 MHz) δ (ppm) 142.51,
139.05. Anal. Calcd for C6H2O6F2·2.5H2O: C, 32.59%; H, 3.19%; N,
0%; found: C, 32.57%; H, 3.15%; N, <0.05%.
In 2015, Long and co-workers described investigations of the
aforementioned (NBu4)2[Fe2(dhbq)3], which indicated that,
unlike (NBu4)2[M2(dhbq)3] (M = Mn, Co, Ni, Zn, and Cd),
the metal centers exist in the +3 oxidation state, according to
Mossbauer spectroscopy.11 A corollary of this is that two-thirds
̈
of the dhbq ligands exist formally as the radical trianion. From a
crystallographic perspective, all dhbq ligands are identical, and
thus the average charge on a dhbq ligand is −2.67. A broad
absorption band in the range of 4000 to 14 000 cm−1 was
attributed to intervalence charge transfer (IVCT) associated
with the dhbq2−/3− bridging ligands. The through-bond mixed
valency due to dhbq2−/3− ligand-based IVCT leads to one of the
highest levels of intrinsic conductivity observed in a 3D
coordination polymer (0.16(1) S cm−1). Also in 2015, Harris et
al.12 identified a +3 oxidation state for the Fe centers within a
2D hexagonal network material of composition
(Me2NH2)2[Fe2(can)3]·2H2O·6DMF (DMF = dimethylforma-
mide). In this case the chloranilate anion may be considered to
have an average oxidation state of −2.67. This work was
followed by a paper, appearing in early 2017, that indicates that
the compound and its desolvated form exhibits electrical
conductivity of 1.4(7) × 10−2 and 1.0(3) × 10−2 S cm−1.13
Harris and co-workers also demonstrated that the compound
can undergo reduction to form a compound in which all ligands
are reduced to the −3 radical form, while the Fe retains its +3
oxidation state. This compound forms a magnetically ordered
phase below Tc = 105 K.
In 2016 Stock et al. reported compounds of composition
(NH2Me2)3[Al4(dhbq)6] and (NH2Me2)3[Al4(can)6], which
each had a similar 2D network to that reported by Harris et al.
but with the bridging/chelating ligands linking Al(III) centers.
In each of the structures reported by Stock, the ligands (dhbq
or can) have an average charge of −2.5.14
We recently published a synthetic, structural, and spectro-
scopic investigation of a series of structurally related
compounds of composition (NEt4)2[M2(can)3] (M = Mg,
Mn, Fe, Co, Ni, Cu, and Zn) and (NEt4)2[Zn2(fan)3], (H2fan =
3,6-difluoro-2,5-dihydroxy-1,4-benzoquinone, fluoranilic
acid).15 Hexagonal anionic networks similar to those identified
by Harris were found for this series; however, the
tetraethylammonium cation plays a different structural role to
t h e d i m e t h y l a m m o n i u m c a t i o n i n H a r r i s ’ s
(Me2NH2)2[Fe2(can)3].12,13 In all cases except for the Fe
compound, divalent metal ions are linked by either can2− or
fan2−. Bond valence sum calculations in addition to near-
infrared spectra suggest that the Fe in (NEt4)2[Fe2(can)3] is in
the +3 oxidation state, while the chloranilate anion has an
oxidation state intermediate between −2 and −3.
Synthesis of (NBu4)2[Fe2(fan)3] (1). A solution of FeIICl2·4H2O
(0.1 mmol), NBu4Br (0.4 mmol), and LiOAc (0.4 mmol) in 3 mL of
H2O was allowed to diffuse into a solution of H2fan (0.15 mmol) in 4
mL of acetone with a 1:1 H2O−acetone buffer solution under ambient
conditions in air. Dark purple tetrahedral crystals suitable for X-ray
diffraction appeared within 4 d (Yield: 17.2 mg, 30.2%). IR (ATR):
2963 (w), 2876 (w), 1656 (w), 1537 (s), 1486 (m), 1347 (s), 1152
(w), 1087 (w), 997 (s) cm−1. Anal. Calcd for Fe2C50H72N2O12F6·
7H2O: C, 48.24%; H, 6.96%; N, 2.25%. Found: C, 48.15%; H, 5.95%;
N, 2.21%.
Synthesis of (NBu4)2[Zn2(fan)3] (2). A solution of Zn(NO3)2·
6H2O (30 mg, 0.1 mmol), NBu4Br (65 mg, 0.2 mmol), and LiOAc (13
mg, 0.2 mmol) in 3 mL of H2O was allowed to diffuse into a solution
of H2fan (32 mg, 0.15 mmol) in 4 mL of acetone with a 1:1 H2O−
acetone buffer solution under ambient conditions in air. Violet
blocklike crystals suitable for X-ray diffraction appeared within 4 d.
(Yield: 39.2 mg, 67.7%). IR (ATR): 2963 (w), 2933 (w), 2876 (w),
1645 (w), 1522 (s), 1495 (m), 1401 (s), 1376 (s), 1037 (w), 1005 (s).
Anal. Calcd for Zn2C50H72N2O18F6·3H2O: C, 50.38%; H, 6.60%; N,
2.27%. Found: C, 50.50%; H, 6.06%; N, 2.32%.
Our current interest lies with further exploration of the
fluorinated form of H2dhbq, H2fan, which we have used with
the intention of incorporating the fluoranilate anion into
framework materials. The choice of fluoranilate as bis-bidentate
“linkers” stems from the rationale that the electron-withdrawing
effect of the fluoro substituents is expected to impact upon the
redox activity of the resulting network. Herein, we report the
Synthesis of Li0.27{(NBu4)2[Fe2(fan)3]} (1·Reduced). A pow-
dered sample of 1 (28 mg, 0.025 mmol) was suspended in a 0.1 M
solution of lithium naphthalenide (0.25 mL, 0.025 mmol) and stirred
for 3 h under an argon atmosphere. The suspension was filtered under
argon and washed with anhydrous THF (3 × 1 mL) to yield a maroon
B
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solid (Yield: 20.8 mg). IR (ATR): 3238 (br), 2962 (w), 2875 (w),
1645 (w), 1532 (s), 1485 (m), 1346 (s), 1086 (w), 996 (w) cm−1.
Anal. Calcd for Fe2C50H72N2O18F6Li0.27: C, 49.36%; H, 5.96%; N,
2.30%. Found: C, 50.40%; H, 6.17%; N, 2.25%.
Attempted Reduction of (NBu4)2[Zn2(fan)3]. A powdered
sample of 2 (114 mg, 0.10 mmol) was suspended in a 0.1 M solution
of lithium naphthalenide (1 mL, 0.1 mmol) and stirred for 3 h under
an argon atmosphere. The suspension was filtered under argon and
washed with anhydrous THF (3 × 1 mL) to yield a yellow solid.
Powder diffraction indicated a loss of crystallinity. IR (ATR): 2962
(w), 2976 (w), 1645 (w), 1519 (m), 1489 (s), 1400 (m), 1373 (s),
1036 (w), 1003 (s) cm−1.
Crystallography. Crystals of (NBu4)2[Fe2(fan)3] and
(NBu4)2[Zn2(fan)3] were transferred directly from the mother liquor
to a protective oil before being mounted on a goniometer in a cooled
stream of nitrogen. Diffraction data for (NBu4)2[Fe2(fan)3] were
collected on an Oxford Diffraction SuperNova diffractometer, while
the MX1 beamline at the Australian Synchrotron was used in the case
of (NBu4)2[Zn2(fan)3]. The structures were solved using SHELXT18
and refined using a full-matrix least-squares procedure based upon
F2.19 Structure solution and refinement were performed within the
WinGX system of programs.20 Crystal information and details relating
to the structural refinements are presented in Table 1. Further
was controlled within the cryo-chamber of the PPMS with a ramp rate
of −2 K min−1. Unit resistivity was calculated according to the
equation ρ = RA/l, where A = width × thickness, and l = 2 mm.
Electrochemistry and Spectroscopy. Cyclic voltammograms
were collected on a BASi Epsilon electrochemical analyzer. All
measurements were recorded in 0.1 M KCl/H2O electrolyte using a
glassy carbon electrode, platinum counter electrode, and a Ag/AgCl
reference electrode in 3 M NaCl(aq). All potentials are reported in volts
versus Ag/AgCl. UV−vis−NIR (NIR = near-infrared) spectra were
collected on an Agilent CARY5000 Spectrometer with a Harrick
Omni-Diff probe. Spectra were collected between 5000 and 25 000
cm−1 and are reported as the Kubelka−Munk transform, where F(R) =
(1 − R)2/2R. The spectra have been smoothed using the Savitzky−
Golay function (30 points).
RESULTS AND DISCUSSION
■
Synthesis and Structure of (NBu4)2[M2(fan)3] (M = Fe,
Zn). The slow diffusion of H2fan in acetone into an aqueous
solution of FeCl2·4H2O and NBu4Br yielded black polyfaceted
crystals of the formula (NBu4)2[Fe2(fan)3] (1) suitable for X-
ray diffraction. When Zn(NO3)2·6H2O was used in place of
FeCl2·4H2O an analogous compound with the formula
(NBu4)2[Zn2(fan)3] (2) was obtained. Investigations of the
stability of these compounds using thermogravimetric analysis
(TGA; Figure S1) indicate that chemical decomposition occurs
at temperatures in excess of 250 °C.
Table 1. Crystal Details and Refinement for 1 and 2
1
2
chemical formula
a, b (Å)
C50H72F6Fe2N2O12
31.2842(7)
19.6429(5)
90, 90, 120
16648.9(9)
12
C50H72F6N2O12Zn2
31.272(4)
19.711(2)
90, 90, 120
16694(4)
12
A single-crystal X-ray diffraction investigation of 1 indicated
the adoption of the trigonal space group R3c. There are two
crystallographically distinct octahedral Fe centers, each of
which is bound to three chelating and bridging fluoranilate
ligands within a 3D coordination network (Figure 1a). One
type of Fe center (Fe1) lies on a general position, while (Fe2)
is located on a threefold axis. There are 3 times as many Fe1
centers as Fe2 centers. All Fe centers within a single network
have the same absolute configuration, and this leads to the
formation of a chiral network with the (10,3)-a topology
(Figure 1b). In its geometrically most symmetrical form the
(10,3)-a net consists of fourfold helices running in three
mutually perpendicular directions; the smallest circuit involving
the 3-connecting nodes is a non-planar decagon. Networks with
the (10,3)-a topology are typically open-type structures with
large intraframework voids and this network is no exception
with the single network occupying only 35% of the crystal
volume, based upon its van der Waals surface. The remaining
space is occupied by a second symmetry-related network with
the opposite configuration and countercations. The two
interpenetrating networks form an enantiomeric pair, and
thus the crystal exists as a racemate, schematically represented
in Figure 1c. In addition to the metal centers possessing
opposite configuration in the two networks, inspection of
Figure 1c reveals that the fourfold helices in the two networks
turn in opposing directions. Disordered tetrabutylammonium
cations are located approximately midway between pairs of Fe
centers belonging to the two networks. A space-filling
representation of part of the two interpenetrating anionic
networks is presented in Figure 1d, while Figure 1e shows a
similar view with the tetrabutylammonium cations filling the
spaces between the networks. A similar structure is obtained for
2 but with Zn in place of Fe. The structures of 1 and 2 provide
an interesting contrast with the structures of
(NBu4)2[M2(dhbq)3] (M = Mn, Fe, Co, Ni, Zn, and Cd)
reported in 2011, which are cubic in the case of M = Fe, Co, Ni,
Zn, and Cd and tetragonal in the case of M = Mn. With the
adoption of the cubic symmetry for (NBu4)2[Fe2(dhbq)3] and
c (Å)
α, β, γ (deg)
V (Å3)
Z
formula weight
space group
T (°C)
1118.79
R3c
1137.83
R3c
−143
1.339
−173
1.358
Dcalcd (g cm−3
μ (cm−1
)
)
4.867
0.940
R(F0) [I > 2σ(I)]
0.0598
0.0777
2
Rw(F0 )
0.1857
0.2180
crystallographic data are presented in Supporting Information, and
CIF files have been deposited with the Cambridge Crystallographic
Data Centre (CCDC 1545608, (NBu4)2[Fe2(fan)3] and 1545609,
(NBu4)2[Zn2(fan)3]).
Mossbauer Spectroscopy. 57Fe Mossbauer spectra were
̈
̈
collected on a spectrometer from Science Engineering & Education
(SEE) Co., MN, equipped with a closed cycle refrigerator system from
Janis Research Co. and Sumitomo Heavy Industries Ltd. Data were
collected in constant acceleration mode in transmission geometry. The
zero velocity of the Mossbauer spectrum refers to the centroid of the
̈
room-temperature spectrum of a 25 μm metallic iron foil. Analysis of
the spectra was conducted using the WMOSS program (SEE Co.,
formerly WEB Research Co. Edina, MN).
Magnetism. Magnetic susceptibility measurements were made
using a Quantum Design MPMS 7 Squid instrument in a direct-
current (dc) field of 1 T, with accurately weighed samples of ∼25 mg
placed in a calibrated gelatin capsule that was held in the center of a
soda straw, the latter fixed to the sample rod. The instrument was
calibrated with a palladium pellet (Quantum Design) or with a
chemical calibrant such as CuSO4·5H2O or HgCo(NCS)4. The
magnetization measurements were made on the same instrument with
scan rate of 680 Oe/step employed between −5000 and 5000 Oe in
Hyst mode, and scan rate of 3000 Oe/step between and −50 000 and
−5000 Oe and between 5000 and 50 000 Oe.
Conductivity. Conductivity measurements were performed on
Quantum Design Physical Property Measurement System (PPMS) on
pelletized samples using a linear four-point electrode. The temperature
C
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Figure 1. Structure of 1 showing (a) the coordination environments of the Fe centers, C black, O red, F green, Fe1 blue, Fe2 pink; Fe2 is located on
a crystallographic threefold axis. (b) One of two (10,3)-a networks. (c) Schematic representation of the two independent (10,3)-a nets; spheres
represent Fe centers (nodes), and connections represent bridging fan anions. (d) Space-filling representation of the two anionic networks (green and
purple). (e) Space-filling representation of the two networks and the tetrabutylammonium countercations (red and blue).
(NBu4)2[Zn2(dhbq)3], all metal centers are symmetry related
with each lying on a threefold axis.
As indicated in the Introduction, Long and co-workers
(NBu4)2[Fe2(dhbq)3] are Fe(III). Estimation of the oxidation
state of the Fe center in (NBu4)2[Fe2(dhbq)3] using bond
valence sum (BVS) calculations21 (+2.98) are consistent with
the spectroscopic assignment of the +3 oxidation state for the
spectroscopically showed that the Fe centers in
D
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Fe centers (see Supporting Information). Similarly, the Fe
centers in (NEt4)2[M2(can)3]15 and (Me2NH2)2[M2(can)3]12
are also close to +3 (+2.97 and +2.96, respectively). The crystal
structure determination performed on (NBu4)2[Fe2(fan)3], 1,
reveal average Fe1−O and Fe2−O distances of 2.046 and 2.065
Å, respectively, which are significantly longer than the average
Fe−O distances in (NBu4)2[Fe2(dhbq)3] of 2.018 Å. BVS
calculations based on the crystal structure of 1 lead to an
estimation of a +2.76 oxidation state for Fe1 and a +2.61
oxidation state for Fe2. Given that there are 3 times as many
Fe1 centers as Fe2 centers, the average oxidation state of the Fe
in 1 is calculated to be +2.72. The average oxidation state of the
anionic ligand must therefore lie somewhere between −2 and
−3. With the BVS calculations for 1 suggesting that all Fe
centers are intermediate between the +2 and +3 oxidation
states, the formula of 1 may be formally represented as
(NBu4)4[FeIIFeIII3(fanII−)3(fanIII−)3]. This formulation would
suggest that the average oxidation state of the fluoranilate is
−2.5 in contrast to (NBu4)2[Fe2(dhbq)3], where the average
oxidation state of the dhbq is −2.67. In the (NBu4)4[FeIIFeIII
-
3
(fanII−)3(fanIII−)3] formulation, the average oxidation state of
the Fe would be +2.75. While the proposed formulation of
(NBu4)4[FeIIFeIII3(fanII−)3(fanIII−)3] is only tentative, the
structural results suggest an average oxidation state for the Fe
of somewhere between +2.5 and +3.0.
While the strong structural similarities that exist between 1
and 2 are clearly apparent, there are subtle but significant
differences between the two structures that largely arise from
the fact that the zinc center is limited to the +2 oxidation state.
To achieve charge balance in (NBu4)2[Zn2(fan)3] the
fluoranilate dianion must be in the −2 oxidation state. Ligand
bond distances for the dianion are consistent with the adoption
of the quinone form of the ligand. In particular, the C−C
separations within the five-membered chelate rings (1.525(10),
1.533(10) Å) are consistent with a formal single bond. In the
case of 1 the corresponding bond distances are 1.494(10) and
1.486(10) Å, which indicate some multiple bond character,
consistent with partial reduction of the dianion.
Figure 2. Variable-temperature 57Fe Mossbauer spectra of 1 measured
̈
in the presence of a weak magnetic field of 47 mT applied parallel to
the γ-rays. The spectra can be deconvoluted into a mixture of slow
relaxing (black dotted, spectrum measured at 5.5 K), intermediate
relaxing (red), and fast relaxing (blue) high-spin iron(III). The total fit
is given by the solid black lines through the hatch marks.
Table 2. Temperature-Dependent 57Fe Mossbauer
̈
a
Parameters Obtained for 1
T (K)
δ (mm/s)
ΔEQ (mm/s)
ΓL(R) (mm/s)
̈
Mossbauer Spectroscopy. The results from the BVS
293
0.56 0.02
0.55 0.05
0.59 0.01
0.55 0.03
0.59 0.01
0.59 0.01
1.22 0.04
0
0.59(0.59)
0.70
calculations coupled with investigations of the related dhbq
network, (NBu4)2[Fe2(dhbq)3], prompted further examination
of the oxidation state of the Fe centers in 1 using variable-
b
b
200
1.24 0.02
0
0.39(0.43)
0.60
temperature 57Fe Mossbauer spectroscopy. The spectra are
̈
100
50
1.27 0.03
1.28 0.02
0.38(0.45)
0.42(0.50)
given in Figure 2, and the parameters are in Table 2. At 293 K
the spectrum shows the presence of at least two components,
namely, a broad quadrupole doublet with parameters (δ = 0.56,
ΔEQ = 1.22 mm/s) that is consistent with high spin iron(III),
and a broad singlet (δ = 0.55 mm/s) that indicates intermediate
relaxation.
a
All spectra contain a proportion of a broad magnetic spectrum caused
b
by ferromagnetic coupling that is fully observed at 5.5 K. This appears
to be caused by intermediate relaxation.
The absence of a subspectrum describing iron(II) suggests
that the system is valence-delocalized and that the high-spin
iron(III) quadrupole doublet actually describes a ca. +2.7
oxidation state as supported by the other data. The large half
height width of the quadrupole doublet could perhaps be
explained by the presence of multiple sites, which cannot be
separated, but as the temperature is lowered, both the
intermediate relaxing singlet and the quadrupole doublet
broadens and turns into the very broad magnetic spectrum
collected when the sample is cooled to 5.5 K. It thus seems
more likely that the broadness is due to the intermediate
relaxation regime that the sample exhibits.
determine relative areas and aid fitting of the other two
components. With this method it can be seen that the
quadrupole doublet decreases in intensity, until at 5.5 K the
system is fully ferromagnetic. This magnetic transition occurs
over a wide temperature range (Figure 3) and is supported by
the magnetic studies (vide infra). The broad magnetic spectrum
collected at ∼5 K is indicative of ferromagnetic coupling with
an internal field B0 ≈ 50 T.
The parameters of the quadrupole doublet vary slightly with
temperature with a slight increase in quadrupole splitting, and
this can be ascribed to the second-order Doppler effect.22
Interestingly, the parameters obtained at 100 K are identical to
those also measured at 100 K for the related unfluorinated
compound described previously11 and similar to those reported
The magnetic spectrum measured at 5.5 K can be used as a
reference and subtracted from all other spectra to help
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dependent on field below 70 K, with a broad maximum of 33
cm3 mol−1 K, at 50 K, found in the 1 T field, becoming sharper
and higher in value, 111 cm3 mol−1 K, at 5 K, in the 0.0087 T
field. Such behavior is indicative of a combination of Zeeman
level thermal depopulation effects combined with either
magnetic coupling or magnetic order. For compound 1, a
χMT of 8.22 cm3 mol−1 K, per Fe2, at 300 K was observed,
which is a little lower than that predicted for the uncoupled
“two Fe site” formulation (NBu4)4[FeIIFeIII3(fanII−)3(fanIII−)3]
(vide supra) containing 1/2[HS FeII, HS FeIII × 3 and S = 1/2
(fanIII−) × 3] of 8.62 cm3 mol−1 K, per Fe2. The 300 K χMT
value observed for (NBu4)2[Fe2(dhbq)3] was 11.11 cm3 mol−1
K, significantly higher than in 1.11 It showed a similar plot to
that of 1, in a 0.1 T field, with a sharp maximum of 150 cm3
mol−1 K at ∼10 K, thus behaving, overall in a similar fashion to
1. However, it was formulated as (NBu4)2[FeIII2(dhbqII−)-
(dhbqIII−)2], which would give a predicted χMT of 9.5 cm3
mol−1 K, per Fe2, at 300 K assuming zero/very weak coupling
at this temperature. Likewise, an even higher χMT of 16.6 cm3
mol−1 K, per Fe2, was reported, at 300 K, for (Me2NH2)2
[FeIII2(canII−)(canIII−)2]·2H2O·6DMF and ascribed to long-
range strong magnetic coupling. Interestingly, it decreased to
9.37 cm3 mol−1 K, at 300 K, for the desolvated species.12
When the temperature was decreased, the gradual increase in
χMT for 1 is usually indicative of ferromagnetic coupling, with
the rapid increase below 70 K indicative of short- or long-range
ordering of Fe and radical spins. The χMT maximum value in
the 1 T field of 33 cm3 mol−1 K, per Fe2, is as predicted for
ferromagnetic coupling yielding a ground-state spin S = [5/2 +
5/2 + 5/2 + 4/2 + 1/2 + 1/2 + 1/2] = 22/2, per Fe4). The
much higher χMT maxima in lower fields lend support for
magnetic ordering occurring. The decrease in χMT below the
maximum can originate from Zeeman level depopulation effects
or from anti-ferromagnetic coupling occurring at very low
temperatures.
Figure 3. Proportion of each species determined by 57Fe Mossbauer is
plotted against temperature. The slow-relaxing iron(III) ( ) is caused
̈
○
by ferromagnetic coupling between iron centers. The fast-relaxing
□
iron(III) ( ) is a quadrupole doublet consistent with high-spin
iron(III), and intermediate relaxation is observed as a broad singlet
△
(
). Both of these species converts into the slow-relaxing
subspectrum. Spectra are presented in Figure 2, and parameters are
given in Table 2.
at 80 K for (Me2NH2)2[Fe2(can)3]·2H2O·6DMF, the latter
showing a lower value of ΔEQ (1.059(2)),12 which increased a
little upon desolvation.
Magnetism. It is well-recognized that the tetraoxolene class
of ligands are able to mediate magnetic interaction between
metals. Discrete binuclear complexes in which metal centers are
linked by anionic forms of H2dhbq and H2can have
demonstrated the ability to exist in a mixed-valence state,
whereby valence-tautomerism occurs via redox modulation of
the ligand via their radical trianion state.23,24 2D coordination
polymers of the form [M2(C6O4X2)3]n+ have been shown to
exhibit long-ranged ferri-25,26 or ferromagnetic coupling
mediated by the di-anionic form of (C6O4X2)2−, while an
example of a 3D framework of diamond-topology with the
formula Na5[HoIII(THBIV−)2]·7H2O (H4THB = 1,2,4,5-
tetrahydroxybenzene), which remarkably incorporates the
fully reduced form of dhbq de novo, was shown to be
ferromagnetic.27 In view of the crystal structure of 1, it was
anticipated that a strong electronic communication between
adjacent iron centers via fan2−/fan3− would give rise to long-
range and ferromagnetic magnetic ordering.
Magnetization isotherms were measured on 1 to help
delineate the type of magnetic order (Figure 4; inset). They
show close to saturation behavior at 2 K, above H ≈ 4 T, with
M ≈ 7.3 Nβ. This is well below the ∼11 Nβ expected for S =
11/2, per Fe2, using the (NBu4)4[FeIIFeIII3(fanII−)3(fanIII−)3]
formulation (see above) and a little lower than 8 Nβ expected
for a S = 4 ground state (S = 5/2 + 5/2−1/2−1/2 = 8/2) if
antiferrimagnetic coupling was occurring in
a
Temperature-dependent magnetic studies of 1 under applied
fields of 0.0087, 0.1, and 1 T were conducted, and the graph of
χMT versus T is presented below (Figure 4). The data are
independent of the field value between 70 and 300 K but
(NBu4)2[FeIII2(dhbqII−)(dhbqIII−)2] type formulation.11 Inter-
estingly, the dhbq compound also showed Msat of 7.2 Nβ. The
solvated chloranilate analog reached M = 8.2 Nβ in H = 7 T at
1.8 K,12 close to the S = 4 ground state value, and showed
hysteresis with a much bigger coercive field of 2630 Oe, at 1.8
K. The coercive field increased to 4650 Oe for the desolvated
chloranilate.
A common way to check for long-range (3D) magnetic order
is to measure the field-cooled (FCM) and zero-field cooled
magnetization (ZFCM) in small dc fields, at low temperatures,
with bifurcation in FCM and ZFCM indicative of long-range
order. These plots (Figure S5) overlay for 1, thus indicating
that only short-range order is occurring down to 2 K. In
contrast, (Me2NH2)2 [FeIII2(canII−)(canIII−)2]·2H2O·6DMF
bifurcation of FCM and ZFCM below 80 K is indicative of
spontaneous magnetization due to dominant intralayer anti-
ferromagnetic interactions.12 Recently, Harris et al. reported the
o n e - e l e c t r o n
r e d u c e d
c o m p l e x
(Cp2Co)1.43(Me2NH2)1.57[Fe2(can)3]·4.9DMF, which contains
Figure 4. Plot of χMT vs temperature and a magnetization vs field plot
(inset) of 1 at 2 and 10 K.
the framework composition [FeIII2(canIII−)3]3−, and showing a
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χMT value of 26.5 cm3 mol−1 K at 300 K much higher than the
9.875 cm3 mol−1 K uncoupled value. This was due to strong
magnetic ordering evidenced by bifurcation of FCM and
ZFCM at 115 K and large coercive fields (4520 Oe at 1.8 K) in
the magnetization isotherms, the coupling being stronger than
that in (Me2NH2)2[FeIII2(canII−)(canIII−)2]·2H2O·6DMF.13
The short-range order in 1, also noted in
(NBu4)2[Fe2(dhbq)3],11 is possibly due to competing ferro-
magnetic and anti-ferromagnetic interactions that prevents 3D
order. It is tempting to ascribe such behavior, in part, to
through-space interactions between the interpenetrated sub-
lattices in 1, but we have no direct evidence for this. The
magnetic interactions occur via the radical-bridged FeII/III
centers in the individual (10,3)-a networks.
oxidation states is neatly illustrated by some elegant work
undertaken by Dei, Gatteschi, and co-workers in 1991.28 In a
discrete binuclear complex in which a pair of Fe(II) centers are
linked by a dhbq dianion, a one-electron oxidation of the
complex resulted in oxidation of both Fe(II) centers to Fe(III)
accompanied by a reduction of the bridging dianion to the
radical trianion. Thus, while it may be tempting to ascribe the
reduction and oxidation processes at −0.80 and −0.65 V,
respectively, to ligand-based processes, such changes in
oxidation state may also be accompanied by changes in the
oxidation state of the Fe centers. The solid-state CV for 2
(Figure S8) fails to provide a useful comparison, as the only
clearly defined process is an irreversible oxidation process at
−0.65 V. The electrochemical results are consistent with
decomposition and dissolution of 2.
Electrochemical and Spectroscopic Properties. Solid-
state dc cyclic voltammetry was performed on 1 in 0.1 M KCl/
H2O electrolyte at multiple scan rate (Figure S7); a
representation of the 200 mV s−1 scan is presented in Figure
5. In the cyclic voltammograms (CVs) of 1, two broad quasi-
The broadness, together with the shifting of processes at
higher scan rates, suggests the observed processes for 1 and 2
are surface-confined as a result of the lack of diffusion of
counterions necessary for charge balance of the framework as
observed in similar systems.10 The results presented here
illustrate the stability of 1 to electrochemical treatment,
whereby the framework was able to undergo multiple redox
cycles while retaining reversibility in the solid state. Thus, 1 was
further studied via solid-state spectro-electrochemistry to probe
the redox modulation in situ.
Solid-state SEC29 studies were conducted on 1 in 0.1 M
KCl/H2O electrolyte to investigate the optical properties. The
UV−vis−NIR spectrum of the as-synthesized species shows
three distinct features as shown in Figure 6; the broad bands
Figure 5. Solid-state CV of 1 at 200 mV in 0.1 M KCl/H2O
electrolyte.
reversible processes are evident from the cathodic sweep at
−0.25 and −0.80 V versus Ag/AgCl. While the process
occurring at −0.25 V is apparent at fast scan rates (1000 mV
s−1), it is barely apparent at slower scan rates, for example, 200
mV s−1. The second reduction process (−0.80 V) is clearly
present at all scan rates. The return anodic sweep shows two
distinct oxidation processes at −0.65 and 0.25 V.
The CVs are somewhat more complicated than that obtained
for the compound (PMePh3)2[Cd2(dhbq)3] reported in 2012
in which octahedral Cd(II) centers are linked by dhbq2− anions
within a 2D hexagonal network.10 In the case of the Cd
network the CV indicated a reduction at −0.68 V and return
oxidation at −0.51 V, which was attributed to the reversible
reduction of the dhbq2− ions in the network to their
tetraanionic form. The solution-state CV for fluoranilic acid
shows similar redox behavior with reduction at −0.19 V and a
return oxidation at 0.01 V (Figure S6). In the case of 1 the
accessibility of two stable oxidation states for Fe and redox
behavior of the fluoranilate makes assignment of the electro-
chemical processes more challenging, not just because an
additional component of the network is redox-active (Fe) but
because the iron centers and fluoranilate are not expected to act
in isolation from each other. The interdependence of the
Figure 6. Solid-state SEC of 1 in 0.1 M KCl/H2O electrolyte and
pictures (inset) of 1 undergoing progressive stages of reduction at the
indium tin oxide interface (right). Arrows indicate the direction of the
spectral progression. The break in the spectrum denotes the region of
the detector changeover, which was removed for clarity.
that appear at ca. 17 500 and 21 000 cm−1 are tentatively
assigned to π−π* and metal-to-ligand charge transfer (MLCT)
transitions, respectively, while the intense band observed in the
near-IR region (ca. 7000 cm−1) is tentatively assigned to a
ligand-based IVCT band. Application of a reductive potential of
−0.9 V leads to the decrease in the bands and overall
absorbance of the sample. The decrease of the IVCT band may
be rationalized as the result of reduction of the fluoranilate
ligand from fan2− to fan4− via the radical trianion form (fan3−).
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A notable color change was observed upon reduction of the
material from dark brown/black to a light yellow/maroon
(Figure 6, inset) providing evidence supporting the reduction
of 1. Attempts to reoxidize the sample through application of
an oxidative potential led to an incomplete regeneration of the
original spectrum (see Supporting Information, Figure S10).
The observation of the IVCT band in 1 is consistent with a
rare mixed-valence state,30 whereby the Fe centers and fan
ligands possess noninteger redox states. Postsynthetic reduction
studies were undertaken to further examine the redox activity of
1.
Chemical Reduction. To verify the SEC experiments,
chemical reduction of 1 and 2 was performed ex situ through
the addition of excess lithium napthalenide (1 equiv) under an
inert atmosphere. ICP-OES studies revealed that 1 could be
reduced to Li0.27{(NBu4)2[Fe2(fan)3]} (1·Reduced). The
powder X-ray diffraction (PXRD) pattern (Figure S2) of 1·
Reduced indicates that the structure remains unchanged with
only minor deterioration in the crystallinity of the sample
apparent upon reduction. The attempted reduction of 2 with
lithium naphthalenide leads to a significant loss in the
crystallinity of the sample as evident from the PXRD (Figure
S3) and a color change of 2 from violet to light yellow. This is
consistent with the formation of the tetraanion form of
fluoranilate and decomposition of the framework to form an
unidentified residual product.
The solid-state UV−vis−NIR spectrum of 1·Reduced
(Figure 7a) revealed spectral changes akin to those found in
the SEC measurements, which showed the IVCT band in the
NIR region diminishing and the broad MLCT bands decreasing
in intensity upon electrochemical reduction. Interestingly, the
vis−NIR spectrum of 2 (Figure 7a) displays a complete absence
of the low-energy IVCT band and MLCT band present in 1,
further supporting the evidence that partial charge transfer
between ligands is mediated by iron, which provides a more
optimal frontier-orbital overlap than zinc. In addition, solid-
state EPR studies (Figure 7b) on 1 revealed the presence of a
radical signal (g = 2.007 35) attributed to the radical trianion,
fan3−. Upon chemical reduction to 1·Reduced, a radical signal
persists (g = 2.000 43). We propose that the intercalation of Li+
ions, for charge balance, is accompanied by the generation of a
greater proportion of fan3−. In contrast, the EPR spectrum of 2
indicates the absence of radical species.
Figure 7. (a) UV−vis−NIR spectrum of 1 (black), 1·Reduced (red),
and 2 (blue). (b) EPR spectrum of 1 (black) and 1·Reduced at room
temperature.
The differences between 1 and 1·Reduced were investigated
using Raman spectroscopy (Figure S11). A spectrum of each
compound was obtained using an 830 nm laser. Very little
difference exists between the spectra for the two compounds
except for a peak at 1650 cm−1 in the case of 1·Reduced. The
spectrum for fluoranilic acid (Figure S12) is significantly
different than 1 and 1·Reduced, although it does show a strong
peak at 1650 cm−1.
Electrical Conductivity. The elucidation of the electro-
chemical and optical properties of 1 encouraged us to explore
the electronic properties of the framework. Investigation of the
temperature-dependent conductivity via the four-point contact
method on a pelletized sample of 1 (Figure 8) revealed
semiconducting behavior at room temperature (ca. 5.5 × 10−3
S/cm). An Arrhenius fit of the conductivity data (Figure S13)
showed that 1 possesses an activation energy of just 124 meV.
I−V curve measurements were conducted via four-point
contact probe on a pressed pellet of 1 (Figure S15). The
room-temperature conductivity was found to be 1.77 × 10−3 S/
cm and Ohmic in the scan range of 10 mV. The bulk
Figure 8. Variable-temperature conductivity of 1 between 165 and 300
K.
conductivity of 1, however, deviates from the compound
reported by Long et al.11 This discrepancy may indicate a larger
number of defects or grain boundaries present in our sample.
H
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We may also postulate the lower conductivity of 1 to arise from
the lower crystal symmetry setting (rhombohedral) as
compared to the cubic structure of (NBu4)2[Fe2(dhbq)3].
From an anisotropy perspective our (10,3)-a framework may
introduce a directional factor, commonly encountered in
“organic metal” complexes that may explain the observed
electrical conductivity. Additionally, the lower conductivity
observed for 1 in contrast to (NBu4)2[Fe2(dhbq)3] could be
the result of air oxidation of fan3− in the as-synthesized
framework of 1 to fan2−. Finally, variation in the devices used
for conductance measurements as well as the environment can
significantly impact upon conductance measurements. As Dinca
and co-workers have clearly stated with respect to conductiv-
ities of MOF-type materials, “accurate measurements of
intrinsic electrical conductivity are challenging and fraught
with potential sources of errors.”31 They further indicate that
caution needs to be exercised when comparing measured
conductivities that vary by up to 2 orders of magnitude.
(Me2NH2)2[FeIII2(can)3] show many similarities but also
significant differences. The similarities in 1 and
(NBu4)2[FeIII2(dhbq)3], that is, short-range magnetic order,
derive from their similar crystal structures and metal/ligand
electronic structures, although the latter also has differences
because of the different Fe oxidation states and bridging ligand
II−/III− states proposed for 1 and for the dhbq analogue.
While (Me2 NH2)2[FeI II 2(can)3]·2H2O·6DMF and
(NBu4)2[FeIII2(dhbq)3] have similar Fe and ligand oxidation
states, the long-range order in (Me2NH2)2 [FeIII2(can)3]·2H2O·
6DMF, a 2D magnet with Tc = 80 K, is much stronger than it is
in (NBu4)2[FeIII2(dhbq)3]. It is even stronger in the compound
(Cp2Co)1.43(Me2NH2)1.57[Fe2(can)3]·4.9DMF that contains
three can3− radicals; a possible bulk 2D ferrimagnet with Tc
= 105 K. Nevertheless, patterns can be discerned in the
magnetic behaviors of these above-mentioned [Fe2L3]n−‑
framework family members that relate to the FeII/III and ligand
LII−/III− oxidation states and relative ratios thereof, as well as to
subtle structural differences. Furthermore, changing the
substituents on the anilate ligand has a significant effect upon
iron site symmetry and the magnetic features of members of
this structurally similar family. In regard to the magnetic
features, we note that Atzori et al.32 reported the influence of
changing the substituent on the bridging, diamagnetic anilate
ligand in isostructural 2D molecular ferrimagnets of type
A[MnIICrIII(X2An)3+]·G (A = [(H3O)(phz)3]+ (phz =
̆
In a discussion of conductivity by Long and co-workers11 it
was suggested that the conductivity of (NBu4)2[Fe2(dhbq)3] is
linked to electron hopping between the dhbq2−/dhbq3−
manifold. The excess of electron-rich dhbq3− ligands over the
electron-deficient dhbq2− ligands is considered to be less than
ideal as far conductivity is concerned. This is supported by the
observation that
a chemically reduced form,
Na0.9(NBu4)1.8[Fe2(dhbq)3], which possesses a higher propor-
tion of the trianion radicals, has a significantly lower
conductivity than (NBu4)2[Fe2(dhbq)3]. Long and co-workers
further proposed that partial oxidation of (NBu4)2[Fe2(dhbq)3]
to generate a material in which the relative proportions of
dhbq3− and dhbq2− are similar may yield a compound with
superior conductivity. On this basis, compound 1, with the
t e n t a t i v e l y p r o p o s e d f o r m u l a t i o n o f
(NBu4)4[FeIIFeIII3(fanII−)3(fanIII−)3], involving equal numbers
of fan2− and fan3− ligands, might have been expected to be a
significantly better conductor than our results indicated.
phenazine) or NBu4 ; X2An2− = C6O4X2 = 2,5-dihydroxy-
2−
1,4-benzoquinone derivative dianion; X = Cl, Br, I, H; G =
water or acetone. The ordering temperature changed from 5.5
to 6.3, 8.2, and 11.0 K (for X = Cl, Br, I, and H, respectively). A
plot of Tc versus the electronegativity of X showed a clear linear
correlation that was explained by the electron-withdrawing
effect of X: as the electronegativity of X increases, the electron
density in the anilato ring decreases, resulting in a weaker
coupling and, therefore, in a lower Tc, thus allowing the tuning
of the magnetic order. Further discussion of the effect of
substituents on anilate ligands is presented in a recent review by
Mercuri et al.33 In the present [Fe2(X2An)3]2− family, involving
X2An2− and X2An3− bridging forms, where X = F (fan; present
work), Cl (can), and H (dhbq) the substituents range from the
most electronegative, F, to the least, H. Unfortunately, because
of the many contributing factors such as the effects of cation
upon network structures, and the mixed-valent nature of the Fe
and bridging ligand centers in family members, it is not possible
to detect clear correlations with variation in X groups. For
example, (NBu4)4[FeIIFeIII3(fanII−)3(fanIII−)], 1, and
(NBu4)2[FeIII2(dhbqII−)(dhbqIII−)2] show similar magnetism
and degree of magnetic coupling.
Also with respect to the magnetic measurements, note that
the coexistence of semiconductivity, with attendant band-
structure, and molecular magnetism in the fan and dhbq
materials means that itinerant electrons do not dominate the
magnetic features.
Electrochemical and spectro-electrochemical investigations in
addition to chemical reduction reveal that the fluoranilate
ligands in both 1 and 2 are susceptible to reduction. In the case
of 1 the reduction is accompanied by a decrease in the intensity
of the absorption bands in the UV−vis−NIR spectrum, which
is consistent with the partial conversion of the fluoranilate ion
to its aromatic tetraanionic form, which is colorless.
CONCLUSIONS
■
Although 1 and 2 share a similar composition and structure in
which octahedral metal centers are linked by fluoranilate
ligands, there is a stark contrast in the physical behavior of the
two compounds. While the Zn(II) centers in
(NBu4)2[Zn2(fan)3] are clearly linked by fluoranilate dianions,
Mossbauer spectroscopy indicates that Fe centers are in an
̈
oxidation state close to +3 in 1, and as a result the average
oxidation state of the fluoranilate lies between −2.5 and −3.0.
While Fe−O bond distances, determined by X-ray crystallog-
raphy, also support the assignment of an oxidation state for the
Fe centers that is clearly greater than +2, they are slightly longer
than the corresponding bond lengths in (NBu4)2[Fe2(dhbq)3]
with BVS calculations suggesting an average oxidation state for
the Fe centers of ca. +2.75. On this basis the formula of the
c o m p o u n d c o u l d b e r e p r e s e n t e d a s
(NBu4)4[FeIIFeIII3(fanII−)3(fanIII−)3]. This formulation con-
trasts with that proposed for the dhbq analogue, which may
be represented as (NBu4)2[FeIII2(dhbqII−)(dhbqIII−)2] accord-
ing to both Mossbauer spectroscopy11 and BVS calculations. In
̈
making these comparisons it is important to recognize that BVS
calculations provide only a crude estimation of oxidation states,
and as a consequence, the assignment of the oxidation states in
(NBu4)2[Fe2(fan)3] should be viewed with caution.
Conductivity measurements have unambiguously indicated
that 1 is a semiconductor; however, the conductivity at room
temperature (5.5 × 10−3 S/cm) is significantly less than that
The magnetic behaviors of 1 and (NBu4)2[FeIII2(dhbq)3],11
( M e 2 N H 2 ) 2 [ F e I I I 2 ( c a n ) 3 ] · 2 H 2 O · 6 D M F , 1 2 a n d
I
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Inorganic Chemistry
Article
found for (NBu4)2[FeIII2(dhbq)3]. Nevertheless, in both
compounds it would seem that the mixed valency of the
ligands is an important feature, and this provides a basis for
designing new coordination polymers that are electrical
conductors.
Present Address
△School of Chemistry, Bio21 Molecular Science and
Biotechnology Institute, 30 Flemington Road, The University
of Melbourne, Parkville Victoria 3052, Australia.
Notes
This current work provides a strong indication that
coordination networks derived from dhbq and related ligands
have outstanding potential as materials possessing unusual and
possibly useful magnetic and electronic properties. The ability
of dhbq-type ligands to exist in multiple oxidation states within
a network offers the prospect of mixed valency, which clearly
impacts upon the electrical conductivity of the solid material.
As this current work has demonstrated, the incorporation of
metal ions that have multiple stable oxidation states will also
have a significant effect on the physical properties of the
network material. The opportunity to include a wide variety of
organic countercations provides a means for controlling
network topology, while the use of a counterion that is
redox-active may allow the cation to participate in electron
transfer processes with the anionic network. Porosity is another
feature that may be controlled by choice of cation, with open
channels allowing the introduction or removal of ions upon
changes in the oxidation state of the anionic network. The
presence of pores in the crystal structure provides an
opportunity for introducing electron donors or acceptors as
guests that may modulate the electronic and magnetic behavior
of the network.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the financial support of the
Australian Research Council. Part of this research was
undertaken on the MX1 beamline at the Australian
Synchrotron.
REFERENCES
■
(1) Batten, S. R., Neville, S. M., Turner, D. R. Coordination Polymers:
Design, Analysis and Application; RSC, 2008.
(2) Sun, L.; Campbell, M. G.; Dinca, M. Electrically Conductive
Porous Metal-Organic Frameworks. Angew. Chem., Int. Ed. 2016, 55
(11), 3566−79.
(3) D’Alessandro, D. M. Exploiting redox activity in metal-organic
frameworks: concepts, trends and perspectives. Chem. Commun.
(Cambridge, U. K.) 2016, 52 (58), 8957−71.
(4) Miyasaka, H. Control of Charge Transfer in Donor/Acceptor
Metal−Organic Frameworks. Acc. Chem. Res. 2013, 46 (2), 248−257.
(5) Faust, T. B.; D’Alessandro, D. M. Radicals in metal-organic
frameworks. RSC Adv. 2014, 4 (34), 17498−17512.
(6) Leong, C. F.; Usov, P. M.; D’Alessandro, D. M. Intrinsically
conducting metal−organic frameworks. MRS Bull. 2016, 41 (11),
858−864.
̆ ́
(7) Dinca, M.; Leonard, F. Metal−organic frameworks for electronics
ASSOCIATED CONTENT
■
and photonics. MRS Bull. 2016, 41 (11), 854−857.
S
* Supporting Information
(8) Kitagawa, S.; Kawata, S. Coordination compounds of 1,4-
dihydroxybenzoquinone and its homologues. Structures and proper-
ties. Coord. Chem. Rev. 2002, 224 (1−2), 11−34.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01038.
(9) Abrahams, B. F.; Hudson, T. A.; McCormick, L. J.; Robson, R.
Coordination Polymers of 2,5-Dihydroxybenzoquinone and Chlor-
anilic Acid with the (10,3)-a Topology. Cryst. Growth Des. 2011, 11
(7), 2717−2720.
Details of experimentation, bond-valence-sum calcula-
tions, tabulated data including R0 and B values, estimated
Fe oxidation states, bond distances, crystal data and
structure refinement, thermogravimetric and differential
(10) Abrahams, B. F.; Bond, A. M.; Thanh, H. L.; McCormick, L. J.;
Nafady, A.; Robson, R.; Nguyen, V. Voltammetric reduction and re-
oxidation of solid coordination polymers of dihydroxybenzoquinone.
Chem. Commun. 2012, 48 (93), 11422−11424.
(11) Darago, L. E.; Aubrey, M. L.; Yu, C. J.; Gonzalez, M. I.; Long, J.
R. Electronic Conductivity, Ferrimagnetic Ordering, and Reductive
Insertion Mediated by Organic Mixed-Valence in a Ferric Semiquinoid
Metal−Organic Framework. J. Am. Chem. Soc. 2015, 137 (50), 15703−
15711.
scanning calorimetric analyses, PXRDs, 57Fe Mossbauer
̈
spectrum, plotted magnetic data, cyclic and square-wave
voltammograms, Raman spectra, Tauc plots, Arrhenius
plots, I−V plots, additional references (PDF)
Accession Codes
CCDC 1545608−1545609 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(12) Jeon, I.-R.; Negru, B.; Van Duyne, R. P.; Harris, T. D. A 2D
Semiquinone Radical-Containing Microporous Magnet with Solvent-
Induced Switching from Tc = 26 to 80 K. J. Am. Chem. Soc. 2015, 137
(50), 15699−15702.
̆
(13) DeGayner, J. A.; Jeon, I.-R.; Sun, L.; Dinca, M.; Harris, T. D. 2D
Conductive Iron-Quinoid Magnets Ordering up to Tc = 105 K via
Heterogenous Redox Chemistry. J. Am. Chem. Soc. 2017, 139 (11),
4175−4184.
(14) Halis, S.; Inge, A. K.; Dehning, N.; Weyrich, T.; Reinsch, H.;
Stock, N. Dihydroxybenzoquinone as Linker for the Synthesis of
Permanently Porous Aluminum Metal−Organic Frameworks. Inorg.
Chem. 2016, 55 (15), 7425−7431.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: bfa@unimelb.edu.au. Phone: +61 3 8344 0341. Fax:
+61 3 9347 5180. (B.F.A.)
*E-mail: deanna.dalessandro@sydney.edu.au. Phone +61 2
9351 3777. Fax: +61 3 9351 3329. (D.M.D.)
*E-mail: r.robson@unimelb.edu.au. Phone: +61 3 8344 6469.
Fax: +61 3 9347 5180. (R.R.)
(15) Kingsbury, C. J.; Abrahams, B. F.; D’Alessandro, D. M.; Hudson,
T. A.; Murase, R.; Robson, R.; White, K. F. Role of NEt4+ in Orienting
and Locking Together [M2lig3]2− (6,3) Sheets (H2lig = Chloranilic or
Fluoranilic Acid) to Generate Spacious Channels Perpendicular to the
Sheets. Cryst. Growth Des. 2017, 17 (4), 1465−1470.
̈
(16) Wallenfels, K.; Friedrich, K. Uber Fluorchinone, II. Zur
ORCID
Hydrolyse und Alkoholyse des Fluoranils. Chem. Ber. 1960, 93 (12),
Brendan F. Abrahams: 0000-0003-2957-860X
3070−3082.
J
DOI: 10.1021/acs.inorgchem.7b01038
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(17) Connelly, N. G.; Geiger, W. E. Chemical Redox Agents for
Organometallic Chemistry. Chem. Rev. 1996, 96 (2), 877−910.
(18) Sheldrick, G. SHELXT - Integrated space-group and crystal-
structure determination. Acta Crystallogr., Sect. A: Found. Adv. 2015,
71, 2053−2733.
(19) Sheldrick, G. Crystal structure refinement with SHELXL. Acta
Crystallogr., Sect. C: Struct. Chem. 2015, 71 (1), 3−8.
(20) Farrugia, L. WinGX and ORTEP for Windows: an update. J.
Appl. Crystallogr. 2012, 45 (4), 849−854.
́
(21) Gagne, O. C.; Hawthorne, F. C. Comprehensive derivation of
bond-valence parameters for ion pairs involving oxygen. Acta
Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2015, 71, 562−578.
(22) Gutlich, P.; Bill, E.; Trautwein, A. X. Mossbauer Spectroscopy and
̈
̈
Transition Metal Chemistry: Fundamentals and Applications; Springer
Verlag, 2011; p 569.
(23) Li, B.; Chen, L.-Q.; Tao, J.; Huang, R.-B.; Zheng, L.-S.
Unidirectional Charge Transfer in Di-cobalt Valence Tautomeric
Compound Finely Tuned by Ancillary Ligand. Inorg. Chem. 2013, 52
(8), 4136−4138.
(24) Min, K. S.; DiPasquale, A. G.; Golen, J. A.; Rheingold, A. L.;
Miller, J. S. Synthesis, Structure, and Magnetic Properties of Valence
Ambiguous Dinuclear Antiferromagnetically Coupled Cobalt and
Ferromagnetically Coupled Iron Complexes Containing the Chlor-
anilate(2−) and the Significantly Stronger Coupling Chlorani-
late(•3−) Radical Trianion. J. Am. Chem. Soc. 2007, 129 (8), 2360−
2368.
(25) Abherve, A.; Manas-Valero, S.; Clemente-Leon, M.; Coronado,
E. Graphene related magnetic materials: micromechanical exfoliation
of 2D layered magnets based on bimetallic anilate complexes with
inserted [FeIII(acac2-trien)]+ and [FeIII(sal2-trien)]+ molecules. Chem.
Sci. 2015, 6 (8), 4665−4673.
(26) Abherve, A.; Clemente-Leon, M.; Coronado, E.; Gomez-Garcia,
C. J.; Verneret, M. One-Dimensional and Two-Dimensional Anilate-
Based Magnets with Inserted Spin-Crossover Complexes. Inorg. Chem.
2014, 53 (22), 12014−12026.
(27) Nakabayashi, K.; Ohkoshi, S.-i. Monometallic Lanthanoid
Assembly Showing Ferromagnetism with a Curie Temperature of 11
K. Inorg. Chem. 2009, 48 (18), 8647−8649.
(28) Dei, A.; Gatteschi, D.; Pardi, L.; Russo, U. Tetraoxolene radical
stabilization by the interaction with transition-metal ions. Inorg. Chem.
1991, 30 (12), 2589−2594.
(29) Usov, P. M.; Fabian, C.; D’Alessandro, D. M. Rapid
determination of the optical and redox properties of a metal-organic
framework via in situ solid state spectroelectrochemistry. Chem.
Commun. 2012, 48 (33), 3945−3947.
(30) D’Alessandro, D. M.; Keene, F. R. Current trends and future
challenges in the experimental, theoretical and computational analysis
of intervalence charge transfer (IVCT) transitions. Chem. Soc. Rev.
2006, 35 (5), 424−440.
̆
(31) Sun, L.; Park, S. S.; Sheberla, D.; Dinca, M. Measuring and
Reporting Electrical Conductivity in Metal−Organic Frameworks:
Cd2(TTFTB) as a Case Study. J. Am. Chem. Soc. 2016, 138 (44),
14772−14782.
(32) Atzori, M.; Benmansour, S.; Mínguez Espallargas, G.; Clemente-
Leon
́
, M.; Abherve,
́
A.; Gom
́
ez-Claramunt, P.; Coronado, E.; Artizzu,
F.; Sessini, E.; Deplano, P.; Serpe, A.; Mercuri, M. L.; Gom
́
ez García,
C. J. A Family of Layered Chiral Porous Magnets Exhibiting Tunable
Ordering Temperatures. Inorg. Chem. 2013, 52 (17), 10031−10040.
(33) Mercuri, M. L.; Congiu, F.; Concas, G.; Sahadevan, S. A. Recent
Advances on Anilato-Based Molecular Materials with Magnetic and/or
Conducting Properties. Magnetochem. 2017, 3 (2), 17.
K
DOI: 10.1021/acs.inorgchem.7b01038
Inorg. Chem. XXXX, XXX, XXX−XXX