Approaching the limit of CuII/CuI mixed valency in a CuIBr2-N-methylquinoxalinium hybrid compound

Article abstract of DOI:10.1039/c5cc04142b

A novel 1D hybrid salt (MQ)[CuBr₂]_∞ (MQ = N-methylquinoxalinium) is reported. Structural, spectroscopic and magnetic investigations reveal a minimal Cu^II doping of less than 0.1%. However it is not possible to distinguish Cu^I and Cu^II. The unusually close packing of the organic moieties and the dark brown colour of the crystals suggest a defect electronic structure.

Full text of DOI:10.1039/c5cc04142b

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COMMUNICATION
II
I
I
Approaching the limit of Cu /Cu mixed valency in a Cu Br /N-
2
methylquinoxalinium hybrid compound
Received 00th January 20xx,
Accepted 00th January 20xx
a
c
d
d
d
Nicolas Leblanc,* Stephen Sproules, Claude Pasquier,* Pascale Auban-Senzier, Helene Raffy
a,b
and Annie K. Powell,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
I
A novel 1D hybrid salt (MQ)[CuBr
reported. Structural, spectroscopic and magnetic investigations reveal
2
]
∞
(MQ= N-methylquinoxalinium) is of (MQ)[BF ] with NaBr and freshly prepared Cu Br leads to a
4
brown solution after cooling to room temperature. Upon slow
vapour diffusion of MeOH, dark brown crystals formed within a
few hours. Phase purity was checked by XRPD (ESI†).
The X-ray crystal structure of (MQ)[CuBr ] was measured at
2 ∞
room temperature (293K) and refined in the space group C2/m.
II
a minimal Cu doping of less than 0.1%. However it is not possible to
I
II
distinguish Cu and Cu . The unusually close packing of the organic
moieties and the dark brown colour of the crystals suggest a defect
electronic structure
.
The structure has infinite 1D [CuBr
sharing [CuBr ] distorted-tetrahedra,
stacks of N-methylquinoxalinium moieties (MQ). These lead to a
slightly distorted chessboard packing by virtue of the ≈97°
angle, when viewed along the b axis (Figure 1a). In the inorganic
2
]
∞
4cbh,5ains composed of edge-
separated by infinite
The chemistry of copper (I) halides has provided an impressive
library of compounds, characterized by their structural
diversity, In some case, owing to the oxidative instability of the
4
1
I
II
β
starting material Cu X (X=Cl, Br, I), pure Cu or mixed valence
Cu /Cu can result. In the case of mixed valency, depending on
I
II
2
I
II
part, the asymmetric unit contains one half copper lying on the
2
how differentiable the two Cu and Cu metal sites are
crystallographically, Robin & Day classified them in terms of
three different classes. In class I, the sites are fully
distinguishable, in class III indistinguishable, and in class II there
-fold axis (0,y,0) and two half bromide lying in the mirror plane
x,0,z) (Figure 1b).
(
3
is an intermediate situation.
In the field of 1D copper bromide based compounds, only a
(a)
(b)
4
few examples of mixed valency have been reported, and of
II
particular interest is the influence that the amount of Cu
present has on the electronic structure.
Here we present a novel 1D copper bromide hybrid compound
(
2 ∞
MQ)[CuBr ] (MQ= N-methylquinoxalinium) which has a tiny
II
amount of Cu (<0.1%), and yet shows properties consistent
with a mixed valent electronic structure.
Colourless crystals of the starting material (MQ)[BF ] were
obtained by recrystallization from MeOH at -20°C. Crystals of
(MQ)[CuBr2]∞ were obtained by the slow vapour diffusion
method. Dissolution in warm DMSO of a stoichiometric amount
Br
1
’
Br
1
4
(c)
(d)
2
Br ’
Br
1
0
a
Cu
Br2’
Br1’
Br1
Br
2
b
Br
2
1
Br ’
Br
2
2
Br ’
c
0
a.
Institut für Nanotechnologie, Karlsruher Institut für Technologie , Hermann- von-
c
Helmholtz-Platz 1,D-76344 Eggenstein-Leopoldshafen, Germany.
E-mail: nicolas.leblanc@partner.kit.edu; Tel: +49 (0)721-608-28922
Institut für Anorganische Chemie, Karlsruher Institut für Technologie,
b.
Figure 1: (a) General view of (MQ)[CuBr
perpendicular to the direction (20-1). The close packing within the organic network
and the two [CuBr chains related by the C-centred Bravais lattice are displayed.
c, d) Overlaid view with the projected symmetry elements of the C2/m space
2 ∞
] along the b axis. (b) - Partial view
Engesserstraβe 15, D-76131, Karlsruhe, Germany. E-mail: annie.powell@kit.edu;
Fax: +049 (0)721-608-48142; Tel: +049 (0)721-608-42135
c.
2 ∞
]
University of Glasgow, School of Chemistry, Joseph Black Building
University Avenue, Glasgow, G12 8QQ, Scotland.
5
A -14,
(
d.
group, along the b axis (c) and the a axis (d).
Laboratoire de Physique des Solides, UMR-CNRS 8502, Bat. 510, Université Paris
Sud, 91405 Orsay Cedex, France
Electronic supplementary information (ESI) available: Tables of crystal and X-Ray
†
The geometry of the coordination sphere is distorted from
ideal tetrahedral symmetry (T ) to give the D d symmetry, as
d 2
confirmed by the data from Table S1. The values of the
crystallographic files in CIF format, XRPD patterns, UV-Vis and IR spectra of
MQ)[CuBr and (MQ)[BF ], the additional EPR spectra of (MQ)[CuBr , and
Cyclic Voltammogram of (MQ)[BF ]. CCDC 1400365 and 1400362. For ESI and
(
]
2 ∞
4
2 ∞
]
4
crystallographic data in CIF or other electronic format see interligand dihedral angles average 80° (from 77.7 to 84.4°)
DOI: 10.1039/x0xx00000x
rather than the expected 90° for a perfect T symmetry. The
d
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values of the edge central and face-edge-face angles
significantly differ from the ideal T values of 109.5° (from 95° to
17°) and 70.53° (from 60.4° to 78.8°) respectively. Significant
d
1
changes also occur in the Cu-Br bond lengths and Cu-Br-Cu
bridging angles (Cu -Br =2.493 Å, Cu -Br -Cu =73.8° and Cu -
1
2
1
2
1
1
1 1 1 1
Br =2.535 Å, Cu -Br -Cu =84.7°). This leads to the Cu metal ion
being off-centred. This, in turn leads to alternating long-short-
long-short Cu-Cu distances of 3.415(1) Å and 2.995(1) Å
respectively, and to an increase of the Br -Cu -Br trans-angle to
1
1
2
Σrvdw(H…N)= 2.75 Å
Σrvdw(H…Br)= 3.05 Å
Σrvdw(Br…N)= 3.40 Å
1
17.5°.
Regarding the organic part of this hybrid structure, the
corresponding bond distances and angles of the MQ moiety are
2 ∞
Figure 2: Partial view of (MQ)[CuBr ] showing the hydrogen bonds (red dashed
listed in Table S2 and compared with those measured in some lines) and the Br-N distance (blue dashed line) between either two MQ or MQ and
Br moieties.
related compounds. According to the Cambridge Structural
Database only two other salts based on the MQ unit,
6
MQ)[TCNQ] and (MQ)
II
-[Fe (CN)
7
]}, have been reported up
(
to now but only (MQ) {H
2
{H
2
6
2 ∞
In terms of the synthesis, crystals of (MQ)[CuBr ] were
]} has been characterized by obtained by heating a solution of Cu Br under aerobic
II
-[Fe (CN)
I
2
2
6
I
II
single crystal X-Ray Diffraction. Because of such a lack of conditions, which tends to destabilize Cu to Cu (ESI†). This is
comparable structures, the simple starting product (MQ)[BF
reinforced by the electron acceptor behaviour of the N-
ESI†) was also characterized by single crystal X-Ray Diffraction, methylquinoxalinium cation, which can be irreversibly reduced
4
]
(
+
and its structural data added for comparison. In (MQ)[CuBr
the MQ moiety is planar. The quinoxaline core is consistent with For these reasons, it is reasonable to suggest that both Cu /Cu
an aromatic π-conjugated system with bond distances (Csp2-Csp2
C
1
been observed in (MQ)[BF
2
]
∞
,
in solution as a neutral radical (E=-0.87 V vs Fc/Fc - Figure S2).
II
I
I
●
,
and MQ /MQ species could be stabilized in the crystalline state.
2 ∞
The single crystal X-Ray structure analysis of (MQ)[CuBr ]
sp2-N) and planar angles, ranging from 1.30 Å to 1.41 Å and
17.1° to 123.3° respectively, which correspond to what has show features that are consistent with the presence of Cu and
I
I
], as well as in some other aromatic MQ in the crystal. In the inorganic part, the distorted
4
8
compounds like naphthalene or pyrazine, but differ significantly tetrahedral environment around the copper is in line with a
II
7
]}.
I 4e,14
II
from those measured in (MQ)
In (MQ)[CuBr
, one half MQ moiety contributes to the geometry.
organic part of the asymmetric unit and is located in a mirror having the same coordination environment,
2
{H
2
-[Fe (CN)
6
four-coordinated Cu ,
while Cu would prefer a planar square
In the specific case of mixed valent compounds
5
c,15
2 ∞
]
4
a-d
the Cu-Br bond
plane perpendicular to the b axis (x,0,z). In the (ac) plane the distances criterion dominates. Here the average Cu-Br distance
I
MQ molecule interacts with its neighbours (X) only through side of 2.51 Å is consistent with the one found in similar pure Cu Br
4
e,14
II
contacts via hydrogen bonds (X…H-Caromatic) (Figure 2, red based salts (2.506 Å),
dashed lines). These occur between either two MQs related by a length is shorter with an average value of 2.422 Å.
symmetry centre (0,0,½) (X=N , dN2…H8-C=2.656 (5) Å,
rvdw (N…H) organic part, the structure, IR vibrations (Figure S3) and the
2.75 Å) or between MQ and its surrounding bromide anions redox state of MQ in (MQ)[CuBr are similar to (MQ)[BF
Table S2), where the cationic state MQ is clear. Thus, according
whereas the comparable Cu Br bond
5
c,15
In the
Σ
2
=
(
2
]
∞
4
]
I
(
X=Br, dBr2’…H5-C=2.962 (1) Å, dBr1’…H9-C=3.036 (1) Å, Σrvdw (Br…H)
=3.05 Å). Additional hydrogen bonds are present between the
methyl group of MQ and the bromide anions either in the (ac)
plane (dBr1…H1C-C=3.054(73) Å) or along the b axis (dBr1’…H1B-
=2.808 (50) Å) and contribute to the reinforcement of the
I
I
to the X-Ray structure analysis, we have a pure (MQ )[Cu Br
composition.
The X-band EPR spectrum of (MQ)[CuBr
2 ∞
]
2
]
∞
shows a broad
signal with g ~ 2.042 (Figure S4). This g-value is higher than
C
typical for organic-based radical spins and thus reveals the
presence of Cu in the polycrystalline material. The fluid solution
packing density.
When viewed perpendicular to the b axis, the structure of
II
9
spectrum in DMSO was recorded at 333 K to improve the
motional tumbling in this viscous solvent. The complicated
sequence of peaks appears to arise from three difference Cu
2 ∞
(MQ)[CuBr ] shows an infinite stack of antiparallel displaced
MQ molecules related by a symmetry centre (¼,¼,½ - Figure 1c
and 1d). The stacking parameters, based on the model of
II
1
0
species; only two four-line patterns can be clearly identified
(Figure S5). There was no evidence of an organic-based radical
GlÓwka and al., lead to an effective stacking surface of about
2% and to an interplanar distance h=b/2 of 3.205 Å (Figure S1).
This distance is surprisingly short compared to the average
value, <h>=3.41 Å ≥ vdw C…C=3.40 Å, observed in (MQ) {H
]} and in some similar cationic π-deficient heterocycles
1
●
●
signal attributable to MQ or to Br in the spectrum. This signal
is consistent with less than 0.4% of the copper ions in the
sample based on spin counting. There is insufficient resolution
to decipher the chemical environment of the copper. Exposure
to the atmosphere sees the growth of a prominent of a signal at
Σ
2
2
-
II
[
Fe (CN)
6
like N-methylphenazinium (NMP), N-methylquinolinium (Q) and
N-methylacridinium (N-MeA), which stack in a similar way (Table
7
,11
2
g ~ 2.17 that perfectly matches the profile of CuBr dissolved in
S3).
interplanar distance of 3.0-3.22 Å (at 173K) has been reported,
this distance we observe in (MQ)[CuBr is by far the shortest
Although in a cationic N-rich aromatic heterocycle an
1
2
DMSO solution (Figure S6).
A Bond Valence Sum (BVS) treatment [equation VCu=2*SCu-Br1
+
*SCu-Br2; S=exp (R -R)/B], using the most recent bond valence
2 ∞
]
2
interplanar distance reported for cationic π-deficient
heterocycles. Such a distance with d<3.30 Å is usually observed
0
6
1
parameter table, leads to the oxidation state VCu=+1.083,
which is in line with the suggestion that the copper in this
compound is nearly exclusively Cu .
1
3
in strongly interacting cation radical or mixed-valence salts.
This raised the question concerning the charge distribution
within the (MQ)[CuBr salt, and especially the respective
oxidation states of all moieties.
I
The magnetic susceptibility of (MQ)[CuBr2]∞ was measured on
a polycrystalline sample (223 mg) from 4.5-300K under an
2 ∞
]
2
| J. Name., 2012, 00, 1-3
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2
applied field of 0.4T and 1T. As shown in Figure 3a, the MV . This is only possible when the orbitals of the halide (X)
+
+
susceptibility is negative which confirms the dominant interact suitably with those of the pyridinium site (N ) of the
+
diamagnetic nature of the compound. The fitted molar electron acceptor, which is characterized by a short X…N
diamagnetic susceptibilities at 0.4 and 1 T gives an average value contact and an almost perpendicular angle between the halide
-
6
-1
=-173.26 10 emu.mol , which is in line with the and the pyridinium plane.
4c,20
χ
approximation 
m
In (MQ)[CuBr
●
fact that there is no evidence of any MQ from the EPR
] , in addition to the
2 ∞
-6
-1
m
=M(MQ)[CuBr]2∞=368.54/2=184.27 10 emu.mol
1
7
and with the one calculated from the Pascal’s Constants
1
(χ
m
=- measurements, such a donor-acceptor interaction is not
present, since the bromide anions are 37° shifted from the ideal
In order to determine the amount of Cu present in the 90° for a “Br…N ” geometry, and the shortest distance (Br
-
40.12 10 emu.mol ).
6
-1
II
+
’…N
)
2
1
compound a global fitting of the difference plots of χ−χdia (T) is much longer than the sum of the Van der Waals radii
(
H=0.4 and 1T) is typical of a paramagnetic species which follows [dBr2’…N1=4.024 (2) Å;
Σ
r
vdw (Br…N) =3.4 Å; Figure 2, blue dashed
I
I
-
a Curie-Weiss law with weak antiferromagnetic interactions line]. This leads to the conclusion that a pure MQ /Cu /Br
-6
-1
(
C=212.766 10 emu.K.mol , θ=-2.404 K - Figure 3b+inset). composition should give colourless crystals of (MQ)[CuBr
2 ∞
] .
Extraction of the Curie constant gives an estimation of about
II
.054% of Cu present in the compound, when comparing the
0
Curie constant of a corresponding pure Cu paramagnet
II
-1
(MQ)[CuBr
(MQ)[BF4]
2 ∞
]
(C=0.391 emu.K.mol with S=1/2, geff=2.042). Thus according to
the X-Ray, EPR, BVS, and SQUID measurements, the amount of
II
Cu within the compound is clearly negligible, and could be
explained as arising from some defects at the surface of the
crystals, which is a well-known phenomenon in the chemistry of
Cu .
I
Experimental (0.4 T)
Experimental (1 T)
Global fit of the data to a Curie Weiss law
4
0
5
b
3
0.20
-
135
140
145
150
155
160
165
170
175
30
0.15
25
20
15
10
5
0
-
Figure 4: Solid state UV-Vis-NIR spectra of (MQ)[CuBr
(dashed line) with their corresponding crystals pictures.
2
]
∞
(solid line) and (MQ)[BF
4
]
0.10
-1
(
χ−χdia
)
=T/C -
θ
/C
-
0.05
0.00
1
-
/
C=0.0047
θ/C=0.0113
-
I
II
9
In the case of Cu /Cu mixed valence the d configuration of
5
10
15
20
25
30
35
40
II
Cu can explain the colours of the compounds.
4a-d
-
Temperature (K)
The
II
absorption bands usually arise from Cu metal centred
transitions and from ligand to metal charge transfer (LMCT)
bands, while an additional transition at lower energy can be
-
0
50
100
150
Temperature (K)
200
250
300
-
4
b
attributed to an intervalence charge transfer band (IVCT).
However, dark colours such as green or deep blue are explained
-
a
-
II
when the amount of Cu is significant in the solid (≥ 50%).
4a,b
In
the case of (pq)[Cu Br ], which has only 2% Cu , the reddish-
0
50
100
150
200
250
300
4
c
II
Temperature (K)
Figure 3: (a) Plot of the experimental Molar susceptibility versus Temperature of
MQ)[CuBr (223mg, 0.4T and 1T). (b+inset) Plot after diamagnetic correction.
The molar paramagnetic susceptibility fits better with
represented by the red line (global fit parameters: C=212.766 10 emu.K.mol , θ=-
2
4
orange colour arises from charge transfer. In our case, with less
than 0.1% Cu and because no charge transfer can occur, we
II
(
2 ∞
]
a
Curie-Weiss law, need to find a different explanation for the dark brown colour
-
6
-1
with such a long absorption range (400-1200 nm).
-
6
-1
-6
-1
2
.404 K with χdia (0.4 T)= -171.85 10 emu.mol , χdia(1T)= -174.68 10 emu.mol )).
Our first interpretation of the results postulated itinerant
electrons mediated by the crystal packing. Single crystal
conductivity measurements were carried out along the b axis,
since this corresponds to the [CuBr2]∞ chain and to the π-
stacking axis of the MQ moieties directions and is expected to
be the preferential conduction path. The temperature
dependence of the conductivity shows an activated behavior,
However this is not consistent with the homogenous dark
brown colour of the crystals (Figure 4) which persists even upon
grinding. Furthermore, the electronic spectrum of (MQ)[CuBr
2 ∞
]
reveals two broad absorption bands, one in the UV-Visible
region (200-800 nm), and the other in the near-infrared (600-
I
1
cation which displays a unique π-π* transition in the UV region
200 nm, λmax≈900 nm). This colour cannot arise from the MQ
-9
and the conductivity at room temperature is estimated as 3.10
S/m (Figure 5). The fit of the data to an Arrhenius law with
exp(-E /T) gives an activation energy E of 7500K i.e. an
energy gap of = 2E = 1.3 eV a little higher than the optical gap.
This value of conductivity is about 300 times smaller than the
σ =
(
species should also give colourless crystals due to the d
λ
max= 335 nm, Figure 4). Similarly a pure copper (I) bromide
σ
0
a
a
1
0
I
electronic configuration of Cu and the impossibility of ligand to
ꢀ
a
5
a,18
metal charge transfer.
charge transfer between the electron donors (Cu or Br ) and the
An electronic transition arising from a
4c
II
I
-
2 4
one reported in (pq)[Cu Br ], containing 2% Cu and acting as
I
acceptor MQ , as originally postulated in the study of some
mediator of the charge transport. This actually in line with the
conductivity value we find giving an estimate of about 0.01%
Cu , acting as a charge carrier in our compound.
2
+
paraquat (MV ) salts
4c,19
2+
is not possible. In the MV salts the
II
authors attribute the colour of the crystals to the presence of
some Cu sites and MV cation radicals in the solid, resulting
II
●+
I
from a halide-mediated electron transfer between the Cu and
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7
8
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1
1
4
,
σ=σ0 exp(-E /kT)
a
8
Ea=7500 K, E = 1.3 eV
g
1
P. Kuppusamy, P. T. Manoharan, C. Mahadevan and M. Seshasayee, J.
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2 ∞
Figure 5: Conductivity of single crystal along the b axis of (MQ)[CuBr ] plotted as a
function of the inverse temperature. The black line is the fit of the data with an
activation law.
4
c
As in the case of Scott & Willet
we conclude that the
1
4
48; (k) T. M. Bockman and J. K. Kochi, J. Am. Chem. Soc., 1989, 111
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,
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Acknowledgments
The authors thank the financial support provided by the 12 R. Centore, M. Causà, S. Fusco and A. Carella, Cryst. Growth Des.,
Alexander von Humboldt Foundation (fellowship to N.L) and
Helmholtz POF “STN”. We also thank Dr. Lionel Sanguinet for the
2013, 13, 3255-3260.
1
1
3 N. Leblanc, N. Mercier, O. Toma, A. H. Kassiba, L. Zorina, P. Auban-
Senzier and C. Pasquier, Chem. Commun., 2013, 49, 10272-10274.
4 (a) J. P. Lang, H. Z. Zhu, X. Q. Xin and M. Q. Chen, Chem. J. Chin. U.,
electrochemistry measurements
.
1
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Notes and references
1
5
‡
Crystal data of (MQ)[CuBr
2
]
∞
: C
9 9
H N
2
Cu
1
Br
2
, M=368.354; monoclinic,
=96.692(12)°,
(F) = 0.029 [1265 unique reflections in the
.4-27.3° θ range (Rint = 0.022), of which 1133 had I > 2σ(I), 92 parameters
refined], wR2(F2) = 0.091 (all data), GOF = 1.32; max/min residual electron
C2/m; a=15.373(2) Å, b=6.4099(7) Å, c= 10.9646(18) Å,
V=1073.1(3) Å ; Z=4; T=293K; R
3
β
3
1
1
-
3
density 0.390/ -0.582 e.A . Single crystals X-Ray diffraction data of
MQ)[CuBr and (MQ)[BF ] were collected at 293K on a STOE IPDS II
(
2
]
∞
4
2
2
243; (c) K. Murray and R. D. Willett, Acta Cryst. C, 1991, 47, 2660-
662; (d) M. R. Bond, H. Place, Z. Wang, R. D. Willett, Y. Liu, T. E.
Diffractometer equipped with a graphite monochromatized Mo Kα
radiation (λ = 0.71073 Å). Structures were solved and refined using the
Shelxl2013 package and molecular diagrams were prepared using
21
Grigereit, J. E. Drumheller and G. F. Tuthill, Inorg. Chem., 1995, 34,
3134-3141; (e) S. Lorenzo, C. Horn, D. Craig, M. Scudder and I. Dance,
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22
Diamond 3.2i. Positions, atomic displacement parameters were refined
2
by full-matrix least-squares routines against F , including hydrogens atoms
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1
1
1
2
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1
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2
1
520.
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(
c) B. Scott, R. Willett, A. Saccani, F. Sandrolini and B. L. Ramakrishna,
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2
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4
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