Structural diversity in hybrid vanadium(iv) oxyfluorides based on a common building block

Article abstract of DOI:10.1039/c3dt52385c

There are only limited reports on vanadium(iv) oxyfluorides (VOFs) with extended crystal structures. Here we expand and enrich the list of existing VOFs with a series of 14 new materials "VOF-n (n = 1-14)" prepared using ionothermal and solvothermal synthesis methods. All of these materials arise from the condensation of a dimeric structural motif. These VOFs can be classified into three groups depending on their key structural features; layer structures: VOF-1 "[HN₂C₇H₆][V ₂O₂F₅]", VOF-2 "[HN₂C ₄H₄][V₂O₂F₅]", VOF-3 "[HN₂C₃H₄][V₂O ₂F₅]" and VOF-4 "V₂(N ₂C₄H₄)O₂F₄", ladder like structures: VOF-5 "[NH₄(HN₂C₃H ₄)][V₂O₂F₆]", VOF-6 "[K(HN₂C₃H₄)][V₂O ₂F₆]", VOF-7 "[HNH₂CH ₂CH₃][VOF₃]", VOF-8 "[HN ₂C₇H₆][VOF₃]", VOF-9 "[H₂N₂C₄H₆][V₂O ₂F₆]", VOF-10 "β-RbVOF₃", VOF-11"α-KVOF₃", VOF-12 "β-KVOF ₃", VOF-13 "[H₂(NH₂) ₂(CH₂)₂][V₂O₂F ₆]", and a chain structure: VOF-14 "[H₂N ₂C₆H₁₂][V₂O₂F ₇]". The crystal structures of VOF-n are presented, and their synthetic and structural relationships are discussed.

Full text of DOI:10.1039/c3dt52385c

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Structural diversity in hybrid vanadium(IV)
oxyfluorides based on a common building block†
Cite this: Dalton Trans., 2014, 43,
568
Farida H. Aidoudi, Cameron Black, Kasun S. Athukorala Arachchige,
Alexandra M. Z. Slawin, Russell E. Morris and Philip Lightfoot*
There are only limited reports on vanadium(IV) oxyfluorides (VOFs) with extended crystal structures. Here
we expand and enrich the list of existing VOFs with a series of 14 new materials “VOF-n (n = 1–14)” pre-
pared using ionothermal and solvothermal synthesis methods. All of these materials arise from the con-
densation of a dimeric structural motif. These VOFs can be classified into three groups depending on
their key structural features; layer structures: VOF-1 “[HN2C7H6][V2O2F5]”, VOF-2 “[HN2C4H4][V2O2F5]”,
VOF-3 “[HN2C3H4][V2O2F5]” and VOF-4 “V2(N2C4H4)O2F4”, ladder like structures: VOF-5 “[NH4(HN2C3H4)]-
[V2O2F6]”, VOF-6 “[K(HN2C3H4)][V2O2F6]”, VOF-7 “[HNH2CH2CH3][VOF3]”, VOF-8 “[HN2C7H6][VOF3]”,
VOF-9 “[H2N2C4H6][V2O2F6]”, VOF-10 “β-RbVOF3”, VOF-11 “α-KVOF3”, VOF-12 “β-KVOF3”, VOF-13
“[H2(NH2)2(CH2)2][V2O2F6]”, and a chain structure: VOF-14 “[H2N2C6H12][V2O2F7]”. The crystal structures of
VOF-n are presented, and their synthetic and structural relationships are discussed.
Received 29th August 2013,
Accepted 4th October 2013
DOI: 10.1039/c3dt52385c
www.rsc.org/dalton
hydrothermal synthesis mainly produces low dimensional
VOFs; monomeric, oligomeric or chain-type structures are all
Introduction
Compounds of vanadium(IV) exhibiting extended lattice struc- accessible hydrothermally at low reaction temperatures. On the
tures are of interest for a variety of properties and applications, other hand more extended VOF structures are accessible using
for example as lithium battery cathodes,1 frustrated magnets2 ionothermal synthesis, where
a
hydrophobic IL, 1-ethyl-
and oxidation catalysts.3 We have focused our own interest 3-methylimidazolium
bis(trifluoromethylsulfonyl)imide
over the past few years on the synthesis of metal (oxy)fluorides4 (EMIM Tf2N) is used as the reaction solvent. Initially, this led
with particular emphasis on developing novel hybrid organic– to the preparation of the first VOF with a 2-D connectivity,
inorganic vanadium (oxy)fluorides using various synthetic [Hpyr]-[V2O2F5],10 and subsequently,
a similar approach
methods, including hydrothermal and ionothermal systems. allowed us to prepare a VOF with a pillared-layer kagome
This has resulted in a wide range of materials containing lattice, [NH4]2[HNC7H13][V7O6F18].11 Very recent studies
vanadium in III, IV or V oxidation states.5–8
demonstrate that this material displays unusual gapless
The many variables involved in both hydrothermal and quantum spin-liquid behaviour.12
ionothermal systems illustrate their complexity, and in the
We have pursued further research on both ionothermal and
course of our exploratory studies we have found that the out- hydrothermal synthesis of VOFs, yielding a variety of further,
comes of these reactions are very sensitive to small changes in novel V(IV)-containing materials with a very rich crystal chem-
conditions. In addition to the vital role of the solvent, many istry. All the materials reported herein contain vanadium in
other factors (e.g. vanadium source, amount of HF, nature and the IV oxidation state, and are based on a common dimeric
amount of the added template and others) may affect the reac- structural motif. The order in which the structures are pres-
tion outcome. Although the direct impact of these parameters ented follows their dimensionality, and the structures are
on the resulting materials is not yet fully understood, there is given the symbols VOF-n.
a clear indication of the propensity of ionothermal systems,9
i.e. using ionic liquids (ILs) as the reaction solvent, to produce
more extended V(IV)-containing fluorides. For instance
Experimental section
Synthesis
EaStChem, School of Chemistry, University of St Andrews, St Andrews,
All reagents were commercially available and used without
further purification, except the ionic liquids EMIM Tf2N and
Fife KY16 9ST, UK. E-mail: pl@st-and.ac.uk
†Electronic supplementary information (ESI) available. CCDC 957929–957939.
N-butylpyridinium bromide (BPB), which were synthesised
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt52385c
according to the literature procedure.13,14
568 | Dalton Trans., 2014, 43, 568–575
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Among the VOFs presented here, only VOF-10, VOF-11 and
The synthesis procedure for the ILs and for each compound
VOF-12 were synthesised solvothermally in a 40 mL Teflon- can be found in full detail in the ESI.†
lined stainless steel autoclave using either a mixture of water–
DMSO or water–ethylene glycol. All the other VOFs were made
ionothermally in a 30 mL Teflon-lined stainless steel autoclave.
X-ray crystallography
All the ionothermally prepared VOFs were synthesised fol-
lowing a typical synthesis procedure: a Teflon-lined autoclave
(volume 30 mL) was charged with VOF3 (0.124 g, 1 mmol,
Sigma Aldrich) (or V2O5 (0.182 g, 1 mmol, Sigma Aldrich) in
the case of VOF-13) and HF (48 wt% in H2O) (0.1 mL,
2.76 mmol, Sigma Aldrich) or (1 mL, 27.6 mmol, Sigma
Aldrich in the case of VOF-13) and then the IL EMIM Tf2N (4 g,
∼10 mmol) (or the IL EPB 2.16 g, ∼10 mmol in the case of
VOF-13) was added along with the added template; benzimida-
zole (VOF-1 and VOF-8), pyrazine (VOF-2 and VOF-4), imidazole
(VOF-3 and VOF-5), imidazole and KNO3 (VOF-6), ethylamine
(VOF-7), 2-methylimidazole (VOF-9), ethylenediamine (VOF-13)
and 1,4-diazabicyclo[2.2.2]octane (DABCO) (VOF-14).
For VOF-1, VOF-2, VOF-3, VOF-5, VOF-6, VOF-7, VOF-8 and
VOF-9 the stainless steel autoclave was then sealed and heated
in an oven at 170 °C for 24 h; for VOF-4 and VOF-13 the auto-
clave was heated at 140 °C for 24 h and for VOF-14 the auto-
clave was heated at 130 °C for 24 h. After the autoclave had
been cooled to room temperature, the product was filtered,
washed with methanol and dried in air for 24 h.
Single crystal X-ray diffraction data for VOF-1 were collected at
station 11.3.1 of the Advanced Light Source at Lawrence Berke-
ley National Laboratories, California using a Bruker APEX II
CCD diffractometer. Single crystal X-ray diffraction data for all
other VOFs were collected using Mo-Kα (0.7107 Å) radiation
utilising a Rigaku rotating anode single-crystal X-ray diffracto-
meter at the University of St Andrews. The structures were
solved with standard direct methods using SHELXS and
refined with least-squares minimisation techniques against F2
using SHELXL within the WinGX packages.
Powder XRD was carried out on a Stoe STADI/P diffracto-
meter using Cu Kα1 X-rays.
VOF-3 displays an inorganic layer that shows an interesting
variation to the one previously seen in [Hpyr][V2O2F5].10 VOF-4,
VOF-13 and VOF-14 display novel polyhedral connectivities
that are unprecedented in this class of materials. For space
reasons only the crystallographic details for these materials are
presented in Table 1; for all the other structures they are tabu-
lated in the ESI.†
A general scheme for the synthesis of VOF-10, VOF-11 and
VOF-12 solvothermally is as follows: a Teflon-lined autoclave
(volume 40 mL) was charged with V2O5 (0.182 g, 1 mmol,
Sigma Aldrich) and RbF (0.209 g, 2 mmol) or K2CO3 (0.136 g,
1 mmol) and HF (48 wt% in H2O) (0.5 mL, 13.8 mmol, Sigma
Aldrich). and then a solution of 3 : 1 H2O–DMSO, 4 : 5 H2O–
ethylene glycol or 7 : 1 H2O–ethylene glycol was added respect-
ively. The stainless steel autoclave was then sealed and heated
in an oven at 160 °C for 24 h or 72 h.
Magnetic measurements
Magnetic susceptibility data for VOF-13 were collected on a
Quantum Design MPMS SQUID. Data were recorded in a
2000 Oe field while warming the sample from 1.8 to 300 K in
4 K steps, following consecutive zero-field cooling (ZFC) and
field cooling (FC) cycles. Data were normalized to the molar
quantity of the sample, and corrected for any diamagnetic
contributions.
Table 1 Crystallographic data for VOF-3, VOF-4, VOF-13 and VOF-14
VOF-3
VOF-4
VOF-13
VOF-14
Formula
[HC3N2H4][V2O2F5]
297.97
V2(N2C4H4)O2F4
289.97
[H2(NH2)2(CH2)2][V2O2F6]
310
[H2N2C6H12][V2O2F7]
381.07
Fw/g mol−1
Space group
a/Å
b/Å
c/Å
P21 (4)
7.170(3)
17.333(7)
7.370(4)
90
P21/c (14)
5.0096(14)
10.8260(3)
7.288(7)
P21/m (11)
8.806(3)
12.100(5)
9.206(4)
C2/c (15)
10.0330(19)
10.0380(17)
11.930(2)
90
α/°
90
90
β/°
118.0531(10)
90
808.32(6)
4
95.9430(10)
90
393.1(4)
106.301(2)
90
941.49(6)
4
110.170(3)
90
1127.8(3)
4
γ/°
V/Å3
Z
2
Crystal size/mm
Crystal shape and colour
F(000)
0.25 × 0.03 × 0.02
Blue needle
576
0.20 × 0.20 × 0.12
Blue platelet
280
0.2 × 0.2 × 0.2
Blue prism
608
0.05 × 0.05 × 0.05
Blue prism
680
Rint
0.0509
0.0247
0.0601
0.1031
Obsd data [I > 2σ(I)]
Data/restraints/parameters
GOOF on F2
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
Largest diff. peak/hole
1875
2153/1/254
1.013
0.0395, 0.0772
0.0468, 0.0815
0.522/−0.504
612
710/0/72
1.069
0.0262, 0.0540
0.0337, 0.0565
0.318/−0.368
1711
1821/0/125
1.241
0.1123, 0.2515
0.1180, 0.2551
2.314/−1.787
891
1046/0/96
1.019
0.0871, 0.2332
0.0970, 0.2486
1.775/−0.847
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Results and discussion
Crystal structures
The VOF structures presented here fall into three major groups:
2-D layers, 1-D ladder-like chains and a 1-D single chain.
Interestingly all these structures can be regarded as arising
from the same structural motif, where in all the structures the
coordination environment around each vanadium atom is
highly distorted (typical characteristic of V(IV) in vanadium oxy-
fluorides) due to the presence of the short vanadyl VvO bond
(∼1.6 Å) and the corresponding elongated trans V–F bond
(∼2.2 Å); the other V–F bonds are within the normal range
(∼1.9 Å).7
Fig. 2 Ladder type [VOF3]nn− chain.
The vanadyl group is terminal in all the structures, with an
additional common feature being two F atoms bridging two
adjacent octahedral units into an edge-shared dimer. These
dimers, are then further linked to others in different ways to
form layers, ladders or chains. The dimer in VOF-4 is slightly
different with the vanadyl group being cis- rather than trans- to
the bridging F atoms, and two N atoms from the pyrazine
ligand taking up one of the trans-positions (V–N distance
2.121(3) Å). These two types of edge-shared-dimers are shown
in Fig. 1.‡
VOF-1 and VOF-2 arise from the condensation of the edge-
shared dimers, described earlier, through bridging F-atoms to
form infinite “ladder” like chains (Fig. 2). These chains are
further linked through F atoms leaving only O atoms terminal,
Fig. 3 [V2O2F5]nn− layer found in VOF-1 and VOF-2.
n−
to form an infinite anionic layer of composition [V2O2F5]n
(Fig. 3), similar to the one previously seen in [Hpyr][V2O2F5].10
These layers are separated via hydrogen-bonded protonated
organic amine moieties, benzimidazolium, or pyrazinium
(Fig. 4).
In VOF-1 there are four vanadium ions in the asymmetric
unit, and in VOF-2 there are two vanadium ions in the
Fig. 4 A view of the layer packing in (a) VOF-1 and (b) VOF-2.
Fig. 1 The dimeric [V2O2F8]4− building units observed in (a) all the VOF-
n structures except VOF-4 and (b) dimeric unit observed in VOF-4.
asymmetric unit, all of which are in the 4+ oxidation state as
confirmed by bond valence sum calculations (see ESI†).
VOF-3 is also constructed from the edge-shared dimer
described earlier, with the dimers sharing F atoms to form
infinite “ladder” like chains in a similar way as VOF-1 and
‡Colour scheme used in all pictures: vanadium (light blue), fluorine (green),
oxygen (red), nitrogen (dark blue), carbon (dark grey), hydrogen (light grey),
potassium (pink) and rubidium (purple).
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Fig. 5 [V2O2F5]nn− layer found in VOF-3.
Fig. 7 A view of the layer packing in VOF-3.
Fig. 8 Inorganic chain found in VOF-4.
n−
Fig. 6 A view of the polymorphic 3 [V2O2F5]n layers: (a) found in
VOF-3 (b) found in VOF-1 and VOF-2.
VOF-2. These chains are then joined through F atoms to form
an infinite anionic layer (Fig. 5). Interestingly, as shown in
Fig. 6, the ladders in VOF-3 are linked in a different way com-
pared to VOF-1 and VOF-2. In VOF-3 the nearest “vanadyl”
bonds from two adjacent ladders point in opposite directions
(Fig. 6(a)); in contrast, in VOF-1 and VOF-2 they point in the
same direction (Fig. 6(b)).
Fig. 9 The V(IV) oxyfluoride/pyrazine layer found in VOF-4.
In VOF-3 the layers are separated via hydrogen-bonded pro-
tonated imidazolium cations (Fig. 7).
It can be seen that the arrangement of the ladders within
two different layers in VOF-1 and VOF-2 follow the same
pattern previously seen in [Hpyr][V2O2F5]: i.e. corrugated layers
arranged parallel to each other while in VOF-3 they are quite
different, and are aligned in nearly anti-parallel fashion.
VOF-4 displays a 2-D coordination polymer motif, built up
from the edge-sharing dimer described earlier (Fig. 1(b)),
which is further linked to other adjacent dimers through two
bridging F-atoms to form an edge-sharing “zigzag” chain
(Fig. 8). These chains are bridged by the pyrazine ligand to
form infinite metal–organic sheets (Fig. 9 and 10).
In VOF-4 there is only one single vanadium ion in the 4+
oxidation state as confirmed by bond valence sum calculations
(∑V1 = 3.77).
Fig. 10 Structure of VOF-4 viewed along the a axis, showing stacks of
layers.
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