Selective and reusable iron(II)-based molecular sensor for the vapor-phase detection of alcohols

Article abstract of DOI:10.1021/ic402816a

A mononuclear iron(II) neutral complex (1) is screened for sensing abilities for a wide spectrum of chemicals and to evaluate the response function toward physical perturbation like temperature and mechanical stress. Interestingly, 1 precisely detects methanol among an alcohol series. The sensing process is visually detectable, fatigue-resistant, highly selective, and reusable. The sensing ability is attributed to molecular sieving and subsequent spin-state change of iron centers, after a crystal-to-crystal transformation.

Full text of DOI:10.1021/ic402816a

Communication
pubs.acs.org/IC
Selective and Reusable Iron(II)-Based Molecular Sensor for the
Vapor-Phase Detection of Alcohols
Anil D. Naik, Koen Robeyns, Christophe F. Meunier, Alexandre F. Leo
́
nard, Aurelian Rotaru,§
†
†
‡
‡
†
†
‡
,†
Bernard Tinant, Yaroslav Filinchuk, Bao Lian Su, and Yann Garcia*
†Institute of Condensed Matter and Nanosciences, Molecules, Solids, Reactivity (IMCN/MOST), Universite
́
Catholique de Louvain,
Place Louis Pasteur 1, 1348 Louvain-la-Neuve, Belgium
‡
Laboratoire de chimie des mater
́
iaux inorganiques, Dep
Department of Electrical Engineering and Computer Science, “Stefan cel Mare” University, Suceava 720229, Romania
S Supporting Information
́ ́
artement de Chimie, Universite de Namur, 5000 Namur, Belgium
§
*
complex, [Fe(trz-tet) (H O) ]·2H O, with a FeN O chromo-
phore with monodentate triazole coordination (Figure 1).
2
2
4
2
2
4
ABSTRACT: A mononuclear iron(II) neutral complex
1) is screened for sensing abilities for a wide spectrum of
(
chemicals and to evaluate the response function toward
physical perturbation like temperature and mechanical
stress. Interestingly, 1 precisely detects methanol among
an alcohol series. The sensing process is visually
detectable, fatigue-resistant, highly selective, and reusable.
The sensing ability is attributed to molecular sieving and
subsequent spin-state change of iron centers, after a
crystal-to-crystal transformation.
here is currently a huge appeal for “chemosensors” based on
metal−organic frameworks (MOFs) for the sensitive and
T
selective detection of gas- and vapor-phase analytes for a range of
applications including chemical threat alerts, medical diagnostics,
1
and environmental monitoring. Among a wide variety of
Figure 1. (Top) Molecular structure of 1. (Bottom) View along the a
axis of the crystal packing, showing the channels of water molecules.
Hydrogen atoms were omitted for clarity.
materials, iron(II) spin-crossover molecular materials, which
respond to a range of external stimuli (temperature, pressure,
light irradiation, hard X-rays, humidity, etc.), have been
developed, given their remarkable spectroscopic, visual/optical,
magnetic, dielectric, or electrical readout signal, which make2s
these materials potentially considered for practical applications.
Among them, iron(II) coordination polymers with several
sorbing species are currently studied including alcohols,
The trz-tet molecule acts as an anionic ligand with
deprotonation of the labile proton on the tetrazole moiety.
Mononuclear units are arranged as parallel sheets in the crystal
packing (Figure 1). These sheets are interpenetrated by similar
neighboring planes wherein tetrazole of the mononuclear units is
3
4
CO (g), SO (g), etc.
2
2
́
brought within 4.47−4.7 Å of the iron centers. This molecular
In this context, we report herein the synthesis and character-
ization of a novel iron(II) mononuclear complex, and its ability to
selectively differentiate and detect alcohols based on molecular
sieving and spin-state change, thanks to an unprecedented
mechanism involving a ligand replacement in the coordination
sphere, provoking a color change, which are both not triggered by
arrangement creates interplanar spaces in the form of zigzag
channels that are occupied by two lattice water molecules. These
noncoordinated species are involved in a dense hydrogen-
bonding network (Table S3 in the SI), being connected to
nitrogen atoms from a neighboring tetrazole moiety but also to
57
coordinated water molecules (Figure S1 in the SI). The Fe
5
temperature, which differs from reported examples of MOFs.
Mossbauer spectrum of 1 at 298 K shows a unique doublet with
an isomer shift δ = 1.17(2) mm/s and a quadrupole splitting ΔEQ
6
Using a simplified transamination reaction, we prepared 5-
II
(4H-1,2,4-triazolyl)-2H-tetrazole (trz-tetH), wherein two azole
= 3.18(3) mm/s, which are typical for high-spin (HS) Fe
7
molecules are connected without a spacer. With Fe(ClO ) ·
(Figure 2a). The asymmetry in the doublet is due to texture. At
4
2
6
H O or Fe(BF ) ·6H O, trz-tetH reacts in water at neutral pH
77 K (Table S2 in the SI), no spin switching is detected, which
2
4 2
2
to give a colorless, microcrystalline, air-stable complex (1; see the
was expected for a FeN O core including water molecules
2 4
Supporting Information, SI). Rectangular colorless crystals
(
∼2−3 mm) of 1 crystallizing in the P1
̅
space group were
Received: November 12, 2013
grown from water (Table S1 in the SI). 1 is a mononuclear
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ic402816a | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
nonreactive, guest molecules that could traverse the empty
channels of such crystals whose internal borders are lined up by
switchable iron centers and a noncoordinated tetrazole moiety. A
wide array of chemicals (alcohols, acids, CHCl , CH Cl ,
3
2
2
CH CN, etc.) were thus exposed to 1, which was not desorbed
3
prior to these experiments. Among them, 1 responds only to
members of the alcohol and acid series. Among physical stimuli, 1
was nonresponsive to a change in temperature (298 to 77 K) and
light irradiation (UV−vis) but instantly changed its color to pale
pink when subjected to mechanical stress (e.g., grinding) and
returned to its original color when friction is halted. The present
account was focused on the responsiveness of 1 toward alcohols.
When the native hydrate 1 is in contact with liquid methanol
[MeOH(l)], at room temperature, it instantly turns to pale pink.
The chromic response is intense when colorless crystals (2−3
mm length) are exposed to MeOH(g) (Figure 3a,b). This
vapochromism is a crystal-to-crystal transformation. Crystals
begin to turn pink within 5 min, but take hours to transform into
dark pink (2) as much as the atmosphere is saturated with
MeOH. This time duration is considerably reduced when
crushed crystals or microcrystals are used. 1 selectively detects
5
7
Figure 2. Spin-state tracking by (a) Fe Mossbauer spectroscopy at 298
K (1) of 1, (2) of 2 after MeOH exchange, (3) of 2 treated with water,
which shows reversibility (b). Diffuse-reflectance spectroscopy of 1 and
2
at 298 K. (c). Thermal variation of χ T for 1 and 2.
M
(
Figure 1). The paramagnetic state is confirmed by SQUID
measurements with a characteristic χ T value for the HS state
Figure 2c). The HS state is also supported by the broad T →
E band centered around 850 nm in the diffuse-reflectance
CH OH over its heavier alcohol analogues by visual, spectral,
3
M
5
optical, and magnetic feedback and reversibly turns back to
colorless (3) when exposed to water vapor/washed with water
for regeneration. This cycle has been repeated more than 100
times without deterioration, demonstrating the robustness of our
molecular sensor. However, 1 loses crystal luster after the first
cycle. The selectivity of detection decreases in the order MeOH
(
2
5
spectrum (Figure 2b).
The thermogravimetric analysis (TGA) profile of 1 (Figure
S2b in the SI) indicates that lattice water molecules are lost
between 313 and 338 K, whereas coordinated water molecules
are lost between 378 and 403 K. Lattice water molecules can also
be removed by pumping-out crystals under vacuum (3 mbar) for
>
EtOH > PrOH > BuOH > ... with increasing alcohol size. These
observations were noted under identical conditions as well as
extension of the solvent vapor exposure period considering the
vapor pressure of different members of the alcohol series. 1 also
shows only a slight color change on the surface of the crystal
when exposed to EtOH vapor, but eventually crystal luster is lost
and detection is restricted. Higher analogues in the alcohol series
were even exposed to 1 for several days; however, neither showed
any color change or deterioration.
3
−4 h at 313 K or at room temperature for 8−10 h. Crystals in
the latter case retain their luster. When observed by scanning
electron microscopy (SEM), periodic cracks on one broad side of
a rectangular crystal are, however, detected (Figure 3c).
Investigation of 1 by mercury intrusion porosimetry (MIP)
shows a pore-size distribution of 80−100 μm and indicates
interparticle porosity (Figure 3e,f).
Among possible mechanisms of such analyte selectivity, size
exclusion (molecular sieving) wherein alcohol molecules that are
smaller than the apertures of the molecular channels of 1 can be
allowed to traverse, whereas higher analogues cannot, is thought
of. The response time of 1 depends on the rate of guest diffusion,
physical state (solution/vapor), and proximity. The driving force
for the striking color change at room temperature is due to a spin-
Such material should be inherently sensitive for host−guest
interactions and constitutes a playground for molecular
recognition. Our goal was to allow polar, hydrophilic, chemically
1
II
state switching of Fe . Indeed, for 1 the FeN O chromophore
2
4
with coordinated water molecules (Figure 1) sets up a weak
ligand-field strength (Figure 2). Thus, eventual substitution of
coordinated water molecules in 1 by MeOH is not expected to
shift the ligand-field strength into the low-spin (LS) region.
However, a comparison of the diffuse-reflectance spectra (Figure
2
b) of 1 and 2 clearly indicates a new band around 500 nm that is
1
1
characteristic of the A → T d−d transition of the LS state of
Fe . SQUID magnetometry also supports this observation by the
1
1
II
lower χ T value for 2. Structural information in addition to a
M
̈
quantitative analysis of spin-state change by Mossbauer spec-
troscopy was obtained. 2 reveals three signals (Figure 2a): a new
quadrupole doublet in the center [δ = 0.41(1) mm/s; ΔE =
Q
0
.19(1) mm/s] of 22%, characteristic of a LS FeN core with
6
Figure 3. (a and b) Images depicting crystal-to-crystal vapochromic
transformation upon MeOH sensing of 1 (colorless) and 2 (red). In part
b, the orientation of crystal bunch was modified because of the handling
of such delicate crystals and the background color was changed during
image recording and processing. (c) SEM image of 1. (d) SEM image of
azole ligands. Indeed, replacement of the oxygen atoms by
nitrogen atoms, which are less electronegative, increases the
electronic density of s electrons in the vicinity of the iron nucleus
and thereby decreases the isomer shift. The two remaining
II
8
2. (e) Intrusion curve. (f) Corresponding pore-size distribution.
contributions correspond to HS Fe sites: HS-1 [δ = 1.28(2)
B
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Inorganic Chemistry
Communication
mm/s; ΔE = 2.48(3) mm/s; 39%] and HS-2 [δ = 0.92(2) mm/
(as evaluated from Mo
̈
ssbauer data), the observed signal
Q
s; ΔE = 2.51(3) mm/s; 39%]. Both correspond to FeN O
transduction is still significant. The main advantage of this
chromogenic system is the striking signal transduction and
operation at ambient temperature without the need for cryogenic
facilities or temperature pretreatment. Undoubtedly, 1 could be
investigated for future theoretical and practical investigations of
spin-state propagation in the crystalline state. In particular, we
could note that 1 not only selectively senses alcohols but also
selectively detects hydrochloric acid among sulfuric, nitric,
perchloric, and acetic acids in the solid state. Interestingly, 1 also
responds to “mechanical friction” with reversible optical
Q
4
2
8
octahedra, which are more distorted than in 1. Not all iron sites
are switched, which could be due to crystallographic
modifications that restrict further MeOH diffusion. Reversibility
̈
was also confirmed by Mossbauer spectroscopy, which shows the
complete disappearance of the LS signal in 3 after having
experienced a water vapor atmosphere (Figure 2a).
The real-time observation of vapochromism and spatiotem-
poral aspects of the spin-state change with subsequent evolution
of phase boundaries could be captured with a high-resolution
optical microscope. A neat crystal of 1 was placed in a small Petri
dish, which was immersed in a large Petri dish containing dry
MeOH. The whole assembly was covered and sealed. The
microscope is focused on the tip of the crystal, and images were
taken at successive intervals of time (Figure 4). The nucleation of
11
responsiveness. The fabrication of alcohol detection strips for
mobile detector applications is planned.
ASSOCIATED CONTENT
Supporting Information
X-ray crystallographic data in CIF format, experimental section
with a reaction scheme, instrumentation, crystallographic data
■
*
S
(
̈
Tables S1−S3), Mossbauer parameters (Table S4), a hydrogen-
bonding network (Figure S1), PXRD and TGA (Figure S2),
FTIR and Raman (Figure S3), and visualization of crystal faces
and Miller indices (Figure S4). This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail: yann.garcia@uclouvain.be. Fax: (+)3210472330.
■
*
Notes
Figure 4. Real-time microscope imaging of a vapochromic spin-state
change in 1 (2−3-mm-long crystal) at room temperature.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was partly funded by the FRS-FNRS (Grants FRFC
.4508.08 and 2.4537.12), RNASR, CNCS−UEFISCDI (Grant
PN-II-RU-TE-2011-3-0307), Romanian Academy, WBI, and the
COST action MP1202.
■
color change begins at a corner, covers the whole corner, and
extends to the sides, and then the phase boundaries/color change
fronts begin to move toward the center. The transformation
time is drastically reduced when microcrystals are used. The
direction of propagation of the color change is consistent with
ligand exchange along the crystallographic a axis, along which the
channel of the coordinated water molecules is extended (Figure
2
9
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FeN core, as detected by Mo
̈
ssbauer spectroscopy. Despite only
6
1
a ∼ / fraction of the molecules undergoing spin-state switching
5
C
dx.doi.org/10.1021/ic402816a | Inorg. Chem. XXXX, XXX, XXX−XXX