Non-Porous Iron(II)-Based Sensor: Crystallographic Insights into a Cycle of Colorful Guest-Induced Topotactic Transformations

Article abstract of DOI:10.1002/anie.201608813

Materials capable of sensing volatile guests at room temperature by an easily monitored set of outputs are of great appeal for development as chemical sensors of small volatile organics and toxic gases. Herein the dinuclear iron(II) complex, [Fe^II₂(L)₂(CH₃CN)₄](BF₄)₄?2 CH₃CN (1) [L=4-(4-methylphenyl)-3-(3-pyridazinyl)-5-pyridyl-4H-1,2,4-triazole], is shown to undergo reversible single-crystal-to-single-crystal (SCSC) transformations upon exposure to vapors of different guests: 1 (MeCN)?2 (EtOH)→3 (H₂O)?1 (MeCN). Whilst 1 and 2 remain dimetallic, SCSC to 3 involves conversion to a 1D polymeric chain (due to a change in L bridging mode), which, remarkably, can undergo SCSC de-polymerization, reforming dimetallic 1. Additionally, SC-XRD studies of two ordered transient forms, 1TF3 and 2TF3, confirm that guest exchange occurs by diffusion of the new guests into the non-porous lattices as the old guests leave. These reversible SCSC events also induce color and magnetic responses. Indeed dark red 1 is spin crossover active (T_1/2↓ 356 K; T_1/2↑ 369 K), whilst orange 2 and yellow 3 remain high spin.

Full text of DOI:10.1002/anie.201608813

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201608813
German Edition: DOI: 10.1002/ange.201608813
Chemical Sensors
Non-Porous Iron(II)-Based Sensor: Crystallographic Insights into
a Cycle of Colorful Guest-Induced Topotactic Transformations
Santiago Rodrꢀguez-Jimꢁnez, Humphrey L. C. Feltham, and Sally Brooker*
Dedicated to Professor Leonard F. Lindoy on the occasion of his 80th birthday and to Marꢀa Trujillo Salmerꢂn
Abstract: Materials capable of sensing volatile guests at room
temperature by an easily monitored set of outputs are of great
appeal for development as chemical sensors of small volatile
organics and toxic gases. Herein the dinuclear iron(II) com-
systems that will not be damaged during cycles of uptake/
release of guests.
Applications similar to those of MOFs have been realized
by molecular systems, including catalytic hydrogenation[8e]
guest sequestration,[10a] chemical sensing,[8d,12] molecular elec-
tronics[9a,10b,13] and opto-electronics.[8c] For chemo-sensor
applications ideally the material should also have easily
monitored readout options.[14] In that regard, spin crossover
(SCO) complexes[15] are excellent candidates, as on applica-
tion of an external stimulus, such as temperature or pressure
variation, light irradiation, or the gain/loss of guest molecules,
they can reversibly switch between high-spin (HS) and low-
spin (LS) electronic states. Importantly, the HS and LS states
plex, [FeII (L)2(CH3CN)4](BF4)4·2CH3CN (1) [L = 4-(4-meth-
2
ylphenyl)-3-(3-pyridazinyl)-5-pyridyl-4H-1,2,4-triazole],
is
shown to undergo reversible single-crystal-to-single-crystal
(SCSC) transformations upon exposure to vapors of different
guests: 1 (MeCN)Ð2 (EtOH)!3 (H2O)Ð1 (MeCN). Whilst
1 and 2 remain dimetallic, SCSC to 3 involves conversion to
a 1D polymeric chain (due to a change in L bridging mode),
which, remarkably, can undergo SCSC de-polymerization,
reforming dimetallic 1. Additionally, SC-XRD studies of two
ordered transient forms, 1TF3 and 2TF3, confirm that guest
exchange occurs by diffusion of the new guests into the non-
porous lattices as the old guests leave. These reversible SCSC
events also induce color and magnetic responses. Indeed dark
ꢀ
usually have very different colors, molecular vibrations, M L
bond lengths, and magnetic responses, so there are multiple
options for easy readout. Crystalline molecular SCO materi-
als are highly sensitive to guest molecule exchange.[9,10b,16]
Herein we report a robust non-porous SCO-active
dinuclear iron(II) complex, [Fe2(L)2(MeCN)4](BF4)4·2MeCN
(1), that undergoes reversible SCSC transformations on
exposure to a variety of solvent vapors, with the associated
color changes providing easy readout of the identity of the
bound guest. 1,2,4-Triazole-type ligands, like L (Scheme 1),
1
1
red 1 is spin crossover active (T = fl 356 K; T = › 369 K), whilst
2
2
orange 2 and yellow 3 remain high spin.
R
obust porous metal–organic frameworks (MOFs)[1] are
a major class of material that can admit, often selectively,
small volatile organics or toxic gases, opening up many
applications from sensors[2] to greenhouse gas capture,[3] gas
storage,[4] increasing the octane rating of fuel,[5] and controlled
catalysis.[6] Most of these applications involve guest exchange
reactions[7] that take advantage of the robust polymeric
network of the MOF. Interestingly, guest exchange is also
possible in non-porous solids, where it occurs by diffusion
through the crystal lattice, and involves either formation/
[8]
ꢀ
cleavage of M L bonds or other reorganization (e.g.
conformational) to accommodate the guest.[9]
When guest exchange occurs in single crystals without
disrupting the crystallographic order, it is a single-crystal-to-
single-crystal (SCSC) transformation. Molecular materials
that undergo such transformations are relatively scar-
ce,[8c–f,9,10] due to the intrinsically larger challenge of main-
taining crystallographic cohesion in a molecular, not poly-
meric, solid. Hence careful design (“crystal engineering”)[11] is
required in order to access robust and flexible molecular
Scheme 1. Synthesis of the new ligand L which features pyridazine
(pink), toluene (green) and pyridine (blue) substituents at the 3, 4 and
5 positions of the 1,2,4-triazole. i) SeO2, pyridine, water; ii) SOCl2,
MeOH; iii) NH2NH2·H2O, EtOH; iv) NaOEt, EtBr; v) BuOH, 5 d reflux.
are known to be very suitable for the production of SCO
active iron(II) materials.[17] Functionalization at the 3- and 5-
positions by pyridazine and pyridine moieties was carried out
in order to facilitate (via favorable intramolecular CH···N;
Figures S7 and S12 in the Supporting Information, SI) the
formation of dimetallic complexes. A tolyl substituent was
employed at the 4-position as the use of that substituent on
a related ligand facilitated the first quantitative guest sensor
based on SCO.[12b] All of these aromatic rings in this new
ligand, L, also contribute to a rich intermolecular network of
[*] S. Rodrꢀguez-Jimꢁnez, Dr. H. L. C. Feltham, Prof. S. Brooker
Department of Chemistry and MacDiarmid Institute for Advanced
Materials and Nanotechnology, University of Otago
PO Box 56, Dunedin 9054 (New Zealand)
E-mail: sbrooker@chemistry.otago.ac.nz
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201608813.
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

Angewandte
Communications
Chemie
Figure 1. Summary of the observed cycle of transformations of 1. Top: photos of single crystals (note: different single crystals were used for each
data collection) of dinuclear complexes: dark red [FeII (L)2(MeCN)4] (BF4)4·2MeCN (1), light red [FeII (L)2(MeCN)4](BF4)4·0.5MeCN·0.5H2O
2
2
(1TF3), orange [FeII (L)2(EtOH)4](BF4)4 (2), light orange [FeII (L)2(H2O)4](BF4)4·2H2O·0.5EtOH (2TF3), and yellow
2
2
polymeric{[FeII (L)2(H2O)4](BF4)4}1 (3). Middle and bottom: two views of two columns of cations (three staggered p–p stacked cations shown in
2
each column) in each of the crystal lattices, viewed: (middle) along and (bottom) down the highlighted column (located between tolyl rings of
cations) of anions and/or lattice solvents (solvents, if present, shown in spacefill). Cations in wireframe; H and anions omitted for clarity. No
lattice solvent in 2 and 3; MeCN in 1; MeCN and H2O in 1TF3; EtOH and H2O in 2TF3.
p–p contacts (see SI). These design features work together to
enable production of millimeter-size single-crystals of dinu-
clear molecules of 1, in a robust yet flexible crystal lattice.
Exposure of dark red crystals of dinuclear [Fe2(L)2-
(MeCN)4](BF4)4·2MeCN (1) to ethanol vapor causes rapid
SCSC conversion to orange crystals of [Fe2(L)2(EtOH)4]-
(BF4)4 (2), both of which have been structurally characterized
(Figures 1, 2 and S14). This is reversible: exposure of 2 to
acetonitrile vapor reforms 1 (Figure 1). Interestingly, when
crystals of 1 or 2 are instead exposed to water vapor, both
undergo another SCSC transformation, producing yellow
crystals of polymeric {[Fe2(L)2(H2O)4](BF4)4}1 (3) (Figures 1,
2 and S27). Furthermore, crystals of dinuclear 1 can be
recovered from polymeric 3 by annealing crystals of 3 at
1208C then re-exposing them to MeCN vapor (see below).
The rate of guest exchange (Figure S37) is related to the
crystal size, with larger crystals taking longer to transform
completely, presumably due to slower guest diffusion into the
interior of the larger crystal, through the non-porous lattice.
The associated changes in color (dark red vs. orange vs.
yellow), bonding (dinuclear vs. polymeric) and magnetic
responses (spin crossover active versus fully high spin, see
later) are dramatic, yet crystallinity is retained during
1 (MeCN)Ð2 (EtOH)!3 (H2O)Ð1 (MeCN).
Two ordered transient forms,[18] between 1 and 3, [FeII -
2
(L)2(MeCN)4](BF4)4·0.5MeCN·0.5H2O (1TF3), and between
2 and 3, [FeII (L)2(H2O)4](BF4)4·2H2O·0.5EtOH (2TF3) were
2
captured and also characterized by single-crystal X-ray
diffraction (SC-XRD), in house (Figures 1, S9–S12 and S18–
S22).
ꢀ
All five of the structures are in the triclinic space group P1
(Table 1), with half of a dinuclear complex in the asymmetric
unit (two halves in the case of 1TF3) and inversion generating
the other half, and six coordinate iron(II) centers (Figure 1).
The octahedral distortion parameter S ranges widely, from 50
to 1128 (Table 1). At 100 K, the Fe–donor bond lengths
(1.954–1.961 ꢀ) are consistent[15a] with [LS-LS] for N6-coor-
dinated 1 and 1TF3, whereas N4O2-coordinated 2, 2TF3, and 3
(2.153–2.161 ꢀ) are fully HS (Tables 1 and S3).
Figure 2. Representation emphasizing the dramatic transformation
between the discrete dinuclear 1 with trans-MeCN co-ligands (left) and
the polymeric 3 with cis-H2O co-ligands (right), upon water vapor
exposure. Fe (green), N (blue), O (red), C (black).
2
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!

Angewandte
Communications
Chemie
II
ꢀ
Table 1: Selected crystallographic data for all five structures. All in the triclinic space group P1, and at 100 K (except 3 at 90 K): [Fe (L)2(MeCN)4]-
2
(BF4)4·2MeCN (1), [FeII (L)2(MeCN)4](BF4)4·0.5MeCN·0.5H2O (1TF3), [FeII (L)2(EtOH)4](BF4)4 (2), [FeII (L)2(H2O)4](BF4)4·2H2O·0.5EtOH (2TF3) and
2
2
2
{[Fe2(L)2(H2O)4](BF4)4}1 (3).
1
1TF3
2
2TF3
3
Solv. cryst./Fe2
Mr
2MeCN
0.5MeCN and 0.5H2O
1281.4
2759.1(8); Z=2
1.542
None
0.5EtOH and 2H2O
1218.77
1274.0(5); Z=1
1.589
0.1149/0.2654
2.153
N4O2
97.8
3.07
3.11
None
1159.7
1085.61(14); Z=1
1.774
0.0473/0.1271
2.161
N4O2
50.04
86.21
12.30
1333.97
1534.38(5); Z=1
1.444
0.0562/0.1751
1.954
N6
52.14
3.52
5.51
1271.91
1318.13(2); Z=1
1.602
0.0777/0.2007
2.158
N4O2
111.37
2.34
4.97
V [ꢂ3]; Z
1calcd [Mgm3]
R1/wR2
0.1486/0.3413
1.961/ 1.960[a]
N6
Av. Fe–donor [ꢂ]
Fe donor set
ꢁ [8][b]
52.20/ 55.60[a]
1.59/ 5.35[a]
7.94/ 7.06[a]
62.91/ 66.29, 89.04[f]
Triaz vs. Pdz [8][c]
Triaz vs. Pyr [8][d]
Tolyl twist [8][e]
79.72
78.46
85.14
88.54
[a] 1TF3 contains two independent iron centers. [b] Octahedral distortion parameter, ꢁ, is the sum of the deviations from 908 of the 12 cis angles in the
coordination sphere.[19] [c] Angle between the mean planes of the triazole ring and attached pyridazinyl ring. [d] Angle between the mean planes of the
triazole ring and attached pyridyl ring. [e] Angle between the mean planes of the triazole ring and attached tolyl ring. [f] 1TF3 contains two independent
iron centers, Fe1 and Fe2. For Fe2 the coordinated L has a 50:50 disordered tolyl ring (Figures S9 and S10).
In all four dinuclear complexes the two iron(II) centers
are sandwiched by two bis-bidentate L ligands which provide
a fairly flat equatorial plane and two triazole bridges between
them (see Figure 1, Table 1, SI). The coordination of each
iron(II) center is completed by a pair of trans (axial) donors:
MeCN for 1 and 1TF3, EtOH for 2, and H2O for 2TF3.
For the polymeric complex, 3, whilst the pyridine pocket
binds as before and the triazole bridges to the second iron(II)
center, the pyridazine pocket of L is very differently
coordinated. The pyridazine is twisted right away (86.218 to
triazole) in order to bind in a monodentate fashion to an
iron(II) center in the next complex, generating a polymeric
chain (Figures 1, 2 and S27). The N4O2 coordination is
completed by cis coordination of two H2O molecules.
Remarkably, these major changes, which convert what was
a discrete dinuclear complex into a polymer, also occur
without destruction of the crystal—and they are reversible:
SCSC de-polymerization reforms 1.
rotation of the tolyl rings with respect to the triazole (Table 1)
to facilitate guest diffusion. The tolyl rings also play an
important role in the packing, fine-tuning the spacing
between cations, which may also enhance the guest diffusion,
and subsequent accommodation of the guests as well as the
anions, whilst maintaining the integrity of the crystal
(Figure 1).
Whilst of modest quality, the structure determinations of
the ordered transient forms 1TF3 and 2TF3 provide impor-
tant crystallographic insights. Specifically, 1TF3 still shares
several features with 1, including MeCN bound LS iron(II)
dinuclear complexes. But instead of only disordered MeCN
molecules of solvation located in between the tolyl rings (as in
1), now there is a mixture of H2O and MeCN present, so some
MeCN has departed and some H2O has entered. This
snapshot indicates that the H2O is diffusing into the lattice,
perhaps aided by rotation of the tolyl rings (as noted above),
so that in the next stage it can coordinate in place of the
MeCN, to give H2O coordinated HS 3. Similarly, 2TF3
remains HS dinuclear iron(II) like in 2, but it already shares
several features with 3, including H2O bound to the HS
iron(II) centers, albeit trans to one another, prior to
polymerization to form cis-H2O coordinated polymer 3.
Unlike both 2 and 3, 2TF3 contains lattice solvent, specifically
the departing EtOH and excess H2O molecules: these are lost
on polymerization to form 3, the most dense of these
compounds (Figure S2). In summary, these snapshots indicate
that during these SCSC transformations the diffusion of new
guests into the non-porous crystal lattice occurs as the old
guests leave (Figure 1, Table 1).
The supramolecular array of p–p, CH···p, anion···p and H-
bonding found in 1 organizes the cations into a staggered
stack (Figures 1 and S8). This is maintained throughout these
SCSC transformations (Figures 1, S11, S15, and S20),
although for polymeric 3 the SCSC transformation from
discrete to polymeric, by Fe–N(pyridazine) bond cleavage/
formation, causes changes to the details (Figures 1, 2 and
S27). Interestingly, all of these transformations occur in an
SCSC (i.e. topotactic[20]) manner, with the cations remaining
ꢀ
stacked in columns, between which the BF4 anions/lattice
solvent are accommodated (Figure 1, colour shaded regions).
Lattice solvent is only present in three cases (1, 1TF3 and
2TF3) and is located in between the tolyl rings of L in the
columns of stacked cations (Figure 1; where lattice solvent is
disordered, both positions are shown).
There are no voids in any of these structures so guest
exchange must be by diffusion through the non-porous
lattices. This mostly occurs in an associative manner, with
new guests entering as the old guests leave, as both types of
guest are present in the transient form structures of 1TF3 and
2TF3. This non-disruptive dynamic process may be aided by
The density increases on SCSC transformation from
1 (MeCN)!2 (EtOH)!3 (H2O), from 1.444 to 1.602 to
1.774 Mgm3 (Table 1). Polymer 3 has the highest density, and
de-polymerization of 3 is the only SCSC transformation
described herein to require a drying step before introduction
of the new guest.
Powder X-ray diffraction (PXRD) revealed distinctive
peaks for 1 at 5.908, for 2 at 8.328, and for 3 at 7.088. These
matched those calculated from the SC-XRD data (Figure 3,
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

Angewandte
Communications
Chemie
Figure 4. cM T (per Fe) versus T (sweep mode at 2 Kminꢀ1) for
1 (maroon trace; 400–450-400 K), 2 (orange trace), and 3 (yellow
trace).
Figure 3. Sequence of PXRD plots showing (left) three cycles of the
reversible SCSC conversions of 1 (maroon trace; main peak: 5.908)Ð2
(orange trace, main peak: 8.328), on exposure to either EtOH or
MeOH vapor, as appropriate. (Right) cycle of SCSC transformations
starting from 1 (top, maroon trace) to 2 (orange trace) to 3 (yellow
trace) and back to 1 [carried out by exposure to EtOH, then H2O,
vapor; then annealing 3 at 1208C for 24 h before exposure to MeCN
vapor for 24 h (blue trace), with conversion complete after 48 h total
exposure to MeCN vapor (bottom, maroon trace)]. The PXRD spectra
calculated from the SC-XRD data for 1–3 are shown at the bottom, and
are a good match for the observed spectra.
by new guest uptake, is seen only for the de-polymerization
of 3.
Remarkably, 1 exhibits reversible SCSC guest molecule
exchange while producing a set of red, orange or yellow
readouts, signalling which of MeCN, EtOH or H2O guests,
respectively, have been taken up. These findings demonstrate
that such systems—non-porous molecular systems—have
great potential as chemical sensors for a range of volatile
guests.
Acknowledgements
bottom, Figures S29–S32). PXRD was therefore able to
monitor guest-exchange activity. Three cycles of dark red
1Ðorange 2 SCSC transformations were completed (Figure 3,
left). Similarly, a 1!2!3!1 cycle of SCSC transformations,
which includes polymerization, 2!3, and de-polymerization,
3!1, was successfully completed (Figure 3, right).
We thank the University of Otago (including award of a PhD
scholarship to S.R.J. and the 2015 purchase of a Versalab
magnetometer which enabled in house 50–400 K magnetic
measurements) and the Marsden Fund (RSNZ; including
a research fellowship to H.L.C.F.) for supporting this
research. We are grateful to Professors Peter J. Steel (Canter-
bury) and Geoffrey B. Jameson (Massey) for helpful dis-
cussions, and thank Lisa Bucke (Otago) for help with the
TOC.
These solid-state SCSC transformations, 1Ð2!3Ð1, can
also be followed by eye, due to the characteristic dark
redÐorange!yellowÐdark red colors (Figure 1).
Dark red 1 was magnetically characterized in a sealed
capsule to prevent solvent loss during the measurements. It
remains [LS-LS] at room temperature, but undergoes a grad-
ual SCO when heated to 400 K, with cM T (per Fe) reaching
2.45 emuKmolꢀ1, corresponding to about 80% [HS-HS].[15a]
Keywords: chemical sensor · crystal engineering ·
reversible guest exchange ·
single-crystal-to-single-crystal transformation · spin crossover
1
On cooling it exhibits a narrow hysteresis loop of 13 K (T = fl =
2
1
356 K; T = › = 369 K, Figure 4), a loop that is retained over at
2
least 8 warming–cooling cycles (Figures S33 and S34). In
contrast, orange 2 and yellow 3 remain HS (consistent with
SC-XRD at 100 and 90 K, respectively).
[1] a) S. R. Batten, R. Robson, Angew. Chem. Int. Ed. 1998, 37,
1460 – 1494; Angew. Chem. 1998, 110, 1558 – 1595; b) M.
Eddaoudi, D. B. Moler, H. Li, B. Chen, T. M. Reineke, M.
OꢁKeefe, O. M. Yaghi, Acc. Chem. Res. 2001, 34, 319 – 330.
[2] a) G. J. Halder, C. J. Kepert, B. Moubaraki, K. S. Murray, C. S.
Cashion, Science 2002, 298, 1762 – 1765; b) A. Galet, A. B.
Gaspar, M. C. MuÇoz, G. V. Bukin, G. Levchenko, J. A. Real,
Adv. Mater. 2005, 17, 2949 – 2953; c) P. D. Southon, L. Llu, E. A.
Fellows, D. J. Price, G. J. Halder, K. W. Chapman, B. Moubaraki,
In summary, these detailed crystallographic investigations,
especially of the two ordered transient forms, improve our
understanding of how SCSC transformations occur in non-
porous molecular solids like 1. Here the non-destructive guest
exchange processes mostly occur associatively, with new
guests coming in at the same time as the old guests leave. The
alternative, dissociative, that is, loss of the old guest followed
4
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!

Angewandte
Communications
Chemie
K. S. Murray, J.-F. Lꢂtard, C. J. Kepert, J. Am. Chem. Soc. 2009,
[11] G. R. Desiraju, Angew. Chem. Int. Ed. 2007, 46, 8342 – 8356;
131, 10998 – 11009; d) R. Ohtani, K. Yoneda, S. Furukawa, N.
Horike, S. Kitagawa, A. B. Gaspar, M. C. MuÇoz, J. A. Real, M.
Ohba, J. Am. Chem. Soc. 2011, 133, 8600 – 8605; e) E. Coronado,
G. Minguez Espallargas, Chem. Soc. Rev. 2013, 42, 1525 – 1539.
[3] a) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D.
Bloch, Z. R. Herm, T.-H. Bae, J. R. Long, Chem. Rev. 2012, 112,
724 – 781; b) M. G. Cowan, R. G. Miller, P. D. Southon, J. R.
Price, O. Yazaydin, J. R. Lane, C. J. Kepert, S. Brooker, Inorg.
Chem. 2014, 53, 12076 – 12224, and front cover.
[4] a) A. R. Millward, O. M. Yaghi, J. Am. Chem. Soc. 2005, 127,
17998 – 17999; b) J. Mason, M. Veenstra, J. R. Long, Chem. Sci.
2014, 5, 32 – 51.
[5] Z. R. Herm, E. D. Bloch, J. R. Long, Chem. Mater. 2014, 26,
323 – 338.
Angew. Chem. 2007, 119, 8492 – 8508.
[12] a) S. Sabbani, S. K. Das, CrystEngComm 2015, 17, 8850 – 8857;
b) R. G. Miller, S. Brooker, Chem. Sci. 2016, 7, 2501 – 2505, and
front cover feature.
[13] M. Nihei, L. Han, H. Oshio, J. Am. Chem. Soc. 2007, 129, 5312 –
5313.
[14] O. S. Wenger, Chem. Rev. 2013, 113, 3686 – 3733.
[15] a) P. Gꢄtlich, H. A. Goodwin, Top. Curr. Chem. 2004, 233 – 235;
b) A. Bousseksou, G. Molnar, L. Salmon, W. Nicolazzi, Chem.
Soc. Rev. 2011, 40, 3313 – 3335; c) T. Matsumoto, G. N. Newton,
T. Shiga, S. Hayami, Y. Matsui, H. Okamoto, R. Kumai, Y.
Murakami, H. Oshio, Nat. Commun. 2014, 5, 3865; d) S.
Brooker, Chem. Soc. Rev. 2015, 44, 2880 – 2892, and front
cover feature.
[6] M. I. Gonzalez, E. D. Bloch, J. A. Mason, S. J. Teat, J. R. Long,
Inorg. Chem. 2015, 54, 2995 – 3005.
[16] a) R.-J. Wei, J. Tao, R.-B. Huang, L.-S. Zheng, Inorg. Chem.
2011, 50, 8553 – 8564; b) E. Coronado, M. Gimꢂnez-Marquꢂs, G.
Mꢃnguez Espallargas, F. Rey, I. J. Vitꢅrica-Yrezꢆbal, J. Am.
Chem. Soc. 2013, 135, 15986 – 15989.
[17] a) O. Kahn, E. Codjovi, Philos. Trans. R. Soc. London Ser. A
1996, 354, 359 – 379; b) J. G. Haasnoot, 1,2,4-Triazoles as
Ligands for Iron(II) High Spin-Low Spin Crossovers, Kluwer,
Dordrecht, 1996; c) J. A. Kitchen, S. Brooker, Coord. Chem.
Rev. 2008, 252, 2072 – 2092; d) G. Aromꢃ, L. A. Barrios, O.
Roubeau, P. Gamez, Coord. Chem. Rev. 2011, 255, 485 – 546.
[18] G. Aromꢃ, C. M. Beavers, J. Sanchez Costa, G. A. Craig, G.
Minguez Espallargas, A. Orera, O. Roubeau, Chem. Sci. 2016, 7,
2907 – 2915.
[7] a) J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen,
J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450 – 1459; b) H.
Kitagawa, Nat. Chem. 2009, 1, 689 – 690; c) L. E. Kreno, K.
Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne, J. T. Hupp,
Chem. Rev. 2012, 112, 1105 – 1125.
[8] a) E. Coronado, M. Gimenez-Marques, G. M. Espallargas, L.
Brammer, Nat. Commun. 2012, 3, 828; b) A. Lennartson, P.
Southon, N. F. Sciortino, C. J. Kepert, C. Frandsen, S. Mørup, S.
Piligkos, C. J. McKenzie, Chem. Eur. J. 2015, 21, 16066 – 16072;
c) M. Albrecht, M. Lutz, A. L. Spek, G. van Koten, Nature 2000,
406, 970 – 974; d) S. Supriya, S. K. Das, J. Am. Chem. Soc. 2007,
129, 3464 – 3465; e) Z. Huang, P. S. White, M. Brookhart, Nature
2010, 465, 598 – 601; f) J. Sundberg, L. J. Cameron, P. D. Southon,
C. J. Kepert, C. J. McKenzie, Chem. Sci. 2014, 5, 4017 – 4025.
[9] a) R.-J. Wei, Q. Huo, J. Tao, R.-B. Huang, L.-S. Zheng, Angew.
Chem. Int. Ed. 2011, 50, 8940 – 8943; Angew. Chem. 2011, 123,
9102 – 9105; b) J. S. Costa, S. Rodrꢃguez-Jimꢂnez, G. A. Craig, B.
Barth, C. M. Beavers, S. J. Teat, G. Aromꢃ, J. Am. Chem. Soc.
2014, 136, 3869 – 3874.
[19] a) F. A. Deeney, J. H. Charles, G. G. Morgan, V. McKee, J.
Nelson, S. J. Teat, W. Clegg, J. Chem. Soc. Dalton Trans. 1998,
1837 – 1843; b) P. Guionneau, C. Brigouleix, Y. Barrans, A. E.
Goeta, J.-F. Lꢂtard, J. A. K. Howard, J. Gaultier, D. Chasseau, C.
R. Acad. Sci. Ser. IIc 2001, 4, 161 – 171.
[20] G. B. Jameson, H. R. Oswald, H. R. Beer, J. Am. Chem. Soc.
1984, 106, 1669 – 1675.
[10] a) J. L. Atwood, L. J. Barbour, A. Jerga, B. L. Schottel, Science
2002, 298, 1000 – 1002; b) B. Li, R.-J. Wei, J. Tao, R.-B. Huang,
L.-S. Zheng, Z. Zheng, J. Am. Chem. Soc. 2010, 132, 1558 – 1566.
Received: September 8, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Chemical Sensors
S. Rodrꢁguez-Jimꢂnez, H. L. C. Feltham,
S. Brooker*
&&&& — &&&&
Non-Porous Iron(II)-Based Sensor:
Crystallographic Insights into a Cycle of
Colorful Guest-Induced Topotactic
Transformations
Three guests, three colours: Reversible
three way-guest sensing by a diiron(II)
complex (blue sphere Fe) results in
interconversion of dark red low spin
1 (red box MeCN) with high spin orange 2
(orange triangle EtOH) and yellow 3
(yellow circle H2O). Remarkably these
guest exchanges are reversible single-
crystal-to-single-crystal (SCSC) transfor-
mations. Crystallography also reveals that
SCSC transformation 1Ð3 involves
a polymerizationÐdepolymerization.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!