A thermal- and light-induced switchable one-dimensional rare loop-like spin crossover coordination polymer

Article abstract of DOI:10.1039/c9dt03285a

Rare loop-like isostructural one-dimensional coordination polymer (1D-CP) systems formulated as {Fe(DPIP)₂(NCSe)₂}_n·4DMF (1) and {Fe(DPIP)₂(NCSe)₂}_n·4DMF (2) were obtained by self-assembling Fe^II and pseudohalide NCX^-(X = S, Se) ions in presence of the V-shaped bidentate bridging ligand, namely, N,N′-dipyridin-4-ylisophthalamide (DPIP), and were characterized by elemental analysis, IR spectroscopy, TGA, single crystal X-ray diffraction and powder X-ray diffraction. The magnetic studies show that complex 2 undergoes a complete thermally induced spin crossover (SCO) behavior centered at T_1/2 = 120 K with ca. 5 K thermal hysteresis loop and light-induced excited spin state trapping effect (LIESST) with T_LIESST = 65 K. However, either the homologous X = S (1) or the desolvated form of complex 2 is high spin at all temperatures, proving further the concerted synergy for the SCO of 2 between the intrinsic ligand field and that indirectly induced via hydrogen bond interaction. The current results provide valuable information for the design of new 1D SCO systems via the rational control of the cooperated effects derived from the intramolecular coordination bond and the intermolecular supramolecular interactions.

Full text of DOI:10.1039/c9dt03285a

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A thermal- and light-induced switchable one-
dimensional rare loop-like spin crossover
coordination polymer†
Cite this: Dalton Trans., 2019, 48,
17014
Wenlong Lan, ‡a Francisco Javier Valverde-Muñoz,‡b Yong Dou,a Xiaoyun Hao,a
b
M. Carmen Muñoz, c Zhen Zhou,a Hui Liu,a Qingyun Liu,d José Antonio Real
*
a
and Daopeng Zhang
*
Rare loop-like isostructural one-dimensional coordination polymer (1D-CP) systems formulated as
{Fe(DPIP)2(NCSe)2}n·4DMF (1) and {Fe(DPIP)2(NCSe)2}n·4DMF (2) were obtained by self-assembling FeII
and pseudohalide NCX−(X = S, Se) ions in presence of the V-shaped bidentate bridging ligand, namely,
N,N’-dipyridin-4-ylisophthalamide (DPIP), and were characterized by elemental analysis, IR spectroscopy,
TGA, single crystal X-ray diffraction and powder X-ray diffraction. The magnetic studies show that
complex 2 undergoes a complete thermally induced spin crossover (SCO) behavior centered at T1/2
=
120 K with ca. 5 K thermal hysteresis loop and light-induced excited spin state trapping effect
(LIESST) with TLIESST = 65 K. However, either the homologous X = S (1) or the desolvated form of complex
2 is high spin at all temperatures, proving further the concerted synergy for the SCO of 2 between the
intrinsic ligand field and that indirectly induced via hydrogen bond interaction. The current results provide
valuable information for the design of new 1D SCO systems via the rational control of the cooperated
effects derived from the intramolecular coordination bond and the intermolecular supramolecular
interactions.
Received 13th August 2019,
Accepted 22nd October 2019
DOI: 10.1039/c9dt03285a
rsc.li/dalton
complexes, because they offer a reversible drastic change
Introduction
2
between the paramagnetic high spin S = 2 (HS, t2g4eg ) and the
0
Responsive switchable materials have always been given wide- diamagnetic low-spin S = 0 (LS, t2g6eg ) electronic states. The
spread attention in the search for new advanced materials transformation between the two types of the spin states can be
with potential applications. One outstanding example of mole- driven by external stimuli (temperature, pressure, light, and
cular switching is featured by the SCO behaviour displayed by analytes), accompanied by concomitant drastic changes of
coordination complexes centred on transition metals with magnetic, optical (colour) and electrical properties as well as
3d4–3d7 electronic configurations. Among these, the most drastic responses in structure, which provide an important set
investigated molecular materials are pseudo-octahedral iron(II) of useful outputs.1
Since the seminal work on the iconic mononuclear com-
plexes [Fe(phen)2(NCS)2] and [Fe(2,2′-bipy)2(NCS)2],2,3 a great
aCollege of Chemical and Chemical Engineering, Shandong University of Technology,
deal of [FeII(L)p(NCX)2]-based (X = S, Se) SCO materials have
Zibo 255049, PR China. E-mail: dpzhang73@126.com
been reported during the past sixty years.1d The most prolific
series corresponds to mononuclear complexes derived from
[phen/2,2′-bipy]-type α-diimine chelate ligands, which often
favour the cis conformation of the ligands.4 Inspired by the
same idea, relevant series of related bipyrimidine-,5,6 pyrazo-
bInstituto de Ciencia Molecular (ICMol), Universitat de València, C/Catedrático José
Beltrán Martínez 2, 46980 Paterna, Valencia, Spain. E-mail: jose.a.real@uv.es
cDepartamento de Física Aplicada, Universitat Politècnica de València, Camino de
Vera s/n, E-46022 Valencia, Spain
dCollege of Chemical and Environmental Engineering, Shandong University of
Science and Technology, Qingdao 266510, PR China
late-,7–9 triazolate-,10 pyridine-11
and 4,4′-bipyridine-
†Electronic supplementary information (ESI) available: Some important struc-
tural parameters for complex 1. The representative cell packing diagram for com-
plexes 1 and 2. The magnetic property for complex 1. The TGA analysis for
complex 2. The calculated and measured pattern of PXRD and the IR spectra for
complexes 1 and 2. Perspective views of the packing of complex 2. CCDC
1897988–1897990. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c9dt03285a
bridged12–14 FeII dinuclear SCO complexes containing ancillary
imine ligands and NCX− groups have also been reported.15–17
The use of bis-monodentate pyridine-like bridging-ligands has
afforded a much more reduced but relevant series of trans-
[FeII(μ-L)2(NCX)2]n 2D-SCO compounds,18–23 some of which
constitute excellent examples of porous coordination polymers
‡These two authors contributed equally.
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0.5 K min−1 using a Quantum Design MPMS2 SQUID suscept-
ometer equipped with a 5.5 T magnet, operating at 1 T and
temperatures in the range 300–10 K. Experimental suscepti-
bilities were corrected from diamagnetism of the constituent
atoms by the use of Pascal’s constants. Powder X-ray measure-
ments were performed on a PANalytical Empyrean X-ray
powder diffractometer (monochromatic MoKα radiation).
Thermogravimetric analysis was performed on a Mettler
Toledo TGA/SDTA 851e instrument in the 298–1290 K tempera-
Scheme 1 The formula of the bridging ligand DPIP.
ture range under
10 K min−1
a nitrogen atmosphere at a rate of
(CPs) made up of interpenetrated 2D layers.24–29 Even less
common is the family of 1D-SCO CPs generically formulated
as [FeII(L)p(NCX)2]n (p = 1, 2). The use of chelate bis-bidentate
bridging ligands based on 4,6-bis(dipyridylamino)-3,4,5-
triazine30–32 and tris-bidentate α-diimine ligand 1,4,5,8,9,12-
hexaazatriphenylene (HAT)33 has provided a series of SCO
chains (p = 1) with NCX− trans- and cis-coordinated FeII
centres, respectively. Interestingly, the HAT derivative is an
.
General procedures and materials
All the reactions were performed at room temperature under
an air atmosphere and all the chemicals and solvents used
were of reagent grade without further purification. FeCl2,
KSCN and KSeCN were purchased from commercial sources.
The ligand N,N′-dipyridin-4-ylisophthalamide (DPIP) was pre-
pared according to the reported method.36
Synthesis of complexes 1 and 2. These two complexes were
prepared using a three-layer diffusion method. The ligand
DPIP (0.2 mmol, 63.6 mg) dissolved in 5 mL DMF was laid in
the bottom of a tube, to which a mixture solvent of DMF and
MeOH in a ratio of 1 : 1 was carefully added. Then, the metha-
nol solution containing Fe(SCN)2 (0.1 mmol) or Fe(SeCN)2
(0.1 mmol), which was freshly prepared by filtering the
mixture of KSCN (0.2 mmol, 19.4 mg) or KSeCN (0.2 mmol,
28.8 mg) and FeCl2 (0.1 mmol, 12.6 mg) in methanol, was care-
fully added to the top of the mixed solvent layer. Single crystals
suitable for X-ray diffraction were obtained about two weeks
later, which were then collected by filtration and dried in air
giving the yield about 50%.
uncommon example of the chiral 1D-SCO complex.
A
rare example of 1D-SCO [FeII(L)(NCX)2]n, exhibiting the
[FeN5S] coordination sphere and based on the tridentate
ligand
L tris-(2-pyridyl)metoxymethane has recently been
reported.34
To the best of our knowledge, only two 1D systems with
formula trans-[FeII(μ-L)2(NCX)2]n, showing SCO described so
far, have been reported by Tao et al. One is based on the brid-
ging ligand L = 5,6-di(pyridin-4-yl)-1,2,3,4-tetrahydropyrazine
formed in situ. Its thiocyanate derivative (X = S) displays an
incomplete (ca. 50%) thermally induced SCO behavior.35a The
second example is afforded by the bridging ligand 4,4′-dipyri-
dyl disulfide (dpds). In this recent study, Tao et al. investigated
NCX− anions with X = C(CN)2, BH3 and Se. The two former
derivatives display complete SCO, while the selenocianate com-
pound is HS at all temperatures.35b
Anal. calcd for C50H56FeN14O8S2 (1). C, 54.54; H, 5.13; N,
17.81. Found: C, 54.48; H, 5.01; N, 17.95. Main IR bands
(cm−1): 2056 (s, νSCN), 1680 (s, νCvO), 3074 (m, νN–H).
Anal. calcd for C50H56FeN14O8Se2 (2). C, 50.26; H, 4.72; N,
16.41. Found: C, 50.12; H, 4.61; N, 16.56. Main IR bands
(cm−1): 2056 (s, νSCN), 1680 (s, νCvO), 3075 (m, νN–H).
As a further step in this line, herein, we report a new rare
loop-like 1D system formulated as {Fe(DPIP)2(NCX)2}n·4DMF
(X = S (1), Se(2)), where DPIP is the V-shaped bridging ligand
L
=
N,N′-dipyridin-4-ylisophthalamide
(Scheme
1).
X-ray data collection and structure refinement. Single crys-
tals of the two complexes for the X-ray diffraction analysis
with suitable dimensions, which were selected from the
mother liquor for direct measurement, were mounted on a
glass rod, and the crystal data were collected on a Bruker
SMART CCD diffractometer using a MoKα sealed tube (λ =
0.71073 Å) at 293 K and 100 K, using a ω scan mode. The
structures were solved by direct method and expanded using
Fourier difference techniques with the SHELXTL-2014
program package. The non-hydrogen atoms were refined ani-
sotropically, while hydrogen atoms were introduced as fixed
contributors. All non-hydrogen atoms were refined with an-
Interestingly, while 1 and the desolvated form of 2 (2*) are
fully paramagnetic in the whole temperature range, the
parent solvated form 2 displays a complete thermal- and
photo-induced SCO behavior, showing the concerted synergy
between the intrinsic ligand field stemming from the DPIP/
NCSe and that indirectly induced through interactions such
as intermolecular interactions from the guest solvent
molecules.
Experimental section
Elemental analyses of carbon, hydrogen, and nitrogen were isotropic displacement coefficients, assigned isotropic dis-
performed using an Elementary Vario El. The infrared spec- placement coefficients U(H) = 1.2U(C) or 1.5U(C) and their
troscopy on KBr pellets was performed on a Magna-IR 750 coordinates were allowed to ride on their respective carbons.
spectrophotometer in the 4000–400 cm−1 region. Variable- CCDC 1897988–1897990 for complexes 1, 2-HS (293 K) and 2-
temperature magnetic susceptibility data were recorded for LS (100 K) are included in the supplementary crystallo-
single crystals of 1 and 2 (10–20 mg) at the scanning rate of graphic data for this paper.† Details of the crystal para-
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Table 1 Details of the crystal parameters, data collection, and refinement for complexes 1 and 2
1 (293 K)
2-HS (293 K)
2-LS(100 K)
Formula
FW
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C50H56FeN14O8S2
1101.06
Monoclinic
C2/c
25.3667(12)
14.6482(5)
17.1303(9)
90
C50H56FeN14O8Se2
1194.86
Monoclinic
C2/c
25.240(2)
14.5659(8)
17.2422(14)
90
C50H56FeN14O8Se2
1194.86
Monoclinic
C2/c
25.1307(14)
14.3256(8)
17.0759(11)
90
α (Å)
β (Å)
118.455(6)
90
5596.2(4)
4
1.307
0.407
2304
3.07 to 25.01
4860
3116
1.035
117.624(10)
90
5616.3(7)
4
1.413
1.628
2448
3.07 to 25.01
4935
3497
1.048
118.119(8)
90
5421.9(6)
4
1.464
1.686
2448
3.10 to 25.01
4758
3836
1.050
γ (Å)
V (Å3)
Z
ρcalcd (g cm−3
)
μ (mm−1
F(000)
θ (°)
)
Unique reflections
Reflections (I > 2σ)
GOF on F2
R1[I > 2σ(I)]
0.0783
0.2336
0.848/−0.659
0.0629
0.1845
1.521/−0.751
0.0569
0.1463
1.615/−0.976
wR2 (all data)
ρmax/ρmin(e Å−3
)
Table 2 Selected bond lengths (Å) and angles (°) for complex 2
meters, data collection, and refinement are summarized in
Table 1.
2(HS 293 K)
2*(LS 100 K)
Synthesis and general characterization
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(1)#1
Fe(1)–N(2)#1
Fe(1)–N(5)#2
Fe(1)–N(5)#3
2.132(4)
2.215(4)
2.132(4)
2.215(4)
2.196(4)
2.196(4)
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(1)#1
Fe(1)–N(2)#1
Fe(1)–N(5)#2
Fe(1)–N(5)#3
1.960(4)
2.013(4)
1.960(4)
2.013(4)
2.024(4)
2.024(4)
The title compounds {Fe(DPIP)2(NCX)2}n·4DMF X = S (1) and
Se (2) were synthesized by a three-layer slow diffusion method
using a DMF solution of DPIP (bottom layer) and a methanol
solution of Fe(NCX)2 (top layer) in stoichiometric molar ratio
2 : 1, for which the two solutions were separated by a
1 : 1 mixture of DMF/Methanol. Single crystal X-ray analysis for
both complexes was performed at 293 K. Furthermore, for the
sake of fully disclosing its LS state, the crystal structure ana-
lysis for 2 was also performed at 100 K (based-on the magnetic
property). Both complexes were characterized by IR spec-
troscopy and X-ray Powder diffraction (XRPD). The IR spectra
(Fig. S1, ESI†) display the presence of a sharp peak character-
istic of the stretching mode CuN at about 2056 cm−1 charac-
teristic of the NCX− (X = S, and Se) groups. The peak at about
1680 cm−1 can be assigned to the stretching vibrations of the
CvO amide group. XRPD (Fig. S2 and S3†) confirmed that the
obtained single crystals and the bulk materials correspond to
the same crystalline phase. Furthermore, the thermal analysis
confirmed the loss of the four DMF in one sharp step at about
450 K (Fig. S4†).
N(1)–Fe(1)–N(5)#2
N(1)#1–Fe(1)–N(5)#2 94.45(15)
N(1)–Fe(1)–N(5)#3 94.45(15)
N(1)#1–Fe(1)–N(5)#3 89.39(15)
N(5)#2–Fe(1)–N(5)#3 90.2(2)
N(1)–Fe(1)–N(2)#1
N(1)#1–Fe(1)–N(2)#1 89.08(16)
N(5)#3–Fe(1)–N(2)#1 90.58(14)
N(1)–Fe(1)–N(2)
N(1)#1–Fe(1)–N(2)
N(5)#2–Fe(1)–N(2)
N(2)#1–Fe(1)–N(2)
89.39(15)
N(1)–Fe(1)–N(5)#2
N(1)#1–Fe(1)–N(5)#2 89.40(15)
N(1)–Fe(1)–N(5)#3 89.40(15)
N(1)#1–Fe(1)–N(5)#3 88.09(15)
N(5)#2–Fe(1)–N(5)#3 89.9(2)
N(1)–Fe(1)–N(2)#1
N(1)#1–Fe(1)–N(2)#1 89.69(15)
N(5)#3–Fe(1)–N(2)#1 89.9(2)
N(1)–Fe(1)–N(2)
N(1)#1–Fe(1)–N(2)
N(5)#2–Fe(1)–N(2)
N(2)#1–Fe(1)–N(2)
88.09(15)
87.04(15)
92.83(15)
89.08(16)
87.04(15)
90.58(14)
88.9(2)
89.69(15)
92.83(15)
90.46(14)
89.3(2)
Symmetry codes: 2: #1 −x, y, −z + 1/2; #2 x, y + 1, z; #3 −x, y + 1, −z +
1/2; #4 x, y − 1, z. 2*: #1 −x, y, −z + 1/2; #2 −x, y − 1, −z + 1/2; #3 x, y −
1, z; #4 x, y + 1, z.
two independent pyridinecarboxamido ligands and the two
axial trans N atoms belong to the N atoms of the XCN groups.
At 293 K, the Fe–Naxial bond lengths of 2.121(6) and 2.132(4) Å
Crystal structure
Selected bond lengths and angles for complexes 1 and 2 are for 1 and 2, respectively, are slightly shorter than the Fe–
given in Table S1 (ESI)† and Table 2, respectively. Compounds Nequatorial bond lengths, 2.207(5), 2.234(5) (1) and 2.215(4),
1 and 2 are isostructural and crystallize in the monoclinic C2/c 2.196(4) (2) Å, indicating a slightly compressed octahedral geo-
space group. Fig. 1 displays the coordination environment of metry for the FeII ion. At 100 K, the average Fe–N bond length
the crystallographically unique FeII centre together with the for complex 2, equal to 1.999(4) Å, is 0.182 Å shorter than that
corresponding atom numbering. The FeII site is situated at the at 293 K (2.181(4) Å). This significant change (over 9%
centre of a slightly distorted [FeN6] octahedron, in which the reduction in bond lengths), consistent with the magnetic
four equatorial positions are occupied by four N atoms from behaviour of 2 (vide infra), is attributed to the occurrence of a
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14.326(2) Å in the LS state and slightly increases until
14.566(1) Å in the HS sate. The chains superpose perfectly
along the a axis, defining layers that stack along the c axis. The
shortest Fe⋯Fe distances between adjacent layers are 9.035(1),
12.40(6), 14.464(1) and 18.70(4) Å in the LS structure and
change, serially, to 9.039(1), 12.39(8), 14.571(1) and 18.85(5) Å
in the HS state (see Fig. S5†). In both complexes, the pseudo-
square grids are stabilized by 4 strong hydrogen bond inter-
actions established between the amido N atoms and the O
atoms of the 4 DMF molecules, which block the windows of
the grids (Fig. 2c). The comparative strong intermolecular N–
H⋯O hydrogen bond interactions are with the D⋯A distances
2.911 and 2.879 Å and the <DHA angles 164.1 and 164.08°
(Fig. 2c). In addition, there also exists weak intermolecular C–
H⋯N interactions between the benzene moiety and the N
atom of the XCN group, especially in LS state of 2 (Fig. S6†).
Fig. 1 ORTEP representation of the coordination environment of
complex 2 (100 K). Thermal ellipsoids are drawn at 50% probability.
Hydrogen atoms have been omitted for clarity.
practically complete spin conversion from HS to LS of the FeII.
This spin conversion also affects the coordination bond angles
around the FeII centre. Indeed, the parameter ∑Fe, corres-
ponding to deviations from the ideal octahedron of the 12
Magnetic and photomagnetic properties
The thermal dependence of χMT product, χM being the mag-
netic susceptibility per mole of the compound and T the temp-
erature, for samples of 1 and 2 constituted of single crystals
were measured under an external magnetic field of 2000 Oe in
the range of 10–300 K at the scan temperature rate 1 K min−1
(Fig. 3). For complex 1, the room temperature χMT value is
3.75 cm3 K mol−1, consistent with the typical value of FeII in
the HS state characterised by relevant orbital contribution.
(Fig. S7†) Upon cooling, the χMT value remains almost con-
stant until about 30 K, indicating that the FeII centres are HS
in the whole range of temperatures.
12
P
“cis” bond angles ( jθi ꢀ 90°j), is 20.34°at 293 K and
i¼1
decreases significantly down to 12.66° at 100 K, which is con-
sistent with the more regular octahedral geometry typically
observed for the LS state of the FeII.
In complexes 1 and 2, the two independent V-shaped equa-
torial DPIP ligands act as bridges connecting the FeII centres,
thus generating a loop-like 1D chain running along the b axis
(Fig. 2a and b). For 2, the intra-chain Fe–Fe distance is
Similarly, the χMT value is 3.72 cm3 K mol−1 in the tempera-
ture range 300–150 K for 2 (Fig. 3). However, below this temp-
erature, χMT drops sharply down to 0.15 cm3 K mol−1 at 50 K,
showing the occurrence of a complete SCO from the HS state
to the LS state. In the heating mode, χMT vs. T plot does not
Fig. 2 Side (a) and top (b) views of the looped-like 1D chain structure
of 1 and 2. (c) Hydrogen bonds (red-white broken lines) between the
four DMF molecules and the corresponding amido groups. The DMF
molecules have been highlighted with black bonds.
Fig. 3 Magnetic and photomagnetic properties of 2. (inset) Pictures of
one single crystal of 2 in the HS state (yellow) and LS state (dark red-
violet).
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match that of the cooling mode, thereby defining a narrow hys-
In addition to strong thermochromism (inset Fig. 3), com-
teresis ca. 5 K wide. The characteristic equilibrium tempera- pound 2 displays a light-induced excited spin state trapping
tures, T1/2, at which the molar fraction of HS and LS centres is effect (LIESST) at a low temperature.38 The photo-generation of
equal to 0.5 are ca. 118.0 K and 123.0 K for the cooling and the metastable HS* state was performed at 10 K, irradiating
heating modes, respectively. This relative low equilibrium the red light (λ = 633 nm) with a microcrystalline sample of 2
temperature (average 120 K) for the SeCN− derivative justifies (ca.3 mg) (Fig. 3). At 10 K, χMT is ca. 0.08 cm3 K mol−1 and
the lack of SCO for the SCN− counterpart, since it is well- increases sharply upon irradiation and reaches a χMT satur-
known that T1/2 values are usually 50–100 K higher for homolo- ation value of about 3.00 cm3 K mol−1 in less than one hour.
gous SeCN− derivatives corroborating the stronger ligand-field Then, the temperature was increased at a rate of 0.3 K min−1
strength induced by the latter. Interestingly, the crystals after the light irradiation was switched off. The χMT value con-
change from light yellow in the HS state to deep red in the LS tinued to slowly increase to a maximum value of ca. 3.56 cm3
state (see inset in Fig. 3).
The SCO behaviour of 2 has been simulated reasonably well light-induced population of the HS* state is complete. As the
K mol−1 up to a temperature of about 57 K, showing that the
(Fig. 4) using the regular solution model expressed in eqn (1):37
temperatures increased further from 57 K, the χMT value
dropped sharply, attaining a value of about 0.26 cm3 K mol−1
at ca. 75 K, indicative of a practically complete relaxation from
the metastable HS* to LS. The characteristic TLIESST tempera-
ture obtained from ∂χMT/∂T in the 10–100 K interval is ca.
67.5 K, perfectly agreeing with the correlation line TLIESST = T0
− 0.3T1/2 with T0 = 100 K, being typical for the less rigid coordi-
nation center [FeIIN6] constituted of monodentate ligands with
small geometrical constraints, as it is the present case.39
ꢀ
ꢁ
1 ꢀ γHS
ΔH þ Γð1 þ γRHS ꢀ 2γHS
Þ
ΔS
R
ln
¼
ꢀ
ð1Þ
γHS ꢀ γRHS
RT
where ΔH, ΔS and Γ are the enthalpy, the entropy variations
and the parameter accounting for the cooperative nature of the
spin conversion, respectively. The molar HS fraction, γHS, is
determined from the magnetic susceptibility through eqn (2):
The strong H-bonding interactions between the loop-like
1D host entity and the co-crystallized DMF guest solvent mole-
cules is denoted by the relatively high temperature, ca. 450 K,
from which the DMF molecules are released in a sharp step
giving 2desolv (see TGA in Fig. S4†). The desolvated phase partly
loses crystallinity, which is clearly observed when comparing
the respective X-ray powder diffraction patterns (see Fig. S8†),
which prevented the structural analysis of 2desolv. The perfect
consistency of the elemental analysis, IR spectra and magnetic
data support the idea that the chains remain despite the loss
of the DMF molecules. However, it seems obvious that in
γHS ¼ ½ðχMTÞ ꢀ ðχMTÞLSꢁ=½ðχMTÞHS ꢀ ðχMT
Þ
LSꢁ
ð2Þ
R
The molar fraction (γHS
)
accounts for the inactive species
that remain HS at low temperatures and is calculated as
follows (eqn (3)):
R
R
ðγHSÞ ¼ ðχMTÞ =½ðχMTÞHSꢁ
ð3Þ
(χMT) is the value of χMT at any temperature, (χMT)HS is the
χMT value of the pure HS state (T → ∞), (χMT)LS ≈ 0 is the χMT
value of the pure LS state and (χMT)R represents the residual χMT
value due to HS species blocked at low temperatures. The simu-
lation of the SCO curve gives the typical thermodynamic para-
meters ΔH = 7.92 kJ mol−1, Γ = 2.4 kJ mol−1 and ΔS = 66 J K−1
mol−1, using the values (χMT)R = 0.26 cm3 K mol−1 (at 80 K) and
(χMT)HS = 3.72 cm3 K mol−1 extracted from the magnetic data.
2
desolv, the packing of the chains must change drastically with
respect to that of 2 since the thermal dependence of χMT for
2desolv (Fig. 3) corresponds to a typical HS behavior at any
temperature, thereby showing the essential role played by the
DMF molecules in the stabilization of the LS state of 2 and,
consequently, in the observation of SCO.
Discussion on the cooperative character in the CP
As described above, the FeII centres remain HS in the S deriva-
tive (1) irrespective of the temperature, while the Se counter-
part (2) displays a 5 K wide hysteretic SCO behaviour centred
at 120.5 K and complete LIESST effect. The paramagnetic
nature of 1 and the relatively low T1/2 values of 2 are related to
the relatively weak ligand field induced by the bridging ligand
DPIP. Indeed, although in general the ligand strength felt by
the FeII centres depend to some extent on the crystal packing,
it is safe to infer that the amide moieties of DPIP have a
marked electron withdrawing effect on the pyridine groups,
thereby decreasing their σ-donor character, and thus stabilis-
ing the HS state of 1. This effect is partially mitigated in 2
owing to the stronger ligand field induced by the NCSe−
anions. These results are consistent with those reported by
Tao et al. for the 1D-SCO CP based on the bridging ligand
Fig. 4 The simulation for the magnetic property of complex 2.
17018 | Dalton Trans., 2019, 48, 17014–17021
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L = 5,6-di(pyridine-4-yl)-1,2,3,4-tetrahydropyrazine (X = S),
Coming back again to complex 2, the structural rigidity
where the pyridine groups, coupled by a CvC bond, lack of conferred by the O⋯H–N hydrogen bonds to the chains
electron withdrawing groups. This thiocyanate derivative vanishes after removing the included DMF molecules. This
undergoes a gradual incomplete (ca. 50%) SCO centred at T1/2 fact obviously provokes profound modifications in the
≈ 160 K, which is ascribed to the rigidity of the bridging crystal packing, which must include the torsion angles of
ligand.35a The same group has investigated for a series of 1D the [Fe2(DPIP)4] loops, thereby stabilising the HS of 2desolv
CPs formulated as [FeII(dpds)2(NCX)2]n·S with X = Se (S = 3.5
.
H2O), BH3 and C(CN)2 and dpds = 4,4′-dipyridyl disulphide. In
this system, the pyridine moieties of dpds seem to generate
Conclusion
weaker ligand strength than DPIP, in contrast to 2, where the
NCSe− derivative is HS at any temperature. However, this weak
This report was undertaken to investigate new loop-like
1D-SCO CPs of the type [FeII(L)2(NCX)2]n based-on the
V-shaped bis-monodentate bridging ligand DPIP (DPIP = N,N′-
dipyridin-4-ylisophthalamide). The solvated SeCN− derivative
presents complete spin crossover behaviour with an obvious
thermal hysteresis loop and the complete photo-switchable
LIESST effect. In contrast, either the solvated NCS− derivative
or the desolvated SeCN− derivative keeps the high spin state of
FeII in the whole temperature range, providing clear infor-
mation that both the coordination field of XCN− and the guest
solvent put cooperative influence on the SCO behaviour. The
{Fe(DPIP)2(NCX)2}n complex is an excellent platform to further
investigate the role of other guests with different polarities,
steric effects and donor/acceptor abilities. This work is under-
way in our lab.
ligand field was compensated using the much stronger
−
pseudo-halide coordinating anions [C(CN)3]− and NCBH3
,
which induce rather gradual SCO behaviours to the corres-
ponding complexes.
As mentioned in the introduction, alternative approaches to
[FeII(L)(NCX)2]n related 1D-SCO CPs were proposed by Neville,
Murray and Real et al. using polydentate chelating ligands,
which were able to induce stronger ligand fields and hence
higher T1/2 characteristic temperatures.30–33 Despite the appar-
ent more robust and rigid nature of the reported chains, the
SCO behaviours are rather gradual compared with the title com-
pound 2. In general, it can safely be stated that the thermo- and
photo-induced SCO properties of 2 are unique in the series of
related compounds [FeII(L)p(NCX)2]n (p = 1, 2). We believe that
the cooperative nature of the SCO observed for 2 is due to the
effective hydrogen bonds O⋯H–N defined between the DMF
and the H–N moieties of DPIP (Fig. 2c). These interactions act
as strips hampering the potential rotation of the DPIP’s amide
arms. Indeed, a superposition of the HS and LS structures
clearly show that the [Fe2(DPIP)4] loops perfectly match in both
states (see Fig. 5), a fact that proves our hypothesis about the
rigidity of the structure as the principal cause of the cooperative
nature of the SCO of 2. The rotational degree of freedom in
related bridging ligands has also been considered as a shock-
absorber mechanism of the elastic interactions between the
SCO centres in a distinct series of 1D-SCO CPs with tetrazole or
triazole donor groups generically formulated as [Fe(L)3](X)2 and
[Fe(L)2(L′)2](X)2 (X = ClO4−, BF4−) where the SCO behaviour is
usually gradual.40,41 A representative number of studies dealing
with this and other aspects influencing the cooperativity in the
latter series of triple- and double-bridged chains is gathered
from ref. 42 and 43.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are very thankful for the support from the National Nature
Science Foundation of China (21671121, 21173006), the Spanish
Ministerio de Economíay Competitividad (MINECO), CTQ2016-
78341-P, Unidad de Excelencia María de Maeztu (MDM-2015-
0538), and Generalitat Valenciana (PROMETEO/2016/
147). F. J. V. M. thanks MINECO for a predoctoral FPI grant.
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