Solid Versus Solution Spin Crossover and the Importance of theFe-N - (X) Angle

Article abstract of DOI:10.1021/acs.inorgchem.7b01338

A new family of mononuclear [Fe^II(Rdpt)₂(NCE)₂] complexes (E = S, Se, or BH₃) is formed by 1:2 reaction of [Fe^II(pyridine)₄(NCE)₂] with the monotopic pyridyl triazole ligand 4-(4-methylphenyl)-3-(2-pyridinyl)-5-phenyl-4H-1,2,4-triazole (tolpyph). The three complexes are obtained as six different solvatomorphs: [Fe^II(tolpyph)₂(NCS)₂]·H₂O (1·H₂O), 1·1.5CH₃OH·0.5H₂O, [Fe^II(tolpyph)₂(NCSe)₂] (2), 2·1.5H₂O, [Fe^II(tolpyph)₂(NCBH₃)₂] (3), and 3·H₂O. Single-crystal X-ray diffraction reveals that 1·1.5CH₃OH·0.5H₂O and 2 are high-spin (HS) at 100 K, while 3 is low-spin (LS) at 100 K and HS at 373 K. Compound 3 is the first structurally characterized example of an [Fe^II(Rdpt)₂(NCE)₂]-type complex with NCBH₃ co-ligand: the crystal packing is dominated by aromatic stacking interactions. Solid-state magnetic measurements show that 1·H₂O and 2·1.5H₂O remain HS down to 50 K, whereas 3·H₂O undergoes spin crossover (SCO) with a T_1/2 of 309 K, slightly above room temperature. A literature survey of analogous trans-[Fe^II(Rdpt)₂(NCX)₂]-type complexes (53 distinct crystal structures) shows that for the complexes that are SCO active in the solid state the Fe-N - C(X) angle is usually close to straight, 162-178°, whereas it is usually lower, 142-159°, for the complexes that remain HS. UV-vis studies in CHCl₃ solution show that in each case the use of a 6:1 ratio of tolpyph/Fe(II) is required to ensure the iron(II) is present in solution as [Fe^II(tolpyph)₂(NCE)₂]. Interestingly, using this ratio, all three compounds are SCO-active in CDCl₃ solution-in dramatic contrast to the solid-state findings. Specifically, while compounds 1 and 2 are not SCO-active in the solid state (they remain HS), they undergo gradual SCO in CDCl₃ solution, with T_1/2 values of 290 and 310 K, respectively. In CDCl₃ solution, compound 3 has a T_1/2 value of 288 K, which is 21 K lower than in the solid state. These results highlight the differences between solid state (ligand field; crystal packing) and solution (ligand field; solvation) effects on SCO, with the latter studies revealing roomerature SCO for all three of these complexes.

Full text of DOI:10.1021/acs.inorgchem.7b01338

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Solid Versus Solution Spin Crossover and the Importance of the
Fe−NC(X) Angle
Santiago Rodríguez-Jimenez and Sally Brooker*
́
Department of Chemistry and the MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Otago, P.O. Box
56, Dunedin 9054, New Zealand
S
* Supporting Information
ABSTRACT: A new family of mononuclear [FeII(Rdpt)2-
(NCE)2] complexes (E = S, Se, or BH3) is formed by 1:2
reaction of [FeII(pyridine)4(NCE)2] with the monotopic pyridyl
triazole ligand 4-(4-methylphenyl)-3-(2-pyridinyl)-5-phenyl-4H-
1,2,4-triazole (tolpyph). The three complexes are obtained as
six different solvatomorphs: [FeII(tolpyph)2(NCS)2]·H2O (1·
H2O), 1·1.5CH3OH·0.5H2O, [FeII(tolpyph)2(NCSe)2] (2), 2·
1.5H2O, [FeII(tolpyph)2(NCBH3)2] (3), and 3·H2O. Single-
crystal X-ray diffraction reveals that 1·1.5CH3OH·0.5H2O and 2
are high-spin (HS) at 100 K, while 3 is low-spin (LS) at 100 K
and HS at 373 K. Compound 3 is the first structurally
characterized example of an [FeII(Rdpt)2(NCE)2]-type complex
with NCBH3 co-ligand: the crystal packing is dominated by aromatic stacking interactions. Solid-state magnetic measurements
show that 1·H2O and 2·1.5H2O remain HS down to 50 K, whereas 3·H2O undergoes spin crossover (SCO) with a T1/2 of 309 K,
slightly above room temperature. A literature survey of analogous trans-[FeII(Rdpt)2(NCX)2]-type complexes (53 distinct crystal
structures) shows that for the complexes that are SCO active in the solid state the Fe−NC(X) angle is usually close to straight,
162−178°, whereas it is usually lower, 142−159°, for the complexes that remain HS. UV−vis studies in CHCl3 solution show
that in each case the use of a 6:1 ratio of tolpyph/Fe(II) is required to ensure the iron(II) is present in solution as
[FeII(tolpyph)2(NCE)2]. Interestingly, using this ratio, all three compounds are SCO-active in CDCl3 solution−in dramatic
contrast to the solid-state findings. Specifically, while compounds 1 and 2 are not SCO-active in the solid state (they remain HS),
they undergo gradual SCO in CDCl3 solution, with T1/2 values of 290 and 310 K, respectively. In CDCl3 solution, compound 3
has a T1/2 value of 288 K, which is 21 K lower than in the solid state. These results highlight the differences between solid state
(ligand field; crystal packing) and solution (ligand field; solvation) effects on SCO, with the latter studies revealing room-
temperature SCO for all three of these complexes.
TCNQ, etc.), as SCO-active mononuclear [FeII(Rdpt)2-
he spin-crossover (SCO) phenomenon can be observed
(NCE)2] complexes are usually obtained.21,23
transition-metal ions.1−4 Systems based on
4
7
Tin d −d
Recently the synthesis of nonsymmetrical Rdpt-type ligands
in which one of the pyridine rings is replaced by a diazine
(RR′pt-type) or a phenyl ring (RR′pht-type) has been
explored as a means of modifying the preferred product type
(i.e., ligand binding mode and/or metal ion nuclearity),12,26,27
as well as tuning the SCO.21,23 For instance, the recently
reported pyridazine-substituted RR′pt-type ligand, Lpdz (Figure
iron(II) are the most often reported. SCO occurs when an
external stimulus is applied to induce a reversible switching
from a high-spin (HS) to a low-spin (LS) electronic state.
These stimuli range from temperature,1−4 pressure,5−8 guest
molecules,9−12 and external magnetic field13 to light irradi-
ation.14−18 To observe SCO, the metal center must have an
appropriate donor environment of intermediate ligand-field
strength. For iron(II) this is normally an octahedral N6
environment.1−4 1,2,4-Triazole-based ligands are appropriate
candidates for this purpose,19−23 and they are extremely
1), facilitated the formation of a dinuclear complex [FeII -
2
(Lpdz)2(MeCN)4](BF4)4·2MeCN that is SCO-active and
undergoes a cycle of single-crystal-to-single-crystal trans-
formations on changing of the guest present, acting as a
“three guests give three colors” sensor.12 Also, the pyrazine-
substituted RR′pt-type ligand Lpz (Figure 1) gave SCO-active
mononuclear [FeII(Lpz)2(NCE)2] with E = S, Se, and BH3, but it
remains ambiguous as to whether the iron(II) center is bound
3
5
versatile, as they can be readily functionalized in the C, C,
and/or 4N positions of the triazole ring (Figure 1). Examples of
such ligands include the bis-terdentate PMRT24 and PSRT25
ligands (Figure 1) and potentially the bis-bidentate Rdpt
(Figure 1) and dpt‑ (Figure 1, R absent) types of ligands,21,23
all of which have provided interesting SCO-active iron(II)
systems. The focus of this study is on the use of Rdpt-type
ligands with anionic co-ligands (i.e., NCS, NCSe, NCBH3,
Received: May 25, 2017
© XXXX American Chemical Society
A
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magnetic behavior of three of the solvatomorphs in the solid
state (two HS and one SCO-active complex) versus solution
(three SCO-active complexes). Complex 3 is the only one that
exhibits SCO in both the solid state and CDCl3 solution, with
T
1/2 values that differ by 21 K. All of the SCO events observed
are centered at approximately room temperature, which is a
highly desirable feature for potential switching, sensing, and
display applications.29−31 Additionally, the findings of a
literature survey of 53 crystal structures of analogous trans-
[FeII(Rdpt-type)2(NCX)2] complexes (X = S, Se, BH3, NCN,
N(CN)2, etc.),21,23,26,27,32,33,35,36,51−64 looking at the relation-
ship between Fe−NC(X) angle and solid-state SCO activity,
are presented.
RESULTS AND DISCUSSION
■
Ligand Synthesis. The synthesis of tolpyph (Scheme 1)
follows the general method described in 2004 by Klingele and
Brooker.28 It is a versatile synthesis, not least as there are two
possible routes to bring the three components together
(Scheme 1). Specifically, either the pyridine or the phenyl
substituent can be introduced as the hydrazide, with the other
instead being introduced as part of the thioamide reactant
(Scheme 1). Regardless of which route is employed, the
thioamides generally need to be activated by ethylation28 and
then promptly reacted (as the ethylated products are usually
unstable) with the appropriate hydrazide to form the triazole
ring. The ring-forming reaction to produce the desired triazole
ligand tolpyph requires a multiday reflux in the high boiling-
point solvent 1-butanol.28 Both routes 1 and 2 (Scheme 1)
readily generate ligand tolpyph in moderate yields (44% and
65%, respectively; route 2 has been previously reported).28
Inorganic Synthesis. The new family of mononuclear bis-
ligand complexes [Fe(tolpyph)2(NCE)2] (E = S, Se, or BH3)
are obtained by 2:1 reaction of tolpyph and [FeII(pyridine)4-
(NCE)2], in 2:1 CHCl3/CH3OH in air (Scheme 1), in
moderate to good yields (37−69%).
Figure 1. Existing bis-terdentate PMRT24 and PSRT25 ligands, and
potentially bis-bidentate Rdpt ligands21,23 (R = amino, methyl, phenyl,
tolyl, hexadecyl, etc.;12,21,23,26,27,32−36 R′ = pyrazinyl,26,27 pyridazinyl,12
quinolinyl, etc.23,34,37,38), as well as the non-symmetrically function-
12,26
alized RR′pt-type (Lpz and Lpdz
)
and RR′pht-type (tolpzph)27
ligands, which have produced many SCO-active iron(II) materials.
The [FeII(L)2(NCE)2]-type complexes of the new ligand tolpyph (in
box) are explored herein.
For the NCS complex, the dark red reaction filtrate gave,
after vapor diffusion of pentane, a few dark red needles of
[FeII(tolpyph)2(NCS)2]·1.5CH3OH·0.5H2O (1·1.5 CH3OH·
0.5H2O). When pentane is instead added directly to the dark
red filtrate, solvatomorph 1·H2O precipitates as an intense dark
red powder in 61% yield.
by the pyrazine or the pyridine ring of this potentially bis-
bidentate ligand.26 This ambiguity is removed by replacing one
of the pyridine rings by a phenyl ring to give a monotopic
bidentate RR′pht-type ligand, tolpzph (Figure 1), from which
the SCO-active and quantitative guest-sensing complex [Fe-
(tolpzph)2(NCS)2] was made.27 It is worth noting that in all
three of these interesting SCO-active examples the 4N-
substituent is a tolyl ring and that this may well also play a
key role in guest access.12,26,27 Because of these findings, R =
tolyl is also employed here, albeit no guest exchange studies
have been undertaken in the present study, as the focus lies
elsewherebut in any case the use of the same R group clearly
facilitates comparisons of magnetic behavior between these
previous complexes and the new generation of very closely
analogous complexes reported herein.
Herein, we report the preparation and structural and
magnetic characterizations of six solvatomorphs of three new
[FeII(L)2(NCE)2] complexes (with E = S, Se, and BH3) of the
parent pyridine/phenyl monotopic bidentate RR′pht-type ligand
4-(4-methylphenyl)-3-(2-pyridinyl)-5-phenyl-4H-1,2,4-triazole,
tolpyph (Figure 1, box).28 Complex [FeII(tolpyph)2-
(NCBH3)2] (3) is the first crystallographically characterized
mononuclear iron(II) Rdpt-type complex with NCBH3 as co-
ligand. Particularly noteworthy is the stark contrast between the
Orange block-like single crystals of [FeII(tolpyph)2(NCSe)2]
(2) and dark red-black block-shaped single crystals of
[FeII(tolpyph)2(NCBH3)2] (3) are obtained, in 37% and
69% yields, respectively, by slow vapor diffusion of either
diethyl ether or pentane, respectively, into the reaction
solutions. However, on standing in air they pick up moisture,
generating solvatomorphs 2·1.5H2O and 3·H2O.
Crystal Structure of tolpyph. Long colorless needle-like
crystals of tolpyph suitable for X-ray diffraction were grown by
slow evaporation of a solution of tolpyph in 5:1 CH2Cl2/
CH3OH (Figure S2). As usual for Rdpt-like ligands, the
pyridine nitrogen atom N(1) is pointing toward the tolyl group
and not toward the N(2) nitrogen atom of the triazole ring due
to the repulsion between the lone pairs of electrons of the
pyridine and triazole nitrogens.26,39−44 In 1,2,4-triazole systems
4
with an aromatic ring as the N-substituent the ring is usually
twisted almost perpendicular to the triazole ring,26,40−42,45−47
3
5
while the other ring substituents at C and C on the triazole
are usually close to being coplanar with the triazole ring. This is
also the case for tolpyph (tolyl vs triazole 69.63°; pyridine vs
triazole 29.63°; phenyl vs triazole 33.89°).
B
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Scheme 1. Two Successful Synthetic Routes to tolpyph
(Route 1, Left and Route 2,28 Right) as well as the Synthesis
of the [FeII(tolpyph)2(NCE)2]-type Complexes Reported
a
Herein
Figure 2. Perspective view of complex [FeII(tolpyph)2(NCS)2]·
1.5CH3OH·0.5H2O (1·1.5CH3OH·0.5H2O) at 100 K. The asym-
metric unit contains one-half iron(II) complex, one 0.75 occupancy
molecule of methanol, and one 0.25 occupancy molecule of water.
Both molecules of solvation are disordered over an inversion center.
Hydrogen atoms are omitted for clarity, except those belonging to the
lattice solvent molecules of water and methanol (green). Symmetry
operation A is −x, −y, −z.
Figure 3. Perspective view of the complex [FeII(tolpyph)2(NCSe)2]
(2) at 100 K. Hydrogen atoms are omitted for clarity. Symmetry
operation A is 1 − x, 2 − y, 1 − z.
a
Note that the numbering of ligand tolpyph corresponds to the peak
assignments of the 1H and 13C NMR spectra described in the
Experimental Section.
Figure 4. Perspective view of the complex [FeII(tolpyph)2(NCBH3)2]
(3) at 373 K. Note that structures of 3 at 100 and 373 K are
isostructural. Hydrogen atoms are omitted for clarity, except those that
belong to NCBH3 co-ligands (green). Symmetry operation A at 100
and 373 K is 1/2 − x, 1/2 − y, 1 − z.
Crystal Structures of Complexes. Single crystals suitable
for X-ray diffraction were obtained for [FeII(tolpyph)2-
(NCS)2]·1.5CH3OH·0.5H2O (1·1.5CH3OH·0.5H2O, Figure
2), [FeII(tolpyph)2(NCSe)2] (2, Figure 3), and [FeII-
(tolpyph)2(NBH3)2] (3, Figure 4). Data were measured at
100 K for all three complexes and, for SCO-active 3, also at 373
K. All three complexes are isostructural, with each iron(II)
center in an octahedral N6 environment composed of two
tolpyph ligands equatorially coordinated in a bidentate manner
(two triazole and two pyridine nitrogen donors); the N6 donor
sphere is competed by to two trans-coordinated NCE anionic
co-ligands (E = S, Se, or BH3). Complexes 1·1.5CH3OH·
center located at the iron(II) center (for 1 there is also lattice
solvent present).
The iron(II) centers of complexes 1·1.5CH3OH·0.5H2O and
2 are stabilized in the HS state at 100 K. In contrast, 3 is
stabilized in the LS state at 100 K and in the HS state at 373 K
(Table 1). The average Fe−N lengths for the HS iron(II)
centers range from 2.124 to 2.170 Å, while for the LS iron(II)
center it is 1.979 Å (Table 1). The average Fe−N values of all
three complexes agree with those of other reported Rdpt-based
analogues.21,23
0.5H O and 2 crystallize in the triclinic space group P1, while
̅
2
compound 3 crystallizes in the monoclinic space group C2/c
(Table 1). In all cases, the asymmetric units contain one-half of
the complex with the other half being generated by an inversion
C
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1·1.5CH3OH·0.5H2O and 3 the phenyl dihedral angles are
much larger (41.65−45.39°), while the tolyl dihedral angles are
smaller (62.61−67.18°).
Table 1. Summary of the Average Fe−N Bond Lengths (Å),
the Octahedral Distortion of the Fe Centers (i.e., Σ, Sum of
the Deviations of the 12 cis Angles from 90°),21 and the
Torsion Angles (deg) of (a) the Coordinated Pyridine Ring,
and (b) the Uncoordinated Phenyl and Tolyl Rings, for the
Structures of 1·1.5CH3OH·0.5H2O and 2 at 100 K, and of 3
at 100 K (Bold Text as LS) and 373 K
The axial NCE ligands in structures 1·1.5CH3OH·0.5H2O
and 2 are significantly bent away from being linear (Figures 2,
3, and 5) with the iron(II) centers (Fe−NC angles: 151−154°),
whereas in the SCO-active complex 3, the Fe−NC angle (HS
175 vs LS 178°) is nearly linear (Figures 4 and 5). The
difference in the Fe−NC angle (Figure 5), which may play an
important role in influencing the spin state stabilized in the
solid state (see Discussion below, including Figure 9),31,48,49 is
likely due to the different weak intermolecular crystal packing
interactions between the complex molecules (Tables S3−S6
and Figures S7−S10) in 1·1.5CH3OH·0.5H2O and 2 as a result
1·1.5CH3OH·0.5H2O
2
3
temp, K
100
100
100
373
space group
average Fe−N
Σ
P1
P1
C2/c
1.979
54.44
5.57
C2/c
2.124
71.76
7.85
̅
̅
2.170
82.04
26.06
45.39
67.18
151
2.159
107.44
8.62
a
pyridine twist
b
phenyl twist
7.42
41.65
62.61
178
42.26
64.94
175
of them having crystallized in a different space group (both P1)
̅
c
from 3 (C2/c).31,50 In the case of 1·1.5CH3OH·0.5H2O the
presence of lattice solvent (Table S3 and Figure S7) also affects
the crystal packing. The packing interactions, in each of these
structures, are discussed in more detail in the following section.
Crystal Packing Comparisons. The crystal packing and
intermolecular interactions in 1·1.5CH3OH·0.5H2O are shaped
by the methanol and water molecules of solvation. The
complexes pack in one-dimensional wave-like chains (Figure 6,
tolyl twist
FeII−NCE
87.58
154
a
The angle between the coordinated triazole ring and the attached
b
pyridine ring. The angle between the coordinated triazole ring and
the attached phenyl ring. The angle between the coordinated triazole
c
ring and the attached tolyl ring.
The values for the octahedral distortion parameter Σ, being
the sum of the deviations of the 12 cis N−Fe−N angles from an
idealized 90°,21 for the HS iron(II) centers of complexes 1·1.5
CH3OH·0.5H2O and 3, are 82.04° and 71.76°, respectively,
whereas for 2 (107.44°) it is higher and lies slightly above the
reported literature range for related HS iron(II) (77.9−
101.1°).21,23 The Σ value for the LS iron(II) center of 3 is
54.44°. As expected, this is significantly lower than for the HS
center in 3 at 373 K (71.76°) and falls within the expected
range for LS complexes (42.5−65.7°).21,23
The pyridine ring is normally close to being coplanar with
the triazole ring due to the restrictions imposed by
coordination of both of them to the iron(II) center;21,23
however, in complex 1·1.5CH3OH·0.5H2O these two rings are
relatively highly rotated away from coplanarity (26.06°)
compared to those found in the structures of 2 and 3 (5.57−
8.62°, including LS and HS structures; Figure 5 and Table 1).
The dihedral angles between the attached phenyl and tolyl rings
and the triazole ring (Figure 5 and Table 1) in complex 2 are
7.42 and 87.58°, respectively, features observed before in
related Rdpt-based complexes.21,23 In contrast, for complexes
Figure 6. Edge-on view, looking down the a-axis, of the wave-like
chains of [FeII(tolpyph)2(NCS)2]·1.5CH3OH·0.5H2O (1·1.5CH3OH·
0.5H2O) highlighted in green, with lattice solvent molecules
accommodated between chains. Note the two 0.75 occupancy
molecules of CH3OH and the two 0.25 molecules of H2O per
complex are shown. Hydrogen atoms are omitted for clarity.
green highlight), with the solvent molecules located between
the chains. Each chain of complexes is offset from its neighbors
by half a complex due to the noncoplanar phenyl and pyridyl
rings (45.39 and 26.06°, respectively) that protrude from the
sides of the chains. Within each chain the complexes interact
with their neighbors via CH···π contacts, whereas the
interactions between neighboring chains occur via pairs (by
symmetry) of non-classical CH···N hydrogen bonds between
pairs of complexes (Figure S7 and Table S3). These packing
interactions result in dihedral angles between the triazole ring
and the attached aromatic rings that differ somewhat from usual
(Table 1). The dihedral angles with the attached pyridyl and
phenyl rings (26.06 and 45.39°) are somewhat larger than
usual, with the difference particularly notable for the
coordinated pyridyl ring, which is usually close to co-planar
with the coordinated triazole to which it is attached. Also, the
dihedral angle between the triazole and tolyl rings (67.18°) is
somewhat smaller than usual (Table 1).
Figure 5. Two perspective views of the same overlay of the three HS
structures of complexes [FeII(tolpyph)2(NCS)2]·1.5CH3OH·0.5H2O
(1·1.5CH3OH·0.5H2O, red) and [FeII(tolpyph)2(NCSe)2] (2, green)
at 100 K and of complex [FeII(tolpyph)2(NCBH3)2] (3, blue) at 373
K. The structures were overlaid by fitting the atoms in the triazole
rings and the iron(II) centers. These representations highlight the
slightly different angles of rotation between the coordinated pyridine
and triazole rings and uncoordinated phenyl and tolyl rings and, more
importantly, the significant differences between the Fe−NC angles.
Hydrogen atoms and lattice solvent molecules are omitted for clarity.
Similar to 1·1.5CH3OH·0.5H2O, the complexes of 2 also
pack in one-dimensional wave-like chains (Figure 7, pink
highlight), which are offset by half a molecule from the adjacent
D
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Web (see Supporting Information, Section S5, for details of the
search), were surveyed (Figures 9 and S21−S24 and Tables S9
and S10).
Figure 7. Edge-on view of the wave-like chains (highlighted in pink) of
[FeII(tolpyph)2(NCSe)2] (2). Hydrogen atoms are omitted for clarity.
chain. However, the packing interactions within chains in this
solvent-free complex, 2, are instead mainly composed of π···π
contacts (Figure S8 and Table S4) between the relatively
electron-rich phenyl and the relatively electron-poor coordi-
nated pyridine and triazole rings in adjacent complexes. This
type of arrangement has been previously observed in similar
Rdpt-type systems.21,23,66 The interactions between the chains
are by CH···π and Se···Se contacts (Figure S8 and Table S4).
The dihedral angles between the triazole ring and all three
attached aromatic rings are as expected (Table 1): close to co-
planar in the case of the attached pyridyl and phenyl rings (8.62
and 7.42°) and close to perpendicular in the case of the tolyl
Figure 9. Survey of the Fe−NC(X) angle in the 53 structures of 36
examples of trans-[FeII(Rdpt-like)2(NCX)2] complexes (X= S, Se,
BH3, NCN, C(CN)2, etc.),21,23,26,27,32,33,35,36,51−64 Of them, 17 were
collected at more than one temperature, versus whether the complex
was SCO-active (35 structures, 66%, in a range of spin states) or not
(18 structures, 34%, all HS). Results shown as (a) bar chart showing
the proportion of structures in each of two distinct Fe−NC(X) angle
ranges that was either SCO-inactive and HS (red; none were LS only)
or SCO-active (black) and as (b) plot showing the actual Fe−NC(X)
angles for each of the 53 crystal structures of complexes that were
SCO-inactive/HS (18) or SCO-active (35). The four structures
4
ring attached to N (87.58°).
Complex 3 has a different crystal packing arrangement than
that of 1·1.5CH3OH·0.5H2O and 2, due to it crystallizing in the
C2/c space group (rather than in P1). This produces alternating
̅
orientations of layers, denoted ABAB (Figure 8). The
○
△
reported herein are shown as solid symbols. (red ) HS, (blue ) LS,
(black ×) mixture of HS/LS.
Of the 53 structures, 35 structures (21 unique complexes)
were identified as SCO-active in the solid state, 31 had a nearly
linear Fe−NC(X) angle, 162−178° (Figure 9 and Table S9),
regardless of spin state [Figure 9b: HS (circle), LS (triangle), or
HS/LS mixture (×)]. For a particular complex, the difference in
the Fe−NC(X) angle between the HS and LS structures is very
small, 0.2−5.1°, in all but one case, where it was somewhat
larger, 11.8°.
However, of the remaining 18 structures (15 unique
complexes), which were SCO-inactive complexes (i.e., all
were HS), 15 structures had a far less linear Fe−NC(X) angle,
from 142 to 159° (Figure 9 and Table S10). Three HS
compounds were measured at different temperatures (Table
S10) with the Fe−NC(X) angles varying only slightly, 0.1−
1.9°.
Figure 8. View of [FeII(tolpyph)2(NCBH3)2] (3), looking down the
b-axis, showing the alternating ABAB arrangement of the layers A
(blue) and B (red). Hydrogen atoms are omitted for clarity.
complexes within each layer are connected by π···π contacts
between the coordinated pyridine and triazole rings (Figure S9
and Tables S5 and S6). Between the layers there are relatively
strong π···π contacts between the almost parallel pyridine and
phenyl rings (Figure S9 and Tables S5 and S6), as well as
BH···π contacts between the NCBH3 and tolyl rings (Figure
S10 and Tables S5 and S6). These features help to explain the
unusual dihedral angles made between the triazole ring and the
attached phenyl (41.65−42.26°; larger than usual) and tolyl
(62.61−64.94°; smaller than usual) rings (Table 1).
Literature Survey. The Fe−NC(X) angles in the 53 crystal
structures determined for the 36 unique examples of analogous
trans-[FeII(Rdpt)2(NCX)2] complexes (X = S, Se, BH3, NCN,
C(CN)2, etc.),21,23,26,27,32,33,35,36,51−64 identified by searching
the Cambridge Structural Database (CSD)65 and SciFinder
Given these findings, it is unsurprising that of the three new
structurally characterized trans-[FeII(Rdpt)2(NCX)2] com-
plexes that are reported herein (shown as solid symbols in
Figure 9b), only the complex with a nearly linear Fe−NC(X)
angle, 3 (Figure 5), is SCO-active.
Magnetic Measurements. While the mononuclear tris-
ligand complexes [FeII(tolpyph)3](X)2 (X = ClO4 or BF4 )
have been reported and are LS,67,68 the sole complex reported
of the analogous pyrazine ligand, [FeII(tolpzph)2(NCS)2], was
spin-crossover active, both when solvent-free (violet, T1/2 = 212
K) and as the tetrahydrofuran (THF) solvate (forest green, T1/2
= 255 K).27 In addition, the symmetrical dipyridyl analogue of
−
−
E
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tolpyph, tolpypy, produced a pair of structurally characterized
polymorphs, trans- and cis-[FeII(tolpypy)2(NCS)2], of which
the cis form remained HS, whereas the trans isomerthe
isomer that is also seen for all of the structurally characterized
complexes reported hereinwas SCO-active.36
The T1/2 of 309 K seen for this pyridine−cyanoborohydride
complex is higher than those seen for the pyrazine−thiocyanate
analogue, [FeII(tolpzph)2(NCS)2] (violet T1/2 = 212 vs forest
green THF solvate T1/2 = 255 K),27 consistent with a stronger
ligand field being imposed by the pyridine−cyanoborohydride
combination than by the pyrazine−thiocyanate combina-
tion.70−72 In the case of the pyridine−thiocyanate and
pyridine−selenocyanate analogues, the absence of SCO could
be explained by the different crystallization conditions and
solvent content of the solvatomorphs: the critical effect of
crystal packing (space group, NCE, and solvent content on the
magnetic behavior in the solid state) is well-
known.4,29,31,49,50,73−75
Hence, it was anticipated that at least some of these new
trans-[FeII(tolpyph)2(NCE)2] complexes should be SCO-
active. Hence, variable-temperature magnetic susceptibility
measurements were performed from 300 to 50 K on a powder
sample of [FeII(tolpyph)2(NCS)2]·H2O (1·H2O), on a
crystalline sample of [FeII(tolpyph)2(NCSe)2]·1.5H2O (2·
1.5H2O), and, from 400 to 50 K, on a crystalline sample of
[FeII(tolpyph)2(NCBH3)2]·H2O (3·H2O). Note that crystal-
line bulk samples of solvatomorph 1·1.5CH3OH·0.5H2O and
solvent-free 2 and 3 could not be measured, since in the case of
1·1.5CH3OH·0.5H2O only a few single-crystals were obtained,
while compounds 2 and 3 are hygroscopic. Within 15 min of
removing them from the mother liquor they formed the stable,
hydrated, solvatomorphs 2·1.5H2O and 3·H2O, as confirmed by
elemental analysis and thermogravimetric analysis (TGA; see
Experimental), so these are the solvatomorphs that were
studied magnetically.
UV−Vis Spectroscopy. Complexes [FeII(tolpyph)2-
(NCS)2] (1), [FeII(tolpyph)2(NCSe)2] (2), and [FeII-
(tolpyph)2(NCBH3)2] (3) were studied by UV−vis spectros-
copy at room temperature in CHCl3 solution and in the solid
state (Figure 11). Speciation in solution was observed for
The powder sample of 1·H2O and crystalline sample of 2·
1.5H2O show similar magnetic behavior (Figure 10): the χMT
Figure 11. UV−vis spectra for all three complexes, both in CHCl3
solution (generated using a tolpyph/Fe(II) ratio of 6:1; solid lines;
extinction coefficient) and in the solid state (dashed lines;
absorbance): 1.52 × 10−4 M [FeII(tolpyph)2(NCS)2] (1, green) and
solid 1·H2O (green dashed line); 1.63 × 10−4 M [FeII(tolpyph)2-
(NCSe)2] (2, orange) and solid 2·1.5H2O (orange dashed line); 1.50
× 10−4 M [FeII(tolpyph)2(NCBH3)2] (3, black) and solid 3·H2O
(black dashed line) at room temperature. The spectrum of blank
double-sided tape, which was used to hold the solid samples, is also
shown (blue dashed line).
Figure 10. χMT vs T plot for powder [FeII(tolpyph)2(NCS)2]·H2O
(1·H2O, green stars), crystalline [FeII(tolpyph)2(NCSe)2]·1.5H2O (2·
1.5H2O, orange circles), and crystalline [FeII(tolpyph)2(NCBH3)2]·
▼
△
H2O (3·H2O; (blue ) cooling mode, (red ) heating mode).
analogous systems;76 so, to find the ratio of tolpyph per
[FeII(pyridine)4(NCE)2] (E= S, Se or BH3) necessary to ensure
the iron(II) is present as the desired complex, a series of ratios,
from 2:1 to 10:1, was tested for each complex (Figures S13−
S15). In all three cases it was found that no further change to
the spectra occurred on adding more than 6 equiv of ligand per
iron(II), so this ratio was used for all subsequent solution
studies (see Evans method measurements in the next section),
including the solution spectra shown in Figure 11 (solid lines).
As a further check, solid-state reflectance measurements on
solid samples of these complexes, attached to double-sided tape
(which itself has a band at 408 nm; Figure 11, blue dashed
line), were performed and are in good agreement with these
solution studies (Figure 11, dashed lines; Table S7).
values start at 3.30 and 3.55 emu·K·mol−1 at 300 K and very
gradually decrease a little, to 2.54 and 3.06 emu·K·mol−1 at 50
K, respectively (Figure 10). On the one hand, these values are
consistent with iron(II) in the HS state, albeit with perhaps a
trace amount undergoing extremely gradual SCO on cooling.69
On the other hand, the crystalline sample of 3·H2O clearly
undergoes SCO (Figure 10). The χMT value at 400 K is 3.51
emu·K·mol−1, consistent with a HS iron(II) center, but on
cooling SCO takes place with a T1/2 of 309 K (Figure S11,
right), resulting in a χMT value of 0.43 emu·mol−1 K at 50 K
consistent with a LS iron(II) center, plus a trace amount of
material trapped in the HS state (a not uncommon
finding).26,55 When the sample is heated back up to 400 K,
the magnetic response follows the same path. Two more
cooling−heating cycles from 373 to 200 K show that the SCO
event is reproducible and does not show a thermal hysteresis
loop (Figure S11).
The observed bands for all three compounds in CHCl3
solution correspond to charge transfer (CT) bands, since the
molar extinction coefficient (ε) values are all larger than 382 L·
mol−1·cm, higher than the expected values for octahedral
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iron(II) d−d bands (Table S7).3 Compound 3 has the highest
ε value: 1565 L·mol−1·cm at 512 nm; compound 1 has an ε
value of 774 L·mol−1·cm at 495 nm, whereas compound 2 has
the lowest ε value: 382 L·mol−1·cm at 492 nm (Table S7).
Solution-Phase Magnetic Measurements. The magnetic
behavior of each complex was studied in CDCl3 solution, by the
variable-temperature (VT) Evans 1H NMR method,77−82 using
a tolpyph-to-iron(II) ratio of 6:1 of to ensure the iron(II) is
present as [FeII(tolpyph)2(NCE)2], as described in the
previous section. Interestingly, the solubility of each compound
in CDCl3 noticeably changes as a function of the anionic NCE
co-ligand. The complex solubility, in order of decreasing
solubility, based on the NCE co-ligand, is NCBH3 ≫ NCSe >
NCS.
a
Table 2. Summary of the Thermodynamic Parameters
b
b
compound
T1/2
ΔH
ΔS
R2
1
2
3
290
310
288
19.8
23.5
32.4
86
90
0.99
0.97
0.99
112
a
ΔH (kJ·mol−1) and ΔS (J·mol−1·K−1) were obtained from fitting the
datasets (Table S8) obtained for complexes [FeII(tolpyph)2(NCS)2]
(1), [FeII(tolpyph)2(NCSe)2] (2), and [FeII(tolpyph)2(NCBH3)2]
(3) with eq 1,25,83,84 with the T1/2 (K) calculated from these values, as
b
well as the R2 for the fit. ΔH and ΔS fall within the expected
literature range of values for solution studies of iron(II) complexes
(ΔH = 4−41 kJ·mol−1 and ΔS = 22−146 J·mol−1·K−1).82,85
All three complexes undergo a gradual SCO in the limited
temperature range able to be studied, that is, from 313 to 243 K
(Figure 12 and Table S8). Interestingly, compounds 1 and 2
be studied. The χMT value for compound 3 at 313 K, 2.57 emu·
K·mol−1 (γHS = 0.74), is significantly higher than for 1 and 2 at
313 K, and this drops to just 0.31 emu·K·mol−1 (γHS = 0.09) at
243 K. The values of enthalpy (ΔH) and entropy (ΔS)
obtained from the data modeling (Table 2) for compounds 1
and 2 (ΔH = 6.3−11.9 kJ·mol−1 and ΔS = 22−38 J·mol−1·K−1)
are lower than the values obtained for compound 3 (ΔH = 32.4
kJ·mol−1 and ΔS = 112 J·mol−1·K−1). Nonetheless, in all three
cases the obtained values of ΔH and ΔS (Table 2) fall within
the expected literature range of values for iron(II) complexes in
other solution studies reported in the literature (ΔH = 4−41
kJ·mol−1 and ΔS = 22−146 J·mol−1·K−1).82,85
By definition ΔG = 0 at the T1/2, so the T1/2 values for the
three complexes can be calculated by simply dividing the
respective ΔH by ΔS (Table 1). This analysis reveals that in
solution all three complexes undergo SCO with T1/2 values that
fall over a narrow range and are close to room temperature,
288−310 K: for complex 3 the T1/2 value (288 K) is somewhat
lower than that of complexes 1 and 2 (290 and 310 K,
respectively). This is surprising, as the anionic NCBH3 co-
ligand should better stabilize, compared to the analogous NCS
and NCSe co-ligands, the LS state at higher temperatures.70−72
In contrast with 3, compounds 1 and 2 follow the expected
trend, as 1 has a lower T1/2 than 2 (290 vs 310 K, Table
2).70−72 The different magnetic behaviors and T1/2 values could
be due to the stronger solvation effects that the NCBH3
complex 3 might experience in CDCl3 solution (i.e., forming
both B−H···Cl and Cl−D···B interactions) compared with NCS
and NCSe complexes 1 and 2, as the latter co-ligands may be
less prone to forming hydrogen bond contacts with the solvent.
The distinctly different solubility in CDCl3 observed for 1 and
2 versus 3 concurs with this. Hydrogen-bonding interactions
have been previously observed to play an important role in
tuning the ligand field strength and hence the T1/2, in
solution.82,86−88 Some recent examples of this have been
reported by Halcrow and co-workers, where the use of a range
Figure 12. Solution-phase magnetic data represented as χMT vs T for
complexes [FeII(tolpyph)2(NCS)2] (1, 2.21 × 10−3 M, green ),
■
[FeII(tolpyph)2(NCSe)2] (2, 3.95 × 10−3 M, orange ), and
●
[FeII(tolpyph)2(NCBH3)2] (3, 5.10 × 10−3 M, black ) in CDCl3
▲
solution from 313 to 243 K. Note that each complex solution was
prepared using a tolpyph-to-iron(II) ratio of 6:1 to ensure it is present
as [FeII(tolpyph)2(NCE)2]. The lines correspond to the best fit found
for each compound using the regular solution model eq 1.25,83,84
have a more gradual SCO than compound 3. Each data set was
modeled as a gradual and complete SCO using the regular
solution model [eq 1; for further details see Section S4 in the
Supporting Information] with good fits obtained (Figure 12
and Table 2).25,83,84
of different solvents with distinct polarities facilitated tuning of
χMT(max)
χMT(T) =
85,87,88
the T1/2
.
1 + exp(−ΔH/RT + ΔS/R)
(1)
In stark contrast to the solid-state measurements in which
compounds 1 and 2 remain HS (Figure 10), all three
complexes undergo SCO in CDCl3 solution (Figure 11). In
the case of compound 3, the T1/2 occurs at a significantly lower
temperature in CDCl3 solution (the T1/2 has decreased by 21
K). These results highlight the pronounced effect that crystal
packing has on the SCO behavior in the solid state.2,82
The χMT values at 313 K for compounds 1 and 2 (Figure 12,
green and orange data sets, respectively) are 1.95 emu·K·mol−1
(HS molar fraction or γHS = 0.56) and 1.72 emu·K·mol−1 (γHS
=
0.49), respectively. When cooled to 243 K, compounds 1 and 2
undergo a very gradual SCO, with the χMT values only
dropping to 1.33 emu·K·mol−1 (γHS = 0.38) and 0.69 emu·K·
mol−1 (γHS = 0.20), respectively.
In comparison to 1 and 2, compound 3 undergoes a more
abrupt SCO in CDCl3 (Figure 12, black data set), which is also
more complete within the limited temperature range that can
CONCLUSION
The 2-pyridine-substituted Rdpt-type ligand tolpyph can be
successfully synthesized by either of two synthetic routes, both
■
G
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min−1 for compounds 1·H2O, 2·1.5H2O, and 3·H2O, then kept
constant until a total elapsed time of 60 min, after which the mass was
constant (Δmass <1% per hour). Magnetic susceptibility measure-
ments on the samples were measured either from 300−50 K for
compounds 1·H2O and 2·1.5H2O or from 400 to 50 K for compound
3·H2O at 5 K·min−1 on settle mode and an applied field of 0.1 T with a
Quantum Design Physical Property Measurement System equipped
with a vibrating sample mount. The magnetic susceptibility was
corrected for the diamagnetism of the sample (−MW × 0.5 × 10−6
emu mol−1, where MW indicates molecular weight).84
Single-crystal X-ray diffraction measurements were collected on an
Oxford Diffraction SuperNova diffractometer with Atlas CCD,
equipped with a Cryostream N2 open-flow cooling device,89 using
mirror monochromated microfocus Cu Kα radiation at 100 and 373 K.
Series of scans were performed in such a way as to collect a complete
set of unique reflections to a maximum resolution of 0.80 Å. Raw
frame data (including data reduction, interframe scaling, unit cell
refinement, and absorption corrections)90 for all structures were
processed using CrysAlis Pro.91 Structures were solved using
SUPERFLIP92 and refined using full-matrix least-squares on F2 and
refined against F2 using all data by full-matrix least-squares techniques
with SHELXS-2014,93 and refined using full-matrix least-squares on F2
within the X-Seed graphical user interface. All hydrogen atoms were
inserted at calculated positions with U(H) = 1.2 U(attached atom)
and rode on the atoms to which they were attached, unless otherwise
stated. Crystallographic data for the structures were deposited with the
Cambridge Crystallographic Data Centre, CCDC Nos. 1551982−
1551986.
of which are based on the well-established multistep literature
procedure for related Rdpt-type ligands.28 A new family of
three mononuclear iron(II) [FeII(tolpyph)2(NCE)2]-type
complexes, where E = S, Se, and BH3, were prepared and
characterized, including three solvatomorphs. Single-crystal X-
ray diffraction characterizations of all three complexes showed
that two tolpyph ligands were equatorially coordinated in a
bidentate manner with two axially coordinated NCE− co-
ligands. The bonding parameters of the iron(II) centers in
[FeII(tolpyph)2(NCS)2]·1.5CH3OH·0.5H2O (1·1.5CH3OH·
0.5H2O) and [FeII(tolpyph)2(NCSe)2] (2) confirmed that
they are both in the HS state at 100 K. In contrast, in the case
of [FeII(tolpyph)2(NCBH3)2] (3), the first structurally
characterized Rdpt-type complex with cyanoborohydride co-
ligands, the iron(II) centers are clearly in the LS state at 100 K
and in the HS state at 373 K. A literature survey of 53
analogous trans-[FeII(Rdpt)2(NCX)2]-type complexes shows
that when the Fe−NC(X) angle is close to linear the complex is
more likely to be SCO-active in the solid state. The findings
presented herein are entirely consistent with this: only 3 has a
nearly linear Fe−NC(X) angle, and it is the only complex that
is SCO-active in the solid state.
Variable-temperature magnetic susceptibility measurements
showed that the hydrates 1·H2O and 2·1.5H2O remain HS
from 300 to 50 K, whereas 3·H2O undergoes SCO with T1/2
=
309 K. UV−vis studies in CHCl3 solution of compounds 1, 2,
and 3 showed that a ratio of 6:1 tolpyph per iron(II) is
required to ensure the iron(II) is present as
[FeII(tolpyph)2(NCE)2]. A subsequent VT CDCl3 solution
study of the three compounds, using the Evans NMR method,
revealed that all three undergo a gradual SCO, with T1/2 values
close to room temperature (288−310 K). Compounds 1 and 2
undergo a more gradual SCO than 3, which could be explained
by 1 and 2 (NCS and NCSe analogues, respectively)
experiencing different solvation effects in CDCl3 compared to
3 (NCBH3 analogue; the only NCE anion herein that is capable
of being a hydrogen-bond donor). The T1/2 value of 3 is 288 K
in solution, which is 21 K lower than when it is in the solid
state. The different magnetic behaviors between the solid and
solution phases highlight the strong impact that crystal packing
has on SCO. These findings also highlight the need for
solution-phase studies to be more commonly performed in
parallel with the solid-state magnetic measurements that are
standardly reported. To this end, further exploration of the
electronic and packing effects that a wider range of azine-
substituted Rdpt-type ligands have on SCO-active iron(II)
complexes is currently underway here.
Solution UV−vis spectra were recorded at room temperature on a
PerkinElmer Lambda 950 UV−vis/NIR (NIR = near-infrared).
Solutions were made using HPLC grade CHCl3 (filtered first through
alumina and then bubbled with N2) of concentrations as follows:
[FeII(tolpyph)2(NCS)2] (1, 1.52 × 10−4 M), [FeII(tolpyph)2-
(NCSe)2] (2, 1.63 × 10−4 M) [FeII(tolpyph)2(NCBH3)2] (3, 1.50
× 10−4 M). Note that the given concentrations are based on the
equivalents of iron(II), and each complex solution was prepared using
2 to 10 equiv of ligand tolpyph per equivalent of the starting material
[FeII(pyridine)4(NCE)2] (E = S, Se, BH3)71,94 (see Section S3). To
prevent any oxidation of the iron(II) starting materials [FeII-
(pyridine)4(NCS)2] (very air-sensitive)95 and [FeII(pyridine)4-
(NCSe)2] (prone to oxidation but less air-sensitive than the NCS
analogue) in CHCl3 solution, the solid samples were weighed and
prepared under argon.
Solid-state visible reflectance data were recorded at room
temperature on a PerkinElmer Lambda 950 UV−vis/NIR spectrom-
eter. Solid samples of complexes [FeII(tolpyph)2(NCS)2]·H2O (1·
H2O), [FeII(tolpyph)2(NCSe)2]·1.5H2O (2·1.5H2O), and
[FeII(tolpyph)2(NCBH3)2]·H2O (3·H2O) were attached to a solid
support of Labsphere reflectance standard using double-sided tape.
Solution magnetic susceptibility data were measured in the
temperature range of 243−313 K in intervals of 5 K for complexes
[FeII(tolpyph)2(NCS)2] (1, 2.21 × 10−3 M), [FeII(tolpyph)2-
(NCSe)2] (2, 3.95 × 10−3 M), and [FeII(tolpyph)2(NCBH3)2] (3,
1
5.10 × 10−3 M) in CDCl3 by H NMR spectroscopy on a Varian500
EXPERIMENTAL SECTION
■
AR spectrometer with a 5 mm OneProbe using Evans’ method.80
Expected error for VT NMR is 1 K. Complex solutions were
prepared, following the same precautions as described above, by
Instrumentation and Measurements. Elemental analyses (C, H,
N) were measured at the Campbell Micro-Analytical Laboratory,
University of Otago. 1H and 13C NMR spectra were recorded on
either a Varian 400-NMR or 500-NMR spectrometer at 298 K.
Chemical shifts are reported in parts per million and referenced to the
residual protonated solvent peak in the 1H NMR spectra and the
residual solvent peak in the 13C NMR spectra [CDCl3: 7.26 ppm (1H)
and 72.16 ppm (13C)]. High-resolution electrospray ionization mass
spectrometry (ESI-MS) spectra were recorded using a Bruker
MicrOTOF-Q mass spectrometer in methanol, and m/z values have
a standard error of 10 ppm at 263 K. IR spectra were recorded on a
Bruker attenuated total reflectance (ATR) IR spectrometer with a
diamond anvil Alpha-P module, within the range between 400 and
4000 cm−1. TGA measurements were collected on a TA Instruments
TGA Q50. In each case the temperature was heated to 393 K at 10 K·
reacting
a precisely known mass of starting material
[FeII(pyridine)4(NCE)2] (E = S, Se, BH3)71,94 in 1.000 mL of
CDCl3 (filtered first through alumina) with 6 equiv of ligand tolpyph
(for further information see Section S3). The derived parameters, that
is, ΔH, ΔS, and the least-squares fittings, were obtained by modeling
each data set as a gradual and complete SCO using the regular solution
model (eq 1)25,83,84 with good fits, and they were performed with
OriginPro version 9.1.0 from OriginLab Corporation. A diamagnetic
correction for the sample84 and a correction for the variation of the
density of the solvent with temperature96 were applied to each data
set.
General Procedures. Benzocarbohydrazide and pyridine-2-
carbohydrazide were prepared as previously described.28 Dry ethanol
H
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was prepared by distilling absolute ethanol from Mg/I2. The iron(II)
starting complexes, [FeII(pyridine)4(NCS)2],71 [FeII(pyridine)4-
(NCSe)2],71 and [FeII(pyridine)4(NCBH3)2],94 were prepared by
mixing [FeII(H2O)6](BF4)2 and NH4(SCN), KSeCN, or NaBH3CN,
respectively, in water containing excess pyridine following analogous
reported methods.71,94 Other chemicals were bought commercially
and used as received without further purification.
[FeII(pyridine)4(NCS)2] (31.4 mg, 0.06 mmol) in CH3OH (1.5
mL) was added dropwise to a pale yellow solution of tolpyph (37.5
mg, 0.12 mmol) in CHCl3 (3 mL) causing the solution to turn dark
red. After it was stirred at RT for 2 h, a purple precipitate formed,
which was filtered off, and the resulting dark red filtrate was exposed to
slow vapor diffusion of pentane to generate a few dark red needlelike
single crystals of [FeII(tolpyph)2(NCS)2]·1.5CH3OH·0.5H2O suitable
for X-ray diffraction.
Organic Synthesis. N-(4-Methylphenyl)benzenethioamide. A
mixture of 4-methylaniline (6.1 g, 0.06 mol), benzaldehyde (6.0 g,
0.06 mol), sulfur (3.8 g, 0.12 mol), and sodium sulfide nonahydrate
(0.3 g, 1.11 mmol, 2 mol % based on benzaldehyde) in
dimethylformamide (DMF; 40 mL) was heated and stirred at 120
°C for 3 d. After it cooled to room temperature (RT), saturated
aqueous NH4Cl (100 mL) was added to the reaction solution, and the
solution was extracted with dichloromethane (DCM; 2 × 100 mL).
The combined organic fractions were filtered, washed with water (4 ×
100 mL) and brine (100 mL), dried over MgSO4, and taken to dryness
to give a brown oil. Column chromatography (silica, DCM/petrol
ether 1:3, Rf = 0.46) of the crude product gave N-(4-methylphenyl)-
benzenecarbothioamide as a mustard solid. The mustard solid was
recrystallized from ethanol (50 mL) to give pure product as a fluffy
intense yellow powder (5.2 g, 41%). C14H13NS (M = 227.33 g mol−1):
calc. C 73.97, H 5.76, N 6.16, S 14.10; found C 73.81, H 6.00, N 6.13,
[FeII(tolpyph)2(NCS)2]·H2O (1·H2O). In air, to a pale yellow solution
of tolpyph (17.5 mg, 0.06 mmol) in CHCl3 (2 mL) a yellow
suspension of [FeII(pyridine)4(NCS)2] (13.4 mg, 0.03 mmol) in
CH3OH (1 mL) was added dropwise producing a dark red solution
that stirred for 2 h at RT. After the purple precipitate was filtered,95
pentane (10 mL) was added to the dark red filtrate resulting in
immediate precipitation of a maroon powder that was filtered, washed
with pentane (5 mL), and air-dried for 30 min to give solvatomorph
[FeII(tolpyph)2(NCS)2]·H2O (15 mg, 0.02 mmol, 61%).
C42H32N10S2Fe·H2O (M = 814.77 g·mol−1): calc. C 61.92, H 4.21,
N 17.19, S 7.87%; found C 61.62, H 3.92, N 17.26, S 7.79%. TGA (6.2
mg, 393 K for 60 min): calculated weight loss for removal of one H2O:
2.2%. Found weight loss on heating: 1.9%. ESI-MS m/z: 738.1887
([FeII(tolpyph)2(NCS)]+ calc. 738.1851), 647.2667 ([Na(tolpyph)2]+
calc. 647.2648), 496.1788 ([Fe(tolpyph)3]2+ calc. 496.1732), 340.1131
([Fe(tolpyph)2]2+ calc. 340.1044), 335.1317 ([Na(tolpyph)]+ calc.
335.1273), 313.1411 ([H(tolpyph)]+ calc. 313.1453). IR (ATR) ν/
cm−1:3058 (w) 2033 (s, CN stretch), 1596 (m), 1463 (m), 1436
(s), 1389 (m), 828 (m), 775 (m), 713 (m), 694(s), 621 (s). χmT
(PPMS with a VSM module) = 3.30 emu·K·mol−1 at 300 K.
1
S 14.04%. H NMR (400 MHz, CDCl3): δ (ppm) = 8.97 (br. S, 1H,
NH), 7.86 (d, J = 8 Hz, 2H, H2 and H6), 7.63 (d, J = 8 Hz, 2H, H8
and H12), 7.49 (t, J = 7 Hz, 1H, H4), 7.44 (t, J = 8 Hz, 2H, H3 and
H5), 7.27−7.23 (m, 2H, H9 and H11), 2.38 (s, 3H, CH3). ESI-MS m/
z: 250.0671 ([Na(C14H13NS)]+ calc. 250.0661). IR (ATR) ν/cm−1:
3153, 3091, 2985, 2916, 1591, 1516, 1447, 1349, 1206, 811, 747, 690,
621, 496.
Ethyl N-(4-Methylphenyl)benzenethiocarboximido-thioate. To a
solution of sodium (0.5 g, 0.02 mol) in dry ethanol (35 mL) N-(4-
methylphenyl)benzenecarbothioamide (1.6 g, 6.89 mmol) was added,
and the mixture was stirred until complete dissolution occurred. To
the subsequent translucent yellow solution, bromoethane (3.7 g, 0.03
mmol) was added, and the reaction mixture was stirred at 50 °C for 4
h, during which time sodium bromide as a white solid precipitate
formed. The resulting dark mustard suspension was cooled to RT,
taken to dryness, and the resulting oil was taken up in DCM (20 mL),
washed with NaHCO3 saturated solution (25 mL), water (2 × 25 mL),
and brine (30 mL), dried over MgSO4, and taken to dryness to yield
crude ethyl N-(4-methylphenyl)benzenecarboximidothioate as a light
mustard oil, which was used without further purification.
4-(4-Methylphenyl)-3-(2-pyridinyl)-5-phenyl-4H-1,2,4-triazole
(tolpyph). (Route 1). Pyridine-2-carbohydrazide (0.97 g, 7.13 mmol)
and the crude product of ethylating N-(4-methylphenyl)-
benzenethiocarboxamide (6.89 mmol based on N-(4-methylphenyl)-
benzenecarbothioamide) were refluxed for 3 d in 1-butanol (50 mL).
The solution cooled, and the volume was reduced, yielding tolpyph as
a fluffy white solid, which was collected, washed with diethyl ether (2
× 25 mL) and then with water (2 × 25 mL), before being dried in
vacuum (0.94 g, 44%). C20H16N4 (M = 312.38 g·mol−1): calc. C 76.90,
H 5.16, N 17.94%, found C 76.95, H 5.09, N 18.01%. 1H NMR
(CDCl3, 500 MHzfor numbering see Scheme 1): δ/ppm = 8.37 (d,
1H, H1), 8.05 (d, 1H, H4), 7.74 (td, 1H, H3), 7.47−7.44(m, 2H, H9),
7.37−7.33 (m, 1H, H11), 7.31−7.27 (m, 2H, H10), 7.21 (dd, 1H, H2),
7.16 (d, 2H, H14), 7.09 (d, 2H, H13), 2.38 (s, 3H, H16). 13C NMR
(CDCl3, 500 MHzfor numbering see Scheme 1): δ/ppm = 155.6
(C6), 153.9 (C7), 149.1 (C1), 147.2 (C5), 139.2 (C15), 136.6 (C3),
133.2 (C12), 129.9 (2 × C14), 129.8 (C11), 129.0 (2 × C9), 128.4 (2 ×
C10), 127.8 (2 × C13), 126.9 (C8), 124.6 (C4), 124.0 (C2), 21.4 (C16).
IR (ATR)/cm−1: 3038 (w), 1583 (m), 1512 (s), 1467 (s), 1450 (m),
1433 (s), 1407 (m), 999 (m), 792 (s), 773 (s), 743 (m), 717 (s), 690
(s), 621 (s), 603 (s), 494 (m). High-resolution ESI-MS (methanol):
m/z = 313.1509 ([H(tolpyph)]+ calc. 313.1453).
[FeII(tolpyph)2(NCSe)2]·1.5H2O (2·1.5H2O). In air, to a pale yellow
solution of tolpyph (31.0 mg, 0.09 mmol) in CHCl3 (2 mL) a yellow
suspension of [FeII(pyridine)4(NCSe)2] (31.0 mg, 0.05 mmol) in
CH3OH (1 mL) was added dropwise causing the solution to turn deep
red. The solution stirred at RT for 1 d, then was filtered through
diatomaceous earth, and exposed to slow diffusion of diethyl ether
vapor to give orange block-like single crystals of
[FeII(tolpyph)2(NCSe)2] suitable for X-ray diffraction. Compound 2
is hygroscopic, and when the crystals are removed from the mother
liquor, they absorb atmospheric water. The orange crystalline solid was
filtered off, air-dried for 30 min, and stored in air to give solvatomorph
[FeII(tolpyph)2(NCSe)2]·1.5H2O (18.0 mg, 0.02 mmol, 37%).
C42H32N10Se2Fe·1.5H2O (M = 917.57 g·mol−1): calc. C 54.98, H
3.84, N 15.26%; found C 54.75, H 3.66, N 15.35%. TGA (2.4 mg, 393
K for 60 min): calculated weight loss for removal of 1.5 H2O: 2.9%.
Found weight loss on heating: 3.1%. ESI-MS m/z: 786.1290
([FeII(tolpyph)2(NCSe)]+ calc. 786.1295), 647.2677 ([Na-
(tolpyph)2]+ calc. 647.2648), 496.1791 ([Fe(tolpyph)3]2+ calc.
496.1732), 340.1133 ([Fe(tolpyph)2]2+ calc. 340.1044), 335.1320
([Na(tolpyph)]+ calc. 335.1273), 313.1526 ([H(tolpyph)]+ calc.
313.1453). IR (ATR) ν/cm−1: 3038 (w), 2062 (s, CN stretch),
1608 (m), 1510 (m), 1466 (m), 1442 (s), 826 (m), 773 (m), 747 (m),
710 (s), 694 (s), 620 (s), 611 (s), 477 (m). χmT (PPMS with a VSM
module) = 3.55 emu·K·mol−1 at 300 K.
[FeII(tolpyph)2(NCBH3)2]·H2O (3·H2O). In air, a yellow suspension of
[FeII(pyridine)4(NCBH3)2] (17.2 mg, 0.04 mmol) in CH3OH (1 mL)
was added dropwise to a pale yellow solution of tolpyph (23.8 mg,
0.08 mmol) in CHCl3 (2 mL) causing the solution to turn dark deep
red. The solution stirred at RT for 1 d and then was filtered and
exposed to slow diffusion of pentane vapor to give reddish-black block-
shaped single crystals of [FeII(tolpyph)2(NCBH3)2] suitable for X-ray
diffraction. Compound 3 is hygroscopic, so when crystals of 3 are
removed from the mother liquor they readily absorb atmospheric
water to give dark deep maroon crystalline solvatomorph
[FeII(tolpyph)2(NCBH3)2]·H2O (20.0 mg, 0.03 mmol, 69%).
C42H38N10B2Fe·H2O (M = 778.31 g·mol−1): calc. C 64.81, H 5.18,
N 18.00%, found C 64.47, H 4.83, N 17.95%. TGA (3.7 mg, 393 K for
60 min): calculated weight loss for removal of one H2O: 2.3%. Found
weight loss on heating: 2.1%. ESI-MS m/z: 720.2450
([FeII(tolpyph)2(NCBH3)]+ calc. 720.2458), 647.2640 ([Na-
(tolpyph)2]+ calc. 647.2648), 496.1768 ([Fe(tolpyph)3]2+ calc.
496.1732), 340.1129 ([Fe(tolpyph)2]2+ calc. 340.1044), 335.1296
Note that the numbering of ligand tolpyph (see Scheme 1)
corresponds to that of the peak assignment in 1H and 13C NMR
described in the Experimental Section.
Inorganic Synthesis. [FeII(tolpyph)2(NCS)2]·1.5CH3OH·0.5H2O (1·
1.5CH3 OH·0.5H2 O). In air,
a yellow suspension of
I
DOI: 10.1021/acs.inorgchem.7b01338
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
([Na(tolpyph)]+ calc. 335.1273), 313.1410 ([H(tolpyph)]+ calc.
313.1453). IR (ATR) ν/cm−1: 3067 (w), 2064 (s, CN stretch),
1599 (m), 1510 (m), 1467 (s), 1425 (m), 1287 (m), 995 (m), 825
(m), 772 (m), 694 (s), 618 (m), 501 (m), 421 (m). χmT (PPMS with
a VSM module) = 3.51 emu·K·mol−1 at 400 K, and 0.43 emu·K·mol−1
at 50 K.
transition-triggered-abrupt vs gradual components of spin crossover.
Dalton Trans. 2015, 44, 20843−20849 and front cover.
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Crossover FeII4 Squares: Two-Step Complete Spin Transition and
Reversible Single-Crystal-to-Single-Crystal Transformation. Angew.
Chem., Int. Ed. 2011, 50, 8940−8943.
ASSOCIATED CONTENT
* Supporting Information
■
S
́
(11) Costa, J. S.; Rodríguez-Jimenez, S.; Craig, G. A.; Barth, B.;
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01338.
Beavers, C. M.; Teat, S. J.; Aromí, G. Three-way crystal-to-crystal
reversible transformation and controlled spin switching by a
nonporous molecular material. J. Am. Chem. Soc. 2014, 136, 3869−
3874.
Additional X-ray crystallographic tables and figures, solid
and solution magnetic data, UV/vis spectra in solution
and solid states, photos of Evans 1H NMR method
solutions (PDF)
́
(12) Rodríguez-Jimenez, S.; Feltham, H. L. C.; Brooker, S. Non-
porous iron(II)-based sensor: crystallographic insights into a cycle of
colorful guest-induced topotactic transformations. Angew. Chem., Int.
Ed. 2016, 55, 15067−15071 and back cover.
(13) Bousseksou, A.; Varret, F.; Goiran, M.; Boukheddaden, K.;
Tuchagues, J. P. The Spin Crossover Phenomenon Under High
Magnetic Field. Top. Curr. Chem. 2004, 235, 65−84.
Accession Codes
CCDC 1551982−1551986 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(14) Hauser, A. Light-Induced Spin Crossover and the High Spin too
Low-Spin Relazations. Top. Curr. Chem. 2004, 234, 155−198.
(15) Absmeier, A.; Bartel, M.; Carbonera, C.; Jameson, G. N. L.;
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Weinberger, P.; Caneschi, A.; Mereiter, K.; Letard, J.-F.; Linert, W.
Both spacer length and parity influence the thermal and light-induced
properties of iron(II) bis(tetrazole-1-yl)alkane coordination polymers.
Chem. - Eur. J. 2006, 12, 2235−2243.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sbrooker@chemistry.otago.ac.nz.
■
(16) Neville, S. M.; Halder, G. J.; Chapman, K. W.; Duriska, M. B.;
Southon, P. D.; Cashion, J. D.; Letard, J.-F.; Moubaraki, B.; Murray, K.
S.; Kepert, C. J. Single-Crystal to Single-Crystal Structure Trans-
formations and Photomagnetism of a Porous Iron(II) Spin-Crossover
Framework. J. Am. Chem. Soc. 2008, 130, 2869−2876.
(17) Marino, A.; Chakraborty, P.; Servol, M.; Lorenc, M.; Collet, E.;
Hauser, A. The Role of Ligand-Field States in the Ultrafast
Photophysical Cycle of the Prototypical Iron(II) Spin-Crossover
Compound [Fe(ptz)6](BF4)2. Angew. Chem., Int. Ed. 2014, 53, 3863−
3867.
ORCID
Santiago Rodríguez-Jimenez: 0000-0002-2979-8525
́
Sally Brooker: 0000-0002-5878-8238
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(18) Decurtins, S.; Gutlich, P.; Kohler, C. P.; Spiering, H.; Hauser, A.
̈
■
Light induced excited state spin state trapping in a transition metal
complex: the hexa-1-propyltetrazole iron(II) tetrafluoroborate spin
crossover system. Chem. Phys. Lett. 1984, 105, 1−4.
We thank the Marsden Fund (RSNZ) and the Univ. of Otago
(including the award of a postgraduate scholarship to S.R.J. and
the 2015 purchase of a Versalab magnetometer) for supporting
this research. We also thank Mr. I. Stewart for assistance with
the variable-temperature NMR measurements and the referees
and coeditor for constructive feedback.
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spin crossovers; Kluwer Academic Publishers: Dordrecht, The Nether-
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