Spin Crossover in Dinuclear N4S2 Iron(II) Thioether-Triazole Complexes: Access to [HS-HS], [HS-LS], and [LS-LS] States

Article abstract of DOI:10.1021/acs.inorgchem.5b02851

Access to a new family of thioether-linked PSRT ligands, 4-substituted-3,5-bis{[(2-pyridylmethyl)sulfanyl]methyl}-4H-1,2,4-triazoles (analogues of the previously studied amino-linked PMRT ligands), has been established. Four such ligands have been prepared, PSPhT, PSⁱBuT, PS^t-BuPhT, and PS^MePhT, with R = Ph, ⁱBu, ^t-BuPh, and ^MePh, respectively. Three dinuclear colorless to pale green iron(II) complexes, [Fe^II₂(PSRT)₂](BF₄)₄·solvent, featuring N₄S₂ donor sets, were prepared. Single-crystal structure determinations on [Fe^II₂(PSPhT)₂](BF₄)₄·2MeCN·H₂O, [Fe^II₂(PSPhT)₂](BF₄)₄·2¹/₂MeCN·¹/₂H₂O·THF, [Fe^II₂(PS^MePhT)₂](BF₄)₄·2MeCN, and [Fe^II₂(PSⁱBuT)₂](BF₄)₄·4MeCN reveal that all four are stabilized in the [HS-HS] state to 100 K and that both possible binding modes of the bis-terdentate ligands, cis- and trans-axial, are observed. Variable-temperature magnetic susceptibility studies of air-dried crystals (solvatomorphs of the single crystal samples) reveal the first examples of spin crossover (SCO) for a dinuclear iron(II) complex with N₄S₂ coordination. Specifically, [Fe^II₂(PSPhT)₂](BF₄)₄·2¹/₂H₂O undergoes a multistep but complete SCO from [HS-HS] to [LS-LS], whereas [Fe^II₂(PS^MePhT)₂](BF₄)₄·1¹/₂MeCN·2H₂O exhibits a half-SCO from [HS-HS] to [HS-LS]. In contrast, [Fe^II₂(PSⁱBuT)₂](BF₄)₄·MeCN·H₂O remains [HS-HS] down to 50 K. The reflectance spectrum of pale green [Fe^II₂(PSPhT)₂](BF₄)₄·¹/₂CHCl₃·2¹/₂H₂O (solvatomorph A) reveals a trace of LS character (572 nm band ¹A_1g → ¹T_1g). Evans' ¹H NMR method and UV-vis spectroscopy studies revealed that on cooling dark green acetonitrile solutions of these complexes from 313 to 233 K, all three undergo SCO centered at or near room temperature. The tendency of the complexes to go LS in solution reflects the electronic impact of R on the σ-donor strength of the PSRT ligand, whereas the opposite trend in stabilization of the LS state is seen in the solid state, where crystal packing effects, of the R group and solvent content, dominate the SCO behavior.

Full text of DOI:10.1021/acs.inorgchem.5b02851

Article
pubs.acs.org/IC
Spin Crossover in Dinuclear N4S2 Iron(II) Thioether−Triazole
Complexes: Access to [HS-HS], [HS-LS], and [LS-LS] States
Ross W. Hogue, Humphrey L. C. Feltham, Reece G. Miller, and Sally Brooker*
Department of Chemistry and MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Otago, PO Box 56,
Dunedin 9054, New Zealand
S
* Supporting Information
ABSTRACT: Access to a new family of thioether-linked PSRT
ligands, 4-substituted-3,5-bis{[(2-pyridylmethyl)sulfanyl]methyl}-
4H-1,2,4-triazoles (analogues of the previously studied amino-
linked PMRT ligands), has been established. Four such ligands
have been prepared, PSPhT, PSiBuT, PSt‑BuPhT, and PSMePhT,
i
with R = Ph, Bu, t‑BuPh, and MePh, respectively. Three dinuclear
colorless to pale green iron(II) complexes, [FeII (PSRT)2](BF4)4·
2
solvent, featuring N4S2 donor sets, were prepared. Single-crystal
structure determinations on [FeII (PSPhT)2](BF4)4·2MeCN·
2
H2O, [FeII (PSPhT)2](BF4)4·21/2MeCN·1/2H2O·THF, [FeII -
2
2
(PSMePhT)2](BF4)4·2MeCN, and [FeII (PSiBuT)2](BF4)4·
2
4MeCN reveal that all four are stabilized in the [HS-HS] state
to 100 K and that both possible binding modes of the bis-terdentate ligands, cis- and trans-axial, are observed. Variable-
temperature magnetic susceptibility studies of air-dried crystals (solvatomorphs of the single crystal samples) reveal the first
examples of spin crossover (SCO) for a dinuclear iron(II) complex with N4S2 coordination. Specifically, [FeII (PSPhT)2](BF4)4·
2
21/2H2O undergoes a multistep but complete SCO from [HS-HS] to [LS-LS], whereas [FeII (PSMePhT)2](BF4)4·11/2MeCN·
2
2H2O exhibits a half-SCO from [HS-HS] to [HS-LS]. In contrast, [FeII (PSiBuT)2](BF4)4·MeCN·H2O remains [HS-HS] down
2
to 50 K. The reflectance spectrum of pale green [FeII (PSPhT)2](BF4)4·1/2CHCl3·21/2H2O (solvatomorph A) reveals a trace of
2
1
1
1
LS character (572 nm band A1g → T1g). Evans’ H NMR method and UV−vis spectroscopy studies revealed that on cooling
dark green acetonitrile solutions of these complexes from 313 to 233 K, all three undergo SCO centered at or near room
temperature. The tendency of the complexes to go LS in solution reflects the electronic impact of R on the σ-donor strength of
the PSRT ligand, whereas the opposite trend in stabilization of the LS state is seen in the solid state, where crystal packing effects,
of the R group and solvent content, dominate the SCO behavior.
[FeII (PMAT)2](BF4)4·DMF complex undergoes an abrupt
INTRODUCTION
2
■
half-SCO, from [HS-HS] to localized [LS-HS] at 224 K, with
the [LS-LS] state inaccessible even under 1.03 GPa pressure at
4 K. Variation of the N4 substituent to give other 4-substituted-
3,5-bis{[(2-pyridylmethyl)amino]methyl}-4H-1,2,4-triazoles
(Figure 1) was shown to greatly affect the SCO properties of
Spin crossover (SCO) complexes are an interesting class of
materials exhibiting molecular bistability with potential
applications in nanotechnological devices as memory storage
units, sensors, or displays.1−5 They can be switched between
two electronic stateshigh spin (HS) and low spin (LS)by
external stimuli, such as a change in temperature or pressure,
light irradiation, or guest presence/absence, in a readily
detectable and reversible way. Dinuclear complexes are of
particular interest, as they can potentially have a two-step
crossover, and intramolecular communication between metal
centers can lead to more abrupt transitions.6
Ligands containing the 1,2,4-triazole moiety are well
documented to have about the right field strength for SCO
in iron(II) complexes, can bridge two iron centers through the
two adjacent nitrogen atoms, and are versatile with respect to
substitution at the N4, C3, and C5 positions.7,8 One such ligand
previously used by us to deliberately generate dinuclear iron(II)
complexes is the bis-terdentate 4-amino-3,5-bis{[(2-
pyridylmethyl)amino]methyl}-4H-1,2,4-triazole ligand
(PMAT; Figure 1 PMRT with R = NH2).9−14 The
the analogous dinuclear iron(II) complexes, [FeII (PMRT)2]-
2
X4, despite R being remote from the donor atoms.15,16 In all of
the structurally characterized dinuclear iron(II) complexes of
the PMRT ligands to date a cis-axial binding mode is observed
(Figure 1). In no case could the second potential SCO event, to
the [LS-LS] state, be accessed, probably because of the steric
restraint of having all 12 donors to the two iron(II) centers, and
two N1N2-triazole bridges between them, provided by just two
ligands.
More recently, the Rentschler group reported a family of
PMRT-like complexes of a thiadiazole-based ligand, 2,5-bis[(2-
pyridylmethyl)amino]methyl-1,3,4-thiadiazole (PMTD, Figure
Received: December 8, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Chart 1. Only Ligands Previously Used in N4S2-Coordinated
Iron(II) Complexes That Are SCO-Active and Structurally
a
Characterized
a
All four ligands feature thioether donors. The six complexes are
mononuclear (5) or polymeric (1): [FeII(bpte)(NCSe)2],
{[FeII(bpte)(μ2-NC(CH2)4CN)](BPh4)2·Me2CO}, [FeII(bpte)-
(MeCN)2], [FeII(bptPh)(MeCN)2], [FeII([9]aneN2S)2][ClO4]2, and
{[PhB(pz)2(CH2SMe)]2FeII}.
Figure 1. Existing PMRT ligands that vary in the choice of R,
including PMAT, where R = NH2, and the existing PMTD ligand, as
well as the new PSRT ligands described herein (in box). Also shown
are the cis-axial (seen for PMRT) and trans-axial (seen for PMTD)
ligand binding modes observed for the literature dinuclear iron(II)
complexes of the PMRT and PMTD ligands.
SCO event upon losing solvent acetone.27 Prior to those
reports, Britovsek and co-workers reported that the closely
related N4S2-coordinated iron(II) complexes [FeII(bpte)-
(MeCN)2] and [FeII(bptPh)(MeCN)2] undergo a gradual,
almost complete SCO below room temperature in acetonitrile
solution.28 Finally, the N4S2-coordinated iron(II) complexes
[FeII([9]aneN2S)2][ClO4]2 and {[PhB(pz)2(CH2SMe)]2FeII},
by Gahan and co-workers29 and by Weber, Holthausen,
Wagner, and co-workers,30 respectively, were both reported
to undergo gradual, incomplete SCO. The former complex,
[FeII([9]aneN2S)2][ClO4]2, transitions between LS in the 4−
150 K range and approximately 30% HS at 300 K (see
Supporting Information) in the solid state and also shows
temperature-dependent magnetic susceptibility in solution.29 In
the s olid stat e t he latter complex, {[PhB-
(pz)2(CH2SMe)]2FeII}, is LS from 5 to 293 K, then undergoes
an incomplete SCO to approximately 10% HS at the limit of
measurement (360 K). Similarly, in toluene solution it
undergoes only partial SCO, as monitored by NMR (203−
298 K) and by UV−vis (283−343 K) spectroscopy.30 To
summarize, five of these six examples of structurally
characterized, SCO-active N4S2-coordinated iron(II) complexes
are mononuclear and one is a 1D polymer. All six feature thioether
S donors.
In order to investigate SCO in dinuclear iron(II) complexes
with mixed NS donor sets, we have incorporated thioether
linkages in place of the amino linkages in the bis-terdentate
PMRT ligand scaffold to give a new family of PSRT ligands
(Figure 1), with two discrete N2S-binding pockets and retaining
a triazole bridge between them. The longer C−S bonds,
compared to C−N bonds, are also expected to give the PSRT
ligands greater flexibility than the amino linkages can in PMRT.
This might give us the “best of both worlds”, i.e., open up
access to the [LS-LS] state in the resulting dinuclear triazole-
bridged complexes, as was seen for the PMTD systems, while
retaining access to both the [LS-HS] and fully [HS-HS] states,
as was seen for the PMRT systems. These new PSRT ligands
remain triazole-based, so retain the advantage of having an R
group off N4 that can be varied. Knowing from our previous
1).17 In contrast to the triazole moieties in the PMRT ligands,
the thiadiazole moiety was shown to produce [FeII (PMTD)2]-
2
X4 complexes with a trans-axial binding mode (Figure 1) and to
facilitate access to the fully [LS-LS] state. Indeed all three
[FeII (PMTD)2]X4 complexes highly favor the [LS-LS] state
2
and undergo a gradual SCO starting above 250 K, not reaching
the [HS-LS] state (or the [HS-HS] state) at the limit of the
measurement, 380 K.
All of the PMAT, PMRT, and PMTD complexes described
above feature N6-coordinated iron(II) centers, which are by far
the dominant class of SCO compounds in the literature.1 Mixed
donor environments for SCO-active iron(II) centers are rare,18
19
but SCO has been reported for iron(II) with N4C2 and
20
P2N2Cl2 coordination spheres, and wide thermal hystere-
sis21−24 and room-temperature bistability21,24,25 have been
demonstrated in N4O2 systems.
SCO in N4S2-coordinated iron(II) complexes has scarcely
been investigated, with only six structurally characterized
examples in the literature (Chart 1), to the best of our
knowledge [CSD search for Fe with 4× any bond to N, 2× any
bond to S (Figure S21); CSD version 5.37 updates (November
2015); 139 hits, 6 of which are SCO active]. Five of these
complexes are mononuclear, while one is polymeric, and all are of
just four ligands (Chart 1), all of which are neutral, feature
thioether (not thiolate) sulfur donors, and provide either N2S2
or N2S donor sets. In 2012, McKenzie and co-workers26
reported a mononuclear iron(II) complex with N4S2 coordina-
tion, [FeII(bpte)(NCSe)2], which crystallized in four poly-
morphs, two of which exhibited four-site SCO, which was
attributed to cooperativity facilitated by the asymmetry of the
mixed donor set at the iron(II) centers being exaggerated upon
an HS → LS transition (average Fe−S 2.54 → 2.24 Å, average
Fe−pyridine 2.18 → 2.00 Å). More recently, they have
developed a 1D polymer, {[FeII(bpte)(μ2-NC(CH2)4CN)]-
(BPh4)2·Me2CO}, which undergoes an abrupt and hysteretic
B
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
studies of the [FeII (PMRT)2](BF4)4 complexes that this R
instead of isolating the thiol, it was more convenient to access it
via hydrolysis of the corresponding thioester, thioacetic acid S-
pyridin-2-ylmethyl ester 4, in situ, then add 3R (Scheme 1).
Initial attempts to isolate 4 by a literature procedure34 proved
unsuccessful. However, simplification of the work up
removing the solid K2CO3 by filtration before extracting into
CH2Cl2 (and not washing with HCl)resulted in 4 in high
2
group has a great influence on the SCO activity,15 four different
R groups were employed in this study in order to generate a
new family of [FeII (PSRT)2](BF4)4 systems, thereby increas-
2
ing our chances of observing and/or fine-tuning SCO.
RESULTS AND DISCUSSION
■
1
yield (89%), but while clean by H NMR spectroscopy and
Ligand Synthesis. The key precursors to the desired bis-
terdentate PSRT ligands are the N4-substituted-3,5-bis-
(chloromethyl)-1,2,4-triazole hydrochlorides (3R) and the
deprotonated thiol 2-pyridinemethanethiolate, as the 1:2
reaction of these precursors should generate the two
thioether-linked (rather than amino-linked for PMRT) “arms”
off the triazole (Scheme 1).
successfully used in the subsequent step, it should be noted that
it was not microanalytically pure.
With thioacetic acid S-pyridin-2-ylmethyl ester 4 in hand, an
analogous protocol to that reported for the synthesis of the
bpte ligand by Nolan in 1970 (obtained as an oil),35 and
improved upon by McKenzie in 2011 (obtained as crystals),36
was employed to add the two “arms” and generate the desired
PSRT ligands (Scheme 1). Specifically, under a N2 atmosphere
in ethanol solution, two equivalents of 4 are deprotected by
stirring with sodium ethoxide for 30 min. The appropriate N4-
substituted-3,5-bis(chloromethyl)-1,2,4-triazole hydrochloride
3R was then added and the mixture stirred at room
temperature for 1 h, followed by a 5 h reflux, and finally by
stirring at room temperature overnight. In all four cases,
workup of the reaction mixtures gave brown oils, which in the
case of the phenyl- and p-tolyl-substituted ligands were
recrystallized from hot toluene to yield PSPhT and PSMePhT
as analytically pure beige powders. After failing to successfully
complex the impure oil of the p-tert-butylphenyl ligand
PSt−BuPhT with iron(II), this ligand was first purified by
column chromatography, which, after recrystallization, gave
PSt−BuPhT as a microanalytically pure microcrystalline solid,
however in poor (13%) yield. The oil obtained in the case of
the isobutyl ligand PSiBuT was clean by 1H NMR spectroscopy
but was not microanalytically pure. However, this crude
material was successfully used in complexations.
Scheme 1. Synthetic Route to the PSRT Ligands
Synthesis of [FeII (PSRT)2](BF4)4 Complexes. Dinuclear
2
iron(II) complexes of the form [FeII (PSRT)2](BF4)4·solvent
Given our established general route to 3R head units,15,16
four N4-substituents, R, were selected for the present study,
three of which, the phenyl, p-tert-butylphenyl, and isobutyl N4-
substituted 3,5-bis(chloromethyl)-1,2,4-triazole hydrochlorides,
were previously reported by us. The fourth 3R head unit, N4-p-
tolyl-3,5-bis(chloromethyl)-1,2,4-triazole hydrochloride, is new
but was synthesized according to our general procedure, in
yields consistent with those found for the other R groups.
While only four such head units, 3R, are employed herein, it
should be noted that there is scope for incorporation of a far
wider range of R groups, as the N4-substituent is readily
introduced in the first step by appropriate choice of a
commercially available primary amine in the synthesis of
triazole 1R (Scheme 1). The reasons that phenyl, tert-
butylphenyl, and isobutyl N4-substituents were chosen for this
initial foray into the development of a new range of PSRT
ligands, PSPhT, PSt−BuPhT, and PSiBuT, respectively (Scheme
2
were synthesized for three of the ligands, PSPhT, PSMePhT,
and PSiBuT. Unfortunately, microanalytically pure complex
was not obtained from PSt‑BuPhT, even after purification of the
ligand (see above).
All three complexes precipitated as white to pale green
powders from the 1:1 reaction of the appropriate PSRT ligand
with iron(II) tetrafluoroborate hexahydrate under argon in
MeOH or MeOH/CHCl3 at room temperature. The solvent
and ratio of solvents used in these three complexations were
varied due to solubility differences between the three complexes
and were chosen in order to precipitate out the desired product.
However, all three powders could be recrystallized, in air, by
the same protocol: vapor diffusion of THF into the respective
MeCN solutions. In all three cases this resulted in the
formation of colorless to pale green single crystals suitable for
X-ray crystal structure determinations (see later). Solvent is
present in all of these samples.
1), is that we previously found that the [FeII (PMRT)2](BF4)4
2
For the PSPhT complex, drying of the initial pale green
powder under a stream of nitrogen yielded microanalytically
analogues with these R groups exhibited SCO from [HS-HS] to
(mostly) [HS-LS]. The fourth R group, tolyl, was selected in
order to access the PSMePhT ligand, as tolyl had been
previously used by our group as an N4-substituent in other
SCO-active iron(II) 1,2,4-triazole systems.31,32
pure [FeII (PSPhT)2](BF4)4·1/2CHCl3·21/2H2O (solvato-
2
morph A), in 52% yield. Recrystallization of this powder gave
colorless block-shaped single crystals of [FeII (PSPhT)2]-
2
(BF4)4·2MeCN·H2O (solvatomorph B, see structure descrip-
tion later), which after air drying lost MeCN and picked up
H2O, in the process also losing crystallinity, resulting in a third
While in principle the triazole head units 3R could be reacted
with two equivalents of 2-pyridinemethanethiol, under basic
conditions, to introduce the two thioether-linked “side arms”
and generate the new PSRT ligands, thiols are prone to
oxidation and often have an unpleasant odor.33 Therefore,
solvatomorph, [FeII (PSPhT)2](BF4)4·21/2H2O (solvatomorph
2
C), obtained in 22% overall yield. An attempt at cocrystalliza-
C
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
tion of the initial precipitate with TCNQ− anions instead
yielded colorless rod-shaped crystals of a fourth solvatomorph,
[FeII (PSPhT)2](BF4)4·21/2MeCN·1/2H2O·THF (solvato-
2
morph D), featuring a different ligand binding mode (see
Figure 1 for binding modes; also see structure description in
SI); however this was obtained only on a small scale, so no
other characterization could be performed.
i
For the other two complexes, with R = MePh and Bu, the
complexation reactions yielded impure off-white precipitates,
which upon recrystallization yielded pale green block-shaped
and colorless block-shaped single crystals of
[FeII (PSMePhT)2](BF4)4·2MeCN and [FeII (PSiBuT)2]-
2
2
(BF4)4·4MeCN, respectively (see structure descriptions later).
On air drying, these crystals also crumbled and lost some
acetonitrile and picked up water, to give the solvatomorphs
[FeI I 2 (PSM e PhT)2 ](BF4 )4 ·11 /2 MeCN·2H2 O and
[FeII (PSiBuT)2](BF4)4·MeCN·H2O, in 18% and 16% overall
2
yields, respectively.
Despite our attempts to produce a set of perfect analogues,
i.e., with exactly the same solvent content, the varied nature of
the packing interactions of each R group involved led to a loss
of control of solvent content; hence differing amounts of
MeCN, H2O, and/or THF were present in the single crystals of
each of these complexes.
Structures of [FeII (PSRT)2](BF4)4 Complexes. X-ray
2
crystal structure determinations were carried out on all three
complexes, including two solvatomorphs of the PSPhT
complex (solvatomorphs B and D), at 100 K. In all four
cases this confirmed that the PSRT ligands had coordinated as
intended, with two of them binding in a bis-terdentate manner
to the two iron(II) centers, giving both iron(II) centers an N4S2
donor set and providing two N1N2-triazole bridges between
them (Figures 2 and 3).
[FeII (PSPhT)2](BF4)4·2MeCN·H2O (solvatomorph B)
2
crystallized in the C2/c space group with the asymmetric unit
constituting half of the complex and the other half generated by
a 2-fold rotation axis along the N3−C9 bond (Figure 2, top).
Hence the two iron(II) centers are crystallographically
−
identical, but half of each ligand is unique. The four BF4
counterions and the two MeCN and one H2O solvent
molecules, per complex, were disordered and could not be
sensibly modeled, so SQUEEZE37 was applied; the electron
density included agreed with the anion and solvent content
deduced by microanalysis.
Figure 2. Perspective view of the cation of (top) [FeII (PSPhT)2]-
2
(BF4)4·2MeCN·H2O (solvatomorph B), [FeII (PSMePhT)2](BF4)4·
2
2MeCN (middle), and (bottom) [FeII (PSiBuT)2](BF4)4·4MeCN.
2
The two equivalent iron(II) atoms are in a distorted
octahedral N4S2 coordination sphere (Figure 2 and Table 1),
with the two PSPhT ligands supplying all 12 donor atoms and
providing two triazole bridges between them. Each bridging
triazole ligand binds the two iron(II) centers in a trans-axial
mode, i.e., with one pyridine arm up and one down relative to
the triazole ring (Figures 1 and 2). The Fe−N bonds
[2.116(16)−2.1569(17) Å] and Fe−S bonds [2.5379(6) and
2.5932(6) Å] are long, the cis angles of the iron(II)
coordination sphere range far from 90° [79.56(5)−
108.06(7)°], and the octahedral distortion parameter is large
(Σ = 109.84°), all of which is consistent with the presence of
HS iron(II) centers at 100 K. For both ligands, the triazole and
attached N4-phenyl ring substituent are almost at right angles to
one another, 68.22° and 78.64°. This precludes any resonance
effect of the R group on the triazole. Also, as R is relatively
remote from the donor atoms, any inductive effect is not
expected to be strong. Hence, in the solid state, the main
impact of the choice of R group is not electronic, but rather is
Note that the PSRT ligands are coordinated in different modes: (top
and middle) “up−down”, i.e., trans-axial mode (see Figure 1) with a 2-
fold symmetry axis along N3−C9···N6; and (bottom) cis-axial mode
(see Figure 1) with an inversion center between the two iron centers.
Color codes: iron, orange; nitrogen, royal blue; sulfur, yellow; carbon,
gray/black (so that the two ligands are easily identified). Hydrogen
atoms are omitted.
on the crystal packing. In this case there is no evidence of π−π
interactions between the aromatic rings of neighboring complex
cations. The central portion of this complex is relatively flat,
with the two triazole ring planes almost coplanar, intersecting at
an angle of just 4.37°.
In a failed attempt to cocrystallize [FeII (PSPhT)2]4+
2
complex cations with TCNQ− anions, colorless rod-shaped
crystals of [FeII (PSPhT)2](BF4)4·21/2MeCN·1/2H2O·THF
2
(solvatomorph D) crystallized in the P1 space group (see the
̅
SI for full description and figures).
D
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
The Fe−N bonds [2.0885(30)−2.1171(35) Å] and Fe−S
bonds [2.4996(12) and 2.5159(14) Å] are shorter than those in
[FeII (PSPhT)2](BF4)4·2MeCN·H2O (solvatomorph B) but
2
are still within the range expected for HS iron(II). The cis
angles of the iron(II) coordination sphere deviate slightly less
from 90° [79.95(9)−104.83(14)°], and Σ is slightly lower, at
87.16°, but these remain consistent with the presence of two
HS iron(II) centers at 100 K. Similar to the previous structures,
the tolyl−triazole angles are near 90° (76.46° and 70.45°), and
there is no evidence of π−π interactions between complex
cations. The central portion of the complex is significantly more
twisted than [FeII (PSPhT)2](BF4)4·2MeCN·H2O (solvato-
2
morph B), with the triazole ring planes at a greater angle
(23.83° PSMePhT vs 4.37° PSPhT).
[FeII (PSiBuT)2](BF4)4·4MeCN crystallized in the P21/c
2
space group. The asymmetric unit contains one iron(II) center,
one complete ligand, two anions, and two MeCN solvent
molecules, with the other half of the complex generated by an
inversion center between the two iron(II) centers (Figures 2 and
3). Hence the two PSiBuT ligands sandwich the two iron(II)
centers in a cis-axial mode; that is, the pyridine arms of each
ligand bind the iron(II) centers from the same face of the
triazole ring (Figure 1).
In common with the other three structures, the iron(II)
centers are in a distorted octahedral N4S2 coordination sphere
(Table 1) comprising two heterocyclic nitrogen donors and
one thioether sulfur donor from each of the two PSiBuT
ligands. The Fe−N [2.1273(45)−2.1683(45) Å] and Fe−S
[2.5448(14) and 2.5822(21) Å] bond distances, cis donor−Fe−
donor angles [79.54(11)−103.16(5)°], and Σ (97.14°) are all
consistent with the complex being in the [HS-HS] state at 100
K. The central portion of this complex is very flat with the
triazole planes parallel by symmetry and offset by just 0.340 Å.
In this structure the anions and solvent molecules were
successfully included in the refinement. As a result, a number of
solvent/anion-π (Figure 3, red dashed lines) and nonclassical
hydrogen bonding (Figure 3, green dashed lines) interactions
Figure 3. Perspective view showing the solvent/anion−π interactions
(red dashed lines) and nonclassical hydrogen bonds (green dashed
lines to main residue, blue dashed lines to neighboring complex
cations) for [FeII (PSiBuT)2](BF4)4·4MeCN. For image clarity only
2
the two unique MeCN and two unique anions are shown (B2 is “twirl”
disordered about the B2−F5 bond; only the 0.6 occupancy component
is shown). Color codes: iron, orange; nitrogen, royal blue; sulfur,
yellow; carbon, gray; fluorine, green; boron, tan; selected hydrogen
atoms, black. Red spheres represent ring centroids.
[FeII (PSMePhT)2](BF4)4·2MeCN (Figure 2, middle) also
2
crystallized in the C2/c space group with half of the complex in
the asymmetric unit and the other half generated by a C2 axis
through the C9−N3 bond. Indeed it is isomorphous with
[FeII (PSPhT)2](BF4)4·2MeCN·H2O (solvatomorph B) (Fig-
2
ure 2, top), so again the iron(II) centers are in an N4S2
distorted octahedral environment (Table 1), and the bridging
triazole ligands bind in the trans-axial mode (Figure 1). Again
SQUEEZE was used to account for the disordered counterions
and solvent molecules: the electron density included agreed
with the anion and solvent content deduced by microanalysis.
Table 1. Comparison of Selected Bond Distances (Å), Angles (deg), and Other Data for the Four [HS-HS] PSRT Complexes
[FeII2(PSPhT)2](BF4)4·
21/2MeCN·1/2H2O·THF
solvatomorph D (see the SI)
[FeII2(PSMePhT)2](BF4)4·
2MeCN
(SQUEEZE applied)
selected
parameters
[FeII2(PSPhT)2](BF4)4·2MeCN·H2O
solvatomorph B (SQUEEZE applied)
[FeII2(PSiBuT)2](BF4)4·
a
4MeCN
spin state at
100 K
HS-HS
HS-HS
HS-HS
HS-HS
PSRT binding
mode
trans-axial
cis-axial
trans-axial
cis-axial
Fe···Fe
4.216
4.231
4.190
4.218
Fe−Nt
Fe−Npy
Fe−S
cis-Nt−Fe−Nt
cis-Nt−Fe−Npy
range; [av]
cis-S−Fe−Nt
cis-S−Fe−Npy
range; [av]
cis-S−Fe−S
2.1166(16), 2.1348(16)
2.1382(17), 2.1569(17)
2.5379(6), 2.5932(6)
96.57(6)
2.1326(52), 2.1564(47)
2.1665(51), 2.1726(50)
2.5397(16), 2.5779(16)
96.95(18)
2.0885(30), 2.0923(33)
2.0892(34), 2.1171(35)
2.4996(12), 2.5159(14)
95.26(12)
2.1273(45), 2.1418(42)
2.1683(45), 2.1578(43)
2.5448(14), 2.5822(21)
96.89(15)
87.07(6)−108.06(7); ]
[98.17]
91.87(18)−105.37(19);
[98.16]
88.51(12)−104.83(14);
[95.86]
88.91(16)−100.86(16);
[96.79]
79.90(4), 79.62(4)
79.56(5)−88.85(5);
[82.58]
79.79(13), 78.99(13)
78.79(14)−85.83(14);
[82.49]
81.09(9), 79.95(9)
81.31(11)−90.08(9);
[84.69]
79.82(11), 80.12(11)
79.54(11)−91.33(11);
[83.64]
104.60(2)
104.41(5)
103.97(4)
103.16(5)
trans-Npy−Fe−
155.21(7)
155.37(19)
162.35(14)
158.87(17)
Npy
trans-S−Fe−Nt
Σ
171.43(5), 173.51(4)
109.84
174.43(13), 175.97(14)
105.25
172.84(9), 174.48(9)
87.16
176.69(11), 177.01(12)
97.54
a
Nt = N donor atom on triazole; Npy = N donor atom on pyridine; Σ = octahedral distortion parameter, defined as the sum of the absolute values of
the difference of each of the 12 cis angles from 90°.
E
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Comparison of Value or Range [Average] of Selected Bond Distances (Å) and Angles (deg) for the Iron(II)
Coordination Spheres in the Average of the Four [HS-HS] PSRT Complexes Reported Herein (See Table S5 for Data on Each),
ab
with that of the [HS-HS] and [LS-HS] PMAT Complex,10 and the [LS-LS] PMTD Complex17
a
Nt = N donor atom on triazole or thiadiazole; Npy = N donor atom on pyridine; X = S or amino N donor atom; Σ = octahedral distortion
parameter, defined as the sum of the absolute values of the difference of each of the 12 cis angles from 90°, τ4 is the four-coordinate geometry index
for the Fe−X−X−Nt−Nt plane of the octahedron, where τ4 = 0 for square planar and τ4 = 1 for a tetrahedron. Data for HS centers in red text; data
for LS centers in blue text. Fe1/Fe2.
b
between them and the cation were able to be identified, and a
sandwiching of a triazole ring, by a solvent lone pair···triazole···
counteranion interaction, was observed. Specifically, there is a
solvent−π interaction15,16,38,39 between the N2 triazole ring
and N7 of a MeCN molecule [centroid···N7 = 3.106 Å;
∠(nitrile CN···centroid) = 169.87°]. On the other face of the
triazole ring there is an anion−π interaction15,16,39−43 with a
tetrafluoroborate counteranion [F2···centroid = 3.199 Å; F4···
centroid = 3.403 Å], where the counteranion sits almost
directly above the triazole ring [∠(B1−F2−centroid) =
PMRT and PMTD complexes, both of the possible binding
modes, cis- and trans-axial (Figure 1), are observed, with two
examples of complexes adopting each mode: evidently both
modes are possible for at least the PSPhT ligand. Analysis of
the octahedral distortion parameter, Σ, of the iron(II) centers
in these PSRT complexes reveals no clear relationship to the
binding mode (cis-axial, Σ = 97.54° or 105.25°; trans-axial, Σ =
87.16° or 109.84°). However, the HS iron(II) centers in these
PSRT complexes are in more regular octahedral environments,
with significantly smaller Σ values in all cases (87.16−109.84°),
−
107.34°; ∠(B1−F4−centroid) = 97.55°]. The B1 BF4
than for the HS iron(II) centers in [FeII (PMAT)2](BF4)4·
2
counterion is also involved in nonclassical hydrogen bonding
to two of the pyridine rings [C2−H···F3 = 3.246(7) Å, ∠(C2−
H···F3) = 125.8°; C15−H···F3 = 3.192(7) Å, ∠(C15−H···F3)
= 123.1°] and to one of the side arm CH2 groups [C10−
DMF in either the [HS-HS] (Σ = 117.5°) or [HS-LS] state (Σ
= 133.1°) structures (Table 2, red data). Examining this more
closely, both the X−Fe−X and X−Fe−Nt (where X = S for
PSRT, X = NH for PMAT and PMTD; Nt = triazole or
thiadiazole) angles are significantly closer to 90° for PSRT than
for HS iron(II) in PMAT (Figure 4) and are in fact more
similar to the corresponding angles of the LS iron(II) centers of
PMAT and PMTD (Table 2, blue data). However, the Nt−Fe−
Nt angles of PSRT are further from right angles than in the HS
centers of PMAT, but interestingly they are similar to those of
LS iron(II) in PMAT and PMTD. For the remaining cis angles,
there is no significant difference between the HS iron(II)
centers of PSRT and PMAT. The trans X−Fe−Nt angles are
closer to 180° in the PSRT complexes than for the HS iron(II)
in the PMAT complexes and are in fact similar to the LS
iron(II) centers in both the PMAT and PMTD complexes.
Thus, the PSRT complexes are in a more square planar
environment through the sulfur and triazole donor atoms, with
the four-coordinate geometry index44 (τ4) for PSRT being
−
H10a···F4 = 3.086(7) Å, ∠(C−H···F) = 130.7°]. The B2 BF4
anion is “twirl” disordered about the B2−F5 bond. Never-
theless it is involved in nonclassical hydrogen bonding [C11−
H11b···F5 = 3.286(7) Å, ∠(C11−H11b···F5) = 157.0°; C6−
H6a···F11 = 3.346(13) Å, ∠(C6−H6a···F11) = 173.4°]. Both
tetrafluoroborate counterions are also involved in extensive
nonclassical hydrogen bonding to neighboring complex cations
throughout the lattice (Figure 3, blue dashed lines; Figures S5
and S6; Tables S3 and S4). The N6 MeCN molecule interacts
with a CH2 group through a nonclassical hydrogen bond [C−
H···N = 3.418(8) Å, ∠(C−H···N) = 150.2°].
All four of the structurally characterized PSRT complexes,
including [FeII (PSPhT)2](BF4)4·21/2MeCN·1/2H2O·THF
2
(described in the SI), are stabilized in the [HS-HS] state at
100 K (Table 1). Interestingly, unlike the case for the related
F
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 4. Schematic showing the average cis-X−Fe−X and cis−S−Fe−
Nt angles in the [HS-HS] structures (left) [FeII (PSRT)2](BF4)4·
2
solvents and (right) [FeII (PMAT)2](BF4)4·DMF at 298 K. These
2
angles, in the iron(II) complexes of these thioether linked (PSRT) and
amino-linked (PMRT) ligands, show the greatest differences. For
clarity, one pyridine donor in each of these N4X2 octahedral iron(II)
coordination spheres is not shown.
Figure 5. χmT per dinuclear complex vs temperature for the samples of
air-dried crystals, [FeII (PSPhT)2](BF4)4·21/2H2O (solvatomorph C),
2
[FeII (PSMePhT)2](BF4)4·11/2MeCN·2H2O, and [FeII (PSiBuT)2]-
closer to a perfect square plane than for the HS PMRT
complexes (Figure 4, Table 2).
2
2
(BF4)4·MeCN·H2O.
In summary, as intended, greater flexibility has been achieved
in these bis-terdentate ligands by attaching the arms by
thioether linkages rather than amino linkages, due to the longer
C−X bonds (av C−S = 1.807 Å in PSRT, av C−NH = 1.488 Å
in [HS-HS] PMAT) and smaller C−X−C angles (Table S6; av
C−S−C in PSRT 102.0°, av C−N(H)−C in PMAT and
PMTD 112.0−114.5°). This has resulted in less constrained
ligand binding in these dinuclear complexes of PSRT, with both
possible ligand-binding modes observed and iron(II) centers
that are more regularly octahedral in the HS state than was the
case for the previously reported amino-linked PMRT ligands.
mol−1 to 3.9 cm3 K mol−1, with T1/2 = 109 K, then leveling out
at 3.5 cm3 K mol−1 at 40 K, consistent with SCO to the [HS-
LS] state.
[FeII (PSiBuT)2](BF4)4·MeCN·H2O has a χmT value of 6.4
2
cm3 K mol−1 at 350 K, which only slightly decreases, to 5.3 cm3
K mol−1 at 50 K, consistent with the complex remaining mainly
[HS-HS] to low temperatures (Figure 5, blue). This is in good
agreement with the crystallographic data for the ·4MeCN
solvatomorph of this complex (Figure 2, bottom).
Next, fresh crystals of [FeII (PSPhT)2](BF4)4·2MeCN·H2O
2
Magnetic Characterization of [FeII (PSRT)2](BF4)4 Com-
(solvatomorph B) and [FeII (PSMePhT)2](BF4)4·2MeCN were
2
2
plexes in the Solid State. Initially, variable-temperature
magnetic susceptibility measurements were performed on the
three air-dried crystal samples, which analyzed as
[FeII2(PSPhT)2](BF4)4·21/2H2O (solvatomorph C),
[FeI I 2 (PSM e PhT)2 ](BF4 )4 ·11 /2 MeCN·2H2 O, and
studied magnetically to investigate the effects of lattice solvent
content on the solid-state magnetic behavior (see the SI).
Recording susceptibility vs temperature data, on samples both
fresh out of the crystallization mother liquor and after drying in
the magnetometer, revealed that loss of MeCN activates SCO
for the former complex and further stabilizes the LS state for
[FeII (PSiBuT)2](BF4)4·MeCN·H2O. These studies revealed
2
that both FeII (PSPhT)2](BF4)4·21/2H2O (solvatomorph C)
the latter complex. In contrast, [FeII (PSiBuT)2](BF4)4·
2
2
and [FeII (PSMePhT)2](BF4)4·11/2MeCN·2H2O are SCO
4MeCN remains [HS-HS] regardless of whether studied
fresh out of solution or after air drying (see the SI).
Unfortunately, due to the highly disordered nature of the
MeCN solvent molecules, a detailed structural analysis of the
interactions between MeCN and the complex for the two SCO-
active complexes could not be performed, and so the effects of
solvent on the SCO that we have described above (and in more
detail in the SI) cannot be fully explained. However, similar
solvent dependence of SCO has been seen for the PMRT
2
active, while [FeII (PSiBuT)2](BF4)4·MeCN·H2O remains in
2
the [HS-HS] state (Figure 5).
At 350 K, the χmT value of [FeII (PSPhT)2](BF4)4·21/2H2O
2
(solvatomorph C) is 6.0 cm3 K mol−1 per dinuclear complex,
which is consistent with the [HS-HS] state (Figure 5, red). As
it is cooled, a gradual SCO occurs, between about 300 and 150
K, with χmT dropping in two steps from 5.5 cm3 K mol−1 to 2.0
cm3 K mol−1. The latter value corresponds to about two-thirds
of the iron(II) centers being in the LS state and the remaining
third remaining in the HS state. This gradual transition appears
to occur in two steps, with the first derivative of χmT with
respect to temperature giving T1/2 values of approximately 265
and 210 K (Figure S8). A second SCO event is observed
between 100 and 70 K, with the value of χmT decreasing from
1.7 cm3 K mol−1 to 1.0 cm3 K mol−1 (T1/2 = 87 K), consistent
with almost all of the iron(II) centers having transitioned to the
LS state.
complex [FeII (PMPhtBuT)2](BF4)4, which is SCO inactive with
2
a solvent content of ·3CH3CN·1/2(C4H10O) but after air drying
becomes SCO active (with scan-rate-dependent thermal
hysteresis) with a solvent content of ·31/2H2O.16
Magnetic Characterization of [FeII (PSRT)2](BF4)4 Com-
2
plexes in Acetonitrile Solution. In order to avoid the
complicating effects of the impact of crystal packing and solvent
of crystallization effects on SCO, noted in the previous section,
the magnetic behavior of the three iron(II) complexes was also
investigated in acetonitrile solution. These solution phase
studies facilitate probing the electronic effects of the R substituent
in these complexes on the SCO events. Evans’ method NMR
spectroscopy and UV−vis spectroscopy have been employed, vs
[FeII (PSMePhT)2](BF4)4·11/2MeCN·2H2O appears to be
2
[HS-HS] at room temperature with χmT = 6.8 cm3 K mol−1 at
300 K (Figure 5, green). Upon cooling, an SCO is observed
between 140 and 85 K, with χmT decreasing from 5.9 cm3 K
G
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
temperature, to probe the spin state vs temperature profile for
each of these three complexes dissolved in acetonitrile solution
(Figures 6 and 7).
Figure 6. χmT per dinuclear complex cation vs temperature for
CD3CN solutions of [FeII (PSPhT)2](BF4)4, [FeII (PSMePhT)2]-
2
2
(BF4)4, and [FeII2(PSiBuT)2](BF4)4, as determined by Evans’
method.45−48 For modern NMR instruments (with superconducting
magnets) there is an amendment to the original Evans’ method
formula49,50 (see Experimental Section for details).
Evans’ Method NMR Study. Magnetic susceptibility
determinations were carried out using Evans’ method.45−48 In
CD3CN, all three complexes appear to undergo gradual
incomplete SCO in the measured temperature range, 264−
313 K (Figure 6, see Experimental Section for details).
Of the three complexes studied in CD3CN solution (Figure
6), [FeII (PSPhT)2]4+ maintains the largest χmT values across
2
the studied temperatures, 313 to 264 K (dropping from 6.3 to
4.8 cm3 K mol−1), so appears to have the lowest T1/2
.
[FeII (PSMePhT)2]4+ undergoes a similar transition, however
2
with lower χmT values (5.5 to 4.1 cm3 K mol−1), which are
indicative of a greater proportion of LS species being present in
solution across all temperatures. Similarly, [FeII (PSiBuT)2]4+
2
has a still greater proportion of LS (4.6 to 2.9 cm3 K mol−1), so
appears to have the highest T1/2
.
In summary, the relative tendency of these three complexes
in MeCN solution to go LS, is in the reverse order of that seen in
the solid state. This is consistent with crystal packing and solvent
content effects dominating in the solid state. In contrast, the
solution results are free from the influence of those factors and
are instead able to reveal the electronic impact of the choice of
R: the results are consistent with the more electron donating
the R substituent at the N4 position on the triazole (iBu > MePh
> Ph), the more the LS state is favored, the octahdedral ligand
field increased, and the T1/2 raised. Hence it appears that for
these complexes in solution the σ-donor strength of the ligand is
the largest contributor to the octahedral ligand field strength.
UV−Vis Spectroscopy (Solid and MeCN Solution). In
Figure 7. Variable-temperature UV−vis absorption spectra of MeCN
solutions of (top) [FeI I 2 (PSPhT)2 ](BF4 )4 , (middle)
[FeII (PSMePhT)2](BF4)4, and (bottom) [FeII (PSiBuT)2](BF4)4.
Note: ε is calculated per mole of dinuclear complex in all cases. For
the 1A1g → 1T1g band, at 574−586 nm, the observed ε value is related
to the product of the LS fraction multiplied by ε of the pure [LS−LS]
state, and therefore indicates the amount of LS Fe(II) present.
2
2
the solid state [FeII (PSPhT)2](BF4)4·1/2CHCl3·21/2H2O
2
(solvatomorph A), [FeII (PSMePhT)2](BF4)4·11/2MeCN·
complex) of 34 and 9 L mol−1 cm−1, respectively, at 293 K
(Figure 7, top). By comparison to typical iron(II) octahedral
complexes,51−55 including those with an N4S2 coordination
sphere,28−30 the band at 586 nm can be attributed to the
2
2H2O, and [FeII (PSiBuT)2](BF4)4·MeCN·H2O are all pale
2
green, but when dissolved in acetonitrile, they give intense dark
green solutions (Figures S13−S15, Figure 7).
Considering the UV−vis spectrum of [FeII (PSPhT)2]-
Laporte-forbidden, spin-allowed d−d transition A1g → T1g of
the LS state, and the weak, broad band at 930 nm to the spin-
allowed d−d transition 5T2g → 5Eg associated with the HS state.
1
1
2
(BF4)4, in MeCN, bands are observed at λmax = 586 and 930
nm with extinction coefficients (per mole of dinuclear
H
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Temperature dependence of the band at 586 nm indicates that
a partial spin crossover occurs in solution over this temperature
range (Figure 7, top), as the extinction coefficient per mole of
dinuclear complex steadily increases with decreasing temper-
ature, from 34 L mol−1 cm−1 at 293 K to 68 L mol−1 cm−1 at
233 K, which is consistent with an increased proportion of LS
iron centers. At 930 nm there is no significant change in
extinction coefficient with temperature, with ε = 9 L mol−1
cm−1 at all measured temperatures: however, this absorption
band of the HS state is very weak and noisy, even at the
solubility limit of the complex in MeCN; thus no spin-state
monitoring can be achieved in this region of the spectrum. The
LS band seen in MeCN solution is also evident in the solid-
state spectrum (λmax = 572 nm) at room temperature (Figures
S13, S14).
CONCLUSIONS
■
An unusual N4S2 coordination environment has been
successfully introduced to the iron(II) centers of a family of
dinuclear PMRT-like complexes by incorporation of thioether
donors into the “arms”, giving rise to the new family of PSRT
ligands. Not only has SCO activity been retained (from the
PMRT systems), but for the first time in complexes of this
kind, all three of the possible spin states[HS-HS], [HS-LS],
and [LS-LS]are accessible for the new PSRT systems.
In contrast, in previous studies of dinuclear iron(II)
complexes of PMAT/PMRT it has not been possible to access
the [LS-LS] state due to the highly constrained nature of the
these amino-linked ligands: on SCO from [HS-HS] to [HS-LS]
in [FeII (PMAT)2](BF4)4·DMF, the iron(II) center, which
2
remains HS, becomes more distorted (HS in [HS-HS] at 293 K
has Σ = 117.5°; HS in [HS-LS] at 123 K has Σ = 133.1°),
which inhibits this center from also transitioning to the LS
state. The PSRT complexes are less constrained due to the
incorporation of a more flexible thioether group, which gives a
more regular octahedral environment at the iron(II) centers in
the HS state, facilitating complete SCO to the [LS-LS] state.
Previously, dinuclear iron(II) systems based on the PMAT
ligand have been observed to bind in either the cis-axial (PMAT
and PMRT) or trans-axial (PMTD) binding modes. Here, the
more flexible nature of the PSRT ligands has given access to
both the cis- and trans-axial binding modes. Interestingly for the
PSRT systems that were studied magnetically, only the trans-
axial binding mode complexes exhibited SCO in the solid state.
However, the sample size is too small to be able to draw firm
conclusions on this point.
Partial SCO also occurs for both of the other complexes in
MeCN as the temperature is lowered (Figure 7). For
[FeII (PSMePhT)2](BF4)4 the extinction coefficient of the 574
2
nm 1A1g → 1T1g band of the iron(II) centers present in the LS
state increases from 53 to 92 L mol−1 cm−1 (per mole of
complex) on cooling from 293 to 233 K (Figure 7, middle).
The position of this band is almost identical to that of
[FeII (PSPhT)2](BF4)4, however with greater extinction
2
coefficients at each temperature, consistent with a greater
proportion of LS iron(II) centers. The very weak and broad
band centered at ca. 900 nm remains at 14 L mol−1 cm−1 across
all studied temperatures.
For [FeII (PSiBuT)2](BF4)4, the LS band is centered at 580
2
nm (Figure 7, bottom). The extinction coefficients (per mole of
complex) are greater than for the other two complexes and
again increase with decreasing temperature (61 L mol−1 cm−1 at
288 K to 139 L mol−1 cm−1 at 233 K). In this spectrum the HS
5T2g → 5Eg band centered at 920 nm was more resolved, and a
slight decrease in extinction coefficient from 7 L mol−1 cm−1 to
5 L mol−1 cm−1 is observed with decreasing temperature.
Four of the temperature points at which UV−vis spectra
were collected also have corresponding Evans’ method data.
The χmT values, and associated HS fraction, γHS, obtained from
the Evans’ method data allow us to “calibrate” the observed ε
values (assigning them γHS values) at these temperatures for
each of the three complexes in MeCN solution. From this we
can go on to estimate the γHS for each complex at the lower
temperatures studied only by UV−vis spectroscopy (see the SI
for details, Table S9, Figures S16 and S17). Overall this analysis
indicates that on cooling these MeCN solutions from 313 K to
Subtle variation of the R substituent at the N4 triazole
position of the PSRT ligands results in a dramatic change in the
magnetic properties of the dinuclear iron(II) complexes in the
solid state. Although the site of variation is relatively remote
from the iron(II) coordination environment, and the aromatic
R groups are far from coplanar with the triazole ring, the
variation of R was still expected to influence the magneto-
chemistry based on our previous PMRT studies. As for those
PMRT studies, the effects of crystal packing and solvato-
morphism on the solid state SCO are again seen to be dominant in
these new PSRT systems.
In contrast to the solid-state magnetic behavior, the UV−vis
and Evans’ method data obtained on the three complexes in
MeCN solution show that all three undergo partial, gradual,
SCO events in the measurable temperature range (313−233
K), with T1/2 values close to room temperature. In solution, the
effects of crystal packing and lattice solvent are eliminated, and
instead the electronic effects of varying the R are able to be
probed. As the magnetic behavior observed in solution is very
different from that seen in the solid state, these studies confirm
that in the solid state the impact of changing R is indeed
dominated by associated changes in the crystal packing and
solvent content. Indeed these effects are so dominant in the
solid state that the complexes actually have the reverse tendency
to go LS, to the order seen in solution. More importantly, these
solution studies also show that the more electron donating the
R substituent at the N4 position on the triazole (iBu > MePh >
Ph), the more the LS state is favored; so for these complexes in
solution the σ-donor strength of the ligand is the largest contributor
to the octahedral ligand field strength.
233 K, [FeII (PSPhT)2](BF4)4 undergoes SCO from γHS ≈ 0.9
2
to γHS ≈ 0.6, [FeII (PSMePhT)2](BF4)4 from γHS ≈ 0.8 to γHS
≈
2
0.5, and [FeII (PSiBuT)2](BF4)4 from γHS ≈ 0.7 to γHS ≈ 0.2.
2
Fitting of the UV−vis54,56 and Evans’ NMR46,47,57,58 data to
the thermodynamic parameters of the spin equilibrium (see the
SI for details, Tables S10−S12, Figures S18−S20) indicates that
in MeCN solution both [FeII2(PSPhT)2](BF4)4 and
[FeII (PSMePhT)2](BF4)4 probably undergo a half SCO, [HS-
2
HS] ↔ [HS-LS], whereas [FeII (PSiBuT)2](BF4)4 appears to
2
undergo a full SCO, [HS-HS] ↔ [LS-LS]. This is consistent
with the estimated high spin fractions, γHS, noted in the
previous paragraph. Determining T1/2 values from these
thermodynamic parameters reveals that the SCO for each
complex in MeCN solution is centered at or near room temperature
(see the SI). From these tentative fits, it also appears that the
1
actual ε values per mole of dinuclear complex, for the LS A1g
→ 1T1g d−d band seen at 574−586 nm, are about 200 and 100
Finally, we note that these are the first dinuclear iron(II)
complexes featuring N4S2 coordination to show SCO and that
L mol−1 cm−1 for the [LS-LS] and [LS-HS] states, respectively.
I
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
−
agrees very well with the four BF4 anions and the proposed solvent
content of two acetonitrile molecules per complex determined by
microanalysis (200 electrons/complex).
all three possible spin states can be accessed; so this new ligand
class shows great promise for further elaboration. Such studies
are currently under way.
[FeII (PSiBuT)2](BF4)4·4MeCN. Colorless block-shaped crystal.
Data were collected using Cu Kα radiation. All non-H atoms were
assigned anisotropic thermal displacement parameters, with the
exception of the C2-rotationally disordered BF4 anion: the shared
2
EXPERIMENTAL SECTION
■
−
General Instrument Details. NMR spectra were recorded on a
Varian 400-MR spectrometer at 400 MHz (1H) or 100 MHz (13C) or
on a Varian 500 MHz VNMRS spectrometer at 500 MHz (1H) or 125
MHz (13C). Mass spectra were recorded on a Bruker MicrOTOF-Q
spectrometer. TGA was recorded on a TA Instruments Q50 with the
samples in a platinum pan heated at 2 °C min−1. Nitrogen gas flow was
set to 40 mL min−1 over the balance and 60 mL min−1 over the
sample. Microanalysis was performed by the Campbell Microanalytical
Laboratory at the University of Otago.
boron atom and fluorine atom were refined anisotropically, and the
three rotationally disordered fluorine atoms, over two sites of
occupancy 60:40, were refined isotropically.
Magnetic data for [FeII (PSPhT)2](BF4)4·21/2H2O (solvatomorph
2
C) were collected on a Quantum Design SQUID magnetometer under
an applied field of 0.1 T at Callaghan Innovation, Lower Hutt, NZ.
The data were collected between 350 and 4 K in settle mode at 5 K
intervals.
Magnetic data for [FeII (PSMePhT)2](BF4)4·11/2MeCN·2H2O were
2
Solid-state visible reflectance data were recorded on a PerkinElmer
Lambda 950 UV−vis/NIR spectrometer in the range 200−1500 nm.
Samples were attached to a solid support of Labsphere reflectance
standard using double-sided tape (Sellotape double-sided).
recorded using a Quantum Design PPMS susceptometer equipped
with a vibrating sample mount, under an applied field of 0.1 T at
Callaghan Innovation, between 300 and 4 K, in sweep mode with a
scan rate of 5 K min−1.
Solution UV−vis spectra were recorded on a PerkinElmer Lambda
950 UV−vis/NIR equipped with a JANIS Research model VNF-100
cryostat and Lake Shore Cryotronics model 335 temperature
controller. Solutions were all in HPLC grade MeCN of concentrations
Magnetic data for [FeII2(PSiBuT)2](BF4)4·MeCN·H2O were
collected in-house on a Quantum Design Versalab, a cryogen-free
PPMS susceptometer, equipped with a vibrating sample mount under
an applied field of 0.1 T. Data were collected in the temperature range
350−50 K in sweep mode with a scan rate of 5 K min−1.
Samples were mounted for the PPMS and Versalab in a
polyethylene capsule, and for the SQUID in a gelatin capsule. Data
were corrected for the diamagnetism of the sample according to the
approximation that χMdia(sample) = −0.5M × 10−6 cm3 mol−1 (where
M = molecular weight of complex),64 and a background correction for
the sample holder was also applied.
(at room temperature) as follows: [FeII (PSPhT)2](BF4)4, 32.66
2
mmol L−1
;
[FeII2(PSMePhT)2](BF4)4, 3.913 mmol L−1
;
[FeII (PSiBuT)2](BF4)4, 6.238 mmol L−1. Temperatures are accurate
2
to 0.5 K. The data were corrected for concentration changes arising
from the temperature dependence of the density of acetonitrile.
X-ray crystallographic data were collected on an Oxford Diffraction
SuperNova diffractometer with an Atlas CCD, equipped with a
Cryostream N2 open-flow cooling device, using mirror monochro-
mated microfocus Mo or Cu Kα radiation at 100 K. 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) for all structures were processed using
CrysAlis Pro.59 Structures were solved using SUPERFLIP60 and
refined against all F2 data using SHELXL-2014.61 Hydrogen atoms
were inserted at calculated positions with U(H) = 1.2U(attached
atom) and rode on the atoms to which they were attached.
Solution magnetic susceptibility data were measured in the
1
temperature range 264−313 K in CD3CN by H NMR spectroscopy
on a Varian 500 MHz VNMRS spectrometer using Evans’ method.48
Samples were prepared by dissolving a precisely known mass in 0.700
mL of CD3CN. Pure CD3CN was placed in a special capillary NMR
tube insert, and the paramagnetic solution was placed in the outer
tube. Temperatures are accurate to 1 K. The shift of the CD3CN
peak in the paramagnetic solution compared to pure CD3CN, Δf in
Hz, can be used to calculate the mass susceptibility of the complexes
by eq 1.
SQUEEZE37 was used for [FeII (PSPhT)2](BF4)4·2MeCN·H2O
2
(solvatomorph B) and [FeII (PSMePhT)2](BF4)4·2MeCN, as the
2
3Δf
4πmf
d0 − ds
m
counteranions and solvent molecules were badly disordered (details
below). High-resolution pictures were prepared using Mercury62 and
POVray63 software. CCDC 1439744−1439747.
χg =
+ χ0 + χ0
(1)
where m is the concentration of the paramagnetic solution in g cm−3
and this was corrected for the temperature dependence of the density
of acetonitrile, f is the spectrometer frequency in Hz, and d0 and ds are
the densities of the solvent and solution, respectively. χ0 is the mass
susceptibility of the solvent in cm3 g−1. However, eq 1 can be
simplified by taking an approximation that ds = d0 + m. This is
reasonable because the solutions used were dilute49,54 (2.5−5.3 mmol
L−1). This approximation leads to the second and third terms in eq 1
canceling out, giving eq 2, which was used in the present analysis.
[FeII (PSPhT)2](BF4)4·2MeCN·H2O (Solvatomorph B). Colorless
2
block-shaped crystal. Data were collected using Mo Kα radiation.
Disorder of the counteranions and solvent molecules could not be
modeled well (large Q peaks could not be sensibly assigned), so all of
them were removed and SQUEEZE37 was applied. This greatly
improved the difference map and (not surprisingly) lowered the R1
(on [I > 2σ(I)] data) from 0.053 (for the best model achieved; one
MeCN molecule disordered over two sites of half-occupancy each and
a half water molecule disordered over two sites of quarter-occupancy
each per asymmetric unit, so 2 MeCN and 1 H2O per complex) to
0.047. The missing electron density found by SQUEEZE was 837
electrons/cell, i.e., 209 electrons per complex (Z = 4), which agrees
very well with the four BF4− anions and the proposed solvent content
of two acetonitrile and one water molecule per complex determined by
microanalysis (210 electrons/complex).
3Δf
4πmf
χg =
(2)
Note that the original Evans’ method was developed using early
generations of NMR spectometers in which the sample axis was
perpendicular to the magnetic field, whereas in modern super-
conducting NMR spectrometers the sample axis is parallel to the
magnetic field. Hence there is a factor of 3/4π in eqs 1 and 2, rather
than the original factor of −3π/2.49,50
Multiplying χg by the molecular weight (M) gave the molar
susceptibility χM. These χM values were then corrected for the
diamagnetic contributions of each sample according to χMdia(sample)
= −0.5M × 10−6 cm3 mol−1.64
General Experimental Details. Ethanol was dried by distillation
over magnesium turnings and iodine. LiTCNQ was synthesized
[FeII (PSMePhT)2](BF4)4·2MeCN. Pale green block-shaped crystal.
2
Data were collected using Mo Kα radiation. Disorder of the
counteranions and solvent molecules could not be modeled well
(large Q peaks could not be sensibly assigned), so all of them were
removed and SQUEEZE37 was applied. This greatly improved the
difference map and (not surprisingly) lowered the R1 (on [I > 2σ(I)]
data) from 0.080 (for the best model achieved; three one-quarter
occupancy MeCN molecules per asymmetric unit, so 11/2 MeCN per
complex) to 0.077. The missing electron density found by SQUEEZE
was 816 electrons/cell, i.e., 204 electrons per complex (Z = 4), which
J
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 3. Crystal Data and Structure Refinement Details for the Complexes [FeII (PSPhT)2](BF4)4·2MeCN·H2O (Solvatomorph
2
B), [FeII (PSPhT)2](BF4)4·21/2MeCN·1/2H2O·THF (Solvatomorph D), [FeII (PSMePhT)2](BF4)4·2MeCN, and
2
2
[FeII (PSiBuT)2](BF4)4·4MeCN
2
[FeII2(PSPhT)2](BF4)4·
[FeII2(PSPhT)2](BF4)4·
21/2MeCN·1/2H2O·THF
[FeII2(PSMePhT)2](BF4)4·
[FeII2(PSiBuT)2](BF4)4·
2MeCN·H2O
2MeCN
4MeCN
empirical formula
Mr
C44H42Fe2N10S4
950.81
C53H58.50B4F16Fe2N12.50O1.50 S4
C46H46Fe2N10S4
978.87
C48H62B4F16Fe2N14S4
1422.29
1481.80
triclinic
cryst syst
space group
a [Å]
monoclinic
C2/c
monoclinic
C2/c
monoclinic
P21/c
P1
̅
15.8987(2)
27.4992(4)
14.3082(2)
90
9.9850(2)
11.9087(3)
15.1801(4)
104.568(2)
97.7804(19)
108.767(2)
1607.40(7)
1
16.0419(6)
27.6191(11)
14.6634(6)
90
10.6772(16)
24.8009(12)
16.296(2)
90
b [Å]
c [Å]
α [deg]
β [deg]
γ [deg]
V [Å]
108.9212(18)
90
110.472(4)
90
134.03(3)
90
5917.52(15)
4
6086.5(4)
4
3102.6(12)
2
Z
T [K]
100(2)
100(2)
100(2)
100(2)
ρcalcd [g/cm3]
1.067
1.531
1.068
1.522
μ [mm−1
]
0.665
5.688
0.648
5.853
F(000)
1968
756
2032
1456
cryst size (mm)
θ range for data collection
reflns collected
0.270 × 0.170 × 0.090
2.963 to 29.670
78 405
0.270 × 0.076 × 0.070
3.095 to 76.311
23 901
0.400 × 0.200 × 0.200
3.215 to 29.593
20 288
0.160 × 0.130 × 0.110
4.173 to 62.619
18 322
indep reflns
7974
6655
7460
4897
R(int)
0.0386
0.1213
0.0376
0.1256
max. and min. transmn
data/restraints/params
Goof (F2)
1.0 and 0.797 44
7974/0/274
1.048
1.000 00 and 0.577 12
6655/55/449
1.058
1.00 and 0.69
7460/0/284
1.035
1.000 and 0.363 27
4897/0/398
1.001
R1 [I > 2σ(I)]
0.0467
0.1068
0.0765
0.0731
wR2 [all data]
0.1331
0.2750
0.2004
0.1979
max./min. res. e density [e·
1.302 and −0.555
2.365 and −1.923
1.364 and −1.528
0.974 and −0.648
Å−3
]
following the reported procedure.65 All other chemicals were
purchased from commercial suppliers and were used as received.
Stepwise Synthesis of the Ligands. N,N-Dimethylformamide azine
was prepared according to the literature method.66 N4-Substituted-
1,2,4-triazoles 1R, N4-substituted-3,5-bis(hydroxymethyl)-1,2,4-tria-
zoles 2R, and N4-substituted-3,5-bis(chloromethyl)-1,2,4-triazole
hydrochlorides 3R were prepared according to the reported general
procedure,15 with the exception of 1MePh, 2MePh, and 3MePh, which
are detailed below.
60.0 mmol) was added. After a further 6 h of heating the product had
precipitated and excess paraformaldehyde had sublimed up the
condenser. Hot filtration of the reaction mixture yielded a white
solid contaminated with paraformaldehyde. This mixture was taken up
in boiling ethanol (100 mL), and solid paraformaldehyde removed by
hot filtration. The filtrate was left to cool to room temperature, and
white crystalline N4-p-tolyl-3,5-bis(hydroxymethyl)-1,2,4-triazole was
isolated by filtration (1.09 g, 83%). Anal. Calcd for C11H13N3O2: C
60.26, H 5.98, N 19.17. Found: C 60.26, H 5.99, N 19.17. δH (400
MHz, d6-DMSO): 7.32−7.36 (m, 4H, PhH2/6 and PhH3/5), 5.35 (t,
2H, J = 5.4 Hz, OH), 4.36 (d, 4H, J = 5.3 Hz, CH2), 2.37 (s, 3H,
CH3). δC (125 MHz, d6-DMSO): 154.9 (tr), 139.2 (PhC4), 131.6
(PhC1), 130.2 (PhC3/5), 127.3 (PhC2/6), 53.6 (CH2), 21.2 (CH3).
N4-p-Tolyl-3,5-bis(chloromethyl)-1,2,4-triazole·1/2HCl (3MePh).
N4-p-Tolyl-3,5-bis(hydroxymethyl)-1,2,4-triazole, 2MePh (1.059 g,
4.83 mmol), was dissolved in 10 mL of SOCl2 and stirred for 2 h.
The reaction mixture was dried first by evaporating SOCl2 under a
stream of compressed air, then in vacuo. Recrystallization from hot
ethanol afforded N4-p-tolyl-3,5-bis(chloromethyl)-1,2,4-triazole hydro-
chloride as a beige powder (1.050 g, 79%). Anal. Calcd for
C11H11.5N3Cl2.5: C 48.16, H 4.22, N 15.32. Found: C 48.51, H 4.19,
N 15.41. δH (400 MHz, CDCl3): 7.37−7.43 (m, 4H, PhH2/6 and
PhH3/5), 4.61 (s, 4H, CH2), 2.50 (s, 3H, CH3). δC (125 MHz,
CDCl3): 152.5 (tr), 141.8 (PhC4), 130.9 (PhC3/5), 128.7 (PhC1),
127.1 (PhC2/6), 32.7 (CH2), 21.4 (CH3).
N4-p-Tolyl-1,2,4-triazole (1MePh). N,N-Dimethylformamide azine
dihydrochloride (4.00 g, 18.6 mmol) and p-toluidine (1.33 g, 12.4
mmol) were refluxed in pyridine (50 mL) for 48 h. After cooling to
room temperature the pyridine was removed in vacuo, and trace
amounts were azeotroped with toluene (50 mL) then methanol (50
mL). The resulting yellow-brown oil was taken up in CH2Cl2 (50 mL),
washed with water (2 × 50 mL), saturated aqueous NaHCO3 (50
mL), and then brine (50 mL), followed by drying with MgSO4 and
taking to dryness in vacuo to give a beige powder. Recrystallization in
hot toluene afforded N4-p-tolyl-1,2,4-triazole as a brown crystalline
solid (1.36 g, 69%). Anal. Calcd for C9H9N3: C 67.91, H 5.70, N
26.40. Found: C 68.12, H 5.69, N 26.50. δH (400 MHz, CDCl3): 8.52
(s, 2H, trH), 7.33−7.35 (m, 2H, phH3/5), 7.26−7.29 (m, 2H, phH2/
6), 2.44 (s, 3H, CH3). δC (100 MHz, CDCl3): 141.5 (tr), 139.4
(PhC4), 131.3 (PhC1), 130.8 (PhC3/5), 122.2 (PhC2/6), 21.1
(CH3).
N4-p-Tolyl-3,5-bis(hydroxymethyl)-1,2,4-triazole (2MePh). N4-p-
Tolyl-1,2,4-triazole (0.956 g, 6.01 mmol) was dissolved in xylenes
(50 mL) with heating to 110 °C. The reaction vessel was placed under
an argon atmosphere, and paraformaldehyde (1.80 g, 60.0 mmol) was
quickly added by briefly removing the condenser, while argon was
flowed into the reaction vessel. The resulting suspension was heated at
125 °C for 2.5 h before a second portion of paraformaldehyde (1.80 g,
Thioacetic Acid S-Pyridin-2-ylmethyl Ester (4). Compound 4 was
prepared using an adapted method from the literature.34 A suspension
of potassium thioacetate (2.30 g, 20.1 mmol), K2CO3 (5.06 g, 36.6
mmol), and 2-picolyl chloride hydrochloride (3.00 g, 18.3 mmol) in
dry DMF (40 mL) was stirred overnight under argon in the dark. The
white precipitate was filtered and washed with CH2Cl2 (50 mL), and
the filtrate was taken to dryness. The dried filtrate was azeotroped with
K
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
toluene (2 × 40 mL) to remove residual DMF. The resulting brown
oil was taken up in CH2Cl2 (40 mL) and washed with water (2 × 40
mL) and brine (40 mL). Drying with MgSO4 before removing the
solvent in vacuo gave 4 as a brown oil (2.72 g, 89%). δH (400 MHz,
CDCl3): 8.54 (d, 1H, J = 5.0 Hz, pyH6), 7.64 (td, 1H, J = 7.7, 1.8 Hz,
pyH4), 7.35 (d, 1H, J = 7.9 Hz, pyH3), 7.17 (ddd, 1H, J = 7.5, 4.9, 1.1
Hz, pyH5), 4.26 (s, 2H, CH2), 2.36 (s, 3H, CH3). δC (125 MHz,
CDCl3): 195.0 (CO), 157.3 (pyC2), 149.5 (pyC6), 136.8 (pyC4),
123.3 (pyC3), 122.2 (pyC5), 35.4 (CH2), 30.3 (CH3). ESI-MS (+):
m/z = 168.0453 [4 + H]+ (calcd = 168.0478), 190.0282 [4 + Na]+
(calcd = 190.0297).
General Procedure for the Preparation of Thioether
Ligands. Sodium (6−8.5 equiv) was dissolved in dry ethanol at 0
°C, and a solution of 4 (2 equiv) in dry ethanol was added and stirred
at 0 °C under N2 for 30 min. N4-Substituted-3,5-bis(chloromethyl-
1,2,4-triazole hydrochloride (1 equiv) was added and stirred at room
temperature for 1 h, refluxed for 5 h, then stirred at room temperature
overnight, all under N2. Water was added and the ethanol removed in
vacuo. The resulting suspension was extracted with CH2Cl2 then
washed with water, then brine, followed by drying with MgSO4 before
taken to dryness in vacuo. Any deviation from this procedure is
detailed below.
powder (0.122 g, 35%). Anal. Calcd for C23H23N5S2: C 63.71, H 5.35,
N 16.15, S 14.79. Found: C, 63.82, H 5.41, N 16.15, S 14.79. δH (500
MHz, CDCl3): 8.56 (d, 2H, J = 5.1 Hz, pyH6), 7.66 (td, 2H, J = 7.6,
1.9 Hz, pyH4), 7.48 (d, 2H, J = 7.8 Hz, pyH3), 7.28−7.31 (m, 2H,
phH3/5), 7.20−7.23 (m, 2H, phH2/6), 7.17 (ddd, 2H, J = 7.6, 4.9, 1.1
Hz, pyH5), 3.92 (s, 4H, py-CH2-S), 3.58 (s, 4H, tr-CH2-S), 2.44 (s,
3H, CH3). δC (125 MHz, CDCl3): 157.5 (pyC2), 152.7 (tr), 149.6
(pyC6), 140.4 (phC4), 136.7 (pyC4), 130.5 (phC3/5), 130.2 (phC1),
127.4 (phC2/6), 123.8 (pyC3), 122.1 (pyC5), 37.3 (py-CH2-S), 24.1
(tr-CH2-S), 21.3 (CH3). ESI-MS (+): m/z = 434.1439 [(PSMePhT) +
H]+ (calcd = 434.1468).
Synthesis of the Iron(II) Complexes. [FeII (PSPhT)2](BF4)4. To a
2
clear yellow solution of PSPhT (122 mg, 0.291 mmol) in 20 mL of
CHCl3 was added a solution of Fe(H2O)6(BF4)2 (98 mg, 0.29 mmol)
in 10 mL of MeOH. This was stirred under argon at room temperature
for 2 h, during which time the clear yellow solution turned cloudy
white. The pale green precipitate was collected by filtration under a
nitrogen stream to yield solvatomorph A, [FeII2(PSPhT)2]-
(BF4)4·1/2CHCl3·21/2H2O (99 mg, 0.076 mmol, 52%), and the solid
was promptly stored under argon. Anal. Calcd for [FeII (PSPhT)2]-
2
(BF4)4·1/2CHCl3·21/2H2O: C 38.10, H 3.41, N 9.99, S 9.14. Found: C
38.17, H 3.32, N 9.72, S 9.11. TGA: calcd 7.5%, found 7.6%. ESI-MS
(+): m/z = 1211.1295 {[FeII2(PSPhT)2](BF4)3}+ (calcd =
PSPhT. Sodium (0.40 g, 17 mmol), 4 (0.954 g, 5.74 mmol), and
3Ph (0.799 g, 2.87 mmol) were reacted in 50 mL of dry ethanol, then
worked up according to the general procedure. Recrystallization of the
brown oily solid in boiling toluene gave PSPhT as a beige powder
(0.979 g, 81%). Anal. Calcd for C22H21N5S2: C 62.98, H 5.05, N 16.69,
S 15.28. Found: C 63.25, H 5.16, N 16.61, S 15.21. δH (500 MHz,
CDCl3): 8.55 (d, 2H, J = 4.9 Hz, pyH6), 7.67 (td, 2H, J = 7.7, 1.8 Hz,
pyH4), 7.50−7.53 (m, 3H, phH3/5 and phH4), 7.49 (d, 2H, J = 7.9
Hz, pyH3), 7.34−7.36 (m, 2H, phH2/6), 7.19 (ddd, 2H, J = 7.7, 4.9,
1.2 Hz, pyH5), 3.93 (s, 4H, py-CH2-S), 3.59 (s, 4H, tr-CH2-S). δC
(125 MHz, CDCl3): 157.3 (pyC2), 152.6 (tr), 149.3 (pyC6), 137.0
(pyC4), 132.9 (phC1), 130.2 (phC4), 129.9 (phC3/5), 127.7 (phC2/
6), 123.9 (pyC3), 122.2 (pyC5), 37.1 (py-CH2-S), 24.1 (tr-CH2-S).
ESI-MS (+): m/z = 442.1104 [(PSPhT) + Na]+ (calcd = 442.1131).
PSiBuT. Sodium (0.30 g, 13 mmol), 4 (0.675 g, 4.06 mmol), and
3iBu (0.525 g, 2.03 mmol) were reacted in 40 mL of dry ethanol, then
worked up according to the general procedure to yield PSiBuT as a
brown oil, which was used for complexation without further
purification (0.649 g, 80%). δH (500 MHz, CDCl3): 8.58 (d, 2H, J
= 4.9 Hz, pyH6), 7.65 (td, 2H, J = 7.6, 1.8 Hz, pyH4), 7.42 (d, 2H, J =
7.8 Hz, pyH3), 7.17 (ddd, 2H, J = 7.6, 4.9, 1.1 Hz, pyH5), 3.84 (s, 4H,
py-CH2-S), 3.82 (s, 2H, iBuCH2) 3.80 (s, 4H, tr-CH2-S), 2.05 (septet,
1H, J = 7.0 Hz, iBuCH), 0.87 (d, 6H, J = 6.6 Hz, iBu(CH3)2). δC (125
MHz, CDCl3): 157.2 (pyC2), 152.2 (tr), 148.9 (pyC6), 137.6 (pyC4),
124.1 (pyC3), 122.4 (pyC5), 51.0 (iBuCH2) 36.7 (py-CH2-S), 29.1
(iBuCH), 24.8 (tr-CH2-S), 19.9 (iBu(CH3)2). ESI-MS (+): m/z =
400.1617 [(PSiBuT) + H]+ (calcd = 400.1624).
1211.1280). A solution of [FeII (PSPhT)2](BF4)4·1/2CHCl3·21/2H2O
2
(59 mg, 0.042 mmol) in 3 mL of MeCN was exposed to THF vapor.
After 1 week, colorless block-shaped crystals of solvatomorph B,
[FeII (PSPhT)2](BF4)4·2MeCN·H2O (25 mg, 43%), had formed. One
2
such experiment gave single colorless block crystals suitable for X-ray
crystallography. Anal. Calcd for [FeII (PSPhT)2](BF4)4·2MeCN·H2O:
2
C 41.23, H 3.60, N 12.02, S 9.17. Found: C 41.29, H 3.54, N 11.95, S
9.03. Upon air drying [FeII (PSPhT)2](BF4)4·2MeCN·H2O, the
2
crystals crumbled and the solvent content changed to give
solvatomorph C, [FeII (PSPhT)2](BF4)4·21/2H2O, as a pale green
2
powder. Anal. Calcd for [FeII (PSPhT)2](BF4)4·21/2H2O: C 39.35, H
2
3.53, N 10.43, S 9.55. Found: C 39.34, H 3.29, N 10.46, S 9.80. TGA:
calcd 3.35%, found 3.40%.
[FeII (PSiBuT)2](BF4)4. To a clear yellow solution of PSiBuT (139
2
mg, 0.348 mmol) in 10 mL of MeOH was added a 5 mL solution of
FeII(H2O)6(BF4)2 (117 mg, 0.348 mmol) in MeOH. Stirring overnight
at room temperature resulted in a brown solution with a white
precipitate. The precipitate was isolated and taken up in 4 mL of
MeCN to give a green solution, which was exposed to THF vapor
diffusion to yield colorless block-shaped crystals. At this point a single
crystal was collected for X-ray diffraction, before the bulk crystalline
sample was air-dried to give [FeII (PSiBuT)2](BF4)4·MeCN·H2O (37
mg, 16%). Anal. Calcd for [FeI2I (PSiBuT)2](BF4)4·MeCN·H2O: C
2
38.30, H 4.21, N 11.70, S 9.74. Found: C 38.20, H 4.29, N 11.98, S
9.35. TGA: calcd 4.48%, found 4.34%. ESI-MS (+): m/z = 1171.1976
PSt−BuPhT. Sodium (0.37 g, 16 mmol), 4 (0.892 g, 5.37 mmol), and
{[FeII (PSiBuT)2](BF4)3}+ (calcd = 1171.1905).
2
3
t−BuPh (0.898 g, 2.68 mmol) were reacted in 50 mL of dry ethanol,
[FeII (PSMePhT)2](BF4)4. To a clear yellow solution of PSMePhT (49
2
then worked up according to the general procedure. Column
chromatography using silica gel and eluting with CH2Cl2 followed
by 5% then 10% MeOH in CH2Cl2 and recrystallization of the
resulting brown oily solid in boiling toluene gave PSt‑BuPhT as a beige
microcrystalline solid (0.171 g, 13%). Anal. Calcd for C26H29N5S2: C
65.65, H 6.15, N 14.72, S 13.48. Found: C 65.78, H 6.34, N 14.71, S
13.25. δH (500 MHz, CDCl3): 8.55 (d, 2H, J = 4.9 Hz, pyH6), 7.64
(td, 2H, J = 7.6, 1.8 Hz, pyH4), 7.47−7.51 (m, 2H, phH3/5), 7.46 (d,
2H, J = 7.7 Hz, pyH3), 7.24−7.27 (m, 2H, phH2/6), 7.15 (ddd, 2H, J
= 7.5, 4.9, 1.2 Hz, pyH5), 3.93 (s, 4H, py-CH2-S), 3.57 (s, 4H, tr-CH2-
S), 1.36 (s, 9H, t-Bu(CH3)3). δC (125 MHz, CDCl3): 157.2 (pyC2),
153.5 (phC4), 152.8 (tr), 148.9 (pyC6), 137.4 (pyC4), 130.0 (phC1),
127.2 (phC2/6), 126.9 (pyC3), 124.1 (phC3/5), 122.3 (pyC5), 36.8
(py-CH2-S), 34.9 (t-BuC), 31.2 (t-Bu(CH3)3), 24.1 (tr-CH2-S). ESI-
MS (+): m/z = 476.1901 [(PSt−BuPhT) + H]+ (calcd = 476.1937).
PSMePhT. Sodium (0.19 g, 8.3 mmol), 4 (0.279 g, 1.68 mmol), and
mg, 0.11 mmol) in 15 mL of CHCl3 was added a 5 mL solution of
FeII(H2O)6(BF4)2 (38 mg, 0.11 mmol) in MeOH. Stirring overnight at
room temperature resulted in a brown solution with a white
precipitate. The precipitate was isolated and taken up in 3 mL of
MeCN to give a green solution, which was exposed to THF vapor
diffusion for 1 week to yield pale green block-shaped crystals. At this
point a few milligrams of single crystals of [FeII (PSMePhT)2](BF4)4·
2
2MeCN was collected for X-ray diffraction and microanalysis, before
the bulk crystalline sample was air-dried to give [FeII (PSMePhT)2]-
2
(BF4)4·11/2MeCN·2H2O (15 mg, 18%; actual yield higher than this, as
a large portion was lost on finding a suitable single crystal for X-ray
analysis). Anal. Calcd for [FeII (PSMePhT)2](BF4)4·2MeCN: C 42.65,
2
H 3.72, N 11.94, S 9.11. Found: C 42.73, H 3.77, N 11.66, S 8.95.
Anal. Calcd for [FeII (PSMePhT)2](BF4)4·11/2MeCN·2H2O: C 41.34,
2
H 3.86, N 11.31, S 9.01. Found: C 41.29, H 3.71, N 11.05, S 9.02.
TGA [FeII (PSMePhT)2](BF4)4·11/2MeCN·2H2O: calcd 6.43%, found
3
MePh (0.238 g, 0.813 mmol) were reacted in 30 mL of dry ethanol,
2
6.85%. ESI-MS (+): m/z = 1239.1737 {[FeII (PSMePhT)2](BF4)3}+
then worked up according to the general procedure. Recrystallization
2
of the brown oily solid in boiling toluene gave PSMePhT as a beige
(calcd = 1239.1594).
L
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(20) Holzhacker, C.; Calhorda, M. J.; Gil, A.; Carvalho, M. D.;
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02851.
■
Ferreira, L. P.; Stoger, B.; Mereiter, K.; Weil, M.; Muller, D.;
̈
̈
S
Weinberger, P.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Dalton Trans.
2014, 43, 11152−11164.
(21) Weber, B.; Bauer, W.; Obel, J. Angew. Chem., Int. Ed. 2008, 47,
10098−10101.
(22) Weber, B.; Kaps, E. S.; Obel, J.; Achterhold, K.; Parak, F. G.
Inorg. Chem. 2008, 47, 10779−10787.
CSD search details (CIF)
Additional X-ray crystallographic tables and figures,
magnetic data, UV−vis spectra, and experimental setup
(PDF)
(23) Bauer, W.; Schlamp, S.; Weber, B. Chem. Commun. 2012, 48,
10222−10224.
(24) Lochenie, C.; Bauer, W.; Railliet, A. P.; Schlamp, S.; Garcia, Y.;
Weber, B. Inorg. Chem. 2014, 53, 11563−11572.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sbrooker@chemistry.otago.ac.nz.
Notes
̀
(25) Iasco, O.; Riviere, E.; Guillot, R.; Buron-Le Cointe, M.;
■
Meunier, J.-F.; Bousseksou, A.; Boillot, M.-L. Inorg. Chem. 2015, 54,
1791−1799.
(26) Lennartson, A.; Bond, A. D.; Piligkos, S.; McKenzie, C. J. Angew.
Chem. 2012, 124, 11211−11214.
The authors declare no competing financial interest.
(27) Lennartson, A.; Southon, P.; Sciortino, N. F.; Kepert, C. J.;
Frandsen, C.; Mørup, S.; Piligkos, S.; McKenzie, C. J. Chem. - Eur. J.
2015, 21, 16066−16072.
ACKNOWLEDGMENTS
■
(28) England, J.; Gondhia, R.; Bigorra-Lopez, L.; Petersen, A. R.;
White, A. J. P.; Britovsek, G. J. P. Dalton Trans. 2009, 5319−5334.
(29) Grillo, V. A.; Gahan, L. R.; Hanson, G. R.; Stranger, R.;
Hambley, T. W.; Murray, K. S.; Moubaraki, B.; Cashion, J. D. J. Chem.
Soc., Dalton Trans. 1998, 2341−2348.
We thank the Marsden Fund (RSNZ) and the University of
Otago (including the award of a postgraduate scholarship to
R.W.H. and the 2015 purchase of a Versalab magnetometer) for
supporting this research.
(30) Reus, C.; Ruth, K.; Tullmann, S.; Bolte, M.; Lerner, H.-W.;
̈
REFERENCES
■
Weber, B.; Holthausen, M. C.; Wagner, M. Eur. J. Inorg. Chem. 2011,
2011, 1709−1718.
(1) Gutlich, P.; Goodwin, H. A. Top. Curr. Chem. 2004, 233, 1−47.
̈
(2) Spin Crossover in Transition Metal Compounds, I-III; Gutlich, P.;
̈
(31) Hogue, R. W.; Miller, R. G.; White, N. G.; Feltham, H. L. C.;
Jameson, G. N. L.; Brooker, S. Chem. Commun. 2014, 50, 1435−1437.
(32) Miller, R. G.; Brooker, S. Chem. Sci. 2016, 7, 2501.
(33) Brooker, S. Coord. Chem. Rev. 2001, 222, 33−56.
(34) van Bommel, K. J. C.; de Jong, M. R.; Metselaar, G. A.;
Verboom, W.; Huskens, J.; Hulst, R.; Kooijman, H.; Spek, A. L.;
Reinhoudt, D. N. Chem. - Eur. J. 2001, 7, 3603−3615.
(35) Livingstone, S. E.; Nolan, J. D. Aust. J. Chem. 1970, 23, 1553−
1558.
Goodwin, H. A., Eds.; Top. Curr. Chem. 2004; Vol. 233−235.
(3) Halcrow, M. A. Spin-Crossover Materials: Properties and
Applications, 1st ed.; John Wiley & Sons, Ltd, 2013.
(4) Gutlich, P.; Gaspar, A. B.; Garcia, Y. Beilstein J. Org. Chem. 2013,
̈
9, 342−391.
(5) Brooker, S. Chem. Soc. Rev. 2015, 44, 2880−2892. , and front
cover feature.
(6) Olguín, J.; Brooker, S. In Spin-Crossover Materials: Properties and
Applications, 1st ed.; Halcrow, M. A., Ed.; John Wiley & Sons, Ltd,
2013; pp 77−120.
(7) Haasnoot, J. G. 1,2,4-Triazoles as Ligands for Iron(II) High Spin-
Low Spin Crossovers; Kluwer Academic Publishers: Dordrecht, 1996.
(8) Kitchen, J. A.; Brooker, S. Coord. Chem. Rev. 2008, 252, 2072−
2092.
(9) Klingele, M. H.; Moubaraki, B.; Murray, K. S.; Brooker, S. Chem. -
Eur. J. 2005, 11, 6962−6973.
(10) Klingele, M. H.; Moubaraki, B.; Cashion, J. D.; Murray, K. S.;
Brooker, S. Chem. Commun. 2005, 987−989 and front cover feature.
(11) Grunert, C. M.; Reiman, S.; Spiering, H.; Kitchen, J. A.; Brooker,
(36) Lennartson, A.; McKenzie, C. J. Acta Crystallogr., Sect. C: Cryst.
Struct. Commun. 2011, 67, o354−o358.
(37) van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A: Found.
Crystallogr. 1990, A46, 194−201.
(38) Mooibroek, T. J.; Teat, S. J.; Massera, C.; Gamez, P.; Reedijk, J.
Cryst. Growth Des. 2006, 6, 1569−1574.
(39) Gamez, P.; Mooibroek, T. J.; Teat, S. J.; Reedijk, J. Acc. Chem.
Res. 2007, 40, 435−444.
(40) Schottel, B. L.; Chifotides, H. T.; Dunbar, K. R. Chem. Soc. Rev.
2008, 37, 68−83.
(41) Salonen, L. M.; Ellermann, M.; Diederich, F. Angew. Chem., Int.
Ed. 2011, 50, 4808−4842.
S.; Gutlich, P. Angew. Chem., Int. Ed. 2008, 47, 2997−2999 and front
̈
cover feature.
(42) Frontera, A. Coord. Chem. Rev. 2013, 257, 1716−1727.
(12) Bhattacharjee, A.; Ksenofontov, V.; Kitchen, J. A.; White, N. G.;
́ ̀
(43) Brooker, S.; White, N. G.; Bauza, A.; Deya, P. M.; Frontera, A.
Brooker, S.; Gutlich, P. Appl. Phys. Lett. 2008, 92, 174104.
̈
Inorg. Chem. 2012, 51, 10334−10340.
(44) Yang, L.; Powell, D. R.; Houser, R. P. Dalton Trans. 2007, 955−
964.
(13) Kitchen, J. A.; White, N. G.; Jameson, G. N. L.; Tallon, J. L.;
Brooker, S. Inorg. Chem. 2011, 50, 4586−4597.
(14) Bhattacharjee, A.; Roy, M.; Ksenofontov, V.; Kitchen, J. A.;
(45) Yatsunyk, L. A.; Walker, F. A. Inorg. Chem. 2004, 43, 757−777.
(46) Weber, B.; Walker, F. A. Inorg. Chem. 2007, 46, 6794−6803.
(47) Weber, B.; Obel, J.; Henner-Vasquez, D.; Bauer, W. Eur. J. Inorg.
Chem. 2009, 36, 5527−5534.
Brooker, S.; Gutlich, P. Eur. J. Inorg. Chem. 2013, 2013, 843−849.
̈
(15) Kitchen, J. A.; Olguín, J.; Kulmaczewski, R.; White, N. G.;
Milway, V. A.; Jameson, G. N. L.; Tallon, J. L.; Brooker, S. Inorg. Chem.
2013, 52, 11185−11199.
(48) Evans, D. F. J. Chem. Soc. 1959, 2003−2005.
(49) Piguet, C. J. Chem. Educ. 1997, 74, 815−816.
(50) Shores, M. P.; Klug, C. M.; Fiedler, S. R. In Spin-Crossover
Materials; John Wiley & Sons Ltd, 2013; pp 281−301.
(51) Hauser, A. Top. Curr. Chem. 2004, 233, 49−58.
(52) Blakesley, D. W.; Payne, S. C.; Hagen, K. S. Inorg. Chem. 2000,
39, 1979−1989.
(16) Kulmaczewski, R.; Olguín, J.; Kitchen, J. A.; Feltham, H. L. C.;
Jameson, G. N. L.; Tallon, J. L.; Brooker, S. J. Am. Chem. Soc. 2014,
136, 878−881.
(17) Herold, C. F.; Carrella, L. M.; Rentschler, E. Eur. J. Inorg. Chem.
2015, 2015, 3632−3636.
(18) Weber, B. In Spin-Crossover Materials; John Wiley & Sons Ltd,
2013; pp 55−76.
(53) Hinek, R.; Spiering, H.; Schollmeyer, D.; Gutlich, P.; Hauser, A.
(19) Costa, J. S.; Balde, C.; Carbonera, C.; Denux, D.; Wattiaux, A.;
̈
Desplanches, C.; Ader, J.-P.; Gutlich, P.; Let
2007, 46, 4114−4119.
́
ard, J.-F. Inorg. Chem.
Chem. - Eur. J. 1996, 2, 1427−1434.
̈
(54) Turner, J. W.; Schultz, F. A. Inorg. Chem. 2001, 40, 5296−5298.
M
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(55) Tafili-Kryeziu, M.; Weil, M.; Muranaka, T.; Bousseksou, A.;
Hasegawa, M.; Jun, A.; Linert, W. Dalton Trans. 2013, 42, 15796−
15804.
(56) Martin, L. L.; Martin, R. L.; Sargeson, A. M. Polyhedron 1994,
13, 1969−1980.
(57) Ortega-Villar, N.; Thompson, A.; Munoz, M. C.; Ugalde-
̃
Saldívar, V. M.; Goeta, A. E.; Moreno-Esparza, R.; Real, J. A. Chem. -
Eur. J. 2005, 11, 5721−5734.
(58) Slichter, C. P.; Drickamer, H. G. J. Chem. Phys. 1972, 56, 2142−
2160.
(59) CrysAlisPro; Agilent Technologies Yarnton: Oxfordshire.
(60) Palatinus, L.; Chapuis, G. J. Appl. Crystallogr. 2007, 40, 786−
790.
(61) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, A64, 112−122.
(62) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.;
McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de
Streek, J.; Wood, P. A. J. Appl. Crystallogr. 2008, 41, 466−470.
(63) POVray; Persistence of Vision Raytracer (Version 3.6); 2004.
(64) Kahn, O. Molecular Magnetism; VCH Publishers Inc.: New York,
1993.
(65) Melby, L. R.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Benson,
R. E.; Mochel, W. E. J. Am. Chem. Soc. 1962, 84, 3374.
(66) Bartlett, R. K.; Humphrey, I. R. J. Chem. Soc. C 1967, C, 1664−
1666.
N
DOI: 10.1021/acs.inorgchem.5b02851
Inorg. Chem. XXXX, XXX, XXX−XXX