Unusual Magneto-Structural Features of the Halo-Substituted Materials [FeIII(5-X-salMeen)2]Y: a Cooperative [HS-HS]?[HS-LS] Spin Transition

Article abstract of DOI:10.1002/chem.201904744

X-ray structures of the halo-substituted complexes [Fe^III(5-X-salMeen)₂]ClO₄ (X=F, Cl, Br, I) [salMeen=N-methyl-N-(2-aminoethyl)salicylaldiminate]at RT have revealed the presence of two discrete HS complex cations in the crystallographic asymmetric unit with two perchlorate counter ions linking them by N?H_amine???O_perchlorate interactions. At 90 K, the two complex cations are distinctly HS and LS, a rare crystallographic observation of this coexistence in the Fe^III-salRen (R=alkyl) spin-crossover (SCO) system. At both temperatures, crystal packing shows dimerization through C?H_imine???O_phenolate interactions, a key feature for SCO cooperativity. Moreover, there are noncovalent contacts between the complex cations through type-II halogen-halogen bonds, which are novel in this system. The magnetic profiles and M?ssbauer spectra concur with the structural analyses and reveal 50 % SCO of the type [HS-HS]?[HS-LS] with a broad plateau. In contrast, [Fe^III(5-Cl-salMeen)₂]BPh₄?2MeOH is LS and exhibits a temperature-dependent crystallographic phase transition, exemplifying the influence of lattice solvents and counter ions on SCO.

Full text of DOI:10.1002/chem.201904744

A Journal of
Accepted Article
Title: Unusual Magneto-structural Features of the Halo-Substituted
Materials
[FeIII(5-X-salMeen)2]Y: a Cooperative [HS-HS] ↔ [HS-LS] Spin
Transition
Authors: Musa Sipho Shongwe, Mariam Al-Azzani, Faizah Al-Mjeni,
Ryoji Mitsuhashi, Masahiro Mikuriya, Craig Robertson,
Imaddin Al-Omari, and Eckhard Bill
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To be cited as: Chem. Eur. J. 10.1002/chem.201904744
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10.1002/chem.201904744
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Unusual Magneto-structural Features of the Halo-Substituted Materials
[FeIII(5-X-salMeen)2]Y: a Cooperative [HS-HS] ↔ [HS-LS] Spin Transition
Mariam A. Al-Azzani,[a] Faizah Al-Mjeni,[a] Ryoji Mitsuhashi,[b] Masahiro Mikuriya,[b] Imad Al-
Omari,[c] Craig C. Robertson,[d] Eckhard Bill,[e] Musa S. Shongwe*[a]
Graphical Abstract
Unusual spin-crossover features of [FeIII(5-X-salMeen)2]+: The series of compounds [FeIII(5-X-
salMeen)2]ClO4 (X = a halogen substituent) exhibit 50% spin crossover with a broad [HS-LS] plateau,
a magnetic profile hitherto not crystallographically elucidated within the FeIII-salRen family (R = alkyl
–
group). As its solvated BPh4 salt, [FeIII(5-Cl-salMeen)2]+ is LS and undergoes a temperature-
dependent crystallographic phase transition.
Keywords
Ferric spin crossover · spin-transition plateau · noncovalent interactions · crystallographic phase
transition · electronic and electrochemical processes
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Ferric Spin Crossover
Unusual Magneto-structural Features of the Halo-Substituted Materials
[FeIII(5-X-salMeen)2]Y: a Cooperative [HS-HS] ↔ [HS-LS] Spin Transition
Mariam A. Al-Azzani,[a] Faizah Al-Mjeni,[a] Ryoji Mitsuhashi,[b] Masahiro Mikuriya,[b] Imad Al-
Omari,[c] Craig C. Robertson,[d] Eckhard Bill,[e] Musa S. Shongwe,*[a]
[a] M. A. Al-Azzani, Dr. F. Al-Mjeni, Prof. M. S. Shongwe
Department of Chemistry, College of Science, Sultan Qaboos University, Private Bag 36, Al-Khod 123, Muscat
(Sultanate of Oman)
E-mail: musa@squ.edu.om
[b] Dr. R. Mitsuhashi, Prof. M. Mikuriya
School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda 669-1337 (Japan)
[c] Prof. I. Al-Omari
Department of Physics, College of Science, Sultan Qaboos University, Private Bag 36, Al-Khod 123, Muscat
(Sultanate of Oman)
[d] Dr. C. C. Robertson
Department of Chemistry, University of Sheffield, Sheffield S3 7HF (United Kingdom)
[e] Dr. E. Bill
Max-Planck-Institut für Chemische Energiekonversion, Stiftstrasse 34-36, D-45470, Mülheim an der Ruhr (Germany)
Abstract:
X-ray structures of the halo-substituted complexes [FeIII(5-X-salMeen)2]ClO4 (X = F, Cl, Br, I) at RT have
revealed the presence of two discrete HS complex cations in the crystallographic asymmetric unit
with two perchlorate counter ions linking them by N–Hamine···Operchlorate interactions. At 90 K, the two
complex cations are distinctly HS and LS, a rare crystallographic observation of this coexistence in
the FeIII-salRen spin-crossover (SCO) system. At both temperatures, crystal packing shows
dimerization through C–Himine···Ophenolate interactions, a key feature for SCO cooperativity. Moreover,
there are noncovalent contacts between the complex cations through type-II halogen-halogen
bonds, which are novel in this system. The magnetic profiles and Mössbauer spectra concur with the
structural analyses and reveal 50% SCO of the type [HS-HS] ↔ [HS-LS] with a broad plateau. In
contrast, [FeIII(5-Cl-salMeen)2]BPh4·2MeOH is LS and exhibits a temperature-dependent
crystallographic phase transition, exemplifying the influence of lattice solvents and counter ions on
SCO.
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Introduction
Virtually four decades ago the first example of an abrupt hysteretic 6A1 ↔ 2T2 spin-crossover complex,
viz. [FeIII(3-OMe-salEen)2]PF6,[1] within the family of mononuclear iron(III) salicylaldimines [FeIII(R1,R2-
salRen)2]+, was reported and its crystallographic analysis[2] was performed three decades later. It was
not until very recently that the literature witnessed two other salEen-based examples of SCO
[3]
materials exhibiting an abrupt spin transition with hysteresis, viz. [FeIII(4-OMe)-salEen)2]NO3 and
[FeIII(5-Br-salEen)2]ClO4.[4] Indeed, in general, there is a paucity of iron(III) spin-crossover complexes
that display the much sought-after unique spin-transition features such as abrupt and hysteretic,[1–
15]
symmetry-breaking,[16–18] stepwise and wide [HS-LS] plateau[5,6,16–20] and structural phase
transition,[21] attributable to cooperativity amongst the paramagnetic centres in the crystal lattice
via noncovalent intermolecular forces. Within the [FeIII(R1,R2-salRen)2]+ system, observation of
cooperative spin crossover is even rarer. The impetus for spin-crossover research derives from the
potential technological applications[22,23] of magnetically bistable molecular materials in the
fabrication of electronic devices as well as the sheer scientific interest in designing and generating
complexes with fascinating magneto-structural properties. Crucial criteria for spin crossover to
materialize in octahedral complexes are: (1) a first-row transition-metal ion with a d4–d7 ground-
state electron configuration, and (2) a coordination donor-atom set that provides a ligand-field
strength capable of stabilizing either spin state of the central metal ion as dictated by external
perturbations. A spin transition is triggered principally by temperature, pressure and
electromagnetic radiation.
The concerted groundbreaking investigation of the spin-crossover curiosities in the series of
complexes [FeIII(R1,R2-salRen)2]Y by Hendrickson, Sinn, Wilson et al.[1,21,24–28] revealed inter alia the
subtle effects of ligand substituent groups, counter ions, lattice solvents, intermolecular forces and
sample type on the nature of spin-state transformations. The tridentate en-based salicylaldimine
ligands of considerable interest were H-R'-salRen (R = Et, H-R'-salEen; Me, H-R'-salMeen and Bz, H-
R'-salBzen), with the substituent groups R' (commonly –OMe, –OEt and –NO2) in either the ortho or
para positions of the phenolate moiety. Just over a decade ago, this system was extended to the
Schiff-base ligands H-R'-salPen and H-R'-salBen (R = n-propyl and n-butyl, respectively).[29]
Modulation of spin-crossover properties in iron(III)-salicylaldimine systems by introducing halogen
substituents into the para position of the phenolate moiety has been demonstrated spectacularly by
the complexes [Fe(qsal-5-X)2]Y (X = halogen; Y ; counter ion).[6,7,11,18,30] Within the family [FeIII(R1,R2-
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salRen)2]Y, only three crystallographically characterized 5-halo-substituted members are known,
[4,32]
namely [FeIII(5-Br-salEen)2]Cl·½H2O,[31] [FeIII(5-Br-salEen)2]ClO4
and [FeIII(5-I-salEen)2]ClO4.[33]
[FeIII(5-Br-salEen)2]Cl·½H2O exists at 150 K as two independent complex cations in the
crystallographic asymmetric unit: one in the HS state and the other in the LS state, a magnetic feature
not crystallographically observed previously in this system. However, structural evidence of such [HS-
LS] coexistence has been reported for a mere handful of other types of iron(III)-based SCO
complexes,[5,16–18,20,30,31,34,35] generally those whose spin transition exhibits a [HS-LS] plateau.
Unfortunately, room-temperature magnetic and crystallographic data on [FeIII(5-Br-salEen)2]Cl·½H2O
[31]
[4,32]
are not available, nor a spin-transition curve for this compound. [FeIII(5-Br-salEen)2]ClO4
exhibits polymorphism induced by the crystallization conditions. The polymorph that crystallizes in
the monoclinic space group P21/c[4] undergoes an abrupt complete S = 5/2 ↔ S = 1/2 crossover
involving one complex cation whereas the other polymorph which adopts the orthorhombic space
group Pbcn[32] is predominantly low spin at room temperature, and its incomplete spin crossover is
characterized by an asymmetric hysteresis loop around room temperature. The magneto-structural
behaviour of the 5-iodo-substituted analogue[33] has a striking resemblance to that of the latter
polymorph save for the irreversibility of the magnetic profile and the narrower hysteresis loop in this
case.
The object of this work is to revisit the [FeIII(X-salRen)2]Y spin-crossover family[24,26] that originated in
the 1970s and extend it with freshly designed members that have the potential to exhibit previously
unobserved rare magnetic behaviour. In the course of this investigation, some aspects of our work
coincided with the research conducted by Martinho et al.[4,31-33] Herein we present a report on a
systematic study of spin-crossover features within the series of halo-substituted complexes [FeIII(5-
X-salMeen)2]Y (X = F, Cl, Br and I; Y = ClO4 and BPh4). Previously, Wilson and Sinn along with their co-
workers demonstrated magnetically and crystallographically the influence of electronic properties of
substituent groups on the spin states of the compounds in the series [FeIII(X-salMeen)2]PF6 (X = 5-
OMe, 3-OMe and 5-NO2).[24,26] In our work the magneto-structural properties of [FeIII(5-X-
salMeen)2]ClO4 (X = halogen) reveal a thermally induced [HS-HS] ↔ [HS-LS] spin transition with a
broad [HS-LS] plateau, a highly unusual magnetic feature in this iron(III)-salicylideneamine spin-
crossover system.
Furthermore, in the compounds [FeIII(5-X-salMeen)2]ClO4 (X = Cl, Br, I) we have encountered halogen-
halogen bonding, which is a dominant feature of the structures. To the best of our knowledge,
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hitherto there has been no crystallographic documentation of the occurrence of halogen-halogen
bonding[36-41] within the [FeIII(X-salRen)2]Y spin-crossover family. Halogen-halogen bonds (C1–X1···X2–
C2, X = halogen) are categorized as types I and II. Geometrically, these types are readily
distinguishable by their characteristic angles θ1 and θ2:[36,37,39–41] whereas for type-I interactions, θ1
and θ2 are obtuse and nearly equivalent; for type-II contacts, θ1 and θ2 are comparable with the
idealized linear and right angles, respectively. Chemically, the type-II halogen-halogen bond differs
from that of type-I in that it is electrostatic in nature (C–Xδ+···Xδ––C), i.e. electrophilic (positive “σ-
hole”)/nucleophilic interaction, and the order of prevalence is Cl < Br < I in accordance with the
polarizability of the halogen atom.[37,41]
A comparison of the structural analyses of the compounds [FeIII(5-Cl-salMeen)2]ClO4 (2) and [FeIII(5-
Cl-salMeen)2]BPh4·2MeOH (5·2MeOH) illustrates the combined influence of intermolecular forces
and crystal packing on the magneto-structural properties of SCO materials. Recently, a π-conjugated
photo-isomerizable azo-phenyl substituent[42] was introduced in the para-position of the phenolic
structural moiety of the salEen ligand to diversify and maximize the noncovalent interactions, leading
to thermally stable iron(III) complexes exhibiting high-temperature spin transitions. In another study,
SCO in the compound [Fe(3-OMe-salEen)2][Ni(dmit)2]·MeOH and the non-solvated compound [Fe(3-
OMe-salEen)2][Ni(dmit)2] (dmit2– = 2-thioxo-1,3-dithiole-4,5-dithiolato) occurred with T values of
½
345 and 275 K, respectively, with the latter compound having lost noncovalent interactions involving
the solvent.[43]
Results and Discussion
Synthetic Strategy and Verification of Chemical Identities
The multifaceted structural design of the compounds of the type [FeIII(3,5-R1,R2-salRen)]Y
necessitates syntheses and isolation of a diverse range of materials to be investigated for SCO
behaviour. Thus it is convenient and cost-effective to produce each iron(III)-salicylideneamine
compound in a short one-pot two-step process that entails generating the ligand by Schiff-base
condensation reaction with subsequent complexation of the ferric ion in situ. In this work, equimolar
amounts of N-methylethylenediamine and the appropriate para-halo-substituted salicylaldehyde
reacted instantaneously to afford the desired Schiff base 5-X-HsalMeen (X = F, Cl, Br or I), but the
reaction mixture was heated under reflux to maximize the yield. In general, in previous studies the
ligands 3,5-R1,R2-HsalRen have been employed merely as intermediate substances in the fabrication
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of SCO materials. In this work, 5-I-HsalMeen (Figure 1) was selected for spectroscopic
characterization and was isolated as an orange-yellow powder; the low yield (~ 0.8%) underscores
the necessity to generate and utilize these ligands in situ. The Schiff-base imine bond in this ligand
is characterized by the stretching frequency 1627 cm–1 in the IR spectrum and the broad visible π →
π* absorption band (λmax ~ 420 nm) in the electronic absorption spectrum, which disappears upon
–
reduction with BH4 .
X
+
Y
6
3
N
X
O
O
N
N
N
H
N
..
H
H
Fe
H
4
O
N
X = F, Cl, Br, I
[FeIII(5-X-salMeen)2]Y
Y = ClO4 , BPh4
X
Figure 1. Designations of Schiff-base ligands and their corresponding complexes
The best 1H-NMR spectrum of 5-I-HsalMeen is that recorded in DMSO-d6 at a radiofrequency of 700
MHz, even though some of the aliphatic proton resonances coincide with those of the solvent; a
portion of the spectrum is displayed in Figure S1. The azomethine proton (–CH=N–) is associated with
the singlet at δ 8.56. The most deshielded proton is that of the phenolic –OH group with a chemical
shift of δ 13.47; crystallographic elucidation of closely related salicylaldimine-based Schiff bases
invariably shows H-bonding interaction between the phenolic proton and the imine nitrogen.[44,45]
The resonances at δ 7.78 (d; J = 2.31 Hz), δ 7.59 (dd; J = 8.68, 2.31 Hz) and δ 6.72 (d; J = 8.61 Hz) are
ascribable to the aromatic protons H6, H4 and H3 (Figure 1), respectively, consistent with the para-
substitution of the phenolic ring.
Treatment of each of the ligands 5-X-HsalMeen with half a molar equivalent of Fe(ClO4)3·xH2O
produced the compounds [FeIII(5-X-salMeen)2]ClO4 (1–4), which were isolated as black needles upon
crystallization from the intense purple solutions in methanol. In the case of [FeIII(5-Cl-
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salMeen)2]BPh4·2MeOH (5·2MeOH), the ferric iron was introduced as its chloride salt and the
tetraphenylborate ion as its sodium salt in three-fold molar excess. The chemical compositions and
structural formulations of the iron(III)-salicylideneamine compounds (Figure 1) were established by
microanalyses (CHN), mass spectrometry, molar conductivity measurements, vibrational
spectroscopy and single-crystal X-ray crystallography. Characteristic IR absorption bands for the
imine (C=N) and secondary amine (N–H) bonds occur in the ranges 1620–1635 and 3130–3330 cm–1,
–
respectively. Conspicuously, the ClO4 counter ion appears as strong vibrations in the range 1180–
–
1060 cm–1 accompanied by a moderate one at 625 cm–1.[34a,45–47] The presence of the BPh4 counter
ion and the solvent molecules of crystallization in [FeIII(5-Cl-salMeen)2]BPh4·2MeOH was evidenced
by the stretching frequencies 730, 710 and 605 cm–1 [45,47] for the former and a broad absorption
band at 3480 cm–1 corresponding to the latter.
Single-Crystal X-ray Analyses of X-salMeen-Based Iron(III) Complexes
Temperature dependence of the X-ray structures of members of the series [FeIII(5-X-salMeen)2]ClO4
(X = F, Cl, Br and I) (1–4) along with the compound [FeIII(5-Cl-salMeen)2]BPh4·2MeOH (5·2MeOH) was
investigated to gain an insight into the magnetochemistry of these materials. Selected
crystallographic data for 1–4 and 5·2MeOH are compiled in Tables 1 and 2, respectively.
X-ray structures of [FeIII(5-X-salMeen)2]ClO4 (X = F, Cl, Br, I) (1–4)
For all the iron(III)-5-X-salMeen compounds, but [FeIII(5-Cl-salMeen)2]ClO4, X-ray diffraction data
were collected on a crystal from the same sample at 298 and 90 K (Tables 1 and 2). Compounds 1–4
crystallized as needles in the monoclinic space group P21/n (Z = 8) independent of the temperature
of measurement. On the whole, these iron(III) compounds are isostructural; hence the same atom
numbering scheme has been adopted. The crystallographic asymmetric unit for each of these
compounds at both temperatures of data collection comprises two discrete complex cations with
two perchlorate counter ions in between connecting them with N–Hamine···Operchlorate hydrogen-
bonding interactions (Tables S1–S4) as illustrated in Figures 2 and 3 for complex 2.
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(a)
(b)
Figure 2. (a) An asymmetric unit and (b) a dimer of [FeIII(5-Cl-salMeen)2]ClO4 at 90 K.
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Figure 3. A portion of a chain of hydrogen-bonded complex cations and counter anions in [FeIII(5-Cl-
salMeen)2]ClO4 at 90 K.
The uninegative tridentate Schiff-base ligands in [FeIII(5-X-salMeen)2]+ are arranged very nearly
orthogonally relative to each other and the phenolate oxygen, imine nitrogen and amine nitrogen
donor atoms adopt the meridional coordination mode. In the distorted octahedral geometry at the
iron(III) centre the imine nitrogen atoms take trans positions whereas the other pairs of donor atoms
of each kind occupy adjacent positions. Selected geometric and distortion parameters for the
coordination spheres of compounds 1–4 are presented in Table 4. Within the [FeIII(X-salRen)2]+
system the literature values of the distances for the bonds FeIII–Oph, FeIII–Nim and FeIII–Nam in the HS
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state lie in the ranges 1.90–1.96, 2.06–2.14 and 2.17–2.25 Å, respectively, whereas the
corresponding ones in the LS state are in the ranges 1.86–1.89, 1.91–1.94 and 2.02–2.06 Å,
respectively.[2-4,26,27,29,31-33,42,43,48] Evidently, both complex cations of compounds 1–4 exist in the HS
state at room temperature; however, at 90 K one complex cation is HS whereas the other is LS (Table
4). For the LT structures of compounds 1–4, the values of Δdave(FeIII–O)HS-LS are 0.049, 0.049, 0.050
and 0.049 Å, respectively, whereas those of Δdave(FeIII–N)HS-LS are 0.159, 0.167, 0.158 and 0.157 Å,
respectively, confirming that one of the two paramagnetic centres converted to the LS state while
the other retained its HS state.[12,13,26,34a]
The values of the distortion parameter of the cis angles, denoted Σ, for the HS and LS complex cations
in compounds 1–4 at 90 K fall in the ranges 68.8–72.9° and 52.7–55.2°, respectively (Table 4). The LT
and RT values for the HS complex cations are comparable. Furthermore, the HS and LS values of Σ for
compounds 1–4 are similar to those observed in other [FeIII(X-salRen)2]+ complex cations.[2-4,26,29,48] It
is noteworthy that for a bis-chelate octahedral iron(III) complex possessing a tridentate Schiff-base
ligand derived from N-alkylethylenediamine, the HS and LS states are reliably distinguishable by the
size of the imine-Fe-amine chelate angle (θim-am : Nim–FeIII–Nam).[45] Literature values of θim-am for the
HS and LS complexes[2-4,26,27,29,31-33,42,43,48] occur in the ranges ~77–80° and ~83–85°, respectively,
which concur with those observed in the series [FeIII(5-X-salMeen)2]ClO4 (1–4) (Table 4). To the best
of our knowledge, this is the first crystallographic characterization of spin crossover of the type [HS-
HS] ↔ [HS-LS] within the [FeIII(5-X-salRen)2]Y family.
Noncovalent intermolecular interactions which are crucial for spin-crossover cooperativity are
readily identifiable in the X-ray structures of the series 1–4 (Figures 2–5, S3 and S4, Tables S1–S4).
The two complex cations in the asymmetric unit are connected by two perchlorate counter ions
through multiple H-bonding interactions (N–Hamine···Operchlorate): each of the secondary amine N–H
donors interacts with the perchlorate ions with bifurcated H-bonds. The H-bonds differ in strength
within the same complex (LS vs. HS) and across the series 1–4 (Tables S1–S4); they are the strongest
of the intermolecular forces present in these compounds, and possibly play a key role in the nature
of the spin-transition profiles. Crystal packing reveals dimers of complex cations,19 formed by C–
Himine···Ophenolate interactions (Figures 2–5 and S2), promoting cooperativity between the
paramagnetic centres. Further cooperativity between the complex cations [FeIII(5-X-salMeen)2]+
derives from type-II halogen-halogen bonding interactions,[36–41] previously not reported for any
member of the [FeIII(X-salRen)2]+ family (Figures 4 and S4). These are most dominant in [FeIII(5-I-
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salMeen)2]ClO4, but nonexistent in [FeIII(5-F-salMeen)2]ClO4, as expected. In the case of [FeIII(5-Cl-
salMeen)2]ClO4, Cl···Cl contacts are observed only at low temperature. Pertinent angles (θ1 = C1–
X1···X2 and θ2 = X1···X2–C2) and X1···X2 contacts (92–95% of the sum of van der Waals radii) are shown
in Table 3. The I···I contacts shown in Figure 4 for [FeIII(5-I-salMeen)2]ClO4 exemplify the halogen-
halogen bonds in this system. [FeIII(5-F-salMeen)2]ClO4 differs also from the other members of this
series in that it has a C–H···F hydrogen-bonding interaction, which contributes to stabilization of the
dimers (Figure 4).
(a)
(b)
Figure 4. (a) Dimerization in [FeIII(5-F-salMeen)2]ClO4 and (b) halogen-halogen bonding interaction in [FeIII(5-
I-salMeen)2]ClO4 at 90 K.
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That noncovalent intermolecular forces influence spin-transition profiles is demonstrated
spectacularly by the closely related complexes [FeIII(5-Br-salMeen)2]ClO4 (3) and [FeIII(5-Br-
salEen)2]ClO4 (the polymorph designated 1a, which undergoes complete and abrupt SCO).4 Although
the ligand anions differ merely by the secondary amine alkyl groups (Me and Et), the magneto-
structural properties of complexes 3 and 1a are markedly different. The major structural differences
at LT are summarized as follows: (a) the crystallographic asymmetric unit of 3 consists of two complex
cations and two perchlorate counter ions packing in the space group P21/n whereas that of 1a
comprises one complex cation and one perchlorate counter ion packing in the space group P21/c; (b)
3 has a C–Himine···Ophenolate interaction forming dimers of the complex cations, which is not replicated
in 1a; (c) in 3, two perchlorate counter ions connect two complex cations in the chains, each of the N–H
donors interacting with the two perchlorate ions using bifurcated H-bonds, as opposed to the bifurcated
single cation-single anion H-bonding in 1a; (d) the Fe···Cl (perchlorate) distance in 1a (4.919 Å) is shorter
than those in 3 (Fe1···Cl = 5.284, 5.297 Å and Fe2···Cl = 5.130, 5.215 Å); (e) whereas 3 exhibits Br···Br type
II X-bonding (Table 3, Figure S4), 1a consists of C–Br···π interactions and Br···Br type I X-bonding [Br2···Br2
= 3.5741(7) Å; θ1 = θ2 = 145.58(6)°, Figure S4].
X-ray structure of [FeIII(5-Cl-salMeen)2]BPh4·2MeOH (5·2MeOH)
The crystal structure of [FeIII(5-Cl-salMeen)2]BPh4·2MeOH (5·2MeOH) was determined at 298 and 90
K on the same crystal. Selected crystallographic data for 5·2MeOH are presented in Table 2.
Surprisingly, this iron(III) compound undergoes a temperature-dependent crystallographic phase
transition from orthorhombic C2221 at RT to monoclinic P21 at LT with retention of the spin state.
The LT packing diagrams of [FeIII(5-Cl-salMeen)2]ClO4 and [FeIII(5-Cl-salMeen)2]BPh4·2MeOH having
–
the same orientation contrast sharply (Figure 5). In the latter compound, the larger BPh4 ions
alternate, but do not interact, with the complex cations; in the former the smaller ClO4– ions allow
close packing of the dimers of the complex cations and promote formation of chains of the dimers
through H-bonding. Given that these entities occupy similar positions at both temperatures, the
most likely cause of the phase transition in 5·2MeOH is the greater thermal motion of the solvent
molecules at room temperature. Whereas in the structure of [FeIII(5-Cl-salMeen)2]ClO4 (2) the two
–
secondary amine groups are hydrogen-bonded to two ClO4 ions, in the structure of [FeIII(5-Cl-
salMeen)2]BPh4·2MeOH (5·2MeOH) these aforementioned ligand functional groups are hydrogen-
bonded to one MeOH molecule which in turn is hydrogen-bonded to the other MeOH molecule.
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(a)
(b)
Figure 5. Crystal packing diagrams for (a) [FeIII(5-Cl-salMeen)2]ClO4 and (b) [FeIII(5-Cl-salMeen)2]BPh4·2MeOH
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As observed in [FeIII(5-Cl-salMeen)2]ClO4, the orientation of the two uninegative tridentate Schiff-
base ligands in [FeIII(5-Cl-salMeen)2]BPh4·2MeOH relative to each other is very nearly orthogonal and
the donor atoms adopt the meridional coordination arrangement. The 3-D structures of the complex
cation [FeIII(5-Cl-salMeen)2]+ in [FeIII(5-Cl-salMeen)2]BPh4·2MeOH at 298 and 90 K differ in that that
at the higher temperature exhibits C2-symmetry with the distances of the bonds FeIII–Oph, FeIII–Nim
and FeIII–Nam being 1.852, 1.921 and 2.037 Å, respectively. For the structure at the lower
temperature, the average distances for the corresponding bonds are 1.865, 1.928 and 2.034 Å,
respectively. The values of Σ and θim-am (39.9° and 83.7°, respectively) for the RT structure are very
similar to those of the LT structure (41.7° and 83.7/83.4°, respectively). These FeIII–L distances and
octahedral distortion parameters (Table 5) point to the LS state of the complex cations at both
temperatures.
(a)
(b)
Figure 6. X-ray structure of [FeIII(5-Cl-salMeen)2]BPh4·2MeOH at (a) 300 K and (b) 90 K.
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Magnetic Susceptibility Measurements and Mössbauer Spectroscopy
Spin-transition profile for [FeIII(5-X-salMeen)2]ClO4 (X = F, Cl, Br, I) (1–4)
Variable-temperature SQUID measurements were carried out principally to determine the spin-
transition profile for the [HS-HS] → [HS-LS] conversion deduced from the single-crystal X-ray analyses
of complexes 1–4. Plots of χMT versus absolute temperature (5–300 K) are presented in Figure 7. The
room-temperature values of χMT for 1–4, which occur in the range 4.06–4.28 cm3 K mol–1, are
consistent with the spin-only value for HS iron(III) (χMT ~ 4.38 cm3 K mol–1). They also concur with
literature values in the range 3.80–4.50 cm3 K mol–1 for closely related ferric spin-crossover
materials;[1-20,24,29,34,35,45,46,48-51] although some of these reported values are somewhat low.[12,14-16,49]
The spin conversion in 1–4, evidenced by the decrease in the values of χMT with temperature,
culminates in a plateau with a width of 115 K (140–25 K) for 2–4 and 85 K (110–25 K) for 1. The sharp
drop of the values of χMT below 25 K is attributable to the zero-field splitting of the 6A1 ground state
for the complex cations trapped in the HS state. The magnetic plateau is characterized by χMT values
between 2.25 and 2.53 cm3 K mol–1 indicative of the coexistence of HS and LS populations in equal
proportions, in accord with the crystallographic analyses of 1–4 at 90 K. This spin-transition profile
featuring a [HS-LS] plateau is rare in the [FeIII(R'-salRen)2]Y family and is reminiscent of that exhibited
by [FeIII(4-OMe-salMeen)2]PF6[24] and the closely related saltrien complexes [FeIIIL]PF6 (H2L = H2-5-iPr-
saltrien or H2-5-tBu-saltrien),[52] which unfortunately were not crystallographically elucidated.
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1-F 2-Cl 3-Br 4-I
1.00
0.50
0.00
0
50
100
150
200
250
300
T / K
Figure 7. Temperature dependence of effective magnetic moment for [FeIII(5-X-salMeen)2]ClO4 (1–4)
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Of the compounds 1–4, [FeIII(5-Cl-salMeen)2]ClO4 (2) undergoes the steepest spin transition; on the
other hand, the spin conversion is most gradual for [FeIII(5-F-salMeen)2]ClO4 (1). The plots of χMT vs.
T for 1–4 give the approximate T values 160, 168, 180 and 195 K, respectively. For [FeIII(5-Cl-
½
salMeen)2]ClO4 (2), magnetic data were collected during cooling and warming of the sample in order
to investigate any hysteretic behaviour. Indeed, this compound does exhibit SCO hysteresis though
modest (Figure 8); the hysteresis loop cannot be determined with much certainty given the high scan
rate (5 K/min) and large temperature interval for the magnetic susceptibility measurements.
Although the overall structures of the compounds [FeIII(5-X-salMeen)2]ClO4 are similar there are
subtle differences in the noncovalent intermolecular forces which might account for the slight
differences in the magnetic profiles, the most significant of which are the H-bonding interactions
between the complex cations and the perchlorate counter ions (Tables S1–S4). There are also
differences in the halogen-halogen bonding interactions (C1–X1···X2–C2) [Table 3, Figures 5(b) & 4S ],
which are most dominant in the structures of [FeIII(5-Br-salMeen)2]ClO4 and [FeIII(5-I-salMeen)2]ClO4.
While [FeIII(5-F-salMeen)2]ClO4 does not exhibit such interactions, it does have a weak C–H···F
hydrogen-bonding interaction between the HS and LS complex cations [Figure 5(a)] at 100 K, the type
of which none of the other members of this series possess.
4.50
4.00
3.50
Cooling
Warming
3.00
2.50
4.00
3.50
2.00
1.50
3.00
2.50
Cooling
Warming
1.00
0.50
0.00
155 160 165 170 175 180
T / K
0
50
100
150
200
250
300
T / K
Figure 8. Hysteretic behaviour of [FeIII(5-Cl-salMeen)2]ClO4.
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The57Fe Mössbauer effect has proven to be an invaluable spectroscopic tool to investigate ferric spin
crossover occurring in pure solid materials. For slow 6A1 ↔ 2T2 interconversions (< 107 s–1) on the 57Fe
Mössbauer time scale (~10–7 s), distinct HS and LS resonances are observable in the Mössbauer
spectrum.[1b,25,27,28,53] The Mössbauer spectra of [FeIII(5-I-salMeen)2]ClO4 (4) recorded at room
temperature (χMT = 4.21 cm3 K mol–1) and at 80 K (χMT = 2.45 cm3 K mol–1) are displayed in Figure 9.
The broad absorption peak[4,14,16,17,29,54] in the RT spectrum with an isomer shift (δ) of 0.32 mm s–1
(relative to α-Fe) is characteristic of HS iron(III), although in some HS iron(III) complexes a narrow
doublet[9,12,15] or a broad asymmetric doublet[5,19,20,34a] is observed instead. To double-check the
purity of the nondegenerate ground state of the complexes in this series at RT, the Mössbauer
spectrum of [FeIII(5-Cl-salMeen)2]ClO4 (2) was also recorded, likewise giving a broad symmetrical
resonance (δ = 0.26 mm s–1) (Figure S2). Given the quenching of the orbital contribution to
paramagnetism in HS FeIII complexes (S = 5/2, L = 0), the broadening of the Mössbauer signal for HS
FeIII is most likely related to spin-spin relaxation[16-18,55] on the Mössbauer time scale. On cooling the
sample of [FeIII(5-I-salMeen)2]ClO4 (4) to liquid-nitrogen temperature, the Mössbauer spectrum
reveals a wide LS (2T2) quadrupole-split doublet (δ = 0.23 mm s–1 and ΔEQ = 2.87 mm s–1) in addition
to the broad HS (6A1) signal with δ = 0.35 mm s–1.
According to the plot of χMT vs. T, the [HS-LS] plateau temperatures lie in the range 30–145 K, where
the HS and LS complex cations exist in equal proportions. The relative intensities of the HS and LS
Mössbauer absorptions (IHS and ILS) at 80 K imply that the amounts of the HS and LS species are in
the ratio of 45 : 55. Minor discrepancies between magnetic susceptibility data and Mössbauer
spectra are quite common;[16,30] in the case of the series [FeIII(X-salEen)2]Y, Hendrickson et al.[1b]
provided an explanation in terms of the Debye-Waller factor which determines the recoilless fraction
for HS and LS species. As always observed crystallographically, the FeIII–L bonds for the LS state are
significantly shorter and stronger than the corresponding HS ones, causing a larger recoilless fraction
for the LS complex cations. In addition, for the [FeIII(X-salEen)2]Y compounds, it was established from
systematic studies that sample grinding shifts the interconversion S = 5/2 ↔ S = 1/2 in favour of the
low-spin state.[25] In the case of [FeIII(5-I-salMeen)2]ClO4 (4), the sample for the magnetic
susceptibility measurements was microcrystalline whereas that for the Mössbauer measurements
was a finely ground powder.
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(a)
(b)
Figure 9. Mössbauer spectra of [Fe(5-I-salMeen)2]ClO4 at (a) 293 K and (b) 80 K.
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Magnetic profile for [FeIII(5-Cl-salMeen)2]BPh4·2MeOH (5·2MeOH)
The magnetic curve for [FeIII(5-Cl-salMeen)2]BPh4·2MeOH is shown in Figure 10(a). The
crystallographic metric parameters for the coordination geometry of this compound point to the low-
spin state. Generally, close to liquid-helium temperature the values of χMT are commonly observed
in the range 0.45–0.50 cm3 K mol–1 whereas at room temperature they can be as high as 0.83 cm3 K
mol–1.[24,26,55] For 5·2MeOH, the values of χMT (0.48–0.85 cm3 K mol–1) suggest the onset of a spin
[24,26]
transition above 220 K. Similar magnetic behaviour was observed for [FeIII(3-OMe-salMeen)2]PF6
whereby the values of χMT rose from 0.67 cm3 K mol–1 at 78 K to 1.22 cm3 K mol–1 at 298 K, and this
compound was estimated to be ~ 70% LS at room temperature. However, crystallographic analysis
revealed a discrepancy in that the octahedral angular distortion parameters and FeIII–L bond
distances were consistent with 100% LS state. Considering the presence of voids in the crystal lattice
of 5·2MeOH into which the solvent molecules can move and the greater thermal motions of the
solvents at higher temperatures, the magnetic properties of this compound are susceptible to
change on sample grinding.[25,34a,52] Indeed the Mössbauer spectrum of 5·2MeOH recorded at 300 K
shows the presence of both the LS (δ = 0.15 mm/s, ΔEQ = 2.66 mm/s) and HS (δ = 0.27 mm/s, ΔEQ =
0.56 mm/s) states34a in a ratio of ~ 80 : 20% [Figure 10(b)].
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
0
50
100
150
200
250
300
T / K
(a)
(b)
Figure 10. Plot of χMT vs. T for 5·2MeOH (a) and Mössbauer spectrum of 5·2MeOH at 300 K (b).
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Solution Spin-State Equilibria
The interconversions between the HS and LS states in the series [FeIII(5-X-salMeen)2]ClO4 (1–4) in
solution were investigated by visible and EPR spectroscopic techniques using methanol as a solvent.
Electronic spectroscopy reveals an overlap of HS and LS absorption spectra for each complex at room
temperature. The LMCT absorptions occur around 360 nm and in the region 460–800 nm. The
charge-transfer spectra for [FeIII(5-Cl-salMeen)2]ClO4 (2) and [FeIII(5-Br-salMeen)2]ClO4 (3), depicted
in Figure 11(a), demonstrate a remarkable correlation between the electronic spectra and the
Hammett parameters (σp, Table 6).[56] The UV LMCT bands (λ ~ 363 nm) are attributable to phenolate
(pπ) → iron(III) (dσ*) electronic transitions whereas the broad lopsided visible LMCT bands are
phenolate (pπ) → iron(III) (dπ) in nature.[34a,45,46,50,51,57-59]
The HS and LS states of the complexes [FeIII(5-X-salMeen)2]ClO4 (1–4) in electronic spectroscopy are
distinguishable by their visible charge-transfer absorptions.[24,45] Given that ΔO < P for the HS state
and ΔO > P for the LS state, there is greater stabilization of the FeIII dπ orbitals in the LS state than in
the HS state. Consequently, the energy separation between the phenolate pπ and the FeIII dπ orbitals
is smaller in the LS complex cations, causing the LS visible charge-transfer absorptions to occur at
longer wavelengths. For complexes 2 and 3 [Figure 11(a)], the HS and LS charge-transfer absorptions
are centred at 546 nm (εmax ~ 2240 dm3 mol–1 cm–1) and 680 nm (εmax ~ 1600 dm3 mol–1 cm–1),
comparable to those reported for the closely related complex [FeIII(salMeen)2]PF6.[24] The existence
of both spin states in methanol solution at room temperature is in sharp contrast to the magnetic
data of the pure solids at the same temperature.
X-band EPR spectroscopy revealed that all the complexes [FeIII(5-X-salMeen)2]ClO4 (1–4) are fully LS
in frozen methanol solution at liquid-nitrogen temperature and their EPR spectra are virtually
indistinguishable from one another (see Experimental Section in Supporting Information). LS FeIII EPR
spectra tend to be either rhombic[28,35,42,49,53,60,62] or axial.[19,27,49,53,58] Those of complexes 1–4,
represented by the spectrum of [FeIII(5-Cl-salMeen)2]ClO4 (2) [Figure 11(b): gꞱ = 2.21 and gǁ = 1.94],
are consistent with the latter. Evidently, the spin-state conversion in the pure solids is different from
that in solution. It is plausible that the hydrogen bonds between the complex cations and between
the complex cations and perchlorate counter ions in compounds 1–4 break during the dissolution in
methanol, and new noncovalent interactions involving the solvent molecules are expected.
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8500
8000
7500
7000
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
Complex 2
Complex 3
0
280 320 360 400 440 480 520 560 600 640 680 720 760 800
λ / nm
(a)
2.21
1.94
1000
2000
3000
4000
5000
B / G
(b)
Figure 11. (a) Charge-transfer spectra of [FeIII(5-Cl-salMeen)2]ClO4 (blue) and [FeIII(5-Br-salMeen)2]ClO4 (red)
in MeOH and (b) X-band EPR spectrum of a frozen MeOH solution of [FeIII(5-Cl-salMeen)2]ClO4 at 80 K.
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Electrochemical Processes
The redox chemistry of the compounds [FeIII(5-X-salMeen)2]ClO4 (1–4) was explored with the aid of
cyclic voltammetry. Given the stabilization of the divalent state of iron in the spin-crossover
[63]
complexes [FeII(3-X,5-NO2-sal-N(1,4,7,10)] (X = H, OMe, OEt)
by the [N4O2]2+ donor set identical
to that in [FeIII(5-X-salMeen)2]ClO4 (1–4), a thermodynamically favourable metal-centred one-
electron reductive process is expected of the FeIII-salMeen complexes. The cyclic voltammograms of
[FeIII(5-X-salMeen)2]ClO4 (X = F, Cl, Br, I), including the unsubstituted complex [FeIII(salMeen)2]ClO4
for comparison, are displayed in Figure 12 and the half-wave potentials (E ) along with other
½
parameters pertinent to the redox processes are presented in Table 6. The potentials of the
compounds (1 mM in CH3CN) at RT were recorded versus Ag/AgCl (reference electrode) under argon
with ferrocene as an internal standard (Fc/Fc+, E = 0.440 V) and [Bu4N]PF6 as supporting electrolyte.
½
The cyclic voltammograms of [FeIII(5-F-salMeen)2]ClO4 measured at different scan rates [Figure 12
(a)] show a reversible wave for the FeIII/FeII couple at –0.35 V (–0.79 V vs. Fc/Fc+). A ligand-centred
redox process at +1.31 V (+0.87 V vs. Fc/Fc+), whose reversibility increases with the scan rate, is
associated with oxidation of the phenolate moiety to a phenoxyl radical.[45,58,64] The cyclic
voltammograms of the rest of the members of this series along with the unsubstituted compound
[FeIII(salMeen)2]ClO4 are exhibited in Figure 12 (b). A quantitative measure of the electronic effect of
a substituent group is given by a Hammett parameter (σp),[56] which comprises inductive and
resonance factors (σI and σR) with opposing effects. The effects of non-halogen electron-withdrawing
and electron-donating substituent groups (e.g. –NO2 and –Me, respectively) on the acidity of
corresponding phenols and the redox potentials of corresponding iron(III)-phenolate complexes are
readily predictable and explicable.
Intriguingly, the patterns of the pKa values of the para-halo-substituted phenols and the Hammett
parameters in relation to the electronegativity values of the corresponding halogens would seem
paradoxical. However, for phenol and parafluorophenol, the comparable values of pKa (9.99 and
9.89, respectively) and σp (H = 0.00 and F = 0.06) demonstrate the importance of the contribution of
the resonance effect (σR) to the overall Hammett constant (σP = σI + σR). The electron-withdrawing
capacity of the fluoro substituent is greatly diminished by the magnitude of σR. Hence the redox
potential of [FeIII(5-F-salMeen)2]ClO4 is comparable with that of the unsubstituted compound
[FeIII(salMeen)2]ClO4. The trend in the redox potentials of the complexes [FeIII(5-X-salMeen)2]ClO4 (X
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= F, Cl, Br, I) is consistent with the Hammett parameters for the respective halogen substituents
(Table 6): the more positive the value of σp, the more spontaneous the FeIII/FeII redox process.
25.00
20.00
15.00
Fc/Fc+
FeIII/FeII
10.00
5.00
0.00
-5.00
100 mV/s
250 mV/s
-10.00
500 mV/s
1000 mV/s
-15.00
-20.00
-0.6
-0.3
0.0
0.3
0.6
0.9
1.2
1.5
E/V vs. Ag/AgCl
(a)
20.00
15.00
10.00
5.00
Fc+/Fc
FeIII/FeII
0.00
-5.00
-10.00
-15.00
-20.00
5-Br
5-Cl
5-I
H
-0.8 -0.6 -0.4 -0.2
0.0
0.2
0.4
0.6
0.8
E / V vs. Ag/AgCl
(b)
Figure 12. Cyclic voltammograms for (a) [FeIII(5-F-salMeen)2]ClO4 at different scan rates and (b) [FeIII(5-X-
salMeen)2]ClO4 (X = H, Cl, Br, I) at a scan rate of 100 mV/s.
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Concluding Remarks
The compounds [FeIII(5-X-salMeen)2]ClO4 (X = F, Cl, Br, I) (1–4) and [FeIII(5-Cl-salMeen)2]BPh4·2MeOH
(5) have been produced straightforwardly and isolated as black crystals. Their chemical identities
have been established by pertinent analytical, spectroscopic and crystallographic techniques. Single-
crystal X-ray analyses of compounds 1–4 shows that at room temperature the asymmetric unit
consists of a pair of discrete complex cations both of which are in the high-spin state [HS-HS].
However, at 90 K one of the complex cations has converted to the low-spin state, resulting in a mixed
spin-state phase [HS-LS]. The packing diagrams at RT and LT reveal chains comprising dimers of
complex cations linked via pairs of perchlorate counter ions through H-bonding interactions.
Moreover, compounds 2–4 exhibit type-II halogen-halogen bonding interactions. Variable-
temperature SQUID measurements revealed a spin-transition profile consistent with half spin
crossover having a wide plateau (χMT values: ~ 2.25–2.53 cm3 K mol–1). In the case of [FeIII(5-I-
salMeen)2]ClO4 (4), this magnetic behaviour was corroborated by Mössbauer spectroscopy. This
cooperative [HS-HS] ↔ [HS-LS] spin transition has not been crystallographically elucidated previously
within the [FeIII(X-salRen)2]Y family. [FeIII(5-Cl-salMeen)2]BPh4·2MeOH (5·2MeOH) undergoes a
temperature-dependent crystallographic phase transition associated with the thermal motions of
the solvent molecules while retaining the LS state. However, as a finely ground powder, 5·2MeOH
exhibits ~20% SCO to the HS state. As homogeneous mixtures with methanol, the compounds 1–4
exhibit different magnetic behaviour than as pure solids. EPR spectroscopy showed that the low-spin
state in all complexes 1–4 is fully accessible at liquid-nitrogen temperature in frozen methanol
solution. Finally, all compounds 1–4 are electroactive and the trend of their metal-centred redox
potentials is in keeping with the values of the Hammett parameters for the halogen substituents.
Experimental Section
Materials, Methods and Physicochemical Measurements
Detailed synthetic procedures for the complexes [Fe(5-X-salMeen)2]ClO4 (1–4) and [Fe(5-X-
salMeen)2]BPh4·2MeOH (5·2MeOH) and data for chemical identification are provided as supporting
information. N-Methylethylenediamine, para-substituted salicylaldehydes, iron(III) perchlorate
hydrate as well as reagents and solvents (HPLC/AR-grade) were purchased from Sigma-Aldrich and
used as received. Caution: salts of perchlorates are notorious for their tendency to explode, hence
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their samples must be handled with due care and in small quantities. Microanalyses (CHN) were
performed on a EuroEA elemental analyzer and mass spectra were recorded on an Agilent 6460
1
Triple Quad LC mass spectrometer in both positive and negative modes. H-NMR spectroscopic
measurements of selected Schiff bases were carried out in DMSO-d6 on a Bruker ASCEN 700 MHz
NMR spectrometer with a Bruker CRYO platform accessory and the chemical shifts are referenced to
TMS (δ = 0) as an internal standard. FT-IR spectra were recorded on an Agilent Cary 600
spectrophotometer (4000–650 cm–1) or a Perkin Elmer spectrophotometer (4000–400 cm–1) with the
samples compressed as KBr discs using a Specac press. Conductivities of the complexes were
measured on a JENWAY 4520 conductivity meter at room temperature using freshly prepared
solutions (1 mM) in MeOH. Electronic absorption spectra were recorded on a Shimadzu 2450 UV-
visible spectrophotometer (190–1000 nm). Measurements of cyclic voltammetry were conducted
with a PalmSense potentiostat (PSTrace 4.5 software) in MeCN under argon using a three-electrode
electrochemical cell comprising a Pt-disc working electrode, a Pt-wire counter (auxiliary) electrode
and a Ag/AgCl reference electrode with 0.1 M [Bu4N]PF6 as supporting electrolyte. Potentials are
quoted with respect to that of the Cp2Fe0/+ couple (E = 440 mV vs. Ag/AgCl under the experimental
½
conditions) using ferrocene as an internal standard.
X-band EPR spectra were recorded on a Bruker ELEXSYS E-500 CW spectrometer (microwave
frequency = 9.43 GHz). Variable-temperature magnetic susceptibility measurements were carried
out on a Quantum Design EverCool SQUID magnetometer operating at an applied field of 8 kOe and
palladium was used as the calibration standard. Diamagnetic corrections were performed the usual
way using Pascal’s constants. 57Fe Mössbauer spectra were recorded on a conventional spectrometer
with alternating current and constant acceleration of the γ-source (57Co/Rh, 1.8 GBq), which was
kept at room temperature. The minimum experimental line width was 0.24 mm s–1 (full width at half-
height). The temperature of the sample was controlled with the aid of an Oxford Instruments Variox
cryostat. Isomer shifts (δ / mm s–1) are quoted relative to α-Fe at room temperature. X-ray diffraction
data were collected using a Bruker SMART APEX-II CCD diffractometer with graphite-
monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were solved with the SHELXT[65]
software using intrinsic phasing and refined on F2 with the SHELXL[66] software using least squares
minimization. The Olex2[67] programme was used for molecular graphics and generation of
publication material.
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Acknowledgements
We owe a debt of gratitude to SQU Deanship of Research for financial support (IG/SCI/CHEM/19/04,
MSS) and Deanship of Postgraduate Studies for a doctoral scholarship (MAA).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
Ferric spin crossover · spin-transition plateau · noncovalent interactions · crystallographic phase
transition · electronic and electrochemical processes
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