Evidence for Reagent-Induced Spin-State Switching in Tripodal Fe(II) Iminopyridine Complexes

Article abstract of DOI:10.1021/acs.inorgchem.9b00340

We present evidence of a spin-state change that accompanies desilylation reactions performed on two related Fe(II) iminopyridine coordination complexes. To probe these systems, we performed titrations with CsF in solution and analyzed the speciation with in situ magnetometry, electrochemistry, and mass spectrometry techniques. We find that pendant tert-butyldimethylsilyl groups are readily cleaved under these conditions, and the resulting desilylated complexes exhibit overall decreased solution magnetic susceptibility values. Density functional theory and ab initio computations probe the impact of substituent identity (prior to- and post-desilylation) on the metal-ligand σ-donor and -acceptor bonding properties. We attribute the observed spin-state changes to the decrease in entropy associated with the conformational freedom of the silylated high-spin complex, resulting in a more favored low-spin state upon desilylation.

Full text of DOI:10.1021/acs.inorgchem.9b00340

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Evidence for Reagent-Induced Spin-State Switching in Tripodal Fe(II)
Iminopyridine Complexes
Tarik J. Ozumerzifon,†,# Robert F. Higgins,†,∥ Justin P. Joyce,† Jacek L. Kolanowski,⊥
Anthony K. Rappe, and Matthew P. Shores*,†
†
́
†Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States
⊥
́
Institute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12/14, 61-704 Poznan, Poland
S
* Supporting Information
ABSTRACT: We present evidence of a spin-state change
that accompanies desilylation reactions performed on two
related Fe(II) iminopyridine coordination complexes. To
probe these systems, we performed titrations with CsF in
solution and analyzed the speciation with in situ magneto-
metry, electrochemistry, and mass spectrometry techniques.
We find that pendant tert-butyldimethylsilyl groups are readily
cleaved under these conditions, and the resulting desilylated
complexes exhibit overall decreased solution magnetic
susceptibility values. Density functional theory and ab initio
computations probe the impact of substituent identity (prior to- and post-desilylation) on the metal−ligand σ-donor and π-
acceptor bonding properties. We attribute the observed spin-state changes to the decrease in entropy associated with the
conformational freedom of the silylated high-spin complex, resulting in a more favored low-spin state upon desilylation.
ligands in Fe(II) cage complexes provides a direct synthetic
route for the alteration of functional groups.34−38
INTRODUCTION
■
Spin crossover (SCO) enables molecular switching functions
due to dramatic contrasts in magnetic, structural, and
spectroscopic properties between spin-states. The low- to
high-spin (LS and HS, respectively) conversion involves a
weakening of the ligand field (ΔH > 0) that is compensated by
an increasing number of electronic microstates and lower-
energy vibrational modes (ΔS > 0). This molecular
phenomenon has been controlled through external perturba-
tions such as temperature, pressure, and light.1−7
Ligand substituents play a complicated role in determining
SCO properties. For example, steric bulk can perturb
equilibrium metal−ligand bond lengths in both molecules
and soft materials,8−15 while hydrogen-bonding interactions
have also been shown to affect ligand donor properties.16,17
Recent reports describe linear relationships between solution-
phase SCO temperatures (T1/2 = ΔH/ΔS) and electronic
character of the substituents for Fe(II) complexes.18−22 These
trends are often system-dependent due to competing impacts
on the ligand field strength via σ-donor and π*-acceptor
interactions.
We chose to explore the feasibility of chemical modulation of
the spin-state in an Fe(II) iminopyridine podand to generate
design principles for future sensing architectures. Building on
our and others’ experience with manipulating spin states in
metal iminopyridine complexes, a model synthetic target is
depicted in Figure 1.8−10,39,40
We envision that cleavage of a fluoride-sensitive silyl ether
moiety will impart a significant change in ligand field, given the
large differences in Hammett parameters (σmeta and σpara) for
the reactant (silyl ether) and product (“phenoxide”)
substituents. Of course, the protonation state of the desilylated
products will depend on reaction conditions, ultimately
affecting the electronic nature of that 5-pyridyl substituent.
In our proposed system, Fe(II) complexes with two different
tripodal ligands will allow us to evaluate the impact of
electronic and other factors on low- and high-spin species. The
spectroscopic, mass spectrometric, electrochemical, and
magnetic data presented herein shed light on the complex
dynamics of these desilylation reactions. Computations probe
fluctuations in ligand σ- and π-bonding properties, to
deconvolute the electronic impacts of substituent changes on
the bidentate iminopyridine ligand set that places the 5-pyridyl
substituents -meta and -para to the pyridine and imine nitrogen
atoms, respectively.
Postsynthetic modification of a ligand scaffold offers an
attractive approach to affect changes to spin-state properties,
with potential to inspire new motifs for environmental sensing,
imaging, and memory devices.23−25 Several groups have
reported reagent-induced spin-state changes through chemical
transformations of the coordinated ligand set.26−33 We are
motivated by the work of Hooley and Nitschke, who have
shown that the distinct reactivity of coordinated iminopyridine
Received: February 5, 2019
© XXXX American Chemical Society
A
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Fe(OTf)2(MeCN)2 (0.119 g, 0.27 mmol) in MeCN (2 mL) was
added dropwise, and the resulting dark red mixture was stirred for an
additional 1 h at 23 °C. The reaction mixture was then passed
through a syringe filter, and the product was precipitated by addition
of Et2O (20 mL) to the filtrate. The mixture was stirred overnight,
and the solid was collected by filtration and washed with Et2O (2 × 5
mL) to afford 2 (0.215 g, 68% yield) as a dark red powder. IR (KBr
pellet): νC=N 1591 cm−1. Absorption spectrum (acetone, Me2CO):
λmax (εM) 354 nm (25 900 M−1 cm−1), 553 nm (7770 M−1 cm−1). MS
(ESI+): m/z calcd for (M − OTf)+ [C44H69N7O9Si3S2F6Fe −
CF3SO3]+: 1008.36, found: 1008.33; m/z calcd for (M − 2OTf)2+
[C44H69N7O9Si3S2F6Fe − C2F6S2O6]2+: 429.71, found: 429.83. 1H
NMR spectra are collected in Figure S39. χMT (298 K, (CD3)2CO):
0.17 cm3 K mol−1; Anal. Calcd for C47H73.5N8.5O9Si3S2F6Fe (2·
1.5MeCN): C, 46.28; H, 6.07; N, 9.76. Found: C, 45.83; H, 5.65; N,
10.09. Note: samples prepared for elemental analysis were crystals
grown from a layering of 2 in acetonitrile (3 mL) with a 1:1
antisolvent layer of Et2O:iPr2O. Small quantities of diffraction-quality
crystals can be grown by diffusion of iPr2O into a concentrated
solution of 2 in acetonitrile; these do not contain co-crystallized
MeCN solvent molecules.
Figure 1. (a, b) Proposed method for spin-state changing in Fe(II)
podand complexes.41 While the expected formation of phenoxide
species is depicted, the actual protonation state of the desilylated
products may depend on reaction conditions.
6-Methyl-5-(tert-butyldimethylsilyloxy)picolinaldehyde (3).
Crude 5-hydroxy-6-methyl-picolinaldehyde (3A,9 see Supporting
Information for preparation), TBSCl (2.22 g, 14.7 mmol), and
DMAP (0.160 g, 1.33 mmol) were charged to a flame-dried flask and
then dissolved in CH2Cl2 (27 mL) and cooled to 0 °C. Then, freshly
distilled triethylamine (2.24 mL, 16.1 mmol) was added dropwise,
and the resulting mixture was warmed to 23 °C and stirred for 1 h.
The reaction mixture was then quenched with water and poured into
a separatory funnel; the product was extracted with CH2Cl2 (2 × 20
mL), washed with brine (20 mL), dried over MgSO4, and filtered to
give crude 3. The desired compound was purified by flash column
chromatography (4:1 hexanes/EtOAc eluent) to give pure silyl ether
3 (2.13 g, 18% yield over three steps; see Supporting Information for
previous two steps), as a pale yellow oil. Rf = 0.67 in 4:1 hexanes/
EXPERIMENTAL SECTION
Note: No unexpected or significant safety hazards occurred in the
present work. The use of perchlorate salts is mentioned in more detail
later (vide infra).
■
Preparation of Compounds. Unless otherwise stated, reactions
were performed under ambient conditions (room temperature,
normal benchtop atmosphere). Air-free manipulations were carried
out in a dinitrogen-filled glovebox (MBRAUN Labmaster 130).
Qualitative thin layer chromatography (TLC) analysis was performed
on 250 mm thick, 60 Å, glass backed, F254 silica (Silicycle, Quebec
City, Canada); samples were visualized with UV light. Flash
chromatography was performed using Silicycle silica gel (230−400
mesh). Syringe filters were purchased from VWR International and
were fitted with 0.2 μm PTFE membranes. Diethyl ether (Et2O),
dichloromethane (CH2Cl2), and acetonitrile (MeCN) were sparged
with nitrogen, passed through alumina columns, and degassed prior to
use. Anhydrous ethanol (EtOH) and methanol (MeOH) were
purchased from Sigma-Aldrich. All other chemicals were purchased
from commercial vendors and used as received.
1
EtOAc). H NMR (400 MHz, CDCl3): δ 9.95 (s, 1H), 7.76 (d, J =
8.2 Hz, 1H), 7.12 (d, J = 8.2 Hz, 1H), 2.53 (s, 3H), 1.03 (s, 9H), 0.28
(s, 6H). 13C NMR (100 MHz, CDCl3): δ 192.3, 154.2, 152.0, 145.5,
124.3, 121.4, 25.5, 19.8, 18.2, −4.3. IR (ATR, film): 2956, 2931, 2859,
1702, 1566, 1456, 1347, 1303, 1278, 1257, 1245, 1170, 1112 cm−1.
HRMS (ESI+): m/z calcd for (M + H)+ [C13H22NO2Si + H]+:
252.1420, found: 252.1391.
[Fe(6-Me-5-OTBSpy)3tren](ClO4)2 (4). Caution! Although we have
not encountered any difficulties with this product, perchlorate-
containing compounds pose explosion hazards and should be handled
with care and in small quantities.
In a dinitrogen glovebox, to a solution of 3 (0.100 g, 0.40 mmol) in
MeCN (2 mL) was added a solution of tren (0.019 g, 0.13 mmol) in
MeCN (2 mL), and the resulting mixture was stirred in the presence
of 3 Å molecular sieves for 1 h at 23 °C. Then, a solution of
Fe(ClO4)2(H2O)12 (0.060 g, 0.13 mmol) in MeCN (2 mL) was
added dropwise, and the resulting red mixture was stirred for an
additional 1 h at 23 °C. The reaction mixture was then passed
through a syringe filter, and the product was precipitated by addition
of Et2O (20 mL) to the filtrate. The mixture was stirred overnight.
The crude product was recrystallized by placing a concentrated
solution of 4 in EtOH into a freezer (−40 °C) overnight, giving pure
4 (0.109 g, 77% yield) as red needle-like crystals that are too brittle
for analysis via single crystal X-ray diffraction. IR (KBr pellet): νC=N
1653 cm−1. Absorption spectrum (Me2CO): λmax (εM) 437 nm (3920
M− 1 cm− 1 ). MS (ESI+ ) m/z calcd for (M - ClO4 )+
[C45H75N7O11Cl2Si3Fe − ClO4]+: 1000.41, found 1000.42, m/z
calcd for (M − 2ClO4)2+ [C45H75N7O11Cl2Si3Fe − Cl2O8]2+: 450.73,
found 450.75. 1H NMR spectra are collected in Figure S40. χMT (298
K, (CD3)2CO): 3.44 cm3 K mol−1. χMT (300 K, solid state): 3.89 cm3
K mol−1. Anal. Calcd for C45H75N7O11Cl2Si3Fe: C, 49.09; H, 6.87; N,
8.90. Found: C, 48.94; H, 6.75; N, 8.97.
5-(tert-Butyldimethylsilyloxy)picolinaldehyde (1). Crude 5-hy-
droxypicolinaldehyde (1A,9 see Supporting Information for prepara-
tion), tert-butyldimethylchlorosilane (TBSCl, 3.00 g, 19.9 mmol), and
N,N-dimethylaminopyridine (DMAP, 0.220 g, 1.8 mmol) were
charged to a flame-dried flask and then dissolved in CH2Cl2 (36
mL) and cooled to 0 °C. Then, freshly distilled triethylamine (3.03
mL, 21.7 mmol) was added dropwise, and the resulting mixture was
warmed to 23 °C and stirred for 1 h. The reaction mixture was then
quenched with water and poured into a separatory funnel; the crude
product was extracted with CH2Cl2 (2 × 20 mL), washed with brine
(20 mL), dried over MgSO4, and filtered. The desired compound was
purified by flash column chromatography (4:1 hexanes/EtOAc
eluent) to give pure silyl ether 1 (1.93 g, 87% yield over two steps;
see Supporting Information for previous step), as a pale yellow oil. Rf
1
= 0.65 in 4:1 hexanes/EtOAc. H NMR (400 MHz, CDCl3): δ 9.99
(s, 1H), 8.34 (d, J = 2.3 Hz, 1H), 7.91 (d, J = 8.6 Hz, 1H), 7.25 (app.
dd, J = 2.3 Hz, 1H), 1.00 (s, 9H), 0.28 (s, 6H). 13C NMR (100 MHz,
CDCl3): δ 192.0, 156.1, 146.7, 142.9, 126.8, 123.2, 25.5, 18.2, −4.4.
IR (ATR, film): 2956, 2931, 2859, 1710, 1574, 1478, 1391, 1363,
1292, 1257, 1210, 1111, 1013 cm−1. HRMS (ESI+): m/z calcd for (M
+ H)+ [C12H20NO2Si + H]+: 238.1263, found: 238.1242.
[Fe(5-OTBSpy)3 tren](OTf)2 (2). In a dinitrogen glovebox, to a
solution of 1 (0.200 g, 0.84 mmol) in MeCN (2 mL) was added a
solution of tris(2-aminoethyl)amine (tren, 0.040 g, 0.27 mmol) in
MeCN (2 mL), and the resulting mixture was stirred in the presence
of 3 Å molecular sieves for 1 h at 23 °C. Then, a solution of
Physical Methods. All methods were performed at room
temperature unless otherwise noted. Absorption spectra were
obtained with a Hewlett-Packard 8453 spectrometer in quartz
B
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cuvettes with a 1 cm path length. Infrared spectra were measured with
a Nicolet 380 FT-IR spectrometer. H NMR spectra were obtained
amounts of colorless insoluble components. A capillary was then
inserted, and the NMR tube was capped, sealed with parafilm, and
quickly transported to a NMR spectrometer.
1
on a Varian 400 MR spectrometer (at 400 MHz) and are reported
relative to SiMe4 (δ 0.00) except the CsF titrations, which were
reported relative to MeOH (δ 3.31). 13C NMR spectra were obtained
on a Varian 400 MR spectrometer (at 100 MHz) and are reported
relative to SiMe4 (δ 0.00). NMR acquisition sequences for metal-
containing species had a 1 ms relaxation delay, 90° pulse angle, and
acquisition time of 1 s. EPR spectra were collected using a Bruker
ESR-300 cooled under a flow of liquid N2. EPR samples were
prepared by dissolving the sample in MeOH (∼0.05 M), transferring
it to an NMR tube, and sealing the cap with parafilm. Elemental
analyses were performed by Midwest Microlab, LLC in Indianapolis,
IN.
Computational Methods. For the computational study, the
trimethylsilyl (TMS) substituent was used in place of TBS since the
electronics are nearly identical, while the size of the calculation is
reduced. The phenoxide substituent is the assumed product for the
complete reaction of 2 with CsF. The singlet states of [Fe(5-
OTMSpy)3tren]2+ and [Fe(5-Opy)3tren]− (Figure 2) were optimized
with the APFD functional44 and the 6-311+g(d) basis set in the
absence of counterions.45,46
Mass Spectrometry Measurements. Samples were prepared in
a dinitrogen-filled glovebox by dissolving the metal complex in MeOH
(to generate approximately 1 mM solutions of Fe complex) and
sealing the vials with a cap. These solutions were transported to the
spectrometer and were injected into the spectrometer as quickly as
possible; brief exposure to air (∼3 s) could not be avoided.
Measurements were performed in the positive-ion mode on a
Thermo LTQ mass spectrometer equipped with an analytical
electrospray ion source and a quadrupole ion trap mass analyzer.
Measurements were also performed in the negative-ion mode, but the
results were not analyzed in detail due to low signal intensities. Unless
otherwise noted, measurements were performed with capillary
temperature = 175 °C, spray voltage = 5 kV, and spray current =
91 μA. All experiments were performed in MeOH, and the instrument
was washed with 1 mL of solvent between scans. The peaks associated
with Fe-containing complexes were identified, and their relative
abundances were summed into categories defined by loss of 0, 1, 2, or
3 TBS groups (See Supporting Information, Tables S1−S4).
Electrochemical Measurements. Electrochemical experiments
were performed in 0.1 M solutions of Bu4NPF6 in MeOH in a
dinitrogen-filled glovebox. Cyclic voltammograms (CVs) and square-
wave voltammograms (SWVs) were obtained with a CH Instruments
potentiostat (model 1230A or 660C) using a 0.25 mm glassy carbon
disk working electrode, Ag+/Ag reference electrode and a Pt wire
counter electrode. Scans were collected at rates between 0.10 V s−1
and 10 V s−1 for CVs and at a step-size of 0.004 V and a frequency of
15 Hz for SWVs. Reported potentials are referenced to the [Cp2Fe]+/
[Cp2Fe] (Fc+/Fc, where Cp = cyclopentadienyl) redox couple and
were determined by adding ferrocenium tetrafluoroborate as an
internal standard at the conclusion of each electrochemical experi-
ment. Note that ferrocene could not be used as the internal standard
as it is insoluble in MeOH.
Magnetic Measurements. Solid-state magnetic susceptibility
data were collected for 4 using a Quantum Design MPMS XL SQUID
magnetometer at temperatures between 2 and 300 K, and DC fields
up to 20 kOe. In a dinitrogen-filled glovebox, powdered microcrystal-
line samples were loaded into polyethylene bags, sealed, and inserted
into drinking straws for measurements. Ferromagnetic impurities were
checked through a variable field measurement (0−20 kOe) of the
magnetization at 100 K; curvature in the M vs H data obtained
between 0 and 2 kOe (Figures S26 and S27) indicates the presence of
minor ferromagnetic impurities. As a result, the magnetic suscepti-
bility measurements were performed at a 5 kOe applied field, where
the M vs H trace was linear. Data were corrected for the diamagnetic
contributions of the sample holder and bag by subtracting empty
containers; corrections for the sample were calculated from Pascal’s
constants.42
Figure 2. LS state geometry optimized model complexes of the
desilylation reaction.
Calculations were performed with the PCM model47 in an implicit
solvent cavity of acetonitrile (ε = 35.688). The APFD functional
accurately reproduces the structural properties of the primary
coordination sphere by comparison to analogous Fe(II) complexes.48
Slight deviation between the structures is shown in the distances
between the tertiary amine of the capping ligand and the Fe(II) center
for the two structures, 3.40 and 3.45 Å for the silyl ether and
phenoxide substituents, respectively. Since those distances exceed
sums of the van der Waals radii, it is unlikely that the bridgehead
nitrogen significantly impacts the ligand field properties of the
complex within the current deviation.49
TD-DFT50 was performed with the ωB97XD functional51 with
settings identical to the optimization procedure. The functional was
selected because of its treatment of charge transfer allows us to better
probe the isolated ligand field states of the system. All DFT
computations were performed with the Gaussian 16 software
package.52
Ab initio ligand field theory (AILFT)53,54 analyses were performed
on the DFT-optimized structures through a CASSCF/NEVPT2
approach, using the ORCA 4.0 software package.55 The AILFT
treatment used a restricted active space consisting of the five 3d
orbitals and with equal weighting of all possible states (5 singlets, 45
triplets, and 50 quintets for a d6 system). The multireference
computations were performed with the CPCM model47 for
acetonitrile and cc-pVDZ and cc-pVTZ56 basis sets for the nonmetal
atoms and Fe center, respectively. The solutions of the ligand field
matrix and the transition states of the CASSCF and NEVPT2 output
were each used to obtain a best fit of the σ- and π-bonding parameters
of the angular overlap model (AOM), using the AOMX software.57 D3
symmetry was applied to the first coordination sphere polyhedra for
[Fe(5-OTMSpy)3tren]2+ and [Fe(5-Opy)3tren]− for the AOM
parametrization.
RESULTS AND DISCUSSION
■
Syntheses and Characterizations of Fe Compounds.
The tripodal complexes 2 and 4 were prepared via Schiff-base
condensation of the appropriate aldehyde (1 or 3, respectively)
with tren, followed by addition of a ferrous salt to the in situ
formed ligand (Scheme 1). The use of iron(II) perchlorate is
crucial to obtain pure 4, as attempts to obtain other salts in
high purity (including triflate, chloride, bromide, tetraphenyl-
borate, and tetrafluoroborate) were unsuccessful. Structural
characterization of 2 was performed by single crystal X-ray
Evans’ method determinations of magnetic susceptibility in
solution for 2 and 4 were carried out using the residual protonated
fraction of the indicated deuterated solvent in a capillary as a
reference.43 (Because of the presence of other silyl peaks in the H
1
NMR spectra for 2 and 4, SiMe4 was not a reliable reference.) To
obtain solution susceptibility data after the addition of CsF, different
aliquots of a stock CsF solution (∼0.3 M) in CD3OD were added to a
solution of 2 or 4 (∼0.01 M) in CD3OD in an NMR tube. The
samples were then passed through a syringe filter to remove small
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To assess the redox properties of 2 and 4, their
electrochemical properties were probed in methanol. The
E1/2 values for the FeIII/II couple of 2 and 4 are +0.95 and
+0.97 V vs Fc+/Fc, as obtained from SWV measurements
(Figures S20 and S23), respectively. These values and overall
behavior regarding reversibility compare well to previously
characterized Fe(II) iminopyridine tripodal systems.8,48,59 On
the basis of cyclic voltammetry (CV) data, 2 displays a
reversible FeIII/II couple at a scan rate of 0.10 V s−1, while the
analogous couple in 4 is apparently irreversible at the same
scan rate (Figure 4). Upon increasing the scan rate to 10 V s−1
Scheme 1. Synthesis of Fe(II) Complexes 2 and 4
diffraction (Figure S25 and accompanying discussion in the
Supporting Information), which confirmed the expected
connectivity throughout the complex, yet contained severe
disorder within the silyl groups, precluding a detailed analysis.
Room temperature (298 K) deuterated acetone solution
magnetic susceptibility values for the Fe-containing com-
pounds were acquired using Evans’ method. For 2, an obtained
χMT value of 0.17 cm3 K mol−1 suggests a LS ground state, and
thus solid-state magnetic susceptibility data were not collected.
On the other hand, the 6-methylated species 4 exhibits a χMT
value of 3.44 cm3 K mol−1, consistent with a HS Fe(II) species
at room temperature, as expected.8−10,12,13 The solid-state
temperature dependence of χMT for 4 is shown in Figure 3.
Figure 4. CVs of 2 (bottom) and 4 (top) measured in 0.10 M
Bu4NPF6 solutions in MeOH, with scan rates of 0.10 V s−1 (solid) or
10 V s−1 (dashed). The asterisks indicate the open circuit potential,
while the arrows indicate the direction of the scan, which in all cases is
anodic.
(red dashed trace), the cyclic voltammogram (CV) shows an
increase in current passed in the cathodic sweep, indicating
that 4 does not exhibit completely irreversible behavior.
After the addition of excess CsF into the electrochemical cell
containing 2, the assigned FeIII/II redox event broadens and
shifts cathodically by approximately 200 mV while still passing
current in both the anodic and cathodic directions of scanning
(Figure S21). This suggests that multiple desilylated species
are formed and the shift in redox potential can be linked to a
significant increase in negative charge on the ligand upon
desilylation. Meanwhile, when excess CsF is added to 4, a
distinct redox event associated with the iron center is not
observed at a scan rate of 0.1 V s−1, although spectroscopic and
ESI-MS data support the presence of Fe coordination
complexes after CsF addition (vide infra). This supports the
notion that solution stability upon desilylation is greater for LS
2 than HS 4.
In Situ Monitoring of Desilylation for 2 and 4. Noting
that compounds 2 and 4 are susceptible to decomposition in
solution over time, we probed their solution stabilities and
summarized the results in the Supporting Information;
importantly, we found that the complexes remained intact
over the time scale of all experiments included herein. In order
to probe the effects of desilylation on Fe(II) spin state in 2 and
4, additional techniques were employed. The results of in situ
mass spectrometry and 1H NMR spectroscopy are presented in
addition to the electrochemical experiments to provide a
clearer picture of the desilylation reaction using CsF.
Figure 3. Temperature dependence of the magnetic susceptibility for
4 measured at an applied dc field of 5000 Oe.
At 300 K, the χMT value for 4 is 3.89 cm3 K mol−1, in
agreement with a quintet state (S = 2). Upon decreasing the
temperature, the χMT product decreases slightly until ∼215 K,
where the decrease becomes more pronounced, indicative of a
spin crossover (SCO) event (T1/2 = 175 K). This decrease
continues until ∼120 K where the χMT product remains
relatively constant at 0.40 cm3 K mol−1. The T1/2 value and
lower temperature magnetic susceptibility behavior are
consistent with other 6-methylated 5-alkoxy Fe(II) tren-
podand complexes that display SCO behavior in the solid
state.9,10 The χMT value does not reach 0 cm3 K mol−1 possibly
due to a paramagnetic impurity that is not separable from 4,
which is fairly common for Fe(II) SCO materials;28,58
alternatively, 4 might undergo incomplete SCO.
Importantly, we find that the identity of the fluoride source
used is critical. We first tried tetrabutylammonium triphenyldi-
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fluorosilicate (TBAT), as it is readily soluble in polar aprotic
solvents. Unfortunately, we found that addition of TBAT to
solutions of 2 or 4 in MeCN induce immediate demetalation
of the complexes, as evidenced by the appearance of free ligand
peaks in the mass spectra (Figures S15 and S16), and a stark
change in color from red to pale yellow (Figure S32).
Alternative fluoride sources, including TBAF in tetrahydrofur-
an, KF/18-crown-6 in acetonitrile, and CsF/18-crown-6 in
acetone, all show similar color changes. When CsF is used in
methanol, however, mass spectrometry experiments indicate
that the ligand was desilylated without significant demetalation.
The results of the CsF titration experiments performed on 2
are presented in Figure 5.
Figure 6. Top: solution-phase magnetic properties for 4 upon
addition of CsF. Bottom: relative abundance of Fe-containing
desilylated ions for 4, as determined by integrations of mass
spectrometry intensities, relative to each individual spectrum. These
data were collected in CD3OD.
stoichiometric. The complete desilylation that is observed for 4
(and not 2) may be rationalized, given that HS complexes are
often more reactive,60,61 and the methyl groups proximal to the
reaction site for 4 may contribute steric effects. Addition of
further equivalents of fluoride leads to an increase in magnetic
susceptibility and the disappearance of the signals correspond-
ing to the fully silylated species. As was the case with 2, there
was also a reduction in the overall intensity of signal in the MS
experiments.
An analysis of the NMR spectra of the desilylation reaction
of 2 offers additional insight into the speciation in solution.
First, there are several sets of pyridyl resonances present prior
to addition of CsF and up until the addition of 5 equiv
(Figures S39 and S40). This suggests there may be multiple
species present in solution until a larger amount of CsF has
been added to the sample. Once five equivalents are added, the
spectrum indicates convergence to one major C3-symmetric
species (Figure S39, bottom). Additionally, upon addition of
CsF, the relative integration of the peaks associated with the
pendant -OTBS moiety relative to the aromatic region
decreases, indicating that this -TBS group is cleaved from
the system. This analysis is more difficult to perform for
paramagnetic 4 as the spectra are extended over a larger range
(δ −14 to 173 ppm).
In conclusion, the experimental results presented indicate
that for both 2 and 4 the titration of CsF leads to the
formation of desilylated species. Further, these in situ formed
Fe-complexes exhibit a decrease in overall magnetic suscept-
ibility, which suggests that the spin-state equilibrium is shifted
toward the LS state.
Computational Results and Interpretation of Ligand
Field Effects. To further understand the observed spin-state
changes, computations were performed on [Fe(5-OTM-
Spy)3tren]2+ and [Fe(5-Opy)3tren]−, models for 2 and its
fully desilylated relative, respectively. We only consider the
phenoxide substituents as the reaction product to probe the
largest possible electronic impact in terms of Hammett
parameter (σmeta = +0.09 → −0.47; σpara = +0.09 → −0.81).
While the primary coordination sphere of the optimized
structures (Fe−Nimine and Fe−Npyridine distances and NNN
torsion angle) is maintained, the desilylation alters the
electronics of the ligand itself as shown in Figure 7. First,
the C−O bond length of the substituent at the 5 position of
the pyridine ring decreases by 0.07 Å upon desilylation. This
Figure 5. Top: solution state magnetic properties for 2 upon addition
of CsF; the line is a guide to the eye. Bottom: relative abundance of
Fe-containing desilylated ions for 2, as determined by integrations of
mass spectrometry intensities, relative to each individual spectrum.
These data were collected in CD3OD.
Here, we observe that the solution-phase χMT values
decrease as CsF is titrated into the solution up to 1.5 equiv.
We note the difference in χMT value at 0 equiv CsF added in
CD3OD as compared to the value provided in (CD3)2CO,
highlighting a difference in complex stability in these two
solvents; see Supporting Information for further discussion (p
S34). This is accompanied by an increase in relative abundance
of peaks associated with desilylated complexes of Fe(II). These
are detected as both protonated and nonprotonated species;
for a full list of identity assignments in the positive ion mode,
see Table S1. Overall low signal intensity in the negative ion
mode precluded full interpretation of anions formed in the
desilylation reactions; notwithstanding, m/z values corre-
sponding to triply desilylated species could be detected.
Upon further addition of fluoride (up to 5 equiv), the magnetic
susceptibility increases, while the trend of increased abundance
of desilylated species continues as before. Noteworthy is that,
at higher loadings of CsF during the mass spectrometry
experiments, the overall intensity of the signals decreases,
suggesting the formation of less ionizable products. In all, the
combined NMR and MS experiments allow us to make a
qualitative link between CsF addition and Fe complex
desilylation, and that these species generally remain in a LS
state over the course of the titration.
Similar effects are observed in the case of complex 4 (Figure
6). As before, addition of up to 1.5 equiv of fluoride leads to a
decrease in magnetic susceptibility and the exclusive detection
of ions identified as triply desilylated species in the ESI-MS.
This suggests that fluoride-mediated cleavage may be super-
E
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 7. Optimized bond lengths (Å) of [Fe(5-OTMSpy)3tren]2+
(blue) and [Fe(5-Opy)3tren]− (green) for the C3 symmetric bidentate
ligand. The hydrogen and remaining atoms of the tren-moiety are
removed for clarity.
Figure 8. ωB97XD/6-311+g(d) NTOs of [Fe(5-OTMSpy)3tren]2+
(top) and [Fe(5-Opy)3tren]− (bottom) depicting the energy and
orbital character associated with a t2g → eg* excitation; hydrogen
atoms are removed for clarity.
suggests that the desilylated functional group has significant
carbonyl character, where the formal negative charge on the
phenoxide is effectively delocalized into the aromatic system.
The electron-donating character of the phenoxide sub-
stituent displays competing impacts on the ligand field strength
of the complex. The anionic charge is expected to increase the
σ-donor ability of the coordinated nitrogen atoms through
increased Lewis basicity. However, delocalization occurs
through the conjugated system, which simultaneously
decreases the π*-acceptor ability of the diimine. Since the
Hammett parameters combine inductive and resonance effects
into a single variable, it is not sufficiently sophisticated to
account for the σ- and π-electronic considerations of the
substituent groups.62
TD-DFT was used to probe the d-orbital splitting through
generation of low-energy electronic transitions of the model
compounds. The long-range corrected (LC) ωB97xd func-
tional was selected because of its treatment of charge transfer
interactions. Natural transition orbitals (NTOs)63 of largely d-
orbital character were obtained (Figure 8). These symmetry-
forbidden ligand field excitations are obscured experimentally
due to the broad and intense MLCT transitions of LS-Fe(II)
compounds, as reported in the Supporting Information. The
comparative energies exhibit a red-shift of 307 cm−1 for the t2g
→ eg* excitation (Figure 8, labels assume local octahedral
symmetry), which suggests the phenoxide substituent weakens
the ligand field strength through dominant π-effects. This
agrees with previous experimental results on Fe(II) ligand sets
with para-substituted pyridine groups.18,32,64,65
complexes66−68 through a CASSCF/NEVPT2 d-orbital active
space, which treats all the available ligand field excitations of
the metal center. The d-orbital energies obtained by AILFT
corroborate the TD-DFT results, with the magnitude of the d-
orbital splitting decreasing only slightly upon desilylation. The
ligand field splitting for [Fe(5-OTMSpy)3tren]2+ is 20360
cm−1 and decreases to 20 195 cm−1 for [Fe(5-Opy)3tren]−.
The AILFT excitation energies were parametrized with the
angular overlap model (AOM) to quantify the fluctuations in
σ- and π-bonding, eσ and eπ, due to substituent identity. The
differences in these values obtained at the NEVPT2 and
CASSCF levels of theory have been previously correlated with
the covalency of the metal−ligand interaction.69 The
eσ(NEVPT2-CASSCF) increases by 115 cm−1, while
eπ(NEVPT2-CASSCF) increases by 100 cm−1, weakening the
bonding character of the t2g orbitals through desilylation. Since
the σ- and π-effects are of comparable magnitude (for
reference, Δ = 3eσ − 4eπ, for complexes with Oh symmetry),
only a minute change is imparted on the overall ligand field
strength, despite the significant change in the electron-
donating character of the substituent group. In total, we find
no large dependence of substituent identity on the electronic
structure of the complex, regardless of the level-of-theory
applied.
We also considered the impact of the overall charge as well
as increased hydrogen bond-accepting ability of the phenoxide
substituent on the ligand field transitions. Additional TD-DFT
calculations were performed on [Fe(5-Opy)3tren]− where a
phenoxide group was docked with (a) solvent methanol
molecule in a hydrogen-bonding orientation, and (b) a sodium
cation in a close ion pair. Coordinates and TD-DFT results of
the structures are included in Tables S12−S13. In both
instances, the ligand field transition maintained a value of 462
nm for the iron-containing moiety [Fe(5-Opy)3tren]−. These
results suggest the differences of charge and hydrogen-bonding
ability do not impact ligand field strength.
We note that a strictly electronic analysis only accounts for
enthalpic contributions of ligand field strength and does not
consider entropic impacts of substituent identity. While the
electronic calculations suggest a preference for the HS state
upon desilylation, the in situ Evans’ method data show a
decrease in solution-phase χMT values upon addition of
fluoride. Although our calculations indicate that the silyl
ether group provides a stronger ligand field, it also has
vibrational degrees of freedom (3N-6, where N is the number
of atoms) which are not present with respect to the phenoxide
group. The stark contrast in the degrees of freedom of these
substituent groups may provide a simultaneous probe for
fluctuations in ΔS between the complexes.
To gain further insight into the character of the d−d
transitions, ab initio ligand field theory (AILFT) was used. This
has previously been successful in the treatment of Fe(II)
The trigonal-pocket formed by the presence of the
substituents at the 5-position of the tren-iminopyridine scaffold
F
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
offers close contacts between the -OTMS groups in the
optimized structure (Figure 9). These van der Waals contacts
discussion of solution stability of 2 and 4; Cartesian
coordinates and NTOs from output of DFT optimiza-
tions; ligand field splitting and AOM parameters from ab
initio output (PDF)
Accession Codes
CCDC 1816444 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: matthew.shores@colostate.edu.
■
Figure 9. [Fe(5-OTMSpy)3tren]2+ (left) and [Fe(5-Opy)3tren]−
(right) oriented along the C3-axis in which the 5-substituents are
modeled with respect to their van der Waals radii.
ORCID
Tarik J. Ozumerzifon: 0000-0001-7568-4679
Robert F. Higgins: 0000-0001-6776-3537
Justin P. Joyce: 0000-0002-8287-622X
Jacek L. Kolanowski: 0000-0002-6779-4736
Anthony K. Rappe: 0000-0002-5259-1186
Matthew P. Shores: 0000-0002-9751-0490
hinder the conformational freedom of the silyl ether groups;
such steric hindrance can be alleviated through the volume
expansion of the complex associated with the increased bond
lengths in the HS state. In addition, the silyl ether-containing
compound possesses more low-frequency vibrational modes
than the desilylated compound. This is anticipated to
significantly contribute to the overall entropy change upon
SCO. Overall, the loss of these vibrational modes and reduced
steric crowding upon desilylation is expected to shift the
equilibrium to the LS state, as is observed in the decreasing
magnetic susceptibility. We note that a favored HS state has
previously been attributed to the conformational disorder
associated with the n-alkyl substituents of analogous Fe(II)
complexes.9,70,71
́
Present Addresses
#(T.J.O.) Isola Group, Chandler, AZ 85224, United States.
∥(R.F.H.) P. Roy and Diana T. Vagelos Laboratories,
Department of Chemistry, University of Pennsylvania,
Philadelphia, Pennsylvania 19104, United States.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by Colorado State University and the
NSF (CHE-1363274 and CHE-1800554). J.L.K. acknowledges
funding from the Polish Ministry of Science and Higher
Education (the KNOW program). We thank Dr. Mihail
CONCLUSIONS
■
This study sheds light on the structural and electronic impacts
of postsynthetic modifications of Fe(II) complexes containing
a silylated ligand scaffold. Specifically, the feasibility of
performing a desilylation reaction to impart a spin-state
change on an Fe(II) center is probed. Our results indicate that
desilylation of 2 and 4 promotes a shift toward a lower overall
spin-state in both cases. For these complexes, the computa-
tional results suggest that changes in substituent identity do
not appreciably alter ligand field strength, implicating entropic
perturbations as the probable source for the observed spin-
state changes. These considerations suggest an alternative
approach to the design of SCO compounds, where release of
steric strain through substituent change may drive spin-state
switching. In the future, we envision that similar desilylation
processes applied on closely related Fe(II) analogs with slightly
weaker ligand fields may find utility in fluoride sensing
applications, as the LS species 2 has significantly stronger
absorption in the visible spectrum than the HS counterpart 4.
Atanasov of the Max Planck Institut fur Kohlenforschung for
̈
providing the AOMX software. We also thank the Central
Instrument Facility at Colorado State University for assistance
with mass spectrometry analysis.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00340.
■
S
Ligand synthesis; ESI-MS spectra and characterization;
electrochemical, NMR, UV−vis, and magnetic analyses;
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DOI: 10.1021/acs.inorgchem.9b00340
Inorg. Chem. XXXX, XXX, XXX−XXX