Spin-State Tuning at Pseudo-tetrahedral d6 Ions: Spin Crossover in [BP3]FeII-X Complexes

Article abstract of DOI:10.1021/acs.inorgchem.6b00066

Low-coordinate transition-metal complexes that undergo spin crossover remain rare. We report here a series of four-coordinate, pseudo-tetrahedral P₃Fe^II-X complexes supported by tris(phosphine)borate P₃ ([PhBP3_R]^-) and phosphiniminato X-type ligands (-N=PR′3) that, in combination, tune the spin-crossover behavior of the system. Most of the reported iron complexes undergo spin crossover at temperatures near or above room temperature in solution and in the solid state. The change in spin state coincides with a significant change in the degree of π-bonding between Fe and the bound N atom of the phosphiniminato ligand. Spin crossover is accompanied by striking changes in the ultraviolet-visible (UV-vis) absorption spectra, which allows for quantitative modeling of the thermodynamic parameters of the spin equilibria. These spin equilibria have also been studied by numerous techniques including paramagnetic nuclear magnetic resonance (NMR), infrared, and M?ssbauer spectroscopies; X-ray crystallography; and solid-state superconducting quantum interference device (SQUID) magnetometry. These studies allow qualitative correlations to be made between the steric and electronic properties of the ligand substituents and the enthalpy and entropy changes associated with the spin equilibria.

Full text of DOI:10.1021/acs.inorgchem.6b00066

Article
pubs.acs.org/IC
Spin-State Tuning at Pseudo-tetrahedral d6 Ions: Spin Crossover in
[BP3]FeII−X Complexes
Sidney E. Creutz and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: Low-coordinate transition-metal complexes that undergo spin crossover remain rare. We report here a series
of four-coordinate, pseudo-tetrahedral P3FeII−X complexes supported by tris(phosphine)borate P3 ([PhBP3R]−) and
phosphiniminato X-type ligands (−NPR′3) that, in combination, tune the spin-crossover behavior of the system. Most of
the reported iron complexes undergo spin crossover at temperatures near or above room temperature in solution and in the solid
state. The change in spin state coincides with a significant change in the degree of π-bonding between Fe and the bound N atom
of the phosphiniminato ligand. Spin crossover is accompanied by striking changes in the ultraviolet−visible (UV-vis) absorption
spectra, which allows for quantitative modeling of the thermodynamic parameters of the spin equilibria. These spin equilibria
have also been studied by numerous techniques including paramagnetic nuclear magnetic resonance (NMR), infrared, and
Mossbauer spectroscopies; X-ray crystallography; and solid-state superconducting quantum interference device (SQUID)
̈
magnetometry. These studies allow qualitative correlations to be made between the steric and electronic properties of the ligand
substituents and the enthalpy and entropy changes associated with the spin equilibria.
spin-crossover transition both in solution and in the solid state.8
INTRODUCTION
■
One advantage of studying molecular systems is that, since the
spin state of a transition-metal complex is dependent on the
balance between the ligand field stabilization energy and spin
pairing energy of the valence d-electrons, spin crossover can
serve as a sensitive reporter of the energetic landscape of the spin
states and valence orbital manifold of a metal complex. Subtle
changes in the primary coordination environment of a molecular
system affect these properties and thus the energetics of the
available spin manifold. Therefore, a molecular system that exhibits
well-defined and tunable spin crossover can provide a great deal of
information about the impact of small changes to the coordination
environment of a metal ion on its electronic structure.
To date, the vast majority of known spin-crossover complexes
are 6-coordinate, octahedral d6 iron(II) complexes, typically
with coordination spheres composed primarily of N, O, and C
donors, because of the often favorable balance of ligand field
strength and spin-pairing energy in these complexes.9,10
Spin-crossover phenomena in molecular systems are of interest,
in part due to their potential applications in magnetic sensing and
information storage.1 Spin-crossover complexes can act as a
type of molecular switch, where properties such as color and
magnetism undergo large changes when appropriate stimuli
for instance heat, pressure, lightare applied. Such responsive
behavior is desirable for materials applications, especially when
the complexes exhibit abrupt spin transitions and/or bist-
ability.2−5 Spin-crossover molecules which exhibit gradual and
nonhysteretic spin equilibria have also shown potential for
applications in sensing.6 More generally, spin crossover has been
implicated as an important factor in chemical processes that
occur at and are facilitated by transition-metal centers such as
those in metalloenzymes.7
Direct and predictable structure−function correlations
between magnetic properties and molecular structure that
would allow for the rational design and synthesis of spin-
crossover systems have yet to be fully realized. For this reason,
chemists continue to pursue a more thorough understanding
of the factors that govern the existence and properties of a
Received: January 10, 2016
© XXXX American Chemical Society
A
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Low-coordinate spin-crossover complexes are more rare.
For example, earlier work in our own laboratory on pseudo-
tetrahedral cobalt(II) complexes established the possibility
of spin crossover in pseudo-tetrahedral d7 ions. This work
capitalized on the electronic structure engendered by strong-field
tripodal “PhBP3” ligands to facilitate spin-state tuning.11−13
Five-coordinate spin-crossover complexes of iron have been
known since the 1970s,14 but the first pseudo-tetrahedral spin-
crossover complex of iron(II) was reported only in 2011,15 with
several other four-coordinate examples having been character-
ized since then.16,17 Most of these pseudo-tetrahedral iron(II)
spin-crossover complexes, and those that will be described
herein, feature electronic structure frameworks related to those
that had been established for the [PhBP3]CoII−X systems.11−13
In 2004, our laboratory reported the partial N atom transfer of
a terminal iron(IV) nitride, [PhBPi3Pr]Fe(N), to triphenylphos-
phine and triethylphosphine to afford iron(II) phosphiniminato
complexes of the type [PhBPi3Pr]Fe−NPR3 (Scheme 1A).18
phosphiniminato ligand that allows the spin-crossover system to
be easily and rationally tuned across a wide range of temperatures,
including near and above room temperature. This ease of
tunability may be of interest for future applications.
RESULTS
■
Synthesis of [PhBPR3]Fe(NPR′3) Complexes. Transition-
metal complexes of phosphiniminato (−NPR3) ligands are
common, especially for early transition metals such as titanium
and other Group 4−6 metals, and are most typically synthesized
either by reaction of an electrophilic metal nitride with a
phosphine, or via metathesis between Me3SiNPR3 and an
M−Cl species via loss of Me3SiCl.19 With one exception21 the
structurally characterized examples of phosphiniminato ligands
on Group 8 or 9 transition metals have generally been synthe-
sized by reaction of terminal nitride complexes with phosphines
(Scheme 1A).15,16,18,20 We sought a more general synthetic
pathway that would be applicable to systems where nitride
species are not readily accessible.
We have determined that salt metathesis between previously
reported [PhBPR3]FeCl precursors22 and LiNPR3′ reagents
affords the desired phosphiniminato complexes in good yields
(Scheme 1B). The lithiated phosphiniminatos are generated
in situ, first by double deprotonation of the phosphiniminium
chlorides, [H2NPR3′]Cl, with n-butyllithium, followed by
addition to the iron(II) chloride precursors at low temperature
in THF. This route was facilitated by the recent report of a
synthesis of [H2NPPh3]Cl by the sequential treatment of
triphenylphosphine with hexachloroethane and ammonia gas;
we have found that this synthetic route can be generalized to
other phosphines, including trialkylphosphines, under anhydrous
conditions.23 The [PhBP3iPr]Fe−NPPh3 (7) and [PhBP3iPr]Fe−
NPEt3 (9) complexes synthesized by this method show
identical solution spectroscopic properties to those previously
generated by reaction of the thermally unstable terminal nitride
[PhBPi3Pr]FeN with PPh3 or PEt3 (Scheme 1A).18
Scheme 1. Synthesis and Numbering Scheme for Complexes
1−9 by (A) Previously Reported Methods and (B) New
a
Methods
Structural Characterization. Complexes 1, 2, 4, 5, and 7−9
have been structurally characterized by X-ray crystallography
(Figure 1). Metrical parameters of interest are tabulated in
Table 1. The Fe−P distances are strongly correlated with the spin
state, lengthening by ∼0.3 Å between the low-spin and high-spin
forms, and the complexes can hence be readily divided into two
categories (high spin or low spin), based on their structural
parameters (pictorially represented in Figure 2). Indeed, the
change in Fe−P distance in response to the spin state makes such
assignments using the solid-state metrical parameters facile and
unambiguous, as was observed for the previously studied
[PhBP3]CoX system, where the Co−P distances are likewise
highly responsive to spin state.11−13 Accordingly, low-spin
complexes 1, 2, 4, and 5 display shorter Fe−P (avg ∼2.15 Å)
and Fe−N (∼1.75 Å) bond lengths, a longer N−P bond
(1.58 Å), and an almost linear Fe−N−P angle. In contrast,
the high-spin complexes 7, 8, and 9 feature long Fe−P bonds
(avg ∼2.45 Å), a longer Fe−N bond (1.85 Å), and a somewhat
shorter N−P bond (1.53 Å), along with a moderately bent
Fe−N−P bond angle (160°−165°). The changes in Fe−N and
N−P bond lengths are consistent with a higher degree of Fe−N
multiple bonding in the low-spin state, concomitant with a
shorter Fe−N bond. The high-spin species, by contrast, feature
stronger and shorter N−P double bonds and likely minimal
multiple bonding character between the Fe and N atoms.
The changes in metrical parameters observed among com-
plexes 1−9 can be compared to those observed by Smith et al.
a
Cy = cyclohexyl; m-ter = meta-terphenyl.
During the course of more recent N atom transfer studies, we
noted that several complexes of the general type [PhBPR3]Fe−
NPR3′ are involved in spin equilibria at room temperature.
These pseudo-tetrahedral d6 complexes benefit from the
electronic properties of the phosphiniminato ligand15,16 to
exhibit spin transitions between diamagnetic S = 0 states and
high-spin S = 2 states. Modifications to the ligand substituents,
both on the trisphosphine borate chelate and on the phos-
phiniminato moiety, allow for multifaceted tuning of the spin
states and crossover temperatures. They can range from
complexes that are high-spin at all temperatures to those with a
spin-crossover critical temperature (T1/2) as high as 405 K.
We introduce a versatile synthetic protocol for installing the
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Figure 1. Crystal structures of complexes 1, 2, 4, 5, 7, and 9. The structure of complex 8 is shown in the Supporting Information. Thermal ellipsoids
represented at 50% probability; solvents and hydrogen atoms are omitted for the sake of clarity.
a
Table 1. Metrical Parameters for the Solid-State Structures of 1, 2, 4, 5, and 7−9
b
b
1
2
4
5
7
8
9
R, R′
Ph, Ph
Ph, Cy
CH2Cy, Cy
2.1682(2)
m-ter, Ph
iPr, Ph
iPr, Cy
iPr, Et
Fe−P (avg, Å)
Fe−N (Å)
N−P (Å)
2.1621(11)
1.757(3)
1.576(3)
174.8(2)
2.1673(5)
1.7382(16)
1.5972(16)
174.99(11)
2.1560(7)
1.7360(18)
1.5866(19)
177.48(14)
2.4629(13)
1.859(3)
1.534(3)
159.8(2)
2.5085(10)
1.842(5)
1.559(6)
163.0(9)
2.4368(5)
1.8325(17)
1.5192(17)
165.30(14)
1.7446(17)
1.5815(17)
174.28(11)
Fe−N−P (deg)
a
b
All data was acquired at 100(2) K. Parameters averaged over two independent molecules in the asymmetric unit.
higher temperature. Notably, the high-spin states (including 7
and 8, which do not undergo spin crossover at any temperature
examined) display completely featureless absorption spectra in
the range from ∼475 nm to 900 nm, whereas the low-spin states
display three clear features in this region (Figure 3). The energies
of these absorptions vary in each complex (Table 2) but the
spectra show the most intense absorption at ∼590 nm with a
discernible shoulder at ∼550 nm, and a slightly weaker, well-
separated absorption at ∼700 nm. In some cases, and most
notably in complex 4 (Figure 3), the high-spin state displays a
discernible absorbance at ∼400 nm that is not present in the low-
spin state; this feature does not interfere with analysis of the
longer-wavelength regime. These spectral properties make it
possible to quantitatively model the respective spin equili-
bria using the absorbance intensities and fitting the data to a
Boltzmann equilibrium, as given in eq 1: xls(T) is the low-spin
Figure 2. (A) Visually exaggerated representation of the changes in
metrical parameters between structurally characterized low- and high-
spin [PhBPR3]Fe(NPR′3) complexes, with representative approximate
bond lengths. (B) Overlay of the representative core structures of 1 (low
spin, blue) and 7 (high spin, red).
for a series of tris(carbene)borate iron phosphiniminato
complexes ([PhB(MesIm)3]Fe(NPR3)). The tris(carbene)
complexes exhibit Fe−C bond lengths of ∼1.88 Å in low-spin
complexes versus 2.07 Å in the high-spin complexes, a somewhat
smaller range than for the Fe−P bonds of the tris(phosphine)-
borate complexes. The Fe−N bonds in the low-spin carbene
complexes are slightly longer (at ∼1.77 Å), compared to the low-
spin phosphine complexes, and the range of Fe−N bond lengths
between high- and low-spin complexes is again slightly smaller
for the carbene ligands. The trends displayed in these two series
of complexes are qualitatively similar.15,16
mole fraction as a function of the temperature T. Values of T1/2
,
ΔH, and ΔS can be extracted from the UV-vis data (see Figure 4
and Table 3). Further details can be found in the Supporting
Information.
1
xls(T) =
⎡
⎤
1 + exp −ΔH °
−
T
1/2
1
T
1
⎢
R
⎣
⎥
)
⎦
(
(1)
Ultraviolet−Visible (UV-vis) Spectroscopy. Spin cross-
over in complexes 1−6 and 9 is accompanied by a striking color
change from deep blue or purple in the low-spin form, populated
at low temperature, to a pale yellow in the high-spin form at
While compounds 7 and 8 ([PhBP3iPr]Fe(NPPh3) and
[PhBPi3Pr]Fe(NPCy3), respectively) do not undergo observable
spin crossover in the temperature range studied, the less sterically
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Figure 3. Temperature-dependent UV-vis spectra of complexes 1−4 in toluene. Arrows represent the direction of change when the temperature is
lowered. Spectra for complexes 5, 6, and 9 can be found in the Supporting Information.
Table 2. Absorption Maxima for Three Ligand-Field Transitions in the Low-Spin Forms of 1−6 and 9 in Toluene Solution
1
2
3
4
5
6
9
R, R′
λmax (188 K, nm)
Ph, Ph
563
Ph, Cy
549
CH2Cy, Ph
552
CH2Cy, Cy
534
m-ter, Ph
557
m-ter, Cy
547
iPr, Et
558
600
734
b
603
596
590
579
604
598
703
698
705
690
713
707
a
ε (M−1 cm−1
)
1700
1400
1100
560
2700
940
a
Extinction coefficients are provided for the lowest energy transition (∼700 nm) of each complex at the lowest temperature measured in each case.
ε was not calculated for 9 because of highly incomplete spin crossover at accessible temperatures.
b
Figure 4. Fractional occupation of the low-spin state modeled according to a spin equilibrium showing Boltzmann behavior, based on UV-vis intensity
data for the compounds 1−6. Dotted lines are fits to eq 1.
hindered triethyl phosphiniminato complex 9 begins to popu-
concomitantly acquire a greenish tinge when cooled below
late a low-spin state at very low temperatures; such solutions
approximately −50 °C. Low-temperature UV-vis data confirms
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Table 3. Thermodynamic Parameters for the Spin Equilibria in Compounds 1−6 Derived from Variable-Temperature UV-vis
a
Measurements in Toluene Solution Based on Fits to eq 1
1
2
3
4
5
6
R, R′
ΔH° (kJ/mol)
Ph, Ph
Ph, Cy
CH2Cy, Ph
36.3 0.5
CH2Cy, Cy
38.4 1.3
m-ter, Ph
m-ter, Cy
32.9 0.5
39.5 0.8
30.3 0.9
38.3 2.5
ΔS° (J/K/mol)
99
1
112
2
139
2
132
5
89
2
95
7
T1/2 (K)
333 0.4
353 0.4
261 0.4
291 0.8
343 0.8
405
5
a
The errors given represent 95% confidence bounds for the fits.
Table 4. Thermodynamic Parameters for the Spin Equilibria in Compounds 1−5 Derived from Variable-Temperature NMR
a
Chemical Shift Measurements in d8-Toluene
1
2
3
4
5
R, R′
ΔH° (kJ/mol)
ΔS° (J/(K/mol))
Ph, Ph
Ph, Cy
CH2Cy, Ph
38.2 1.3
147 0.6
259 0.5
CH2Cy, Cy
39.8 4.6
139 17
286 1.4
m-ter, Ph
31.8 1.3
35.6 1.3
27.4 0.5
96
4
99
4
79
1
T1/2 (K)
331 0.5
361 1.0
349 1.0
a
Thermodynamic parameters are derived from the average of the fitted parameters from at least two different resonances in the NMR spectra (see
text and the Supporting Information for additional details). Uncertainties are derived from the 95% confidence bounds of the fits.
that a species with spectral parameters similar to the other
low-spin complexes grows in at very low temperature in toluene
solution.24 T1/2 for this complex is well below that observed for
complexes 1−6.
NMR Characterization and Solution Magnetic Suscept-
ibility Measurements. All of the complexes studied display
paramagnetically shifted and broadened NMR spectra at room
temperature, consistent with at least partial occupation of a
paramagnetic state. As the temperature is reduced, for complexes
1−6 and 9, the 1H NMR chemical shifts approach the expected
region for diamagnetic complexes (∼0−9 ppm), and, in most
cases, broad 31P NMR peaks become discernible. This behavior
is consistent with spin crossover to a diamagnetic state at low
temperature. For a paramagnetic complex showing Curie
behavior, the chemical shift range is expected to expand as the
temperature is reduced, because of the inverse relationship
between temperature and magnetization. Deviations from the
expected Curie behavior for the chemical shifts can be quan-
titatively accounted for by the spin equilibrium and modeled to
extract the thermodynamic parameters of the Boltzmann
equilibrium, as given in eq 2,25
C
δ = δls +
⎧
⎫
⎬
⎭
⎡
⎤
ΔH °
R
1
T
1
⎨
T 1 + exp −
−
T
1/2
⎢
⎥
⎦
(
)
⎣
⎩
(2)
where δ is the measured chemical shift, δls the corresponding shift
in the diamagnetic state, and C the appropriate Curie’s law
constant. This method has been used to model the solution-state
spin-crossover properties of other spin-crossover complexes in
previous work.16,26 The values extracted from these fits are
given in Table 4. A representative example of the temperature-
dependent chemical shifts of 3, and the resulting fits to eq 2
are shown in Figure 5; corresponding data for complexes 1, 2,
4, and 5 can be found in the Supporting Information. For
complexes 6 and 9, although variable-temperature NMR did
show evidence of spin crossover through changes in the chemical
shifts and the variation in the solution magnetic moment (see
below, Figure 6), the temperature range accessible was inadequate
to allow for reliable fitting and extraction of thermodynamic
parameters.
Figure 5. (A) Variable-temperature 1H NMR spectra of 3 in d8-toluene
(0.044 M) from 188 K (bottom) to 368 K (top), in 5 K increments from
188 to 298 and 10 K increments above 298 K. (B) Change in the NMR
chemical shift for several resonances from the 1H NMR spectra of 3 with
temperature from 188 K to 368 K (red circles). Low-temperature data
are omitted when the peak becomes too broad to be discerned clearly.
Data were fit to a Curie Law/Boltzmann equilibrium expression (eq 2)
to model spin crossover (blue lines). Data above 300 K were omitted
from the fits (see the Supporting Information).
trends in ΔH° and ΔS° extracted from the UV-vis data.
Quantitatively, the parameters are also in good agreement; the
ΔH° values derived from the two methods agree within 10% in
all cases and within 5% in most cases; the ΔS° values agree within
12% in all cases and within 6% in most. The largest deviation is
The thermodynamic parameters derived from the fits to the
chemical shifts, according to eq 2, qualitatively reproduce the
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Figure 6. Variable-temperature magnetic susceptibility data, measured
using the Evans method in d8-toluene, for compounds 1, 2, 3, 5, and 9.
Susceptibility values have been corrected for diamagnetic contributions
using Pascal’s constants.
Figure 7. Solid-state magnetic moments of compounds 1−4, 7, and 8
measured by SQUID magnetometry. All samples were neat micro-
crystalline solids, and the magnetic susceptibility was corrected for the
approximate diamagnetic contribution derived from Pascal’s constants.
Measurements were carried out from low temperature to high
temperature, following initial cooling in zero field.
observed for complexes 2 and 5. These complexes also exhibit
the highest T1/2 of the compounds shown in Table 4 and thus
undergo the most incomplete conversion to the high-spin form
under the conditions studied.
The solution paramagnetism of these complexes was further
examined by variable-temperature Evans method measurements,
which more directly probed the change in the paramagnetic
susceptibility and effective magnetic moment with tempera-
ture.27 The variable-temperature Evans method results qual-
itatively confirm the change in spin state and the corresponding
change in susceptibility of complexes 1−6 and 9 with tem-
perature; representative data is shown in Figure 6. While the
data could be fit to a Boltzmann-equilibrium expression, the fit
parameters suffered from large uncertainties because of the
relatively small range of temperatures for which χT could be
reliably measured using this method (see the Supporting
Information for further details).
Solid-State (SQUID) Magnetometry. Spin-crossover
molecules frequently show different behavior in the solid state
than in solution. Often, the spin-state change in the solid state is
no longer well described as a simple thermodynamic equilibrium
and, instead, becomes dependent on cooperative interactions in
the solid state, formation of domains, crystallinity, and other
solid-state effects, which can cause either very abrupt or very
gradual and incomplete spin crossover.28 These factors some-
times lead to thermal hysteresis of the magnetic response,
depending on the direction of temperature change. Although
neither hysteresis nor a very abrupt spin crossover were observed
for any of the complexes studied herein, they did show more-
complex behavior in the solid state than in solution.
The magnetic moments of solid samples of complexes 1−4, 7,
and 8 were measured using SQUID magnetometry in the
temperature range from 4 K to 400 K (see Figure 7). Compounds
7 and 8 do not appear to undergo a spin transition at any
temperature recorded. For compounds 1−4, spin crossover is
gradual and incomplete; both the degree and rate of crossover
proved to be extremely dependent on sample preparation and
particularly on the degree of crystallinity, which is a phenomenon
that is frequently observed in the solid-state magnetic behavior of
mononuclear spin-crossover complexes and often rationalized
on the basis of domain formation and grain-size effects.28,29 For
instance, grinding a microcrystalline sample with a mortar and
pestle typically resulted in a more-incomplete spin crossover
(Figure 8). In the most striking example of these effects, a yellow,
crystalline sample of 3 exhibited no spin crossover, maintaining a
Figure 8. Variable-temperature solid-state SQUID magnetometry
showing the effect of sample preparation on the measured properties
of compounds 3 and 4. Measurements were carried out from low
temperature to high temperature, following initial cooling in zero field.
magnetic moment near 5.4 μB at least down to 20 K, despite the
fact that, in solution, this species has a T1/2 value of 255 K.
However, if the sample is lyophilized from benzene instead
of crystallized, it takes on a greenish-blue color and a gradual and
incomplete spin crossover is observed (see Figure 8).
Many of the complexes display irreversible changes in their
magnetic behavior after heating above 300 K, which is a phe-
nomenon that is attributed to a loss of co-crystallized solvent
upon heating under vacuum in the magnetometer; except for
compounds 7−9, all of the crystallographically characterized
compounds contain co-crystallized solvent, which is not lost
upon drying the crystals at room temperature under vacuum (see
the Supporting Information for details and illustrative data).
However, the change in the behavior of compound 3 upon
lyophilization is not due to solvent loss, as neither crystalline nor
lyophilized 3 includes co-crystallized solvent molecules (as
determined by NMR). Other than the effects attributed to
solvent loss upon heating, no change in the magnetic behavior is
observed upon repeatedly cycling the temperature. The fact that
the crystal structures of most of these compounds were refined
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with static disorder in both complex and solvent moieties (see
Supporting Information for details) complicates the analysis of
intermolecular interactions and their impact on the solid-state
magnetic properties.
Solid-State Infrared Spectroscopy. Phosphiniminato
ligands typically show a strong ν(NP) vibration in their
infrared spectra near 1200 cm−1; this is observed for complexes 3,
7, and 9 (Figure 9). However, no strong vibration in this region is
Figure 10. Representative 80 K Mossbauer spectra and parameters for
̈
high-spin 7 (crystalline, panel (A)) and 3 (crystalline, panel (B)); low-
spin 1 (crystalline, panel (C)) and 4 (Me-THF glass, panel (D)); and
lyophilized 3, exhibiting a mixture of high- and low-spin populations
(panel (E)).31
Table 5. Mossbauer Parameters for Complexes 1, 3, 4, and 7
̈
Figure 9. Representative solid-state infrared transmission spectra of
complexes 7, 9, 3, 5, and 6, illustrating the presence of an ν(NP)
vibrational band at ∼1207 cm−1 in the high-spin complexes (7, 9, 3) that
is absent in low-spin complexes 5 and 6. Spectra were recorded at room
temperature.
isomer shift, δ quadrupole splitting, composition
(mm/s)
ΔEQ (mm/s)
(%)
1 (crystalline)
4 (2-MeTHF glass)
7 (crystalline)
3 (crystalline)
3 (lyophilized)
0.008
−0.025
0.617
0.559
0.392
1.373
1.449
0.603
1.252
0.820
present in the low-spin states of the respective [PhBPR3]Fe-
(NPR3′) complexes, as is evident in the room-temperature solid-
state IR spectra of complexes 1, 2, and 4−6 (see Figure 9 for 5
and 6). This is consistent with weakening of the NP bond due
to increased π-bonding between N and Fe in the low-spin state
(vide infra). This data corroborates the important role of the
electronic flexibility of the phosphiniminato ligand in facilitating
access to both low- and high-spin states across this series of
pseudo-tetrahedral iron(II) complexes.
0.000
51
49
0.607
in these complexes is qualitatively similar to that of an octahedral
complex13 with three low-lying, primarily nonbonding orbitals and
an antibonding, doubly degenerate orbital set at higher energy.
This analogy is noteworthy, given that the overwhelming
majority of spin-crossover compounds are octahedral. This orbital
̈
2
Mossbauer Spectroscopy. High- and low-spin states of
schemean approximate 2-over-3 splitting with a low-lying dz
an iron complex typically show distinct Mossbauer parameters
̈
orbitalis well-established both theoretically and experimentally
(isomer shift and quadrupole splitting),30 providing another
convenient method of characterization of the iron compounds
for pseudo-tetrahedral Fe and Co complexes with [PhBPR3]−
2
ligands. It can be attributed, in part, to the mixing of dz with the
presented in this study. Mossbauer spectra were hence collected
̈
metal-based 4pz and 4s orbitals, and also to the 90° P−Fe−P bond
angles and, correspondingly, >120° P−Fe−N angles, favored by
the ligand chelate; these angles lower the relative energy of the a1
for examples of both predominantly low-spin (1 as a micro-
crystalline solid and 4 as a glassed solution in 2-MeTHF)
and high-spin (microcrystalline 7 and 3) complexes at 80 K, as
well as for one example (lyophilized 3) for which both spin forms
can be distinguished across a series of temperatures (Figure 10;
additional variable-temperature data is in the Supporting
Information). The relevant parameters are provided in Table 5.
The high-spin species show a higher isomer shift and a larger
quadrupole splitting relative to the low-spin species.
11,32,33
2
orbital of primarily dz parentage.
For the Fe(II) complexes of present interest, the lowest
unoccupied molecular orbital (LUMO) in the low-spin state is an
approximately degenerate set of orbitals with dxz/dyz character
and π* symmetry, with respect to the apical ligand. Spin cross-
over to the S = 2 state involves population of this level with two
electrons. Therefore, the thermodynamic parameters (ΔH°) of
the spin equilibrium are expected to be strongly dependent on
the energy of this level.
The qualitative orbital picture (Figure 11) is supported by
DFT calculations34 carried out using the crystallographically
determined structures of complexes 4 and 8 in their low- and
high-spin forms, respectively (Figure 12). In 4 the high-lying
DISCUSSION
■
A sketch of the d-orbital manifold (Figure 11) for Fe(II) in
[PhBPR3]Fe(NPR′3) in the high-spin and low-spin states provides
some context for interpreting the spin-crossover behavior and
trends delineated in the Results section. The ligand field splitting
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2
2
the near-degenerate nonbonding orbitals of dxy/dx −y parentage
(HOMO−1 and HOMO−2).
Based on the variation of the spin-crossover equilibrium
parameters among compounds 1−9, some trends can be gleaned
(Figure 13). There is a strong correlation between the magnitude
Figure 11. (A) Schematic of the d-orbital manifold for the low- and
high-spin states of pseudo-tetrahedral [PhBPR3]Fe(NPR3′) complexes.
(B) Limiting representations of the bonding interactions in the low-spin
(a) and high-spin (b) states.
Figure 13. Comparison of ΔH°, ΔS°, and T1/2 for [PhBPR3]Fe(NPR3′)
complexes 1−6. Average values from Tables 3 and 4 are plotted.
of ΔH° and the identity of the phosphiniminato substituent
R′ (Ph or Cy); among the [PhBPR ]Fe(NPR′3) complexes
3
that undergo spin-crossover, for a given R (−CH2Cy, Ph, or
m-terphenyl), the complex where R′ is the more electron-donating
Cy has a larger ΔH° and undergoes spin-crossover at a higher
temperature than the complex with R′ = Ph, although, in the case
of 3 and 4, the difference in ΔH° is within our margin of error.
This observation can be rationalized; the more electron-donating
phosphine (R′ = Cy) engenders better π-donation from N to Fe,
raising the energy of the π*-orbitals to favor the low-spin state.
Consistent with this idea, the metrical parameters (e.g., N−Fe and
N−P bond lengths; Table 1) suggest a stronger bond between Fe
and N in the low-spin state, and a compensatory weakening of the
multiple bonding between N and P; the presence of two stronger
π-bonding interactions is also consistent with the near-linear
Fe−N−P angles of the low-spin complexes. This can be
represented as a more important contribution from an electronic
structure picture involving Fe−N multiple bonding in the low-
spin state (Figure 11B, structure a). This electronic structure is
reminiscent of the previously characterized anionic Fe(II) imido
complex, {[PhBPP3h]Fe(NR)}−.33
The influence of the [PhBPR3]− substituent R on the spin
equilibrium parameters is less straightforward to interpret. It
seems evident by comparison of 3 and 4 (R = CH2Cy) versus the
electronically similar 7 and 8 (R = iPr), that steric factors play
a large role. It is difficult to rationalize the difference between
these pairs of complexes purely on the basis of electronic con-
siderations; 3 and 4 have T1/2 values of 266 and 290 K, while 7
and 8 are high spin at all temperatures. As observed in these
complexes, increased steric crowding is expected to favor the
Figure 12. Valence molecular orbitals calculated for (A) low-spin
[PhBP3CH Cy]Fe(NPCy3) (4) and (B) high-spin [PhBP3iPr]Fe(NPCy3) (8).
2
π-symmetry d-orbitals are mixed with P−N π-bonding orbitals
and are unoccupied. In 8, the related orbitals in the α manifold
are occupied, consistent with the observed shorter P−N bond
due to a higher P−N bond order. The HOMO of low-spin 4 is an
2
a1 orbital of dz parentage that lies slightly higher in energy than
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high-spin state, because of the much longer Fe−P bond lengths,
resulting in a less-strained environment around the metal center.
The fact that 9 ([PhBPi3Pr]Fe(NPEt3)) undergoes spin crossover,
albeit at a relatively low temperature, further corroborates the
role of steric crowding. Steric effects were also previously hypo-
thesized to play a role in the greater propensity of [PhBPi3Pr]CoX
complexes to occupy high-spin states (versus the corresponding
[PhBPP3h]CoX complexes); the data presented here bolster this
hypothesis.13
Time-dependent density functional theory (TD-DFT) calcu-
lations on complex 4 support this model (see Supporting
Information for details). The lack of similar bands in the high-
spin forms of the complexes likely results from the fact that the
d−d transitions in the high-spin molecules are shifted farther into
the near-infrared because of the smaller ligand field splitting.
Qualitative crystal field considerations in idealized C3v
symmetrywhich approximates well the local geometry around
the iron centers in the low-spin complexesallow a plausible
assignment of these three optical bands (see Figure 14). Considering
There is a strong trend in the ΔS° values for the different
[PhBPR3]− ligands. For a given R, the pairs of complexes with
R′ = Ph or Cy have similar ΔS° values, and among the three
ligands (R = m-terphenyl, Ph, or −CH2Cy), ΔS° is smallest for
the more-rigid R = m-terphenyl (∼90 J/K/mol) and largest for
the less rigid R = −CH2Cy (∼144 J/K/mol). This trend can be
qualitatively rationalized; the less sterically crowded environ-
ment of the high-spin state allows the more “floppy” substituents
(−CH2Cy) to move more freely, thus contributing substantially
more to the vibrational entropy of the molecule. In contrast, the
entropic contributions of the m-terphenyl-substituted ligand do
not change as much due to the inherent rigidity of the aryl groups
and the steric demands of the bulky terphenyls, which likely
constrain the molecular geometry, even in the high-spin state.
Electronic considerations with respect to R likely also con-
tribute to ΔH°, but the trend is less clear. Among complexes 1, 3,
and 5 (with R′ = Ph), ΔH° increases significantly, going from
R = m-ter < Ph < CH2Cy. One likely rationale is that the more
strong-field alkyl phosphine donors in the [PhBPR3]− ligand help
raise the energy of the e(b) orbital set, which is σ* antibonding,
with respect to the phosphines. The R′ = Cy complexes (2, 4,
and 6) do not follow quite the same trend, but the absolute
differences between the ΔHo values for this series are smaller and
may be within the error of our measurements.
One aspect of our analysis that should be underscored is that it
is only upon distilling the equilibrium parameters into ΔS° and
ΔH° that the trends imparted by the ligand substituents become
clear (Figure 13); although these parameters are correlatedfor
instance, both are affected by changes in bond lengthstheir
independent consideration can reveal new information. Studies
of spin equilibria often attempt to extract trends by considering
only T1/2, which is a parameter that convolutes both entropic and
enthalpic contributions. Such an analysis could, in some cases,
lead to misinterpretations or make it difficult to identify robust
trends concerning the effect of the primary coordination envi-
ronment on the spin equilibria. While it is sometimes more
challenging to determine reliable values for ΔS° and ΔH°, the
present results emphasize that it is worthwhile to do so when
possible.
Further analysis of the UV-vis spectra of the low-spin forms of
complexes 1−6 and 9 can provide additional insight into the
respective ligand fields of these molecules. In all cases, the spectra
display three bands of appreciable intensity in the visible range: a
peak at ∼600 nm with a high-energy shoulder at ∼550 nm, and a
second peak at ∼700 nm. The exact positions of these peaks, as
derived from spectral decomposition, have been tabulated above
in Table 1. The energies and intensities of these bands, as well as
the fact that they are present in all complexes at similar energies,
regardless of the identities of R and R′, corroborates their
assignment as primarily ligand field d-d transitions (which are
Laporte-allowed in C3v symmetry, with strengthened inten-
sity by d-p mixing induced by strong covalency) rather than
charge-transfer transitions to the ligand backbone. Significant
mixing, especially with the P−N π-orbitals, also contributes.
Figure 14. Proposed assignments of UV-vis transitions in low-spin
phosphiniminato complexes: (top) qualitative orbital diagrams and
electron configurations of ground state and singlet one-electron excited
states; (bottom) absorption spectra of 1 at 188 K with proposed
assignments labeled.
1
only spin-allowed transitions to singlet excited states, the A1
low-spin ground state (e(a))4(a1)2(e(b))0 can undergo one-electron
excitations to either a (e(a))4(a1)1(e(b))1 or a (e(a))3(a1)2(e(b))1
configuration. The former is a 1E state, while the latter has 1E, 1A1,
and 1A2 components. In C3v symmetry, 1A1 → 1E and 1A1 → 1A1
optical transitions are orbital-symmetry allowed, while the 1A1 →
1A2 transition is forbidden. Therefore, three ligand-field transi-
tions are expected for this state, consistent with the observed
spectra. Assigning these reliably is nontrivial; we tentatively assign
the lower-energy, well-separated band at ∼700 nm to the
1
(e(a))4(a1)2(e(b))0 → (e(a))4(a1)1(e(b))1 transition (1A1 → E),
and the two higher-energy, closely spaced bands to the allowed
(e(a))4(a1)2(e(b))0 → (e(a))3(a1)2(e(b))1 transitions (1A1 → 1E and
1A1 → 1A1). In the lowered symmetry (C1) that more rigorously
describes the solid-state structure of these complexes, a quali-
tatively similar assignment can be proposed; the 700 nm band
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measurements were taken in toluene solution using a Cary 50 UV-vis
spectrophotometer using a 1 cm two-window quartz cell, with a Unisoku
CoolSpek cryostat for temperature control. Thin-film infrared (IR)
spectra were obtained on a Bruker Alpha spectrometer equipped with a
diamond ATR probe. Solid-state magnetic data was obtained using a
Quantum Designs SQUID magnetometer running MPMSR2 software
(Magnetic Property Measurement System Revision 2) at a field strength
of 5000 G. Samples were inserted into the magnetometer in plastic
straws sealed under nitrogen with polycarbonate capsules. Loaded
samples were centered within the magnetometer using the DC centering
scan at 35 K and 500 gauss. The magnetic susceptibility was adjusted for
diamagnetic contributions using the constitutive corrections of Pascal’s
constants as well as a diamagnetic correction due to the holder dia-
magnetism. Microcrystalline samples for SQUID magnetometry were
prepared as detailed in the synthetic methods below and dried in vacuo at
room temperature before being loaded into the sample capsules.
likely arises from transitions between the valence orbital of dz2
parentage to the unoccupied, dxz/dyz-derived π-symmetry orbitals,
while the higher-energy bands derive from transitions from the
near-degenerate dxy and dx −y nonbonding orbitals. Notably, d6
metallocenes such as ferrocene, which have qualitatively similar
valence d-orbital electronic structures, similarly show three
symmetry-allowed optical d−d transitions, although the precise
assignment of these differs, because of the different point-group
symmetry.35
2
2
Therefore, the energies of the visible transitions can provide
some information about the ligand field splitting of the d-orbitals
in the low-spin complexes. Again, the clearest trend is in the
dependence of the transition energies on the identity of the
phosphiniminato substituent. In all cases, for a given R, all three
absorptions are lower in energy for R′ = Ph than R′ = Cy, which is
consistent with the conclusions discussed above; the e(b) π*
orbital is raised higher in energy in the case of the more strongly
electron-donating and π-bonding alkyl phosphiniminato. Inter-
estingly, for complex 9, the transition energies are substantially
lower than for the other alkyl phosphiniminato complexes; this
may suggest that, although this complex is able to populate a
low-spin state, steric strain still enforces longer bond lengths and,
therefore, a weaker ligand field than in complexes 2, 4, and 6.
The correlations and trends discussed above suggest that, in
these spin-crossover iron(II) complexes, both the specific pro-
perties of the [PhBPR3] ligands, enforcing a pseudo-tetrahedral
geometry and a corresponding “pseudo-octahedral” orbital
arrangement, and the ability of the phosphiniminato ligand to
modulate its π-donation between two possible limiting elec-
tronic structures, are important to the spin-equilibrium behavior
observed. Tuning the electron-donating ability of the phosphi-
niminato phosphine is a straightforward way to strongly alter the
Mossbauer spectra were recorded on a spectrometer from SEE Co
̈
(Edina, MN) operating in the constant acceleration mode in a trans-
mission geometry. Spectra were recorded with the temperature of the
sample maintained at 80 K, except as otherwise noted. The sample was
kept in a Model SVT-400 dewar (Janis, Wilmington, MA) at zero field.
The quoted isomer shifts are relative to the centroid of the spectrum of a
metallic foil of α-Fe at room temperature. Solid samples were prepared by
mounting in a cup fitted with a screw cap as a boron nitride pellet. Data
analysis was performed using the program WMOSS (www.wmoss.org),
and quadrupole doublets were fit to Lorentzian lineshapes.
Computations. Single-point DFT energy calculations on 4 (S = 0)
and 8 (S = 2) were calculated using the Gaussian 09 software package37
and the hybrid B3LYP functional. The 6-311G(df) basis set was used
for the Fe and P atoms, and the 6-31G(d) basis set was used on the
remaining atoms.
X-ray Crystallography. XRD studies were carried out at the
Beckman Institute Crystallography Facility on a Bruker Kappa Apex II
diffractometer or Bruker Photon 100 diffractometer (Mo Kα radiation).
Structures were solved using SHELXS and refined against F2 on all data
by full-matrix least-squares with SHELXL.38 The crystals were mounted
on a wire loop. All crystals were measured at a temperature of 100 K.
Methyl group H atoms not involved in disorder were placed at calcu-
lated positions starting from the point of maximum electron density. All
other H atoms were placed at geometrically calculated positions and
refined using a riding model. The isotropic displacement parameters of
the H atoms were fixed at 1.2 (1.5 for methyl groups) times the Ueq value
of the atoms to which they are bonded. 1,2- and 1,3-rigid bond restraints
were applied to all non-hydrogen atoms. Further details for each
structure can be found in the Supporting Information.
T
1/2 and thermodynamic properties of the complexes, which can
be further fine-tuned by modifying the steric and electronic
properties of the tris(phosphine)borate ligand. Further modi-
fication of such complexes along these two independent axes
could give rise to molecules with more desirable properties for
magnetic memory applications, such as strongly cooperative or
hysteretic spin crossover, or light-induced spin-state trapping
(LIESST).36
Tricyclohexylphosphiniminium Chloride. Tricyclohexylphos-
phine (1.00 g, 3.57 mmol) and hexachloroethane (844 mg, 3.57 mmol)
were combined in 80 mL of THF and stirred for 2 h at room temperature,
during which time a white precipitate develops. The suspension was then
cooled to −20 °C and anhydrous gaseous ammonia was bubbled through
the solution. The reaction mixture was allowed to warm to room
temperature overnight under NH3 and then concentrated to dryness. The
white residue was taken up in 500 mL of dry dichloromethane, filtered
through Celite, and concentrated to dryness. The residue was redissolved
in minimal MeOH, then 200 mL of Et2O were added and the mixture
stored in the freezer (−40 °C) for 2 h. The resulting white crystalline
solids were collected on a sintered glass frit and washed with Et2O, giving
977 mg of the desired product as a white solid (83%). 31P{1H} NMR
(CDCl3, 162 MHz, 25 °C): δ 53.4 (s) ppm. 1H NMR (CDCl3, 400 MHz,
25 °C): δ 5.70 (br s, 2H, NH2), 2.33 (q, J = 12 Hz, 3H), 2.01 (d, J = 12 Hz,
6H), 1.86 (d, J = 12 Hz, 6H), 1.72 (d, J = 12 Hz, 3H), 1.56 (q, J = 12 Hz,
6H), 1.37−1.22 (m, 9H) ppm. 13C NMR (CDCl3, 101 MHz, 25 °C) δ
32.2 (d, J(P) = 54 Hz), 26.4 (d, J(P) = 13 Hz), 26.0 (d, J(P) = 3 Hz), 25.6
(s) ppm. Anal. Calcd for C18H35NPCl: C, 65.14; H, 10.63; N, 4.22.
Found: C, 65.38; H, 11.05; N, 4.14.
EXPERIMENTAL DETAILS
■
General Considerations. All syntheses and measurements, unless
otherwise stated, were carried out under an inert atmosphere (N2) in a
glovebox or using standard Schlenk techniques, and solvents were dried
and degassed by thoroughly sparging with N2 and then passing through
an activated alumina column in a solvent purification system supplied by
SG Water, LLC. Nonhalogenated solvents were tested with a standard
purple solution of sodium benzophenone ketyl in tetrahydrofuran, to
confirm effective moisture removal. [PhBPP3h]FeCl,22a [PhBP3m‑ter]FeCl,22d
[PhBP3iPr]FeCl,22b [PhBP3CH Cy]FeCl,22c and [Ph3PNH2]Cl23 were
2
prepared according to literature procedures. All other reagents were
purchased from commercial vendors and used without further puri-
fication unless otherwise stated.
Physical Methods. Elemental analyses were performed by Midwest
Microlab (Indianapolis, IN). Deuterated solvents were purchased from
Cambridge Isotope Laboratories, Inc., degassed, and dried over active
3 Å molecular sieves prior to use. 1H and 13C chemical shifts are reported
in ppm, relative to tetramethylsilane, using residual proton and 13C
resonances from solvent as internal standards. 31P chemical shifts
are reported in ppm, relative to 85% aqueous H3PO4. Solution-phase
magnetic measurements were performed by the method of Evans in
d8-toluene solution.27 The NMR spectrometer temperature was
calibrated using 100% methanol (from 25 °C to −85 °C) or 100%
ethylene glycol (from 25 °C to 105 °C). Optical spectroscopy
Triethylphosphiniminium Chloride. Triethylphosphine (0.500 g,
4.23 mmol) was dissolved in THF and hexachloroethane (1.00 g,
4.22 mmol) was added dropwise as a solution in THF. White precipitate
formed immediately, and the reaction was stirred for 2 h at room
temperature and then cooled to −20 °C. Anhydrous gaseous ammonia
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was bubbled through the solution, and the reaction mixture was allowed
to warm to room temperature and stir overnight under NH3. The
mixture was then concentrated to dryness and the white residue was
extracted with dichloromethane, filtered through Celite, and concen-
trated to a volume of 3 mL. This concentrated solution was layered with
10 mL of Et2O and stored at −40 °C for 2 h. The resulting white crystals
were isolated atop a sintered glass frit and washed with Et2O. The
desired product was obtained as 683 mg of a white solid (95%). 31P{1H}
NMR (CDCl3, 162 MHz, 25 °C): δ 59.8 (s) ppm. 1H NMR (CDCl3,
400 MHz, 25 °C): δ 5.65 (br s, 2H, NH2), 2.18 (dq, J = 8, 15 Hz, 6H),
1.22 (dt, 8, 18 Hz, 9H) ppm. 13C NMR (CDCl3, 101 MHz, 25 °C) δ 15.9
(d, J(P) = 61 Hz), 5.4 (d, J(P) = 5 Hz) ppm. Anal. Calcd for
C6H17ClNP: C, 42.48; H, 10.10; N, 8.26. Found: C, 42.41; H, 9.94; N,
8.08.
18.1, 8.9, −0.5, −1.4, −3.1, −6.5, −7.8, −13.7, −20.8, −21.8, −29.9,
−40.6 ppm. 31P{1H} NMR (C6D6, 202 MHz, −75 °C): δ 77.3 (br s, 3P),
41.7 (br s, 1P) ppm. Anal. Calcd for C69H104BFeNP4: C, 72.82; H, 9.21;
N, 1.23. Found: C, 72.62; H, 9.27; N, 1.22.
[PhBPC3H Cy]Fe(NPCy3) (4). [H2NPCy3][Cl] (86 mg, 0.259 mmol)
2
was suspended in THF (5 mL) and cooled to −78 °C with stirring.
n-Butyllithium (323 μL, 1.6 M in hexane, 0.518 mmol) was added to the
solution, which was stirred at low temperature for 30 min, then allowed
to warm to room temperature for 30 min, then cooled back to −78 °C.
This solution was added dropwise to a separately cooled solution of
[PhBPC3 H Cy]FeCl (200 mg, 0.246 mmol) in THF (2 mL), and the
2
reaction mixture was stirred at low temperature for 10 min, resulting in a
dark blue solution, before being allowed to warm to room temperature.
The solution was stirred for an additional 15 min and then concentrated
to dryness, extracted with pentane, and filtered through Celite, and
concentrated to dryness. The blue residue was taken up in pentane and
recrystallized by slow evaporation of the pentane solution. The resulting
blue crystals were washed with cold pentane and dried, giving 179 mg of
the desired product (63%). Crystals suitable for XRD were grown by
slow evaporation of a concentrated pentane solution into HMDSO.
1H NMR (C6D6, 300 MHz, 25 °C): δ 44.1, 29.2, 22.8, 21.3, 15.8, 14.5,
11.5, 7.5, 5.1, 3.2, −0.7, −1.2, −2.2, −3.0, −4.3, −5.2, 7.6, −8.4, −14.3,
−15.8, −19.0, −24.0 ppm. 31P{1H} NMR (C6D6, 202 MHz, −75 °C): δ
82.2 (br s, 3P), 62.2 (br s, 1P) ppm. Anal. Calcd for C69H122BFeNP4: C,
71.67; H, 10.64; N, 1.21. Found: C, 71.30; H, 10.25; N, 0.91.
[PhBPPh3]Fe(NPPh3)·THF (1·THF). [H2NPPh3][Cl] (88.9 mg,
0.283 mmol) was suspended in THF (5 mL) and cooled to −78 °C
with stirring. n-Butyllithium (354 μL, 1.6 M in hexane, 0.566 mmol) was
added to the solution, which was stirred at low temperature for 30 min,
then allowed to warm to room temperature for 30 min, then cooled back
to −78 °C. This solution was added dropwise to a separately cooled
solution of [PhBPP3h]FeCl (200 mg, 0.257 mmol) in THF (5 mL), and
the reaction mixture was stirred at low temperature for 30 min before
being allowed to warm to room temperature and stirred overnight. The
resulting blue solution was concentrated to dryness, extracted with
benzene, and filtered through Celite. The filtrate was concentrated to
give a blue residue that was recrystallized by taking up in THF (5 mL),
layered with pentane (15 mL), and allowing to stand at room tem-
perature overnight. The resulting blue crystals were thoroughly washed
with pentane and 1:1 THF/pentane, giving 251 mg of compound 1 as its
THF solvate (95%). Crystals suitable for X-ray diffraction (XRD) were
grown by vapor diffusion of pentane into a concentrated THF solution
at room temperature. 1H NMR (C6D6, 300 MHz, 25 °C): δ 42.1 (br),
15.4, 10.4, 10.1, 9.9, 7.3, 6.0, 2.7, 2.5 ppm. Anal. Calcd for C67H64BFeNP4O
(1·THF): C, 73.84; H, 5.92; N, 1.29. Found: C, 73.01; H, 5.96; N, 1.06.
[PhBPP3h]Fe(NPCy3) (2). [H2NPCy3][Cl] (88.3 mg, 0.266 mmol)
was suspended in THF (5 mL) and cooled to −78 °C with stirring.
n-Butyllithium (333 μL, 1.6 M in hexane, 0.533 mmol) was added to the
solution, which was stirred at low temperature for 30 min, then allowed
to warm to room temperature for 30 min, then cooled back to −78 °C.
This solution was added dropwise to a separately cooled solution of
[PhBPP3h]FeCl (196.8 mg, 0.253 mmol) in THF (2 mL), and the
reaction mixture was stirred at low temperature for 10 min before being
allowed to warm to room temperature for 15 min and then concen-
trated. The resulting blue solution was concentrated to dryness,
extracted with benzene, and filtered through Celite. The filtrate was
concentrated to give a blue residue which was recrystallized by taking up
in minimal THF, layering with pentane (15 mL), and allowing to stand
at −40 °C overnight. The resulting blue crystals were thoroughly
washed with pentane, giving 213 mg of the desired product (81%).
Crystals suitable for XRD were grown by layering pentane over a
[PhBPm3 ‑ter]Fe(NPPh3) (5). [H2NPPh3][Cl] (40.2 mg, 0.128 mmol)
was suspended in THF (5 mL) and cooled to −78 °C with stirring.
n-Butyllithium (160 μL, 1.6 M in hexane, 0.256 mmol) was added to the
solution, which was stirred at low temperature for 30 min, then allowed
to warm to room temperature for 30 min, then cooled back to −78 °C.
This solution was added dropwise to a separately cooled solution of
PhBPm3 ‑terFeCl (206.1 mg, 0.122 mmol) in THF (2 mL), and the reac-
tion mixture was stirred at low temperature for 10 min before being
allowed to warm to room temperature for 2 h and then concentrated.
The resulting blue solution was concentrated to dryness, extracted with
benzene, and filtered through Celite. The filtrate was concentrated to
2 mL, layered with 15 mL of pentane and allowed to stand at room
temperature overnight. The resulting blue crystals were thoroughly
washed with pentane, giving 132 mg of the desired product (56%).
Crystals suitable for XRD were grown by vapor diffusion of pentane into
a concentrated benzene solution. 1H NMR (C6D6, 300 MHz, 25 °C): δ
39.4 (br), 14.7, 14.6, 10.0, 9.9, 9.2, 7.0, 6.9, 6.7, 5.9, 3.3, −1.3 ppm.
31P{1H} NMR (C6D6, 152 MHz, −75 °C): δ 100.2 (br s, 3P), 50.0 (br s,
1P) ppm. Anal. Calcd for C135H104BFeNP4: C, 83.98; H, 5.43; N, 0.73.
Found: C, 83.33; H, 5.57; N, 0.54.
[PhBPm3 ‑ter]Fe(NPCy3) (6). [H2NPPh3][Cl] (41.2 mg, 0.124 mmol)
was suspended in THF (5 mL) and cooled to −78 °C with stirring.
n-Butyllithium (155 μL, 1.6 M in hexane, 0.248 mmol) was added to the
solution, which was stirred at low temperature for 30 min, then allowed
to warm to room temperature for 30 min, then cooled back to −78 °C.
This solution was added dropwise to a separately cooled solution of
PhBPm3 ‑terFeCl (200 mg, 0.118 mmol) in THF (2 mL), and the reaction
mixture was stirred at low temperature for 10 min before being allowed
to warm to room temperature for 2 h and then concentrated. The
resulting blue solution was concentrated to dryness, extracted with
benzene, and filtered through Celite. The filtrate was concentrated to
2 mL, layered with 15 mL of pentane and allowed to stand at room
temperature overnight. The resulting blue crystals were thoroughly
washed with pentane, giving 170 mg of the desired product (87%).
1H NMR (C6D6, 300 MHz, 25 °C): δ 9.0, 7.7, 7.5, 7.08, 7.05, 6.99, 3.2,
2.4, 1.7, 1.3, 0.7, 0.5 ppm. 31P{1H} NMR (C6D6, 202 MHz, −75 °C): δ
100.5 (br s, 3P), 66.7 (br s, 1P) ppm. We have had difficulty obtaining
elemental analysis data on this compound, likely due to its high air
sensitivity. The best results that we have obtained are as follows: Anal.
Calcd for C135H122BFeNP4: C, 83.19; H, 6.31; N, 0.72. Found: C, 81.94;
H, 7.07; N, 0.96.
1
concentrated THF solution at −40 °C. H NMR (C6D6, 300 MHz,
25 °C): δ 14.1 (br), 10.5, 8.6, 8.2, 6.7, 5.6, 4.4, 4.2, 3.6, 2.8, 1.9 ppm. Anal.
Calcd for C63H74BFeNP4: C, 73.05; H, 7.20; N, 1.35. Found: C, 72.85;
H, 7.17; N, 1.24.
[PhBPCH2Cy3]Fe(NPPh3) (3). [H2NPPh3][Cl] (81 mg, 0.258 mmol)
was suspended in THF (5 mL) and cooled to −78 °C with stirring.
n-Butyllithium (321 μL, 1.6 M in hexane, 0.616 mmol) was added to the
solution, which was stirred at low temperature for 30 min, then allowed
to warm to room temperature for 30 min, then cooled back to −78 °C.
This solution was added dropwise to a separately cooled solution of
PhBPC3 H CyFeCl (199.1 mg, 0.245 mmol) in THF (2 mL), and the
2
reaction mixture was stirred at low temperature for 10 min, resulting in a
dark blue solution, before being allowed to warm to room temperature.
The solution, which is dark green at room temperature, was stirred for
1 h and then concentrated to dryness, extracted with benzene, and
filtered through Celite. The filtrate was concentrated to dryness, taken
up in minimal THF, layered with pentane, and allowed to stand
overnight at room temperature. The resulting yellow crystals were
thoroughly washed with pentane, giving 172 mg of the desired product
(62%). 1H NMR (C6D6, 300 MHz, 25 °C): δ 163.2 (br), 40.6, 40.0, 20.0,
[PhBPi3Pr]Fe(NPPh3) (7). [H2NPPh3][Cl] (56.4 mg, 0.180 mmol)
was suspended in THF (5 mL) and cooled to −78 °C with stirring.
n-Butyllithium (224 μL, 1.6 M in hexane, 0.360 mmol) was added to the
solution, which was stirred at low temperature for 30 min, then allowed
K
DOI: 10.1021/acs.inorgchem.6b00066
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
to warm to room temperature for 30 min, then cooled back to −78 °C.
This solution was added dropwise to a separately cooled solution of
[PhBPi3Pr]FeCl (99.0 mg, 0.173 mmol) in THF (2 mL), and the reaction
mixture was stirred at low temperature for 10 min before being allowed
to warm to room temperature for 2 h and then concentrated. The
resulting yellow solution was concentrated to dryness, extracted with
benzene, and filtered through Celite, and concentrated to dryness again.
The yellow-orange residue was taken up in minimal ether, layered with
pentane, and stored at −40 °C overnight, resulting in the formation of
yellow crystals. The isolated material displayed the same spectroscopic
characteristics as previously reported for this compound.18 Crystals
suitable for XRD were grown by layering pentane over a concentrated
ether solution at −40 °C. 1H NMR (C6D6, 300 MHz, 25 °C): δ 198.2,
53.0, 49.3, 23.6, 21.2, 8.3, 2.0, 1.8, −5.8, −27.2, −48.6 ppm. Anal. Calcd
for C45H68BFeNP4: C, 66.43; H, 8.42; N, 1.72. Found: C, 65.96; H, 8.41;
N, 2.45.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jpeters@caltech.edu.
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Gordon and Betty Moore
Foundation and through the NSF via a GRFP award to S.E.C.
Larry Henling and Michael Takase are thanked for their
assistance with X-ray crystallography. The Molecular Materials
Research Center (MMRC) at Caltech is thanked for the use of
the SQUID magnetometer.
[PhBPi3Pr]Fe(NPCy3) (8). [H2NPCy3][Cl] (62.9 mg, 0.190 mmol)
was suspended in THF (5 mL) and cooled to −78 °C with stirring.
n-Butyllithium (237 μL, 1.6 M in hexane, 0.380 mmol) was added to the
solution, which was stirred at low temperature for 30 min, then allowed
to warm to room temperature for 30 min, then cooled back to −78 °C.
This solution was added dropwise to a separately cooled solution of
[PhBPi3Pr]FeCl (104.5 mg, 0.181 mmol) in THF (2 mL), and the
reaction mixture was stirred at low temperature for 10 min before being
allowed to warm to room temperature for 2 h and then concentrated.
The resulting yellow solution was concentrated to dryness, extracted
with benzene, and filtered through Celite, and concentrated to dryness
again. The yellow-orange residue was taken up in minimal ether, layered
with pentane, and stored at −40 °C overnight, resulting in the formation
of yellow crystals which were collected, washed with cold pentane,
and dried to give 69 mg of the desired compound (46%). Crystals
suitable for XRD were grown by slow evaporation of a pentane solution.
1H NMR (C6D6, 300 MHz, 25 °C): δ 194.2, 84.7, 50.3, 42.5, 23.7, 21.4,
20.1, 13.9, 7.7, 6.0, −7.7, −26.2, −49.3 ppm. Anal. Calcd for
C45H86BFeNP4: C, 64.98; H, 10.42; N, 1.68. Found: C, 64.79; H,
10.42; N, 1.61.
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ASSOCIATED CONTENT
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* Supporting Information
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The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.6b00066.
Additional spectral data for novel compounds, crystallo-
graphic details, and further details concerning the analysis
of variable-temperature UV-vis and NMR data (PDF)
Crystallographic data for compounds 1, 2, 4, 5 and 7−9.
(CIF)
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