Polymorphism in Fe[(p-IC6H4)B(3-Mepz) 3]2 (pz = Pyrazolyl): Impact of supramolecular structure on an Iron(ll) electronic Spin-state crossover

Article abstract of DOI:10.1021/ic048406q

The new ligands Na[(p-IC₆H₄)B(3-Rpz)₃] (R = H, Me) have been prepared by converting I₂C₆H ₄ to IC₆H₄SiMe₃ with Li ⁺Bu and SiMe₃Cl, and then to IC₆H ₄BBr₂ with BBr₃ and subsequent reaction with 3 equiv of (un)substituted pyrazole and 1 equiv of NaO⁺Bu. These new ligands react with FeBr₂ to give either purple, low-spin Fe[(p-IC₆H₄)B(pz)₃]₂ or colorless, high-spin Fe[(p-IC₆H₄)B(3-Mepz)₃]₂. Depending upon the crystallization conditions, Fe[(p-IC₆H ₄)B(3-Mepz)₃]₂ can exist both as two polymorphs and as a methylene chloride solvate. An examination of these polymorphs by variable-temperature X-ray crystallography, magnetic susceptibility, and Moessbauer spectroscopy has revealed different electronic spin-state crossover properties for each polymorph and yields insight into the influence of crystal packing, independent of other electronic perturbations, on the spin-state crossover. The first polymorph of Fe[(p-IC₆H ₄)B(3-Mepz)₃]₂ has a highly organized three-dimensional supramolecular structure and does not undergo a spin-state crossover upon cooling to 4 K. The second polymorph of Fe[(p-IC ₆H₄)B(3-Mepz)₃]₂ has a stacked two-dimensional supramolecular structure, a structure that is clearly less well organized than that of the first polymorph, and undergoes an abrupt iron(II) spin-state crossover from high spin to low spin upon cooling below ca. 130 K. The crystal structure of the methylene chloride solvate of Fe[(p-IC ₆H₄)B(3-Mepz)₃]₂ has a similar stacked two-dimensional supramolecular structure, but the crystals readily lose the solvate. The resulting desolvate undergoes a gradual spin-state crossover to the low-spin state upon cooling below ca. 235 K. It is clear from a comparison of the structures that the longrange solid-state organization of the molecules, which is controlled by noncovalent supramolecular interactions, has a strong impact upon the spin-state crossover, with the more highly organized structures having lower spincrossover temperatures and more abrupt spin-crossover behavior.

Full text of DOI:10.1021/ic048406q

Inorg. Chem. 2005, 44, 1852−1866
Polymorphism in Fe[(p-IC6H4)B(3-Mepz)3]2 (pz
) Pyrazolyl): Impact of
Supramolecular Structure on an Iron(II) Electronic Spin-State Crossover
Daniel L. Reger,*,† James R. Gardinier,† Mark D. Smith,† Ahmed M. Shahin,‡ Gary J. Long,*,‡
Leila Rebbouh,§ and Fernande Grandjean§
Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia,
South Carolina 29208, Department of Chemistry, UniVersity of MissourisRolla,
Rolla, Missouri 65409-0010, and Department of Physics, B5, UniVersity of Lie`ge,
B-4000 Sart-Tilman, Belgium
Received November 12, 2004
The new ligands Na[(p-IC6H4)B(3-Rpz)3] (R
) H, Me) have been prepared by converting I2C6H4 to IC6H4SiMe3 with
LitBu and SiMe3Cl, and then to IC6H4BBr2 with BBr3 and subsequent reaction with 3 equiv of (un)substituted pyrazole
and 1 equiv of NaOtBu. These new ligands react with FeBr2 to give either purple, low-spin Fe[(p-IC6H4)B(pz)3]2 or
colorless, high-spin Fe[(p-IC6H4)B(3-Mepz)3]2. Depending upon the crystallization conditions, Fe[(p-IC6H4)B(3-Mepz)3]2
can exist both as two polymorphs and as a methylene chloride solvate. An examination of these polymorphs by
variable-temperature X-ray crystallography, magnetic susceptibility, and Mo¨ssbauer spectroscopy has revealed different
electronic spin-state crossover properties for each polymorph and yields insight into the influence of crystal packing,
independent of other electronic perturbations, on the spin-state crossover. The first polymorph of Fe[(p-IC6H4)B-
(3-Mepz)3]2 has a highly organized three-dimensional supramolecular structure and does not undergo a spin-state
crossover upon cooling to 4 K. The second polymorph of Fe[(p-IC6H4)B(3-Mepz)3]2 has a stacked two-dimensional
supramolecular structure, a structure that is clearly less well organized than that of the first polymorph, and undergoes
an abrupt iron(II) spin-state crossover from high spin to low spin upon cooling below ca. 130 K. The crystal structure
of the methylene chloride solvate of Fe[(p-IC6H4)B(3-Mepz)3]2 has a similar stacked two-dimensional supramolecular
structure, but the crystals readily lose the solvate. The resulting desolvate undergoes a gradual spin-state crossover
to the low-spin state upon cooling below ca. 235 K. It is clear from a comparison of the structures that the long-
range solid-state organization of the molecules, which is controlled by noncovalent supramolecular interactions,
has a strong impact upon the spin-state crossover, with the more highly organized structures having lower spin-
crossover temperatures and more abrupt spin-crossover behavior.
Introduction
biomedical imaging,2 temperature threshold indicators,3 and
optical elements in display devices.3 The temperature de-
pendence of the transition between the highly colored low-
Octahedral iron(II) complexes that undergo a thermal spin-
state crossover1 have emerged from the realm of laboratory
curiosities to the precipice of applied chemistry with such
diverse applications as “intelligent” contrast agents for
spin (LS) A1g (t2g6eg*0) state and the typically colorless or
1
lightly colored high-spin (HS) T2g (t2g4eg*2) state of octa-
5
hedral 3d6 iron(II) complexes is exploited in the development
of such devices. Generally, the temperature dependence of
the spin transition shows any one of five behaviorssgradual,
abrupt, stepwise, incomplete, or one with hysteresissand
each type of behavior dictates the ultimate application, if
any, of the material. In this sense, there has been a
* To whom correspondence should be addressed. E-mail: reger@
mail.chem.sc.edu (D.L.R.); glong@umr.edu (G.J.L.).
† University of South Carolina.
‡ University of Missouri-Rolla.
§ University of Lie`ge.
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1852 Inorganic Chemistry, Vol. 44, No. 6, 2005
10.1021/ic048406q CCC: $30.25
© 2005 American Chemical Society
Published on Web 02/23/2005

Polymorphism in Fe[(p-IC6H4)B(3-Mepz)3]2
considerable emphasis toward developing new examples of
spin-transition materials in order to more fully understand
the nature of the temperature dependence of the spin
transitions. Complexes that contain the FeN6 coordination
environment are, by far, the most studied class of spin-
transition complexes. Thus, reports of the spin-state crossover
behavior of iron(II) derivatives of triazoles,4 tetrazoles,5
isoxazoles,6 polypyridyls,7 picolylamines,8 cyanopyridines,9
poly(pyrazolyl)borates,10 and some pyrazolyl/pyridyl mixed
ligand systems11 are available. Our group has recently
reported the unusual spin-state crossover behavior exhibited
by both homoleptic and heteroleptic tris(pyrazolyl)methane
iron(II) complexes.12 In these complexes, the temperature
dependence of the magnetic susceptibility was shown to be
dependent on the nature of the counteranion, the included
solvent, and the groups bound to the pyrazolyl rings. These
results are consistent with other systems in that steric
interactions involving groups proximal to the metal center
favor the HS ground state and tend to decrease the temper-
ature at which the resulting complexes undergo the spin-
state crossover, and in some cases, the LS state may not even
be accessible.
Several different and acceptable models have been put
forth from a physical chemistry perspective, to describe the
nature of spin-state transition behavior of various complexes.1
These models, which incorporate terms such as “chemical
pressure”, “elastic interactions”, and “lattice forces”, account
for the nature of the cooperativity in polymeric species
connected by covalent bonding interactions (that give rise
to abrupt rather than gradual spin transitions). However, the
true meaning of these terms and, hence, the nature of the
transitions in discrete molecular species or “isolated” cationic
iron(II) species really becomes apparent only on the rare
occurrence that two polymorphs of the same spin-transition
compound can be found and examined in detail. To the best
of our knowledge, the occurrence, identification, and char-
acterization of iron(II) spin-crossover polymorphs has been
limited to the heavily studied tetraaza complexes of
Fe(NCS)2.13a-h Of these, the recently described Fe(N-2-
pyridylmethylene-4-aminobiphenyl)2(NCS)2 system,13c which
crystallizes as two polymorphs (I and II), provides a nice
example of how small and seemingly unimportant structural
changes can lead to different magnetic behavior. Form I
undergoes an abrupt HS-to-LS transition at 170 K, while
the second (form II) undergoes a gradual transition centered
around 205 K. The main difference in the two structures is
that form I has a slightly more distorted FeN6 octahedron
and shorter, weak, intermolecular S‚‚‚HC hydrogen-bonding
interactions (between the sulfur of one NCS group on one
iron molecule with a phenyl ring hydrogen from a neighbor-
ing molecule) compared to form II. Other related systems
show even more diverse behavior. For instance, only one of
the two polymorphs of Fe(2,2′-di-2-thiazoline)2(NCS)2 shows
a HS-LS transition centered at 175 K (with hysteresis),
while the second form remains HS at all temperatures.13g
Also, Fe[(3-aminopropyl)bis(2-pyridylmethyl)amine](NCS)2
exists as three polymorphs that show either an abrupt
transition with hysteresis (TcV 112 K, Tcv 120 K), a gradual
transition without hysteresis (Tc 176 K), or no transition (HS
only), depending on the extent of intermolecular π-stacking
of pyridine rings and deformation of the FeN6 octahedral
coordination spheres.13d The possibility of using noncovalent
interactions to control magnetic properties of molecular spin-
crossover compounds is alluring from a supramolecular
synthetic chemist’s viewpoint and has seen some promise
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Reger et al.
between 4.2 and 295 K on a constant-acceleration spectrometer
that utilized a room-temperature rhodium matrix cobalt-57 source
and was calibrated at room temperature with R-iron foil. The
estimated absolute errors are (0.005 mm/s for the isomer shifts,
(0.01 mm/s for the quadrupole splittings and line widths, and
(0.2% for the relative spectral absorption areas.
from the studies of arylazoanilinedi(isothiocyanato)iron(II)
complexes.13j
During the course of a recent comprehensive systematic
study that we have undertaken to design systems that promote
specific covalent and noncovalent interactions with the goal
of determining how these forces impact the spin-state
transitions in both tris(pyrazolyl)methane and tris(pyrazolyl)-
borate iron(II) complexes, we have discovered that the
magnetic properties of Fe[(p-IC6H4)B(3-Mepz)3]2 exhibited
an unusual sample dependence. Some samples exhibited spin-
state transitions, as first indicated by a change in color on
cycling the temperature of the sample between 294 and 78
K, whereas other samples did not. In this contribution, we
outline the differences in the ground-state magnetic properties
of the Fe[(p-IC6H4)B(3-Rpz)3]2 (R ) H, Me) complexes. By
studying the unusual sample-dependent magnetic, structural,
and Mo¨ssbauer behavior observed with two polymorphs and
a methylene chloride solvate form of the 3-methylpyrazolyl
derivative, we have been able to correlate the supramolecular
structure of discrete molecular poly(pyrazolyl)borate iron-
(II) complexes with the temperature dependence of their spin-
state crossover behavior. This foundation will be expanded
upon in future papers with the goal of more precisely defining
both the intramolecular and intermolecular forces that are
responsible for the cooperative interactions and whether the
cooperative interactions lead to or fail to lead to an electronic
spin-state crossover.
Syntheses. p-IC6H4SiMe3. This complex was prepared by a
modification of the literature procedure.14 An 88.6 mL (151.5 mmol)
portion of Li(tBu) as a 1.7 M solution in pentane was added
dropwise over the period of 20 min to a cold suspension (-78 °C)
of 25.0 g (75.8 mmol) of p-I2C6H4. A yellow solution formed after
one-half of the Li(tBu) was added, and after complete addition the
resulting yellow suspension was stirred for 1.5 h at -78 °C. A 15
mL (118.2 mmol) aliquot of SiMe3Cl was added by syringe to the
cold suspension, and the mixture was stirred for an additional hour.
The mixture was then stirred to room temperature for 1 h and was
poured into 100 mL of a 4% NaHCO3 solution. The organic and
aqueous layers were separated, and the aqueous layer was extracted
twice with 100 mL of Et2O. The combined organics were dried
over MgSO4, solvent was removed by rotary evaporation, and the
residual yellow oil was passed through a silica gel plug (eluting
with hexanes) to remove colored impurities (alternatively, the crude
yellow oil could be distilled under vacuum at 83-85 °C/1 mmHg).
The solvent was removed by rotary evaporation to leave 20.23 g
(97%, if pure) of crude p-IC6H4SiMe3 as a colorless liquid. As is
typical for this reaction, this crude liquid was found by NMR to
be contaminated with 3% of I2C6H4 (δH 7.42 in CDCl3) and 3% of
(Me3Si)2C6H4 (δH 7.53, 0.29 in CDCl3). These impurities are very
difficult to separate from the desired compound by either distillation
or chromatography. However, if the colorless liquid is cooled to
-20 °C for 2 days and filtered cold, the impurity I2C6H4 can be
removed, while that of (Me3Si)2C6H4 can be reduced to about
1-2%. The product obtained in this fashion was used successfully
Experimental Section
General Considerations. All operations were carried out either
under a nitrogen atmosphere using standard Schlenk techniques or
in a Vacuum Atmospheres HE-493 inert atmosphere drybox, unless
otherwise specified. Solvents for synthetic procedures and spec-
troscopic studies were dried by conventional methods and distilled
under a N2 atmosphere immediately prior to use. BBr3 was used as
received from Strem Chemicals. All other chemicals were purchased
from Aldrich Chemicals. Pyrazole was sublimed and 3-methylpyra-
zole was distilled under vacuum before use. Robertson Microlit
Laboratories performed all elemental analyses. Melting point
determinations were made on samples contained in glass capillaries
by using an Electrothermal 9100 apparatus and are uncorrected.
Mass spectrometric measurements recorded in ESI(() mode were
obtained on a Micromass Q-Tof spectrometer, whereas those
performed by direct probe analyses were made on a VG 70S
instrument. NMR spectra were recorded by using either a Varian
Gemini 300 or a Varian Mercury 400 instrument, as noted within
the text. Chemical shifts were referenced to solvent resonances at
δH 2.05 and δC 29.8 for acetone-d6, δH 7.27 and δC 77.23 for CDCl3,
and δH 3.31 and δC 49.15 for CD3OD.
1
in all further experiments. H NMR (300 MHz, CDCl3): δ 7.71
(part of AA′BB′ 2 H), 7.26 (part of AA′BB′ 2 H), 0.27 (s, 9H,
SiMe3). 13C NMR (75.4 MHz, CDCl3): δ 140.0 (Ci-Si), 136.9,
135.2, 95.9(Ci-I), -1.1 (SiCH3).
Combined NMR/Synthetic Scale Preparation of p-IC6H4BBr2.
In a drybox, 1.30 g (5.20 mmol) of BBr3 was added by pipet to a
solution of 1.25 g (4.51 mmol) of p-IC6H4SiMe3 in 5-10 mL of
CDCl3 contained in a Schlenk flask. After the mixture was stirred
for 1 min to ensure complete mixing, a 0.7 mL aliquot was
transferred to an NMR tube which was subsequently sealed with
several layers of Parafilm over the NMR cap. The Schlenk flask
and NMR tube were placed in a 60 °C water bath, and both were
removed at intervals during the course of monitoring the reaction.
It took 8 h of heating for complete reaction, as is indicated by the
appearance of proton resonances for SiMe3Br (δH 0.61) and for
the desired compound p-IC6H4BBr2 (vide infra) at the expense of
resonances for p-IC6H4SiMe3. After the reaction was complete, the
volatile components were removed from the Schlenk flask by
vacuum distillation at room temperature to leave 1.39 g (82%) of
crude p-IC6H4BBr2 as a nearly colorless (tannish) solid. 1H NMR:
δ 7.91 (part of AA′BB′), 7.86 (part of AA′BB′). 13C NMR: δ 138.8,
137.8, 105.5. 11B NMR: δ 56.6.
Magnetic susceptibilities were measured at 0.5 T using a
Quantum Design MPMS XL SQUID magnetometer. Gelatin
capsules were used as sample containers for measurements taken
in the temperature range 5-300 K. The very small diamagnetic
contribution of the gelatin capsule had a negligible contribution to
the overall magnetization, which was dominated by the sample.
The molar magnetic susceptibilities were corrected for the diamag-
netism of the complexes, which was calculated from tables of
Pascal’s constants to be -454 × 10-6 emu/mol for Fe[(p-IC6H4)B-
(3-Mepz)3]2.
Na[(p-IC6H4)B(pz)3]. A 100 mL Schlenk flask with a Teflon
stopcock equipped with a magnetic stir bar was charged with a
mixture of 1.44 g (5.23 mmol) of p-IC6H4SiMe3 and 1.50 g (5.99
mmol) of BBr3 and was sealed with a glass stopcock. After the
flask had been subjected to a freeze-pump-thaw cycle, the
The Mo¨ssbauer spectral absorber contained ca. 50 mg/cm2 of
powder mixed with boron nitride, and the spectra were measured
(14) Stephens, E. B.; Kinsey, K. E.; Davis, J. F.; Tour, J. M. Macromol-
ecules 1993, 26, 3519.
1854 Inorganic Chemistry, Vol. 44, No. 6, 2005

Polymorphism in Fe[(p-IC6H4)B(3-Mepz)3]2
apparatus was placed in a 60 °C water bath for 8 h, and then the
SiMe3Br and the excess BBr3 were removed by vacuum distillation
at room temperature to leave p-IC6H4BBr2 as a nearly colorless
(pale pink) solid. The p-IC6H4BBr2 was dissolved in 35 mL of
toluene, and the solution was added via cannula to a solution of
1.07 g (15.7 mmol) of pyrazole and 1.5 mL (10.4 mmol) of NEt3
in 100 mL of toluene. After the resulting cloudy suspension was
allowed to stir at room temperature for 12 h, the toluene solution
[presumably containing p-IC6H4B(pz)2(pzH)] was transferred to a
500 mL Schlenk flask by cannula filtration, thereby removing the
colorless precipitate of [HNEt3][Br] (1.88 g, 99%). A solution of
0.503 g (5.23 mmol) of NaOtBu in 50 mL of 1:1 toluene/THF was
added via cannula to the solution of “p-IC6H4B(pz)2(pzH)”, the
resulting suspension was stirred at 60 °C (external water bath) for
4 h, and solvent was removed by vacuum distillation. The residue
was triturated with 50 mL of Et2O, filtered, and air-dried to leave
1.00 g of the desired compound as a colorless powder. The filtrate
was a 1:1 mixture of pyrazole and the desired compound (NMR);
thus, solvent was removed by vacuum distillation, and the residue
was triturated with 25 mL of Et2O, filtered, and dried in an oven.
The yield of the combined Et2O-insoluble portions was 1.49 g or
65% based on IC6H4SiMe3. Mp: 310 °C (slow dec), 328-330 °C
filtered, and solvent was removed to yield 1.14 g (87%) of Fe[(p-
IC6H4)B(pz)3]2 as a purple solid. The product was dissolved in 80
mL of CH2Cl2 and filtered, and 160 mL of methanol was layered
on the purple solution, which, after the solvents were allowed to
diffuse slowly over 2 days, yielded 0.975 g (74%) of lustrous purple
needles. Mp: 295-298 °C (dec, black liquid). Anal. Calcd for
C30H26B2FeI2N12: C, 40.67; H, 2.96; N, 18.97. Found: C, 40.30;
H, 2.47; N,18.70. 1H NMR (300 MHz, CDCl3): δ 8.14, 8.05, 7.61,
6.13, 6.00.13C NMR (75.4 MHz): δ 148.1 (Ar), 139.4 (Ar), 137.5
(C3/5-pz), 105.7 (C4-pz), 95.2 (Ci-I). High resolution MS, direct
probe, calcd (obsd): 886.0039 (886.0044).
Bis{(p-iodophenyl)[tris(3-methylpyrazolyl)]borato}iron(II),
Fe[(p-IC6H4)B(3-Mepz)3]2. The addition of 100 mL of an aqueous
solution of 0.284 g (1.32 mmol) of FeBr2 to a methanol solution
of 1.27 g (2.64 mmol) of Na[(p-IC6H4)B(3-Mepz)3] yielded a
copious amount of colorless solid that was isolated by filtration.
The solid was then taken up in CH2Cl2, the solution was dried over
MgSO4 and filtered, and the MgSO4 was extracted twice with 100
mL of refluxing benzene and filtered hot. After cooling, and solvent
was removed from the combined organic fractions by rotary
evaporation to yield 1.03 g (80% yield) of pure Fe[(p-IC6H4)B(3-
Mepz)3]2 as a colorless solid. Recrystallization by layering a CH2-
Cl2 solution with methanol and allowing the solvents to diffuse
slowly at room temperature over 2 days yielded 0.912 g (71% yield)
of X-ray-quality colorless block crystals (form InSCO, where the
superscript nSCO stands for “no spin crossover”). Mp (form
InSCO): 280 °C (slowly darkens); 335-338 °C (dec, red-black
liquid). Anal. Calcd for C36H38B2FeI2N12: C, 44.57; H, 3.95; N,
17.33. Found: C, 44.65; H, 3.56; N, 17.25. A second polymorph
of Fe[(p-IC6H4)B(3-Mepz)3]2 (form IISCO, “spin crossover”) is
formed within 20 min to 1 h at room temperature as small
crystallites at the boundary between the slowly diffusing MeOH
and CH2Cl2 layers. This second polymorph can be formed
preferentially [possibly contaminated by a trace of the (desolvated)
solvate, form IIISCO] by allowing approximately 2-4 mM CH2Cl2
solutions contained in 5 dram vials to evaporate in the hood
(maintained at a flow rate of 80-100 ft3/min) over a period of
8-12 h. The reproducibility of the evaporative growth of the second
polymorph is somewhat problematic, as slower evaporation rates
tended to lead to the formation of quantities of the first polymorph,
while faster rates may lead to more of the desolvated third form.
Leaving crystals of IISCO in the mother liquid causes them to convert
to form InSCO over time. The platelike block crystals of InSCO are
resistant to redissolution in CH2Cl2, which is fortuitous because
the “as-formed” crystalline samples may contain a small quantity
of the second, more soluble polymorph IISCO that can be removed
by washing with a minimal quantity of CH2Cl2. Mp (form IISCO):
250-260 °C (slowly darkens); 338-345 °C (dec, red-black liquid).
Anal. Found: C, 44.43; H, 3.62; N, 17.12. If the methanol/CH2Cl2
crystallization is carried out at -20 °C by using a precooled
apparatus, cold CH2Cl2 solutions, and a cold diffusing solvent, large
blocks of the methylene chloride solvate, Fe[(p-IC6H4)B(3-Mepz)3]2‚
CH2Cl2 (IIISCO), are formed preferentially (ca. 78%, vide infra),
where the minor impurity is the form InSCO polymorph. These
crystals lose solvent and concomitantly turn opaque and fracture
within seconds of exposure to air. Even though it is possible to
mechanically separate the three forms as single crystals, it has not
yet been possible to grow pure bulk phases of the last two forms
of Fe[(p-IC6H4)B(3-Mepz)3]2. Mp (form IIISCO, air-dried, desolvated
crystals): 265-275 °C (slowly darkens); 340-345 °C (dec, red-
black liquid). Anal. Found: C, 44.57; H, 3.99; N, 17.24. 1H NMR
(300 MHz, CDCl3): δ 45.8, 43.8, 4.3, 2.1, -6.3. High resolution
MS, direct probe, calcd (obsd): 970.0980 (970.0526).
1
(black liquid). H NMR (300 MHz, CD3OD): δ 7.55 (d, J ) 1
Hz, H3-pz, 3H), 7.49 (part of AA′BB′, 2H), 7.11 (d, J ) 2 Hz,
H5-pz, 3H), 7.05 (part of AA′BB′, 2H), 6.15 (dd, J ) 2,1 Hz, 3H).
13C NMR (75.4 MHz, CD3OD): δ 140.7 (C5-pz), 137.5 (aryl), 136.8
(aryl), 135.9 (C3-pz), 134.7 (Ci-B), 104.5 (C4-pz), 93.2 (Ci-I).
High resolution ESI(-) MS, calcd (obsd) for anion: 415.0340
(457.0351).
Na[(p-IC6H4)B(3-Mepz)3]. Similar to the parent derivative
above, the reaction between 2.36 g (8.56 mmol) of p-IC6H4SiMe3
and 2.51 g (10.0 mmol) of BBr3 neat was used to produce p-IC6H4-
BBr2, which was then dissolved in situ in 50 mL of toluene. The
p-IC6H4BBr2 solution was added via cannula to a solution of 2.11
g (25.7 mmol) of 3-methylpyrazole and 2.4 mL (17.2 mmol) of
NEt3 in 200 mL of toluene. After the resulting cloudy suspension
was allowed to stir at room temperature overnight, the toluene
solution [presumably containing p-IC6H4B(3-Mepz)2(3-MepzH)]
was transferred to a 500 mL Schlenk flask by cannula filtration,
thereby removing the colorless precipitate of [HNEt3][Br] (3.00 g,
96%). A solution of 0.823 g (8.56 mmol) of NaOtBu in 50 mL of
1:1 toluene/THF was added by cannula to the solution of “p-
IC6H4B(3-Mepz)2(3-MepzH)”, and the resulting suspension was
stirred for 12 h. The colorless solid was separated by filtration,
washed with two 100 mL portions of hexanes, and dried under
vacuum to leave 3.08 g (75%) of Na[(p-IC6H4)B(3-Mepz)3] as a
colorless solid. Mp: 310 °C (sinters), 348-350 °C (dec, black liquid
with effervescence). Anal. Calcd for C18H19BIN6Na: C, 45.03; H,
3.99; N, 17.50. Found: C, 45.30; H, 4.31; N, 17.29. 1H NMR (300
MHz, CD3OD): δ 7.46 (part of AA′BB′, 2H), 7.07 (part of AA′BB′,
2H), 6.98 (d, J ) 2 Hz, H5-pz, 3H), 5.88 (d, J ) 2 Hz, H4-pz, 3H),
2.23 (s, 9H, CH3). 13C NMR (75.4 MHz, CD3OD): δ 149.9, 137.7,
136.8, 136.6 (aryl, C3- and C5-pz), 104.1 (C4-pz), 92.9 (C-I), 13.6
(CH3) (C-B not obsd). High resolution ESI(-) MS, calcd (obsd)
for anion: 457.0812 (457.0811).
Bis{(p-iodophenyl)[tris(pyrazolyl)]borato}iron(II), Fe[(p-
IC6H4)B(pz)3]2. A solution of 1.30 g (2.97 mmol) of Na[(p-
IC6H4)B(pz)3] in 100 mL of 1:1 MeOH:H2O was added to 100 mL
of an aqueous solution of FeBr2 (0.320 g, 1.48 mmol), resulting in
the immediate precipitation of a purple solid. The suspension was
stirred for 30 min and filtered, and the purple solid was washed
with two 50 mL portions of H2O. The resulting solid was then taken
up in CH2Cl2, the purple solution was dried over MgSO4 and
Inorganic Chemistry, Vol. 44, No. 6, 2005 1855

Reger et al.
Crystallography. X-ray intensity data from a reddish-purple bar
of Fe[(p-IC6H4)B(pz)3]2 and a colorless block of Fe[(p-IC6H4)B-
(3-Mepz)3]2 (form InSCO) were measured at 150(1) K, whereas those
from a prism of Fe[(p-IC6H4)B(3-Mepz)3]2 (form IISCO) were
measured at 294(1) K (colorless), 200(1) K (pink), and 100(1) K
(purple), and those from a boxlike crystal of Fe[(p-IC6H4)B(3-
Mepz)3]2‚CH2Cl2 (IIISCO) were measured at 200(1) K (pink) and
subsequently at 100(1) K (purple) on a Bruker SMART APEX
CCD-based diffractometer (Mo KR radiation, λ ) 0.71073 Å).15
Raw data frame integration and Lp corrections were performed with
SAINT+.15 Final unit cell parameters were determined by least-
squares refinement of 7428 reflections from the data set of Fe[(p-
IC6H4)B(pz)3]2, of 7459 reflections from the data set of Fe[(p-
IC6H4)B(3-Mepz)3]2 (form InSCO), and of 9522 reflections from the
data set of Fe[(p-IC6H4)B(3-Mepz)3]2 (form IISCO), each with I >
5σ(I). Analyses of the data showed negligible crystal decay during
all data collection. An empirical absorption correction was applied
to the data sets of the first two complexes with SADABS,15 whereas
an analytical absorption correction was applied to the data of the
last complex using six indexed faces, {011}, {01-1}, and {100}
with XPREP.16 A face-indexed absorption correction was applied
to the last crystal due to its well-defined faces. The faces of other
crystals in the paper could not be indexed, so an empirical correction
was necessary. Direct methods structure solution, difference Fourier
calculations, and full-matrix least-squares refinements against F2
were performed with SHELXTL.16
Each step in Scheme 1 requires comment, as deviations from
the described protocol severely undermine the reproducibility
of the syntheses. The preparation of p-IC6H4SiMe3 followed
a modification of Tour’s procedure.14 In the current modi-
fication, it was found necessary to keep the reaction mixture
of “LiC6H4I” at -78 °C until after an excess of SiMe3Cl
has been added and the mixture had been stirred for a short
period of time. If the mixture is allowed to warm to room
temperature, even momentarily, and is then recooled before
the addition of SiMe3Cl, significant quantities of soluble
oligo- or polyphenylene products are formed. Even with the
above modification, the product obtained after flash chro-
matography (SiO2/hexanes) was frequently contaminated
with minor quantities (5-10%) of I2C6H4 and/or (Me3-
Si)2C6H4. These impurities conveniently crystallize from
p-IC6H4SiMe3 when the mixture is held at -20 °C and can
be separated by filtration. The preparation of p-IC6H4BBr2
was achieved by exploiting the well-known reactions between
boron trihalides and trimethylsilyl aryls.19,20 Unlike these
latter reactions, in which complete conversion to ArBX2 (X
) Cl, Br) is fairly rapid and occurs at or below room
temperature, the preparation of p-IC6H4BBr2 diluted in CDCl3
required 8 h of heating for a complete reaction, as determined
by an NMR study. Further studies showed that more
concentrated samples reacted within 1.5 h at 60 °C. Finally,
the choice of toluene as a solvent is critical to the success
of the subsequent reaction between p-IC6H4BBr2 and pyra-
zole, because it was found in separate experiments that the
use of THF resulted in products derived from cleavage of
the furan ring.
Fe[(p-IC6H4)B(pz)3]2 crystallizes in the Pbcn space group, as
determined by the pattern of systematic absences in the intensity
data. Systematic absences in the intensity data indicated the P21/c
space group for each form of Fe[(p-IC6H4)B(3-Mepz)3]2. All three
complexes are situated on crystallographic inversion centers. All
non-hydrogen atoms were refined with anisotropic displacement
parameters; hydrogen atoms were placed in idealized positions and
included as riding atoms.
Attempts to prepare the Na[(p-IC6H4)B(pz3R)3] compounds
by alternative routes using the potentially attractive reagent
p-IC6H4B(OH)2, which is commercially available and air
stable, were also investigated, but mixtures were always
obtained. Thus, the reaction between p-IC6H4B(OH)2, pyra-
zole, and sodium/potassium pyrazolide21 gave M[(p-IC6H4)B-
(3-Rpz)3] (M ) Na, K; R ) H, Me), contaminated with
boroxine [p-IC6H4B(µ-O)]3 or its pyrazolyl adducts [(p-
IC6H4)B(µ-O)]3‚(Hpz3R) (R ) H, Me).22 The reaction be-
tween p-IC6H4B(OH)2 and LiAlH4 [giving Li(THF)n[(p-
IC6H4)BH3 (n ) 1-3)],23 followed by reaction with the
Results
Syntheses. The Fe[(p-IC6H4)B(3-Rpz)3]2 complexes, where
R is H or Me, were prepared in good yield by the metathetical
reaction between Na[(p-IC6H4)B(3-Rpz)3] and iron(II) ha-
lides. The preparation of high-purity Na[(p-IC6H4)B(3-Rpz)3]
(R ) H, Me) was critical to the success of the reaction, and
a number of routes were explored. By far, the most efficient
route to the compound was that depicted in Scheme 1, which
was inspired by the synthesis reported both by Wagner and
co-workers17 in their preparation of ferrocenyltris(pyrazolyl)-
borate compounds and by Janiak and co-workers,18 who
adapted the procedure to alkylborato derivatives.
(15) SMART, Version 5.625, SAINT+, Version 6.22, and SADABS, Version
2.05; Bruker Analytical X-ray Systems, Inc.: Madison, WI, 2001.
(16) Sheldrick, G. M. SHELXTL, Version 6.1; Bruker Analytical X-ray
Systems, Inc.: Madison, WI, 2000.
(17) Ja¨kle, F.; Polborn, K.; Wagner, M. Chem. Ber. 1996, 129, 603.
(18) Janiak, C.; Braum, L.; Girgsdies, F. J. Chem. Soc., Dalton Trans. 1999,
3133.
(19) (a) Kaufmann, D. Chem. Ber. 1987, 120, 853. (b) Kaufmann, D. Chem.
Ber. 1987, 120, 901. (c) Gross, U.; Kaufmann, D. Chem. Ber. 1987,
120, 991. (d) Rottlander, M.; Palmer, N.; Knochel, P. Synlett 1996,
573. (e) Hupe, E.; Calaza, M. I.; Knochel, P. Chem. Commun. 2002,
1390.
Scheme 1. High-Yield Route to Na[(p-IC6H4)B(pz3R)]
(R ) H, Me)
THF
p-I2C6H4 + 2LitBu -78 °C8 “LiC6H4I” + LiI + HCMe3 +
H2CCMe2 i. SiMe Cl, -78 °C, 1 h8 Me3SiC6H4I + LiCl (A)
3
ii. RT, 1.5 h
(20) Haubold, W.; Herdtle, J.; Gollinger, W.; Einholz, W. J. Organomet.
Chem. 1986, 315, 1.
neat or CDCl3
(21) White, D.; Faller, J. W. J. Am. Chem. Soc. 1982, 104, 1548.
(22) Similar to (a) [PhB(µ-O)]3‚(Hpz): Niedenzu, K.; Bielawski, J. Inorg.
Chem. 1986, 25, 1771. Niedenzu, K.; Bielawski, J. Inorg. Chem. 1986,
25, 85. (b) [PhB(µ-O)]3‚(7-azaindole): Wu, Q. G.; Wu, G.; Brancaleon,
L.; Wang, S. Organometallics 1999, 18, 2553. (c) [PhB(µ-O)]3‚(py):
Beckmann, J.; Dakternieks, D.; Duthie, A.; Lim, A. E. K.; Tiekink,
E. R. T. J. Organomet. Chem. 2001, 633, 149.
Me3SiC6H4I + 1.2BBr3
8 IC6H4BBr2 + SiMe3Br
60 °C, 8 h
(B)
toluene
3Hpz3R
(R ) H, Me)
IC6H4BBr2 +
+ 2NEt3 -2[HNEt ][Br]8
3
+NaOtBu
IC6H4B(pz3R)2(Hpz3R)
8 [Na][IC6H4B(pz3R)3] (C)
(23) Kisko, J. L.; Haseall, T.; Kimblin, C.; Parkin, G. J. Chem. Soc., Dalton
Trans. 1999, 1929.
-HOtBu
1856 Inorganic Chemistry, Vol. 44, No. 6, 2005

Polymorphism in Fe[(p-IC6H4)B(3-Mepz)3]2
Table 1. Crystallographic Data for Fe[(p-IC6H4)B(pz)3]2, Fe[(p-IC6H4)B(3-Mepz)3]2 (Form InSCO), Fe[(p-IC6H4)B(3-Mepz)3]2 (Form IISCO), and
Fe[(p-IC6H4)B(3-Mepz)3]2‚CH2Cl2 (Form IIISCO
)
Fe[(IC6H4)B(3-Rpz)3]2
R ) Me
R ) H
form IIISCO
(2CH2Cl2)
form IIISCO
(2CH2Cl2)
form InSCO
form IISCO
form IISCO
form IISCO
empirical
formula
formula wt
temperature (K)
space group
a (Å)
C30H26B2-
FeI2N12
885.90
150(1)
C36H38B2-
FeI2N12
970.05
150(1)
P21/c
11.1677(6)
13.0473(7)
13.1043(7)
90
90.0800(10)
90
1909.40(18)
2
1.687
C36H38B2-
FeI2N12
970.05
294(1)
P21/c
11.0501(5)
15.7119(7)
11.8054(5)
90
102.9270(10)
90
1997.68(15)
2
1.613
C36H38B2-
FeI2N12
970.05
200(1)
P21/c
10.7587(5)
15.7861(8)
11.6703(6)
90
102.0760(10)
90
1938.20(17)
2
1.662
C36H38B2-
FeI2N12
970.05
100(1)
P21/c
10.4732(5)
15.8022(7)
11.5601(5)
90
101.1200(10)
90
1877.27(15)
2
1.716
C38H42B2Cl4-
FeI2N12
1139.91
200(1)
C38H42B2Cl4-
FeI2N12
1139.91
100(1)
Pbcn
P21/c
P21/c
18.0250(9)
8.7070(4)
20.7543(10)
90
90
90
3257.3(3)
4
1.807
12.0760(7)
14.6548(9)
12.7920(8)
90
93.5690(10)
90
2259.4(2)
2
1.676
11.8468(7)
14.5537(9)
12.7124(8)
90
93.3170(10)
90
2188.1(2)
2
1.730
b (Å)
c (Å)
R (°)
â (°)
γ (°)
volume (Å3)
Z
F
calcd (g/cm3)
abs. coeff. (mm-1
final R indices
[I > 2σ(I)]
)
2.402
R1 ) 0.0337,
wR2 ) 0.0750
2.057
R1 ) 0.0272,
wR2 ) 0.0726
1.966
R1 ) 0.0366,
wR2 ) 0.0954
2.026
R1 ) 0.0355,
wR2 ) 0.0831
2.092
R1 ) 0.0275,
wR2 ) 0.0662
1.980
R1 ) 0.0318,
wR2 ) 0.0772
2.045
R1 ) 0.0272,
wR2 ) 0.0663
desired pyrazoles, produced intractable mixtures of Li[B(3-
Mepz)4], Li[HB(3-Rpz)3], and trace amounts of Li[(p-
IC6H4)B(3-Rpz)3]. In either route, the yields of the desired
products suffered poor reproducibility and never exceeded
longer than in LS t2g6eg*0 complexes. Because the bite angles
of tris(pyrazolyl)borate ligands are generally fixed, there are
other subtle distortions of the ligand framework that occur
on binding to metals of different size. We have previously
noted12g in tris(pyrazolyl)methane iron(II) complexes that the
extent of tilting of the pyrazolyl rings from ideal C3V
symmetry depends largely upon the size of the metal, and
thus the spin state of the iron(II) complexes. The M-N(n1)-
N(n2)-B and the M-N(n1)-N(n2)-C(n1) torsion angles
(see Figure 1), where n is the pyrazolyl ring number, describe
the degree of ligand tilting. This tilting is best visualized in
the bottom right views of Figures 1 and 4, which show the
molecules oriented down the pseudo-three-fold axis. When
these ligands are bound to smaller metal ions, e.g., LS iron-
(II), the M-N-N-B and M-N-N-C torsion angles
approach their ideal values of 0° and 180°, respectively.
When the ligands bind larger metal ions, e.g., HS iron(II),
the M-N-N-B angle increases and the M-N-N-C angle
decreases in magnitude.
The X-ray structural study of Fe[(p-IC6H4)B(pz)3]2 showed
it was LS in the solid state. The ORTEP diagram of Fe[(p-
IC6H4)B(pz)3]2 is given in Figure 1, and selected bond
distances and angles are given in Table 2. The average Fe-N
bond distance was found to be 1.98 Å, and the average Fe-
N-N-C and Fe-N-N-B torsion angles are 173° and 5°,
respectively. The short Fe-N bond distance and the torsion
angles are consistent with the related LS iron(II) pyra-
zolylborate compounds, such as Fe[PhB(pz)3]2 (1.99 Å, 171°/
5°) and Fe[pzB(pz)3]2 (1.97 Å, 174°/2.4°).24
1
25%. H NMR spectra of violet Fe[(p-IC6H4)B(pz)3]2 in
CDCl3 showed broadened resonances in the 8-6 ppm range,
indicating a LS diamagnetic complex at room temperature.
The broadened nature of the resonances is attributed to a
paramagnetic impurity, possibly a result of the slow decom-
position of the complex in CDCl3. In contrast, the 1H NMR
spectra of colorless CDCl3 solutions of Fe[(p-IC6H4)B(3-
Mepz)3]2 exhibit broad resonances in the range of -6 to 50
ppm, a range that is indicative of the HS, paramagnetic nature
of this complex in solution.
Solid State Structures. The Fe[(p-IC6H4)B(3-Rpz)3]2 (R
) H, Me) complexes can be obtained as exquisite crystals
by layering CH2Cl2 solutions with MeOH and allowing the
solvents to slowly diffuse. The crystallizations are best
performed in the absence of light (foil-covered vials were
used in the present case), which prevents the solutions from
turning either brown when R is H or orange when R is Me
as a result of the formation of unidentified iron(III) impurities
(as is revealed by Mo¨ssbauer spectroscopy in the latter case).
Simple inspection of the color of the crystals can be used
as a first indication of the spin states of the iron(II) poly-
(pyrazolyl)borate complexes because the LS complexes are
purple whereas the HS complexes are colorless. In the case
of Fe[(p-IC6H4)B(3-Rpz)3]2, the R ) H derivative is purple,
whereas the R ) Me derivative is colorless at room
temperature. Once formed, the crystals of each derivative
are resistant to redissolution in CH2Cl2 and exhibit only
modest solubility in CDCl3. The as-formed powders of these
complexes exhibit much greater solubility.
The supramolecular structure of Fe[(p-IC6H4)B(pz)3]2,
shown in Figure 2, is that of corrugated sheets. The
corrugated sheets result from two types of CH-π interactions
and a set of π-π interactions. The molecules of Fe[(p-
IC6H4)B(pz)3]2 are arranged in chains that are oriented along
b by one set of CH-π interactions between the acidic
hydrogens H(12) at the 4-position of the pyrazolyl ring of
The electron configurations of the iron(II) poly(pyrazolyl)-
borate complexes can be readily determined from the Fe-N
bond distances in the solid-state structures because the
population of the antibonding eg* orbitals in HS t2g4eg*2
complexes causes the Fe-N bond distances to be ca. 0.2 Å
(24) Sohrin, Y.; Kokusen, H.; Matsui, M. Inorg. Chem. 1995, 34, 3928.
Inorganic Chemistry, Vol. 44, No. 6, 2005 1857

Reger et al.
Figure 1. View of crystals (bottom left) and ORTEP diagrams of Fe[(p-IC6H4)B(pz)3]2 showing atom labeling (top) and (bottom right) a view emphasizing
the Fe-N-N-C torsion angles. Thermal ellipsoids are drawn at the 50% probability level.
distances between the sheets; they are bound by simple van
der Waals interactions.
The Fe[(p-IC6H4)B(3-Mepz)3]2 complex crystallizes in any
one of three colorless forms, depending on the crystallization
conditions (see Experimental Section). Two of the forms are
polymorphs (forms InSCO and IISCO), whereas the third form
(IIISCO) is a methylene chloride solvate. A comparison of
the unit cell parameters and other details of the three
crystalline forms of Fe[(p-IC6H4)B(3-Mepz)3]2 is summarized
in Table 1. Because the first two crystalline forms were
originally visually indistinguishable at room temperature, it
was fortunate that these polymorphs were easily distinguish-
able at lower temperatures, as is illustrated in Figure 3.
Polymorph InSCO remains colorless (or pale blue-green) upon
cooling to 78 K, whereas polymorph IISCO changes from
colorless to violet upon cooling to 78 K, a change that is
Figure 2. Supramolecular structure of Fe[(p-IC6H4)B(pz)3]2. (Top) View
indicative of a change of spin state of the iron(II) ion.
depicting stacking of corrugated sheets held together by π-π (blue lines)
Crystals of form IIInSCO slowly turn violet as the temperature
and CH-π interactions (red lines). (Bottom) View showing the stacking
is lowered.
along the b axis as a result of CH-π (red lines) and π-π (blue lines)
interactions.
Single-crystal X-ray structural studies of each form of
Fe[(p-IC6H4)B(3-Mepz)3]2 revealed that the iron(II) ions are
HS at the highest temperatures at which diffraction data were
one borate ligand and the π-cloud of a neighboring iodo-
phenyl ring {CH(12)-arene centroid distance (Ct) of 3.29
Å, CH(12)-arene centroid angle of 126.5°} working in
tandem with a π-π interaction involving neighboring
pyrazolyl rings that contain N(11) {centroid-centriod dis-
tance (Ct[N(11)]-Ct[N(11)]) of 3.87 Å, perpendicular in-
terplane separation of 3.44 Å, parallel arene planes with a
slip angle between plane centroids, â, of 27.3°}. The chains
are connected in a second dimension to form a sheet as a
result of a second set of CH-π interactions between
neighboring iodophenyl substituents [CH(5)-Ct(arene) 3.42
Å, 155.4°]. The geometry of each type of interaction is within
typical values.25 There are no unusually short interatomic
collected, i.e., 150, 294, and 200 K for InSCO, IISCO, and IIISCO
,
respectively. Due to rapid desolvation even when encased
in a protective coat of epoxy resin, crystals of form IIISCO
did not survive data collection at temperatures above 200
K. Figure 4 shows a numbered ORTEP for form InSCO as
well as a view emphasizing the Fe-N-N-C torsion angles.
The average iron-nitrogen bond distances are 2.18, 2.18,
and 2.16 Å for forms InSCO, IISCO, and IIISCO, respectively.
(25) (a) Takahashi, H.; Tsuboyama, S.; Umezawa, Y.; Honda, K.; Nishio,
M. Tetrahedron 2000, 56, 6185. (b) Janiak, C. J. Chem. Soc., Dalton
Trans. 2000, 3885. (c) Reger, D. L.; Gardinier, J. R.; Semeniuc, R.
F.; Smith, M. D. J. Chem. Soc., Dalton Trans. 2003, 1712.
1858 Inorganic Chemistry, Vol. 44, No. 6, 2005

Polymorphism in Fe[(p-IC6H4)B(3-Mepz)3]2
Figure 3. (Top) Temperature dependence of the crystal color of the two polymorphs of Fe[(p-IC6H4)B(3-Mepz)3]2 alternately epoxied on a glass fiber.
(Middle) As-grown intergrowth twin aggregation of form InSCO (colorless) and form IISCO (purple) at 150 K. (Bottom) Temperature dependence of the crystal
color for the CH2Cl2 solvate of Fe[(p-IC6H4)B(3-Mepz)3]2.
the LS Fe[(p-IC6H4)B(pz)3]2 complex. Moreover, the average
Fe-N-N-C and Fe-N-N-B torsion angles observed for
Fe[(p-IC6H4)B(3-Mepz)3]2 are 165 and 9° for form InSCO, 166
and 9° for form IISCO, and 164 and 10° for form IIISCO. In
contrast, these angles are 173 and 5° in the LS Fe[(p-
IC6H4)B(pz)3]2 complex. The torsion angles in the HS
complexes are much smaller than the 177 and 178° angles
observed for the two crystallographically independent HS
iron(II) sites found26 in Fe[HB(3-Mepz)3]2. The steric
interactions involving the hydrogens at the 5-position of the
pyrazolyl rings and the atoms of the aryl group in complexes
with the [(p-IC6H4)B(3-Mepz)3]- moieties direct the defor-
mation of the ligands such that the pyrazolyl rings must tilt
to accommodate binding to a larger metal ion compared to
the HS iron(II) complex of HB(3-Mepz)3, whose ligands
deform in other ways (about the boron, for instance).
Variable-temperature studies revealed the spin-crossover
behavior of forms IISCO and IIISCO. It should be noted that,
Figure 4. View of crystals (bottom left) and ORTEP diagrams of Fe[(p-
IC6H4)B(3-Mepz)3]2 showing atom labeling (top) and a view emphasizing
the Fe-N-N-C torsion angles (bottom right). Thermal ellipsoids are drawn
at the 50% probability level, and hydrogen atoms have been removed for
in the case of the higher temperature structures of IISCO, the
displacement ellipsoids for the iodine atoms are abnormally
clarity.
large and elongated perpendicular to the plane of the phenyl
ring (Figure 5). The ring carbon ellipsoids near the iodine
atom (especially C4) also show a similar elongation. This
elongation can be attributed to a slight positional disorder
of the -C6H4I group at higher temperatures, a displacement
that becomes more pronounced at the molecule’s extremity.
A two-component disorder model was used for optimal
refinement of this structural disorder. As can be seen, this
These distances can be compared with the average Fe-N
distances found for other HS iron(II) borate complexes, such
as the 2.20 Å distance found26 in Fe[HB(3-Mepz)3]2, and
contrasted with the 1.96 Å average Fe-N distance found in
(26) Calogero, S.; Lobbia, G. G.; Cecchi, P.; Valle, G.; Friedl, J. Polyhedron
1994, 13, 87.
Inorganic Chemistry, Vol. 44, No. 6, 2005 1859

Reger et al.
Figure 5. Changes in the crystal color, molecular structure, and Fe-N bond lengths of form IISCO of Fe[(p-IC6H4)B(3-Mepz)3]2 at different temperatures.
Table 2. Selected Bond Lengths and Angles for Fe[(p-IC6H4)B(pz)3]2 and Fe[(p-IC6H4)B(3-Mepz)3]2 (Forms InSCO, IISCO, and IIISCO
)
Fe[(IC6H4)B(3-Rpz)3]2
R ) H
R ) Me
form InSCO
150 K
form IISCO
294 K
form IISCO
form IISCO
form IIISCO
200 K
form IIISCO
100 K
temperature
150 K
200 K
100 K
Bond Distances (Å)
Fe-N(11)
Fe-N(21)
Fe-N(31)
1.963(2)
1.952(2)
1.974(2)
2.137(2)
2.206(2)
2.208(2)
2.124(2)
2.207(2)
2.195(2)
2.049(2)
2.109(2)
2.097(2)
1.967(2)
2.009(2)
2.000(2)
2.188(2)
2.137(2)
2.162(2)
2.002(2)
1.9804(19)
1.9953(19)
Bond Angles (°)
85.37(7)
88.46(7)
85.00(7)
180
180
180
94.63(7)
91.54(7)
95.00(7)
N(11)-Fe(1)-N(21)
N(21)-Fe(1)-N(31)
N(31)-Fe(1)-N(11)
N(11a)-Fe(1)-N(11)
N(21a)-Fe(1)-N(21)
N(31a)-Fe(1)-N(31)
N(11)-Fe(1)-N(21a)
N(21)-Fe(1)-N(31a)
N(31)-Fe(1)-N(11a)
88.22(10)
88.09(10)
88.63(10)
180
180
180
91.78(10)
91.91(10)
91.37(10)
85.18(6)
87.63(6)
85.07(6)
180
180
180
94.82(6)
92.37(6)
94.93(6)
87.40(8)
89.18(8)
87.07(8)
180
180
180
92.60(8)
90.82(8)
92.93(8)
89.39(8)
89.99(8)
89.56(8)
180
180
180
90.61(8)
90.01(8)
90.44(8)
85.79(9)
84.91(9)
88.10(9)
180
180
180
94.21(9)
95.09(9)
91.90(9)
89.24(8)
89.27(8)
89.95(8)
180
180
180
90.76(8)
90.73(8)
90.05(8)
“disorder” is alleviated at lower temperatures. Over the
studied temperature range, the average Fe-N bond distances
and torsion angles are not affected by the treatment and
subsequent refinement of the disorder. On lowering the
temperature to 200 K, the crystal of form IISCO turns pink
(middle of Figure 5), and the observed average Fe-N bond
distance of 2.09 Å is intermediate between the typical HS
and LS Fe-N distances of 2.20 and 1.98 Å, respectively,
indicative of a significant population of the LS state in the
crystal at this temperature. The associated average Fe-N-
N-C and Fe-N-N-B torsion angles are 169 and 7°. Upon
a further lowering of the temperature to 100 K, the crystal
becomes purple, the average Fe-N distance is 1.99 Å, and
the associated average Fe-N-N-C and Fe-N-N-B
torsion angles are 172 and 5°, indicating both that the fraction
of HS iron(II) is negligible in the crystal at this temperature
and that the spin transition is complete or nearly complete.
Likewise, the 100 K structure of form IIISCO shows a shorter
Fe-N bond distance of 1.99 Å and more “ideal” Fe-N-
N-C and Fe-N-N-B torsion angles of 171 and 5° than
at 200 K, indicating that it has changed over to the LS form
at this temperature.
A comparison of the highest temperature, HS supra-
molecular structures of the polymorphs of Fe[(p-IC6H4)B-
(3-Mepz)3]2 is provided in Figure 6. Overall, the supra-
molecular structure of InSCO is that of tightly stacked planar
sheets, whereas the other two forms are made up of loosely
packed corrugated sheets. The first polymorph, InSCO, crystal-
lizes with a highly ordered three-dimensional supramolecular
structure organized by weak CH-π and CH‚‚‚I inter-
actions25,28 between neighboring molecules of Fe[(p-IC6H4)B-
(3-Mepz)3]2. As may be seen in the bottom left of Figure 6,
(27) Cecchi, P.; Berrottoni, M.; Giorgetti, M.; Calogero, S.; Lobbia, G.
G.; Stievano, L. Inorg. Chim. Acta 2001, 318, 67.
(28) Ugozzoli, F.; Arduini, A.; Massera, C.; Pochini, A.; Secchi, A. New
J. Chem. 2002, 26, 1718.
1860 Inorganic Chemistry, Vol. 44, No. 6, 2005

Polymorphism in Fe[(p-IC6H4)B(3-Mepz)3]2
Figure 6. Comparison of the supramolecular structures of the HS forms InSCO (left), IISCO (center), and IIISCO (right) of Fe[(p-IC6H4)B(3-Mepz)3]2. Crystals
of form InSCO have a three-dimensional supramolecular structure based on CH-π interactions (red lines) that form sheets and on CH‚‚‚I interactions (green
lines) that hold sheets together. Crystals of forms IISCO and IIISCO have two-dimensional corrugated sheet structures based on CH-π interactions (red lines),
and there are no short contacts between adjacent sheets.
a CH-π interaction between the acidic hydrogen at the
4-position of a pyrazolyl ring and the π-electron cloud of a
neighboring C6H4I ring organizes the molecules into poly-
meric chains along the a-axis [CH(22)-Ct 3.05 Å, 122.0°],
whereas short CH‚‚‚I interactions28 between the halogen and
the acidic hydrogen at the 5-position of the pyrazolyl ring
of a neighboring molecule [CH(11)‚‚‚I: 3.14 Å, 142.3°]
further organize the chains into sheets in the ac-plane. It
should be noted that the closest iodine-iodine distance is
4.79 Å, which is much larger than 3.96 Å, twice the van der
Waals radius of iodine. As is best seen in the top drawing
in Figure 6, additional CH-π interactions involving the
methyl group hydrogens H(14b) and H(24c) and the π-clouds
of the pyrazolyl rings that contain N(31) and N(11),
respectively {CH(14b)-Ct[N(31)], 3.37 Å, 147.6°; CH-
(24c)-Ct[N(11)], 3.23 Å, 141.1°}, help to form sheets in
the bc-plane, thereby providing cohesion of the overall
supramolecular structure in three dimensions.
The second polymorph, IISCO, crystallizes as loosely
stacked corrugated sheets, as is shown in the middle of Figure
6, and exhibits a packing arrangement similar to that
observed in the Fe[(p-IC6H4)B(pz)3]2 complex. In this case,
however, the sheets are found in the bc-plane as a result of
three sets of CH-π interactions, one involving CH(22) of a
pyrazolyl ring directed to the π-cloud of a second pyrazolyl
ring containing N(21) {CH(22)-Ct[N(21)] 3.15 Å, 126.6°}
and two interactions involving H(5) and H(6) of the
iodophenyl group which interact with the π-clouds of the
pyrazolyl rings containing N(31) and N(11), respectively
{CH(5)-Ct[N(31)], 3.18 Å, 168.0°; CH(6)-Ct[N(11)], 3.30
Å, 152.2°}. The corrugated sheets, shown edge-on in the
middle of Figure 6, stack along the a-axis, but with no
unusually short interatomic distances.
Crystals of IIISCO exhibit a loosely stacked corrugated sheet
structure (see the right portion of Figure 6) that is similar to
form IISCO, but in this case, the sheets are organized by only
one rather than three sets of CH-π interaction. In these
crystals, the symmetry operations on the interacting asym-
metric units permit the interaction to propagate in more than
one dimension. This set of CH-π interactions, which occur
between phenyl group hydrogen donors and the π-clouds of
the pyrazolyl rings containing N(11) {CH(45)-Ct[N(11)],
3.38 Å and 142.7°}, are near the upper limit for such
interactions.25 Solvent molecules reside in the voids produced
by the loose packing arrangement of molecules both between
and within the sheet structure.
Magnetic Studies. The temperature dependence of the
effective magnetic moment was measured for bulk crystalline
samples of Fe[(p-IC6H4)B(3-Mepz)3]2 grown under the three
distinct crystallization conditions mentioned earlier, and the
temperature dependence of the inverse molar magnetic
susceptibility and µeff for each is shown in Figure 7. At room
temperature, the observed magnetic moments for InSCO, IISCO
,
and the desolvated crystals of IIISCO are 5.1, 5.2, and 5.0 µB,
Inorganic Chemistry, Vol. 44, No. 6, 2005 1861

Reger et al.
Figure 7. Plots of the (a) inverse molar magnetic susceptibility and (b) effective magnetic moment versus temperature for InSCO, IISCO, and the desolvated
crystals of IIISCO
.
respectively. These moments are slightly higher than the spin-
only moment of 4.9 µB expected for a HS iron(II) complex
and indicate the expected31 presence of a small orbital
contribution to the moment. These room-temperature mo-
ments are also in good agreement with the moments observed
in related HS iron(II) complexes such as Fe[HB(3,5-Me2-
pz)3]2 and Fe[HB(3,4,5-Me3pz)3]2, which exhibit10b-d mo-
ments of 5.0 and 5.2 µB at 298 K, respectively, {Fe[2,6-
di(pyrazol-3-yl)pyridine]2}[Fe(CN)5(NO)], which exhibits11a
a moment of 5.4 µB at 298 K, and {Fe[2,6-di(pyrazol-1-yl)-
pyridine]2}[ClO4]2(CH3CN), which exhibits11b a moment of
ca. 5.5 µB at 298 K.
in the spin state of the iron(II) ion, i.e., InSCO is a HS
paramagnetic iron(II) complex at all temperatures above
4 K.
In contrast to the behavior of InSCO, as may be seen in
Figure 7b, the magnetic moment of IISCO slowly decreases
to 4.7 µB upon cooling until, at ca. 131 K, the moment
abruptly decreases to 1.0 µB at 121 K; the moment then
decreases to a constant value of about 0.8 µB on further
cooling. The residual moment of ca. 0.8 µB is due to both
some temperature-independent paramagnetism10 and the
presence of traces of either some HS iron(II) or HS iron(III)
impurity. If one assumes a 500 × 10-6 emu/mol contribution
from the temperature-independent paramagnetic susceptibility
in IISCO, a reasonable value for a LS complex at temperatures
just below a spin crossover, one finds the presence of either
1.1 wt % of an iron(III) impurity that is assumed to have a
moment of 5.92 µB or 1.5 wt % of an iron(II) impurity that
is assumed to have a moment of 5.20 µB. Such a low level
of impurity will not be observed in most studies but will be
apparent in the magnetic properties of an essentially dia-
magnetic LS iron(II) complex. Further, as would be expected,
both above and below the spin crossover, the inverse molar
magnetic susceptibility is linear (see Figure 7a). The rather
negative intercept observed for the inverse susceptibility of
the HS state is not a consequence of the impurity, but rather
is a consequence of a small admixture of the LS state into
the predominately HS state above the spin crossover. Such
an admixture would also account for the small decrease in
the magnetic moment observed between ca. 125 and 225 K.
Upon cooling of the sample, the magnetic moment of InSCO
decreases slightly from 5.0 µB at 295 K to ca. 4.8 µB at 50
K, and then decreases more substantially below ca. 40 K.
However, as may be seen in Figure 7a, 1/ø versus T is linear
even at the lowest temperatures. The decrease in the moment
below ca. 40 K is exactly that expected31,32 of a magnetically
dilute HS iron(II) complex in a highly distorted octahedral
coordination environment. Thus, the decrease in moment at
the lowest temperatures is not associated with any change
(29) Long, G. J.; Hutchinson, B. B. Inorg. Chem. 1987, 26, 608.
(30) (a) Hayami, S.; Gu, Z.-z.; Yoshiki, H.; Fujishima, A.; Sato, O. J. Am.
Chem. Soc. 2001, 123, 11644. (b) Hayami, S.; Gu, Z.-z.; Shiro, M.;
Einaga, Y.; Fujishima, A.; Sato, O. J. Am. Chem. Soc. 2000, 122,
7126. (c) Boe`a, R.; Boe`a, M.; Dlha´o`, L.; Falk, K.; Fuess, H.; Haase,
W.; Jarosˇcˇiak, R.; Papa´nkova´, B.; Renz, F.; Vrbova´, M.; Werner, R.
Inorg. Chem. 2001, 40, 3025.
(31) Figgis, B. N. Introduction to Ligand Fields; Wiley-Interscience: New
York, 1966; p 274.
(32) Long, G. J.; Baker, W. A., Jr. J. Chem. Soc., A 1971, 2956.
1862 Inorganic Chemistry, Vol. 44, No. 6, 2005

Polymorphism in Fe[(p-IC6H4)B(3-Mepz)3]2
Table 3. Mo¨ssbauer Spectral Hyperfine Parameters for Samples of Fe[(p-IC6H4)B(pz)3]2 and Fe[(p-IC6H4)B(3-Mepz)3]2 Crystallized under Different
Conditions
compound
(crystallization conditions)
T,
K
δ,
∆EQ,
mm/s
Γ,
mm/s
area,
%
mm/sa
assignment
Fe[(p-IC6H4)B(pz)3]2
295
78
295
78
295
225
0.380
0.447
1.004
1.112
1.002
1.045
0.451
1.096
0.512
1.112
0.542
0.539
0.999
1.110
0.538
1.132
0.543
0.32
0.33
3.69
3.84
3.68
3.78
0.42
3.79
0.42
3.82
0.42
0.43
3.69
3.83
0.42
3.80
0.42
0.25
0.25
0.23
0.26
0.22
0.24
0.27
0.26
0.30
0.24
0.24
0.31
0.27
0.24
0.25
0.25
0.26
100
100
100
100
100
83.5
16.5
71.8
28.2
9.8
90.2
100
100
21.3
78.7
21.6
78.4
LS iron(II)
LS iron(II)
HS iron(II)
HS iron(II)
HS iron(II)
HS iron(II)
LS iron(II)
HS iron(II)
LS iron(II)
HS iron(II)
LS iron(II)
LS iron(II)
HS iron(II)
HS iron(II)
LS iron(II)
HS iron(II)
LS iron(II)
Fe[(p-IC6H4)B(3-Mepz)3]2
(form InSCO)
Fe[(p-IC6H4)B(3-Mepz)3]2
(form IISCO
)
155
78
4.2
295
78
Fe[(p-IC6H4)B(3-Mepz)3]2
(form IIISCO, dried)
4.2
a The isomer shifts are given relative to room-temperature R-iron foil.
The effective magnetic moment of the sample that was
grown from CH2Cl2/MeOH at -20 °C and then air-dried to
represent the desolvated form IIISCO undergoes a gradual
decrease from 4.8 µB above 235 K to a constant value of
2.2 µB beginning near 140 K (see Figure 7b). The relatively
high residual magnetic moment in this sample below 140 K
is due predominately to the presence of ca. 20% of InSCO as
an impurity that is clearly observed in the Mo¨ssbauer spectra
of the samples (see below) and, perhaps to a lesser extent,
to the expected temperature-independent paramagnetism of
the LS fraction of the desolvated IIISCO complex. Once again,
as would be expected and may be seen in Figure 7a, the
inverse molar magnetic susceptibility is linear both above
and below the spin-state crossover.
Mo1ssbauer Spectral Studies. The same samples used for
the magnetic studies were subsequently used for Mo¨ssbauer
spectral studies, and the resulting Mo¨ssbauer spectra are
shown in Figures 8-10. The spectra have been fit with
symmetric quadrupole doublets, and the resulting hyperfine
parameters are given in Table 3.
Figure 8. Mo¨ssbauer spectra of polymorph InSCO of Fe[(p-IC6H4)B(3-
Mepz)3]2 obtained at 295 and 78 K. The solid line corresponds to a fit to
the experimental data, +, with the hyperfine parameters given in Table 3.
The 295 K spectrum of crushed but unground crystals of
Fe[(p-IC6H4)B(3-Mepz)3]2 (InSCO, Figure 8) indicates that this
polymorph is 100% HS with an isomer shift, δ, of 1.004
mm/s and a quadrupole splitting, ∆EQ, of 3.69 mm/s. These
values are similar to the respective 295 K values of 1.06
and 3.8 mm/s observed26,27 for Fe[HB(3-Mepz)3]2, 1.03 and
3.7 mm/s observed29 for Fe[HB(3,5-Me2pz)3]2, and 1.04 and
3.7 mm/s observed29 for Fe[HB(3,4,5-Me3pz)3]2.
The lower isomer shift observed for Fe[(p-IC6H4)B(3-
Mepz)3]2 as compared with that for Fe[HB(3-Mepz)3]2
indicates a somewhat higher s-electron density at the iron-
57 nucleus in the former complex. It would be tempting to
associate this increased s-electron density with electron
donation by the p-IC6H4 substituent on the ligand, but this
substituent is surely too remote to have a significant influence
on the s-electron density at the iron(II) ion. Thus, it seems
more likely that the higher s-electron density is a conse-
quence of the shorter average Fe-N bond distance of 2.18
Å observed in Fe[(p-IC6H4)B(3-Mepz)3]2 as compared with
the distance of 2.20 Å observed26 in Fe[HB(3-Mepz)3].
Further, the slightly smaller quadrupole splitting observed
The 295 and 78 K spectra of crushed but unground crystals
of Fe[(p-IC6H4)B(pz)3]2 indicate that this complex is LS at
both temperatures. The observed hyperfine parameters are
almost identical to those observed10,29,33a for the LS iron(II)
in Fe[HB(pz)3]2 at the same temperatures. The only signifi-
cant difference is an increase in the quadrupole splitting from
0.21 mm/s in Fe[HB(pz)3]2 to 0.32 mm/s in Fe[(p-IC6H4)B-
(pz)3]2, a difference that is reflected in the slightly less
distorted coordination environment observed10a in the former
complex as compared to the latter. By using the structural
results10 of Oliver et al. for Fe[HB(pz)3]2, the average
percentage bond distance and bond angle distortion param-
eters33b are found to be 0.24 and 0.18, respectively, whereas
the results given in Table 2 for Fe[(p-IC6H4)B(pz)3]2 yield
the analogous percentage distortions of 0.37 and 0.24, values
which are significantly higher.
(33) (a) Grandjean, F.; Long, G. J.; Hutchinson, B. B.; Ohlhausen, L.; Neill,
P.; Holcomb, J. D. Inorg. Chem. 1989, 28, 4406. (b) Renner B.;
Lehmann, G. Z. Krist. 1986, 175, 43.
Inorganic Chemistry, Vol. 44, No. 6, 2005 1863

Reger et al.
Figure 10. Mo¨ssbauer spectra of the desolvated form of IIISCO, Fe[(p-
IC6H4)B(3-Mepz)3]2, obtained at 295, 78, and 4.2 K. The solid line
corresponds to a fit to the experimental data, +, with the hyperfine
parameters given in Table 3.
K. The relative area of this doublet increases to 28.2% at
155 K, predominates with a relative area of 90.2% at 78 K,
and is the only component observed at 4.2 K; the resulting
4.2 K isomer shift of 0.539 mm/s and quadrupole splitting
of 0.43 mm/s are typical of a LS iron(II) complex. This
behavior is completely consistent with an iron(II) complex
undergoing a complete HS-to-LS state crossover and agrees
with the magnetic properties shown in Figure 7.
Figure 9. Mo¨ssbauer spectra of polymorph IISCO of Fe[(p-IC6H4)B(3-
Mepz)3]2 obtained at several temperatures between 295 and 4.2 K. The
solid line corresponds to a fit to the experimental data, +, with the hyperfine
parameters given in Table 3.
The variable-temperature Mo¨ssbauer spectra (see Figure
10) of the sample obtained at -20 °C from MeOH/CH2Cl2
and then air-dried and containing predominately desolvated
crystals of IIISCO are very similar to the spectra observed
in Fe[(p-IC6H4)B(3-Mepz)3]2 as compared with that ob-
served26,27 in Fe[HB(3-Mepz)3]2 is probably a consequence
of the smaller distortions observed in the former complex
(see above).
for InSCO
and IISCO. Thus, the 295 K spectrum is fully HS,
but at both 78 and 4.2 K, the spectra exhibit the presence of
ca. 80% by area of a LS quadrupole doublet that may be
associated with IIISCO and 20% by area of a HS quadrupole
Upon cooling of the sample to 78 K, the isomer shift of
the InSCO form of Fe[(p-IC6H4)B(3-Mepz)3]2 increases to
1.112 mm/s as a result of the second-order Doppler shift,
but no other spectral changes are observed, and InSCO remains
completely HS, as would be expected from its observed
magnetic moment at low temperature.
doublet that may be associated with the impurity of InSCO
.
It is worth noting that the LS iron(II) isomer shift of 0.54
mm/s observed at 78 K for both IISCO and IIISCO is
significantly larger that the 0.47 mm/s value observed29 in
Fe[HB(pz)3]2. Further, the LS quadrupole splitting of 0.42
mm/s observed at 78 K for both IISCO and IIISCO is
significantly larger that the 0.22 mm/s splitting observed29
in Fe[HB(pz)3]2 and the 0.33 mm/s splitting observed in
Fe[(p-IC6H4)B(pz)3]2. All of these differences are those
expected from a comparison of the average bond distances
and the average percentage bond distortion parameters33b
observed for Fe[HB(pz)3]2, Fe[(p-IC6H4)B(pz)3]2, and both
IISCO and IIISCO. In all cases, the percentage distortion
parameters for IISCO and IIISCO are larger than for the other
two complexes.
The Mo¨ssbauer spectra of an absorber containing crushed
but unground crystals of the IISCO polymorph of Fe[(p-
IC6H4)B(3-Mepz)3]2 are shown in Figure 9. As expected from
the observed magnetic moment (see Figure 7), the 295 K
spectrum of IISCO shows a single HS iron(II) quadrupole
doublet with an isomer shift of 1.002 mm/s and a quadrupole
splitting of 3.68 mm/s, parameters which are virtually
identical to those of InSCO. However, upon cooling of the
sample, the behavior of IISCO is very different from that of
InSCO, as may be seen by a comparison of Figures 8 and 9.
Upon cooling, a second quadrupole doublet corresponding
to LS iron(II) appears with a relative area of 16.5% at 225
1864 Inorganic Chemistry, Vol. 44, No. 6, 2005

Polymorphism in Fe[(p-IC6H4)B(3-Mepz)3]2
Discussion
The Fe[(p-IC6H4)B(pz)3]2 complex, like the related Fe-
[PhB(pz)3]2 complex, is LS at room temperature. In contrast,
the 3-methyl-substituted Fe[(p-IC6H4)B(3-Mepz)3]2 complex
is HS at room temperature as a consequence of the added
steric interactions involving the six added 3-methyl substit-
uents.
Initially, the spin-state crossover exhibited by Fe[(p-
IC6H4)B(3-Mepz)3]2 appeared to be sample dependent, but
it was later found to be a consequence of the changing
relative distributions of the two polymorphs, InSCO and IISCO
,
and/or the (desolvated) solvate, IIISCO, in the samples. Once
conditions were found that produced each polymorph in
relatively high purity, it was found that the polymorphs
exhibited remarkably different electronic properties as a result
of differences in their crystal packing. Form InSCO crystallizes
with a highly ordered three-dimensional supramolecular
structure and does not undergo a spin-state crossover upon
cooling. In contrast, form IISCO, with a stacked two-
dimensional supramolecular structure, undergoes a reversible
and abrupt spin-state crossover between 126 and 131 K.
Qualitatively, the temperature dependence of the spin-state
crossover in single crystals of the third form, IIISCO, is
different from that of the second form, as can be seen by
the gradual crystal color changes in Figure 3 over similar
temperature ranges. Quantitative magnetic or Mo¨ssbauer
measurements on the solvated form were not possible due
to the rapid loss of the solvent. Bulk samples of the third
form were grown at -20 °C and were air-dried to completely
remove the solvent in the lattice. The magnetic data obtained
on these desolvated crystals showed a gradual transition
between about 140 and 235 K from the HS to the LS state,
as is confirmed by the Mo¨ssbauer spectra. The structure of
desolvated form IIISCO is unknown but is likely only a weakly
associated structure, at best, given that even the crystalline
form of the solvate is a loosely packed two-dimensional
structure organized by only weak CH-π interaction.
An examination of the temperature dependence of the
effective magnetic moment of the three forms reveals several
features. First, form InSCO does not undergo any electronic
spin-state crossover at temperatures above 4 K; second, form
IISCO exhibits an abrupt crossover between 126 and 131 K;
and, third, the desolvated form III shows a gradual transition
between 140 and 235 K. The solid-state structure of each of
the first two polymorphs is organized by several noncovalent
interactions, whereas it is likely that the desolvated third form
is not highly organized. The abrupt nature of the spin-state
crossover in IISCO can be envisioned to result from the
geometry changes associated with the spin-state crossover,
changes that are propagated throughout the crystal via the
noncovalent interactions that occur in the bc-plane of the
crystal.
Figure 11. Temperature dependence of the relative cell parameters, a/a294
,
b/b294, c/c294, and â/â294, and the cell volume in the InSCO (top) and IISCO
(bottom) polymorphs of Fe[(p-IC6H4)B(3-Mepz)3]2.
noncovalent sheets structures are loosely stacked, exhibits
the largest change, an ca. 5.2% compression from 11.05 Å
at 294 K to 10.47 Å at 100 K. In comparison, the changes
in the other two axes, where the noncovalent network is
strong, are much less dramatic. Indeed, the b-axis, which
contains the CH(pz)-π(pz) interactions, actually expands by
0.6% on cooling, from 15.71 Å at 294 K to 15.80 Å at 100
K, whereas the c-axis, which contains the CH(arene)-π-
(pz) interactions, contracts by only ca. 2.1% on cooling, from
11.81 Å at 294 K to 11.56 Å at 100 K. These large,
directionally oriented changes are contrasted by the changes
observed for InSCO, where the dimensions in all three
directions slowly contract linearly as the temperature is
lowered, the result expected for this compound with a three-
dimensional supramolecular structure that exhibits no spin
crossover.
The observation that form IISCO, with the less organized
structure, undergoes a crossover from the HS to the LS state,
whereas no such change is observed in InSCO, is in agreement
with the general observations by others30 that the “spin-state
crossover phenomenon depends strongly on intermolecular
interactions.” 30b The unique result reported herein is that the
comparison is made with polymorphs in which comparisons
of the supramolecular structures can be made on two different
crystalline polymorphs of the same molecular compound. It
is clear from these results that the organization of the
molecules in the solid state controlled by the noncovalent
interactions has a significant impact on the spin-crossover
temperature; the more highly organized structure better resists
the crossover to the LS state at low temperature. The third
form, the desolvated form of IIISCO, also supports this trend
in that the solid-state form used in the magnetic and
Mo¨ssbauer studies certainly has a less organized structure
The relative robustness of the sheet structure is also
manifested in changes in the crystal geometry upon cooling
(see Figure 11). For IISCO there is a 6% decrease in unit cell
volume, from 1998 Å3 at 294 K to 1877 Å3 at 100 K. Of the
axes involved in the change, the a-axis, along which the
Inorganic Chemistry, Vol. 44, No. 6, 2005 1865

Reger et al.
than the other two forms and it has the highest T1/2. In
addition, as revealed by the magnetic results shown in Figure
7, IISCO exhibits a much more “cooperative” spin-state
crossover changeover than does IIISCO, as would be expected
because of its more highly organized solid-state structure.
These results are consistent with previous observations that
the driving force for the spin-state crossover from LS to HS
iron(II) is dominated by the entropy term associated with
the electronic reorganization and the associated bond expan-
sion and ligand deformation processes. One can envision that
the HS state of form InSCO is energetically “locked-in” by
the multiple noncovalent interactions that prevent the spin-
state crossover from occurring; the net result would be a
stabilization of the HS state. Calorimetry measurements have
provided transition enthalpies, ∆H, of 6-15 kJ/mol and
entropies, ∆S, of 40-50 J/Kmol for charge-neutral iron(II)
spin-state crossover complexes.1 If the polymorphs of Fe-
[(p-IC6H4)B(3-Mepz)3]2 fall into the middle of these ranges,
i.e., ∆H ) 10 kJ/mol and ∆S ) 45 J/Kmol, and if the
crossover temperature of InSCO is at least 130 K lower than
that of IISCO, then the energy difference in the spin crossover
of IISCO and a hypothetical transition for InSCO are more
favorable in the former polymorph by about 6 kJ/mol. This
value appears to correlate well with the proposed interaction
energy25,28 (0.5-15 kJ/mol) and number of weak noncovalent
interactions stabilizing the crystals of InSCO and IISCO, where
more interactions are observed in the three-dimensional
structure of InSCO. Likewise, the energy difference in the spin
transitions between IISCO, with a T1/2 of 128 K, and the
disorganized, and possibly amorphous, desolvated IIISCO, with
a T1/2 of 175 K, is of the order of 2 kJ/mol.
supramolecular structures, as is shown herein through a
comparison of solid-state structures and magnetic and
Mo¨ssbauer spectral studies of the polymorphs of Fe[(p-
IC6H4)B(3-Mepz)3]2. Noncovalent interactions, such as π-π,
CH-π, and other weak hydrogen-bonding interactions, such
as CH-halide interactions, provide a vehicle for controlling
the geometric changes and can result in abrupt spin-state
crossover temperature ranges and even prevent the changeover
from occurring at low temperatures. From the current study,
we find that, for a given donor set, the more highly organized
structures will have lower spin-crossover temperatures and
more abrupt spin crossovers than those with less highly or
more “loosely” organized structures.
Acknowledgment. We thank Dr. Matthew J. Davis and
William Gemmill for assistance with the measurements made
with the SQUID magnetometer. We acknowledge with
thanks the National Science Foundation (CHE-0414239) and
the Petroleum Research Fund (36368-AC3) for support. The
Bruker CCD single-crystal diffractometer was purchased
using funds provided by the NSF Instrumentation for
Materials Research Program through Grant DMR:9975623.
L.R. and F.G. acknowledge with thanks the financial support
of the Fonds National de la Recherche Scientifique, Belgium,
through grants 9.456595 and the Ministe`re de la Re´gion
Wallonne for grant RW/115012.
Supporting Information Available: CIF files for Fe[(p-
IC6H4)B(3-Rpz)3]2 (R ) H and R ) Me, three forms at various
temperatures). This material is available free of charge via the
Internet at http://pubs.acs.org.
Conclusions
The spin-state crossover behavior of poly(pyrazolyl)borate
iron(II) compounds can be dramatically affected by their
IC048406Q
1866 Inorganic Chemistry, Vol. 44, No. 6, 2005