Tris(2-pyridylborate) (Tpyb) metal complexes: Synthesis, characterization, and formation of extrinsically porous materials with large cylindrical channels

Article abstract of DOI:10.1021/ic4010664

Sandwich-like metal complexes (Tpyb)₂M (M = Mg, Fe, Mn) that are based on the novel t-butylphenyltris(2-pyridyl)borate ligand were prepared and fully characterized by multinuclear NMR spectroscopy, high-resolution matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, and single crystal X-ray crystallography. The unique steric and electronic nature of the Tpyb ligand led to structural parameters and properties that are quite different to those of previously reported tris(pyrazolyl)borate and tris(2-pyridyl)aluminate complexes. Most importantly, depending on the crystallization procedure, supramolecular structures could be generated with relatively smaller (ca. 4-5 A) or larger (ca. 8 A) diameter pores propagating throughout the crystal lattice. Although the supramolecular structures are held together only by weak intermolecular C-H...π interactions, the solvent in the larger channels could be completely removed without any loss of crystallinity or degradation of the framework. Surface area and gas uptake measurements on the Mg complex further confirmed the permanent porosity, while the calculated non-localized density functional theory (NLDFT) pore diameter of 8.6 A proved to be in excellent agreement with that obtained from single crystal X-ray crystallography. Our new materials are remarkably thermally stable as degradation did not occur up to about 400 C based on thermogravimetric analysis (TGA), and a sample of the Mg complex showed no loss of crystallinity even after heating to 140 C under high vacuum for 72 h according to single crystal X-ray diffraction data.

Full text of DOI:10.1021/ic4010664

Article
pubs.acs.org/IC
Tris(2-pyridylborate) (Tpyb) Metal Complexes: Synthesis,
Characterization, and Formation of Extrinsically Porous Materials
with Large Cylindrical Channels
Chengzhong Cui, Patrick R. Shipman, Roger A. Lalancette,* and Frieder Jakle*
̈
Department of Chemistry, Rutgers UniversityNewark, 73 Warren Street, Newark, New Jersey 07102, United States
S
* Supporting Information
ABSTRACT: Sandwich-like metal complexes (Tpyb)2M (M =
Mg, Fe, Mn) that are based on the novel t-butylphenyltris(2-
pyridyl)borate ligand were prepared and fully characterized by
multinuclear NMR spectroscopy, high-resolution matrix-assisted
laser desorption/ionization (MALDI) mass spectrometry, and
single crystal X-ray crystallography. The unique steric and
electronic nature of the Tpyb ligand led to structural parameters
and properties that are quite different to those of previously
reported tris(pyrazolyl)borate and tris(2-pyridyl)aluminate com-
plexes. Most importantly, depending on the crystallization
procedure, supramolecular structures could be generated with
relatively smaller (ca. 4−5 Å) or larger (ca. 8 Å) diameter pores
propagating throughout the crystal lattice. Although the supra-
molecular structures are held together only by weak intermolecular C−H···π interactions, the solvent in the larger channels could
be completely removed without any loss of crystallinity or degradation of the framework. Surface area and gas uptake
measurements on the Mg complex further confirmed the permanent porosity, while the calculated non-localized density
functional theory (NLDFT) pore diameter of 8.6 Å proved to be in excellent agreement with that obtained from single crystal X-
ray crystallography. Our new materials are remarkably thermally stable as degradation did not occur up to about 400 °C based on
thermogravimetric analysis (TGA), and a sample of the Mg complex showed no loss of crystallinity even after heating to 140 °C
under high vacuum for 72 h according to single crystal X-ray diffraction data.
are attached to boron through strong and only moderately
polar B−C bonds. Moreover, the fact that pyridine is a better σ
donor than pyrazole and that the steric requirements are
different because of the use of six- rather than five-membered
heterocycles has implications on potential applications not only
in catalysis8 but also in supramolecular chemistry, where
tridentate ligands have had a remarkable impact.9,10
In this paper we describe in detail the complexation behavior
of the tris(2-pyridyl)borate (Tpyb) ligand and the character-
ization of a range of octahedral metal complexes (Tpyb)2M (M
= Mg, Fe, Mn). We also discuss the unexpected discovery of
extended 3-dimensional structures with large channels. The
creation of new porous materials that contain extended
channels or cavities is among the most important targets in
the field of crystal engineering because of their utility in the
capture and storage of gases and other small molecules,
separation science, catalysis and even fuel cells.11 Typically
these materials are stabilized by metal coordination, hydrogen
bonding, or halogen-bonding interactions.12 We introduce here
a very rare example of “permanently porous”13−16 molecular
materials that, although held together solely based on C−H···π
INTRODUCTION
■
Tris(1-pyrazolyl)borates (Tp, Chart 1) have evolved into one
of the most useful classes of ligands in modern coordination
chemistry.1 Commonly referred to as “scorpionates” to reflect
their distinct coordination geometry, they are potent ligands
that complex essentially every metal in the periodic table. As
uninegative tridentate six-electron donors, tris(1-pyrazolyl)-
borates may also be viewed as analogues of cyclopentadienyls.
One of the major advantages of pyrazolylborate ligands is their
versatility derived from the different steric and electronic effects
that are readily attained by varying the nature, number, and
position of the substituents in the pyrazole rings. These second
generation Tp ligands,2 in which substituents on the pyrazolyl
groups are introduced for steric and electronic fine-tuning, have
been followed by a third generation ligand class, in which
functional groups on the terminal (noncoordinated) substituent
on boron provide for even greater diversity and tunability
(Chart 1).3,4 Consequently, these and related ligands have
found widespread use in fields ranging from homogeneous
catalysis to bioinorganic chemistry and materials science.4,5
We have recently introduced an alternative ligand design in
which the pyrazolyl moieties in Tp are replaced with 2-pyridyl
groups to form tris(2-pyridyl)borate (Tpyb) ligands.6,7 An
advantage of this new type of ligand is that the pyridyl groups
Received: April 29, 2013
© XXXX American Chemical Society
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identified by single crystal X-ray diffraction (vide infra) as the
complex 2-Mg. This complex can be obtained in higher yields
of about 35% when the reaction is performed in toluene, the
crude mixture evaporated to dryness, washed with hot MeOH
and then cyclohexane. Alternatively, metal complexes of 1 can
be generated by treatment of the free ligand with an
appropriate metal salt in the presence of a base (Scheme 2).
Chart 1
a
Scheme 2. Synthesis of Metal Complexes 2-Fe and 2-Mn
a
(i) MCl2, NEt3.
Treatment of 1 with FeCl2 in THF/MeOH mixture containing
about 3% NEt3 gave 2-Fe as a red solid that was purified by
column chromatography and recrystallized from toluene.
Similarly, reaction of 1 with MnCl2 resulted in 2-Mn, which
was isolated as a light yellow solid. A small amount of hydrazine
was added to the reaction mixture to prevent air oxidation of
the metal ion.
interactions, feature large cylindrical channels (diameters of ca.
8 Å) that proved to be stable even at temperatures in excess of
100 °C.
While the 11B NMR shifts of 1, 2-Mg, and 2-Fe are very
1
similar (Figure 1), the pattern of the pyridyl rings in the H
RESULTS AND DISCUSSION
■
The Tpyb ligand 1 was prepared by reaction of t-butylphenyl
dibromoborane with 2-PyMgCl in CH2Cl2 at room temper-
ature (RT) (Scheme 1). An aqueous Na2CO3 solution was
Scheme 1. Synthesis of Ligand 1 and Magnesium Complex 2-
Mg
Figure 1. Overlay of 11B NMR spectra of ligand 1, complexes 2-Mg
and 2-Fe in CDCl3 (the broad features in 2-Mg and 2-Fe are due to
borosilicate glass).
NMR changed dramatically as a result of the mutual shielding
effect of the 6 pyridyl rings attached to the central Mg2+ ion. An
1
overlay of the H NMR spectra of the free ligand 1 and the
Mg(II) and Fe(II) complexes is shown in Figure 2. The pyridyl
protons in the ortho- and meta-positions to the ligating N
experience characteristic upfield shifts from 8.49 to 7.58/7.11
(ortho) and 7.10 to 6.64/6.44 ppm (meta) upon going from
the free ligand to the Mg/Fe complexes. Simultaneously, the
phenylene protons are strongly shifted downfield. The
assignments were confirmed by 2D NOESY NMR spectrosco-
py. The 13C NMR spectra exhibit characteristic quartets at
150.3 (J(11B,13C) = 55 Hz)/149.2 (J(11B,13C) = 57 Hz) and
185.0 (J(11B,13C) = 50 Hz)/188.0 (J(11B,13C) = 50 Hz) ppm,
which are due to coupling of the 11B nucleus to the ipso-phenyl
and ipso-pyridyl carbons, respectively. Complex 2-Mn is
paramagnetic, thus experiencing large shifts in both the 11B
(−109.6 ppm) and the 1H NMR spectra (Supporting
Information). The identity of each of the complexes was
further confirmed by matrix-assisted laser desorption/ioniza-
tion-time of flight (MALDI-TOF) mass spectrometry.
added, and the product extracted into CH2Cl2. The crude
product was purified by column chromatography on NEt3-
deactivated silica gel using a mixture of acetone and hexanes as
the eluent. Crystallization from toluene gave the analytically
pure ligand 1 in its protonated form as a colorless solid in 54%
yield.
During the synthesis of 1, we noticed that a small amount of
insoluble material remained after acetone extraction. This
fraction showed a sharp peak in the 11B NMR spectrum at a
slightly different chemical shift (−9.2 vs −10.8 ppm for 1).
Extraction of the residue with dichloromethane and recrystal-
lization from toluene afforded colorless crystals, which were
B
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1
Figure 2. Comparison of H NMR spectra (aromatic region) of ligand 1, complexes 2-Mg, and 2-Fe in CDCl3.
Figure 3. Ball-and-stick representations of the X-ray structures of 2-Mg, 2-Fe, and 2-Mn (hydrogen atoms are omitted).
To study the coordination environment about the metal ion
we performed single crystal X-ray diffraction analyses on
crystals obtained by slow evaporation of toluene solutions
(Figure 3). For 2-Fe additional crystal samples were grown by
slow solvent evaporation from CH2Cl2 and cyclohexane,
respectively. The geometric features of the main molecules
are quite similar to those of the toluene-grown crystals and
hence not further discussed here. For all complexes the metal
center lies on a crystallographic inversion center and adopts a
slightly distorted octahedral geometry. Two of the M-N bonds
are of similar length in each case (2-Mg: 2.1932(12) and
2.1916(11) Å, 2-Fe: 1.9902(13) and 1.9880(13) Å, 2-Mn:
2.268(2) and 2.265(2) Å), whereas the third one (2-Mg:
2.1322(16), 2-Fe: 1.9685(13), 2-Mn: 2.179(2) Å) is
significantly shorter (see Table S1, Supporting Information).
This distortion is also reflected in the ligand architecture. While
for the free ligand 1 the C−B−C angles fall into a narrow range
from 106.2−113.5°,6 upon complexation a smaller angle is
consistently observed between the more tightly bound pyridyl
groups (C1−B1−C6 from 100.5(1)−101.8(1)°); the angles
C1−B1−C11 and C1−B1−C16 (112.7(2)−116.0(2)°) to the
phenyl group are correspondingly wider than the expected 109°
in a perfect tetrahedral environment. These distortions appear
to be related to steric interactions of the pyridyl hydrogens in
ortho-position to the boryl group (H2, H7) with the ortho-
hydrogens on the phenyl ring (H17, H21). The latter is
adopting a position orthogonal to the more weakly bound
pyridyl ring (N3 ring) but is positioned at relative small angles
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to the N1 and N2 rings. The N−Fe−N angles are close to 90°
with a maximum deviation of 0.7°, whereas the variation of the
N−Mg−N (86.0 to 94.0°) and N−Mn−N (84.0 to 96.0°)
angles is somewhat larger, indicative of further geometric
distortions. The pyridine nitrogens of one ligand are arranged
coparallel to those of the second ligand with distances of 2.66 Å
(2-Mg), 2.29 Å (2-Fe), and 2.80 Å (2-Mn), respectively, which
increase with elongation of the M-N bonds. The phenyl rings
are also positioned in a coparallel arrangement at fairly similar
distances of 1.06 (2-Mg), 0.96 (2-Fe), and 1.08 Å (2-Mn) from
one another.
A comparison of the M-N bond lengths to related complexes
is insightful. For 2-Mn the Mn−N bond distances of 2.179(2)−
2.268(2) Å are somewhat shorter than those reported by
Wright for the homologous aluminate complex [Mn{MeAl(2-
py)3}2]17 (2.291(3)−2.305(3) Å). Nonetheless, they are well
within in the range of Mn−N bond distances reported in the
literature for Mn(II) d5 high-spin complexes with tridentate N
ligands.18 A high spin configuration is also consistent with the
light yellow color of the compound and the large chemical
shifts in the 1H NMR spectra. As in the case of the Mn species,
the Fe−N bonds of 1.9685(13)−1.9902(13) Å for 2-Fe are
shorter than those for the corresponding charge-neutral high-
spin complex [Fe{MeAl(2-py)3}2] (Fe−N average of 2.054(3)
Å17). They are, however, comparable to or only slightly longer
than the ones in diamagnetic tripodal iron complexes with N or
C as the bridgehead atom ([Fe{N(2-py)3}2]2+: Fe−N =
1.970(5)−1.995(5) Å19; [Fe{HC(2-py)3}2]2+: Fe−N =
1.947(2)−1.954(2) Å20). Also on the basis of the NMR
evidence, we conclude that both in solution and in the solid
state 2-Fe adopts a low spin configuration. Our results further
indicate that the size of the bridgehead atom (Al vs B) has a
critical influence on the electronic structure of these Fe(II)
complexes.
Supramolecular Structures. Based on the discussions
above the structural features of complexes 2-M (M = Mg, Fe,
Mn) are quite similar; however, 2-Fe crystallized from toluene
in the monoclinic space group C2/c whereas 2-Mg and 2-Mn
crystallized in the more highly symmetric hexagonal space
Figure 4. Perspective views of the extended structure of 2-Fe(C7H8)
along the crystallographic b-axis (top) and a-axis (bottom); B: green,
N: blue, Fe: orange, toluene molecules: red; H atoms are omitted.
group R3. When further examining the crystal packing, we were
̅
surprised to discover that all these complexes adopt supra-
molecular structures with channels that run through the crystal
lattice along the crystallographic b- (2-Fe) or c-axis (2-Mg and
2-Mn). In the case of the toluene solvate 2-Fe(C7H8), a layered
structure is evident in which the metal complexes alternate with
layers made up of the t-butylphenyl groups and the cocrystal-
lized toluene molecules (Figure 4). The solvent channels are
oriented perpendicular to these layers; they are rhomboid-
shaped and have relatively small diameters (the shortest H···H
distances across the channels are about 5.5 and 4.4 Å). There
are 4 channels per unit cell, each of them accommodating one
toluene molecule per main molecule.
In contrast, for 2-Mg and 2-Mn large hexagonal channels (3
channels per unit cell, 9 main molecules) with a diameter of
about 8 Å were observed (Figure 5).17,21,22 In an attempt to
generate a porous structure of 2-Fe similar to the one adopted
by 2-Mg and 2-Mn, we examined a range of different solvents
and conditions for crystallization. Indeed, when we crystallized
2-Fe by slow solvent evaporation of a solution in CH2Cl2 or
The different complexes are isostructural and the channel
sizes vary only slightly with changes in the central metal ion or
the nature of the cocrystallized solvent. The pore sizes were
estimated by measuring the shortest H···H distances across the
channels to be 8.99 Å for 2-Mg(C7H8)x; 8.98 Å for 2-
Mn(C7H8)x; 8.88 Å for 2-Fe(C6H12)x; and 8.89 Å for 2-
Fe(CH2Cl2)x (the effective diameters are somewhat smaller
when taking the H van der Waals radii into consideration). The
individual channels are generated by alternate packing of 3
metal complexes that are rotated by 60° from layer to layer. As
illustrated in Figure 6, the third layer (light blue) is positioned
in the next unit cell and coincides with the positioning in the
first layer (dark blue). There is some overlap between the
phenyl rings in one layer and the pyridyl rings of the next layer
and vice versa. This leads to sandwich-like arrangements, in
which two pyridyl rings (blue) show C−H···π contacts to a
phenyl ring (green), whereas two phenyl rings (blue) of the
adjacent molecules in the same layers show C−H···π contacts
to a pyridyl ring (green) positioned in-between them. The Ph-
to-Py C−H···π distances differ slightly from compound to
compound, which is primarily due to slight differences in the
cyclohexane, we succeeded in obtaining single crystals in the R3
space group that feature pores similar to those in 2-Mg and 2-
Mn.
̅
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Figure 5. Ball-and-stick and space filling views of the extended structures of 2-Mg(C7H8)x (left) and 2-Fe(C6H12)x (right) along the crystallographic
c-axis; diffuse solvent molecules were removed using the SQUEEZE routine in Platon.23
occupation of every other site. The incorporated solvent
molecules in 2-Fe(C6H12)x proved to be diffuse and had to be
treated using the “SQUEEZE” routine in the program Platon.23
Similarly, the solvent molecules in 2-Mg(C7H8)x and 2-
Mn(C7H8)x were not well localized. Importantly, the solvent-
accessible volume based on the Platon SQUEEZE routine is
similar in 2-Mg(C7H8)x (1829 Å3) and 2-Mn(C7H8)x (1864
Å3), and just slightly larger than in 2-Fe(C6H12)x (1643 Å3).
We performed thermogravimetric analyses (10 °C/min) on
all complexes to quantitate the amount of solvent that is
present and to determine suitable conditions for solvent
removal without decomposition. The TGA trace of 2-
Fe(toluene) showed a sharp 10.8% weight loss at an onset
temperature of 130 °C (midpoint of 135 °C; see Figure 7).
This result is consistent with loss of all cocrystallized toluene
molecules, indicative of facile desorption from the crystal at the
boiling point of toluene because of a lack of specific
interactions. In contrast, the loss of toluene from the hexagonal
channel-like structures of 2-Mg(toluene)x and 2-Mn(toluene)x
occurred much more gradually, over a temperature range from
about 100 to 250 °C. The weight loss for 2-Mg(toluene)x and
2-Mn(toluene)x corresponded to about 5 and 6 toluene
molecules per unit cell (9 main molecules), respectively,
which correlates well with an expected 2 toluene molecules in
each of the 3 channels when the structure is fully solvated.
Similarly, for 2-Fe(CH2Cl2)x, the TGA measurements showed a
very gradual loss of about 1 molecule of CH2Cl2 per main
molecule, which is significantly more than the amount
determined by X-ray crystallography (x = 0.5). This apparent
discrepancy is readily explained by partial desolvation of the
crystal used for X-ray structure determination. The sample used
for TGA was isolated immediately prior to the measurement,
and the TGA trace indicates facile partial solvent loss close to
room temperature.
Figure 6. Illustration of the formation of hexagonal channels in 2-
Mg(C7H8)x.
relative orientation of the aromatic groups (2-Mg(C7H8)x
3.155, 3.341; 2-Mn(C7H8)x 3.121, 3.381; 2-Fe(C6H12)x 3.228,
3.348; 2-Fe(CH2Cl2)x 2.877, 3.284 Å; the C−H distances were
fixed at 0.95 Å in all cases). They are in the range of, or slightly
longer that, typically observed for strong C−H···π interactions
(≤3.05 Å).24
In the case of the crystals grown from CH2Cl2, 18 solvent
molecules per unit cell were found in well-defined positions in
the channels. Their occupancy was refined to about 0.25; a
positional disorder is expected given the relative orientation
and very close contact between the CH2Cl2 molecules (see
Figure S6, Supporting Information, and Table 1). Considering
the crystal symmetry, it is possible that partial solvent loss
occurred, and the actual occupancy of a fully solvated crystal
could well be larger, up to a theoretical maximum of one
CH2Cl2 molecule per main molecule, which corresponds to full
Over the course of our studies we discovered that the single
crystals that adopt channel-like structures retained their
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Table 1. Comparison of Crystal Data and Thermogravimetric Analysis (TGA) Results for Microporous Channel-Like Structures
of 2-M (M = Mg, Fe, Mn)
2-Fe(C7H8)
2-Mg(C7H8)x
2-Mn(C7H8)x
2-Fe(CH2Cl2)x
2-Fe(C6H12)x
Z (main molecule)
MW (main molecule)
MW (solvent)
4
9
9
9
9
812.43
(92.14)1
1.320
4553.1(1)
e
780.89
(92.14)x
1.062
10984.6(6)
1829
ca. 100
190
811.52
(92.13)x
1.100
11021.9(2)
1864
ca. 100
210
812.43
812.43
d
(84.93)x
(84.16)x
a
d
ρcalc, g cm−3
1.188
1.133
V, Å3
10759.9(7)
e
10717.6(7)
solvent-accessible V (Platon), Å3
1643
b
TGA Tonset (°C, 1st step)
TGA T1/2 (°C, 1st step)
TGA % weight loss (1st step)
130
ca. 50
195
e
e
e
e
e
e
c
135
10.8
6.3
7.6
10.2
f
TGA solvent molecules x per main molecule
0.95
0.54
0.67
0.98
b
TGA Tonset (°C, 2nd step)
TGA T1/2 (°C, 2nd step)
430
440
395
410
425
c
410
430
430
a
The crystal density is calculated based on solvent-filled channels for 2-Fe(C7H8) and 2-Fe(CH2Cl2)0.5 and for empty channels for all other
b
c
d
structures. Onset of solvent loss (1st step) or thermal degradation (2nd step). 50% weight loss temperature for the specified step. For 2-
Fe(CH2Cl2)x the refined occupancy is about 0.25, corresponding to 0.25 × 18 solvents per 9 main molecules in the unit cell; i.e., x = 0.5. Not
determined. Crystals were freshly prepared prior to measurement.
e
f
115 °C for 24 h and subsequently to 140 °C for 72 h.
Comparison of the X-ray data demonstrated the integrity of the
crystal and channel structure upon removal of solvent as
illustrated by a very small change in the cell parameters with
decreasing amount of trapped solvent (Supporting Information,
Table S3). While the trapped solvent could not be refined, we
were able to judge the loss of solvent based on the residual
electron density during refinement. For example, for the freshly
crystallized sample, the strongest residual peak upon refinement
of the main molecule was due to the solvent (toluene) and had
an intensity of 1.41, whereas the strongest H of the main
molecule had an intensity of 0.51. After heating to 115 °C for
Figure 7. Comparison of TGA data for 2-Mg(C7H8)x, 2-Fe(C7H8), 2-
Mn(C7H8)x, and 2-Fe(CH2Cl2)x; all data acquired at a scan rate of 10
°C/min.
24 h both the residual solvent and the strongest hydrogen on
the main molecule had the same intensity at 0.73 with no
change in the cell parameters. The intensity of the solvent peak
decreased further to 0.63 after evacuation at 140 °C for 72 h,
indicating significant loss of trapped solvent with little change
in the cell parameters. The remaining intensity might be due to
uptake of gas or small molecules from the atmosphere during
crystal mounting (the crystals were mounted without the use of
oil in a glass capillary).
We further examined the porosity of 2-Mg by means of gas
sorption measurements (Figure 8). For this purpose, the
solvent of a sample of 2-Mg(C7H8)x was exchanged with n-
pentane (3 × 24 h) and the material was then evacuated for 20
appearance even at elevated temperatures and after prolonged
exposure to air (>1 year in some cases). This suggested that
solvent loss occurred without degradation, a phenomenon
commonly referred to as permanent porosity. Permanently
porous materials are relatively common for metal−organic
frameworks (MOFs) and covalent organic frameworks (COFs),
but they remain exceedingly rare for porous materials based on
molecular compounds. This is especially true when the
molecular structure itself does not feature any void spaces:
so-called extrinsic porous materials.14,25 The main reason is that
the lack of strong intermolecular interactions typically results in
facile structural collapse upon evaporation of cocrystallized
solvent molecules. The phenomenon of extrinsic permanently
porous organic solids was first confirmed for tris-
(phenylenedioxy)-cyclophosphazene (TPP).26 A focused
search by several research groups for other examples of such
materials has resulted in the discovery of a range of new
compounds over the past several years, including dipeptides,
silylated biphenyl species, phthalocyanines, and triptycene
derivatives.16,27−31 The observation of relatively large pores in
some of these materials has attracted considerable interest
regarding not only gas sorption but also separation and even
catalytic applications.27,32
To establish the permanent porosity of our compounds we
acquired X-ray diffraction data of a freshly crystallized sample of
2-Mg(C7H8)x before and after heating under high vacuum to
●
Figure 8. Nitrogen sorption isotherm of 2-Mg at 77K. adsorption,
○
desorption. The inset shows the NL-DFT pore-size distribution.
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h while heating to 100 °C. Based on 77 K N2 sorption
measurements, the Brunauer−Emmett−Teller (BET) surface
area was 290 m2/g and the Langmuir surface area 431 m2 g−1. A
micropore volume Vmp = 0.168 mL g−1 was determined. The
surface area and pore volume are in a similar range as for the
relatively few other previously reported extrinsically porous
molecular organic solids.16,27,28 They are however lower than
those of McKeown’s phthalocyanine unsolvated nanoporous
crystals (PUNCs)29 and the record of about 2796 m2 g−1 and
Vmp = 1.02 mL g−1 for triptycenetrisbenzimidazolone (TTBI)
very recently reported by Mastalerz.31
Pore size analysis using the non-localized density functional
theory (NLDFT) model revealed a single narrow distribution
of pores with a diameter of 8.6 Å, which is in excellent
agreement with the crystal structure data. This is larger than for
most reported molecular organic solids and even larger than the
predominant pores reported for TTBI,31 although the latter
shows some additional pores that are >10 Å in diameter.
Noteworthy is also that the structures in our workare clearly
microporous with large channels, despite the fact that they do
not satisfy the criterion of a calculated density of <0.9 g cm−3
put forward by McKeown and co-workers33 in their search for
new microporous organic solids. This can be understood when
considering the incorporation of (heavier) metal atoms in the
actual molecular structure and the relatively dense packing of
the molecules around the hexagonal channels that does not give
rise to additional smaller void spaces. We also performed
preliminary studies on the sorption of other gases (Figure S7,
Supporting Information). Adsorption of about 8 mL g−1 CO2
(1.55 wt %) at 25 °C and about 4 mL g−1 of CH4 (0.28 wt %)
at 0 °C was observed, respectively.
presence of metal centers that provide for potentially
interesting optical, redox, electronic, magnetic, and sensory
properties. Further efforts at taking advantage of these
properties are in progress, as are attempts at introducing
metal complexes based on appropriately substituted “2nd and
3rd generation” pyridylborate ligands to create new functional
materials.
EXPERIMENTAL SECTION
■
Materials and General Methods. Mg, 2-bromopyridine, and
NEt3 were purchased from Fisher Scientific, MnCl2 and FeCl2
(anhydrous) from Sigma-Aldrich. 4-t-Butylphenyl dibromoborane
was prepared according to a literature procedure.34 Solvents were
purchased from Pharmco and used as received unless noted otherwise.
Ether solvents were distilled from Na/benzophenone prior to use.
Hydrocarbon and chlorinated solvents were purified using a solvent
purification system (Innovative Technologies; alumina/copper
columns for hydrocarbon solvents). Chlorinated solvents were distilled
from CaH2 and degassed via several freeze−pump−thaw cycles.
Reactions and manipulations involving boron halide species were
carried out under an atmosphere of prepurified nitrogen using either
Schlenk techniques or an inert-atmosphere glovebox (Innovative
Technologies). All other procedures were carried out under ambient
conditions.
1
The 499.9 MHz H NMR, 125.7 MHz 13C NMR, and 160.4 MHz
11B NMR spectra were recorded on a Varian INOVA 500 MHz
spectrometer equipped with a boron-free probe. All 1H NMR and 13
C
NMR spectra were referenced internally to the solvent peaks and 11B
NMR spectra to BF3·Et2O (δ = 0) in C6D6. The assignments are based
on the numbering scheme shown here.
CONCLUSIONS
■
Utilizing the novel pyridylborate ligand Tpyb we prepared and
fully characterized a range of sandwich-like complexes with
different central metal ions. We introduced two different
synthetic pathways to these novel metal complexes. In
comparison to previously reported pyrazolylborate and
pyridylaluminate complexes significantly different structures
and properties were observed. Interestingly, we discovered the
formation of supramolecular 3-D structures with unusually
large channels. Extrinsic porous molecular crystals have
attracted much interest in recent years, but remain exceedingly
rare, because the interactions that hold the framework together
are typically weak and structural collapse tends to occur
readily.14−16 We succeeded in completely removing the solvent
by direct thermal treatment under high vacuum or after
exchange of the solvent with n-pentane. Compared to most
other extrinsically porous molecular materials the channel sizes
in our compounds are significantly larger. The surface area and
gas uptake measurements on 2-Mg further confirmed the
permanent porosity, while the calculated NL-DFT channel size
proved to be consistent with that from single crystal X-ray
crystallography. The thermal stability of our new materials is
remarkable: thermal degradation of compounds 2-M did not
occur up to about 400 °C based on TGA, and a sample of 2-Mg
showed no loss of crystallinity after heating to 140 °C under
high vacuum for 72 h according to X-ray diffraction analysis
data. This stands in sharp contrast to most previously reported
molecular organic solids, which tend to either display low
melting points and/or undergo phase changes at relatively low
temperatures. Finally, a unique aspect of our materials in
comparison to those based on purely organic materials is the
The MALDI-TOF measurements were performed on an Applied
Biosystems 4800 Proteomics Analyzer in reflectron (+) or (−)-mode
with delayed extraction. For data acquisition in (+)-mode benzo[α]-
pyrene was used as the matrix (20 mg/mL in toluene) and in
(−)-mode α-cyano-4-hydroxycinnamic acid (50% acetonitrile, 0.1%
TFA in deionized water). The sample was dissolved in toluene or
MeOH (ca. 10 mg/mL), mixed with the matrix in a 1:10 ratio, and
then spotted on the wells of a sample plate. High resolution MALDI-
MS data (benzo[α]pyrene matrix) were obtained on an Apex Ultra 7.0
Hybrid FTMS (Bruker Daltonics).
TGA data were acquired on a Perkin-Elmer Pyris 1 Thermogravi-
metric Analyzer at a scan rate of 10 °C/min. Elemental analyses were
performed by Quantitative Technologies, Inc., Whitehouse, NJ.
Surface area and gas sorption measurements were performed on a
Quantachrome Autosorb-1C instrument.
Single crystal X-ray diffraction intensities were collected on a Smart
Apex2 CCD diffractometer at 100 K using Cu Kα (1.54178 Å)
radiation and details of the X-ray diffraction experiments and crystal
structure refinements are given in Supporting Information, Tables S1
and S2. The crystals were mounted on a loop using Paratone-N oil
with the exception of the structure analysis of 2-Mg described in
Supporting Information, Table S3 (sealed capillary). The structures
were solved by direct methods and refined by full-matrix least-squares
based on F2 with all reflections.35 Non-hydrogen atoms were refined
with anisotropic displacement coefficients, and hydrogen atoms were
treated as idealized contributions. SADABS36 numerical face-indexed
absorption was applied. Diffuse solvent contributions were treated
using the program Platon.23 Crystallographic data for the structures of
2-Fe(C7H8), 2-Mg(C7H8)x, 2-Fe(CH2Cl2)0.5, 2-Fe(C6H12)x, and 2-
G
dx.doi.org/10.1021/ic4010664 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Mn(C7H8)x have been deposited with the Cambridge Crystallographic
Data Center as supplementary publications CCDC-878728, 931934−
931937, respectively. Copies of the data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.
(fax: (+44) 1223-336-033; email: deposit@ccdc.cam.ac.uk).
A solution of t-butylphenyltris(2-pyridyl)borate free acid (0.30 g, 0.79
mmol) and triethylamine (2.0 mL, 14 mmol) in methanol (10 mL)
was then added. The reaction mixture was kept stirring for 3 h and
subsequently filtered to give a red solution. The volatile components
were removed on a rotary evaporator to give a red solid, which was
purified by column chromatography on silica gel with hexanes as the
eluent. Solvent evaporation gave the product as a red solid, which was
dried under high vacuum at RT for 1 h. Yield: 0.22 g (60%). 1H NMR
(499.973 MHz, CDCl3) δ = 8.09 (d, 3J = 7.5 Hz, 4H, tPh-H2,6), 7.60
(d, 3J = 8.0 Hz, 4H, tPh-H3,5), 7.59 (d, 3J = 9.0 Hz, 6H, pyridyl-H3),
Synthesis of t-Butylphenyltris(2-pyridyl)borate Free Acid
(1). t-Butylphenyl dibromoborane (5.00 g, 16.5 mmol) in CH2Cl2 (30
mL) was added dropwise to a solution of pyridyl Grignard (14.5 g,
25.7 mmol) in CH2Cl2 (50 mL). The resulting dark red mixture was
kept stirring for 5 h. The reaction mixture was poured into an aqueous
Na2CO3 solution (30 g in 250 mL H2O) to give a slurry which was
stirred for 30 min. Extraction with CH2Cl2 (3 × 200 mL) gave a brown
organic phase that was dried over Na2SO4. The solvents were removed
under vacuum to give an oil that was redissolved in acetone (100 mL),
filtered, and brought to dryness. The product was further purified by
chromatography on silica gel with a 1:1 mixture of hexanes and
acetone containing 1% (v/v) triethylamine as the eluent. The product
was dried under high vacuum at 60 °C for 10 h to give a white solid.
3
3
7.27 (pst, J = 7.5 Hz, 6H, pyridyl-H4), 7.11 (d, J = 5.5 Hz, 6H,
3
pyridyl-H6), 6.44 (pst, J = 6.5 Hz, 6H, pyridyl-H5), 1.52 (s, 18H, t-
Bu). 13C NMR (125.718 MHz, CDCl3) δ = 188.0 (q, 1JC−B = 49.6 Hz,
1
pyridyl-C2), 158.3 (pyridyl-C6), 149.2 (q, JC−B = 57.3 Hz, tPh−C1),
147.4 (tPh−C4), 136.7 (tPh−C2,6), 132.0 (pyridyl-C4), 125.7
(pyridyl-C3), 124.5 (tPh−C3,5), 119.7 (pyridyl-C5), 34.7
(C(CH3)3), 32.0 (C(CH3)3). 11B NMR (160.411 MHz, CDCl3) δ =
−7.5 (w1/2 = 21 Hz). UV−vis (25 °C, CH2Cl2): λmax = 480 nm (ε =
16400 M−1 cm−1), 425 (shoulder, ε = 11000 M−1 cm−1). Cyclic
voltammetry: E1/2 = −350 mV, ΔEp = 84 mV (1 mM, CH2Cl2
containing 0.1 M [Bu4N]PF6 as the supporting electrolyte; scan rate
1
Yield: 3.4 g (54%). H NMR (499.973 MHz, CDCl3) δ = 19.5 (br s,
1H, pyridyl N−H), 8.49 (d, 3J = 5.0 Hz, 3H, pyridyl-H6), 7.58 (pst, 3J
= 7.5 Hz, 3H, pyridyl-H4), 7.40 (d, 3J = 7.5 Hz, 3H, pyridyl-H3), 7.16
(d, 3J = 7.5 Hz, 2H, tPh-H3,5), 7.10 (pst, 3J = 6.3 Hz, 3H, pyridyl-H5),
100 mV/s). MALDI-TOF MS (benzo[α]pyrene): m/z = 812.356 (M+,
12
3
6.96 (br d, J = 6.0 Hz, 2H, tPh-H2,6), 1.27 (s, 9H, t-Bu). 13C NMR
calcd for
C
50
1H5011B256Fe14N6 812.363). Elemental analysis for
1
crystals obtained from toluene: calcd for C50H50B2FeN6·C7H8: C
75.68, H 6.46, N 9.29%; found C 75.61, H 6.46, N 9.24%. Single
crystals of complex 2-Fe for X-ray diffraction analysis were obtained by
slow evaporation of a solution in toluene, CH2Cl2, or cyclohexane
(heated to achieve dissolution).
(125.718 MHz, CDCl3) δ = 184.2 (q, JC−B = 53.1 Hz, pyridyl-C2),
152.3 (q, 1JC−B = 50.0 Hz, tPh−C1), 146.9 (tPh−C4), 143.6 (pyridyl-
C6), 136.3 (pyridyl-C4), 134.2 (tPh−C2,6), 131.8 (pyridyl-C3), 124.2
(tPh−C3,5), 119.8 (pyridyl-C5), 34.3 (C(CH3)3), 31.7 (C(CH3)3).
11B NMR (160.386 MHz, CDCl3) δ = −10.8 (w1/2 = 13 Hz). MALDI-
TOF MS (benzo[α]pyrene): m/z = 380.233 (MH+, calcd for
Synthesis of Bis(t-butylphenyltris(2-pyridyl)borate)
Manganese(II) (2-Mn). To a solution of t-butylphenyltris(2-pyridyl)-
borate (0.30 g, 0.79 mmol) in methanol (10 mL) was added a solution
of MnCl2 (54 mg, 0.43 mmol) in methanol (5 mL). Triethylamine
(1.0 mL) and hydrazine monohydrate (0.1 mL) were sequentially
added into the reaction mixture. The resulting suspension was stirred
for 5 min and then filtered. A yellow solid was collected, which was
dried under high vacuum at RT and then extracted into toluene (120
mL). A small amount of insoluble components was removed by
filtration and the product was obtained as a light yellow solid by
solvent evaporation and drying under high vacuum. Yield: 0.20 g
(62%). 1H NMR (499.973 MHz, CDCl3) δ = 35.0 (br, 6H, pyridyl-H),
17.6 (br, 6H, pyridyl-H), 9.6 (br, 4H, tPh), 7.5 (br, 4H, tPh), 1.34 (s,
18H, t-Bu), −6 (very br, 6H, pyridyl-H), −42.5 (br, 6H, pyridyl-H).
11B NMR (160.411 MHz, CDCl3) δ = −109.6 (w1/2 = 150 Hz). High-
12
C
1H2711B114N3 380.313). Elemental analysis: calcd for
25
C25H26B1N3: C 79.16, H 6.91, N 11.08%; found C 78.92, H 6.94, N
11.08%. Single crystals for X-ray diffraction analysis were obtained by
slow evaporation of a toluene solution.
Synthesis of Bis(t-butylphenyltris(2-pyridyl)borate) Magne-
sium (2-Mg). A solution of t-butylphenyl dibromoborane (1.0 g, 3.3
mmol) in toluene (15 mL) was slowly added to a solution of pyridyl
Grignard (2.9 g, 5.4 mmol) in toluene (40 mL). The reaction mixture
was stirred for 12 h and then brought to dryness under high vacuum to
give a white solid residue. Methanol (300 mL) was added, and the
mixture was refluxed for 1 h. The product was collected by filtration,
washed with cyclohexane, and then dried under high vacuum. Yield:
1
3
0.45 g (35%). H NMR (499.973 MHz, CDCl3) δ = 7.97 (d, J = 7.5
Hz, 4H, tPh-H2,6), 7.70 (d, 3J = 8.0 Hz, 6H, pyridyl-H3), 7.58 (d, 3J =
3
resolution MALDI-MS (+ mode, benzo[α]pyrene): m/z = 811.3657
5.5 Hz, 6H, pyridyl-H6), 7.56 (d, J = 8.0 Hz, 4H, tPh-H3,5), 7.39
1H5011B2 Mn14N6 811.3658), 733.3318
12
55
(M+, 50%, calcd for
C
(pst, 3J = 7.5 Hz, 6H, pyridyl-H4), 6.64 (pst, 3J = 6.3 Hz, 6H, pyridyl-
H5), 1.51 (s, 18H, t-Bu). 13C NMR (125.718 MHz, CDCl3) δ = 185.0
50
12
55
(M+-Py, 100%, calcd for
C
45
1H4611B2 Mn14N5 733.3314). Single
1
1
crystals for X-ray diffraction analysis were obtained by slow
evaporation of a saturated toluene solution.
(q, JC−B = 50 Hz, pyridyl-C2), 150.3 (q, JC−B = 55 Hz, tPh−C1),
148.4 (pyridyl-C6), 147.2 (tPh−C4), 137.0 (tPh−C2,6), 134.9
(pyridyl-C4), 129.9 (pyridyl-C3), 124.3 (tPh−C3,5), 118.7 (pyridyl-
C5), 34.6 (C(CH3)3), 32.0 (C(CH3)3). 11B NMR (160.411 MHz,
CDCl3) δ = −9.2 (w1/2 = 18 Hz). MALDI-TOF (benzo[α]pyrene):
Surface Area, Porosity, and Gas Uptake Measurement on 2-
Mg. Following recrystallization of 2-Mg from toluene, a portion of the
crystals was subjected to solvent exchange in n-pentane to remove
toluene from within the crystal lattice (following a procedure similar to
that reported by Mastalerz31). The crystals were immersed in n-
pentane for 24 h three times, and the solvent exchanged for fresh n-
pentane each time. The sample was then evacuated for 20 h at 100 °C
prior to performing gas sorption measurements.
m/z = 780.415 (M+, calcd for
C
50
1H5011B2 Mg114N6 780.413).
12
24
Elemental analysis (dried under vacuum at 80 °C overnight): calcd for
C50H50B2Mg1N6: C 76.90, H 6.45, N 10.76%; found C 76.44, H 6.45,
N 10.68%. Single crystals for X-ray diffraction analysis were obtained
by slow evaporation of solutions prepared in dry toluene at 50 °C or in
CH2Cl2 at room temperature, respectively.
The methanol filtrate from the above workup procedure was
concentrated to about 10 mL and then CH2Cl2 (100 mL) was added.
The resulting solution was poured into an aqueous Na2CO3 solution
(20 g, 150 mL), the organic layer was isolated and the aqueous layer
was extracted with CH2Cl2 (2 × 50 mL). The combined organic layers
were dried over Na2SO4, concentrated and purified by column
chromatography on silica gel with solvent gradients ranging from
hexanes to acetone containing 1% (v/v) triethylamine. The product
was isolated by evaporation of the solvents and identified by NMR
spectroscopy to be the free acid 1. Yield: 0.25 g (20%).
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional experimental details. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: fjaekle@rutgers.edu (F.J.), rogerlal@rutgers.edu
(R.A.L.). Fax: +1 973 353 1264.
Synthesis of Bis(t-butylphenyltris(2-pyridyl)borate) Iron(II)
(2-Fe). A 100 mL Schlenk flask was charged with anhydrous FeCl2
powder (0.10 g, 0.79 mmol) and anhydrous tetrahydrofuran (50 mL).
Notes
The authors declare no competing financial interest.
H
dx.doi.org/10.1021/ic4010664 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation under Grant CHE-0956655. The authors
acknowledge partial support by NSF-CRIF Grant 0443538 for
acquisition of the X-ray diffractometer used in these studies. We
thank Prof. Jing Li, Haohan Wu, and Hao Wang at Rutgers
University, New Brunswick campus, for acquisition of porosity
and gas sorption data and Prof. John Sheridan for helpful
discussions. F.J. thanks the Alfred P. Sloan foundation for a
research fellowship.
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