Externally and Internally Functionalized Copper(II) β-Diketonate Molecular Squares

Article abstract of DOI:10.1021/acs.inorgchem.5b00792

Five functionalized bis(β-diketones) and their Cu(II) molecular squares are described. The new bis(β-diketones), m-pbhxH₂ (3), 5-MeO-m-pbaH₂ (4), 5-BuO-m-pbaH₂ (5), 2-MeO-m-pbaH₂ (6), and 2-MeO-m-pbprH₂

Full text of DOI:10.1021/acs.inorgchem.5b00792

Article
pubs.acs.org/IC
Externally and Internally Functionalized Copper(II) β‑Diketonate
Molecular Squares
Jackson K. Cherutoi,† Jace D. Sandifer, Uttam R. Pokharel,‡ Frank R. Fronczek, Svetlana Pakhomova,
and Andrew W. Maverick*
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States
S
* Supporting Information
ABSTRACT: Five functionalized bis(β-diketones) and their
Cu(II) molecular squares are described. The new bis(β-
diketones), m-pbhxH2 (3), 5-MeO-m-pbaH2 (4), 5-BuO-m-
pbaH2 (5), 2-MeO-m-pbaH2 (6), and 2-MeO-m-pbprH2 (7),
were prepared by reaction of the corresponding aldehydes with
phospholenes, as we previously reported for m-pbaH2 (1) and
m-pbprH2 (2). Ligand 3 has long alkyl chains in its β-diketone
moieties, while ligands 4−7 have alkoxy substituents on their
aromatic rings. When treated with Cu2+, the new bis(β-
diketones) 3, 4, 5, and 7 afford molecular squares, Cu4(m-
pbhx)4 (10), Cu4(5-MeO-m-pba)4 (11), Cu4(5-BuO-m-pba)4
(12), and Cu4(2-MeO-m-pbpr)4 (13), respectively. Two of the
new molecular squares, 10 and 12, contain longer-chain
substituents and are soluble in a wider range of organic solvents. The other squares, 11 and 13, contain external and internal
methoxy groups, respectively, and they show smaller changes in solubility. Single-crystal X-ray analyses are reported for three of
the molecular squares without guest molecules, and for five adducts of the squares with σ- (polypyridine) and π-bonded
(fullerene) guests. The Cu···Cu distances in the “empty” squares range from 14.047 to 14.904 Å; those in the adducts vary over a
wider range depending on the guest molecule involved.
with improved solubility: modification of the alkyl groups in the
β-diketone moieties of m-pbaH2, and functionalization of the
m-phenylene moieties. Herein, we describe the new bis(β-
diketones) 3−7 (Figure 1) and their Cu(II) molecular squares.
For modification of the β-diketone alkyl groups, we
considered commonly employed alternatives such as t-Bu (as
in dipivaloylmethane, dpmH) and Ph (as in dibenzoylmethane,
dbmH); see Figure 2. Indeed, in our earlier work with m-xbaH2
and its complexes, we prepared such analogs, m-xbpH2 and m-
xbdH2. However, we were unable to prepare metal complexes
of either of these substituted bis(β-diketones).17 Earlier
investigators also noted that sterically hindered β-diketones
react only weakly or not at all with metal ions such as Fe3+ and
Cu2+. For example, 3-propyl-2,4-pentanedione reacts readily
with Fe3+ and Cu2+, but 3-isopropyl-2,4-pentanedione does
not;18 the binding constants for β-diketones with Cu2+ are
significantly smaller when the central C atom is substituted.19,20
A general rule appears to be that if the 3 position in
acetylacetone is substituted,21 then branched substituents in the
1, 3, or 5 positions are likely to interfere with Cu2+
complexation. On the basis of this information, we chose n-
pentyl groups for our experiments with longer-chain alkyl
groups in the β-diketone moieties of m-pbaH2.
INTRODUCTION
■
Porous supramolecular metal−organic assemblies have at-
tracted great attention in recent years because of their potential
applications in areas such as catalysis, separation, gas storage,
and host−guest chemistry.1−10 Among the various assemblies
known, molecular squares have been studied extensively. Many
molecular squares have been constructed from linear 4,4′-
bipyridine (4,4′-bpy) ligands and cis-protected square planar
metal centers; octahedral metal centers and other ligands have
been employed as well.11−15 These square planar and
octahedral metal centers provide the 90° corners needed for
the squares. We have reported molecular squares prepared from
Cu2+ and the bis(β-diketone) ligands m-pbaH2 (1) and m-
pbprH2 (2);16 see Figure 1. Our molecular squares are different
from others that are commonly studied in two ways: (1) the
ligands occupy the corners and the metals form the edges; and
(2) the Cu metal centers are not protected and are
coordinatively unsaturated, which means they can interact
with guest molecules. The squares bind 4,4′-bpy and C60; 4,4′-
bpy is coordinated through its N atoms, while C60 is held
through π−π interactions. Unfortunately, the squares are
soluble only in a small number of solvents, which limits their
usefulness.
One of our goals in the present work was to prepare Cu β-
diketonate molecular squares that are soluble in a wider range
of solvents. We pursued two routes toward molecular squares
Received: April 8, 2015
© XXXX American Chemical Society
A
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example, have reported coordination cages (“nanoballs”) whose
interiors are decorated with polyethylene glycol (PEG) or
fluorocarbon substituents.23,24
RESULTS AND DISCUSSION
■
Ligands. a. m-Phenylenebis(dihexanoylmethane), m-
pbhxH2 (3). Our synthetic route for m-pbhxH2 (the analog of
m-pbaH2 containing pentyl substituents) is shown in Scheme 1.
Scheme 1. Synthesis of m-pbhxH2 (3)
Figure 1. Conversion of bis(β-diketones) (1−7) into molecular
squares (8−13) by reaction with [Cu(NH3)4]2+. The new substituted
bis(β-diketones) 3−7, and their molecular squares 10−13, are
reported here.
The α-diketone required for this route, dodecane-6,7-dione
(14), has been prepared via a Grignard reaction.25 We
experimented with this method, but found the general method
of Zibuck and Seebach26 to be more convenient. Reaction of 6-
dodecyne with NaIO4 and a catalytic amount of RuO2·xH2O in
a CCl4/CH3CN/H2O solvent mixture gave yellow solid 14.
Treatment of 14 with trimethyl phosphite afforded 2,2,2-
trimethoxy-4,5-dipentyl-1,3,2-dioxaphospholene (15), which
reacted with isophthalaldehyde to yield m-pbhxH2 (3), as a
colorless oil after column chromatography.
b. 5-MeO-m-pbaH2 (4), 5-BuO-m-pbaH2 (5), 2-MeO-m-
pbaH2 (6), and 2-MeO-m-pbprH2 (7). Scheme 2 shows the
syntheses of these new bis(β-diketones).
Copper(II) Molecular Squares. Treatment of dichloro-
methane solutions of the β-diketones 3, 4, 5, and 7 with
aqueous [Cu(NH3)4]2+ afforded green solutions, and upon
evaporation of the solvent, the squares (10−13) were isolated
as dark green powders in moderate to high yields (see Figure
1). These results are similar to those that we previously
reported for the conversion of 1 and 2 into their molecular
squares (8 and 9). Analogous experiments involving 6 yielded
an insoluble dark green product. This may be a molecular
square, but we were unable to characterize it further due to its
lack of solubility.
Solubility. Table 1 shows the solubility of the molecular
squares in various solvents. Squares 10 and 12 are soluble in
nine organic solvents. We attribute the greater solubility of
these two squares to the presence of long alkyl chains (pentyl)
in the β-diketone moieties of 3 and the long alkoxy (butoxy)
groups in the 5-position on the aromatic rings of 5, respectively.
Squares 11 and 13, with methoxy groups in the 5- and 2-
positions on the ligand aromatic rings, respectively, are soluble
in about the same number of solvents as the unsubstituted
square Cu4(m-pba)4,16 though with a different range of
polarities.
Figure 2. β-Diketones on the left generate metal complexes readily;
those on the right do not, because of steric hindrance among their
bulkier substituents.
For functionalization of the m-phenylene moiety in m-pbaH2,
we introduced substituents in the 2- and 5-positions, leading to
internally and externally functionalized squares, respectively. Yu
and co-workers prepared a molecular square from a 5-
substituted m-pbaH2 ligand (R5 = PhCH2O in Figure 1);22
however, they discussed only its solutions in CH2Cl2. The
preparation of 3-substituted β-diketones, and their use in
supramolecular structures, has been reviewed by Ziessel et al.21
Adding functional groups to the 2-position in particular is
attractive for two reasons: (1) if the substituents are large
enough, steric factors may lead to formation of larger
macrocycles such as molecular pentagons or hexagons, and
(2) we could study the effects of changing the chemical
environment inside the squares. Fujita and co-workers, for
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Scheme 2. Syntheses of New Bis(β-diketones): (i) 4, (ii) 5, and (iii) 6 and 7
Table 1. Solubility of the Cu(II) Molecular Squares
a
solvent
CH2Cl2
Cu4(m-pba)4, 8 Cu4(m-pbpr)4, 9 Cu4(m-pbhx)4, 10 Cu4(5-MeO-m-pba)4, 11 Cu4(5-BuO-m-pba)4, 12 Cu4(2-MeO-m-pbpr)4, 13
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
CHCl3
Y
Y
N
N
N
N
N
N
Y
Y
C6H6
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Toluene
Y (hot)
CS2
N
N
N
N
N
1,2-dichlorobenzene
chlorobenzene
THF
bromobenzene
N
a
All of the molecular squares are insoluble in methanol, acetone, acetonitrile, hexane, and diethyl ether.
Structures. Cu4(m-pbhx)4 (10) crystallizes as a solvate with
these crystals, disordered water molecules are also bound on
the inside of the squares (occupancy 0.5, for 2 water molecules
per Cu4 square), so that the overall coordination of the Cu
atoms is approximately octahedral. The molecules of Cu4(5-
MeO-m-pba)4 have crystallographically imposed 4/m (C4h)
symmetry.
methanol (see Figure 3). The molecules have approximate
inversion symmetry; two of the Cu atoms are square pyramidal,
with apical CH3OH, and the other two are square planar. The
pyramidal distortion at Cu2 and Cu4 leads to a longer Cu···Cu
distance than that between the four-coordinate Cu1 and Cu3.
The apical Cu−O distances at Cu2 and Cu4 (2.262(4) and
2.263(3) Å) are similar to that reported by Clegg and co-
workers for a Cu(II) β-diketonate with apical THF (2.342(1)
Å).27
The structure of Cu4(2-MeO-m-pbpr)4 (13) is shown in
Figure 5. In this crystal, the square has a puckered
conformation, with crystallographically imposed 4 (S )
̅
4
symmetry. The internal methoxy groups alternately point
toward opposite sides of the Cu4 ring. Steric interactions
between the methoxy groups and the nearby portions of the
square may be strong enough to cause distortion away from the
Compound 11, Cu4(5-MeO-m-pba)4, crystallizes as a
coordination polymer (Figure 4), in which the external MeO
groups are coordinated to Cu atoms in adjacent molecules. In
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Figure 3. Crystal structure of Cu4(m-pbhx)4(CH3OH)2, from 10(CH3OH)2·12CHCl3, Cu1···Cu3 14.047(1) Å, Cu2···Cu4 14.904(1) Å. In this and
all other crystal-structure illustrations, ellipsoids are at the 50% probability level, and hydrogen atoms and uncoordinated solvent molecules are
omitted for clarity.
Figure 4. Structure of [Cu4(5-MeO-m-pba)4(H2O)2], from 11(H2O)2·12CHCl3·8H2O, Cu···Cu 14.322(2) Å. Portions of adjacent molecules in the
crystal are shown (with lighter coloring), to illustrate close Cu−O(methoxy) bonds (2.608(8) Å).
normally coplanar arrangement of aromatic rings and Cu
atoms.
Host−Guest Chemistry of the Squares. a. Adducts with
Pyridine Derivatives. Several of the new squares react with
pyridine derivatives, forming adducts with internally coordi-
nated guests. The reaction of Cu4(m-pbhx)4 (10) with the
guest molecules 1,2-bis(4-pyridyl)ethylene (bpe) and 1,2-bis(4-
pyridyl)ethane (bpa) is confirmed by formation of the adducts
[Cu4(m-pbhx)4(μ-bpe)(MeOH)2] (23; Figure 6) and [Cu4(m-
pbhx)4(μ-bpa)(MeOH)2] (24; Supporting Information Figure
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Figure 5. Crystal structure of Cu4(2-MeO-m-pbpr)4, from (13)·6.75CH3CN, Cu···Cu′ 14.3862(5) Å. Minor components of disordered C atoms
omitted for clarity. (a) Face view. (b) Edge view, approximately along the blue diagonal line in part a, showing puckering of the square. The CH3O
groups on the downward-facing aromatic rings also point downward, with corresponding upward displacements for the other two ligands.
S1). In both of these adducts, the Cu atoms bridged by the
guest molecule are approximately 1 Å closer together than the
unbridged Cu atoms. This indicates that the bpe and bpa guest
molecules are slightly shorter (N···N′ 9.425(4) and 9.278(5) Å,
respectively) than would be ideal for the Cu4(m-pbhx)4 host.
Cu4(5-BuO-m-pba)4 (12) reacts with 4,4′-bpy to produce
adduct 25, [Cu4(5-BuO-m-pba)4(μ-bpy)]. The structure of 25
(Figure 7) is similar to the one we previously reported for
Cu4(m-pba)4(μ-bpy),16 with the shorter 4,4′-bpy guest
molecule leading to a greater difference in Cu···Cu distances
(ca. 4 Å) than in the above bpe (23) and bpa (24) adducts.
In all three of these adducts, the unbridged Cu atoms have O
atoms coordinated externally, either from solvent molecules (in
23 and 24) or from an adjacent square (in 25).This leads to
outward-pointing square pyramidal coordination geometry,
which also contributes to the larger unbridged Cu···Cu
distances.
pure or crystalline form, so they were not studied further. In
contrast, square 13, Cu4(2-MeO-m-pbpr)4, with internal
methoxy groups, remained green in solution even in the
presence of large excesses of potential guest molecules, and no
adducts could be isolated. This suggests that 13 has a much
lower affinity for guests than the other Cu molecular squares we
have studied. We attribute this low guest affinity to steric
interference from its internal OCH3 groups.
b. Adduct with C60 (26). Treatment of square 10 with C60
produced Cu4(m-pbhx)4(μ-C60) (26), whose crystal structure is
shown in Figure 8. This structure is similar to that of our
previously reported Cu-square-fullerene adduct, i.e., Cu4(m-
pbpr)4(μ-C60),16 except for the longer β-diketone alkyl groups
in the present structure. In both structures, several of the β-
diketone alkyl groups are oriented inward, toward the face of
the C60 guest. Multiple weak interactions between the alkyl
groups and the guest are likely to provide some stabilization for
the adducts, in addition to π interactions between the fullerene
and unsaturated portions of the host.
Square 11, Cu4(5-MeO-m-pba)4, also changed color from
green to blue on treatment with pyridine derivatives, which
indicates that similar host−guest chemistry is occurring.
However, we could not obtain the products in analytically
The longer pentyl groups in Cu4(m-pbhx)4(μ-C60) (26)
could lead to guest−host attractions that are stronger than
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Figure 6. Crystal structure of Cu4(m-pbhx)4(μ-bpe)(CH3OH)2, from (23)(CH3OH)2·10CHCl3. Cu1···Cu1′ 14.667(1) Å, Cu2···Cu2′ 13.9893(8)
Å.
Figure 7. Crystal structure of Cu4(5-BuO-m-pba)4(μ-4,4′-bpy), from (25)·16CHCl3; Cu1···Cu1′ 11.9537(7), Cu2···Cu2′ 15.9851(9) Å. Portions of
adjacent molecules in the crystal are shown (with lighter coloring), to illustrate close Cu2···O4′ contacts (2.491(2) Å).
those in the previously described adduct Cu4(m-pbpr)4(μ-
C60).16 However, it should be noted that, in both the present
structures and the previous one, not all of the alkyl groups are
directed inward. Molecular modeling of these adducts indicates
that there is not enough room around the Cu square to
accommodate all 16 β-diketonate alkyl groups (R in Figure 1)
pointing inward, even with the smaller ethyl groups of Cu4(m-
pbpr)4 (9). A recently reported hemicarceplex containing six
−OC10H20O− bridging groups is an effective host for C60, also
relying on multiple alkyl···fullerene interactions.28
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Figure 8. Crystal structure of [Cu4(m-pbhx)4(μ-C60)], from (26)·4C6H4Cl2: Cu1···Cu1′ 13.4326(9) Å; Cu2···Cu2′ 15.146(1) Å. The C60 molecule
makes short contacts with Cu1 and neighboring atoms (minimum Cu1···C 3.40 Å) and with some of the alkyl C atoms (minimum C···C 3.57 Å).
The Cu4(m-pbhx)4 host makes short contacts with adjacent molecules: Cu2···O4′ 2.460(3) Å.
In our previous report of the binding of C60 to Cu4(m-pbpr)4
(9),16 we were unable to study the reaction in detail, because
the host and guest are not readily soluble in the same solvents.
However, the new host Cu4(m-pbhx)4 (10) is soluble in several
solvents that are also good solvents for fullerenes. Among the
good solvents for Cu4(m-pbhx)4 (10), 1,2-dichlorobenzene is
the best solvent for fullerenes,29,30 and toluene is also
commonly used for fullerene binding studies. Thus, we used
these two solvents for our experiments.
Host−guest reactions are often studied by electronic
absorption spectroscopy. Often, the guest molecules do not
absorb in the region of interest, so the changes in the spectrum
of the host on binding guest molecules are easy to observe.
However, in the present case, both Cu4(m-pbhx)4 (10) and C60
are colored, and the absorption of C60 is more intense than that
Figure 9. Portions of the electronic absorption spectra of solutions of
of the Cu square. Thus, in these experiments, we used a fixed
Cu4(m-pbhx)4 and C60 in 1,2-dichlorobenzene. All solutions contain
concentration of fullerene, and varied the concentration of the
0.167 mM C60. Concentrations of added Cu4(m-pbhx)4 increase in
Cu square. Figure 9 shows a series of absorption spectra of 0.17
mM C60 in 1,2-C6H4Cl2, with increasing concentrations of
Cu4(m-pbhx)4 (10). Analysis of these spectra with the global
analysis program SPECFIT/32 gives a binding constant of 5400
800 M−1. Experiments in toluene solution gave a binding
constant about 6 times as large (see Table 2); binding of C60
and C70 to hosts is usually stronger in toluene than in 1,2-
C6H4Cl2, because toluene solvates fullerenes more weakly.31−34
The new binding constants are larger than those we previously
observed for σ-donating guests such as dabco and pyrazine with
the binuclear host Cu2(nba)2.35 Still, they are significantly
smaller than values reported (>105 M−1) for binding of C60 to
other macrocyclic polynuclear metal complexes.36−38
order from the bottom spectrum upward: 0, 0.0922, 0.184, 0.461,
0.738, 1.01, 1.29, and 1.57 mM.
Table 2. Binding Constants for the Reaction of Cu4(m-
pbhx)4 (10) with Fullerene Guests
solvent
guest = C60
guest = C70
1,2-dichlorobenzene
toluene
5.4 0.8 × 103
3.4 0.7 × 104
3.5 0.8 × 104
7
3 × 105
suggested C70 is also capable of binding as a guest. However, we
were unable to isolate either of the C70 adducts. These
reactions, like those with C60, were complicated by the lack of a
common solvent for host and guest. We now report that 1,2-
dichlorobenzene solutions of Cu4(m-pbhx)4 (10) and C70 react
c. Adduct with C70 (27). In our previous work with Cu4(m-
pba)4 and Cu4(m-pbpr)4,16 we observed color changes that
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Figure 10. Crystal structure of [Cu4(m-pbhx)4(μ-C70)], from (27)·3.25C6H4Cl2. Only one of the four molecules in the asymmetric unit is shown.
The (approximate) 5-fold axis of the C70 guest is shown by the two lavender balls (centroids of 5-membered rings). Cu1···Cu3 13.315(3), Cu2···Cu4
15.260(3) Å; short contacts with adjacent molecules are shown in lighter colors. For an illustration of all four [Cu4(m-pbhx)4(μ-C70)] molecules in
the asymmetric unit, see the Supporting Information.
readily. The structure of the resulting adduct (27) is shown in
Figure 10.
a thermodynamic preference for these orientations, and
suggests that the next larger host, [12]CPP, prefers an
intermediate “half-lying” orientation of C70 (Figure 11b).40
If our Cu molecular squares are considered to be ring-shaped
hosts, their interactions with fullerenes may be similar to those
of CPPs. In the crystal structure of Cu4(m-pbhx)4(μ-C70) (27),
molecules A, B, C, and D (see Figure 10 and Supporting
Information Figure S2) in the asymmetric unit are all in the
“half-lying” orientation, with angles θ of 50.2°, 33.6°, 50.2°, and
50.0°, respectively. According to the logic of Zhao et al., this
suggests that our Cu4 host is slightly too large for optimal
interaction with C70. Thus, a host that is slightly smaller than
Cu4(m-pbhx)4 might be a better match for guests such as C60
and C70, with correspondingly higher binding constants.
A molecular square “nanobarrel” has been prepared from
four nickel-porphyrin units by Osuka et al.,41 and the structure
of its C60 adduct was determined. The binding constant
between C60 and this Ni4 host is 5.3 × 105 M−1 in toluene,
about 15 times as large as that with our Cu4(m-pbhx)4 host.
And yet the crystal structure of the Osuka Ni4−C60 adduct
shows that the fullerene guest is not fully inserted into its host.
This may be because the Ni···Ni distances in the host are
slightly too short for the optimum interaction with C60. Thus,
the best metal−metal distance in a molecular square for
forming a C60 adduct may be somewhere between those in the
“nanobarrel” (13.7 Å) and those we have observed in the
present study (average 14.25 Å).
Crystals of Cu4(m-pbhx)4(μ-C70) (27) contain four crystallo-
graphically independent molecules in the asymmetric unit. This
gave us an opportunity to investigate whether the C70 guest has
a similar orientation in each molecule. Whereas C60 is
approximately spherical, C70 has the approximate shape of a
prolate spheroid, slightly longer than C60 but with the same
diameter. It appeared from the structures of our C60 adducts as
if the size of C60 is well-matched with the internal cavity of the
host molecule, Cu4(m-pbhx)4 (Figure 8) or Cu4(m-pbpr)4.16 If
the hosts have the ideal cavity size for C60, then they should
also be ideal for the equatorial diameter of C70. If this is true,
then C70 should bind to our molecular squares in a preferred
orientation, with its 5-fold symmetry axis parallel to the 4-fold
axis of the Cu4 host. This is sometimes called the “lying”
orientation of a C70 guest; see schematic illustration in Figure
11a (θ = 0°).
The preferred orientation of C70 guest molecules in ring-
shaped [n]CPP hosts (CPP = cycloparaphenylene, with n =
number of p-phenylene moieties in the ring) has been studied
experimentally and computationally. Yamago and co-workers
determined the crystal structure of the C70 adducts of two
CPPs.39 When bound in a 10-ring host ([10]CPP), the C70
guest prefers the “lying” orientation, i.e., with its 5-fold axis
parallel to the principal axis of the CPP ring (Figure 11a, θ =
0°). In the next larger host, [11]CPP, on the other hand, the
C70 symmetry axis is perpendicular to the CPP axis (i.e., in the
plane of the ring); this is the “standing” orientation (Figure 11c,
θ = 90°). A recent computational study by Zhao et al. supports
We also determined the equilibrium constants for the
binding of C70 to Cu4(m-pbhx)4 from UV−vis spectra; see
Table 2. The values obtained were substantially larger than
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Figure 11. Three possible orientations of a C70 guest molecule (purple) in a ring-shaped host (black). In each diagram, the centroids of the two
opposite C5 rings of the C70 guest (coincident with its 5-fold symmetry axis) are marked by small lavender spheres. (a) “Lying” orientation, with the
C70 and ring axes coincident, defined as θ = 0°. (b) “Half-lying” orientation, with 0° < θ < 90° (θ ≈ 54° shown). (c) “Standing” orientation, with the
C70 and ring axes perpendicular (θ = 90°).
those for C60, in both 1,2-dichlorobenzene and toluene. Other
metal-based hosts also tend to show higher affinities for C70
than for C60.37,38 Calculated spectra for Cu4(m-pbhx)4(μ-C60)
and Cu4(m-pbhx)4(μ-C70) derived from this analysis are
included in Supporting Information.
internal substituent (2-OCH3) is introduced. All but one of the
new squares react with guest molecules such as 1,2-bis(4-
pyridyl)ethylene (bpe) (σ) and C60 and C70 (π). The structure
of the C70 adduct suggests that it may be possible to build a
molecular square that binds fullerenes more strongly than
Cu4(m-pbhx)4 does. We observed no reaction between the
internally substituted molecular square, Cu4(2-MeO-m-pbpr)4
(13), and potential guest molecules. This is likely due to steric
interference. In order to permit both internal substitution and
guest binding, the hosts will need to be made larger. We are
now preparing bis(β-diketones) with longer bridging groups so
as to allow for both functions.
CONCLUSION
■
Externally and internally substituted Cu(II) molecular squares
have been prepared from appropriate derivatives of the bis(β-
diketone) m-pbaH2. Substituted squares exhibit modified
solubility, with longer substituents (OC4H9, C5H11) leading
to solubility in a variety of common organic solvents. The
solubility of the molecular square changes only slightly when an
I
DOI: 10.1021/acs.inorgchem.5b00792
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Inorganic Chemistry
Article
1
(96%). H NMR: δ 10.06 (s, 2H, CHO); 7.97 (s, 1H); 7.66 (s, 2H);
3.94 (s, 3H, OCH3).
EXPERIMENTAL SECTION
■
General Considerations. Reagents were used as received, from
Sigma-Aldrich (except for 6-dodecyne, which was obtained from GFS
Chemicals, and fullerenes, from MER Corporation). Silica gel
(Sorbent Technologies, 230−450 mesh) was used for column
chromatography. Dry solvents were obtained from a commercial
solvent purification system. Mass spectra were taken with Agilent 6210
(ESI) and Varian Saturn 2200 GC/MS, and UV−vis spectra with an
Aviv 14DS instrument. Binding constants were calculated from UV−
vis spectra (25.5 °C) by using the global-analysis program SPECFIT/
32 (Spectrum Software Associates). Elemental analyses were
performed by M-H-W Laboratories (Phoenix, AZ). NMR spectra
were recorded on Bruker 250 or 400 MHz spectrometers with CDCl3
as solvent. Microwave reactions were performed using a CEM MARS
microwave oven. The phospholenes 2,2,2-trimethoxy-4,5-dimethyl-42
and 2,2,2-trimethoxy-4,5-diethyl-1,3,2-dioxaphospholene16 were pre-
pared by literature methods.
5-Methoxy-m-phenylenebis(acetylacetone), 5-MeO-m-
pbaH2 (4). Compound 4 was prepared using the same procedure as
3. 5-Methoxyisophthalaldehyde (16) (0.30 g, 1.8 mmol) and 2,2,2-
trimethoxy-4,5-dimethyl-1,3,2-dioxaphospholene (0.76 g, 3.6 mmol)
were mixed together. After the reaction was complete, the crude
material was purified by column chromatography (ethyl acetate−
1
hexane, 1:5 v/v) to obtain a white solid, 0.19 g (34%). H NMR: δ
16.6 (s, 2H); 6.71 (s, 2H); 6.62 (s, 1H), 3.84 (s, 3H, OCH3), 1.92 (s,
12H). 13C NMR: δ 190.5, 160.1, 138.7, 126.1, 115.6, 114.6, 55.2, 23.9.
Anal. Calcd for C17H20O5 (M = 304.34): C 67.09, H 6.62. Found: C
66.13, H 6.93. Although this material is somewhat impure, as judged
by microanalysis, we saw no evidence for impurities in its NMR or
mass spectra (m/z 305 [M + H]).
Dimethyl 5-Butoxyisophthalate (17). Dimethyl 5-hydroxyisoph-
thalate (4.50 g, 21.4 mmol), powdered K2CO3 (7.40 g, 53.6 mmol),
and 1-bromobutane (3 mL, 27.6 mmol) were placed in a round-
bottom flask. Then, the flask was flushed with N2 for about 10 min,
and dry DMF (50 mL) was added to the flask via syringe. The reaction
mixture was heated to ca. 80 °C for 7 h, with stirring and under
nitrogen. It was allowed to cool to room temperature and extracted
with ethyl acetate and water; the organic phase was dried over Na2SO4
and evaporated. The tan solid residue was purified by column
chromatography (ethyl acetate−hexane, 1:4 v/v) to yield a white solid,
Precursors and β-Diketones: Dodecane-6,7-dione (14). This
compound was prepared following the reported procedure for making
other α-diketones.26 6-Dodecyne (5.0 g, 30.0 mmol) was added to
CCl4/CH3CN/H2O (2:2:3 v/v; 140 mL) and the mixture stirred for
10 min at room temperature. To this mixture was added solid NaIO4
(25.72 g, 120.0 mmol), and stirring continued vigorously at room
temperature until the solid dissolved, giving two colorless phases.
Then RuO2·xH2O (0.088 g, 0.66 mmol) was added; immediately, the
mixture turned black. Ten minutes later, the color of the mixture
changed to dark green, and a white solid precipitated out. As stirring
continued, the mixture gradually changed to lighter green and finally to
yellow. After 12 h, CH2Cl2 (50 mL) was added, and the mixture was
separated into two phases; the organic phase was dried over Na2SO4,
filtered, and evaporated to yield a yellow solid. Column chromatog-
raphy (hexane−ethyl acetate, 70:30 v/v) gave a yellow solid, 3.57 g
1
3.13 g (62%). GC/MS: m/z 265.9 [M]+. H NMR: δ 8.25 (s, 1H),
7.74 (s, 2H), 4.04 (t, 2H, OCH2CH2CH2CH3), 3.94 (s, 6H,
CO2CH3); 1.79 (quintet, 2H, OCH2CH2CH2CH3); 1.51 (sextet,
2H, OCH2CH2CH2CH3); 1.00 (t, 3H, O(CH2)3CH3)).
5-Butoxy-1,3-benzenedimethanol (18). Compound 18 was
prepared according to a procedure developed by Hayama et al. for
forming 5-methoxybenzenebis(methanol-α,α-d2) from dimethyl 5-
methoxyisophthalate and lithium aluminum deuteride.45 Dry THF (30
mL) was added to a three neck round-bottom flask with an addition
funnel under N2. Then, 5.0 mL (10 mmol) of lithium aluminum
hydride (2 M in THF) was added via syringe. Into this solution was
added dropwise dimethyl 5-butoxyisophthalate (17; 1.097 g, 4.12
mmol) in dry THF (20 mL). The mixture was allowed to stir for 22 h
at room temperature. It was then cooled to 0 °C and 16 mL of 1 M
H2SO4(aq) was added slowly, followed by water (30 mL) and ethyl
acetate (100 mL). The organic layer was separated and washed with
100 mL of saturated NaCl, dried over Na2SO4, and evaporated to give
25
1
(60%). H NMR confirmed the identity of the product.
2,2,2-Trimethoxy-4,5-dipentyl-1,3,2-dioxaphospholene (15).
Trimethyl phosphite (2.06 g, 16.6 mmol) was cooled to 0 °C.
Dodecane-6,7-dione (14; 3.0 g, 15 mmol) dissolved in dry CH2Cl2 (20
mL) was added dropwise and the reaction mixture allowed to warm to
room temperature under nitrogen; stirring was continued for 24 h.
Completion of the reaction was indicated by the disappearance of the
yellow color. The crude product was isolated as an oil by evaporation
of solvent; its 1H NMR spectrum indicated that it was sufficiently pure
for use in the next step. 1H NMR: δ 3.59 (d, 9H, OCH3), 2.17 (t, 4H),
1.51 (q, 4H), 1.34−1.26 (m, 8H), 0.89 (t, 6H)).
m-Phenylenebis(dihexanoylmethane), m-pbhxH2 (3). This
procedure is similar to those previously reported for o-, m-, and p-
phenylenebis(acetylacetone) (o-, m-, and p-pbaH2).16,43 Isophthalalde-
hyde (0.60 g, 4.47 mmol) was dissolved in dry CH2Cl2 (20 mL),
followed by addition of 2,2,2-trimethoxy-4,5-dipentyl-1,3,2-dioxaphos-
pholene (15; 2.88 g, 8.94 mmol). The mixture was stirred at room
1
a light yellow solid, 0.690 g (80%). H NMR: δ 6.93 (s, 1H), 6.85 (s,
2H), 4.66 (s, 4H, CH2OH); 3.98 (t, 2H, OCH2CH2C2H5), 1.77 (m,
2H, OCH2CH2CH2CH3); 1.48 (m, 2H, OCH2CH2CH2CH3), 0.97 (t,
3H, O(CH2)3CH3). ESI-MS: m/z 193.12 [M − OH]. Anal. Calcd for
C12H18O3 (M = 210.27): C 68.54, H 8.63. Found: C 67.33, H 8.06.
The microanalysis indicated that the product is not quite pure, and
1
some small impurity peaks were observed in the H NMR spectrum.
However, it was used successfully to prepare pure 19 (see below).
5-Butoxyisophthalaldehyde (19). Compound 19 was prepared
according to a procedure developed by Bennani et al. to convert 5-tert-
butyl-1,3-benzenedimethanol to 5-tert-butylisophthalaldehyde.46 Pyr-
idinium chlorochromate (PCC; 1.54 g, 7.15 mmol) and 3 g of Celite
were added to a 250 mL flask equipped with an addition funnel under
N2. Then, about 10 mL of dry CH2Cl2 was added, and the mixture was
vigorously stirred. A solution of 5-butoxy-1,3-benzenedimethanol (18;
0.520 g, 2.47 mmol) in CH2Cl2 (10 mL) was added dropwise into the
mixture. Stirring was continued for 3 h, and the material was filtered
through a short pad of silica gel using CH2Cl2 and ethyl acetate.
Volatiles were removed in vacuo, leaving a yellow liquid, 0.420 g
1
temperature under nitrogen and the reaction monitored by H NMR.
Reaction was judged to be complete when the isophthalaldehyde
CHO peak (10.1 ppm) had disappeared (ca. 12 h). Then, methanol
(30 mL) was added and the mixture refluxed for 3 h; solvent was
removed to yield a light-brown oily product. Column chromatography
(hexane−ethyl acetate, 7:3 v/v) yielded a colorless oil, 1.00 g (45%).
GC/MS: m/z 498 [M]. 1H NMR: δ 16.81 (s, 2H), 7.42 (t, 1H), 7.14
(d, 2H), 7.00 (s, 1H), 2.10 (t, 8H), 1.52 (quintet, 8H), 1.25−1.12 (m,
16H), 0.82 (t, 12H). 13C NMR: δ 193.7, 137.4, 134.7, 130.8, 129.4,
114.5, 36.8, 31.7, 25.4, 22.5, 14.0.
5-Methoxyisophthalaldehyde (16). 5-Methoxy-1,3-benzenedi-
methanol (0.502 g, 2.98 mmol), p-toluenesulfonic acid (2.29 g, 12.0
mmol), and NaNO3 (0.523 g, 6.15 mmol) were placed in a microwave
reaction vessel and CH3CN (2 mL) added. Then the solvent was
evaporated and the vessel was placed in the microwave. The mixture
was heated for a total of 6 min, via 30-s intervals separated by
approximately 10 s of cooling time.44 After cooling, the solid was taken
up in a mixture of CH2Cl2 and water and the organic phase separated,
dried over Na2SO4, and evaporated to yield a yellow solid, 0.470 g
1
(82%). H NMR: δ 9.97 (s, 2H, CHO), 7.86 (s, 1H), 7.56 (s, 2H),
4.00 (t, 2H, OCH2CH2C2H5), 1.74 (quintet, 2H, OCH2CH2-
CH2CH3), 1.42 (sextet, 2H, OCH2CH2CH2CH3), 0.91 (t, 3H,
O(CH2)3CH3). 13C NMR: δ 190.82, 160.19, 138.15, 123.74, 119.71,
68.41, 30.84, 18.98, 13.60. ESI-MS: m/z 207.1 [M + H]. Anal. Calcd
for C12H14O3 (M = 206.24): C 69.88, H 6.84. Found: C 69.80, H 6.66.
5-Butoxy-m-phenylenebis(acetylacetone), 5-BuO-m-pbaH2
(5). This compound was prepared using the same procedure as for
compound 3. 5-Butoxyisophthalaldehyde (0.40 g, 1.9 mmol) was
J
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Article
treated with 2,2,2-trimethoxy-4,5-dimethyl-1,3,2-dioxaphospholene
(0.81 g, 3.8 mmol). After the reaction was complete, the crude
product was purified by column chromatography (ethyl acetate−
hexane, 1:2 v/v) which yielded a yellow-brown solid, 0.29 g, (44%). 1H
NMR: δ 16.6 (s, 2H), 6.70 (s, 2H), 6.60 (s, 1H), 3.97 (t, 2H,
OCH2CH2C2H5), 1.92 (s, 12H, CH3); 1.80 (m, 2H,
OCH2CH2CH2CH3), 1.53 (m, 2H, OCH2CH2CH2CH3), 1.00 (t,
3H, O(CH2)3CH3). 13C NMR: δ 190.76, 190.55, 163.86, 138.62,
125.90, 116.15, 114.70, 67.79, 31.01, 26.06, 19.07, 13.71. GC/MS: m/z
347 [M + H].
[Cu4(5-MeO-m-pba)4] (11). CuSO4·5H2O (0.18 g, 0.72 mmol); 4
(0.219 g, 0.72 mmol). Yield: 0.24 g of dark green powder (96%). ESI-
MS: m/z 1461.18 [M + H]. Blue crystals suitable for X-ray analysis
were grown by layering CHCl3 onto a solution of the compound in a
CH3OH−CH2Cl2 mixture. Anal. Calcd for C68H76Cu4O22 ([Cu4(5-
MeO-m-pba)4]·2H2O, M = 1499.50): C 54.47, H 5.11. Found: C
54.54, H 5.61.
[Cu4(5-BuO-m-pba)4] (12). CuSO4·5H2O (0.18 g, 0.72 mmol); 5
(0.250 g, 0.72 mmol). Yield: 0.28 g of an olive green powder (97%).
ESI-MS: m/z 1629.32 [M + H]. Anal. Calcd for C80H96Cu4O20·2H2O
(M = 1667.82): C 57.61, H 6.04. Found: C 57.88, H 6.41.
[Cu4(2-MeO-m-pbpr)4] (13). CuSO4·5H2O (0.138 g, 0.55 mmol);
7 (0.20 g, 0.55 mmol). Yield: 0.14 g of a dark green powder (60%).
ESI-MS: m/z 1685.45 [M + H]+. Anal. Calcd for C84H104Cu4O20 (M =
1687.90): C 59.77, H 6.21. Found, C 59.64, H 6.24. Crystals suitable
for X-ray analysis were grown from a solution in hot toluene by
layering with acetonitrile.
Adducts with Guest Molecules: [Cu4(m-pbhx)4(μ-bpe)-
(MeOH)2]·10CHCl3 (23). Molecular square 10 (0.020 g, 0.008
mmol) was dissolved in chloroform (2 mL); this was layered with
1,2-bis(4-pyridyl)ethylene (bpe) (0.006 g, 0.03 mmol) in methanol (3
mL). After 10 days, blue block-shaped crystals had formed. Anal. Calcd
for C143H211Cl3Cu4N2O18 ([Cu4(m-pbhx)4(μ-bpe)(MeOH)2]·CHCl3,
M = 2606.75): C 65.89, H 8.16, N 1.07. Found: C 66.16, H 8.20, N
0.80.
[Cu4(m-pbhx)4(μ-bpa)(MeOH)2] (24). This adduct was prepared
using the same procedure as for 23 above except that 1,2-bis(4-
pyridyl)ethane (bpa) was used in place of bpe. After 7 days, blue
crystals had formed.
Dimethyl 2-Methoxyisophthalate (20). 2-Methoxyisophthalic
acid (3.00 g, 15 mmol) was dissolved in MeOH (20 mL) and 0.5 mL
of conc H2SO4, and the mixture was refluxed for ca. 16 h. The mixture
was neutralized with NH3(aq), and the solvent was removed under
reduced pressure to give a yellowish-white solid. This was taken up in
CH2Cl2 (100 mL) and the solution washed with water, 0.5 M Na2CO3,
and brine (50 mL). The organic phase was dried over Na2SO4 and
1
evaporated to yield a colorless oil (3.02 g, 88%). H NMR: δ 7.32 (d,
2H), 7.18 (t, 1H), 3.92 (s, 3H, OCH3), 3.91 (s, 6H, CO2CH3).
2-Methoxy-1,3-benzenedimethanol (21). This procedure is
similar to one recently reported by Ay and co-workers.47 Lithium
aluminum hydride (LiAlH4) (1.84 g, 48 mmol) was suspended in dry
THF (40 mL). Into this suspension a solution of 20 (3.03 g, 13.5
mmol) in THF (20 mL) was added dropwise under N2. The mixture
was stirred at room temperature for 24 h, and then cooled to 0 °C, and
an aqueous solution of 1 M H2SO4 (47 mL) was added slowly,
followed by ethyl acetate (100 mL). The organic layer was washed
with brine, dried over Na2SO4, and evaporated to give a white solid
1
(1.96 g, 86%). GC/MS: m/z 168. H NMR: δ 7.33 (d, 2H), 7.15 (t,
[Cu4(5-BuO-m-pba)4(μ-4,4′-bpy)] (25). A solution of square 12
(0.015 g, 0.009 mmol) in chloroform (2 mL) was layered with 4,4′-
bpy (0.0057 g, 0.036 mmol) in methanol (3 mL). Blue crystals formed
after several days.
[Cu4(m-pbhx)4(μ-C60)] (26). A solution of square 10 (0.015 g,
0.006 mmol) and C60 (0.005 g, 0.006 mmol) in 1,2-dichlorobenzene
(2 mL) was layered with acetonitrile (3 mL) and left at −20 °C. After
4 days, dark brown crystals had formed.
1H), 4.75 (s, 4H, CH2OH); 3.87 (s, 3H, OCH3).
2-Methoxyisophthalaldehyde (22). Compound 21 (1.00 g, 5.9
mmol) was dissolved in CH2Cl2 (30 mL) and added all at once to a
mixture of PCC (3.83 g, 17.8 mmol) and Celite (8 g) suspended in
CH2Cl2 (30 mL). The mixture was stirred vigorously for 4 h, and then
filtered over a short pad of silica gel, followed by a rinse with CH2Cl2/
ethyl acetate (1:1 v/v). Solvent was removed in vacuo to yield a white
1
solid (0.95 g, 97%). The H NMR spectrum of the product matches
[Cu4(m-pbhx)4(μ-C70)] (27). The same procedure was followed as
with the C60 adduct (26), except that C70 was used instead. Dark
brown crystals formed over a period of 7 days.
that reported for the authentic compound.48
2-Methoxy-m-phenylenebis(acetylacetone), 2-MeO-m-
pbaH2 (6). This compound was prepared in the same manner as
compound 3. 2-Methoxyisophthalaldehyde (22; 0.40 g, 2.4 mmol) and
2,2,2-trimethoxy-4,5-dimethyl-1,3,2-dioxaphospholene (1.02 g, 4.8
mmol) were mixed together. After the reaction was complete, the
crude product was purified by column chromatography (ethyl
ASSOCIATED CONTENT
* Supporting Information
■
S
Illustrations of the crystal structure of Cu4(m-pbhx)4(μ-
bpa)(CH3OH)2 (24), and of the orientations of all four C70
guest molecules in [Cu4(m-pbhx)4(μ-C70)] (27); spectral data
for Cu4(m-pbhx)4(μ-C60) and Cu4(m-pbhx)4(μ-C70); and X-ray
crystallographic data for 10, 11, 13, 23, 24, 25, 26, and 27, as
summary table and in CIF format. The Supporting Information
is available free of charge on the ACS Publications website at
DOI: 10.1021/acs.inorgchem.5b00792.
1
acetate−hexane, 1:4 v/v), yielding a white solid, 0.33 g (45%). H
NMR: δ 16.76 (s, 2H), 7.27−7.16 (m, 3H), 3.47 (s, 3H), 1.94 (s,
12H). Anal. Calcd for C17H20O5 (M = 304.34): C 67.09; H 6.62.
Found: C 67.31; H 6.75.
2-Methoxy-m-phenylenebis(dipropionylmethane), 2-MeO-
m-pbprH2 (7). This compound was prepared following the same
procedure as described for 3. Compound 22 (0.40 g, 2.4 mmol) and
2,2,2-trimethoxy-4,5-diethyl-1,3,2-dioxaphospholene (1.15 g, 4.8
mmol) were combined together. Column chromatography of the
crude product (ethyl acetate−hexane, 1:4 v/v) afforded a colorless oil
(0.26 g, 30%). 1H NMR: δ 16.84 (s, 2H), 7.17−7.10 (m, 3H), 3.46 (s,
3H), 2.17 (q, 8H), 1.07 (t, 12H).
AUTHOR INFORMATION
Corresponding Author
*E-mail: maverick@lsu.edu.
■
Syntheses of Copper Molecular Squares: [Cu4(m-pbhx)4]
(10). The procedure described here was followed for all four squares,
10−13. Aqueous CuSO4·5H2O (0.401 g, 1.6 mmol, in 30 mL H2O)
was converted to [Cu(NH3)4]2+ by treatment with conc NH3(aq). A
solution of m-pbhxH2 (3) (0.8 g, 1.6 mmol) in CH2Cl2 (40 mL) was
added, and stirring continued for 4 h; the green organic phase was
dried over Na2SO4 and the solvent removed to yield a dark green
powder, 0.87 g (97%). ESI-MS: m/z 2238.15 [M + H]. Anal. Calcd for
C128H192Cu4O16 (M = 2241.07): C 68.60, H 8.64. Found: C 68.42, H
8.51. Crystals for X-ray analysis were grown by layering methanol on a
chloroform solution. Anal. Calcd for C130H200Cu4O18 ([Cu4(m-
pbhx)4]·2CH3OH, M = 2305.15): C 67.73, H 8.75. Found: C 68.00,
H 8.64.
Present Addresses
†Moi University, Eldoret, Kenya.
‡Department of Physical Sciences, Nicholls State University,
Thibodaux, Louisiana 70310, United States.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the U.S. Department of Energy (Grant DE-
FG02-01ER15267) for partial support of this research. We
thank Ms. Samantha Ingalls and Dr. Robert P. Hammer for
K
DOI: 10.1021/acs.inorgchem.5b00792
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Article
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DOI: 10.1021/acs.inorgchem.5b00792
Inorg. Chem. XXXX, XXX, XXX−XXX