Syntheses and characterization of copper(II) carboxylate dimers formed from enantiopure ligands containing a strong π???π stacking synthon: Enantioselective single-crystal to single-crystal gas/solid-mediated transformations

Article abstract of DOI:10.1021/ic201238n

Tri- and tetrafunctional enantiopure ligands have been prepared from 1,8-naphthalic anhydride and the amino acids l-alanine, d-phenylglycine, and l-asparagine to produce (S)-2-(1,8-naphthalimido)propanoic acid (HL _ala), (R)-2-(1,8-naphthalimido

Full text of DOI:10.1021/ic201238n

ARTICLE
pubs.acs.org/IC
Syntheses and Characterization of Copper(II) Carboxylate Dimers
Formed from Enantiopure Ligands Containing a Strong
π π Stacking Synthon: Enantioselective Single-Crystal to
Single-Crystal Gas/Solid-Mediated Transformations
3 3 3
Daniel L. Reger,* Jacob J. Horger, Agota Debreczeni, and Mark D. Smith
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States
S Supporting Information
b
ABSTRACT: Tri- and tetrafunctional enantiopure ligands have
been prepared from 1,8-naphthalic anhydride and the amino
acids L-alanine, D-phenylglycine, and L-asparagine to produce
(S)-2-(1,8-naphthalimido)propanoic acid (HLala), (R)-2-(1,8-
naphthalimido)-2-phenylacetic acid (HLphg), and (S)-4-amino-
2-(1,8 naphthalimido)-4-oxobutanoic acid (HLasn), respec-
tively. Reactions of Lalaꢀ with copper(II) acetate under a variety
of solvent conditions has led to the formation and characteriza-
tion by X-ray crystallography of three similar copper(II) pad-
dlewheel complexes with different axial ligands, [Cu2(Lala)4(THF)2] (1), [Cu2(Lala)4(HLala)] (2), and [Cu2(Lala)4(py)(THF)]
(3). A similar reaction using THF and Lphgꢀ leads to the formation of [Cu2(Lphg)4(THF)2] (4). With the exception of a disordered
component in the structure of 4, the naphthalimide groups in all of these compounds are arranged on the same side of the square,
central paddlewheel unit, forming what is known as the chiral crown configuration. A variety of π π stacking interactions of the
3 3 3
1,8-naphthalimide groupꢀs organize all of these complexes into supramolecular structures. The addition of the amide group
functionality in the Lasn ligand leads to the formation of tetrameric [Cu4(Lasn)8(py)(MeOH)] (5), where reciprocal axial
coordination of one of the amide carbonyl oxygen atoms between two dimers leads to the tetramer. Extensive supramolecular
interactions in 5, mainly the π π stacking interactions of the 1,8-naphthalimide supramolecular synthon, support an open three-
3 3 3
dimensional structure containing large pores filled with solvent. When crystals of [Cu4(Lasn)8(py)(MeOH)] are exposed to (S)-
ethyl lactate vapor, the coordinated methanol molecule is replaced by (S)-ethyl lactate, bonding to the copper ion through the
carbonyl oxygen, yielding [Cu4(Lasn)8(py)((S)-ethyl lactate)] (6) without a loss of crystallinity. With the exception of the
replacement of the one axial ligand, the molecular structures of 5 and 6 are very similar. In a similar experiment of 5 with vapors of
(R)-ethyl lactate, again a change occurs without a loss of crystallinity, but in this case the (R)-ethyl lactate displaces only slightly more
than half of the axial methanol molecules forming [Cu4(Lasn)8(py){((R)-ethyl lactate)0.58(MeOH)0.42}] (7). Importantly, in 7, the
(R)-ethyl lactate coordinates through the hydroxyl group. When crystals of [Cu4(Lasn)8(py)(MeOH)] are exposed to vapors of
racemic ethyl lactate, the coordinated methanol molecule is displaced without a loss of crystallinity exclusively by (S)-ethyl lactate,
yielding a new form of the tetramer [Cu4(Lasn)8(py)((S)-ethyl lactate)], in which the ethyl lactate in the pocket bonds to the
copper(II) ion through the carbonyl oxygen as with 6. Exposure of [Cu4(Lasn)8(py){((R)-ethyl lactate)0.58(MeOH)0.42}] to
racemic ethyl lactate yields a third form of [Cu4(Lasn)8(py)((S)-ethyl lactate)], where the three forms of [Cu4(Lasn)8(py)((S)-ethyl
lactate)] have differences in the number of ordered (S)-ethyl lactate molecules located in the interstitial sites. These results
demonstrate enantioselective bonding to a metal center in the chiral pocket of both 5 and 7 during single-crystal to single-crystal
gas/solid-mediated exchange reactions.
’ INTRODUCTION
One important structural type that has been extensively
studied is formed by rigid metallic building blocks, referred to
as secondary building units (SBUs), organized into ordered
structures by robust organic linkers, compounds generally
referred to as metal-organic frameworks (MOFs).4,5 Multidentate
linkers such as carboxylates are used to prepare the SBU in the
formation of these rigid MOFs; these ligands have the added
Investigations of framework complexes comprised of metal
centers linked by organic spacers are important in many fields of
chemistry and materials science because of potential applications
as catalysts,1 molecular sensors,2 and molecular sieves3 and in the
design of materials with interesting physical properties. These
types of complexes can have a range of rigidity depending on the
makeup of the organic linkers and the types of bonding interac-
tions, both covalent and noncovalent, from which the structures
are built.
Received: June 9, 2011
Published: September 15, 2011
r
2011 American Chemical Society
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Scheme 1. Bifunctional Carboxylate Ligands Containing the
1,8-Naphthalimide Group
convenience—the 1,8-naphthalimide group can easily be intro-
duced into different types of ligand systems.
More recently, we have prepared bifunctional carboxylate
ligands containing this supramolecular synthon (Scheme 1).10
Using these ligands to prepare metal complexes with the well-
known copper(II) carboxylate dimers (paddlewheels) as the
SBU produced interesting three-dimensional structures. The
copper(II) carboxylate system was chosen for its “square” pad-
dlewheel architecture and its ability to strongly coordinate to
axial ligands.11
We report here that the introduction into these types of
ligands of an enantiopure chiral center from an amino acid,
Scheme 2, leads to the syntheses of copper(II) carboxylate
dimers and asparagine tetramers that have interesting molecular
and supramolecular structures. We anticipated the preparation of
homochiral complexes, with unique structures organized by the
square-shaped SBU and the 1,8-naphthalimide supramolecular
synthon, that have unusual properties.12ꢀ15 Of particular interest
in the results reported here are the tetrameric complexes
prepared from the asparagine ligand where the open supramo-
lecular structures combined with the chirality of the ligands lead
to the observation of enantioselective exchange of axial ligands
during single-crystal to single-crystal gas/solid-mediated trans-
formations. Chiral rhodium(II) complexes of enantiopure car-
boxylate ligands related to those reported here (mainly
phthalimide derivatives) have been used previously as catalysts
to support asymmetric cyclopropanation and CꢀH insertion
reactions,16,17 and only recently have a number of these com-
plexes been structurally characterized, including one example of a
copper(II) dimer.16g,17b,18 We have previously communicated
some of these results.19
Scheme 2. Enantiopure Trifunctional (Lalaꢀ, Lphgꢀ) and
Tetrafunctional (Lasnꢀ) Ligands Containing the 1,8-
Naphthalimide Group
advantage of producing neutral metal clusters that do not
contain counterions that would fill potential open spaces in the
structures.
Monodentate ligand to metal coordination linkages (using
mostly rigid pyridine-based ligands) are also widely used to
support the formation of these types of structures. Frameworks
built partially or completely by these interactions can be viewed
as somewhat different from MOFs, as the stability of the network
is reduced such that the framework is more likely to collapse
upon the removal of solvent molecules occupying spaces in the
structure.6
Much less rigid but still highly organized structures can be
built with noncovalent supramolecular interactions. Hydrogen
bonding is the most widely used organizational synthon in these
materials, which has the potential advantage (and disadvantage
in some cases) of creating more flexible materials that are
more “elastic” in their properties.7 Other types of noncovalent
’ EXPERIMENTAL SECTION
General Considerations. Unless otherwise specified, all opera-
tions were carried out with no special precautions to exclude air or
moisture. All solvents and reagents were used as purchased. (R)-ethyl
lactate was produced by transesterification of commercially available
(R)-methyl lactate with ethanol and purified by kugelrohr distillation;
enantiopurity was verified via 1H NMR by the addition of a small amount
of the chiral shift reagent ErFOD. Analytical figures for the metal
complexes were carried out on crystals dried to a constant weight.
General Procedure for Preparation of Ligands. To a stirred
solution of the desired amino acid (22 mmol) in water (25 mL) was
added solid potassium hydroxide (1.12 g, 20 mmol). This solution was
allowed to stir for 20 min, and 1,8-naphthalic anhydride (3.96 g, 20
mmol) and ethanol (75 mL) were added. The solution was heated at
reflux for 6ꢀ8 h, during which time the cream-colored suspension of 1,8-
naphthalic anhydride dissolved. The heat was removed, and a solution of
1 M aqueous HCl (20 mL, 20 mmol) was added. The stirring was
stopped, and the reaction mixture was allowed to stand undisturbed
overnight. The solid product that formed was filtered and washed with 4
ꢁ 50 mL portions of water followed by 50 mL of ice-cold anhydrous
ethanol. The solid was dried under vacuum conditions overnight to
afford analytically pure 1,8-naphthalimide-substituted amino acids.
(S)-2-(1,8-Naphthalimido)propanoic Acid (HLala). Prepared as
above, starting from L-alanine (1.96 g, 22 mmol). Yield: 4.59 g (85%).
1H NMR (CDCl3): δ 8.62 (d, J = 7.3 Hz, 2 H, 2-nphth), 8.24 (d, J = 8.2
Hz, 2 H, 4-nphth), 7.77 (t, J = 7.6 Hz, 2 H, 3-nphth), 5.84 (q, J = 7.0 Hz, 1
H, α-CH), 1.71 (d, J = 7.1 Hz, 3 H, β-CH3). Anal. Calcd for
C15H11NO4: C, 66.91; H, 4.12; N, 5.20. Found: C, 66.85; H, 3.91;
N, 4.94.
interactions, most notably π π stacking of aromatic groups,
3 3 3
have been used in the construction of flexible materials.8
We are attempting to construct framework complexes that
combine the rigidity of the central SBU core with the supramo-
lecular organization provided by unusually strong π π stacking
3 3 3
interactions. Given that the established order of stability of
π
π interactions is π-deficient ꢀ π-deficient > π-deficient
3 3 3
ꢀ π-rich > π-rich ꢀ π-rich,8a we have recently incorporated the
π-deficient, extended aromatic 1,8-naphthalimide group as an
important noncovalent bonding synthon into multifunctional
poly(pyrazolyl)methane ligands. We have shown that metal
complexes of these ligands are organized into highly ordered
structures by this functional group using strong noncovalent
interactions, interactions that persist in solution.9 An additional
important advantage of this supramolecular synthon is synthetic
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(R)-2-(1,8-Naphthalimido)-2-phenylacetic Acid (HLphg). Prepared
as above, starting from D-phenylglycine (3.33 g, 22 mmol). Yield: 5.04 g
(76%). 1H NMR (CDCl3): δ 8.97 (dd, J = 7.4, 0.9 Hz, 2 H, 2-nphth),
8.56 (dd, J = 7.9, 0.3 Hz, 2 H, 4-nphth), 8.10 (m, 4 H, 2,5-Ph, 3-nphth),
7.67 (m, 4 H, 3,4,5-Ph, α-CH). Anal. Calcd for C20H13NO4: C, 72.50 H,
3.95; N, 4.23. Found: C, 72.63; H, 3.81; N, 3.48.
Scheme 3. General Synthesis of Enantiopure 1,8-Naphthali-
mide Ligands from Amino Acids
(S)-4-Amino-2-(1,8-naphthalimido)-4-oxobutanoic Acid (HLasn).
Prepared as above, starting from L-asparagine (2.91 g, 22 mmol). Yield:
4.83 g (75%). Due to the extremely low solubility of this compound in
organic solvents, the NMR spectrum was measured as the potassium salt
of the ligand. 1H NMR (CD3OD): δ 7.02 (d, J = 7.3 Hz, 2 H, 2-nphth),
6.80 (d, J = 8.5 Hz, 2 H, 4-nphth), 6.26 (t, J = 7.3 Hz, 2 H, 3-nphth), 4.44
(dd, J = 5.5, 8.8 Hz, 1 H, α-CH), 1.84, 1.45 (m, 2 H, β-CH2). Anal. Calcd
for C16H12N2O5: C, 61.54; H, 3.87; N, 8.97. Found: C, 61.29; H, 3.82;
N, 8.79.
obtained after 4ꢀ7 days. Anal. Calcd for C134H97N17O41Cu4: C, 56.36;
H, 3.42; N, 8.34. Found: C, 56.53; H, 3.58; N, 8.12.
Preparation of Paddlewheel Complexes. [Cu2(Lala)4(THF)2]
(1). To a powdered sample of HLala (1.07 g, 4.0 mmol) suspended in
water (40 mL) was added a solution of 1 M KOH in methanol (4 mL, 4.0
mmol). This solution was stirred until the suspension had completely
cleared (ca. 5 min). To this solution was added a solution of copper(II)
acetate (400 mg, 1.0 mmol) in water (40 mL). A purple precipitate
formed immediately. This mixture was stirred for 2 h, and the purple
precipitate was isolated by filtration and washed twice with each water,
absolute ethanol, and diethyl ether. After drying under vacuum cond-
tions, this powder was dissolved in a minimal amount of 1:1 THF/
CH2Cl2. An equal volume of hexane was added, and the solvents were
removed via rotary evaporation to give the title compound (1.02 g, 76%)
as a green powder. Single crystals were grown by layering hexane on top
of a THF solution of the title compound (ca. 5 mg/mL) and allowing the
solutions to rest undisturbed in a quiet location. Crystals suitable for
X-ray analysis were obtained after 1ꢀ3 days. Anal. Calcd for
C68H56N4O18Cu2: C, 60.76; H, 4.20; N, 4.17. Found: C, 60.61; H,
4.07; N, 4.25.
Vapor-Phase Single-Crystal to Single-Crystal Ligand Ex-
change. For the vapor-phase single-crystal to single-crystal ligand
exchange, the crystallization solvent was carefully removed from a test
tube containing a freshly grown sample of [Cu4(Lasn)8(py)(MeOH)]
(without disturbing the crystals adhered to the walls of the tube), and the
crystals were washed with a small amount of a 10:9:1 diethyl ether/
dichloromethane/methanol mixture. The crystals were briefly allowed
to air-dry (∼30ꢀ60 s), and a small test tube lined with filter paper
containing ∼1 mL of the desired ligand ((S)-ethyl lactate (S-EtLac),
racemic ethyl lactate, or (R)-ethyl lactate (R-EtLac)) was carefully
placed inside the larger test tube. The system was sealed with a rubber
septum and allowed to sit undisturbed for approximately 48 h. Single
crystals were selected from the tube for X-ray crystallographic analysis
and, for the cases of (S)-ethyl lactate and racemic ethyl lactate, shown to
be [Cu4(Lasn)8(py)(S-EtLac)]. Anal. Calcd for C138H103N17O43Cu4: C,
56.35; H, 3.53; N, 8.09. Found: C, 56.12; H, 3.43; N, 8.09. For the cases
of (R)-ethyl lactate, they were shown to be [Cu4(Lasn)8(py){(R-
EtLac)0.58(MeOH)0.42}]
[Cu2(Lala)4(HLala)] (2). This compound was prepared by the same
procedure as for [Cu2(Lala)4(THF)2], with the intermediate purple
product being redissolved in a minimal amount of pure CH2Cl2 instead
of a mixed CH2Cl2/THF system. Yield: 882 mg, 60%. Single crystals
were grown by layering hexane on top of a CH2Cl2 solution of the title
compound (ca. 5 mg/mL) and allowing the solutions to rest undisturbed
in a quiet location.
[Cu2(Lala)4(py)(THF)] (3). This compound was prepared by the same
procedure as for [Cu2(Lala)4(THF)2], but with the addition of pyridine
(0.2 mL) to the aqueous solution of copper acetate. Yield: 1.05 g, 78%.
Single crystals were grown by layering hexane on top of a THF solution
of the title compound (ca. 5 mg/mL) and allowing the solutions to rest
undisturbed in a quiet location. Anal. Calcd for C69H53N5O17Cu2: C,
61.33; H, 3.95; N, 5.18. Found: C, 61.17; H, 4.23; N, 4.96.
[Cu2(Lphg)4(THF)2] (4). This compound was prepared by the same
procedure as [Cu2(Lala)4(THF)2], starting with HLphg (1.33 g, 4.0
mmol). The title compound (1.29 g, 81%) was isolated as a green
powder. Single crystals were grown by layering hexane on top of a THF
solution of the title compound (ca. 5 mg/mL) and allowing the solutions
to rest undisturbed in a quiet location. Anal. Calcd for C88H64N4O18Cu2:
C, 66.37; H, 4.05; N, 3.52. Found: C, 64.24; H, 4.01; N, 3.26.
[Cu4(Lasn)8(py)(MeOH)] (5). This compound was prepared by the
same procedure as [Cu2(Lala)4(THF)2], except a small amount of
pyridine was added to the copper(II) acetate solution, starting with
HLasn (1.25 g, 4.0 mmol), with the intermediate purple product being
redissolved in approximately 400 mL of 1:9 MeOH/CH2Cl2 instead of a
mixed CH2Cl2/THF system. Yield: 1.26 g, 88%. Single crystals were
grown by layering diethyl ether on top of a 1:9 MeOH/CH2Cl2 solution
of the title compound (ca. 5 mg/mL) and allowing the solutions to rest
undisturbed in a quiet location. Crystals suitable for X-ray analysis were
X-Ray Crystallographic Data Collection and Refinement.
All X-ray intensity data were measured at 150(2) K using a Bruker
SMART APEX diffractometer (Mo Kα radiation, λ = 0.71073 Å).20 Raw
area detector data frame reduction and corrections for absorption effects
were performed with the SAINT+ and SADABS programs.20 Final unit
cell parameters were determined by least-squares refinement of large
sets of reflections from each data set. Direct methods structure solution,
difference Fourier calculations, and full-matrix least-squares refinement
against F2 were performed with SHELXTL.21 X-ray crystallographic data
are given in Table S1 (Supporting Information), and detailed explana-
tions of the refinement procedures, including the use of SQUEEZE22 in
certain structures, are given in the Supporting Information.
’ RESULTS
Syntheses. The tri- and tetrafunctional ligands were synthe-
sized from commercially available amino acids in one pot
(Scheme 3). In the presence of ethanolic potassium hydroxide,
the amino acids are heated at reflux with 1,8-naphthalic anhy-
dride to give (after precipitation with acid) the protonated form
of the chiral carboxylate ligand, HLaminoacid. HLala and similar
ligands using other amino acid analogues have been previously
prepared using the conditions of refluxing 1,8-naphthalic anhy-
dride in DMSO, DMF, or pyridine.23 These reactions in DMSO
or DMF, in our hands, could not reliably ensure the retention of
configuration of the chiral center. The reaction conditions
described here both prevent racemization of the amino acid chiral
center (even in the case of the base-sensitive D-phenylglycine) and
eliminate the need for chromatographic purification.
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Scheme 4. Preparation of Paddlewheel Compounds from
Enantiopure Carboxylate Ligands
As has been pointed out elsewhere,24 although there are many
conformations that complexes 1ꢀ4 can adopt, four possible
arrangements of the substituents on the chiral ligands in relation-
ship to the copper dimer core have mainly been considered. The
large 1,8-naphthalimide groups can be arranged such that two are
on each side of the square, central paddlewheel unit, with either a
cis or trans orientation; three could be on one side, or all four
could be on one side, Figure 1. Though the precise arrangement
of the naphthalimide groups varies from complex to complex, all
but part of one of the structures reported here show the
naphthalimide groups all on one side (Figure 1d), adopting what
has been termed in the rhodium dimer literature the “chiral
crown” configuration.16g,18a Following that literature, the
naphthalimide groups are oriented on the same side, forming a
chiral, hydrophobic pocket defined as the α face, and the side
chain “arms” of the ligand located at the chiral carbon are
oriented on the opposite side defined as the β face.
Structure of [Cu2(Lala)4(THF)2] (1). Figure 2 shows a num-
bered diagram for [Cu2(Lala)4(THF)2] and the top view from
the α face that illustrates the “chiral crown” configuration. On the
α face, the naphthalimide groups are all canted in the same
direction, a consequence of the enantiopure center in the ligands
and the steric bulk of the naphthalimide groups. Four separate
carboxylate ligands bridge the two copper(II) centers equato-
rially around the paddlewheel dimer. Molecules of THF coordi-
nate to the copper ion sites at both the α and β faces. The chiral
pocket formed by the naphthalimide groups is nearly symme-
trical. Measuring the size of the pocket by taking the distance
between the outside carbon (labeled C(10) in Figure 2) of
naphthalimide groups trans across the pocket gives values of
13.9 Å and 13.8 Å.
Figure 1. Four possible ligand configurations in chiral-ligand paddle-
wheel complexes.
Three copper(II) dimers of 1, colored in Figure 3 as green,
blue, and purple, are organized into trimeric units by π
π
3 3 3
To prepare the copper(II) paddlewheel complexes, the
potassium salt of the ligand is generated in situ with KOH and
allowed to react with copper(II) acetate (Scheme 4). If pyridine
is desired as an axial ligand, it is added to the reaction mixture in
slight excess. A water-insoluble purple intermediate product that
has not been characterized precipitates from these reaction
mixtures. Subsequent wꢀorkup of this solid that precipitated in
the reaction of the Lala ligand with a THF/CH2Cl2 mixture
followed by crystallization by layering a THF solution with
hexane yields [Cu2(Lala)4(THF)2] (1). A similar workup using
pure CH2Cl2 in both steps yields [Cu2(Lala)4(HLala)] (2). In a
stacking interactions from two naphthalimide groups on each
dimer. The interactions occur via the overlap of the outside face
of one naphthalimide group in the chiral crown of one dimer with
the inside face of a second naphthalimide group in the chiral
crown on a second dimer. Each dimer makes one inside the
pocket and one outside the pocket interaction using naphthali-
mide groups located on adjacent sides of the chiral pocket. These
trimers are arranged so that the α faces of the bimetallic units are
all oriented inward toward each other, while the β faces all point
out. The π π stacking interactions are strong; the planes of
3 3 3
the two rings are separated vertically by 3.42 Å, and the rings are
close to parallel with an interplanar angle of 5.7°. The rings also
have substantial overlap, as measured by the “slippage”parameterχ,
which is the third side of the right triangle formed with the
average perpendicular distance between the rings and the line
joining the central fused ring carbon atoms of the two rings. In
this case, χ is 1.67 Å; we have shown previously that slippage in
the range between 0.43 Å and 2.4 Å is indicative of a strong
interaction.10 The angle made by the two naphthalimide dipole
vectors (running through the central ring carbon atoms, pointing
toward the nitrogen), an additional parameter of interest in these
systems, is 148°.
ꢀ
Lala reaction to which pyridine had been added, the workup
as described above with a THF/CH2Cl2 mixture yields [Cu2-
(Lala)4(py)(THF)] (3). A similar reaction and workup that was
ꢀ
used in 1 but using Lphg yields [Cu2(Lphg)4(THF)2] (4).
ꢀ
Finally, a reaction similar to that discussed for 3 using Lasn
and MeOH/CH2Cl2 instead of a mixed THF/CH2Cl2 system
(for solubility reasons) in the workup and ether as the layering
solvent yields [Cu4(Lasn)8(py)(MeOH)] (5).
Structural Analysis. In all of the new complexes of the three
ligands reported here, the core of each complex is the anticipated
square, dimeric Cu2(O2CR)4 “paddlewheel” SBU motif. Each
carboxylate ligand surrounds the two copper atoms with μ2-
bonding placing the oxygen donor atoms in each of the four
equatorial positions about the square pyramidal, five-coordinate
copper ion sites. The fifth site is filled by the axial ligands. The
The π-stacked trimers pack close in the crystal into staggered
zigzag layers in a repeating AB pattern; neighboring trimers do
not have any substantial π π stacking or other important
3 3 3
noncovalent interactions with one another. Solvent-filled pores
are formed by this AB repeating unit, which corresponds to the
crystallographic c axis. The pores measure approximately 9.5 Å
in diameter and contain ordered hexane and disordered THF
Cu Cu nonbonding distances are between 2.6 and 2.7 Å.
3 3 3
Selected bond distances and angles are given in Tables S2 and S3
(Supporting Information), respectively.
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Figure 2. Left: Partially numbered diagram of [Cu2(Lala)4(THF)2] (1). Right: Top view of α-face chiral pocket.
Figure 3. Naphthalimide π π stacking interactions in [Cu2(Lala)4-
3 3 3
(THF)2] organizing the bimetallic units into a trimer.
molecules. A view of the pores formed by packed trimers can be
seen in Figure 4, where the two layers are color-coded. The
molecular surface of each layer is shown on the right-hand side of
the figure. Solvent molecules have been omitted for clarity.
Structure of [Cu2(Lala)4(HLala)] (2). When the same purple
intermediate initially formed in the reaction of the Lalaꢀ ligand
(used to grow crystals of 1) is dissolved in the noncoordinating
solvent dichloromethane so as to avoid adding potential axial
ligands, [Cu2(Lala)4(HLala)] crystallizes. This dimer contains an
additional protonated form of the ligand coordinated to the
copper ion on the α face, as shown in Figure 5. Again, four
carboxylate ligands bridge the equatorial sites of the paddlewheel.
The carbonyl oxygen of the acid group of HLala is coordinated to
the copper ion inside the α-face pocket with the OH of this group
hydrogen-bonded to one of the equatorial carboxylate oxygen
Figure 4. Top: Section of the overall structure of [Cu2(Lala)4(THF)2]
showing the pores, viewed down the c axis. Layer A trimers are located
above and are shown in shades of blue; layer B trimers are below in red
and yellow. The molecular surface is shown on the right half of the image
to more clearly indicate the pores. Bottom inset: edge-on view of the
section on top showing the AB pattern in the stacking.
ligands on an adjacent dimer, forming a one-dimensional zigzag
coordination polymer. Figure 6 shows six paddlewheel units of
the polymer chain. Only a carbonyl group on one naphthalimide
(from ligand A) in each dimer is involved in the chain formation;
the other three μ2-carboxylate ligands (B, C, and D) and the fifth,
axial HLala (E), do not participate. We note that a coordination
atoms. The chiral pocket measures 15.2 Å (B D) by 13.4 Å
3 3 3
(A C).
3 3 3
The axial ligand for the copper ion located at the β face is
a naphthalimide carbonyl oxygen from one of the four μ2-carboxylate
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polymer composed of dirhodium paddlewheel dimers derived
from a similar enantiopure phthalimide-based ligand, linked by
1,4-dicyanobenzene, has been reported previously.18a
(B E) are also typical of strong interactions. All three planes
3 3 3
are nearly parallel, with both B E and B D interplanar
3 3 3
3 3 3
angles of 6°. This interaction forms one-dimensional zigzag
chains, which run perpendicular to the direction of the coordina-
tion polymer.
Two of the remaining equatorial naphthalimide groups (B, D)
and the axial naphthalimide E participate in π π stacking
3 3 3
interactions. Four π-stacked dimers are shown in Figure 7—the
ligands not participating in the π-stacking interaction have been
omitted for clarity. Naphthalimide groups D and E from one
dimer “clamp” a central equatorially coordinated naphthalimide
(B0) from a neighboring dimer between them; similarly, naphtha-
limide B is itself clamped by naphthalimide groups from a
The combination of the one-dimensional coordination poly-
mer and the one-dimensional π-stacking interaction results in a
fully three-dimensional structure. Each coordination polymer is
π-stacked with four adjacent coordination polymers; Figure 8 is a
cartoon representation of the framework showing these features.
The inefficient packing of this structure leaves a considerable
amount of space occupied by disordered dichloromethane
different dimer. Both π π stacking parts of the interaction
3 3 3
are strong, with reasonable χ values between the central ring and
the axial (χBE = 1.64 Å) and equatorial (χBD = 1.44 Å) arms of the
“clamp.” The vertical distances of 3.42 Å (B D) and 3.48 Å
3 3 3
Figure 7. Naphthalimideꢀnaphthalimide π stacking in [Cu2(Lala)4-
(HLala)]. Separate dimeric units are shown in different colors. On the
left, stacking interactions are indicated with orange dashed lines. The
drawing on the right is a 90° rotation that shows the overlap of the
naphthalimide rings.
Figure 5. One [Cu2(Lala)4(HLala)] (2) unit, viewed toward the α face
where the axial ligand is HLala. The hydrogen-bonding interaction of this
axial ligand is shown in magenta.
Figure 6. The polymeric chain formed by adjacent [Cu2(Lala)4(HLala)] dimers. Nonbridging naphthalimide groups are shown as transparent discs in
the lower figure, colored to match the paddlewheel core to which they belong; the axial HLala units have a hue complementary to the rest of the
paddlewheel.
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Figure 10. Three π-stacked units in a chain of 3. The green-colored
(central) dimer π-stacks with the red-colored (left) and blue-colored
(right) dimers. Top: side view of the chain. Bottom: chain viewed from
Figure 8. Simplified representation of the framework in [Cu2(Lala)4-
(HLala)]. Thick colored cylinders indicate separate polymeric chains,
where the SBU central dimeric unit is represented by a single blue
the α face of the green-colored dimer. The CꢀH π interactions are
3 3 3
shown as dashed magenta lines.
sphere. Each polymer chain is π π stacked with four other polymer
3 3 3
chains, as indicated by the dashed cylinders. The network of dichlor-
omethane molecules occupying the channels is indicated by transparent
light green cylinders.
Figure 9. Two views of the dimer in [Cu2(Lala)4(py)(THF)] (3). Left:
view from the side of the dimer. Right: view from the α-face pocket. The
CꢀH π interactions are shown in both as dashed magenta lines.
3 3 3
molecules. The dichloromethane molecules (light green lines)
occupy three-dimensional channels that interpenetrate the net-
work formed by the π-stacked coordination polymers.
Structure of [Cu2(Lala)4(py)(THF)] (3). When pyridine is
present in the reaction mixture that formed the precursor for 1
and 2, workup with a THF/CH2Cl2 mixture yields [Cu2(Lala)4-
(py)(THF)], a product that has two different ligands in the axial
sites, one THF and one pyridine, as shown in Figure 9. The
pyridine is bonded to the copper(II) ion inside the naphthali-
mide pocket of the α face with the THF molecule capping the β-
face copper. The preference of pyridine for the chiral pocket
Figure 11. The two independent molecules in the structure of
[Cu2(Lphg)4(THF)2] (4). (a) The ABCD dimer, viewed from the side
(top) and from the α face (bottom). The phenyl rings are shown in blue.
(b) A side view of the EFGH dimer (top) and the EFGI dimer (bottom).
The phenyl rings associated with E, F, and G naphthalimide rings are shown in
green, and the phenyl rings associated with H and I naphthalimide rings in red.
appears to be driven by two CꢀH π interactions from the 2
The two naphthalimide groups not participating in the
3 3 3
and 6 position hydrogens of the pyridine ring (shown as dashed
magenta lines) with the nitrogen-containing ring of two of the
naphthalimide groups (H centroid = 3.01, 3.02 Å, CH
CꢀH π stacking interactions with pyridine π π stack
3 3 3
3 3 3
with naphthalimide groups on adjacent dimeric units with both
rings interacting on the inside of the α-face pocket. These
interactions are strong, being nearly parallel (2.8°) with a vertical
separation of 3.39 Å, a χ value of 1.63 Å, and a naphthalimideꢀ
3 3 3
3 3 3
centroid = 179, 170°). These interactions cause asymmetry in
the size of the naphthalimide pocket—the two naphthalimide
groups that participate in the CꢀH π interaction are 11.6 Å
naphthalimide dipole vector angle of 119°. Three π π stacked
units (colored red, green, and blue) are shown in Figure 10; these
interactions form one-dimensional chains where the dimers
3 3 3
3 3 3
apart, as compared to 16.1 Å for the other two naphthalimide
groups.
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Figure 12. ABCD EFG(H/I) π π stacking interactions in the
3 3 3
3 3 3
structure of 4. Arms A, B, C, and D are shown in blue; arms E, F, and G in
green; and arm I in red. Disorder component H is not shown.
alternate orientation (in the top figure, the αface goes up, down, up).
There are no additional significant supramolecular interactions,
yielding a one-dimensional structure.
ꢀ
Structure of [Cu2(Lphg)4(THF)2] (4). The complex of the Lphg
ligand, prepared analogously to [Cu2(Lala)4(THF)2], crystallizes
with two independent [Cu2(Lphg)4(THF)2] dimers in the unit
cell, pictured in Figure 11. One dimer is fully ordered (with
naphthalimide arms labeled A, B, C, and D) and displays the
standard “chiral crown” conformation. It is important to note that
the naphthalimide groups are canted in the opposite direction with
respect to the other dimers discussed above—this change is due to
the absolute configuration of the ligand (R), which was derived from
a D-amino acid (D-phenylglycine) rather than an L-amino acid. The
ABCD chiral pocket (like that observed in [Cu2(Lala)4(THF)2])
is almost symmetrical, measuring 13.4 Å ꢁ 13.2 Å. The other
independent dimer has three ordered arms (E, F, and G), which
are all on the same side (α-face), and a fourth arm, which exhibits
a 2-fold rotational disorder over two equally occupied positions
(H, α-oriented/I, β-oriented). The EFGH disorder component is
almost isostructural with the ABCD dimer, with the chiral pocket
formed by this component measuring 13.6 Å ꢁ 13.9 Å. The EFGI
disorder component, unlike the rest of the complexes, has its
naphthalimide groups arranged as Figure 1c, where only three of
the naphthalimide groups are on one side. This disorder does not
affect the conformation of the other naphthalimide groups but
causes minor changes to the orientations of the phenyl side chains
on arms E and G.
Figure 13. Axial view of one sheet formed by [Cu2(Lphg)4(THF)2]
paddlewheels. Ligands E, F, and G are shown in green; ligand I is shown
in red. The ABCD paddlewheel is colored blue. Naphthalimide groups
are represented by colored hoops.
The ABCD dimer π π stacks with the EFG(H/I) dimer via
3 3 3
the naphthalimide groups of ligands B and D (Figure 12). One
interaction occurs with naphthalimide groups using the inside
faces of the chiral pockets (D E) and one on the outside faces
3 3 3
(B G). As the π π stacking of these dimers does not
3 3 3
3 3 3
involve arm H/I, it is unaffected by the disorder. The values for
Figure 14. Drawing of the [Cu4(Lasn)8(py)(MeOH)] (5) tetramer.
The pyridine-coordinated dimeric subunit is shown in orange and the
methanol-coordinated dimeric subunit in purple. Intramolecular hydro-
gen bonds are shown in green; hydrogen atoms attached to carbon have
been omitted for clarity.
these interactions are comparable to those seen for the other
systems, with the “inside” π π stacking having a vertical
3 3 3
separation of 3.43 Å, a χ value of 1.91 Å, naphthalimide dipole
vector angles oriented at 153°, and a very small interplanar angle
(3°). The “outside” interactions have a similar vertical separation
(3.42 Å) and are less slipped (χ = 1.01 Å). The naphthalimi-
deꢀnaphthalimide dipole angles are oriented at 138°, and the
rings are almost coplanar (1° interplanar angle).
Structure of [Cu4(Lasn)8(py)(MeOH)] (5). The complex
ꢀ
formed from the tetrafunctional Lasn ligand (the additional
In addition to the ABCD EFG(H/I) interaction, there are a
functionality being the amide substituent located on the side
chain) retains the paddlewheel SBU central core, but two non-
equivalent paddlewheel dimers are linked together via reciprocal
axial coordination of one of the amide carbonyl oxygen atoms
present in the ligand side chains of each dimer subunit to form
tetramer [Cu4(Lasn)8(py)(MeOH)]. Figure 14 shows the struc-
ture of this tetramer. One dimeric subunit (shades of purple) has
four carboxylate ligands (labeled E, F, G, and H) bridging the
3 3 3
number of other, weaker interactions involving the phenyl
groups on the ligands (see the Supporting Information). These
interactions, taken in combination with those outlined above,
arrange the dimers into two-dimensional sheets, which have no
significant additional supramolecular interactions. One sheet, as
viewed from the β face of the ABCD paddlewheel, is shown in
Figure 13. Only the EFGI disorder component is pictured.
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Table 1. Hydrogen Bonding Parameters in
[Cu4(Lasn)8(py)(MeOH)]
DꢀH
(Å)
H
A
D
A
<DHA
(deg)
3 3 3
3 3 3
DꢀH
A
(Å)
(Å)
3 3 3
N(2A)ꢀH(2A1) O(5E)
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
2.35
2.27
2.06
2.15
2.05
2.40
2.04
2.08
3.09
3.13
2.93
3.00
2.81
2.88
2.83
2.88
141
165
170
164
144
114
148
149
3 3 3
N(2A)ꢀH(2A2) O(3B)
3 3 3
N(2B)ꢀH(2B1)...O(3F)
N(2F)ꢀH(2F1) O(3B)
3 3 3
Figure 15. Naphthalimide π π stacking interactions of the pyridine-
3 3 3
N(2E)ꢀH(2E1) O(5D)
3 3 3
coordinated subunit of [Cu4(Lasn)8(py)(MeOH)]. The α face of the
central dimeric subunit (viewed from the β face) π-stacks with the
pyridine-coordinated subunits (viewed from the α faces) of adjacent
subunits (shown in orange).
N(2E)ꢀH(2E2) O(5E)
3 3 3
N(2E)ꢀH(2E2) O(1H)
3 3 3
N(2H)ꢀH(2H2) O(3D)
3 3 3
equatorial positions of the paddlewheel with a molecule of
methanol (from the crystallization solvent) coordinated to the
copper ion on the α-face pocket. The chiral pocket formed by
these rings is more open than in the dimers described above, with
a size of 16.5 Å (F H) by 15.7 Å (E G). The other dimeric
3 3 3
3 3 3
subunit (shades of orange) also has four carboxylate ligands
(labeled A, B, C, and D) present in its equatorial sites and has a
pyridine coordinated to the copper ion in the α-face pocket. This
axially coordinated pyridine shows the same pair of CH
interactions as seen in [Cu2(Lala)4(py)(THF)] (see Figure 9).
π
3 3 3
The pocket is also similarly distorted by the pyridine, with the
naphthalimide rings participating in the CH π interaction (A
3 3 3
and C) separated by 12.2 Å, compared to 15.2 Å for the
noninteracting rings (B and D). The copper ions at the β face
of both dimeric subunits are coordinated to an amide carbonyl
oxygen from the other dimeric subunit through ligands A and E.
These interactions are further supported by amide NꢀH
O
3 3 3
3 3 3
(coordinated carboxylate) hydrogen bonds (E D and A E)
3 3 3
from the axially coordinated amide NH2 group. These bridging
interactions bring the interdimer Cu Cu distances down from
3 3 3
over 10 Å in the dimeric structures described above to the shortest
now being 6.56 Å. Additionally, there are a number of intramole-
cular hydrogen bonds between the “arms” of the ligands; the amide
group from arm B hydrogen-bonds to amides from arms F and A,
the amide from arm E to the naphthalimide group on arm H, and
the arm D amide to the arm H amide. Parameters for these
hydrogen-bonding interactions can be seen in Table 1.
Figure 16. Naphthalimide π π stacking interactions of the methanol-
coordinated subunit of [Cu4(Lasn)8(py)(MeOH)], viewed from the α face
of the central subunit and the β faces of the peripheral subunits. All four
3 3 3
rings participate in π π stacking interactions with neighboring subunits,
3 3 3
3 3 3
shown in dark (G H “inner” interactions) and light (E F “outer”
interactions) purple.
3 3 3
These tetramers are organized into a complex supramolecular
structure, mainly by π π stacking interactions of the
between arms E and F, while two more form interactions on the
inside of the pocket between arms G and H. The latter interac-
tions are the stronger of the two. The vertical separations are
shorter (G H = 3.41 Å, E F = 3.71 Å). The χ values are
3 3 3
naphthalimide groups, where each dimeric subunit participates
in different types of π-stacking motifs. These interactions link
the tetramers into a fully three-dimensional structure. The
pyridine-coordinated dimeric subunit of one tetramer π-stacks
with the pyridine-coordinated subunit of two adjacent tetra-
mers via the two naphthalimide groups not participating in the
3 3 3
3 3 3
lower (G H = 1.42 Å, E F = 2.39 Å), and the naphthalimide
3 3 3
3 3 3
rings are closer to coplanar (interplanar angles for G H =
3 3 3
5°; for E F = 16°). The naphthalimideꢀnaphthalimide dipole
3 3 3
coordinated pyridine CꢀH π interactions (B and D). This
angles for the G H interactions are 100°, and 133° for the
3 3 3
3 3 3
π
πstack is analogous to that seen with [Cu2(Lala)4(py)(THF)],
E
F interactions. Figure 16 shows these interactions where the
3 3 3
3 3 3
with both the interacting naphthalimide groups from adjacent
units interacting on the inside of the α-face pocket (Figure 15).
The metrics for the stacking interactions are also similar, with a χ
value of 1.84 Å, a vertical separation of 3.51 Å, and a naphtha-
limideꢀnaphthalimide dipole angle of 122°.
central dimeric subunit is interacting with all four neighboring
similar subunits (shades of purple) via π π stacking interactions.
3 3 3
The structure is further supported by two of the amide arms
participating in intermolecular hydrogen bonds between arm C
on one pyridine coordinated subunit and arms G and F on an
adjacent methanol coordinated subunit, as shown in Figure 17,
which join neighboring tetramers in a one-dimensional chain
that runs perpendicularly to the π-stacking interaction of the
pyridine-coordinated dimeric subunit. One diethyl ether and
two methanol molecules are also held into the framework by
In the case of the methanol-coordinated subunit, all four of the
naphthalimide groups participate in π π stacking interactions
3 3 3
with the MeOH-coordinated subunit of four adjacent tetramers.
There are pairs of symmetrically equivalent interactions: two
adjacent arms forming interactions on the outside of the pockets
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hydrogen bonding; the ether (green) and one methanol (light
orange) are hydrogen-bonded to the remaining free arm amide
hydrogen atoms, while the other methanol (red) is hydrogen-
bonded to the axial-coordinated methanol (yellow) of the
MeOH-coordinated (purple) unit.
In the overall supramolecular structure, each tetramer π-stacks
with six other tetramers and forms hydrogen bonds with two
additional tetramers. Taken together, these noncovalent inter-
actions organize the tetramers into a three-dimensional network,
C(O)OEt) vapor, the coordinated methanol molecule and the
methanol hydrogen-bonded to it as well as the ordered solvent in
the pores are replaced by (S)-ethyl lactate (S-EtLac), yielding
[Cu4(Lasn)8(py)(S-EtLac)] (6) without a loss of crystallinity. The
overall structure is closely similar to that observed for [Cu4-
(Lasn)8(py)(MeOH)]; one tetrameric unit is shown in Figure 20.
The copper ion located at the α face of the purple subunit
now is coordinated to the ester carbonyl oxygen of ethyl lactate
held together by the naphthalimide π π stacking interactions
3 3 3
supported by amideꢀamide hydrogen bonding. Figure 18 shows
a simplified representation of this network, a drawing that shows
that the structure has large pores.
The ether and one of the methanol molecules shown in
Figure 17 are located in the pores formed by the surface of the
tetrameric units. These pores also contain additional solvent—in
addition to the ether and the methanol that hydrogen-bond to
the arms of the dimers, there are three fully ordered dichlor-
omethane molecules and a region containing disordered solvent,
which refined to a mixture of dichloromethane, ether, and
methanol. Figure 19 illustrates the pore, where the disordered
solvent has been removed (the “empty” space). An axial view of a
pore can be seen on the left (this view matches the pore seen in
the center of Figure 18); the molecular surface of the tetramers is
shown and the dichloromethane (yellow), ether (green), and
methanol (red) molecules that occupy it. A cutaway view of the
same pore is shown on the right. The disordered solvent volume
occupies a total of 3973.1 Å3 (23.2% of the unit cell volume), and
the pore measures approximately 8.5 ꢁ 7.5 Å.
Figure 18. Simplified representation of the overall structure in
[Cu4(Lasn)8(py)(MeOH)]. Individual dimeric subunits are represented
by single blue spheres and the dimerꢀdimer covalent interaction by
thick bicolored cylinders (orange = pyridine-coordinated, purple =
MeOH-coordinated). The π π stacking interactions (four from each
3 3 3
Single-Crystal to Single-Crystal Transformations. [Cu4-
(Lasn)8(py)(S-EtLac)] (6). When crystals of [Cu4(Lasn)8(py)-
(MeOH)] are exposed to (S)-ethyl lactate (CH3C(H)(OH)-
purple and two from each orange subunit) are represented by thin
orange and purple dashed cylinders. Intramolecular hydrogen bonds are
shown as thin green dashed cylinders.
Figure 17. Chain of three hydrogen-bonded tetrameric units (top) in [Cu4(Lasn)8(py)(MeOH)]. Hydrogen bonds shown as green and red dots. The
methanol-coordinated dimeric subunit (coordinated methanol as yellow space filling model) is shown in purple and the pyridine-coordinated subunit in
orange. The methanol molecule hydrogen-bonded to the coordinated methanol is colored red and the ether and additional methanol molecules green
and light orange, respectively, are shown as space filling models. Naphthalimide groups are shown as transparent hoops. Inset a: view of the same chain
rotated by 90°. Inset b: expansion of hydrogen bonding region.
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(shown in yellow and red). This axial ligand exchange changes
the dimensions of the chiral pocket, which now measures 16.0 Å
by 15.2 Å, a decrease of 0.5 Å in each dimension from [Cu4-
(Lasn)8(py)(MeOH)]. The chiral pocket of the pyridine-coordi-
nated dimer is not affected by the exchange.
shown on the right. The disordered solvent volume occupies a
total of 3232.5 Å3, or 19% of the total unit cell volume.
[Cu4(Lasn)8(py){(R-EtLac)0.58(MeOH)0.42}] (7). When crystals
of [Cu4(Lasn)8(py)(MeOH)] are exposed to vapors of (R)-ethyl
lactate for the same 48 h period as in the (S)-ethyl lactate
experiment, again a change occurs without a loss of crystallinity,
but in this case, the (R)-ethyl lactate coordinates via the ꢀOH
group, rather than through the carbonyl oxygen. After 48 h of
exposure, the ethyl lactate has only partially displaced the axial
methanol ligand; a 58/42 disordered mixture of (R)-ethyl lactate
and methanol was located in the pocket (as determined by the
refinement fixing the pocket occupancy to unity, this ratio does not
change after seven days of exposure to (R)-ethyl lactate), giving
[Cu4(Lasn)8(py){(R-EtLac)0.58(MeOH)0.42}] (7). As above, the
overall structure of the tetramer is the same, and there is no change
The pyridine-coordinated dimeric subunit (orange) has dis-
order in one arm (arm A\Z) and the pyridine ring. The Z
disorder in arm A occurs at the CH2ꢀCONH2 portion of the
amide—this fragment is disordered equally over two positions.
One of the disordered positions is analogous to the position of
the arm A amide in [Cu4(Lasn)8(py)(MeOH)], and the other
(referred to as amide Z) has the amide NH2 group hydrogen-
bonding with the coordinated carboxylate from arm H and the
naphthalimide carbonyl oxygen of arm B. In both cases, the
amide carbonyl oxygen is still coordinated with the copper ion
located on the β face of the other dimeric subunit. Though the
values of the intramolecular hydrogen bonds have changed, the
overall connectivity of arms AꢀG is the same; these values and
the new values for arm Z are given in Table 2.
The overall supramolecular structure of 6 is the same as that
discussed above for 5; it is dominated by each tetramer π-
stacking with six other tetramers and forming hydrogen bonds
with two additional tetramers. Overall, the π π stacking is the
3 3 3
same, but the metric parameters have changed somewhat,
Table 3. Though some of the changes could be considered
significant (>0.1 Å), none fall outside of the range for a strong
naphthalimide π-stacking interaction.
Two of the amide arms participate in intermolecular hydrogen
bonds that join the tetrameric units into a chain, analogous to
those shown in Figure 17, inset b. The remaining free arm
amide hydrogen atoms form hydrogen bonds with two additional
molecules of (S)-ethyl lactate, one of which in turn hydrogen
bonds to a third (S)-ethyl lactate. These interactions are illu-
strated in Figure 21.
The three ordered noncoordinated (S)-ethyl lactate molecules
shown in Figure 21 are located in the pores formed by the surface
of the tetrameric units. These ethyl lactates occupy the same
region of the pore as the ordered solvents in [Cu4(Lasn)8(py)-
(MeOH)]. Figure 22 illustrates the pore formed by the molecular
surface of the tetramers and the ethyl lactate contained within
(green spacefilling models); the disordered solvent has been
removed by SQUEEZE22 (the “empty” space). An axial view of a
pore is shown on the left, and a cutaway view of the same pore is
Figure 20. Drawing of one of the two disordered forms (disorder is in
the A arm, only the A component is shown) of the tetramer
[Cu4(Lasn)8(py)(S-EtLac)] (6), the form that matches the structural
arrangement of [Cu4(Lasn)8(py)(MeOH)] shown in Figure 14. The
pyridine-coordinated dimeric subunit is shown in orange and the (S)-
ethyl lactate coordinated subunit in purple. Intramolecular hydrogen
bonds are shown as green dots; hydrogen atoms attached to carbon have
been omitted for clarity.
Figure 19. Two views of one pore in 5. The molecular surface of the tetramers is shown and has been color-coded on the basis of the subunit (purple =
axial ligand methanol, orange = axial ligand pyridine). The ordered solvent occupying the pores is also shown (green = ether, yellow = dichloromethane,
red = methanol). Left: view down the axis of the pore; right, view from the side of the pore, with the obscuring units removed for clarity.
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Table 2. Hydrogen Bonding Parameters for
[Cu4(Lasn)8(py)(S-EtLac)]
Table 3. Naphthalimide π π Stacking Parameters in
[Cu4(Lasn)8(py)(S-EtLac)], as compared with
3 3 3
[Cu4(Lasn)8(py)(MeOH)]
DꢀH
(Å)
H
A
D
A
<DHA
(deg)
3 3 3
3 3 3
planes Δ planes
planes Δ planes dipole Δ dipole
χ Δ χ angle angle angle angle
DꢀH
A
(Å)
(Å)
3 3 3
rings distance distance
N(2A)ꢀH(2A1) O(5E)
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
2.37
2.44
2.07
1.91
2.12
2.11
2.01
2.59
2.27
2.13
3.07
3.25
2.94
2.79
2.89
2.88
2.86
3.17
3.01
2.91
137.4
153.3
170.7
176
3 3 3
G
E
B
H
3.38
3.90
3.40
ꢀ0.03 1.70 +0.28
+0.19 2.29ꢀ0.10 19
ꢀ0.11 1.71ꢀ0.13
6
+1
+3
98
134
123
ꢀ2
+1
+1
N(2A)ꢀH(2A2) O(3B)
3 3 3
3 3 3
3 3 3
3 3 3
F
N(2B)ꢀH(2B1) O(3F)
3 3 3
D
7
ꢀ2
N(2F)ꢀH(2F1) O(3B)
3 3 3
N(2E)ꢀH(2E1) O(5D)
146.8
145.7
160.4
124.1
142.1
147.5
3 3 3
N(2E)ꢀH(2E2) O(1H)
3 3 3
N(2H)ꢀH(2H2) O(3D)
3 3 3
N(2A)ꢀH(2A1) O(5F)
3 3 3
N(2Z)ꢀH(2Z1) O(5H)
3 3 3
N(2Z)ꢀH(2Z2) O(1D)
3 3 3
in the chiral pocket of the pyridine-coordinated dimer. Unlike the
(S)-ethyl lactate experiment, no ordered solvent could be located
within the pores. Both chiral pockets are similar to those in
[Cu4(Lasn)8(py)(MeOH)], with the pyridine-coordinated di-
mer measuring 15.3 by 12.5 Å, and the (R)-ethyl lactate-
coordinated dimer measuring 16.5 by 15.3 Å.
The overall supramolecular structure of 7 is the same as
discussed above for 5 and 6; it is dominated by each tetramer
π-stacking with six other tetramers and forming hydrogen bonds
Figure 21. Chain of three hydrogen-bonded tetrameric units in
[Cu4(Lasn)8(py)(S-EtLac)] (6), with the hydrogen bonds shown as
red dots. The ethyl lactate-coordinated dimeric subunit is shown in
purple and the pyridine-coordinated subunit in orange. The metal-
bonded ethyl lactate is shown as a yellow space filling model, and the
three hydrogen-bonded ethyl lactate molecules are shown as green space
filling models. Naphthalimide groups are shown as transparent hoops.
with two additional tetramers. Again, the π π stacking is the
3 3 3
same, but the metric parameters have changed slightly, Table 4.
None of the naphthalimide π-stacking interactions fall outside of
the range to be considered strong.
16.0 Å by 15.5 Å, close to that seen in 6. This form differs from
that obtained by exposing [Cu4(Lasn)8(py)(MeOH)] to pure
(S)-ethyl lactate or racemic ethyl lactate in that one noncoordi-
nated (S)-ethyl lactate molecule was clearly located in the pores
formed by the tetrameric units, whereas three were clearly
located in the formation of 6 (direct exposure of [Cu4(Lasn)8-
(py)(MeOH)] to (S)-ethyl lactate) and none in the formation of
8. We note another difference in the structure of 9 is that the
arrangement the (S)-ethyl lactate molecule located in the pores is
“trans” with respect to the carbonyl and alcohol oxygen atoms,
whereas all three in the pores of 6 are “cis”.
Crystals of [Cu4(Lasn)8(py)(MeOH)] were exposed to
vapors of (R)-ethyl lactate, yielding again [Cu4(Lasn)8(py)-
{(R-EtLac)0.58(MeOH)0.42}] (7), as confirmed by X-ray crys-
tallography. The (R)-ethyl lactate was removed from this
sample tube followed by exposure of these crystals (now
compound 7) to racemic ethyl lactate. X-ray crystallography again
confirmed that a single-crystal to single-crystal transformation
had taken place, yielding the same form of [Cu4(Lasn)8(py)(S-
EtLac)] (9) as observed in the above double exchange experi-
ment. This experiment clearly demonstrates that the (S)-ethyl
lactate in a racemic mixture has displaced the disordered (R)-ethyl
lactate and MeOH in 7 and is now exclusively located in the chiral
pocket.
[Cu4(Lasn)8(py)(S-EtLac)] (8). When crystals of [Cu4(Lasn)8-
(py)(MeOH)] are exposed to vapors of racemic ethyl lactate, the
coordinated methanol molecule is displaced without a loss of
crystallinity exclusively by (S)-ethyl lactate, yielding a new form of
the tetramer [Cu4(Lasn)8(py)(S-EtLac)] (8). The overall con-
nectivity is very similar to that of 6 (one tetramer still interacts
with six other tetramers via π π stacking and two others
3 3 3
through hydrogen bonding). The major difference between the
structure of 8 and the structure of 6, obtained from the vapor
diffusion of pure (S)-ethyl lactate, is that the solvent residing in
the pores in 8 is so disordered as to be essentially featureless. This
disordered solvent occupies the entire volume of the pores,
measuring 5875.3 Å, or a total of 33.9% of the overall unit cell
volume. The values for the naphthalimide π π stacking
3 3 3
interactions in 8 have changed somewhat compared to 6, Table 5,
and the chiral pocket now measures 16.2 Å by 15.3 Å, close to
that seen in 6.
Double Exchange Experiments. Crystals of [Cu4(Lasn)8-
(py)(MeOH)] were exposed to vapors of racemic ethyl lactate
yielding [Cu4(Lasn)8(py)(S-EtLac)] (8), as confirmed by X-ray
crystallography, again with fully disordered solvent residing in
the pores. The racemic ethyl lactate was removed from this
sample tube followed by exposure of these crystals of 8 to (S)-
ethyl lactate. X-ray crystallography again confirmed that a single-
crystal to single-crystal transformation had taken place yielding a
new form of [Cu4(Lasn)8(py)(S-EtLac)] (9). The overall con-
nectivity of 9 is very similar to that of 6 (one tetramer still
We note that a number of other chiral substrates were briefly
screened for analogous exchange properties, but to date none
have been successful. Similar exchange experiments with 2-bu-
tanol, 2-butylamine, and 1-phenylethylamine all cause the crys-
tals of [Cu4(Lasn)8(py)(MeOH)] to dissolve. An exchange
experiment with N,N-dimethyl-1-phenylethylamine caused the
crystals to change color, but the crystals were of poor quality for
X-ray analysis.
interacts with six other tetramers via π π stacking and two
3 3 3
others through hydrogen bonding). The values for the naphtha-
limide π π stacking interactions in 9 have changed some-
3 3 3
what compared to 6, Table 6, and the chiral pocket now measures
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ARTICLE
Figure 22. Two views of one pore in [Cu4(Lasn)8(py)(S-EtLac)] (6). The molecular surface of the tetramer is shown and has been color-coded on the
basis of the subunit (purple = axial ligand ethyl lactate, orange = axial ligand pyridine). The (S)-ethyl lactate coordinated to the copper ion is shown in
yellow, and the three ordered (S)-ethyl lactates occupying the pores are shown in green. Left: view down the axis of the pore; right, view from the side of
the pore, with the obscuring units removed for clarity.
Table 4. Naphthalimide π π Stacking Parameters in
[Cu4(Lasn)8(py)(R-EtLac)] (7), As Compared with Those in 6
Table 5. Naphthalimide π π Stacking Parameters in
[Cu4(Lasn)8(py)(S-EtLac)] (8) Compared to Those in 6
3 3 3
3 3 3
planes Δ planes
planes Δ planes dipole Δ dipole
planes Δ planes
planes Δ planes dipole Δ dipole
rings distance distance
χ
Δχ
angle
angle
angle
angle
rings distance distance
χ
Δχ
angle
angle
angle
angle
G
E
B
H
3.43
3.70
3.43
+0.05 1.63 ꢀ0.07
ꢀ0.20 2.54 +0.25
+0.03 1.49 ꢀ0.22
11
19
1
+5
+0
101
138
123
+3
+4
+0
G
E
B
H
3.44
3.70
3.46
+0.06
ꢀ0.20 2.43 +0.14
+0.06
1.48 ꢀ0.23
1.61 ꢀ0.09
6
20
2
+0
+1
100
138
121
+2
+4
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
3 3 3
F
F
D
ꢀ6
D
ꢀ5
ꢀ2
’ DISCUSSION
side chain, the substituent on the other face of the dimers, is not
bulky, including in three cases (complexes 1ꢀ3) where this
group is methyl. The partial exception to this observation is the
arrangement in the structure of the EFG(H/I) dimer from
[Cu2(Lphg)4(THF)2], where one naphthalimide group is
oriented toward the β face (Figure 1c) 25% of the time. Given
that the naphthalimide group appears to favor the chiral crown
configuration and that in asymmetric synthesis the chiral pocket
is certainly the key to selectivity, the use of the naphthalimide
group in enantiopure ligands is particularly attractive for the
development of catalysts, a result already observed in some,23b,e
but not all,23d catalytic studies. A very recent publication has
highlighted the use of the naphthalimide group in important
enantioselective reactions catalyzed by rhodium dimers.16i
The shape of the chiral pocket is also of interest, with the
asymmetry noticed in previous studies being indicated as im-
portant to the catalytic selectivity.16g In the structures reported
here, the size and symmetry of the chiral pocket varies greatly. In
While a main intent of the research described here was to
determine the properties of copper(II) carboxylate dimers built
from enantiopure carboxylate ligands containing the strong
π
π stacking 1,8-naphthalimide synthon, the structures
3 3 3
reported here clearly also relate to research in asymmetric rhodium
dimer catalysis. The use of rhodium dimers formed from
enantiopure phthalimide and naphthalimide-substituted carbox-
ylate ligands as a catalyst in enantioselective cyclopropanation16
and CꢀH insertion reactions15 has been extensive, but only one
catalyst had been structurally characterized until a recent series of
publications was published. Two similar structures (crystallized from
different solvents) have been carried out where the substituents on
the chiral carbon are phthalimide and the relatively small benzyl
group; both have the adjacent two up, two down structure shown
in Figure 1a.16a,18a In all other published structures, the side chain
is bulky, either tert-butyl or iso-propyl, and generally phthalimide
is the other group at the chiral carbon. All of these structures that
have the smaller phthalimide substituent (one of which is a
dicopper compound18a) have the chiral crown configuration
(twisted in one case18a), Figure 1d, with all of the groups on the
same side of the metal dimer.16a,17b,18a In addition, two dirhodium
compounds where the ligand contains naphthalimide and tert-
butyl substituents have this chiral crown structure.18
[Cu2(Lala)4(THF)2] and in the two forms of [Cu2(Lphg 4-
)
(THF)2], where a relatively small THF group is bonded to the
copper ions, the pocket is nearly symmetric and relatively small,
averaging 13.6 Å across the pocket with a difference in the two
values for each complex not greater than 0.3 Å. The pocket is
larger and more asymmetric for [Cu2(Lala)4(HLala)] and
the pockets in the tetramers that do not coordinate pyridine as
the axial ligand. The size of these pockets is likely influenced by
the axial ligand, which varies from HLala, methanol, and ethyl
lactate. The importance of the axial ligand in the chiral pocket is
more clearly indicated by the six cases where it is pyridine. In
Essentially, in all of the dicopper complexes reported here, the
chiral crown configuration is observed, despite that fact that the
side chain on the chiral carbon is small. In contrast to the two
rhodium structures with phthalimide and the small benzyl group
substituents at the chiral carbon, in the results reported here with
the bulky naphthalimide group bonded to the chiral carbon, the
chiral crown configuration is observed, even in cases where the
these cases, the pocket is very asymmetric due to the CꢀH
π
3 3 3
interaction from the pyridine 2- and 6-position hydrogen atoms
that draw in the naphthalimide groups involved in the interaction
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Inorganic Chemistry
ARTICLE
Table 6. Naphthalimide π π Stacking Parameters in
[Cu4(Lasn)8(py)(S-EtLac)] (9) Compared to Those in 6
Given this open structure combined with the “soft” π
π
3 3 3
3 3 3
stacking and hydrogen bonding interactions organizing the
supramolecular structure,26 it is not surprising that we have been
able to observe a gas/solid-mediated single-crystal to single-
crystal transformation when 5 is exposed to (S)-ethyl lactate
vapor. In this transformation, the coordinated methanol mole-
cule, the methanol molecule hydrogen-bonded to it, and the
ordered (and likely disordered) solvent in the pores are replaced
by (S)-ethyl lactate (S-EtLac), yielding [Cu4(Lasn)8(py)(S-
EtLac)] (6) without a loss of crystallinity. There is no change in
the chiral pocket containing the pyridine ligand. The low vapor
pressure of ethyl lactate (2 mmHg at 20 °C) combined with a
clear lack of change in the appearance of the crystals shows that
this reaction is a single-crystal to single-crystal transformation taking
place by gas phase exchange. These types of exchanges are rare.27
In fact, the whole field of single-crystal to single-crystal transfor-
mations is relatively new.27aꢀc,28
planes Δ planes
planes Δ planes dipole Δ dipole
rings distance distance
χ
Δχ
angle
angle
angle
angle
G
E
B
H
3.45
3.62
3.46
+0.07 1.57 ꢀ0.13
9
18
1
+3
+1
+6
104
140
123
+6
+6
+0
3 3 3
3 3 3
3 3 3
F
ꢀ0.28
2.78 +0.49
D
+0.06
1.35 ꢀ0.36
to an average distance of 12.0 Å, while the other two rings are
separated by an average of 15.5 Å. This impact of the axial ligand
may be a model for substrate interactions in the catalytic
processes, as has been suggested by others.16ꢀ18,23b
At least in the solid state, the arrangement of the π
π
3 3 3
stacking of the 1,8-naphthalimide groups, especially whether the
interaction is on the inside or outside of the pocket, also impacts
the size of the chiral pocket. Of course, it could be the other way
around where the size of the pocket (as impacted by the axial
Importantly, this chiral pocket in the tetramer is selective for
coordination of (S)-ethyl lactate. While (R)-ethyl lactate vapor
does cause a single-crystal to single-crystal transformation with 5,
it coordinates to the metal via the hydroxyl group rather than the
ester carbonyl and does not displace all of the axially coordinated
methanol ligands yielding [Cu4(Lasn)8(py){(R-EtLac)0.58(Me-
OH)0.42}] (7). The selectivity for (S)-ethyl lactate is established
by the exclusive formation of [Cu4(Lasn)8(py)(S-EtLac)] (8) in
the single-crystal to single-crystal transformation of [Cu4(Lasn)8-
(py)(MeOH)] with racemic ethyl lactate vapor. Confirmation of
this selectivity is demonstrated by the double single-crystal to
single-crystal experiment where 5 is converted to 7 in the
presence of (R)-ethyl lactate followed by the exposure of 7 to
racemic ethyl lactate vapor that results in the formation of
[Cu4(Lasn)8(py)(S-EtLac)] (9). Although the tetrameric units
in the structures of 8 and 9 have the same composition and very
similar unit cell parameters as with 6 and each other, the overall
structures are different in the ordering of the ethyl lactate in the
pores. We believe this selective exchange with racemic ethyl
lactate vapor to be the first example of a single-crystal to single-
crystal gas/solid transformation resulting in enantioselective bond-
ing to a metal center.
ligand) favors one type of π π stacking or the other. For
3 3 3
example, in the asymmetric chiral pockets containing the pyr-
idine ligands, the two well-separated naphthalimide groups not
involved in the CꢀH π interaction show π π stacking on
3 3 3
3 3 3
the inside of the pocket. Analysis of the data reported here does
not support this idea that the size of the pocket strongly favors
one type of π π stacking where, for example, in [Cu2(Lala)4-
3 3 3
(THF)2], one of the rings does an “inner” and one an “outer”
π
π stacking interaction with its neighbors on adjacent
3 3 3
naphthalimide groups, but the across-ring distances are short
and essentially the same for both cases.
The chirality built into these complexes is manifested in these
chiral pockets rather than the overall supramolecular structure.
The various noncovalent interactions, mainly π π stacking,
3 3 3
that organize these solids into supramolecular structures do not
result in any helical type arrangements, an arrangement we have
observed with analogous enantiopure ligands that have longer
connections between the 1,8-naphthalimide synthon and the
amino acid.25 In fact, many different types of supramolecular
arrangements are observed in the work reported here. [Cu2-
(Lala)4(THF)2] forms trimers that are arranged in the solid state
by simple AB close packing. As can be seen in Figure 4, there are
large pores in the structure, but the chiral pockets are not
pointing toward these pores. [Cu2(Lala)4(HLala)] has a complex
three-dimensional structure with large pores filled by solvent
molecules, but both the structural arrangement and the large axial
ligand isolate the chiral pocket from the pores. In contrast,
[Cu2(Lala)4(py)(THF)] has a simple one-dimensional structure
built by only two naphthalimide groups on each dimer interact-
ing on the inside of the chiral face, and [Cu2(Lphg)4(THF)2] has
a two-dimensional sheet structure.
The preference for the occupation of the chiral pocket by (S)-
ethyl lactate can be seen in Figures 20 and 21 and in the space
filling drawing shown in Figure 23. In Figure 23, the napthala-
mide groups that form the chiral pocket are shown in purple, and
the two naphthalimide groups from two other tetramers that
π
π stack on the inside of the pocket (see Figure 16) are
3 3 3
shown in two shades of blue. The hydrogen atom bonded to the
chiral center of the ethyl lactate molecule(not visible inFigure 23,
but clearly indicated in Figures 20 and 21) is pointed directly
between the two naphthalimide groups not involved in the
The major structural change in these complexes is caused by
the introduction of additional functionality, an amide group, to
the side chain of the Lasn ligand, forming complexes where
interior π π stacking interactions. In this arrangement, the
3 3 3
substrate clearly fits well in the pocket. Although an analogous
drawing using crystallographic data cannot be generated for the R
isomer (it bonds differently), reversal of the stereochemistry at
the chiral center, leaving the hydrogen atom in the same sterically
hindered position, would force the methyl group at the chiral
center that is now located on the outside of the pocket to be
located on the inside of the pocket (where the OH group is
located), causing steric interference. Importantly, for these solid-
state structures, the supramolecular arrangement is very impor-
tant to any discussion relating to the chiral pocket—removal of
ꢀ
reciprocal axial coordination of one of the amide carbonyl oxygen
atoms between two dimers leads to the tetramer [Cu4(Lasn)8-
(py)(MeOH)] (5). X-ray-quality crystals were only obtained in
the presence of pyridine, which acts as the axial ligand for one of
the two chiral pockets of the tetramers, causing substantial
asymmetry in that chiral pocket. Tetramer 5 has extensive
supramolecular interactions, mainly caused by the 1,8-naphtha-
limide supramolecular synthon, which support an open structure
containing large pores, where in this case the chiral pockets line
the pores.
the naphthalimide groups that π π stack on the inside of the
3 3 3
pocket change the pocket considerably. How this issue would
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Inorganic Chemistry
ARTICLE
by π π interactions of the naphthalimide groups, interactions
3 3 3
that impact the size and shape of the chiral pocket in the solid
state. Introduction of additional functionality, an amide group,
ꢀ
to the side chain in the Lasn ligand leads to the formation of
tetrameric [Cu4(Lasn)8(py)(MeOH)], where reciprocal axial
coordination of amide carbonyl oxygen atoms link the paddle-
wheel dimers. This complex has extensive supramolecular orga-
nization that forms an open structure. The structure contains one
chiral pocket occupied by a pyridine ligand and another by a
methanol molecule derived from the crystallization solvent. This
methanol molecule exchanges enantioselectively with the (S)-
ethyl lactate component of a racemic mixture of ethyl lactate
vapor, yielding [Cu4(Lasn)8(py)(S-EtLac)] without a loss of
crystallinity, apparently the first example of a single-crystal to
single-crystal gas/solid transformation resulting in enantioselec-
tive bonding to a metal center.
’ ASSOCIATED CONTENT
Figure 23. Space filling model of the chiral pocket of 6 showing the
orientation of the (S)-ethyl lactate molecule. The naphthalimide groups
S
Supporting Information. X-ray crystallographic files in
b
CIF format for the structural determinations; structural refine-
ment descriptions; tables of crystallographic information, bond
distances, and bond angles; number drawings for complexes
2ꢀ5; and a supramolecular structural description for compound
4. This information is available free of charge via the Internet at
http://pubs.acs.org.
colored in blue are from two other tetramers that π π stack on the
3 3 3
inside of the pocket of the purple-colored tetramer.
affect asymmetric homogeneous catalysis by these types of com-
plexes is unclear, although we have shown in different systems
that the π π stacking of the naphthalimide groups can be
3 3 3
retained in solution.9a,c
’ AUTHOR INFORMATION
There is no change in space group for the single-crystal
transformations reported here; the chiral pocket is large enough
to bond the (S)-ethyl lactate without a large structural change. As
can be seen in Figures 19 and 22, individual solvent or (S)-ethyl
lactate molecules are smaller than the pores; the channels in the
structures are clearly large enough to allow the exchange process
of this relatively large molecule. An additional important issue is
the “flexibility” imparted to these solid state structures by the
Corresponding Author
*E-mail: reger@chem.sc.edu.
’ ACKNOWLEDGMENT
The authors acknowledge with thanks the financial support of
the U.S. National Science Foundation through grant CHE-
1011736.
π
π stacking interactions of the 1,8-naphthalimide supramo-
3 3 3
lecular synthon. As can be seen in Tables 3ꢀ6, these interactions
can change their orientations somewhat without apparently
changing the strength of the interaction to any appreciable
degree, allowing the structures to change slightly in these gas/
solid phase exchange reactions without destroying the crystal-
linity. To support this idea, we have previously observed inter-
esting single-crystal to single-crystal transformations in very
different types of metal complexes whose structures are also
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