Influencing the size and anion selectivity of dimeric Ln 3+[15-Metallacrown-5] compartments through systematic variation of the host side chains and central metal

Article abstract of DOI:10.1021/ic202347j

Dimeric Ln³⁺[15-metallacrown-5] compartments selectively recognize carboxylates through guest binding to host metal ions and intermolecular interactions with the phenyl side chains. A systematic study is presented on how the size, selectivity, and number of encapsulated guests in the dimeric containers is influenced by the Ln³⁺[15-metallacrown _Cu(II)-5] ligand side chain and central metal. Compartments of varying heights were assembled from metallacrowns with S-phenylglycine hydroxamic acid (pgHA), S-phenylalanine hydroxamic acid (pheHA), and S-homophenylalanine hydroxamic acid (hpheHA) ligands. Guests that were examined include the fully deprotonated forms of terephthalic acid, isonicotinic acid, and bithiophene dicarboxylic acid (btDC). X-ray crystallography reveals that the side-chain length constrains the maximum and minimum length guest that can be encapsulated in the compartment. Compartments with heights ranging from 9.7 to 15.2 A are formed with different phenyl side chains that complex 4.3-9.2 A long guests. Up to five guests are accommodated in Ln ³⁺[15-metallacrown_Cu(II)-5] compartments depending on steric effects from the host side chains. The nine-coordinate La³⁺ central metal promotes the encapsulation of multiple guests, while the eight-coordinate Gd³⁺ typically binds only one dicarboxylate. Electrospray ionization mass spectrometry reveals that the dimerization phenomenon occurs beyond the solid state, suggesting that these containers can be utilized in solid-state and solution applications.

Full text of DOI:10.1021/ic202347j

Article
pubs.acs.org/IC
3+
Influencing the Size and Anion Selectivity of Dimeric Ln [15-
Metallacrown-5] Compartments through Systematic Variation of the
Host Side Chains and Central Metal
Joseph Jankolovits, Choong-Sun Lim, Gellert Mezei, Jeff W. Kampf, and Vincent L. Pecoraro*
Department of Chemistry, University of Michigan, Ann Arbor, 930 North University Avenue, Ann Arbor, Michigan 48109,
United States
*
S Supporting Information
3
+
ABSTRACT: Dimeric Ln [15-metallacrown-5] compartments selec-
tively recognize carboxylates through guest binding to host metal ions
and intermolecular interactions with the phenyl side chains. A systematic
study is presented on how the size, selectivity, and number of
encapsulated guests in the dimeric containers is influenced by the
3
+
Ln [15-metallacrownCu(II)-5] ligand side chain and central metal.
Compartments of varying heights were assembled from metallacrowns
with S-phenylglycine hydroxamic acid (pgHA), S-phenylalanine hydroxa-
mic acid (pheHA), and S-homophenylalanine hydroxamic acid (hpheHA)
ligands. Guests that were examined include the fully deprotonated forms
of terephthalic acid, isonicotinic acid, and bithiophene dicarboxylic acid (btDC). X-ray crystallography reveals that the side-chain
length constrains the maximum and minimum length guest that can be encapsulated in the compartment. Compartments with
heights ranging from 9.7 to 15.2 Å are formed with different phenyl side chains that complex 4.3−9.2 Å long guests. Up to five
3
+
guests are accommodated in Ln [15-metallacrown
-5] compartments depending on steric effects from the host side chains.
Cu(II)
3
+
3+
The nine-coordinate La central metal promotes the encapsulation of multiple guests, while the eight-coordinate Gd typically
binds only one dicarboxylate. Electrospray ionization mass spectrometry reveals that the dimerization phenomenon occurs
beyond the solid state, suggesting that these containers can be utilized in solid-state and solution applications.
INTRODUCTION
Encapsulation of guests in molecular containers has implica-
tions for chemical transformations, sensing, stimuli-responsive
MCs are a unique class of self-assembled macrocycles with a
28,29
■
metal-rich topology that resembles crown ethers.
These
30,31
molecules formed some of the first molecular triangles,
3
2−34
35−37
1
−10
squares,
recognize anions through their open metal sites,
suggesting that novel molecular recognition chemistry could
and pentagons.
Many MCs selectively
devices, separations, and the design of molecular materials.
3
8−40
The expansive body of research on anion encapsulation focuses
largely on organic hosts that recognize anions through
noncovalent interactions such as hydrogen bonds and van der
Waals forces. Molecular containers with open metal sites have
drawn interest for the promise of novel guest selectivity,
catalytic activity, and addressable properties that can arise from
3
+
arise from embedding the MC in hydrophobic cavity. Ln [15-
41
MCCu(II)-5] complexes (Scheme 1a) have been developed
into chiral metallocavitands through synthesis with chiral α-
amino hydroxamic acid ligands bearing hydrophobic side
chains, such as S-phenylalanine hydroxamic acid (S-pheHA,
Scheme 1c). These metallocavitands display remarkable
solution stability and bind anions to the Ln central metal or
11
metal ions and concave cavities acting in concert. Metalated
container molecules display strong anion binding in aqueous
solution due to strong electrostatic interactions between anions
42
3+
2+
43−45
3+
12−14
Cu ring metals.
The Ln [15-MCCu(II)-5] hydrophobic
and the metal ions.
Furthermore, metal ions can impart
cavity selectively recognizes carboxylate guests through hydro-
spectroscopic, electrochemical, catalytic, magnetic, and struc-
tural properties to the host−guest complex not observed in
traditional organic assemblies.
4
6−48
3+
phobic interactions with the phenyl side chains.
Gd [15-
3+
15−20
MCCu(II), S‑pheHA-5] containers display enantioselectivity for
Rebek and co-workers
particular α-amino acid analogs and show a strong thermody-
prepared a Zn(II) metallocavitand that strongly binds
49
2
1
22
namic preference for α-hydroxy-substituted carboxylates. One
phosphocholine esters and catalyzes hydrolysis and
3+
2
3
promising application of Ln [15-MCCu(II)-5] cavitands is using
guest binding to prepare soft, chiral solids with functional
acetylation reactions. Another benchmark example is
Kersting’s binuclear thiophenolate macrocyclic cavitand,
which displays addressable magnetic and electrochemical
3
+
topologies or addressable properties. In this regard, Ln [15-
50
2
4
MCCu(II)-5] complexes have been realized as resolved helices,
properties. Bound carboxylates undergo selective bromina-
2
5
26
tion and Diels−Alder reactions within the metallocavitand.
Furthermore, the host forms dimeric capsules with bridging
Received: October 30, 2011
Published: February 6, 2012
2
7
dicarboxylates.
©
2012 American Chemical Society
4527
dx.doi.org/10.1021/ic202347j | Inorg. Chem. 2012, 51, 4527−4538

Inorganic Chemistry
Article
52
3
+
3+
Scheme 1. ChemDraw Diagrams of (a) Ln [15-
MCCu(II), S‑α‑aminoHA-5], (b) S-pgHA, (c) S-pheHA,
Gd [15-MC
-5](isonicotinate) complex. The
Cu(II), pheHA
3+
longest is the 12.1 Å Gd [15-MCCu(II), pheHA-5](muconate)
5
6
(
(
d) S-hpheHA, (e) Isonicotinate, (f) Terephthalate, and
g) btDC
compartment. Despite the differences in height, both
compartments display π-stacking interactions between phenyl
side chains that contribute to the integrity of the dimeric
a
3
+
container. Notably, the maximum height of the Ln [15-
MCCu(II), pheHA-5] compartment is limited. Naphthalene
dicarboxylate and bithiophene dicarboxylate (btDC) (guest
lengths = 8.00 and 9.24 Å, respectively) disrupt compartment
formation by preventing formation of associative π-stacking
5
6,57
interactions between the two MCs.
guests in Ln [15-MC
Inclusion of multiple
3+
-5] hosts has been observed in both
Cu(II)
46
52,55
monomeric MC complexes and dimeric compartments.
3+
Monomeric La [15-MCCu(II)-5] complexes often bind two
carboxylates in the hydrophobic cavity, which is afforded by the
3
+
additional coordination site on the 9-coordinate La central
58
3+
metal. Gd [15-MC
-5] hosts typically bind only one
Cu(II)
3
+
carboxylate guest that coordinates bidentate to the Gd ion.
3
+
Given these observations in Ln [15-MC
pheHA-5]
Cu(II),
compartments we sought to establish a theoretical framework
for considering how the side-chain length influences the
compartment size and number of encapsulated guests in
3+
dimeric Ln [15-MC-5] containers. We hypothesized that the
constraints on compartment height originate from the length of
3
+
the phenyl side chain. Considering that the Ln [15-
MCCu(II), pheHA-5] compartment encapsulates terephthalate, we
suspected that an MC with a shorter side chain would be
incapable of forming a compartment with the 5.8 Å long guest.
Following similar reasoning, we expected that a MC with a
longer side chain would effectively encapsulate long guests that
3+
disrupt the Ln [15-MCCu(II), pheHA-5] compartment. Lastly, we
predicted that the side chain would limit the extent that a
compartment can compress to encapsulate a short guest.
Concerning inclusion of multiple guests in the compartments,
we suspected that the nine-coordinate La3+ central metal would
promote the inclusion of two guests based on the precedent in
monomeric Ln [15-MCCu(II), pheHA-5] complexes. Furthermore,
we hypothesized that steric effects from the phenyl side chain
would influence the number of encapsulated guests.
a
The length of e corresponds to the pyridyl nitrogen to carboxylate
carbon distance, while f and g are the carboxylate carbon to
carboxylate carbon distances, as determined from crystal structures.
51
mesostructured assemblies, dipolar solids displaying second-
52
53
harmonic generation, and dimeric molecular containers. In
3+
54
addition, the Dy3+ analogue is a single-molecule magnet. The
3
+
supramolecular and chemical versatility of Ln [15-MCCu(II)-5]
complexes in solution and the solid state has prompted
thorough studies of their guest recognition behavior that would
aid in the application of these metallocavitands for a variety of
applications, including catalysis, stimuli-responsive devices, and
solid-state materials.
To address these issues of compartment size and selectivity, a
3+
series of Ln [15-MCCu(II)-5] complexes has been synthesized
from ligands bearing phenyl side chains that differ in the
number of methylene carbons: S-phenylglycineHA (pgHA),
S-pheHA, and S-homopheHA (hpheHA) (Scheme 1b, 1c, and
Dimeric compartments have been a consistent feature of
1
d, respectively). The size and selectivity of this series of
3
+
Ln [15-MCCu(II)-5] complexes in the solid state. These
dimeric compartments assemble through supramolecular
interactions between the Ln [15-MCCu(II)-5] side chains.
dimeric containers were assessed by examining the crystal
structures of their inclusion complexes with a set of bridging
guests that serve as molecular rulers: isonicotinate, tereph-
thalate, and btDC (Scheme 1e, 1f, and 1g, respectively).
3
+
With S-tyrosine hydroxamic acid (tyrHA) ligands, the
3+
Ln [15-MCCu(II), tyrHA-5] complexes dimerize through coordi-
nation of the phenol to a copper ring metal, resulting in a 7.6 Å
tall cylindrical compartment that selectively encapsulates nitrate
anions. S-pheHA side chains associate through hydrophobic
interactions, generating a larger dimeric compartment that
encapsulates unsaturated dicarboxylate guests such as tereph-
thalate (Figure 1f). These compartments show guest selectivity
in the solid state, as all saturated dicarboxylates with a carbon
chain length up to six have been excluded from the host’s
hydrophobic face.
Previous characterization of Ln [15-MC
compartments reveals that the height of the compartment is
guest dependent. The shortest compartment was the 10.3 Å tall
3+
3+
Ln [15-MC
-5] hosts with the 9-coordinate La and 8-
Cu(II)
3
+
coordinate Gd central metals have been employed to consider
how the central metal coordination number influences the
selectivity of the compartments. This work builds off of
previous studies to establish a theoretical framework for
predicting the structure of compartment inclusion complexes.
The results presented herein reveal that the phenyl side chain
dictates the maximum and minimum guest length that can be
encapsulated in the compartment. The work also shows that
steric effects from the side chain and the central metal
coordination number can lead to the inclusion of up to five
carboxylate guests in the compartments.
5
5
4
2,56
3
+
pheHA-5]
Cu(II),
4
528
dx.doi.org/10.1021/ic202347j | Inorg. Chem. 2012, 51, 4527−4538

Inorganic Chemistry
Article
3
+
Metallacrown Syntheses. La [15-MC
-5](NO ) and
EXPERIMENTAL SECTION
Cu(II), S‑pheHA
3
■
3+
Gd [15-MC
-5](NO ) were prepared as previously
Cu(II), S‑pheHA
3
All reagents were obtained from commercial sources and used as
received unless specified. CHN analysis was performed by Atlantic
Microlabs Inc. Bithiophene dicarboxylic acid was synthesized as
58
described.
3+
La [15-MC
-5](NO ). S-phenylglycine hydroxamic acid
5 mmol, 0.83 g), copper acetate monohydrate (5 mmol, 1.00 g), and
Cu(II), S‑pgHA
3
(
59
previously described. Sodium salts of isonicotinate and bithiophene
dicarboxylic acid were prepared by neutralizing the respective
carboxylic acid with sodium hydroxide in a water/methanol solution
and precipitating with ether. The isolated solid was rinsed with ether
and air dried. ESI-MS was performed with a Micromass LCT time-of-
flight electrospray mass spectrometer at cone voltages ranging from 10
to 75 V on samples containing ∼1 mg of compound dissolved in 1 mL
of 4:1 methanol/water (v/v). Samples were injected via syringe pump.
ESI-MS data was processed with MassLynx 4.0 software. Crystal
structure images were prepared with Mercury 2.2.
lanthanum nitrate hexahydrate (1 mmol, 0.43 g) were stirred overnight
in 60 mL of methanol. The solution was filtered through a 2.5 cm
shortpad of basic alumina (Brockman 1, Sigma-Aldrich) and rinsed
through with 10 mL of methanol. The filtrate was dried under vacuum,
2+
yielding a dark purple solid. Yield: 1.293 g, 89%. ESI-MS: m/z 668.8
2+
3+
−
2+
+
(
(
668.9 calcd for [La [15-MC
1398.8 calcd for [La [15-MC
-5](NO )] ), 1399.8
Cu(II), pgHA 3
+
3+
−
+
-5](NO ) ] ). CHN
3 2
Cu(II), pgHA
found (calcd) for LaCu C H N O (NO ) (OH) (H O) : C,
5
40 40 10 10
3
1.5
1.5
2
3.5
3
2.93 (32.96); H, 3.35 (3.24); N, 11.04 (11.04).
3+
Gd [15-MC
-5](NO ). This compound was synthesized via
Cu(II), S‑pgHA
3+
3
Ligand Syntheses. Hydroxamic acid ligands were synthesized
the procedure for La [15-MC
-5](NO ), substituting
3
Cu(II), pgHA
from the hydrochloric acid salts of their corresponding methyl esters
gadolinium nitrate hexahydrate for lanthanum nitrate hexahydrate.
6
0
following a modification of previously reported general procedures.
2+
2+
Yield: 0.8213 g, 89%. ESI-MS: m/z 676.9 (678.4 calcd for
The procedure is outlined below for S-phenylglycine hydroxamic acid.
The esters were obtained by refluxing a solution of the carboxylic acid
and 1.5 equiv of thionyl chloride in dry methanol for 3 h followed by
removal of the solvent under vacuum.
S-Phenylglycine Hydroxamic Acid. Potassium hydroxide (0.045
mol, 2.88 g) was dissolved in 75 mL of degassed methanol and added
to (S)-(+)-2-phenylglycine methyl ester hydrochloride (0.045 mol,
3+
-5](NO )] ), 1418.8 (1417.8+ calcd for
−
2+
+
[
Gd [15-MC
Cu(II), pgHA
3
3+
−
+
[
Gd [15-MC
-5](NO ) ] ). CHN found (calcd) for
Cu(II), pgHA
3 2
GdCu C H N O (NO ) (OH) (H O) : C, 33.43 (33.54); H,
5
40 40 10 10
3
0.5
2.5
2
3.5
3
.32 (3.48); N, 10.27 (10.30).
3+
La [15-MC
-5](NO ). This compound was synthesized
Cu(II), S‑hpheHA
3+
3
via the procedure for La [15-MC
-5](NO ), substituting S-
3
Cu(II), pgHA
homopheHA for S-phenylglycineHA. Yield: 1.3899 g, 87%. ESI-MS:
9.0 g). White potassium chloride quickly precipitated and was filtered
2+
2+
3+
−
2+
m/z 738.9 (740.0 calcd for [La [15-MC
1
-5](NO )] ),
Cu(II), hpheHA
-5](NO ) ] ).
Cu(II), hpheHA 3 2
3
after 10 min of cooling in an ice bath. Separately, hydroxylamine
hydrochloride (0.134 mol, 9.30 g) was dissolved in 150 mL of
degassed methanol, and another sample of potassium hydroxide
+
+
3+
−
+
540.0 (1541.0 calcd for [La [15-MC
CHN found (calcd) for LaCu C H6 0 N1 0 O1 0 (NO3 )1 . 5
5
5 0
(
OH) (H O) : C, 37.49 (37.55); H, 4.11 (4.32); N, 10.11 (10.07).
1.5 2 3.5
3+
(
0.134 mol, 8.6 g) was dissolved in 125 mL of degassed methanol. The
Gd [15-MC
-5](NO ). This compound was synthesized
3
Cu(II), S‑hpheHA
3+
two solutions were combined and cooled in an ice bath under a stream
of nitrogen to generate hydroxylamine and precipitate potassium
chloride. The precipitate was filtered off, and the two filtrates were
combined and stirred in a nitrogen atmosphere at room temperature.
After 1 day, the hydroxamic acid precipitated as a white powder and
was filtered. The filtrate was subjected to another portion of
hydroxylamine and stirred overnight under nitrogen, and the
hydroxamic acid precipitate was filtered. This filtrate was reduced to
a volume of 40 mL under vacuum and allowed to sit for 4 days.
Additional hydroxamic acid was isolated by filtration. The filtered
solids were combined, stirred in 100 mL of water for 2 h as a fine
suspension, and filtered. The solid was then stirred in 100 mL of
dichloromethane for 1 h as a fine suspension and filtered. The solid
via the procedure for La [15-MC
-5](NO ), substituting
Cu(II), hpheHA
3
gadolinium nitrate hexahydrate for lanthanum nitrate hexahydrate.
2
+
2+
Yield: 1.4726 g, 92%. ESI-MS: m/z 746.9 (748.5 calcd for
3+
-5](NO )] ), 1558.1 (1558.0+ calcd for
-5](NO ) ] ). CHN found (calcd) for
3 2
−
2+
+
[
[
Gd [15-MC
Gd [15-MC
Cu(II), hpheHA
3
3
+
−
+
Cu(II), hpheHA
GdCu C H N O (NO ) (OH) (H O) : C, 37.44 (37.44); H,
5
50 60 10 10
3
2
2
4
4.07 (4.44); N, 9.63 (9.66).
3
+
Synthesis of Host−Guest Complexes. Gd [15-MC
Cu(II),
S‑pheHA-5](iso-
4
2
3+
-5](terephthalate) and Gd [15-MC
S‑pheHA
Cu(II),
5
2
nicotinate) were synthesized as previously described.
3+
3+
La [15-MC
-5](isonicotinate). La [15-MC
pgHA-5]
Cu(II),
Cu(II), S‑pgHA
(
1
NO ) (0.021 mmol, 30 mg) and sodium isonicotinate (0.069 mmol,
0 mg) were dissolved in 5 mL of a 2:1 methanol/water mixture and
3
1
was washed with ether and vacuum dried. Yield: 5.86 g (71%). H
filtered. Slow evaporation of the solvent yielded crystals within 1 month.
2
+
2+
NMR [(CD ) SO, 400 MHz]: δ 8.8 (br s, 1H, NH) 7.35 (d, J = 7 Hz,
3
2
Yield: 9.1 mg, 24%. ESI-MS: m/z 698.8 (698.9 calcd for
3+
−
2+
3+
3+
2
4
H, o-Ph), 7.31 (t, J = 7 Hz, 2H, m-Ph), 7.22(t, J = 7 Hz, 1H, p-Ph),
{
La [15-MC
-5](isonicotinate )} ), 973.2 (973.3 calcd for
-5]) (isonicotinate ) } ), 1521.9 (1520.9 calcd
Cu(II), pgHA
−1
3+
−
3+
+
+
.22 (s, 1H, C H). MP = 172 °C. IR (KBr, cm ): 3032 br (NH, NH2,
α
{(La [15-MC
Cu(II), pgHA 2 3
3+
−
+
OH), 2878 br (CH), 2646 s (CH), 1612 s (CO), 1542 m, 1467 m,
for {La [15-MC
-5](isonicotinate ) } ). CHN found (calcd for
2
Cu(II), pgHA
1362 m, 1312 w, 1276 w, 1199 w, 1017 w, 922 m, 819w, 776 w, 716 w,
LaCu C H N O (C H NO ) (OH) (H O) ): C, 39.32 (39.50);
5
40 40 10 10
6
4
2 2.5
0.5
2
4.5
6
93 m, 609 w. CHN found (calcd) for C H N O : C, 58.11 (57.82);
H, 3.38 (3.59); N, 10.33 (10.47).
8
10
2
2
3+
3+
H, 6.17 (6.07); N, 16.75(16.86).
S-Phenylalanine Hydroxamic Acid. Reaction was run on a 0.08
La [15-MC
-5](isonicotinate). La [15-MC
pheHA-
Cu(II),
Cu(II), S‑pheHA
5](NO ) (0.018 mmol, 30 mg) and sodium isonicotinate (0.069
3
mol S-phenylalanine methyl ester hydrochloride scale. Yield: 12.19 g
mmol, 10 mg) were mixed in 6 mL of a 2:1 methanol/water mixture.
The solution was filtered, and crystals were obtained within 2 months
upon slow evaporation of the solvent. Yield: 14.6 mg, 46%. ESI-MS:
1
(
85%). H NMR [(CD ) SO, 400 MHz]: δ 7.23 (m, 5H, Ph), 3.25 (t,
3
2
J = 7 Hz, 1H, C H), 2.84 (q, J = 13 Hz, J = 6 Hz, 1H, C H), 2.61 (q,
α
1
2
β
−1
2+
2+
3+
J = 14 Hz, J = 8 Hz, 1H, C H). MP = 190 °C. IR (KBr, cm ): 3028
m/z 733.9 (735.0 calcd for{La [15-MCCu(II),pheHA-5](isonico-
1
2
β
−
2+
3+
3+
br (NH, NH , OH), 2857 br (CH), 1615 s (CO), 1557 m, 1534 m,
tinate )} ), 1019.9 (1020.0 calcd for {(La [15-MC
(isonicotinate ) } ), 1590.0 (1591.0 calcd for{La [15-
MCCu(II), pheHA-5](isonicotinate ) } ). CHN found (calcd for
-5])2
Cu(II), pheHA
3+
2
−
3+
+
+
1466 m, 1383 s, 1288 s, 1174 w, 1072 w, 913 m, 886 m, 698 m. CHN
3
−
+
found (calcd) for C H N O : C, 60.14 (59.97); H, 6.77 (6.71); N,
9
12
2
2
2
15.50(15.55).
LaCu C H N O (C H NO ) (NO )(H O) ): C, 38.74 (38.85);
5
45 50 10 10
6
4
2
2
3
2
6
S-Homophenylalanine Hydroxamic Acid. Reaction was run on a
H, 3.98 (4.01); N = 10.08 (10.34).
La 15-MCCu(II), hpheHA-5](isonicotinate). La [15-MC
3+[
3+
0
.075 mol (S)-homophenylalanine methyl ester hydrochloride scale.
hpheHA-
Cu(II),
1
Yield: 12.61 g (87%). H NMR [(CD ) SO, 400 MHz]: δ 7.27 (t, J =
5](NO ) (0.019 mmol, 30 mg) and sodium isonicotinate (0.069
3
2
3
7
Hz, 2H, m-Ph), 7.17 (m, 3H, o-Ph, m-Ph) 3.02 (t, J = 7 Hz, 1H,
mmol, 10 mg) were combined in 8 mL of a 3:1 methanol/water
C H), 2.62 (m, 1H, C H), 2.54 (m, 1H, C H), 1.78 (m, 1H, C H), 1.63
mixture. The solution was filtered and set to slowly evaporate. Crystals
α
γ
γ
β
−1
2+
2+
3+
(
m, 1H, C H). MP = 177.5 °C. IR (KBr, cm ): 3029 br (NH, NH ,
formed in 2 weeks. Yield: 9.8 mg, 29%. ESI-MS: m/z 768.9 (770.0
β
2
3
+
−
2+
OH), 2859 br (CH), 1614 s (CO), 1543 m, 1468 m, 1454 m, 1365
calcd for{La [15-MC
-5](isonicotinate )} ), 1066.7
Cu(II), hpheHA
3 +
(1067.43 calcd for {(La [15-MC
(isonicotinate ) } ), 1660.1 (1661.0 calcd for {La [15-
MCCu(II), hpheHA-5](isonicotinate ) } ). CHN found (calcd for
+
-5])2
s, 1315 w, 1059 w, 942 w, 876 w, 775 w, 744 w, 696 w. CHN found
C u ( I I ) , h p h e H A
−
3+
+
+
3+
(
calcd) for C H N O : C, 62.07 (61.84); H, 7.41 (7.27); N, 14.32
10
14
2
2
3
−
+
(
14.42).
2
4
529
dx.doi.org/10.1021/ic202347j | Inorg. Chem. 2012, 51, 4527−4538

Inorganic Chemistry
Article
LaCu C H N O (C H NO ) (NO )(H O) : C, 41.50 (41.67);
GdCu C H N O (C H O S ) (H O) ): C, 40.05 (40.23); H,
5
50 60 10 10
6
4
2
2
3
2
3.5
5
50 60 10 10
10
4
4
2
1.5
2
7
H, 4.15 (4.23); N, 9.72 (10.19).
4.13 (4.16); N, 7.37 (7.22).
3
+
3+
3+
3+
La [15-MC
-5](terephthalate). La [15-MCCu(II), pheHA-5]
La [15-MC
-5](btDC). La [15-MC
pgHA-5](NO3)
Cu(II),
Cu(II), pheHA
Cu(II), pgHA
(
NO ) (0.018 mmol, 30 mg) and terephthalic acid sodium salt (0.027
(0.021 mmol, 30 mg) and 2,2′-bithiophene dicarboxylic acid disodium
salt (0.034 mmol, 10 mg) were mixed in 12 mL of a 2:2:1 methanol/
water/acetonitrile mixture. A blue precipitate formed and was
redissolved upon the addition of 3 mL of DMF. The solution was
filtered and set to slowly evaporate, yielding crystals in 1 month. Yield:
3
mmol, 5.7 mg) were dissolved in 8 mL of a 3:1 methanol/water
mixture. The solution was set to slowly evaporate, yielding crystals
within 1 month. Yield: 22.7 mg, 76%. ESI-MS: m/z 755.4 (756.5
calcd for {La [15-MC
2+
3
+
−
2+
3+
-5](terephthalateH )]} ), 973.3
Cu(II), pheHA
3+
3
+
2‑
2+
2+
3+
(
(
(
{
973.3 calcd for {(La [15-MC
NO )} ), 1007.6 (1007.6 calcd for [La [15-MC
-5]) (terephthalate )-
8.4 mg, 22%. ESI-MS: m/z 765.3 (765.4 calcd for {La [15-
Cu(II), pheHA
3+
2
−
3+
3+
3+
−
2+
2+
2+
3+
-5]
MCCu(II), pgHA-5](btDCH )} ), 801.9 (801.9 calcd for {La [15-
3
Cu(II), pheHA
2
‑
−
3+
+
+
-5](btDCH )(DMF)} ), 1529.8 (1528.8+ calcd for
−
2+
+
terephthalate )(terephthalateH )} ), 1511.0 (1509.9 calcd for-
MC
{La [15-MC
Cu(II), pgHA
3
+
2‑
+
2+
2+
3+
2‑
+
2+
2+
La [15-MC
-5](terephthalate )} ), 1511.0 (1511.0
-5](btDC )} ), 1529.8 (1528.8 calcd for
-5]) (btDC ) } ). CHN found (calcd for
Cu(II), pgHA 2 2
Cu(II), pheHA
Cu(II), pgHA
3
+
2‑
2+
3+
2‑
2+
calcd for [(La {15-MC
-5]) (terephthalate ) } ). CHN
{(La [15-MC
Cu(II), pheHA
2
2
found (calcd for LaCu C H N O (C H O )(NO )(H O) ): C,
LaCu C H N O (C H O S ) (H O) ): C, 36.23 (36.34); H,
5
45 50 10 10
8
4
4
3
2
5
5 40 40 10 10 10 4 4 2 1.5 2 9
3
8.38 (38.26); H, 3.77 (3.88); N, 9.11 (9.26).
3.62 (3.55); N, 7.88 (7.70).
Crystallography. Intensity data for La [15-MC
3+
3+
3+
La [15-MC -5](terephthalate). La [15-MC
Cu(II), hpheHA
hpheHA-5]
pgHA-5]-
Cu(II),
Cu(II),
(
(
NO ) (0.019 mmol, 30 mg) and terephthalic acid sodium salt
0.048 mmol, 10 mg) were mixed in 8 mL of a 3:1 methanol/water
(btDC) was collected at 150 K on a D8 goniostat equipped with a
Bruker APEXII CCD detector at Beamline 11.3.1 at the Advanced
Light Source (Lawrence Berkeley National Laboratory) using
synchrotron radiation tuned to λ = 0.7749 Å. A series of 3 s frames
measured at 0.2° increments of ω were collected to calculate a unit
cell. For data collection frames were measured for a duration of 3 s for
low-angle data and 5 s for high-angle data at 0.3° intervals of ω with a
maximum 2θ value of ∼60°. Data frames were collected using APEX2
3
fixture and filtered. The solvent slowly evaporated, yielding crystals in
two weeks. Yield: 16.3 mg, 49%. ESI-MS: m/z 790.5 (791.5 calcd
2
+
2+
+
−
2+
3+
for {La [15-MC
-5](terephthalateH )} ), 1055.0
3
Cu(II), hpheHA
3+
1055.03 calcd for {(La [15-MC
terephthalateH )} ), 1061.7 (1062.3 calcd for {(La [15-
+
-5]) (terephthalate )-
2‑
(
(
Cu(II), hpheHA
2
−
3+
3+
3+
2
‑
−
3+
MC
-5]) (terephthalate )(terephthalateH )(H O)} ),
Cu(II), hpheHA
2
2
581.0+ (1581.6 calcd for{La [15-MC
+
3+
-5]-
61
1
and processed using the program SAINT routine within APEX2.
Cu(II), hpheHA
3+
2
‑
+
2+
2+
(
terephthalate )} ), 1581.0 (1581.6 calcd for {(La [15-
Data were corrected for absorption and beam corrections based on the
2
‑
2+
62
MCCu(II), hpheHA-5]) (terephthalate ) } ). CHN found (calcd for
multiscan technique as implemented in SADABS. The structure was
2
2
LaCu C H N O (C H O ) (H O) ): C, 42.36 (42.46); H, 4.17
solved and refined with the Bruker SHELXTL (version 2008/4)
5
50 60 10 10
8
4
4
1.5
2
5
6
3
(
4.37); N, 7.95 (7.99).
software package.
3
+
3+
3+
Gd [15-MC
-5](terephthalate). Gd [15-MCCu(II), pgHA-5]
Intensity data for La [15-MCCu(II), pheHA-5](isonicotinate) was
collected at 85(2) K on a standard Bruker SMART-APEX CCD-based
X-ray diffractometer equipped with a low-temperature device and fine
focus Mo-target X-ray tube (λ = 0.71073 A) operated at 1500 W
power (50 kV, 30 mA). Indexing was performed by use of the
Cu(II), pgHA
(
NO ) (0.021 mmol, 30 mg) was dissolved in 5 mL of water.
3
Terephthalic acid sodium salt (0.06 mmol, 12.6 mg) was dissolved in
about 2 mL of water. The solution was filtered, and crystals were
obtained upon slow evaporation of the solvent. Yield: 3.4 mg, 8.7%.
2
+
2+
3+
64
ESI-MS: m/z 729.9 (729.9 calcd for {Gd [15-MC
-5]
CELL_NOW program, which indicated that the crystal was a two-
Cu(II), pgHA
3+
−
2+
3+
3+
(
MC
(
1
terephthalateH )} ), 972.8 (973.6 calcd for {Gd [15-
component, nonmerohedral twin. Analysis of the data showed
negligible decay during data collection; data were processed with
2
‑
−
3+
+
-5](terephthalate )(terephthalateH )} ), 1460.0
Cu(II), pgHA
+
3+
2‑
+
65
1458.9 calcd for [Gd [15-MC
-5](terephthalate )} ,
TWINABS and corrected for absorption. The domains are related by
Cu(II), pgHA
3+
460.02 (1460.0 calcd for [(Gd [15-MC
+
2+
-5])2
a rotation of 2.3° about the reciprocal [0.660 1.000 0.274] axis or
direct [0.505 1.000 −0.169] axis. For this refinement, single and
composite reflections from the primary domain were used. Merging of
Cu(II), pgHA
3+
2
‑
2+
+
+
(
terephthalate ) } ), 1625.0 (1624.9 calcd for {Gd [15-
2
−
+
MCCu(II), pgHA-5](terephthalateH ) } ). CHN found (calcd for
2
6
5
GdCu C H N O (C H O )(C H O )(H O) ): C, 35.53
the data was performed in TWINABS and an HKLF 4 format file
5
40 40 10 10
8
4
4
8
5
4
2
13
(
36.18); H, 3.51 (4.07); N, 8.14 (7.53).
used for refinement. Structures were solved and refined with the
Bruker SHELXTL (version 2008/4) software package.
Gd [15-MCCu(II), hpheHA-5](terephthalate). Gd3+[15-
3
+
63
MCCu(II), hpheHA-5](NO ) (0.019 mmol, 30 mg) and terephthalic acid
Intensity data for all other samples were collected at 85(2) K on a
standard Bruker SMART-APEX CCD-based X-ray diffractometer
equipped with a low-temperature device and fine focus Mo-target
X-ray tube (λ = 0.71073 A) operated at 1500 W power (50 kV,
30 mA). Frames were integrated with the Bruker SAINT software
3
sodium salt (0.027 mmol, 5.7 mg) were dissolved in 10 mL of ethanol
and 3 mL of water. The solution was filtered, and deep purple crystals
were obtained upon slow evaporation of the solvent. Yield: 8.7 mg,
2
MC
7%. ESI-MS: m/z 800.02 (800.0
+
2+
calcd for {Gd [15-
3+
−
2+
3+
3+
61
-5](terephthalateH )} ), 1067.7 (1067.0 calcd for
package with a narrow-frame algorithm. Data were processed with
Cu(II), hpheHA
3+
2‑
−
3+
62
{
(Gd [15-MC
-5]) (terephthalate )(terephthalateH )} ),
SADABS and corrected for absorption. Structures were solved
Cu(II), hpheHA
3+
2
1
(
074.43 (1073.0 calcd for{(Gd [15-MC
+
3+
-5])2
and refined with the Bruker SHELXTL (version 2008/4) software
Cu(II), hpheHA
2
−
−
3+
+
+
63
terephthalate )(terephthalateH )(H O)} , 1600.1 (1600.0 calcd
package.
2
3+
2−
+
2+
2+
for{Gd [15-MC
calcd for{(Gd [15-MC
-5](terephthalate )} , 1600.1 (1600.0
Cu(II), hpheHA
Additional crystallographic details are presented in Table 1. CIF
files and crystallographic tables for all structures are presented in the
Supporting Information.
Cu(II), hpheHA
3+
2−
2+
-5]) (terephthalate ) } ). CHN
2
2
found (calcd for GdCu C H6 0 N1 0 O1 0 (C H O )(OH)-
5
5 0
8
4
4
(
C H O) (H O) ): C, 41.47 (41.39); H, 4.12 (4.47); N, 7.83 (8.18).
2
6
0.5
2
4
3
+
3+
Gd [15-MC
-5](btDC). Gd [15-MCCu(II), hpheHA-5](NO3)
Cu(II), hpheHA
RESULTS AND DISCUSSION
(
0.019 mmol, 30 mg) and 2,2′-bithiophene dicarboxylic acid disodium
■
salt (0.027 mmol, 8.2 mg) were mixed in 6 mL of a 2:1 DMF/water
mixture. The solution was filtered. Water (4 mL) was slowly added
until the blue precipitate persisted. Methanol (1 mL) was added to
clarify the solution, which was slowly evaporated to yield crystals in
under 2 weeks. Yield: 13.5 mg, 37%. ESI-MS: m/z 843.9 (845.0
calcd for {Gd [15-MC
Compartments. In terms of their covalent structure,
3
+
Ln [15-MC
pheHA-5] complexes are monomeric metal-
Cu(II),
locavitands. Crystal structures of the hosts with certain bridging
carboxylate guests reveal dimeric compartments that form
through associative hydrophobic interactions between phenyl
rings on the ligand side chains. We recognized certain structural
2
+
2+
2+
3+
3
+
−
2+
2+
-5](btDCH )} ), 880.5 (881.5
Cu(II), hpheHA
3
+
−
2+
calcd for {Gd [15-MC
-5](btDCH )(DMF)} ), 1125.6
Cu(II), hpheHA
3+
42,48,56
1125.72 calcd for {(Gd [15-MC
+
3+
2‑
criteria for Ln [15-MC
S‑pheHA-5] compartments.
(
(
5
-5]) (btDC )-
Cu(II),
Cu(II), hpheHA
3+
2
−
3+
+
+
First, compartments contain hydrophobic interactions between
phenyl side chains on adjacent LnMCs, effectively creating the
walls of the container. Second, compartments typically possess
btDCH )} , 1688.0 (1688.0 calcd for {Gd [15-MCCu(II), hpheHA-
2
‑
+
2+
2+
3+
](btDC )} ), 1688.0
(1688.0
calcd for {(Gd [15-
2
‑
+
MCCu(II),
-5]) (btDC ) } ). CHN found (calcd for
hpheHA
2
2
4
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Inorganic Chemistry
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as the compartment does not form with naphthalene
5
6,57
3+
dicarboxylate or btDC.
Complexes between the Ln [15-
MCCu(II), pgHA-5] host and terephthalate and btDC guests
presented herein establish further that long guests lead to
compartment disruption by associative intermetallacrown
interactions between the phenyl side chains that are necessary
for compartment integrity.
3+
In the Gd [15-MC
pgHA-5](terephthalate) complex
Cu(II),
(
Figure 1) a monomeric host is observed with terephthalate-
bound bidentate on the hydrophilic face. The other carboxylate
on this terephthalate forms hydrogen bonds with lattice water
3
+
molecules. On the hydrophobic face a water is bound to Gd .
An unbound terephthalate forms a hydrogen bond with this
coordinated water and π-stacking interactions with pgHA
phenyl rings. Unlike other MC structures, the saturated guest is
not bound to the lanthanide central metal on the hydrophobic
face. This is attributed to the orientation of the inflexible phenyl
side chain toward the periphery of the MC, which limits how
effectively the phenyl side chains can recognize a guest bound
to the central metal. Importantly, this structure shows that the
short pgHA side chain prevents compartment formation with
the 5.8 Å long terephthalate. In contrast, a compartment is
3
+
Figure 1. Thermal ellipsoid plot of Gd [15-MC
(
pgHA-5]-
Cu(II),
terephthalate). Color scheme: pink = Gd, orange = Cu, red = O,
dark blue = N, gray = MC C, green = guest C. Lattice solvent
molecules and hydrogen atoms were removed for clarity. Thermal
ellipsoids are displayed with 50% probability.
3
+
observed in the Ln [15-MC
-5](terephthalate)
Cu(II), pheHA
an approximately cylindrical shape in order to maximize
associative contacts between phenyl side chains and completely
complexes with La3+ (vide infra) and Gd , which further
demonstrates that the maximum allowed compartment height
is constrained by side-chain length.
3+
3
+
encapsulate the guest. Third, the Ln [15-MCCu(II)-5] phenyl
side chains recognize guests through directional, convergent
intermolecular interactions. As a whole, these features define
the compartment as a dimeric container that recognizes guests
through both dative bonds and intermolecular interactions. The
solid-state guest recognition described herein has straightfor-
Unsurprisingly, the prototypical cylindrical compartment is
3+
also not observed in the La [15-MC
pgHA-5](btDC)
Cu(II),
3+
complex. The two La [15-MC
pgHA-5]s dimers in the
Cu(II),
asymmetric unit are bridged by either a cis- or a trans-btDC
(
Figure 2a or 2b, respectively). A second btDC is bound to the
3
+
ward implications for application of the Ln [15-MCCu(II)-5]
compartment as a crystalline molecular flask for solid-state
hydrophobic face of each MC. This guest extends bridges a
2+
Cu ring metal on the hydrophilic face of another MC in the
6
6
reactions. Importantly, the associative phenyl interactions
between ligand side chains would contribute to compartment
formation in the absence of solid-state packing forces,
suggesting the possibility of solution applications for these
hosts.
Compartment height is a significant characteristic of
Ln [15-MCCu(II)-5] dimers. In this work, we report compart-
ment heights (or MC−MC distance) as the distance between
the centroid of the oxygen-mean planes of each MC. Given
that the location of the central metal varies from approximately
lattice, resulting in a coordination polymer. In this structure the
length of btDC prevents formation of the cylindrically shaped
compartment, as the MCs in the dimer are horizontally
displaced by over 8 Å. Importantly, the pgHA phenyl rings on
opposite MCs do not associate through π-stacking interactions.
While some van der Waals interactions between pgHA side
chains are observed, the association of the side chains primarily
occurs through the packing of interstitial DMF solvent
molecules. Overall, this structure does not fit the criteria of a
compartment (vide supra), further demonstrating how long
guests can disrupt intermetallacrown interactions.
3+
6
7
0
.2 to 0.7 Å above or below the center of the oxygen-mean
5
8
plane, referencing compartment dimensions to the center of
the oxygen-mean plane allows for more direct comparisons of
different structures. Given the dependence of the compart-
ment’s dimensions on guest size, we investigated guests of
varying length in order to assess how side-chain variation effects
compartment size and selectivity. Guest length is defined as the
distance between carboxylate carbons for dicarboxylates or
carboxylate carbon to pyridyl nitrogen for isonicotinate.
Referencing guest length to the carboxylate carbon eliminates
the disparity between endo and exo carboxylate binding. In
the specific case of the btDC guest, the length can vary by
over 0.3 Å depending on the torsion angle between the two
thiophene rings. The trans rotamer distance is reported in
Scheme 1.
As expected, the longer hpheHA side chain renders the
Ln [15-MCCu(II), hpheHA-5] host capable of encapsulating longer
3+
guests than hosts with pheHA or pgHA side chains. The crystal
3+
structure of the Gd [15-MCCu(II), hpheHA-5](btDC) complex
reveals a 15.2 Å cylindrical compartment that encapsulates the
9
.2 Å btDC guest (Figure 3). The btDC guest is bound
3+
bidentate to 8-coordinate Gd central metals. Many π-stacking
interactions are observed between ligands across the hydro-
phobic faces and the aromatic rings on the guest. As a whole,
these structures reveal that the maximum height of the
compartment is dictated by the side-chain length. Encapsula-
tion of the 5.8 Å long terephthalate is observed with the pheHA
side chain and not with the shorter pgHA. Similarly,
encapsulation of the 9.2 Å long btDC is observed with the
hpheHA side chain and not with pheHA or pgHA.
Side-Chain Length Constrains the Compartment
Height. It has previously been demonstrated that the 5.8 Å
long terephthalate guest is effectively encapsulated in the
3+
Interestingly, the La [15-MC
hpheHA-5](isonicotinate)
Cu(II),
complex reveals that compartments can also be disrupted
by excessively short guests. Isonicotinate is effectively
3+
42
dimeric Gd [15-MCCu(II), pheHA-5] container. It has also been
shown that the maximum compartment height is constrained,
3+
encapsulated in compartments with Gd [15-MCCu(II), pheHA- 5],
4
531
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4
532
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Inorganic Chemistry
Article
Figure 4. Thermal ellipsoid plot and compartment diagram of
3
+
La [15-MC
hpheHA-5](isonicotinate). Crystal structure color
Cu(II),
scheme: pink = La, orange = Cu, red = O, dark blue = N, gray =
MC C, green = guest C. Guests on the hydrophilic face, unbound
counteranions, solvent molecules, and hydrogen atoms were removed
for clarity. Thermal ellipsoids are displayed at the 50% probability
level.
3+
La [15-MC
hpheHA-5] hydrophobic cavity (Figure 4).
Cu(II),
Inclusion of two guests is afforded by an additional co-
ordination site on the nine-coordinate La3+ central metal. The
isonicotinates are bound through their carboxylates, with one
Figure 2. Thermal ellipsoid plots and compartment diagrams for
III
3+
bidentate to La and the other bridging the central metal and a
La [15-MCCu(II), pgHA-5](btDC) showing the MC dimers bridged by
a) cis-btDC and (b) trans-btDC. Crystal structure color scheme: pink
La, orange = Cu, red = O, dark blue = N, gray = MC C, green =
II
Cu ring metal. The pyridyl nitrogens are unbound, extending
(
=
out of the hydrophobic cavity and hydrogen bonding to lattice
water molecules. The two monomeric MCs associate via
π-stacking interactions with a 16.1 Å MC−MC distance. How-
ever, the monomers are significantly offset with a 7.3 Å
horizontal displacement relative to each other. This demon-
strates that though compartments can compress to accom-
modate short guests, the extent that they will compress is
limited by the side-chain length.
guest C, purple = sulfur. Solvent molecules and hydrogen atoms were
removed for clarity. Thermal ellipsoids are displayed at the 50%
probability level.
3
+
As a whole, the Ln [15-MC
-5] compartment structures
Cu(II)
4
2,48,52,56−58
described heretofore and in previous work
reveal
that the side-chain length constrains the maximum and
minimum compartment height. The assembly of the compart-
ment through nondirectional hydrophobic interactions between
the flexible side chains gives the dimeric compartment a margin
of a few Angstroms where it can adapt to a certain guest.
However, the compartment can be readily disrupted by
extremely long or short guests. These results suggest that as a
3
+
guideline for designing Ln [15-MCCu(II)-5] compartment
inclusion compounds the side-chain length should be
complementary with the length of the guest.
Factors Influencing the Number of Encapsulated
Guests. Inclusion of multiple guests is appealing from the
viewpoint of multimolecular reactions in the interior of a host
or the design of materials. We sought to establish the factors
3
+
Figure 3. Thermal ellipsoid plot and compartment diagram of
that promote inclusion of multiple guests in dimeric Ln [15-
3+
Gd [15-MC
hpheHA-5](btDC). Crystal structure color scheme:
Cu(II),
MCCu(II)-5] compartments. Analysis of the crystal structures
pink = Gd, orange = Cu, red = O, dark blue = N, gray = MC C, green =
guest C, purple = S. Guests on the hydrophilic face, unbound
counteranions, solvent molecules, and hydrogen atoms were removed
for clarity. Thermal ellipsoids are displayed at the 50% probability
level.
3+
of the series of Ln [15-MC
-5] host−guest complexes
Cu(II)
reveals that steric interactions with the side chain, compart-
ment compression or elongation, and central lanthanide
coordination number all influence the number of encapsu-
lated guests.
The effect of side-chain sterics is most evident by comparing
3
+
3+
3
+
3+
La [15-MC
(
-5], and La [15-MCCu(II), pgHA-5] hosts
Cu(II), pheHA
the La [15-MC
-5](isonicotinate) and La [15-
Cu(II), pheHA
3
+
vide infra). However, the La [15-MCCu(II), hpheHA- 5] does not
form a compartment with the 4.3 Å long isonicotinate. In the
structure, two isonicotinate guests are bound in a monomeric
MCCu(II), pgHA-5](isonicotinate) structures. Three isonicotinate
3+
guests are encapsulated in the La [15-MC
pheHA-5]
Cu(II),
compartment (Figure 5), while five are encapsulated in the
4
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the compartment. One carboxylate bridges a nine-coordinate
3+
La and a ring metal, while the remaining four are bound
monodentate to a ring metal. In component b, only four of the
isonicotinates are parallel. The fifth is roughly perpendicular,
with the pyridine group oriented to the periphery of the host.
The distinction between the inclusion of three or five guests
3+
3+
in the La [15-MC
-5](isonicotinate) and La [15-
Cu(II), pheHA
MCCu(II), pgHA-5](isonicotinate) compartments suggests that the
length and orientation of the phenyl side chain can influence
the number of encapsulated guests. With pgHA, the phenyl side
chain is rigidly extended to the periphery of the MC. Thus,
there are minimal steric effects obstructing inclusion of multiple
isonicotinates. In contrast, the pheHA side chains are no longer
more flexible. They are observed converging on guests bound
in the hydrophobic cavity. This recognition by the more flexible
side chain likely introduces steric interactions which hinder the
binding of additional guests. This effect of side-chain
convergence limiting the guest accessibility to the host is
most evident in compartment volumes measurements.
Figure 5. Thermal ellipsoid plot and compartment diagram of
3
+
La [15-MC
pheHA-5](isonicotinate). Crystal structure color
Cu(II),
scheme: pink = La, orange = Cu, red = O, dark blue = N, gray =
MC C, green = guest C. Guests on the hydrophilic face, unbound
counteranions, solvent molecules, and hydrogen atoms were removed
for clarity. Thermal ellipsoids are displayed at the 50% probability
level.
68
Compartment volumes were estimated with Platon by
comparing the void space of the crystal structure with and
without encapsulated guests. The solvent-accessible void space
of the structure was measured with and without the
encapsulated substrates in the interior of the compartment,
which entails the carboxylate guests and other counterions or
solvent molecules. The difference in the void spaces of the
complete structure from that of the empty compartment gave
the estimate for the volume of the host. These values are
tabulated in Table 2 for complexes with well-defined compart-
ments. The guest volume was measured in Discovery Studio
Visualizer by rolling a 1.4 Å probe over the surface of the guest.
3
69
The volume of a water molecule was set as 11.5 Å . The
compartment and guest volumes were used to determine the
packing coefficient, defined as the percentage of the host
volume filled by the guests. A previous study has reported that
the optimal packing coefficient for guest inclusion is 55 ± 9%,
though this is expected to be higher in assemblies with highly
70
polar host−guest interactions. Packing coefficients for select
3+
Ln [15-MC-5] compartments are listed in Table 2. The
3
+
volume of the La [15-MC
-5](isonicotinate) com-
Cu(II), pgHA
Figure 6. Thermal ellipsoid plots and compartment diagram of
3
3+
partment is 710.3 Å . The volume of the La [15-
MCCu(II), pheH A3 -5](isonicotinate) compartment is over 30%
less at 480.3 Å . The compartment volumes reflect how longer,
more flexible side chains can converge on substrates in the
hydrophobic cavity and limit the number of encapsulated
guests.
3
+
La [15-MC
pgHA-5](isonicotinate). Structure is disordered
Cu(II),
between two compartment orientations, with all isonicotinates aligned
a) and one isonicotinate perpendicular (b). Color scheme: pink = La,
(
orange = Cu, red = O, dark blue = N, gray = MC C, green = guest C.
Unbound counteranions, solvent molecules, and hydrogen atoms were
removed for clarity. Thermal ellipsoids are displayed at the 50%
probability level.
The inclusion of multiple guests also seems to be promoted
by compartment compression. When compartments compress
with a short bridging guest, the side chains must splay out
toward the periphery of the MC, which widens the compart-
ment and increases the volume of the host. The effects of
compartment compression are evident when comparing the
3+
3+
La [15-MC
-5] complex (Figure 6). The La [15-
Cu(II), pgHA
MCCu(II), pheHA-5](isonicotinate) complex closely resembles the
3
+
52
previously reported structure that had a Gd central metal.
3+
The La [15-MCCu(II), pheHA-5] compartment has a height of
0.5 Å. Two isonicotinates are aligned antiparallel, with the
pyridine coordinated to a Cu ring metal and at least one
3+
1
inclusion complexes of the Ln [15-MC
pheHA-5] hosts
Cu(II),
2
+
with isonicotinate and terephthalate. As previously described,
3
+
3+
carboxylate oxygen atom bound to a La central metal. The
the La [15-MC
pheHA-5](isonicotinate) compartment
Cu(II),
3
+
third isonicotinate binds bidentate to a La and does
not span the dimer, instead extending toward the periphery
of the MC.
The La [15-MCCu(II), pgHA-5](isonicotinate) compartment is
.7 Å in height and encapsulates 5 guests. The structure is
compresses to a mere 10.5 Å in height. This compression
promotes inclusion of three guests3.
+
In contrast, the taller Ln [15-MC
pheHA-5]-
Cu(II),
3+
(terephthalate) compartments are consequentially narrower,
which likely explains why only one guest is encapsulated. The
9
3
+
disordered between two components with 75% and 25%
occupancy (Figure 6a and 6b, respectively). In component a,
the five isonicotinate guests are aligned parallel as they bridge
structure of the La [15-MC
-5](terephthalate) com-
Cu(II), pheHA
3
+
plex (Figure 7) resembles the previously reported Gd com-
plex. The La [15-MCCu(II), pheHA-5](terephthalate) compartment
3+
4
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Inorganic Chemistry
Table 2. Size and Guest Inclusion Parameters for Ln [15-MC-5] Compartments
Article
3
+
MC
La [15-MCCu(II),pgHA-5]
guest
Vhost (Å3)
Vguest (Å3)
P.C. (%)
no. of guests
Ln3+ coord. no.a
compartment height (Å)
3
+
isonicotinate
isonicotinate
isonicotinate
isonicotinate
terephthalate
terephthalate
terephthalate
terephthalate
terephthalate
btDC
710.3
480.3
392.0
292.4
d
463.8
306.3
257.6
171.7
d
65.3
68.5
65.7
58.7
d
5
3
3
2
0
1
1
2
1
9, 9, 9, 8
9, 8
9.7
3+
La [15-MCCu(II),pheHA-5]
10.5
10.3
c
3+
52
Gd [15-MCCu(II),pheHA-5]
8, 7
3+
b
La [15-MCCu(II),hpheHA-5]
9
3+
Gd [15-MCCu(II),pgHA-5]
8
c
3+
La [15-MCCu(II),pheHA-5]
380.0
375.3
526.7
277.5
d
175.4
217.6
325.7
170.6
d
48.3
58.0
61.8
61.5
d
8, 8
12.4
11.9
12.2
11.9
15.2, 15.9
c
3+
42
Gd [15-MCCu(II),pheHA-5]
8, 8
3+
La [15-MCCu(II),hpheHA-5]
9, 9
3+
Gd [15-MCCu(II),hpheHA-5]
8, 8
3+
e
La [15-MCCu(II),pgHA-5]
2
e
9, 9, 9, 9
9, 9
3+
57
La [15-MCCu(II),pheHA-5]
btDC
d
d
d
2
3+
Gd [15-MCCu(II),hpheHA-5]
btDC
495.7
332.5
67.1
1
8, 8
15.2
a
b
c
Listed for each distinct MC in the asymmetric unit. Volume of the monomeric cavity is reported. Structure did not contain a formal dimeric MC
d
e
complex. Structure did not contain a discrete, complete compartment. Number of guests bound on the hydrophobic face of each MC.
the hard carboxylate has a higher affinity for the hard Ln3+
central metal. This drives isonicotinate to bind further toward
the periphery of the compartment, while terephthalate has the
highest affinity for the center of the compartment. The
peripheral binding of isonicotinate can also factor into inclusion
of multiple guests.
The effect of compartment compression is further demon-
3+
strated in the La [15-MCCu(II), hpheHA-5](terephthalate) com-
plex (Figure 8), which contains two terepthalate guests
Figure 7. Thermal ellipsoid plot and compartment diagram of
3+
La [15-MCCu(II), pheHA-5](terephthalate). Color scheme: pink = La,
orange = Cu, red = O, dark blue = N, gray = MC C, green = guest C.
Unbound counteranions, solvent molecules, and hydrogen atoms were
removed for clarity. Thermal ellipsoids are displayed at the 50%
probability level.
has a height of 12.4 Å, with one encapsulated terephthalate
3
+
bridging the La central metals. With terephthalate, the
compartment is elongated, which forces the side chains
toward the center in order to maintain associative interac-
tions with the opposite MC in the dimer. The narrow com-
partment can only accommodate one guest. Thus, these
structures demonstrate that compressed compartments can
have wider interiors, which promote inclusion of multiple
guests.
Differences in guest charge likely do not influence the
number of encapsulated guests because MCs typically achieve
charge balance with unbound anions in the lattice and by
binding anions on the hydrophilic face. Though the overall +6
charged MC dimer would be expected to bind more −1
charged isonicotinates than −2 charged terephthalates, guest
binding on the hydrophilic face prevents guest charge from
factoring significantly in guest inclusion in the interior of the
Figure 8. Thermal ellipsoid plots and compartment diagrams of
3+
3+
(
a) La [15-MC
-5](terephthalate) and (b) Gd [15-
Cu(II), hpheHA
MCCu(II), hpheHA-5](terephthalate). Crystal structure color scheme:
pink = Gd or La, orange = Cu, red = O, dark blue = N, gray = MC C,
green = guest C. Guests on the hydrophilic face, unbound
counteranions, solvent molecules, and hydrogen atoms were removed
for clarity. Thermal ellipsoids are displayed at the 50% probability
level.
3
+
compartment. All MCs in La [15-MC
pheHA-5]-
Cu(II),
3
+
encapsulated in a 12.2 Å compartment. The terephthalates
are bound with one carboxylate bidentate to the central nine-
(
isonicotinate) and La [15-MC
pheHA-5](terephthalate)
Cu(II),
have two bound carboxylates. This suggests the number of
encapsulated guests in the hydrophobic compartment arises
largely from the supramolecular behavior of the container and
not merely electrostatic effects. It should also be noted that the
3+
coordinate La and the other bridging the central metal and a
2+
Cu ring metal. The terephthalate guests engage in π-stacking
interactions and a CH−O interaction with the phenyl side
chains. The compartment is roughly cylindrical, with the MC
2
+
soft pyridyl group prefers to bind to the Cu ring metal, while
4
535
dx.doi.org/10.1021/ic202347j | Inorg. Chem. 2012, 51, 4527−4538

Inorganic Chemistry
Article
3+
faces offset by 2.7 Å. The effect of compartment compression is
complexes between Ln [15-MC-5]s and their dicarboxylate
guests were taken to assess whether or not dimerization occurs
in solution with these complexes.
3
reflected in the 526.7 Å host volume. This volume is almost
3
3+
1
50 Å larger than the La [15-MCCu(II), pheHA-5](terephthalate)
compartment, which encapsulates only one terephthalate guest.
Spectra were taken of the crystalline host−guest complexes
dissolved in 4:1 methanol/water and injected via a syringe
pump at a range of cone voltages (10−75 mV). For all host−
guest complexes, monomeric 1:1 and 1:2 MC−guest species
were observed, consistent with previous results. More
importantly, dimeric MC−guest species were present for all
host−guest complexes. These peaks typically had weaker
intensity than the monomeric MC species, though they were
discernible over the background noise. With isonicotinate, 2:3
3
+
Comparison with the Ln [15-MC
terephthalate) complexes with La and Gd central metals
hpheHA-5]-
Cu(II),
3+
3
+
(
demonstrates that the central metal coordination number is
also a factor in the number of encapsulated guests. In the
3+
Gd [15-MC
hpheHA-5](terephthalate) structure (Figure
Cu(II),
8b), one terephthalate guest is encapsulated in an 11.9 Å tall
compartment. The terephthalate is bound bidentate at each
3
+
carboxylate group to an 8-coordinate Gd central metal. The
3
3+
volume of the compartment is merely 277.5 Å , about one-half
MC−guest species are observed as 3+ cations. 2:2 Ln [15-
MC-5]-guest peaks are observed with terephthalate and btDC
(Figure 9). These species correspond to 3+ charged MC dimers
3+
of the La [15-MCCu(II), hpheHA-5](terephthalate) compartment.
The disparity between the number of encapsulated guests with
3+
3+
Gd and La complexes is from the different coordination
number on the metal ions. Similar to the structures of
3
+
monomeric MC−guest complexes, the eight-coordinate Gd
preferentially accommodates one bidentate carboxylate; thus,
3
+
Gd [15-MC-5] compartments are likely to bind one
dicarboxylate guest. In contrast, the nine-coordinate La3+ can
bind one bidentate carboxylate and one carboxylate that bridges
with a Cu2 ring metal, leading to inclusion of two carboxylate
guests.
+
It is worth considering why La3+ central metals are
occasionally 8 coordinate in certain compartment complexes.
Lanthanide ions are known to adopt lower coordination
numbers due to steric clashes with bound substrates. It is likely
that steric effects from substituents and the MC face, such as
side chains converging at the MC center or bound guests, lead
3+
Figure 9. ESI-MS spectra of La [15-MCCu(II), pheHA-5](terephthalate)
taken in 4:1 methanol/water (v/v) injected via syringe pump.
2
+
3+
Important peaks are observed at m/z = 755.5 for La [15-
2
+
3+
3+
3+
MCCu(II), -5](terephthalateH)} , 973.9
for {La [15-
to lower coordination numbers. The La [15-MC
pheHA-5]-
pheHA
Cu(II),
3+
3+
3+
MCCu(II), pheHA-5]} (terephthalate)(NO ) , 1008.0 for {La [15-
2
3
(
terephthalate) compartment is slightly stretched in order to
3+
+
MC
-5]} (terephthalate)(terephthalateH) , and 1511.7 for
-5](terephthalate) and {La [15-
-5]} (terephthalate)2 . (Inset) Isotope model of
Cu(II), pheHA
3+
2
accommodate terephthalate, which forces the side chains
inward. Steric effects from this side-chain conformation cause
+
3+
La [15-MCCu(II),
MC
pheHA
2+
Cu(II), pheHA
3+
2
3+
a La ion to be 8-coordinate. Due to the somewhat constrained
volume of the compartment, guest inclusion can also reduce
the central metal coordination number. For example, some of
3+
the{La [15-MC
-5]} (terephthalate)(terephthalateH)
2
Cu(II), pheHA
peak at 1008.03 m/z.
+
the La3 central metals in the La [15-MC
+
3+
pheHA-5]-
Cu(II),
with two dicarboxylates and one proton. Peaks corresponding
to 3+ charged MC dimers with a mixture of dicarboxylate
guests and nitrate, hydroxide, or methoxide anions were also
observed. In addition, peaks corresponding to 2+ charged MC
dimers with two completely deprotonated dicarboxylate guests
were also observed. The 2+ charged 2:2 host−guest species
have the same m/z as the 1+ charged 1:1 host−guest complex,
though observation of half-integer peaks proves that the 2+ 2:2
species is present. Re-examination of the previously reported
3
+
(
isonicotinate), La [15-MC
La [15-MC
pgHA-5](isonicotinate), and
pheHA-5](ferrocene dicarboxylate) compart-
Cu(II),
3+
Cu(II),
ments adopt 8-coordinate geometries due to the steric bulk of
3
+
the guests. Similarly, the Gd [15-MC
(
-5]-
Cu(II), pheHA
3+
isonicotinate) complex likely has a 7-coordinate Gd central
metal due to steric effects.
Considering these structures together reveals that the
3
+
number of encapsulated guests in Ln [15-MC-5] compart-
ments is influenced by steric effects from the size of the side
chain, steric effects from compression or elongation of the com-
partment, and preferred coordination number of the central
lanthanide ion. Due to the flexibility of the host, the com-
partment volumes can vary significantly from one host−guest
complex to another, making extraction of firm rules difficult.
However, these results generate the guidelines that larger lanthanide
central metals, compressed compartments, and the orientation of
inflexible ligands to the periphery of the compartment promote the
inclusion of multiple guests.
3
+
56
3+
Gd [15-MC
-5](terephthalate) and Gd [15-
52
Cu(II), pheHA
MCCu(II), pheHA-5](isonicotinate) complexes also revealed
3
+
dimeric MC complexes. ESI-MS spectra of Ln [15-MCCu(II)-5]-
(NO ) complexes contain a 2:3 MC-nitrate species. Bridging
3
nitrates have been observed crystallographically across the
5
5,58
hydrophilic and hydrophobic faces of the MC.
Considering
the disparate findings presented herein and from previous
studies, it is evident that injection via a syringe pump, as
opposed to high-performance liquid chromatography as
previously utilized, was necessary to observe the dimers. This
difference in conditions explains why the dimers were not
originally observed.
3+
ESI-MS of Ln [15-MC-5](guest) Complexes. There is
3
+
interest in utilizing Ln [15-MC-5] compartments in solution
applications such as supramolecular catalysts or in solution-
processed materials. ESI-MS is a useful technique for
qualitatively assessing host−guest interactions. While it is a
gas-phase technique, experimentally observed ions correspond
Unfortunately ESI-MS does not provide structural informa-
tion on whether the guests bridge across the hydrophobic or
3+
hydrophilic faces of the Ln [15-MC-5] dimer. On the basis of
the crystal structures, the simplest explanation is that
isostructural species exist in solution. Overall, the ESI-MS
71
to species that persist in solution. ESI-MS spectra of the
4
536
dx.doi.org/10.1021/ic202347j | Inorg. Chem. 2012, 51, 4527−4538

Inorganic Chemistry
Article
data supports that MC dimerization occurs in solution.
ACKNOWLEDGMENTS
■
3
+
Therefore, in the development of Ln [15-MC-5] complexes
for solution applications, it is appropriate to consider these
species as dimers in solution with the appropriate guest.
We thank the National Science Foundation (CHE-0717098
and CHE-1057331) for funding. Crystallographic data for
La [15-MCCu(II), pgHA-5](btDC) were collected through the
3+
SCrALS (Service Crystallography at Advanced Light Source)
program at the Small-Crystal Crystallography Beamline 11.3.1
at the Advanced Light Source (ALS), Lawrence Berkeley
National Laboratory. The ALS is supported by the U.S.
Department of Energy, Office of Energy Sciences Materials
Sciences Division, under contract DE-AC02-05CH11231.
CONCLUSIONS
■3
+
Ln [15-MC-5] compartments with pgHA, pheHA, and
hpheHA ligands are a series of metalated supramolecular
containers with distinct molecular recognition behavior.
Differences in the size and selectivity of the compartments
arise from how the length and flexibility of the phenyl side
chains influence host dimerization and the degree that the side
chain can converge on a guest. Systematic inclusion of bridging
carboxylate guests in these hosts reveals that the compartment
height is constrained by the length of the ligand side chain.
Longer side chains allow formation of taller containers that
encapsulate longer guests, as hpheHA ligands can form a 15.2 Å
tall compartment. Considering the previously reported 7.70 Å
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dx.doi.org/10.1021/ic202347j | Inorg. Chem. 2012, 51, 4527−4538