Encapsulation of a metal complex within a self-assembled nanocage: Synergy effects, molecular structures, and density functional theory calculations

Article abstract of DOI:10.1021/ic402539x

A novel palladium-based metallacage was self-assembled. This nanocage displayed two complementary effects that operate in synergy for guest encapsulation. Indeed, a metal complex, [Pt(NO₂)₄] ^2-, was hosted inside the cavity, as demonstrated by solution NMR studies. Single-crystal X-ray diffraction shows that the guest adopts two different orientations, depending on the nature of the host-guest interactions involved. A density functional theory computational study is included to rationalize this type of host-guest interaction. These studies pave the way to a better comprehension of chemical interaction and transformation within confined nanospaces.

Full text of DOI:10.1021/ic402539x

Article
pubs.acs.org/IC
Encapsulation of a Metal Complex within a Self-Assembled
Nanocage: Synergy Effects, Molecular Structures, and Density
Functional Theory Calculations
Christophe Desmarets,*,†,‡ Geoffrey Gontard,†,‡ Andrew L. Cooksy,§ Marie Noelle Rager,⊥
and Hani Amouri*,†,‡
†Institut Parisien de Chimie Molec
Curie (UPMC), 4 place Jussieu, 75252 Paris Cedex 05, France
́ ́ ́ ́
ulaire (IPCM), UMR 8232, Sorbonne Universites, Universite Paris 06, Universite Pierre et Marie
‡IPCM, UMR 8232, Centre National de la Recherche Scientifique (CNRS), 4 place Jussieu, 75252 Paris Cedex 05, France
§Department of Chemistry and Biochemistry, San Diego State University, 5500 Campanile Drive, San Diego California 92182-1030,
United States
⊥Institut de Recherche de Chimie Paris, CNRS−Chimie ParisTech, 11 rue Pierre et Marie Curie, 75005 Paris, France
S
* Supporting Information
ABSTRACT: A novel palladium-based metallacage was self-
assembled. This nanocage displayed two complementary
effects that operate in synergy for guest encapsulation. Indeed,
a metal complex, [Pt(NO2)4]2−, was hosted inside the cavity,
as demonstrated by solution NMR studies. Single-crystal X-ray
diffraction shows that the guest adopts two different
orientations, depending on the nature of the host−guest
interactions involved. A density functional theory computa-
tional study is included to rationalize this type of host−guest
interaction. These studies pave the way to a better
comprehension of chemical interaction and transformation
within confined nanospaces.
Pursuing our research in this area, we sought other types of
templating guests, notably organometallic complexes. Prior to
this work, only a few examples of inorganic receptors capable of
encapsulating a metal complex had been reported: for example,
a coordination box encapsulating discrete stacks of square-
planar complexes, described by Fujita et al.10 Therrien et al
constructed a large cationic hexanuclear metallaprism capable
of hosting a square-planar metal complex.11 A coordination
cage encapsulating a stacked platinum complex of the Magnus
salt type has been reported by the group of Clever, Shionoya,
and co-workers.12 Furthermore, a dinuclear palladium molec-
ular complex that encapsulates two cis-platin molecules inside
the metallosupramolecular cavity was reported by Crowley et
al.13
INTRODUCTION
■
The rational design of inorganic artificial receptors for host−
guest chemistry is one of the most attractive areas in
contemporary supramolecular chemistry.1 Self-assembly is
now well-established as an elegant “bottom-up” method for
fabricating elaborate architectures2 such as helicates,3 cages,4
metallacryptands,5 metallacycles,6 and coordination polymers.7
This approach becomes particularly powerful when the ease of
control offered by the self-assembly of organic components is
combined with the electronic, ion-sensing, catalytic, magnetic,
or photonic properties of inorganic components. In the past
decade, there has been intensive research in the preparation of
metallacages, which have shown particular promise in the
chemistry of host−guest interactions.8
Herein, we report a rational, high-yield (>90%) strategy for
the preparation of nanocages (Figure 1) based on palladium
coordination chemistry, and we demonstrate their properties as
metal complex hosts. These novel nanocages incorporate
unsaturated palladium centerswhich may act as electrostatic
anchorsand bridging ligands containing endohedral amino
Research activities in our group have been focused on the
assembly of cagelike systems that incorporate a central cavity
where anions can be encapsulated. Thus, we have developed
syntheses of some metallacryptands and discrete 3D capsules
that are able to encapsulate weakly coordinated anions.5a,9 For
instance, our metallacages of the type M2L4 [M = Co; L =
bis(benzimidazole)-1,3-phenylene] were perfectly designed to
house BF4 anions, where the encapsulated anion interacts with
the two cobalt centers through M−F contacts.
Received: October 8, 2013
Published: April 11, 2014
© 2014 American Chemical Society
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Article
mL) were introduced. The reaction mixture was then heated to 60 °C
and allowed to stir for 48 h. After cooling, solvents were removed
under vacuum, and the crude product was purified by column
chromatography with AcOEt (100%): 710 mg of yellow solid (75%).
IR (ATR; ν, cm−1): 3452 (NH2), 3367, 2932, 2838, 2200 (CC),
1730, 1572 (N−H), 1643, 1435, 1403, 1361, 1309, 1273, 1231, 1214,
1182, 1127, 1044, 1020, 940, 855, 837, 793, 696, 626, 582, 539, 491,
445, 396, 286, 212. 1H NMR (300 MHz, CD3CN): δ 8.81 (s, 2H, Hk),
8.58 (d, J = 4.9 Hz, 2H, Hj), 7.94 (d, J = 7.9 Hz, 2H, Hh), 7.42 (dd, J =
7.9 and 4.9 Hz, 2H, Hi), 7.06 (s, 2H, Hb), 4.99 (bs, 2H, NH2), 3.77 (s,
3H, OMe). 13C NMR (75 MHz, CD3CN): δ 151.5 (Ck), 150.3 (Cj),
148.6 (Ca), 144.3 (Cd), 137.9 (Cb), 123.0 (Ci), 119.6 (Cg), 118.6 (Cb),
107.0 (Cc), 91.1 (Ce), 88.0 (Cf), 55.2 (COMe). ESI-MS. Calcd for
[2H]+: m/z 326.12. Found: m/z 326.13. Anal. Calcd for C21H15N6O:
C, 77.52; H, 4.65; N, 12.91. Found: C, 76.21; H, 4.84; N, 12.07.
Synthesis of [OTf⊂Pd2(2)4][OTf]3 ([3][OTf]3). Ligand 2 (100 mg,
0.30 mmol) was added to a solution of [Pd(CH3CN)4][OTf]2 (87 mg,
0.15 mmol) in CH3CN (15 mL). The yellow solution was stirred at
room temperature 30 min, during which time the initially yellow
solution turns to orange. The solvent was removed under vacuum.
Hexane (10 mL) was then added. The solids were collected by
filtration from the n-hexane solution, washed with CH2Cl2 (5 mL) and
diethyl ether (3 × 5 mL), and dried under vacuum: 147 mg of orange
solid (93%). This supramolecular cage was recrystallized from
CH3CN/Et2O to afford quantitatively yellow crystals and was
characterized as [3][OTf]3. IR (ATR; ν, cm−1): 3367 (NH2), 3078,
2205 (CC), 1590 (N−H), 1465, 1439, 1417, 1364, 1236 (OTf),
1218, 1155, 1131, 1025, 946, 855, 809, 758, 693, 634, 573, 548, 515,
396, 354, 319. 1H NMR (400 MHz, DMSO-d6): δ 9.46 (d, J = 5.8 Hz,
8H, Hj), 9.23 (s, 8H, Hk), 8.26 (d, J = 8.0 Hz, 8H, Hh), 7.86 (dd, J =
8.0 and 5.8 Hz, 8H, Hi), 7.07 (s, 8H, Hb), 5.78 (bs, 8H, NH2), 3.67 (s,
12H, OMe). 13C NMR (100 MHz, DMSO-d6): δ 152.8 (Ck), 150.2
(Cj), 150.1 (Ca), 145.7 (Cd), 142.6 (Ch), 127.6 (Ci), 123.1 (Cg), 119.9
(Cb), 106.2 (Cc), 92.7 (Ce), 90.2 (Cf), 55.8 (COMe). ESI-MS. Calcd
for [Pd2(L2)4]4+: m/z 378.07. Found: m/z 378.5. Calcd for
[Pd2(L2)4(CF3SO3)]3+: m/z 553.75. Found: m/z 554.7. Calcd for
[Pd2(L2)4(CF3SO3)2]2+: m/z 905.10. Found: m/z 906.3. Calcd for
[Pd2(L2)4(CF3SO3)3]+: m/z 1961.12. Found: m/z 1961.10. Anal.
Calcd for [C88H60F12N12O16Pd2S4].4H2O: C, 48.43; H, 3.14; N, 7.70.
Found: C, 47.97; H, 3.21; N, 7.30.
Figure 1. Schematic drawing of the Pd2L4 nanocage displaying both
functionalities that operate in synergy for guest encapsulation.
functionality for hydrogen bonding. Both effects operate in a
synergistic manner to enhance host−guest interactions.
EXPERIMENTAL SECTION
■
General Information and Materials. All solvents used were
reagent grade or better. Commercially available reagents were used as
received. 2,6-Dibromo-4-methoxyaniline14 and [Pd(CH3CN)4]-
15
[OTf]2 were prepared according to published methods. All
experimental manipulations were carried out under argon using
Schlenk techniques. IR spectra were recorded on a Bruker Tensor 27
spectrometer equipped with a Harrick ATR instrument. Elemental
analyses were performed by the microanalytical laboratory of ICSN,
Gif-sur Yvette, France. Positive-mode electrospray ionization mass
spectrometry (ESI-MS) spectra were obtained using a triple-
quadrupole mass spectrometer (Quattro I Micromass). Automatic
data acquisition was processed using the software Masslynx 3.4. NMR
experiments were carried out on a Bruker Avance II 300 MHz or a
Bruker Avance III HD 400 MHz spectrometer operating at 297 K with
chemical shifts referenced to residual solvent peaks. Chemical shifts are
reported in parts per million (ppm) and coupling constants (J) in
hertz (Hz). Standard abbreviations indicating the multiplicity were
used as follows: m = multiplet, t = triplet, d = doublet, s = singlet, and
b = broad.
Synthesis of [[Pt(NO2)4]⊂Pd2(2)4][OTf]2 ([4][OTf]2). K2Pt-
(NO2)4 (32 mg, 0.07 mmol) was added to a solution of [3][OTf]3
(147 mg, 0.07 mmol) in CH3CN (5 mL). The solution was stirred at
room temperature for 30 min, during which time an orange precipitate
formed. The solids were collected by filtration, washed with diethyl
ether (3 × 5 mL), and dried under vacuum: 142 mg of orange solid
(93%). This supramolecular cage was recrystallized from low diffusion
of Et2O to a CH3CN/N,N-dimethylformaimde (DMF) solution,
affording quantitatively yellow crystals, and was characterized as
[4][OTf]2. IR (ATR; ν, cm−1): 3352 (NH2), 3074, 2201 (CC),
2140, 1590 (N−H), 1565, 1464, 1404, 1331 (NO2), 1311 (NO2),
1279, 1235 (OTf), 1218, 1161, 1130, 1027, 946, 855, 807, 741, 696,
Synthesis of 2,6-Diethynyl-4-methoxyaniline (1). A Schlenk
flask was charged with 2,6-dibromo-4-methoxyaniline (1 g, 3.55
mmol), CuI (67 mg, 0.355 mmol), and [Pd(PPh3)4] (410 mg, 0.355
mmol). The Schlenk flask was vacuum-purged and refilled with argon
(three times). Distilled tetrahydrofuran (THF; 7 mL) and Et3N (7
mL) were introduced. Then trimethylsilylacetylene (5.05 mL, 35.5
mmol) was added. The reaction mixture was then heated to 60 °C and
allowed to stir for 48 h. After cooling, solvents were removed under
vacuum, and the crude product was first purified by column
chromatography (hexane/AcOEt = 90/10) to afford a yellow oil. A
KOH/MeOH solution was then prepared (1 g into 25 mL) and added
to the crude mixture previously obtained. The solution was then
stirred overnight. The solvent was removed by reduced pressure, and
the product was purified via column chromatography (hexane/AcOEt
= 70/30): 590 mg of yellow solid (97%). IR (ATR; ν, cm−1): 3433
(NH2), 3328 (CC−H), 3265, 3004, 2947, 2829, 2094 (CC),
1682, 1620, 1593 (N−H), 1460, 1431, 1325, 1290, 1264, 1230, 1188,
1
636, 585, 549, 516, 420, 398, 344, 313, 295. H NMR (400 MHz,
DMSO-d6): δ 9.32 (d, J = 5.9 Hz, 8H, Hj), 9.22 (s, 8H, Hk), 8.22 (d, J
= 8.1 Hz, 8H, Hh), 7.80 (dd, J = 8.1 and 5.9 Hz, 8H, Hi), 7.08 (s, 8H,
Hb), 6.22 (bs, 8H, NH2), 3.69 (s, 12H, OMe) 13C NMR (100 MHz,
DMSO-d6): δ 151.9 (Ck), 150.1 (Cj), 149.9 (Ca), 146.0 (Cd), 142.5
(Ch), 127.6 (Ci), 123.0 (Cg), 119.9 (Cb), 105.7 (Cc), 91.9 (Ce), 89.8
(Cf), 55.9 (COMe). ESI-MS. Calcd for [[Pt(NO2)4]⊂Pd2(2)4]4+: m/
z) 946. 61. Found: m/z 946. 12. Anal. Calcd for
[C88H60F12N12O16Pd2S4]·2H2O: C, 46.35; H, 2.90; N, 10.06. Found:
C, 44.63; H, 2.72; N, 10.18.
X-ray Crystal Structure Determination of Metallacages
[3][OTf]3 and [4][OTf]2. Data were collected on a Bruker Kappa
APEXII. Determinations of the unit-cell parameters, data collection
strategy, and integration were carried out with the Bruker APEX2 suite
of programs. A multiscan absorption correction was applied.16 The
structures were solved using SIR9217 (3) and SUPERFLIP18 (4) and
refined anisotropically by full-matrix least-squares methods using
SHELXL-2013.19 Crystallographic data (excluding structure factors)
1
1140, 1051, 936, 873, 840, 609, 472, 349, 235. H NMR (300 MHz,
CD2Cl2): δ 6.95 (s, 2H, Hb), 4.60 (bs, 2H, NH2), 3.74 (s, 3H, OMe),
3.50 (s, 2H, Hf). 13C NMR (75 MHz, CD3CN): δ 150.9 (Ca), 145.5
(Cd), 119.6 (Cb), 107.4 (Cc), 83.2 (Ce), 80.1 (Cf), 56.3 (COMe). ESI-
MS. Calcd for [1H]+: m/z 172.07. Found: m/z 172.2. Anal. Calcd for
C11H9NO: C, 77.17; H, 5.30; N, 8.18. Found: C, 76.89; H, 5.33; N,
7.97.
Synthesis of 2,6-(3-Pyridylethynyl)-4-methoxyaniline (2). A
Schlenk flask was charged with 1 (500 mg, 2.9 mmol), CuI (55 mg,
0.29 mmol), [Pd(PPh3)4] (337 mg, 0.29 mmol), and 3-bromopyridine
(0.7 mL, 0.725 mmol). The Schlenk flask was vacuum-purged and
refilled with argon (three times). Distilled THF (8 mL) and Et3N (8
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Scheme 1. Preparation of the Assembling Ligand 2
Scheme 2. Synthesis of the Palladium-Based Metallacage [3][OTf]3
Figure 2. Partial ESI-MS spectrum illustrating the sequence of peaks corresponding to the intact metallacage [3][OTf]3 associated with different
numbers of triflate anions. Experimental and theoretical isotope distributions of [{[OTf⊂Pd2(2)4][OTf]2}]+.
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Figure 3. 19F NMR spectrum of the metallacage [3][OTf]3 recorded in acetone-d6 at 25 °C.
Figure 4. (a) X-ray crystal structure of the cationic part of [3][OTf]3·5CH3CN showing the triflate anion within the cavity. Color code: C, black; O,
red; N, blue; Pd, violet. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd1−Pd1
11.812(2), Pd−N1 2.012(2), Pd1−N4 2.031(2), Pd1−N3 2.028(3), Pd1−N6 2.022(3); N1−Pd1−N3 179.82, N4−Pd1−N6 178.49. (b) 2D
networks of metallacapsules [3][OTf]3·5CH3CN generated by π−π contacts among individual units.
for the structures reported in this paper have been deposited at the
Cambridge Crystallographic Data Centre as CCDC 949856 (3) and
949857 (4). These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk. See
the Supporting Information (SI).
Crystal Data for [3][OTf]3·5CH3CN: yellow crystals,
C H F N O Pd S , triclinic, P1, a = 13.0241(12) Å, b =
̅
98 75 12 17 16
2 4
13.3619(14) Å, c = 17.873(3) Å, α = 98.136(7)°, β = 97.312(7)°, γ
= 109.328(5)°, V = 2854.6(6) Å3, Z = 1, T = 200(1) K, μ = 0.473
mm−1, 71465 reflections measured, 18246 independent (Rint
=
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0.0414), 14624 observed [I > 2σ(I)], 674 parameters, final R indices
R1 [I > 2σ(I)] = 0.0720 and wR2 (all data) = 0.2394, GOF (on F2) =
1.077, max/min residual electron density = 2.632/−1.196 e Å−3.
Crystal Data for [4][OTf]2·3CH3CN·2H2O: yellow crystals,
average distance between two facing phenyl rings is 9.502(2) Å.
Furthermore, one triflate anion is present inside the cage cavity
through a weak hydrogen-bonding interaction with the amine
groups, while the three other anions are disordered and located
outside the cavity. Of interest, we note further π−π interactions
between the central ring and the acetylene groups of two M2L4
neighbors to generate a 2D network constituted of 3D capsules
and in a manner similar to that reported by Hooley and co-
workers20a for a metallacage with endohedral amine groups
(Figure 4b). It is well established that noncovalent π−π
interactions maintain the coherence of coordination assemblies
with different geometrical forms and influence their proper-
ties.20c,d
On the other hand, the presence of an encapsulated anion
inside the cavity led us to investigate the host−guest properties
of this metallacage with endohedral amine groups using more
challenging guest molecules.
We reasoned that recognition could be enhanced through
synergetic interactions (mainly hydrogen bonding) provided by
the amine groups pointing toward the interior of the cavity and
electrostatic interactions of the positively charged palladium(II)
cations. Thus, the metal complex guest [Pt(NO2)4]2− was
chosen. Only a few examples are known where a square metal
complex is encapsulated within a metallacage.10−13
C H F N O Pd PtS , triclinic, P1, a = 12.2300(5) Å, b =
̅
92 73
6
19 20
2
2
13.5899(5) Å, c = 16.4293(7) Å, α = 90.570(2)°, β = 100.104(2)°,
γ = 105.592(2)°, V = 2584.65(18) Å3, Z = 1, T = 200(1) K, μ = 1.819
mm−1, 118345 reflections measured, 15880 independent (Rint
=
0.0212), 13465 observed [I > 2σ(I)], 780 parameters, final R indices
R1 [I > 2σ(I)] = 0.0490 and wR2 (all data) = 0.1623, GOF (on F2) =
1.140, max/min residual electron density = 1.975/−1.807 e Å−3.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Assembling
Ligand 2. To demonstrate the viability of our strategy, we first
prepared ligand 2 from 2,6-dibromo-4-methoxyaniline through
a palladium/copper Sonogashira cross-coupling reaction to give
the related diacetylene aniline derivative 1 (Scheme 1). A
subsequent palladium/copper Sonogashira cross-coupling
reaction with bromopyridine provided the assembling ligand
2 in 73% overall yield, as fully characterized by IR, NMR, mass
spectrometry, and elemental analysis.20b
Synthesis and Characterization of Metallacages [3]-
[OTf]3 and [4][OTf]2. Ligand 2 was treated with freshly
prepared [Pd(CH3CN)4][OTf]2 in an acetonitrile solution to
afford the cage complex [3][OTf]3. The tetragonal cationic
cage was isolated and characterized as its triflate salt in 90%
yield (Scheme 2). The X-ray molecular structure confirmed the
presence of one encapsulated triflate anion (vide infra).
The IR spectrum of [3][OTf]3 shows the presence of triflate
anions at 1236 cm−1 and displays the alkyne stretching
transition at 2205 cm−1. The ESI-MS spectrum clearly indicates
the formation of [Pd2L4] species in association with varying
numbers of triflate counterions (Figure 2).
Indeed, when the metallacage compound [3][OTf]3 was
titrated with 1 equiv of K2[Pt(NO2)4] in DMSO-d6, immediate
1
and significant shifts were observed in the H NMR spectrum,
which suggested the formation of a novel complex of
stoichiometry {[Pt(NO2)4]⊂[Pd2(2)4][OTf]2} ([4][OTf]2;
Figure S2 in the SI). For instance, the signals attributed to
hydrogen atoms Hj and Hk moved from 9.45 and 9.23 ppm in
the metallacage [3][OTf]3 to 9.32 and 9.22 ppm in [4][OTf]2.
1
Moreover, no changes in the H NMR spectrum of [4][OTf]2
occurred upon allowing the solution to stand over time,
suggesting that this host−guest system is kinetically and
thermodynamically stable.
Moreover, the molecular capsule was found to be stable both
in the solid state and in solution and to be soluble in common
polar solvents such as acetonitrile, acetone, and methanol. The
1H NMR spectrum of [3][OTf]3 was recorded in CD3CN and
displayed a symmetric pattern similar to that of the ligand 2.
However, we note that upon coordination to the palladium
center a downfield shift is observed for [3][OTf]3, particularly
for Hj, Hk, and the two N−H amino protons (Scheme 2 and
Figure S1 in the SI).
To confirm the formation of the host−guest system
[4][OTf]2, the reaction was repeated in CH3CN, and an
orange precipitate was then isolated and fully characterized as
[4][OTf]2. The IR spectra showed the triflate anion bands at
1235 cm−1 and the alkyne and nitro group stretching bands at
2201 cm−1 and 1311 and 1331 cm−1, respectively. The
existence of the assembly [4][OTf]2 in solution was
corroborated by ESI-MS. The 2+ charge state of the
{[Pt(NO2)4]⊂[Pd2(2)4]} fragment appears at m/z 946.12
and was verified by a comparison of the observed and
theoretical isotopic patterns (Figure 5).
X-ray Molecular Structure of [4][OTf]2. Gratifyingly,
using slow vapor diffusion of diethyl ether into an acetonitrile/
DMF (1:2) solution of [4][OTf]2, we were successful in
growing single crystals suitable for X-ray crystal structure
determination. The molecular cage [4][OTf]2 crystallized with
three molecules of CH3CN and two molecules of H2O. As
expected from the solution experiments, all M2L4 cages were
found to contain one molecule of the anionic square-planar
[Pt(NO2)4]2− complex (Figure 6).
Remarkably, the 19F NMR spectrum of [3][OTf]3 recorded
at room temperature in acetone-d6 showed two singlets in a
roughly 3:1 intensity ratio: a singlet at −78.7 ppm attributed to
free triflate anions and a weaker, broad signal at −75.8 ppm,
which we assign to the encapsulated anion (Figure 3). This
result confirms the presence of a triflate anion inside the cavity
in solution slowly exchanging on the NMR time scale and
corroborates the findings obtained by X-ray crystallography
(vide infra).21
X-ray Molecular Structure of [3][OTf]3. To ascertain the
molecular structure of [3][OTf]3, an X-ray structural
determination was carried out. Crystals were grown by vapor
diffusion of diethyl ether into a solution of the complex in
CH3CN. The molecular cage [3][OTf]3 crystallized as
The Pd−N bond lengths lie in the range of 2.073(1)−
2.034(1) Å, very close to that observed in the previous cage
[3][OTf]3. The Pd−Pd distance is 11.417(1) Å, slightly smaller
than that of [3][OTf]3. Moreover, unlike [3][OTf]3, the
distances between adjacent facing phenyl rings are non-
equivalent and lie in the range of 8.573(1)−10.815(1) Å,
which average to 9.694(1) Å. This might be due to the nature
5CH CN solvate in the triclinic space group P1. The structure
̅
3
shows the formation of a tetragonal cage, where each palladium
adopts a square-planar geometry as a result of coordination to
four pyridine arms of the bridging ligands 2 (Figure 4a). The
Pd−N bond distances lie in the range of 2.012−2.030(3) Å and
are comparable to those reported for related palladium
cages.4a,f,20 The Pd−Pd distance is 11.892(2) Å and the
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Interestingly, the two other NO2 units of the anionic guest
interact within the frame of the cage via hydrogen bonding to
the amino functions −NH2 (as hydrogen donor sites), while
the −NO2 oxygen atoms act as hydrogen acceptor sites. As
shown in Figure 6a, the hydrogen-bonding interactions with the
encapsulated guest involve the four endohedral amines. In
molecule 4b, two Pd−O interactions were also detected
involving −NO2 oxygen atoms, with a Pd−O bond distance
of 2.964 Å, as found in molecule 4a. However, stronger
hydrogen-bonding interactions were observed between the two
−NH2 groups of the cage and the two −NO2 oxygen atoms of
the guest. We note, in this case, that two of the capsule’s amino
groups are not engaged in the formation of hydrogen bonds
with the encapsulated guest (Figure 6b).
Density Functional Theory (DFT) Calculations on the
Host−Guest System [4][OTf]2. To shed light on the nature
of the host−guest interactions within [4a][OTf]2 and [4b]-
[OTf]2, DFT calculations were carried out. Geometry
optimizations and natural bond order analyses were carried
out on the two observed configurations by means of B3LYP
theory,22a,b using a cc-pVDZ basis set22c for the main-group
atoms and CEP-121G22d for the transition metals. Using the
structures determined by X-ray diffraction (XRD) as initial
geometries, the optimizations converged to structures qual-
itatively consistent with the two observed forms, 4a and 4b.
Gross features of the host cavity are well-predicted by DFT
calculations. In 4a, for example, the Pd−Pd separation of 11.5 Å
determined by XRD is predicted at 11.7 Å, and the calculated
and measured separations between amino nitrogen atoms on
opposite sides of the Pd−Pt−Pd axis agree to within 0.1 Å. For
4b, the macrocycle in XRD is more asymmetric than that in the
calculated structure, and one of the observed amino N−N
distances is larger by nearly 0.5 Å than the calculated value,
whereas the Pd−Pd distance and the other N−N distance from
experiment and from calculation agree to within 0.1 Å.
Figure 5. Calculated and observed isotope patterns for the intact
metallacage {[Pt(NO2)4]⊂[Pd2(2)4]2+ (4).
and size of the encapsulated square-planar metal guest.
Furthermore, the structure revealed the presence of two
molecules, 4a and 4b, in the unit cell in a 6:4 ratio. These
correspond to two different orientations adopted by the anionic
complex guest within the cavity, determined by the nature of
the host−guest interactions. For instance, in molecule 4a
(Figure 6a), [Pt(NO2)4]2− is located in the center of the cavity.
Each palladium center of the cage binds to the platinum
complex through a metal−oxygen coordination bond of the
NO2 moiety with a Pd−O bond distance of 2.964 Å.
Figure 6. X-ray molecular structure of [4][OTf]2·3CH3CN·2H2O showing the encapsulated metal complex [Pt(NO2)4]2− displaying two different
orientations in molecules 4a and 4b. Color code: C, black; O, red; N, blue; Pd, violet; Pt, gray. Hydrogen atoms and solvent molecules are omitted
for clarity. Selected bond lengths (Å) and angles (deg): Pd1−Pd1 11.417(1), Pd−N1 2.027(1), Pd1−N4 2.030(1), Pd1−N3 2.034(1), Pd1−N6
2.027(1); N1−Pd1−N3 179.82, N4−Pd1−N6 178.49. (a) Molecule 4a: Pt1A−N7 1.8816(1), Pt1A−N8A 1.9992(1), Pd1−O3 2.964. (b) Molecule
4b: Pt1B−N7 2.248(1), Pt1B−N8B 2.241(1), Pt1B−N9B 2.089(1), Pd1−O3 2.964.
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Inorganic Chemistry
Article
Figure 7. Representative oxygen atom lone pair and unoccupied Pd d orbitals in 4a. Color code: O, red; N, blue; C, gray; Pd and Pt, blue-green.
While the XRD and DFT geometries of the cage itself agree
well, the calculated large-scale geometries of the isolated
(effectively gas-phase) species differ perceptibly from the
structure determined by X-ray in the crystal. The largest of
these differences are roughly 10° deviations in the dihedral
angles formed by the planes of the two macrocycles that form
the cage. However, as these differences do not appear to greatly
impact the solvent cage geometry otherwise, we expect this gas-
phase model to still serve as an adequate model of the host−
guest interactions.
The relative energies of 4a and 4b are correctly predicted to
be similar, but with 4b more stable than 4a by 4.8 kcal/mol, the
observed relative abundances indicate that 4a should be slightly
more stable than 4b. Because the calculations neglect lattice
effects and vibrational corrections, an error in the calculated
relative energy of some 5 kcal/mol is not surprising.
For both configurations, at least two distinct binding
mechanisms are available to anchor the guest nitro groups:
(i) complexation to the host palladium atoms and (ii) hydrogen
bonding to the host amino groups. The geometries indicate
that 4a and 4b may each take advantage of both mechanisms
but to varying degrees. In the DFT structures, 4a aligns one
N−Pt−N axis at an angle of just 19° from the Pd−Pt−Pd axis,
attaining the minimum possible separation of 3.0 Å between
two of the nitro oxygen atoms and the neighboring palladium
atoms. The other two nitro groups are positioned with their
oxygen atoms at 2.1−2.2 Å from amino group hydrogen atoms,
to be stabilized by hydrogen bonding. In 4b, the guest is rotated
to give 45° angles of the N−Pt−N axes with respect to the Pd−
Pt−Pd axis. This positions the nitro groups so that six of the
eight oxygen atoms lie 2.0−2.3 Å from an amino hydrogen
atom. The remaining two oxygen atoms are part of a nitro
group, which has been twisted 90° to take advantage of oxygen
atom complexation to the palladium atom rather than the
available hydrogen bonding to the amino groups.
In summary, form 4a relies primarily on oxygen−metal
complexation for host−guest binding, whereas form 4b relies
primarily on hydrogen bonding. The orbital analysis of both
configurations find the energy differences between the lowest
unoccupied Pd d orbital and the highest occupied lone-pair
orbital on the neighboring oxygen to be only 811 kcal/mol,
consistent with favorable complexation. Two of the relevant
orbitals for form 4a are shown in Figure 7.
CONCLUSION
■
In conclusion, we have reported a member of a novel class of
palladium-based nanocages. This nanocage displayed two
complementary effects that operate in synergy for the
encapsulation of a kinetically labile metal complex, [Pt-
(NO2)4]2−. Single-crystal XRD measurements show that the
guest adopts two different orientations depending on the nature
of the host−guest interactions involved. DFT calculations
support our conclusions regarding the nature of the host−guest
interactions. These studies pave the way to a better
comprehension of chemical interaction and transformation
within confined nanospaces.
ASSOCIATED CONTENT
* Supporting Information
Detailed information in CIF format on the crystal structures of
[3][OTf]3 and [4][OTf]2, NMR spectra titration data (Figures
S1−S3), and DFT calculations (Tables S1 and S2). This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
*E-mail: christophe.desmarets@upmc.fr.
*E-mail: hani.amouri@upmc.fr.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank CNRS and UPMC for supporting this work. A.L.C.
thanks the NSF for equipment support.
■
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