Supramolecular Paradigm for Capture and Co-Precipitation of Gold(III) Coordination Complexes

Article abstract of DOI:10.1002/chem.202003680

A new supramolecular paradigm is presented for reliable capture and co-precipitation of haloauric acids (HAuX₄) from organic solvents or water. Two classes of acyclic organic compounds act as complementary receptors (tectons) by forming two sets of directional non-covalent interactions, (a) hydrogen bonding between amide (or amidinium) NH residues and the electronegative X ligands on the AuX₄^?, and (b) electrostatic stacking of the electron deficient Au center against the face of an aromatic surface. X-ray diffraction analysis of four co-crystal structures reveals the additional common feature of proton bridged carbonyls as a new and predictable supramolecular design element that creates one-dimensional polymers linked by very short hydrogen bonds (CO???OC distance <2.5 ?). Two other co-crystal structures show that the amidinium-π???XAu interaction will reliably engage AuX₄^? with high directionality. These acyclic compounds are very attractive as co-precipitation agents within new “green” gold recovery processes. They also have high potential as tectons for controlled self-assembly or co-crystal engineering of haloaurate composites. More generally, the supramolecular paradigm will facilitate the design of next-generation receptors or tectons with high affinity for precious metal square planar coordination complexes for use in advanced materials, nanotechnology, or medicine.

Full text of DOI:10.1002/chem.202003680

Accepted Article
Accepted Article
Title: Supramolecular Paradigm for Capture and Co-Precipitation of
Gold(III) Coordination Complexes
Authors: Bradley D. Smith, Cassandra C. Shaffer, Wenqi Liu, and Allen
G. Oliver
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202003680
Link to VoR: https://doi.org/10.1002/chem.202003680
01/2020

10.1002/chem.202003680
Chemistry - A European Journal
FULL PAPER
Supramolecular Paradigm for Capture and Co-Precipitation of
Gold(III) Coordination Complexes
Cassandra C. Shaffer, Wenqi Liu, Allen G. Oliver, and Bradley D. Smith*
Abstract: A new supramolecular paradigm is presented for reliable
capture and co-precipitation of haloauric acids (HAuX4) from organic
solvents or water. Two classes of acyclic organic compounds act as
complementary receptors (tectons) by forming two sets of directional
non-covalent interactions, (a) hydrogen bonding between amide (or
amidinium) NH residues and the electronegative X ligands on the
assembly of four-coordinate Au(III) complexes is harder to control
because the aurophilic interactions are inherently weaker.[8]
There is a particular need for new self-assembly methods that
capture and immobilize industrially important Au(III) compounds
such as chloroauric (HAuCl4) or bromoauric (HAuBr4) acid which
can be produced from ores and waste streams by straightforward
processes.[9] Emerging research suggests that “green”
haloaurate recovery methods can be developed by applying
modern supramolecular improvements to large scale extraction
or co-precipitation technologies.[10] Similarly, effective methods
for precise supramolecular recognition of haloaurate complexes
are likely to spur new approaches to core/shell gold
nanoaprticles,[11] spatially immobilized catalytic gold,[12] or
controlled delivery of medicinal gold.[13] Common to these
potential applications are supramolecular strategies that treat
anionic square planar AuX4- complexes as guest molecules or
building blocks (tectons),[14] with distinctive sizes, shapes and
electrostatic surfaces. In recent years, a number of macrocyclic
-
AuX4 , and (b) electrostatic stacking of the electron deficient Au center
against the face of an aromatic surface. X-ray diffraction analysis of
four co-crystal structures reveals the additional common feature of
proton bridged carbonyls as a new and predictable supramolecular
design element that creates one-dimensional polymers linked by very
short hydrogen bonds (CO···OC distance < 2.5 Å). Two other co-
crystal structures show that the amidinium-π···XAu interaction will
-
reliably engage AuX4 with high directionality. These acyclic
compounds are very attractive as co-precipitation agents within new
“green” gold recovery processes. They also have high potential as
tectons for controlled self-assembly or co-crystal engineering of
haloaurate composites. More generally, the supramolecular paradigm
will facilitate the design of next-generation receptors or tectons with
high affinity for precious metal square planar coordination complexes
for use in advanced materials, nanotechnology, or medicine.
[18]
host molecules, such as -cyclodextrin,[15,16] cucurbiturils,[17]
[19] and cationic cyclophanes,[20] have been reported as additives
that can induce co-precipitation of specific haloaurate salts. X-ray
Previous Work:
Introduction
t-Bu
Not only is gold a common precious metal in jewels and coins,
it is also employed increasingly within modern industries such as
telecommunications, nanotechnology, chemical synthesis, and
medicine. As a result, world demand for gold continues to
increase which raises interest in developing new methods of
extracting it from the earth or recovering it from societal waste
H-bond
donor
-
O
O
O
O
H
H
H
H
X
NH HN
+
-electron
donor
Au
NH HN
[2]
streams.[1] In addition, there is a growing need for advanced
X
H-bond
donor
materials that are comprised of gold containing nanocomposites
-
[4]
with nanoscale architecture.[3] A rational, bottom-up synthesis
t-Bu
of these nanocomposites has the attraction of precise atomic-
level control, but successful implementation requires self-
assembly paradigms that can produce predictable lattices and
+
(quite common)
Counter cation: H3O H2O
This Work:
super structures. In addition to the impressive advances in
[4] [5]
controlled assembly of elemental gold,
there is a scattered
R
R
body of work on self-assembly and crystal engineering of gold
coordination complexes. Most success has involved two-
coordinate Au(I) complexes which are known to regularly engage
in Au···Au and Au···π interactions that promote formation of
continuous atomic networks.[6] [7] [8] At present, programmed self-
-
H-bond
donor
X
R
H
H
X
NH
+
-electron
Au
donor
X
HN
H
H-bond
donor
H
-
[a]
C. C. Shaffer, Dr. W. Liu, Dr. A. G. Oliver, Dr. B. D. Smith
Department of Chemistry & Biochemistry
University of Notre Dame
236 Nieuwland Science Hall, University of Notre Dame, IN., 46545,
USA. Email: smith.115@nd.edu
O
R
R
R
+
(very rare)
C=O
H
O=C
Counter cation:
Scheme 1. Side-view of square planar AuX4- associated with macrocyclic
(top) or acyclic (bottom) compounds. Two sets of directional non-covalent
interactions (hydrogen bonding and Au aromatic stacking) are central to
the supramolecular paradigm.
Supporting information for this article is given via a link at the end of
the document
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10.1002/chem.202003680
Chemistry - A European Journal
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acyclic tectons that were easier to prepare on a large scale and
more amenable to conjugation. We hypothesized that effective
acyclic tectons must contain a combination of amide NH residues
O
CH3
NH
O
O
O
-
for strong directional hydrogen bonding to the X ligands of AuX4
80 °C
99%
and a π-electron rich aromatic surface to stack against the Au
center (Scheme 1).[23] Moreover, we reasoned that acyclic tectons
NH2
HN
H3C
1
based on the readily available starting compound, 1,4-
O
O
[24]
bis(aminomethyl)durene (BAMD),[21]
would produce suitably
preorganized molecular shapes that were complementary
(CH2)4CH3
NH
-
receptors for square planar AuX4 . Here, we report evidence that
H2N
O
strongly supports our hypothesis by disclosing three new acyclic
tectons that are very effective as co-precipitation compounds for
haloauric acids. We describe a series of six co-crystal structures
that each show hydrogen bonding and aromatic stacking as
reliable drivers of the haloauric acid phase transfer and solid-state
assembly. In addition, the crystal structures reveal two new types
of non-covalent interactions that further facilitate the rational
design of receptors or tectons for co-precipitation, nanocomposite
self-assembly, or crystal engineering of haloauric acids.
Cl
(CH2)4CH3
BAMD
Et3N, DCM
69%
HN
CH3(CH2)4
2
O
NHBoc
1. Boc Anhydride,
Et3N, CHCl3
BAMD
2. Acetyl chloride,
Et3N, CHCl3
HN
H3C
3
38%
O
Results and Discussion
1. TFA / CHCl3
2. 0.1 M NaOH
Cl-
The two symmetrical bis(amides) 1 and 2, and the unsymmetric
structure 5, were each prepared from BAMD using
straightforward synthetic chemistry (Scheme 2). The first set of
co-precipitation experiments mixed HAuX4 in dibutyl carbitol (a
common industrial organic solvent used often for gold
94%
NH2
H2N
CH3
• HCl
NH
NH
O
extraction[25]
) with a molar equivalent of the symmetrical
EtOH
0 °C
compounds 1 or 2. Small-scale gold recovery experiments were
conducted using gravimetric analysis as detailed in the
experimental section. Co-precipitation of the gold occurred almost
instantaneously with > 85% recovery (Figure 1).
HN
H3C
HN
H3C
Scheme 2. Synthesis of compounds 1 – 5.
4
98%
5
O
O
diffraction analysis of crystals from these co-precipitates has
revealed intricate networks of weakly attractive interactions such
as CH···XAu hydrogen bonds and ion-dipole interactions, and in
some cases hydrogen bonding with coordinated water molecules.
But the general picture from these crystal structures indicates no
common set of relatively strong, directional non-covalent
interactions that can be exploited to reliably induce a specific
super structure or lattice assembly.
Recently, our group reported that tetralactam macrocycles,
such as the structure shown in Scheme 1, were capable of co-
precipitating square planar complexes of MX4-n, where M = Au,
Pd, Pt and X = Cl, Br from organic solution.[21] X-ray crystal
structures of the co-precipitated complexes showed that, in each
Figure 1. Illustration of typical co-precipitation experiment.
-n
case, the square planar MX4 was encapsulated inside the
macrocyclic cavity. Present in each structure were two key non-
covalent interactions, (a) hydrogen bonding between the inward-
directed amide NH residues of the macrocycle and the
electronegative X ligands on the MX4-n, and (b) electrostatic
attraction between the two π-electron rich aromatic sidewalls of
the macrocycle and the electron deficient central M.[22] We
decided that a successful transition towards practical applications
required a move away from organic macrocycles and towards
Recrystallization of the co-precipitates yielded single crystals
that were suitable for analysis by X-ray diffraction. Shown in
Figure 2 are illustrations of four separate solid-state structures;
pure 1, 1•HAuCl4, 1•HAuBr4, 2•HAuCl4, and 2•HAuBr. In the
crystal-state, pure 1 adopts a slipped stacking motif in which each
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Figure 2 X-ray crystal structures of pure 1, 1•HAuCl4, 1•HAuBr4, 2•HAuCl4, and 2•HAuBr4. The red oval highlights a “proton bridged carbonyls” motif
(CO···H+···OC). For improved clarity, disorder is removed and there is inexact stoichiometry.
amide NH residue forms an intermolecular hydrogen bond with
the carbonyl oxygen of an adjacent molecule. The crystal
structures of 1•HAuCl4 and 1•HAuBr4 are very similar to each
other, with the two amide groups on the same durene ring
directed in opposite directions each forming a hydrogen bond with
a stacked tetrahaloaurate anion. The resulting lattice is a
continuous alternating stack of tetrahaloaurate anions and
durene rings. For the 1•HAuCl4 complex, the average NH···ClAu
distance is 2.97 Å; whereas, in the case of 1•HAuBr4 the average
NH···BrAu distance is 3.04 Å (the values refer to the H···X
distance, see Table S1 for further details about these hydrogen
bonds). The separation between parallel stacked durene rings is
7.069(3) Å for 1•HAuCl4 and 7.293(3) Å for 1•HAuBr4. Thus,
consistently found a repeating lattice of macrocycle/haloaurate
pairs bridged by a hydrated hydronium cation, a relatively
common lattice location for H+.[21,27] In striking contrast, a common
feature of the four co-crystal structures in Figure 2 is a naked H+
bridging the carbonyls of adjacent tectons (Figure 3). In each
case, this bridging proton was located in the Fourier difference
map and further refined as described in the Supporting
Information. The proton bridge creates a series of a linear, one-
dimensional polymers that stack to form sheets (Figures S23-
S26) that in turn are layered to create the three dimensional lattice.
Though uncommon, there are examples of proton bridged amide
[27] [28] [29] [30] [31]
carbonyls in the crystal structure literature.
A
recent computational modeling study by Love and coworkers
proposed this motif as part of a supramolecular complex that
-
compound 1 can form a co-crystal with AuCl4- or the larger AuBr4
-
without any major change in molecular conformation. This is not
case with compound 2 which has two longer hexamide chains
appended to a durene ring. The structure of 2•HAuBr4 is very
similar to those of 1•HAuX4 with the two amide groups directed in
opposite directions, but in the case of 2•HAuCl4 both amide
groups are directed to the same face of the common durene ring
and the lattice packing is altered to accommodates this
significantly very molecular shape. We attribute the change in
solid-state molecular conformation to small differences in crystal
formed when HAuCl4 was extracted by a simple primary amide
compound into an organic liquid phase,[32] and it was observed
-
packing energies caused by the size difference of AuCl4 and
-
AuBr4 . Overall, the four X-ray structures in Figure 1 confirm our
hypothesis that the hydrogen bonding and aromatic stacking
motifs shown in Scheme
1 are dominant intermolecular
interactions that reliably drive co-precipitation and co-crystal
crystal structure.[26]
An intriguing aspect with these four haloauric acid co-crystal
structures is the location of the H+ counter cation. Our previous
work on co-crystals that included tetralactam macrocycles
Figure 3. Selected average crystal structure bond lengths (Å).
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Figure 4. X-ray co-crystal structures of
5•HAuCl4 and
5•HAuBr4, along with a schematic picture of the structures. Disorder removed for clarity.
within the X-ray structure of an analogous [SnCl6]2- complex.[31] It
is notable that the four co-crystal structures containing
bis(acetamide) 1 and bis(hexamide) 2 exhibit large differences in
solid state packing due to changes in molecular size and shape,
but in all cases the proton bridged amide carbonyl motif is
maintained indicating it is a very robust and reliable packing
interaction. Provided in Figure 3 is a comparison of bond
distances for the amide groups in each of the four co-crystal
structures as well as the crystal structure of pure bis(acetamide)
1. The average C=O bond length for the co-crystals is 1.272(8) Å,
which is significantly longer than the 1.236(2) Å for pure 1;
likewise, the average C-N bond length for the co-crystals is
1.308(9) Å, which is shorter than the 1.336(2) Å for pure 1. These
bond differences suggest that the amide groups in each of the co-
crystal structures have a higher fraction of the dipolar resonance
contributor (confirmed by comparing differences in the calculated
Mulliken charge distribution; see, Figure S37). The enhanced
amide polarization in the co-crystal structures is due to
organic solvents, we felt that co-precipitation from water was
likely to be more valuable as a potential gold recovery process.[40]
Thus, we synthetically substituted one of the neutral acetamide
groups in symmetrical 1 with an isosteric, cationic acetamidinium
group to create unsymmetrical compound 5 (Scheme 2). Co-
precipitation experiments added an equimolar amount of 5 to
separate aqueous solutions of HAuCl4 or HAuBr4 and in each
case a co-precipitate formed instantly with >85% recovery by
gravimetric analysis. Essentially identical co-precipitation results
were obtained with HAuCl4 in 0.1 M HCl, and subsequent
treatment of solid 5•HAuCl4 with Na2S2O5 or N2H2 produced gold
metal as a precipitate leaving 5 in the filtrate for recrystallization
and recycling.
Recrystallization of 5•HAuCl4 and 5•HAuBr4 yielded single
crystals that were suitable for analysis by X-ray diffraction. Shown
in Figure 4 is a representation of each crystal structure along with
a
schematic picture indicating the major non-covalent
interactions. Consistent with our supramolecular paradigm, each
-
-
cooperative hydrogen bonding with H+ at one end and AuX4 and
AuX4 in the crystal was stacked between two aromatic durene
the other end.[33,34] Thus, the co-crystals can be considered as
rings and the peripheral X ligands on the square planar AuX4
-
-
packed arrays of repeating [H+···amide···AuX4 ···amide···] units.
formed a network of hydrogen bonds with amide and amindinium
NH residues.[26] In addition, the X ligands were in close contact
with the methyl CH residues on an adjacent molecule and also
with the electron-deficient π-orbital of an amidinium group on a
second adjacent molecule. The latter contact is especially
interesting; although Lewis acid/base interactions between
electron rich ligands on metal complexes and π-acids are well
Another noteworthy fact in Figure 3 is the extremely short average
distance between proton bridged carbonyl oxygens which is
2.432(6) Å. Historically, a CO···OC distance < 2.5 Å has been
called a short or low-barrier hydrogen bond, and there is
considerable evidence that the free energy of formation in the
solid state is in the range of -15 to -40 kcal mol−1.[35] [36] At present,
short hydrogen bonds are not rationally incorporated into the
designs of supramolecular materials, but the results shown in
Figures 2 and 3 highlight the usefulness of haloauric acids as a
source of “anhydrous H+” to induce the proton bridged carbonyl
motif.[29,37,38] A recent study demonstrated that the motif persists
in the solution state;[27] thus, it seems plausible that bis(amides)
like 1 and 2 can be employed to produce new classes of self-
assembled polymers with “proton bridged carbonyls” as a
reversible linker with very high bond strength.[39]
X
X
X
X
Au
CH3
NH2
HN
X
X
X
Au
X
Figure 5. Amidinium group as L-shaped heteroditopic tecton that
simultaneously forms directional amidinium-NH···XAu and amidinium-
π···XAu interactions oriented at about 90°.
Although it is conceivable that compounds 1 and 2 could be
used practically for co-precipitation of haloauric acids from
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[20] [23]
documented,[41] Au-X···π interactions are quite rare.[8]
Moreover, each amidinium group in Figure
presented in ppm and referenced by residual solvent peak. High-resolution
mass spectrometry (HRMS) was performed using a Bruker micro TOP II
spectrometer. Column chromatography was performed using Biotage
SNAP Ultra columns with silica gel. 1,4-Bis(aminomethyl)durene, BAMD,
was synthesized according to a previously published procedure.[21]
4 acts as a
heteroditopic L-shaped bridge that simultaneously engages one
-
-
AuX4 by an amidinium-NH···XAu interaction and a second AuX4
by an amidinium-π···XAu interaction with the vectors of the two
non-covalent interaction oriented at about 90° (Figure 5). [26] Like
the structures described above, the lattice packing for 5•HAuCl4
and 5•HAuBr4 can be viewed as a stack of one-dimensional
polymers, this time in a zig-zag orientation with each tecton
connected by a hydrogen bond between an amidinium-NH and
an amide carbonyl oxygen on an adjacent molecule (Figures S27-
S30). This hydrogen bonded polymer network promotes the self-
assembly and co-crystallization processes.
Synthesis
Compound 1
BAMD,[21] (0.1 g, 0.52 mmol) was stirred in acetic anhydride at 80 °C
overnight. The solvent was removed to yield 1 as an off-white solid (0.141
g, 99%). 1H-NMR (400 MHz, CDCl3/MeOD): δ 4.44 (s, 4H), 2.26 (s, 12H),
1.97 (s, 6H) ppm. 13C-NMR (400 MHz, CDCl3/MeOD): δ 170.9, 133.6,
133.0, 39.0, 22.0, 15.8 ppm. HRMS (ESI): calcd for C16H25N2O2 [M+H]+
277.1911, found 277.1907.
Compound 2
BAMD[21] (0.1 g, 0.52 mmol) and triethylamine (0.11 mL, 0.78 mmol) were
combined in DCM (6 mL) with stirring. Hexanoyl chloride (0.21 mL, 1.56
mmol) was then added to the flask, upon which a white precipitate formed.
The reaction mixture was allowed to stir for one hour at room temperature
before the solid was collected by vacuum filtration. The pure product 2 was
obtained as a white solid in 69% yield (0.14 g). 1H-NMR (400 MHz, CDCl3):
δ 5.18 (s, 2H), 4.42 (d, J = 4.5 Hz, 4H), 2.20 (s, 12H), 2.08 (t, J = 7.6 Hz,
4H), 1.62-1.40 (m, 4H), 1.28-1.17 (m, 8H), 0.81 (t, J = 6.9 Hz, 6H) ppm.
13C-NMR (400 MHz, CDCl3): δ 172.9, 134.2, 133.9, 39.3, 36.7, 31.5, 25.5,
22.4, 16.4, 14.0 ppm. HRMS (ESI): calcd for C24H41N2O2 [M+H]+ 389.3163,
found 389.3166.
Conclusions
The results of this study strongly support
a
new
supramolecular paradigm for reliable capture and co-precipitation
of HAuX4 from organic solvents or water. Two families of acyclic
organic compounds were designed to act as complementary
supramolecular receptors (tectons) for HAuX4. They all induce co-
precipitation of HAuX4 by providing a combination of amide (or
amidinium) NH residues for hydrogen bonding to the X ligands of
AuX4- and a π-electron rich aromatic surface to stack against the
electron-deficient Au center (Scheme 1).[22] X-ray diffraction
analysis of four co-crystal structures that include bis(amides) 1 or
2 show the additional common feature of proton bridged
carbonyls (CO···H+···OC) as a new way to create linear
supramolecular polymers with very high hydrogen bond strength.
Likewise, two co-crystal structures that include the unsymmetrical
amidinium 5 reveal lattice packing of linear hydrogen bonded
polymers. In addition, the amidinium-π···XAu interaction is
observed as a new L-shaped heteroditopic tecton with high
directionality. The convenient synthesis of these acyclic
compounds, or their conjugates, makes them very attractive for
incorporation within “green” gold recovery processes.
Furthermore, they have high potential as tectons for controlled
self-assembly or co-crystal engineering of composites containing
HAuX4. Equally important, the underlying supramolecular
paradigm can be exploited as a general molecular design
platform that produces next-generation receptors or tectons with
high affinity for Au(III) or closely related precious metal square
planar coordination complexes for use in advanced materials,
nanotechnology, or medicine.[4] [11] [12] [13]
Compound 3
BAMD[21] (0.4 g, 2.08 mmol) was suspended in CHCl3 (150 mL) with
triethylamine (1 mL). Boc anhydride (0.454 g, 2.08 mmol) was dissolved
in CHCl3 (50 mL) and added slowly by addition funnel. The reaction
mixture was allowed to stir overnight before the solvent was removed
under reduced pressure. The resulting crude residue was purified by
column chromatography (SiO2, 0-10% MeOH in CHCl3) to yield an off-
white solid (40-50% yield). This intermediate compound, t-butyl (4-
(aminomethyl)-2,3,5,6-tetramethylbenzyl)carbamate (1.66 g, 5.68 mmol),
was dissolved in CHCl3 (50 mL) with triethylamine (0.8 mL). Acetyl chloride
(0.45 mL, 6.3 mmol) was added to the reaction flask which was then
allowed to stir overnight. The solvent was removed under reduced
pressure and the resulting crude solid was purified by column
chromatography (SiO2, CHCl3) to yield 3 as a pale yellow solid (1.45 g,
76% yield). 1H-NMR (400 MHz, CDCl3): δ 5.18 (s, 1H), 4.43 (s, 2H), 4.32
(s, 2H), 2.22 (s, 6H), 2.20 (s, 6H), 1.91 (s, 3H), 1.38 (s, 9H) ppm. 13C-NMR
(400 MHz, CDCl3): δ 169.7, 155.5, 134.4, 134.1, 134.0, 133.6, 79.4, 40.0,
39.4, 28.4, 23.1, 22.4, 16.4, 16.4 ppm. HRMS (ESI): calcd for C19H30N2O3
[M+H]+ 335.2203, found 335.2219.
Compound 4
Compound 3 (1.45 g) was stirred in 20% TFA in CHCl3 (60 mL) for 2 hours
before the solvent was removed. The product was partitioned between
NaOH (0.1 M, 90 mL) and DCM (80 mL). The aqueous phase was washed
with DCM (3 x 50 mL) then the organic phases were combined, dried over
MgSO4, and removed by evaporation to yield 4 as an off-white solid (0.96
g, 94%). 1H-NMR (400 MHz, MeOD): δ 4.43 (s, 2H), 4.22 (s, 2H), 2.34 (s,
6H), 2.28 (s, 6H), 1.93 (s, 3H) ppm. 13C-NMR (400 MHz, MeOD): δ 171.6,
134.7, 134.3, 133.9, 130.5, 398.7, 37.9, 20.8, 15.2 ppm. HRMS (ESI):
calcd for C14H23N2O [M+H]+ 235.1805, found 235.1822.
Experimental Section
Materials
All chemicals and solvents were purchased as reagent grade and used
without further purification unless otherwise noted. Chloroauric acid was
purchased from Oakwood Chemical while bromoauric acid was purchased
from Strem Chemicals. Reactions were monitored by analytical thin-layer
chromatography (TLC) on silica gel 60-F254 plates, visualized by
ultraviolet (254, 365 nm). NMR spectra (1H, 13C) were recorded on Bruker
AVANCE III HD 400 MHz spectrometer at 25 °C. Chemical shift was
Compound 5
Compound 4 (0.96 g, 4.08 mmol) was stirred in ice-cold EtOH (50 mL).
Ethyl acetimidate hydrochloride (0.6 g, 4.9) was added, upon which the
reaction solution became opaque. The reaction was left to stir overnight
before the solvent was removed. The crude solid was resuspended in H2O
and washed with Et2O (4 x 60 mL). The aqueous layer was removed by
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10.1002/chem.202003680
Chemistry - A European Journal
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evaporation to yield 5 as a white solid (1.25 g, 98%). 1H-NMR (400 MHz,
D2O): δ 4.28 (s, 2H), 4.23 (s, 2H), 2.18 (s, 6H), 2.11 (s, 6H), 2.10 (s, 3H),
1.86 (s, 3H) ppm. 13C-NMR (400 MHz, MeOD): δ 171.5, 164.7, 135.1,
134.5, 134.4, 134.1, 133.8, 129.7, 129.3, 42.0, 38.7, 37.7, 20.8, 17.1, 15.3,
15.2, 15.2, 15.1 ppm. HRMS (ESI): calcd for C16H25N3O[M+H]+ 276.2070,
found 276.2061.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
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Gold Co-Precipitation
The organic-soluble co-precipitation experiments were performed as
follows. Stock solutions were prepared of 1 or 2 (4 mM) in CHCl3, and
HAuCl4 or HAuBr4 (4 mM) in dibutyl carbitol. An aliquot (0.3 mL) of the
receptor solution was combined with an equal volume of each gold
solution in a centrifuge tube, and the solution became cloudy almost
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give a black precipitate containing the gold metal.
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Acknowledgements
DOI 10.1021/acsami.0c09673.
We are grateful for funding support from the NSF
(CHE1708240).
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with the electropositive Au atom. The methyl CH residues
also provide a periphery of partial positive charge for
Conflict of Interest
The authors declare no conflict of interest.
Keywords: Supramolecular chemistry • Host-Guest systems •
Self-assembly • Crystal Engineering • Gold
-
electrostatic interaction with the X ligands of AuX4 .
[22]
The attractive electrostatic interaction between the Au
center and aromatic surface is not a classic metal cation-π
interaction because the four X ligands are covalently
bonded to the Au center. See: K. Theilacker, H. B.
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This article is protected by copyright. All rights reserved.

10.1002/chem.202003680
Chemistry - A European Journal
FULL PAPER
FULL PAPER
Acyclic organic compounds reliably form a set of directional non-covalent interactions
with HAuX4 that enable gold recovery or crystal engineering of gold composites.
Cassandra C. Shaffer, Wenqi Liu, Allen
G. Oliver, and Bradley D. Smith*
Page No. – Page No.
H
R
Supramolecular Paradigm for Capture
and Co-Precipitation of Gold(III)
Coordination Complexes
R
H-bond
donor
H
d+
Au
d-
d-
X
X
H-bond
donor
H
R
R
+
H
p-electron
donor
C=O
H
O=C
(very rare)
This article is protected by copyright. All rights reserved.