Reversible Self-Assembly of Water-Soluble Gold(I) Complexes

Article abstract of DOI:10.1021/acs.inorgchem.7b02343

The reaction of the gold polymers containing bipyridyl and terpyridyl units, [Au(C≡CC₁₅H₁₀N₃)]_n and [Au(C≡CC₁₀H₇N₂)]_n, with the water-soluble phosphines 1,3,5-triaza-7-phosphatricyclo[3.3.1.13.7]decane and 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane gives rise to the formation of four gold(I) alkynyl complexes that self-assemble in water (H₂O) and dimethyl sulfoxide (DMSO), through different intermolecular interactions, with an impact on the observed luminescence displayed by the supramolecular assemblies. A detailed analysis carried out by NMR studies performed in different DMSO/deuterated H₂O mixtures indicates the presence of two different assembly modes in the aggregates: (i) chain assemblies, which are based mainly on aurophilic interactions, and (ii) stacked assemblies, which are based on Au···π and π···π interactions. These different supramolecular environments can also be detected by their intrinsic optical properties (differences in absorption and emission spectra) and are predicted by the changes in the relative binding energy from density functional theory calculations carried out in DMSO and H₂O. Small-angle X-ray scattering (SAXS) experiments performed in the same mixture of solvents are in agreement with the formation of aggregates in all cases. The aromatic units chosen, bipyridine and terpyridine, allow the use of external stimuli to reversibly change the aggregation state of the supramolecular assemblies. Interaction with the Zn²⁺ cation is observed to disassemble the aggregates, while encapsulating agents competing for Zn²⁺ complexation revert the process to the aggregation stage, as verified by SAXS and NMR. The adaptive nature of the supramolecular assemblies to the metal-ion content is accompanied by significant changes in the absorption and emission spectra, signaling the aggregation state and also the content on Zn²⁺.

Full text of DOI:10.1021/acs.inorgchem.7b02343

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Article
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
pubs.acs.org/IC
Reversible Self-Assembly of Water-Soluble Gold(I) Complexes
†
‡
†
§
∥
Elisabet Aguilo,
́
Artur J. Moro, Raquel Gavara, Ignacio Alfonso, Yolanda Per
́
ez,
⊥
⊥,#
,†,∇
¶
†
†
Francesco Zaccaria, Cel
Joao Carlos Lima,* and Laura Rodríguez*
̃
Departament de Química Inorgan
8028 Barcelona, Spain
́
ia Fonseca Guerra, Marc Malfois, Clara Baucells, Montserrat Ferrer,
,‡
†
̀ ̀ ́ ̀ ̀
ica i Organica, Seccio de Química Inorganica, Universitat de Barcelona, Martí i Franques 1-11,
0
‡
LAQV-REQUIMTE, Departamento de Química, CQFB, Universidade Nova de Lisboa, 2829-516 Monte de Caparica, Portugal
§
∥
Departamento de Química Biolog
́
ica y Modelizacion
́
Molecular and NMR Facility, IQAC−CSIC, Jordi Girona 16−26, E-08034
Barcelona, Spain
⊥
Department of Theoretical Chemistry, Amsterdam Center for Multiscale Modeling, 1081 HV Amsterdam, The Netherlands
#
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, 2311 EZ Leiden, The Netherlands
¶
̀
ALBA Synchrotron Light Laboratory (CELLS), Carrer de la Llum 2−26, 08290 Cerdanyola del Valles, Barcelona, Spain
Institut de Nanocien
∇
2
̀
cia i Nanotecnologia (IN UB), Universitat de Barcelona, 08028 Barcelona, Spain
*
S Supporting Information
ABSTRACT: The reaction of the gold polymers containing bipyridyl
and terpyridyl units, [Au(CCC H N )] and [Au(C
1
5
10
3
n
CC H N )] , with the water-soluble phosphines 1,3,5-triaza-7-
1
0
7
2
n
phosphatricyclo[3.3.1.13.7]decane and 3,7-diacetyl-1,3,7-triaza-5-
phosphabicyclo[3.3.1]nonane gives rise to the formation of four
gold(I) alkynyl complexes that self-assemble in water (H O) and
2
dimethyl sulfoxide (DMSO), through different intermolecular
interactions, with an impact on the observed luminescence displayed
by the supramolecular assemblies. A detailed analysis carried out by
NMR studies performed in different DMSO/deuterated H O mixtures
2
indicates the presence of two different assembly modes in the
aggregates: (i) chain assemblies, which are based mainly on aurophilic interactions, and (ii) stacked assemblies, which are based
on Au···π and π···π interactions. These different supramolecular environments can also be detected by their intrinsic optical
properties (differences in absorption and emission spectra) and are predicted by the changes in the relative binding energy from
density functional theory calculations carried out in DMSO and H O. Small-angle X-ray scattering (SAXS) experiments
2
performed in the same mixture of solvents are in agreement with the formation of aggregates in all cases. The aromatic units
chosen, bipyridine and terpyridine, allow the use of external stimuli to reversibly change the aggregation state of the
2+
supramolecular assemblies. Interaction with the Zn cation is observed to disassemble the aggregates, while encapsulating agents
2
+
competing for Zn complexation revert the process to the aggregation stage, as verified by SAXS and NMR. The adaptive nature
of the supramolecular assemblies to the metal-ion content is accompanied by significant changes in the absorption and emission
2+
spectra, signaling the aggregation state and also the content on Zn .
INTRODUCTION
phosphine (compounds A and B in Chart 1), self-assemble
to form long fibers and hydrogels through intermolecular π-
■
The self-assembly of small molecules by the establishment of
noncovalent interactions has received great attention in the past
decade as a way to build supramolecular structures with a large
number of specific functions and morphologies.
supramolecular chemistry has matured from a conceptually
8
,9
stacking and metallophilic interactions.
Replacing the
pyridine moiety by a different aromatic unit such as coumarin
(compound C in Chart 1) does not hamper the assembling
1
−4
As a result,
1
0
ability of these molecules. On the other hand, the
introduction of a positive charge on the chemical structure
either at the phosphine (complexes 1 and 4 in Chart 1) or at
the pyridyl unit (complexes 2, 3, 5, and 6) switches the fibrillary
self-assembly into rods, spherical vesicles, or squarelike
marvelous scientific curiosity to a technologically relevan5t
science encompassing a broad area of advanced materials.
Within this field, gold(I) complexes represent an emerging area
of investigation in the last years because they show weak Au ···
Au interactions that can modulate and govern the resulting
I
11
I
morphologies.
assemblies and properties in very different potential applica-
6
,7
tions. It was recently shown that small molecules of the type
Received: September 12, 2017
R-gold(I) alkynylpyridine, where R is a water-soluble
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02343
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Chart 1. Molecular Structures of the Gold(I) Alkynyl Complexes Previously Studied
Novel supramolecular nanostructures constituted by the
noncovalent self-organization of small metallophilic molecules
with tunable properties are underexplored within the field of
self-assembling materials. Research interest has increased in the
last years in the construction of stimuli-responsive metal-
losupramolecular architectures with diverse sizes, shapes, and
symmetries because of their promising applications such as
molecular transporting devices, chemosensors, optoelectronic
devices, drug delivery, tissue engineering, biomaterials, surface
1% H
2
O in deuterated H
2
O (D
2
O). Samples 2a and 2b were dissolved
in the corresponding deuterated solvent containing 3-(trimethylsilyl)-
propionic-2,2,3,3-d acid sodium salt (TSP-d ). TSP-d was used as an
4
4
4
1
13
internal standard both for calibrating the H and C chemical shifts
and for estimating the relative concentration changes upon
aggregation. Bruker TopSpin 3.5pl6 standard pulse sequences were
used for the 1D and 2D experiments. To obtain the highest receiver
gain and to apply the minimum amount of scans and increments using
low sample concentrations (≤1 mM), the reduction of residual water
in the NMR spectra (coming from the sample and/or on deuterated
solvents) was needed, so pulse sequences with a soft suppression of
the water resonance were used. 1D 1H NMR experiments were
acquired using the zgesgppe pulse sequence, which incorporates water
suppression using excitation sculpting with gradients and perfect echo.
5
,12−16
science, displays, etc.
Stimuli-responsive systems are
capable of modulating their behavior in a differential manner
depending on the external stimulus to which they are exposed,
being a considerable challenge for supramolecular chemists.
The modular chemistry of gold(I) alkynyl complexes,
furthermore, allows for a rational design where solubility and
function can be added to program the supramolecular assembly
into nanostructures that can be tuned by changes in the charge
or polarity, among others.
In this work, we extend the development of these small
molecules by adding a chelating unit able to trigger the
assembly/disassembly process by external stimuli (e.g., cations
and chelating molecules). At the same time, the increase in the
number of aromatic rings will allow a change in the balance
between Au···Au and π···π interactions, which will impact the
stability of the aggregates. Complexation with cations will
significantly change the electrostatic balance of the assemblies,
leading to dissociation. The disassembly can be reversed by the
addition of molecules that compete for the metal cation, leading
to reassembly of the nanostructures.
1
1
1
1
For 2D H− H NMR experiments, excitation sculpting for H− H
NOESY (noesyesgpph pulse program) and 3−9−19 pulse train for
DOSY (stebpgp1s19 pulse program) were applied for water
suppression. 2D 1H−13C (HSQC and HMBC) experiments were
also acquired. Spin−lattice (T ) and spin−spin (T ) relaxation times
1
2
were measured for complex 2b using a standard inversion−recovery
2
1
pulse sequence for T and PROJECT-CPMG for T .
1
2
2
2
2D DOSY experiments were performed using a stimulated-echo
sequence incorporating bipolar gradient pulses and one spoil gradient.
The gradient strength was logarithmically incremented in 18−24 steps
from 5 to 98% of its maximum value. Diffusion times and gradient
pulse durations were optimized for each experiment. Typically,
diffusion times between 60 and 120 ms and bipolar rectangular
gradient pulses of 2.0 ms were employed. Diffusion NMR data were
23
processed and represented using the free software DOSY Toolbox.
The translational diffusion coefficient obtained for different samples
was used to estimate the “apparent” sizes of the gold(I) complexes in
different solvent compositions. For calculation of the hydrodynamic
radii, the diffusion coefficients were obtained from the monoexpo-
nential fitting of the integral values versus gradient strength using the
TopSpin 3.5pl6 relaxation module. When calculating and interpreting
the values of the hydrodynamic radii, we have taken into account two
facts: (1) the Stokes−Einstein equation is only valid for spherical
particles with sizes larger that the solvent molecules; (2) the effects of
chemical exchange in diffusion spectra. In our case, for DMSO-d6
and DMSO-d with low amounts of D O, we expected to have mainly
monomer or dimer molecules in solutions, so our gold(I) complexes
have slightly larger sizes than the solvent. Therefore, for our r
calculations, we followed the method described by Macchioni et al.
using the expression of factor c as a function of rsolv/r derived by
EXPERIMENTAL SECTION
General Procedures. All manipulations have been performed
■
under prepurified N using standard Schlenk techniques. All solvents
2
have been distilled from appropriated drying agents. Commercial
reagents 1,3,5-triaza-7-phosphatricyclo[3.3.1.13.7]decane (PTA; 97%,
Aldrich) and 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane
2
2
(
DAPTA; 97%, Aldrich) have been used as received. Literature
6
2
1
7
methods have been used to prepare 5-ethynyl-2,2′-bipyridine, 4′-
1
8
19
ethynyl-2,2′,6′,2′′-terpyridine,
[Au(CCC H N )] ,
and
H
1
5
10
3
n
24
20
[
Au(CCC H N )] .
10
7
2
n
Physical Measurements. IR spectra have been recorded on a
H
2
5
1
Nicolet 520 Fourier transform infrared spectrophotometer.
H
Chen et al. and the approximation that our complexes were
spherical.
1
[
δ(TMS) = 0.0 ppm] and 31P{ H} [δ(85% H PO ) = 0.0 ppm]
3 4
NMR spectra have been obtained on Varian Mercury 400 and Bruker
00 spectrometers. One- and two-dimensional (1D and 2D) NMR
experiments were performed also at 298 K on a 500 MHz Bruker
AVANCEIII-HD spectrometer equipped with a z-gradient (65.7 G/
cm) inverse TCI cryoprobe. The gradient strength was calibrated using
Positive-mode electrospray ionization mass spectrometry [ESI-
MS(+)] spectra were recorded on a Fisons VG Quatro spectrometer.
Absorption spectra were recorded on a Varian Cary 100 Bio UV
spectrophotometer and emission spectra on Horiba-Jobin-Ybon SPEX
Fluorolog 3.22 and Nanolog spectrofluorimeters. Fluorescence
microscopy was recorded on a Zeiss Axioplan 2ie imaging microscope
equipped with a Nikon DXM1200F digital camera and a Leica DMIRB
4
the diffusion coefficient of water (H O) in a standard solution of 0.1
2
mg/mL GdCl , 0.1% 4,4-dimethyl-4-silapentane-1-sulfonic acid, and
3
B
DOI: 10.1021/acs.inorgchem.7b02343
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
fluorescence microscope. Optical microscopy images were acquired on
a Leica ICC50 W microscope. Dynamic light scattering (DLS)
measurements were carried out on a Horiba SZ-100 nanoparticle
analyzer instrument operating at 22 °C.
mL), and n-hexane (10 mL) was added to precipitate a pale-yellow
solid (82 mg, 85%). 31P NMR (161.9 MHz, CDCl , ppm): −23.0. H
1
3
NMR (400 MHz, CDCl , ppm): 8.75 (dd, J = 4.0 Hz, J = 0.6 Hz, 1H,
3
H ), 8.66 (dd, J = 8.0 Hz, J = 0.8 Hz, 1H, H ), 8.37 (dt, J = 12.0 Hz, J
9
6
Small-Angle X-ray Scattering (SAXS). SAXS was performed on
the NCD beamline at the ALBA Synchrotron at 12.4 keV, and the
distance sample/detector was 2.2 m to cover the range of momentum
= 0.8 Hz, 1H, H ), 8.32 (dd, J = 11.8 Hz, J = 0.4 Hz, 1H, H ), 7.82
1
2
3
(m, 2H, H + H ), 7.28 (ddd, J = 12.0 Hz, J = 7.2 Hz, J = 0.6 Hz, 1H,
4
11
H ), 5.78 (d, J = 20.0 Hz, 1H, H , NCH N), 5.66 (dd, J = 20.0 Hz, J
10
19a
2
−1
transfer of 0.09 < q (nm ) < 5.6. The data were collected on an
= 12.0 Hz, 1H, H23a, NCH P), 4.94 (d, J = 16.0 Hz, 1H, H21a,
2
2
ImXPad S1400 detector with a pixel size of 130.0 × 130.0 μm . The
NCH
2
N), 4.70−4.64 (m, 2H, H17a, NCH
2
P, + H21b, NCH
P), 4.06 (d, J = 20.0 Hz,
P), 3.57 (dt, J = 20.0 Hz, J
2
N), 4.15
exposure time was 10 s. The q-axis calibration was obtained by
(dt, J = 20.0 Hz, J = 4.0 Hz, 1H, H17b, NCH
1H, H19b, NCH
= 4.0 Hz, 1H, H23b, NCH
2
26
27
measuring silver behenate. The program pyFAI was used to
integrate the 2D SAXS data into 1D data.
2
N), 3.87 (s, 2H, H24, NCH
2
−1
P), 2.17 (s, 6H, COCH ). IR (KBr, cm ):
2
3
The data were then subtracted by the background using PRIMUS
3425 (C−H), 2104 (CC), 1640 (CN). ESI-MS(+): m/z 606.132
28
+
+
software. The maximum particle dimension Dmax and the pair-
([M + H] ; calcd m/z 606.133), 1233.233 ([2M + Na] ; calcd m/z
1233.241).
2
9
distance distribution function P(r) were determined with GNOM.
The low-resolution structure of the aggregates was reconstructed ab
initio from the initial portions of the scattering patterns using the
Computational Details. All quantum-level calculations were
31
performed with the Amsterdam Density Functional (ADF) program
3
0
program DAMMIN.
using the dispersion-corrected density functional ZORA-BLYP-D3-
Solutions of 1 × 10 and 1 × 10−5 M of complexes 1a, 1b, 2a, and
b were prepared in different H O/dimethyl sulfoxide (DMSO)
−4
32
(BJ). A previous benchmark study on noncovalent interactions
33
2
showed that it is essential to incorporate the dispersion correction.
2
mixtures of 0% H O to 100% (0, 25, 50, 75, and 90%) DMSO.
Geometry optimizations of monomers and dimers were obtained with
the TZ2P basis set. Geometry optimizations of tetramers were
obtained with the TZP basis set, and the bond energies were
computed with the TZ2P basis set (single point with the optimized
structures). The small frozen-core approximation was applied.
Symmetry was applied and is denoted in Table 1 and in the
2
Synthesis of [Au(CCC H N )(PTA)] (1a). Solid PTA (21 mg,
1
5 10 3
0
.13 mmol) was added to a suspension of [Au(CCC H N )] (60
15
10
3
n
mg, 0.13 mmol) in CH Cl (20 mL). After 45 min of stirring at room
2 2
temperature, the resulting pale-yellow solution was concentrated (5
mL), and n-hexane (15 mL) was added to precipitate a pale-yellow
solid (66 mg, 81%). 31P NMR (161.9 MHz, CDCl , ppm): −12.8. H
1
Supporting Information (SI). Solvent effects in H
O and DMSO were
3
2
34
NMR (400 MHz, CDCl , 298 K, ppm): 8.62 (dq, J = 4.8 Hz, J = 0.8
estimated using the conductor-like screening model (COSMO), as
3
3
5,36
Hz, 2H, H ), 8.53 (dt, J = 12.0 Hz, J = 1.2 Hz, 2H, H ), 8.46 (s, 2H,
implemented in the ADF program.
According to work by Riley et
9
12
3
7
H + H ), 7.84 (td, J = 11.9 Hz, J = 1.4 Hz, 2H, H ), 7.30 (ddd, J =
al., the dispersion correction does not need to be modified for
solvated systems. The bond energy ΔEbond of the dimers and tetramers
is defined by eqs 1 and 2, respectively:
3
5
11
1
2.0 Hz, J = 4.8 Hz, 2H, H ), 4.60−4.48 (AB q, J = 13.0 Hz, 6H,
10
−1
NCH N), 4.30 (s, 6H, NCH P). IR (KBr, cm ): 3425 (C−H), 2114
2
2
+
+
(
CC), 1636 (CN). ESI-MS(+): m/z 611.137 ([M + H ] ; calcd
+
+
a
b
m/z 611.139), 1243.250 ([2M + Na ] ; calcd m/z 1243.252).
Synthesis of [Au(CCC H N )(DAPTA)] (1b). Solid DAPTA (30
n
Table 1. Au−Au Distances (Å) and Bond Energies (kcal/
1
5
10
3
mol) of the Tetramers of 1a, 1b, 2a, and 2b in H O
2
mg, 0.13 mmol) was added to a suspension of [Au(CCC H N )]
15
10
3
(60 mg, 0.13 mmol) in CH Cl (20 mL). After 45 min of stirring at
Compound 1a
2
2
room temperature, the resulting pale-yellow solution was concentrated
stacked, C2h
double dimer, C2h
chain, C2
(
10 mL), and n-hexane (10 mL) was added to precipitate a pale-yellow
31
1
Au1−Au2
Au2−Au3
Au3−Au4
ΔEbond
5.99
3.27
4.13
solid (75 mg, 85%). P NMR (161.9 MHz, CDCl , ppm): −10.2. H
NMR (400 MHz, CDCl , ppm): 8.68 (dq, J = 5.0 Hz, J = 0.8 Hz, 2H,
3
10.44
5.99
11.59
3.27
3.16
3
4.13
H ), 8.56 (dt, J = 12.0 Hz, J = 0.8 Hz, 2H, H ), 8.48 (s, 2H, H + H ),
9
12
3
5
7
4
=
.82 (td, J = 11.8 Hz, J = 1.6 Hz, 2H, H ), 7.30 (ddd, J = 12.0 Hz, J =
−56.8
−41.8
−65.0
11
.8 Hz, 2H, H ), 5.78 (d, J = 20.0 Hz, 1H, H , NCH N), 5.66 (dd, J
Compound 2a
10
19a
2
20.0 Hz, J = 12.0 Hz, 1H, H23a, NCH P), 4.94 (d, J = 16.0 Hz, 1H,
2
stacked, C2h
double dimer, C1
chain, C1
H21a, NCH N), 4.70−4.64 (m, 2H, H , NCH P, + H , NCH N),
2
17a
2
21b
2
Au1−Au2
Au2−Au3
Au3−Au4
ΔEbond
5.96
3.29
3.07
4
.15 (dt, J = 20.0 Hz, J = 4.0 Hz, 1H, H1 , NCH P), 4.06 (d, J = 20.0
7b 2
11.10
5.96
6.97
3.00
Hz, 1H, H19b, NCH N), 3.87 (s, 2H, H , NCH P), 3.57 (dt, J = 20.0/
2
24
2
−1
3.28
3.10
4
3
.0 Hz, 1H, H23b, NCH P), 2.17 (s, 6H, COCH ). IR (KBr, cm ):
2
3
441 (C−H), 2116 (CC), 1640 (CN). ESI-MS(+): m/z 683.158
−57.9
−42.0
−67.6
+
+
(
[M + H] ; calcd m/z 683.160), 705.139 ([M + Na] ; calcd m/z
Compound 1b
7
05.142).
stacked, Ci
double dimer, C2
chain, C1
Synthesis of [Au(CCC H N )(PTA)] (2a). Solid PTA (25 mg,
1
0 7 2
Au1−Au2
Au2−Au3
Au3−Au4
ΔEbond
9.73
3.26
3.18
0
.16 mmol) was added to a suspension of [Au(CCC H N )] (60
10
7
2
n
6.77
11.48
3.26
3.13
mg, 0.16 mmol) in CH Cl (20 mL). After 45 min of stirring at room
temperature, the resulting yellow solution was concentrated (10 mL),
2
2
9.73
3.16
and n-hexane (10 mL) was added to precipitate a pale-yellow solid (77
−60.2
−42.5
−81.4
mg, 90%). 31P NMR (161.9 MHz, CDCl , ppm): −48.2. H NMR
1
Compound 2b
3
(
400 MHz, CDCl , ppm): 8.73 (d, J = 5.0 Hz, 1H, H ), 8.65 (dd, J =
3
9
stacked, Ci
double dimer, Ci
chain, C1
1
2.0 Hz, J = 0.8 Hz, 1H, H ), 8.36 (dt, J = 12.0 Hz, J = 4.8 Hz, 1H,
6
Au1−Au2
Au2−Au3
Au3−Au4
ΔEbond
9.64
3.59
3.22
H ), 8.29 (dd, J = 8.4 Hz, J = 2.5 Hz, 1H, H ), 7.80 (m, J = 8.0 Hz, J =
12
3
12.35
9.64
11.46
3.59
3.07
1
1
6
.6 Hz, 2H, H + H ), 7.27 (ddd, J = 8.0 Hz, J = 4.0 Hz, J = 2.0 Hz,
4 11
3.23
H, H ), 4.60−4.48 (AB q, J = 13.0 Hz, 6H, H , NCH N), 4.30 (s,
10
19b
2
−
1
H, NCH P). IR (KBr, cm ): 3425 (C−H), 2104 (CC), 1640
−56.1
−36.0
−70.0
2
+
+
(
CN). ESI-MS(+): m/z 534.111 ([M + H ] ; calcd m/z 534.112),
+
+
a
5
56.092 ([M + Na ] ; calcd m/z 556.094).
Structures of tetramers computed at the ZORA-BLYP-D3(BJ)/TZP
Synthesis of [Au(CC−C H N )(DAPTA)] (2b). Solid DAPTA (36
level of theory with the COSMO approximation for the solvent
1
0 7 2
b
mg, 0.16 mmol) was added to a suspension of [Au(CCC H N )]
(H O). Bond energies computed at the ZORA-BLYP-D3(BJ)/TZ2P
10
7
2
n
2
(
60 mg, 0.16 mmol) in CH Cl (20 mL). After 45 min of stirring at
level of theory with the COSMO approximation for the solvent
2
2
room temperature, the resulting yellow solution was concentrated (10
(H O).
2
C
DOI: 10.1021/acs.inorgchem.7b02343
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Synthesis of [Au(CCC H N )(PR )] [PR = PTA (1a) and DAPTA (1b)] Complexes
1
5
10
3
3
3
Scheme 2. Synthesis of [Au(CCC H N )(PR )] [PR = PTA (2a) and DAPTA (2b)] Complexes
1
0
7
2
3
3
1
−4
Figure 1. H NMR spectra (400 MHz) of compound 1a in D O (top) and CDCl (bottom) at 5 × 10 M concentration and 298 K.
2
3
ΔEbond = Edimer − 2Emonomer
ΔEbond = Etetramer − 4Emonomer
(1)
(2)
characteristic patterns of the phosphines PTA (1a and 2a) or
8
−11
31
1
DAPTA (1b and 2b)
(see the SI). P{ H} NMR spectra
carried out in CDCl typically show one signal between −10
3
and −50 ppm, with a 50 ppm (for PTA derivatives) or a 30
ppm (for DAPTA derivatives) downfield shift in comparison
with the free phosphines. The corresponding CC and CN
vibrations of the chromophoric units were also observed in the
IR spectra in all cases.
where Etetramer is the energy of the optimized tetramer, Edimer is the
energy of the optimized dimer, and Emonomer is the energy of one of the
four different optimized monomers used.
RESULTS AND DISCUSSION
Synthesis and Characterization. Complexes 1 and 2 were
obtained by the same methodology as that previously described
■
Aggregation in Water and Water/DMSO Mixtures. As
8
−10
was previously found for neutral gold(I) alkynyl complexes,
8
aged solutions of 1a, 1b, 2a, and 2b in H O give origin to the
in the literature for [Au(CCC H N)(PTA)] and [Au(C
2
5
4
9
formation of luminescent fibers (Figure S9), apparently
CC H N)(DAPTA)]. A CH Cl suspension of the previously
5
4
2
2
10
originating from the merging of smaller aggregates (see the
SAXS and DLS discussion below). The fibers are not observed
from DMSO and chloroform solutions in the same time scale.
synthesized [Au(CCC H N )] (for 1a and 1b) or
1
5
10
3
n
[
Au(CCC H N )] (for 2a and 2b) polymer was stirred
10
7
2
n
with the phosphine PTA or DAPTA in a 1:1 ratio (Schemes 1
and 2). The reaction was maintained under stirring at room
temperature for 45 min. Coordination of these phosphines
In D O, terpyridyl and bipyridyl protons are not observed in
H NMR, even under highly diluted conditions (Figure 1),
2
1
(
PTA and DAPTA) immediately gave yellow solutions that,
while the phosphine protons are clearly seen, even if they are
after concentration and the addition of n-hexane, yielded the
corresponding complexes quantitatively.
slightly broader than those in CDCl . The absence of the
3
terpyridyl and bipyridyl proton signals is an expected result
from aggregation because of the slower mobility of the
aggregates. What is unexpected is that the phosphine protons
are still detected in the aggregates because the restriction in
mobility should apply to both parts of the molecule.
Characterization of complexes 1 and 2 by H and 31P NMR
and IR spectroscopies and mass spectrometry verified the
successful formation of these products. Mass spectrometry
measurements were able to detect the corresponding molecular
1
1
peaks of the protonated species in all cases (see the SI). H
In order to obtain deeper knowledge of the self-assembling
process at the molecular level, we performed a series of NMR
spectroscopy studies in D O/DMSO-d mixtures because the
NMR spectra of the complexes recorded in CDCl show the
3
38−41
characteristic protons of the terpyridine
(1a and 1b) and
2
6
2
0,42
of the bipyridine
(2a and 2b), together with the
good miscibility of both solvents and the complete solubiliza-
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1
Figure 2. (a) Chemical structure of 2b with labeling numbers used in the NMR assignment. (b) H NMR spectra (aromatic region) of 1 mM 2b at
1
different times after dissolution in D O/DMSO-d (1:1). (c) H NMR spectra of 1 mM 2b in different solvent conditions. The experiments were
2
6
acquired at 500 MHz and 298 K, with a relaxation delay of 20 s. Below the peaks, the numbers in red are the normalized peak area values (integral).
Figure 3. Schematic representation of the possible aggregates observed in solution.
tion of the molecules in DMSO-d6 (Figure S10), having
acetyl of the DAPTA moiety, suggesting that the minor species
should locate the bipyridyl and DAPTA moieties in relatively
close proximity (Figure S13).
behavior comparable to that previously observed in CDCl . We
3
must point out that the very large aggregates are invisible in
conventional solution NMR experiments because of the large
line broadening produced by the very efficient transverse
The DOSY experiment (Figures S14 and S15) rendered an
apparent molecular size corresponding to a dimer structure for
the two observed species (Table S2; hydrodynamic radii of 7.3
and 8.6 Å for the minor and major species, respectively).
Accordingly, we assigned the minor species to the chain dimer
and the major one to the stacked dimer (Figure 3). Very
interestingly, the measured longitudinal and transverse
relaxation times (Table S1) are unexpectedly long for discrete
organic molecules in solution. Moreover, the magnetic
relaxation of the signals from the bipyridyl moiety showed to
be less efficient than the relaxation of the DAPTA moiety. This
1
relaxation in large molecular entities. The H NMR spectrum of
2
b in pure DMSO-d showed well-resolved signals, implying
6
the major presence of discrete species (Figures 2c, black trace,
1
13
and S9) suitably assigned by 2D H− C HSQC and HMBC
experiments (Figures S11 and S12). However, two sets of
signals were clearly observed in a 63:37 proportion, suggesting
the presence of two species in solution (with notation H and
H′ in Figure 2). The NOESY spectrum of this sample showed a
cross-peak between the H ′ signal and the methyl signal of the
6
E
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Article
could be due to some rigidity of the bipyridyl group, although
no definitive conclusions could be drawn due to the implication
of the DAPTA moiety in a cis/trans dynamic process of the
corresponding tertiary amide groups. Actually, the molecular
system is implicated in the different exchange processes that
occurred at very different time scales, as observed by the EXSY
peaks in the NOESY experiments (Figure S13).
Thus, the amide cis/trans isomerization of DAPTA is slow in
the chemical shift NMR time scale and can be observed by
exchange cross-peaks in the NOESY experiments at 250 ms.
Additionally, the exchange between the two dimeric species
effects in agreement with the formation of very large aggregates
in solution where the bipyridine would be less accessible to the
solvent, leading to a large broadening up to undetectable
signals. A faster dynamic and solvent exposure of the DAPTA
fragments allowed the detection of their NMR signals. The
same behavior was observed for complex 2a in pure DMSO-d6
and a 1:1 DMSO- d /D O mixture (Figures S21−S26).
6
2
31
P NMR experiments recorded in different DMSO- d /D O
6 2
proportions are in agreement with these data. A broad signal is
observed in pure DMSO-d , implying the existence of different
6
conformations in slow equilibrium. Moreover, the increase of
(chain/stacked; Figure 3) is slow in both the NMR chemical
the amounts of D O shifts to a sharp and clearly defined
2
shift and NOESY time scales (exchange slower than 0.5 s). The
complexity of the systems is even higher when looking at the
integrals of the H NMR signals. The H NMR spectrum
acquired at quantitative conditions (with a relaxation delay of
downfield peak, as expected when the equilibrium between the
chain and stacked conformations becomes faster in D O
2
1
1
(Figures S27 and S28).
Considering all of these NMR results, a putative self-
assembling mechanism for gold(I) complexes 2a and 2b is
proposed in Figure 3.
The aggregation has a visible impact on the optical properties
of the solutions in both the absorbed and emitted light. With an
20 s) showed that the intensities of the bipyridine signals are
lower than the ones for DAPTA, once again suggesting that the
bipyridyl ring is closely implicated in the self-assembling
process. Besides, a comparison of the integrals of the observed
NMR signals with that of an internal standard showed that
about 10% of the molecule remains undetected in solution
NMR, suggesting the presence of large aggregates even in pure
increase in the H O content, there is a clear change of the
2
absorption toward the visible region, displayed by the yellowing
of the solutions, and at the same time, the strong blue emission
DMSO-d . The apparent lower motion of the bipyridyl rings is
observed in DMSO is quenched at increasing amounts of H O
6
2
also observed in the negative NOE cross-peaks, which implies a
longer apparent tumbling time of this part of the molecule.
and replaced by a red emission (Figures 4 and S29).
To study the effect of H O on the self-assembly, we
2
performed a titration of the sample initially prepared in DMSO-
d with increasing amounts of D O (Figure S16). The increase
6
2
of the polarity of the medium produced changes in the
bipyridyl proton signals, which is in agreement with the
43
previously reported behavior. All the signals broadened, and
the protons corresponding to the two dimeric species started to
approach their chemical shifts (without an apparent variation in
the proportion of the species). This suggests that the presence
of H O accelerates the exchange between the two dimeric
2
species. The DOSY experiment at 20% D O in DMSO-d6
2
showed the clear presence of two dimeric species with apparent
sizes similar to that observed in pure DMSO (Table S2 and
1
Figure S17). However, the H NMR signals coalesce at a 1:1
DMSO-d /D O mixture, in which a single set of signals was
6
2
observed for the bipyridyl moiety (Figure 2c, red trace). The
NOESY spectrum of this sample showed peaks that are
compatible with the two dimeric structures, implying that a fast
exchange must occur between the chain and stacked species in
this solvent mixture (Figure S18). This is also supported by the
negative sign of the NOESY peaks, implying an apparent larger
size of the species in solution rendering transferred NOESY
effects between the dimeric species and larger aggregates. The
1
integrals of the protons in the 1D H NMR spectrum suggested
a higher proportion of invisible aggregates, while the DOSY
experiments (Figure S19) rendered values compatible with the
presence of dimeric species and in equilibrium with larger
aggregates (hydrodynamic radii of 7.8 and 15.0 Å for different
signals of the spectrum; Table S2). Moreover, acquisition of the
Figure 4. (top) Variation of the color of 1 × 10− M D
4
O/DMSO-d
2
6
solutions of 2b under visible and UV light. (bottom) Normalized
emission of 2b at different compositions of DMSO-d /D O (λ = 360
6
2
exc
nm).
1
H NMR spectrum after longer preparation times showed a
decrease in the intensity signals with time (Figure 2b), implying
the slow formation of very large self-assemblies (6 days).
Finally, the H NMR spectrum of a sample prepared in pure
The lower-energy band at ca. 630 nm is assigned to the
aggregates, where the presence of excimers due to π−π or
Au−π stacking can contribute to both the broad shape and red
1
11
D O showed a further decrease in the intensity of the protons
2
shift of the band. These interactions seem to be more favored
of DAPTA and the complete disappearance of the signals from
the bipyridyl moiety (Figure 2c, blue trace). The NOESY
spectrum of this sample (Figure S20) showed spin-diffusion
in H O, in agreement with theoretical calculations (see below).
2
The higher emission band at 430 nm is assigned to the singlet
1
metal-to-ligand charge-transfer ( MLCT)/singlet ligand-to-
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Figure 5. DAMMIN low-resolution structures reconstructed from SAXS patterns for 1 × 10−4 M (top) and 1 × 10−5 M (bottom) solutions of 2a in
DMSO (left), 1:1 H O/DMSO (center), and 3:1 H O/DMSO (right).
2
2
Figure 6. Structures of the computed dimers.
1
ligand charge-transfer ( LLCT) transition, being more favored
in DMSO, because of the presence of a chain conformation,
which allows free movement of the phosphane and
polypyridine moieties of the compounds in the aggregates
increase in the H O concentration. There is no significant
2
difference in the sizes between the terpyridyl and bipyridyl
derivatives (Figures 5 and S30−S32). This result suggests that
aggregates exist in the different H O/DMSO mixtures of the
2
(
mainly aurophilic interactions).
SAXS measurements were performed for 1 × 10 and 1 ×
solvents, and the differences observed may arise from internal
reordering of the aggregates (chain/stacked aggregates detected
by NMR and luminescence). Additionally, the temperature
does not significantly affect the size or the shape of the resulting
structures (Figures S33−S36).
−5
−4
1
0
M solutions of the compounds in different H O/DMSO
2
proportions. The low-resolution structures were reconstructed
ab initio from the scattering patterns using the DAMMIN
program and are shown in Figures 5 and S30−S32 for two
concentrations and two solvent compositions.
The observation is that, in all mixtures, the large aggregates
show comparable dimensions at the same concentration,
independent of the solvent composition and, in general, a
slight increase in the sizes of the aggregates is shown with an
3
0
DLS measurements were in agreement with the presence of
aggregates in DMSO at 1 × 10− M concentration. The
presence of ca. 100 nm size supramolecular structures is
observed in all cases (Figures S37−S40). As expected, the sizes
of the aggregates obtained by DLS and SAXS are much larger
than the hydrodynamic radii previously calculated by NMR
4
G
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Figure 7.
DOSY experiments (see above). SAXS/DLS and NMR
techniques are complementary toward the detected assembly
size: because of the slow tumbling of larger macromolecules in
solution leading to faster relaxation of transverse magnetization,
only the soluble dimers and small aggregates in equilibria with
the larger assemblies (detected by SAXS/DLS) are observed by
NMR (the larger structures are “invisible” in solution NMR).
Computational Study of the Aggregation Modes. We
have computationally analyzed the structures of 1a, 1b, 2a, and
The Au−Au distances between successive monomeric units in
the tetramers and the bond energies are summarized in Table 1.
Note that, with no exception, there is a nonnegligible
stabilization of the chain conformation in the tetramers. The
same is not observed for the bond energies of the dimers,
collected in Table 2. For the dimers, the conformations with
shorter Au−Au distances (dimers 2 and 4, Au···Au interactions)
are not more stable than the conformations with favored
ligand−ligand interactions, either in head-to-tail (dimer 1; π···
Au interaction) or head-to-head (dimer 3; π−π interaction)
conformations (Figure 6). That is, the formation of the dimers
2b (Figure S41) and their autoassociation properties to form
different dimers and tetramers (as a model for larger
aggregates).
We have analyzed different dimerization and tetramerization
possibilities for each of the monomers in Figure S41, involving
different arrangements of ligand−ligand, Au−Au, and ligand−
Au interactions (Figures 6 and 7).
in H O in the stacked conformation is more favored according
2
to these calculations (in agreement with the luminescence
data), while the predicted stability of the larger structures does
not seem to be so straightforward because entropic factors (not
considered here) should be more important in the presence of
π···Au interactions.
An important difference between the tetramers calculated is
that, in the stacked conformation, the main contribution arises
from Au−π interactions, in the double dimer, there are
contributions from Au−Au and π−π interactions, while in the
chain conformation, the main contribution is a continuous
chain of Au−Au interactions that grows with the aggregate size.
Another distinct aspect of the aggregation modes is related to
the impact of the aggregates on the observables from either
NMR, SAXS, and optical spectroscopies. For all of the
tetramers, the favored aggregation modes are either stacked
or chain aggregation. On the contrary, while chain aggregation
minimizes hydrophobic interactions of the chromophoric unit
H
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a
Table 2. Au−Au Distances (Å) and Bond Energies (kcal/
number of Au atoms, these interactions become more
important in stabilization of the aggregates.
mol) of the Tetramers of 1a, 1b, 2a, and 2b in H O
2
We cannot forget that theoretical calculations take into
consideration the bonding energy, while some entropic aspects,
like hydrophobic interactions (expected to be more important
for terpyridyl complexes), have a direct impact on the
aggregation.
dimer 1
dimer 2
dimer 3
dimer 4
Compound 1a
Au−Au distance
ΔEbond
6.20
−21.1
Compound 2a
3.39
4.72
3.22
−11.1
−20.2
−18.5
Reversible Aggregation/Dissaggregation Processes.
The presence of N-donor atoms in the chemical structure of
the complexes makes these systems ideal candidates for cation
coordination. The main important point in this work is that the
coordination of cations may have a direct influence on the
resulting aggregates being able to externally modulate the
formation/dissolution of the supramolecular structures. The
initial solutions display the typical π−π* (CCC H N ; for
Au−Au distance
ΔEbond
11.67
3.24
5.33
3.34
−21.2
−14.5
−22.2
−15.5
Compound 1b
b
Au−Au distance
ΔEbond
9.50
−25.0
Compound 2b
3.30
4.41
3.17
b
−14.7
−18.7
−11.7
Au−Au distance
ΔEbond
11.42
−20.0
3.86
5.41
3.25
−8.6
−22.3
−13.0
15 10
3
a
1a and 1b) or π−π* (CCC H N ; for 2a and 2b)
10
7
2
Bond energies of the dimers computed at the ZORA-BLYP-D3(BJ)/
TZP level of theory with the COSMO approximation for the solvent
absorption transitions of chromophores between 310 and 360
20,45,46
b
(
H O). The structure of dimer 2 of 1b is close to the structure of
nm.
Additionally, a band at higher energy (ca. 280 nm)
2
dimer 4.
due to the intraligand absorption of the phosphine is also
recorded, and another band at lower energy and with very low
intensity between 360 and 400 nm is also present, which is
assigned to a σ*Au−Au → π* transition, in agreement with
and maximizes exposure to the solvent, the stack aggregation
maximizes the hydrophobic Au···π interaction, pulling the
chromophoric unit toward the core of the aggregate while
allowing the phosphine unit full exposure to the solvent. The
stack aggregation thus is compatible with the excitonic
interaction displayed in both the absorbed (yellowing) and
emitted (quenching and red shift) light and can be the major
previous data based on theoretical and thermodynamic
44,47
calculations
(Figure S42). In order to check the ability of
the complexes to coordinate metal cations, aqueous solutions
−4
2+
(
1 × 10 M) of 1a−b, 2a−b were titrated with Zn , chosen as
a divalent ion that can coordinate to N atoms. Absorption and
emission variations in the presence of increasing amounts of
Zn were detected, and they are displayed in Figures 8 and
S43−S45.
An interesting feature is observed with an increase in the
amount of Zn . The broad intraligand absorption band turns
to some vibronic resolution profile with higher absorption.
Meanwhile, the red emission band disappears, and a blue
2+
species observed with an increase in the amount of D O in
2
luminescence studies. By the same argument, chain aggregation
is more compatible with the optical spectroscopy data obtained
in DMSO, where both aromatic and phosphine protons are
exposed to the solvent and the chromophore absorption in the
UV, together with the blue emission, is recovered. At the same
time, a second species (chain conformers) is detected in H
NMR spectra recorded in pure DMSO-d . Calculations
performed in DMSO support this fact (Table S3). It can be
seen that the contribution of the chain conformation becomes
more important in this solvent, mainly in the bipyridyl
complexes. In fact, the charge-transfer (CT) contribution
observed in the case of the Au···Au interactions increases with
the number of Au atoms involved in the chain and while a
single Au···Au bond is on the same order of magnitude as other
interactions (Au···π and π···π stacking). With growth of the
2+
1
1
fluorescence, previously assigned to MLCT, rises for DMSO
solutions. Emission bands resulting from Zn2+ coordination
present some vibronic resolution, typical of nonaggregated
chromophores. Additionally, the resulting blue emission is
much higher in intensity than the red aggregated emission, as
6
observed in the H
The plot of the emission variations against the Zn
O/DMSO solutions of the compounds.
2
44
2+
concentration displays a change in the slope at ca. 0.3 equiv
(Figure 8, right inset), which is an indication of 1:3 (divalent
cation/gold complex) coordination (Figure S46).
Figure 8. Absorption (left) and emission (right) spectra of a 1 × 10−4 M aqueous solution of 2a in the presence of increasing amounts of Zn2+.
I
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1
Figure 9. H NMR spectra of 0.9 mM 2b in D O and with increasing amounts of metal (1, 2, or 3 equiv of ZnCl ) and after the addition of 6 equiv
2
2
2
+
of cryptand to the sample with 3 equiv of Zn .
−4
Figure 10. DAMMIN low-resolution structures reconstructed from SAXS patterns for 1 × 10 M of 2a (left) in H O, in the presence of 1 equiv of
2
2+
2+
Zn (middle), and in the presence of 1 equiv of Zn and cryptand (right).
Interestingly, the red emission band appears again when a
Zn2 encapsulating agent (1,2-[bis(3-aminopropyl)amine]-
ethane or 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]-
SAXS experiments performed in the presence of Zn2+
followed by the addition of an encapsulating agent show that
+
the enclosed structures of gold(I) complexes in H
0, left) are disturbed in the presence of the divalent cation
O (Figure
2
1
hexacosane cryptand) is added to the solution (Figure S47),
as a result of Zn2+ entrapment. This means that the aggregation
(Figure 10, middle), giving rise to smaller structures and going
back to aggregation when the encapsulating agent traps the
cation and allows the gold(I) complex to self-assemble again
state can be directly modulated, being an effective example of
adaptive supramolecular systems modulated by external stimuli.
As mentioned before, the intensity reduction of bipyridine
(
Figures 10, right, and S48−S50).
These processes were observed also for samples in solution
resonances for pure 2b in D O are related to the longer
2
under optical microscopy. Well-organized fibrillar aggregates
rotational correlation times (τ ) and shorter T associated with
C
2
can be detected for an aqueous solution of complexes that
the assembly process. Therefore, this assembly/disassembly
process could be followed by monotoring the intensity changes
2+
disaggregate in the presence of Zn and come back to
aggregation when an encapsulating agent is added to the
solution (Figure S51).
All in all, we have demonstrated for the first time the self-
2
+
on the NMR spectra. The addition of Zn to a solution of 2b
1
in D O produced the intensity recovery of the H NMR signals
2
of the bipyridyl moiety. The observed chemical shifts of this
aggregation of discrete gold(I) complexes in H O that can be
2
fragment are in agreement with coordination of the Zn2+ ion to
externally modulated by the presence/absence of a divalent
cation. This process is reversible and has been detected by
complementary techniques: NMR, absorption, emission, SAXS,
DLS, and optical microscopy in solution.
48
the bipyridine ring in a syn disposition of the N atoms. The
2
+
complexation of Zn cations with an excess of complexing
agent cryptand partially reverted the disassembly effects of
2+
Zn , as observed by the concomitant decrease of the intensity
of the aromatic signals in the upper spectrum of Figure 9. The
CONCLUSIONS
■
2
+
presence of an excess of Zn also shifted the DAPTA proton
signals, suggesting a marginal interaction with the tertiary N
atoms or the amide carbonyls of DAPTA (Figure 9).
Gold(I) alkynyl complexes containing bipyridine and terpyr-
idyne π systems and water-soluble phosphines (PTA and
DAPTA) self-assemble in H O and DMSO, as shown by SAXS,
2
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NMR, and DLS, with very distinct luminescence displayed in
both solvents.
An exhaustive H NMR study performed with different
staff. A.J.M. thanks the FCT for a postdoctoral grant (SFRH/
BPD/69210/2010). This research was supported by a Marie
Curie Intra European Fellowship within the 7th European
Community Framework Programme (to R.G.). We also thank
1
D O/DMSO mixtures indicates the presence of two different
2
assembly modes in the aggregates: chain assemblies, which are
based mainly on aurophilic interactions, and stacked assemblies,
which are based on Au···π interactions, which are confirmed by
density functional theory calculations carried out in both
solvents. The initial aggregates of nanometer size merge with
time to the formation of fibers. This process is promoted by the
L. Gimen
ligands.
́
ez and I. Brea for their help on the synthesis of the
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AUTHOR INFORMATION
Corresponding Authors
E-mail: lima@fct.unl.pt.
E-mail: laura.rodriguez@qi.ub.es. Tel.: +34 934039130.
■
Rissanen, K.; Lima, J. C.; Rodríguez, L. A coumarin based gold(I)-
alkynyl complex: a new class of supramolecular hydrogelators. Org.
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*
*
ORCID
the effect of counterion, positive charge and solvent. Dalton Trans.
Ignacio Alfonso: 0000-0003-0678-0362
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016, 45, 7328.
12) Chan, A. K.-W.; Lam, W. H.; Tanaka, Y.; Wong, K. M.-C.; Yam,
Yolanda Per
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ez: 0000-0003-3767-5346
Joao
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Carlos Lima: 0000-0003-0528-1967
V. W.-W. Multiaddressable molecular rectangles with reversible host−
guest interactions: Modulation of pH-controlled guest release and
capture. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 690.
Laura Rodríguez: 0000-0003-1289-1587
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The authors are grateful to the Ministry of Economy, Industry
and Competitiveness of Spain (AEI/FEDER, UE Projects
CTQ2016-76120-P, CTQ2015-65040-P, and CTQ2015-
̈
S.; Duedal, T.; Sastre-Santos, A.; Jeppesen, J. O.; Sessler, J. L. Design
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0117-R). This work was also supported by the Associated
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16) (a) Arcau, J.; Andermark, V.; Aguilo, E.; Gandioso, A.; Moro, A.;
Laboratory for Sustainable Chemistry, Clean Processes and
Technologies, LAQV, which is financed by national funds from
FCT/MEC (UID/QUI/50006/2013) and cofinanced by the
ERDF under the PT2020 Partnership Agreement (POCI-01-
145-FEDER-007265). The IQAC NMR Facility 500 MHz
spectrometer and cryoprobe were funded by the CSIC13-4E-
076 MINECO-FEDER grant. SAXS experiments were
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activity. Dalton Trans. 2014, 43, 4426. (b) Gavara, R.; Aguilo, E.;
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016, 446, 189. (c) Meyer, A.; Gutierrez, A.; Ott, I.; Rodríguez, L.
performed at the NCD-BL11 beamline of the ALBA
Synchrotron Light Facility in collaboration with the ALBA
Phosphine-bridged dinuclear gold(I) alkynyl complexes: Thioredoxin
reductase inhibition and cytotoxicity. Inorg. Chim. Acta 2013, 398, 72.
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