Directing the crystal packing in triphenylphosphine gold(I) thiolates by ligand fluorination

Article abstract of DOI:10.1021/acs.inorgchem.9b03131

We explore herein the supramolecular interactions that control the crystalline packing in a series of fluorothiolate triphenylphosphine gold(I) compounds with the general formula [Au(SRF)(Ph3P)] in which Ph3P = triphenylphosphine and SRF = SC6F5, SC6HF4-4, SC6F4(CF3)-4, SC6H3F2-2,4, SC6H3F2-3,4, SC6H3F2-3,5, SC6H4(CF3)-2, SC6H4F-2, SC6H4F-3, SC6H4F-4, SCF3, and SCH2CF3. We use for this purpose (i) DFT electronic structure calculations and (ii) the quantum theory of atoms in molecules and the non-covalent interactions index methods of wave function analyses. Our combined experimental and computational approach yields a general understanding of the effects of ligand fluorination in the crystalline self-assembly of the examined systems, in particular, about the relative force of aurophilic contacts compared with other supramolecular interactions. We expect this information to be useful in the design of materials based on gold coordination compounds.

Full text of DOI:10.1021/acs.inorgchem.9b03131

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Directing the Crystal Packing in Triphenylphosphine Gold(I)
Thiolates by Ligand Fluorination
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Guillermo Moreno-Alcantar,* Luis Turcio-García, Jose M. Guevara-Vela, Eduardo Romero-Montalvo,
́
́
́
́
Tomas Rocha-Rinza, Angel Martín Pendas, Marcos Flores-Alamo, and Hugo Torrens
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ABSTRACT: We explore herein the supramolecular interactions that control
the crystalline packing in a series of fluorothiolate triphenylphosphine gold(I)
compounds with the general formula [Au(SRF)(Ph3P)] in which Ph3P =
triphenylphosphine and SRF = SC6F5, SC6HF4-4, SC6F4(CF3)-4, SC6H3F2-2,4,
SC6H3F2-3,4, SC6H3F2-3,5, SC6H4(CF3)-2, SC6H4F-2, SC6H4F-3, SC6H4F-4,
SCF3, and SCH2CF3. We use for this purpose (i) DFT electronic structure
calculations and (ii) the quantum theory of atoms in molecules and the non-
covalent interactions index methods of wave function analyses. Our combined
experimental and computational approach yields a general understanding of
the effects of ligand fluorination in the crystalline self-assembly of the
examined systems, in particular, about the relative force of aurophilic contacts
compared with other supramolecular interactions. We expect this information to be useful in the design of materials based on gold
coordination compounds.
insights.14−17 In these cases, the exploitation of other
theoretical options to the quantitative analysis of supra-
molecular interactions in inorganic compounds, such as wave
creation of complex architectures stabilized by weak inter-
INTRODUCTION
■
The development of supramolecular chemistry has led to the
function analyses, might be valuable.
actions among organic and inorganic molecules.1 The
Concerning the noncovalent interactions which involve gold
fascinating assemblies displayed by inorganic compounds in
atoms, Au(I) compounds show an interesting tendency to
the solid state are often responsible for desirable physical
form short Au···Au contacts, especially in the solid state.18−20
(optical, magnetic and mechanical)2−4 as well as chemical
The existence of these contacts is often related to interesting
properties (gas storage and separation as well as catalysis).5,6
properties of gold-based materials.21−28 Theoretical studies
Synthetic chemists and crystal engineers strive for the rational
suggest that these contacts, widely known as aurophilic
control of these properties. Nevertheless, without fully
interactions, result from dispersion interactions and therefore
understanding the forces that underlie crystal packaging,
from electron correlation ultimately.18,20,29−31 Nonetheless,
these efforts might be futile. The probability that a compound
there is also evidence which indicates that the covalent
displays any of these desired properties relies on favoring of a
character of Au···Au interactions is important.32−34 The
given assembly of building blocks over other possibilities. This
molecular tectonics of gold(I) linear compounds is frequently
favoring crucially depends on understanding the relative
directed by such gold−gold contacts.35−37 However, the
formation energies of competing aggregates.7,8 The strategies
occurrence of competing interactions promoted by the
aimed to build a predetermined assembly are frequently based
structure of the ligands can impair aurophilicity.7 Then, the
on the qualitative and, in some cases, intuitive examination of
modification of the backbones of the ligands attached to gold
extensive experimental evidence.9,10 However, the experimen-
atoms is a feasible strategy to tune the interactions which direct
tal quantitative analyses of the relative strength of intermo-
the formation of supramolecular architectures. Such mod-
lecular interactions is complicated. For instance, observations
ifications can be exploited in the control of aurophilic
are repeatedly nontransferable from one system to others.
Therefore, the use of theoretical tools to complement the
experimental analysis of noncovalent interactions has been a
Received: October 25, 2019
common and usually successfully alternative in systems
containing representative elements.11−13 Nevertheless, the
inclusion of heavy atoms and the intricacy of supramolecular
assemblies, particularly for inorganic systems, requires the use
of expensive computational methods to get accurate energetic
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Scheme 1. Compounds Addressed in This Work
aggregates.36 For example, electron-withdrawing substituents
which decrease the electron density of Au centers weaken the
gold−gold contacts.38 The study of systems wherein the Au···
Au interactions are disfavored or overwhelmed by other
synthons could underestimate the real significance of these
interactions.39 In contrast, the analyses of systems in which the
Au···Au contacts are favored indicate that they are as strong as
other archetypical supramolecular interactions such as π-
stacking and hydrogen bonds.37,40 Concerning the character-
ization of aurophilic contacts, we have already used descriptors
defined in the theoretical framework of the Quantum Theory
of Atoms in Molecules (QTAIM)41 and the Non-Covalent
Interactions (NCI) index42 to successfully analyze NCIs in
gold compounds.32,43 Indeed, the QTAIM and NCI-index
methodologies have also given valuable insights into the study
of different noncovalent interactions, such as π-stacking and
hydrogen bonds, which are also relevant for the systems
addressed herein.44−46
Here, we characterize thoroughly the prevalent interactions
in the crystalline supramolecular architectures of a dozen
compounds with the formula [Au(SRF)(Ph3P)] where Ph3P =
triphenylphosphine and SRF = SC6F5 (1), SC6HF4-4 (2),
SC6F4(CF3)-4 (3), SC6H3F2-2,4 (4), SC6H3F2-3,4 (5),
SC6H3F2-3,5 (6), SC6H4(CF3)-2 (7), SC6H4F-2 (8),
SC6H4F-3 (9), SC6H4F-4 (10), SCF3 (11), and SCH2CF3
(12) (Scheme 1). We point out that the X-ray structures of
compounds 3−12 have not been reported before. The analysis
of these structures reveals that changes in the fluorination of
the thiolate group coordinated to the gold atom affect very
strongly the prevalence of aurophilic interactions as a result of
two effects: first, by means of the electronic modulation that
the electronegative fluorine atoms exert over the metal centers
and, second, via the promotion of different supramolecular
arrangements wherein the fluorinated moieties act as synthons,
in this case, π-stacking and hydrogen-bonding. The QTAIM
and NCI-index electron density analyses allow us to quantify
and visualize the gradual variation in strength of the different
interactions directing the crystal packing in the investigated
compounds. Overall, this research reveals how the structural
modulation of aurophilic and other significant supramolecular
interactions can be achieved by means of fluorination.
diffraction structures of all 12 compounds were determined
by analyzing samples obtained by slow evaporation from
acetone solutions. Although the structures of compounds 1
and 2 were previously reported,47,48 we have obtained and
analyzed our own X-ray diffraction data. We proceeded in this
way to maintain the same experimental conditions of
crystallization and analysis in all the compounds addressed in
this investigation. Our determined structures of 1 and 2 agree
with previous reports.
In general, all compounds present the expected molecular
geometry, in which the gold atom exhibits a linear
coordination environment, with P−Au−S angles in the interval
of 169° to 180°. The largest deviations from linearity are
observed in the compounds with Au···Au contacts. Bond
distances are also in the typical ranges. Molecular ORTEP
diagrams and structural information are available in the SI.
Despite these similarities in molecular structure, the crystalline
arrangements of the examined compounds present notorious
diversity in the modes of interaction between molecular vicinal
pairs. Our theoretical analyses of noncovalent interactions have
allowed us to identify the most prominent contacts in the
investigated compounds.
The main interactions which drive the crystalline arrange-
ments in the 12 synthesized compounds are π-stacking,
hydrogen bonds, and aurophilic contacts. They contend in
the determination of the crystal packing of the systems under
examination. We analyze the crystalline structures by
identifying the main noncovalent interactions in dimers of
the molecules considered herein. This identification is made
via X-ray diffraction structures and NCI-index maps. Our
analyses also provide insights about the factors promoting the
observed interactions and include a quantitative assessment of
the strength of the different interactions using several
descriptors defined within the theoretical framework of the
QTAIM, e.g., the delocalization index (DI). We proceed now
to discuss the architectures assembled mainly by π-stackings
then by H-bonds to finally consider those bonded by
aurophilic contacts.
Architectures Directed by π-Stacking. One of the most
significant manifestations of π-stacking in our compounds is
the π−πF interaction, which involves aryl-fluoroaryl contacts
whose associated formation energies are reported to be about
20−25 kJ·mol−1. Nevertheless, weaker (10−15 kJ·mol−1) π−π
and πF−πF interactions can also be found.49−52 In turn, π−πF
interactions can possibly impair aurophilic contacts. The highly
fluorinated phenylthiolate derivatives 1 and 2 show crystalline
dispositions determined by the formation of π-stacking
RESULTS AND DISCUSSION
■
Compounds 1−12 were synthesized by substitution of chlorine
with the corresponding thiolate group from the previously
reported compound [AuCl(Ph3P)] (synthetical details are
given in the Experimental Section). Single crystal X-ray
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interactions, and hence aurophilic contacts are totally sup-
pressed. The left side of Figure 1 shows the formation of
Figure 2. Top: Dimeric unit observed in the crystalline arrangement
of compound 3. The figure displays two main types of interaction,
π−πF and aurophilic contacts. Hydrogen atoms are omitted for clarity.
Distances are indicated in Å. Bottom: General view along the b
crystallographic axis showing the packing of four of those dimeric
units mainly directed by H···F contacts. Fluorinated and unfluorinated
rings are displayed in red and blue, respectively.
Figure 1. Left: Formation of πF−π stacked dimers of compounds 1
(top) and 2 (bottom). Hydrogen atoms are omitted for clarity. Right:
View along the c crystallographic axis showing the packing of four of
those dimeric πF−π bonded assemblies held together also by π−π and
πF−πF stackings. Fluorinated and nonfluorinated rings are displayed in
red and blue colors, respectively. Distances are indicated in Å.
allows for other noncovalent interactions within the system
such as C−H···F and aurophilic contacts to stand out.
We consider now the wave function analyses of the dimers
of 1−3. The NCI-index maps reveal the locations where weak
interactions take place within these systems (Figure 3). On one
hand, the most important interaction surfaces in the dimers of
compounds 1 and 2 are located in regions between fluorinated
and unfluorinated phenyl groups. On the other, the analysis of
the compound 3 dimer shows notably more disperse surfaces
for the π−πF regions, an indication of weaker interactions. The
NCI-index analysis reveals several additional interactions in 3.
First, there is an extra π-stacking which involves the phosphine
phenyl rings (Figure 3, inset i). Second, there is a clear NCI
surface between the Au atoms which confirms a weak
aurophilic contact (Figure 3, bottom-left). In short, the
strength of the Au···Au interactions investigated herein is
modulated by their competition with π−πF contacts. Third, we
identify an intramolecular Au···F contact that can be equally
observed in 1 and 2 (Figure 3, inset ii). The Au···F distances as
well as the descriptors of these interactions based on the
topology of the electron density are in agreement with our
previous reports concerning the dominant closed-shell nature
of these contacts (Figure S3 and Table S1).32,32
dimeric units linked by two symmetry equivalent π−πF
interactions. The fluorophenyl ring on the thiolate fragment
interacts with one of the vicinal phosphine phenyl groups. This
arrangement was previously analyzed in terms of the
quadrupolar model of π-stacking interactions.47,48 Additionally,
these dimeric units are held together via other stacking
interactions of the types πF−πF and π−π as observed on the
right side of Figure 1. Table 1 reports some selected distances
used to characterize these interactions within the crystal.
Hereof, compound 3 which holds the relatively steric −CF3
group would not, in principle, present an effective π-stacking
such as those observed in systems 1 and 2. Nevertheless, the
crystalline network of 3 has a similar pattern to 1 and 2.
Indeed, π−πF contacts occur in the dimeric units of 3, although
displacements and angles between the stacked rings reveal a
weaker interaction than those displayed in 1 and 2 (Figure 2).
While compounds 1 and 2 show larger Au···Au distances
(7.6253(7) and 7.4596(7) Å), the impairment of the π−πF
stacking interactions in compound 3 leads to a very substantial
shortening in the gold−gold distance (dAu···Au = 3.5068(6) Å)
which stands in the frontier of aurophilic distances (≤3.5 Å).18
The secondary π−π and πF−πF interactions displayed in the
dimers of 1 and 2 almost disappear in system 3, whose dimers
are instead held together by several F···H contacts between
fluorinated and unfluorinated rings. The weakening of the π-
stacking interactions in compound 3 due to steric hindrance
Concerning compounds 4−10, π-stacking interactions are
not the principal factors in the crystalline packing in these
systems. Figure 4 shows the NCI-index analyses of dimeric
aggregates of compounds 6, 7, and 9, which display π−πF
stacking interactions that are clearly weaker than those found
in systems 1−3. Nevertheless, the reduction of the fluorination
Table 1. Distances Used to Characterize the π-Stacking Interactions in Compounds 1 and 2
compound 1
compound 2
πF−πF
distances /Å
πF−π
πF−πF
π−π
πF−π
π−π
dcentroids
dsplit
dinterplanar
3.665(3)
1.612(2)
3.291(2)
3.624(3)
1.152(1)
3.436(2)
4.260(3)
2.281(2)
3.598(2)
3.621(3)
1.357(1)
3.365(2)
3.581(3)
1.232(2)
3.373(2)
4.356(3)
2.289(1)
3.706(1)
C
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Figure 3. NCI-index isosurfaces which characterize the interactions
within the dimers of 1 (top) and 3 (bottom-left). The insets on the
bottom-right show a detail of (i) the secondary π···π interactions in 3
and (ii) the Au···F intramolecular contact in 2. The isosurface value of
the reduced density gradient is s = 0.5 au, and the values of ρ satisfy
the condition ρ ≤ 0.015 au. The scale for the relative magnitude of
the interactions is presented at the bottom of the figure.
degree of the πF rings in systems 6, 7, and 9 increases the
strength of H-bond interactions. On the other hand, the
increase of the electron density in the gold atoms within
compounds 4, 5, 8, and 10 results in architectures directed by
aurophilicity (vide infra).
Figure 4. NCI-index isosurfaces (left) and ORTEP diagrams (right)
which characterize the π···πF-stacking interactions within the dimers
of 6 (top), 7 (medium), and 9 (bottom). The isosurface value of s
equals 0.5 au, and the electron density is such that ρ ≤ 0.015 au. The
scale for the relative magnitude of the interactions is presented at the
bottom of the figure. Distances are indicated in Å.
The analysis of the electron delocalization index (DI) as
defined by QTAIM allows us to evaluate the covalent
contributions of the corresponding interactions. The delocal-
ization index quantifies the number of electron pairs (and
hence covalency) shared between two atoms or groups of
atoms in real space. The DI values for the most important
interactions observed in the dimers presenting π-stacking are
reported in Table 2. The π−πF interactions in 1 and 2 are
associated with high values of DI, while those measured for
compound 3 are considerably smaller, in agreement with the
NCI-index analyses. The determined DIs reveal also that the
weakening of the π−πF stacking in compound 3 results in other
noncovalent interactions. For example, the DIs associated with
the π−π stacking which involves the lateral phosphines of 3
(0.19 au) are comparable with those of the π−πF moieties in 1
and 2 (Table 2). Most importantly, the DI between the gold
centers in 3 (0.12 au) indicates a weak aurophilic contact.
Furthermore, if we consider the whole (Au···S)2 fragment, the
total intermolecular DI is significantly larger (0.46 au). This
result indicates a significant contribution of the Au···S
interactions to the intermolecular bonding of the 3 dimer.
The DI values for the πF−π-stacking interactions in the
dimers of compounds 4−10 are considerably lower than those
in systems 1−3, in agreement with the weaker interaction in
the former set of molecules due to the smaller fluorination
degree of their πF rings. The fluorination pattern in compounds
6, 7, and 9 is suitable for the occurrence of H-bonded
structures. The transfer of electron density to the gold centers
Table 2. Delocalization Indices and Geometrical Parameters
Used to Characterize πF−π-Stacking Interactions in the
Dimers of 1−10
a
DI πF−π /
Θ πF−π/
b
compound
a.u.
d
centroids/Å
deg
main interaction
1
2
3
4
5
6
7
8
9
0.298
0.285
0.246
0.121
0.178
0.129
0.139
0.124
0.130
0.103
3.665(3)
3.621(3)
3.836(3)
4.214(4)
3.983(3)
4.225(4)
4.277(3)
4.209(2)
4.276(2)
4.812(3)
1.6
2.3
πF−π
πF−π
πF−π /aurophilic
aurophilic
aurophilic
H-bond
H-bond
aurophilic
H-bond
21.1
25.1
8.97
27.5
34.9
23.9
29.9
42.1
10
aurophilic
a
This value takes into account all the C, H, and F atoms in the
b
interacting phenyl groups. Dihedral angle between the πF and π
planes.
in compounds 4, 5, 8, and 10 strengthens the Au···Au contacts
even to the point that aurophilic interactions overcome π−πF
stacking. Our results point out that 3 is a borderline case in
which the π−πF interactions having DIs values of around 0.24
au compete with aurophilic contacts. The nature of the Au···Au
contact in 3 will be further described below.
D
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Hydrogen-Bonded Architectures. Compounds 6, 7, 9,
11, and 12 present C−H···F, C−H···S, or C−H···π53 H-bonds.
Systems 6 and 9 present the fluorination of the thiolate ring
only in the meta positions. The motif formed in the dimeric
structures of compounds 6 and 9 is similar to that found by
Desiraju54 in fluorinated benzenes. The acidity of the ortho
hydrogen of the phenyl of the thiolate ligand in these
molecules is increased by the α influence of fluorine and
sulfur. Therefore, cyclic structures occur that give rise to
dimeric units with H···F contacts as revealed by NCI-index
analyses (Figure 5). Other noncovalent contacts that are
Table 3. QTAIM Indicators and Distances for Selected H···
A (A = F, S, π) Interactions in Dimers of the Systems 6, 7, 9,
11, and 12
a
compound contact DIH···A/au ρ(rBCP)/ a.u. ∇2ρ(rBCP)/ au
dH···A/Å
6
H···F
H···π
0.027
0.063
0.032
0.015
0.023
0.019
0.056
0.025
0.026
0.032
0.020
0.065
0.007
0.0312
2.455
2.664
3.145
2.732
3.068
2.631
2.752
3.155
2.507
3.024
2.654
2.642
b
H···S
H···F
H···S
H···F
H···π
0.007
0.004
0.006
0.005
0.0206
0.0130
0.0169
0.0202
7
9
b
H···S
H···F
H···S
H···F
H···π
0.006
0.007
0.006
0.005
0.0204
0.0318
0.0179
0.0185
11
12
a
When the system presents more than one of the indicated types of
interaction, we report only the value which corresponds to the contact
b
with the largest DI. The indicated contact is an S···σC−H interaction
rather than an H-bond.
observation can be rationalized in terms of the enhanced
acidity of the protons because of the inductive effects of the
double fluorination in 6. Table 3 also presents the quantum
chemical topology parameters for other kinds of H···A contacts
obtained for the dimers of compounds 7, 11, and 12. System 7
shows several weak C−H···F interactions (Figure S4). Given
the expected greater proton affinity of aliphatic over aromatic
fluorine atoms,54,55 11 and 12 display even stronger H···F
contacts (Figures 6 and S5).56 The NCI-index analysis in
Figure 5. C−H···F/C−H···π bridged motifs observed in the dimer of
6. We show the crystal structure of this compound (top-left), a
schematic representation of the interactions within the corresponding
dimer (top-right) and the NCI-index isosurfaces within a dimer of 6
(bottom). The s isosurface value equals 0.5 au. The values of the
electron density are such that ρ ≤ 0.015 au. The scale for the relative
magnitude of the interactions is presented at the bottom of the figure.
Similar results are found for the dimer of 9. Distances are indicated in
Å.
relevant for the formation of the dimers of 6 and 9 are H···π
interactions between one phenyl ring on the phosphine and the
para hydrogen of the fluorothiolate. These interactions are
revealed as cone-shaped surfaces in the NCI-index analysis
shown in Figure 5.
Table 3 shows selected quantum chemical topology
parameters for the A···H (A = F, S, π) interactions in the
dimers of compounds 6 and 9. Besides DIs, the electron
density at the Bond Critical Point (ρ(rBCP)) has also been used
to quantitatively assess the strength of chemical interactions.
The H···F contacts observed in the dimer of the difluorinated
compound 6 show a DI of 0.027 au, with a value of ρ(rBCP) =
0.007 au, while in the dimer of the monofluorinated compound
9 the corresponding values of DI are 0.019 au with ρ(rBCP) =
0.005 au. Thus, the crystalline arrangement is most compact
and presents stronger H···F contacts in system 6. This
Figure 6. Top: Principal interactions revealed by the NCI-index
analysis in compound 12. The isosurface value of s equals 0.5 au, and
the electron density is such that ρ ≤ 0.015 au. The scale for the
relative magnitude of the interactions is presented in the top of the
figure. Bottom: Selected H···A distances in the system. The distances
are reported in Ångstroms.
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compound 12 reveals another H···π contact as typical cone-
shaped surfaces among different rings of the PPh3 fragments
(Figure 6). The distance of the H atom to the phenyl plane in
that interaction is 2.642 Å, and the calculated DI for the
contact is 0.65 au, which indicates a particularly strong
interaction, comparable to that among water monomers in
H2O clusters.57 The Laplacians of the electron density at the
bond critical points (∇2ρ(rBCP)) corresponding to all the
above-mentioned A···H interactions are small and positive, a
condition which indicates the closed shell character of these
contacts.
Architectures with Aurophilic Interactions. Aurophilic
contacts turn out to be the prevalent driving force in the
stabilization of the crystalline arrangement of the moderately
fluorinated compounds 4, 5, 8, and 10. The blue surfaces in the
NCI-index analyses of the dimers of these compounds indicate
strong Au···Au interactions (Figure 8). These analyses also
The dimers of systems 6, 7, 9 and 11 present H···S contacts.
These interactions in 6 and 9 are not hydrogen bonds but
agostic-like S···σC−H interactions as the bond paths associated
with these contacts finish at the C−H bond critical points58
(Figure 7). Although the DIs between H and S are small, the
Figure 8. X-ray diffraction structures and NCI-index analyses of the
dimers of compounds 4, 5, 8, and 10. The NCI plots indicate
aurophilic contacts and secondary π-stacking interactions. The Au···
Au distances are 3.0787(5), 3.1010(4), 3.1019(5), and 3.1140(5) Å
for systems 4, 5, 8, and 10, respectively. The isosurface value of the
reduced density gradient is s = 0.5 au, and the values of ρ satisfy the
condition ρ ≤ 0.015 au. The scale for the relative magnitude of the
interactions is presented at the bottom of the figure.
Figure 7. S···H interactions in the dimer of 6 (top) along with the
corresponding bond paths (bottom). The indicated H···S distance is
reported in Å.
reveal π−πF contacts that, as previously mentioned, are not
strong enough to impair the formation of aurophilic contacts in
these systems. As opposed to the nearly parallel arrangement of
the previously discussed dimers of predominant π-stacked and
H-bonded architectures, the conformations of systems 4, 5, 8,
and 10 in their dimers match those expected for aurophilic
arrangements.40 Namely, we observe S/Au/Au/S torsion
angles equal to 87.1°, 93.5°, 90.3°, and 94.9° for 4, 5, 8, and
10, respectively. Table 4 reports some topological properties of
the electron density used to characterize these aurophilic
contacts. The QTAIM analyses reveal that the examined Au···
total DIs between the S atom and the C−H fragments are not
negligible (0.057 and 0.044 au for 6 and 9, respectively).
Similarly to the H···F contacts, these H···S interactions are
slightly stronger and shorter in 6 than they are in 9 (Table 3).
The values of ∇2ρ(rBCP) indicate that the interactions are
predominantly closed shell in nature. There are bond paths in
the dimers of compounds 7 and 11 associated with H···S
contacts, which are also closed shell interactions.
Overall, H-bonds and π-stacking contacts prevent the
formation of aurophilic interactions which otherwise are
formed with other fluorination patterns as discussed in the
following section.
Au interactions are moderately covalent: the values of ρ(rBCP
)
and ∇2ρ(rBCP) are generally small; the total energy densities
H(rBCP) are slightly negative; and the ratios |V(rBCP)|/G(rBCP
)
are greater than 1.59 The incipient Au···Au contact in the
dimer of 3 is weaker than those in the rest of the investigated
F
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Table 4. QTAIM Descriptors and Experimental Distances for the Au···Au Interactions Observed in This Work
system
qAu1/a.u.
qAu2/a.u.
DIAu···Au
ρ(rBCP
)
∇2ρ(rBCP
)
H(rBCP
)
|V(rBCP)|/G(rBCP
)
G(rBCP)/ρ(rBCP
)
d
Au···Au/Å
3
4
5
8
10
SPh
−0.01
−0.05
−0.05
−0.04
−0.05
−0.04
−0.01
−0.04
−0.05
−0.04
−0.05
−0.04
0.119
0.280
0.260
0.277
0.257
0.246
1.34 × 10−02
2.80 × 10−02
2.69 × 10−02
2.69 × 10−02
2.62 × 10−02
2.43 × 10−02
3.13 × 10−02
7.12 × 10−02
6.86 × 10−02
6.84 × 10−02
6.70 × 10−02
6.15 × 10−02
+0.0005
−0.0011
−0.0009
−0.0008
−0.0007
−0.0004
0.93
1.06
1.05
1.05
1.04
1.03
0.54
0.68
0.67
0.67
0.67
0.65
3.5068(6)
3.0787(5)
3.1010(4)
3.1019(5)
3.1140(5)
3.155 (2)
a
[Au(SPh)(PPh3)].65
a
compounds, and it has a different character, as revealed by the
corresponding smaller values of DI, ρ(rBCP), and ∇2ρ(rBCP
modulation of the interactions discussed in the paper via the
fluorinated moiety.
)
and the fact that the quotient |V(rBCP)|/G(rBCP) is less than 1.
Accordingly, the small, yet negative, charges of the gold atoms
of 3 explain the incipient formation of the Au···Au contact in
this dimer. This interaction does not prevail over the rest of
intermolecular contacts in this system, and thus, its role is
secondary in the mainly π-stacked architecture discussed
previously.
CONCLUSIONS
■
The analysis of a series of gold(I) compounds bearing
triphenylphosphine and fluorinated thiolates illustrates how
to rationally modulate the formation of aurophilic contacts via
their supramolecular competition with π−πF stacking and H-
bond interactions. While highly fluorinated systems favor the
formation of strong π-stacking interactions unless strongly
steric groups interfere as they do in compound 3, a decrease in
the fluorination degree allows the formation of aurophilic
contacts except if the fluorination pattern promotes the
formation of C−H···F motifs. QTAIM analyses indicate that
π-stacked architectures prevail when large electron delocaliza-
tion occurs for the involved rings (between 0.24 and 0.30 au).
When this electron delocalization decreases, H···F and
aurophilic contacts arise. The latter become dominant when
the DI between the gold atoms is larger than 0.2 au. In general,
the preference in interactions follows the order π−πF > H···F >
Au···Au > π−π. The fluorination of the ligands exerts
electronic effects that contribute to the modulation of
aurophilic contacts and other noncovalent interactions. Besides
the previous evidence which supports that electron rich gold
centers form strong Au···Au contacts, our results indicate that
the acceptor character of the gold centers might also
strengthen these interactions. This donor−acceptor interplay
between the gold centers can also be moderated by the degree
of fluorination of the ligands. Overall, the results of this
investigation yield valuable insights into the design of
supramolecular building blocks of Au(I) complexes.
As recently stated by Dem’yanov et al.,38 electron-rich gold
centers tend to form relatively strong Au···Au contacts, but the
relation between charge-density and the energetics of
aurophilic interactions is not straightforward. The system
with the largest DI(Au···Au), and correspondingly the stronger
interaction, is the dimer of the difluorinated compound 4. We
note that the strength of the interaction decreases with the
change in the fluorination pattern in 5 and with the removal of
a fluorine atom in 8 and 10. Furthermore, the dimer of the
unfluorinated derivative [Au(SPh)(PPh3)] has a larger Au···Au
distance (3.155(2) Å)60 and smaller values of DI, ρ(rBCP), and
∇2ρ(rBCP). These conditions indicate a weaker aurophilic
interaction in the dimer of the last-mentioned compound.
Therefore, the strengthening of aurophilic contacts might
result not only from an increase of the electron population over
the gold centers but also from a slight degree of fluorination in
the system. This observation can be understood by considering
Au···Au contacts as donor−acceptor interacting pairs.33,34 In
this way, the electronic population of gold atoms characterizes
the capabilities of these centers as donors while the light
fluorination of the thiolate improves their efficiency as
acceptors.
In summary, we analyzed how the fluorination in thiolate
ligands can modify the crystalline self-association of gold(I)
compounds. The relative strength of the different interactions
in each system has been quantified by means of several
QTAIM descriptors, in particular by DIs. While phenyl groups
are present in all compounds. Thus, π-stacking interactions
may occur; simple phenyl π−π stacking motifs (DI < 0.2 au)
are secondary with respect to H···F and Au···Au contacts. The
introduction of stronger π-stacking synthons as the highly
fluorinated rings allows the formation of stronger π−πF
interactions (DI > 0.2 au) which overcome the H····F and
Au···Au contacts. These observations provide a valuable tool
for the tuning of aurophilic contacts and illustrate how the
experimental and theoretical approach followed in this research
proves useful in the quantitative assessment of the strength of
noncovalent interactions in compounds containing gold atoms.
Furthermore, the examined systems are of broad interest for
inorganic chemists as PPh3 is a model for the wide range of
phosphines used as ligands for gold complexes. Additionally,
the thiolate pseudohalogen nature not only provides stability
to the studied compounds but it also allows for an electronic
EXPERIMENTAL SECTION
All the chemicals and deuterated solvents were purchased from
commercial sources and used without further purification. Regular
solvents were dried using standard Schlenk techniques.
■
Caution! Lead is a highly toxic heavy metal. It must be used carefully,
and residual derivatives should be properly disposed of. Thiols and
thiolates are highly smelly and must be manipulated in fume hoods.
Instrumentation. Infrared spectra were measured on a
PerkinElmer FTIR/FIR Spectrum 400 spectrometer in the range of
4000 to 400 cm−1 via Attenuated Total Reflectance (ATR). Elemental
analyses were determined utilizing a Thermo Scientific Flash 2000
Analyzer at 950 °C. 1H and 13C NMR spectra were recorded on a 9.4
T Varian VNMRS spectrometer, while 19F and 31P NMR were
obtained on a 7.0 T Oxford Spectrometer. Chemical shifts are in parts
per million relative to TMS with a δ = 0 ppm internal reference for 1H
and 13C and to external references of H3PO4 for 31P and CFCl3 for 19
F
at 0 ppm. The coupling constants values are given in hertz. Positive-
ion fast atom bombardment spectra (MS-FAB+) were measured on an
MStation JMS-700 mass spectrometer operated at an acceleration
voltage of 10 kV. Samples were desorbed from a 3-nitrobenzyl alcohol
matrix by 3 keV xenon atoms employing the matrix ions as the
reference material.
G
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Synthesis. Lead thiolates Pb(SRF)2 used in the synthesis of the
compounds examined herein were obtained by the reaction of
stoichiometrical amounts of aqueous lead acetate (Pb(CH3COO)2)
with a concentrated solution of the corresponding thiol in methanol,
yielding a yellow or white (SRF = SC6HF4, SCH2CF3) solid that was
thoroughly washed with water and hexane.61 Silver(I) trifluorome-
thylthiolate, obtained as described in ref 62, was used in the synthesis
of 12 because of the instability of the lead analogue. The compound
[AuCl(PPh3)] was obtained by the direct reaction of K[AuCl4] with
PPh3 as reported by Fackler et al.63 The synthesis and characterization
of compounds 1, 3, 11, and 12 were previously reported.64,65 The
syntheses of all the new compounds were carried in a similar manner,
thus only the synthesis of 2 is described in detail.
Compound 2 [Au(SC6HF4)(PPh3)]. A total of 200 mg (0.404
mmol) of [AuCl(PPh3)] was dissolved in 20 mL of CH2Cl2 in a 50
mL round-bottom flask. Later, a stoichiometric amount (0.202 mmol)
of solid lead thiolate was added, and the mixture was stirred at room
temperature for 12 h. Afterward, a white precipitate powder (PbCl2)
was formed, and the reaction mixture was filtered. The liquid fraction
volume reduced to 5 mL via low pressure evaporation. The product
precipitates as a pale yellowish crystalline powder after the addition of
20 mL of hexane to the system. Yield: 89.1% (0.2307 g, 0.360 mmol);
mp: 165−167 °C. Anal. Calcd for C24H16AuF4PS: C, 45.01; H, 2.52;
S, 5.01. Found: C, 45.11; H, 2.57; S, 4.98. IR (cm−1): 1478.83,
1428.41, 1163.40, 1100.16, 910.79, 884.15, 689.98. MS (FAB+; m/Z):
[M]+ 640 (13%), [M−SC6F4H]+ 459 (100%), [M+Au(PPh3)]+ 1099
(100%). 1H NMR (CDCl3, 400 MHz): δ 7.58−7.43 (m, 15H, PPh3),
6.69 (m, 1H, H−C6F4H). 19F-NMR (CDCl3, 282.4 MHz): δ −132.94
(m, 2F, o-F-C6F4H), −141.27 (m, 2F, m-F-C6F4H). 31P NMR
(CDCl3, 121.5 MHz): δ 37.7 (s, 1P, PPh3).
Compound 4 [Au(SC6H3F2-2,4)(PPh3)]. Yield: 90.0% (0.2174 g,
0.360 mmol). mp: 140−142 °C. Anal. Calcd for C24H18AuF2PS: C,
47.69; H, 3.00; S, 5.30. Found: C, 47.66; H, 3.05; S, 5.26. IR (cm−1):
3072.17, 3058.66, 1477.49, 1434.46, 1132.73, 1100.85, 746.86,
689.27. MS (FAB+; m/Z): [M]+ 604 (22%), [M−SC6F2H3]+ 459
(84%), [M+Au(PPh3)]+ 1063 (100%). 1H NMR (CDCl3, 400 MHz):
δ 7.58 (m, 1H), 7.57−7.42 (m, 15H, PPh3), 6.75 (m, 1H), 6.67 (m,
1H). 19F-NMR (CDCl3, 282.4 MHz): δ −100.37 (m, 1F, o−F-
C6H3F2), −116.86 (m, 1F, p−F-C6H3F2). 31P NMR (CDCl3, 121.5
MHz): δ 38.2 (s, 1P, PPh3).
Compound 5 [Au(SC6H3F2-3,4)(PPh3)]. Yield: 89.7% (0.2192 g,
0.363 mmol). mp: 140−142 °C. Anal. Calcd for C24H18AuF2PS: C,
47.69; H, 3.00; S, 5.30. Found: C, 47.71; H, 3.10; S, 5.25. IR (cm−1):
3070.24, 3046.34, 1590.55, 1490.02, 1480.20, 1434.67, 1267.45,
1099.30, 744.72, 690.38. MS (FAB+; m/Z): [M]+ 604 (20%), [M−
SC6F2H3]+ 459 (100%), [M+Au(PPh3)]+ 1063 (86%). 1H NMR
((CD3)2SO, 400 MHz): δ 7.80−7.57 (m, 15H, PPh3), 7.44 (m, 1H),
7.30 (m, 1H), 7.22 (m, 1H). 19F-NMR (CDCl3, 282.4 MHz): δ
−148.37 (m, 1F, m-F-C6H3F2), −141.58 (m, 1F, p-F-C6H3F2). 31P
NMR (CDCl3, 121.5 MHz): δ 36.0 (s, 1P, PPh3).
Compound 8 [Au(SC6H4F-2)(PPh3)]. Yield: 88.6% (0.2100 g,
0.358 mmol). mp: 144−145 °C. Anal. Calcd for C24H19AuFPS: C,
49.16; H, 3.27; S, 5.47. Found: C, 49.18; H, 3.33; S, 5.49. IR (cm−1):
1467.99, 1461.46, 1434.46, 1208.36, 1099.69, 1067.84, 736.21,
688.35. MS (FAB+; m/Z): [M]+ 586 (39%), [M-SC6H4F]+ 459
(100%), [M+Au(PPh3)]+ 1045 (88%). 1H NMR (CDCl3, 400 MHz):
δ 7.67 (m, 1H), 7.58−7.44 (m, 16H), 6.96 (m), 6.90 (m, 1H). 19F-
NMR (CDCl3, 282.4 MHz): δ −105.09 (s, 1F). 31P NMR (CDCl3,
121.5 MHz): δ 38.2 (s, 1P, PPh3).
Compound 9 [Au(SC6H4F-3)(PPh3)]. Yield: 80.1% (0.1900 g,
0.324 mmol). mp: 151−153 °C. Anal. Calcd for C24H19AuFPS: C,
49.16; H, 3.27; S, 5.47. Found: C, 49.09; H, 3.31; S, 5.47. IR (cm−1):
1594.38, 1567.01, 1463.10, 1433.05, 1098.57, 871.62, 742.62, 689.45.
MS (FAB+; m/Z): [M]+ 586 (39%), [M−SC6H4F]+ 459 (100%), [M
1
+Au(PPh3)]+ 1045 (95%). H NMR (CDCl3, 400 MHz): δ 7.60−
7.41 (m, 16H), 7.33 (m, 1H), 7.03 (m, 1H), 6.67 (m, 1H). 19F-NMR
(CDCl3, 282.4 MHz): δ −114.57 (s, 1F). 31P NMR (CDCl3, 121.5
MHz): δ 38.5 (s, 1P, PPh3).
Compound 10 [Au(SC6H4F-4)(PPh3)]. Yield: 89.2% (0.2115 g,
0.361 mmol). mp: 159−160 °C. Anal. Calcd for C24H19AuFPS: C,
49.16; H, 3.27; S, 5.47. Found: C, 49.20; H, 3.25; S, 5.54. IR (cm−1):
1477.77, 1434.79, 1213.42, 1099.36, 1084.49, 823.18, 746.59, 691.64.
MS (FAB+; m/Z): [M]+ 586 (19%), [M−SC6H4F]+ 459 (65%), [M
1
+Au(PPh3)]+ 1045 (100%). H NMR (CDCl3, 400 MHz): δ 7.58−
7.43 (m, 17H), 6.80 (m, 1H). 19F-NMR (CDCl3, 282.4 MHz): δ
−121.17 (s, F). 31P NMR (CDCl3, 121.5 MHz): δ 37.8 (s, 1P, PPh3).
COMPUTATIONAL DETAILS
■
Electron densities and approximate pair densities of the
systems examined herein were computed via Density Func-
tional Theory using the X-ray experimental structures with the
aid of the Orca program.66 More specifically, we used the BP86
exchange-correlation functional67,68 along with the TVZ-
ZORA basis set69 under the Zeroth Order Regular
Approximation (ZORA).70,71 This method has proved to be
accurate in its description of metal−metal and metal−ligand
interactions in coordination and organometallic compounds.
The resulting electron and pair densities were analyzed with
the Quantum Theory of Atoms in Molecules (QTAIM), which
provides a partition of the three-dimensional space in disjoint
regions that are identified with atoms and functional groups in
chemistry.41,72,73 The QTAIM is based on the electron
distribution, a scalar field that equals the expectation value of
a Dirac observable, i.e., ρ(r) = ⟨∑Ni=1δ(ri − r)⟩ and therefore is
invariant under orbital rotations. This condition enables
QTAIM to examine chemical bonding in different systems,
e.g., π−π, H-bonded, and donor−acceptor complexes74 under
the same physically sound footing. The QTAIM analysis was
performed with the AIMAll software.75 Noncovalent inter-
actions were further examined by considering the NCI index42
(a quantity based on the reduced gradient of the electron
density), with the NCIPlot program.76 The visualization of
chemical structures was made with the program VMD.77
Compound 6 [Au(SC6H3F2-3,5)(PPh3)]. Yield: 90.0% (0.2171 g,
0.360 mmol). mp: 161−162 °C. Anal. Calcd for C24H18AuF2PS: C,
47.69; H, 3.00; S, 5.30. Found: C, 47.70; H, 3.08; S, 5.33. IR (cm−1):
3072.52, 3059.52, 1602.32, 1575.69, 1479.25, 1433.57, 1101.19,
979.64, 746.83, 690.07. MS (FAB+; m/Z): [M]+ 604 (25%), [M−
SC6F2H3]+ 459 (100%), [M+Au(PPh3)]+ 1063 (100%). 1H NMR
(CDCl3, 400 MHz) δ 7.77−7.60 (m, 15H, PPh3), 7.15 (m, 2H, o−H-
C6H3F2), 6.86 (m, 1H, p-H-C6H3F2). 19F-NMR (CDCl3, 282.4
MHz): δ −114.88 (m, 2F, F−C6H3F2). 31P NMR (CDCl3, 121.5
MHz): δ 33.1 (s, 1P, PPh3).
Compound 7 [Au(SC6H4(CF3)-2)(PPh3)]. Yield: 86.9% (0.2238
g, 0.314 mmol). mp: 149−151 °C. Anal. Calcd for C25H19AuF3PS: C,
47.18; H, 3.01; S, 5.04. Found: C, 47.22; H, 2.97; S, 4.99. IR (cm−1):
1433.88, 1308.72, 1166.98, 1100.90, 1030.18, 997.30, 748.10, 691.90.
MS (FAB+; m/Z): [M]+ 637 (30%), [M−SC6H4F]+ 459 (100%), [M
+Au(PPh3)]+ 1095 (57%). 1H NMR (CDCl3, 400 MHz): δ 7.98 (m,
1H), 7.56 (m, 1H), 7.60−7.41 (m, 15H, PPh3), 7.16 (m, 1H), 7.03
(m, 1H). 19F-NMR (CDCl3, 282.4 MHz): δ −62.02 (s, 3F). 31P
NMR (CDCl3, 121.5 MHz): δ 38.3 (s, 1P, PPh3).
CRYSTAL STRUCTURE DETERMINATION
■
Suitable single crystals of compounds 1−12 were mounted on
glass fibers, and crystallographic data were collected at 130 K
with an Oxford Diffraction Gemini “A” diffractometer with a
CCD area detector with a monochromator of graphite for
λMoKα = 0.71073 Å. CrysAlisPro and CrysAlis RED software
packages were used for data collection and integration.78 The
double pass method of scanning was used to exclude any noise.
The collected frames were integrated by using an orientation
matrix determined from the narrow frame scans. Final cell
constants were determined by global refinement. The
H
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Inorg. Chem. XXXX, XXX, XXX−XXX

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pubs.acs.org/IC
Article
́
absorbance in the collected data was considered via analytical
and numeric corrections using a multifaceted crystal model
based on expressions from the Laue symmetry with equivalent
reflections.79 Structure solutions and refinements were carried
out with the SHELXS-2014 and SHELXL-2014 packages.80,81
The WinGX v2018.382 software was used to prepare material
for publication. Full-matrix least-squares refinements were
Marcos Flores-Alamo − School of Chemistry, National
Autonomous University of Mexico, Circuito Escolar, Ciudad
Universitaria, Coyoacan, 04510 Mexico City, Mexico
Hugo Torrens − School of Chemistry, National Autonomous
University of Mexico, Circuito Escolar, Ciudad Universitaria,
́
́
Coyoacan, 04510 Mexico City, Mexico
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03131
2
carried out by minimizing (Fo − Fc2)2. All non-hydrogen
atoms were refined anisotropically. H atoms attached to C
atoms were placed in geometrically idealized positions and
refined as bonded on their parent atoms, with the C−H bond
length equal to 0.95 and 0.99 Å for aromatic and methylene
groups respectively and with Uiso(H) = 1.2Ueq(C). In the
structures of compounds 4 and 8, F1 and F1A are disordered
over two sites with occupancies 0.63:0.37 and 0.51:0.49,
respectively. Crystallographic data for all the investigated
complexes are presented in Tables S1−S36 in the Supporting
Information.
Author Contributions
G.M.-A. and H.T. designed the experimental work. G.M.-A.
and L.T.G. carried out the corresponding experiments. M.F.A.
determined the crystal structures. J.M.G.-V., E.R.-M., A.M.P.,
and T.R.-R. carried out the quantum chemical analyses. All
authors contributed to the writing of the paper. All authors
have given approval to the final version of the manuscript.
J.M.G.-V. and E.R.-M. contributed equally to the work
presented herein.
Funding
ASSOCIATED CONTENT
■
We acknowledge financial support from DGAPA-UNAM
IN210818 and CONACYT-Mexico CB-2012/177498 as well
as Ph.D. scholarships 270993 and 472432 for J.M.G.-V. and
E.R.-M respectively along with the postdoctoral grant 740732
for G.M.-A. The Spanish government grant (PGC2018-
095953-I100) is also gratefully acknowledged.
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03131.
Supplementary figures and tables, Au−F interaction
characterization, experimental NMR spectra, and
crystallographic tables (PDF)
Notes
The authors declare no competing financial interest.
Accession Codes
CCDC 1955689−1955700 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
ACKNOWLEDGMENTS
■
We acknowledge the instrumental support of the Unit for
Support to Research and Industry (USAII) at the School of
Chemistry at UNAM. We also acknowledge the experimental
́
advice of Hugo Hernandez-Toledo. The authors are also
thankful for the DGTIC/UNAM project LANCAD-DGTIC-
UNAM-250 for computer time. G.M.-A. wishes to thank Prof.
Luisa De Cola for helpful discussions.
AUTHOR INFORMATION
■
Corresponding Author
́
Guillermo Moreno-Alcantar − School of Chemistry, National
ABBREVIATIONS
■
Autonomous University of Mexico, Circuito Escolar, Ciudad
́
Universitaria, Coyoacan, 04510 Mexico City, Mexico; Institut
QTAIM, Quantum Theory of Atoms in Molecules; NCI, Non-
Covalent Interactions; Ph, Phenyl; BCP, Bond Critical Point;
TMS, Tetramethylsilane; DI, Delocalization Index
́
́
de Science et d’ Ingenierie Supramoleculaires (ISIS), University
of Strasbourg, 67000 Strasbourg, France; orcid.org/0000-
0001-9836-4694; Email: morenoalcantar@unistra.fr, lgma@
comunidad.unam.mx
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