Unprecedented [d9]Cu?[d10]Au coinage bonding interactions in {Cu(NH3)4[Au(CN)2]}+[Au(CN)2]?salt

Article abstract of DOI:10.1039/d1cc02709c

The X-ray structure of the {Cu(NH₃)₄[Au(CN)₂]}⁺[Au(CN)₂]^?salt is reported showing an unprecedented [d⁹]Cu?[d¹⁰]Au coinage bond. The physical nature of the interaction

Full text of DOI:10.1039/d1cc02709c

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Unprecedented [d9]CuÁ Á Á[d10]Au coinage bonding
interactions in {Cu(NH3)4[Au(CN)2]}+[Au(CN)2]À
salt†
Cite this: Chem. Commun., 2021,
57, 7268
Received 23rd May 2021,
Accepted 23rd June 2021
a
c
Emanuele Priola,
Antonio Frontera
Ghodrat Mahmoudi,*b Jacopo Andreo
and
d
*
DOI: 10.1039/d1cc02709c
rsc.li/chemcomm
The X-ray structure of the {Cu(NH3)4[Au(CN)2]}+[Au(CN)2]À salt is Lewis bases and anions, where the coinage atoms act as Lewis
reported showing an unprecedented [d9]CuÁ Á Á[d10]Au coinage acids. These NCIs involving Group 11 elements acting as
bond. The physical nature of the interaction has been studied using electron acceptors are named either regium bonds10 or coinage
DFT calculations, including the quantum theory of atoms-in- bonds13 in the literature.
molecules, the noncovalent interaction plot and the natural bond
Herein we report experimental and theoretical results that
orbital analysis, revealing the nucleophilic role of the [d10]Au metal reveal the existence of coinage-bond (CiB) between two cationic
and the electrophilic role of [d9]Cu metal.
species where Cu(II) functions as an acceptor of electron density
and Au(I) as a donor of electron density. The [d9]CuÁ Á Á[d10]Au
Noncovalent metalÁÁÁmetal interactions (metallophilicity) are well contacts involving Au and Cu atoms of different units drive the
known forces that can be utilized to generate unique structures formation of supramolecular cationic polymers. Theoretical
with advanced functionalities.1–4 Homo-metallophilicity, such as calculations prove the electrophilic role of copper and nucleo-
aurophilicity,2 argentophilicity,3 and cuprophilicity,4 has been philic role of gold as well as the attractive nature of the
traditionally utilized for the construction of metal clusters. On [d9]CuÁ Á Á[d10]Au interaction. These findings show that CiBs
the other hand, heterometallophilicity allows the generation of involving two coinage atoms (one as the donor and another
diverse structures and functionalities, particularly in the d10–d10 as the acceptor) are robust enough to drive the formation of
heterometallic complexes.5 Among them, supramolecular archi- attractive cationÁ Á Ácation NCIs, which determine the crystal
tectures through AuÁÁÁCu interactions have been developed to packing of the salt. To the best of our knowledge, there is no
construct phosphorescent materials.5,6 The dispersive nature of previous report in the literature describing Cu(II)Á Á ÁAu(I) attrac-
metallophilic interactions, which take place between closed elec- tive NCIs. In contrast, linear 1D polymeric assemblies formed
7
¨
tron shell atoms, has been proposed by Pyykko. An alternative by alternating [CuBr2]ÁÁÁ[AuL2] units (see Scheme 1) guided by
orbital explanation was also proposed by Hoffmann8 and rein- [d10]CuÁ Á Á[d10]Au NCIs have been described.17
forced by the investigation of Fonseca-Guerra’s group.9
As part of our continuous investigations on NCIs involving
Several theoretical and experimental investigations10–15 have coinage elements, we obtained the single-crystal X-ray structure
shown that nanoparticles16 and halide salts of Cu, Ag, and Au of dicyanoaurate salt of {Cu(NH3)4[Au(CN)2]}+ (1), see Fig. 1a. It
form attractive noncovalent interactions (NCIs) with a variety of was synthesized in a good yield (99%) by mixing [Cu(NH3)4]2+
with K[Au(CN)2] in water (see the ESI† for further experimental
a
´
Department of Chemistry, Universita di Torino, Via Pietro Giuria 7, Torino 10125,
details). In the solid state, the Cu(II) metal centre of the cationic
unit is coordinated to four ammonia ligands in the basal plane
and to a dicyanoaurate metalloligand in the apical position.
The cationic and anionic units of 1 are connected through quite
long N–HÁ Á ÁC contacts. The N–HÁ Á ÁC angles are 146.31 and
150.31 and the HÁ Á ÁC distances are 2.811 Å and 2.927 Å, which
could be classified as long H-bonds or C–HÁ Á Áp(CRN) interac-
tions. Fig. 1b depicts the location of the five [Au(CN)2]À anions
that surround the cation. Two of them are located opposite to
the coordinated [Au(CN)2]À metalloligand, establishing the
aforesaid long N–HÁ Á ÁC contacts. The other three anions are
Italy
b Department of Chemistry, Faculty of Science, University of Maragheh,
P.O. Box 55136-83111, Maragheh, Iran. E-mail: ghodratmahmoudi@gmail.com
c BCMaterials, Basque Center for Materials, UPV/EHU Science Park, Leioa 48940,
Spain
d Department of Chemistry, Universitat de les Illes Balears,
Crta de Valldemossa km 7.6, Palma de Mallorca 07122, Baleares, Spain.
E-mail: toni.frontera@uib.es
† Electronic supplementary information (ESI) available: Synthesis, X-ray details,
tables with distances and computational details. CCDC 2085287. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1cc02709c
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[SRvdw = 1.73 (Cu) + 1.86 (Au) = 3.59 Å] and quite longer than the
sum of covalent radii (SRcov = 2.68 Å), thus suggesting the
formation of a CuÁ Á ÁAu NCI. The Cu1 ion in the cationic unit
adopts the classical square-pyramidal arrangement and the Au2
atom of another cationic unit gets close to the copper centre,
along the direction of the apical CN–Cu bond (N5–Cu1Á Á ÁAu2 is
186.71) and almost orthogonal to the CuN4 mean plane (i.e., the
N5–Cu1Á Á ÁAu2 angle is 84.91 and the N6–Cu1Á Á ÁAu2 angle is
86.71). Therefore, the interaction might be rationalized as
s–hole CiB. These CiBs dictate the formation of an infinite
zig-zag supramolecular chain (see Fig. 1c) bearing the anions as
appended residues.
First, the theoretical study (see the ESI† for details) was
focused on the analysis of the molecular electrostatic potential
(MEP) surfaces of the {Cu(NH3)4[Au(CN)2]}+ cationic unit and
the ion-pair. The MEP results are summarized in Fig. 2. As
expected, the MEP values of the isolated cation are positive
almost over the entire surface because of its net positive charge.
Only the non-coordinated N-atom of the cyano ligand remains
negative (À29 kcal molÀ1). The MEP maxima are located at the
Cu-atom opposite to the coordinated cyano group (s–hole) and
the H-atoms of coordinated ammonia ligands (+144 kcal molÀ1),
thus explaining the location of the counterions in the solid state.
The MEP surface analysis of the salt (the closest anion has been
used in this model) shows that the value of the potential at the
s–hole of the Cu center is reduced to +88 kcal molÀ1. The MEP
maxima are located at the H-atoms of the coordinated ammonia
ligands (+105 kcal molÀ1). The MEP minimum is located between
Scheme 1 Left: Reported supramolecular polymer based on heterome-
tallophilic Cu[d10]Á Á ÁAu[d10
] interactions. Right: First example of a
Cu[d9]Á Á ÁAu[d10] CiB supramolecular polymer.
Fig. 1 (a) X-ray structure of compound 1 with the atom numbering
scheme. (b) Partial view of the X-ray structure showing the cationic
{Cu(NH3)4[Au(CN)2]}+ unit surrounded by five [Au(CN)2]À anions. Aurophilic
interactions indicated using red dashed lines and N–HÁ Á ÁC,N contacts
using black dashed lines. (c) Representation of the zig-zag supramolecular
polymer of 1 containing CiBs (shown in black dashed lines) and aurophilic
interactions in red. Distances in Å.
located close and parallel to the coordinated [Au(CN)2]À metal-
loligand, forming a combination of N–HÁ Á ÁN bonds (HÁ Á ÁN
distances ranging 2.27 to 2.34 Å) and classical [d10]AuÁ Á Á[d10]Au
aurophilic interactions (AuÁ Á ÁAu separation 3.37 and 3.67 Å).
In the crystal packing, the Cu(II) atom of {Cu(NH3)4
[Au(CN)2]}+ forms a short contact with the Au(I) atom of the
adjacent cationic unit (see Fig. 1c). The AuÁ Á ÁCu separation is
Fig. 2 MEP surfaces of the {Cu(NH3)4[Au(CN)2]}+ cationic unit (a) and the
ion-pair (b) using 0.00 a.u. isovalues at the PBE0-D3/def2-TZVP level of
3.329 Å, shorter than Batsanov’s18 sum of van der Waals radii theory. The values are given in kcal molÀ1 at some points of the surface.
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the terminal N-atoms of the coordinated and non-coordinated NCIplot analysis, which reveals the existence of light blue
[Au(CN)2]À moieties (À100 kcal molÀ1). It is interesting to high- isosurfaces located between the interacting atoms. The com-
light that the MEP at the Au-atoms becomes negative, and thus parison of the different colours of the isosurfaces reveals that
adequate for acting as an electron donor.
CuÁ Á ÁAu shows the strongest interaction. The interaction
Second, for both cationÁ Á Ácation and cationÁ Á Áanion dimers, energy of the cationÁ Á Ácation dimer is repulsive (+23 kcal molÀ1
,
the CuÁ Á ÁAu and AuÁ Á ÁAu contacts were further characterized by see Fig. 3a), as expected. However, it is significantly smaller
using the quantum theory of ‘‘atoms-in-molecules’’ (QTAIM)19 than the electrostatic repulsion originating from two point
and the noncovalent interaction index (NCIplot).20 By using the charges of the same sign located at the centre of masses of
X-ray geometries, Fig. 3 shows the results of using QTAIM and both molecules (+58.9 kcal molÀ1), thus confirming the attrac-
NCIplot index computational tools combined in the same tive nature of the NCIs (CuÁ Á ÁAu and N–HÁ Á ÁC contacts).
representation. The interaction energies are also indicated in The interaction energy of the ion-pair dimer is attractive
Fig. 3. The QTAIM analysis confirms the existence of both the (À67.4 kcal molÀ1) and slightly larger (in absolute value) than
AuÁ Á ÁAu and CuÁ Á ÁAu contacts, each one characterized by a the electrostatic attraction of two point charges of opposite
bond critical point (CP) and a bond path connecting the Au2 sign located at the centre of masses of both molecules
atom to the Cu1 or Au1 atom (cationÁ Á Ácation or cationÁ Á Áanion, (À64.9 kcal molÀ1), thus suggesting that the AuÁ Á ÁAu aurophilic
respectively). The analysis also reveals the existence of four interaction is weaker than that in the CuÁ Á ÁAu one, in line with
N–HÁ Á ÁC contacts in the cationÁ Á Ácation complex (distances the NCIplot analysis.
range from 2.72 to 2.82 Å). The shortest ones are characterized
We have also studied if orbital contributions are important
by the corresponding bond critical points (CPs) and bond paths in the aforementioned CuÁ Á ÁAu interaction. For this purpose, a
connecting the N–H bonds to the C atoms (see Fig. 3). The natural bond orbital (NBO) second-order perturbation analysis
longest ones are characterized by green (attractive) NCIplot has been performed, since it is adequate for analysing donor–
index isosurfaces located between the H and C-atoms. The acceptor interactions.21 Remarkably, the NBO analysis reveals
presence of CuÁ Á ÁAu and AuÁ Á ÁAu is also confirmed by the two orbital contributions in the cationÁ Á Ácation dimer, which
are also indicated and represented in Fig. 3. These contribu-
tions come from electron donations from one lone pair (LP) of
electrons located on the 5dx2Ày2 Au atomic orbital to the empty
4s and 4p atomic orbitals of Cu. The second-order stabilization
energies E(2) associated with the orbital donor–acceptor inter-
actions are also shown in Fig. 3. The LP(Au) - 4p(Cu) donation
(2.79 kcal molÀ1) is larger than the LP(Au) - 4s(Cu) donation
(1.80 kcal molÀ1). Therefore, the total stabilization energy due
to the orbital contribution to the CuÁ Á ÁAu contact is
4.59 kcal molÀ1. This NBO analysis evidences the CiB nature
of the CuÁ Á ÁAu interaction, where the Cu atom acts as an
acceptor and the Au atom acts as a donor of electron density.
These results are in line with recent theoretical investiga-
tions that have demonstrated that gold in the oxidation state +1
can behave as a Lewis base and, particularly, as an excellent
HaB acceptor.22 It has been estimated that the strength of the
halogen bonds involving I2 as a s–hole donor and Au(I) as an
electron donor in the cationic [(H3N–Au–NH3)Á Á ÁI2]+ adduct is
À5.9 kcal molÀ1 23
,
which is similar to the CiB energy derived
from the NBO analysis in 1. In addition, experimental findings
and DFT calculations have recently shown that cationic species
(iodonium cations, L–I+–L) are able to act as nucleophiles in
co-crystals with Ag(I) complexes, establishing I+Á Á ÁAg+
interactions.24 This NCI has been termed as ‘‘nucleophilic
iodonium interaction’’ (NII).
In conclusion, experimental evidence is provided for the
first time demonstrating that {Cu(NH3)4[Au(CN)2]}+ cations
form short CuÁ Á ÁAu interactions in the solid state. DFT calcula-
tions show that these interactions are attractive, and their
directionality is consistent with s–hole CiBs. The combined
experimental and theoretical results reported here prove that
cationÁ Á Ácation CiB interactions involving {Cu(NH3)4[Au(CN)2]}+
are relevant in guiding the crystalline structure and are
Fig. 3 QTAIM distribution of the bond and ring CPs (red and yellow
spheres, respectively) and bond paths for the cationÁ Á Ácation (a) and
cationÁ Á Áanion (b) dimers of compound 1. The superimposed NCIplot
isosurfaces (RDG isovalue = 0.4) are shown. The cut-off r = 0.04 a.u.
has been used. Colour range: À0.035 a.u. r(signl2)r r 0.035 a.u. Level of
theory: PBE0-D3/def2-TZVP. The second-order perturbation energies E(2)
along with the donor and acceptor orbitals obtained from the NBO
analysis are indicated.
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mers. The unprecedented Cu[d9]Á Á ÁAu[d10] CiBs reported here
À
in addition to CiBs recently described in anionÁ Á Áanion AuCl4
salts25 are important extensions of the potential of gold inter-
actions, which are currently dominated by aurophilic bonds.2c
We gratefully acknowledge the MICIU/AEI of Spain
(ref. CTQ2017-85821-R, FEDER funds), the University of
Maragheh and the Iran Science Elites Federation (ISEF) for
financial support of this research.
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