1Department of Chemistry, Key Laboratory of Functional
Inorganic Materials of Anhui Province, Anhui University, Hefei, 230601,
PR China
2Key Laboratory of Structure and Functional Regulation
of Hybrid Materials (Anhui University), Ministry of Education, Hefei,
230601, PR China
Correspondence
Qinqin Yuan and Longjiu ChengDepartment of Chemistry, Key Laboratory of Functional
Inorganic Materials of Anhui Province, Anhui University,Hefei, 230601, PR ChinaKey Laboratory of Structure and Functional Regulation of Hybrid
Materials (Anhui University), Ministry of Education, Hefei, 230601, PR
ChinaEmail:qinqinyuan@ahu.edu.cn (Q. Yuan) andclj@ustc.edu (L. Cheng)
Funding information
the National Natural Science Foundation of China (21873001), and by the
Foundation of Distinguished Young Scientists of Anhui Province.
1 | INTRODUCTION
Noble gas (Ng) atoms have proven
capable of forming chemical interactions ranging from weak van der Waals
forces to strong covalent bonds and ionic bonds, though they are
traditionally considered to feature chemical inertness arising from the
fully occupied valence shell. In fact, since XePtF6 was
first synthesized by Bartlett in 1962,[1] numerous
Ng complexes have been discovered and investigated, triggering ongoing
and growing attentions. Ng complexes are important in understanding and
developing the bonding theory and related chemistry, as well as in
various research and industrial applications.
For instance, XeF2and XeF4 are exploited as key agents to remove
impurities in alloy smelting,[2] while
XeO3 and XeO4 serve as useful explosive
components.[3]From a chemical bonding perspective, the Ng complexes uncovered so far
can be generally divided into two types according to the formation
mechanism differentiated by how the Ng atoms donating electrons. The
former refers to those formed from that Ng atoms interact with highly
electronegative atoms (F, Cl, Br, O, N) or groups (C≡CH), constituting
traditional covalent or ionic bonds.[4-12] At
present, Ng atoms have been found to bind to electronegative parts with
a coordination number up to 6 and Ng atom itself is insp3d2 hybridization, as in
both inorganic XeFn (n = 2, 3, 4,
6)[2, 13, 14] and organic
Xe(CCH)n(n =1, 4, 6).[11, 12] The latter type,
however,
represents
those where Ng atoms donate electrons to metal atoms (Cu, Ag, Au) to
form coordinated structures through van der Waals interactions or
covalent bonds with Ng in sp3 hybridization. In
2000, by reduction of AuF3 with elemental xenon, the
first complex containing noble gas-noble metal (Ng-Nm) interactions
(AuXe42+[Sb4F11]2)
was synthesized experimentally,[15] in which two
kinds of noble element, i.e., Au and Xe, formed a strong bonding
of the σ-donor type. This unique Ng-Nm bond stimulated further
search for similar complexes. Soon after, Gerry et al. observed a
series of linear molecules comprising such bonds via laser ablation of
metals in explosion of buffer Ng. These species share a general formula
of NgNmX (Ng = Ar, Kr, Xe; Nm = Cu, Ag, Au; X = F, Cl, Br)[16, 17-23] and comprise Ng-Nm bonds that are
short and rigid. In particular, the
Xe-Au bond in XeAuF ranked the strongest Ng-Nm bond and was specially
characterized with covalent character.[21]Given that a Ng atom like Xe owns
fully occupied valence shell and can thus afford as many as eight
electrons to form four equivalent sp3 orbitals,
it is reasonable to forge multi-coordinated Ng structures via
interactions with metals, yet such multi-coordinated complexes have
hardly been reported.
Gold outstands as a promising role in the study of Ng chemistry and in
constructing brand-new multi-coordinated Ng structures. As a heavy atom,
gold possesses strong relativistic effects[24-27]that induce the expanding of 5d orbital and the shrink of the
6s orbital. Accordingly, Au exhibits a higher electron affinity
compared to other noble metals like Ag or Cu, which was readily verified
in complexes like NgNmX, where Ng-Au bond is saliently stronger than
Ng-Ag and Ng-Cu bond. Another unique property of gold is the
Au(I)···Au(I) aurophilic interaction, a special attractive force between
Au atoms with closed-shell electronic configuration
(5d10) that is thought to be a joint result of
strong relativistic effects and dispersion-type electron correlations.[28-38] Aurophilic interactions exert profound
effects on the configuration of related molecular structures. For
example, in a prior synergetic photoelectron spectroscopy (PES) and
theoretical study of gaseous
Au2I3- cluster,
aurophilic interaction was found to compete with Au-I covalent bonding,
leading to the formation of two “bond-bending isomers” with
approximately degenerate energies.[39]Therefore, equipped with these
peculiar characteristics, Au indubitably proves an ideal candidate for
further pursuing novel multi-coordinated Ng-metal structures.
However, although many efforts have been devoted to generating and
investigating Ng complexes, the understanding of relevant structures and
bonding remains controversial and obscure. In fact, it has become one of
key issues to search for new Ng complexes, especially the
multi-coordinated Ng-metal species, and explore the underlying bonding
mechanism. Ng atoms, in particular
those having larger radiuses, are capable of coordinating several atoms
or groups simultaneously.[14] One is nature to
wonder, if the strongest Ng-Nm bond, i.e. Xe-Au bond, can be
extended from already obtained XeAuF to heretofore unknown
Xe(AuF)n (n ≥ 2), in light of the fact
that the diameter of Xe atom is large enough to spatially satisfy more
AuF groups. To address this question and gain more insights into the
Ng-metal bonding nature, this work theoretically predicts and
characterizes a series of novel multi-coordinated Ng complexes,
Xe(AuF)n (n = 2-4), by performing quantum
chemical calculations, serving as
an essential complement to the research field of Ng chemistry. To the
best of our knowledge, such multi-coordinated
Xe(AuF)n(n = 2-4) complexes have not
been reported yet.
2 | COMPUTATIONAL
DETAILS
In order to select an appropriate theoretical methodology, two key
structural and energetic parameters of linear XeAuF molecules,i.e., the length and dissociation energy of Xe-Au bond obtained
separately from previous experimental work and CCSD(T) calculations, are
employed to benchmark several computational approaches including DFT
with different functionals (e.g., PBE,[40]B3LYP,[41] M06-2X,[42]CAM-B3LYP,[43] ωB97X,[44]and LC-ωPBE-D3 [45]) and MP2. During the
evaluation, def2TZVPP basis set were used for DFT calculations, while
MP2 and CCSD(T) computations are performed with def2-QZVP basis set. All
the dissociation energies are acquired based on the LC-ωPBE-D3/def2TZVPP
optimized geometry. According to the comparison results presented inTable S1 and for the sake of balancing the accuracy and cost of
calculations, the LC-ωPBE-D3/def2TZVPP method, whose outputs are very
close to the judgment criteria, was chosen for further geometry
optimization and single-point energy calculations of
Xe(AuF)n (n = 2-4). Geometry optimizations
were carried out without any symmetry constraints. Vibrational frequency
analysis was performed at the same level to ensure that the optimized
structures were true minima, and obtained frequencies were used to
calculate the zero-point vibrational energies (ZPEs).
All the DFT, MP2 and CCSD(T) calculations and natural bonding orbital
(NBO) analysis [46, 47] were performed with
Gaussian 16 suites.[48] Basis set superposition
error (BSSE) corrections are included when computing dissociation
energies.[49] To deeper understand the electron
structures and chemical bonding characteristics, the electron and
bonding analyses including electron localization function
(ELF),[50] extended transition state–natural
orbitals for chemical valence (ETS-NOCV),[51, 52]reduced density gradient (RDG)[53] and quantum
theory of atoms in molecules (QTAIM)
methods[54-56] were conducted with the aid of
Multiwfn codes.[57]
3 | RESULTS AND
DISCUSSION
3.1 | Geometries and Binding
Motifs
Multiple optimized structures of
Xe(AuF)n(n = 2-4) were theoretically located at the LC-ωPBE-D3/def2TZVPP
level and alphabetically labeled from na tonc according to their relative energies, as shown inFigure 1. The key geometrical parameters for all these
predicted structures along with XeAuF were summarized in Table
S2. For Xe(AuF)2, two near-degenerate isomers,2a and 2b, were found to adopt a common bent geometry
with C2v symmetry, in which the center Xe atom
binds to both AuF units but with different sizes of Au-Xe-Au angle. A
prior case study of
Au2I3- shows similar
bond-bending isomerism that was characterized by an intramolecular
competition between aurophilic interaction and Au-I covalent
bonds.[39] Here, the slightly high-lying isomer2b simply contains two strong covalent Xe-Au bonds with a
length of 2.56 Å, almost identical to that in linear
XeAuF.[21] This covalent bonding favors a obtuse
Au-Xe-Au bent angle of 104.4°, a value that resembles the angle of water
molecules and supports the water-like bent configuration. Isomer2a is more stable with 0.077 eV lower in energy due to the
additional intramolecular Au···Au aurophilic interaction arising from
the saliently smaller Au-Au distance of
2.82 Å compared to that of 4.05 Å in2b. From 2b to 2a, the bond angle of Au-Xe-Au
pronouncedly shrinks from 104.4° to 64.9°, the bond length of Xe-Au is
mildly elongated from 2.56 Å to 2.63 Å, while the length of Au-F bonds
maintains almost unchanged.FIGURE 1 The optimized structures of
Xe(AuF)n (n = 2-4) at the level of
LC-ωPBE-D3/def2TZVPP with key bond lengths (Å) and angles (°) noted in
black. The natural population analysis (NPA) charges on Xe, Au and F are
shown in red. The structural symmetries and relative energies (in eV)
are given under each structure (Xe, pink; Au, dark cyan; F, purple). The
structures framed by dotted lines have imaginary frequencies at this
level.
The n = 3 complex presents three triangular conical geometries.
The lowest-lying 3a isomer features C3vsymmetry and three intense aurophilic interactions between every two of
the three Au atoms but is calculated to have an
imaginary
frequency of -29.05 cm-1. TheCs symmetric 3b structure lies 0.241 eV
higher in energy and contains one single aurophilic interaction between
the Au1 and Au2 atoms that are separated by 2.81 Å and exhibit a bending
angle of 64.5° with respect to the center Xe atom. The other Au3 atom
resides 4.05 Å apart from each of these two Au atoms, with two Au-Xe-Au
angles equaling to 102.7°. The rest 3c structure devoid of
aurophilic interactions has a C3v symmetry,
displaying three identical Au-Au distances (4.02 Å) and bending angles
(104.0°). By comparing 3b and 3c, the absence of one
intramolecular aurophilic interaction raises the energy of 3cby nearly 0.1 eV. It is also worth mentioning that the Au-Au distance
corresponding to the aurophilic interaction keeps almost identical for2a (2.82 Å) and 3b (2.81 Å), hinting that these
aurophilic interactions are considerably close in intensity.
For n = 4, the 4a (Cs) and4c (D2d) structures are both calculated
to have imaginary frequencies albeit featuring aurophilic interactions,
while 4b (Td) is determined as the
stable local minima, which may be attributed from the synergistic effect[58] that all eight outer electrons of Xe
participate in interacting with four AuF ligands in a perfectly
symmetrical manner, forming a regular tetrahedral configuration like
that of well-known methane.
Overall, these predicted structures informally fall into two types based
on whether intramolecular aurophilic interactions exist. One type is
that Au simply forms conventional covalent bonding patterns to Xe and
that different AuF moieties are connected through a central Xe atom via
Xe-Au interactions. The calculated Au-Xe bond length for 2b,3c, and 4b structures equal to 2.56, 2.55 and 2.53
Å,
respectively, appreciably shorter compared to the sum of covalent radii
of bare Au and Xe atoms, suggesting the covalent Xe-Au bonding nature.
By comparison, the other type features both Xe–Au covalent and Au···Au
aurophilic interactions. The isomers of this type, e.g.,2a and 3b, shows a much closer Au-Au distance (2.82
and 2.81Å), in contrast to the former type with Au-Au distance up to 4.0
Å. The lengths of Xe-Au bonds for the Au involved in aurophilic
interaction are also slightly elongated, while the Au-F bonds maintain
basically identical in length. For the sake of simplicity of discussion,
these two types of isomers are each named as traditional andaurophilic structures.FIGURE 2 Electron and bonding analysis of
Xe(AuF)n (n = 2-4). The left shows results
of the extended transition state (ETS) method combined with the natural
orbitals for chemical valence (ETS-NOCV), while the right visualizes the
electron localization function (ELF) pictures.
3.2 | Electronic structures and chemical bonding
analysis
Noting that some of the structures are computed to be unstable with
imaginary frequencies, only the rest stable isomers, namely 2a,2b, 3b, 3c and 4b, were used to
in-depth analyze the electronic structure, while the analysis results of
the unstable species were simply listed in Figure S1-S3 for
necessary reference. As a matter of fact, for all these
multi-coordinated structures, Au
atoms are in the +1 oxidation state. The 6s electron of each Au
atom is transferred to the adjacent F atom, forming an intense Au-Fσ covalent bond with each Au atom in
5d10 electron configuration. Meanwhile, the
highly electronegative F atom induces a σ-hole of Au atom on the
side away from the F atom (Figure S4). The electron on Xe atom
will be further pushed to this σ-hole and constitute covalent
Xe-Au bonding. This generalizes how the electron donation-feedback
mechanism works within the formation of Xe-Au bonds.
To better understand the chemical bonding nature and to obtain a
qualitative bonding picture, ETS-NOCV analysis and EFL analysis were
jointly performed (Figure 2) to elucidate the electronic
structures. ELF directly visualize the probability of finding an
electron in the vicinity of another reference electron. On the other
hand, ETS-NOCV method has been long established useful in investigating
orbital interactions by decomposing the orbital interaction energy into
the contribution of each NOCV pair. The density of each NOCV pair
corresponds to its contribution to the electron density. Here,
Xe(AuF)n were divided into n+1 groups (Xe
and nAuF, n = 2-4) to directly probe the interactions of
Xe and Au. Figure 2 shows the dominating NOCV pairs selected to
characterize Xe-Au bonding. Apparently, all the Xe-Au bonds are formed
upon p electron transferred from Xe to Au (see pair 1) and the
feedback from d orbital of Au atom to Xe (see pair 2). No orbital
interactions were observed between any two Au atoms in 2b,3c and 4b, indicating the absence of aurophilic
interactions. By contrast, the NOCV pairs in 2a and 3bclearly present the d orbital interaction between the specific Au
atoms that forms aurophilic interaction, which was also visually
confirmed by the rendered ELF image in Figure 2. Moreover, the ETS-NOCV
analysis suggests stronger d orbital feedback of Au in the
aurophilic species, interpreting why the Xe-Au bonding intensity differs
when aurophilic interactions are engaged. The hyperconjugation ofnNg → σ*Nm, on the other hand, results
in electron density transferring from the lone pair electron of Ng to
the antibonding orbital of NmX moiety, thereby endowing the Ng atom
moderate positive charge with the X atom getting slightly negative,
aligning with the NPA analysis in Figure 1. When the number of
AuF groups increases, the positive charge on Xe atom of traditional
structures (2b→3c→4b) gradually increases
from 0.35 to 0.45 and then to 0.49, implying a rise in the contribution
of electron pairs of Xe, while the positive charge on Au and F keeps
nearly unchanged.FIGURE 3 Reduced density
gradient (RDG) analysis of Xe(AuF)n (n =
2-4) with non-covalent
interactions (NCI) isosurface set at s = 0.3.
In addition, to quantitatively investigate the related chemical bonding
strength, Reduced density gradient (RDG) analysis was conducted to
obtain the peak spike values of non-covalent interactions (NCI) (seeFigure 3). Generally, the more negative the spike value, the
stronger the corresponding interaction. Three types of spike values were
captured for Xe(AuF)n (n = 2,3,4) from the
crossing point of the RDG image with horizontal axis. The two side
values approximately equal to -0.140, and -0.045, respectively, whereas
the middle value occupies an appreciably broader range from -0.066 to
-0.082. Noting that the spike value in neutral AuF was reported as
-0.138[38], the most negative value of nearly
-0.140 was reasonably attributed to the Au-F bond, which is moderately
stronger in Xe(AuF)n due to the feedback fromd orbital of Au to Xe atom. The close spike values of -0.045 and
-0.048 respectively in 2a and 3b arise from the
isosurface denoting Au···Au interaction on account of they only showing
up in these aurophilic structures. Given above, the middle spike value
that spans from -0.066 to -0.082 is concluded to represent Au-Xe
bonding, in which the smaller values refer to those where Au
participates in aurophilic interactions. Obviously, the Xe-Au bonding
strength in 2a is mildly weaker than that in 2b by
comparing the corresponding spike values of -0.070 and -0.078, in good
agreement with the bond lengths in 2a (2.63Å) and 2b(2.56Å). The 3b structure comprises two kinds of Xe-Au spike
values respectively of -0.080 and -0.066, representing two different
Xe-Au interactions classified according to whether the Au atom is
involved in aurophilic interactions. Moreover, the spike values of Xe-Au
bonding in traditional structures slightly reduce from -0.078 to -0.080
and then to -0.082 as the number of AuF increases, consistent with the
decrease in bond length of Xe-Au (from 2.56 to 2.55 and to 2.53 Å) and
the enhancement of Xe-Au bonding intensities. Furthermore, by comparing
the largest spike value of Xe-Au bond equaling to -0.066 in 3bwith the corresponding value of -0.070 in 2a, we are convinced
the aurophilic interaction is more competitive in 3b than that
in 2a.TABLE 1 The Xe-Au bond lengths, along with electron densityρ(rc), Laplacian of electron density
▽2ρ(rc), electron local
energy density H(rc), local kinetic energy
density G(rc), and the ratio ofG(rc) andρ(rc) at the bond critical points (BCPs)
of the Xe-Au bonds in Xe(AuF)n (n = 1-4)
calculated at the LC-ωPBE-D3/def2TZVPP level.