Structural Snapshots of π‐Arene Bonding in a Gold Germylene Cation

Abstract Heavier group 14 element cations exhibit a remarkable reactivity that has typically hampered their isolation. For the few available examples, the role of π‐arene interactions is crucial to provide kinetic stabilization, but dynamic and structural information on those contacts is yet limited. In this study we have accessed the metalogermylenium cation [(PMe2ArDipp2)AuGe(ArDipp2)Cl]+ (4+) (ArDipp2=C6H3‐2,6‐(C6H3‐2,6‐iPr2)2) that has been structurally characterized with three different non‐coordinating counter anions. These studies provide for the first time dynamic information about the conformational rearrangement that characterizes π‐arene bonding thorough a series of X‐ray diffraction structural snapshots. Computational studies reveal the weak character of the π‐arene bonding (ca. 2 kcal mol−1) that can be described as the donation from a πC=C bond toward the empty p valence orbital of germanium.


Solution NMR analysis of compounds of type 4.
Cation 4 + has been prepared with four weakly coordinating counteranions, namely   Figure S1. This representation, as well as the δ values included in Table S1, provides some evidence for the existence of cation-anion interactions, since all other parameters (temperature, solvent or concentration) are strictly kept identical. This is not surprising considering the existence of similar interactions in both transition and main group metal systems.
For instance, triflimidate is known to be weakly coordinating, with abundant solid-state structures in which it remained bound to transition 2,4 and main group metals. 5 The same would be expected for the related [NTf 2 ·GaCl 3 ]anionic Lewis pair, though its predicted reduced basicity and increased steric profile may lead to weaker interactions or alternative coordination modes. There are also numerous examples where GaCl 4 anion interacts with metals, 6 and even for the very weakly coordinating BAr F fragment precedents exist, 7 though the latter would likely be forbidden for a highly congested system as 4 + .

Main packing interactions involving counteranions in the structures of 4-NTf 2 , 4-NTf 2 ·GaCl 3 and 4-GaCl 4 .
A range of weak interactions in the solid state and dominated by the three investigated counteranions is most likely responsible for the different conformations observed for cation 4 + . Figure S5 collects those interactions and their defining geometric parameters. In 4·NTf 2 there are two C-H···O interactions for each triflimidate anion involving C(sp 2 )-H termini of terphenyl rings of two different 4 + cations. One of the latter additionally participates in a C-H···F contact with the same triflimidate anion which connects by the same type of interaction a third cation 4 + . The structure of 4·NTf 2 ·GaCl 3 involves pairing of the uncommon [NTf 2 ·GaCl 3 ]anion to a single 4 + cation through two C-H···O contacts, as well as a C-F···F-C interaction with an adjacent anion. Finally, the structure of 4·GaCl 4 is characterized by three C-H···Cl interactions between two of the chlorine atoms of GaCl 4 with three 4 + cations. All C-H···X and F···F interactions exhibit normal interatomic distances and angles. 12

Computational details
Geometry optimization of minima and transition states was carried out with the Gaussian software package. 13 Optimizations were carried out without symmetry restrictions using DFT methods. The PBE0 functional 14 was used with empirical dispersion taken into account by adding the D3 version of Grimme's dispersion with Becke-Johnson damping. 15 The 6-31g(d,p) basis set 16 was used for non-metal atoms and the Au atoms were described with the SDD basis and associated electron core potential (ECP). 17 Bulk solvent effects (benzene) were included during optimization with the SMD continuum model. 18 The extended wavefunction .wfx and NBO .47 files were calculated on previously optimized geometries but using the triple-ζ basis set def2-TZVP 19 basis for all atoms, which includes and ECP for Au. 20 Wavefunction analysis and NBO analysis were performed with the Multifwn code 21 and the NBO6.0 22 software respectively. The CYLview visualization software has been used to create some of the figures. 23 The X-Ray structure of the cation of 4-NTf 2 was optimized freely in bulk benzene resulting in a geometry, 4s + , with a shortest Ge···C aryl distance of 2.40Å, which is 0.09 Å shorter than the experimental result. Inclusion of one explicit triflimidate anion in the calculation yields the expected ion pair and its formation is exergonic by 17.1 kcal·mol -1 (with respect to the ions at infinite distance in bulk benzene solution). Also, this interaction further shortened the Ge···C aryl distance to 2.32 Å, evincing the influence of the counteranion in the geometry of the cation. Furthermore, N-coordination of this anion to the Ge atom is exergonic by 1.02 kcal·mol -1 with respect to the ion pair in agreement with the diffusion NMR experiments.
When the X-Ray structures of 4-NTf 2 ·GaCl 3 and 4-GaCl 4 were used as starting points, free optimizations of the cations, the shortest Ge···C aryl distances collapsed to values significantly shorter than the experimental. Indeed, fully relaxed Potential Energy Surface scans along Ge···C aryl distance coordinates revealed one absolute minimum at shortest Ge···C aryl distances close to that found in 4-NTf 2 and no local minima for scenarios with longer Ge···C aryl distances. Figure S5 shows the results of two of these scans. The blue dots are energies for geometries starting from that of 4s + , as a function of the Ge···C aryl distance. The orange dots correspond to results obtained starting from a geometry with longer Ge···C aryl distances. One striking result from these PES scans is that the shortest Ge···C aryl distance at the absolute minimum is dependant of the initial geometry used in the calculation, which hints at subtle equilibria between attractive and repulsive interactions within the cation. In addition, geometries with longer Ge···C aryl distances must be stabilized by intramolecular interactions, not considered in the present study. Figure S6. Fully relaxed Potential Energy Scan along Ge···C aryl distance coordinates.

Since the calculations could not account for geometries with intermediate and long
Ge···C aryl distances when no geometric constrains were imposed, new optimizations starting from the X-ray structures of the cations of 4-NTf 2 ·GaCl 3 and 4-GaCl 4 were performed, but this time the shortest Ge···C aryl distance was fixed to the experimental value in each case to yield cations 4i + and 4l + . The resulting geometries, together with that of 4s + , were used in subsequent localized orbital (NBO) and electron density (AIM) analysis. Selected data are shown in Tables S4-S6.     Interestingly, the ellipticity at the bcp is high at 1.27. The ellipticity at the bcp is defined as ε = λ 1 /λ 2 -1, where λ 1 and λ 2 are the curvatures (or the eigenvalues of the Hessian) of the electron density perpendicular to the bond path at the bcp, ρ b . For cylindrical bonds, this curvature is expected to be cero, since λ 1 = λ 2 . As can be seen in Figure S7    Ge···Caryl distance fixed in the calculation corresponds to the same atoms as in 4s + and 4i + , and not to the shortest Ge···Caryl distance found in the X-Ray analysis of 4-GaCl 4 .