Aurophilic Interactions in Cationic Three-Coordinate Gold(I) Bipyridyl/Isocyanide Complex

Abstract

Gold(I) isocyanide complexes featuring Au···Au interactions attract considerable attention because of their tunable photophysical properties. Although the synthetic exploration of isocyanide gold(I) complexes seems reasonable, their structural diversity is mainly limited to linear gold(I) derivatives. The synthesis and structural characterization of cationic three-coordinate gold(I) mixed 2,2′-bipyridyl/isocyanide complex are presented here for the first time. Cationic gold species form supramolecular dimers in the solid state via attractive Au···Au interactions. The nature and energies of aurophilic contacts, which are responsible for dimerization in the solid state, were studied by DFT calculations together with QTAIM, ELF, RDG, and NCI techniques and Hirshfeld surface analysis. The estimated energy of the aurophilic interactions was 6.3 kcal/mol.

1. Introduction

Metallophilic interactions in soft materials have recently emerged as a topic of growing interest since they have an impact on material properties, such as photophysical [1,2] and magnetic properties [3], electrical conductivity [4,5], sensing [6], etc. Several reports have demonstrated that the strength of aurophilic contacts is comparable to that of hydrogen bonding [7,8].

Aurophilic interactions have been implicated in the intriguing luminescent features of metal complexes. The isocyanide complexes of gold(I) are versatile luminescent compounds that have attracted significant attention and often feature aurophilic contacts [9,10]. These attractive Au···Au interactions in isocyanide complexes are important since they influence the self-assembly of these materials in the solid state. Neutral linear aryl gold(I) isocyanide derivatives showed potential in the context of the exploration of their tunable photophysical characteristics in the solid state, which could be affected by aurophilic interactions [11,12,13]. Cationic linear diisocyanide gold(I) complexes also showed a propensity to self-assemble in the solid state in higher-order emissive aggregates via attractive Au···Au interactions [1,9,10]. Although the synthetic exploration of isocyanide gold(I) complexes is an attractive area, their structural diversity seems to be limited to linear gold(I) derivatives. Here, we describe the synthesis and structural characterization of the cationic three-coordinate gold(I) 2,2′-bipyridyl/isocyanide complex, which forms supramolecular dimers in the solid state via attractive Au···Au interactions, for the first time. In addition, we performed DFT calculations together with QTAIM, ELF, RDG, and NCI techniques and Hirshfeld surface analysis, which supported the presence of intermolecular aurophilic interactions in the solid state.

2. Materials and Methods

General remarks. All manipulations were carried out in air unless specified otherwise. All the chemicals employed in this work were purchased from commercial suppliers (Aldrich (Schnelldorf, Germany), TCI-Europe (Zwijndrecht, Belgium), Strem (Bischheim, France), and ABCR (Karlsruhe, Germany)). The solvents were distilled using standard methods. Nuclear magnetic resonance spectra were obtained on a Bruker Avance neo 700 (Karlsruhe, Germany); chemical shifts (δ) are given in ppm, and coupling constants (J) are given in Hz. C, H, and N elemental analyses were performed on a Euro EA 3028HT CHNS/O analyzer (Pavia, Italy).

Computational details. The quantum chemical calculations based on the X-ray geometry of 4 were carried out using the ωB97XD [14] DFT functional in the Gaussian-09 program [15]. The Douglas–Kroll–Hess 2nd order scalar relativistic calculations with the relativistic core Hamiltonian were carried out using the DZP–DKH basis sets [16,17] for all atoms. The topological analysis of the electron density distribution (quantum theory of atoms in molecules, QTAIM), reduced density gradient (RDG), electron localization function (ELF), and noncovalent interactions (NCI) analyses were performed using the Multiwfn program (version 3.7) [18]. The VMD program [19] was additionally utilized for visualization. The Cartesian atomic coordinates for the model structure are presented in Table S1 (Supplementary Materials). The Hirshfeld molecular surface was generated by the CrystalExplorer program (version 17.5) [20] using the normalized contact distance d_norm [21] based on Bondi’s van der Waals radii [22].

Synthesis of 4. Silver bis(trifluoromethanesulfonyl)imide (AgNTf₂) (0.055 mmol, 22 mg) was added to a solution of chloro(2,6-dimethylphenyl isocyanide)gold(I) (0.055 mmol, 20 mg) in CH₂Cl₂ (1 mL). The reaction mixture was placed in an ultrasound bath for 20 min. After that, the reaction mixture was filtered, and then 2,2′-bipyridyl (0.055 mmol, 9 mg) was added. The solvent was evaporated to obtain a colorless solid (yield 80%, 21 mg). ¹H NMR (700 MHz, CDCl₃) δ 8.83 (d, J = 4.7 Hz, 2H, from bipy), 8.45 (d, J = 8.1 Hz, 2H, from bipy), 8.19 (t, J = 7.2 Hz, 2H, from bipy), 7.74 (s, 2H, from bipy), 7.37 (t, J = 7.7 Hz, 1H, from Xyl), 7.22 (d, J = 7.7 Hz, 2H, from Xyl), 2.52 (s, 6H, 2CH₃). ¹³C NMR (176 MHz, CDCl₃) δ 151.25, 141.34, 136.18 (2C_quat. from Xyl), 131.29 (CH from Xyl), 128.73 (2CH from Xyl), 127.00, 124.01 (C_quat. from Xyl), 121.03, 119.21, 29.83, 18.91 (2 CH₃). IR (ν): 2184 cm⁻¹ (N≡C). MS (ESI⁺), found: 484.1083 [M–NTf₂]⁺; calculated for C₁₉H₁₇AuN₃: 484.1088.

3. Results and Discussion

Following our interest in the isocyanide complexes [23,24,25,26,27,28,29,30], triple CN bond activation [31,32,33,34,35,36], and noncovalent interactions [37,38,39,40,41,42,43,44], the gold(I) complex 4 was prepared in a high yield by the sequential addition of AgNTf₂ (2) and 2,2′-bipyridyl (3) to chloro(2,6-dimethylphenyl isocyanide)gold(I) (1) in CH₂Cl₂ as a solvent (Scheme 1).

The structure and purity of compound 4 were established by elemental analyses (C, H, N), high-resolution electrospray ionization mass spectrometry (ESI-MS), ¹H and ¹³C NMR spectroscopies, and X-ray diffraction structural analysis. The ESI⁺ mass spectrum of 4 contained a peak corresponding to the cationic part of the complex ([M–NTf₂]⁺) with the characteristic isotopic distribution. The IR spectrum of 4 exhibited one peak at 2184 cm⁻¹ due to C≡N stretching vibration.

Compound 4 produced colorless crystals, which were suitable for analysis by single-crystal X-ray crystallography, when it was recrystallized from CH₂Cl₂ layered by hexane. The structural investigations confirmed the formation of the xylylisocyanide bipyridyl complex (Figure 1).

The C–N–C–Au moiety in 4 is almost linear (N–C–Au and C–N–C for 1 are 178.3º and 178.1º, respectively). Interestingly, the C–Au bond distance (1.92 Å) is shorter than that of gold carbene complexes, which were described earlier [45,46]. This indicates the stronger back donation of the isocyanide to the metal center compared with the diarylcarbene ligand. Another interesting structural peculiarity of 4 is that the N–Au–C angles around the metal center are noticeably different (148.0º vs. 139.4º). The N–Au distances are also slightly different (2.26 Å vs. 2.24 Å), which means that the bypyridyl ligand is asymmetrically coordinated with the gold metal center. All other bond distances in 4 are unremarkable.

The crystallographic data analysis suggests the existence of the aurophilic interactions Au(I)···Au(I) in the crystal structure 4. Indeed, the distance between appropriate metal centers (viz. 3.216 Å) is significantly lower than the sum of their shortest van der Waals (vdW) radii (viz. 3.32 Å; note that currently, several values of vdW radii for gold are proposed, from the “classic” 1.66 Å radii [22] to “modern” 2.32 Å radii [47]. In order to confirm or disprove the hypothesis of the existence of such intramolecular noncovalent interactions in the X-ray structure of 4, the DFT calculations together with QTAIM, ELF, RDG, and NCI techniques [48] were carried out at the ωB97XD/DZP-DKH level of theory for the model structure (see Computational details and Table S1 in Supporting Information). The QTAIM analysis of the model structure revealed the presence of the bond critical point (3, –1) for the aurophilic interactions Au(I)···Au(I) in the X-ray structure of 4 (Figure 2 and

Table 1. The density of all electrons, ρ(r); Laplacian of electron density, ∇²ρ(r); appropriate λ₂ eigenvalue; energy density, H_b; potential energy density, V(r); and Lagrangian kinetic energy, G(r) (a.u.) at the bond critical point (3, –1) corresponding to aurophilic interactions Au(I)···Au(I) in the X-ray structure of 4.

ρ(r)	∇2ρ(r)	λ2	Hb	V(r)	G(r)
0.023	0.058	–0.023	–0.003	–0.020	0.017

). The values of the electron density, Laplacian of electron density, and energy density in the bond critical point (3, –1) for the aurophilic interactions Au(I)···Au(I) in the X-ray structure of 4 are typical for metallophilic interactions in similar chemical systems [49,50,51,52]; the ratio –G(r)/V(r) < 1 reveals some small covalent contribution in this short contact [53], and the sign of λ₂ (λ₂ < 0) suggests that these interactions are attractive [54,55]. The estimated energy of the aurophilic interactions Au(I)···Au(I) in 4 (E_int ≈ –V(r)/2) [56] is 6.3 kcal/mol. The visualization of intermolecular contacts in 3D using the noncovalent interaction analysis (NCI analysis) technique and the scatter graph of RDG vs. the product of sign of λ₂ (second largest eigenvalue of the Hessian matrix of electron density) and ρ (electron density) (NCI plot [54]) for 4 are shown in Figure 3. To understand what kinds of intermolecular contacts have the largest contributions to crystal packing, we performed the Hirshfeld surface analysis for the X-ray structure of 4 (Figure 4). We found that intermolecular contacts involving hydrogen atoms (viz. H···H, C···H, O···H, and F···H) have the largest contributions to crystal packing. The fingerprint plots from the Hirshfeld surface analysis for the X-ray structure of 4 are given in the attached zip archive (Supplementary Materials).

In summary, we described the synthesis and structural characterization of the cationic three-coordinate gold(I) 2,2′-bipyridyl/isocyanide complex for the first time. Two cationic gold derivatives form supramolecular dimers in the crystal via attractive Au···Au contacts. The estimated energy of the aurophilic interactions in 4 is 6.3 kcal/mol.