Synthesis and crystal structure of benzo[b]thiophen-2-ylgold(I) tert-Butyl Isocyanide

Abstract

Introduction

Organogold(I) chemistry, particularly complexes featuring isocyanide ligands, has been a field of active research for over half a century [1-4]. These compounds are of significant interest due to their diverse applications, notably as phosphorescent materials [5-8] and efficient catalysts in organic synthesis [9-13]. The Lewis acidic nature of the gold(I) center and its carbophilicity is key to its reactivity, enabling the activation of unsaturated substrates such as isocyanides, which serve as classic σ-donor ligands in coordination chemistry.

The conventional synthetic route to aryl gold(I) isocyanide complexes relies on the transmetallation from organomagnesium reagents [8,14], a method often complicated by anhydrous conditions and the sensitivity of the reagents. In contrast, transmetallation from boronic acids [11,12,15] presents a more robust and versatile alternative, a reaction step also identified as a key intermediate in gold-catalyzed Sonogashira-type couplings. This boronic acid pathway potentially facilitates access to a wider range of functionalized aryl complexes, enabling the systematic exploration of structure-property relationships.
While the molecular structures of such complexes are often linear, their solid-state supramolecular architectures are dictated by weak, yet highly influential, non-covalent interactions. Among these, aurophilic interactions [4,16,17] are a hallmark of gold(I) chemistry and are known to profoundly impact photophysical properties [18,19]. However, the predictable formation of specific oligomeric species, such as discrete tetramers, through a combination of aurophilic and other weak forces remains an area of keen interest.
In this context, we report the synthesis and detailed structural analysis of a novel benzo[b]thiophen-2-yl gold(I) tert-butyl isocyanide complex. Utilizing the practical boronic acid transmetallation route, we obtained the target complex and uncovered its solid-state structure.

Synthesis and crystallization

The title compound was prepared according to a literature procedure15(Fig. 1). Chloro(dimethyl sulfide)gold(I) (15.0 mg, 0.051 mmol, 1.0 eq) and tert-butyl isocyanide (4.2 mg, 0.051 mmol, 1.0 eq) were dissolved in dichloromethane (2 ml) in a 10 ml vial equipped with a magnetic stir bar. Benzo[b]thiophene-2-boronic acid (9.1 mg, 0.051 mmol, 1.0 eq) was dissolved in methanol (2 ml) and added to a solution under stirring, then sodium carbonate (54.0 mg, 0.51 mmol, 10 eq) was added. After 1 hour of stirring the mixture was filtered, the solution was evaporated and the residue was purified by column chromatography (eluent: DCM/hexane, 1:1), then evaporated and dried under vacuum. White solid. Yield: 7.8 mg (37%). Crystals suitable for X-ray analysis were obtained by layering hexane over a dichloromethane solution of the target complex.

Figure 1. Synthesis of the title gold(I) complex

Full size image

X-ray single crystal crystallography

A suitable single crystal was carefully selected and mounted on a glass fiber prior to data acquisition. Diffraction data were collected using a Bruker SMART APEX II diffractometer equipped with Mo-Kα radiation (λ = 0.71073 Å) at ambient temperature. Standard corrections for Lorentz, polarization, and absorption effects were applied to the collected intensities. The crystal structure was determined employing direct methods as implemented in the SHELXT [24] program and subsequently refined using full-matrix least-squares procedures on F² within the SHELXL [25] framework. Hydrogen atoms were placed in calculated positions and refined using a riding model, with C–H distances of 0.95–0.98 Å and Uiso(H) values set to 1.5 Ueq(C) for methyl groups and 1.2 Ueq(C) for aromatic carbon atoms.
The thio­phene ring (S2/C19–C22) in mol­ecule II is rotationally disordered (flip disorder) by ca 180° (around the single C14—Au2 bond, to which it is attached) over two sites with the site-occupation factors of 0.55 and 0.45 (fixed after refinement cycles). For these rings, FLAT, SADI and EADP instructions were also used. Due to weak agreement between the measured and calculated intensities, nineteen reflections were excluded from the final refinement. (0 1 1, 0 -1 1, -15 8 2, -14 10 6, -12 9 -7, -8 2 12, -11 2 6, -1 6 16, 2 -2 -14,   -7 7 16, -3 -3 -3, -3 -17 -1, -13 9 -3, -11 9 7, 1 6 -2, -2 0 -16, 4 2 -17, 0 11 11, 11 2 -8) were omitted during the final refinement cycle. ORTEP-3 [26] and Platon [29] software was used for graphics. The experimental parameters and their particulars are itemized in table 1.

Figure 4. Calculated real and imaginary part of dielectric function of bulk bilayer and monolayer structure of GaAs

Full size image

Structural commentary and supramolecular features

Figure 2. The molecular structure of the compound, showing the atom- numbering scheme. Displacement ellipsoids are drawn at the 30 % probability level. The minor component of the disorder has been omitted for clarity

Full size image

The complex crystallizes in the monoclinic P21/n space group with Z = 8 (figure 1). Figure 2 shows an overlay plot of two mol­ecules in the asymmetric unit, with an r.m.s. deviation of 0.725 Å. Except for the atoms of the minor part of the disordered mol­ecule and the butyl group, all atoms of two mol­ecules are quite compatible and coincide with each other. The asymmetric unit contains one-half of the tetrameric molecule (figure 3), the center of which coincides with an inversion center.

Figure 3. Superposition of two molecules in the asymmetric unit

Full size image

Figure 4. Molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level

Full size image

The tetramer adopts a zigzag configuration, which is stabilized by aurophilic interactions between the gold atoms with Au···Au distances of 3.1586(4) and 3.1837(6) Å and an Au-Au-Au angle of 107.259(14)° (table 2). Each gold(I) center exhibits the typical linear coordination geometry, with C–Au–C angles of 175.6(3)° and 175.9(3)° (table 2). The Au–C bond lengths to the isocyanide and aryl carbon atoms are 1.991(8)/1.975(8) Å and 2.021(8)/2.023(8) Å, respectively (table 1). The central units of the tetramer are arranged in a strictly head-to-tail fashion, as evidenced by a C1–Au1···Au1'–C1' torsion angle of 180.0°. In contrast, the terminal fragments are oriented with a C1–Au1···Au2–C14 torsion angle of 114.2°. The stability of the tetrameric framework is further enhanced by a pair of C–H···π interactions (figure 4, table 3). The other bond lengths and angles are comparable to those in the similar structures discussed in a recently published study [25].

Figure 5. Calculated DOS for bulk bilayer and monolayer of GaAs

Full size image

Figure 1. Biomass yield of the bacterial consortium when grown on brominated phenolic and aromatic compounds: a- ortho-bromophenol, b- para-bromophenol, c- meta-bromophenol, d- bromobenzene, e- tetrabromopyrocatechol, f- para-bromotoluene, meta-bromotoluene, ortho-bromotoluene

Full size image

In the crystal lattice, the tetramers are interconnected primarily via weak C–H···S hydrogen bonds (figures 5 to 8, table 2). Additional stabilization is provided by C–H···π contacts involving the central and terminal benzothiophene fragments of adjacent molecules (figure 6, table 3).

Figure 5. Molecular packing of the compound, viewed down the a axis

Full size image

Figure 6. Molecular packing of the compound, viewed down the b axis

Full size image

Figure 7. Molecular packing of the compound, viewed down the c axis

Full size image

Figure 8. A partial view of the C---H… p interactions in the unitcell

Full size image

Hirshfeld surface analysis

Hirshfeld surfaces and two-dimensional fingerprint plots have been generated to visualize the intermolecular contacts of the compound using Crystal Explorer program [26]. These intercontacts are featured by normal mapping of dnorm on molecular Hirshfeld surfaces (figure 9a). The 3D Hirshfeld Surface mapped over dnorm between -0.1282 a.u (blue) and 1.3969 a.u (red). White denotes contacts with distances equal to the van der Waals (vdW) radii. Connections that are short of the vdW radii are represented in red, while those that exceed the vdW radii are shown in blue. In figure 7a, dark-red spots represent strong inter­molecular C—H⋯S interactions. In addition, the shape-index is used to identify complementary hollows (red) and bumps (blue) where two mol­ecular surfaces touch one another (figure 9b). This includes π–π stacking, represented by adjacent red and blue triangles. In this structure there are C---H… π interactions and no π–π stacking interaction. Curvedness is a tool for pinpointing planar stacking configurations and how neighbouring mol­ecules inter­act. Figure 9c shows relatively large green planes in the nine-membered ring systems separated by blue edges. These green planes give us an idea of the flatness of complexes, and the fragment patch (figure 9d) is designed to indicate the nearest neighbouring mol­ecule [7]. 
In the crystal structure of the compound, intermolecular C—H···S and C—H···p interactions were observed. The two-dimensional fingerprint plots show that the main contributors to crystal packing are H…H contacts (54.8 %, figure 10b), C…H/H…C (25.9 %, figure 10c), S…H/H…S (7.2 %, figure 10d) and Au…H/H…Au (6.0 %, figure 10e). The remaining small contributions are C…C (1.5 %), S…C/C…S (1.3 %), Au…C/C…Au (1.0 %), Au…Au (0.8 %), N…C/C…N (0.5 %), N…H/H…N (0.5 %), Au…N/N…Au (0.3 %), and S…N/N…S (0.2 %). The optimized molecular geometry corresponded well to the observed solid-state geometry. This supports the crystallographic findings and implies that the molecular structure is stable.

Figure 9. Hirshfeld surface mapped over dnorm for compound, showing intermolecular contacts. Hirshfeld surfaces for the compound mapped with a) dnorm b) shape index, c) curvedness and fragment patch for the compound

a)                                                  b)

Full size image

f9cd

Full size image

Figure 1. Crystal structure of bulk GaAs

Full size image

Figure 2. Calculated phonon band structure and phonon dencity of state for bulk structure of GaAs

Full size image

Figure 3. Calculated band structure bulk bilayer and monolayer structure of GaAs

Full size image

Conclusion

In conclusion, we have successfully synthesized a novel benzo[b]thiophen-2-yl gold(I) tert-butyl isocyanide complex through a practical and robust transmetallation reaction from the corresponding boronic acid. This method offers a valuable alternative to the more sensitive organomagnesium-based routes, potentially broadening access to functionalized aryl gold(I) isocyanide complexes. Single-crystal X-ray diffraction analysis revealed that the complex does not exist as discrete monomers in the solid state but instead self-assembles into a well-defined zigzag-shaped tetramer. This supramolecular architecture is primarily stabilized by significant aurophilic interactions (Au···Au 3.1586(4) - 3.1837(6) Å), which dictate the tetrameric arrangement.

References

About this article