Gold(iii)–arene complexes by insertion of olefins into gold–aryl bonds

The synthesis and characterization of the first gold(iii)–arene complexes are described.


Generation of cationic Gold(III) aryl complex 3
In a glovebox, a screw-cap NMR tube was charged with AgSbF6 (8.7 mg, 0.025 mmol) in dichloromethane-d 2 (0.35 mL). Complex 2 (16.3 mg, 0.025 mmol) was transferred into a small glass vial and solubilized in dichloromethane-d 2 (0.35 mL). The prepared solution was loaded into a plastic syringe equipped with stainless steel needle. The syringe was closed by blocking the needle with a septum. Outside of the glovebox, the NMR tube was put under positive argon pressure and cooled down to -80 °C (Acetone/N2 coldbath). At this temperature, the solution of complex 2 was added. The tube was kept at -60 °C and immediately introduced into the NMR machine for analysis. A mixture of two cationic species in a 70/30 ratio was observed by 31 P NMR spectroscopy at -60°C (δ 31 P: 77.4 ppm for the major species and 80.8 ppm for the minor species), corresponding to two forms of complex 3. Complete NMR characterization of the major species is described hereafter.

Synthesis of Gold(III)-arene complex 4
A solution of AgSbF6 (9.6 mg, 0.028 mmol) in 0.2 mL of CD2Cl2 was added at -80°C to a mixture of complex 2 (15.0 mg, 0.023 mmol) and NB (0.028 mmol) in CD2Cl2 (0.4 mL). The reaction was allowed to warm up to room temperature and kept for 10 min at this temperature (the reaction medium turns from colorless to yellow). Complex 4 was characterized at -20°C without purification.

Synthesis of Gold(III)-(p-methoxyphenyl) complex 4'
A solution of AgSbF6 (61.1 mg, 0.178 mmol) in 1.2 mL of DCM was added at -80°C to a mixture of complex 2' (100.0 mg, 0.148 mmol) and NB (16.8 mg,0.178 mmol) in DCM (2.8 mL). The reaction was allowed to warm up to room temperature and stirred for 30 min at this temperature (the reaction medium evolved from colorless to yellow). The resulting suspension was filtered via filtrating cannula, evaporated to dryness and washed with pentane. Drying under vacuum afforded the crude compound 4' as a yellow powder in 65% yield (74 mg). The solid was further purified by crystallization at -20°C in a saturated DCM/pentane mixture to give single crystals suitable for X-ray diffraction analysis (19% yield, 18 mg).
The reaction was allowed to warm up to room temperature and stirred for 30 min at this temperature (the reaction medium evolved from colorless to yellow). A solution of NBu4Cl (24.7 mg, 0.089 mmol) in DCM (0.6 mL) was then added and the medium immediately turned colorless. After filtration on a celite pad, the solvent was removed in vacuo and the crude mixture was purified by flash column chromatography (silica, eluent: Pentane/DCM 100:0 to 50:50) to afford compound as a white solid in 56% yield (28 mg).

Characterization of Gold(III) complex 6
In a glovebox, a pressure tube was charged with AgSbF6 (8.4 mg, 0.024 mmol) in dichloromethane-d 2 (0.35 mL). Complex 2 (15.6 mg, 0.024 mmol) was transferred into a small glass vial and dissolved in dichloromethane-d 2 (0.35 mL). The prepared solution was loaded into a plastic syringe equipped with stainless steel needle. The syringe was closed by blocking the needle with a septum. Outside of the glovebox, the NMR tube was put under positive argon pressure and cooled down to -80 °C (Acetone/N2 coldbath). At this temperature, the solution of complex 2 was added. The tube was kept at -80 °C and the solution was degased 3 times using the Freeze-Pump-Thaw degassing technique and 2 bars of ethylene were added. Complex 5 was immediately formed at -80 °C and was characterized at 0 °C. Complex 5 decomposes at room temperature after 1h to give styrene and diphosphine cationic gold(I) complex.

Crystallographic data
Crystallographic data of 2 and 4' were collected at 193(2) K on diffractometers equipped with air-cooled microfocus sources: a Bruker-AXS PHOTON100 D8 VENTURE diffractometer with CuKα radiation, λ= 1.54178 Å for 2 and a Bruker-AXS APEX II Quazar diffractometer with MoKα radiation,  = 0.71073Å for 4'. Phi-and omega-scans were used. Space groups were determined on the basis of systematic absences and intensity statistics. Empirical absorption correction was employed. 2 The structures were solved by direct methods 3 and refined using the least-squares method on F 2 . All non-H atoms were refined with anisotropic displacement parameters. Hydrogen atoms were refined isotropically at calculated positions using a riding model except the H atom on C9 for 4' which was located in difference Fourier maps and refined with the isotropic displacement parameter Uiso(H) = 1,2 Ueq(C).

Computional details
All calculations were performed using the Gaussian 09 package [4] and the B3PW91 hybrid functional on the real experimental systems. [5] The weakly coordinating counter-anion SbF6has not been considered in the calculations since we previously showed that even a more coordinating counter-anion like NTf2has no significant impact on the reaction profile (migratory insertion of norbornene and ethylene, -hydride elimination) in similar processes than those described in this work. [6] The gold atom was described with the relativistic electron core potential SDD and associated basis set, [7] augmented by a set of f-orbital polarization functions. [8] The 6-31G** basis set were employed for other atoms. All stationary points involved were fully optimized. Frequency calculations were undertaken to confirm the nature of the stationary points, yielding one imaginary frequency for transition states (TS), corresponding to the expected process, and all of them positive for minima. The connectivity of the transition states and their adjacent minima was confirmed by intrinsic reaction coordinate (IRC) [9] calculations. Natural Bond Orbital [10] calculations (NBO, 5.9 version) [11] have been carried to analyze the bonding situation, in particular for the description of -interaction in arene gold complexes.
Natural Localized Molecular Orbital (NLMO) were plotted with Molekel 4.3 [12] and all the geometrical structures with Gaussview 5.0. [13] Coupling constants JCP and 13 C NMR chemical shifts were evaluated by employing the direct implementation of the Gauge Including Atomic Orbitals (GIAO), [14] with the IGLOII [15] basis set on C, H and P atoms, using as reference the corresponding SiMe4 shielding constant calculated at the same level of theory.  Figure S26. DFT optimized structures and relative energy stability (in kcal.mol -1 ) for the cis and trans isomers of complex 4, computed at the B3PW91/SDD+f(Au),6-31G**(other atoms) level of theory. The transition state for the trans -cis isomerization is also shown. Structure Cis-4i corresponds to a local minimum with -coordination of CiCo' (instead of CiCo in Cis-4). E values are given in brackets.

Trans-4
Cis - 4.6% C m 1.5% C p Figure S27. a) NLMO plot of Ci-Co (cutoff 0.05 au) associated with the -arene interaction in complex cis-4. Stabilizing energy E(2) found at the second order perturbation theory is around 30.7 kcal/mol. Atomic contributions of main atoms in the corresponding C-C NLMO are shown as well. b) Superposition of the donor Ci-Co and acceptor *CAr-Au (chicken-wire) NBOs (cutoff: 0.07 au) outlining the -arene interaction. Figure S28. DFT optimized structures and relative energy stability (in kcal.mol -1 ) for the cis and trans isomers of complex 4', computed at the B3PW91/SDD+f(Au),6-31G**(other atoms) level of theory.  Table S2. Main geometrical parameters with distances in Å and bond angles in (°) and computed NMR data (main 13 C NMR chemical shifts in ppm and JPC coupling constants in Hz) for complex cis-4'.  Figure S30. a) DFT optimized structures and relative energy stability (ΔG in kcal.mol -1 ) for the cis and trans isomers of complex 6, computed at the B3PW91/SDD+f(Au),6-31G**(other atoms) level of theory. b) Energy profile (∆G (∆E) values in kcal·mol -1 ) computed at B3PW91/SDD+f(Au)/6-31G** (other atoms) level of theory for the trans to cis isomerization of complex 6. 1.3% C para 3.5% C meta 1.0% C meta' 6.3% P Figure S31. NLMO plot of Cipso-Cortho (cutoff 0.05 au) orbital in complex trans-6, associated with the -arene interaction. Stabilizing energy E(2) found at the second order perturbation theory is around 38.6 kcal·mol -1 (Cipso-Cortho→σ*PAu). Atomic contributions of main atoms in the corresponding Cipso-Cortho NLMO are shown as well. Table S3. Main geometrical parameters with distances in Å and bond angles in (°) and computed NMR data (main 13 C NMR chemical shifts in ppm and JPC coupling constants in Hz) for complex trans-6.  Figure S32. Energy profile (∆G (∆E) values in kcal·mol -1 ) computed at B3PW91/SDD+f(Au)/6-31G** (other atoms) level of theory for the reaction of the cationic complex 3 with ethylene: a) migratory insertion and trans→cis isomerization, b) -hydride elimination, c) styrene rotation and d) re-insertion into the Au-H bond.