Gold(I) α‐Trifluoromethyl Carbenes: Synthesis, Characterization and Reactivity Studies

Abstract Aryl trifluoromethyl diazomethanes 2‐R (R=Ph, OMe, CF3) are readily decomposed by the (o‐carboranyl)diphosphine gold(I) complex 1. The resulting α‐CF3 substituted carbene complexes 3‐R have been characterized by multi‐nuclear NMR spectroscopy as well as X‐ray crystallography (for 3‐Ph). The bonding situation was thoroughly assessed by computational means, showing stabilization of the electrophilic carbene center by π‐donation from the aryl substituent and backdonation from Au, as enhanced by the chelating P^P ligand. Reactivity studies under stoichiometric and catalytic conditions substantiate typical carbene‐type behavior for 3‐Ph.


Materials and methods
Unless otherwise stated, all reactions and manipulations were carried out under an atmosphere of dry argon using standard Schlenk techniques or in a glovebox under an inert atmosphere. Dry, oxygen-free solvents were employed. Solution 1 H, 13 C, 31 P, 19 F and 11 B NMR spectra were recorded on Bruker Avance 300, 400 or 500 spectrometers at 298K unless otherwise stated. Chemical shifts are expressed with a positive sign, in parts per million, calibrated to residual 1 H and 13 C solvent signals. External 85% H3PO4, CFCl3 and BF3.OEt2 were used as reference for 31 P, 19 F and 11 B NMR, respectively. The following abbreviations and their combinations are used: br, broad; s, singlet; d, doublet; t, triplet; q, quartet; quin, quintuplet; m, multiplet. The 1 H and 13 C resonance signals were attributed by means of 2D HSQC and HMBC. Mass spectra were recorded on a Waters UPLC Xevo G2 Q TOF apparatus. The UV-vis spectrum was recorded on an Agilent Cay 60 UV-Vis apparatus. Dilithio-1,2-dicarba-closo-dodecaborane, 1 2-chloro-1,3-diisopropyl-1,3,2-diazaphospholidine, 2 1,2bis(diaminophosphino)-1,2-dicarba-closododecaborane, 3 hydrazone 4 were prepared according to reported procedures. All other starting materials were purchased from Aldrich and used as received unless otherwise stated.

Experimental procedures and analytical data 2.1 Synthesis of the gold complex
Prepared in a similar way than the AuCl complex, 3 with AuI instead of (DMS)AuCl. 31 P{ 1 H} NMR (121 MHz, CDCl3): δ 141.0 (s).

Crystallographic data
Crystallographic data were collected at low temperature (193(2) K) on a Bruker-AXS APEX II Quazar diffractometer equipped with a 30W air-cooled microfocus or on a Bruker-AXS PHOTON100 D8 VENTURE diffractometer, using MoKα radiation (λ = 0.71073 Å). Phiand omega-scans were used. An empirical absorption correction was performed with SADABS. 8 The structures were solved by direct intrinsic phasing method (SHELXT), 9 and refined using the least-squares method on F 2 . 10 All H atoms on carbon atoms were refined isotropically at calculated positions using a riding model. CCDC 2124447 (3-Ph), 2124452 (4') and 2124272 (6) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.   (4) 89.58(4) Cortho-Cipso-C-Au 0.6(5) -3.4(6) Table S2. Key geometric features of the two independent molecules of the crystal structure of 3-Ph.

Computational details
All calculations were performed using the Gaussian 09 package 11 and the B3PW91 hybrid functional 12 on the real experimental systems. The gold atom was described with the relativistic electron core potential SDD and associated basis set, 13 augmented by a set of f-orbital polarization functions. 14 The 6-31G** basis set was employed for all other atoms. 15 Frequency calculations were undertaken to confirm the nature of the stationary points, yielding zero imaginary frequency for minima and one imaginary frequency for transition states (TS), corresponding to the expected process (rotation of phenyl ring). The connectivity of the transition states and their adjacent minima was confirmed by intrinsic reaction coordinate (IRC) 16 calculations. All the geometrical structures were plotted with Chemcraft program. 17 For the frontier orbitals, the atomic orbital compositions of each MO (%) have been computed thanks to Multiwfn 3.6 package. 18 The bonding situation in all systems was studied using Natural Bond Orbital 19 analyses (NBO, 7.0 version). 20 Charge transfer between the carbene and the metallic fragment has been calculated using atomic NPA charges. The Natural Localized Molecular Orbitals (NLMO) associated to the interactions involving the vacant of the carbene, i.e. the dxz(Au)→2p π (Ccarbene) and πC=CAr→2p π (Ccarbene) interactions, have been analyzed. NLMO plots associated to the Au→Ccarbene back-donation or Aryl→Ccarbene interaction were drawn (cutoff : 0.04) with Chemcraft program. 17

S28
For each system, a charge decomposition analysis (CDA) was carried out with the CDA 2.2 program developed by G. Frenking. 21 The orbital contributions to the charge distributions are divided into four parts: (i) the mixing of the occupied orbitals of the ligand (carbene) and the unoccupied MOs of the metal fragment (P,P)Au + (Ligand→Au donation d), (ii) the mixing of the unoccupied orbitals of the ligand and the occupied MOs of the metal fragment (Ligand←Au back-donation b), (iii) the mixing of the occupied orbitals of the ligand and the occupied orbitals of the metal fragment (Ligand↔Au repulsive polarization r), and (iv) the mixing of the unoccupied orbitals of the ligand and the unoccupied orbitals of the metal fragment (residual term Δ).
The absorption spectrum was calculated at SMD(DCM)/B3PW91/SDD+f(Au), 6-31G**(other atoms)// B3PW91/SDD+f(Au), 6-31G**(other atoms) level by using time-dependent density functional theory (TD-DFT) 22 method on the geometry of the ground-state. Solvents effects (DCM: dichloromethane) was included by means of the universal Solvation Model based on Density (SMD). 23 For the NMR calculations of the carbene gold complexes, the 13 Ccarbene NMR chemical shift ( in ppm) and JPC and JPF coupling constants (in Hertz) were computed by employing the direct implementation of the Gauge Including Atomic Orbitals (GIAO), 24 with the IGLOII 25 basis set on B, C, H, N, F and P atoms and using as reference SiMe4, optimized at the same level of theory.  Table S3. Main geometrical parameters (distances in Å and bond and dihedral angles in °) and NMR data ( 13 Ccarbene chemical shifts  in ppm and JPC, JPF coupling constants in Hz) for the complexes 3-OMe, 3-Ph and 3-CF3 calculated at the B3PW91/SDD+f(Au), 6-31G**(other atoms) level of theory. a NMR calculations carried out at GIAO-B3PW91/IGLO(II) for H, B, C, N, F, P atoms and SDD+f for Au.

3-CF3
For all complexes, the plot of the frontier orbitals evidences interaction of the 2p π (C) orbital with both the metal and the aryl substituent. The energy gap between the frontier orbitals is relatively small in all cases: 2.34 eV for 3-Ph, 2.46 eV for 3-OMe and 2.44 eV for 3-CF3.  Table S5. TD-DFT calculations for complex 3-Ph: absorption wavelengths λ (in nm) and transition energies (in eV) corresponding to the main π → π* absorptions for the 9 first excited states, oscillator strength f, associated electronic transitions. a With HOMO-1 and HOMO-2 centered on the P^P ligand; HOMO-3, HOMO-4, HOMO -5 and HOMO-6 centered on the biphenyl group; HOMO-7 and LUMO+1 centered on Au-C and Au-P bonds. HOMO-1 LUMO+1 Figure S40. Plots of the molecular orbitals (cutoff : 0.04) corresponding to the electronic transitions described in Table S5. -0.007 -0.006 0.007 a another TS for the rotation about the Au-C bond with dissociation of one P arm was also found on the PES. Table S7. Main geometrical parameters and bonding situation (NBO and CDA analyses) for the transition states associated to the rotation of the phenyl ring (TSrot Ph), the rotation of the Au=C bond with the 2 phosphines coordinated at gold (TSrot AuC) or only 1 phosphine coordinated at gold (TSrot AuC-dissoc P), calculated at the B3PW91/SDD+f(Au),6-31G**(other atoms) level of theory. Distances are given in angstroms, bond angles and bond dihedral angles in degrees. Charge transfer (CT) from carbene to gold fragment accounting for NBO calculation. Contributions of the main atoms (in percent) in the NLMO associated to dxz(Au) and πC=CAr. Figure S41. Energy profiles for the rotations around the Au=C bond (TSrot AuC) and Ccarbene-CPh bond (TSrot Ph), with the 2 phosphine arms coordinated to Au, calculated at the B3PW91/SDD+f(Au),6-31G**(other atoms) level of theory. Selected bond lengths in Å. G(S/T) a +12.7 0 +9.9 0 +5.7 0 a The triplet state is more stable than the singlet state. Table S8. Free carbenes (singlet and triplet states) associated to 3-CF3, 3-Ph and 3-OMe. Main geometrical parameters (distances in Å, bond angles and dihedral angles in °). Relative stability of the singlet and triplet states in kcal/mol (G).