Comparing Metal–Halide and −Oxygen Adducts in Oxidative C/O–H Activation: AuIII–Cl versus AuIII–OH

High-valent metal–halides have come to prominence as highly effective oxidants. A direct comparison of their efficacy against that of traditional metal–oxygen adducts is needed. [AuIII(Cl)(terpy)](ClO4)2 (1; terpy = 2,2′:6′,2-terpyridine) readily oxidized substrates bearing O–H and C–H bonds via a hydrogen atom transfer mechanism. A direct comparison with [AuIII(OH)(terpy)](ClO4)2 (2) showed that 1 was a kinetically superior oxidant with respect to 2 for all substrates tested. We ascribe this to the greater thermodynamic driving force imbued by the Cl ligand versus the OH ligand.

Crystals suitable for single crystal X-ray diffraction measurements were grown over two days from a N,N-dimethylformamide (DMF)/ diethyl ether (Et 2 O) mixture. Data were measured (λ = 0.71073 Å) at 100(2)K on a Bruker D8 Quest Eco with an Oxford Cryostream low temperature device using a MiTeGen micromount. Bruker APEX 5 software was used to collect the data and correct for Lorentz and polarization effects. Data were integrated with SAINT 6 and corrected for absorption effects using the Multi-Scan method, SADABS. 7 The structure was solved with the XT 8 structure solution program using Intrinsic Phasing and refined with the XL 9 refinement package using Least Squares minimization with Olex2, 10 using the space group P2 1 , with Z = 2. This confirms the desired structure with formula unit, C 15 H 11 AuCl 3 N 3 O 8 , with slightly different cell parameters with respect to those already reported 11 . [6] The structure was refined as an inversion twin, with a refined Flack parameter of 0.011(6). One perchlorate displays disorder in one oxygen site which was modelled in two orientations with the majority population 0.58(4) using geometric (SADI) and displacement (ISOR, SIMU) restraints. The growth of such crystals over two days showed that was 1 stable in dry DMF, allowing its reactivity to be explored in such conditions. Crystallographic data for this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no.

EPR analysis
Electron paramagnetic resonance (EPR) spectra of frozen solutions were acquired on a Bruker EMX X-band EPR, equipped with an Oxford Instruments CE 5396, ESR9 Continuous Flow Cryostat, a precision Temperature Controller and an Oxford Instruments TTL20.0/13 Transfer Tube. EPR samples were prepared by freezing the EPR tubes containing the analyte solutions, previously prepared at the UV-Vis spectrophotomer, in liquid nitrogen. EPR spectra of the 2,4,6-tris-tert-butylphenoxyl radical were recorded at 77 K, 9.2 GHz, 0.2 mW microwave power, with a 10 mT field sweep in 84 s, and 0.2 mT field modulation amplitude. EPR spectra of 4-X-TEMPO were recorded at 77 K, 9.2 GHz, 2.02 mW microwave power, with a 60 mT field sweep in 84 s, and 0.3 mT field modulation amplitude. Integration, simulation, and fitting were performed with Matlab and the easySpin computational package. 12 The simulation for the phenoxyl radicals spectra was modelled as an S = ½ electron spin with an isotropic g tensor. The oxidation yield of the samples was calculated by quantification of the concentration of spin in the samples. This was obtained by comparison of the double integral S4 of the signals to that of a frozen reference 3 mM solution of (2,2,6,6-tetramethyl-piperidin-1yl)oxyl (TEMPO), measured under the same conditions.

Electrochemistry
Cyclic voltammetry (CV) experiments were conducted with a CH Instruments 600E electrochemical analyzer, using a glassy carbon working electrode, a platinum wire counter

Gas chromatography-Flame Ionisation Detector (GC-FID) analysis
Gas chromatography experiments have been performed using a ThermoFisher TRACE™ Graphics were generated using ChemCraft. 14

Equilibrium constant calculations
We adopted a method reported by Mayer and co-workers to determine K a . 15 Table S1. Crystal data and structure refinement for 1. Complex                  Figure S25. 1 H-NMR spectra (400 MHz, DMSO-D 6 ) of free terpyridine after the addition of 2 equiv. of trifluoroacetic acid.  Figure S26. 1 H-NMR spectra (400 MHz, DMSO-D 6 ) of 2 (blue trace, 2.6 mM), of 2 upon addition of 5 equiv. of PyHOTf (2H + , green trace) and of 2H + upon addition of 5 equiv. of 2,6-lutidine (red trace). The resonance at δ = 6.35 ppm is assigned to the proton in the hydroxide ligand. Figure S27. Change in the electronic absorption spectra of 2 upon the progressive addition of 1 equiv. of pyridinium triflate to yield 2H + (red trace, 15 equiv. pyridinium triflate). Figure S28. Change in the region between 400 and 700 nm of the electronic absorption spectra of 2 (black trace) upon the progressive addition of 1 equiv. of pyridinium triflate to yield 2H + (red trace, 15 equiv. pyridinium triflate).