Pincer Complexes : Determination of the Au II – Au II Bond Energy

All manipulations were performed using standard Schlenk techniques under dry nitrogen or a Saffron Scientific glove box. Nitrogen was purified by passing through columns of supported P2O5, with moisture indicator, and activated 4 Å molecular sieves. Anhydrous solvents were freshly distilled from appropriate drying agents. LAuCl, LAuOH, LAuH and LAuAuL([L = 2,6-(C6H3Bu )2-pyridine) were prepared using literature methods.

S5] All electrochemical measurements were performed at ambient temperatures under an inert N 2 atmosphere in CH 2 Cl 2 containing 0.05 M [ n Bu 4 N][B(C 6 F 5 ) 4 ] as the supporting electrolyte, and iR-compensated using positive-feedback to within 85 ± 5% of the uncompensated solution resistance.S6] Data were recorded with Autolab NOVA software.CV simulations were performed using DigiElch -Professional (v 7.030) software.

Digital simulation of cyclic voltammetry
In order to obtain kinetic and thermodynamic parameters for the electrochemical processes observed, and to support the proposed mechanisms postulated above, digital simulations were performed for the metal centered reductions of 1 and 2 and the oxidation of 4.
Diffusion coefficients, D, were first estimated by application of the Randles-Sevcik method for 1 and 2, in which the peak current for the reduction is plotted vs. the square root of scan rate.This assumes a single electron transfer (n = 1).These values gave a starting point for the simulations and produce values for the diffusion coefficients of 1, 2 and 4 that are in good agreement with those obtained from DOSY experiments.The diffusion coefficient for 4 was determined by performing potential-step chronoamperometry at a microdisk electrode.The current transients (i) were then fitted using the empirical procedure of Shoup and Szabo, [S7] which describes the current transient at a microelectrode on both short (τ < 1) and long (τ > 1) timescales according to equation 1: in which n is the number of electrons transferred, F is the Faraday constant (96 485 C mol -1 ), C* is the bulk concentration (mM), D is the diffusion coefficient (m 2 s -1 ), r is the radius of the electrode (m), and f(τ) is given by: f(τ) = 0.7854 + 0.8863τ -1⁄2 + 0.2146 exp(-0.7823τ-1⁄2 ) (2) where τ ≡ 4Dt/r 2 is the dimensionless time.The Shoup-Szabo best fit simultaneously gives optimized parameters for D and nC* (Figure S2).As C* is known, the other parameters can then be deduced.The Shoup-Szabo best fit for 3 yielded a value for the diffusion coefficent of 7 ± 1 x 10 -6 cm 2 s -1 and gave the number of electrons transferred during the oxidation of the dimer, n, as being equal to 1.

DFT calculations
To reduce computational complexity, simulations were performed using modified ligand structures in which the t-Bu group were replaced by a hydrogen atom.Initial geometries were derived from X-ray data, where available, with appropriate manual modification.S14] Geometry optimisation and frequency calculations were carried out using the pure functional of Perdew, Burke and Ernzerhof [S15,16] made into a hybrid by Adamo, [S17] using 25% exchange and 75% correlation weighting ('PBE0').S20,21] Structures were geometry optimised in the gas phase with the default convergence criteria and confirmed as minima through frequency calculations.Zero-point energies and theromodynamic properties were calculated at 298.15 K and 1 atm.Bond enthalpies were calculated from the difference in energy of complete molecules and the sum of the energy of the appropriate optimised fragments obtained by homolytic bond cleavage.Final atomic coordinates for the optimised structures are given in the appendix.
The electrochemical process was modelled for the following steps, using NMe 4 + ions to take account of the presence of supporting electrolyte and counterbalance the charges (Table S3).
For X = OH the optimized geometry of (C^N^C)Au II -X-Au III (C^N^C) is highly asymmetric, and a reduced hydroxide-bridged dimer [(C^N^C)Au II -OH-Au II (C^N^C)] -could not be modelled: the energy given for this case is that of the {(C^N^C)Au II -OH
•• Au II (C^N^C)} -assembly.The relative energy profile for the reduction processes for (C^N^C)Au III -H and (C^N^C)Au III -OH are shown in Figure S5.Electronic Supplementary Material (ESI) for Chemical Communications This journal is © The Royal Society of Chemistry 2013 (C^N^C)Au-OH-Au(C^N^C)