nter ions on the far-infrared spectra of tris ( triphenylphosphinegold ) oxonium dimer salts †

Department of Chemistry, University of Ad E-mail: greg.metha@adelaide.edu.au; Fax: The MacDiarmid Institute for Advanced Ma of Chemistry, University of Canterbury, Chr Chemistry Department, University of Malay Flinders Centre for NanoScale Science and T SA 5001, Australia † Electronic supplementary information (E between the crystallographic and DFT-opti for the [[(Ph3PAu)3O]BF4]2 and [[(Ph3PAu between the theoretical spectra for the species. See DOI: 10.1039/c5ra11599j Cite this: RSC Adv., 2015, 5, 74499


Introduction
3][4][5][6][7] The discovery of the tris(triphenylphosphinegold)oxonium salts of the form [(Ph 3 PAu) 3 O]X (X ¼ BF À 4 , MnO À 4 , CF 3 COO À ), which have been shown to exist as dimers in the crystalline phase, represented an addition to the few known species which incorporate direct Au-O bonding. 2,3hese oxonium salts have seen employment throughout the literature due to their ability to readily decompose in situ into highly reactive Ph 3 PAu + species, 7 capable of reacting with a wide array of organic species to yield corresponding aurated derivatives.Oxonium salts have been reported to react with cyclic structures such as cyclopentadiene, tetraphenylcyclopentadiene, and ferrocene, resulting in the substitution of H by AuPPh 3 . 5,6,8,9They have been reported to react with carboncontaining species such as methyl ketones, vinyl esters and ethers, and chloroform, yielding a-aurated species such as aurated acetone, aurated acetaldehyde, and trichloromethyl gold. 71][12] The oxonium salts have also been reported to form mixed-metal ligated clusters, with, for example, the species Au 3 CoRu 3 (CO) 12 (PPh 3 ) 3 having been synthesized utilizing tris(triphenylphosphinegold) oxonium species as a reagent. 13,14nfrared vibrational spectroscopy is a standard technique for probing the unique vibrational ngerprint of a given molecule, which is widely utilized to identify functional groups present within a molecule of interest.For the tris(triphenylphosphategold)oxonium salts, only a rudimentary assignment of the infrared absorption features have been reported, 3 with no reports of spectra in the far-infrared region below 800 cm À1 .This region is of importance to ligated metal cluster species as vibrational modes involving metal-metal stretching appear within this region. 15,16We have previously reported the far-infrared (50-650 cm À1 ) vibrational spectra of a range of gold, ruthenium, and gold-ruthenium species, which were accompanied by computational investigations to give an insight into the specic vibrational modes which give rise to each feature within the infrared spectra. 15,16For all previous calculations involving species with counter-ions, the counter-ions were removed and the charge balanced for the primary cluster of interest, i.e. the counter-ions were not explicitly calculated.
The present work reports the vibrational spectra of the oxonium salts [{{Au(PPh 3 )} 3 (m 3 ÀO)} 2 ] 2+ (BF À 4 ) 2 and [{{Au(PPh 3 )} 3 (m 3 ÀO)} 2 ] 2+ (MnO À 4 ) 2 , and utilize density functional theory (DFT) based calculations to assign the infrared features of each of these species to specic molecular vibrational modes.Herein, we demonstrate the importance of including counter-ions within the calculations to explain all observed features within the experimental spectrum.As well, the energies of association of the counter-ions to the gold oxonium species have also been calculated, and are reported.Finally, consistent with literature reports that the salts exist as dimers, 3,7 the energy of dimerization for both oxonium salts are calculated and presented.
The far-IR absorption spectra were recorded using the IFS125 Bruker FT spectrometer located at the far-IR beamline, at the Australian Synchrotron.The transmission spectrum for each sample was recorded from 50 to 800 cm À1 , at 1 cm À1 resolution utilizing the synchrotron light source (200 mA in top-up mode), with a 6 micron thick multilayer Mylar beamsplitter in combination with a Si bolometer detector.This bolometer was equipped with an 800 cm À1 far-IR cut-on cold-lter consisting of a 13 micron PE lm overlaid with a 6 micron diamond scatter layer.All spectra were recorded at room temperature, and have been baseline corrected.
Once the [{{Au(PPh 3 )} 3 (m 3 ÀO)} 2 ] 2+ structure was optimized, the respective counter-ions (either BF À 4 or MnO À 4 ) were added and this structure was further optimized at the same level of theory.Several potential starting counter-ion positions were investigated, and for each species the global minimum structure was utilized for frequency calculations and analysis.The charge was explicitly held on each molecule within all calculations which included counter-ions, i.e. the BF À 4 or MnO À 4 was explicitly held at À1 formal charge, and the [{{Au(PPh 3 )} 3 -(m 3 ÀO)} 2 ] 2+ species was held at +2 formal charge.All calculations were performed as closed-shell species.The optimizations were all performed in the C 1 point group, with no potential symmetry identied for all systems.
The calculations were carried out using a larger than default grid (Gaussian keyword "int ¼ ultrane") for numerical integral evaluation, with all other cut-offs being le at the default.Each geometry optimization was followed by a harmonic frequency calculation to conrm that the geometry was a true minimum with no imaginary frequencies.To obtain the predicted IR spectra, each predicted stick spectrum was convoluted with a Gaussian line shape function with 8 cm À1 full width at half maximum using the GaussView 5 program, to best match the experimental spectra.Full geometric information for each optimized structure is provided in the accompanying ESI.†

50-400 cm À1 region
The experimental spectra of the [{{Au(PPh 3 )} 3 -(m 3 ÀO)} 2 ] 2+ (BF À 4 ) 2 cluster in the 50-150 cm À1 region (Fig. 1a) contains two main features of interest.Below 100 cm À1 , there is a broad, featureless peak with a maximum intensity at 65 cm À1 , which we have previously attributed to phenyl group motions that are dampened in the solid phase compared with the gas phase calculation. 15The second feature, with maximum intensity at 107 cm À1 , is assigned to peak #4 in the predicted spectrum.This peak arises from a combination of thirteen vibrational motions of varying infrared activity involving motions of the Au core.Based upon our previous work, 15,16 this peak is expected to exhibit strongly within the experimental spectrum due the high Au core contribution component of 10.6%.The experimental spectrum of the [{{Au(PPh 3 )} 3 (m 3 ÀO)} 2 ] 2+ (MnO À 4 ) 2 cluster below 150 cm À1 (Fig. 2a) is obscured by a moderate amount of fringing.These
For each feature in the experimental spectrum, this is the average contribution by the Au/O core atoms toward each vibrational mode, weighted by the predicted infrared activity.This feature was not observed, possibly due to occlusion from neighbouring features (see text), however is included for comparison.
a bThis feature was not able to be resolved due to experimental noise.c d These features are not assigned due to poorly resolved peaks, due to their proximity to the polyethylene window (see text).