Investigation of Molecular Alkali Tetrafluorido Aurates by Matrix‐Isolation Spectroscopy

Abstract Molecular alkali tetrafluorido aurate ion pairs M[AuF4] (M=K, Rb, Cs) are produced by co‐deposition of IR laser‐ablated AuF3 and MF in solid neon under cryogenic conditions. This method also yields molecular AuF3 and its dimer Au2F6. The products are characterized by their Au–F stretching bands and high‐level quantum‐chemical calculations at the CCSD(T)/triple‐ζ level of theory. Structural changes in AuF4 − associated with the coordination of the anion to different alkali cations are proven spectroscopically and discussed.

Isolation of molecular ions and ion pairs has alwaysb een a challenge in matrix-isolation spectroscopy.O ne method that is capable of producing mostly radicals but also small amounts of anionsa nd cations is by passingg as-mixtures through am icrowaved ischarge. [1] Another way to produce ions is to use laser-ablation of metal targets whichp roduces electrons and thereby anionic species like the free trifluoride ion (F 3 À ). [2] A long known methodt oi solate molecular ion pairs is by evaporation of as alta th igh temperatures in aK nudsen cell. [3] However,t his approachi se laborate and needs long deposition times. Recently,f ree ions in addition to ion pairs were isolated in rare-gas matrices by pulsed IR-laser deposition of salt targets. [4,5] This method produces significantly higher yields in a fraction of the time needed for thermale vaporation and is much simpler to control.
Herein, we report as imple methodt hat allows for the reaction of two crystalline nonvolatile reactants by laser-ablation of am ixed salt target material. With that methodi tw as possible to produce and characterize molecular alkali tetrafluorido aurates (M[AuF 4 ], M= K, Rb, Cs) for the first time by reactiono f laser-ablated alkali fluorides (MF) with gold trifluoride (AuF 3 ) under cryogenic conditions. The Au-F stretches of such molecules are slightly dependento nt he alkali metal and are shown to be in excellent agreement with high level quantum-chemical calculations. To the best of our knowledge,R aman and IR studies have so far only been published on the crystalline bulk materialofM [AuF 4 ]. [6,7] In IR spectra recorded after co-depositiono fl aser-ablated MF/AuF 3 (M = K, Rb, Cs) with excess neon at 6K several bands were observed in the Au-F stretching region ( Figure 1). The position of some of those bands were found to be alkali metal-dependent and some are metal-independent. Two metal-independent bands at 694 and 692 cm À1 are, according to Wang et al.,a ssigned to AuF 3 . [8,9] Twof urtherw eakb ands at 655 and 494 cm À1 in this previous work were attributed to Au 2 F 6 obtained by evaporation of solid AuF 3 in aK nudsen cell. [8,9] In our spectra, the 655 cm À1 band is not present.I nstead we observed four strong bands at 665, 660, 494, and 492 cm À1 associated with the four strongest stretching bands of Au 2 F 6 (D 2h ): the in-phasea ntisymmetric( b 2u )a nd the out-ofphase symmetric (b 3u )s tretching modes of the terminal F atoms, and the in-phase antisymmetric (b 3u )a nd the out-ofphase symmetric (b 2u )s tretching modes of the bridging F atomsi nd escending order.Ac omparison of the calculated Au 2 F 6 vibrational spectrum with our experimental resultsi s showni nT able S1 in the Supporting Information. Both sets of bands, due to AuF 3 andA u 2 F 6 diminish under UV light (l =  273 nm), whereby the AuF 3 bands are more sensitive to irradiation.
Prior to irradiation ab and at 562 cm À1 was present in all spectra,e ven after deposition of pure alkali metal fluorides MF and is known to be associated with the antisymmetric F 3 Àstretch in MF 3 . [4] The presence of MF 3 ion pairs indicates the sequenceo fr eactions (1)-(3)d uring laser-ablationa nd matrixisolation of alkali fluorides. The MF 3 band disappears completely by irradiation with UV light (l = 273 nm, 5min). Its behavior is therefore very different from the irradiation resistant bands at 563-564 cm À1 (cf. Figures S1 and S2, Supporting Information).
These latter bands show as light shift depending on the alkali metal Ma nd they are part of as et of four bandst hat did not appear in the spectra obtained with pure alkali metal fluorides (MF). These findings suggest that the carriero ft he four bands is ar eaction product of the reactants MF and AuF 3 , most likelym olecular alkali tetrafluorido aurate.
Calculationsa tt he CCSD(T) level of theory suggest that molecular M[AuF 4 ]h as a C 2v minimum structure of ad istorted square-planar tetrafluorido aurate with two bridging fluorido ligands to the alkali metal (Figure2). Compared with the free tetrafluorido aurate anion, the angle between the terminal fluorine atoms in the ion pairs almost remains 908,w hile the angle betweent he m 2 -F atoms is decreased (878). The terminal AuÀFb onds in the ion pairs are shortened whereas the AuÀF' bonds to the bridging fluorine atoms are elongated. This trend is also reflectedi nt he calculated harmonic frequencies of the ion pairs M[AuF 4 ]( Ta ble 1): The terminal Au-F stretches (a 1 ,b 1 ) are predicted to appear 45-50 cm À1 higher and the bridging Au-F' stretches (a 1 ,b 1 )4 0-60 cm À1 lower than the AuF 4 À (e u ) stretch. In the series M[AuF 4 ]( M= K, Rb, Cs) the terminal Au-F bonds slightly decrease from Cs to K, whereas the Au-F' bonds get longer, followingt he increaseo ft he Lewis acidity of the alkali metal cations(Cs + < Rb + < K + ). The observed blueshift of the terminal Au-F stretches at 640-642 and 632-634 cm À1 and the redshift of the Au-F' bands at 563-564, and 542-543 cm À1 in the series from Cs to Ki se xcellently matched by the calculated spectra (Figure 1, Ta ble1). The IR spectra of crystalline Cat[AuF 4 ]( Cat + = Cs + ,M e 4 N + ,E t 4 N + ) [7] show only one AuF 4 À band at 598 cm À1 indicating that in the crystal the D 4h symmetry of the anion is preserved.
The free anionsA uF 4 À ,p redicted at 615cm À1 ,a nd AuF 6 À , predicted at 651 cm À1 (CCSD(T)/def2-TZVPPD, Ta ble 1), were not observed in any of the experiments. It is, however,possible that the e u stretcho ft he free   In the present study,w er eport the complete sets of experimental IR stretching bands of molecular Au 2 F 6 and M[AuF 4 ]i on pairs with M = K, Rb, and Cs for the first time. These species were produced by laser-ablation of solid mixtures of MF salts with AuF 3 and isolated in solid neon under cryogenic conditions. The metal dependence of the IR active Au-F stretches in the ion pairs for the different alkali metals is fully consistent with their structural changes obtained by high-level quantumchemicalc alculations. With these resultsa th and, we have shown that pulsed-laser deposition from am ixed salt target is an excellent method to facilitate the reactiono ft wo crystalline nonvolatile reactants under cryogenic conditions. By this new approach, using mixed salt targets,t he investigation of larger ion paired speciesbecomes viable.

Experimental Section
Matrix-isolation experiments were performed using as elf-built matrix chamber in which av acuum of at least 6 10 À6 mbar was maintained by an oil diffusion pump connected to ar otary vane pump. The matrix support was kept at at emperature of 6K using a Sumitomo Heavy Industries cold head with ah elium compressor unit. IR spectra were recorded using 1000 scans at ar esolution of 0.5 cm À1 on a Bruker Vertex 80v FTIR vacuum spectrometer equipped with aK Br beam splitter and al iquid nitrogen cooled MCT detector (4000-350 cm À1 ). In at ypical experiment 97-98 %M F (M = K, Rb, Cs) and % 2-3 %o fafluorido gold species (AuF 3 , M[AuF 4 ], or M[AuF 6 ]) were mixed and ground under an argon atmosphere and subsequently pressed into ac ylindric pellet using a hydraulic lab press. The target was mounted onto ar otatable target holder and transferred into the matrix chamber.T he solid MF/AuF 3 mixture was evaporated using af ocused pulsed Nd:YAG IR laser (1064 nm) with pulse energies of 50 mJ and ap ulse length of 3-7 ns, and co-deposited with pure neon at 6K using deposition times varying between 90 and 180 min. AuF 3 and M[AuF 4 ]w ere prepared as published. [7] For the Cs[AuF 6 ] preparation, Cs[AuF 4 ]( 200 mg, 0.49 mmol) was dissolved in anhydrous HF.F luorine (2 bar,2 5equiv) was added and the mixture was irradiated with UV light for 12 hu nder constant stirring. Finally,e xcess F 2 and anhydrous HF were removed at low pressure to obtain the product Cs[AuF 6 ]i nquantitative yields. Calculations were carried out at the B3LYP, [10] SCS-MP2, [11] and CCSD(T) levels (M[AuF 4 ], M= K, Rb, Cs) using the Orca 4.0.1 [12] program package. CCSD(T) calculations for the free anions [AuF 4 ] À and [AuF 6 ] À were performed using Molpro 2015.1. [13] The frozen core approximation was applied in all SCS-MP2 and CCSD(T) calculations. The ion pairs M[AuF 4 ]w ere calculated using def2-TZVPP [14] basis sets of triple-z quality for all atoms. These basis sets include effective core potentials for Rb (ECP-28), [15] Cs (ECP-46), [15] and Au (ECP-60). [16] For the free anions AuF 4 À and AuF 6 À ,d ef2-TZVPPD [17] basis sets with additional diffuse functions were used for Au and F.