Structure of a new chloroauric acid

The new tetrachloroaurate ethanol hydrate HAuCl₄·0.65C₂H₅OH·1.35H₂O was prepared from chloroauric acid trihydrate in ethanol. The compound crystallizes in the triclinic space group (No. 2). The [AuCl₄]^- units in the structure have approximately square-planar symmetry, forming chains parallel to the crystallographic b-axis direction.

S1. Introduction

The new HAuCl₄·0.65C₂H₅OH·1.35H₂O compound was crystallized from anhydrous ethanol by dissolving HAuCl₄·3H₂O and slow evaporation in argon atmosphere. HAuCl₄·3H₂O was obtained by dissolving gold in aqua regia at 80 °C. HNO₃ was removed from the solution by repeated evaporation, while adding more HCl. After the final evaporation of HCl and subsequent coagulation by cooling, crystalline material of HAuCl₄·3H₂O was obtained.

A yellow single crystal of HAuCl₄·0.65C₂H₅OH·1.35H₂O was sealed in a capillary for intensity measurements using an X-ray single crystal diffractometer (Stoe, IPDS, Darmstadt, Germany), equipped with (graphite) monochromated Mo-K_α radiation (λ = 71.073 pm). The intensity data were corrected for Lorentz factors, polarization effects by the IPDS software. Crystal structure solutions were performed with direct methods (SHELXS), followed by full matrix least square structure refinements (SHELXL-97) (Sheldrick, 1997). The hydrogen atoms of the hydro­nium ion were calculated geometrically and refined using AFIX 1. Further details of the crystal structure investigation may be obtained free of charge via https://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: [phone]; or e-mail: deposit @ccdc.cam.ac.uk, on quoting the depository number CCDC 1016070. ATR-FTIR measurement was carried on a Perkin–Elmer FTIR spectrometer (Spectrum 1000).

S2. Experimental

The new HAuCl₄·0.65C₂H₅OH·1.35H₂O compound was crystallized from anhydrous ethanol by dissolving HAuCl₄·3H₂O and slow evaporation in argon atmosphere

S2.1. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1.

S3. Results and discussion

The tetra­hydrate and trihydrate of tetra­chloro­auric acid are commercially available and commonly used as starting material in the synthesis of gold complexes and gold nanoparticles. A yellow cubic-shaped single crystal of HAuCl₄·0.65C₂H₅OH·1.35H₂O was chosen for X-ray single crystal structure determination. Atomic coordinates and isotropic equivalent displacement parameters are reported in Table 1. A summary of selected bond lengths is listed in Table 2. HAuCl₄·0.65C₂H₅OH·1.35H₂O crystallizes with two formula units in the triclinic system with the space group P (no. 2) with a = 804.5 (2), b = 804.6 (2), c = 1029.1 (2) pm, and α = 94.31 (3)°, β = 104.08 (2)°, γ = 109.21 (3)°.

The crystal structure presented in Figure 1 contains gold atoms surrounded by four chlorine atoms. The [AuCl₄]^- ion in the structure has approximately square-planar geometry (Bonamico & Dessy, 1973) with a maximum deviation less than 2 pm (Tab. 2). [AuCl₄]^- units are arranged in chains parallel to the crystallographic b-axis direction. Ethanol and water molecules occupy spaces in-between the chains. The oxygen atoms from ethanol and water molecules share the same lattice site in the crystal structure of HAuCl₄·0.65C₂H₅OH·1.35H₂O. The Au–Au contacts within chains are 405.8 (2) and 631.6 (2) pm and can be regarded as nonbonding (Wells, 1971; Mingos, 1996; Mendizabal & Pyykkö, 2004). The shorter distance is comparable with [NH(C₂H₄OH)₃]⁺[AuCl₄]^–·H₂O, where Au–Au contacts are about 409 and 425 pm (Sharutin et al., 2010). The chain structure of the gold(III) complexes where the [AuCl₄]^- ions are stabilized by different ligands present for example in [EtC(OEt)NH₂]⁺[AuCl₄]^- (Potts et al., 1991), [NH(C₂H₄OH)₃]⁺[AuCl₄]^-·H₂O (Sharutin et al., 2010) or [C₄H₁₂N₂]²⁺[AuCl₄]₂^2-·2H₂O (Polishchuk et al., 2009).

The cohesion in the structure between [AuCl₄]^- ions and protonated water with ethanol molecules is given by a network of hydrogen bridges. Nearest neighbor Cl???O distances are 330.1 (1) pm for Cl(1), 327.7 (1) pm for Cl(2), 322.9 (1) for Cl(3), and 316.9 (1) pm for Cl(4). The values are similar to the closest distance between non-hydrogen atoms in [NH(C₂H₄OH)₃]⁺[AuCl₄]^-·H₂O where Cl???O distance is about 326 pm for water molecule (O???O distances between water and different ammonium cations are about 280 pm) (Sharutin et al., 2010) and in [EtC(OEt)NH₂]⁺[AuCl₄]^- where Cl???N distance is 335 pm (Potts et al., 1991). Nuclear quadrupole resonance studies performed for NaAuCl₄·2H₂O and NaAuCl₄·2D₂O suggest that chlorine atoms form two weak hydrogen bonds (Needham, 2013) with two adjacent water molecules at O···Cl(1) distances of approximately 336 pm (Fryer & Smith, 1968).

A closer inspection of the refined crystal structure of HAuCl₄·0.65C₂H₅OH·1.35H₂O leads to the conclusion that H⁺ ions are present in the structure and the formula can be expressed as [(0.65EtOH·1.35H₂O)H]⁺[AuCl₄]^-. A di­aqua­ted proton was described in the structure of the tetra­hydrated acid (O?Reilly et al., 1971). From theoretical calculations the minimum energy of the O···O distance of a protonated (H₅O₂)⁺ ion was found to be 238 pm (O?Reilly et al., 1971), and thus shorter than in the water dimer (H₂O)₂ (298 pm) (Buckingham et al., 2008). For HAuCl₄·4H₂O this distance is about 248 pm (O?Reilly et al., 1971), in HAuCl₄·3H₂O is 246 pm, and in HAuCl₄·2H₂O is 238 pm (Büchner & Wickleder, 2005). For the new HAuCl₄·0.65C₂H₅OH·1.35H₂O compound the O···O distance is 240.67 (7) pm. The differences in distances for the chloro­auric acid hydrates are proportional to the hydrogen bonds of the next water molecules connected to the (H₅O₂)⁺ ion, for the new compound the same as for HAuCl₄·2H₂O there are no chains of water molecules present in the structure. For comparison, in (H₅O₂)[Au(NO₃)₄]·H₂O where [Au(NO₃]₄]^- units are linked by (H₅O₂)⁺ ions the corresponding distance is 243 pm (Büchner & Wickleder, 2004).

The chain structure of HAuCl₄·0.65C₂H₅OH·1.35H₂O can also be stabilized by weak C–H···Cl hydrogen bonds (Aullon et al., 1998; Bourosh et al., 2007) with the distances C(1)–Cl(4) 347.9 (1), C(3)–Cl(1) 359.2 (1), and C(4)–Cl(3) 369.0 (1)pm.

The infrared spectrum of HAuCl₄·0.65C₂H₅OH·1.35H₂O was recorded in the range of 400-4000 cm^-1. The broad absorption band in the 400-3655 cm^-1 region can be assigned to the vibration of O–H···O groups (Vol?pin, 1981; Varnek et al., 2002). The spectrum was additionally characterized by sharp bands in the range from 400 to 1865 cm^-1, with the three most intensive peaks at 774, 922 and 1502 cm^-1. The band for ethanol ascribed to CH₃ asymmetric stretching vibration was at 2976 cm^-1, the band related to the stretching vibration of the O–H bond was observed at 3297 cm^-1. The band at lower energies associated to the stretch vibration of C–O bond in ethanol molecule was difficult to distinguish.

The infrared spectrum of HAuCl₄·3H₂O was also recorded for comparison. The broad absorption band in the 400-3668 cm^-1 region confirmed the presence of the (H₅O₂)⁺ ion in the crystal structure and was assigned to the vibration of O–H···O groups (Vol?pin, 1981; Varnek et al., 2002). The band, related to the stretching vibration of the O–H bond for the water molecules, was observed at 3424-3504 cm^-1. The broad band between 1358 and 1921 cm^-1 with two maximums at 1604 and 1700 cm^-1 was assigned to the HOH bending vibrations for the water molecules and one of O–H···O bending vibrations in the (H₅O₂)⁺ ions. The asymmetric stretching vibrations and another bending motions of the O–H···O fragment were ascribed to the broad band at 1077 cm^-1 (Vener et al., 2001; Vener & Sauer, 2005; Stoyanov & Reed, 2006). At low frequency, band between 400 and 807 with maximum at 492 cm^-1, was due to the wagging and rocking modes of the terminal water molecules (Vener & Sauer, 2005; Vendrell et al., 2007).

Computing details

Data collection: STOE IPDS-Software; cell refinement: STOE IPDS-Software; data reduction: STOE IPDS-Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997).

Figures

Fig. 1.

(I)

Crystal data

C1.29H7.59AuCl4O2	Z = 2
Mr = 393.97	F(000) = 357
Triclinic, P1	Dx = 2.176 Mg m^−3
a = 8.0454 (18) Å	Mo Kα radiation, λ = 0.71073 Å
b = 8.0457 (18) Å	Cell parameters from 6932 reflections
c = 10.291 (2) Å	θ = 2.7–25.0°
α = 94.31 (3)°	µ = 13.07 mm^−1
β = 104.08 (3)°	T = 235 K
γ = 109.21 (3)°	Bloc, yellow
V = 601.3 (2) Å^3	0.5 × 0.5 × 0.3 mm

Data collection

STOE IPDS I diffractometer	1986 independent reflections
Radiation source: fine-focus sealed tube	1719 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.078
Oscillation scans	θmax = 25.0°, θmin = 2.7°
Absorption correction: numerical STOE X-RED/X-SHAPE	h = −9→9
Tmin = 0.058, Tmax = 0.123	k = −9→9
6932 measured reflections	l = −12→12

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.041	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.087	H atoms treated by a mixture of independent and constrained refinement
S = 0.99	w = 1/[σ^2(Fo^2) + (0.0529P)^2] where P = (Fo^2 + 2Fc^2)/3
1986 reflections	(Δ/σ)max < 0.001
101 parameters	Δρmax = 1.10 e Å^−3
24 restraints	Δρmin = −1.16 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Au1	0.07963 (4)	0.70101 (4)	0.15911 (2)	0.05237 (15)
Cl1	−0.1837 (4)	0.7499 (4)	0.1545 (2)	0.0731 (6)
Cl2	0.3459 (4)	0.6563 (4)	0.1649 (2)	0.0713 (6)
Cl3	0.0733 (4)	0.7847 (3)	−0.04779 (19)	0.0698 (6)
Cl4	0.0874 (4)	0.6162 (4)	0.3669 (2)	0.0769 (7)
O1	0.2993 (10)	0.3478 (8)	0.3579 (5)	0.0722 (18)
H5A	0.4366	0.4447	0.4088	0.087*	0.68
H5B	0.2383	0.4367	0.3088	0.087*
H5C	0.3321	0.2737	0.2821	0.087*
C1	0.181 (4)	0.232 (4)	0.426 (3)	0.068 (8)	0.324 (13)
H1A	0.0630	0.1611	0.3599	0.081*	0.324 (13)
H1B	0.1593	0.3034	0.4963	0.081*	0.324 (13)
C2	0.272 (3)	0.112 (2)	0.487 (2)	0.044 (5)	0.324 (13)
H2A	0.3906	0.1834	0.5499	0.053*	0.324 (13)
H2B	0.2889	0.0385	0.4160	0.053*	0.324 (13)
H2C	0.1960	0.0367	0.5353	0.053*	0.324 (13)
O2	0.3273 (9)	0.1898 (9)	0.1611 (6)	0.0706 (17)
H6A	0.2027	0.1619	0.0770	0.085*
H6B	0.2907	0.0650	0.1593	0.085*	0.676 (13)
C3	0.512 (2)	0.233 (2)	0.1464 (16)	0.025 (4)	0.324 (13)
H3A	0.5096	0.1565	0.0668	0.031*	0.324 (13)
H3B	0.5610	0.3580	0.1343	0.031*	0.324 (13)
C4	0.623 (3)	0.205 (3)	0.267 (2)	0.040 (5)	0.324 (13)
H4C	0.7461	0.2278	0.2585	0.049*	0.324 (13)
H4B	0.5706	0.0825	0.2797	0.049*	0.324 (13)
H4A	0.6292	0.2856	0.3442	0.049*	0.324 (13)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Au1	0.0621 (3)	0.05993 (18)	0.03438 (18)	0.02198 (16)	0.01269 (14)	0.00686 (13)
Cl1	0.0721 (16)	0.1048 (16)	0.0585 (12)	0.0469 (14)	0.0221 (11)	0.0278 (12)
Cl2	0.0686 (15)	0.0937 (16)	0.0646 (13)	0.0377 (13)	0.0258 (11)	0.0272 (12)
Cl3	0.0763 (15)	0.0981 (15)	0.0360 (9)	0.0299 (13)	0.0152 (9)	0.0257 (10)
Cl4	0.111 (2)	0.1089 (17)	0.0346 (9)	0.0596 (16)	0.0291 (10)	0.0305 (10)
O1	0.107 (5)	0.092 (4)	0.034 (3)	0.049 (4)	0.024 (3)	0.034 (3)
C1	0.071 (11)	0.073 (11)	0.070 (11)	0.033 (8)	0.031 (8)	0.007 (8)
C2	0.054 (10)	0.036 (7)	0.046 (8)	0.011 (6)	0.020 (7)	0.037 (6)
O2	0.065 (4)	0.093 (4)	0.051 (3)	0.030 (4)	0.009 (3)	0.016 (3)
C3	0.028 (8)	0.029 (6)	0.028 (7)	0.016 (5)	0.026 (6)	0.014 (5)
C4	0.026 (8)	0.056 (8)	0.045 (8)	0.012 (7)	0.021 (6)	0.018 (7)

Geometric parameters (Å, º)

Au1—Cl1	2.268 (3)	O1—C1	1.45 (3)
Au1—Cl2	2.274 (3)	C1—C2	1.48 (3)
Au1—Cl3	2.2749 (19)	O2—C3	1.458 (18)
Au1—Cl4	2.286 (2)	C3—C4	1.42 (3)
Cl1—Au1—Cl2	179.18 (9)	Cl2—Au1—Cl4	90.67 (9)
Cl1—Au1—Cl3	90.92 (9)	Cl3—Au1—Cl4	179.64 (9)
Cl2—Au1—Cl3	88.98 (9)	O1—C1—C2	109 (2)
Cl1—Au1—Cl4	89.43 (10)	C4—C3—O2	107.4 (12)