Hg₃AlF₆O₂H, a structure with a short interpolyhedral O...O distance

The crystal structure of Hg₃AlF₆O₂H, trimercury(II) alu­minium hydrogen hexafluoride dioxide, can be derived from a slightly distorted cubic close-packed (ccp) arrangement of the metal atoms, where three quarters of the positions are occupied by Hg atoms and one quarter by Al atoms. The F and O atoms are considerably dislocated from the tetrahedral voids of this arrangement, thus forming [HgO₂F₆] polyhedra, with two short Hg—O distances, two intermediate Hg—F distances and four longer Hg—F distances, and nearly ideal [AlF₆] octahedra. The H atoms are presumably located close to the inversion centre. Their positions were derived from crystal chemical arguments, and they take part in the formation of O—H...O hydrogen bonds between two O atoms, with an O...O distance of 2.562 (9) Å.

S1. Comment

Only a few crystal structures of fluorinated and hydrated compounds containing Hg in different oxidation states are known, namely HgF(OH) (Stålhandske, 1979), HgF₂(H₂O)₂ (Bukvetskii et al., 1976), Hg₂AlF₅(H₂O)₂ (Fourquet et al., 1981), Hg₂SiF₆(H₂O)₂ (Dorm, 1971), Hg₂(OH)₂SiF₆(H₂O)₂ (Golovastikov, 1984), HgFeF₅(H₂O)₂ (Fourquet et al., 1985) and Hg₂FeF₅(OH)₂(H₂O) (Courant et al., 1985). The present work on Hg₃AlF₆O₂H forms part of a project to prepare such phases under different conditions.

The crystal structure of the title compound can be derived from a cubic close-packed (CCP) arrangement of the metal atoms, where three quarters of the positions are occupied by Hg atoms and one quarter by Al atoms. The appropriate rhombohedral cell, with lattice parameters a = 5.4491 Å and α = 83.572°, corresponds to a slight distortion of the cubic close-packing. F and O atoms are considerably dislocated from the tetrahedral voids of this arrangement, thus forming [HgO₂F₆] polyhedra and [AlF₆] octahedra (Fig. 1). The corresponding coordination number (CN) around the Hg atom (CN = 8) might be described as being between a distorted cube and a triangulated dodecahedron (Fig. 2).

The bond lengths in the title compound agree with those found in other fluorinated and hydrated Hg compounds, where the Hg—O bonds are considerably shorter than the Hg—F bonds. The [AlF₆] group has a nearly perfect octahedral geometry, with bond lengths within the typical range for other fluoroaluminates and in good agreement with the mean Al—F value of 1.801 Å for the more irregular [AlF₆] octahedron observed in Hg₂AlF₅(H₂O)₂ (Fourquet et al., 1981). The F atoms are surrounded by one Al and three Hg atoms, resulting in a strongly distorted tetrahedral arrangement. The O atoms are three-coordinate and are located at the apices of trigonal [OHg₃] pyramids [Hg—O—Hg 115.55 (8)°].

Tests on the occupancy factors of Hg, Al, F and O did not indicate the presence of vacancies. Taking into account the fact that, if only the atoms given in the chemical formula were present, the charge sum would be negative, and that the remaining electron density map showed no significant peaks corresponding to other atoms, we are led to the conclusion that H atoms must be present in the structure. However, it was not possible to determine the position of the H atom(s) unambiguously by difference Fourier synthesis of the present X-ray data.

Even though the interpolyhedral F···O distance is within the range for the formation of a weak hydrogen bond, it is rather a normal distance between non-bonded O and F atoms. On the other hand, the O···O distance is too short to be an ordinary interpolyhedral contact. Moreover, this is an ideal length for strong hydrogen bonding. Since this distance is too long for the formation of a symmetrical O—H—O bond, with O—H 1.281 Å and the H atom situated at the inversion centre [position 3(a), with site symmetry 3 m and full occupancy], it is most likely that the H atom lies along the O···O line on position 6(c) (00z) with half-occupancy, to form an asymmetrical O—H···O hydrogen bond with a linear O—H···O angle.

The bond-valence sums for the atoms, calculated using the bond-valence parameters provided by Brese & O'Keefe (1991), are equal to 1.88 for Hg, 2.98 for Al, 1.87 for O and 0.814 for F. This does not strongly support the suggested model, but it has to be considered that the Hg—F parameters are not well tested and therefore the valences of the Hg—F bonds are unreliable.

Another common method to derive the position(s) of the H atom(s) within the structure is the application of IR spectroscopy. A very useful correlation of O—H stretching frequencies and O—H···O hydrogen bond length has recently been published (Libowitzky, 1999), and therefore conventional IR measurements on a powder specimen using the KBr and KCl technique, and single-crystal measurements of selected crystals, were performed. However, although they were expected to give useful results, these IR experiments were not useful because of the reaction of the powder with the matrix and the small size of the measured single crystals.

A comparison between Hg₃AlF₆O₂H and the mineral eglestonite, (Hg₂)₃Cl₃O₂H (Mereiter et al., 1992), shows a similar situation concerning the hydrogen bonding. In the latter mineral, short O—O distances [2.494 (11) Å] and the location of the O atoms at the apices of trigonal [OHg₃] pyramids [Hg—O—Hg 113.7 (1)°; Fig. 3] are also observed. The H-atom position in eglestonite was determined by means of neutron measurements performed on a microcrystalline sample and, by considering half-occupancy for the H atom, it was refined close to the inversion centre, thus resulting in asymmetric hydrogen bonding with a linear O—H···O angle.

Further studies to determine the H-atom position(s) of the title compound experimentally, preferably by means of neutron scattering, need to be carried out in the future.

S2. Experimental

Single crystals of Hg₃AlF₆O₂H were prepared under hydrothermal conditions. Stoichiometric amounts of Hg(NO₃)₂·2H₂O (Merck, p·A.) and AlF₃ (Merck, Patinal; milled before application), with an Hg:Al molar ratio of 1:1, were placed in a 5 cm³ Teflon inlay which was two-thirds filled with 32wt% HF solution (Merck, p·A.). The inlay was then closed, placed in a steel autoclave and heated in a laboratory furnace at 543 K for 10 d. Besides small light-yellow plates of Hg₃AlF₆O₂H with mostly hexagonal habit, unreacted AlF₃ and yellow hexagonal columns of an as yet unknown compound were obtained. The latter could be indexed with hexagonal lattice parameters of a = 6.9705 (4) Å and c = 7.2809 (4) Å. Thermal analysis of selected crystals of this compound and subsequent X-ray powder diffraction analysis of the remaining solid provided an Hg:Al molar ratio of ≈ 3:1, but due to the poor quality of the single crystals a structure solution has not so far been possible. By working in more dilute HF solutions (24wt%), mainly colourless tetragonal columns of Hg₂AlF₅(H₂O)₂ (Fourquet et al., 1981) and unreacted AlF₃ have been observed.

Computing details

Data collection: SMART (Siemens, 1996); cell refinement: SAINT (Siemens, 1996); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS for Windows (Dowty, 1998); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A projection of the crystal structure of Hg3AlF6O2H along [110]. Hg atoms are plotted as dark grey spheres, F atoms as open spheres and O atoms as hatched spheres. The [AlF6] octahedra are light grey. The assumed hydrogen bonding between two O atoms is indicated with dashed bonds.
	Fig. 2. A plot of the [HgO2F6] polyhedron, with displacement ellipsoids drawn at the 75% probability level [symmetry codes: (i) 2/3 - x, 1/3 - y, 1/3 - z; (ii) 1/3 + x, y - 1/3, z - 1/3; (iii) 2/3 + x - y, 1/3 + x, 1/3 - z; (iv) 1/3 - x + y, -1/3 - x, z - 1/3; (v) -x, -y, -z. (vi) 1 + x, y, z; (vii) -y, x - y, z; (viii) 1 + y, y - x, -z].
	Fig. 3. A diagram of the [Hg3O] pyramids in a) eglestonite (Mereiter et al., 1992) and b) Hg3AlF6O2H. Hg atoms are dark grey, O atoms are white and H atoms are light grey. The inversion centre is denoted by a small black point. The broken bonds in a) correspond to the connection to neighbouring Hg atoms of the Hg2^2+ dumb-bell.

trimercury aluminium hydrogen hexafluoride dioxide

Crystal data

Hg3AlF6O2H	Dx = 8.104 Mg m^−3
Mr = 775.76	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3m	Cell parameters from 1547 reflections
a = 7.2621 (6) Å	θ = 3.8–29.8°
c = 10.4415 (9) Å	µ = 72.47 mm^−1
V = 476.89 (7) Å^3	T = 293 K
Z = 3	Plate, light yellow
F(000) = 972	0.14 × 0.08 × 0.08 mm

Data collection

Siemens SMART CCD area-detector diffractometer	192 independent reflections
Radiation source: fine-focus sealed tube	192 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.048
ω scans	θmax = 30.0°, θmin = 3.8°
Absorption correction: numerical using indexed crystal faces (SHELXTL; Siemens, 1995)	h = −9→10
Tmin = 0.034, Tmax = 0.496	k = −9→10
1744 measured reflections	l = −14→14

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)] = 0.014	w = 1/[σ^2(Fo^2) + (0.0134P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.027	(Δ/σ)max < 0.001
S = 1.19	Δρmax = 1.71 e Å^−3
192 reflections	Δρmin = −1.10 e Å^−3
17 parameters	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.00150 (12)

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
Hg	0.5000	0.0000	0.0000	0.01440 (12)
Al	0.0000	0.0000	0.0000	0.0148 (7)
F	−0.2353 (4)	−0.1177 (2)	0.0987 (2)	0.0247 (5)
O	0.0000	0.0000	0.3773 (4)	0.0151 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Hg	0.01606 (14)	0.01281 (14)	0.01324 (16)	0.00640 (7)	0.00130 (3)	0.00260 (6)
Al	0.0142 (9)	0.0142 (9)	0.0161 (15)	0.0071 (5)	0.000	0.000
F	0.0192 (12)	0.0299 (10)	0.0214 (12)	0.0096 (6)	0.0039 (10)	0.0019 (5)
O	0.0148 (12)	0.0148 (12)	0.016 (2)	0.0074 (6)	0.000	0.000

Geometric parameters (Å, º)

Hg—O^i	2.1461 (10)	F—Hg^xvi	2.526 (3)
Hg—O^ii	2.1461 (10)	F—Hg^xv	2.6703 (19)
Hg—F^iii	2.526 (3)	F—Hg^xii	2.6703 (19)
Hg—F^iv	2.526 (3)	F—O^xvii	2.772 (3)
Hg—F^v	2.6703 (19)	F—O	3.264 (5)
Hg—F^vi	2.6703 (19)	F—O^xviii	3.364 (4)
Hg—F^vii	2.6703 (19)	F—Hg^xix	3.961 (2)
Hg—F^viii	2.6703 (19)	F—Hg^xx	3.961 (2)
Hg—Hg^ix	3.6311 (3)	F—O^xxi	3.973 (2)
Hg—Hg^x	3.6311 (3)	F—O^xxii	3.973 (2)
Hg—Al	3.6310 (3)	F—Hg^vii	4.0544 (11)
Hg—Al^vi	3.6310 (3)	O—Hg^xix	2.1461 (10)
Al—F^xi	1.804 (2)	O—Hg^xvi	2.1461 (10)
Al—F^xii	1.804 (2)	O—Hg^xx	2.1461 (10)
Al—F	1.804 (2)	O—O^xxiii	2.562 (9)
Al—F^xiii	1.804 (2)	O—F^xxiv	2.772 (3)
Al—F^vii	1.804 (2)	O—F^xxv	2.772 (3)
Al—F^v	1.804 (2)	O—F^xxvi	2.772 (3)
Al—Hg^xiv	3.6311 (3)	O—F^vii	3.264 (5)
Al—Hg^xii	3.6311 (3)	O—F^xii	3.264 (5)
Al—Hg^xv	3.6310 (3)	O—F^xxvii	3.364 (4)
Al—Hg^x	3.6310 (3)	O—F^xviii	3.364 (4)
Al—Hg^vii	3.6310 (3)
O^i—Hg—O^ii	180.0 (2)	F^xi—Al—F^vii	89.44 (12)
O^i—Hg—F^iii	88.23 (13)	F^xii—Al—F^vii	90.56 (12)
O^ii—Hg—F^iii	91.77 (13)	F—Al—F^vii	90.56 (12)
O^i—Hg—F^iv	91.77 (13)	F^xiii—Al—F^vii	180.00 (18)
O^ii—Hg—F^iv	88.23 (13)	F^xi—Al—F^v	90.56 (12)
F^iii—Hg—F^iv	180.00 (3)	F^xii—Al—F^v	89.44 (12)
O^i—Hg—F^v	69.31 (6)	F—Al—F^v	180.0 (2)
O^ii—Hg—F^v	110.69 (6)	F^xiii—Al—F^v	90.56 (12)
F^iii—Hg—F^v	107.87 (5)	F^vii—Al—F^v	89.44 (12)
F^iv—Hg—F^v	72.13 (5)	Al—F—Hg^xvi	138.99 (13)
O^i—Hg—F^vi	110.69 (6)	Al—F—Hg^xv	106.90 (9)
O^ii—Hg—F^vi	69.31 (6)	Hg^xvi—F—Hg^xv	102.84 (7)
F^iii—Hg—F^vi	72.13 (5)	Al—F—Hg^xii	106.90 (9)
F^iv—Hg—F^vi	107.87 (5)	Hg^xvi—F—Hg^xii	102.85 (7)
F^v—Hg—F^vi	180.00 (11)	Hg^xv—F—Hg^xii	85.67 (8)
O^i—Hg—F^vii	110.69 (6)	Al—F—Hg^xix	80.20 (8)
O^ii—Hg—F^vii	69.31 (6)	Hg^xvi—F—Hg^xix	63.64 (5)
F^iii—Hg—F^vii	72.13 (5)	Hg^xv—F—Hg^xix	161.51 (7)
F^iv—Hg—F^vii	107.87 (5)	Hg^xii—F—Hg^xix	108.97 (2)
F^v—Hg—F^vii	56.76 (10)	Al—F—Hg^xx	80.20 (8)
F^vi—Hg—F^vii	123.24 (10)	Hg^xvi—F—Hg^xx	63.64 (5)
O^i—Hg—F^viii	69.31 (6)	Hg^xv—F—Hg^xx	108.97 (2)
O^ii—Hg—F^viii	110.69 (6)	Hg^xii—F—Hg^xx	161.51 (7)
F^iii—Hg—F^viii	107.87 (5)	Hg^xix—F—Hg^xx	54.57 (4)
F^iv—Hg—F^viii	72.13 (5)	Al—F—Hg^vii	63.58 (3)
F^v—Hg—F^viii	123.24 (10)	Hg^xvi—F—Hg^vii	109.62 (4)
F^vi—Hg—F^viii	56.76 (10)	Hg^xv—F—Hg^vii	61.343 (17)
F^vii—Hg—F^viii	180.00 (10)	Hg^xii—F—Hg^vii	137.58 (8)
F^xi—Al—F^xii	180.00 (14)	Hg^xix—F—Hg^vii	109.56 (5)
F^xi—Al—F	89.44 (12)	Hg^xx—F—Hg^vii	60.91 (2)
F^xii—Al—F	90.56 (12)	Hg^xix—O—Hg^xvi	115.55 (8)
F^xi—Al—F^xiii	90.56 (12)	Hg^xix—O—Hg^xx	115.55 (8)
F^xii—Al—F^xiii	89.44 (12)	Hg^xvi—O—Hg^xx	115.55 (8)
F—Al—F^xiii	89.44 (12)

Symmetry codes: (i) −x+2/3, −y+1/3, −z+1/3; (ii) x+1/3, y−1/3, z−1/3; (iii) x−y+2/3, x+1/3, −z+1/3; (iv) −x+y+1/3, −x−1/3, z−1/3; (v) −x, −y, −z; (vi) x+1, y, z; (vii) −y, x−y, z; (viii) y+1, −x+y, −z; (ix) −y+1, x−y, z; (x) −y, x−y−1, z; (xi) x−y, x, −z; (xii) −x+y, −x, z; (xiii) y, −x+y, −z; (xiv) −x+y+1, −x+1, z; (xv) x−1, y, z; (xvi) −y−1/3, x−y−2/3, z+1/3; (xvii) x−2/3, y−1/3, z−1/3; (xviii) −x−2/3, −y−1/3, −z+2/3; (xix) −x+y+2/3, −x+1/3, z+1/3; (xx) x−1/3, y+1/3, z+1/3; (xxi) −x−1/3, −y−2/3, −z+1/3; (xxii) −x−1/3, −y+1/3, −z+1/3; (xxiii) −x, −y, −z+1; (xxiv) x+2/3, y+1/3, z+1/3; (xxv) −x+y−1/3, −x−2/3, z+1/3; (xxvi) −y−1/3, x−y+1/3, z+1/3; (xxvii) x−y+1/3, x+2/3, −z+2/3.