Diiso­propyl­ammonium hydrogen difluoride

By the reaction of diiso­propyl­amine with concentrated hydro­fluoric acid, the title compound, C₆H₁₆N⁺·HF₂⁻, was synthesized. The cations and anions are connected via hydrogen bonds to form chains along [100]. The interlocked polymeric zigzag chains are hexagonally surrounded by six neighbouring chains. The F⁻ anion is strongly hydrogen bonded to HF [F—H 0.98 (4), H...F 1.34 (4), F...F 2.3125 (16) Å and F—H...F 174 (3)°]; a better description is that of a non-centred hydrogen difluoride anion.

S1. Comment

Solutions of alkylamines with an excess of hydrogen fluoride have been reported as convenient reagents for selective fluorination reactions (Haufe, 1996). The structure of the hydrogen difluoride anion (Emsley, 1980) and polyhydrogen fluorides, H_n-1F_n^- (e.g. Wiechert et al., 1998) have been intensively studied in the last decades.

The diisopropylammonium cation is well known to stabilize various anionic systems (Reiss, 1999 and literature cited there). The structures of all yet known diisopropylammonium halogenides (dipHal) have been found isostructural but not isotypic. A systematic study on diisopropylammonium halogenides showed that at least dimorphism - a monoclinic and an orthorhombic modification are known - occurs for dipCl and dipBr (Reiss, 2000).

In the course of this investigation the reaction of diisopropylamine with concentrated hydrofluoric acid yields a compound, (I), that is best described as the salt of the diisopropylammonium cation and the hydrogen difluoride anion. The found hydrogen bonding within the hydrogen difluoride is very similar to that in pyridinium hydrogen difluoride (Boenigk & Mootz, 1988). The hydrogen atom of the HF₂^- anion is found in a non-centered position, closer to the F atom not involved in further hydrogen bonds (F2), but still donating a strong hydrogen bond to the F atom (F1) accepting also two N—H···F hydrogen bonds (Table 2). The donor acceptor distance within the HF₂^- anion is with a value of 2.3125 (16) Å approximately as short as that found in the pyridinium salt mentioned above. Significant shorter F···F donor acceptor distances in the range of 2.26–2.28 Å are reported for the alkali and ammonium hydrogen difluorides (Emsley, 1980). A new unique finding is the extremely short F···F distance [2.233 (2) Å] reported for L-argininium hydrogen difluoride (Ramos Silva et al., 2000). \sch

All C—C and N—C bond lengths within the diisopropylammonium cations (Table 1) are as expected. Each diisopropylammium cation donates two hydrogen bonds to HF₂^- anions and the fluorine atom F1 of every HF₂^- anion accepts two hydrogen bonds from the neighbouring cations. Related to the strong F—H···F hydrogen bond within the complex anion the hydrogen bonds between the cations and the anions are as expected significantly weaker (Tab. 2). Due to this connectivity the structure of the title compound consists of hydrogen bonded, interlocked zigzag chains along the [100] direction (Fig. 2a). The hydrogen-bonding motif may be classified according to Etter's nomenclature (Etter et al., 1990) as C(4). Each polymeric chain is hexagonally surrounded by six further chains (Fig. 2 b). Related to the structures of the other known diisopropylammonium halogenides the title compound is found to be isostructural, with the HF₂^- anion replacing the single halogen atoms in the corresponding chloride and bromide salts.

Related literature

For related literature, see: Boenigk & Mootz (1988); Emsley (1980); Etter et al. (1990); Haufe (1996); Ramos Silva, Paixão, Matos Beja & Alte da Veiga (2000); Reiss (1999, 2000); Wiechert et al. (1998).

S2. Experimental

Diisopropylamine readily reacts with concentrated hydrofluoric acid to give a colourless solution. From this solution, very small thin platelets can be grown within a few weeks at room temperature.

S3. Refinement

All non-hydrogen atoms were refined freely using anisotropic displacement parameters. Atomic coordinates of all H atoms were taken from successive difference Fourier syntheses.

X-ray diffraction data collected using an area-detector suffer more or less from the principle disadvantage that all reflections are exposed with the same time. To overcome this pitfall an appropriate weighting scheme is always recommended.

More serious problems may arise when pseudo-symmetry problems are additionally present or weak diffracting crystals are under investigation. Doing the structure refinement of the title compound using different weighting schemes the wR2 values vary between 4.42% and 12.32%. The structure model shows significant distorsions if a w = 1/s² weighting scheme used (small wR2 value). An analysis of bond lengths variations showed that the N—H distances are mainly affected. Weighting up weak reflections (higher R values) gave equilibrated bond lengths and a more stable refinement. According to nearly identical donor acceptor distances of 2.683 (2) and 2.697 (2) Å of the nitrogen atom of the ammonium group to the two neighbouring hydrogen difluoride anions equilibrated N—H distances are most plausible. To stabilize the latest stages of refinement the N—H distances have been restrained to one refined common value.

The atomic coordinates and individual U_iso values of all hydrogen atoms involved in hydrogen bonds and the H atoms of the C—H groups were refined freely. The hydrogen atoms of the methyl groups were refined riding on their corresponding carbon atom, allowed to rotate by the C—C bond and with a common U_iso value for each group. As expected the Flack parameter is inconclusive.

Computing details

Data collection: 'IPDS-Software (Stoe & Cie, 1998)'; cell refinement: 'IPDS-Software'; data reduction: 'IPDS-Software'; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: Diamond (Brandenburg, 1998); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The diisopropylammonium and the complex hydrogen difluoride anion are connected via hydrogen bonds to form chains along [100]. (The ellipsoids are drawn at the 50% probability level)
	Fig. 2. (a) The interlocked hydrogen-bonded zigzag chains are viewed towards [100] and (b) the hexagonal closed rod packing of the hydrogen-bonded chains in bc plane is shown.

Diisopropylammonium hydrogen difluoroide

Crystal data

C6H16N^+·HF2^−	Dx = 1.050 Mg m^−3
Mr = 141.21	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, P212121	Cell parameters from 650 reflections were selected from the whole dataset. reflections
a = 7.726 (2) Å	θ = 4.7–24.9°
b = 8.057 (3) Å	µ = 0.09 mm^−1
c = 14.352 (6) Å	T = 293 K
V = 893.4 (6) Å^3	Platelet, colourless
Z = 4	0.20 × 0.05 × 0.01 mm
F(000) = 312

Data collection

Stoe IPDS diffractometer	1195 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.029
Graphite monochromator	θmax = 25.0°, θmin = 4.6°
Detector resolution: 50 pixels mm^-1	h = −8→8
ω scans	k = −9→9
3729 measured reflections	l = −17→12
1510 independent reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.030	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.071	w = 1/[σ^2(Fo^2) + (0.040P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.01	(Δ/σ)max = 0.001
1510 reflections	Δρmax = 0.10 e Å^−3
112 parameters	Δρmin = −0.10 e Å^−3
2 restraints	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.052 (11)

Special details

Experimental. 277 exposures were taken in the phi range of 0–360° with a crystal to detector distance of 60 mm and an exposure time of nine minutes. Dynamic integration profiles (13–19 pixels) not allowed to overlap were used for integration.

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
F1	0.87124 (13)	0.72043 (12)	0.56375 (6)	0.0630 (3)
N1	1.21155 (18)	0.68303 (13)	0.59436 (9)	0.0412 (3)
H12	1.266 (2)	0.7152 (16)	0.5418 (11)	0.045 (4)*
H11	1.099 (2)	0.6972 (19)	0.5831 (13)	0.054 (5)*
F2	0.71062 (17)	0.80572 (16)	0.69100 (9)	0.0911 (4)
H1	0.776 (4)	0.763 (3)	0.638 (2)	0.130 (9)*
C1	1.2482 (2)	0.50143 (17)	0.60616 (13)	0.0516 (4)
H1A	1.189 (2)	0.4745 (18)	0.6614 (13)	0.050 (4)*
C2	1.2644 (2)	0.79684 (18)	0.67198 (10)	0.0499 (4)
H2A	1.384 (3)	0.7845 (18)	0.6776 (13)	0.055 (4)*
C11	1.1649 (3)	0.4123 (2)	0.52519 (16)	0.0825 (7)
H11A	1.1817	0.2949	0.5320	0.097 (4)*
H11B	1.2169	0.4490	0.4680	0.097 (4)*
H11C	1.0433	0.4364	0.5241	0.097 (4)*
C12	1.4392 (3)	0.4700 (2)	0.61334 (18)	0.0755 (6)
H12A	1.4848	0.5280	0.6664	0.096 (4)*
H12B	1.4955	0.5089	0.5578	0.096 (4)*
H12C	1.4595	0.3532	0.6204	0.096 (4)*
C21	1.1723 (3)	0.7528 (3)	0.76166 (12)	0.0719 (6)
H21A	1.2113	0.6462	0.7830	0.095 (4)*
H21B	1.0497	0.7490	0.7508	0.095 (4)*
H21C	1.1974	0.8351	0.8081	0.095 (4)*
C22	1.2251 (3)	0.97138 (19)	0.64089 (15)	0.0713 (6)
H22A	1.1023	0.9848	0.6344	0.090 (4)*
H22B	1.2802	0.9921	0.5821	0.090 (4)*
H22C	1.2679	1.0485	0.6864	0.090 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
F1	0.0470 (6)	0.0918 (7)	0.0503 (5)	0.0096 (4)	−0.0050 (4)	0.0020 (5)
N1	0.0387 (8)	0.0453 (6)	0.0395 (7)	0.0014 (5)	0.0004 (6)	0.0024 (5)
F2	0.0744 (8)	0.1262 (9)	0.0726 (7)	−0.0011 (7)	0.0123 (6)	−0.0284 (7)
C1	0.0546 (11)	0.0477 (8)	0.0526 (8)	0.0029 (6)	−0.0010 (8)	0.0089 (7)
C2	0.0365 (11)	0.0621 (8)	0.0509 (9)	−0.0016 (6)	−0.0037 (7)	−0.0075 (7)
C11	0.1074 (18)	0.0489 (8)	0.0913 (15)	0.0010 (9)	−0.0226 (13)	−0.0078 (9)
C12	0.0631 (14)	0.0648 (9)	0.0985 (16)	0.0180 (8)	−0.0020 (11)	0.0061 (9)
C21	0.0711 (14)	0.0993 (13)	0.0454 (9)	−0.0064 (9)	0.0000 (8)	−0.0086 (9)
C22	0.0839 (15)	0.0553 (8)	0.0747 (11)	−0.0003 (9)	0.0040 (11)	−0.0119 (9)

Geometric parameters (Å, º)

F1—H1	1.34 (4)	C11—H11A	0.9600
N1—C1	1.500 (2)	C11—H11B	0.9600
N1—C2	1.500 (2)	C11—H11C	0.9600
N1—H12	0.903 (14)	C12—H12A	0.9600
N1—H11	0.893 (15)	C12—H12B	0.9600
F2—H1	0.98 (4)	C12—H12C	0.9600
C1—C12	1.501 (3)	C21—H21A	0.9600
C1—C11	1.510 (3)	C21—H21B	0.9600
C1—H1A	0.940 (19)	C21—H21C	0.9600
C2—C22	1.506 (2)	C22—H22A	0.9600
C2—C21	1.513 (3)	C22—H22B	0.9600
C2—H2A	0.93 (2)	C22—H22C	0.9600
C1—N1—C2	117.46 (13)	C1—C11—H11C	109.5
C1—N1—H12	106.6 (9)	H11A—C11—H11C	109.5
C2—N1—H12	108.5 (9)	H11B—C11—H11C	109.5
C1—N1—H11	109.2 (10)	C1—C12—H12A	109.5
C2—N1—H11	108.8 (11)	C1—C12—H12B	109.5
H12—N1—H11	105.6 (15)	H12A—C12—H12B	109.5
N1—C1—C12	110.97 (14)	C1—C12—H12C	109.5
N1—C1—C11	107.25 (14)	H12A—C12—H12C	109.5
C12—C1—C11	113.07 (19)	H12B—C12—H12C	109.5
N1—C1—H1A	103.2 (9)	C2—C21—H21A	109.5
C12—C1—H1A	112.3 (11)	C2—C21—H21B	109.5
C11—C1—H1A	109.5 (10)	H21A—C21—H21B	109.5
N1—C2—C22	107.22 (13)	C2—C21—H21C	109.5
N1—C2—C21	111.12 (13)	H21A—C21—H21C	109.5
C22—C2—C21	112.11 (16)	H21B—C21—H21C	109.5
N1—C2—H2A	105.6 (10)	C2—C22—H22A	109.5
C22—C2—H2A	108.9 (9)	C2—C22—H22B	109.5
C21—C2—H2A	111.6 (11)	H22A—C22—H22B	109.5
C1—C11—H11A	109.5	C2—C22—H22C	109.5
C1—C11—H11B	109.5	H22A—C22—H22C	109.5
H11A—C11—H11B	109.5	H22B—C22—H22C	109.5
C2—N1—C1—C12	−62.6 (2)	C1—N1—C2—C22	176.26 (16)
C2—N1—C1—C11	173.42 (16)	C1—N1—C2—C21	−60.9 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H11···F1	0.89 (2)	1.79 (2)	2.6827 (18)	178 (2)
N1—H12···F1^i	0.90 (1)	1.80 (1)	2.6974 (18)	179 (2)
F2—H1···F1	0.98 (4)	1.34 (4)	2.3125 (16)	174 (3)

Symmetry code: (i) x+1/2, −y+3/2, −z+1.