1,1′-(Hexane-1,6-di­yl)dipyridinium bis­(hexa­fluoro­phosphate)

Liang, J.; Jin, D.; Gao, X.; Wang, J.

doi:10.1107/S1600536808039639

organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 64| Part 12| December 2008| Page o2495

doi:10.1107/S1600536808039639

Open

access

1,1′-(Hexane-1,6-diyl)dipyridinium bis(hexafluorophosphate)

Jin-hua Liang,^a Dong Jin,^b Xiao-feng Gao ^a and Jin-tang Wang ^a ^*

^aDepartment of Applied Chemistry, College of Science, Nanjing University of Technology, Nanjing 210009, People's Republic of China, and ^bDepartment of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, People's Republic of China
^*Correspondence e-mail: wjt@njut.edu.cn

(Received 18 November 2008; accepted 25 November 2008; online 29 November 2008)

The asymmetric unit of the title compound, C₁₆H₂₂N₂²⁺·2PF₆⁻, contains one half-molecule and a hexafluorophosphate anion. In the crystal structure, intermolecular C—H⋯F hydrogen bonds link the molecules. The F atoms in the hexafluorophosphate anion are disordered over two positions and were refined with occupancies of 0.43 (2) and 0.57 (2).

Related literature

For general background, see: Jared et al. (2005 ). For bond-length data, see: Allen et al. (1987 ).

Experimental

Crystal data

C₁₆H₂₂N₂²⁺·2PF₆⁻
M_r = 532.30 (3)
Triclinic, $[P \overline 1]$
a = 7.9140 (16) Å
b = 9.2930 (18) Å
c = 9.4870 (19) Å
α = 65.13 (3)°
β = 65.46 (3)°
γ = 74.37 (3)°
V = 572.0 (3) Å³
Z = 1
Mo Kα radiation
μ = 0.29 mm⁻¹
T = 298 (2) K
0.30 × 0.30 × 0.20 mm

Data collection

Enraf–Nonius CAD-4 diffractometer
Absorption correction: ψ scan (North et al., 1968 ) T_min = 0.917, T_max = 0.944
2172 measured reflections
2014 independent reflections
1499 reflections with I > 2σ(I)
R_int = 0.047
3 standard reflections frequency: 120 min intensity decay: none

Refinement

R[F² > 2σ(F²)] = 0.065
wR(F²) = 0.166
S = 1.00
2014 reflections
200 parameters
H-atom parameters constrained
Δρ_max = 0.30 e Å⁻³
Δρ_min = −0.38 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C1—H1A⋯F4′ⁱ	0.93	2.48	3.333 (17)	153
C2—H2A⋯F2′ⁱⁱ	0.93	2.53	3.267 (18)	137
C3—H3A⋯F3′ⁱⁱ	0.93	2.47	3.257 (15)	142
C4—H4A⋯F1′ⁱⁱⁱ	0.93	2.52	3.287 (14)	140
Symmetry codes: (i) -x+1, -y+1, -z+1; (ii) x, y, z-1; (iii) -x, -y+2, -z+1.

Data collection: CAD-4 Software (Enraf–Nonius, 1989 ); cell refinement: CAD-4 Software; data reduction: XCAD4 (Harms & Wocadlo, 1995 ); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL.

Supporting information

Crystal structure: contains datablocks I, global, x1. DOI: 10.1107/S1600536808039639/hk2581sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536808039639/hk2581Isup2.hkl

checkCIF report

Comment top

The title compound is a dicationic ionic liquid, which has high thermal stability. Applications of the dicationic ionic liquid are found in biochemistry as well as many areas of chemistry (Jared et al., 2005). We report herein the crystal structure of the title compound.

The asymmetric unit of the title compound (Fig. 1) contains one-half molecule and a hexafluorophosphate molecule, where the bond lengths (Allen et al., 1987) and angles are within normal ranges.

In the crystal structure, intermolecular C-H···F hydrogen bonds (Table 1) link the molecules (Fig. 2), in which they may be effective in the stabilization of the structure.

Related literature top

For general background, see: Jared et al. (2005). For bond-length data, see: Allen et al. (1987).

Experimental top

For the preparation of the title compound, 1,6-dibromide hexane (12.2 g, 0.05 mol) was added to acetonitrile solution (50 ml) of dehydrate pyridine (7.91 g, 0.10 mol) at 353 K. After stirring for 24 h, the mixture was cooled to room temperature and filtered. The solid was washed with ethyl acetate and dried. Then, the solid (2.01 g, 5 mmol) was dissolved in distilled water (50 ml) and potassium hexafluorophosphate (1.84 g, 10 mmol) was added. After stirring at room temperature for 3 h, the colorless solid formed was collected by filtration, washed with distilled water (50 ml) and dried. The product was purified by repeated crystallization. Crystals suitable for X-ray analysis were obtained by slow evaporation of acetone (yield; 3.08 g, 80%, m.p. 513 K).

Computing details top

Data collection: CAD-4 Software (Enraf–Nonius, 1989); cell refinement: CAD-4 Software (Enraf–Nonius, 1989); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top

	Fig. 1. The asymmetric unit of the title compound, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level.
	Fig. 2. A partial packing diagram of the title compound. Hydrogen bonds are shown as dashed lines.

1,1'-(Hexane-1,6-diyl)dipyridinium bis(hexafluorophosphate) top

Crystal data top

C₁₆H₂₂N₂²⁺·2PF₆⁻	Z = 1
M_r = 532.30 (3)	F(000) = 270
Triclinic, P1	D_x = 1.545 Mg m⁻³
Hall symbol: -P 1	Melting point: 513 K
a = 7.9140 (16) Å	Mo Kα radiation, λ = 0.71073 Å
b = 9.2930 (18) Å	Cell parameters from 25 reflections
c = 9.4870 (19) Å	θ = 10–12°
α = 65.13 (3)°	µ = 0.29 mm⁻¹
β = 65.46 (3)°	T = 298 K
γ = 74.37 (3)°	Block, colorless
V = 572.0 (3) Å³	0.30 × 0.30 × 0.20 mm

Data collection top

Enraf–Nonius CAD-4 diffractometer	1499 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	R_int = 0.047
Graphite monochromator	θ_max = 25.1°, θ_min = 2.4°
ω/2θ scans	h = −8→9
Absorption correction: ψ scan (North et al., 1968)	k = −9→10
T_min = 0.917, T_max = 0.944	l = 0→11
2172 measured reflections	3 standard reflections every 120 min
2014 independent reflections	intensity decay: none

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.065	H-atom parameters constrained
wR(F²) = 0.166	w = 1/[σ²(F_o²) + (0.06P)² + 0.95P] where P = (F_o² + 2F_c²)/3
S = 1.01	(Δ/σ)_max < 0.001
2014 reflections	Δρ_max = 0.30 e Å⁻³
200 parameters	Δρ_min = −0.38 e Å⁻³
Primary atom site location: structure-invariant direct methods

Crystal data top

C₁₆H₂₂N₂²⁺·2PF₆⁻	γ = 74.37 (3)°
M_r = 532.30 (3)	V = 572.0 (3) Å³
Triclinic, P1	Z = 1
a = 7.9140 (16) Å	Mo Kα radiation
b = 9.2930 (18) Å	µ = 0.29 mm⁻¹
c = 9.4870 (19) Å	T = 298 K
α = 65.13 (3)°	0.30 × 0.30 × 0.20 mm
β = 65.46 (3)°

Data collection top

Enraf–Nonius CAD-4 diffractometer	1499 reflections with I > 2σ(I)
Absorption correction: ψ scan (North et al., 1968)	R_int = 0.047
T_min = 0.917, T_max = 0.944	3 standard reflections every 120 min
2172 measured reflections	intensity decay: none
2014 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.065	200 parameters
wR(F²) = 0.166	H-atom parameters constrained
S = 1.01	Δρ_max = 0.30 e Å⁻³
2014 reflections	Δρ_min = −0.38 e Å⁻³

Special details top

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 (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
P	0.16995 (14)	0.79005 (13)	0.78487 (13)	0.0539 (4)
N	0.6355 (4)	0.7536 (4)	0.2462 (4)	0.0472 (8)
F1	−0.006 (3)	0.696 (3)	0.842 (2)	0.156 (6)	0.43 (2)
F2	0.369 (2)	0.8457 (19)	0.721 (2)	0.104 (5)	0.43 (2)
F3	0.034 (2)	0.8894 (16)	0.8982 (15)	0.091 (4)	0.43 (2)
F4	0.215 (2)	0.645 (2)	0.939 (2)	0.076 (4)	0.43 (2)
F5	0.126 (2)	0.936 (2)	0.635 (2)	0.078 (4)	0.43 (2)
F6	0.223 (3)	0.677 (3)	0.684 (2)	0.076 (4)	0.43 (2)
F1'	−0.0338 (9)	0.7809 (16)	0.8155 (14)	0.118 (4)	0.57 (2)
F2'	0.375 (2)	0.785 (2)	0.775 (2)	0.144 (5)	0.57 (2)
F3'	0.127 (2)	0.9036 (12)	0.8910 (13)	0.098 (3)	0.57 (2)
F4'	0.150 (2)	0.6338 (18)	0.9501 (17)	0.090 (4)	0.57 (2)
F5'	0.195 (2)	0.9495 (17)	0.6211 (17)	0.094 (4)	0.57 (2)
F6'	0.2820 (19)	0.6854 (19)	0.6687 (18)	0.082 (4)	0.57 (2)
C1	0.6255 (6)	0.6893 (5)	0.1468 (5)	0.0607 (11)
H1A	0.7227	0.6159	0.1113	0.073*
C2	0.4732 (7)	0.7312 (6)	0.0972 (6)	0.0735 (13)
H2A	0.4661	0.6866	0.0288	0.088*
C3	0.3292 (6)	0.8420 (6)	0.1517 (6)	0.0751 (14)
H3A	0.2260	0.8736	0.1179	0.090*
C4	0.3402 (6)	0.9030 (6)	0.2537 (6)	0.0721 (13)
H4A	0.2428	0.9751	0.2913	0.087*
C5	0.4934 (5)	0.8603 (5)	0.3029 (5)	0.0543 (10)
H5A	0.5003	0.9029	0.3730	0.065*
C6	0.7977 (5)	0.7070 (5)	0.3014 (5)	0.0580 (10)
H6A	0.8387	0.8022	0.2896	0.070*
H6B	0.8998	0.6558	0.2307	0.070*
C7	0.7536 (5)	0.5942 (5)	0.4801 (5)	0.0563 (10)
H7A	0.7184	0.4969	0.4908	0.068*
H7B	0.6475	0.6434	0.5502	0.068*
C8	0.9189 (5)	0.5519 (5)	0.5406 (5)	0.0602 (11)
H8A	0.9628	0.6498	0.5183	0.072*
H8B	0.8761	0.4963	0.6595	0.072*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
P	0.0470 (6)	0.0550 (6)	0.0538 (6)	0.0051 (4)	−0.0114 (5)	−0.0263 (5)
N	0.0332 (15)	0.0466 (17)	0.0475 (17)	−0.0002 (13)	−0.0093 (13)	−0.0107 (14)
F1	0.136 (9)	0.160 (11)	0.165 (9)	−0.050 (8)	−0.033 (7)	−0.049 (8)
F2	0.082 (6)	0.104 (7)	0.133 (9)	−0.045 (5)	−0.035 (6)	−0.030 (6)
F3	0.101 (8)	0.077 (5)	0.074 (5)	0.016 (5)	−0.007 (5)	−0.048 (4)
F4	0.085 (8)	0.067 (5)	0.077 (7)	−0.003 (6)	−0.043 (6)	−0.016 (4)
F5	0.070 (6)	0.080 (7)	0.059 (5)	0.015 (5)	−0.024 (5)	−0.015 (4)
F6	0.097 (9)	0.078 (5)	0.083 (8)	−0.014 (6)	−0.038 (7)	−0.046 (5)
F1'	0.041 (3)	0.113 (7)	0.173 (7)	−0.002 (3)	−0.039 (3)	−0.027 (5)
F2'	0.118 (6)	0.180 (10)	0.147 (8)	−0.033 (7)	−0.069 (6)	−0.036 (7)
F3'	0.126 (7)	0.088 (4)	0.086 (4)	0.000 (5)	−0.021 (5)	−0.058 (3)
F4'	0.090 (7)	0.068 (4)	0.074 (4)	0.010 (5)	−0.009 (5)	−0.022 (3)
F5'	0.124 (8)	0.077 (4)	0.069 (4)	−0.014 (6)	−0.023 (6)	−0.023 (3)
F6'	0.074 (6)	0.078 (5)	0.075 (4)	0.002 (4)	0.003 (4)	−0.043 (3)
C1	0.064 (3)	0.052 (2)	0.057 (2)	0.0000 (19)	−0.015 (2)	−0.022 (2)
C2	0.084 (3)	0.086 (3)	0.066 (3)	−0.017 (3)	−0.040 (3)	−0.023 (3)
C3	0.051 (3)	0.095 (4)	0.066 (3)	−0.015 (2)	−0.031 (2)	−0.002 (3)
C4	0.042 (2)	0.083 (3)	0.066 (3)	0.014 (2)	−0.014 (2)	−0.021 (2)
C5	0.050 (2)	0.056 (2)	0.053 (2)	0.0118 (18)	−0.0171 (18)	−0.0268 (19)
C6	0.0340 (19)	0.066 (3)	0.066 (3)	−0.0019 (17)	−0.0187 (18)	−0.017 (2)
C7	0.0339 (19)	0.074 (3)	0.057 (2)	0.0055 (18)	−0.0156 (17)	−0.027 (2)
C8	0.044 (2)	0.082 (3)	0.060 (2)	0.009 (2)	−0.0230 (19)	−0.035 (2)

Geometric parameters (Å, º) top

P—F1'	1.535 (7)	C3—C4	1.350 (7)
P—F6	1.567 (19)	C3—H3A	0.9300
P—F2'	1.575 (15)	C4—C5	1.379 (6)
P—F3	1.582 (11)	C4—H4A	0.9300
P—F2	1.583 (14)	C5—N	1.372 (5)
P—F5	1.588 (16)	C5—H5A	0.9300
P—F6'	1.604 (14)	N—C6	1.476 (5)
P—F4'	1.611 (14)	C6—C7	1.518 (6)
P—F4	1.612 (18)	C6—H6A	0.9700
P—F5'	1.617 (14)	C6—H6B	0.9700
P—F1	1.619 (15)	C7—C8	1.534 (5)
P—F3'	1.631 (9)	C7—H7A	0.9700
C1—N	1.347 (5)	C7—H7B	0.9700
C1—C2	1.375 (6)	C8—C8ⁱ	1.518 (7)
C1—H1A	0.9300	C8—H8A	0.9700
C2—C3	1.397 (7)	C8—H8B	0.9700
C2—H2A	0.9300

F1'—P—F6	85.7 (10)	F4—P—F1	83.6 (8)
F1'—P—F2'	173.4 (6)	F5'—P—F1	116.5 (8)
F6—P—F2'	95.7 (10)	F1'—P—F3'	97.5 (6)
F1'—P—F3	70.4 (7)	F6—P—F3'	176.5 (11)
F6—P—F3	156.1 (13)	F2'—P—F3'	80.9 (7)
F2'—P—F3	107.8 (9)	F2—P—F3'	78.8 (9)
F1'—P—F2	164.7 (6)	F5—P—F3'	92.5 (8)
F6—P—F2	98.4 (9)	F6'—P—F3'	160.7 (10)
F3—P—F2	105.1 (10)	F4'—P—F3'	90.4 (7)
F1'—P—F5	76.2 (6)	F4—P—F3'	86.1 (8)
F6—P—F5	89.5 (10)	F5'—P—F3'	88.0 (6)
F2'—P—F5	110.2 (6)	F1—P—F3'	112.6 (7)
F3—P—F5	86.5 (8)	N—C1—C2	120.6 (4)
F2—P—F5	89.1 (7)	N—C1—H1A	119.7
F1'—P—F6'	101.7 (9)	C2—C1—H1A	119.7
F2'—P—F6'	80.1 (9)	C1—C2—C3	118.7 (4)
F3—P—F6'	172.1 (12)	C1—C2—H2A	120.7
F2—P—F6'	82.4 (8)	C3—C2—H2A	120.7
F5—P—F6'	91.2 (9)	C4—C3—C2	119.8 (4)
F1'—P—F4'	86.0 (5)	C4—C3—H3A	120.1
F6—P—F4'	88.5 (9)	C2—C3—H3A	120.1
F2'—P—F4'	87.5 (6)	C3—C4—C5	121.1 (4)
F3—P—F4'	88.2 (7)	C3—C4—H4A	119.4
F2—P—F4'	108.7 (6)	C5—C4—H4A	119.4
F5—P—F4'	162.2 (6)	N—C5—C4	118.6 (4)
F6'—P—F4'	91.8 (7)	N—C5—H5A	120.7
F1'—P—F4	104.3 (6)	C4—C5—H5A	120.7
F6—P—F4	91.8 (10)	C1—N—C5	121.2 (3)
F2'—P—F4	69.2 (7)	C1—N—C6	120.7 (3)
F3—P—F4	92.5 (9)	C5—N—C6	118.1 (3)
F2—P—F4	90.3 (7)	N—C6—C7	112.5 (3)
F5—P—F4	178.6 (11)	N—C6—H6A	109.1
F6'—P—F4	90.0 (9)	C7—C6—H6A	109.1
F1'—P—F5'	95.7 (6)	N—C6—H6B	109.1
F6—P—F5'	93.0 (9)	C7—C6—H6B	109.1
F2'—P—F5'	90.7 (6)	H6A—C6—H6B	107.8
F3—P—F5'	91.0 (7)	C6—C7—C8	112.7 (3)
F2—P—F5'	69.5 (6)	C6—C7—H7A	109.0
F6'—P—F5'	89.2 (8)	C8—C7—H7A	109.0
F4'—P—F5'	177.8 (7)	C6—C7—H7B	109.0
F4—P—F5'	159.8 (6)	C8—C7—H7B	109.0
F6—P—F1	69.9 (12)	H7A—C7—H7B	107.8
F2'—P—F1	149.1 (8)	C8ⁱ—C8—C7	113.4 (4)
F3—P—F1	87.2 (7)	C8ⁱ—C8—H8A	108.9
F2—P—F1	166.6 (10)	C7—C8—H8A	108.9
F5—P—F1	97.2 (9)	C8ⁱ—C8—H8B	108.9
F6'—P—F1	85.7 (11)	C7—C8—H8B	108.9
F4'—P—F1	65.6 (7)	H8A—C8—H8B	107.7

Symmetry code: (i) −x+2, −y+1, −z+1.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1A···F4′ⁱⁱ	0.93	2.48	3.333 (17)	153
C2—H2A···F2′ⁱⁱⁱ	0.93	2.53	3.267 (18)	137
C3—H3A···F3′ⁱⁱⁱ	0.93	2.47	3.257 (15)	142
C4—H4A···F1′^iv	0.93	2.52	3.287 (14)	140

Symmetry codes: (ii) −x+1, −y+1, −z+1; (iii) x, y, z−1; (iv) −x, −y+2, −z+1.

Experimental details

Crystal data
Chemical formula	C₁₆H₂₂N₂²⁺·2PF₆⁻
M_r	532.30 (3)
Crystal system, space group	Triclinic, P1
Temperature (K)	298
a, b, c (Å)	7.9140 (16), 9.2930 (18), 9.4870 (19)
α, β, γ (°)	65.13 (3), 65.46 (3), 74.37 (3)
V (Å³)	572.0 (3)
Z	1
Radiation type	Mo Kα
µ (mm⁻¹)	0.29
Crystal size (mm)	0.30 × 0.30 × 0.20

Data collection
Diffractometer	Enraf–Nonius CAD-4 diffractometer
Absorption correction	ψ scan (North et al., 1968)
T_min, T_max	0.917, 0.944
No. of measured, independent and observed [I > 2σ(I)] reflections	2172, 2014, 1499
R_int	0.047
(sin θ/λ)_max (Å⁻¹)	0.598

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.065, 0.166, 1.01
No. of reflections	2014
No. of parameters	200
No. of restraints	?
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.30, −0.38

Computer programs: CAD-4 Software (Enraf–Nonius, 1989), XCAD4 (Harms & Wocadlo, 1995), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1A···F4'ⁱ	0.93	2.48	3.333 (17)	153.00
C2—H2A···F2'ⁱⁱ	0.93	2.53	3.267 (18)	137.00
C3—H3A···F3'ⁱⁱ	0.93	2.47	3.257 (15)	142.00
C4—H4A···F1'ⁱⁱⁱ	0.93	2.52	3.287 (14)	140.00

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) x, y, z−1; (iii) −x, −y+2, −z+1.

Acknowledgements

The authors thank Professor Hua-qin Wang of the Analysis Centre, Nanjing University, for carrying out the X-ray crystallographic analysis.

References

Allen, F. H., Kennard, O., Watson, D. G., Brammer, L., Orpen, A. G. & Taylor, R. (1987). J. Chem. Soc. Perkin Trans. 2, pp. S1–19. CrossRef Web of Science Google Scholar
Enraf–Nonius (1989). CAD-4 Software. Enraf–Nonius, Delft. The Netherlands. Google Scholar
Harms, K. & Wocadlo, S. (1995). XCAD4. University of Marburg, Germany. Google Scholar
Jared, L. A., Ding, R. F., Arkady, E. & Daniel, W. A. (2005). J. Am. Chem. Soc. A, 127, 593–604. Google Scholar
North, A. C. T., Phillips, D. C. & Mathews, F. S. (1968). Acta Cryst. A24, 351–359. CrossRef IUCr Journals Web of Science Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar