N,N′-Di­methyl­piperazine, grown at 270 K and studied at 220 K

The crystal structure of the title compound, MeNC₄H₈NMe or C₆H₁₄N₂, which is liquid under ambient conditions, has been determined at low temperature. The mol­ecule occupies a special position on a crystallographic inversion centre. The piperazine ring has an almost ideal chair conformation, with the absolute values of the endocyclic torsion angles in the range 57.4–58.6°. No close C—H...N contacts are observed in the crystal structure.

Key indicators

• Single-crystal X-ray study
• T = 220 K
• Mean [sigma](C-C) = 0.002 Å
• R factor = 0.038
• wR factor = 0.107
• Data-to-parameter ratio = 16.6

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

N,N'-Dimethylpiperazine has been observed in 36 crystal structures in the April 2002 Release (Version 5.23) of the Cambridge Structural Database (Allen & Kennard, 1993), although the structure of the pure compound has not previously been determined. Of these database structures, 24 are observed to exist in the diammonium cationic form [e.g. CSD refcodes GOPSEJ, GOPSOT, GOPSUZ and GOPTEK (Troyanov et al., 1999)], and six have both amino groups ligated to a metal [e.g. CUCSAU (Clegg et al., 2000) and MPIPDC (Hassel & Pedersen, 1959)]. Three additional structures have one amino N atom protonated and the second ligated to a metal cation [QEFXOO (Clemente et al., 1999), QISFED and QISFON (Marzotto et al., 2001)]. This leaves three structures [DOSKAX (Gall et al., 1985) and GERWEF and GERWIJ (Yabuki et al., 1988)] with the compound in its neutral form, of which two are cocrystals of different enantiomers (GERWEF and GERWIJ). In all three structures, the amino groups are involved as hydrogen-bond acceptors.

The molecule of N,N'-dimethylpiperazine, (I), as determined by the present study (Fig. 1), occupies a special position in the crystallographic inversion centre. The piperazine ring has an almost ideal chair conformation with the endocyclic torsion angles showing alternating signs and absolute values within the narrow interval of 57.4–58.6°.

In pure N,N'-dimethylpiperazine, no hydrogen-bond donors exist. Nevertheless, it might be expected that close C—H···N contacts would be observed from the methyl groups. This proves not to be the case, with no contacts of less than the sum of the van der Waals radii of the closest atoms observed in this structure. The shortest non-bonded contact to the amino N atom is N1···H1Aⁱ—C1ⁱ [symmetry code (i): x, 1/2 − y, −1/2 + z] is 2.88 Å to the geometrically placed H atom and 3.6646 (17) Å to the C atom. The crystal packing process can therefore be said to be governed by weak van der Waals interactions. This is entirely consistent with the previously reported crystal structure of trimethylamine, determined by Blake et al. (1984) and redetermined by Boese et al. (1998), where voids are observed in the crystal structure in the expected position of the N-atom lone-pair. The lower than expected density of 1.053 Mg m⁻³ of (I) is comparable with that of 0.858 Mg m⁻³ of trimethylamine observed by Blake et al. (1984).

S2. Experimental

N,N'-Dimethylpiperazine was used as received from Aldrich without any further purification. An X-ray quality crystal was grown in situ at 270 K from a small seed obtained by melting back the sample of the frozen liquid in a thin capillary of 0.30 mm diameter.

Computing details

Data collection: DIF4 (Stoe & Cie, 1990); cell refinement: DIF4; data reduction: REDU4 (Stoe & Cie, 1990); program(s) used to solve structure: SHELXTL (Sheldrick, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL.

Figures

Fig. 1. A view of N,N'-dimethylpiperazine. Displacement ellipsoids are drawn at the 30% probability level. The unlabelled atoms are related by an inversion centre in the centre of the molecule.

(I)

Crystal data

C6H14N2	Dx = 1.053 Mg m^−3
Mr = 114.19	Melting point: 270 K
Monoclinic, P21/c	Cu Kα radiation, λ = 1.54178 Å
a = 5.8083 (9) Å	Cell parameters from 85 reflections
b = 10.9298 (19) Å	θ = 15.0–22.0°
c = 5.7575 (8) Å	µ = 0.50 mm^−1
β = 99.903 (12)°	T = 220 K
V = 360.06 (10) Å^3	Cylinder, colourless
Z = 2	0.45 × 0.30 × 0.30 × 0.15 (radius) mm
F(000) = 128

Data collection

Stoe Stadi-4 four-circle diffractometer	590 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.027
Graphite monochromator	θmax = 69.7°, θmin = 7.7°
ω–2θ scans	h = −7→7
Absorption correction: multi-scan [MULABS (Spek, 1998), based on the method of Blessing (1995)]	k = −13→13
Tmin = 0.761, Tmax = 0.840	l = −6→6
2463 measured reflections	3 standard reflections every 30 min
646 independent reflections	intensity decay: 6%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.038	H-atom parameters constrained
wR(F^2) = 0.107	w = 1/[σ^2(Fo^2) + (0.053P)^2 + 0.0372P] where P = (Fo^2 + 2Fc^2)/3
S = 1.10	(Δ/σ)max < 0.001
646 reflections	Δρmax = 0.16 e Å^−3
39 parameters	Δρmin = −0.10 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.087 (9)

Special details

Experimental. Crystal grown at 270 K.

A full sphere of data were collected to give a fourfold multiplicity.

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
C1	0.3206 (2)	0.13775 (12)	0.3157 (2)	0.0669 (5)
H1A	0.2967	0.2256	0.3085	0.100*
H1B	0.4847	0.1198	0.3192	0.100*
H1C	0.2725	0.1058	0.4571	0.100*
N1	0.18206 (17)	0.08079 (9)	0.10903 (17)	0.0530 (4)
C2	−0.0636 (2)	0.11075 (10)	0.0942 (2)	0.0573 (4)
H2A	−0.0836	0.1997	0.0840	0.069*
H2B	−0.1175	0.0827	0.2374	0.069*
C3	−0.2093 (2)	0.05179 (11)	−0.1179 (2)	0.0569 (4)
H3A	−0.3741	0.0728	−0.1229	0.068*
H3B	−0.1615	0.0833	−0.2617	0.068*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0666 (8)	0.0617 (8)	0.0698 (9)	−0.0056 (6)	0.0045 (6)	−0.0049 (6)
N1	0.0539 (6)	0.0454 (6)	0.0593 (7)	−0.0006 (4)	0.0084 (4)	0.0005 (4)
C2	0.0607 (8)	0.0447 (6)	0.0667 (8)	0.0078 (5)	0.0112 (6)	−0.0011 (5)
C3	0.0551 (7)	0.0491 (7)	0.0652 (8)	0.0079 (5)	0.0066 (5)	0.0039 (5)

Geometric parameters (Å, º)

C1—N1	1.4566 (15)	C2—C3	1.5054 (17)
C1—H1A	0.9700	C2—H2A	0.9800
C1—H1B	0.9700	C2—H2B	0.9800
C1—H1C	0.9700	C3—N1^i	1.4577 (15)
N1—C2	1.4523 (15)	C3—H3A	0.9800
N1—C3^i	1.4577 (15)	C3—H3B	0.9800
N1—C1—H1A	109.5	C3—C2—H2A	109.4
N1—C1—H1B	109.5	N1—C2—H2B	109.4
H1A—C1—H1B	109.5	C3—C2—H2B	109.4
N1—C1—H1C	109.5	H2A—C2—H2B	108.0
H1A—C1—H1C	109.5	N1^i—C3—C2	110.78 (10)
H1B—C1—H1C	109.5	N1^i—C3—H3A	109.5
C2—N1—C1	110.54 (10)	C2—C3—H3A	109.5
C2—N1—C3^i	109.02 (9)	N1^i—C3—H3B	109.5
C1—N1—C3^i	110.79 (10)	C2—C3—H3B	109.5
N1—C2—C3	111.15 (10)	H3A—C3—H3B	108.1
N1—C2—H2A	109.4

Symmetry code: (i) −x, −y, −z.