Crystal structure of 4-(prop-2-yn­yloxy)-2,2,6,6-tetra­methyl­piperidin-1-ox­yl

The structure of a TEMPO derivative with a propyn­yloxy substituent at the 4-position of the piperidine ring is reported. The crystal packing features an unusual C—H...π inter­action involving the triple bond of the propyne group which combines with C—H...O hydrogen bonds to stack the mol­ecules along the b-axis direction.

Key indicators

• Single-crystal X-ray study
• T = 100 K
• Mean [sigma](C-C) = 0.004 Å
• R factor = 0.043
• wR factor = 0.123
• Data-to-parameter ratio = 16.5

checkCIF/PLATON results

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Alert level C
PLAT906_ALERT_3_C Large K value in the Analysis of Variance ......      2.156 Check
PLAT911_ALERT_3_C Missing # FCF Refl Between THmin & STh/L=  0.600          6 Report
PLAT934_ALERT_3_C Number of (Iobs-Icalc)/SigmaW > 10 Outliers ....          1 Check

Alert level G
PLAT003_ALERT_2_G Number of Uiso or Uij Restrained non-H Atoms ...         15 Report
PLAT032_ALERT_4_G Std. Uncertainty on Flack Parameter Value High .      0.300 Report
PLAT850_ALERT_4_G Check Flack Parameter Exact Value 0.00 and su ..       0.30 Check
PLAT860_ALERT_3_G Number of Least-Squares Restraints .............         90 Note
PLAT912_ALERT_4_G Missing # of FCF Reflections Above STh/L=  0.600         24 Note

   0 ALERT level A = Most likely a serious problem - resolve or explain
   0 ALERT level B = A potentially serious problem, consider carefully
   3 ALERT level C = Check. Ensure it is not caused by an omission or oversight
   5 ALERT level G = General information/check it is not something unexpected

   0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   1 ALERT type 2 Indicator that the structure model may be wrong or deficient
   4 ALERT type 3 Indicator that the structure quality may be low
   3 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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S1. Chemical context

TEMPO, 2,2,6,6-tetra­methyl­piperidin-1-oxyl, and its derivatives have attracted significant inter­est in recent years as functional organic radicals with considerable chemical stability (Soegiarto et al., 2011). They are known to exhibit both ferromagnetism and anti­ferromagnetism at low temperatures (Togashi et al., 1996; Ishida et al., 1995), and the effect of inter­molecular contacts on their magnetic properties has been examined (Iwasaki et al., 1999a,b). TEMPO and its derivatives have been utilized in applications as diverse as catalysis in organic synthesis (Zhao et al., 2005), pulsed electron–electron double-resonance (PELDOR) spectroscopy (Bode et al., 2007), and use as qubits (quantum bits) in quantum computing (Nakazawa et al., 2012).

Our inter­est in TEMPO derivatives is as reversible redox-active subunits in polymer-gel actuators (Goswami et al., 2013). In particular, the alkyne group present in the title compound, (1), allows us to utilize the versatile CuAAC `click' cyclo­addition with organic azides (Hein & Fokin, 2010; Lewis et al., 2013) as a means to attach the TEMPO unit to the gel skeleton.

S2. Structural commentary

The structure of (1) and its atom numbering are shown in Fig. 1. The molecule comprises a standard TEMPO unit with a propynyl­oxy substituent at the 4-position. The N1/C2–C6 ring adopts a flattened chair conformation with the C4 atom 0.706 (4) Å from the best fit plane through the remaining four C atoms, while N1 lies only 0.384 (4) Å from the plane in the opposite direction. The propynyl C7–C9 unit points away from this plane in the same direction as C4, with C7—C8—C9 = 178.6 (3)°. The N—O bond is 1.289 (3) Å long, which compares favorably with the average value of 1.285 (18) Å for other TEMPO structures (Macrae et al., 2008).

S3. Supra­molecular features

In the crystal structure of (1), C9—H9···O1 hydrogen bonds link molecules into C(9) chains along b (Table 1). Additional C61—H61A···O1 contacts form R₃²(16) rings, resulting in double chains of molecules along b (Fig. 2). In an almost orthogonal direction, C7—H7B···O2 hydrogen bonds form C(3) chains along a. An inter­esting feature of these latter contacts is the support provided by C5—H5B···Cg inter­actions (Cg is the mid-point of the C8—C9 bond) involving the alkyne unit (Fig. 3). Such contacts are often overlooked, but they have been reported previously for both terminal and non-terminal alkyne systems (Banerjee et al., 2006; Thakur et al., 2010; McAdam et al., 2012). Overall, these contacts generate a three-dimensional network with molecules stacked in inter­connected columns along the b axis (Fig. 4).

S4. Database survey

The Cambridge Structural Database (CSD; Version 5.35, November 2013 with 2 updates; Allen, 2002) reveals a total of 175 structures of TEMPO and its derivatives. However, structures of alk­oxy-TEMPO derivatives are rare with only a single example, albeit in two separate papers in which Polovyanenko et al. (2008) and Soegiarto et al. (2011) report the structure of 4-(meth­oxy)-TEMPO, 4-(meth­oxy)-2,2,6,6-tetra­methyl­piperidin-1-oxyl. The first paper examines the TEMPO derivative as an inclusion complex of p-hexanoyl calix[4]arene (C₆OH), and investigates the magnetism and orientation dependent motion of the encapsulated radical. In the second, the molecule is included in the cavities of two porous frameworks derived from guanidinium cations and two organodi­sulfonate anions; the magnetic behaviour of the radical guest is investigated. Aryl­oyl­oxy-TEMPO derivatives are more abundant with 19 entries in the CSD (see, for example, Pang et al., 2013; Nakazawa et al., 2012; Akutsu et al., 2005). Again, the focus is very much on the magnetic properties of the materials.

S5. Synthesis and crystallization

Synthesis and characterisation (IR and mass spectroscopy) are as previously described (Gheorghe et al., 2006; Kulis et al., 2009). Colourless blocks were obtained from di­ethyl ether solution at room temperature. Analysis calculated for C₁₂H₂₀NO₂: C 68.54, H 9.59, N 6.66%; found: C 68.57, H 9.66, N 6.68%.

S6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. With no heavy atom in the non-centrosymmetric structure, the absolute structure could not be reliably determined. Friedel opposites were not, however, merged. All H atoms were refined using a riding model, with C—H = 0.99 Å and U_iso(H) = 1.2U_eq(C) for methyl­ene H atoms, C—H = 1.00 Å and U_iso(H) = 1.2U_eq(C) for methine H atoms, C—H = 0.98 Å and U_iso(H) = 1.5U_eq(C) for methyl H atoms, and C—H = 0.95 Å and U_iso(H) = 1.2U_eq(C) for the terminal alkyne H atom. Anisiotropic refinement of the non-H atoms was constrained using the ISOR command in SHELXL to prevent atoms becoming non-positive definite. 10 reflections with F_o >> F_c were omitted from the final refinement cycles.

Related literature

For related literature, see: Akutsu et al. (2005); Allen (2002); Banerjee et al. (2006); Bode et al. (2007); Gheorghe et al. (2006); Goswami et al. (2013); Hein & Fokin (2010); Ishida et al. (1995); Iwasaki et al. (1999a, 1999b); Kulis et al. (2009); Lewis et al. (2013); Macrae et al. (2008); McAdam et al. (2012); Nakazawa et al. (2012); Pang et al. (2013); Polovyanenko et al. (2008); Soegiarto et al. (2011); Thakur et al. (2010); Togashi et al. (1996); Zhao et al. (2005).

Computing details

Data collection: CrysAlis PRO (Agilent, 2013); cell refinement: CrysAlis PRO (Agilent, 2013); data reduction: CrysAlis PRO (Agilent, 2013); program(s) used to solve structure: SIR2011 (Burla et al., 2012); program(s) used to refine structure: SHELXL2013 (Sheldrick, 2008) and TITAN2000 (Hunter & Simpson, 1999); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2013 (Sheldrick, 2008), enCIFer (Allen et al., 2004), PLATON (Spek, 2009) and publCIF (Westrip 2010).

Figures

	Fig. 1. The structure of (1), showing the atom numbering and with displacement ellipsoids drawn at the 50% probability level.
	Fig. 2. Double chains formed from molecules of (1) along b. In this and subsequent Figures, C—H···O hydrogen bonds are drawn as dashed lines and H atoms bound to atoms not involved in hydrogen bonding are not shown.
	Fig. 3. Zigzag chains formed along a from C—H···O and C—H···π (green dotted lines) contacts. The mid-point of the C8═C9 triple bond is shown as a red sphere.
	Fig. 4. The overall packing for (1), viewed along the b axis.

4-(Prop-2-ynyloxy)-2,2,6,6-tetramethylpiperidin-1-oxyl

Crystal data

C12H20NO2	Dx = 1.167 Mg m^−3
Mr = 210.29	Cu Kα radiation, λ = 1.54184 Å
Orthorhombic, P212121	Cell parameters from 4862 reflections
a = 7.94506 (13) Å	θ = 5.3–74.2°
b = 10.17919 (16) Å	µ = 0.63 mm^−1
c = 14.8052 (3) Å	T = 100 K
V = 1197.36 (4) Å^3	Block, colourless
Z = 4	0.18 × 0.15 × 0.08 mm
F(000) = 460

Data collection

Agilent SuperNova (Dual, Cu at zero, Atlas) diffractometer	2307 independent reflections
Radiation source: SuperNova (Cu) X-ray Source	2203 reflections with I > 2σ(I)
Mirror monochromator	Rint = 0.046
Detector resolution: 5.1725 pixels mm^-1	θmax = 74.3°, θmin = 5.3°
ω scans	h = −9→9
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2013)	k = −12→12
Tmin = 0.522, Tmax = 1.000	l = −18→13
6622 measured reflections

Refinement

Refinement on F^2	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H-atom parameters constrained
R[F^2 > 2σ(F^2)] = 0.043	w = 1/[σ^2(Fo^2) + (0.0376P)^2 + 1.029P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.123	(Δ/σ)max < 0.001
S = 1.15	Δρmax = 0.22 e Å^−3
2307 reflections	Δρmin = −0.28 e Å^−3
140 parameters	Absolute structure: Flack x determined using 858 quotients [(I^+)-(I^-)]/[(I^+)+(I^-)] (Parsons & Flack, 2004)
90 restraints	Absolute structure parameter: 0.0 (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.

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

	x	y	z	Uiso*/Ueq
O1	0.3936 (3)	0.73046 (19)	0.71556 (15)	0.0174 (5)
N1	0.3444 (3)	0.8315 (2)	0.67027 (16)	0.0107 (5)
C2	0.2036 (4)	0.9093 (3)	0.71116 (19)	0.0116 (6)
C21	0.2698 (4)	0.9827 (3)	0.7945 (2)	0.0169 (6)
H21A	0.3310	0.9214	0.8335	0.025*
H21B	0.1750	1.0205	0.8280	0.025*
H21C	0.3457	1.0533	0.7753	0.025*
C22	0.0670 (4)	0.8118 (3)	0.7398 (2)	0.0179 (7)
H22A	0.0301	0.7611	0.6872	0.027*
H22B	−0.0289	0.8599	0.7651	0.027*
H22C	0.1125	0.7520	0.7856	0.027*
C3	0.1325 (3)	1.0046 (3)	0.6408 (2)	0.0119 (6)
H3A	0.0654	0.9543	0.5964	0.014*
H3B	0.0558	1.0671	0.6714	0.014*
C4	0.2663 (4)	1.0812 (3)	0.59126 (19)	0.0099 (6)
H4	0.3331	1.1364	0.6341	0.012*
C5	0.3796 (4)	0.9843 (3)	0.54137 (19)	0.0112 (6)
H5A	0.4664	1.0342	0.5078	0.013*
H5B	0.3110	0.9357	0.4966	0.013*
C6	0.4674 (4)	0.8850 (3)	0.6036 (2)	0.0114 (6)
C61	0.6167 (4)	0.9461 (3)	0.6539 (2)	0.0152 (6)
H61A	0.5803	1.0270	0.6840	0.023*
H61B	0.7065	0.9665	0.6108	0.023*
H61C	0.6587	0.8839	0.6991	0.023*
C62	0.5300 (4)	0.7698 (3)	0.5464 (2)	0.0168 (6)
H62A	0.5934	0.7087	0.5846	0.025*
H62B	0.6031	0.8028	0.4982	0.025*
H62C	0.4335	0.7240	0.5197	0.025*
O2	0.1772 (3)	1.16202 (19)	0.52724 (14)	0.0140 (5)
C7	0.2733 (4)	1.2682 (3)	0.4916 (2)	0.0149 (6)
H7A	0.2201	1.2992	0.4350	0.018*
H7B	0.3877	1.2362	0.4765	0.018*
C8	0.2872 (4)	1.3793 (3)	0.5551 (2)	0.0160 (6)
C9	0.2975 (4)	1.4705 (3)	0.6048 (2)	0.0198 (7)
H9	0.3058	1.5435	0.6445	0.024*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0281 (11)	0.0083 (9)	0.0159 (11)	0.0025 (9)	0.0015 (10)	0.0060 (8)
N1	0.0172 (11)	0.0066 (10)	0.0085 (10)	0.0001 (9)	0.0007 (10)	0.0011 (8)
C2	0.0163 (13)	0.0110 (12)	0.0075 (12)	−0.0005 (11)	0.0037 (12)	0.0002 (10)
C21	0.0245 (15)	0.0169 (14)	0.0092 (14)	−0.0002 (12)	−0.0002 (13)	−0.0026 (11)
C22	0.0208 (15)	0.0190 (15)	0.0140 (15)	−0.0056 (12)	0.0028 (13)	0.0044 (12)
C3	0.0145 (12)	0.0110 (12)	0.0101 (13)	0.0020 (10)	0.0011 (11)	−0.0008 (10)
C4	0.0148 (13)	0.0087 (12)	0.0061 (12)	0.0005 (10)	0.0000 (11)	0.0012 (9)
C5	0.0160 (12)	0.0092 (12)	0.0084 (13)	−0.0001 (10)	0.0009 (11)	0.0009 (10)
C6	0.0161 (12)	0.0090 (12)	0.0090 (13)	0.0008 (10)	−0.0008 (11)	0.0015 (10)
C61	0.0165 (13)	0.0132 (13)	0.0160 (15)	−0.0004 (11)	−0.0003 (13)	0.0025 (11)
C62	0.0229 (14)	0.0122 (13)	0.0153 (16)	0.0036 (11)	0.0048 (13)	−0.0019 (11)
O2	0.0178 (10)	0.0102 (9)	0.0140 (10)	0.0000 (8)	−0.0032 (9)	0.0041 (7)
C7	0.0209 (14)	0.0105 (12)	0.0133 (14)	0.0004 (11)	−0.0017 (12)	0.0040 (10)
C8	0.0178 (13)	0.0139 (13)	0.0161 (14)	−0.0007 (11)	−0.0008 (12)	0.0057 (11)
C9	0.0275 (15)	0.0155 (15)	0.0164 (15)	−0.0025 (12)	−0.0026 (14)	0.0022 (12)

Geometric parameters (Å, º)

O1—N1	1.289 (3)	C5—C6	1.535 (4)
N1—C6	1.492 (4)	C5—H5A	0.9900
N1—C2	1.498 (4)	C5—H5B	0.9900
C2—C3	1.531 (4)	C6—C62	1.530 (4)
C2—C22	1.531 (4)	C6—C61	1.532 (4)
C2—C21	1.536 (4)	C61—H61A	0.9800
C21—H21A	0.9800	C61—H61B	0.9800
C21—H21B	0.9800	C61—H61C	0.9800
C21—H21C	0.9800	C62—H62A	0.9800
C22—H22A	0.9800	C62—H62B	0.9800
C22—H22B	0.9800	C62—H62C	0.9800
C22—H22C	0.9800	O2—C7	1.424 (3)
C3—C4	1.509 (4)	C7—C8	1.475 (4)
C3—H3A	0.9900	C7—H7A	0.9900
C3—H3B	0.9900	C7—H7B	0.9900
C4—O2	1.441 (3)	C8—C9	1.187 (5)
C4—C5	1.526 (4)	C9—H9	0.9500
C4—H4	1.0000
O1—N1—C6	115.9 (2)	C4—C5—C6	113.8 (2)
O1—N1—C2	116.0 (2)	C4—C5—H5A	108.8
C6—N1—C2	124.3 (2)	C6—C5—H5A	108.8
N1—C2—C3	109.6 (2)	C4—C5—H5B	108.8
N1—C2—C22	107.3 (2)	C6—C5—H5B	108.8
C3—C2—C22	109.7 (2)	H5A—C5—H5B	107.7
N1—C2—C21	109.1 (2)	N1—C6—C62	107.4 (2)
C3—C2—C21	111.4 (2)	N1—C6—C61	109.5 (2)
C22—C2—C21	109.6 (2)	C62—C6—C61	109.2 (2)
C2—C21—H21A	109.5	N1—C6—C5	109.9 (2)
C2—C21—H21B	109.5	C62—C6—C5	108.7 (2)
H21A—C21—H21B	109.5	C61—C6—C5	112.1 (2)
C2—C21—H21C	109.5	C6—C61—H61A	109.5
H21A—C21—H21C	109.5	C6—C61—H61B	109.5
H21B—C21—H21C	109.5	H61A—C61—H61B	109.5
C2—C22—H22A	109.5	C6—C61—H61C	109.5
C2—C22—H22B	109.5	H61A—C61—H61C	109.5
H22A—C22—H22B	109.5	H61B—C61—H61C	109.5
C2—C22—H22C	109.5	C6—C62—H62A	109.5
H22A—C22—H22C	109.5	C6—C62—H62B	109.5
H22B—C22—H22C	109.5	H62A—C62—H62B	109.5
C4—C3—C2	113.5 (2)	C6—C62—H62C	109.5
C4—C3—H3A	108.9	H62A—C62—H62C	109.5
C2—C3—H3A	108.9	H62B—C62—H62C	109.5
C4—C3—H3B	108.9	C7—O2—C4	114.4 (2)
C2—C3—H3B	108.9	O2—C7—C8	112.7 (2)
H3A—C3—H3B	107.7	O2—C7—H7A	109.1
O2—C4—C3	105.6 (2)	C8—C7—H7A	109.1
O2—C4—C5	109.9 (2)	O2—C7—H7B	109.1
C3—C4—C5	108.5 (2)	C8—C7—H7B	109.1
O2—C4—H4	110.9	H7A—C7—H7B	107.8
C3—C4—H4	110.9	C9—C8—C7	178.6 (3)
C5—C4—H4	110.9	C8—C9—H9	180.0
O1—N1—C2—C3	−166.8 (2)	O1—N1—C6—C62	49.9 (3)
C6—N1—C2—C3	36.2 (4)	C2—N1—C6—C62	−153.1 (3)
O1—N1—C2—C22	−47.7 (3)	O1—N1—C6—C61	−68.6 (3)
C6—N1—C2—C22	155.3 (3)	C2—N1—C6—C61	88.4 (3)
O1—N1—C2—C21	71.0 (3)	O1—N1—C6—C5	167.9 (2)
C6—N1—C2—C21	−86.0 (3)	C2—N1—C6—C5	−35.1 (4)
N1—C2—C3—C4	−47.6 (3)	C4—C5—C6—N1	45.1 (3)
C22—C2—C3—C4	−165.2 (2)	C4—C5—C6—C62	162.3 (2)
C21—C2—C3—C4	73.2 (3)	C4—C5—C6—C61	−76.9 (3)
C2—C3—C4—O2	178.6 (2)	C3—C4—O2—C7	163.0 (2)
C2—C3—C4—C5	60.8 (3)	C5—C4—O2—C7	−80.2 (3)
O2—C4—C5—C6	−174.6 (2)	C4—O2—C7—C8	−77.8 (3)
C3—C4—C5—C6	−59.5 (3)

Hydrogen-bond geometry (Å, º)

Cg is the mid-point of the C8–C9 bond.

D—H···A	D—H	H···A	D···A	D—H···A
C9—H9···O1^i	0.95	2.28	3.205 (4)	163
C7—H7B···O2^ii	0.99	2.52	3.298 (4)	135
C61—H61A···O1^iii	0.98	2.56	3.481 (4)	157
C5—H5B···Cg^iv	0.99	2.93	3.885 (4)	156

Symmetry codes: (i) x, y+1, z; (ii) x+1/2, −y+5/2, −z+1; (iii) −x+1, y+1/2, −z+3/2; (iv) −x−1, y+5/2, −z+3/2.