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.


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 redoxactive 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.

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).

Supramolecular features
In the crystal structure of (1), C9-H9Á Á ÁO1 hydrogen bonds link molecules into C(9) chains along b (Table 1). Additional C61-H61AÁ Á ÁO1 contacts form R 2 3 (16) rings, resulting in double chains of molecules along b (Fig. 2). In an almost orthogonal direction, C7-H7BÁ Á ÁO2 hydrogen bonds form C(3) chains along a. An interesting feature of these latter contacts is the support provided by C5-H5BÁ Á ÁCg 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 nonterminal 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).

Figure 2
Double chains formed from molecules of (1) along b. In this and subsequent Figures, C-HÁ Á ÁO hydrogen bonds are drawn as dashed lines and H atoms bound to atoms not involved in hydrogen bonding are not shown.

Figure 1
The structure of (1), showing the atom numbering and with displacement ellipsoids drawn at the 50% probability level.

Figure 3
Zigzag chains formed along a from C-HÁ Á ÁO and C-HÁ Á Á (green dotted lines) contacts. The mid-point of the C8 C9 triple bond is shown as a red sphere.
(C 6 OH), 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.

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.    The overall packing for (1), viewed along the b axis.

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 (Å 2 )