Comment

Persulfate oxidation of iminooxyacetic acids, R(R')C=NOCH₂COOH, provides a useful route to iminyl radicals (Forrester et al., 1979). The subsequent reactions of the iminyl radicals thus generated depend greatly on the substituents, and important species including nitrogen-containing heterocycles can result. We report here the crystal structure of the title compound, (I) (Fig. 1), and we compare (I) with the simpler analogues (II) and (III), whose structures we reported recently (Glidewell et al., 2004).

The C-O distances in the carboxyl group of (I) (Table 1) are consistent with the fully ordered location of the carboxyl H atom as deduced from a difference map. The distance O3-N4 corresponds exactly with the mean value for the -O-N= bond in oximes (Allen et al., 1987). The other distances are all typical of their types. The interbond angle at O3 is less than the idealized tetrahedral value, and those at N4, N11 and N12 are all significantly less than 120°, pointing in every case to the stereochemical influence of the unshared pairs of electrons. The angles at C5 indicate planarity at this atom. While the azobenzene fragment is effectively planar, as indicated by the key torsion angles (Table 1), the conformation of the side chain between O1 and C11 (Fig. 1) is not amenable to simple explanation. Compounds (I) and (III) have configurations at the C=N double bond [viz. (E) in (I) and (Z) in (III)] such that the sterically smaller substituent at C5 is on the same side as the OCH₂COOH substituent.

The molecules of (I) are linked by paired, and nearly linear, O-HO hydrogen bonds (Table 2) into a centrosymmetric R₂²(8) (Bernstein et al., 1995) dimer, selected for the sake of convenience to lie across the inversion centre at (½, ½, ½) (Fig. 2). In this respect, the primary aggregation in (I) is the same at that in (II) and (III). The structure of (I) differs from that of (III), however, in that there are no direction-specific interactions between the dimers in (I), whereas those in (III) are linked into chains by a single aromatic - stacking interaction.

Figure 1 View of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2 Part of the crystal structure of (I), showing the formation of an R22(8) dimer centred at (0.5, 0.5, 0.5). For the sake of clarity, H atoms bonded to C atoms have been omitted. Atoms marked with an asterisk (*) are at the symmetry position (1 - x, 1 - y, 1 - z).

Experimental

The title compound was prepared by reaction of ClCH₂CO₂H with the oxime derived from 2-(PhN=N)C₆H₄COCH₃ (Forrester et al., 1979), following the general procedure described recently (Glidewell et al., 2004). Crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation of a solution in ethanol (m.p. 420-421 K).

Crystal data

C16H15N3O3 Mr = 297.31 Monoclinic, P2/c a = 16.927 (2) Å b = 4.5244 (5) Å c = 19.673 (2) Å = 105.828 (5)° V = 1449.5 (3) Å3 Z = 4 Dx = 1.362 Mg m-3 Mo K radiation Cell parameters from 3256 reflections = 3.7-27.6° = 0.10 mm-1 T = 120 (2) K Block, colourless 0.24 × 0.18 × 0.10 mm

Data collection

Nonius KappaCCD diffractometer scans, and scans with offsets Absorption correction: multi-scan (SORTAV; Blessing, 1995, 1997) Tmin = 0.967, Tmax = 0.990 13 640 measured reflections 3256 independent reflections 1443 reflections with I > 2(I) Rint = 0.114 max = 27.6° h = -21 21 k = -5 5 l = -25 23

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.069 wR(F2) = 0.187 S = 0.95 3256 reflections 201 parameters H-atom parameters constrained w = 1/[2(Fo2) + (0.0887P)2] where P = (Fo2 + 2Fc2)/3 (/)max < 0.001 max = 0.33 e Å-3 min = -0.30 e Å-3

Table 1 Selected geometric parameters (Å, °) C1-O1 1.306 (4) C1-O2 1.216 (4) C1-C2 1.497 (5) C2-O3 1.419 (4) O3-N4 1.416 (3) N4-C5 1.321 (4) C2-O3-N4 107.6 (2) O3-N4-C5 107.8 (2) N12-N11-C12 114.1 (2) N11-N12-C21 113.9 (3) N4-C5-C6 126.8 (3) N4-C5-C11 114.1 (3) C6-C5-C11 119.0 (3) O1-C1-C2-O3 157.5 (3) C1-C2-O3-N4 83.0 (3) C2-O3-N4-C5 174.9 (3) O3-N4-C5-C11 176.4 (2) N4-C5-C11-C12 114.9 (3) C11-C12-N11-N12 -171.6 (3) C12-N11-N12-C21 -177.0 (2) N11-N12-C21-C22 -169.2 (3)

Table 2 Hydrogen-bonding geometry (Å, °) D-HA D-H HA DA D-HA O1-H1O2i 0.82 1.84 2.655 (3) 175 Symmetry code: (i) 1-x,1-y,1-z.

All H atoms were located in difference maps and then treated as riding atoms in idealized positions, with distances C-H = 0.95 (aromatic), 0.98 (CH₃) or 0.99 Å (CH₂), and O-H = 0.82 Å, and with U_iso(H) = 1.2U_eq(C,O) or 1.5U_eq(C_methyl). The low proportion (ca 44%) of the reflections labelled observed, even at 120 (2) K, together with the rather high merging index (0.11), are suggestive of poorly diffracting crystals, which may in turn be associated with the very limited intermolecular aggregation.

Data collection: KappaCCD Server Software (Nonius, 1997); cell refinement: DENZO-SMN (Otwinowski & Minor, 1997); data reduction: DENZO-SMN; program(s) used to solve structure: OSCAIL (McArdle, 2003) and SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: OSCAIL and SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97 and PRPKAPPA (Ferguson, 1999).