Crystal structure of trans-N,N′-bis(3,5-di-tert-butyl-2-hydroxyphenyl)oxamide methanol monosolvate

In the title solvate, the oxamide derivative presents the same conformation as in the unsolvated molecule, but the lattice solvent introduces disorder for various functional groups.


Chemical context
1,2-Bis-(3,5-di-tert-butyl-2-hydroxyphenyl)oxamide has been synthesized by two different routes, reported in the literature (Jímenez-Pé rez et al., 2000;Beckmann et al., 2003). For several oxamide derivatives, NMR and crystallographic studies have shown that these compounds have the same conformation in the solid state and in solution: a planar structure stabilized by an intramolecular three-centre hydrogen bond forming two five-membered rings (Martínez-Martínez et al., 1993, 1998. Other studies of the polymerization of ethylene showed that Zr complexes bearing oxamide ligands are active as catalyst (Gü izado-Rodríguez et al., 2007). Phenyloxamides have also been reported as light stabilizers for plastics (Burdet et al., 1972).
While attempting to coordinate 1,2-bis-(3,5-di-tert-butyl-2hydroxyphenyl)oxamide to first-row transition metals in ISSN 2056-9890 methanol, we obtained crystals of the title solvate, for which we report here the molecular and crystal structures.

Structural commentary
The trans-oxamide derivative lies on an inversion centre, placed at the midpoint of the central C1-C1 i bond [symmetry code: (i) 1 À x, Ày, 1 À z], and the methanol molecule is placed close to the twofold axis of the C2/c space group, and was then refined with its occupancy constrained to 1/2 (Fig. 1). The dimensions for the oxamide molecule are very similar to those reported for the unsolvated crystal (Jímenez-Pé rez et al., 2000).
The molecular conformation is not planar, and can be described using the dihedral angle between the oxamide core C1/O1/N1 and the benzene ring C2-C7. In the title solvate, this angle is 32.4 (2) , slightly smaller than the same angle observed in the unsolvated crystal, 38.4 . A comparison of conformations stabilized for this molecule shows that a planar conformer is obtained only if amine and hydroxy groups are deprotonated to form a tetraanion, which is then able to coordinate a metal centre (e.g. Beckmann et al., 2003). The twisted conformation for the neutral molecule is probably a consequence of the formation of an intramolecular hydrogen bond between hydroxy and carbonyl groups (Table 1, entry 1). The resulting motif is an S(7) self-associating pattern having an envelope shape, in order to bring the O-HÁ Á ÁO angle as close as possible to 180 . The involved OH group is disordered over two chemically equivalent positions on the benzene ring, C7 and C3. However, the most populated site, O3A, which has a site occupancy factor of 0.696 (4), is that forming this contact (Fig. 2). Because of the centrosymmetric character of the molecule, two occurrences of the S(7) motif are stabilizing the twisted conformation.
Other potential intramolecular hydrogen bonds starting from the amine groups N1 are present in the molecule, forming other S rings with lower degree. However, these contacts, N1-H1Á Á ÁO1 and N1-H1Á Á ÁO3B, are not relevant for the molecular conformation, since their D-HÁ Á ÁA angles are close to 100 , corresponding to a stabilization energy close to 0 kJ mol À1 (Wood et al., 2009).

Supramolecular features
The introduction of methanol changes the original P1 crystal symmetry (Jímenez-Pé rez et al., 2000) to C2/c ( Table 2). The methanol molecule is located in close proximity to the oxamide, and behaves both as a donor and acceptor for hydrogen bonding (Table 1, entries 2-4). Discrete O-HÁ Á ÁO(methanol) weak bonds are formed with the disordered hydroxy group O3B of the oxamide, as well as N-HÁ Á ÁO(methanol) with the amine groups. As a result, R 1 2 (7) rings are formed (Fig. 2). The last heteroatom involved in hydrogen bonding is the carbonyl O atom O1, acting as an acceptor (Table 1, entry 4), to form R 2 1 (7) rings.

Figure 1
The structure of the title solvate, with atom labelling and displacement ellipsoids drawn at the 30% probability level. A single site for the disordered groups is shown, and non labelled atoms are generated by inversion symmetry. The inset represents the resolved disorder in the tertbutyl group C8/C9/C10/C11 (colour code: red, green and blue ellipsoids are for sites A, B and C, respectively). Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x; Ày; z þ 1 2 ; (ii) Àx þ 1; Ày; Àz þ 1.

Figure 2
S and R motifs formed via hydrogen bonding in the title solvate. Disordered sites O3A and O3B are retained, since they participate in different patterns. All hydrogen bonds listed in Table 1 are represented by dashed lines.

Database survey
The oxamidate derived from the title oxamide has been used extensively for coordination chemistry. It is possible to find one report in the literature for zinc clusters with 1,2-bis-(3,5di-tert-butyl-2-hydroxyphenyl)oxamidate (Rufino-Felipe et al., 2016). In these complexes, the crystal structures exhibit an octanuclear Zn 8 cage and a hexanuclear Zn 6 cage, where the nuclearity of the cages is driven by the solvent. Other compounds with Si or Ge (Jimé nez-Pé rez et al., 2007) are described as bimetallic hexacyclic symmetric heterocycles, with hypervalent Si and Ge centres. For Sn compounds (Jímenez-Pé rez et al., 2000;Contreras et al., 2000), two pentaor hexa-coordinated Sn atoms are arranged in a hexacyclic symmetric planar array. For Fe and Ga complexes (Beckmann et al., 2003;Bill et al., 2002), the metal ions Ga 3+ and Fe 3+ are five-coordinate, with a distorted trigonal-bipyramidal geometry in a hexacyclic symmetric planar array. Finally, in Ti, Zr and Hf complexes (Gü izado- Rodríguez et al., 2007), the metal displays a planar structure similar to that observed in Sn complexes, but no X-ray structures were determined. On the other hand, several phenol-oxamides have shown different conformations, ranging from completely flat (Weiss et al., 2015) to arrangements where the oxamide group presents a tilt angle, or is even almost completely perpendicular to the plane of the aromatic rings (Wen et al., 2006;Piotrkowska et al., 2007). Piotrkowska's group made a good analysis of the phenyl-oxamides and explained how the substituent groups on the aromatic rings and the presence of solvent influence the conformation of the oxamide group: hydrogen bonds andstacking between aromatic rings are the main forces responsible for the assembly of molecules within the crystal lattice. Thus, the steric effects of the bulky o-substituents cause twisting of the aryl ring from the oxamide plane, and interfere with the formation of hydrogen bonds (Piotrkowska et al., 2007).

Synthesis and crystallization
The reaction of 100 mg (0.171 mmol) of disodium bis(4,6-ditert-butyl-1-oxo-phenyl)oxamido and 81 mg (0.342 mmol) of NiCl 2 Á6H 2 O in methanol with a molar ratio 1:2 afforded a dark-brown solution. An amount of maleic acid (79 mg, 0.684 mmol), intended as a bridging ligand, was then added, changing the colour of the solution to light green. After a few minutes under stirring, a cottony precipitate formed. The solution was filtered and the filtrate allowed to crystallize by solvent evaporation, affording needle-shaped green crystals. The green colour is due to a thin layer of nickel chloride covering the crystals. Some of these crystals were washed with methanol, giving colourless crystals (m.p. 496-497 K), used for X-ray crystallography.  PRO (Agilent, 2013); program(s) used to solve structure: SHELXS2013 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2008); software used to prepare material for publication: CifTab (Sheldrick, 2015).