Crystal structure of tetrakis(μ3-2-{[1,1-bis(hydroxymethyl)-2-oxidoethyl]iminomethyl}-6-methoxyphenolato)tetrakis[aquacopper(II)]: a redetermination at 200 K

Using a predesigned Schiff base tripodal ligand, a cubane-type tetranuclear copper(II) cluster has been synthesized and its structure redetermined at 200 K.


Chemical context
During the last few years, we have been exploring the chemistry of transition metal complexes of Schiff base ligands with the aim of preparing heterometallic polynuclear compounds with diverse potential advantages. In these studies, we continued to apply the direct synthesis of coordination compounds based on spontaneous self-assembly, in which one of the metals is introduced as a powder (zerovalent state) and oxidized during the synthesis (typically by dioxygen from the air) (Pryma et al., 2003;Nesterova et al., 2008;Nesterov et al., 2012). The main advantage of this approach is the generation of building blocks in situ, in one reaction vessel, thus eliminating separate steps in building-block construction. Reactions of a metal powder and another metal salt in air with a solution containing a pre-formed Schiff base ligand have yielded a number of novel Co/Fe and Cu/Fe compounds (Chygorin et al., 2015;Nesterova et al., 2013).
The title compound was prepared in studies of the coordination behavior of the versatile multidentate Schiff base ligand 2-{[(2-hydroxy-3-methoxyphenyl)methylene]amino}-2-(hydroxymethyl)-1,3-propanediol (H 4 L) (Odabaşog lu et al., 2003) which results from the condensation between o-vanillin and tris(hydroxymethyl)aminomethane. In the syntheses, the condensation reaction was utilized without isolation of the resulting Schiff base. In an attempt to prepare a heterometallic assembly we reacted Cu powder and Zn(CH 3 COO) 2 with a methanol solution of the Schiff base in a 1:1:2 molar ratio. However, the isolated green microcrystalline product was identified crystallographically to be the tetranuclear Cu II Schiff base complex Cu 4 (H 2 L) 4 (H 2 O) 4 (1) of a hetero-cubane type.
The crystal structure of (1) has been reported previously at room temperature by Back et al. (2015) (refcode IGOSUU). In that report of the structure, no standard uncertainties are recorded for the oxygen atoms of the deprotonated hydroxymethyl group, O2, and the water molecule coordinating to the metal atom, O6, indicating that they were not refined. The hydrogen atoms of some OH groups and water molecules have also not been positioned accurately. It is clear from the checkCIF output that at least one of the water molecule hydrogen atoms, H6B, and one OH hydrogen atom, H4, are incorrectly positioned. Since the present structure was determined at a lower temperature, all atoms, including these hydrogen atoms, have been determined more accurately, resulting in improved standard uncertainties in the bond lengths and angles.

Figure 1
The molecular structure of the title complex, showing the atomnumbering scheme. Non-H atoms are shown with displacement ellipsoids at the 50% probability level. H atoms are not shown.
the oxygen atom of one hydroxymethyl group. A further intramolecular hydrogen bond involves the other hydroxymethyl group (O12). Bifurcated intermolecular hydrogen bonds are also present, involving the remaining hydrogen atom of water molecule and the phenolic and methoxyl oxygen atoms. These hydrogen-bond contacts are of weak-tomoderate strength [2.736 (12)-2.892 (7) Å ], Table 2.
The title compound appears to be a new solvatomorph of the blue copper(II) complex with the same ligand, [Cu 4 (C 12 H 15 NO 5 ) 4 (H 2 O)]Á3.75CH 3 OHÁ2H 2 O (refcode SUGKUC; Tabassum & Usman, 2015). Monoclinic SUGKUC crystallizes in the P2 1 /n space group and has no crystallographically imposed symmetry. It is also a cubane-type complex but with some of the coordinating water molecules replaced by other solvents. The bond lengths and angles of (1) are comparable to those in the Ni II analogue (refcode ZEHGUQ; Guo et al., 2008) and a Cu II complex with a similar ligand (refcode AFIMUY; Dong et al., 2007). The ligand of the latter does not have the methoxy group and the copper atom is five-coordinate, the structure lacking the coordinating water molecule of (1).

Supramolecular features
Interactions between [Cu 4 (H 2 L) 4 (H 2 O) 4 ] molecules in the crystal lattice are weak, the closest CuÁ Á ÁCu inter-cluster separation exceeds 8.43 Å . The hydrogen on the hydroxymethyl group (O13) is involved in an intermolecular hydrogen bond to the water molecule on the cluster related by a crystallographic twofold axis (Table 2), forming a hydrogenbonded polymer propagating along the b axis (Fig. 2). Nostacking is observed.

Database survey
In the solid state, the H 4 L ligand adopts the keto-amine tautomeric form, with the formal aryl-OH H atom relocated to the N atom, and the NH group and phenolic O atom forming a strong intramolecular N-HÁ Á ÁO hydrogen bond (Odabaşog lu et al., 2003). Crystal structures of about 30 metal complexes of this ligand are found in the Cambridge Database (CSD Version 5.36 with one update; Groom & Allen, 2014). These comprise five homometallic mononuclear Mn, Ni and Mo complexes, polynuclear Co 2 , V 2 , Cu 4 , Mn 4 , Ni 4 , Ln 9 and Ln 10 assemblies and heterometallic 1s-3d and 3d-4f clusters of 4-20 nuclearity. The ligand molecules exist in either doubly or triply deprotonated forms and adopt a chelating-bridging mode, forming five-and six-membered rings. Obviously, the H 4 L ligand favours formation of polynuclear paramagnetic clusters due to the presence of the tripodal alcohol functionality. At the same time, the lack of heterometallic structures with two kinds of 3d metal supported by H 4 L is also evident. This perhaps explains the failure of the preparation of a Cu/Zn compound in the present study.

Synthesis and crystallization
2-Hydroxy-3-methoxy-benzaldehyde (0.30 g, 2 mmol), tris-(hydroxymethyl)aminomethane (0.24 g, 2 mmol), NEt 3 (0.3 ml, 2 mmol) were added to methanol (20 ml) and stirred magnetically for 30 min. Next copper powder (0.06 g, 1 mmol) and Zn(CH 3 COO) 2 (0.19 g, 1 mmol) were added to the yellow solution and the mixture was heated to 323 K under stirring until total dissolution of the copper powder was observed (1 h). The resulting green solution was filtered and allowed to stand at room temperature. Dark-green rhombic prisms of the title compound were formed in several days. They were collected by filter-suction, washed with dry Pr i OH and finally dried in vacuo (yield: 59% based on copper).
The IR spectrum of (1) in the range 4000-400 cm À1 shows all the characteristic Schiff base ligand frequencies: (OH), (CH) and (C N) at 3400, 3066-2840, and 1604 cm À1 , respectively. A strong peak at 1628 cm À1 that is due to the Part of the crystal structure with intra-and intermolecular hydrogen bonds shown as blue dashed lines. C-H hydrogens have been omitted for clarity.
bending of H 2 O molecule provides evidence of the presence of water in (1).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Diffraction data were collected at 200 K, rather than the more usual 100 K, due to apparent disordering at lower temperatures. Water molecule hydrogen atoms were refined with geometries restrained to ideal values; the OH hydrogen atoms H12 and H13 were refined using a riding model. All hydrogen atoms bound to carbon were included in calculated positions and refined using a riding model with isotropic displacement parameters based on those of the parent atom [C-H = 0.95 Å , U iso (H) = 1.2U eq (C) for CH and CH 2 , 1.5U eq (C) for CH 3 ). Anisotropic displacement parameters were employed for the non-hydrogen atoms.

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. Refinement. Diffraction data were collected at 200 K, rather than the more usual 100 K, due to apparent disordering at lower temperatures. Water molecule hydrogen atoms were refined with geometries restrained to ideal values. Geometric parameters (Å, º)