A bis-chelate o-vanillin-2-ethanolamine copper(II) complex bearing both imine and amine forms of the ligand

The copper(II) atom in the molecular title complex has a distorted square-pyramidal coordination environment by three O and two N atoms from a bidentate and a tridentate ligand.


Chemical context
Over the last decade, research on transition-metal complexes with salicylidene-type Schiff bases (SB) gained a new impetus after a number of highly effective and simple M-(SB) catalysts were obtained, where M = Cu, Co, Al, etc (Payne et al., 2020;Mitra et al., 2015;Fei et al., 2014;Saha et al., 2013). It has been shown that incorporation of partially or fully reduced Schiff bases (RSB) into the coordination spheres of metal cations can significantly increase their catalytic activities (Liu et al., 2020;Huo et al., 2021;Adã o et al., 2014;Sreenivasulu et al., 2005). Despite the fact that complexes with RSB ligands are supposed to be very promising objects for the creation of new catalysts, information about their syntheses and structures is rather limited. Continuing our work on the elaboration of alternative methods for the synthesis of coordination compounds , we have investigated the following system: zinc (powder) -copper (powder) -H 2 Lammonium thiocyanate -methanol, to prepare heterometallic Cu/Zn complexes with the Schiff base H 2 L im , which is formed in situ upon condensation of o-vanillin and 2-aminoethanol. The complex [Cu(HL im )(HL am )] (where H 2 L im = 2-[(2-hydroxyethyl)iminomethyl]-6-methoxyphenol; H 2 L am = 2-[(2hydroxyethyl)aminomethyl]-6-methoxyphenol) was formed in the reaction mixture as an unintended by-product for which only a few crystals suitable for X-ray analysis were isolated. ISSN 2056-9890 Herein, we report the crystal structure of the title compound, [Cu(HL im )(HL am )] (I), which represents the first example of a mixed (SB/RSB) complex derived from salicylidene-2-aminoethanol type ligands.

Structural commentary
The asymmetric unit of (I) comprises one neutral molecular complex [Cu(HL im )(HL am )] (Fig. 1). The copper(II) ion has an O 3 N 2 coordination set defined by two monodeprotonated molecules of the organic ligands realizing their bidentate (N,O) and tridentate (O,N,O) functions for the SB and RSB forms, respectively. This difference in coordination behavior of the ligands can be explained by a higher flexibility of the amine ligand, and is observed in similar bis-chelate copper(II) complexes with salicylidene-2-aminoethanol type ligands.  (2000). It is worth noting that such a dependence was not found for similar Ni II complexes, which have an octahedral shape via both tridentate imino and amino ligands. For [Ni(SB) 2 ], see: Floyd et al. (2005); Wang et al. (2011a,b); for [Ni(RSB) 2 ], see: Zhang et al. (2007). The shape of the coordination polyhedron of the Cu II ion in (I) can be described as distorted [4 + 1] square-pyramidal. The equatorial Cu-O(N) bond lengths vary from 1.923 (2) to 2.030 (3) Å and are in accordance with those found in related complexes (Stetsiuk et al., 2018;Xie et al., 2000;Zabierowski et al., 2013).  (11) , respectively] are caused by the steric hindrances that are typical for chelate rings. According to the criterion for five-coordinate complexes (Addison et al., 1984;O'Sullivan et al., 1999), the distortion of the CuN 2 O 3 coordination polyhedron is about 26% along the pathway from regular square-pyramidal to regular trigonal-bipyramidal. The bond-valence sums calculated for Cu II with CN = 4 (1.86 valence units) and CN = 5 (1.99 valence units) (Allmann, 1975;Shields et al., 2000) can serve as an additional argument in favor of the coordination number of 5 for Cu II in (I).

Figure 1
Molecular structure of (I), with the numbering scheme and displacement ellipsoids drawn at the 50% probability level (carbon-bound H atoms are omitted for clarity).

Database survey
Among the 33 deposited crystal structures of bis-complexes with a salicylidene-2-aminoethanol-type ligand (CSD, version 5.42, last update February 2021; Groom et al., 2016), there are 30 hits for complexes with SBs and three hits for complexes with RSBs (Xie et al., 2000(Xie et al., , 2003Zhang et al., 2007). M(SB)(RSB) complexes including both forms of a ligand are not known up to now.

Synthesis and crystallization
o-Vanillin (0.3 g, 0.002 mol) and 2-aminoethanol (0.12 ml, 0.002 mol) were dissolved in methanol and then stirred magnetically at 323-333 K for 20 mins. Copper powder (0.06 g, 0.001 mol), zinc powder (0.07 g, 0.001 mol) and NH 4 SCN (0.15 g, 0.002 mol) were added to the hot yellow solution with further stirring until total dissolution of powder was observed (about 4 h). The resulting brown solution was filtered and left for 1 d. A green powdery precipitate with a few green crystals available for X-ray crystallographic analysis was collected by filtration.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Carbon-bound H atoms were placed in idealized positions and refined using a riding model. H atoms of the NH and OH groups were located in a difference-Fourier map. For the final model they were also treated as riding on their parent atoms. Computer programs: CrysAlis PRO (Rigaku OD, 2015), SHELXS (Sheldrick, 2008), SHELXL (Sheldrick, 2015), OLEX2 (Dolomanov et al., 2009) and publCIF (Westrip, 2010).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.