A copper complex of an unusual hydroxy–carboxylate ligand: [Cu(bpy)(C4H4O6)]

A five-coordinate copper(II) complex with bpy and 2-(hydroxymethyl)tartronate ligands forms centrosymmetric dimers via Cu⋯O contacts [2.703 (2) Å].

As a common reducing reagent, ascorbic acid has also been investigated in complex synthesis and redox reactions (Creutz, 1981;Niemelä , 1987;Sorouraddin et al., 2000). For example, we have recently observed that mixtures of Cu complexes and ascorbate react with O 2 to produce Cu II oxalate complexes (Khamespanah et al., 2021). However, to our knowledge, the particular degradation product of ascorbic acid observed here, 2-(hydroxymethyl)tartronic acid [2-(hydroxymethyl)-2-hydroxy-1,3-propanedioic acid], has been reported only a few times. It was identified by mass spectrometry as a product of oxidation of ascorbic acid (Niemelä , 1987;Lö wendahl & Petersson, 1976) and two carbohydrates (Lö wendahl et al., 1975a,b). We have now isolated compound (I), a copper(II) complex of the 2-(hydroxymethyl)tartronate anion (see Scheme), and its crystal structure is reported here. ISSN 2056-9890 The preparation of the title complex is shown in Fig. 1. A solution of [(bpy) 2 Cu(ONO 2 )]NO 3 and Dabco (1,4-diazabicyclo[2.2.2]octane) turned from blue to dark brown on addition of ascorbic acid, suggesting reduction of Cu II to Cu I . The solution was then exposed to air. It turned green over a period of several days, and the title compound (I) could be crystallized (Fig. 2).
In this procedure, Dabco also crystallizes, in its doubly protonated form as colorless [DabcoH 2 ](NO 3 ) 2 (II). We could not isolate the title compound (I) when Dabco was omitted from the reaction mixture. We determined the structure of (II) as well (Gao et al., 2020). Although this structure was reported previously by Knope & Cahill (2007), the new structure provides improved resolution.

Structural commentary
The Cu atom in (I) adopts a square-pyramidal geometry, with coordination to two bpy N atoms and three O atoms from the 2-(hydroxymethyl)tartronate anion (C 4 H 4 O 6 2-). The two inversion-related complexes in the unit cell make a dimer via two CuÁ Á ÁO contacts: Cu1Á Á ÁO1 0 = 2.703 (2) Å . This kind of dimerization (see inset in Fig. 2) is commonly observed in 4-and 5-coordinate Cu II complexes. It is discussed further in the Database survey section.

Supramolecular features
The structure of (I) includes two O-HÁ Á ÁO hydrogen bonds, one intramolecular and one intermolecular; see Table 1. The intermolecular hydrogen bonds form centrosymmetric hydrogen-bonded dimers with graph set R 2 2 (12) (Etter et al., 1990). These dimers are linked into chains in the [100] direction, as illustrated in Fig. 3.

Figure 2
Crystal structure of (I). Ellipsoids are drawn at the 50% probability level; hydrogen atoms are displayed but not labeled. Primed and unprimed atoms are related by an inversion center, which brings the two squarepyramidal Cu(bpy)(C 4 H 4 O 6 ) moieties into contact [CuÁ Á ÁO1 0 = 2.703 (2) Å ]. The inset is a schematic illustration of the dimerization. Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
Packing structure of (I), showing the intermolecular O2-H2OÁ Á ÁO4 hydrogen bonds. This arrangement is similar to that observed in the Cu(bpy)(C 4 H 4 O 6 ) moiety of (I), except that (I) contains an apical alcohol ligand rather than H 2 O. Because the alcohol in (I) is part of a small chelate ring, its coordination is bent slightly away from perpendicularity to the CuO 2 N 2 plane [N1-Cu1-O2 104.04 (9), N2-Cu1-O2 91.77 (9) ]; the average N-Cu-OH 2 angle in the above seven published structures is 93 (3) .

Synthesis and crystallization
General procedures. Reagents were used as received, from Sigma-Aldrich. FTIR spectra were recorded on a Bruker Tensor 27 spectrometer in attenuated total reflectance mode.

Refinement
Crystal data, data collection, and structure refinement are summarized in Table 2. All H atoms were visible in difference-Fourier maps. Coordinates of those on O were refined with O-H distances restrained to 0.88 (2) Å . Those on C were positioned geometrically (C-H = 0.95 Å for aromatic C, 0.99 Å for CH 2 ) and treated as riding. Displacement parameters for H were assigned as U eq (H) = 1.2U eq (C) and 1.5U eq (O).

(2,2′-Bipyridine-κ 2 N,N′)[2-hydroxy-2-(hydroxymethyl-κO)propanedioato-κ 2 O 1 ,O 3 ]copper(II)
Crystal data 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.