Crystal structure of bis(pivaloylhydroxamato-κ2 O,O′)copper(II)

The mononuclear discrete complex [Cu(pivHA)2] contains two pivaloyl hydroxamates (pivHA−) coordinated to a copper(II) ion in a centrosymmetric trans mode through hydroxamate and carbonyl O atoms. The copper(II) O4 coordination is square planar.


Chemical context
Numerous studies over the past decade of various hydroxamate complexes with 3d and 4f metal ions have been inspired by their potential applications in molecular magnetism, luminescence, adsorption and catalysis (Ostrowska et al., 2016;Pavlishchuk et al., 2015). The ability of further functionalized hydroxamic acids to serve as bridging ligands and to form polynuclear species with different structural motifs has been comprehensively examined in recent years (Mezei et al., 2007;Pavlishchuk et al., 2018;Odarich et al., 2016;McDonald et al., 2014McDonald et al., , 2015Gaynor et al., 2002). Studies of simple unsubstituted hydroxamic acids have been undertaken because of their possible application as mimics of mononuclear iron(III) siderophores (Marmion et al., 2004). As a result of the potentially multiple coordination modes of unsubstituted hydroxamic acids, they can also lead to the formation of polynuclear assemblies (Tirfoin et al., 2014). However, reactions of unsubstituted hydroxamic acids with transition metal ions lead mainly to the formation of octahedral 1:3 (Abu- Dari et al., 1979) or square-planar 1:2 (Baughman et al., 2000) complexes with the hydroxamate in an O,O 0 -coordination mode. The ability of pivalic acid itself to form polynuclear metallamacrocyclic complexes with various metal ions is well known (Vitó rica-Yrezá bal et al., 2017; Garlatti et al., 2018). The aim of the current work was to investigate if a tert-butyl-substituted hydroxamic acid (i.e. the hydroxamate analogue of pivalic acid) could be used as a scaffold for the preparation of polynuclear copper(II) complexes. ISSN 2056-9890

Structural commentary
Crystals of the title compound 1 were obtained by reaction of copper(II) nitrate hexahydrate with pivaloylhydroxamic acid in methanol.
Complex 1 crystallizes in the space group I4 1 /a, with eight [Cu(pivHA) 2 ] complex molecules per unit cell. The [Cu(pivHA) 2 ] molecules are centrosymmetric, with the copper ion located on an inversion center. Each [Cu(pivHA) 2 ] molecule contains one copper(II) ion in a square-planar coordination environment generated by the coordination of two pivaloylhydroxamate monoanions, forming five-membered chelate rings through both the carbonyl and hydroxamate O atoms (Fig. 1). The centrosymmetric nature of the complex forces the copper(II) ions to be exactly coplanar with the four donor O atoms, O1O2O1 i O2 i [symmetry code: (i) Àx, 1 À y, Àz], and the two pivHA À monoanions in [Cu(pivHA) 2 ] are necessarily mutually trans-coordinated. The axial positions of the copper(II) ions remain unoccupied. The Cu-O carbonyl and Cu-O hydroxamate bond lengths are typical for copper(II) hydroxamate or oximate complexes (Buvailo et al., 2012;Pavlishchuk et al., 2017a,b) (Table 1). The hydroxamate N-H groups remain protonated and are not involved in metal coordination. Deprotonation of the N-H groups could possibly be achieved at higher pH without hydrolysis of hydroxamic acid, which might aid in the formation of polynuclear complexes.

Supramolecular features
Adjacent [Cu(pivHA) 2 ] complexes are connected to each other via N1-H1Á Á ÁO1 ii hydrogen bonds between the hydroxamate N-H group of one complex molecule and a deprotonated hydroxamate oxygen of an adjacent [Cu(pivHA) 2 ] molecule (Table 2, Fig. 2). Four of these N-HÁ Á ÁO hydrogen bonds connect molecules into tetramers arranged around a fourfold rotoinversion center. The N-H group of the second hydroxamate ligand of each complex creates an equivalent tetramer trans across the copper ion, thus creating an infinite three-dimensional network of corner-connected tetramers (with the copper ions acting as the bridging element, Fig. 3). The molecular structure of complex 1 showing the neutral centrosymmetric fragment [Cu(pivHA) 2 ], along with the atom labelling. Displacement ellipsoids are at the 50% probability level. Symmetry code: (') Àx, 1 À y, Àz.

Figure 2
A fragment of the lattice of complex 1, showing the intramolecular hydrogen-bonding connections (dashed lines) between the [Cu(pivHA) 2 ] molecules. The tert-butyl groups are omitted for clarity.

Synthesis and crystallization
A solution of pivaloylhydroxamic acid (23.4 mg, 0.20 mmol) in 5 mL of methanol was added to copper(II) nitrate hexahydrate (29.6 mg, 0.10 mmol) in 5 mL of methanol. The resulting blue solution was stirred for 30 min. at room temperature, filtered and left for slow evaporation. After a week, blue crystals suitable for single crystal X-ray analysis had formed.

Bis(pivaloylhydroxamato-κ 2 O,O′)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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )