Crystal structure of bis-(pivaloyl-hydroxamato-κ2O,O')copper(II).

Reaction of copper(II) nitrate with pivaloyl-hydroxamic acid yielded the title compound, [Cu(pivHA)2] (where pivHA- is pivaloyl hydroxamate, C5H10NO2). The centrosymmetric mononuclear complex consists of a CuII ion, which is located on a center of inversion, with two coordinated pivaloyl hydroxamate monoanions. The CuII ion has a square-planar coordination environment consisting of four O atoms - two carbonyl O atoms and two hydroxamate O atoms from two hydroxamate pivHA- ligands. The pivHA- anions are coordinated to copper(II) in a trans-mode, forming two five-membered O,O'-chelate rings.

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., 2014 ▸, 2015 ▸; Gaynor 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′-coordination mode. The ability of pivalic acid itself to form polynuclear metallamacrocyclic complexes with various metal ions is well known (Vitórica-Yrezábal 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.

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 I41/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, O1O2O1iO2i [symmetry code: (i) −x, 1 − y, −z], and the two pivHA− monoanions in [Cu(pivHA)2] are necessarily mutually trans-coordinated. The axial positions of the copper(II) ions remain unoccupied. The Cu—Ocarbon­yl and Cu—Ohydroxamate 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.

Figure 1

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: (’) −x, 1 − y, −z.

Table 1

Selected geometric parameters (Å, °)

C1—O2	1.2821 (13)	O1—Cu1	1.8899 (8)
C1—N1	1.3066 (14)	O2—Cu1	1.9244 (8)
N1—O1	1.3764 (12)
O1—Cu1—O1i	180 (5)	O1—Cu1—O2i	95.16 (3)
O1—Cu1—O2	84.84 (3)	O1i—Cu1—O2i	84.84 (3)
O1i—Cu1—O2	95.16 (3)	O2—Cu1—O2i	180

Symmetry code: (i) .

Supra­molecular features

Adjacent [Cu(pivHA)2] complexes are connected to each other via N1–H1⋯O1ii 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—H⋯O 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 ▸).

Table 2

Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1⋯O1ii	0.88	1.90	2.7185 (13)	154

Symmetry code: (ii) .

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.

Figure 3

A fragment of the packing of complex 1, showing the formation of supra­molecular tetra­mers [Cu(pivHA)2]4 formed by hydrogen bonds. The tert-butyl groups are omitted for clarity.

Database survey

The Cambridge Structural Database (CSD, Version 5.27, updated in August 2012; Groom et al., 2016 ▸) contains one report with structural information for pivaloyl­hydroxamic acid (CCDC 1155138; Due et al., 1987 ▸). Though the survey did not contain any information about complexes with pivaloyl­hydroxamic acid, there are two reports devoted to structural studies of Th4+ (1180613 and 1180614; Smith & Raymond, 1981 ▸) and MoO2 2+ (763210–763214; Dzyuba et al., 2010 ▸) complexes with structurally similar ligands (N-isopropyl-2,2-di­methyl­propane­hydroxamate, N-isopropyl-3,3-di­methyl­butane­hydroxamate and decano-, N-methyl-decano-, N-methyl-hexano-, N-methyl-1-adamantano- or N-tert-butyl-hexa­nohydroxamates, respectively). It should be mentioned that coordination of hydroxamate ligands in the O,O′-chelating mode is quite typical (Tedeschi et al., 2003 ▸; Seitz et al., 2007a ▸,b ▸; Brewer & Sinn, 1981 ▸) and the CSD contains many records with such binding in various mononuclear bis-hydroxamate complexes (e.g. Drovetskaia et al., 1996 ▸; Li et al., 2004 ▸; Fisher et al., 1989 ▸; Harrison et al., 1976 ▸), which are usually coordinated in the trans- mode with respect to each other (Gaynor et al., 2001 ▸; Lasri et al., 2012 ▸; Casellato et al., 1984 ▸).

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. Yield: 23 mg (78%). Elemental analysis C:H:N Expected (calculated): 40.75 (40.60): 7.03 (6.81): 9.22 (9.47). IR in KBr pellets (cm−1): 3400 (νN–H); 3196–3040 (νO–H, likely due to the presence of N1—H1⋯O1ii hydrogen bonds); 1595 and 1503 (νamid I); 1330, 1220 and 1053 (νC–C and ν-C-N); 963 (νN–O).

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3 ▸. H atoms attached to carbon and nitro­gen atoms were positioned geometrically and constrained to ride on their parent atoms: C—H =0.98 Å with U iso(H) = 1.5U eq(C) and N—H = 0.88 Å with U iso(H) = 1.2U eq(N). Methyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density.

Table 3

Experimental details

Crystal data
Chemical formula	[Cu(C5H10NO2)2]
M r	295.82
Crystal system, space group	Tetragonal, I41/a
Temperature (K)	100
a, c (Å)	12.8059 (5), 17.7051 (8)
V (Å3)	2903.5 (3)
Z	8
Radiation type	Mo Kα
μ (mm−1)	1.51
Crystal size (mm)	0.35 × 0.35 × 0.29
Data collection
Diffractometer	Bruker D8 Quest CMOS
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 ▸)
T min, T max	0.656, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections	24433, 2764, 2444
R int	0.035
(sin θ/λ)max (Å−1)	0.769
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.029, 0.074, 1.19
No. of reflections	2764
No. of parameters	82
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	0.46, −0.48

Computer programs: APEX2 and, SAINT (Bruker, 2014 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2018 (Sheldrick, 2015 ▸), shelXle (Hübschle et al., 2011 ▸), Mercury (Macrae et al., 2006 ▸) and publCIF (Westrip, 2010 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989018012227/ex2012sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989018012227/ex2012Isup2.hkl

Click here for additional data file.

Supporting information file. DOI: 10.1107/S2056989018012227/ex2012Isup3.cdx

Additional supporting information: crystallographic information; 3D view; checkCIF report

[Cu(C5H10NO2)2]	Dx = 1.353 Mg m−3
Mr = 295.82	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I41/a	Cell parameters from 9939 reflections
a = 12.8059 (5) Å	θ = 3.2–33.2°
c = 17.7051 (8) Å	µ = 1.51 mm−1
V = 2903.5 (3) Å3	T = 100 K
Z = 8	Prism, blue
F(000) = 1240	0.35 × 0.35 × 0.29 mm

Bruker AXS D8 Quest CMOS diffractometer	2764 independent reflections
Radiation source: I-mu-S microsource X-ray tube	2444 reflections with I > 2σ(I)
Laterally graded multilayer (Goebel) mirror monochromator	Rint = 0.035
ω and phi scans	θmax = 33.2°, θmin = 3.2°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −19→19
Tmin = 0.656, Tmax = 0.747	k = −19→19
24433 measured reflections	l = −27→27

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.029	Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.074	H-atom parameters constrained
S = 1.19	w = 1/[σ2(Fo2) + (0.0257P)2 + 3.2993P] where P = (Fo2 + 2Fc2)/3
2764 reflections	(Δ/σ)max < 0.001
82 parameters	Δρmax = 0.46 e Å−3
0 restraints	Δρmin = −0.48 e Å−3

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.

	x	y	z	Uiso*/Ueq
C1	0.13160 (8)	0.34258 (8)	−0.02113 (6)	0.01465 (18)
C2	0.20901 (9)	0.26772 (9)	−0.05643 (7)	0.01747 (19)
C3	0.30109 (12)	0.33273 (12)	−0.08492 (10)	0.0327 (3)
H3A	0.349813	0.287558	−0.112390	0.049*
H3B	0.337056	0.364691	−0.041874	0.049*
H3C	0.275434	0.387597	−0.118744	0.049*
C4	0.15406 (12)	0.21475 (12)	−0.12341 (8)	0.0284 (3)
H4A	0.130218	0.268097	−0.159159	0.043*
H4B	0.093923	0.174854	−0.105007	0.043*
H4C	0.203005	0.167530	−0.148793	0.043*
C5	0.24745 (12)	0.18527 (12)	−0.00063 (8)	0.0274 (3)
H5A	0.187982	0.144200	0.017576	0.041*
H5B	0.281569	0.219691	0.042206	0.041*
H5C	0.297448	0.139024	−0.025866	0.041*
N1	0.10376 (7)	0.33335 (8)	0.04955 (5)	0.01550 (17)
H1	0.128862	0.282995	0.078083	0.019*
O1	0.03387 (7)	0.40497 (7)	0.07791 (5)	0.01837 (16)
O2	0.09269 (7)	0.41676 (7)	−0.06079 (5)	0.01830 (16)
Cu1	0.000000	0.500000	0.000000	0.01360 (6)

	U11	U22	U33	U12	U13	U23
C1	0.0150 (4)	0.0146 (4)	0.0143 (4)	−0.0005 (3)	0.0017 (3)	0.0009 (3)
C2	0.0180 (5)	0.0179 (5)	0.0166 (5)	0.0020 (4)	0.0038 (4)	0.0011 (4)
C3	0.0242 (6)	0.0314 (7)	0.0424 (8)	−0.0013 (5)	0.0165 (6)	0.0028 (6)
C4	0.0311 (7)	0.0303 (6)	0.0238 (6)	0.0075 (5)	−0.0015 (5)	−0.0098 (5)
C5	0.0321 (7)	0.0275 (6)	0.0227 (6)	0.0144 (5)	0.0056 (5)	0.0046 (5)
N1	0.0168 (4)	0.0156 (4)	0.0142 (4)	0.0034 (3)	0.0031 (3)	0.0026 (3)
O1	0.0227 (4)	0.0186 (4)	0.0138 (3)	0.0077 (3)	0.0063 (3)	0.0033 (3)
O2	0.0241 (4)	0.0172 (4)	0.0136 (3)	0.0045 (3)	0.0037 (3)	0.0035 (3)
Cu1	0.01670 (10)	0.01244 (9)	0.01165 (9)	0.00085 (6)	0.00182 (6)	0.00171 (6)

C1—O2	1.2821 (13)	C4—H4B	0.9800
C1—N1	1.3066 (14)	C4—H4C	0.9800
C1—C2	1.5141 (16)	C5—H5A	0.9800
C2—C5	1.5275 (18)	C5—H5B	0.9800
C2—C3	1.5290 (18)	C5—H5C	0.9800
C2—C4	1.5368 (18)	N1—O1	1.3764 (12)
C3—H3A	0.9800	N1—H1	0.8800
C3—H3B	0.9800	O1—Cu1	1.8899 (8)
C3—H3C	0.9800	O2—Cu1	1.9244 (8)
C4—H4A	0.9800
O2—C1—N1	119.04 (10)	H4A—C4—H4C	109.5
O2—C1—C2	119.84 (10)	H4B—C4—H4C	109.5
N1—C1—C2	121.12 (10)	C2—C5—H5A	109.5
C1—C2—C5	112.43 (10)	C2—C5—H5B	109.5
C1—C2—C3	107.24 (10)	H5A—C5—H5B	109.5
C5—C2—C3	109.95 (12)	C2—C5—H5C	109.5
C1—C2—C4	107.35 (10)	H5A—C5—H5C	109.5
C5—C2—C4	109.97 (11)	H5B—C5—H5C	109.5
C3—C2—C4	109.81 (11)	C1—N1—O1	117.82 (9)
C2—C3—H3A	109.5	C1—N1—H1	121.1
C2—C3—H3B	109.5	O1—N1—H1	121.1
H3A—C3—H3B	109.5	N1—O1—Cu1	108.18 (6)
C2—C3—H3C	109.5	C1—O2—Cu1	110.11 (7)
H3A—C3—H3C	109.5	O1—Cu1—O1i	180.00 (5)
H3B—C3—H3C	109.5	O1—Cu1—O2	84.84 (3)
C2—C4—H4A	109.5	O1i—Cu1—O2	95.16 (3)
C2—C4—H4B	109.5	O1—Cu1—O2i	95.16 (3)
H4A—C4—H4B	109.5	O1i—Cu1—O2i	84.84 (3)
C2—C4—H4C	109.5	O2—Cu1—O2i	180.0
O2—C1—C2—C5	179.56 (11)	C2—C1—N1—O1	179.33 (10)
N1—C1—C2—C5	−0.14 (16)	C1—N1—O1—Cu1	−0.49 (12)
O2—C1—C2—C3	58.59 (15)	N1—C1—O2—Cu1	1.02 (13)
N1—C1—C2—C3	−121.11 (13)	C2—C1—O2—Cu1	−178.69 (8)
O2—C1—C2—C4	−59.37 (14)	N1—O1—Cu1—O2	0.79 (7)
N1—C1—C2—C4	120.94 (12)	N1—O1—Cu1—O2i	−179.21 (7)
O2—C1—N1—O1	−0.37 (16)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O1ii	0.88	1.90	2.7185 (13)	154