Distrontium oxalate tetra­hydroxidocuprate(II)

Kunisawa, H.; Yamaura, J.; Nomura, T.

doi:10.1107/S241431462600043X

metal-organic compounds

IUCrDATA

ISSN: 2414-3146

Volume 11| Part 1| January 2026| x260043

https://doi.org/10.1107/S241431462600043X

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Distrontium oxalate tetrahydroxidocuprate(II)

Hibiki Kunisawa,^a Jun-ichi Yamaura ^b and Toshihiro Nomura ^a ^*

^aDepartment of Physics, Shizuoka University, Shizuoka 422-8529, Japan, and ^bInstitute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
^*Correspondence e-mail: [email protected]

Edited by M. Weil, Vienna University of Technology, Austria (Received 2 January 2026; accepted 16 January 2026; online 20 January 2026)

The crystal structure of distrontium oxalate tetrahydroxidocuprate(II) or poly[tetra-μ-hydroxido-μ₆-oxalato-copperdistrontium], Sr₂(C₂O₄)[Cu(OH)₄] or [Sr₂Cu(C₂O₄)(OH)₄]_n, has been determined in the triclinic space group P1. The asymmetric unit contains one Sr, one Cu, two hydroxide groups, and half of an oxalate anion. By application of inversion symmetry, a square-planar {Cu(OH)₄} unit and a complete oxalate anion are generated. The structure consists of a three-dimensional framework of edge-sharing {SrO₄(OH)₄} polyhedra decorated by {Cu(OH)₄} units and oxalate groups. Only weak hydrogen bonds are observed within the framework.

Keywords: crystal structure; inorganic; oxalate.

CCDC reference: 2523251

Structure description

The title compound, Sr₂(C₂O₄)[Cu(OH)₄], was obtained serendipitously during attempts to synthesize SrCu₂(BO₃)₂ (Kageyama et al., 1999 ) under hydrothermal conditions. Although numerous crystal structures containing Cu^II and oxalato ligands or oxalate anions have been reported, the combination with alkaline-earth ions is surprisingly rare. According to the Inorganic Crystal Structure Database (ICSD; version 2025–1; Zagorac et al., 2019 ), only a few related compounds such as Sr₂(Cu(C₂O₄)₃)(H₂O)₇ (Insausti et al., 1994 ) and BaCu(C₂O₄)₂·6H₂O (Hallock et al., 1990 ; Bouayad et al., 1995 ; Kasthuri et al., 1996 ; Nenwa et al., 2008 ) have been reported. Insausti and coworkers also reported the thermal analysis of CaCu(C₂O₄)₂·2H₂O and SrCu(C₂O₄)₂·4H₂O, yet the crystal structures of these compounds have not been determined (Insausti et al., 1993 ). Here, we describe the crystal structure of Sr₂(C₂O₄)[Cu(OH)₄].

The crystal structure of Sr₂(C₂O₄)[Cu(OH)₄] consists of a three-dimensional framework built from Sr^II cations coordinated by {Cu(OH)₄}^2– and oxalate (C₂O₄)^2– units (Figs. 1 and 2). The asymmetric unit comprises one Sr^II, one Cu^II, two (OH) groups, and half of an oxalate anion. By application of inversion symmetry, a {Cu(OH)₄} square-planar unit and the full oxalate anion are generated. The coordination environment around Sr is an {SrO₄(OH)₄} polyhedron, which resembles a square antiprism but is significantly distorted in the triclinic lattice. Each oxalate anion bonds to six Sr^II cations, with four bridging and two chelating modes. The {Cu(OH)₄} units are oriented nearly perpendicular to the crystallographic [111] direction (Fig. 3). The OH⁻ groups of the {Cu(OH)₄} unit do not form obvious hydrogen bonds with the surrounding oxygen atoms of the oxalate anions. The O1⋯O4′ and O2⋯O3′ distances are around 3.0 Å, however, the O—H⋯O angles are strongly bent from 180° (104 and 121°, respectively), indicating that these hydrogen bonds are rather weak.

Figure 1
The asymmetric unit of the title compound expanded to visualize the complete {Cu(OH)₄}^2– unit and the oxalate anion. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (iii) −x + 1, −y, −z + 1, (iv) −x, −y + 1, −z + 2.]

Figure 2
Three-dimensional framework of the crystal structure with polyhedral representation of the {SrO₄(OH)₄} (green) and {Cu(OH)₄} (blue) building units, as viewed along the (a) a and (b) b axes.

Figure 3
The expanded unit cell of the title compound viewed along the [111] direction; color codes are as in Fig. 2

Synthesis and crystallization

Sr(OH)₂·8H₂O (1.4 g), Cu(OH)₂ (0.1 g), acetylacetone (C₅H₈O₂, 0.2 ml), H₃BO₃ (0.03 g), and distilled water (10 ml) were placed in a Teflon-lined stainless-steel autoclave and heated at 473 K for 24 h. All reagents were purchased from FUJIFILM Wako and used without further purification. The oxalate ions are likely generated through the oxidative decomposition of acetylacetone under the alkaline reaction conditions. Violet, rhombic plates were obtained, and a single crystal was selected for X-ray diffraction at room temperature.

Refinement

Crystal data, data collection, and structure refinement details are summarized in Table 1. H atoms were located from difference syntheses and were refined using a riding model (AFIX 147 instruction; Sheldrick, 2015b ).

Table 1
Experimental details

Crystal data
Chemical formula	[Sr₂Cu(C₂O₄)(OH)₄]
M_r	394.84
Crystal system, space group	Triclinic, P $[\overline{1}]$
Temperature (K)	293
a, b, c (Å)	6.0754 (3), 6.5442 (3), 6.5466 (2)
α, β, γ (°)	103.712 (3), 117.235 (4), 106.601 (4)
V (Å³)	200.06 (2)
Z	1
Radiation type	Cu Kα
μ (mm⁻¹)	20.54
Crystal size (mm)	0.08 × 0.06 × 0.01

Data collection
Diffractometer	XtaLAB Synergy R, HyPix
Absorption correction	Multi-scan (CrysAlis PRO; Rigaku OD, 2023 )
T_min, T_max	0.705, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	1612, 785, 774
R_int	0.019
(sin θ/λ)_max (Å⁻¹)	0.631

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.018, 0.049, 1.07
No. of reflections	785
No. of parameters	64
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.49, −0.48
Computer programs: CrysAlis PRO (Rigaku OD, 2023), SHELXT (Sheldrick, 2015a ), SHELXL (Sheldrick, 2015b), VESTA 3 (Momma & Izumi, 2011 ) and publCIF (Westrip, 2010 ).

Structural data

CCDC reference: 2523251

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S241431462600043X/wm4243sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S241431462600043X/wm4243Isup2.hkl

checkCIF report

Computing details top

Poly[tetra-µ-hydroxido-µ₆-oxalato-copperdistrontium] top

Crystal data top

[CuSr₂(C₂O₄)(OH)₄]	Z = 1
M_r = 394.84	F(000) = 185
Triclinic, P1	D_x = 3.277 Mg m⁻³
a = 6.0754 (3) Å	Cu Kα radiation, λ = 1.54184 Å
b = 6.5442 (3) Å	Cell parameters from 1429 reflections
c = 6.5466 (2) Å	θ = 7.8–75.7°
α = 103.712 (3)°	µ = 20.54 mm⁻¹
β = 117.235 (4)°	T = 293 K
γ = 106.601 (4)°	Plate, translucent, violet
V = 200.06 (2) Å³	0.08 × 0.06 × 0.01 mm

Data collection top

XtaLAB Synergy R, HyPix diffractometer	785 independent reflections
Radiation source: Rotating-anode X-ray tube	774 reflections with I > 2σ(I)
Detector resolution: 10.0000 pixels mm^-1	R_int = 0.019
ω scans	θ_max = 76.6°, θ_min = 7.8°
Absorption correction: multi-scan (CrysAlisPro; Rigaku OD, 2023)	h = −7→7
T_min = 0.705, T_max = 1.000	k = −4→8
1612 measured reflections	l = −8→8

Refinement top

Refinement on F²	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H-atom parameters constrained
R[F² > 2σ(F²)] = 0.018	w = 1/[σ²(F_o²) + (0.0306P)² + 0.1268P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.049	(Δ/σ)_max < 0.001
S = 1.07	Δρ_max = 0.49 e Å⁻³
785 reflections	Δρ_min = −0.48 e Å⁻³
64 parameters	Extinction correction: SHELXL2019/2 (Sheldrick 2015b), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0151 (9)

Special details top

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.

Refinement. Hydrogen atoms were located from difference syntheses. All O—H hydrogen atoms were refined using the AFIX 147 riding model (Sheldrick, 2015b).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Sr	0.25216 (5)	0.13033 (4)	0.87805 (4)	0.01415 (14)
Cu	0.500000	0.000000	0.500000	0.01451 (17)
C	0.0730 (5)	0.5793 (4)	0.9551 (5)	0.0137 (5)
O1	0.5900 (4)	0.1511 (4)	0.3047 (4)	0.0187 (4)
O2	0.2604 (4)	0.1402 (4)	0.4994 (4)	0.0194 (4)
O3	0.0218 (5)	0.7498 (4)	0.9402 (4)	0.0228 (4)
O4	0.2296 (5)	0.5243 (4)	0.9062 (5)	0.0234 (5)
H1	0.733912	0.275791	0.402865	0.028*
H2	0.299239	0.256363	0.471123	0.029*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Sr	0.01757 (18)	0.01766 (18)	0.01888 (19)	0.01191 (12)	0.01421 (14)	0.01195 (13)
Cu	0.0152 (3)	0.0194 (3)	0.0155 (3)	0.0094 (2)	0.0114 (3)	0.0095 (2)
C	0.0144 (12)	0.0129 (12)	0.0151 (13)	0.0064 (10)	0.0088 (11)	0.0070 (10)
O1	0.0232 (10)	0.0207 (10)	0.0206 (10)	0.0102 (8)	0.0162 (9)	0.0128 (8)
O2	0.0220 (10)	0.0264 (11)	0.0246 (11)	0.0154 (9)	0.0175 (9)	0.0181 (9)
O3	0.0315 (11)	0.0201 (10)	0.0370 (12)	0.0174 (9)	0.0268 (10)	0.0193 (9)
O4	0.0291 (11)	0.0220 (10)	0.0416 (13)	0.0169 (9)	0.0297 (11)	0.0197 (10)

Geometric parameters (Å, º) top

Sr—O2	2.518 (2)	Sr—Srⁱⁱ	3.9910 (5)
Sr—O1ⁱ	2.539 (2)	Cu—O2ⁱⁱⁱ	1.9287 (18)
Sr—O2ⁱⁱ	2.583 (2)	Cu—O2	1.9287 (18)
Sr—O4	2.588 (2)	Cu—O1ⁱⁱⁱ	1.9603 (19)
Sr—O1ⁱⁱⁱ	2.600 (2)	Cu—O1	1.9603 (19)
Sr—O3^iv	2.630 (2)	C—O4	1.248 (3)
Sr—O3^v	2.708 (2)	C—O3	1.254 (3)
Sr—O4^vi	2.714 (2)	C—C^iv	1.565 (5)
Sr—Cu	3.5020 (2)	O1—H1	0.8200
Sr—Cu^vii	3.7623 (3)	O2—H2	0.8200
Sr—Sr^viii	3.7886 (4)

O2—Sr—O1ⁱ	135.38 (6)	O4—Sr—Srⁱⁱ	88.07 (5)
O2—Sr—O2ⁱⁱ	77.03 (7)	O1ⁱⁱⁱ—Sr—Srⁱⁱ	74.17 (5)
O1ⁱ—Sr—O2ⁱⁱ	137.35 (6)	O3^iv—Sr—Srⁱⁱ	106.34 (5)
O2—Sr—O4	80.99 (6)	O3^v—Sr—Srⁱⁱ	99.03 (5)
O1ⁱ—Sr—O4	113.62 (7)	O4^vi—Sr—Srⁱⁱ	115.39 (5)
O2ⁱⁱ—Sr—O4	95.78 (7)	Cu—Sr—Srⁱⁱ	59.857 (6)
O2—Sr—O1ⁱⁱⁱ	63.16 (6)	Cu^vii—Sr—Srⁱⁱ	53.604 (6)
O1ⁱ—Sr—O1ⁱⁱⁱ	85.00 (7)	Sr^viii—Sr—Srⁱⁱ	115.634 (11)
O2ⁱⁱ—Sr—O1ⁱⁱⁱ	91.06 (6)	O2ⁱⁱⁱ—Cu—O2	180.0
O4—Sr—O1ⁱⁱⁱ	140.92 (6)	O2ⁱⁱⁱ—Cu—O1ⁱⁱⁱ	92.84 (8)
O2—Sr—O3^iv	132.05 (6)	O2—Cu—O1ⁱⁱⁱ	87.16 (8)
O1ⁱ—Sr—O3^iv	89.39 (7)	O2ⁱⁱⁱ—Cu—O1	87.16 (8)
O2ⁱⁱ—Sr—O3^iv	77.40 (7)	O2—Cu—O1	92.84 (8)
O4—Sr—O3^iv	62.15 (6)	O1ⁱⁱⁱ—Cu—O1	180.00 (6)
O1ⁱⁱⁱ—Sr—O3^iv	156.01 (6)	O2ⁱⁱⁱ—Cu—Sr	135.56 (6)
O2—Sr—O3^v	128.48 (7)	O2—Cu—Sr	44.44 (6)
O1ⁱ—Sr—O3^v	68.85 (7)	O1ⁱⁱⁱ—Cu—Sr	47.09 (6)
O2ⁱⁱ—Sr—O3^v	68.65 (7)	O1—Cu—Sr	132.91 (6)
O4—Sr—O3^v	138.09 (6)	O2ⁱⁱⁱ—Cu—Srⁱⁱⁱ	44.44 (6)
O1ⁱⁱⁱ—Sr—O3^v	79.93 (6)	O2—Cu—Srⁱⁱⁱ	135.56 (6)
O3^iv—Sr—O3^v	76.30 (7)	O1ⁱⁱⁱ—Cu—Srⁱⁱⁱ	132.91 (6)
O2—Sr—O4^vi	76.83 (7)	O1—Cu—Srⁱⁱⁱ	47.09 (6)
O1ⁱ—Sr—O4^vi	69.84 (7)	Sr—Cu—Srⁱⁱⁱ	180.0
O2ⁱⁱ—Sr—O4^vi	152.23 (6)	O2ⁱⁱⁱ—Cu—Srⁱⁱ	140.54 (6)
O4—Sr—O4^vi	71.00 (7)	O2—Cu—Srⁱⁱ	39.46 (6)
O1ⁱⁱⁱ—Sr—O4^vi	85.41 (6)	O1ⁱⁱⁱ—Cu—Srⁱⁱ	86.22 (6)
O3^iv—Sr—O4^vi	114.46 (7)	O1—Cu—Srⁱⁱ	93.78 (6)
O3^v—Sr—O4^vi	137.07 (7)	Sr—Cu—Srⁱⁱ	66.539 (7)
O2—Sr—Cu	32.44 (4)	Srⁱⁱⁱ—Cu—Srⁱⁱ	113.461 (8)
O1ⁱ—Sr—Cu	106.01 (4)	O2ⁱⁱⁱ—Cu—Sr^ix	39.46 (6)
O2ⁱⁱ—Sr—Cu	92.68 (4)	O2—Cu—Sr^ix	140.54 (6)
O4—Sr—Cu	107.57 (5)	O1ⁱⁱⁱ—Cu—Sr^ix	93.78 (6)
O1ⁱⁱⁱ—Sr—Cu	33.52 (4)	O1—Cu—Sr^ix	86.22 (6)
O3^iv—Sr—Cu	164.31 (5)	Sr—Cu—Sr^ix	113.461 (7)
O3^v—Sr—Cu	111.68 (4)	Srⁱⁱⁱ—Cu—Sr^ix	66.539 (8)
O4^vi—Sr—Cu	69.58 (5)	Srⁱⁱ—Cu—Sr^ix	180.0
O2—Sr—Cu^vii	87.84 (5)	O4—C—O3	126.4 (2)
O1ⁱ—Sr—Cu^vii	136.43 (5)	O4—C—C^iv	116.8 (3)
O2ⁱⁱ—Sr—Cu^vii	28.33 (4)	O3—C—C^iv	116.8 (3)
O4—Sr—Cu^vii	71.54 (5)	Cu—O1—Sr^x	127.46 (10)
O1ⁱⁱⁱ—Sr—Cu^vii	119.12 (5)	Cu—O1—Srⁱⁱⁱ	99.39 (8)
O3^iv—Sr—Cu^vii	53.30 (5)	Sr^x—O1—Srⁱⁱⁱ	95.00 (7)
O3^v—Sr—Cu^vii	79.79 (5)	Cu—O1—H1	109.5
O4^vi—Sr—Cu^vii	141.23 (4)	Sr^x—O1—H1	115.8
Cu—Sr—Cu^vii	113.461 (7)	Srⁱⁱⁱ—O1—H1	103.2
O2—Sr—Sr^viii	99.63 (4)	Cu—O2—Sr	103.13 (8)
O1ⁱ—Sr—Sr^viii	43.12 (4)	Cu—O2—Srⁱⁱ	112.21 (9)
O2ⁱⁱ—Sr—Sr^viii	120.37 (4)	Sr—O2—Srⁱⁱ	102.97 (7)
O4—Sr—Sr^viii	143.24 (5)	Cu—O2—H2	109.5
O1ⁱⁱⁱ—Sr—Sr^viii	41.88 (5)	Sr—O2—H2	126.4
O3^iv—Sr—Sr^viii	128.30 (5)	Srⁱⁱ—O2—H2	102.5
O3^v—Sr—Sr^viii	68.79 (5)	C—O3—Sr^iv	120.51 (17)
O4^vi—Sr—Sr^viii	73.39 (4)	C—O3—Sr^xi	133.28 (17)
Cu—Sr—Sr^viii	67.216 (6)	Sr^iv—O3—Sr^xi	103.70 (7)
Cu^vii—Sr—Sr^viii	145.022 (10)	C—O4—Sr	122.20 (17)
O2—Sr—Srⁱⁱ	39.09 (5)	C—O4—Sr^vi	117.75 (17)
O1ⁱ—Sr—Srⁱⁱ	157.66 (5)	Sr—O4—Sr^vi	109.00 (7)
O2ⁱⁱ—Sr—Srⁱⁱ	37.93 (4)

O4—C—O3—Sr^iv	−170.3 (2)	O3—C—O4—Sr	−170.1 (2)
C^iv—C—O3—Sr^iv	9.5 (4)	C^iv—C—O4—Sr	10.2 (4)
O4—C—O3—Sr^xi	−11.5 (5)	O3—C—O4—Sr^vi	49.9 (4)
C^iv—C—O3—Sr^xi	168.3 (2)	C^iv—C—O4—Sr^vi	−129.9 (3)

Symmetry codes: (i) x, y, z+1; (ii) −x, −y, −z+1; (iii) −x+1, −y, −z+1; (iv) −x, −y+1, −z+2; (v) x, y−1, z; (vi) −x+1, −y+1, −z+2; (vii) x−1, y, z; (viii) −x+1, −y, −z+2; (ix) x+1, y, z; (x) x, y, z−1; (xi) x, y+1, z.

Acknowledgements

The XRD experiment was performed as joint research at the Institute for Solid State Physics, UTokyo (Project No. 202410-MCBXG-0002) and using the Rigaku XtaLAB Synergy-R at the Molecular Structure Analysis Section, Shizuoka Instrumental Analysis Center, Shizuoka University.

Funding information

Funding for this research was provided by: Japan Society for the Promotion of Science.

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