Crystal structure of di-μ-hydroxido-κ(4) O:O-bis[bis(acetyl-acetonato-κ(2) O,O')cobalt(III)].

The dinuclear title complex, [Co2(C5H7O2)4(μ-OH)2] or [Co(acac)2(μ-OH)]2, where acac is acetyl-acetonate, is centrosymmetric with half of the mol-ecule per asymmetric unit. The mol-ecular structure is a dimer of octa-hedrally coordinated Co(III) atoms with four O atoms from two chelating acac ligands and two O atoms from bridging hydroxide ligands. The crystal packing features weak C-H⋯O inter-actions between neighboring mol-ecules, leading to the formation of chains normal to the ac plane. The hydroxide H atoms are not involved in hydrogen bonding because of the bulky acac ligands. This is the first crystal structure reported of a dimeric transition metal bis-acac complex with OH(-) as the bridging group.

Chemical context

Well-defined cobalt(III) hydroxide complexes are relatively rare, especially in the absence of amine ligands (Bryndza & Tam, 1988 ▸). One of the earliest examples is [Co(acac)2(μ-OH)]2 (acac is acetyl­acetonate, C5H7O2), (I), which was prepared by oxidation of Co(II)(acac)2 with hydrogen peroxide. The complex reacts with 2,4-penta­nedione to form CoIII(acac)3 and may serve as a useful model for hydration and oxidation catalysts (Masłowska & Baranovski, 1978 ▸; Bergquist et al., 2003 ▸; Zinn et al., 2007 ▸; Wang et al., 2009 ▸) Boucher and Herrington characterized the complex according to IR and 1H NMR spectra (Boucher & Herrington, 1971 ▸). These data indicated a single diastereoisomer, the identity of which was not clear from the spectra. We now report its crystal structure, confirming that it is centrosymmetric.

Structural commentary

The structure of (I) contains one crystallographically independent CoIII atom with an approximately octa­hedral coord­in­ation environment. The coordination sphere of Co1 is filled by the oxygen atoms of two κ2-O,O′ acac ligands [Co1—O2 = 1.8830 (16) Å, Co1—O3 = 1.8770 (16) Å, Co1—O4 = 1.8814 (16) Å, Co1—O5 = 1.8820 (17) Å) and two μ2-hydroxyl groups [Co1—O1 = 1.9131 (16) Å, Co1—O1i = 1.9087 (17) Å; symmetry code: (i) −x, −y+2, −z+1]. The angles around Co1 are distorted slightly from the ideal 90° and 180° of a perfect octa­hedron. The cis angles range from 82.07 (7) to 95.92 (7)° while the trans angles range from 173.53 (7) to 178.37 (6)°.

The mol­ecular structure of (I) contains a [Co2(μ2-OH)2] motif with each metal coordinated by two acac ligands in a κ2-O,O′ mode (Fig. 1 ▸). The two halves of the dimer are related via inversion symmetry. The Co1⋯Co1i distance is 2.8829 (7) Å. This distance falls within the range (2.696–3.355 Å) of all Co⋯Co distances reported in the Cambridge Crystallographic Database (Groom & Allen, 2014 ▸) for OH−-bridged Co complexes in which the metals are coordinated by six oxygen atoms. It is well below the average Co⋯Co distance of 3.108 Å.

Figure 1

The mol­ecular structure of (I), showing displacement ellipsoids at the 35% probability for non-H atoms and spheres of arbitrary size for H atoms. The unlabeled atoms are related by the symmetry operator (−x, −y + 2, −z + 1).

Supra­molecular features

There are no significant supra­molecular features to discuss with the extended structure of (I). There are weak C—H⋯O inter­molecular inter­actions (Table 1 ▸) between one methyl group of an acac ligand and the hydroxide oxygen atom. These inter­actions result in the formation of chains normal to the ac plane (Fig. 2 ▸). It should be noted that the hydroxyl H atom does not participate in hydrogen bonding. Examination of the packing diagram shows that the bulky acac ligands prevent any hydrogen-bonding inter­actions with neighboring mol­ecules.

Table 1

Hydrogen-bond geometry (, )

DHA	DH	HA	D A	DHA
C1H1BO1i	0.98	2.42	3.395(3)	174

Symmetry code: (i) .

Figure 2

A view along the a axis of the crystal structure of (I), showing extended chains normal to the ac plane. The weak C—H⋯O inter­actions are shown as red dashed lines. All H atoms except the hydroxide H atom (H1) and the inter­acting H atoms (H1B) have been omitted for clarity. Color code: blue = Co, red = O, gray = C, green = H.

Database survey

One closely related crystal structure, [Co(L)2(μ-OH)]2; L = 1-(di­benzyl­amino)-5,5-dimethyl-1,4-dioxohex-2-en-2-olate, has been reported previously (Wang et al., 2009 ▸). The ligand in this complex is a modified acac with a tert-butyl group in place of one methyl and a {CON(CH2Ph)2} group in place of the other methyl group. The coordination environment of the CoIII atoms is the same as in (I). The average Co—O distance of 1.890 Å is similar to the average Co—Oacac distance in (I) of 1.881 Å. The average Co—OH distance of 1.907 Å is also comparable to that of (I) (1.911 Å).

A search of the Cambridge Crystallographic Database (Groom & Allen, 2014 ▸) returned 13 dimeric complexes with the general formula [TM(acac)2(μ-X)]2; TM = transition metal, and X = O, OR, NO, or S (Bottomley et al., 1982 ▸; Nakahanada et al., 1992 ▸; Smith et al., 1972 ▸; Sokolov et al., 1999 ▸). Complex (I) is the first crystal structure reported that fits this general formula in which the bridging group is OH−.

Synthesis and crystallization

The title complex was synthesized according to the procedures reported by Boucher & Herrington (1971 ▸). To a mixture of Co(acac)2·2H2O (2 g, 7.27×10 −3 mol, 1 equiv) and KOAc (3.2 g, 3.26×10 −2 mol, 4.5 equiv) in methanol (125 ml) was added a solution of H2O2 in water (30%wt, 2 ml). The resulting solution changed color from pink to green. The reaction was stirred at room temperature for 1 h under an ambient atmos­phere. The reaction was then concentrated to dryness on a rotary evaporator. The residual green solid was washed with water (3 × 20 ml) and then acetone (3 × 20 ml), and then dried in air, leaving the product (0.85 g, 1.55×10 −3 mol, 43% yield). Crystals, suitable for X-ray diffraction, were grown by slow diffusion of pentane into chloro­form solutions of the green product.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. The hydroxyl H atom was located in a difference map and its position was allowed to refine freely. Methyl H atom positions, R-CH3, were optimized by rotation about R—C bonds with idealized C—H, R—H and H⋯H distances. Remaining H atoms were included as riding idealized contributors. Methyl and hydroxide H atom U iso’s were assigned as 1.5U eq of the carrier atom; remaining H atom U iso’s were assigned as 1.2U eq of the carrier atom.

Table 2

Experimental details

Crystal data
Chemical formula	[Co2(C5H7O2)4(OH)2]
M r	548.30
Crystal system, space group	Triclinic, P
Temperature (K)	173
a, b, c ()	7.8610(11), 8.2481(11), 9.8372(13)
, , ()	100.786(8), 106.708(8), 99.492(9)
V (3)	583.67(14)
Z	1
Radiation type	Mo K
(mm1)	1.47
Crystal size (mm)	0.23 0.19 0.04
Data collection
Diffractometer	Bruker Kappa APEXII CCD
Absorption correction	Integration (SADABS; Bruker, 2012)
T min, T max	0.776, 0.945
No. of measured, independent and observed [I > 2(I)] reflections	16164, 2617, 2228
R int	0.083
(sin /)max (1)	0.646
Refinement
R[F 2 > 2(F 2)], wR(F 2), S	0.036, 0.093, 1.05
No. of reflections	2617
No. of parameters	152
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
max, min (e 3)	0.39, 0.53

Computer programs: APEX2 and SAINT (Bruker, 2013 ▸), SHELXS and SHELXTL (Sheldrick, 2008 ▸), SHELXL2013/4 (Sheldrick, 2015 ▸), CrystalMaker (CrystalMaker, 1994 ▸), XCIF (Bruker, 2013 ▸) and publCIF (Westrip, 2010 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989015013663/wm5186sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989015013663/wm5186Isup2.hkl

Click here for additional data file.

Supporting information file. DOI: 10.1107/S2056989015013663/wm5186Isup3.cdx

CCDC reference: 1413567

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

[Co2(C5H7O2)4(OH)2]	Z = 1
Mr = 548.30	F(000) = 284
Triclinic, P1	Dx = 1.560 Mg m−3
a = 7.8610 (11) Å	Mo Kα radiation, λ = 0.71073 Å
b = 8.2481 (11) Å	Cell parameters from 3791 reflections
c = 9.8372 (13) Å	θ = 2.6–25.5°
α = 100.786 (8)°	µ = 1.47 mm−1
β = 106.708 (8)°	T = 173 K
γ = 99.492 (9)°	Plate, blue
V = 583.67 (14) Å3	0.23 × 0.19 × 0.04 mm

Bruker Kappa APEXII CCD diffractometer	2617 independent reflections
Radiation source: fine-focus sealed tube	2228 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.083
profile data from φ and ω scans	θmax = 27.3°, θmin = 2.2°
Absorption correction: integration (SADABS; Bruker, 2012)	h = −10→10
Tmin = 0.776, Tmax = 0.945	k = −10→10
16164 measured reflections	l = −12→12

Refinement on F2	0 restraints
Least-squares matrix: full	Hydrogen site location: mixed
R[F2 > 2σ(F2)] = 0.036	H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.093	w = 1/[σ2(Fo2) + (0.0366P)2 + 0.1984P] where P = (Fo2 + 2Fc2)/3
S = 1.05	(Δ/σ)max = 0.001
2617 reflections	Δρmax = 0.39 e Å−3
152 parameters	Δρmin = −0.53 e Å−3

Experimental. One distinct cell was identified using APEX2 (Bruker, 2013). Fourteen frame series were integrated and filtered for statistical outliers using SAINT (Bruker, 2013) then corrected for absorption by integration using SAINT/SADABS, v2012/1 (Bruker, 2012) to sort, merge, and scale the combined data. No decay correction was applied.

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

Refinement. Structure was phased by direct methods (Sheldrick, 2008). Systematic conditions suggested the ambiguous space group. The space group choice was confirmed by successful convergence of the full-matrix least-squares refinement on F2. The final map had no significant features. A final analysis of variance between observed and calculated structure factors showed little dependence on amplitude and resolution.

	x	y	z	Uiso*/Ueq
Co1	0.08356 (4)	0.96491 (4)	0.63894 (3)	0.01839 (12)
O1	0.0955 (2)	1.1492 (2)	0.54669 (18)	0.0197 (4)
H1	0.182 (4)	1.158 (4)	0.530 (3)	0.030*
O2	0.0442 (2)	0.7772 (2)	0.71880 (18)	0.0244 (4)
O3	0.2682 (2)	0.9024 (2)	0.56955 (18)	0.0220 (4)
C1	0.0467 (4)	0.5042 (3)	0.7559 (3)	0.0294 (6)
H1A	0.1238	0.5263	0.8584	0.044*
H1B	0.0576	0.3969	0.7002	0.044*
H1C	−0.0808	0.4967	0.7508	0.044*
C2	0.1074 (3)	0.6460 (3)	0.6920 (2)	0.0216 (5)
C3	0.2293 (3)	0.6301 (3)	0.6154 (3)	0.0239 (5)
H3	0.2605	0.5236	0.5962	0.029*
C4	0.3092 (3)	0.7594 (3)	0.5647 (2)	0.0204 (5)
C5	0.4588 (3)	0.7360 (3)	0.5010 (3)	0.0291 (6)
H5A	0.4703	0.8171	0.4411	0.044*
H5B	0.4290	0.6201	0.4398	0.044*
H5C	0.5745	0.7556	0.5805	0.044*
O4	−0.1041 (2)	1.0298 (2)	0.70365 (18)	0.0211 (3)
O5	0.2764 (2)	1.0871 (2)	0.81009 (18)	0.0258 (4)
C6	−0.2501 (3)	1.1417 (4)	0.8641 (3)	0.0302 (6)
H6A	−0.2682	1.2526	0.8490	0.045*
H6B	−0.2371	1.1397	0.9658	0.045*
H6C	−0.3556	1.0527	0.7980	0.045*
C7	−0.0812 (3)	1.1114 (3)	0.8328 (3)	0.0216 (5)
C8	0.0873 (3)	1.1769 (3)	0.9434 (3)	0.0265 (5)
H8	0.0877	1.2323	1.0373	0.032*
C9	0.2534 (3)	1.1666 (3)	0.9258 (3)	0.0245 (5)
C10	0.4268 (4)	1.2530 (4)	1.0500 (3)	0.0383 (7)
H10A	0.4992	1.1698	1.0733	0.057*
H10B	0.3972	1.3033	1.1363	0.057*
H10C	0.4973	1.3424	1.0215	0.057*

	U11	U22	U33	U12	U13	U23
Co1	0.01960 (18)	0.01881 (19)	0.02005 (19)	0.00636 (13)	0.01007 (13)	0.00540 (13)
O1	0.0196 (8)	0.0184 (8)	0.0237 (9)	0.0052 (7)	0.0118 (7)	0.0033 (7)
O2	0.0309 (9)	0.0253 (9)	0.0253 (9)	0.0113 (7)	0.0164 (7)	0.0102 (7)
O3	0.0223 (8)	0.0223 (9)	0.0267 (9)	0.0086 (7)	0.0130 (7)	0.0083 (7)
C1	0.0407 (15)	0.0259 (13)	0.0245 (13)	0.0060 (11)	0.0144 (11)	0.0090 (10)
C2	0.0227 (12)	0.0217 (12)	0.0162 (11)	0.0041 (9)	0.0015 (9)	0.0035 (9)
C3	0.0276 (12)	0.0198 (12)	0.0253 (13)	0.0091 (10)	0.0091 (10)	0.0041 (10)
C4	0.0199 (11)	0.0214 (12)	0.0166 (11)	0.0057 (9)	0.0023 (9)	0.0018 (9)
C5	0.0292 (13)	0.0297 (14)	0.0366 (15)	0.0149 (11)	0.0175 (11)	0.0099 (11)
O4	0.0210 (8)	0.0233 (9)	0.0218 (8)	0.0066 (7)	0.0114 (7)	0.0042 (7)
O5	0.0216 (9)	0.0323 (10)	0.0231 (9)	0.0069 (7)	0.0088 (7)	0.0033 (7)
C6	0.0301 (13)	0.0399 (15)	0.0278 (14)	0.0147 (12)	0.0167 (11)	0.0085 (12)
C7	0.0292 (12)	0.0188 (12)	0.0231 (12)	0.0075 (10)	0.0155 (10)	0.0080 (9)
C8	0.0317 (13)	0.0303 (14)	0.0196 (12)	0.0094 (11)	0.0120 (10)	0.0034 (10)
C9	0.0282 (13)	0.0260 (13)	0.0214 (12)	0.0074 (10)	0.0098 (10)	0.0074 (10)
C10	0.0313 (14)	0.0479 (18)	0.0277 (15)	0.0049 (13)	0.0074 (12)	−0.0021 (13)

Co1—O3	1.8770 (16)	C4—C5	1.506 (3)
Co1—O4	1.8814 (16)	C5—H5A	0.9800
Co1—O5	1.8820 (17)	C5—H5B	0.9800
Co1—O2	1.8830 (16)	C5—H5C	0.9800
Co1—O1i	1.9087 (17)	O4—C7	1.268 (3)
Co1—O1	1.9131 (16)	O5—C9	1.279 (3)
Co1—Co1i	2.8829 (7)	C6—C7	1.495 (3)
O1—Co1i	1.9087 (17)	C6—H6A	0.9800
O1—H1	0.74 (3)	C6—H6B	0.9800
O2—C2	1.278 (3)	C6—H6C	0.9800
O3—C4	1.269 (3)	C7—C8	1.395 (3)
C1—C2	1.502 (3)	C8—C9	1.380 (3)
C1—H1A	0.9800	C8—H8	0.9500
C1—H1B	0.9800	C9—C10	1.502 (3)
C1—H1C	0.9800	C10—H10A	0.9800
C2—C3	1.387 (3)	C10—H10B	0.9800
C3—C4	1.393 (3)	C10—H10C	0.9800
C3—H3	0.9500
O3—Co1—O4	178.37 (6)	C2—C3—C4	124.5 (2)
O3—Co1—O5	85.06 (7)	C2—C3—H3	117.8
O4—Co1—O5	95.92 (7)	C4—C3—H3	117.8
O3—Co1—O2	95.73 (7)	O3—C4—C3	125.0 (2)
O4—Co1—O2	85.55 (7)	O3—C4—C5	115.1 (2)
O5—Co1—O2	91.96 (7)	C3—C4—C5	120.0 (2)
O3—Co1—O1i	90.26 (7)	C4—C5—H5A	109.5
O4—Co1—O1i	88.67 (7)	C4—C5—H5B	109.5
O5—Co1—O1i	173.53 (7)	H5A—C5—H5B	109.5
O2—Co1—O1i	92.95 (7)	C4—C5—H5C	109.5
O3—Co1—O1	88.10 (7)	H5A—C5—H5C	109.5
O4—Co1—O1	90.54 (7)	H5B—C5—H5C	109.5
O5—Co1—O1	93.29 (7)	C7—O4—Co1	124.45 (16)
O2—Co1—O1	173.75 (7)	C9—O5—Co1	123.82 (16)
O1i—Co1—O1	82.07 (7)	C7—C6—H6A	109.5
O3—Co1—Co1i	88.91 (5)	C7—C6—H6B	109.5
O4—Co1—Co1i	89.47 (5)	H6A—C6—H6B	109.5
O5—Co1—Co1i	134.10 (5)	C7—C6—H6C	109.5
O2—Co1—Co1i	133.93 (6)	H6A—C6—H6C	109.5
O1i—Co1—Co1i	41.09 (5)	H6B—C6—H6C	109.5
O1—Co1—Co1i	40.98 (5)	O4—C7—C8	125.1 (2)
Co1i—O1—Co1	97.93 (7)	O4—C7—C6	116.0 (2)
Co1i—O1—H1	103 (2)	C8—C7—C6	118.9 (2)
Co1—O1—H1	106 (2)	C9—C8—C7	124.4 (2)
C2—O2—Co1	123.65 (15)	C9—C8—H8	117.8
C4—O3—Co1	124.41 (15)	C7—C8—H8	117.8
C2—C1—H1A	109.5	O5—C9—C8	125.7 (2)
C2—C1—H1B	109.5	O5—C9—C10	114.7 (2)
H1A—C1—H1B	109.5	C8—C9—C10	119.6 (2)
C2—C1—H1C	109.5	C9—C10—H10A	109.5
H1A—C1—H1C	109.5	C9—C10—H10B	109.5
H1B—C1—H1C	109.5	H10A—C10—H10B	109.5
O2—C2—C3	125.1 (2)	C9—C10—H10C	109.5
O2—C2—C1	115.2 (2)	H10A—C10—H10C	109.5
C3—C2—C1	119.6 (2)	H10B—C10—H10C	109.5

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1B···O1ii	0.98	2.42	3.395 (3)	174