The neutral cluster amminehexa-μ₂-chlorido-μ₄-oxido-tris­(1,4,6-triaza­bicyclo­[3.3.0]oct-4-ene)tetra­copper(II)

The title compound, [Cu₄Cl₆O(C₅H₉N₃)₃(NH₃)], is a neutral conformationally chiral cluster which crystallizes under the conditions described in this paper as a racemic conglomerate. It contains four Cu^II atoms in a tetra­hedral coordination with a central O atom lying on a crystallographic threefold axis. Six chloride anions bridge the four Cu^II atoms. Three Cu^II atoms are bound by an N atom of a monodentate 1,4,6-triaza­bicyclo­[3.3.0]oct-4-ene (Htbo) ligand and the remaining Cu^II atom is bound by a terminal ammine ligand. The geometry at each copper center is trigonal bipyramidal, produced by the bound N atom of Htbo or ammonia, the O atom in the axial position, and three chloride ions in the equatorial plane. The chloride anions form an octa­hedron about the oxygen center. The copper–ammonia bond lies along the crystallographic threefold axis, along which the mol­ecules are packed in a polar head-to-tail fashion.

S1. Comment

Clusters with four copper(II) atoms arranged tetrahedrally around oxygen(II) have been studied for several years for their magnetic properties in materials science (Atria et al., 1999; Reim et al., 1995; Dickinson et al., 1977), as well as in bioinorganic chemistry (El-Sayed et al., 1992) and in anticorrosion studies (Skorda et al., 2005). They have been reported to be good catalysts in the oxidation of aromatic compounds and in some biological systems (Sun et al., 2004; Weinberger et al., 1998).

Other non-bridging ligands may be attached to copper(II), such as halides (Jackson et al., 1996; Belford et al., 1972; Fu & Chivers, 2007; Harlow Simonsen, 1977), oxygen-bonded ligands (Churchill & Rotella, 1979; Bertrand & Kelley, 1966), nicotine (Haendler, 1990), oxoamines (Weinberger et al., 1998), pyridine (Näther & Jeß, 2002; Gill & Sterns, 1970; Duncan et al., 1996; Kilbourn & Dunitz, 1967), imidazole (Zhang et al., 2003; Norman et al., 1989; Erdonmez et al., 1990; Clegg et al., 1988; Cortés et al., 2006), pyrazole (Keij et al., 1991; Liu et al., 2003), triazole (Skorda et al., 2005), tetrazole (Lyakhov et al., 2004), azaindole (Poitras & Beauchamp, 1992), phosphine (Bertrand, 1967), thiazole (Bolos & Christidis, 2002), sulfimide (Kelly et al., 1999), phosphite (Churchill et al., 1975) or sulfoxide (Brownstein et al., 1989; Guy et al., 1988).

We report here an example of an oxo-centered cluster with guanidinate (1,4,6-triazabicyclo[3.3.0]oct-4-ene; Htbo) and ammonia ligands. Although guanidinates are commonly found as deprotonated (anionic) bidentate ligands, in this cluster Htbo behaves as a terminal neutral ligand (see scheme). Only two other reported compounds have this terminal ligand feature (Khalaf et al., 2008), both with the metal lithium. This is the first structure in which a transition metal atom is coordinated by a terminal Htbo ligand. Table 3 presents the C—N distances and N—C—N angles for Htbo in the present compound, in neutral, unligated Htbo, and in previously reported bridging and terminal Htbo ligands.

When Htbo is a terminal ligand there is a significant difference between the C1—N1 and C1—N3 bond lengths. This difference has been interpreted as localization of the electron density from the double bond over the non-protonated N1 (Khalaf et al., 2008). The bond angle N1—C1—N3 is also noticeably larger for terminal Htbo, similar to the angle found in the free ligand.

In the cluster reported here, three copper(II) ions are bound by as many terminal Htbo ligands, with the fourth Cu atom bonded to ammonia. The result is a conformationally chiral compound which crystallizes in space group R3 (Fig. 1). This can be contrasted with the formation of racemic crystals, which contain equal numbers of both enantiomers in the same unit cell and usually crystallize in a centrosymmetric group (Li et al., 2008). Since the preparation and crystallization of this compound involve no factor that would favor one conformational chirality sense, and since our refinement gave a clear indication of the absolute structure, we conclude that the bulk sample is a racemic conglomerate (Flack & Bernardinelli, 1999).

The structure contains intramolecular non-covalent interactions that could be described as hydrogen bonds. The contact N3—H3···Cl1 (Table 2) stabilizes the tilt of the Htbo ligand, which is ultimately responsible for the conformational chirality of the molecule. There are no intermolecular hydrogen bonds, but there are some short contacts between H atoms and chloride anions (Table 2; vide infra).

The central oxo atom has a slightly distorted tetrahedral coordination geometry [with unique Cu—O—Cu angles 111.51 (14) and 107.34 (15)°]. The three Cl—Cu2—Cl angles, identical by symmetry, reveal only the slightest distortion from ideality in the equatorial plane of the trigonal–bipyramidal coordination about this metal (Table 1). The Cl—Cu—Cl angles at Cu1, however, reveal a degree of distortion. The Cu1—Cl distances also present significant differences (Table 1), varying over a range of about 0.18 Å. The unique Cu2—Cl2 distance is within the range presented by the Cu1—Cl distances. The Cu1 units are positioned out of the Cl2/Cl1/Cl1ⁱ plane by 0.216 Å toward the Htbo ligand [symmetry code: (i) -y + 1, x - y, z], while atom Cu2 is 0.166 Å out of the (Cl2)₃ plane toward the ammonia ligand.

The space group R3 not only accommodates an enantiopure molecular conformation, but being polar also hosts a purely head-to-tail packing arrangement (Fig. 2). All of the Cu2→N4 vectors point toward the negative c direction (the polar axis direction is established by the absolute structure parameter). Three very well defined intermolecular N—H··· Cl interactions, from the three symmetry related N—H bonds of the ammonia ligand to the three Cl1 congeners of the molecule at (x, y, z - 1), mediate the polar organization of the structure. Thus, both the chirality and the polarity of space group R3 play a role in enabling important features of this structure.

Related literature

For related literature, see: Belford et al. (1972); Bertrand (1967); Bertrand & Kelley (1966); Bolos & Christidis (2002); Brownstein et al. (1989); Churchill & Rotella (1979); Churchill et al. (1975); Clegg et al. (1988); Cortés et al. (2006); Cotton et al. (2006); Dickinson et al. (1977); Duncan et al. (1996); El-Sayed, Ali, Davies, Larsen & Zubieta (1992); Erdonmez et al. (1990); Flack & Schwarzenbach (1988); Fu & Chivers (2007); Gill & Sterns (1970); Guy, Cooper, Gilardi, Flippen-Anderson & George (1988); Haendler (1990); Harlow & Simonsen (1977); Jackson et al. (1996); Keij et al. (1991); Kelly et al. (1999); Khalaf et al. (2008); Kilbourn & Dunitz (1967); Li et al. (2008); Liu et al. (2003); Lyakhov et al. (2004); Näther & Jeß (2002); Norman et al. (1989); Poitras & Beauchamp (1992); Reim et al. (1995); Skorda et al. (2005); Sun et al. (2004); Weinberger et al. (1998); Zhang et al. (2003).

S2. Experimental

The Htbo ligand was prepared by the procedure described by Cotton et al. (2006). A solution of 0.057 g (0.51 mmol) of Htbo in 20 ml of tetrahydrofuran (THF) was placed in a 100 ml round-bottomed flask. Solid anhydrous copper(I) chloride (0.05 g, 0.51 mmol) was added. A condenser was fitted with a fritted drying tube containing magnesium carbonate and mounted on the reaction flask. The reaction mixture was refluxed for 48 h. After removing the condenser, the solution was evaporated to dryness by keeping the flask open and heated. After cooling, the solid product was extracted with THF and then the remaining solid was extracted with acetonitrile. The THF solution was layered with ether, rendering after one week deep-orange block-shaped crystals. The crystal used for data collection was mounted on a loop with Paratone oil. The THF fraction gave 0.050 g of product (0.060 mmol), a yield of 47%. The acetonitrile fraction was layered with ether but gave poor quality crystals of (µ₄-O)(µ₂-Cl)₆(CuHtbo)₄ (structure not reported) in 20% yield. The remaining solid contained decomposition products.

S3. Refinement

Since the crystal has a polar space group with a floating origin on c, SHELXL97 (Sheldrick, 2008) automatically generated the appropriate restraint (Flack & Schwarzenbach, 1988). All H atoms were placed in calculated positions and refined using a riding model. The C—H disatances were fixed at 0.97 Å and the N—H distances at 0.86–0.89 Å. The U_iso(H) parameters were fixed at 1.2U_eq(N,C). The H atoms of the ammonia ligand are related to each other by the threefold axis; their positions were first established using a local difference map and rotating rigid group constraint (AFIX 137), and for the final refinement these H atoms were treated as riding (AFIX 3). Their final positions were checked with an omit map, using PLATON (Spek, 2009).

Computing details

Data collection: APEX2 (Bruker–Nonius, 2008); cell refinement: APEX2 (Bruker–Nonius, 2008); data reduction: APEX2 (Bruker–Nonius, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXS97 (Sheldrick, 2008); molecular graphics: X-SEED (Barbour, 2001); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. Figure 1 A molecule of (I), lying on a threefold axis, with 50% probability displacement ellipsoids for the non-H atoms. [Symmetry codes: (i) -x + y + 1, -x + 1, z; (ii) -y + 1, x - y, z.] Do not match tables
	Fig. 2. Head-to-tail packing in the extended crystal structure, with N—H···Cl contacts between molecules. )

amminehexa-µ₂-chlorido-µ₄-oxido-tris(1,4,6-triazabicyclo[3.3.0]oct-4- ene)tetracopper(II)

Crystal data

[Cu4Cl6O(C5H9N3)3(NH3)]	Dx = 1.971 Mg m^−3
Mr = 833.35	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3	Cell parameters from 2221 reflections
Hall symbol: R3	θ = 2.3–25.0°
a = 17.548 (9) Å	µ = 3.59 mm^−1
c = 7.898 (4) Å	T = 110 K
V = 2106.2 (17) Å^3	Block, orange
Z = 3	0.11 × 0.08 × 0.06 mm
F(000) = 1248

Data collection

Bruker APEXII diffractometer	1972 independent reflections
Radiation source: fine-focus sealed tube	1772 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.061
ω scans	θmax = 26.7°, θmin = 2.3°
Absorption correction: multi-scan (SADABS; Sheldrick, 2006)	h = −21→22
Tmin = 0.696, Tmax = 0.811	k = −21→21
6295 measured reflections	l = −9→9

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.033	H-atom parameters constrained
wR(F^2) = 0.063	w = 1/[σ^2(Fo^2) + (0.0255P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 0.94	(Δ/σ)max < 0.001
1972 reflections	Δρmax = 0.42 e Å^−3
109 parameters	Δρmin = −0.31 e Å^−3
1 restraint	Absolute structure: Flack (1983), 983 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.019 (19)

Special details

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. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Cu1	0.72127 (3)	0.26784 (3)	0.35495 (6)	0.01685 (14)
Cu2	0.6667	0.3333	0.04015 (11)	0.0178 (2)
Cl1	0.58657 (7)	0.17217 (7)	0.49138 (13)	0.0202 (2)
Cl2	0.75559 (8)	0.26465 (8)	0.06120 (13)	0.0223 (3)
O1	0.6667	0.3333	0.2827 (6)	0.0133 (11)
N1	0.7872 (2)	0.2140 (2)	0.4352 (5)	0.0205 (9)
N2	0.8474 (3)	0.1481 (3)	0.5876 (5)	0.0217 (9)
N3	0.7198 (3)	0.1269 (3)	0.6852 (5)	0.0275 (10)
H3	0.6700	0.1251	0.6886	0.033*
N4	0.6667	0.3333	−0.2058 (9)	0.0301 (17)
H4A	0.6356	0.3575	−0.2434	0.036*	0.33333
H4B	0.6427	0.2784	−0.2434	0.036*	0.33333
H4C	0.7217	0.3645	−0.2434	0.036*	0.33333
C1	0.7794 (3)	0.1647 (3)	0.5622 (6)	0.0192 (10)
C2	0.8745 (3)	0.2400 (3)	0.3603 (6)	0.0244 (11)
H2A	0.9063	0.3024	0.3339	0.029*
H2B	0.8681	0.2070	0.2577	0.029*
C3	0.9227 (3)	0.2181 (3)	0.4981 (6)	0.0244 (11)
H3A	0.9599	0.1973	0.4497	0.029*
H3B	0.9577	0.2681	0.5711	0.029*
C4	0.8473 (3)	0.1284 (4)	0.7683 (6)	0.0296 (12)
H4D	0.8839	0.1813	0.8332	0.035*
H4E	0.8673	0.0865	0.7866	0.035*
C5	0.7509 (3)	0.0892 (3)	0.8126 (6)	0.0295 (12)
H5A	0.7200	0.0255	0.8049	0.035*
H5B	0.7436	0.1061	0.9257	0.035*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu1	0.0177 (3)	0.0186 (3)	0.0170 (3)	0.0112 (3)	0.0020 (2)	0.0030 (2)
Cu2	0.0204 (4)	0.0204 (4)	0.0125 (5)	0.01021 (18)	0.000	0.000
Cl1	0.0175 (6)	0.0194 (6)	0.0221 (6)	0.0081 (5)	0.0028 (4)	0.0062 (5)
Cl2	0.0267 (6)	0.0278 (7)	0.0176 (6)	0.0175 (6)	0.0027 (5)	−0.0010 (5)
O1	0.0143 (16)	0.0143 (16)	0.011 (3)	0.0071 (8)	0.000	0.000
N1	0.023 (2)	0.021 (2)	0.022 (2)	0.0147 (19)	0.0050 (17)	0.0057 (17)
N2	0.030 (2)	0.026 (2)	0.018 (2)	0.021 (2)	−0.0005 (17)	0.0023 (17)
N3	0.028 (2)	0.037 (3)	0.024 (2)	0.021 (2)	0.0066 (17)	0.0115 (18)
N4	0.033 (3)	0.033 (3)	0.024 (4)	0.0167 (13)	0.000	0.000
C1	0.026 (3)	0.017 (3)	0.019 (2)	0.014 (2)	0.000 (2)	−0.0025 (19)
C2	0.029 (3)	0.025 (3)	0.027 (3)	0.020 (2)	0.005 (2)	0.006 (2)
C3	0.022 (3)	0.024 (3)	0.031 (3)	0.014 (2)	−0.003 (2)	−0.002 (2)
C4	0.036 (3)	0.035 (3)	0.024 (3)	0.023 (3)	−0.004 (2)	0.000 (2)
C5	0.041 (3)	0.029 (3)	0.025 (3)	0.022 (3)	0.003 (2)	0.004 (2)

Geometric parameters (Å, º)

Cu1—O1	1.9146 (18)	N3—H3	0.8600
Cu1—N1	1.930 (4)	N4—H4A	0.8913
Cu1—Cl1	2.3660 (15)	N4—H4B	0.8885
Cu1—Cl2	2.4046 (16)	N4—H4C	0.8902
Cu1—Cl1^i	2.5423 (16)	C2—C3	1.540 (6)
Cu2—O1	1.916 (5)	C2—H2A	0.9700
Cu2—N4	1.943 (7)	C2—H2B	0.9700
Cu2—Cl2	2.4075 (16)	C3—H3A	0.9700
N1—C1	1.286 (6)	C3—H3B	0.9700
N1—C2	1.486 (6)	C4—C5	1.516 (7)
N2—C1	1.377 (6)	C4—H4D	0.9700
N2—C3	1.460 (6)	C4—H4E	0.9700
N2—C4	1.468 (6)	C5—H5A	0.9700
N3—C1	1.336 (6)	C5—H5B	0.9700
N3—C5	1.452 (6)
O1—Cu1—N1	173.71 (13)	Cu2—N4—H4C	109.5
O1—Cu1—Cl1	86.53 (8)	H4A—N4—H4C	109.3
N1—Cu1—Cl1	97.69 (11)	H4B—N4—H4C	109.6
O1—Cu1—Cl2	86.14 (15)	N1—C1—N3	134.0 (4)
N1—Cu1—Cl2	95.00 (12)	N1—C1—N2	116.4 (4)
Cl1—Cu1—Cl2	126.57 (5)	N3—C1—N2	109.5 (4)
O1—Cu1—Cl1^i	81.65 (8)	N1—C2—C3	104.6 (4)
N1—Cu1—Cl1^i	92.19 (12)	N1—C2—H2A	110.8
Cl1—Cu1—Cl1^i	119.56 (6)	C3—C2—H2A	110.8
Cl2—Cu1—Cl1^i	111.49 (5)	N1—C2—H2B	110.8
O1—Cu2—N4	180.000 (1)	C3—C2—H2B	110.8
O1—Cu2—Cl2	86.04 (3)	H2A—C2—H2B	108.9
N4—Cu2—Cl2	93.96 (3)	N2—C3—C2	100.0 (4)
Cl2—Cu2—Cl2^i	119.528 (8)	N2—C3—H3A	111.8
Cu1—Cl1—Cu1^ii	80.23 (4)	C2—C3—H3A	111.8
Cu1—Cl2—Cu2	79.77 (4)	N2—C3—H3B	111.8
Cu1—O1—Cu1^i	111.51 (14)	C2—C3—H3B	111.8
Cu1—O1—Cu2	107.34 (15)	H3A—C3—H3B	109.5
C1—N1—C2	105.3 (4)	N2—C4—C5	101.6 (4)
C1—N1—Cu1	134.2 (3)	N2—C4—H4D	111.5
C2—N1—Cu1	119.8 (3)	C5—C4—H4D	111.5
C1—N2—C3	105.5 (3)	N2—C4—H4E	111.5
C1—N2—C4	107.0 (4)	C5—C4—H4E	111.5
C3—N2—C4	124.2 (4)	H4D—C4—H4E	109.3
C1—N3—C5	110.6 (4)	N3—C5—C4	102.8 (4)
C1—N3—H3	124.7	N3—C5—H5A	111.2
C5—N3—H3	124.7	C4—C5—H5A	111.2
Cu2—N4—H4A	109.4	N3—C5—H5B	111.2
Cu2—N4—H4B	109.5	C4—C5—H5B	111.2
H4A—N4—H4B	109.5	H5A—C5—H5B	109.1
O1—Cu1—Cl1—Cu1^ii	1.77 (13)	Cl2^ii—Cu2—O1—Cu1^ii	−7.24 (3)
N1—Cu1—Cl1—Cu1^ii	177.07 (12)	Cl2^i—Cu2—O1—Cu1^ii	112.76 (3)
Cl2—Cu1—Cl1—Cu1^ii	−80.82 (6)	Cl1—Cu1—N1—C1	−22.1 (5)
Cl1^i—Cu1—Cl1—Cu1^ii	80.14 (5)	Cl2—Cu1—N1—C1	−150.1 (5)
O1—Cu1—Cl2—Cu2	−5.58 (3)	Cl1^i—Cu1—N1—C1	98.1 (5)
N1—Cu1—Cl2—Cu2	−179.38 (11)	Cl1—Cu1—N1—C2	169.0 (3)
Cl1—Cu1—Cl2—Cu2	77.20 (5)	Cl2—Cu1—N1—C2	41.0 (3)
Cl1^i—Cu1—Cl2—Cu2	−85.04 (4)	Cl1^i—Cu1—N1—C2	−70.8 (3)
O1—Cu2—Cl2—Cu1	5.58 (3)	C2—N1—C1—N3	174.2 (5)
N4—Cu2—Cl2—Cu1	−174.42 (3)	Cu1—N1—C1—N3	4.2 (9)
Cl2^ii—Cu2—Cl2—Cu1	−77.60 (6)	C2—N1—C1—N2	−2.3 (6)
Cl2^i—Cu2—Cl2—Cu1	88.76 (6)	Cu1—N1—C1—N2	−172.3 (3)
Cl1—Cu1—O1—Cu1^i	122.9 (2)	C5—N3—C1—N1	−170.8 (5)
Cl2—Cu1—O1—Cu1^i	−110.1 (2)	C5—N3—C1—N2	5.9 (5)
Cl1^i—Cu1—O1—Cu1^i	2.34 (18)	C3—N2—C1—N1	20.5 (6)
Cl1—Cu1—O1—Cu1^ii	−2.49 (19)	C4—N2—C1—N1	154.5 (4)
Cl2—Cu1—O1—Cu1^ii	124.55 (19)	C3—N2—C1—N3	−156.9 (4)
Cl1^i—Cu1—O1—Cu1^ii	−123.0 (2)	C4—N2—C1—N3	−22.8 (5)
Cl1—Cu1—O1—Cu2	−119.80 (3)	C1—N1—C2—C3	−15.7 (5)
Cl2—Cu1—O1—Cu2	7.24 (3)	Cu1—N1—C2—C3	156.0 (3)
Cl1^i—Cu1—O1—Cu2	119.64 (4)	C1—N2—C3—C2	−27.2 (5)
Cl2—Cu2—O1—Cu1	−7.24 (3)	C4—N2—C3—C2	−151.0 (4)
Cl2^ii—Cu2—O1—Cu1	112.76 (3)	N1—C2—C3—N2	26.1 (5)
Cl2^i—Cu2—O1—Cu1	−127.24 (3)	C1—N2—C4—C5	29.1 (5)
Cl2—Cu2—O1—Cu1^i	112.76 (3)	C3—N2—C4—C5	152.2 (4)
Cl2^ii—Cu2—O1—Cu1^i	−127.24 (3)	C1—N3—C5—C4	12.6 (5)
Cl2^i—Cu2—O1—Cu1^i	−7.24 (3)	N2—C4—C5—N3	−24.5 (5)
Cl2—Cu2—O1—Cu1^ii	−127.24 (3)

Symmetry codes: (i) −y+1, x−y, z; (ii) −x+y+1, −x+1, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N3—H3···Cl1	0.86	2.54	3.210 (4)	135
N4—H4B···Cl1^iii	0.89	2.64	3.423 (5)	147

Symmetry code: (iii) x, y, z−1.