A linear cyclic oximate-bridged ­tetracopper(II) complex

The crystal structure of the title complex, tetrakis­[[mu]-6-amino-3-methyl-4-aza­hex-3-en-2-one oximato(1-)-[kappa]⁴N,N',N'':O]tetracopper(II) tetraperchlorate 0.6-hydrate, [Cu₄(C₆H₁₂N₃O)₄](ClO₄)₄·0.6H₂O, shows the cation to be an oximate-bridged tetramer composed of four 6-amino-3-methyl-4-aza­hex-3-en-2-one oxime ligands and four copper(II) ions and to have crystallographically imposed symmetry. Each Cu^II atom is four-coordinated by the three N atoms of one oxime ligand and by the O atom of another oxime ligand in a distorted square-planar geometry.

S1. Comment

Studies on multinuclear copper(II) complexes have been focused on the magneto-structural relationships and characterization of the active site in multicopper proteins (Alagi et al., 1997). Linear, di- and trinuclear copper(II) complexes with oximate groups (C = N—O^-) have been widely studied (Cervera et al., 1997; Dominguezvera et al., 1997; Luneau et al., 1989), but tetranuclear copper(II) complexes occur less frequently. In the present study, the isolation and X-ray structure of the tetranuclear copper(II) complex tetrakis[µ-6-amino-3-methyl-4-azahex-3-en-2-one oximato(1-)-κ⁴N,N',N'':O]- tetracopper(II) tetraperchlorate 0.6-hydrate, (I), is reported.

Each Cu ion in the structure is coordinated by three N atoms from one 6-amino-3-methyl-4-aza-hex-3-en-2-one oximate ligand (L^-) and by the oxime O atom from a second ligand (L^-) in a distorted square-planar arrangement. The O1, N1, N2 and N3 donor atoms are planar within 0.124 (2) Å, and the Cu atom deviates by 0.129 (2) Å from this plane. Four of these CuL subunits are linked into a discrete tetrameric cation which has 4 crystallographic symmetry (Fig. 1) and which contains a 12-membered heterocyclic ring of composition (Cu,O1,N3)₄. The unit cell contains two of these tetramers.

Unlike the structures of square tetranuclear metal complexes in which the four metal ions lie in a plane (Chaudhuri et al., 1993; Maekawa et al., 1999), the central tetranuclear copper(II) core of the title complex has an `open-butterfly' configuration with the four copper(II) ions located at the corners of a flattened tetrahedron and is much different from rhombic structures (Raper, 1997; Castro et al., 1995; Gomez-Garcia et al., 1992; Sletten et al., 1990; Tandon et al., 1991). The degree of distortion of the tetrahedron is indicated by the two Cu···Cu distances of 4.244 (1) Å perpendicular to the c axis and the four other Cu···Cu distances of 3.469 (1) Å within the tetramer. The oxime group adopts an out-of-plane coordination mode as a diatomic (µ-1,2)-bridging ligand between copper(II) ions (Ruiz et al., 1998). The orientations of the oxime ligands in the tetramer are alternately up and down.

The Cu/N1/N2/C1/C2 and Cu/N1/N2/C1/C2 five-membered rings are in the skew form and are planar within 0.158 (5) and 0.051 (3) Å, respectively. The Cu— Nbond lengths, ranging from 1.939 (4) to 2.004 (4) Å, are considered normal covalent bonds. The order of the Cu—N distances, amine [2.004 (4) Å] > oxime [1.989 (3) Å] > imine [1.941 (3) Å], is the same as that observed for other oximato-bridged Cu^II complexes (Nasakkala et al., 1981; Butcher et al., 1979). The Cu—O(bridging oxime) distance [1.941 (3) Å] is slightly longer than that found in these oximate-bridged Cu^II complexes because of the steric effect of the bulky folded conformation (Nasakkala et al., 1981; Butcher et al., 1979). Owing to the weak metal–ligand interactions in the title complex, the N—O distance [1.339 (4) Å] is longer than that found in these complexes with strongly coordinated bridging-oxime groups [1.311 (3) Å; Raston et al., 1978; Butcher et al., 1979]. The bond angles of the oxime and imine N atoms are approximately 120°, in keeping with the expected sp²-hybridization stabilized by π-bonding. The N3ⁱ—O1—Cu [112.7 (2)°; symmetry code: (i) 3/2 - y, x, 1/2 - z] angle indicates a variation from sp² hybridization for the oxime O atom, resulting from coordination to another Cu atom. The N—O distance does show some decrease in going from the free ligand (1.375 Å) to complexes with weak interactions [1.339 (4) Å, this study] to complexes with strong interactions [1.311 (3) Å; Raston et al., 1978; Butcher et al., 1979].

There appears to be a water molecule on a partially occupied site [occupancy = 0.30 (3)] on a twofold axis. This is external to the tetramer and is loosely held by hydrogen bonding to the perchlorate ion (Table 2). There are two equivalent sites on the axis that are 2.80 (3) Å apart. Other hydrogen bonds are detailed in Table 2.

Related literature

For related literature, see: Alagi et al. (1997); Butcher et al. (1979); Castro et al. (1995); Cervera et al. (1997); Chaudhuri et al. (1993); Dominguezvera et al. (1997); Gomez-Garcia, Coronado & Borras-Almenar (1992); Luneau et al. (1989); Maekawa et al. (1999); Nasakkala et al. (1981); Raper (1997); Raston et al. (1978); Ruiz et al. (1998); Singh et al. (1977); Sletten et al. (1990); Tandon et al. (1991).

S2. Experimental

The ligand 6-amino-3-methyl-4-azahex-3-en-2-one oxime was prepared according to the method of Singh et al. (1977). Equimolar quantities of copper perchlorate hexahydrate and the ligand were mixed in methanol and the solution was heated under reflux for 2 h. The resulting solution was cooled to room temperature and evaporated to dryness by rotatory evaporation. The crude product was washed with acetonitrile and then recrystallized from methanol by slow evaporation.

S3. Refinement

H atoms were placed geometrically and refined as riding, with U_iso(H) = 1.2U_eq(C, N or O). The water H atoms were not located. The largest peak in the difference map is near Cu.

Computing details

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: NRCVAX (Gabe et al., 1989); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990) and NRCVAX; program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: NRCVAX; software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. The structure of the title compound showing 30% probability displacement ellipsoids. Only the asymmetric part of the tetramer is labelled. The water molecule and the perchlorate ions have been omitted for clarity.

(I)

Crystal data

[Cu4(C6H12N8O)4](ClO4)4·0.6H2O	Dx = 1.790 Mg m^−3
Mr = 1232.32	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P42/n	Cell parameters from 25 reflections
a = 12.440 (3) Å	θ = 5.4–17.3°
c = 14.851 (7) Å	µ = 2.15 mm^−1
V = 2298.2 (12) Å^3	T = 293 K
Z = 2	Square bipyramidal, black
F(000) = 1260	0.47 × 0.34 × 0.29 mm

Data collection

Nonius CAD-4 diffractometer	1710 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.000
Graphite monochromator	θmax = 30.0°, θmin = 2.1°
ω–2θ scans	h = 0→17
Absorption correction: empirical (using intensity measurements) (North et al., 1968)	k = 0→17
Tmin = 0.37, Tmax = 0.54	l = 0→20
3349 measured reflections	3 standard reflections every 60 min
3349 independent reflections	intensity decay: 3%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.066	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.183	H-atom parameters constrained
S = 1.09	w = 1/[σ^2(Fo^2) + (0.1P)^2] where P = (Fo^2 + 2Fc^2)/3
3349 reflections	(Δ/σ)max = 0.031
153 parameters	Δρmax = 1.31 e Å^−3
0 restraints	Δρmin = −0.74 e Å^−3

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)
Cu	0.57965 (4)	0.75901 (4)	0.19143 (3)	0.0455 (2)
Cl	0.36504 (11)	−0.05427 (12)	0.27656 (10)	0.0689 (4)
O1	0.6149 (3)	0.7823 (2)	0.31724 (18)	0.0464 (7)
O2	0.4544 (3)	−0.0699 (4)	0.2175 (3)	0.0825 (12)
O3	0.3256 (6)	−0.1522 (5)	0.3059 (5)	0.174 (3)
O4	0.2841 (4)	0.0031 (6)	0.2328 (4)	0.149 (2)
O5	0.3975 (5)	0.0029 (6)	0.3523 (4)	0.139 (2)
OW	0.7500	0.7500	0.6558 (15)	0.221 (14)	0.30 (3)
N1	0.4520 (3)	0.6624 (3)	0.2065 (3)	0.0566 (10)
H1A	0.4722	0.5996	0.2314	0.068*
H1B	0.4031	0.6936	0.2428	0.068*
N2	0.5295 (3)	0.7720 (3)	0.0683 (3)	0.0552 (10)
N3	0.6919 (3)	0.8554 (3)	0.1407 (2)	0.0423 (8)
C1	0.4056 (6)	0.6438 (6)	0.1174 (4)	0.103 (3)
H2A	0.4331	0.5760	0.0947	0.123*
H2B	0.3284	0.6361	0.1241	0.123*
C2	0.4252 (5)	0.7224 (6)	0.0538 (4)	0.087 (2)
H3A	0.3696	0.7769	0.0572	0.104*
H3B	0.4230	0.6909	−0.0059	0.104*
C3	0.5803 (4)	0.8361 (4)	0.0179 (3)	0.0574 (12)
C4	0.5456 (6)	0.8705 (6)	−0.0739 (4)	0.103 (3)
H4A	0.4747	0.8435	−0.0857	0.123*
H4B	0.5947	0.8426	−0.1179	0.123*
H4C	0.5449	0.9476	−0.0770	0.123*
C5	0.7629 (5)	0.9393 (5)	0.0051 (4)	0.0780 (18)
H5A	0.7460	1.0145	0.0068	0.094*
H5B	0.7630	0.9149	−0.0563	0.094*
H5C	0.8327	0.9278	0.0310	0.094*
C6	0.6812 (4)	0.8782 (4)	0.0573 (3)	0.0487 (11)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu	0.0509 (3)	0.0544 (4)	0.0312 (3)	−0.0061 (2)	−0.0043 (2)	0.0055 (2)
Cl	0.0638 (8)	0.0758 (9)	0.0669 (8)	0.0017 (7)	0.0049 (7)	−0.0011 (7)
O1	0.0584 (18)	0.0494 (17)	0.0315 (14)	0.0025 (13)	−0.0040 (13)	0.0049 (12)
O2	0.081 (3)	0.090 (3)	0.076 (3)	0.015 (2)	0.023 (2)	0.014 (2)
O3	0.171 (7)	0.103 (4)	0.249 (9)	−0.026 (4)	0.125 (6)	−0.014 (5)
O4	0.092 (4)	0.259 (8)	0.094 (4)	0.063 (5)	−0.003 (3)	0.014 (5)
O5	0.119 (4)	0.217 (7)	0.081 (3)	−0.030 (5)	0.003 (3)	−0.062 (4)
OW	0.25 (3)	0.18 (2)	0.23 (2)	−0.080 (17)	0.000	0.000
N1	0.063 (2)	0.058 (2)	0.049 (2)	−0.0108 (19)	−0.0078 (18)	0.0078 (18)
N2	0.060 (2)	0.067 (3)	0.039 (2)	−0.011 (2)	−0.0141 (17)	0.0044 (18)
N3	0.050 (2)	0.0439 (18)	0.0328 (17)	−0.0005 (15)	−0.0055 (15)	0.0037 (14)
C1	0.103 (5)	0.139 (6)	0.066 (4)	−0.059 (5)	−0.027 (4)	0.013 (4)
C2	0.075 (4)	0.128 (6)	0.058 (3)	−0.041 (4)	−0.021 (3)	0.017 (4)
C3	0.077 (3)	0.060 (3)	0.035 (2)	−0.007 (2)	−0.012 (2)	0.005 (2)
C4	0.123 (6)	0.139 (6)	0.046 (3)	−0.038 (5)	−0.034 (4)	0.032 (4)
C5	0.096 (4)	0.097 (4)	0.041 (3)	−0.031 (3)	−0.003 (3)	0.019 (3)
C6	0.063 (3)	0.052 (3)	0.032 (2)	−0.005 (2)	−0.0030 (19)	0.0029 (17)

Geometric parameters (Å, º)

Cu—N2	1.939 (4)	N3—O1^ii	1.339 (4)
Cu—O1	1.941 (3)	C1—C2	1.382 (8)
Cu—N3	1.989 (3)	C1—H2A	0.9700
Cu—N1	2.004 (4)	C1—H2B	0.9700
Cl—O3	1.383 (6)	C2—H3A	0.9700
Cl—O5	1.391 (5)	C2—H3B	0.9700
Cl—O4	1.395 (6)	C3—C6	1.480 (6)
Cl—O2	1.429 (4)	C3—C4	1.492 (6)
O1—N3^i	1.339 (4)	C4—H4A	0.9600
N1—C1	1.462 (7)	C4—H4B	0.9600
N1—H1A	0.9000	C4—H4C	0.9600
N1—H1B	0.9000	C5—C6	1.487 (7)
N2—C3	1.264 (6)	C5—H5A	0.9600
N2—C2	1.452 (6)	C5—H5B	0.9600
N3—C6	1.278 (5)	C5—H5C	0.9600
N2—Cu—O1	165.50 (16)	C2—C1—H2B	108.3
N2—Cu—N3	79.54 (15)	N1—C1—H2B	108.3
O1—Cu—N3	96.66 (15)	H2A—C1—H2B	107.4
N2—Cu—N1	84.26 (16)	C1—C2—N2	110.9 (5)
O1—Cu—N1	99.28 (16)	C1—C2—H3A	109.5
N3—Cu—N1	163.78 (15)	N2—C2—H3A	109.4
O3—Cl—O5	107.4 (5)	C1—C2—H3B	109.4
O3—Cl—O4	110.0 (4)	N2—C2—H3B	109.5
O5—Cl—O4	108.9 (4)	H3A—C2—H3B	108.0
O3—Cl—O2	110.4 (3)	N2—C3—C6	114.4 (4)
O5—Cl—O2	109.9 (3)	N2—C3—C4	125.3 (5)
O4—Cl—O2	110.2 (3)	C6—C3—C4	120.3 (5)
N3^i—O1—Cu	112.7 (2)	C3—C4—H4A	109.5
C1—N1—Cu	107.9 (3)	C3—C4—H4B	109.5
C1—N1—H1A	110.1	H4A—C4—H4B	109.5
Cu—N1—H1A	110.1	C3—C4—H4C	109.5
C1—N1—H1B	110.2	H4A—C4—H4C	109.5
Cu—N1—H1B	110.1	H4B—C4—H4C	109.5
H1A—N1—H1B	108.4	C6—C5—H5A	109.5
C3—N2—C2	128.8 (4)	C6—C5—H5B	109.5
C3—N2—Cu	116.8 (3)	H5A—C5—H5B	109.5
C2—N2—Cu	113.1 (3)	C6—C5—H5C	109.5
C6—N3—O1^ii	118.6 (3)	H5A—C5—H5C	109.5
C6—N3—Cu	115.3 (3)	H5B—C5—H5C	109.5
O1^ii—N3—Cu	125.4 (2)	N3—C6—C3	113.1 (4)
C2—C1—N1	115.9 (5)	N3—C6—C5	123.2 (4)
C2—C1—H2A	108.3	C3—C6—C5	123.7 (4)
N1—C1—H2A	108.3

Symmetry codes: (i) −y+3/2, x, −z+1/2; (ii) y, −x+3/2, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O2^iii	0.90	2.31	3.181 (6)	164
N1—H1B···O3^iv	0.90	2.34	3.158 (8)	151
OW—(H)···O4^v			3.305 (10)

Symmetry codes: (iii) −y+1/2, x, −z+1/2; (iv) x, y+1, z; (v) −y+1, x+1/2, z+1/2.