Bis{1-n-hex­yl-3-meth­yl-4-[1-(phenyl­imino)prop­yl]-1H-pyrazol-5-olato}­copper(II): a new copper(II) complex with a chelating alkyl­pyrazolone-based enamine

The title compound, [Cu(C₁₉H₂₆N₃O)₂], is the first reported complex of the alkyl­pyrazolone-derived ligand 1-n-hexyl-3-methyl-4-[1-(phenylimino)propyl]-1H-pyrazol-5(4H)-one. The most notable feature is the imine-enol character presented by the ligand due to coordination, in spite of its enamine-ketone structure in the free state. The ligand chelates through N and O atoms, resulting in a square-planar coordination around the Cu^II atom, which lies on an inversion centre.

S1. Comment

Pyrazolones constitute an important group of organic compounds (Elguero, 1996), for both theoretical and practical reasons. Their application fields include analgesic and anti-inflamatory drugs (Gürzov et al., 2000), dyes (Emeleus et al., 2001), extractants for several ions (Petinari et al., 2000), etc. Pyrazolones have attracted much attention because they exhibit prototropic tautomerism (Elguero et al., 1976; Uraev et al., 2000; Gilchrist, 2001) and they have been extensively studied both in solution and in the crystalline phase (Chmutova et al., 2001).

Pyrazolones have usually been obtained by the same synthetic procedure, a condensation between an acyl acetate and a hydrazine, for more than one century (Knorr, 1884). In spite of the many advantages of 1-alkylpyrazolone derivatives (viz. their greater solubility), most literature reports deal with 1-phenylpyrazolones or N1 unsubstituted pyrazolones. This situation may result from the fact that the few commercially available alkylhydrazines are very expensive. Furthermore, there are no convenient syntheses to obtain them. Emeleus et al. (2001) described the use of some alkylpyrazolones that were obtained following a different procedure (Butler & de Wald, 1971). However, alkylhydrazines were still one of the required reagents.

More recently, with the aim of studying the tautomerism involved, it was shown that pyrazolone could be easily alkylated at N1 with primary alkylhalides (Bartulin et al., 1992, 1994; Belmar et al., 1997, 1999). This procedure finally allowed the obtention of several enamines derived from 4-acyl-1-(n-hexyl)-3-methyl-5-pyrazolones (Belmar et al., 2004, 2005) and even some nitrido complexes using these ligands (Pérez et al., 2005). In addition to these reports, papers by Maurya et al. (1992) and Dey et al. (1999) described some complexes using 1-phenylpyrazolone-based imines. In this paper, we present the structure of the title compound, (I), Cu(L)₂ (L is the C₁₉H₂₆N₃O⁻ anion), which is the first copper complex ever reported using a chelating 1-alkylpyrazolone-based enamine ligand.

Compound (I) is monomeric and consists of a tetracoordinated Cu^II atom lying on a symmetry centre, coordinated to two chelating (symmetry-related) bidentate L ligands (Fig. 1). The resulting CuO₂N₂ core is perfectly planar, due to the restraints imposed by symmetry. The coordination bond lengths do not depart significantly from average values taken from a selected subset of 80 structures with a similar coordination scheme found in the November 2004 release of the Cambridge Structural Database (CSD; Allen, 2002), viz. Cu—N 2.036 (3) and Cu—O 1.879 (2) Å in this work, compared with 2.001 (20) and 1.887 (21) Å, respectively, from the CSD. The ligand L does not twist appreciably because of chelation. Fig. 2 presents a superposition diagram of the free ligand (Belmar et al., 2004) and the ligand in (I); both cores overlap almost entirely, the larger departure being found in the phenyl ring [average discrepancies are 0.08 (8) Å for non-benzyl atoms and 1.8 (11) Å for benzyl atoms]. The free ligand forms an N—H···O intramolecular hydrogen bond, which rocks the conformation.

The pyrazine ring and its two substituents at C1 and C2 determine a planar group [mean deviation 0.01 (1) Å], which binds in a slightly slanted way through the outermost atoms O1 and N3 to the Cu coordination plane [dihedral angle 16.8 (1)°]. Thus, the six-membered ring O1/C1/C2/C5/N3/Cu1 presents an envelope conformation, with the Cu atom puckering 0.38 (1) Å away from the planar group defined by the remaining five atoms.

There are four lateral substituents attached to the planar main frame of (I), namely Me at C3, Et at C5, n-hexyl at N1 and Ph at N3. All of them, with the obvious exception of the methyl group, are almost perpendicular to the quasi-planar coordination core, the Ph group subtending an angle of 77.4 (1)° and the two aliphatic chains deviating from the vertical by 17 (1)°. The hexyl group presents a striking unperturbed almost planar zigzag conformation, with torsion angles in the range 177.1 (1)–179.2 (1)°. A survey of the CSD showed this to be a rather infrequent conformation: in 842 cases (89% out of a total of 947 hexyl groups reported), the aliphatic chain presented greater deviations from a perfect unperturbed zigzag state, as measured by the largest torsion angle deviation from an expected 180°. Incidentally, only one out of the 947 structures surveyed showed a symmetry-forced perfectly planar conformation.

This particular disposition of the sustituents in (I) introduces a strong steric limitation to the approach of planar groups from different molecules, and thus precludes the most noteworthy intermolecular interaction found in the structure of the free ligand (Belmar et al., 2004), viz. the π–π contact between aromatic rings.

Previously reported structural work on complexes derived from related ligands strongly suggests an `enamine-to-imine' shift of the character of the ligand upon coordination, viz. a nitridomanganese(V) complex with N,N'-bis{[1-(n-hexyl)-3-methyl-5-oxo-2-pyrazolin-4-yl]propyliden-1-yl}-ethylenediamine (Belmar et al., 2005). In those cases, however, the comparison of conformations `before' and `after' coordination could only be made through the use of similar (but not the same) ligands, as the present study is the first case where both structures (free ligand and a derived complex) are known from an X-ray analysis. A comparison of selected ligand bond distances for (I) and their homologues in the free moiety (taken from Belmar et al., 2004) is shown in Fig. 3. Although individual differences are subtle enough to be considered not relevant, the overall trend of the bond lengths changes in the N3—C5—C2—C1—O1 chain clearly points to an enamine-to-imine shift of the ligand character after complexation.

S2. Experimental

The protonated ligand LH (C₁₉H₂₇N₃O; Belmar et al., 2004, 2005; 0.1 g, 0.32 mmol) and Cu(O₂CCH₃)₂·H₂O (Quantity?) were dissolved in ethanol (10 ml) and heated to reflux for 1 h. The solution was then evaporated to a final volume of 5 ml and allowed to cool down to room temperature. The solid was filtered and crystallized by slow evaporation from a chloroform–hexane mixture (Ratio?) [yield 0.08 g, 73%; m.p. 396 (2) K].

S3. Refinement

H atoms were placed in their theoretical positions, with C—H_aromatic = 0.93, C—H₂ = 0.97 and C—H₃ = 0.96 Å, and allowed to ride on their parent atoms, with U_iso(H) = 1.2U_eq(C) for the first two cases and 1.5U_eq(C) for the third. The methyl groups were also allowed to rotate around their C—C axis.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL/PC (Sheldrick, 1994); software used to prepare material for publication: SHELXTL/PC.

Figures

	Fig. 1. A molecular diagram of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 40% probability level and H atoms are shown as small spheres of rabitrary radii. Shaded ellipsoids denote the independent part of the molecule and open ones the symmetry-related moiety at (1 − x, 1 − y, 1 − z).
	Fig. 2. A superposition diagram, showing the similarities between the nuclei in (I) and in the free ligand (Belmar et al., 2004). All H atoms have been omitted for clarity.
	Fig. 3. A comparison of selected bond distances for the coordinated ligand L in (I) (underlined bold type) and the uncoordinated ligand in Belmar et al. (2004) (regular type).

Bis{1-(n-hexyl)-3-methyl-4-[1-(phenylimino)propyl]-2-pyrazol-5-olato}copper(II)

Crystal data

[Cu(C19H26N3O)2]	Z = 1
Mr = 688.40	F(000) = 367
Triclinic, P1	Dx = 1.258 Mg m^−3
Hall symbol: -P 1	Melting point: 396(2) K
a = 8.0366 (10) Å	Mo Kα radiation, λ = 0.71073 Å
b = 8.8767 (11) Å	Cell parameters from 1870 reflections
c = 14.3438 (17) Å	θ = 3.6–23.5°
α = 94.121 (2)°	µ = 0.64 mm^−1
β = 95.416 (2)°	T = 298 K
γ = 115.946 (2)°	Prism, brown
V = 908.67 (19) Å^3	0.30 × 0.16 × 0.14 mm

Data collection

Bruker SMART CCD area-detector diffractometer	3530 independent reflections
Radiation source: fine-focus sealed tube	2449 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.047
ϕ and ω scans	θmax = 26.0°, θmin = 2.6°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −9→9
Tmin = 0.86, Tmax = 0.91	k = −10→10
7009 measured reflections	l = −17→17

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) = 0.139	H-atom parameters constrained
S = 1.02	w = 1/[σ^2(Fo^2) + (0.0558P)^2] where P = (Fo^2 + 2Fc^2)/3
3530 reflections	(Δ/σ)max = 0.003
217 parameters	Δρmax = 0.49 e Å^−3
0 restraints	Δρmin = −0.31 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
Cu1	0.5000	0.5000	0.5000	0.0370 (2)
O1	0.5640 (4)	0.4339 (3)	0.38683 (16)	0.0498 (7)
N1	0.7606 (4)	0.3657 (4)	0.3059 (2)	0.0458 (8)
N2	0.8911 (4)	0.3055 (4)	0.3202 (2)	0.0477 (8)
N3	0.6102 (4)	0.3711 (3)	0.57545 (19)	0.0365 (7)
C1	0.6876 (5)	0.3793 (4)	0.3852 (3)	0.0388 (9)
C2	0.7755 (5)	0.3259 (4)	0.4561 (2)	0.0342 (8)
C3	0.9011 (5)	0.2823 (4)	0.4096 (3)	0.0410 (9)
C4	1.0364 (5)	0.2195 (5)	0.4442 (3)	0.0563 (11)
H4A	1.1069	0.2137	0.3948	0.084*
H4B	0.9699	0.1091	0.4626	0.084*
H4C	1.1198	0.2952	0.4976	0.084*
C5	0.7292 (5)	0.3166 (4)	0.5493 (3)	0.0372 (9)
C6	0.8135 (5)	0.2364 (5)	0.6155 (3)	0.0498 (10)
H6A	0.8165	0.2791	0.6801	0.060*
H6B	0.9412	0.2675	0.6048	0.060*
C7	0.7035 (6)	0.0449 (5)	0.6019 (3)	0.0643 (13)
H7A	0.7672	−0.0022	0.6412	0.096*
H7B	0.6929	0.0028	0.5370	0.096*
H7C	0.5810	0.0133	0.6189	0.096*
C8	0.5561 (5)	0.3405 (5)	0.6682 (3)	0.0428 (9)
C9	0.3911 (6)	0.2032 (5)	0.6772 (3)	0.0590 (12)
H9A	0.3144	0.1333	0.6235	0.071*
C10	0.3390 (7)	0.1688 (6)	0.7648 (4)	0.0719 (14)
H10A	0.2287	0.0744	0.7699	0.086*
C11	0.4473 (8)	0.2715 (7)	0.8440 (4)	0.0762 (15)
H11A	0.4116	0.2474	0.9031	0.091*
C12	0.6092 (7)	0.4108 (7)	0.8359 (3)	0.0692 (14)
H12A	0.6830	0.4818	0.8899	0.083*
C13	0.6642 (6)	0.4470 (5)	0.7484 (3)	0.0556 (11)
H13A	0.7735	0.5426	0.7436	0.067*
C14	0.7233 (6)	0.4135 (5)	0.2151 (3)	0.0560 (11)
H14A	0.8412	0.4844	0.1943	0.067*
H14B	0.6553	0.4800	0.2219	0.067*
C15	0.6118 (7)	0.2645 (6)	0.1400 (3)	0.0678 (13)
H15A	0.5973	0.3061	0.0806	0.081*
H15B	0.6813	0.1997	0.1319	0.081*
C16	0.4214 (7)	0.1497 (6)	0.1628 (3)	0.0738 (14)
H16A	0.3544	0.2160	0.1740	0.089*
H16B	0.4364	0.1042	0.2207	0.089*
C17	0.3049 (7)	0.0050 (6)	0.0865 (3)	0.0758 (14)
H17A	0.2868	0.0509	0.0292	0.091*
H17B	0.3742	−0.0586	0.0737	0.091*
C18	0.1176 (8)	−0.1137 (7)	0.1092 (4)	0.0977 (18)
H18A	0.0481	−0.0503	0.1218	0.117*
H18B	0.1356	−0.1597	0.1665	0.117*
C19	0.0025 (8)	−0.2575 (7)	0.0330 (4)	0.111 (2)
H19A	−0.1172	−0.3245	0.0517	0.166*
H19B	0.0656	−0.3264	0.0232	0.166*
H19C	−0.0144	−0.2137	−0.0245	0.166*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu1	0.0349 (4)	0.0432 (4)	0.0360 (4)	0.0203 (3)	0.0049 (3)	0.0056 (3)
O1	0.0602 (18)	0.0717 (19)	0.0367 (15)	0.0467 (16)	0.0076 (13)	0.0070 (14)
N1	0.051 (2)	0.053 (2)	0.041 (2)	0.0291 (17)	0.0115 (16)	0.0081 (16)
N2	0.046 (2)	0.051 (2)	0.053 (2)	0.0269 (17)	0.0146 (17)	0.0056 (17)
N3	0.0362 (17)	0.0400 (18)	0.0348 (17)	0.0173 (15)	0.0080 (14)	0.0090 (14)
C1	0.040 (2)	0.036 (2)	0.042 (2)	0.0182 (18)	0.0094 (18)	0.0028 (17)
C2	0.033 (2)	0.032 (2)	0.041 (2)	0.0163 (17)	0.0090 (17)	0.0062 (16)
C3	0.035 (2)	0.035 (2)	0.051 (3)	0.0134 (18)	0.0072 (18)	0.0051 (18)
C4	0.052 (3)	0.056 (3)	0.072 (3)	0.033 (2)	0.017 (2)	0.011 (2)
C5	0.033 (2)	0.031 (2)	0.046 (2)	0.0131 (17)	0.0021 (17)	0.0019 (17)
C6	0.056 (3)	0.057 (3)	0.048 (3)	0.035 (2)	0.004 (2)	0.009 (2)
C7	0.083 (3)	0.065 (3)	0.063 (3)	0.046 (3)	0.017 (3)	0.022 (2)
C8	0.045 (2)	0.047 (2)	0.043 (2)	0.026 (2)	0.0067 (19)	0.0100 (19)
C9	0.059 (3)	0.056 (3)	0.054 (3)	0.017 (2)	0.013 (2)	0.009 (2)
C10	0.081 (4)	0.068 (3)	0.074 (4)	0.031 (3)	0.040 (3)	0.027 (3)
C11	0.106 (4)	0.100 (4)	0.050 (3)	0.063 (4)	0.030 (3)	0.035 (3)
C12	0.088 (4)	0.096 (4)	0.038 (3)	0.057 (3)	−0.001 (3)	0.005 (3)
C13	0.058 (3)	0.069 (3)	0.043 (3)	0.032 (2)	0.002 (2)	0.007 (2)
C14	0.065 (3)	0.065 (3)	0.044 (3)	0.032 (2)	0.013 (2)	0.013 (2)
C15	0.082 (3)	0.084 (3)	0.040 (3)	0.039 (3)	0.010 (2)	0.011 (2)
C16	0.086 (4)	0.076 (3)	0.055 (3)	0.032 (3)	0.014 (3)	0.009 (3)
C17	0.084 (4)	0.084 (4)	0.058 (3)	0.037 (3)	0.005 (3)	0.010 (3)
C18	0.097 (4)	0.089 (4)	0.084 (4)	0.019 (4)	0.018 (3)	0.013 (3)
C19	0.093 (4)	0.096 (4)	0.107 (5)	0.012 (4)	0.002 (4)	0.011 (4)

Geometric parameters (Å, º)

Cu1—O1	1.879 (2)	C9—C10	1.374 (5)
Cu1—O1^i	1.879 (2)	C9—H9A	0.9300
Cu1—N3	2.036 (3)	C10—C11	1.360 (6)
Cu1—N3^i	2.036 (3)	C10—H10A	0.9300
O1—C1	1.282 (4)	C11—C12	1.370 (6)
N1—C1	1.349 (4)	C11—H11A	0.9300
N1—N2	1.375 (4)	C12—C13	1.383 (5)
N1—C14	1.445 (5)	C12—H12A	0.9300
N2—C3	1.314 (4)	C13—H13A	0.9300
N3—C5	1.317 (4)	C14—C15	1.513 (5)
N3—C8	1.446 (4)	C14—H14A	0.9700
C1—C2	1.409 (5)	C14—H14B	0.9700
C2—C5	1.420 (5)	C15—C16	1.505 (6)
C2—C3	1.430 (5)	C15—H15A	0.9700
C3—C4	1.486 (5)	C15—H15B	0.9700
C4—H4A	0.9600	C16—C17	1.509 (6)
C4—H4B	0.9600	C16—H16A	0.9700
C4—H4C	0.9600	C16—H16B	0.9700
C5—C6	1.506 (5)	C17—C18	1.495 (6)
C6—C7	1.521 (5)	C17—H17A	0.9700
C6—H6A	0.9700	C17—H17B	0.9700
C6—H6B	0.9700	C18—C19	1.502 (6)
C7—H7A	0.9600	C18—H18A	0.9700
C7—H7B	0.9600	C18—H18B	0.9700
C7—H7C	0.9600	C19—H19A	0.9600
C8—C9	1.378 (5)	C19—H19B	0.9600
C8—C13	1.381 (5)	C19—H19C	0.9600
O1—Cu1—O1^i	180.00 (7)	C8—C9—H9A	119.7
O1—Cu1—N3	92.24 (11)	C11—C10—C9	120.6 (5)
O1^i—Cu1—N3	87.76 (11)	C11—C10—H10A	119.7
O1—Cu1—N3^i	87.76 (11)	C9—C10—H10A	119.7
O1^i—Cu1—N3^i	92.24 (11)	C10—C11—C12	119.4 (4)
N3—Cu1—N3^i	180.00 (13)	C10—C11—H11A	120.3
C1—O1—Cu1	122.5 (2)	C12—C11—H11A	120.3
C1—N1—N2	112.1 (3)	C11—C12—C13	120.7 (4)
C1—N1—C14	127.7 (3)	C11—C12—H12A	119.6
N2—N1—C14	120.1 (3)	C13—C12—H12A	119.6
C3—N2—N1	105.9 (3)	C8—C13—C12	119.7 (4)
C5—N3—C8	116.8 (3)	C8—C13—H13A	120.1
C5—N3—Cu1	126.6 (2)	C12—C13—H13A	120.1
C8—N3—Cu1	116.6 (2)	N1—C14—C15	113.5 (3)
O1—C1—N1	121.9 (3)	N1—C14—H14A	108.9
O1—C1—C2	131.7 (3)	C15—C14—H14A	108.9
N1—C1—C2	106.4 (3)	N1—C14—H14B	108.9
C1—C2—C5	122.8 (3)	C15—C14—H14B	108.9
C1—C2—C3	104.4 (3)	H14A—C14—H14B	107.7
C5—C2—C3	132.8 (3)	C16—C15—C14	113.5 (4)
N2—C3—C2	111.2 (3)	C16—C15—H15A	108.9
N2—C3—C4	116.8 (3)	C14—C15—H15A	108.9
C2—C3—C4	132.0 (4)	C16—C15—H15B	108.9
C3—C4—H4A	109.5	C14—C15—H15B	108.9
C3—C4—H4B	109.5	H15A—C15—H15B	107.7
H4A—C4—H4B	109.5	C15—C16—C17	114.1 (4)
C3—C4—H4C	109.5	C15—C16—H16A	108.7
H4A—C4—H4C	109.5	C17—C16—H16A	108.7
H4B—C4—H4C	109.5	C15—C16—H16B	108.7
N3—C5—C2	120.6 (3)	C17—C16—H16B	108.7
N3—C5—C6	121.3 (3)	H16A—C16—H16B	107.6
C2—C5—C6	118.1 (3)	C18—C17—C16	114.9 (4)
C5—C6—C7	111.9 (3)	C18—C17—H17A	108.6
C5—C6—H6A	109.2	C16—C17—H17A	108.6
C7—C6—H6A	109.2	C18—C17—H17B	108.6
C5—C6—H6B	109.2	C16—C17—H17B	108.6
C7—C6—H6B	109.2	H17A—C17—H17B	107.5
H6A—C6—H6B	107.9	C17—C18—C19	114.6 (5)
C6—C7—H7A	109.5	C17—C18—H18A	108.6
C6—C7—H7B	109.5	C19—C18—H18A	108.6
H7A—C7—H7B	109.5	C17—C18—H18B	108.6
C6—C7—H7C	109.5	C19—C18—H18B	108.6
H7A—C7—H7C	109.5	H18A—C18—H18B	107.6
H7B—C7—H7C	109.5	C18—C19—H19A	109.5
C9—C8—C13	118.8 (4)	C18—C19—H19B	109.5
C9—C8—N3	119.5 (3)	H19A—C19—H19B	109.5
C13—C8—N3	121.7 (3)	C18—C19—H19C	109.5
C10—C9—C8	120.6 (4)	H19A—C19—H19C	109.5
C10—C9—H9A	119.7	H19B—C19—H19C	109.5

Symmetry code: (i) −x+1, −y+1, −z+1.