Dibromo(pyridoxal semicarbazone-κ³N¹,O³,O^3′)copper(II)

The title compound, di­bromo(3-hydroxy-5-hydroxy­methyl-2-methyl-4-pyridine­carbox­aldehyde semicarbazone-κ³N¹,O³,O^3′)copper(II), [CuBr₂(C₉H₁₂N₄O₃)], consists of discrete complex units with the tridentate pyridoxal semicarbazone ligand as a zwitterion in an almost planar configuration. The Cu^II ions are in a distorted square-pyramidal coordination, with the equatorial Br atom at a distance of 2.4017 (6) Å and the apical Br atom at a distance of 2.6860 (6) Å.

S1. Comment

Semicarbazones (sc) and thiosemicarbazones (tsc) are excellent chelating ligands of different denticity, and a great number of transition metal complexes containing these ligands have been described in the literature (Campbell, 1975; Padhye & Kauffman, 1985; Casas et al., 2000). Many of these sc and tsc complexes, especially those with tsc, possess a broad spectrum of biological activity (West et al., 1991). In addition, since both kinds of ligands form very stable and intensely coloured complexes, some of them are suggested as analytical reagents (Singh et al., 1978).

The synthesis, properties and structures, as well as the biological activities, of some transition metal complexes with pyridoxal (3-hydroxy-5-hydroxymethyl-2-methyl-pyridine-4-carbaldehyde) thiosemicarbazone (Pxtsc) have already been described (Belicchi Ferrari et al., 1987, 1994, 1995, 1998). Most of them are copper(II) complexes, where Pxtsc in the neutral or deprotonated form acts as a tridentate chelate ligand.

It would be of interest to compare copper(II) complexes containing pyridoxal thiosemicarbazone as an O,N,S ligand with related complexes containing pyridoxal semicarbazone (Pxsc) as a potential O,N,O ligand. However, to the best of our knowledge, no structures of Pxsc complexes have been described to date. In order to make possible the comparison of Pxsc and Pxtsc complexes, the title compound, (I), has been prepared and its crystal structure is described in this paper. \sch

Compound (I) consists of discrete complex units, with Pxsc coordinated as a neutral tridentate ligand (Fig. 1). As expected, the phenolic O atom (O2) is deprotonated, but this H atom is shifted to N4; therefore, Pxsc is zwitterionic. Similar behaviour has already been observed in Cu^II-Pxtsc complexes (Belicchi Ferrari et al., 1987, 1995, 1998), while in only one case is Pxtsc deprotonated (Belicchi Ferrari et al., 1987). Nearly all the bond distances in the Pxsc ligand in (I) are very close to the corresponding values found previously in Pxtsc ligands. One of the exceptions is O3—C7 [1.362 (4) Å], which seems a little short. This can be attributed to the unresolved disorder (see Experimental) of the alcoholic group, since a riding motion calculation (Busing & Levy, 1964) gives a quite reasonable value of 1.412 Å. The other exception is N2—C9, which is about 0.02 Å longer than the mean value in corresponding Pxtsc complexes. This can be a consequence of the absence of an S atom in the vicinity.

The whole ligand in (I) is very close to coplanarity; if only skeletal atoms are considered, the greatest deviation from the least-squares plane is only 0.031 (2) Å for atom O2. At the same time, atoms C6, C7 and Cu deviate from this plane by 0.046 (3), 0.206 (4) and −0.1757 (4) Å, respectively.

The dihedral angles between the pyridoxal moiety (A), and the six- (B) and five-membered (C) chelate rings (Fig. 1) are as follows: A—B 4.70 (6), A—C 4.27 (7) and B—C 6.57 (5)°. On average, these values are smaller than the corresponding values in Cu^II-Pxtsc complexes, which, very probably, can be ascribed to the more extended electron delocalization in the Pxsc ligand.

The Cu^II atoms in (I) are in a distorted square-pyramidal environment, with atoms Br1, N1, O1 and O2 in the basal plane (Fig. 1). The Cu—O2 (phenolic) bond is shorter than Cu—O1 and Cu—N1 (Table 1). Both these observations are in agreement with similar features found in Cu^II-Pxtsc complexes. The Cu—O2 bond distance is close to the value found recently in one Cu^II N-salicylidene-rac-alaninato complex [1.965 (2) Å; Warda, 1998], where the ligand has a very similar skeleton. The basal and apical Cu—Br distances are significantly different (Table 1) and, using the criteria of Orpen et al. (1989), they belong to the classes of short and long Cu—Br bonds, respectively. Our experimental distances are very close to the mean values listed in the paper cited above.

The basal plane of the coordination polyhedron of (I) is tetrahedrally deformed, with a maximum distance from the least-squares plane of 0.209 (2) Å for N1. As expected, the Cu atom is displaced towards the apical Br2 atom by 0.3585 (4) Å. With respect to the plane defined by atoms N1, O1 and O2, the basal Br1 atom is displaced outwards by 0.6405 (4) Å.

Possible hydrogen bonds are listed in Table 2. Due to the presence of the N2—H2N···O3 bond, a centrosymmetric dimer with almost coplanar chelate ligands is formed. The O3—H3O···Br1 and N3—H3AN···Br1 hydrogen bonds further join such dimers into sheets approximately parallel to the plane (355). However, the apically coordinated Br2 atoms belong to neighbouring sheets, where they take part in two additional hydrogen bonds, connecting the sheets and stabilizing the overall structure (Table 2).

S2. Experimental

The title complex was prepared by the reaction of CuBr₂·2H₂O and Pxsc (in a 1:1 molar ratio). Warm MeOH was used as the solvent. After standing overnight, green single crystals of (I) of suitable size were obtained.

S3. Refinement

With the exception of the H atoms belonging to the CH₂OH group (Fig. 1), it was possible to find all H atoms in the ΔF maps. However, at the final stage of the refinement, the H atoms were positioned geometrically (N—H 0.86, O—H 0.82 and C—H 0.93–0.97 Å) Is this added text OK? and refined using a riding model, with fixed isotropic displacement parameters 20% larger than those of the attached atoms. The H atom of the OH group was modelled using option HFIX 143 in SHELXL97 (Sheldrick, 1997). It is worth noting that an attempt to model this H atom with the program HYDROGEN (Nardelli, 1999) resulted in a nearly identical geometry. Two electron-density peaks on the final ΔF map, 1.16 and 0.85 e Å⁻³, located near atoms O3 and C7, respectively, together with an elongated O3 ellipsoid (Fig. 1), indicated disorder of the alcoholic group. However, all attempts to model this disorder were unsuccessful. Similar disorder has also been observed in Pxtsc complexes (Belicchi Ferrari et al., 1987).

Computing details

Data collection: SMART-NT (Bruker, 1998); cell refinement: SAINT (Siemens, 1996); data reduction: SHELXTL (Bruker, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEX8a (McArdle, 1995; Burnett & Johnson, 1996); software used to prepare material for publication: SHELXL97 and PARST (Nardelli, 1983, 1995).

Figures

Fig. 1. The molecular structure of (I), with the atom- and ring-numbering schemes. Displacement ellipsoids are plotted at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.

Dibromo(pyridoxal semicarbazone-κ³N¹,O³,O^3')copper(II)

Crystal data

[CuBr2(C9H12N4O3)]	Z = 2
Mr = 447.59	F(000) = 434
Triclinic, P1	Dx = 2.218 Mg m^−3
a = 7.5764 (10) Å	Mo Kα radiation, λ = 0.71073 Å
b = 8.7614 (11) Å	Cell parameters from 769 reflections
c = 10.8154 (14) Å	θ = 4.9–56.7°
α = 92.965 (3)°	µ = 7.60 mm^−1
β = 97.881 (2)°	T = 298 K
γ = 108.709 (3)°	Plate, green
V = 670.05 (15) Å^3	0.18 × 0.14 × 0.07 mm

Data collection

Bruker Model CCD area-detector diffractometer	4057 independent reflections
Radiation source: fine-focus sealed tube	2678 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.037
Detector resolution: 81.92 pixels mm^-1	θmax = 30.5°, θmin = 3.0°
ϕ and ω scans	h = −10→10
Absorption correction: empirical (using intensity measurements) (XPREP in SHELXTL; Bruker, 1997)	k = −12→12
Tmin = 0.291, Tmax = 0.587	l = −15→15
7646 measured reflections

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) + (0.0213P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 0.85	(Δ/σ)max = 0.001
4057 reflections	Δρmax = 1.16 e Å^−3
174 parameters	Δρmin = −0.56 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: heavy-atom method	Extinction coefficient: 0.0016 (3)

Special details

Experimental. Preliminary cell constants are calculated from 119 strong reflections (120 frames, each 0.3° in ω). Final cell parameters are determined using 769 reflections in a global refinement of data obtained after integration of intensities. During data collection, five sets of exposures in combination of ω and ϕ scans nominally covered over a hemisphere of the reciprocal space. Each exposure was collected 20 s and covered 0.25° in ω. The crystal-to-detector distance was 3.8 cm.

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
Br1	1.06532 (5)	0.76179 (4)	0.71736 (4)	0.04243 (11)
Br2	0.68362 (5)	0.95380 (4)	0.84096 (3)	0.03304 (10)
Cu	0.73454 (5)	0.72847 (4)	0.68497 (4)	0.02544 (10)
O1	0.7549 (3)	0.8643 (2)	0.5434 (2)	0.0292 (5)
O2	0.6829 (3)	0.5569 (2)	0.7888 (2)	0.0339 (5)
O3	−0.1085 (4)	0.3557 (3)	0.6695 (4)	0.0867 (12)
H3O	−0.0496	0.4293	0.7246	0.130*
N1	0.4704 (3)	0.6393 (2)	0.5941 (2)	0.0223 (5)
N2	0.4442 (3)	0.7228 (3)	0.4936 (2)	0.0280 (6)
H2N	0.3364	0.7016	0.4465	0.034*
N3	0.5843 (4)	0.9275 (3)	0.3789 (3)	0.0325 (6)
H3AN	0.6816	1.0025	0.3630	0.039*
H3BN	0.4761	0.9085	0.3329	0.039*
N4	0.3654 (4)	0.2429 (3)	0.9149 (2)	0.0283 (6)
H4N	0.3695	0.1848	0.9763	0.034*
C1	0.5263 (4)	0.3524 (3)	0.8985 (3)	0.0238 (6)
C2	0.5213 (4)	0.4534 (3)	0.7997 (3)	0.0232 (6)
C3	0.3467 (4)	0.4293 (3)	0.7224 (3)	0.0218 (6)
C4	0.1826 (4)	0.3048 (3)	0.7442 (3)	0.0275 (7)
C5	0.1973 (4)	0.2158 (3)	0.8434 (3)	0.0317 (7)
H5C	0.0906	0.1368	0.8610	0.038*
C6	0.7024 (4)	0.3646 (4)	0.9819 (3)	0.0326 (7)
H6AC	0.7670	0.4756	1.0146	0.049*
H6BC	0.7818	0.3265	0.9354	0.049*
H6CC	0.6729	0.2998	1.0500	0.049*
C7	−0.0058 (5)	0.2559 (4)	0.6541 (4)	0.0379 (8)
H71C	0.0192	0.2549	0.5686	0.045*
H72C	−0.0806	0.1466	0.6663	0.045*
C8	0.3292 (4)	0.5236 (3)	0.6186 (3)	0.0249 (6)
H8C	0.2122	0.4999	0.5680	0.030*
C9	0.6012 (4)	0.8416 (3)	0.4739 (3)	0.0237 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.02325 (17)	0.0475 (2)	0.0602 (3)	0.01147 (14)	0.01120 (16)	0.02805 (18)
Br2	0.0451 (2)	0.03091 (16)	0.02657 (18)	0.01427 (13)	0.01006 (14)	0.01159 (13)
Cu	0.02213 (19)	0.02415 (17)	0.0257 (2)	0.00150 (14)	0.00129 (15)	0.01071 (15)
O1	0.0267 (11)	0.0275 (10)	0.0274 (12)	0.0010 (8)	0.0013 (10)	0.0109 (9)
O2	0.0239 (11)	0.0326 (11)	0.0382 (14)	−0.0003 (9)	−0.0010 (10)	0.0201 (10)
O3	0.0464 (18)	0.0639 (19)	0.137 (3)	0.0231 (15)	−0.027 (2)	−0.026 (2)
N1	0.0249 (12)	0.0187 (11)	0.0225 (14)	0.0057 (9)	0.0032 (10)	0.0081 (10)
N2	0.0252 (13)	0.0275 (12)	0.0290 (15)	0.0062 (10)	−0.0015 (11)	0.0159 (11)
N3	0.0339 (15)	0.0317 (13)	0.0330 (16)	0.0094 (11)	0.0071 (12)	0.0185 (12)
N4	0.0356 (15)	0.0283 (12)	0.0221 (14)	0.0092 (11)	0.0073 (12)	0.0142 (11)
C1	0.0296 (16)	0.0204 (13)	0.0205 (16)	0.0070 (11)	0.0034 (13)	0.0033 (12)
C2	0.0239 (15)	0.0199 (13)	0.0234 (16)	0.0033 (11)	0.0054 (12)	0.0032 (12)
C3	0.0243 (15)	0.0177 (12)	0.0228 (16)	0.0049 (10)	0.0051 (12)	0.0063 (11)
C4	0.0245 (15)	0.0239 (14)	0.0321 (19)	0.0047 (12)	0.0040 (14)	0.0068 (13)
C5	0.0270 (17)	0.0295 (15)	0.0352 (19)	0.0018 (12)	0.0082 (14)	0.0141 (14)
C6	0.0346 (18)	0.0327 (16)	0.0294 (18)	0.0116 (13)	−0.0013 (14)	0.0075 (14)
C7	0.0308 (18)	0.0288 (16)	0.051 (2)	0.0018 (13)	0.0129 (16)	0.0077 (15)
C8	0.0223 (14)	0.0231 (13)	0.0268 (17)	0.0046 (11)	0.0015 (13)	0.0062 (12)
C9	0.0292 (16)	0.0210 (13)	0.0217 (16)	0.0079 (11)	0.0068 (13)	0.0037 (12)

Geometric parameters (Å, º)

Br1—Cu	2.4017 (6)	N4—C5	1.339 (4)
Br2—Cu	2.6860 (6)	N4—H4N	0.8600
Cu—O2	1.895 (2)	C1—C2	1.426 (4)
Cu—O1	1.980 (2)	C1—C6	1.474 (4)
Cu—N1	1.990 (2)	C2—C3	1.411 (4)
O1—C9	1.247 (3)	C3—C4	1.424 (4)
O2—C2	1.294 (3)	C3—C8	1.444 (4)
O3—C7	1.362 (4)	C4—C5	1.372 (4)
O3—H3O	0.8200	C4—C7	1.534 (5)
N1—C8	1.286 (3)	C5—H5C	0.9300
N1—N2	1.368 (3)	C6—H6AC	0.9600
N2—C9	1.358 (3)	C6—H6BC	0.9600
N2—H2N	0.8600	C6—H6CC	0.9600
N3—C9	1.321 (4)	C7—H71C	0.9700
N3—H3AN	0.8600	C7—H72C	0.9700
N3—H3BN	0.8600	C8—H8C	0.9300
N4—C1	1.329 (3)
O2—Cu—O1	165.90 (9)	O2—C2—C1	115.0 (3)
O2—Cu—N1	89.77 (9)	C3—C2—C1	118.4 (2)
O1—Cu—N1	80.23 (9)	C2—C3—C4	119.4 (3)
O2—Cu—Br1	91.72 (7)	C2—C3—C8	122.2 (2)
O1—Cu—Br1	93.06 (6)	C4—C3—C8	118.4 (3)
N1—Cu—Br1	154.38 (7)	C5—C4—C3	118.8 (3)
O2—Cu—Br2	97.85 (7)	C5—C4—C7	118.4 (3)
O1—Cu—Br2	93.30 (6)	C3—C4—C7	122.5 (3)
N1—Cu—Br2	97.30 (7)	N4—C5—C4	119.9 (3)
Br1—Cu—Br2	107.81 (2)	N4—C5—H5C	120.1
C9—O1—Cu	113.6 (2)	C4—C5—H5C	120.1
C2—O2—Cu	128.5 (2)	C1—C6—H6AC	109.5
C7—O3—H3O	109.5	C1—C6—H6BC	109.5
C8—N1—N2	118.9 (2)	H6AC—C6—H6BC	109.5
C8—N1—Cu	129.9 (2)	C1—C6—H6CC	109.5
N2—N1—Cu	111.2 (2)	H6AC—C6—H6CC	109.5
C9—N2—N1	114.9 (2)	H6BC—C6—H6CC	109.5
C9—N2—H2N	122.6	O3—C7—C4	113.8 (3)
N1—N2—H2N	122.6	O3—C7—H71C	108.8
C9—N3—H3AN	120.0	C4—C7—H71C	108.8
C9—N3—H3BN	120.0	O3—C7—H72C	108.8
H3AN—N3—H3BN	120.0	C4—C7—H72C	108.8
C1—N4—C5	125.1 (3)	H71C—C7—H72C	107.7
C1—N4—H4N	117.4	N1—C8—C3	122.0 (3)
C5—N4—H4N	117.4	N1—C8—H8C	119.0
N4—C1—C2	118.3 (3)	C3—C8—H8C	119.0
N4—C1—C6	119.2 (3)	O1—C9—N3	122.5 (3)
C2—C1—C6	122.6 (3)	O1—C9—N2	119.8 (3)
O2—C2—C3	126.6 (3)	N3—C9—N2	117.7 (3)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3O···Br1^i	0.82	2.77	3.358 (3)	130
N2—H2N···O3^ii	0.86	1.90	2.752 (4)	174
N3—H3AN···Br1^iii	0.86	2.61	3.457 (2)	170
N3—H3BN···Br2^iv	0.86	2.65	3.338 (3)	138
N4—H4N···Br2^v	0.86	2.39	3.226 (3)	165

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