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Structural Study of a Binuclear Zinc Complex of 2,6-Diacetylpyridine Bis{N(4)-dimethylthiosemicarbazone}, [Zn(2,6Ac4DM)]2 • MeCNElena Labisbal3, Alfonso Castiñeiras3, Christine A. Brownb, and Douglas X. Westb3 Departamento de Quimica Inorgänica, Universidade de Santiago de Compostela,

E -15706 Santiago de Compostela, Galicia, Spain b Department of Chemistry, Illinois State University, Normal, IL-61790-4160, USA

Reprint requests to Dr. E. Labisbal. Fax: +34-81-547163. E-mail: qilabis@correo.lugo.usc.es

Z. Naturforsch. 56 b, 229-232 (2001); received December 4, 2000Zinc, 2,6-Diacetylpyridine Bis{N(4)-dimethylthiosemicarbazone}, Electrochemical Synthesis

Zinc metal was oxidized in the presence of 2,6-diacetylpyridine bis{N(4)-dimethylthiosemi- carbazone} (2 ,6H2Ac4DM) in an acetonitrile solution, to produce a complex of the formula [Zn(2 ,6Ac4DM)]2-CH3CN. One of the zinc atoms is distorted octahedrally 6-coordinate and the coordination involves both pyridyl nitrogen atoms of the two ligands, the imine nitrogen atom and the thiolato sulfur atom of one thiosemicarbazone moiety of each ligand. The other zinc center is distorted tetrahedrally 4-coordinate with NS coordination of the two remaining thiosemicarbazone moieties. The complex crystallizes in the monoclinic space group C2/c with a = 14.952(2), b = 17.656(3), c = 15.553(4) A, ß = 111.456(16)°, V= 3821.5(13) A andZ = 4.

Introduction

There is considerable interest in 2,6-diacetyl- pyridine-bis(thiosemicarbazone) and its metal com­plexes [1], Crystal structures have been reported for three different zinc complexes, two of which are binuclear [2]. Of these two binuclear complexes, one features distorted octahedral coordination of the zinc atoms that are nearly equivalent. The second binuclear complex contains a distorted octahedrally and a tetrahedrally coordinated zinc center. More re­cently, the former of these binuclear complexes has been prepared under different conditions and a new crystal structure has been reported [3]. Although spectral and biological studies have been carried out on metal complexes of 2 ,6 -diacetylpyridine-bis- {N(4)-substituted thiosemicarbazones} [4, 5] until recently [6 ] structural information has been lack­ing. The N(4)-substituent in other types of thiosemi­carbazones, including bis(thiosemicarbazones), has been shown to affect their biological activity [7].

Since zinc(II) complexes [1 ,8] have significant pharmacological activity and little research has been done on N-heterocyclic bis[N(4)-substituted thio­semicarbazones], we report in this work the X- ray crystal structure of [Zn(2 ,6 Ac4 DM)]2 -CH3CN (2,6Ac4DM is the dianion of 2,6-diacetylpyridine- bis(N(4)-dimethylthiosemicarbazone) and compare

its structural properties to [Zn(2,6Achexim) ] 2

where 2 ,6 H2Achexim is 2 ,6 -diacetylpyridine-bis- (3-hexamethyleneiminylthiosemicarbazone) [6 ]. A probable structural formula for 2 ,6 H2 Ac4DM based on spectral properties similar to 2 ,6 H2Achexim (its structure was recently reported [6 ]):

Me

ExperimentalReagents, materials and syntheses

2 ,6H2Ac4DM was prepared by condensation un­der reflux in EtOH of 2,6-diacetylpyridine (0.005 mol, 0.815 g, Aldrich) with 0.01 mol (1.19 g) of N(4)-dime- thylthiosemicarbazide, which was prepared as described by Scovill [9]. The resulting yellow product (yield = 76%) was washed with isopropanol and diethyl ether and stored in a vacuum desiccator over calcium sul­fate. [Zn(2 ,6Ac4DM)]2-CH3CN was prepared by electro-

0932-0776/01/0300-0229 S 06.00 © 2001 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com K

230 E. Labisbai et al. • Structural Study o f a Binuclear Zinc Complex

Table 1. Crystal data and structure refinement for [Zn(2,6Ac4DM)]2-CH3CN.

Table 2. Selected bond distances [A] and angles (°) for [Zn(2,6Ac4DM)]2-CH3CN.

Empirical formula C32H45Ni5S4Zn2 Znl-N12 2.029(6) Znl-Nl 2.476(6)Color, habit yellow, prism Znl-Sl 2.509(2) Zn2-N22 1.989(6)Formula weight 898.89 Zn2-S2 2.338(3) S1-C17 1.706(8)Temperature [K] 293(2) S2-C27 1.742(9) N12-C16 1.306(10)Crystal system monoclinic N22-C26 1.338(9) N12-N13 1.396(8)Space group C2/c (No. 15) N22-N23 1.409(8) N13-C17 1.324(10)a [A] 14.952(2) N23-C27 1.325(10) N14-C17 1.341(10)MA] 17.656(3) N24-C27 1.330(11)c [A] 15.533(4) N12-Znl-N12a 176.6(4) N12-Znl-Nl 71.5(2)/n ° ] „ 111.456(16) N12-Znl-Nla 106.3(2) N12-Znl-Sl 80.48(19)Volume [A' ] 3821.5(13) N12-Znl-Sla 101.73(19) N l-Z nl-N la 101.3(3)Z 4 N l-Znl-Sl 151.96(16) N l-Z nl-S la 86.15(16)Density [mg/m3] 1.562 S l-Z nl-S la 99.93(13) N22-Zn2-N22a 135.2(4)Absorption coeff. [mm-1] 3.973 N22-Zn2-S2 85.54(19) N22-Zn2-S2a 117.18(19)F(000) 1864 S2-Zn2-S2a 120.30(6) Znl-Sl-C17 95.7(3)Crystal size [mm] 0.20 x 0.16 x 0.08 Znl-N12-C16 127.0(6) Znl-N12-N13 120.9(5)9 Range for data coll. [°] 4.04 - 75.96 Zn2-S2-C27 95.1(3) Zn2-N22-C26 126.9(5)Index ranges -18 < /i < 0 , 0 < 22 , Zn2-N22-N23 117.1(5) C16-N12-N13 111.3(6)

-18 < / < 19 N12-N13-C17 120.5(6) N13-C17-N14 115.9(8)Collected reflections 4151 N13-C17-S1 122.3(6) N14-C17-S1 121.8(7)Independent reflections 3998 [/?,nt = 0.038] C26-N22-N23 113.3(6) N22-N23-C17 119.7(7)Absorption correction t/j-scan N23-C27-N24 119.4(8) N23-C27-S2 121.5(7)Max. / min. transmission 0.966/0.887 N24-C27-S2 119.0(6)Data / parameters Goodness-of-Fit on F 2 Final R indices [/ > 2cr(/)] R Indices (all data)Largest diff. peak/hole [eA- 3

1.028R{ = 0.077, wR2 = 0.209 Ri = 0.190, wR2 = 0.267 1.065/-0.570

chemical oxidation of zinc in a solution of 2,6H2 Ac4DM (0.093 mmol, 31.4 mg) in CH3CN (40 ml) containing 10 mg of tetraethylammonium perchlorate for 1 h at 5 mA. A loss of 5.8 mg of the anode resulted.

X-ray data collection and processing

A yellow prismatic crystal of [Zn(2,6Ac4DM)]2 • CH3CN, grown from a 1:1 v:v mixture of MeOH and CH3CN, was mounted on a glass fiber and used for data collection. Reflections were collected with a Non­ius CAD4 diffractometer, Cu-KQ (A = 1.54184 Ä). The cell constants and an orientation matrix for data collection were obtained by least-squares refinement of the diffrac­tion data of 15 reflections from 9.61 ° < 9 < 21.65°. The structure was solved by direct methods [10] and sub­sequent difference Fourier maps and refined on F 2 by a full-matrix least-squares procedure using anisotropic displacement parameters [11]. All hydrogen atoms were located in their calculated positions (C-H 0.93 - 0.97 A) and were and refined using a riding model. A disordered CH3CN molecule was located on a difference map and re­fined (1:1). Despite large correlation coefficients between

the parameters of the three atoms, this model appeared to be the best possible and no further attempt was made to improve it. Atomic scattering factors are from the “Inter­national Critical Tables for X-ray Crystallography” [12], and molecular graphics are from PLATON99 [13]. Sum­maries of X-ray data collection, experimental details and refinement results are listed in Table 1.

Results and Discussion

Table 2 lists selected bond distances and an­gles for [Zn(2 ,6 Ac4 DM)]2 -CH3CN. In contrast to [Zn(2 ,6 Achexim)]2 [6 ], which has two dis­torted tetrahedral zinc centers, [Zn(2 ,6 Ac4 DM )]2

has 6 -coordinate and 4-coordinate zinc centers, as was found for one of the binuclear zinc com­plexes of 2 ,6 -diacetylpyridine-bis(thiosemicarb- azone), H22,6Ac4DH [2]. In contrast to [Zn(2,6Ac- 4 DH)]2 [2 ] the bond distances for the two thiosemi­carbazone moieties coordinating to each zinc have equal bond distances, as do the pyridine nitrogen atoms (designated NI and N la in Fig. 1), but the octahedrally coordinated zinc has longer coordinate bonds than the tetrahedrally coordinated one. As ex­pected, the Znl-Sl bonds (i. e., octahedrally coordi­nate zinc), 2.509(3) A, are somewhat longer than the Zn2-S2 bonds, 2.338(3) A, and the Znl -N bonds are

E. Labisbai et al. • Structural Study o f a Binuclear Zinc Complex 231

Table 3. Rms planes for [Zn(2 ,6Ac4DM)]2-CH3CN.

Compound Plane rms dev. [A] largest deviation z [ ° ] a

[Zn(2,6Ac4DM)]2 C16a-N 12a-N 13a-C 17a-S 1 a-N 14a C16-N12-N13-C17-S1-N14 C26-N22-N23-C27-S2-N24 N 1 -C1 -C14-C15-C24-C25 C16-N12-N13-C17-S1-N14

0.07470.07470.11860.0090b

N12a, 0.0941(0.0056) N12, 0.0941(0.0056) N22, 0.2077(0.0056)C l5, 0.0133(0.0060)b

76.10(0.12)73.97(0.12)64.25(0.17)19.42(0.31)

a Angle with previously listed plane;b shown above.

Fig. 1. Perspective view of [Zn(2 ,6Ac4DM)]2, draw at 50% probability displacement ellipsoids.

much longer than the imine Znl-N12, 2.029(6) A. The pyridine Zn-N bond for the octahedral center of [Zn(2,6Ac4DH) ] 2 has an average bond distance of 2.36(1) A [2] suggesting that the N(4)-dimethyl function may exert a small steric effect. Similarly, the Zn-S bonds, 2.509(2) A, in [Zn(2,6Ac4DM) ] 2

are longer than the average bond in [Zn(2,6Ac- 4 DH)]2 , 2.421(4) A, but the imine Zn-N bonds are shorter, 2.029(6) A compared to 2.11(1) A.

Consistent with a lower coordination number the Zn-N and Zn-S bonds for the tetracoordi- nate zinc center are substantially shorter than the analogous bonds to the hexacoordinate zinc of [Zn(2 ,6 Ac4 DM)]2 . A comparison of the average Zn-S bonds with the tetracoordinate zinc center(s)

in [Zn(2 ,6 Achexim)]2 [6 ] and [Zn(2 ,6 Ac4 DH)]2 [2 ] shows that the Zn-S bond distance is affected by the bulkiness of the substituents on the thiosemi- carbazone moiety: [Zn(2 ,6 Achexim)]2 , 2.358(2) A; [Zn(2,6Ac4DM)]2, 2.338(3) A and [Zn(2,6Ac- 4DH)]2, 2.325(4) A. However, [Zn(2,6Ac4DM) ] 2

has a considerably shorter imine Zn-N bond dis­tance, 1.989(6) A, compared to [Zn(2 ,6 Achexim)]2 , 2.052(6) A [6 ] and [Zn(2,6Ac4DH)]2, 2.07(1) A [2].

The imine C-N and thiolato C-S bond dis­tances of the thiosemicarbazone moieties bonded to the octahedral and tetrahedral zinc centers in [Zn(2,6Ac4DM) ] 2 (Table 2 ) indicate the stronger coordinate bonds to the latter. Similar differences exist for the remaining thiosemicarbazone bond distances, as well as with the tetrahedral centers of [Zn(2 ,6 Achexim)]2 , and [Zn(2 ,6 Ac4 DH)]2 . The longer N-N bond distance of is [Zn(2 ,6 Ac4 DM )]2

consistent with stronger coordination by the imine nitrogen and thiolato sulfur atoms.

The distortion from tetrahedral symmetry is substantial; the largest bond angle is N(imine)- Zn-N(imine), 135.2(4)°, the S-Zn-S angle is 120.30(6)°, and the smallest is a chelating S- Zn-N(imine) angle, 85.54(19)°. This compares to 159.2(2)°, 114.6(1)° and 82.9(2)° for N-Zn-N, S- Zn-S and S-Zn-N, respectively of [Zn(2,6Achex- im)]2 - For the octahedral-tetrahedral binuclear com­plex, [Zn(2,6Ac4DH)]2, the angles around the tetrahedral zinc center are as follows: N-Zn-N, 140.8(4)°, S-Zn-S 118.3(2)° and S-Zn-N, 83.8(4)°[2 ], which are somewhat closer to the angles of [Zn(2,6Ac4DM)]2.

The mean plane deviations, the atom with the greatest deviation and the angle between the planes are compiled in Table 3. The two thiosemicarbazone moieties at the octahedrally coordinated zinc cen­ter, have identical rms deviations of 0.0747 A, with N12 (and N12a) having the greatest devia­

232 E. Labisbai e t al. ■ Structural Study o f a Binuclear Zinc Complex

tions from the plane, and the two planes are at an angle of 76.10(0.12)°. The difference from 90° in­dicates the distortion from octahedral symmetry. The plane of the second thiosemicarbazone moi­ety of the bis(thiosemicarbazone) that is attached to the tetrahedrally coordinated zinc forms an angle of 73.97(0.12)° with the aforementioned thiosemi­carbazone plane. The pyridyl ring forms an angle of 64.35(0.17)° with the thiosemicarbazone moiety attached to Zn2 and 19.42(0.31)° with the thiosemi­carbazone moiety attached to Znl.

Preparations of other bis(thiosemicarbazones) with zinc, as well as cadmium, are underway, in order to establish a potential structure/biological ac­tivity relationship of the resulting complexes.

Supplem entary m ateria l

Crystallographic data (excluding structure fac­tors) have been deposited with the Cambridge Crys­tallographic Data Centre as supplementary publi­cation No. CCDC-152184. Copies of data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CD2 1EZ, UK (Fax: +44- 1223/336-033; E-mail: deposit@ccdc.cam.ac.uk).

Acknowledgements

E. L. thanks the Universidade de Santiago de Com­postela for financial support to carry out research at Illi­nois State University.

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