Structural Studies of 2 , 6-Diacetyl-and 2 , 6-Diformylpyridine Bis ( thiosemicarbazones )

Embora um grande número de estruturas tiosemicarbazonas heterocíclicas tenham aparecido na literatura recentemente, poucas estruturas de bis(tiosemicarbazonas) heterocíclicas e dos seus complexos metálicos foram descritas. Complexos de ferro (II), índio (II), estanho (IV) e bismuto (III) contêm bis(tiosemicarbazonas) coordenadas na forma de ligantes pentadentados N 3 S 2 , freqüentemente resultando em complexos heptacoordenados. Em comparação, os complexos de zinco (II) são freqüentemente binucleares e contêm as frações tiosemicarbazonas do ligante bis(tiosemicarbazona) coordenadas a dois diferentes centros de zinco. Foi também incluída neste estudo a estrutura do primeiro complexo de níquel (II) que contém um ligante 2,6-diformilpiridina bis(tiosemicarbazona) em uma coordenação incomum.


Iron(II) complexes
The first transition metal complex of a heterocyclic bis(thiosemicarbazone) reported was an iron(II) complex, [Fe(H 2 2,6Ac4DH)(NCS) 2 ], where H 2 2,6Ac4DH is 2,6diacetylpyridine bis(thiosemicarbazone). 30 The complex, which crystallized in the tetragonal P4 1 2 1 2 space group with 4 molecules in the unit cell, is an approximate pentagonal bipyramid with the N-thiocyanato ligands in the apical positions, Figure 3.The NCS groups are linear with FeNC angles of 178.7(7)º and interact with the water of crystallization (non-bonding O⋅⋅⋅SCN distance of 3.4 Å).It should be noted that this complex was obtained by reacting iron(III) with H 2 2,6Ac4DH, indicating that reduction, more than likely by the bis(thiosemicarbazone), takes place during synthesis.Although a number of other transition metal complexes have been prepared and tested for biological activity, 31 to our knowledge no other crystal structures have been reported.However, structures of transition metal complexes with 2,6-diacetylpyridine bis(semicarbazone) continue to be an area of study. 32

Indium(III) complexes
Two complexes involving the monoanion {i.e., one of the thiosemicarbazone moieties has lost N(3)H} of 2,6diacetylpyridine bis(thiosemicarbazone), H 2 2,6Ac4DH, are included in a recent article. 33Crystals of [In(H2,6Ac4DH)Cl 2 ]⋅DMSO, which is sparingly soluble  in most solvents, were obtained by dissolution in boiling dimethyl sulfoxide followed by cooling the solution at 10º/h.The complex has monoanionic H2,6Ac4DH coordinating as a pentadentate ligand via the pyridine nitrogen, two imine nitrogen atoms and the thione and thiolato sulfur atoms with chloro ligands in the apical positions of an approximate pentagonal bipyramid, Figure 3.This complex provides a comparison of two different thiosemicarbazone moieties in the same complex.It should be noted that there are several complexes with heterocyclic tridentate thiosemicarbazone ligands that have two forms of different charge attached to the same metal, 34,35 as well as inequivalent coordination of the same form. 36Listed in Table 1 are selected bond distances for the two forms, and there is considerable difference.Bonds to indium(III) for the anionic thiosemicarbazone moiety are shorter than the analogous bonds of the neutral moiety.The thione C-S bond in the neutral moiety remains a formal double bond making it shorter than the C-S bond of the anionic moiety, and the N(3)-C bond (Figure1) which formally becomes a double bond in the anionic moiety, is longer in the neutral moiety.The In-Npy bond is 2.330(5) Å, the apical chloro distances show a small difference, 2.497(2) and 2.535(2) Å, and the Cl-In-Cl angle is 166.02(5)ºindicating significant distortion from a regular stereochemistry.
An interesting second complex with the monoanionic bis(thiosemicarbazone) of 2,6-diacetylpyridine was prepared with In(NO 3 ) 3 ⋅5H 2 O. 33 With monoanionic H2,6Ac4DH coordinating as a planar pentadentate ligand as in [In(H2,6Ac4DH)Cl 2 ], {O[In(H2,6Ac4DH)(OH)] 2 } ⋅5MeOH consists of two In(III) centers bridged by an oxido ligand and hydroxo ligands at the terminal, apical positions.The two In(III) centers, bridged by the oxido ligand, are like the center shown in Figure 3, and are rotated ca.180º to each other, the In-O bonds are about 0.1 Å shorter than the In-OH bonds and the In-O-In angle is 179.8(5)º.It was not possible to locate all of the hydrogens in the final Fourier map, but the differences in bond distances in the anionic and neutral thiosemicarbazone moieties are similar to those of [In(H2,6Ac4DH)Cl 2 ].

Tin(IV) complexes
Complexes of the unsubstituted 2,6-diacetylpyridine bis(thiosemicarbazone), H 2 2,6Ac4DH, have received considerable attention and a number of 7-coordinate tin(IV) complexes have been reported.The hydrochloride salt of H 2 2,6Ac4DH was dissolved in dry MeOH and the various Sn(IV) species also dissolved in MeOH were added and the mixtures refluxed for an hour.Neutral complexes with the dianionic bis(thiosemicarbazone) resulted on loss of HCl and cationic complexes with the monoanionic thiosemicarbazone/thiosemicarbazonato ligand {i.e., only one of the moieties loses N(3)H} are formed and 2,6Ac4DH and H2,6Ac4DH both coordinate about the Sn in an approximate pentagonal plane.Two apical positions are occupied by monodentate aryl, alkyl or halo ligands and raise the coordination number to seven for approximate pentagonal bipyramids.The bond distances involving tin(IV) and the bis(thiosemicarbazone) ligands are listed in Table 2 for the complexes discussed below.

Cationic tin(IV) complexes
Crystals of [Sn(H2,6Ac4DH)(Me)Cl]Cl⋅MeOH 38 were acquired by slow evaporation of the preparative solution, and the structure was solved to be monoclinic P2 1 /c with Z = 4.With the monoanionic ligand this is another complex that provides an interesting contrast between its two thiosemicarbazone moieties, as well as tin(IV) complexes with dianionic 2,6Ac4DH.Tin(IV)-ligand bond distances, Table 2, are listed together for the two moieties to allow easy comparison.Also included in this article is the bis(semicarbazone) complex, [Sn(2,6AcO4DH)(Me)Cl], which has the following bond distances: Sn-N(Py) = 2.262(2) Å and Sn-N(im) = 2.284(7) and 2.252(7) Å.

Bismuth(III) complexes
Reaction between BiCl 3 and H 2 2,6Ac4DH in acetone, evaporation to dryness followed by dissolution of the powder in DMSO, addition of excess NaN 3 and slow evaporation of this solution produced red crystals of [Bi(2,6Ac4DH)(N 3 )]⋅0.5DMSO,P2 1 /n and Z = 2. 40 2,6Ac4DH coordinates via the usual pyridine nitrogen, imine nitrogen atoms and thiolato sulfur atoms and the essentially linear azido ligand, ∠N-N-N = 178(3)°, brings the coordination number to 6 resulting in an approximate pentagonal pyramid structure.The azido ligand coordinates in the expected fashion with ∠Bi-N-N = 121(1)°.Bi-S bond distances are 2.717(8) and 2.685(7) Å, Bi-N(2) = 2.58(2) and 2.46(2) Å and Bi-N(1) is listed as 2.44(2) Å although it would seem based on other complexes of 2,6Ac4DH that this value should be longer {e.g., 2.58(2) Å}.The Bi-N(azido) distance is 2.25(2) Å.The packing of the molecules is by means of contacts with the other terminal nitrogen of the azido ligand and the unoccupied apical position on the Bi of an adjacent molecule.

Zinc(II) complexes
Like zinc complexes of the 2,6-diacetylpyridine bis(semicarbazone), 41 heterocyclic bis(thiosemicarbazone) complexes with zinc show greater variation in their mode of coordination than the previously discussed complexes.For example, an early article 42 contains structures of both mononuclear and binuclear zinc complexes with two different modifications of the latter.

Binuclear zinc(II) complexes
Two binuclear zinc(II) complexes were reported in the same article. 42One of the complexes, [Zn(2,6Ac4DH)] 2 ⋅MeOH⋅2DMF, features two zinc centers reported to have distorted octahedral symmetry, Figure 4a.This complex crystallizes from DMF in the monoclinic C2/c space group with four molecules in the unit cell, but only half a [Zn(2,6Ac4DH)] 2 in the asymmetric unit with the other half related to it by a crystallographic two-fold axis.Each zinc atom in the binuclear complex is coordinated to two sulfur and two nitrogen {i.e., N(2)} atoms from two different 2,6Ac4DH ligands.The two pyridine nitrogens have weak interactions that "bridge" the zinc centers of the binuclear unit to bring the coordination number to six.The pyridine nitrogens are closer to one zinc, 2.571(4) Å, than the other, 2.705(5) Å and the Zn-N-Zn angle is 94.2(2)º with a Zn-Zn separation of 3.866(1) Å.The other Zn-ligand bond distances are listed in Table 3, and it appears based on the other complexes that "coordination" of the pyridine nitrogens could be described as a weak interaction and that the two zinc(II) centers could be considered as 5coordinate, or even 4-coordinate.The S-Zn-S angle is 110.75 (7)º suggesting tetrahedral symmetry, but the N(2)-Zn-N(2) angle is 152.5(1)º, more consistent with distorted octahedral or trigonal bipyramid symmetry (i.e., N(2)'s in apical positions).This same complex was reported by another group with essentially the same structural characteristics a few years later. 43second binuclear complex, [Zn(2,6Ac4DH)] 2 ⋅MeOH⋅H 2 O, is a structural isomer of the aforementioned [Zn(2,6Ac4DH)] 2 .MeOH .2DMF in terms of the zinc centers.42 One zinc atom is coordinated to two sulfur and four nitrogen donors {i.e., two N(2) atoms and two inequivalently coordinated pyridine nitrogen atoms, Table 3} in a distorted octahedral arrangement, Figure 4b.The second zinc(II) is coordinated to two sulfur and two imine nitrogen donors in a distorted tetrahedral arrangement {i.e., the pyridine nitrogens are 2.737(9) and 2.909(9) Å from a)Averages are listed when the coordination is considered equivalent; b)The two bonds are identical for both centers and 2,6Ac4DM is the dianion of 2,6-diacetylpyridine bis(N4-dimethylthiosemicarbazone); c)2,6Achexim is the dianion of 2,6-diacetylpyridine 3hexamethyleneiminylthiosemicarbazone.this zinc center}.[Zn(2,6Ac4DH)] 2 ⋅MeOH⋅H 2 O crystallizes from methanol in the triclinic P-1 space group with two molecules per unit cell and has a rather large R value of 0.070.The N(2)-Zn-N(2) angle for the "octahedral zinc(II)" is 169.5(4)º, which is larger than the analogous angle, 154.61(18)º, found for octahedral bis(2-pyridineformamide 3-piperidylthiosemicarbazonato)zinc(II).29 The tetrahedral zinc(II) center has S-Zn-S and N(2)-Zn-N(2) angles of 118.3(2) and 140.8(4)º, which are not very different from the "octahedral" center of [Zn(2,6Ac4DH)] 2 ⋅MeOH⋅2DMF.The other two angles that involve the N(2)-N(3)-C-S-Zn chelate rings, the N(2)-Zn-S angles, are 111.6(4) an 120.8(3)º.

Nickel(II) complexes
We include here the first example of a metal complex with the dianion of a 2,6-diformylpyridine bis{N(4)-dimethylthiosemicarbazone} ligand, [Ni(2,6Fo4DM)], Figure 5 (the numbers without parentheses used in this section are those of Figure 5 and Table 5).This complex was prepared by reacting equimolar amounts of nickel(II) acetate and 2,6-diformylpyridine N(4)-dimethylthiosemicarbazone in refluxing ethanol.The crystal, grown from a 1:1 by volume mixture of methanol and acetonitrile, was mounted on a glass fiber and used for data collection on a Nonius CCD automatic diffractometer, MoKa (l = 0.71073 Å).The structure was solved with direct methods and missing atoms were found by difference-Fourier synthesis.All nonhydrogen atoms were refined with anisotropic temperature factors and the hydrogens, except for the methyl hydrogens, were found on the difference map and refined isotropically.The methyl hydrogens were fixed at d = 0.96 Å, allowed to ride on the C atoms and assigned fixed isotropic temperature factor, U = 0.05 Å 2 .Refinement of the structures was made by full-matrix least-squares on F 2 .Scattering factors are from Wassmaire and Kirfel, 45 calculations were done by maXus, version 2.0 46 and graphics are Platon for Windows. 47rystallographic and refinement data are shown in Table 4 for [Ni(2,6Fo4DM].Figure 5 shows that the two moieties coordinate in a different fashion; one is the typical coordination of a heterocyclic thiosemicarbazonato ligand, N(imine) nitrogen and thiolato sulfur atoms along with the pyridine nitrogen atom.The other thiosemicarbazone moiety coordinates via its hydrazinic nitrogen to make the complex 4-coordinate with a 5-5-6 trichelate system, rather than the expected 5-5  2) has also been found for the aldehyde thiosemicarbazone moiety of a planar palladium(II) complex of 1-phenylglyoxal bis{N(4)-diethylthiosemicarbazone} to give a 5-6-4 trichelate system, 11 and we recently have solved the nickel(II) complex of the same ligand with an identical bonding pattern. 49

Conclusion
Coordination of heterocyclic bis(thiosemicarbazones) as neutral, monoanionic and dianionic ligands is possible depending on the metal center and the particular metal salt selected for complex preparation.To date, these ligands have been shown to coordinate as planar pentadentate and tetradentate ligands to one metal center or bridging ligands in binuclear complexes.Further structural studies of complexes of these ligands will likely expand the field with new modes of coordination.Substitution on the pyridine ring, as well as more complexes with substituted