To study the substituent and solvent effect on solid state reaction, we have done the reaction of zinc(II) sulphate with sodium salts of 3-methylbenzoic acid or 2-methylbenzoic acids, followed by dissolution of the reaction mixturre in methanol or dimethylsulfoxide in the presence or absence of pyridine. A variety of complexes, as listed in Scheme 4.2, are obtained
terized by X-ray crystallography.
Thermogravimetric curve of the com
and each of the complexes is charac
ZnSO4. 7H2O
2) Pyridine, DMSO
1) 2-Methyl benzoic acid, NaOH, solid-state mixing 2) Pyridine,
Methanol
[Zn2(L2)4 dine)2]
[Zn2(L1)4(pyridine)2] [Zn4(L1)6(O)(DMSO)2] [Zn5(L2)6(L2)2(OH)2(pyridine)2]
L1= 2-Methyl benzoate
(pyri
1) 3-Methyl benzoic acid, NaOH, solid-state mixing
2) Pyridine, Methanol
2) Pyridine, DMSO
L2= 3-Methyl benzoate
4.3 4.5 4.4 4.6
Scheme 4.2
The reaction of zinc(II) sulphate heptahydrate with sodium salts of 3-methylbenzoic acid or 2- methylbenzoic acids in a methanolic solution containing pyridine gave dinuclear zinc complexes 4.3 and 4.4 having a paddle-wheel type structure, with two pyridine ligands at axial positions (Figure 4.4A and 4.4B). In each case there are four carboxylate ligands bridged between two zinc centers. On a careful look at the geometry of the complexes, it is observed that in the case of complex 4.3 the two pyridine rings lies in one plane, whereas in the case of 4.4 the two rings are perpendicular to each other. The orientation of the rings is attributed to the steric effect of the methyl groups on crystal packing. The complex 4.3 embles in the lattice through C19–H····O2 [dC19····O2 3.480 Å, <D-H····A 166.5°] and C–
truct a herringbone type
H····π [dC6····π 3.669 Å] interaction. It is also ported that non-symmetric carboxylato bridged complexes of zinc can be prepared with ass
H····π [dC18····π 3.680 Å, and dC20····π 3.603 Å] interactions and cons
hydrogen bonded network (Figure 4.5A). The packing pattern of 4.4 is shown in Figure 4.5B, which has a grid-like structure through C6–
re
pyridine as an ancillary ligand88.
A B
Figure 4.4 A) The crystal structure of complex 4.3, B) Structure of complex 4.4
ange 8.62 to 7.18 pm (Figure 4.9). The methyl protons appear at 2.45 ppm. In case of the complex 4.4 also, the methyl protons appear at 2.45 ppm and the protons of pyridine and benzoate group appear in the region 8.84-7.46 ppm (Figure 4.10).
The FT-IR spectra of the complexes 4.3 and 4.4 show absorption band at 1568 and 1583 cm-1 due to asymmetric C=O stretching of bridging carboxylate groups. The symmetric C=O stretching band appears at 1403 cm-1 in both the complexes. 1H-NMR spectra of the complex 4.3 shows that the protons of pyridine and benzoate groups appear in the r
p
A B
ethanol paddle-wheel pe structures are formed, whereas, similar reactions in DMSO lead to high nuclearity rboxylate clusters. It is clear that methanol has no role in the formation of these dinuclear the nuclearity.
remaining zinc centers fulfil the octahedral geometry by three bridging carboxylate, one oxy ligand and two dimethylsulfoxide ligands. The complex 4.5 has two symmetry non-equivalent molecules per Figure 4.5 A) Solid state assembly of the complex 4.3 through weak C–H····O and C–H····π interaction, B) Grid like packing structure of the complex 4.4
The reaction of zinc(II) sulphate heptahydrate with sodium salts of 3-methylbenzoic acid and 2-methylbenzoic acid in dimethylsulfoxide gave tetranuclear and pentanuclear zinc complexes 4.5 and 4.6, respectively. It should be noted here that, in m
ty ca
structures while DMSO being a coordinating solvent leads to an expansion of
Complex 4.5 is a tetranuclear complex having one oxy-ligand shared by four zinc centers (Figure 4.6A). The oxy anion acts as a central point in a tetrahedral geometry formed by four zinc ions and one oxide anion. Among the four zinc centers three zinc centers are equivalent with tetrahedral geometry as shown in Figure 4.6B. These three zinc centers are anchored by three carboxylate bridges and are attached to the central oxygen. The
unit cell. This is a rare example of a polynuclear complex having symmetry non-equivalent molecules in the unit cell.
benzoate rings. The important bond distances and angles of this complex are listed in Table 17 (Appendix).
Table 4.1: Hydrogen bond geometry(Å, °) in 4.5
D−H···A d(D−H) d(H···A) d(D···A) <D−H···A
A B
Figure 4.6 A) Crystal structure of complex 4.5, B) Structure of oxo centered tetranuclear zinc core
The two symmetry non equivalent units are assembled through weak C29–H····O4 [dC29···O4
3.569 Å, <D–H···A 154.9°], C65–H····O25 [dC65···O25 3.295 Å, <D–H···A 142.3°], C89–
H····O6 [dC89···O6 3.427 Å, <D–H···A 156.4°] and C103–H····O26 [dC103···O26 3.642 Å, <D–
H···A 165.8°] interactions (Table 4.1). Apart from this, the hydrogens of benzoate goups and dimethylsulphoxide ligands are involved in weak C50–H····π [dC50···π 3.597 Å] and C44–H····π [dC50···π 3.440 Å] interactions with
C(29)–H(29)····O(4) 0.93 2.71 3.569 154.9
C(65)–H(65)····O(25) 0.93 2.51 3.295 142.3
C(89)–H(89)····O(6) 0.93 2.55 3.427 156.4
C(103)–H(103)····O(26) 0.96 2.70 3.642 165.8
The complex 4.5 is similar to the tetranuc ic zinc core reported301 by Straughan rdinated to solvent
molecu motif
lear symmetr et
al., but with the difference that, in our case, one of the zinc centers is coo
le making the complex less symmetric. Thus, our cluster possesses a new
ear species.
The crystal structure of the complex 4.6 shows that, it is a pentanuclear zinc carboxylate complex in which the zinc centers are interconnected by six bridging benzoate groups and two hydoxo ligands. The Complex 4.6 has a symmetric structure and a mirror plane bisects the two halves containing two tetrahedral units anchored by hexa-coordinated zinc center. The crystal structure is shown in Figure 4.7A.
coordinated by two carboxylate oxygens, one hydroxyl group and a pyridine molecule. Other containing four coordinated zinc centers along with six coordinated zinc centers. Recently, a tetranuclear oxo-cluster of zinc has been reported and it is observed that molecular oxygen is responsible for the formation of such clusters from organo-zinc reactions302. In our reactions the oxo or hydroxo species are generated from water molecules in the solvent and hydrated salts. Dimethylsulphoxide being an aprotic coordinating solvent allows the deprotonation of water in basic medium whereas in methanol the labile hydrogen is in a large excess, thus facilitates the formation of neutral dinucl
A B
Figure 4.7 A) Crystal structure of pentanuclear zinc complex 4.6, B) Structure of pentanuclear zinc core
The complex has two hydroxyl ligands which are bridged between the three zinc centers. The hydroxyl groups are intramolecularly hydrogen bonded with two free carboxyl groups of the monodentate carboxylate through O9–H····O2 [dO9····O2 2.740 Å, <D−H····A 171.5°]
interaction. The peripheral four zinc centers have tetrahedral geometry and the central zinc center have an octahedral geometry (Figure 4.7B). For the central zinc four of the coordination sites are occupied by four carboxylate oxygen and the other two sites are occupied by two hydroxyl groups. Among the four peripheral zinc centers two of them are
ex 4.6 self-assembles in the solid state through weak C12–H····π [dC12····π 3.727 Å], C24–H····π [dC24····π 3.730 Å] and C36–H····π [dC36····π 3.666 Å] interactions.
Some important bond distances and angles of complex 4.6 are listed in Table 18 (Appendix).
Similar type of pentanuclear zinc(II) benzoate complex with 2,6-dimethyl pyrazine is reported by Kim and coworkers which is an effective catalyst for transesterification reactions303.
The monodentate carboxylate of 4.6 shows IR absorption at 1623cm-1 whereas the bridging carboxylate of the complex has νaCOO at 1578cm-1 and νs COO stretchingappears at 1428cm-1. The C-O stretching of monodentate carboxylate group appears at 1404 cm-1. On the other hand, the complex 4.5 has all bridging carboxylate ligands and IR absorptions at 1566 cm-1 (νs COO) and 1437cm-1 (νa COO) are observed. The S=O stretching of coordinated DMSO appears at 1004cm-1 (Figure 4.11). The 1H-NMR spectra of the complex 4.5 shows the benzoate protons in the region 7.15 to 7.75 ppm and the methyl protons appears at 2.52 ppm.