*2.3.1 Crystal structure of L4*

The molecular structure of **L<sup>4</sup>** was established with the single-crystal X-ray diffraction studies. It was observed that the N–N, N–C, and C–S bond distances are shorter than the corresponding single bond distances indicating a partial double bond character due to resonance stabilization using π-bonded electrons. The morpholine moiety preferred chair conformation in the solid state. The distance between the nonbonded SS is 2.738 Å, and weak interactions (**Figure 6**) were observed between them in the solid state. A two dimensional supramolecular network is formed by the H-bonding and π- π interactions [34].

#### **3. Synthesis of metal complexes**

#### **3.1 Metal complexes with mono-imine Schiff base (L<sup>1</sup> ) ligand**

With this new neutral N4 donor ligand system, a crystalline Ni(II) complex has been synthesized. Two routes of synthesis of the nickel complex (**complex 1**) were reported (**Figure 7**). According to **Route 1**, the nickel complex was synthesized by in situ reaction of an equimolar mixture of diacetylmonoxime (1.01 g, 10 mmol) and diethylenetriamine (1.04 g, 10 mmol) and Ni(SCN)2.4H2O (2.47 g, 10 mmol) in methanol under reflux condition, which produces [Ni(L<sup>1</sup> )(SCN)2] (**complex 1**). In **Route 2**, **complex 1** was synthesized by following the same steps, but nickel chloride (NiCl2.6H2O) (2.37 g, 10 mmol) was used instead of Ni(SCN)2 and then by treating the product with NH4SCN (1.52 g, 20 mmol) in aqueous-methanol (1:1, 10 ml). Pure crystals (single) were obtained from the slow evaporation of the

*Crystal Structure and Solid-State Properties of Metal Complexes of the Schiff Base Ligands… DOI: http://dx.doi.org/10.5772/intechopen.90171*

mother liquor. Yields of the **complex 1** in the routes 1 and 2 are 85% and 82% respectively [29].

#### **3.2 Metal complex with di-imine Schiff base (H2L<sup>2</sup> ) ligand**

The di-imine Schiff base ligand (H2L<sup>2</sup> ) was employed for the synthesis of a oxovanadium complex, the PXRD of which have also been determined. Reflux of equimolecular mixture of **H2L<sup>2</sup>** (1.28 g, 5 mmol) and vanadyl acetate (0.93 g, 5 mmol) in methanol (30 ml) (**Figure 8**) afforded the greenish-gray complex of vanadium (**complex 2**), having composition [VO(L<sup>2</sup> )]. In this complex the ligand is found to act as a dibasic N4 donor system [33].

#### **3.3 Metal complexes with thio-hydrazone Schiff base (H2L<sup>3</sup> ) ligand**

The thio-hydrazone Schiff base ligand (H2L<sup>3</sup> ) is very interesting, and the thiol form of the Schiff base is always observed for the binding purposes (see **Figure 5**) with the metal systems during complexation. Most interestingly, this ligand is found to show two types of binding modes (**Figure 9**), one is observed through N,S donor atoms and another one through N,N,S donor atoms. An efficient control over the ligand for binding through either N,S or N,N,S mode has been achieved though specific choice of metal systems [34–38].

**Figure 7.** *Scheme of preparation of Ni(II) complexes.*

**Figure 8.** *Scheme of preparation of VO(IV) complex.*

**Figure 9.** *Different modes of binding of H2L3 ligand.*

The hydrogen atom attached with N atom of hydrazide group can undergo thione-thiol tautomerism (**Figure 5**). Thus NNS coordination mode is facilitated

*Perspective view of the ligand L4 with atom number scheme (hydrogen atoms are omitted for clarity).*

The molecular structure of **L<sup>4</sup>** was established with the single-crystal X-ray diffraction studies. It was observed that the N–N, N–C, and C–S bond distances are shorter than the corresponding single bond distances indicating a partial double bond character due to resonance stabilization using π-bonded electrons. The morpholine moiety preferred chair conformation in the solid state. The distance between the nonbonded SS is 2.738 Å, and weak interactions (**Figure 6**) were observed between them in the solid state. A two dimensional supramolecular net-

With this new neutral N4 donor ligand system, a crystalline Ni(II) complex has been synthesized. Two routes of synthesis of the nickel complex (**complex 1**) were reported (**Figure 7**). According to **Route 1**, the nickel complex was synthesized by in situ reaction of an equimolar mixture of diacetylmonoxime (1.01 g, 10 mmol) and diethylenetriamine (1.04 g, 10 mmol) and Ni(SCN)2.4H2O (2.47 g, 10 mmol) in

**Route 2**, **complex 1** was synthesized by following the same steps, but nickel chloride (NiCl2.6H2O) (2.37 g, 10 mmol) was used instead of Ni(SCN)2 and then by treating the product with NH4SCN (1.52 g, 20 mmol) in aqueous-methanol (1:1, 10 ml). Pure crystals (single) were obtained from the slow evaporation of the

**) ligand**

)(SCN)2] (**complex 1**). In

work is formed by the H-bonding and π- π interactions [34].

**3.1 Metal complexes with mono-imine Schiff base (L<sup>1</sup>**

methanol under reflux condition, which produces [Ni(L<sup>1</sup>

during the formation of complexes [34].

*Stability and Applications of Coordination Compounds*

**3. Synthesis of metal complexes**

*2.3.1 Crystal structure of L4*

**Figure 5.**

**Figure 6.**

**4**

*Thione-thiol tautomerism.*

An organometallic complex, [PhHg(HL<sup>3</sup> )] (**complex 3**), and the zinc(II) complex [Zn(HL<sup>3</sup> )(OAc)(H2O)] (**complex 4**) were prepared by the gentle reflux of the equimolar quantities of diacetylmonoxime (0.5 g, 5 mmol) and morpholine Nthiohydrazide (0.8 g, 5 mmol) (**Figure 10**) in the presence of the respective metal salts, [PhHg(OAc)] (1.68 g, 5 mmol) and [Zn(OAc)2].2H2O (1.1 g, 5 mmol). Yields of the complexes are 65% (**3**) and 75% (**4**). The pure single crystals of the complexes were obtained from their chloroform solutions [34, 35].

**4. Crystal structures, PXRD, and some interesting properties of the**

*Crystal Structure and Solid-State Properties of Metal Complexes of the Schiff Base Ligands…*

**4.1 Crystal structure and catalytic properties of nickel(II) complex with the**

**)**

octahedral geometry where central Ni(II) is coordinated by the neutral ligand **L<sup>1</sup>**

The coordination environment around the nickel(II) ion is surrounded by N6 fashion (four N from ligand and two N from thiocyanate ions) tending towards distorted octahedral geometry. The Ni2+ center is not lying exactly within the equatorial plane of N4 moiety, and unequal axial and equatorial bond distances (2.112 Å and 2.072 Å, respectively) confirm the distortion. The non-coordinated O–H groups on the ligand L<sup>1</sup> are engaged in H-bonding interactions with thiocyanate S atoms (**Figure 6**) which lead to 1D supramolecular sheet-like arrangement (**Figure 13**). These H-bonding interactions lead to OS separations of 3.132 Å

Analytical grade reagents and freshly distilled solvents, viz., water, acetonitrile,

methanol, and dichloromethane, were used to check the catalytic activity. The oxidation reaction was carried out in liquid phase under vigorous stirring in twonecked round bottom flask fitted with a water condenser and placed in an oil bath at 60°C. Substrate (5 mmol) was taken in 10 ml solvent(s) for different sets of reactions along with 5 mg catalyst, to which 10 mmol of *tert*-Butyl hydrogen peroxide

*Perspective view of complex 1 with atom numbering scheme (hydrogen atoms are omitted for clarity).*

*SO interaction in the crystal of complex 1 (hydrogen atoms are omitted for clarity).*

with tetradentate N4 binding mode and two N-bonded thiocyanate ions occupying the *cis-*position. **Figure 12** depicts the molecular structure of the

)(NCS)2] (**1**) has been found to possess

**metal complexes**

nickel (II) complex **1** [29].

**mono-imine Schiff base ligand (L<sup>1</sup>**

*DOI: http://dx.doi.org/10.5772/intechopen.90171*

and play prominent role in crystal packing.

*4.1.1 Catalytic activity of complex 1*

**Figure 12.**

**Figure 13.**

**7**

The neutral monomeric complex [Ni(L<sup>1</sup>

The Cd(II), Cr(III), and Fe(III) complexes, i.e., [Cd(HL<sup>3</sup> )2)] (**complex 5**), [Cr(HL<sup>3</sup> )2]Cl.3H2O (**complex 6**), and [Fe(HL<sup>3</sup> )2]Cl.3H2O (**complex 7**), were prepared by the gentle reflux of the mixture of diacetylmonoxime (0.5 g, 5 mmol) and morpholine N-thiohydrazide (0.8 g, 5 mmol) and the respective metal salts ([Cd(OAc)2].2H2O (0.67 g, 2.5 mmol), CrCl3.6H2O (0.67 g, 2.5 mmol), and FeCl3.6H2O (0.68 g, 2.5 mmol) in the ratio of 2:2:1 (**Figure 10**). Yields of the complexes had been recorded as 75% for each complex. The pure single crystals of the complexes were obtained from their chloroform solutions [35, 36].

Another zinc(II) complex (**complex 8**) was synthesized by the reflux of the mixture of diacetylmonoxime (0.5 g, 5 mmol) and morpholine N-thiohydrazide (0.8 g, 5 mmol) in the presence of Zn(OAc)2.2H2O (0.55 g, 2.5 mmol) in the molar ratio of 2:2:1 in water–methanol (1:1, v/v) mixture (**Figure 11**). The yellow colored complex of composition [Zn(HL3)2]. 2H2O had been separated with 60% yield and recrystallized from chloroform [37].

The complexes of Ni(II) (**complex 9**), Co(II) (**complex 10**), and Cu(II) (**complex 11**) were synthesized by gentle reflux of three sets of equimolar quantities of diacetylmonoxime (0.5 g, 5 mmol) and morpholine N-thiohydrazide (0.8 g, 5 mmol) in the presence of the metal salts (Ni(OAc)2.4H2O (1.24 g, 5 mmol), Co (OAc)2.4H2O (1.25 g, 5 mmol), and Cu(OAc)2.4H2O (1.28 g, 5 mmol), respectively) in ethanol (50 ml) (**Figure 11**) [29]. General molecular formula of the metal complexes **9–11** is [MII(HL3 )(OAc)] where M stands for Ni, Co, and Cu.

**Figure 10.** *Scheme of formation of metal complexes using H2L<sup>3</sup> ligand indicating different binding pattern.*

**Figure 11.** *Scheme of preparation of other metal complexes using H2L3 ligand.*

*Crystal Structure and Solid-State Properties of Metal Complexes of the Schiff Base Ligands… DOI: http://dx.doi.org/10.5772/intechopen.90171*
