**1. Introduction**

The synthesis and characterization of metal complexes of Schiff bases have been started since 1865. But the importance of Schiff base ligands in several fields compelled us to consider it as a "privileged ligand" even in recent days [1, 2]. Currently, there has been considerable interest in the chemistry of Schiff base metal complexes, primarily because of their tremendous biochemical activity, viz*.*, antibacterial [3, 4], antimalarial [5, 6], antiviral [7, 8], and antitumor activities [9–11]. Besides, some transition metal Schiff base complexes were found to be efficient catalysts in organic synthesis [12–18]. Such types of Schiff base metal complexes are also very interesting for opening of a new pathway in crystal engineering [19, 20]. Various kinds of supramolecules of diverse fascinating structures are being

**2.2 Di-imine Schiff base (**H2L2

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

**2.3 Thio-hydrazone Schiff base (**H2L<sup>3</sup>

diffraction) of its metal complexes [34–36].

*.*

*.*

*Scheme of formation of ligand H2L3 and zwitterionic heterocycle (L4*

4-carbathio-ate (abbreviated as **L4**

*Scheme of formation of ligand L1*

*Scheme of formation of ligand H2L2*

reported yet.

**Figure 2.**

**Figure 3.**

**Figure 4.**

**3**

base 3,30

dioxime (**H2L<sup>2</sup>**

**) ligand**

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

) (**Figure 3**). Yield is 65% [33].

The condensation reaction of 1,3-diaminopropane-2-ol (0.45 g, 5 mmol) (dapol) with diacetylmonoxime (1.01 g, 10 mmol) in 1:2 molar ratio in methanol (25 ml) under gentle reflux for 2 h yielded the tetradentate bicondensed di-imine Schiff


**) ligand**

The condensation reaction of diacetylmonoxime (1.01 g, 10 mmol) with morpholine N-thiohydrazide (mth) (1.6 g, 10 mmol) in 1:1 molar ratio in ethanol (30 ml) on refluxing for 2 h afforded a gummy mass with very low yield. The isolation of solid ligand in pure form with high yield is still a challenge. Considering the low yield of the ligand, all the complexation reactions with this ligand were carried out under in situ condition. However, the molecular thiol form of the ligand **H2L<sup>3</sup>** (**Figure 4**) was established from the structural analysis (single-crystal X-ray

A slight modification of synthetic procedure by continuing the refluxing procedure for 16 h with few drops of water in 1:1 molar ratio in ethanol (50 ml) (shown in the left part of **Figure 4**) yielded a light yellow crystalline solid compound (yield 45%). X-ray diffraction study of single crystal along with other analytical data of this compound inferred a zwitterionic structure of a nitrogen–sulfur heterocyclic compound, N-(3,4-dimethyl-1,2,5-thiadiazole-2-ium-2-yl)morphine-

) [25]. Metal complexes with **L<sup>4</sup>** have not been

*).*

**Figure 1.** *Structural drawing of the ligand systems.*

synthesized by different Schiff bases [20, 21]. Different types of weak force interactions (e.g., H-bonding, ππ, C–Hπ, etc.) are responsible for construction of such new metal organic frameworks (MOF) [22–24]. Multinuclear metal complexes of Schiff base are of real importance on the field of magnetochemistry [25–28]. Various ferro- and antiferro-type magnetic interactions are responsible for the generation of different magnetic materials. Besides, the solid-state properties, e.g., variable temperature conductivity, and optical properties of such complexes are also producing very interesting results which are extremely important in materials chemistry. Very recently the comparison of different observed physical properties with the theoretically predicted values is being done by the use of DFT approach [29–32].

In the above context, the development of new pathways for the synthesis of new Schiff base ligands and their metal complexes is of immense significance. The strategic pathway becomes more important when the Schiff base ligands and their corresponding metal complexes are produced in a controlled approach fulfilling the main objectives of the synthesis. We have chosen easily available, exceptionally economical, and full of exciting properties organic molecule, diacetylmonoxime, as our precursor molecule for the synthesis of many new Schiff base ligands by reacting it with different molecular amine systems. One of the main advantages of such Schiff base ligands is the change in their ligational behavior depending on the metallic systems and the stoichiometry.

In this review, we have selected only three Schiff base ligands (**Figure 1**) derived from diacetylmonoxime and three different amine systems. Though a huge number of metal complexes have been synthesized and characterized using such ligand systems, only the structure of the ligands and metal complexes for which single crystal or PXRD have been determined, are discussed in this mini-review including the weak force interactions depicted therein. Some of the solid-state properties, viz., electrical and optical properties of such complexes, are also discussed to enlighten their fascinating material properties.

#### **2. Synthesis of the ligands**

#### **2.1 Mono-imine Schiff base (L1 ) ligand**

The stoichiometrically controlled condensation reaction of diacetylmonoxime (dam) (1.01 g, 10 mmol) and diethylenetriamine (dien) (1.04 g, 10 mmol) in 1:1 molar ratio in methanol (15 ml) on constant stirring for 45 min at room temperature and then refluxing for 2 h on water bath (**Figure 2**) afforded the monocondensed amine-imine-oxime Schiff base ligand 3-((2-((2-aminoethyl)-amino)ethyl)imino) butan-2-one oxime (**L<sup>1</sup>** ). Yield is 55% [29].

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

#### **2.2 Di-imine Schiff base (**H2L2 **) ligand**

The condensation reaction of 1,3-diaminopropane-2-ol (0.45 g, 5 mmol) (dapol) with diacetylmonoxime (1.01 g, 10 mmol) in 1:2 molar ratio in methanol (25 ml) under gentle reflux for 2 h yielded the tetradentate bicondensed di-imine Schiff base 3,30 -((2-hydroxypropane-1,3-diyl)bis(azanylylidene))bis(butan-2-one) dioxime (**H2L<sup>2</sup>** ) (**Figure 3**). Yield is 65% [33].

#### **2.3 Thio-hydrazone Schiff base (**H2L<sup>3</sup> **) ligand**

The condensation reaction of diacetylmonoxime (1.01 g, 10 mmol) with morpholine N-thiohydrazide (mth) (1.6 g, 10 mmol) in 1:1 molar ratio in ethanol (30 ml) on refluxing for 2 h afforded a gummy mass with very low yield. The isolation of solid ligand in pure form with high yield is still a challenge. Considering the low yield of the ligand, all the complexation reactions with this ligand were carried out under in situ condition. However, the molecular thiol form of the ligand **H2L<sup>3</sup>** (**Figure 4**) was established from the structural analysis (single-crystal X-ray diffraction) of its metal complexes [34–36].

A slight modification of synthetic procedure by continuing the refluxing procedure for 16 h with few drops of water in 1:1 molar ratio in ethanol (50 ml) (shown in the left part of **Figure 4**) yielded a light yellow crystalline solid compound (yield 45%). X-ray diffraction study of single crystal along with other analytical data of this compound inferred a zwitterionic structure of a nitrogen–sulfur heterocyclic compound, N-(3,4-dimethyl-1,2,5-thiadiazole-2-ium-2-yl)morphine-4-carbathio-ate (abbreviated as **L4** ) [25]. Metal complexes with **L<sup>4</sup>** have not been reported yet.

**Figure 2.** *Scheme of formation of ligand L1 .*

synthesized by different Schiff bases [20, 21]. Different types of weak force interactions (e.g., H-bonding, ππ, C–Hπ, etc.) are responsible for construction of such new metal organic frameworks (MOF) [22–24]. Multinuclear metal complexes of Schiff base are of real importance on the field of magnetochemistry [25–28]. Various ferro- and antiferro-type magnetic interactions are responsible for the generation of different magnetic materials. Besides, the solid-state properties, e.g., variable temperature conductivity, and optical properties of such complexes are also producing very interesting results which are extremely important in materials chemistry. Very recently the comparison of different observed physical properties with the theoretically predicted values is being done by the use of DFT

In the above context, the development of new pathways for the synthesis of new

In this review, we have selected only three Schiff base ligands (**Figure 1**) derived from diacetylmonoxime and three different amine systems. Though a huge number of metal complexes have been synthesized and characterized using such ligand systems, only the structure of the ligands and metal complexes for which single crystal or PXRD have been determined, are discussed in this mini-review including the weak force interactions depicted therein. Some of the solid-state properties, viz., electrical and optical properties of such complexes, are also discussed to

**) ligand**

). Yield is 55% [29].

The stoichiometrically controlled condensation reaction of diacetylmonoxime (dam) (1.01 g, 10 mmol) and diethylenetriamine (dien) (1.04 g, 10 mmol) in 1:1 molar ratio in methanol (15 ml) on constant stirring for 45 min at room temperature and then refluxing for 2 h on water bath (**Figure 2**) afforded the monocondensed amine-imine-oxime Schiff base ligand 3-((2-((2-aminoethyl)-amino)ethyl)imino)

Schiff base ligands and their metal complexes is of immense significance. The strategic pathway becomes more important when the Schiff base ligands and their corresponding metal complexes are produced in a controlled approach fulfilling the main objectives of the synthesis. We have chosen easily available, exceptionally economical, and full of exciting properties organic molecule, diacetylmonoxime, as our precursor molecule for the synthesis of many new Schiff base ligands by reacting it with different molecular amine systems. One of the main advantages of such Schiff base ligands is the change in their ligational behavior depending on the

approach [29–32].

**Figure 1.**

*Structural drawing of the ligand systems.*

*Stability and Applications of Coordination Compounds*

metallic systems and the stoichiometry.

enlighten their fascinating material properties.

**2. Synthesis of the ligands**

butan-2-one oxime (**L<sup>1</sup>**

**2**

**2.1 Mono-imine Schiff base (L1**

**Figure 3.** *Scheme of formation of ligand H2L2*

*).*

**Figure 4.** *Scheme of formation of ligand H2L3 and zwitterionic heterocycle (L4*

*.*

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

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

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

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

**) ligand**

) was employed for the synthesis of a

)]. In this complex the ligand is

**) ligand**

) is very interesting, and the thiol

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

vanadium (**complex 2**), having composition [VO(L<sup>2</sup>

The thio-hydrazone Schiff base ligand (H2L<sup>3</sup>

specific choice of metal systems [34–38].

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

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

*Different modes of binding of H2L3 ligand.*

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

found to act as a dibasic N4 donor system [33].

The di-imine Schiff base ligand (H2L<sup>2</sup>

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

respectively [29].

**Figure 7.**

**Figure 8.**

**Figure 9.**

**5**

**Figure 6.** *Perspective view of the ligand L4 with atom number scheme (hydrogen atoms are omitted for clarity).*

The hydrogen atom attached with N atom of hydrazide group can undergo thione-thiol tautomerism (**Figure 5**). Thus NNS coordination mode is facilitated during the formation of complexes [34].
