**1. Introduction**

160 Current Trends in X-Ray Crystallography

Stylianou, M., Keramidas, A.D. & Drouza, C., (2010). PH-potentiometric investigation

Tanski, J.M., Vaid, T.P., Lobkovsky, E.B. & Wolczanski, P.T., (2000). Covalent metal-organic

Tanski, J.M. & Wolczanski, P.T., (2001). A covalent vanadium(III) 2-dimensional network

Ung, V.A., Bardwell, D.A., Jeffery, J.C., Maher, J.P., McCleverty, J.A., Ward, M.D. &

Ung, V.A., Couchman, S.M., Jeffery, J.C., McCleverty, J.A., Ward, M.D., Totti, F. &

Vaid, T.P., Lobkovsky, E.B. & Wolczanski, P.T., (1997). Covalent 3- and 2-dimensional

Vaid, T.P., Sydora, O.L., Douthwaite, R.E., Wolczanski, P.T. & Lobkovsky, E.B., (2001).

Vaid, T.P., Tanski, J.M., Pette, J.M., Lobkovsky, E.B. & Wolczanski, P.T., (1999). Covalent

Zhang, X.Q., Huang, F.P., Bian, H.D., Yu, Q. & Liang, H., (2009). Aqua N-(2,5-

*Structure Reports Online*, Vol.65, (Nov), pp. M1393-U1393, ISSN 1600-5368 Zharkouskaya, A., Buchholz, A. & Plass, W., (2005). A new coordination polymer

Vol.35, No.18, (Aug), pp. 5290-5299, ISSN 0020-1669

*Communications*, No.14, pp. 1300-1301, ISSN 1359-7345

No.14, (Jul), pp. 3394-3405, ISSN 0020-1669

1565-3633

4756-4765, ISSN 0020-1669

pp. 346-353, ISSN 0020-1669

369, ISSN 0020-1669

(Sep), pp. 8742-8743, ISSN 0002-7863

towards chelating tendencies of p -hydroquinone and phenol iminodiacetate copper(II) complexes. *Bioinorganic Chemistry and Applications*, Vol.2010, pp. ISSN

networks: Pyridines induce 2-dimensional oligomerization of (mu-OC6H4O)(2)Mpy(2) (M = Ti, V, Zr). *Inorganic Chemistry*, Vol.39, No.21, (Oct), pp.

and vanadyl chains linked by aryldioxides. *Inorganic Chemistry*, Vol.40, No.2, (Jan),

Williamson, A., (1996). Dinuclear oxomolybdenum(V) complexes showing strong interactions across diphenol bridging ligands: Syntheses, structures, electrochemical properties, and EPR spectroscopic properties. *Inorganic Chemistry*,

Gatteschi, D., (1999). Electrochemical and magnetic exchange interactions in trinuclear chain complexes containing Oxo-Mo(V) fragments as a function of the topology of the bridging ligand. *Inorganic Chemistry*, Vol.38, No.2, (Jan), pp. 365-

titanium-quinone networks. *Journal of the American Chemical Society*, Vol.119, No.37,

Hydrogen bonds between polyphenol (p-HOC6H4O)(6)W and bipyridines: (4,4 ' bipy center dot HOC6H4O)(6)W and 3-D networks {4,4 '- (nC(5)H(4))(2)(CH2CH2)}(n){(HOC6H4O)(6)W} (infinity) (n=2, 3). *Chemical* 

three-dimensional titanium(IV)-aryloxide networks. *Inorganic Chemistry*, Vol.38,

dihydroxybenzyl)iminodiacetato copper(II). *Acta Crystallographica Section E-*

architecture with (10,3)-a network containing chiral hydrophilic 3-D channels. *European Journal of Inorganic Chemistry*, No.24, (Dec), pp. 4875-4879, ISSN 1434-1948

Azomethines (known as Schiff bases), are perspective materials for wide spectrum of applications, particularly for anion sensor [1], antimicrobial agents [2-4] and nonlinear optical materials [5,6]. There has been considerable interest in some Schiff bases derived from salicylaldehyde because they show photochromism and thermochromism in the solid state [7]. The preparation of these compounds is simple and elegant. Since their discovery by Hugo Schiff in 1864 [8], they are prepared by condensing an active carbonyl compounds (ketone or aldehyde) with an amine, generally in refluxing alcohol [9-15]. Schiff bases are often used as ligands in inorganic chemistry [16-22].

In recent years, there has been a growing interest in the synthesis, characterization and crystal structures of copper(I) Schiff base complexes, not only because they have interesting properties and structural diversity [23-25] but also because they have found important application in catalysis for the coupling of phenylacetylene with halobenzene [26], preparation of supramolecular assemblies [27,28], the design of single and double-stranded architectures [29,30] and the grid complexes [31,32]. Then, Many efforts have been devoted to the design and synthesis of new Schiff base ligands that would be able to control the crystal structure of copper(I) complexes [33-40]. The purpose of this chapter is to present the current status of chemistry of copper(I) Schiff base complexes.

### **2. Schiff base ligands**

Schiff bases are functional groups that contain a carbon-nitrogen double bond (C=N) with the nitrogen atom connected to an aryl or alkyl group, but not hydrogen. They are of the general formula R1R2C=N-R3, where R3 is an aryl or alkyl group that makes the Schiff base a stable imine. Schiff base compounds can be synthesized from an amine and a carbonyl compound by nucleophilic addition, followed by a dehydration to generate an imine [9-15], and are broadly classified as bidnetate and bis-bidentate Schiff bases.

#### **2.1 Bidentate schiff-bases**

#### **2.1.1 Symmetric bidentate schiff bases**

The basic symmetric bidentate Schiff base ligands (Scheme 1) have different R1 and R2 substituents [41-62]. Schiff bases based on aldehydes have hydrogen atom as one of the

Structural Diversity on Copper(I) Schiff Base Complexes 163

L10 [49] L11 [50]

**R3 R3**

**R1 N N R1**

R1 = R2 = H R3 = CH3 L16 [57] R1 = R2 = R3 = Ph L17 [57]

> **N N R1 R1**

R1 = H L18 [58] R1 = NO2 L19 [59]

**R2 R3**

**R1**

**R2**

**R2 N N**

**R1**

 R1 = R2 = R3 = H L12 [51,52] R1 = R2 = H R3 = NO2 L13 [53] R1 = R3 = H R2 = Ph L14 [54,55] R2 = R3 = H R1 = CH3 L15 [56]

**N N**

**N N**

Scheme 4.

Scheme 5.

Scheme 6.

substituents (R1) at carbon atom of azomethine group, while second substituent R2 may be an alkyl, an aryl or a heterocyclic group. Schiff bases based on ketones have an alkyl, an aryl or a heterocyclic group in both the substituents at carbon atom of azomethine group, which may be same or different.

Scheme 1.

The symmetric bidentate Schiff base ligands have two arms connected via a ring, or C-C bond, as for example, shown in Schemes 2-8.

Scheme 2.

Scheme 3.

Scheme 4.

162 Current Trends in X-Ray Crystallography

substituents (R1) at carbon atom of azomethine group, while second substituent R2 may be an alkyl, an aryl or a heterocyclic group. Schiff bases based on ketones have an alkyl, an aryl or a heterocyclic group in both the substituents at carbon atom of azomethine group, which

The symmetric bidentate Schiff base ligands have two arms connected via a ring, or C-C

**R3 R4 R4 R3**

R1 = R2 = R4 = H R3 = CH3 L1 [41]

R1 = R3 = R4 = H R2 = CH3O L3 [43] R1 = R2 = R4 = H R3 = CH3O L4 [44] R2 = R3 = R4 = H R1 = Cl L5 [45] R1 = R2 = R3 = R4 = H L6 [46]

**N**

**R1**

**N**

**N**

R1 = H R2 = R3 = R4 = CH3O L2 [41,42]

**N**

**R2 R2** R1 = Ph R2 = H L7 [47] R1 = CH3 R2 = Cl L8 [48] R1 = CH3 R2 = NO2 L9 [48]

**R1**

**R1**

**R2**

**H**

bond, as for example, shown in Schemes 2-8.

**R2**

**R1**

**N**

**N**

**R2**

**H**

**R2**

**R1**

**R1**

may be same or different.

Scheme 1.

Scheme 2.

Scheme 3.

Scheme 5.

R1 = R2 = R3 = Ph L17 [57]

Scheme 6.

Structural Diversity on Copper(I) Schiff Base Complexes 165

The basic bis-bidentate Schiff base ligands have two arms connected via a ring, or C-C bond,

L31 [35,66] L32 [36]

**CH3**

L33 [29, 67] L34 [68]

**H**

**<sup>O</sup> <sup>O</sup>**

**N N N N**

**N N**

**H H**

**N N**

**N**

**H**

**N**

**Ph Ph**

**S S**

**N N**

as for example, shown in Schemes 10 – 14 [27-30,32,35-40,66-72].

**S S**

**N N**

**N N N N**

**N**

 R = H L35 [30, 38] L37 [67] R = CH3 L36 [39,69]

**N**

**N N**

**R R**

**N N**

**Ph Ph**

**2.2 Bis-bidentate schiff-bases** 

Scheme 10.

Scheme 11.

**H3C**

Scheme 8.

#### **2.1.2 Asymmetric bidentate schiff bases**

Asymmetric Schiff base ligands (Scheme 9) have been synthesized from an amine and pyridinecarboxaldehyde. They can be classified by different R substituent [6365].


Scheme 9.

#### **2.2 Bis-bidentate schiff-bases**

The basic bis-bidentate Schiff base ligands have two arms connected via a ring, or C-C bond, as for example, shown in Schemes 10 – 14 [27-30,32,35-40,66-72].

Scheme 10.

164 Current Trends in X-Ray Crystallography

**N N**

**R1 R1**

**R2 R2**

R1 = Ph , R2 = H L20 [60,61] R1 = H, R2 = NO2 L21 [62]

Asymmetric Schiff base ligands (Scheme 9) have been synthesized from an amine and

R L [Ref] R L [Ref] R L [Ref]

**H3C CH3**

L23 [63] **N O** L26 [64] L29 [65]

L24 [63] L27 [65] L30 [65]

pyridinecarboxaldehyde. They can be classified by different R substituent [6365].

**N N R**

L25 [63] L28 [65]

Scheme 7.

Scheme 8.

**H3C CH3**

**H3C**

Scheme 9.

**2.1.2 Asymmetric bidentate schiff bases** 

**CH3**

**CH3**

L22 [63]

Scheme 11.

Structural Diversity on Copper(I) Schiff Base Complexes 167

**N N**

**R**

**R**

**3. Bonding modes of schiff base ligands** 

**4. Copper(I) schiff base complexes** 

= halide (Cl-, Br-, I-), pseudohalide (NCS-

complexes with Schiff base ligands. Some of them are:

4. Precipitation and re-crystallization from a mixture of solvents

, BPh4-

be distorted tetrahedral.

3. Layering technique

Scheme 14.

anions (ClO4-

modes, I - VIII (Scheme 15).

**N N**

**R R**

R = H L47 [28,70,71] R = CH3 L48 [72] R = C2H5 L49 [27]

**N N N N**

L50 [40]

A number of bonding modes have been observed for the Schiff bases in their neutral form. The binding occurs via nitrogen atom of azomethine group in chelating and bridging

Copper(I) Schiff base complexes were generally prepared by the reaction of a CuX, where X

by room temperature stirring under an N2 or air atmosphere [41-62]. In these complexes, Schiff-bases act as chelating ligands and cause the geometry around the copper(I) atom will

Several different crystallization techniques have been used to grow crystals of copper(I)

1. Slow diffusion of Et2O into the concentrated solution of complex at room temperature 2. Very slow evaporation of the solvent at ambient temperatures for several days

) with a suitable Schiff base ligands in an acetonitrile solvent, followed

), or [Cu(CH3CN)4]Y (Y = the non-coordinating

, N3 -

Scheme 12.

Scheme 13 [32].

Scheme 14.

166 Current Trends in X-Ray Crystallography

**N Nh**

**N N H H**

L38 [37]

**R N N N N R**

R L R L

**H3C**

**F**

**F**

**Cl**

**Cl**

L43

L44

L45

L46

**H3C**

**OMe** L39 **CH3**

**OH** L40

**CH3** L41

L42

**CH3**

**H3C**

Scheme 12.

Scheme 13 [32].

**H**
